Junpei Shimada1, Atsushi Tani1, Chihiro Hanazato2, Takashi Masuyama2, Yohsuke Yamamoto1, Atsushi Kawachi1,2. 1. Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan. 2. Faculty of Bioscience and Applied Chemistry, Hosei University, 3-7-2 Kajino-cho, Koganei, Tokyo 184-8584, Japan.
Abstract
Herein, we report detailed studies on a series of o-(silyl)(boryl)benzenes (1-4), in which the two Lewis acid centers consisting of silicon and boron atoms are linked via an o-phenylene skeleton. o-(Fluorosilyl)(dimesitylboryl)benzenes 1 and 2 were prepared by the reaction of fluorodimesitylborane with [o-(fluorodimethylsilyl)phenyl]lithium (7) and [o-(fluorodiphenylsilyl)phenyl]lithium (8), respectively. o-(Difluorosilyl)(dimesitylboryl)benzenes 3 and 4 were also prepared by the reaction of fluorodimesitylborane with o-{[di(methoxy)methylsilyl]phenyl}lithium (11) and o-{[di(methoxy)phenylsilyl]phenyl}lithium (12), respectively, and their subsequent treatment with HF·pyridine. Compounds 1-4 readily capture a fluoride ion in the presence of 18-crown-6 or [2.2.2]cryptand to afford their corresponding μ-fluoro-bridged ate complexes (15-18). The structures of 15-18 were revealed by NMR spectroscopy and X-ray crystallography. DFT studies and natural bond orbital analysis of 15-18 were conducted to elucidate the nature of the Si-F and B-F bonding interactions in the μ-fluoro-bridges. The fluoride ion affinities of 1-4 were investigated by 1H NMR spectroscopy to monitor their competitive reactions. The dynamic behaviors of 15-18 at variable temperatures were monitored using 19F NMR spectroscopy.
Herein, we report detailed studies on a series of o-(silyl)(boryl)benzenes (1-4), in which the two Lewis acid centers consisting of silicon and boron atoms are linked via an o-phenylene skeleton. o-(Fluorosilyl)(dimesitylboryl)benzenes 1 and 2 were prepared by the reaction of fluorodimesitylborane with [o-(fluorodimethylsilyl)phenyl]lithium (7) and [o-(fluorodiphenylsilyl)phenyl]lithium (8), respectively. o-(Difluorosilyl)(dimesitylboryl)benzenes 3 and 4 were also prepared by the reaction of fluorodimesitylborane with o-{[di(methoxy)methylsilyl]phenyl}lithium (11) and o-{[di(methoxy)phenylsilyl]phenyl}lithium (12), respectively, and their subsequent treatment with HF·pyridine. Compounds 1-4 readily capture a fluoride ion in the presence of 18-crown-6 or [2.2.2]cryptand to afford their corresponding μ-fluoro-bridged ate complexes (15-18). The structures of 15-18 were revealed by NMR spectroscopy and X-ray crystallography. DFT studies and natural bond orbital analysis of 15-18 were conducted to elucidate the nature of the Si-F and B-F bonding interactions in the μ-fluoro-bridges. The fluoride ion affinities of 1-4 were investigated by 1H NMR spectroscopy to monitor their competitive reactions. The dynamic behaviors of 15-18 at variable temperatures were monitored using 19F NMR spectroscopy.
Bidentate Lewis acids bearing two Lewis
acid centers have received
a great deal of attention in the fields of organic synthesis, molecular
recognition, and main group chemistry because they efficiently accept
Lewis bases via a reversed chelation mode.[1]Since the Lewis acidity of tricoordinate boron[2] and tetracoordinate silicon[3] atoms are well recognized, the introduction of these elements into
an organic framework is a promising route to construct high-performance
bidentate Lewis acids. Thus, homonuclear B/B bidentate Lewis acids I–V(4) and Si/Si bidentate
Lewis acids VI–VIII(5) have been extensively investigated, as shown in Chart .
Chart 1
Previous Examples
of B/B, Si/Si, and B/Si Bidentate Lewis Acids
Heteronuclear B/Si bidentate Lewis acids IX–XI consisting of silicon and boron centers have
attracted an increasing
amount of research interest.[6] Katz et al.
have applied a 1,8-naphthalene skeleton to construct a B/Si bidentate
Lewis acid but found that the skeleton was not suitable for capturing
a fluoride ion due to steric congestion.[6a] Aldridge et al. applied a ferrocene skeleton to construct a B/Si
bidentate Lewis acid.[6c] Some groups applied
several aromatic frameworks to construct a variety of bidentate Lewis
acids bearing B/heteroatom centers for fluoride ion recognition.[7,8]In our previous work, we constructed two B/Si bidentate Lewis
acids, o-(fluorosilyl)(dimesitylboryl)benzenes 1 and 2, using a o-phenylene
skeleton (Chart ).[9,10] In
this context, we have extended this chemistry to the difluorosilyl
system, o-(difluorosilyl)(dimesitylboryl)benzenes 3 and 4. Herein, we report the preparation, structure,
Lewis acidity, and dynamic behavior of monofluorosilyl and difluorosilyl
systems 1–4. The chemistry of mixed Si/B Lewis
acids is disclosed in detail and systematically more than ever before.
Chart 2
B/Si Bidentate Lewis Acids Described in This Study
Results and Discussion
Preparation of o-C6H4(SiR2F)BMes21 (R = Me) and 2 (R = Ph)
Li–Br exchange of o-(fluorosilyl)bromobenzenes 5 and 6 using tert-BuLi in Et2O at −78 °C affords
aryllithium 7 and 8, which were subsequently
reacted with Mes2BF to yield 1 and 2, respectively (Scheme ).[9,10]1 and 2 were obtained
as colorless crystals via recrystallization from hexane.
Scheme 1
Preparation
of B/Si Bidentate Lewis Acids 1 and 2
Preparation of o-C6H4(SiRF2)(BMes2) 3 (R = Me) and 4 (R = Ph)
Treatment of [di(methoxy)silyl]bromobenzenes 9 and 10 with tert-BuLi at −78
°C gave aryllithium 11 and 12, which
were allowed to react with Mes2BF at −60 °C
for 24 h to form [di(methoxy)silyl]borylbenzenes 13 and 14, respectively (Scheme ).[11] Reacting 13 and 14 with an excess amount of HF·pyridine in
Et2O at 0 °C produces (difluorosilyl)borylbenzenes 3 and 4, respectively.[12]3 and 4 were obtained as colorless crystals
via recrystallization from hexane.
Scheme 2
Preparation of B/Si Bidentate Lewis
Acids 3 and 4
Formation of μ-Fluoro-bridged Si/B Ate Complexes
Reaction of 1 and 2 with KF to Form
[o-C6H4(SiR2F)(BMes2)(μ-F)−][K+L] 15 (R = Me) and 16 (R = Ph)
(Fluorosilyl)borylbenzenes 1 and 2 capture a fluoride ion from KF in the presence
of [2.2.2]cryptand (a) or 18-crown-6 (b)
(= L) in toluene at room temperature (Scheme ).[9] The resulting μ-fluoro-bridged ate complexes (15 and 16) were isolated as colorless crystals via recrystallization
from THF–hexane.
