Synthesis of alkali metal salts of borylsilyl anions utilizing highly crowded silylboranes and their properties

Article abstract of DOI:10.1021/om7008974

The lithium salt of a borylsilyl anion, [Tbt(Mes)SiBpin]^-Li ⁺ (11: Tbt = 2,4,6-tris[bis(trimethylsilyl)-methyl]phenyl, Mes = mesityl, pin = pinacolato), was synthesized by the boron-metal exchange reaction of a diborylsilane, Tbt(Mes)Si(Bpin)₂ (8), which was synthesized by the insertion of a highly crowded diarylsilylene, Tbt(Mes)Si: (1), into the B-B bond of a diborane(4) compound, B₂pin₂. Although a salt of the borylsilyl anion 11 could not be isolated due to its thermal instability, the investigation of the reactivity of the anion revealed that 11 is a possible precursor for Si-functionalized silylboranes as well as Si,Si-difunctionalized silanes. Furthermore, the borylsilyl anion [Tbt(Mes)SiBScat]^- (27: Scat = dithiocatecholato), which was synthesized by a procedure similar to that used for 11, showed a reactivity different from that of 11. Theoretical calculations for the borylsilyl anions revealed that the anion 27 has a boratasilene character; namely, the Si-B bond has a double-bond character due to the donation of an electron pair of the negatively charged silicon atom into the vacant p-orbital of the adjacent boron atom.

Full text of DOI:10.1021/om7008974

880
Organometallics 2008, 27, 880–893
Synthesis of Alkali Metal Salts of Borylsilyl Anions Utilizing Highly
Crowded Silylboranes and Their Properties#
Takashi Kajiwara, Nobuhiro Takeda,‡ Takahiro Sasamori, and Norihiro Tokitoh*
Institute for Chemical Research, Kyoto UniVersity, Uji, Kyoto 611-0011, Japan
ReceiVed September 7, 2007
The lithium salt of a borylsilyl anion, [Tbt(Mes)SiBpin]–Li+ (11: Tbt ) 2,4,6-tris[bis(trimethylsilyl)-
methyl]phenyl, Mes ) mesityl, pin ) pinacolato), was synthesized by the boron–metal exchange reaction
of a diborylsilane, Tbt(Mes)Si(Bpin)2 (8), which was synthesized by the insertion of a highly crowded
diarylsilylene, Tbt(Mes)Si: (1), into the B–B bond of a diborane(4) compound, B2pin2. Although a salt
of the borylsilyl anion 11 could not be isolated due to its thermal instability, the investigation of the
reactivity of the anion revealed that 11 is a possible precursor for Si-functionalized silylboranes as well
as Si,Si-difunctionalized silanes. Furthermore, the borylsilyl anion [Tbt(Mes)SiBScat]– (27: Scat )
dithiocatecholato), which was synthesized by a procedure similar to that used for 11, showed a reactivity
different from that of 11. Theoretical calculations for the borylsilyl anions revealed that the anion 27 has
a boratasilene character; namely, the Si–B bond has a double-bond character due to the donation of an
electron pair of the negatively charged silicon atom into the vacant p-orbital of the adjacent boron atom.
respectively,8 and the anions thus generated have been shown
Introduction
to be versatile intermediates in organic syntheses.9–13 Further-
In recent decades, the chemistry of silyl anions, which are
versatile reagents for the synthesis of various organosilicon
compounds, has been greatly developed.1 In particular, func-
tionalized silyl anions2 such as (fluorosilyl)lithiums (R2SiFLi),3
(alkoxysilyl)lithiums (R2Si(OR′)Li),4 and (aminosilyl)lithiums
(R2Si(NR′2)Li)5 have attracted attention from the viewpoints
of both fundamental and applied chemistry. In contrast to these
actively explored species, borylsilyl anions, silyl anions sub-
stituted by boron atoms, have not been investigated until very
recently.
The carbon analogue, the carbanions substituted by boron
atoms, have been known as boron-stabilized carbanions or
borataalkenes.6 They were first postulated in 1960s as interme-
diates in the hydrolysis reactions of gem-diborylalkanes.7 In the
1970s, boron-stabilized carbanions were found to be generated
by the removal of R- and γ-protons of alkyl- and vinylboranes,
more, isolable boron-stabilized carbanions were synthesized and
their structural characterization showed them to have signifi-
cantly short B–C bond lengths.14–16
The chemistry of boron-stabilized carbanions is comparable
to that of carbonyl-stabilized carbanions (enolates),17,18 which
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# Dedicated to Emeritus Prof. Renji Okazaki on the occasion of his 70th
birthday.
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* To whom correspondence should be addressed. E-mail: tokitoh@
boc.kuicr.kyoto-u.ac.jp.
‡ Present address: Department of Chemistry and Chemical Biology,
Graduate School of Engineering, Gunma University, 1-5-1 Tenjin-cho,
Kiryu, Gunma 376-8515, Japan.
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10.1021/om7008974 CCC: $40.75
2008 American Chemical Society
Publication on Web 02/15/2008

Synthesis and Properties of Borylsilyl Anions
Organometallics, Vol. 27, No. 5, 2008 881
are one of the most important reagents in organic synthesis.
Silicon analogues of enolates (2-silenolates) have been known
since 1989,19 and the first isolable example was reported in
2003,20 while, as mentioned above, silicon analogues of boron-
stabilized carbanions have been unexplored.
