Rearrangement/Fragmentation Reactions of Oligosilanes with Aluminum Chloride.

Reinvestigation of the Lewis acid catalyzed rearrangement of some open-chain permethyloligosilanes with the Al(Fe)Cl(3) catalyst system exhibited several cases of additional reactivity: namely, a fragmentation/cyclization reaction. Introduction of (trimethylsilyl)methyl substituents into the oligosilane substrates strongly facilitated this reaction, yielding cyclic or bicyclic carbacyclosilanes. Investigations concerning the composition of the catalyst system indicated that the incorporation of about 0.1% FeCl(3) into the AlCl(3) lattice provided an effective catalyst.

Introduction

Over the last few years our group has been strongly involved in the study of the chemistry of oligosilanes.[1,2] In particular, the development of a convenient synthetic access to structurally diverse oligosilanyl anions[3−6] allowed the preparation of a huge variety of previously unknown small polysilanes.[1,2]

Some 40 years ago Ishikawa and Kumada discovered the AlCl3-catalyzed rearrangement of linear methylated oligosilanes to branched isomers.[7,8] This chemistry can be considered to be complementary to our oligosilanyl anion chemistry, as it produces the strongly branched oligosilanes[9,10] that are required as precursors for effectively stabilized oligosilanyl anions.

We became aware of this potential during the synthesis of an all-silicon analogue of the adamantane structure.(11) In a subsequent study we investigated AlCl3-catalyzed rearrangement reactions of already branched cyclosilanes.(12) One of the main conclusions we could draw from this work was that these rearrangement reactions always lead to substituted cyclopentasilanes.(12)

More recently, we showed that the introduction of trimethylgermyl groups into oligosilanes followed by AlCl3-catalyzed rearrangement exclusively led to the formation of isomers with germanium atoms possessing the highest possible silylation degree.(13) In the course of this study a computational investigation of the reaction mechanism demonstrated its similarity to the well-known Wagner–Meerwein reaction: starting with the abstraction of a methyl group from the oligosilane, a silylium ion forms, which undergoes a series of 1,2-methyl and -silyl shifts until the most stable ion is obtained. Eventually remethylation of the ion concludes the rearrangement process.(13)

Results and Discussion

The rearrangement processes of methylated oligosilanes can conveniently be monitored by NMR spectroscopy. 1H NMR gives a good impression of the degree of conversion and in addition can be used to judge the molecular symmetry of the formed products from the number and integration of methyl resonances. 29Si NMR spectroscopy provides even more insight. Not only is the direct methylation degree of silicon atoms reflected by their chemical shift but also to a lesser extent some information about the neighboring silicon atoms can be obtained. In general the rearrangement process leads to highly branched structures, and by analysis of the corresponding 29Si NMR spectra we can distinguish between nonmethylated (δ(SiSi4): ca. −110 to −140 ppm), monomethylated (δ(MeSiSi3): ca. −70 to −100 ppm), dimethylated (δ(Me2SiSi2): ca. −25 to −60 ppm), and trimethylated silicon atoms (δ(Me3SiSi): ca. −5 to −20 ppm). In addition, tetraalkylated silicon atoms resonate close to a chemical shift value of 0 ppm. A high degree of branching of a neighboring silicon atom usually results in an additional downfield shift. How general this assignment scheme is can be estimated by studying Schemes 5, 6, and 8–13 of this report. 29Si NMR shifts of the involved oligosilanes are shown with the compounds. In addition, a short discussion of the chemical shift behavior of silicon atoms connected to methylene groups is given after the synthetic part.

Scheme 5

Rearrangement Reactions of (Trimethylsilyl)methyl-Substituted Tri-, Tetra-, and Pentasilanes

29Si NMR chemical shifts (ppm) of substrates and rearrangement products have been added to the structures.

Scheme 6

Cyclization Reactions of (Trimethylsilyl)methyl-Substituted Hexa-, Hepta-, and Octasilanes

29Si NMR chemical shifts (ppm) of substrates and cyclized products have been added to the structures.

Scheme 8

Cyclization Reaction of a (Trimethylsilyl)methyl-Substituted Branched Nonasilane To Give an Intermediate Monocyclic Molecule and Further To Give a Bicyclic Molecule

29Si NMR chemical shifts (ppm) of substrates and cyclized products have been added to the structures.

Scheme 13

Cyclization Reaction of a Tris[(trimethylsilyl)methyl]-Substituted (Trimethylsilyl)cyclohexasilane To Give a Bicyclic Molecule with Two Methylene Bridges

29Si NMR chemical shifts (ppm) of substrates and cyclized products have been added to the structures.

Although much of the relevant analytical data required to describe the course of reaction can be obtained from NMR spectroscopy, reactions can also be followed by GC/MS analysis. While MS fragmentation patterns of rearranged oligosilanes are usually indistinguishable from the starting material, the retention behavior often changes and the method is suitable to detect fragmentation/cyclization reactions. Isolated products are finally subjected to standard characterization techniques such as NMR and UV spectroscopy, elemental analysis or HRMS, and (if possible) X-ray single-crystal diffraction analysis.

For our studies concerning the rearrangement of branched cyclosilanes(12) we used a cosublimate of AlCl3 and FeCl3 as described by Blinka and West(14) as the catalyst. Attempts to use plain AlCl3 proved to give less reproducible results in comparison to those for the cosublimate. However, the fact that in the original papers by Ishikawa, Kumada, and co-workers[7−10] only the use of AlCl3 was reported prompted us to study the rearrangement of some materials isomeric with their noncyclic oligosilane substrates with the Al(Fe)Cl3 cosublimate catalyst.

Acyclic Methylated Oligosilanes

In the course of our studies we found that in general Al(Fe)Cl3-catalyzed reactions of branched oligosilanes which are isomeric with materials already studied by the Kumada group[7−10] led to the same results. However, a few reactions showed an additional twist.

For example, the AlCl3-mediated rearrangement of n-eicosamethylnonasilane was reported to yield 2,2,5-tris(trimethylsilyl)undecamethylhexasilane (1) and 2,2,4,4-tetrakis(trimethylsilyl)octamethylpentasilane in a ratio of 4:1 in high yield.(9) Explicitly it was noted that no other isomers were detected by spectroscopic analysis.(9) In contrast to that, the rearrangement of 2,2,5-tris(trimethylsilyl)undecamethylhexasilane (1) with Al(Fe)Cl3 led to the almost quantitative formation of 1,1,3-tris(trimethylsilyl)heptamethylcyclopentasilane (2)[10,12] and tetramethylsilane (Scheme 1).

Scheme 1

Cyclization Reaction of a Branched Methylated Nonasilane To Give 1,1,3-Tris(trimethylsilyl)heptamethylcyclopentasilane

Similarly Ishikawa et al. reported that the rearrangement of n-octadecamethyloctasilane gave a mixture of 2,2,4-tris(trimethylsilyl)nonamethylpentasilane (4) and 2,2,3,3-tetrakis(trimethylsilyl)hexamethyltetrasilane (5) in a ratio of 81:19.(9) Our attempts to rearrange the isomeric compound 2,5-bis(trimethylsilyl)dodecamethylhexasilane (3) with Al(Fe)Cl3 revealed initial formation of the two aforementioned compounds 4 and 5 in addition to 2,2-bis(trimethylsilyl)dodecamethylhexasilane (6) and 1,1-bis(trimethylsilyl)octamethylcyclopentasilane (7).[10,12] However, after a prolonged reaction time the cyclic compound 7 and tetramethylsilane became the major reaction products (Scheme 2).

Scheme 2

Rearrangement/Cyclization Reaction of a Branched Methylated Octasilane To Give Branched Isomers and Further To Give 1,1-Bis(trimethylsilyl)octamethylcyclopentasilane

A related result was obtained when 2,2,9,9-tetrakis(trimethylsilyl)octadecamethyldecasilane (8) was subjected to the rearrangement conditions with Al(Fe)Cl3. After initial formation of 2,2,5,8,8-pentakis(trimethylsilyl)pentadecamethylnonasilane (9) again the occurrence of 1,1,3-tris(trimethylsilyl)pentamethylcyclopentasilane (2) was observed, this time accompanied by 2,2-bis(trimethylsilyl)octamethyltetrasilane(9) (10) as an elimination product (Scheme 3).

Scheme 3

Cyclization/Fragmentation Reaction of a Branched Methylated Tetradecasilane To Give 1,1,3-Tris(trimethylsilyl)Heptamethylcyclopentasilane and 2,2-Bis(trimethylsilyl)octamethyltetrasilane

As this desilylative cyclization behavior was never mentioned in the Ishikawa/Kumada group reports,[7−10] we attribute it to the enhanced reactivity of Al(Fe)Cl3 compared to that of commercially available AlCl3. Mechanistically the reaction can be interpreted as a migration/fragmentation reaction. We assume that the cyclization occurs via a 1,5-silyl shift. In the course of this shift the cationic charge is not retained in the molecule, as in the usual rearrangement reaction, but is eliminated as a trimethylsilylium ion which upon methylation with [MeAlCl3]− is converted to tetramethylsilane. There seems to be a steric component of this reaction, as it was not possible to cyclize 2,2,5,5-tetrakis(trimethylsilyl)decamethylhexasilane, which is likely sterically too hindered at the 5-position to undergo a nucleophilic attack at the silylium ion (Scheme 4).