Scheme 3
Reaction of 1 and 2 with KF to Form 15 and 16
Reaction of 3 and 4 with KF to Form [o-C6H4(SiR2F)(BMes2)(μ-F)−][K+L] 17 (R = Me) and 18 (R
= Ph)
(Difluorosilyl)borylbenzenes 3 and 4 also captured a fluoride ion under the same reaction conditions
used for 1 and 2 (Scheme ). The resulting μ-fluoro-bridged ate
complexes (17 and 18) were isolated as colorless
crystals via recrystallization from hexane.
Scheme 4
Reaction of 3 and 4 with KF to Form 17 and 18
Solid-State Structures
Fluorosilyl Derivatives 15 and 16
The binding mode for the fluoride ion in μ-fluoro-bridged
ate complexes 15 and 16 was revealed using
X-ray crystallography.[13] The bond lengths
and bond angles in 15 and 16 and their NMR
data are summarized in Table , and their molecular structures are shown in Figures and 2, respectively. 15a, 16a, and 16b exist as separated ion pairs, in which the potassium cation (K+) is fully coordinated to the cryptand (15a and 16a) or crown ether and a THF molecule (16b).[14] On the other hand, 15b forms a
contact ion pair including an interaction between the terminal fluorine
(F1) and the potassium cation with a bond distance of 2.81 Å.
The bridging fluorine (F2) is tightly bonded to the boron atom (B–F2
= 1.49–1.51 Å) and is also weakly coordinated to the silicon
atom (Si–F2 = 2.25–2.53 Å). The two fluorine atoms
(F1 and F2) occupy apical positions with a linear alignment (F2–Si–F1
= 175–176°) in a pseudo trigonal bipyramidal (TBP) structure
centered at the silicon atom.
Table 1
Summary of the Crystallographic and
Spectroscopic Data Obtained for 15 and 16
Crystal structure of the anionic part of 15a (30%
thermal ellipsoids).
Figure 2
Crystal structure of the anionic part of 16a (30%
thermal ellipsoids).
Crystal structure of the anionic part of 15a (30%
thermal ellipsoids).Crystal structure of the anionic part of 16a (30%
thermal ellipsoids).
Difluorosilyl Derivatives 17 and 18
The structures of difluorosilyl derivatives 17 and 18 (L = [2.2.2]cryptand (a) and 18-crown-6
(b)) were also revealed using X-ray crystallography.[13] The bond lengths and the bond angles of 17 and 18 are summarized in Table , and the molecular structures
are shown in Figures and 4, respectively.
Table 2
Summary of the Crystallographic and
Spectroscopic Data Obtained for 17 and 18
–SiR3–nFn
–SiMeF2 (17a)
–SiMeF2 (17b)
–SiPhF2 (18a)
–SiPhF2 (18b)
Si–F1 [Å]
1.6300(12)
1.633(2)
1.6263(10)
1.642(5)
Si···F3 [Å]
2.2101(11)
2.2472(17)
2.1701(9)
2.021(4)
B–F3 [Å]
1.5124(19)
1.511(3)
1.5192(16)
1.568(8)
α/β [deg]
122/117
121/119
120/118
120/116
Σ(C–Si–C(F2))3 [deg]
354
352
353
357
% TBPe[5b]
81.0
74.6
77.8
90.5
δ(11B)
[ppm]
7 (br)
7 (br)
8 (br)
9 (br)
δ(19F)
[ppm]
–139.8
(1F), −132.3
(2F)
–138.8
(1F), −132.1
(2F)
–134.1
to −137.0
(3F)
–134.9
(3F)
δ(29Si)
[ppm]
–32.2
–32.8
–58.2a
–58.2a
Δδ(29Si) [ppm]
–20.1
–20.1
–28.5a
–28.6a
1JSi–F1 [Hz]
276
276
273a
273a
1JSi–F2 [Hz]
38
39
64a
65a
Observed at 181 K.
Figure 3
Crystal structure of
the anionic part of 17a (30%
thermal ellipsoids).
Figure 4
Crystal structure of the anionic part of 18a (30%
thermal ellipsoids).
Crystal structure of
the anionic part of 17a (30%
thermal ellipsoids).Crystal structure of the anionic part of 18a (30%
thermal ellipsoids).Observed at 181 K.Compounds 17a, 17b, and 18a exist as solvent-separated ion pairs. The potassium cation
is coordinated
to the cryptand (17a and 18a) or crown ether
and a THF molecule (17b).[14] On the other hand, 18b is a contact ion pair, in which
the two fluorine atoms (F1 and F2) on the silicon atom are coordinated
to the potassium cation (F1/F2–K+ = 2.801(4)/2.712(4)
Å). Furthermore, the potassium cation interacts with the C–H
moiety of the o-phenylene of an adjacent molecule,
forming an infinite chain in the solid state. The bridging fluorine
(F3) is tightly bound to the boron atom (B–F3 = ca. 1.51–1.57
Å) and weakly coordinated to the silicon atom (Si–F3 =
ca. 2.02–2.25 Å). F1 and F3 occupy apical positions with
a linear alignment (F1–Si–F3 = 174–179°)
in a pseudo TBP structure centered at the silicon atom, while F2 occupies
the equatorial position.
General Tendency in the Solid-State Structures of 15–18
The structural parameters in 15–18 are
shown in Tables and 2, and their general trends deserve comment. The
bridging fluorines (F2) in 15 and 16 and
(F3) in 17 and 18 are designated as Fbr. The Si–Fbr bond lengths in 15–18 are within the distance of the minimal non-bonded approach[15] between Si and F (2.63 Å) and are comparable
to the bridging Si–F bond lengths in μ-fluoro-bis(silicates)
(1.700(3)–2.369(3) Å)[5b] and
hexakis(fluorodimethylsilyl)benzene (2.39 Å).[5d]The Si–Fbr bond length is decreased
upon replacing a methyl group with a phenyl group (15 → 16; 17 → 18) and increasing the number of fluorine atoms (15 → 17; 16 → 18). As the Si–Fbr bond length decreases, the sum of the bond angles between
the three equatorial bonds increases and thus, the pentacoordination
character (% TBPe)[5b] of the
silicon center is increased. The % TBPe is defined as follows:
[(sum of three equatorial-to-equatorial bond angles/3) – 109.5°/(120°
– 109.5°)] × 100 (%)].It was also found that
the Si–Fbr bond length
is related to the in-plane angles of the Cipso–Si
and Cipso–B bonds. When the sum of ∠Si–C1–C2
(α°) and ∠B–C2–C1 (β°)
is reduced, the Si–Fbr bond lengths in 15–18 are decreased, while keeping the B–Fbr bonds intact.The calculated %TBPe ranges
from 65.1 to 81.0 in the
cryptand complexes and from 74.6 to 90.5 in the crown ether complexes.[5b] Thus, 18b exhibits the shortest
Si–Fbr bond length (2.021(4) Å) with the largest
equatorial bond angle (357°; % TBPe = 90.5). These
features are consistent with the general tendency that aryl substitution
and fluoro substitution increase the Lewis acidity of the silicon
center.[4,14]It has been reported that potassium
(aryl)(dialkyl)difluorosilicates
are relatively unstable and cannot be isolated even with the aid of
a cryptand ligand.[14] The present o-borylphenyl framework enables the isolation of aryl(dialkyl)difluorosilicates 15a and 15b.