Although silylboranes have been extensively investigated,21–23
methods for their preparation are severely limited24–27 and the
routes to silylboranes bearing leaving groups on their silicon
atoms have not been established. On the other hand, we have
recently investigated the reactions of a highly crowded diaryl-
silylene (1) with various boron compounds in a search for a
novel synthetic method for silylborane derivatives. Insertion of
silylene 1 into B–H and B–X (X ) halogen) bonds proceeded
to give the corresponding (hydrosilyl)boranes and (halosilyl)bo-
ranes, respectively, which are difficult to synthesize by other
methods.28 We describe here a study of the synthesis of
borylsilyl anions taking advantage of the silylboranes prepared
by our method. During the course of this study, we found that
silylene 1 is reactive toward B–B bonds as well as B–H and
B–X bonds and that the resulting diborylsilanes are good
precursors for borylsilyl anions.29 Although salts of the borylsilyl
anions could not be isolated in the present study,30 some
reactivities of the anions were examined. Theoretical calculations
on the structure of model molecules of the borylsilyl anions
are also described.
The absence of reports on borylsilyl anions is mainly due to
the lack of appropriate synthetic methods for their precursors.
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882 Organometallics, Vol. 27, No. 5, 2008
Kajiwara et al.
Results and Discussion
Attempted Synthesis: (a) Deprotonation. First, we exam-
ined the deprotonation of (hydrosilyl)borane 228b with a base.
Although the deprotonation of hydrosilanes to form the corre-
sponding silyl anions is rather exceptional due to their bond
polarity (i.e., Siδ+–Hδ- whereas it is Cδ––Hδ+ in hydrocarbons),
the sila-metalation reaction of trialkylsilyl-substituted hydrosi-
lanes has been reported very recently.31 Inspired by the effect
of electropositive silyl groups, we thought that the deprotonation
of boryl-substituted hydrosilanes might be possible, because of
the electropositive character of boron.32 Accordingly, (hydrosi-
lyl)borane 228b was treated with a base (Scheme 1).
The reaction of 2 with LDA (lithium diisopropylamide) for
1 h followed by treatment with chlorotrimethylsilane resulted
in formation of hydrodisilane 3 in 15% yield together with
recovered starting material (85%, Scheme 1). The structure of
3 was confirmed spectroscopically and crystallographically
(Figure 1). This reaction was significantly affected by the
reaction time. Thus, after the addition of Me3SiCl, reaction for
6 h increased the yield of 3 to 35% and decreased the amount
of unreacted 2 (11%). In this case, 1,2-dihydrobenzo[b]silete
433 and dihydrosilane 534 also were obtained in 7 and 18%
yields, respectively (Scheme 1). Much longer reaction time (32
h) afforded 4 as the main product (65%), and no 2 and 3 were
obtained (Scheme 1).
Figure 1. ORTEP drawing of 3 with thermal ellipsoid plots (50%
probability).
Scheme 1. Reaction of (Hydrosilyl)borane 2 with LDA
The reaction course is reasonably interpreted in terms of the
boron–lithium exchange reaction of silylborane 2 (Scheme 2).
The reaction of hydrosilyl anion 6 thus generated with chlo-
rotrimethylsilane gives the corresponding trimethylsilylated
hydrosilane 3, while protonation of the anion leads to dihy-
drosilane 5. The formation of 4 is explained by the R-elimination
of lithium hydride from the anion 6 giving silylene 1 followed
by intramolecular C–H insertion33,34 (Scheme 2). Boron–metal
exchange reaction of silylboranes has recently been reported as
a new route to silyl anions, where more than 2 molar amounts
of alkyllithiums, alkylmagnesium bromide, or potassium alkox-
ide were used.22a Our results show that the boron–metal
exchange reaction also proceeds with only an equimolar amount
of LDA.
Scheme 2. Mechanism for the Reaction of 2 with LDA
To suppress the nucleophilic attack of the base at the boron
atom of 2, the non-nucleophilic base LiTMP (lithium 2,2,6,6-
tetramethylpiperidide) was used (Scheme 3). As expected, the
Scheme 3. Reaction of 2 with LiTMP
(31) Iwamoto, T.; Okita, J.; Kabuto, C.; Kira, M. J. Am. Chem. Soc.
2002, 124, 11604–11605.
(32) In ref 31, the effects of silyl substituents have been interpreted in
terms of the calculated atomic charges on the central silicon atom (RSi)
and the Si–H hydrogen atoms (RH) for R2SiH2 molecules (R ) Me, Ph,
and SiH3). Our calculations for (H2B)2SiH2 molecule revealed that the RSi
and RH values of (H2B)2SiH2 are quite different from those of Me2SiH2
and Ph2SiH2 and are rather similar to those of (H3Si)2SiH2. See Supporting
Information for details.
yield of products derived from the anion 6 decreased; that is,
hydrodisilane 3 was obtained in 26% yield and 4 and 5 were
not obtained even after a 12 h reaction time (Scheme 3).
However, no products derived from Si–H deprotonation were
observed.
Synthesis of Diborylsilane: Silylene Insertion into a B–B
Bond. Treatment of (hydrosilyl)borane 2 with a lithium base
led to boron–lithium exchange rather than deprotonation (Vide
supra). This result suggested that a borylsilyl anion might be
(33) Suzuki, H.; Tokitoh, N.; Okazaki, R. Bull. Chem. Soc. Jpn. 1995,
68, 2471–2481.