Scheme 4

Mechanistic Rationale for the Cyclization Behavior Involving a 1,5-Silyl Migration

Acyclic Methylated Oligosilanes Containing One Me3SiCH2 Group

In a recent study we investigated the conformational behavior of acyclic methylated oligosilanes containing (trimethylsilyl)methyl groups.(15) These compounds provided us with the opportunity to study their rearrangement behavior. Initial expectations to obtain homogeneous product distributions were not too high, as it was demonstrated by Blinka and West that the introduction of an ethyl group into an otherwise methylated oligosilane led to a scrambling of ethyl groups upon rearrangement to give non-, mono-, di-, tri-, and higher ethylated products.(14) We found the same behavior after introducing single phenyl groups into methylated oligosilanes. However, after preliminary results showed a clean rearrangement course, it was decided to study the reactions of a series of branched compounds of the general composition Me3SiCH2(SiMe2)SiMe3, with n ranging from 2 to 8.

The series starts with the linear trisilane 11, where the (trimethylsilyl)methyl substituent is located in the 2-position. The rearrangement involved a 1,2-shift of this group to a terminal silicon atom, thus minimizing steric interaction and affording 1-[(trimethylsilyl)methyl]heptamethyltrisilane (12) as the product (Scheme 5). The rearrangement of the next higher homologue, 2-(trimethylsilyl)-2-[(trimethylsilyl)methyl]hexamethyltrisilane (13), proceeded to 2-(trimethylsilyl)-1-[(trimethylsilyl)methyl)]hexamethyltrisilane (14) (Scheme 5). Starting material 13 and product 14 both contain isotetrasilane structures. Again a 1,2-shift of the (trimethylsilyl)methyl group moved the more bulky substituent from the branching point of the silane to a terminal position. Steric interaction at the branching point is thus diminished. In a similar way the next member of the series, 2-(trimethylsilyl)-2-[(trimethylsilyl)methyl]octamethyltetrasilane (15), was converted to 2,2-bis(trimethylsilyl)-3-[(trimethylsilyl)methyl]pentamethyltrisilane (16) (Scheme 5). The driving force behind this transformation is the typical branching mechanism which favors the formation of neopentasilanyl units such as in 16.

All reactions depicted in Scheme 5 show the expected rearrangement behavior, featuring a series of 1,2-shifts to the most stable silylium ion, yielding the most stable branched oligosilane isomers.

The next higher homologues 17–19 possess enough formal dimethylsilylene units to exhibit a cyclization reaction behavior (Scheme 6) similar to what was already outlined above for compounds 1, 3, and 8. It is interesting to see that in compounds 21 and 22 the branching points are located in 1,2-relationships. While a 1,3-disilyl substitution pattern would be sterically less hindered, it would also be associated with a lower silylation degree at the branching points.

For cyclooligosilane rearrangement reactions a general trend favoring the formation of five-membered rings was observed.(12) This trend was also found for the reactions of 17–19 (Scheme 6). In all cases carbacyclopentasilanes were formed. The incorporated methylene group was always connected to dimethylsilylene units and was never attached to a branching point of the oligosilane backbone (Scheme 6).

It seems that the first step of these cyclization processes is a rearrangement similar to what was observed for 11, 13, and 15. The preferentially formed acyclic intermediate would feature the (trimethylsilyl)methyl group at a terminal position. Such a structure can then undergo a cyclization reaction in a way similar to that pointed out above. The feasibility of this can be judged from the fact that in none of the rearrangement reactions of 17–19 could any intermediate be detected by NMR spectroscopy, indicating that once the reaction started, it would invariably lead to the desilylative cyclization. Scheme 7 depicts the assumed cyclization path for compound 17.

Scheme 7

Mechanistic Rationale for the Cyclization Behavior of a (Trimethylsilyl)methyl-Substituted Hexasilane Involving a 1,5-Methylene Shift

The next higher homologue, 23, was expected to follow a similar path. Spectroscopic evidence suggested initially a related cyclization. Supposedly a carbacyclohexasilane was formed to avoid attachment of the methylene group to a disilylated silicon atom. In addition to this product the formation of another compound was detected right from the beginning of the reaction, which eventually became the only product. After isolation, structural and spectroscopic characterization this product was found to be 3-carba-1-(trimethylsilyl)undecamethylbicyclo[3.2.1]nonasilane (25), which was formed by elimination of another 1 equiv of tetramethylsilane (Scheme 8). It was surprising to find 25 as the final product, as we previously recognized the formation of tetrasilylated silicon atoms to be a major driving force. Whereas compound 24 possesses two of these structural elements, only one of them is found in the structure of 25. An explanation for this apparent problem would be that, while the outcome of most of the rearrangements is governed by the thermodynamic stability of the formed cations, the elimination of tetramethylsilane would introduce a nonreversible kinetic component to the reaction.

Acyclic Methylated Oligosilanes with Two Me3SiCH2 Groups

After the investigation of the rearrangement behavior of oligosilanes containing one (trimethylsilyl)methyl group a second series of substrates containing two of these substituents was investigated. Rearrangement of the first two compounds of this series, (Me3Si)2Si(CH2SiMe3)2 (26) and [(Me3Si)2Si(CH2SiMe3)]2 (27), did not give clean reactions. In contrast to this, the next higher homologue, 28, gave a single product. Elimination of 2 equiv of tetramethylsilane led to the bicyclic compound 3,7-dicarba-1,5-bis(trimethylsilyl)octamethylbicyclo[3.3.0]octasilane (29) (Scheme 9).

Scheme 9

Cyclization Reaction of a Bis[(trimethylsilyl)methyl]-Substituted Branched Octasilane To Give a Bicyclic Molecule

29Si NMR chemical shifts (ppm) of substrates and cyclized products have been added to the structures.

Increasing the number of dimethylsilylene units by one unit led to a reaction which was less clean but still formed one major product that could be isolated and characterized. The structure of the resulting compound is related to 29 with the additional SiMe2 unit inserted into the central Si–Si bond, thus representing 3,7-dicarba-1,5-bis(trimethylsilyl)decamethylbicyclo[3.3.1]nonasilane (31) (Scheme 10).

Scheme 10

Cyclization Reaction of a Bis[(trimethylsilyl)methyl]-Substituted Branched Nonasilane To Give a Bicyclic Molecule

29Si NMR chemical shifts (ppm) of substrates and cyclized products have been added to the structures.

Rearrangement of 2,9-bis(trimethylsilyl)-2,9-bis[(trimethylsilyl)methyl]octadecamethyldecasilane (32), where in comparison to 30 the number of dimethylsilylene units was increased by another three units, was also not clean.

Cyclic Methylated Oligosilanes Containing Me3SiCH2 Substituents

Considering the enhanced cyclization potential of compounds containing (trimethylsilyl)methyl groups, we assumed that cyclic oligosilanes with this substituent should exhibit a pronounced tendency to form bicyclic compounds under rearrangement conditions. The reaction of the easily available cyclohexasilane 33 confirmed this assumption, and 3-carba-1,5-bis(trimethylsilyl)decamethylbicyclo[3.2.1]octasilane (34) was obtained (Scheme 11).

Scheme 11

Cyclization Reaction of a (Trimethylsilyl)methyl-Substituted Tris(trimethylsilyl)cyclohexasilane To Give a Bicyclic Molecule

29Si NMR chemical shifts (ppm) of substrates and cyclized products have been added to the structures.

Also, the rearrangement of a mixture of cis- and trans-35 followed a selective pathway leading to the already known bicyclic compound 31 (see above) (Scheme 12).

Scheme 12

Cyclization Reaction of an Isomeric Mixture of 1,4-Bis[(trimethylsilyl)methyl]-Substituted 1,4-Bis(trimethylsilyl)cyclohexasilanes To Give a Bicyclic Molecule with Two Methylene Bridges

29Si NMR chemical shifts (ppm) of substrates and cyclized products have been added to the structures.

The rearrangement of 36, which contains three (trimethylsilyl)methyl substituents, led cleanly to the formation of 29 (Scheme 13). This was unexpected, for it was assumed that the product would contain three methylene units corresponding to the three (trimethylsilyl)methyl groups of the starting material. However, different from most of the reactions described so far, the elimination of tetramethylsilane was not observed but rather that of bis(trimethylsilyl)methane. The course of reaction is therefore similar to what was observed with compound 8, where also a more complex silylium ion was eliminated.

Finally, the reactions of 1,1,4,4-tetrakis[(trimethylsilyl)methyl]octamethylcyclohexasilane (37) and the bicyclic compound 1-(trimethylsilyl)-4-[(trimethylsilyl)methyl]dodecamethylbicyclo[2.2.2]octasilane (38) did not give clean conversions to single products.

X-ray Crystallography

The cyclohexasilanes 33, 35-, and 37 as well as the bicyclic compounds 25, 29, 31, 34, and 38 could be subjected to crystal structure analyses.

The molecular structure of 33 (Figure 1), which crystallizes in the monoclinic space group C2/c, resembles that obtained for 1,1,4,4-tetrakis(trimethylsilyl)octamethylcyclohexasilane.(16) Both structures are rare cases of six-membered rings in a stable twist conformation. Both 35- (Figure 2) and 37 (Figure 3) crystallize in the monoclinic space group P21/c, and their six-membered rings show the expected chair conformation. Si–Si bond distances in all three cyclohexasilanes are in the normal range between 2.35 and 2.37 Å, and the same is true for the Si–C distances, except for that between the ring silicon atoms and the attached CH2 groups in 33 and 35-. These distances are somewhat elongated to a value of 1.91 Å, whereas they are around 1.89 Å for 37. The two CH2–SiMe3 groups of 35- do not adopt the expected equatorial but rather axial positions, as was reported for similar compounds in the literature.(17)

Figure 1

Molecular structure and numbering of 33. Selected bond lengths (Å) and bond angles (deg) with esd's: Si(1)–C(12) = 1.913(2), Si(1)–Si(7) = 2.3572(9), Si(1)–Si(6) = 2.3640(10), Si(1)–Si(2) = 2.3683(9), Si(2)–Si(3) = 2.3443(10), Si(8)–C(12) = 1.877(2); Si(6)–Si(1)–Si(2) = 110.83(3), Si(3)–Si(2)–Si(1) = 111.29(3), Si(2)–Si(3)–Si(4) = 112.29(3), Si(3)–Si(4)–Si(5) = 110.95(3), Si(6)–Si(5)–Si(4) = 112.68(3), Si(5)–Si(6)–Si(1) = 112.94(3).