Structures in Solution
The structures of 15 and 16 in THF were
investigated using multinuclear NMR spectroscopy. The NMR data obtained
for 15 and 16 are summarized in Table .[16] The cryptand and crown ether complexes exhibit almost the
same spectra. This indicates that 15 and 16 exist as solvent-separated ion pairs in THF solution, although 15b forms a contact ion pair in the solid state as observed
by X-ray crystallography. The 11B chemical shifts (δ
= 6–7) fall in the region typical for tetracoordinate borate
compounds.[17] In the 19F NMR
spectra, the terminal fluorine (F1) is observed at δ = −145
to −147 with F–F coupling (2JF–F = 9–18 Hz). On the other hand, the bridging
fluorine (F2 = Fbr) shows a broad signal at δ = −148
to −152 without F–F coupling. The coordination of the
bridging fluorine to the silicon atom is reflected in the 29Si NMR spectra:[18] the 29Si
signals are shifted upfield relative to their precursors (Δδ
= −15 for 15 and −29 for 16). The Si–F1 coupling constants (260 Hz for 15 and 262–266 Hz for 16) are reduced when compared
to their precursors (278 Hz for 1 and 284 Hz for 2). The Si–F2 coupling constant of 16 (17
Hz) is larger than that of 15 (7 Hz), which corresponds
to the shorter Si–F2 bond of 16 in the solid state.The structures of difluorosilyl derivatives 17 and 18 in THF were also investigated using multinuclear
NMR spectroscopy. The NMR data are summarized in Table .[16] The spectra of the cryptand complexes are almost the same as those
obtained for the crown ether complex. This means that 17 and 18 exist as a solvent-separated ion pair in THF,
although 18b forms a contact ion pair in the solid state.
The 11B chemical shifts (δ = 7 for 17 and δ = 8 for 18) fall in the region typical
for tetracoordinate borate compounds.[17] In the 19F NMR spectra, the two terminal fluorine nuclei
(F1 and F2) in 17 are observed at the same chemical shift
(δ = −132), which is due to the rapid intramolecular
exchange of the two fluorines at ambient temperature. The bridging
fluorine (F3) (= Fbr) shows a broad signal at δ =
−139. On the other hand, 18 exhibits a single
peak at δ = −135 due to the rapid exchange of the three
fluorines (F1, F2, and F3) at ambient temperature.The coordination
of the bridging fluorine to the silicon atom is also reflected in
the 29Si NMR spectra:[18] (a)
the 29Si signals are shifted upfield relative to those
of the precursors (Δδ = −20 for 17 and −28 for 18); (b) the Si–F1/F2 coupling
constants (276 Hz for 17 and 273 Hz for 18) are reduced when compared to their precursors (289 Hz for 3 and 293 Hz for 4) due to the elongation of
the Si–F bonds in the pentacoordinate structures observed in
the solid state; (c) the Si–Fbr coupling constant
of 18 (64 Hz at 181 K) is larger than that of 17 (38 Hz), which is consistent with the fact that the Si–Fbr bond in 18 is shorter than that in 17 in the solid state (see Table ).
Reaction and Structure of the o-Fluorosilyl
Derivatives with KCN
(Fluorosilyl)borylbenzene 1 captures the cyanide ion from KCN in the presence of 18-crown-6
in toluene at room temperature (Scheme ). The cyanide complex (19) was isolated
as colorless crystals via recrystallization from THF.[19] (Difluorosilyl)borylbenzene 3 also captures
a cyanide ion under similar reaction conditions to afford 20.
Scheme 5
Reactions of 1 and 3 with KCN to
Form 19 and 20
X-ray crystallography of 19 reveals
that the carbon
atom in the cyanide group is bonded to the boron atom (B–CN
= 1.620(8) Å), while the nitrogen atom of the cyanide ion is
coordinated to the potassium cation (N···K+ = 2.931(5) Å), as shown in Figure .[20] Furthermore,
the potassium cation interacts with the fluorine atom of an adjacent
molecule (K+···F = 2.765(4) Å), forming
an infinite chain in the solid state (see the Supporting Information).
Figure 5
Crystal structure of 19 (30%
thermal ellipsoids).
Crystal structure of 19 (30%
thermal ellipsoids).
Preparation, Reaction, and Structure of p-Fluorosilyl
Derivatives
p-(Fluorosilyl)borylbenzene
(21) was prepared for comparison with its ortho-derivative
(1). Treating p-bromo(dimethylsilyl)benzene
(22) with tert-BuLi at −78 °C
gave aryllithium 23, which was reacted with dimesitylfluoroborane
to afford p-(dimethylsilyl)borylbenzene (24) (Scheme ). Treating 24 with Ph3C+BF4– produced 21.[21]
Scheme 6
Preparation
and Reaction of p-Derivative 21
The reaction of 21 with KF in the
presence of [2.2.2]cryptand
or 18-crown-6 in toluene furnished fluoroborates 25.
Cryptand complex 25a yielded no crystals in spite of
several attempts, whereas crown complex 25b provides
colorless crystals suitable for X-ray crystallography.[22] The crystal structure of 25b revealed
that fluoride F1 was coordinated to the boron atom (F1–B =
1.493(7) Å) and also interacts with the potassium cation (F1–K+ = 2.623(3) Å), as shown in Figure . There are no intra- or intermolecular interactions
observed between the fluorine and silicon atoms.
Figure 6
Crystal structure of 25b (30% thermal ellipsoids).
Crystal structure of 25b (30% thermal ellipsoids).
Competition Reactions of o-Fluorosilyl Derivatives
The fluoride ion affinity of fluorodimethylsilyl derivative 1 and fluorodiphenylsiyl derivative 2 were estimated
using competition reactions with triarylborane 26 and
(fluoro)triarylborate 27, as shown in Scheme .[23,24] Mixing equimolar amounts of 1 and 27 in
THF at 25 °C gave an equilibrium mixture consisting of 1, 27, 15b, and 26.
The ratio of 27/15b was determined to be 5:95 (K298 = 3.6 × 102) using 1H NMR spectroscopy. Mixing fluorodiphenylsilyl derivative 2 and 27 in THF forms an equilibrium mixture
consisting of 2, 27, 16b, and 26. The ratio of 27/16b was determined to be
22:78 (K298 = 13). The reactions in the
reverse direction were executed by mixing 15b (or 16b) and 26 in THF, providing almost the same
equilibrium mixture (27/15b = 2:98; 27/16b = 21:79). Thus, the fluorosilyl groups
ortho to the boron atom increase the fluoride ion affinity of the
triarylborane compounds.[3] These results
suggest that the fluoride ion affinity of 1 is higher
than that of 2. In fact, competition reactions between 1 and 16b as well as 15b and 2 preferably form 15b over 16b (15b/16b = 82:18; K298 = 21) (Scheme ).