(34) (a) Tokitoh, N.; Suzuki, H.; Okazaki, R.; Ogawa, K. J. Am. Chem.
Soc. 1993, 115, 10428–10429. (b) Suzuki, H.; Tokitoh, N.; Okazaki, R.;
Harada, J.; Ogawa, K.; Tomoda, S.; Goto, M. Organometallics 1995, 14,
1016–1022.

Synthesis and Properties of Borylsilyl Anions
Organometallics, Vol. 27, No. 5, 2008 883
Scheme 4. New Strategy for the Generation of a Borylsilyl
Anion
Scheme 5. Synthesis of Diborylsilane 8
Figure 2. ORTEP drawing of 8 with thermal ellipsoid plots (50%
probability).
skeleton.38,39 However, silyl-bridged boranes and silaboranes,
i.e., cage compounds containing three-center B–Si–B interac-
tions, are well-known.40
The 29Si NMR spectrum of diborylsilane 8 shows a broad
signal, which is characteristic of a nucleus attached to a boron
atom, at –67.6 ppm. This value is similar to those of the related
hydrosilanes, i.e., (hydrosilyl)borane 2 (–62.7 ppm) and dihy-
drosilane 5 (–61.3 ppm). The molecular structure of 8 was
determined by X-ray crystallographic analysis, and the ORTEP
drawing of 8 is shown in Figure 2. The Si–B bond lengths (2.038(3)
Å for Si1–B1 and 2.054(3) Å for Si1–B2) are comparable to those
of previously reported silylborane derivatives.24h,25,26a,28,39c,41
Interestingly, one of the two dioxaborolane rings in 8 is slightly
distorted, while the other is almost planar. The ring puckering
coordinates defined by Cremer and Pople42 were q2 ) 0.04 Å
and ø2 ) 115.80° for the O1–B1–O2–C2–C1 ring and q2 )
0.31 Å and ø2 ) 128.00° for the O3–B2–O4–C8–C7 ring. The
ø2 value of the latter is close to the value (126°) appropriate to
one of the twist forms with the twist axis through B2. Similarly
distorted BO2C2 rings have been reported for R2C[B(pin)]2
compounds, though both of the two rings are distorted.43
Attempted Synthesis: (b) Boron–Metal Exchange
Reaction. With the diborylsilane in hand, we examined the
reaction of 8 with a nucleophile. Although the reactions of 8
with LDA, lithium diethylamide, or potassium hydride did not
generated if a diborylsilane was used instead of a (hydrosi-
lyl)borane (Scheme 4). Accordingly, the synthesis of a dibor-
ylsilane was explored.
When a THF or benzene suspension of disilene 734 and
bis(pinacolato)diborane(4)35 was heated at 60 °C, the original
orange color faded very slowly, giving a colorless solution in 5
days. Diborylsilane 8 was isolated from the reaction mixture
by silica gel column chromatography in 45% yield (Scheme
5).29 The formation of 8 is reasonably interpreted in terms of
the insertion of silylene 1, generated from disilene 7, into the
B–B bond of bis(pinacolato)diborane(4). Thus, silylene 1 was
found to be reactive toward a B–B bond as well as toward B–H
and B–halogen bonds, although B–B bond insertion was much
slower than B–H and B–Cl insertion.28b The reaction of
silylene–isocyanide complex 936 with bis(pinacolato)diborane(4)
also gave diborylsilane 8, but the reaction required a higher
temperature and the yield was much lower (Scheme 5).29 This
result agrees with the reduced reactivity of a silylene–Lewis
base complex compared with a free silylene.37
(39) (a) Kirk, R. W.; Timms, P. L. J. Am. Chem. Soc. 1969, 91, 6315–
6318. (b) Wilcsek, R. J.; Matteson, D. S.; Douglas, J. G. J. Chem. Soc.,
Chem. Commun. 1976, 401–402. (c) Pfeiffer, J.; Maringgele, W.; Meller,
A. Z. Anorg. Allg. Chem. 1984, 511, 185–192. (d) Wong, W. C.; Wong,
C. M.; Pritzkow, H.; Siebert, W. Z. Naturforsch. 1990, 45b, 1597–1599.
(40) For examples, see: (a) Gaines, D. F.; Iorns, T. V. J. Am. Chem.
Soc. 1967, 89, 4249–4250. (b) Calabrese, J. C.; Dahl, L. F. J. Am. Chem.
Soc. 1971, 93, 6042–6047. (c) Tabereaux, A.; Grimes, R. N. J. Am. Chem.
Soc. 1972, 94, 4768–4770. (d) Hosmane, N. S.; de Meester, P.; Siriwardane,
U.; Islam, M. S.; Chu, S. S. C. J. Chem. Soc., Chem. Commun. 1986, 1421–
1422. (e) Schubert, D. M.; Rees, W. S., Jr.; Knobler, C. B.; Hawthorne,
M. F. Organometallics 1990, 9, 2938–2944. (f) Seyferth, D.; Büchner, K. D.;
Rees, W. S., Jr.; Wesemann, L.; Davis, W. M.; Bukalov, S. S.; Leites, L. A.;
Bock, H.; Solouki, B. J. Am. Chem. Soc. 1993, 115, 3586–3594. (g)
Wesemann, L.; Englert, U.; Seyferth, D. Angew. Chem., Int. Ed. Engl. 1995,
34, 2236–2238. (h) Wesemann, L.; Englert, U. Angew. Chem., Int. Ed. Engl.
