Platinum-Catalyzed Reduction of DMF by 1,1,3,3-Tetramethyldisiloxane, HMeSi2OSiMe2H: New Intermediates HSiMe2OSiMe2OCH2NMe2 and HSiMe2(OSiMe2)3OCH2NMe2 and Their Further Chemical Reactivity.

The use of Karstedt's catalyst to study the reduction of Me2NCHO (DMF) by the popular "dual SiH"-containing tetramethyldisiloxane, HMe2SiOSiMe2H (1), has revealed that the first step in the process involves an initial single hydrosilylation to form HSiMe2OSiMe2OCH2NMe2 (3). This intermediate is readily isolated and purified via distillation. In the continued presence of the catalyst, 3 transforms to the transient tetrasiloxane HMe2SiOSiMe2OSiMe2OSiMe2OCH2NMe2 (4), along with the formation of Me3N. The tetrasiloxane 4 itself transforms to Me3N and (Me2SiO) n (n = 4-6). Despite the demonstrated reactivity of 3, it can also be used to perform the expected metal-catalyzed hydrosilylation chemistry of the SiH group as well as reactions of the SiOCH2NMe2 functionality involving siloxane chain extension and is thus an important new reagent for siloxane chemistry.

Since the initial, and generally uncited, report by the Voronkov group that silanes, R3SiH, could effectively reduce DMF to Me3N with concomitant formation of disiloxanes,[1] their use as reducing agents to transform amides to amines has been well-studied and widely used due to the generally mild reaction conditions employed.[2] Using monosilanes, R3SiH, to reduce DMF as a model amide, we recently demonstrated that such reductions proceed via an initial hydrosilylation reaction to produce siloxymethylamines R3SiOCH2NMe2 (O-silylated hemi-aminals).[3] This class of compounds can further react with silanes in the presence of catalysts, and also in excess DMF, to form the amine and appropriate disiloxane (eq 1).

A particularly versatile silane for such reductions is 1,1,3,3-tetramethyldisiloxane, HSiMe2OSiMe2H (1), and using a range of catalysts, including Karstedt’s catalyst (bis[1,3-bis(η2-ethenyl)-1,1,3,3-tetramethyldisiloxane]platinum), this reagent possesses some unique reduction characteristics.[4−6] For example, it has been demonstrated that 1 can effectively reduce amides under conditions where other silanes are ineffective and this special property has been associated with a “dual SiH effect”.[5a] Furthermore, the same disiloxane has been noted as a poor reagent for other reactions.[5c] We have previously reported that using 1 in the presence of (Me3N)Mo(CO)5 as catalyst, for the reduction of DMF, a double hydrosilylation occurs to form Me2NCH2OSiMe2OSiMe2OCH2NMe2, (2), which can be isolated and characterized.[7] As a continuation of our studies in this arena we have now used Karstedt’s catalyst to study the reaction of 1 with DMF, monitoring the reaction with 29Si, 13C, and 1H NMR spectroscopy. Typical monitoring sequences of such a reaction in C6D6 at room temperature (∼295 K) are presented in Figures 1 (29Si) and 2 (13C). Along with the disappearance of the 29Si resonance at −4.6 ppm due to 1, there is a growth of two resonances at −6.4 and −12.2 ppm, and under the reaction conditions used, no further significant chemistry takes place.

Figure 1

29Si NMR monitoring of the reaction between HSiMe2OSiMe2H (−4.6 ppm) and DMF (1:5 molar ratio) catalyzed by 1 mol % of Karstedt’s catalyst, illustrating the formation of 3 (−6.4 and −12.2 ppm).

Figure 2

13C NMR monitoring of the reaction between HSiMe2OSiMe2H (0.1 ppm) and DMF (1:5 molar ratio) catalyzed by 1 mol % of Karstedt’s catalyst, illustrating the formation of 3 (81.7 (CH2), 41.0 (NMe2), 0.84 and −0.89 ppm (SiMe)). Me resonances for DMF appear at 30.6 and 35.2 ppm.

The related 13C spectral sequence (Figure 2) exhibits the appearance of new resonances at 81.7, 41.0, 0.84, and −0.89 ppm typical of the SiOCH2NMe2 group and two new Me2Si units. The new material is the single hydrosilylation product, Me2NCH2OSiMe2OSiMe2H (3; eq 2).

The same chemistry takes place on a larger preparative scale using either hexane or benzene as solvent, and we have been able to isolate this material by distillation at 49 °C/15 mmHg in excellent yield. Compound 3 is relatively stable at room temperature; however, when it is subjected to the presence of Karstedt’s catalyst the 29Si resonances at −6.4 and −12.2 ppm transform to four new signals at −6.7, −13.9, −19.8, and −21.5 ppm and via 13C NMR we observe the concurrent formation of Me3N. The new silicon-containing material is the tetrasiloxane HSiMe2OSiMe2OSiMe2OSiMe2OCH2NMe2 (4). While we have been unable to obtain 4 as an analytically pure material, we have been able to prepare and isolate it in ∼95% purity and study its further chemistry, as described in the Supporting Information. Continued exposure of 4 to the catalytic conditions results in formation of more Me3N and a mixture of cyclic dimethylpolysiloxanes, (Me2SiO) (n = 4, D4; n = 5, D5; n = 6, D6), as noted by comparison of their 29Si NMR and GC/MS spectra with those of known materials (Figures S-5–S-8, respectively (Supporting Information)). The 29Si and 13C monitoring of this transformation is presented in Figures 3 and 4, respectively.