Scheme 7
Competition Reactions of 1 or 2 vs 26
Scheme 8
Competition Reactions of 1 vs 2
Competition Reactions of o-Difluorosilyl Derivatives
The fluoride ion affinity of difluorosilyl derivatives 3 and 4 was also estimated using competition reactions
between fluorosilyl derivatives 1 and 2 and
their corresponding fluoroborates 15b and 16b, as shown in Scheme .[23,24] Mixing equimolar amounts of 3 and 15b in THF at 25 °C yielded an equilibrium
mixture of 3, 15b, 17b, and 1. The ratio of 15b/17b was determined to be
21:79 (K298 = 14) using 1H
NMR spectroscopy. Difluorophenylsilyl derivative 4 also
captures a fluoride ion from 15b to provide an equilibrium
mixture of 4, 15b, 18b, and 1. The ratio of 15b/18b was determined to be
27:73 (K298 = 7.0). The reactions in the
reverse direction were carried out by mixing 17b (or 18b) with 1 and exhibited almost similar equilibrium
positions (17b/15b = 18:82; 18b/15b = 25:75). Thus, the fluoride ion affinities of
difluorosilyl derivatives 3 and 4 were higher
than that observed for monofluorosilyl derivative 1.
The competition reactions between 3 and 18b as well as 17b and 4 in THF preferably
formed 17b over 18b (17b/18b = 55:45; K298 = 1.5) (Scheme ).
Scheme 9
Competition
Reactions of 3 or 4 vs 1
Scheme 10
Competition Reactions of 3 vs 4
Competition Reactions of p-Fluorosilyl Derivatives
The fluoride ion affinity of p-(fluorosilyl)(boryl)benzene 21 was estimated using competition reactions with triarylborane 26 and (fluoro)triarylborate 27, as shown in Scheme .[23,24] Mixing equimolar amounts of 21 and 27 in
THF at 25 °C gave an equilibrium mixture of 21, 27, 25b, and 26. The ratio of 27/25b was determined to be 30:70 using 1H NMR
spectroscopy. The reaction in the reverse direction was carried out
mixing 25b and 26 and provides the same
equilibrium mixture. Thus, the para fluorosilyl group
also increases the fluoride ion affinity of the triarylborane but
less than the ortho fluorosilyl group.
Scheme 11
Competition
Reactions of 3 or 4 vs 1
Fluoride Ion Affinity of 1–4
From
the results of the competition reactions, the order of the fluoride
ion affinity of o-(silyl)borylbenzenes 1–4 and triarylborane 26 was determined
to be 26 < 2 < 1 < 4 < 3, as shown in Chart .[3a] Difluorosilyl
derivatives exhibit a higher fluoride ion affinity than the monofluorosilyl
derivatives. It is reasonable that the electronegative fluorine atoms
increase the Lewis acidity of the silicon center. The methylsilyl
derivatives exhibit a higher fluoride ion affinity than the phenylsilyl
derivatives.
Chart 3
Order of Fluoride Ion Affinity Observed for 1–4 and 26
The total stability of o-(silyl)borylbenzenes 1–4 can be determined by the total balance
between the two interactions: (a) the electrostatic attraction between
the silicon center and bridging fluoride atom and (b) steric repulsion
between the substituents on the silicon atom and the mesityl groups
on the boron atom, which destabilize the μ-fluoro-bridged ate
complex.
Computational Studies on 1–4 and 15–18
To obtain further insight into the B-μ-F–Si
bonding interactions in 15–18, DFT
calculations,[25] GIAO calculations,[26] and natural bond orbital (NBO) analysis[27] were performed.Since most of compounds 15–18 exist as separated ion pairs both in the solid
state and in solution, it is reasonable that the initial coordinates
were generated from the crystallographic data of the anion parts in 15a, 16b, 17a, and 18a.[28]
Geometry Optimization and GIAO NMR Shifts
We carried
out the geometry optimization at the B3PW91/6-31G(d) level of theory.[25] The input coordinates were generated from the
X-ray crystallographic data of the anions in 15a, 16b, 17a, and 18a. The optimized
geometries were designated as 15, 16, 17, and 18 for those obtained at the B3PW91/6-31G(d) level of theory.
The geometrical parameters are summarized in Table . Their NMR shifts were calculated using
the GIAO method at the RHF/6-311+G(2d,p) level of theory.[26] The calculated geometries and NMR shifts are
well matched to those observed experimentally, which is evidence for
the validity of the calculations for the anion parts only.
Table 3
Calculated Structural Parameters of 15–18
15opt
16opt
17opt
18opt
Si–Fap [Å]
1.664
1.668
1.653
1.655
Si–Feq [Å]a
1.620
1.619
Si···Fbr [Å]a
2.508
2.355
2.273
2.175
B–Fbr [Å]a
1.488
1.510
1.508
1.528
α/β [deg]a
124.3/120.1
123.5/119.2
121.9/118.1
120.7/117.6
δ(29Si)
[ppm]b
4.7
–28.6
–36.9
–61.3
δ(11B)
[ppm]b
3.8
4.9
4.8
6.3
δ(19F)
[ppm]b
–146.3
–148.8
–135.7
–137.6
–143.5
–126.1
–135.6
–131.1
–131.1
–114.3
Optimized at B3PW91/6-31G(d).
GIAO at RHF/6-311+G(2d,p)//B3PW91/6-31G(d).
Optimized at B3PW91/6-31G(d).GIAO at RHF/6-311+G(2d,p)//B3PW91/6-31G(d).
NBO Analysis
In order to shed light on the bonding
interactions in 15–18, NBO analysis was executed
at the B3PW91/6-31G(d) level of theory.[27] Fap corresponds to F1 in 15, 16, 17, and 18; Feq corresponds
to F2 in 17 and 18; Fbr corresponds
to F2 in 15 and 16 and F3 in 17 and 18.The second-order perturbation theory
analysis reveals intramolecular coordination from the donor LPF to the acceptor LPSi*. The sum of the stabilization energy E(2) was calculated to be 18–31 kcal/mol, as shown
in Table . One of
the lone pairs of the bridging fluorine atom (LP[F]) mainly coordinates
to the σ*(Si–F) orbital (LP*[Si]).
Table 4
Selected NBO Analysis Data of 15–18a
donor NBOb
acceptor
NBOc
E(2)d(kcal/mol)
15opt
LP(1)[F4]
LP*(1)[Si]
4.88
LP(3)[F4]
LP*(1)[Si]
13.84
16opt
LP(1)[F4]
LP*(1)[Si]
5.86
LP(3)[F4]
LP*(1)[Si]
23.24
17opt
LP(1)[F3]
LP*(2)[Si]
5.34
LP(3)[F3]
LP*(2)[Si]
23.12
18opt
LP(3)[F5]
LP*(2)[Si]
31.27
Optimized at B3PW91/6-31G(d).
LP[F] = lone pair at the bridging
fluorine atom.
LP*[Si] =
vacant orbital at the
silicon atom.
Stabilization
energy associated
with the delocalization of electrons from LP[F] to LP*[Si] according
to second-order perturbation theory analysis of the fock matrix in
NBO basis. The threshold for listing is 5.0 kcal/mol.