1996, 35, 527. (i) Dopke, J. A.; Bridges, A. N.; Schmidt, M. R.; Gaines,
D. F. Inorg. Chem. 1996, 35, 7186–7187.
Although the formal insertion of a silylene, t-Bu2Si:, into a
B–B bond of an azadiboriridine has been reported, the reaction
was explained in terms of nucleophilic attack of the silylenoid,
t-Bu2SiClLi, on the boron atom, because a silylene Me2Si:,
generated by the photolysis of the corresponding cyclohexasilane
Si6Me12, did not show such reactivity.38 Thus, the formation of
diborylsilane 8 by the reaction of 7 or 9 with bis(pinacolato)-
diborane(4) should be of great interest as the first example of
the insertion of a silylene into a B–B bond. Moreover, there
have been only few reports of compounds containing a B–Si–B
(35) Ishiyama, T.; Murata, M.; Ahiko, T.-a.; Miyaura, N. Org. Synth.
2000, 77, 176–181.
(36) (a) Takeda, N.; Suzuki, H.; Tokitoh, N.; Okazaki, R.; Nagase, S.
J. Am. Chem. Soc. 1997, 119, 1456–1457. (b) Takeda, N.; Kajiwara, T.;
Suzuki, H.; Okazaki, R.; Tokitoh, N. Chem.-Eur. J. 2003, 9, 3530–3543.
(37) Belzner, J.; Ihmels, H. AdV. Organomet. Chem. 1999, 43, 1–42.
(38) Luckert, S.; Eversheim, E.; Englert, U.; Wagner, T.; Paetzold, P.
Z. Anorg. Allg. Chem. 2001, 627, 1815–1823.
(41) Lippert, W.; Nöth, H.; Ponikwar, W.; Seifert, T. Eur. J. Inorg.
Chem. 1999, 817–823.
(42) Cremer, D.; Pople, J. A. J. Am. Chem. Soc. 1975, 97, 1354–1358.
(43) Abu Ali, H.; Goldberg, I.; Kaufmann, D.; Burmeister, C.; Srebnik,
M. Organometallics 2002, 21, 1870–1876.

884 Organometallics, Vol. 27, No. 5, 2008
Kajiwara et al.
Scheme 7. Reaction of (Chlorosilyl)boranes 12 and 13 with
Na/K Alloy
Scheme 8. Generation of Borylsilyl Anion 11 at a Higher
Temperature
Scheme 9. Reactions of Borylsilyl Anion 11 with Electrophiles
Figure 3. ORTEP drawing of 10 with thermal ellipsoid plots (50%
probability, one of the two crystallographically independent mol-
ecules).
Scheme 6. Reaction of Diborylsilane 8 with n-BuLi
proceed at all, the reaction of 8 with 4 molar equiv of n-BuLi
in hexane at –78 °C afforded a deep red solution. Addition of
chlorotrimethylsilane gave Si-trimethylsilylated silylborane 10
in 61% yield (Scheme 6).29 The formation of trimethylsilyl-
substituted silylborane 10 is most likely explained in terms of
the reaction of borylsilyl anion 11, which is generated by
boron-lithium exchange between n-BuLi and 8,22a with chlo-
rotrimethylsilane. The formation of 11 by the boron-lithium
exchange reaction of 8 is the first example of the generation of
a borylsilyl anion.29,30
ylsilane to give 10 in 59% yield (Scheme 7). In contrast, the
reaction of 12 with Na/K alloy did not proceed (Scheme 7).
Reactions of Borylsilyl Anion 11 with Electrophiles.
Although spectroscopic observation of the lithium salt of
borylsilyl anion 11 has been unsuccessful due to its thermal
instability (Vide infra), the reactions of 11 with several elec-
trophiles were examined. Borylsilyl anion 11 was found to be
stable for hours at –40 °C in diethyl ether or at 0 °C in hexane,
respectively, to give the corresponding Si-trimethylsilylated
silylborane after the addition of chlorotrimethylsilane (Scheme
8). In the latter case, 2 molar equiv of n-BuLi were sufficient
to consume diborylsilane 8, while 4 molar equiv were required
in the reaction at –78 °C (Scheme 6).
When borylsilyl anion 11 was treated with iodomethane, the
corresponding Si-methylated silylborane 14 was obtained in 58%
yield (Scheme 9), the structure of which was confirmed
spectrographically and crystallographically (Figure 4). The
reaction of borylsilyl anion 11 with dimethyl sulfate also
afforded 14 in 77% yield (Scheme 9). On the other hand, the
reaction of 11 with methanol-d gave a different type of product,
dideuteriosilane 5-d2, in 65% yield (Scheme 9).
The 29Si NMR spectrum of boryldisilane 10 shows a broad
signal at –57.8 ppm. This value is roughly the same range as
that of the boron analogue, i.e., diborylsilane 8 (–67.6 ppm), as
documented in the literature.44 The molecular structure of 10
was determined by X-ray crystallographic analysis, and the
ORTEP drawing of 10 is shown in Figure 3.