Figure 2

Molecular structure and numbering of 35-. Selected bond lengths (Å) and bond angles (deg) with esd's: Si(1)–Si(2) = 2.3484(18), Si(1)–Si(3) = 2.3658(13), Si(2)–Si(3A) = 2.3541(15), Si(3)–Si(2A) = 2.3541(15), Si(5)–C(1) = 1.860(3); Si(2)–Si(1)–Si(3) = 107.03(6), Si(1)–Si(2)–Si(3A) = 111.58(6), Si(2A)–Si(3)–Si(1) = 111.45(5).

Figure 3

Molecular structure and numbering of 37. Selected bond lengths (Å) and bond angles (deg) with esd's: Si(1)–C(2) = 1.889(3), Si(1)–C(1) = 1.897(3), Si(1)–Si(3A) = 2.3570(11), Si(1)–Si(2) = 2.3592(12), Si(2)–Si(3) = 2.3495(11), Si(3)–Si(1A) = 2.3571(11); Si(3A)–Si(1)–Si(2) = 105.83(4), Si(3)–Si(2)–Si(1) = 119.18(4), Si(2)–Si(3)–Si(1A) = 109.49(5).

Compound 38 (Figure 4) crystallizes in the monoclinic space group P21/c, whereas the parent compound without the CH2 spacer was found to crystallize in a tetragonal space group due to the high degree of symmetry.(18) The molecule consists of a regular [2.2.2]cyclooctasilane framework with all Si–Si bond lengths within a range of 2.34–2.36 Å and all Si–Si–Si bond angles close to the ideal tetrahedral angle (106–113°). The distance between the silicon atoms and the CH2 spacer is again at 1.91 Å longer than all the other Si–C distances (1.89 Å).

Figure 4

Molecular structure and numbering of 38. Selected bond lengths (Å) and bond angles (deg) with esd's: Si(1)–C(16) = 1.909(3), Si(1)–Si(7) = 2.3537(10), Si(1)–Si(6) = 2.3583(11), Si(1)–Si(2) = 2.3599(10), Si(2)–Si(3) = 2.3767(10), Si(3)–Si(4) = 2.3373(10), Si(4)–Si(5) = 2.3367(10), Si(4)–Si(8) = 2.3422(9), Si(10)–C(16) = 1.855(3); Si(7)–Si(1)–Si(6) = 107.89(4), Si(7)–Si(1)–Si(2) = 106.26(3), Si(6)–Si(1)–Si(2) = 106.75(3), Si(1)–Si(2)–Si(3) = 108.17(4), Si(4)–Si(3)–Si(2) = 111.87(4), Si(5)–Si(4)–Si(3) = 106.81(4), Si(5)–Si(4)–Si(8) = 108.03(4), Si(3)–Si(4)–Si(8) = 107.59(4), Si(4)–Si(5)–Si(6) = 107.12(4), Si(5)–Si(6)–Si(1) = 113.13(3), Si(1)–Si(7)–Si(8) = 108.45(4), Si(4)–Si(8)–Si(7) = 110.81(4).

The [3.2.1]-3-carba-cyclooctasilane compounds 25 (Figure 5) and 34 (Figure 6) both crystallize in the triclinic space group P1̅. For both structures no elongation of the bonds between the Si atoms and the CH2 spacer was observed, but in the case of 34 the exo-methyl groups next to the spacer are somewhat longer (Si(6)–C(7) and Si(7)–C(10) both 1.90 Å). In contrast to this, Si–Si bond lengths are all fairly short, in the ranges of 2.34–2.37 Å for 34 and 2.34–2.35 Å for 25.

Figure 5

Molecular structure and numbering of 25. Selected bond lengths (Å0 and bond angles (deg) with esd's: Si(1)–Si(5) = 2.344(3), Si(1)–Si(7) = 2.346(3), Si(1)–Si(2) = 2.354(3), Si(2)–Si(3) = 2.356(3), Si(3)–Si(4) = 2.355(3), Si(4)–Si(5) = 2.336(3), Si(4)–Si(6) = 2.346(3), Si(6)–C(10) = 1.865(6), Si(7)–C(10) = 1.892(6); Si(5)–Si(1)–Si(7) = 107.91(10), Si(5)–Si(1)–Si(2) = 101.05(10), Si(7)–Si(1)–Si(2) = 108.35(10), Si(1)–Si(2)–Si(3) = 103.96(10), Si(4)–Si(3)–Si(2) = 105.08(10), Si(5)–Si(4)–Si(6) = 108.70(10), Si(5)–Si(4)–Si(3) = 100.94(10), Si(6)–Si(4)–Si(3) = 108.54(10), Si(4)–Si(5)–Si(1) = 98.18(10), Si(6)–C(10)–Si(7) = 121.0(3).

Figure 6

Molecular structure and numbering of 34. Selected bond lengths (Å) and bond angles (deg) with esd's: Si(1)–Si(5) = 2.3372(10), Si(1)–Si(2) = 2.3389(11), Si(2)–Si(7) = 2.3459(12), Si(2)–Si(3) = 2.3538(11), Si(3)–Si(4) = 2.3722(11), Si(4)–Si(5) = 2.3498(10), Si(5)–Si(6) = 2.3455(12), Si(6)–C(9) = 1.882(2), Si(7)–C(9) = 1.877(2); Si(5)–Si(1)–Si(2) = 100.51(4), Si(1)–Si(2)–Si(7) = 106.82(4), Si(1)–Si(2)–Si(3) = 100.35(4), Si(7)–Si(2)–Si(3z) = 108.11(4), Si(2)–Si(3)–Si(4) = 104.73(4), Si(5)–Si(4)–Si(3) = 105.42(4), Si(1)–Si(5)–Si(9) = 114.10(4), Si(1)–Si(5)–Si(6) = 107.20(4), Si(1)–Si(5)–Si(4) = 100.21(4), Si(6)–Si(5)–Si(4) = 107.28(4), Si(7)–C(9)–Si(6) = 121.32(13).

Compound 29 (Figure 7) also crystallizes in the triclinic space group P1̅. All Si–Si bonds, even the central one, Si(2)–Si(3), are in the range of regular bond lengths (2.34–2.37 Å). The previously studied all-silicon compound 1,5-bis(trimethylsilyl)dodecamethylbicyclo[3.3.0]octasilane(19) exhibited a cisoid configuration of the two rings with a torsion angle of the two SiMe3 substituents along the bridge of 40.3°.(19) In 29 the replacement of two SiMe2 units by CH2 led to diminished strain, resulting in a torsion angle of 14.5°.

Figure 7

Molecular structure and numbering of 29. Selected bond lengths (Å) and bond angles (deg) with esd's: Si(1)–C(3) = 1.876(3), Si(1)–Si(2) = 2.3625(10), Si(2)–Si(6) = 2.3508(12), Si(2)–Si(3) = 2.3719(11), Si(3)–Si(4) = 2.3573(11), Si(3)–Si(7) = 2.3653(11), Si(4)–C(3) = 1.884(3), Si(6)–C(11) = 1.881(3); C(3)–Si(1)–C(2) = 111.55(12), C(3)–Si(1)–C(1) = 109.04(12), C(3)–Si(1)–Si(2) = 105.22(9), Si(6)–Si(2)–Si(1) = 111.95(5), Si(6)–Si(2)–Si(3) = 98.03(4), Si(1)–Si(2)–Si(3) = 99.89(4), Si(4)–Si(3)–Si(7) = 111.32(4), Si(4)–Si(3)–Si(2) = 98.11(4), Si(7)–Si(3)–Si(2) = 100.02(4), Si(1)–C(3)–Si(4) = 113.21(13), Si(6)–C(11)–Si(7) = 113.09(13).

The parent all-silicon compound of 31 (Figure 8), hexadecamethylbicyclo[3.3.1]nonasilane, was found to crystallize in the monoclinic space group P21/c with two molecules in the asymmetric unit,(20) while 31 crystallized in the triclinic space group P1̅. In the all-silicon compound the two six-membered rings featured a chair and a twist conformation. The replacement of two SiMe2 units by CH2 also changed this system, in which both rings have perfect chair conformations.

Figure 8

Molecular structure and numbering of 31. Selected bond lengths (Å) and bond angles (deg) with esd's: Si(1)–C(10) = 1.884(3), Si(1)–Si(2) = 2.3552(11), Si(2)–Si(7) = 2.3404(12), Si(2)–Si(3) = 2.3534(11), Si(3)–C(5) = 1.882(3), Si(4)–Si(5) = 2.3522(11), Si(5)–Si(7) = 2.3382(13), Si(5)–Si(6) = 2.3556(11), Si(6)–C(10) = 1.877(3); Si(7)–Si(2)–Si(3) = 106.86(5), Si(7)–Si(2)–Si(1) = 106.22(4), Si(3)–Si(2)–Si(1) = 109.43(5), Si(7)–Si(5)–Si(4) = 106.51(4), Si(8)–Si(5)–Si(4) = 111.15(5), Si(7)–Si(5)–Si(6) = 106.79(5), Si(4)–Si(5)–Si(6) = 109.75(4), Si(5)–Si(7)–Si(2) = 104.12(5), Si(4)–C(5)–Si(3) = 122.29(14), Si(6)–C(10)–Si(1) = 123.51(14).