Optimized at B3PW91/6-31G(d).LP[F] = lone pair at the bridging
fluorine atom.LP*[Si] =
vacant orbital at the
silicon atom.Stabilization
energy associated
with the delocalization of electrons from LP[F] to LP*[Si] according
to second-order perturbation theory analysis of the fock matrix in
NBO basis. The threshold for listing is 5.0 kcal/mol.
Computed Fluoride Ion Affinity
In order to assess the
strength of the interactions involving the bridging fluoride in 15–18, we have computed the gas-phase fluoride ion
affinities (FIAs) of 1–4.[4h,29] COF2 was used as a reference compound to simplify the
calculations (Scheme ). The resulting relative FIAs were converted to an absolute scale
using the experimentally known value (49.9 kcal/mol) for the FIA of
COF2. The obtained FIAs are as follows: −81.1 kcal/mol
for 1, −80.2 kcal/mol for 2, −81.1
kcal/mol for 3, −84.2 kcal/mol for 4, and −72.5 kcal/mol for 26 (see the Supporting Information). Thus, the order of the
calculated FIAs is 4 > 3 > 1 > 2 > 26. The calculated order is
partly
inconsistent with the observed one: the order of 3 and 4 is reversed. Although the reason is not clear, it may be
plausible that the competition reactions of 3 versus 4 did not completely achieve the equilibria or the effect
of the countercation cannot be negligible.
Scheme 12
Isodesmic Reactions
for FIA Calculations of 1, 2, 3, and 4
Dynamic Behavior of μ-Fluoro-Bridged Ate Complexes 15–18: Site-Exchange Process Monitored Using 19F NMR Spectroscopy
The site exchange between the two or
three fluorine atoms in 15–18 was observed using
variable-temperature 19F NMR spectroscopy. The changes
in the NMR spectra can be rationalized using four processes described
below.[5b]
General Outline of the Site-Exchange Processes
Process a: Rotation around the Si–Cipso Bond
Rotation around the Si–Cipso bond causes site
exchange between Fap and Feq, resulting in the
coalescence of the peaks observed for Fap and Feq. This rotation is retarded with an increase in the strength of the
Si–Fbr bond.
Process b: Fbr Migration between the Boron and Silicon
Atoms
The migration of Fbr between the boron and
silicon atoms results in the formation of tetracoordinate boron (fluoroborate)
or pentacoordinate silicon (fluorosilicate), resulting in the broadening
of the peak corresponding to Fbr. The B–Fbr bond is strong when compared to the Si–Fbr bond
(interaction), as observed in the crystal structures. Thus, Fbr migration may be a high-energy process when compared to
Si–Cipso bond rotation.
Process c: Pseudorotation at the Pentacoordinate Silicon Center
Pseudorotation[3c,3d] at the pentacoordinate silicon
causes site exchange among Fap, Feq, and Fbr, resulting in the coalescence of these peaks.
Process d: Flipping of the Five-Membered Ring
Flipping
of the C–Si−μ-F–Si–C five-membered
ring in the o-bis(silyl)benzene complex has been
discussed by Tamao et al.[5b] However, flipping
of the C–Si−μ-F–B–C five-membered
ring in the o-(silyl)borylbenzene complexes may not
cause large motion because the five-membered ring is only slightly
puckered.
FMe2Si Derivatives 15
Two peaks
corresponding to Fap (=F1) (δ = −147) and
Fbr (=F2) (δ = −152) were observed at 298
K (Figures S2–S5, see the Supporting
Information).[9] These peaks exhibit no noticeable
change up to 383 K.
FPh2Si Derivatives 16
Two distinct
peaks corresponding to Fap (=F1) (δ = −145)
and Fbr (=F2) (δ = −148) were observed at
298 K in DMSO-d6 (Figure S2-S6, see the Supporting Information).[9] These peaks broadened with increasing temperature and then
coalesced at 361 K. The peaks finally appeared as a single peak at
373 K. This was attributed to Fbr migration from the boron
to silicon atoms (process b) and subsequent pseudorotation of the
resulting pentacoordinate silicon (process c), as shown in Scheme . The two processes
may occur coincidentally in the same temperature range. The possibility
of rotation around the Si–Cipso bond (process a)
rather than pseudorotation cannot be excluded because the two processes
lead to the same structure in 16.
Scheme 13
Plausible F–F
Exchange Process between B and Si for 15 and 16: (b) F– Migration
between B and Si; (c) Pseudorotation at Si
F2MeSi Derivatives 17
Three
peaks corresponding to Fap (=F1) (δ = −125.9),
Feq (=F2) (δ = −137.0), and Fbr (=F3) (δ = −138.8) were observed at 193 K in THF-d8, as shown in Figure a. Increasing the temperature broadened the
two peaks corresponding to Fap and Feq, which
then coalesce at 258 K, while the Fbr peak remained unchanged.
This was attributed to the site exchange between Fap and
Feq caused by Si–Cipso bond rotation
(process a). The combined peak corresponding to Fap/eq (δ
= −132) and Fbr (δ = −138) was observed
at 290 K. As shown in Figure b, increasing the temperature broadened the Fap/eq and Fbr peaks, which then coalesce at 373 K and finally
appear as a single peak at 413 K. This was attributed to site exchange
among Fap, Feq, and Fbr caused by
Fbr migration (process b) from boron to silicon and subsequent
pseudorotation (process c) of the resulting pentacoordinate silicon,
as shown in Scheme . It is notable that the Fbr peak was completely broadened
prior to its coalescence at 359 K, which indicates that Fbr migration and pseudorotation occur in a stepwise manner.
Figure 7
Variable-temperature 19F NMR spectra of 17a recorded (a) at lower temperatures
in THF-d8 and (b) at higher temperatures
in DMSO-d6.
Scheme 14
Plausible F–F Exchange Process between B and
Si for 17 and 18: (a) Si–C Rotation,
(b) F– Migration between B and Si, and (c) Pseudorotation
at Si
Variable-temperature 19F NMR spectra of 17a recorded (a) at lower temperatures
in THF-d8 and (b) at higher temperatures
in DMSO-d6.
F2PhSi Derivatives 18
The dynamic
behavior of F2PhSi derivatives 18 was rather
complicated. The NMR spectra recorded in THF-d8 at 181 K showed two sets of peaks in a 2:1 ratio (Figure ). On the base of
the peak intensities, we tentatively assigned the peaks at δ
−123.4 (dd, JF–F = 30 and
15 Hz, Fap(maj)), −135.8 (broad, Fbr(maj)),
and −146.9 (t, Jav = 33 Hz, Feq(maj)) to major isomer 18 and the peaks at δ −128.7 (dd, JF–F = 23 and 11 Hz, Fap(min)), −139.1
(broad, Fbr(min)), and −140.1 (dd, J = 65 and 25 Hz, Feq(min)) to minor isomer 18.
Figure 8
Variable-temperature 19F NMR spectra
of 18a recorded in THF-d8.