Attempted Synthesis: (c) Reductive Dehalogenation. We
examined the synthesis of borylsilyl anions by the reduction of
the corresponding (halosilyl)boranes. The reduction of (chlo-
rosilyl)catecholborane 1228b with Li, LiNaph, K, or KC8
afforded complicated mixtures. Treatment of pinacolato deriva-
tive 1328b with K or KC8 also gave a complicated mixture, while
Na/K alloy was found to be an appropriate reductant for the
reductive dehalogenation of 13. Thus, the reaction of 13 with
Na/K alloy in diethyl ether at –78 °C resulted in the formation
of borylsilyl anion 11, which was trapped with chlorotrimeth-
One might think that the formation of 5-d2 is the result of
the reaction of dianion species 15 with MeOD. However, the
dianion mechanism is excluded because the reaction with other
electrophiles gave no disubstituted product. Indeed, the reaction
of diborylsilane 8 with an excess (even 10 molar equiv) of
n-BuLi gave (methylsilyl)borane 14 rather than the dimethyl
(44) Araujo Da Silva, J. C.; Birot, M.; Pillot, J.-P.; Pétraud, M. J.
Organomet. Chem. 2002, 646, 179–190.

Synthesis and Properties of Borylsilyl Anions
Organometallics, Vol. 27, No. 5, 2008 885
Scheme 12. Reaction of (Hydrosilyl)borane 2 with Sodium
Methoxide
Scheme 13. Reaction of Borylsilyl Anion 11 with Pivaloyl
Chloride
Scheme 14. Boron–Metal Exchange Reaction of (Methylsilyl)-
borane 14
Figure 4. ORTEP drawing of 14 with thermal ellipsoid plots (50%
probability).
Scheme 10. Reaction of Diborylsilane 8 with an Excess of
n-BuLi
Scheme 15. Synthesis of Hydrosilane 17
Scheme 11. Plausible Mechanism for the Production of
Dideuteriosilane 5-d2
of (chlorosilyl)borane 13 and (hydrosilyl)borane 2 in 31 and
25% yields, respectively (Scheme 13). Nucleophilic attack of
11 at the carbonyl carbon atom of pivaloyl chloride was
suppressed due to the extreme steric congestion, while electron
transfer occurred readily to give 13.
Diborylsilane as a Masked Silicon Dianion. The boron–
metal exchange of (methylsilyl)borane 14, the product of the
reaction of borylsilyl anion 11 with dimethyl sulfate, was
examined. Such a reaction did not proceed when alkyllithiums
were used. The reaction of 14 with 4 molar equiv of potassium
tert-butoxide followed by treatment with dimethyl sulfate (12
equiv) afforded not dimethylsilane 16 but hydrosilane 17 (41%)
together with a 59% recovery of starting material (Scheme 14).
These results are due to the severe steric hindrance of both
(methylsilyl)borane 14 and methylsilyl anion 18.
The treatment of 14 with an excess of sodium methoxide in
methanol-d gave hydrosilane 17-d in 81% yield (Scheme 15).
Furthermore, boron–lithium exchange between (hydrosilyl)bo-
rane 2 and n-BuLi and subsequent treatment with dimethyl
sulfate also resulted in formation of 17 in 30% yield together
with dihydrosilane 5 (30%) and a 29% recovery of 2 (Scheme
15). The ORTEP drawing of 17 is shown in Figure 5.
derivative 16 after treatment with an excess (23 molar equiv to
8) of dimethyl sulfate (Scheme 10). Thus, the formation
mechanism of 5-d2 is interpreted in terms of the stepwise process
as follows. The reaction of borylsilyl anion 11 with MeOD
initially afforded the corresponding Si-deuteriated silylborane
2-d and lithium methoxide. Nucleophilic attack of LiOMe thus
formed in situ at the boron atom of 2-d resulted in generation
of a deuteriosilyl anion 6-d by boron–lithium exchange and
anion 6-d was trapped by MeOD to give the final product 5-d2
(Scheme 11). Although the formation of the intermediary
silylborane 2-d could not be confirmed, this mechanism was
supported by the fact that the reaction of the isolated (hydrosi-
lyl)borane 2 with sodium methoxide in methanol gave the
corresponding dihydrosilane 5 in 74% yield (Scheme 12).
The reaction of 11 with pivaloyl chloride did not afford the
corresponding (acylsilyl)borane but resulted in the formation

886 Organometallics, Vol. 27, No. 5, 2008
Kajiwara et al.
Figure 6. ORTEP drawing of 19 with thermal ellipsoid plots (50%
probability).
1H and 13C NMR spectra showed the unsymmetrical structure
of the pinacolyl group. In the 1H NMR spectrum, 7 signals due
to methyl groups (4 signals assignable to the pinacolyl and 3
signals assignable to the mesityl group) were observed. In
addition, the signal at 2.57 ppm disappeared when D2O was
added to the solution of 19. In the 13C NMR spectrum, 7 signals
due to methyl groups and 2 signals due to the quaternary carbon
atoms attached to oxygen were observed. The original diox-
aborolane ring of 8 obviously had been opened, giving a C–OH
group. The presence of the O–H bond was shown by an
absorption at 3588 cm–1 in the IR spectra. The 11B NMR
spectrum showed a signal at 65.4 ppm, suggesting that at least
one of the two B–O bonds had been cleaved. The 29Si NMR
spectrum showed a very broad and weak signal at –55.3 ppm,
strongly suggestive of a B–Si bond, together with signals
assignable to trimethylsilyl groups. The molecular structure of
19 was determined by X-ray crystallographic analysis, and the
ORTEP drawing of 19 is shown in Figure 6. The Si–B bond
length (2.036(5) Å) is comparable to those of the previously
reported silylborane derivatives.24h,25,26a,28,39c,41 The five-
membered ring in the 2,3-dihydro-1H-benzo[d][1,2]silaborole
skeleton is nearly planar (the ring puckering coordinates:42q2 )
0.10 Å and ø2 ) 214.30° for the Si1–B1–C22–C17–C16 ring).