NMR Spectroscopy

1H, 13C, and 29Si NMR spectra of the compounds described in this study were measured. To some extent the spectroscopic characteristics of oligosilanes with attached (trimethylsilyl)methyl units have already been discussed recently.(15) As 29Si NMR spectra are the most significant for structural assignments of oligosilanes, a short discussion seems appropriate. Especially cyclosilanes with endocyclic methylene groups were found to exhibit some interesting behavior. For compound 20 (Chart 1), with two SiMe2 groups attached to the CH2 unit, we would expect their 29Si resonances around −9.0 ppm, which is typical for a trimethylsilyl group attached to a silicon atom. In fact, we find indeed one resonance (−9.0 ppm) (for the SiMe2 group attached to another SiMe2) in the expected region, whereas the other resonance (which is connected to a quaternary silicon atom) is shifted downfield to +0.6 ppm (Chart 1). For compound 21 the situation is similar, but in this case the first SiMe2 (now attached to a tertiary Si atom) resonance is now observed at −3.3 ppm while the second resonance is further shifted to +2.5 ppm. For the symmetrical compound 22 both SiMe2 groups (attached to quaternary Si atoms) were found to resonate at +0.6 ppm.

Chart 1

29Si NMR Chemical Shifts (ppm) of Selected Compounds

Data used in the discussion are shown in boldface type.

Analysis of the SiMe2CH2 resonances of acyclic compounds 12 (−15.1 ppm) and 16 (−8.1 ppm) showed also that a higher silylation degree of the attached silicon atom causes a downfield shift (Chart 1). However, the extent of this shift seems also to be connected to some type of steric strain, which is particularly effective in the cyclopentasilanyl unit. This is seen by a comparison of the bicyclic compounds 29, 31, and 34. While the last two (31, 34) exhibit SiMe2CH2 resonances in the expected range around −8.0 ppm, for 29, which has structural features resembling those of 20–22, again a marked downfield shift to +2.9 ppm was observed (Chart 1).

The influence of ring strain on SiMe2 groups of homocyclic polysilanes can be seen also from a comparison of 1,1,3,3-tetrakis(trimethylsilyl)hexamethylcyclopentasilane[14,21] (SiMe2SiMe2 –24.8 ppm) and 1,1,4,4-tetrakis(trimethylsilyl)octamethylcyclohexasilane(6) (SiMe2SiMe2 −40.0 ppm).

Catalyst

It is somewhat peculiar that some of our results are different from the findings of Ishikawa, Kumada, and co-workers.[7−10] The additional cyclization reactivity observed by us is most likely caused by the modified catalyst. While Ishikawa et al. reported the use of commercially available AlCl3, they also noticed that on use of larger amounts of the catalyst chlorination of the products was a frequently occurring side reaction. For this reason they recommended treatment of the reaction mixture with methyl Grignard reagent in order to obtain permethylated products. Large catalyst amounts, which are common if the reaction does not start properly, were also reported to lead to polymerization or decomposition. Blinka and West(14) pointed out two types of catalysts that worked in their hands: either sublimed technical AlCl3 (with a specified FeCl3 content of 0.1%) or a cosublimate of pure AlCl3 and 1% FeCl3. They further noticed that pure sublimed AlCl3, nonsublimed technical AlCl3, and a mixture of pure AlCl3 and 1% FeCl3 (without prior cosublimation) did not show catalytic activity.(14)

For these reasons it seemed a worthwhile goal to establish conditions which allow a reliable start of the reaction with reasonably small amounts of catalyst. To find these optimized conditions, we used the known rearrangement reaction of dodecamethylcyclohexasilane to (trimethylsilyl)nonamethylcyclopentasilane[10,14] as a test system. Cosublimates of AlCl3 and FeCl3 were prepared with 1%, 3%, 5%, and 10% FeCl3 content. The 1% FeCl3 product had a pale yellow color, whereas the other products were bright yellow. The almost identical colors and the fact that the 5% and 10% batches led to a larger sublimation residue (of FeCl3) seemed to indicate that these samples have a similar stoichiometry. This was supported by the fact that there was no marked difference in the reactivity of these two samples. The minimum catalyst amount to achieve complete conversion was for all samples between 0.03 and 0.09 mol equiv. Experiments to study the activity of the catalyst were conducted in dichloromethane at room temperature. This solvent was found useful for an easy rearrangement of substrates but occasionally led to chlorination side reactions at longer reaction times. For difficult cases, therefore, cyclohexane was the preferred solvent, even though it requires elevated temperatures.

As additional alternative catalysts a cosublimate of AlBr3 and FeBr3 and samples of technical AlBr3 and Al(OTf)3 were studied. The AlBr3/FeBr3 cosublimate was of equal value to the AlCl3/FeCl3 system in dichloromethane with respect to both minimum catalyst amount and reaction time, whereas technical AlBr3 required the use of at least 0.15 mol equiv for complete conversion. However, the visible inhomogeneity of this material made conclusive statements difficult. Finally, Al(OTf)3 did not show any activity as a catalyst at all. In conclusion, the system reported by Blinka and West(14) was found to be the best one.

To find a mechanistic rationale for the requirement of the FeCl3 cosublimation procedure, we were interested in the type of incorporation of the FeCl3 into the AlCl3 lattice. As it is not clear in what way this incorporation occurs, we decided to study the material by X-ray powder diffraction, electron microscopy (SEM, EDXS), and atomic emission spectroscopy (ICP-OES). The powder diffraction measurement of a sample prepared by cosublimation of AlCl3 and 1% FeCl3 did not indicate any presence of FeCl3 at all and gave a pattern indistinguishable from that of pure AlCl3. The virtually identical cell constants thus suggested a very low concentration of FeCl3, which likely is incorporated in a very regular way into the lattice.

For the SEM measurements two samples were chosen. The first one was a cosublimate of AlCl3 and 3% FeCl3, which was heated in refluxing cyclohexane for several hours in order to mimic the rearrangement conditions. As the EDXS measurements of this sample did not indicate the presence of any iron (see the Supporting Information for details), we tried to increase the FeCl3 content by preparing a sample by cosublimation of AlCl3 and 10% FeCl3. The SEM images of Figure 9 show the size and shape of the particles of this sample. Rod-shaped particles with a length of up to several 100 μm and spherical particles with diameters of approximately 1 μm can be seen. EDXS was again used for the chemical analysis of the particles and to detect whether any strong accumulation of iron somewhere at the particles or at specific particles could be found. The detection limit of EDXS is around 0.1 wt %, the lateral resolution is around 1 μm, and the analysis depth is also around 1 μm. Thus, in the case of the big particles the EDXS analysis definitely corresponds to a surface analysis. No iron could be detected, either in an overall analysis or in the analysis of individual particles (see Figure 10). Thus, the iron concentration in the catalyst particles was, despite the high initial concentration of 10% FeCl3, below the detection limit of EDXS. ICP-OES analysis eventually proved that the iron concentration in the sample was close to 0.1 wt %. When the results from EDXS and ICP-OES are combined, it follows that the iron is homogenously distributed both across the individual particles and also across the whole sample. No local accumulation of iron could be detected. This was also confirmed by the backscattered electron images (see Figure 10 left), where the contrast increases with increasing mean atomic number and therefore iron-enriched areas should appear much brighter than the rest of the surface. The small contrast variations observed in the images proved to be only due to variations in the particle topography. Quantitative evaluation of the spectra showed that in case of the large particles the atom ratio Al:Cl corresponds within measurement accuracy to AlCl3. Additionally some oxygen could be detected. Much higher oxygen concentrations were found in the small spherical particles. Possibly, despite great care during the transfer of the samples, air admission could not be completely avoided.

Figure 9

SEM images of the catalyst particles: (left) backscattered electron image, image width 1.2 mm; (right) secondary electron image, image width 11.5 μm.

Figure 10

EDX spectrum for the left-hand image in Figure 9.

Conclusion

On investigation of some rearrangement reactions of branched oligosilanes with catalytic amounts of Al(Fe)Cl3, a novel cyclization reaction was found in addition to the already known rearrangement behavior.[7−12,14] The introduction of (trimethylsilyl)methyl substituents into oligosilanes strongly facilitates the rearrangement/cyclization behavior leading to rearranged branched oligosilanes and in the case of a sufficient number of silicon atoms to carbacyclosilanes with endocyclic methylene units. Depending on the number of (trimethylsilyl)methyl groups and the structure of the starting material, either mono- or bicyclic compounds were obtained. The bicyclic nature of these compounds restricts the conformational flexibility of the polysilane skeleton, making them interesting building blocks for the construction of longer chains with all-transoid conformation.[22−27]

Investigations concerning the composition of the catalyst system indicated that the cosublimation of AlCl3 even with 10% FeCl3 leads to the incorporation of only about 0.1% FeCl3 into the AlCl3 lattice.

Experimental Section

General Remarks

All reactions involving air-sensitive compounds were carried out under an atmosphere of dry nitrogen or argon using either Schlenk techniques or a glovebox. Solvents were dried using a column solvent purification system. All chemicals bought from different suppliers were used without further purification.

1H (300 MHz), 13C (75.4 MHz), and 29Si (59.3 MHz) NMR spectra were recorded on a Varian INOVA 300 spectrometer. If not noted otherwise, C6D6 was used for all samples or, in the case of reaction samples, they were measured with a D2O capillary in order to provide an external lock frequency signal. To compensate for the low isotopic abundance of 29Si, the INEPT pulse sequence was used for the amplification of the signal.[28,29] Elementary analysis was carried out using a Heraeus Vario Elementar instrument . EI high-resolution mass spectra (HRMS) were obtained using a Waters GCT premier instrument.