Variable-temperature 19F NMR spectra
of 18a recorded in THF-d8.Increasing the temperature led to line broadening
and coalescence
of the peaks. Fbr(maj) and Fbr(min) coalesced
at 223 K. At that temperature, Fap(maj), Feq(maj), Fap(min), and Feq(min) underwent massive
broadening and then appeared as a single peak (Fap/eq)
at 273 K. Finally, Fbrand Fap/eq coalesced to
a single peak (Fap/eq/br) at 343 K. It was confirmed that
these behaviors were reversible between 181 and 343 K.The two
isomers 18 and 18 may be assignable to conformational
isomers associated to the flipping of the C–Si−μ-F–B–C
five-membered ring. The optimized structures of the two isomers were
obtained at the B3PW91/6-31G(d) level of theory, as shown in Scheme (see the Supporting Information in detail). The flipping
causes change in the conformations of the two Mes groups on the boron
atom. Thus, one of the Mes groups and the Ph group on the silicon
atom are almost parallel to each other in more stable 18, whereas the Mes group and the Ph group
are perpendicular to each other, which causes the steric repulsion,
in less stable 18. It was
found that 18 is the same
as 18 in terms of the geometrical
parameters and the thermal parameters.
Scheme 15
Plausible Flipping
of the Puckered C–Si−μ-F–B–C
Five-Membered Ring in 18 and 18 (above) and Their Optimized
Structures at the B3PW91/6-31G(d) Level of Theory (below)
The calculated energy difference between 18 and 18 (ΔG181°) is 0.184 kcal/mol, and the derived
equilibrium constant K is 1.67, which is close to
the observed ratio of the two
isomers at 181 K (2:1). The unusual behavior of 18a remains
unclear and still under investigation.
Conclusions
A series of o-(fluorosilyl)(boryl)benzenes 1–4 were prepared using [o-(silyl)phenyl]lithium anions 7, 8, 11, and 12 and characterized using NMR spectroscopy
and X-ray crystallography. Compounds 1–4 capture
a fluoride ion in the presence of [2.2.2]cryptand or 18-crown-6 to
afford their corresponding μ-fluoro-bridged ate complexes (15–18), which were also characterized in solution and
the solid state. DFT studies and NBO analysis of 15–18 revealed the nature of the Si–F and B–F bonding interactions
in the μ-fluoro-bridges. The fluoride ion affinities of 1–4 were demonstrated using competitive reactions,
and the order of the fluoride ion affinity was determined to be 3 > 4 > 1 > 2. The
dynamic behavior of 15–18 at variable temperatures
mainly consists of (a) rotation around the Si–Cipso bond, (b) Fbr migration between the boron and silicon
atoms, and (c) pseudorotation at the pentacoordinate silicon center.
Experimental Section
General Procedures
1H (400 MHz), 11B (128.3 MHz), 13C (100 MHz), 19F (376 MHz),
and 29Si (79.4 MHz) NMR spectroscopy was recorded on a
JEOL EX-400 or AL-400 spectrometer. The 1H and 13C NMR chemical shifts were referenced to the residual NMR solvent
signal (CDCl3: δ(1H) = 7.26 ppm, δ(13C) = 77.0 ppm; C6D6: δ(1H) = 7.20 ppm, δ(13C) = 128.0 ppm). The 1H and 13C chemical shifts in THF-d8 were referenced to tetramethylsilane as an internal standard
(δ(1H) = 0 ppm; δ(13C) = 0 ppm).
The 11B chemical shifts were referenced to BF3·Et2O as an external standard (δ = 0 ppm).
The 19F chemical shifts were referenced to CFCl3 as an external standard (δ = 0 ppm). The 29Si chemical
shifts were referenced to tetramethylsilane as an external standard
(δ = 0 ppm). Mass spectroscopy (EI) was recorded at 70 eV using
a JEOL SX-102A mass spectrometer located at the Natural Science Center
for Basic Research and Development (N-BARD), Hiroshima University.
Melting points were measured using a Yanaco micro melting point apparatus
and were uncorrected. Elemental analysis was performed on a PerkinElmer
2400CHN elemental analyzer located at Prof. Yamamoto’s Laboratory.
Column chromatography on silica gel was performed using Kieselgel
60 (230–400 mesh) (Merck). All reactions were carried out under
an inert gas atmosphere otherwise noted.Hexane was distilled
under a nitrogen atmosphere over calcium hydride, or the dehydrated
solvent (<10 ppm; Kanto Chemical Co., Inc.) was dried using a solvent
dispensing system (GlassContour) under a nitrogen atmosphere (99.999%).
THF and Et2O were distilled under a nitrogen atmosphere
over sodium diphenylketyl, or the dehydrated solvents (<10 ppm
for THF and <50 ppm for Et2O; Kanto Chemical Co., Inc.)
were dried using a solvent dispensing system (GlassContour) under
a nitrogen atmosphere (99.999%). THF-d8, toluene, and benzene were distilled under a nitrogen atmosphere
over sodium diphenylketyl. CCl4, CHCl3, CH2Cl2, CH3CN, and MeOH were distilled
under a nitrogen atmosphere over calcium hydride.The following
reagents were used as received: tert-BuLi in pentane
(Kanto Chemical Co., Inc), n-BuLi
in hexane (Kanto Chemical Co., Inc.), [2.2.2]cryptand (Wako Pure Chemical
Industries, Ltd.), 1,2-dibromobenzene (Tokyo Chemical Industry Co.,
Ltd.), 1,4-dibromobenzene (Sigma-Aldrich), HF·pyridine (Tokyo
Chemical Industry Co., Ltd.), AgF (Mitsuwa Chemicals Co., Ltd.), CuF2 (98%) (Sigma-Aldrich), PdCl2 (Wako Pure Chemical
Industries, Ltd.), KCN (Katayama), KCI (Wako Pure Chemical Industries,
Ltd.), BCl3 in heptane (Sigma-Aldrich), BCl3 in CH2Cl2 (Wako Pure Chemical Industries,
Ltd.), chloro(dimethoxy)methylsilane (Shin-Etsu Chemical Co., Ltd.),
trimethylphenylsilane (Shin-Etsu Chemical Co., Ltd.), chlorotrimethylsilane
(Tokyo Chemical Industry Co., Ltd.), chlorodimethylsilane (Tokyo Chemical
Industry Co., Ltd.), and phenylsilane (Tokyo Chemical Industry Co.,
Ltd.).KF (spray-dried grade (98%)) (Wako Pure Chemical Industries,
Ltd.)
was dried in vacuo at 150 °C for 1 d. BF3·OEt2 (Kanto Chemical Co., Inc.) was distilled over CaH2. B(OMe)3 (Tokyo Chemical Industry Co., Ltd.) was distilled
under a nitrogen atmosphere. Ph3CBF4 (Sigma-Aldrich)
was recrystallized from CH3CN and dried in vacuo. n-Bu4NCl (Tokyo Chemical Industry Co., Ltd.)
was recrystallized from acetone and dried in vacuo. n-Bu4NBr (Sigma-Aldrich) was recrystallized from ethyl
acetate and dried in vacuo. n-Bu4NI (Tokyo
Chemical Industry Co., Ltd.) was recrystallized from ethyl acetate
and dried in vacuo. 18-crown-6 (Tokyo Chemical Industry Co., Ltd.)
was recrystallized from CH3CN and dried in vacuo. 12-Crown-4
(Merck) was distilled under a nitrogen atmosphere over CaH2.Fluorodimesitylborane[30] and dimesitylphenylborane[31] (26) were prepared according to
previously reported procedures. The preparation of compounds 1, 2, 13, and 14 has
been reported in our previous papers.[9,10]
A solution of 1 (202 mg,
0.50 mmol), 18-crown-6 (136 mg, 0.51 mmol), and KF (30 mg, 0.51 mmol)
in toluene (1.5 mL) was stirred at room temperature for 5 h forming
a white precipitate. The solvent was removed in vacuo, and the resulting
white solid was dissolved in a minimum amount of THF at room temperature.