Figure 3

Transformation of 4 (−6.7, −13.9, −19.8, and −21.5 ppm) to predominantly D4 (−19.1 ppm) and D6 (−21.6 ppm) monitored by 29Si NMR.

Figure 4

Transformation of 4 (81.8 ppm (CH2); 41.1 ppm (Me2N) to Me3N (47.5 ppm) and D4–6 monitored by 13C NMR. The minor resonances at ∼20 ppm are associated with the xylene solvent of the catalyst solution.

To prove the structure and formulation, we have reacted 4 with Me3SiCl. This procedure results in the formation and high-yield isolation of the expected pentasiloxane HSiMe2OSiMe2OSiMe2OSiMe2OSiMe3 (5), a known compound.[8]

Overall during the reduction of DMF by 1, the initially formed hydrosilylation product 3 reacts with itself to liberate Me3N and 4, which further eliminates Me3N along with formation of D, both reactions demonstrating the reactivity of the siloxymethylamines with SiH species involving formation of Me3N and siloxanes (Scheme 1).[3] This is a further illustration of the capacity of hydrosilanes to reduce siloxymethylamines (O-silylated hemi-aminals), as illustrated in eq 1, and the more general capacity to reduce aminals.[9]

Scheme 1

Sequence of Reactions Leading to Production of Me3N and D

The transformation of 4 to Me3N poses the question as to whether this is an intramolecular elimination reaction or involves a bimolecular reaction with a second SiH functionality. Since the silicon-containing compound formed is predominantly D4 and we see no significant amounts of long-chain polysiloxanes, we favor the intramolecular amine elimination. We attempted to answer that question by D-labeling experiments using deuteriotetramethyldisiloxane, DSiMe2OSiMe2D[10] (1D). Use of 1D to reduce DMF resulted, as expected, in the formation of Me2NCHD2. However, using an equimolar mixture of 1 and 1D resulted predominantly in the formation of Me2NCH2D and Me3N and smaller amounts of Me2NCHD2: i.e., a significant H/D scrambling had occurred (Figure S-4 (Supporting Information)). Separate experiments mixing 1D and Et3SiH resulted in a very rapidly established equilibrium illustrating the metal-catalyzed H/D exchange in hydrosilanes,[11] thus ruling out the mechanistic clarification we sought.

As we previously reported, the use of (Me3N)Mo(CO)5 as catalyst for the reduction of DMF by 1 proceeds via a different route, involving the intermediacy of the double-hydrosilylation intermediate (Me2NCH2OSiMe2)2O.[7] We have treated the new intermediate 3 with DMF in the presence of the molybdenum catalyst and observed the rapid formation of (Me2NCH2OSiMe2)2O (2) with no initial Me3N formation, illustrated in Figure 5. The two catalysts clearly have distinctive properties in this chemistry, and studies to tease out these distinctions, along with those of other catalysts, are in progress.

Figure 5

29Si NMR monitoring of the reaction between DMF and HSiMe2OSiMe2OCH2NMe2 (3) catalyzed by 1 mol % of (Me3N)Mo(CO)5 leading to Me2NCH2OSiMe2OSiMe2OCH2NMe2, (2) at −13.9 ppm.

Treatment of 3 under varying conditions of the functional group reactivity was performed to illustrate that the two terminal silyl groups (SiH and SiOCH2NMe2) retain their established chemistry in the presence of each other, even in the presence of metal catalysts. Thus, the reaction of 3 with Me3ECl (E = Si, Ge) led to the high-yield formation of the siloxane chain extension products, resulting in either trisiloxane 6a or disiloxygermoxane 6b (eq 5).

The trisiloxane 6a is a useful, commercially available reagent,[12] whereas the germanium analogue is unreported. All analytical and spectroscopic data are in accord with either the published data or those expected. For example, the 29Si NMR data for 6a exhibits three resonances at 7.4, −6.9, and −19.4 ppm for the Me3SiO, HMe2SiO, and Me2SiO silicon atoms, respectively. The Ge analogue 6b exhibits 29Si resonances at −8.2 and −17.6 ppm, in accord with expectation.

The chemistry of the SiH functionality of 3 was initially expected to be complicated, since the hydrosilylation reaction normally needs a catalytic species similar to that required for the amine elimination/amide reduction product (eq 2). Hence, a competition between hydrosilylation and amine elimination was anticipated. However, in a test hydrosilylation reaction we treated 3 with Me3SiCH=CH2 in the presence of Karstedt’s catalyst and obtained a high yield of the “expected” hydrosilylation product 7 (eq 6). No amine elimination chemistry was observed and the hydrosilylation appears to be regiospecific.

Compound 7 can be readily reacted with chlorosilanes for further siloxane chain extension (eq 7).

Thus, the newly observed and isolated material 3 has three distinct and very useful modes of chemistry: elimination of the reduced amide as the amine, hydrosilylation using the SiH group, and siloxane chain extension using the SiOCH2NMe2 functionality (Scheme 2).

Scheme 2

Reactivity of 3

Legend: (a) Me3SiCH=CH2/Karstedt’s catalyst; (b) (Me3N)Mo(CO)5/DMF; (c) Me3ECl, E = Si, Ge; (d) Karstedt’s catalyst.