2,5-Bis(trimethylsilyl)dodecamethylhexasilane (3),(30) 2,2,3,3-tetrakis(trimethylsilyl)hexamethyltetrasilane (5),[3,31] 2,2,9,9-tetrakis(trimethylsilyl)octadecamethyldecasilane (8),(32) 2-(trimethylsilyl)-2-[(trimethylsilyl)methyl]hexamethyltrisilane (13),(15) 2-trimethylsilyl-2-[(trimethylsilyl)methyl]octamethyltetrasilane (15),(15) 2,2,3-tetrakis(trimethylsilyl)-3-[(trimethylsilyl)methyl]hexamethyltetrasilane (18),(15) 2,2,5,tris(trimethylsilyl)-5-[(trimethylsilyl)methyl]decamethylhexasilane (23),(15) 2,2-bis[(trimethylsilyl)methyl]hexamethyltrisilane (26),(15) 2,3-bis(trimethylsilyl)-2,3-bis[(trimethylsilyl)methyl]hexamethyltetrasilane (27),(15) 2,5-bis(trimethylsilyl)-2,5-bis[(trimethylsilyl)methyl]decamethylhexasilane (28),(15) 2,6-bis(trimethylsilyl)-2,6-bis[(trimethylsilyl)methyl]dodecamethylheptasilane (30),(15) 2,9-bis(trimethylsilyl)-2,9-bis[(trimethylsilyl)methyl]octadecamethyldecasilane (32),(15) 2,2,5,5-tetrakis(trimethylsilyl)decamethylhexasilane,[21,33,34] 2,2,3-tris(trimethylsilyl)heptamethyltetrasilane,(35) 2,2,4,4-tetrakis(trimethylsilyl)octamethylpentasilanes,(3) pentamethylchlorodisilane,(36) 1,1,4,4-tetrakis(trimethylsilyl)octamethylcyclohexasilane,(6) and 1,4-bis(trimethylsilyl)dodecamethylbicyclo[2.2.2]octasilane(6) were prepared by employing published procedures.

X-ray Structure Determinations

For X-ray structure analyses the crystals were mounted onto the tip of glass fibers, and data collection was performed with a Bruker-AXS SMART APEX CCD diffractometer using graphite-monochromated Mo Kα radiation (0.710 73 Å). The data were reduced to Fo2 and corrected for absorption effects with SAINT(37) and SADABS,(38) respectively. The structures were solved by direct methods and refined by full-matrix least-squares methods (SHELXL97).(39) If not noted otherwise, all non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were located in calculated positions to correspond to standard bond lengths and angles. All diagrams were drawn with 30% probability thermal ellipsoids, and all hydrogen atoms were omitted for clarity. Unfortunately, the obtained crystal quality of some substances was poor. This fact is reflected by quite high R and low θ values. Crystallographic data for compounds 25, 29, 31, and 33 are given in Table 1 and for compounds 34, 35-, 37, and 38 in Table 2.

Table 1

Crystallographic Data for Compounds 25, 29, 31, and 33

	25	29	31	33
empirical formula	Si8C15H44	Si8C16H46	Si9C18H52	Si10C21H62
Mw	449.22	463.25	541.40	595.61
temp (K)	100(2)	100(2)	100(2)	100(2)
size (mm)	0.30 × 0.22 × 0.20	0.34 × 0.28 × 0.20	0.32 × 0.22 × 0.18	0.35 × 0.30 × 0.10
cryst syst	triclinic	triclinic	triclinic	monoclinic
space group	P1̅	P1̅	P1̅	C2/c
a (Å)	9.793(2)	9.6431(19)	9.3855(19)	16.278(3)
b (Å)	9.793(2)	10.100(2)	11.547(2)	10.248(2)
c (Å)	15.025(3)	15.544(3)	16.262(3)	46.368(9)
α (deg)	89.15(3)	75.42(3)	71.75(3)	90
β (deg)	80.67(3)	87.67(3)	80.80(3)	94.92(3)
γ (deg)	88.36(3)	78.90(3)	73.88(3)	90
V (Å3)	1421(2)	1438(2)	1603(2)	7706(3)
Z	2	2	2	8
ρcalcd (g cm–3)	1.050	1.070	1.080	1.027
abs coeff (mm–1)	0.377	0.375	0.378	0.351
F(000)	492	508	572	2624
θ range (deg)	2.08 < θ < 25.00	1.17 < θ < 24.00	1.32 < θ < 26.36	1.76 < θ < 26.37
no. of collected/unique rflns	10 251/4975	11 183/5711	12 830/6442	30 040/7837
completeness to θ (%)	99.2	97.3	98.5	99.7
no. of data/restraints/params	4975/0/222	5711/0/231	6442/0/260	7837/0/300
goodness of fit on F2	0.97	1.09	1.10	1.16
final R indices (I > 2σ(I))	R1 = 0.080, wR2 = 0.133	R1 = 0.049, wR2 = 0.132	R1 = 0.054, wR2 = 0.131	R1 = 0.043, wR2 = 0.098
R indices (all data)	R1 = 0.170, wR2 = 0.166	R1 = 0.052, wR2 = 0.135	R1 = 0.061, wR2 = 0.136	R1 = 0.050, wR2 = 0.100
largest diff peak/hole (e/Å3)	0.57/-0.42	0.73/-0.43	0.59/-0.35	0.60/-0.22

Table 2

Crystallographic Data for Compounds 34, 35-37, and 38

	34	35-trans	37	38
empirical formula	Si9C17H50	Si10C22H64	Si10C24H68	Si10C19H56
Mw	507.38	609.63	637.68	565.54
temp (K)	150(2)	100(2)	100(2)	100(2)
size (mm)	0.38 × 0.25 × 0.20	0.33 × 0.28 × 0.16	0.30 × 0.28 × 0.20	0.30 × 0.30 × 0.15
cryst syst	triclinic	monoclinic	monoclinic	monoclinic
space group	P1̅	P21/c	P21/c	P21/c
a (Å)	9.866(2)	9.7457(19)	9.7384(19)	19.710(4)
b (Å)	12.667(3)	17.373(4)	23.464(5)	10.492(2)
c (Å)	13.420(3)	11.849(2)	9.820(2)	18.068(4)
α (deg)	72.56(3)	90	90	90
β (deg)	89.26(3)	97.06(3)	114.63(3)	101.12(3)
γ (deg)	86.28(3)	90	90	90
V (Å3)	1597(2)	1991(2)	2040(2)	3667(2)
Z	2	2	2	4
ρcalcd (g cm–3)	1.055	1.017	1.038	1.025
abs coeff (mm–1)	0.378	0.341	0.335	0.366
F(000)	556	672	704	1260
θ range (deg)	1.59 < θ < 26.37	2.09 < θ < 25.00	1.74 < θ < 26.37	2.11 < θ < 26.38
no. of collected/unique rflns	12 807/6436	13 146/3466	16 073/4171	28 568/7477
completeness to θ (%)	98.4	98.8	100	99.8
no. of data/restraints/params	6436/0/251	3466/0/155	4171/0/164	7477/0/280
goodness of fit on F2	1.14	1.05	1.09	1.05
final R indices (I > 2σ(I))	R1 = 0.045, wR2 = 0.103	R1 = 0.060, wR2 = 0.152	R1 = 0.053, wR2 = 0.112	R1 = 0.047, wR2 = 0.116
R indices (all data)	R1 = 0.050, wR2 = 0.105	R1 = 0.089, wR2 = 0.173	R1 = 0.073, wR2 = 0.120	R1 = 0.053, wR2 = 0.120
largest diff peak/hole (e/Å3)	0.55/-0.22	0.88/-0.44	0.63/-0.26	1.45/-0.38

Crystallographic data (excluding structure factors) for the structures of compounds 25, 29, 31, 33, 34, 35-, 37, and 38 reported in this paper have been deposited with the Cambridge Crystallographic Data Center as Supplementary Publication Nos. CCDC-720006 (33), 720005 (35-), 720009 (37), 720007 (38), 720003 (34), 720002 (31), 720004 (29), and 720008 (25). Copies of the data can be obtained free of charge at http://www.ccdc.cam.ac.uk/products/csd/request/.

SEM/EDXS: Analysis of the Catalyst

The catalyst particles from the cosublimate of AlCl3 and FeCl3 with 5% FeCl3 were investigated by energy dispersive X-ray spectrometry (EDXS: Noran Voyager, Thermo Fisher Scientific Inc., Waltham, MA, USA) in the scanning electron microscope (SEM: Leo DSM 982 Gemini, Carl Zeiss SMT Inc., Oberkochen, Germany). The samples were mounted in the glovebox, where they had also been synthesized, on an adhesive attached to a copper plate. Subsequently they were transferred in an airtight specimen holder to a carbon coater, where they were coated with a thin carbon layer (thickness ∼20 nm) and finally transferred again in the airtight specimen holder to the SEM. Both the carbon coater and the SEM are equipped with purpose-built airlocks to avoid contact between sample and air as far as possible. The coating of the samples with a thin conductive carbon layer is necessary to avoid charging of the electrically insulating specimens during electron irradiation in the SEM.

General Procedures for the Preparation of Silyl Anions

Method A for Monoanions in THF

The corresponding oligosilane together with KOtBu (1–1.05 equiv) was dissolved in a minimum amount of THF. The completeness of the conversion could be confirmed by 29Si NMR spectroscopy and was usually achieved within a few hours.

Method B for Dianions in THF

The corresponding oligosilane together with KOtBu (2–2.05 equiv) was dissolved in a minimum amount of THF. The completeness of the conversion could be confirmed by 29Si NMR spectroscopy and could require, depending on the starting material, some days. Heating to 60 °C allows to increase the reaction rate.

Method C for Monoanions in Benzene

The corresponding oligosilane together with KOtBu (1–1.05 equiv) and 18-crown-6 (1–1.05 equiv) was dissolved in a minimum amount of benzene. The completeness of the conversion could be confirmed by 29Si NMR spectroscopy and should be achieved within 1 h.