The formation of 19 is interesting because only a few examples
of cyclic silylboranes are known.38,39d,46
Figure 5. ORTEP drawing of 17 with thermal ellipsoid plots (50%
probability).
Scheme 16. Possible Route to Si,Si-Difunctionalized Silanes
Starting from Diborylsilanes
Scheme 17. Formation of 2,3-Dihydro-1H-benzo[d][1,2]silabo-
role 19
These results indicate that borylsilyl anion 11 is a good
precursor for gem-Si-difunctionalized silanes as well as Si-
functionalized silylboranes.45 In this context, a diborylsilane may
be regarded as a “masked silicon dianion”, whose reactions
provide difunctionalized silanes (Scheme 16).
On the other hand, a different type of product was obtained
when MeOD was used as the trapping agent. The reaction of
diborylsilane 8 with MeLi in THF at room temperature after
quenching with MeOD afforded deuteriohydrosilane 20-d (62%),
which has a (pinacolato)boryl group at the o-benzyl position of
the Tbt group(Scheme 18). The reaction of (chlorosilyl)borane
Rearrangement of Borylsilyl Anion 11. Although borylsilyl
anion 11 is stable at low temperature, it was found that 11 is
unstable at room temperature, undergoing a migration reaction.
When the mixture of borylsilyl anion 11 generated from 8 and
4 molar equiv of MeLi at –78 °C was warmed to room
temperature and quenched with dimethyl sulfate (16 equiv), 2,3-
dihydro-1H-benzo[d][1,2]silaborole 19 was obtained as the main
product (74%, Scheme 17).
(46) (a) Hengge, E.; Wolfer, D. Angew. Chem., Int. Ed. Engl. 1973, 12,
315–316. (b) Hengge, E.; Wolfer, D. J. Organomet. Chem. 1974, 66, 413–
424. (c) Van Bonn, K.-H.; Schreyer, P.; Paetzold, P.; Boese, R. Chem. Ber.
1988, 121, 1045–1057. (d) Paetzold, P.; Hahnfeld, D.; Englert, U.;
Wojnowski, W.; Dreczewski, B.; Pawelec, Z.; Walz, L. Chem. Ber. 1992,
125, 1073–1078. (e) Markov, J.; Fischer, R.; Wagner, H.; Noormofidi, N.;
Baumgartner, J.; Marschner, C. Dalton Trans. 2004, 2166–2169. (f)
Suginome, M.; Noguchi, H.; Hasui, T.; Murakami, M. Bull. Chem. Soc.
Jpn. 2005, 78, 323–326. (g) Nakata, N.; Izumi, R.; Lee, V. Ya.; Ichinohe,
M.; Sekiguchi, A. Chem. Lett. 2005, 34, 582–583. (h) Nakata, N.; Sekiguchi,
A. Chem. Lett. 2007, 36, 662–663.
The structure of 19 was supported by NMR, IR, and mass
spectra and determined by X-ray crystallographic analysis. The
(45) Very recently, the alternative synthetic method for functionalized
silylboranes was reported; see: Ohmura, T.; Masuda, K.; Furukawa, H.;
Suginome, M. Organometallics 2007, 26, 1291–1294.

Synthesis and Properties of Borylsilyl Anions
Organometallics, Vol. 27, No. 5, 2008 887
Table 1. Calculated Structures of Boron-Stabilized Silyl Anions and Carbanions (1)a
X
Y
Si–B (Å)
X–Si–B (deg)
X–Si–X (deg)
∑Si (deg)b
δSi
δB
1J(29Si–11B)
H
H
F
H
F
H
F
1.880
2.008
1.907
2.101
1.466
1.457
126.0
100.3
120.8
98.9
123.8
122.7
106.9
99.6
98.8
98.3
112.4
114.7
358.8
300.2
340.5
296.2
360.0
360.0
–52.5
–222.1
150.9
33.6
50.6
–6.5
25.6
19.2
32.5
146.3
43.2
16.4
F
113.3
1.9
[H2C–BH2]–
[H2C–BF2]–
(77.8)c
(0.1)c
(90.6)d
(137.5)d
a Calculated at the B3LYP/6-311+G(2d,p)//B3LYP/6-31+G(d) level with Cs symmetry. b Sum of angles around the central silicon atom. c δC.
d 1J(13C–11B).
Table 2. Energy Differences between the Highest and the Lowest
Energy Isomers Varing the Torsion Anglea
o-benzyl proton of the Tbt group by the silyl anion in borylsilyl
anion 11 gives benzyl anion 21. Similar intramolecular proton
abstraction from the o-benzyl position of the Tbt group has been
found in the group 14 dianionic species [Tbt(Dip)M]2– (Dip )
2,6-diisopropylphenyl, M ) Si, Ge, and Sn).48 Nucleophilic
attack of benzyl anion 21 at the intramolecular boron atom leads
to cyclic silylborate 22. The latter is unreactive toward dimethyl
sulfate, and the protonation of alkoxide 23, which exists in the
equilibrium with 22, affords the cyclic silylborane 19, bearing
a hydroxy group (Scheme 19, path a). On the other hand, the
formation of hydrosilane 20 might be interpreted in terms of
boron–metal exchange by the methoxide anion generated in situ,
as in the case of the reaction of borylsilyl anion 11 with MeOH.