A small amount of hexane was then slowly added over the saturated
solution, resulting in a two-layer solution. The two-layer solution
was allowed to stand at room temperature for 1 d to give 15b·THF (332 mg, 83% yield) as colorless crystals.mp 155–157
°C (decomposed). 1H NMR (THF-d8): δ 0.05 (br, 6H), 1.87 (br, 12H), 2.08 (s, 6H), 3.53
(s, 24H, crown), 6.36 (br, 4H), 6.78–6.80
(m, 2H), 7.05–7.08 (m, 1H), 7.60–7.62 (m, 1H). 13C NMR (THF-d8): δ 2.7 (br),
21.2, 26.3, 71.1 (crown), 122.7, 127.2, 128.6, 130.9,
133.3 (d, 3JF–C = 8
Hz), 134.0 (d, 3JF–C = 9 Hz), 141.6 (dd, 2JF–C = 15 Hz, 2JF–C = 7
Hz), 142.0 (br); the peaks corresponding to the ipso and para carbon atoms in the mesityl groups were
not observed. 11B NMR (THF-d8): δ 6 (br). 19F NMR (THF-d8): δ −151.7 (br), −146.6 (d of sept, 3JF–H = 9 Hz, 2JF–F = 9 Hz). 29Si
NMR (THF-d8): δ 6.4 (d, 1JSi–F = 260 Hz). Anal. Calcd for
C42H64BF2KO7Si (15b·THF): C, 63.30; H, 8.09. Found: C, 63.02; H, 8.45.
A solution of 1 (201 mg, 0.50
mmol), 18-crown-6 (87 mg, 0.33 mmol), and KCN (22 mg, 0.32 mmol) in
toluene (3.0 mL) was stirred at room temperature for 20 h. The solvent
was removed in vacuo, and the resulting white solid was dissolved
in THF (ca. 5 mL) at room temperature. The solution was filtered through
a membrane filter; the filtrate was partly concentrated and cooled
to −31 °C to give 19·THF as colorless
crystals (81 mg, 36%).mp 195–200 °C (decomposed/sealed). 1H NMR (THF-d8): δ 0.22–0.33
(br, 6H), 1.89 (br, 12H), 2.11 (s, 6H), 3.59 (s, 24H, crown), 6.40–6.49 (br, 4H), 6.77 (t, 3J = 7 Hz, 1H), 6.87 (t, 3J = 7 Hz, 1H),
7.07 (d, 3J = 8 Hz, 1H), 7.62 (d, 3J = 8 Hz, 1H). 13C NMR (THF-d8): δ −3.2 (d, 2JF–C = 18 Hz, CH3), 21.1 (CH3), 26.4 (CH3), 71.1 (crown, CH2), 123.0 (CH), 127.8 (CH), 129.4 (CH), 131.7 (C), 134.1 (d, 3JF–C = 7 Hz, CH), 137.0
(CH), 143.5 (C), 144.3 (d, 3JF–C = 11 Hz, C); the peaks corresponding to the ipso and para carbon atoms in the mesityl groups and
the carbon atom in the cyano group were not observed. 11B NMR (THF-d8): δ −14 (br). 19F NMR (THF-d8): δ −152.1
(sept, 3JF–H = 9 Hz). 29Si NMR (THF-d8): δ 20.6
(d, 1JSi-F = 266 Hz).
Anal. Calcd for C43H64BFKNO7Si (19·THF): C, 64.24; H, 8.02; N, 1.74. Found: C, 64.17;
H, 8.12; N, 1.67.
A solution of 26 (163 mg, 0.50 mmol), KF
(30 mg, 0.51 mmol), and 18-crown-6 (134 mg, 0.51 mmol) in toluene
(1.5 mL) was stirred at room temperature for 3.5 h. The solvent was
removed in vacuo, and the resulting white solid was dissolved in a
minimum amount of THF at room temperature. A small amount of hexane
was slowly added over the saturated solution, resulting in a two-layer
solution. The two-layer solution was allowed to stand at room temperature
for 1 d to give 27·THF (304 mg, 94% yield) as colorless
crystals.mp 173–174 °C (decomposed). 1H NMR (THF-d8): δ 1.90 (s, 12H),
2.10 (s, 6H), 3.54 (s, 24H, crown), 6.42 (s, 4H),
6.79 (t, 1H, 3J = 7 Hz), 6.70–7.20
(br, 3H), 7.90 (br, 1H). 13C{1H} NMR (THF-d8): δ 21.3, 25.8 (d, 4JF-C = 4 Hz), 70.9 (crown), 123.2, 126.2, 128.9, 131.1, 134.6, 142.0 (d, 3JC–F = 3 Hz), 156.0 (br), 164.1 (br). 11B{1H} NMR (THF-d8):
δ 5.5. 19F NMR (THF-d8): δ −173.2. Anal. Calcd for C40H59O7BFK (27·THF): C, 66.65; H, 8.25. Found:
C, 66.42; H, 8.52.
p-Bromo(dimethylsilyl)benzene (22)
n-BuLi in hexane (1.66 M, 20.0 mL, 33.2
mmol) was added dropwise over 11 min to a solution of p-dibromobenzene (7.22 g, 30.0 mmol) in THF (60 mL) and Et2O (60 mL) at −78 °C. The resulting reaction mixture was
stirred at −78 °C for 40 min. A solution of chlorodimethylsilane
(4.0 mL, 36.0 mmol) in Et2O (15 mL) was added dropwise
over 13 min to the reaction mixture at −78 °C. The resulting
reaction mixture was stirred at −78 °C for 1.5 h and then
allowed to warm to room temperature. The solvent was evaporated, and
the resulting residue was dissolved in hexane (30 mL). The solution
was filtered through a glass fiber pad, and the filtrate was concentrated.
The resulting residue was distilled under reduced pressure (43–46
°C/0.4 mmHg) to give 22 as a colorless liquid (4.49
g, 70% yield).1H NMR (CDCl3): δ
0.33 (d, 3J = 4 Hz, 6H), 4.39 (sept, 3J = 4 Hz, 1H), 7.40 (dt, 3J = 8 Hz, 4J = 2 Hz, 2H), 7.50
(dt, 3J = 8 Hz, 4J = 2 Hz, 2H). 13C NMR (CDCl3): δ −3.9
(CH3), 123.9 (C), 131.0 (CH), 135.5 (CH), 136.1 (C). 29Si NMR (CDCl3): δ −16.6 (s).
p-(Dimethylsilyl)(dimesitylboryl)benzene (24)
A solution of tert-BuLi in pentane
(1.59 mol/L, 3.3 mL, 5.25 mmol) was added to a solution of 22 (1.11 g, 5.17 mmol) in Et2O (9 mL) at −78 °C
over 3 min. The resulting reaction mixture was stirred at −78
°C for 2 h, forming a white suspension of 23. A
solution of fluorodimesitylborane (1.41 g, 5.26 mmol) in Et2O (8 mL) was added to the suspension at −78 °C over 8
min, and the reaction mixture was stirred at −78 °C for
0.5 h and 0 °C for 0.5 h and then allowed to warm to room temperature.