Method D for Dianions in Benzene

The corresponding oligosilane together with KOtBu (2 to 2.05 equiv) and 18-crown-6 (2–2.05 equiv) was dissolved in a minimum amount of benzene. The completeness of conversion could be confirmed by 29Si NMR spectroscopy and was usually complete within 18 h.

For the derivatization of the obtained anions, the obtained solutions were treated with an electrophile and after complete conversion subjected to an aqueous workup, which included washing with dilute H2SO4 and saturated NaHCO3 solution. Extraction of the product with pentane was followed by drying over Na2SO4 and removal of the solvents.

Synthesis of Starting Materials

2,2,5-Tris(trimethylsilyl)undecamethylhexasilane (1)

Compound 1 was obtained from 2,2,5,5-tetrakis(trimethylsilyl)decamethylhexasilane (395 mg, 0.65 mmol) and KOtBu (72 mg, 0.64 mmol) using method A and dimethyl sulfate (61 μL, 0.64 mmol). Colorless crystals (350 mg, 98%) were obtained from pentane (mp 68–71 °C). NMR data (δ in ppm): 29Si, −9.8, −12.1, −31.9, −33.4, −81.3, −129.1; 1H, 0.47 (s, 6H), 0.42 (s, 6H), 0.31 (s, 27H), 0.24 (s, 18H), 0.23 (s, 3H); 13C, 3.5, 1.1, 1.0, −1.1, −11.1. Anal. Calcd for C20H60Si9 (553.46): C, 43.40; H, 10.93. Found: C, 43.26; H, 10.91.

2-[(Trimethylsilyl)methyl]heptamethyltrisilane (11)

Compound 11 was obtained from tris(trimethylsilyl)methylsilane (465 mg, 1.77 mmol) and KOtBu (219 mg, 1.95 mmol) using method A and (trimethylsilyl)methyl chloride (224 mg, 1.82 mmol), yielding a colorless oil (468 mg, 96%). NMR data (δ in ppm): 29Si, 1.7, −16.3, −47.3; 1H, 0.21 (s, 3H), 0.15 (s, 18H), 0.09 (s, 9H), −0.17 (s, 2H); 13C, 1.7, −1.0, −3.8, −6.0. Anal. Calcd for C11H32Si4 (276.72): C, 47.75; H, 11.66. Found: 46.67; H, 10.96. HRMS for C11H32Si4: m/z calcd 276.1581, found 276.1595.

2,3-Bis(trimethylsilyl)-2-[(trimethylsilyl)methyl]heptamethyltetrasilane (17)

Compound 17 was obtained from 2,2,3-tetrakis(trimethylsilyl)-3-[(trimethylsilyl)methyl]hexamethyltetrasilane (18; 406 mg, 0.80 mmol) and KOtBu (89 mg, 0.79 mmol) using method A and dimethyl sulfate (101 mg, 0.80 mmol). After aqueous workup 17 was obtained as a colorless oil (315 mg, 88%). NMR data (δ in ppm): 29Si, 1.4, −11.8, −12.8, −76.1, −78.9; 1H, 0.32 (s, 3H), 0.30 (s, 18H), 0.27 (s, 18H), 0.19 (s, 2H), 0.14 (s, 9H); 13C, 2.3, 1.9, 1.7, 0.4, −8.5. UV absorption (pentane, λmax in nm (ε in M–1 cm–1)): 216 (1.4 × 104). Anal. Calcd for C17H50Si7 (451.18): C, 45.26; H, 11.17. Found: C, 45.26; H, 10.75.

2,3,3-Tris(trimethylsilyl)-2-[(trimethylsilyl)methyl]octamethylpentasilane (19)

Compound 19 was obtained from 2,2,3-tetrakis(trimethylsilyl)-3-[(trimethylsilyl)methyl]hexamethyltetrasilane (18; 696 mg, 1.37 mmol) and KOtBu (158 mg, 1.38 mmol) using method A and chloropentamethyldisilane (228 mg, 1.37 mmol). After aqueous workup colorless crystals (720 mg, 93%) of 19 were obtained after removing the solvent (mp 138–140 °C). NMR data (δ in ppm): 29Si, 1.2, −10.0, 13.1, −13.2, −38.2, −69.7, −115.2; 1H, 0.39 (s, 6H), 0.35 (s, 18H), 0.33 (s, 18H), 0.20 (s, 9H), 0.14 (s, 9H), 0.09 (s, 2H); 13C, 4.6, 3.3, 2.1, 1.3, 0.8, 0.1. UV absorption (pentane, λmax in nm (ε in M–1 cm–1)): 216 (1.9 × 104). Anal. Calcd for C21H62Si9 (567.49): C, 44.45; H, 11.01. Found: C, 44.46; H, 10.83.

1,1,4-Tris(trimethylsilyl)-4-[(trimethylsilyl)methyl]octamethylcyclohexasilane (33)

To an ice-cold solution of (chloromethyl)trimethylsilane (116 mg, 0.94 mmol, 1.0 equiv) in THF (5 mL) were added the monoanion of 1,1,4,4-tetrakis(trimethylsilyl)octamethylcyclohexasilane (550 mg, 0.94 mmol) and KOtBu (106 mg, 0.94 mmol ) (Method A) dropwise over a period of 10 min. After it was stirred for 16 h at room temperature, the reaction mixture was subjected to an aqueous workup. Recrystallization from Et2O/acetone yielded colorless 33 (230 mg, 41%; mp 131–137 °C). NMR data (δ in ppm): 29Si, 1.2, −7.8, −8.7, −11.9, −37.2, −40.0, −79.0, −131.8; 1H, 0.41 (s, 6H), 0.39 (s, 6H), 0.38 (s, 6H), 0.37 (s, 6H), 0.33 (s, 9H), 0.32 (s, 9H), 0.31 (s, 9H), 0.16 (s, 9H), 0.06 (s, 2H); 13C, 4.0, 3.9, 2.5, 1.9, −0.2, −1.0, −2.0, −3.3, −6.1. UV absorption (pentane, λmax in nm (ε in M–1 cm–1)): 206 (4.8 × 104), 249 (s, 6 × 7 × 103), 267 (s, 2.7 × 102). Anal. Calcd for C21H62Si10 (595.57): C, 42.35; H, 10.49. Found: C, 41.11; H, 10.49. HRMS for C21H62Si10: m/z calcd 594.2545, found 594.2549.

cis-/trans-1,4-Bis(trimethylsilyl)-1,4-bis[(trimethylsilyl)methyl]octamethylcyclohexasilane (35)

The reaction was carried out analogously to the preparation of 33 using the dianion prepared from 1,1,4,4-tetrakis(trimethylsilyl)octamethylcyclohexasilane (424 mg, 0.73 mmol) and KOtBu (164 mg, 1.46 mmol) (Method B) and chloromethyltrimethylsilane (179 mg, 2 equiv). The mixture of cis- and trans-35 (430 mg, 97%) was obtained as a colorless solid. By recrystallization with Et2O/acetone the trans isomer was separated (mp 157–164 °C). Data for the trans-35 isomer are as follows. NMR data (δ in ppm): 29Si, 1.4, −10.9, −36.9, −77.2; 1H, 0.41 (s, 12H), 0.39 (s, 12H), 0.32 (s, 18H), 0.16 (s, 18H), 0.09 (s, 4H); 13C, 2.7, 2.0, −1.1, −3.2, −5.7. UV absorption (pentane, λmax in nm (ε in M–1 cm–1)): 211 (3.8 × 104), 269 (s, 4.5 × 103). Data for the cis-2 isomer are as follows. NMR data (δ in ppm): 29Si, 1.3, −11.9, −38.2, −78.6; 1H, 0.37 (s, 12H), 0.35 (s, 12H), 0.30 (s, 18H), 0.15 (s, 18H), 0.06 (s, 4H); 13C, 2.5, 1.3, −2.2, −2.7, −6.1. Anal. Calcd for C22H64Si10 (609.60): C, 43.35; H, 10.58. Found: C, 43.92; H, 10.55.

1-(Trimethylsilyl)-1,4,4-tris[(trimethylsilyl)methyl]octamethylcyclohexasilane (36)

The reaction was carried out analogously to the preparation of 33 using the monoanion obtained from 35 (334 mg, 0.54 mmol), KOtBu (62 mg, 1.0 equiv), and 18-crown-6 (145 mg, 1.0 equiv) (Method C) and chloromethyltrimethylsilane (68 mg, 1 equiv). Compound 36 (285 mg, 83%) was obtained as a colorless solid after recrystallization with Et2O/acetone (mp 103–107 °C). NMR data (δ in ppm): 29Si, 1.4, 1.3, −11.0, −36.1, −37.1, −40.0, −78.0; 1H, 0.40 (s, 6H), 0.37 (s, 6H), 0.36 (s, 6H), 0.35 (s, 6H), 0.32 (s, 9H), 0.32 (s, 9H), 0.22 (s, 2H), 0.19 (s, 9H), 0.18 (s, 9H), 0.16 (s, 9H), 0.13 (s, 2H), 0.09 (s, 2H); 13C, 2.6, 2.4, 2.3, 2.0, −0.1, −0.7, −1.5, −2.9, −3.2, −4.3, −5.7. UV absorption (pentane, λmax in nm (ε in M–1 cm–1)): 233 (s, 1.2 × 104), 255 (s, 5.8 × 103). Anal. Calcd for C23H66Si10 (623.63): C, 44.30; H, 10.67. Found: C, 44.81; H, 10.76.