Hydrosilyl anion 24, having a borate moiety at the o-benzyl
position of the Tbt group, was trapped by MeOH to give 20
(Scheme 19, path b).
molecule
energy difference (kcal/mol)
[H2Si–BH2]–
[H2Si–BF2]–
[F2Si–BH2]–
[F2Si–BF2]–
[H2C–BH2]–
[H2C–BF2]–
H3Si–BH2
60.2
10.2
61.8
5.2
57.3
32.1
1.1
H2SidCH2
[H2Si–CH3]–
H3Si–CH3
53.1
1.7
1.4
a Calculated at the B3LYP/6-31+G(d) level.
Scheme 18. Formation of Dihydrosilane 20 and Deuteriohydro-
silane 20-d
When the isolated alcohol 19 was allowed to react with
potassium hydride and subsequently treated with MeOD,
deuteriohydrosilane 20-d was obtained in 37% yield (Scheme
20). This result strongly supports the existence of an equilibrium
between cyclic silylborate 22 and alkoxide 23.
The thermal instability of borylsilyl anion 11 is probably
attributable to localization of the anionic charge at the silicon
atom due to the strong electron-donating ability of pinacolato
group attached to the boron atom. To elucidate the substituent
effect, theoretical calculations were carried out.
Theoretical Calculations on the Structure of Boron-
Stabilized Silyl Anions. In this section, we use the term “boron-
stabilized silyl anion” for borylsilyl anion according to the
carbon analogue, boron-stabilized carbanion. As described in
the Introduction section, boron-stabilized silyl anions have two
canonical forms: one has a silicon–boron single bond and the
localized anion charge at the silicon atom and the other has a
silicon–boron double bond and the formal charge at the boron
atom. To distinguish them, the former form is referred to as
“borylsilyl anion” and the latter is denoted as “boratasilene”
(Chart 1).
13 with Na/K alloy in diethyl ether at room temperature
followed by addition of MeOH or MeOD also afforded
dihydrosilane 20 or its deuterio analogue 20-d, respectively, in
81% yield (Scheme 18).
The structure of 20 was fully supported by its NMR, IR, and
1
mass spectra. The H and 13C NMR spectra showed signals
assignable to a (pinacolato)boryl group; that is, the original
dioxaborolane ring was not opened in contrast to the case of
19. The 11B NMR spectrum showed a signal at 33.5 ppm, which
is in the range for the chemical shifts of (pinacolato)borane
derivatives.47 The 29Si NMR spectrum showed a sharp signal
at –53.8 ppm together with signals due to trimethylsilyl groups.
The sharp signal strongly suggests the absence of a B–Si bond.
Chart 1. Canonical Forms for Boron-Stabilized Silyl Anions
A plausible but speculative mechanism for the formation of
(48) (a) Tokitoh, N.; Hatano, K.; Sadahiro, T.; Okazaki, R. Chem. Lett.
1999, 28, 931–932. (b) Tokitoh, N.; Hatano, K.; Sasaki, T.; Sasamori, T.;
Takeda, N.; Takagi, N.; Nagase, S. Organometallics 2002, 21, 4309–4311.
(c) Tajima, T.; Takeda, N.; Sasamori, T.; Tokitoh, N. Eur. J. Inorg. Chem.
2005, 4291–4300.
19 and 20 is shown in Scheme 19. The deprotonation of the
(47) Biedrzycki, M.; Scouten, W. H.; Biedrzycka, Z. J. Organomet.
Chem. 1992, 431, 255–270.

888 Organometallics, Vol. 27, No. 5, 2008
Kajiwara et al.
Table 3. Calculated Structures of Boron-Stabilized Silyl Anions(2)a
Z
Z′
Me
OMe
OMe
OH
Si–B (Å)
C–Si–B (deg)
C–Si–C (deg)
∑Si(°)b
δSi
δB
1J(29Si–11B)
Me
Me
OMe
OH
1.929
1.993
2.043
2.048
2.051
2.064
2.020
1.999
1.977
117.3/117.3
107.8/108.5
105.6/102.1
101.9/101.9
106.1/110.8
100.5/100.4
102.5/103.0
103.9/103.9
106.8/106.8
104.5
102.6
99.7
100.6
98.8
339.1
318.9
307.4
304.3
315.7
299.9
306.8
310.2
316.8
31.3
0.4
–34.2
–34.6
–9.4
–27.6
–36.9
–40.3
–1.0
63.6
67.8
43.5
39.9
47.1
41.7
44.6
48.0
68.4
95.7
62.9
40.9
37.8
48.0
28.5
41.1
47.2
56.7
NMe2
NH2
pinc
NMe2
NH2
99.0
101.3
102.4
103.3
catd
Scate
a Calculated at the B3LYP/6-311+G(2d,p)//B3LYP/6-31+G(d) level without any symmetry constraints. b Sum of angles around the central silicon
atom. c Pinacolato. d Catecholato. e Dithiocatecholato.