The reaction mixture was concentrated in vacuo; the residue was diluted
with hexane (20 mL), and the insoluble precipitate was removed via
filtration through a glass fiber pad. The filtrate was concentrated
in vacuo, and the crude product (2.19 g) was subjected to column chromatography
on silica gel eluted with hexane (R = 0.43) to give 22 (1.51 g, 76%) as a white solid.mp 97–99 °C (air). 1H NMR (C6D6): δ 0.22 (dd, 3J =
4 Hz, 4J = 1 Hz, 6H), 2.15–2.30
(br, 18H), 4.65 (sept, 3J = 4 Hz, 1H),
6.85 (s, 4H), 7.56 (d, 3J = 7 Hz, 2H),
7.74 (d, 3J = 7 Hz, 2H). 13C NMR (C6D6): δ −4.0 (CH3), 21.3 (CH3), 23.8 (CH3), 128.8 (CH), 134.1
(CH), 135.8 (CH), 138.9 (C), 141.0 (C), 142.2 (br, C), 142.3 (C),
147.2 (br, C). 11B NMR (C6D6): δ
75 (br). 29Si NMR (C6D6): δ
−16.8 (s). Anal. Calcd for C26H33BSi:
C, 81.23; H, 8.65. Found: C, 80.84; H, 8.88.
A solution of 24 (1.93 g,
5.02 mmol) in CH2Cl2 (15 mL) was added dropwise
over 5 min to a solution of Ph3CBF4 (1.68 g,
5.09 mmol) in CH2Cl2 (13 mL) at 0 °C. The
reaction mixture was stirred at 0 °C for 1.5 h and then allowed
to warm to room temperature. The reaction mixture was concentrated
in vacuo. The residue was diluted with hexane (30 mL), and the insoluble
precipitate was removed via filtration through a glass fiber pad.
The filtrate was partly concentrated and cooled to −18 °C.
The precipitated triphenylmethane (917 mg) was removed via filtration.
The filtrate was concentrated, and the residue was subjected to bulb-to-bulb
distillation under reduced pressure (190 °C (bath temp.)/0.1
mmHg) to give the remaining triphenylmethane (282 mg). The residue
was recrystallized from hexane at −18 °C to give 21 (1.31 g, 65%) as a white solid.mp 93–97 °C
(air). 1H NMR (THF-d8): δ
0.47 (dd, 3J = 8 Hz, 4J = 1 Hz, 6H), 1.98 (s, 12H), 2.28 (s, 6H), 6.81 (s, 4H),
7.49 (d, 3J = 7 Hz, 2H), 7.60 (d, 3J = 7 Hz, 2H). 13C NMR (THF-d8): δ −1.2 (d, 2JF–C = 16 Hz, CH3), 21.4 (CH3), 23.8 (CH3), 129.1 (CH), 133.4 (d, 3JF–C = 2 Hz, CH), 136.1 (CH),
139.6 (C), 141.4 (C), 141.5 (C), 142.5 (br, C), 148.8 (br, C). 11B NMR (THF-d8): δ 75 (br). 19F NMR (THF-d8): δ −161.8
(sept, 3JF–H = 7 Hz). 29Si NMR (THF-d8): δ 20.4
(d, 1JSi–F = 278 Hz).
Anal. Calcd for C26H32BFSi: C, 77.60; H, 8.01.
Found: C, 77.68; H, 8.32.
A solution
of 1 (20 mg, 0.050 mmol) and 27 (33 mg,
0.050 mmol) in THF (1.0 mL) in a Schlenk flask was stirred at room
temperature for 24 h. The reaction mixture was transferred into a
J. Young NMR tube, and its 19F NMR spectrum was recorded
at 298 K in an unlocked mode. The molar ratio (95:5) was determined
using the ratio of the integrals for 15b and 27: K298 = [15b][26]/[1][27] = [95][95]/[5][5] = 3.6 ×
102.
Reaction of 1 and 16b in THF (Scheme )
A solution
of 1 (20 mg, 0.050 mmol) and 16b (46 mg,
0.050 mmol) in THF (1.0 mL) in a Schlenk flask was stirred at room
temperature for 24 h. The reaction mixture was transferred into a
J. Young NMR tube, and its 19F NMR spectrum was recorded
at 298 K in an unlocked mode. The molar ratio (95:5) was determined
using the ratio of the integrals for 15b and 16b: K298 = [15b][2]/[1][16b] = [82][82]/[18][18] = 21.
Reaction of 3 and 15b in THF (Scheme )
A solution
of 3 (20 mg, 0.050 mmol) and 15b (40 mg,
0.050 mmol) in THF (1.0 mL) in a Schlenk flask was stirred at room
temperature for 24 h and then heated at reflux for 24 h to complete
the equilibrium. The reaction mixture was transferred into a J. Young
NMR tube, and its 19F NMR spectrum was recorded at 298
K in an unlocked mode. The molar ratio (79:21) was determined using
the ratio of the integrals for 15b and 17b: K298 = [17b][1]/[3][15b] = [79][79]/[21][21] = 14.
Reaction of 3 and 18b in THF (Scheme )
A solution
of 3 (20 mg, 0.050 mmol) and 18b (40 mg,
0.050 mmol) in THF (1.0 mL) in a Schlenk flask was stirred at room
temperature for 24 h. The reaction mixture was transferred into a
J. Young NMR tube, and its 19F NMR spectrum was recorded
at 298 K in an unlocked mode. The molar ratio (45:55) was determined
using the ratio of the integrals for 3 and 4: K298 = [17b][4]/[3][18b] = [55][55]/[45][45] = 1.5.
Dynamic Behavior at Variable Temperatures
Typical Procedure
A solution of 15a (20
mg, 0.024 mmol) in THF-d8 (0.60 mL) was
injected into a J. Young NMR tube, and its 19F NMR spectrum
was recorded at variable temperatures.
Authors: Andrey Y Khalimon; Zi Hua Lin; R Simionescu; Sergei F Vyboishchikov; Georgii I Nikonov Journal: Angew Chem Int Ed Engl Date: 2007 Impact factor: 15.336
Chart 1
Chart 2
Scheme 1
Scheme 2
Scheme 3
Scheme 4
Figure 1
Figure 2
Figure 3
Figure 4
Scheme 5
Figure 5
Scheme 6
Figure 6
Scheme 7
Scheme 8
Scheme 9
Scheme 10
Scheme 11
Chart 3
Scheme 12
Scheme 13
Figure 7
Scheme 14
Figure 8
Scheme 15