1,1,4,4-Tetrakis[(trimethylsilyl)methyl]octamethylcyclohexasilane (37)

The reaction was carried out analogously to the preparation of 35 using the dianion obtained from 35 (493 mg, 0.79 mmol), KOtBu (183 mg, 1.63 mmol), and 18-crown-6 (432 mg, 1.63 mmol) (Method D) and chloromethyltrimethylsilane (203 mg, 1.65 mmol). Compound 37 (461 mg, 89%) was obtained as a colorless solid after recrystallization with Et2O (mp 173–179 °C). NMR data (δ in ppm): 29Si, 1.4, −36.6, −40.5; 1H, 0.36 (s, 24H), 0.19 (s, 36H), 0.16 (s, 8H); 13C, 2.3, −1.2, −3.8. UV absorption (pentane, λmax in nm (ε in M–1 cm–1)): 231 (s, 8.7 × 103), 261 (s, 1.2 × 103). Anal. Calcd for C24H68Si10 (637.66): C, 45.21; H, 10.75. Found: C, 45.23; H, 10.74.

1-(Trimethylsilyl)-4-[(trimethylsilyl)methyl]dodecamethylbicyclo[2.2.2]octasilane (38)

The reaction was carried out analogously to the preparation of 33 using the monoanion of 1,4-bis(trimethylsilyl)dodecamethylbicyclo[2.2.2]octasilane (641 mg, 1.16 mmol) and KOtBu (131 mg, 1.17 mmol) (Method A) and chloromethyltrimethylsilane (147 mg, 1.20 mmol, 1.03 equiv). Compound 38 (630 mg, 96%) was obtained as a colorless solid (mp 168–175 °C). NMR data (δ in ppm): 29Si, 1.0, −6.1, −37.8, −39.9, −79.2, −131.5; 1H, 0.38 (s, 18H), 0.36 (s, 18H), 0.31 (s, 9H), 0.15 (s, 9H), 0.05 (s, 2H); 13C, 3.7, 1.5, −0.9, −2.4, −2.4. Anal. Calcd for C19H56Si10 (565.50): C, 40.35; H, 9.98. Found: C, 39.99; H, 9.97.

General Procedure for the Rearrangement Reactions

Solutions of the corresponding oligosilanes (50–1000 mg) and approximately 0.15 equiv of Al(Fe)Cl3 in cyclohexane in a Schlenk tube with a Teflon screw cap were heated to 80 °C. After 16 or 24 h the rate of conversion was checked by 29Si NMR spectroscopy. If no or insufficient conversion was observed, additional Al(Fe)Cl3 was added. After completion of the reaction, acetone (10–20 mL) was added and the white precipitate that formed was removed by either filtration or centrifugation. After removal of the solvent the residue was subjected to crystallization conditions. In the case of air-sensitive compounds the workup procedure was carried out under a nitrogen atmosphere and simple filtration through a path of Celite. Especially when higher catalyst loads were used, chlorinated byproducts were detected by mass spectroscopy. Treatment of these mixtures with methylmagnesium bromide or methyllithium could be used to obtain homogeneous product distributions.

Synthesis of the Catalysts

A mixture of AlCl3 and FeCl3 (3%) was sublimed at a temperature of 100 °C and a pressure of 0.3–0.5 mbar for 3–4 h. A bright yellow powder was obtained, which was stored under nitrogen. For the characterization experiments a sample of the sublimate was refluxed for 12 h in cyclohexane in order to mimic the reaction conditions (see Figures S2 and S3 in the Supporting Information). ICP analysis of this sample indicated a value of 0.36 mg of Fe/g (0.104%). For all other measurements a sublimate (AlCl3 2.441 g, FeCl3 297 mg (10%)) was used which according to ICP contained 1.30 mg of Fe/g (0.37%).

Rearrangement of 2,2,5-Tris(trimethylsilyl)undecamethylhexasilane (1) to 1,1,3-Tris(trimethylsilyl)heptamethylcyclopentasilane (2)

Following the general procedure, the reaction of 1 (386 mg, 0.70 mmol)) was carried out with Al(Fe)Cl3 (25 mg, 0.27 equiv) for 12 h in cyclohexane. After 16 h complete conversion was detected and compound 2(10,12) (303 mg, 93%) was isolated.

Rearrangements of 2,5-Bis(trimethylsilyl)dodecamethylhexasilane (3) and 2,2,3,3-Tetrakis(trimethylsilyl)hexamethyltetrasilane (5)

Following the general procedure, the reaction of 3 (332 mg, 0.63 mmol) was started with Al(Fe)Cl3 (25 mg, 0.30 equiv) at 80 °C in cyclohexane. After 16 h a mixture of 4 (48%), 5 (12%), 6 (34%), and 7 (6%) was detected. Another batch of Al(Fe)Cl3 (25 mg, 0.30 equiv) was added, and almost no change of composition was detected after an additional 24 h. After 6 days a mixture of compounds, with 6 (30%) and 7 (30%) being the main products, was observed. Additional Al(Fe)Cl3 (25 mg, 0.30 equiv) was added. After another 6 days compound 7 and tetramethylsilane were found to be the main products along with numerous smaller signals in the NMR spectrum. An increasingly worse signal/noise ratio indicates some decomposition process. Assignment of the structure of compound 6 was based on the 29Si NMR data (δ in ppm, cyclohexane, D2O capillary): 29Si, −9.5, −14.8, −32.8, −37.8, −42.0, −129.1.[10,12]

Repeating the reaction with 5 gave essentially the same result.

Rearrangement of 2,9,9-Tetrakis(trimethylsilyl)octadecamethyldecasilane (8) to 2,2,5,9,9-Pentakis(trimethylsilyl)pentadecamethylnonasilane (9) and Further to 1,1,3-Tris(trimethylsilyl)pentamethylcyclopentasilane (2) and 2,2-Bis(trimethylsilyl)octamethyltetrasilane (10)

Following the general procedure, the reaction of 8 (379 mg, 0.45 mmol) was started with Al(Fe)Cl3 (20 mg, 0.33 equiv) at 80 °C in cyclohexane. Two additional batches of Al(Fe)Cl3 (2 × 25 mg, 0.83 equiv) were required before extensive conversion of 8 to 9 was observed. Addition of another batch of Al(Fe)Cl3 (25 mg, 0.42 equiv) led to further conversion of 8 but was also accompanied by the formation of additional products. Workup allowed the isolation of 9 (46 mg, 13%) still contaminated with some 17% of 8. Purification was achieved by recrystallization from acetone/isopropyl alcohol and further from diethyl ether at −30 °C (mp 89–92 °C). NMR data of 9 (δ in ppm): 29Si, −9.8, −11.7, −30.6, −32.7, −72.6, −128.5; 1H, 0.56 (s, 6 H), 0.55 (s, 6H), 0.55 (s, 6H), 0.51 (s, 6H) 0.34 (s, 54H), 0.31 (s, 9H), 0.29 (s, 3H); 13C, 3.5, 1.6, 1.4, 1.3, −0.1, −0.2, −8.6. UV absorption (pentane, λmax in nm (ε in M–1 cm–1)): 216 (6.1 × 104); 258 (4.8 × 104); 270 (2.4 × 104). Anal. Calcd for C30H90Si14 (844.24): C, 42.68; H, 10.74. Found: C, 41.90; H, 10.11.

If the reaction was not stopped, complete conversion of 8 and 9 was observed. In addition to a small amount of side products compounds 2 and 10 in approximately equal quantities were the major reaction products.

Rearrangement of 2-[(Trimethylsilyl)methyl]heptamethyltrisilane (11) to 1-[(Trimethylsilyl)methyl]heptamethyltrisilane (12)

The reaction was carried out by following the general procedure, using 11 (186 mg, 0.672 mmol) and Al(Fe)Cl3 (2 × 25 mg, 0.56 equiv) in cyclohexane at 80 °C. After 48 h no conversion was detected and a second batch of Al(Fe)Cl3 was added. Clean and complete conversion to 12 was detected after 120 h. After workup and removal of solvent 12 (173 mg, 93%) was obtained as a colorless oil. NMR data (δ in ppm): 29Si, 1.3, −15.1, −16.3, −48.2; 1H, 0.21 (s, 6H), 0.15 (s, 9H), 0.15 (s, 6H), 0.09 (s, 9H), −0.19 (s, 2H); 13C, 1.8, 1.7, −0.5, −1.2, −6.8. Anal. Calcd for C11H32Si4 (276.72): C, 47.75; H, 11.66. Found: C, 44.04; H, 10.26.

Rearrangement of 2-(Trimethylsilyl)-2-[(trimethylsilyl)methyl]hexamethyltrisilane (13) to 2-(Trimethylsilyl)-1-[(trimethylsilyl)methyl]hexamethyltrisilane (14)

The reaction was carried out by following the general procedure, using 13 (421 mg, 1.26 mmol) and Al(Fe)Cl3 (120 mg, 0.72 equiv) in cyclohexane. Completion was reached after 24 h. 14 (380 mg, 90%) could be obtained as a colorless oil. NMR data (δ in ppm): 29Si, 1.3, −11.3, −12.9, −86.5; 1H, 0.29 (s, 6H), 0.21 (s, 18H), 0.13 (s, 3H), 0.10 (s, 9H), −0.08 (s, 2H); 13C, 3.7, 1.7, 1.2, 0.6, −13.1. Anal. Calcd for C13H38Si5 (334.87): C, 46.63; H, 11.44. Found: C, 44.66; H, 10.86.

Rearrangement of 2-(Trimethylsilyl)-2-[(Trimethylsilyl)methyl]octamethyltetrasilane (15) to 2-(Trimethylsilyl)-2-[((trimethylsilyl)methyl)dimethylsilyl]hexamethyltrisilane (16)

The reaction was carried out by following the general procedure using 15 (240 mg, 0.611 mmol) and Al(Fe)Cl3 (20 mg, 0.25 equiv) in cyclohexane. Completion was reached after 16 h. 16 (227 mg, 95%) could be obtained as a colorless oil. NMR data (δ in ppm): 29Si, 1.2, −8.1, −10.3, −132.8; 1H, 0.36 (s, 6H), 0.27 (s, 27H), 0.09 (s, 9H), 0.02 (s, 2H); 13C, 6.4, 3.5, 2.9, 1.8. Anal. Calcd for C15H44Si6 (393.03): C, 45.84; H, 11.28. Found: C, 46.28; H, 11.21.

Rearrangement of Tetrakis(trimethylsilyl)-1-[(trimethylsilyl)methyl]-2-methyldisilane (17) to 1,1-Bis(trimethylsilyl)-3-carba-2,2,4,4,5,5-hexamethylcyclopentasilane (20)

The reaction was carried out by following the general procedure using 17 (253 mg, 0.561 mmol) and Al(Fe)Cl3 (60 mg, 0.80 equiv) in cyclohexane. Completion was reached after 36 h. 20 (186 mg, 91%) could be obtained as a colorless liquid. NMR data (δ in ppm): 29Si, 0.6, −8.8, −9.0, −34.6, −136.7; 1H, 0.35 (s, 6H), 0.30 (s, 6H), 0.27 (s, 18H), 0.20 (s, 6H), −0.08 (s, 2H); 13C, 7.1, 3.2, 1.6, 0.4, −2.2. Anal. Calcd for C13H38Si6 (362.96): C, 43.02; H, 10.55. Found: C, 37.67; H, 8.71.

Rearrangement of Pentakis(trimethylsilyl)[(trimethylsilyl)methyl]disilane (18) to 1,1,5-Tris(trimethylsilyl)-3-carba-2,2,4,4,5-pentamethylcyclopentasilane (21)

The reaction was carried out by following the general procedure using 18 (227 mg, 0.446 mmol) and Al(Fe)Cl3 (20 mg, 0.34 equiv) in cyclohexane. Completion was reached after 16 h. White crystalline 21 (68 mg, 36%) could be obtained after recrystallization from pentane/2-propanol at 0 °C (oil at room temperature). NMR data (δ in ppm): 29Si, 2.5, −3.3, −8.1, −9.9, −11.5, −78.8, −131.5; 1H, 0.34 (s, 3H), 0.31 (s, 3H), 0.29 (s, 3H), 0.28 (s, 9H), 0.27 (s, 9H), 0.26 (s, 3H), 0.25 (s, 9H), 0.24 (s, 3H), −0.02 (d, 1H, J = 13 Hz), −0.11 (d, 1H, J = 13 Hz); 13C, 7.5, 4.3, 3.6, 3.4, 3.3, 1.8, 0.5, 0.2, −7.4. Anal. Calcd for C15H44Si7 (421.11): C, 42.78; H, 10.53. Found: C, 42.00; H, 10.24.

Rearrangement of 1-Pentamethyldisilanyltetrakis(trimethylsilyl)-2-[(trimethylsilyl)methyl]disilane (19) to 1,1,5,5-tTetrakis(trimethylsilyl)-3-carba-2,2,4,4-cyclopentasilane (22)

The reaction was carried out by following the general procedure using 19 (252 mg, 0.444 mmol) and Al(Fe)Cl3 (50 mg, 0.84 equiv) in cyclohexane. Completion was reached after 24 h. White crystalline 22 (110 mg, 52%) could be obtained after recrystallization from methanol/2-propanol at −30 °C (oil at room temperature). NMR data (δ in ppm): 29Si, 1.6, −9.1, −128.7; 1H, 0.31 (s, 36H), 0.31 (s, 12H), −0.05 (s, 2H); 13C, 8.7, 4.0, 3.5. Anal. Calcd for C17H50Si8 (479.27): C, 42.60; H, 10.52. Found: C, 39.95; H, 9.61. HRMS for C17H50Si8: m/z calcd 478.2067, found 478.2099.

Rearrangement of Pentakis(trimethylsilyl)[(trimethylsilyl)methyl]-2,2,3,3-tetramethyltetrasilane (23) to 1-(Trimethylsilyl)-3-carbaundecamethylbicyclo[3.2.1]octasilane (25)

The reaction was carried out by following the general procedure using 23 (291 mg, 0.465 mmol) and Al(Fe)Cl3 (60 mg, 0.97 equiv) in cyclohexane. After 24 h a 1:1 mixture of 25 and 1,1,5,5-tetrakis(trimethylsilyl)-3-carbahexamethylcyclohexasilane (24) (NMR data (δ in ppm, D2O capillary): 29Si −8.1, −9.6, −30.2, −132.6.) could be observed. Complete conversion was reached after 48 h. White crystalline 25 (110 mg, 53%) could be obtained after recrystallization from pentane/2-propanol (mp 100–110 °C). NMR data (δ in ppm): 29Si, −6.0, −6.9, −8.9, −27.9, −33.7, −39.4, −77.8, −129.6; 1H, 0.54 (s, 3H), 0.39 (s, 3H), 0.37 (s, 3H), 0.36 (s, 3H), 0.33 (s, 3H), 0.31 (s, 3H), 0.30 (s, 3H), 0.29 (s, 9H), 0.28 (s, 3H), 0.26 (s, 3H), 0.24 (s, 3H), 0.05 (d, 1H, J = 13 Hz), −0.16 (d, 1H, J = 13 Hz); 13C, 8.6, 5.3, 3.6, 3.4, 2.5, 1.0, 0.1, −1.1, −1.1, −1.8, −3.7, −4.0, −14.1. Anal. Calcd for C15H44Si6 (449.20): C, 40.11; H, 9.87. Found: C, 38.52; H, 9.65.

Rearrangement of 33 to 3-Carba-1,5-bis(trimethylsilyl)decamethylbicyclo[3.2.1]octasilane (34)

The reaction was carried out by following the general procedure using 33 (202 mg, 0.339 mmol) and Al(Fe)Cl3 (20 mg, 0.44 equiv) in cyclohexane. Completion was reached after 16 h, and white crystalline 34 (117 mg, 68%) could be obtained after recrystallization from dichloromethane at room temperature (mp 162–169 °C). NMR data (δ in ppm): 29Si, −6.2, −7.3, −22.5, −31.4, −126.8; 1H, 0.63 (s, 3H), 0.43 (s, 3H), 0.37 (s, 6H), 0.36 (s, 6H), 0.32 (s, 6H), 0.30 (s, 6H), 0.29 (s, 18H), 0.02 (d, 1H, J = 13 Hz), −0.14 (d, 1H, J = 13 Hz); 13C, 9.7, 5.4, 3.6, 3.4, 2.6, 0.8, −1.0, −2.1. UV absorption (pentane, λmax in nm (ε in M–1 cm–1)): 218 (4.9 × 104), 243 (1.8 × 104). Anal. Calcd for C17H50Si9 (507.35): C, 40.25; H, 9.93. Found: C, 39.67; H, 9.71. HRMS for C17H50Si9: m/z calcd 506.1836, found 506.1824.

Rearrangement of 35 or 1,1,5,5-Tetrakis(trimethylsilyl)-1,5-bis[(trimethylsilyl)methyl]hexamethylpentasilane (30) to 3,7-Dicarba-1,5-bis(trimethylsilyl)decamethylbicyclo[3.3.1]nonasilane (31)

The reaction was carried out by following the general procedure using 35 (425 mg, 0.697 mmol) and Al(Fe)Cl3 (60 mg, 0.65 equiv) in cyclohexane. Completion was reached after 24 h. White crystalline 31 (280 mg, 77%) could be obtained after recrystallization from benzene at room temperature (mp 189–195 °C). NMR data (δ in ppm): 29Si, −7.2, −8.4, −31.3, −132.8; 1H, 0.59 (s, 6H), 0.33 (s, 12H), 0.32 (s, 12H), 0.29 (s, 18H), −0.04 (d, 2H, J = 13 Hz), −0.10 (d, 2H, J = 13 Hz); 13C, 11.0, 5.9, 3.9, 3.6, 3.3. UV absorption (pentane, λmax in nm (ε in M–1 cm–1)): 216 (3.1 × 104), 244 (2.1 × 104). Anal. Calcd for C18H52Si9 (521.38): C, 41.47; H, 10.05. Found: C, 41.47; H, 10.00.

Starting from 30 (362 mg, 0.519 mmol) and Al(Fe)Cl3 (125 mg, 1.81 equiv), complete conversion was reached after 4 days but some unidentified byproducts were detected in this case.

Rearrangement of 36 or 1,1,4,4-Tetrakis(trimethylsilyl)-1,4-bis[(trimethylsilyl)methyl]tetramethyltetrasilane (28) to 3,7-Dicarba-1,5-bis(trimethylsilyl)decamethylbicyclo[3.3.0]octasilane (29)

The reaction was carried out by following the general procedure using 36 (153 mg, 0.251 mmol) and Al(Fe)Cl3 (15 mg, 0.45 equiv) in cyclohexane. Completion was reached after 16 h, and white crystalline 29 (65 mg, 56%) was obtained after recrystallization from dichloromethane at −30 °C (sublimation from 280 °C). NMR data (δ in ppm): 29Si, 2.9, −8.7, −127.9; 1H, 0.36 (s, 12H), 0.33 (s, 18H), 0.31 (s, 12H), −0.04 (d, 2H, J = 13 Hz), −0.26 (d, 2H, J = 13 Hz); 13C, 9.3, 3.5, 3.2, 3.1. UV absorption (pentane, λmax in nm (ε in M–1 cm–1)): 224 (2.1 × 104). Anal. Calcd for C16H46Si8 (463.22): C, 41.49; H, 10.01. Found: C, 39.65; H, 9.81.

Starting from 28 (130 mg, 0.203 mmol) and Al(Fe)Cl3 (30 mg, 1.11 equiv), 29 (44 mg, 47%) was obtained after 48 h.