Scheme 19. Plausible Mechanism for the Formation of Cyclic Silylborane 19 and Hydrosilane 20
Scheme 20. Production of Hydrosilane 20 Starting from
Alcohol 19
borylsilyl anion form becomes dominant. These results are
explained as follows: (a) the σ-electron-withdrawing fluorine
substituents on the silicon atom stabilize the borylsilyl anion
form; (b) the π-electron-donating fluorine substituents on the
boron atom destabilize the boratasilene form and thus the
borylsilyl anion form becomes more favored. Interestingly,
the substitution of hydrogen by fluorine atoms on the boron
atom does not affect the structure of boron-stabilized carbanions,
in contrast to the corresponding silyl anions. This difference
may be attributed to the electropositive character of the silicon
atom and/or the lesser tendency of silicon to sp2 hybridization.
A rotational barrier around the Si–B bond in boron-stabilized
silyl anions is an index of its double-bond character. Although
rotational transition states were not calculated, rotational barriers
were approximately estimated by PES (potential energy surface)
scans.53 The results are summarized in Table 2. The parent
molecule ([H2Si–BH2]–) has a large energy difference between
the highest and lowest energy isomers (60.2 kcal/mol). Since
this value is much larger than that for the parent silylborane
(H3Si–BH2, 1.1 kcal/mol), which has an Si–B single bond, it is
suggested that the Si–B bond of the parent molecule
([H2Si–BH2]–) has a double-bond character. In contrast, the
fluorine analogue ([F2Si–BF2]–) shows only a small energy
We started from the calculations for the parent molecule
([H2Si–BH2]–) and its fluorine analogues.49 The results are
shown in Table 1, together with those of related carbanions
(boratamethylenes).17a The parent molecule ([H2Si–BH2]–) has
a planar geometry at the silicon atom and the Si–B distance is
quite short (1.880 Å).50 This value is comparable to the
calculated values for the parent borasilene, a neutral silicon–
boron doubly bonded molecule (H2SidBH, 1.819 Å)51 and
for the 1,3-disila-2-borataallenic anion ([(H3Si)2SidBd
Si(SiH3)2]–, 1.858 Å).52 However, the substitution of hydrogen
atoms by fluorine atoms leads to longer Si–B distances and
pyramidalized geometry at the silicon atom, indicating that the
(49) All calculations were carried out using Gaussian 98 or Gaussian
03 programs. (a) Frisch, M. J.; et al. Gaussian 98, Revision A.11.2;
Gaussian, Inc.: Pittsburgh, PA, 2001. (b) Frisch, M. J.; et al. Gaussian 03,
ReVision B.05; Gaussian, Inc.: Pittsburgh, PA, 2003.
(53) For a recent review, see: (a) Ziegler, T.; Autschbach, J. Chem. ReV.
2005, 105, 2695–2722. For recent examples of the application of PES scans,
see:. (b) Barder, T. E.; Biscoe, M. R.; Buchwald, S. L. Organometallics
2007, 26, 2183–2192. (c) Barder, T. E.; Buchwald, S. L. J. Am. Chem.
Soc. 2007, 129, 12003–12010. (d) Ikawa, T.; Barder, T. E.; Biscoe, M. R.;
Buchwald, S. L. J. Am. Chem. Soc. 2007, 129, 13001–13007.
(50) Hopkinson, A. C.; Lien, M. H. Tetrahedron 1981, 37, 1105–1112.
(51) Schleyer, P. v. R.; Kost, D. J. Am. Chem. Soc. 1988, 110, 2105–
2109.
(52) Hsiao, J.; Su, M.-D. Organometallics 2007, 26, 4432–4438.

Synthesis and Properties of Borylsilyl Anions
Organometallics, Vol. 27, No. 5, 2008 889
Scheme 21. Synthesis of Diborylsilane 25
Scheme 22. Boron–Lithium Exchange Reaction of Diboryl-
silane 25
difference (5.2 kcal/mol). However, this value is still larger than
that of the parent silylborane. This suggests a small extent of
double-bond character in the Si–B bond of the fluorine analogue.
It is noteworthy that the methylsilyl anion ([H2Si–CH3]–) has a
small energy difference (1.7 kcal/mol), and the value is
comparable to that of methylsilane (H3Si–CH3, 1.4 kcal/mol).
The results suggested from the PES scans are consistent with
those indicated by the calculated structures.
Next, boron-stabilized silyl anions bearing methyl groups on
the silicon atom and C, N, O, and S substituents on the boron
atoms were calculated; the results are summarized in Table 3.
Due to their π-electron donating property, the introduction of
nitrogen or oxygen substituents makes the borylsilyl anion form
that has longer Si–B distances and pyramidalized geometry at
the silicon atom preferable. In contrast, the tetramethyl-
substituted derivative ([Me2Si–BMe2]–) has a slightly pyrami-
dalized geometry at the silicon atom, and its Si–B distance is
rather short (1.929 Å). This molecule can be regarded as a
boratasilene. It is noteworthy that the molecule bearing one
methyl and one methoxy group on the boron atom still shows
a slightly shortened Si–B distance (1.993 Å). Indeed, the isolable
example of a boron-stabilized silyl anion, recently reported by
Nakata and Sekiguchi, is substituted by one carbon substituent
(ethynyl) and one amino group on the boron atom, and the anion
has a short Si–B distance.30
While the pinacolato derivative favors the borylsilyl anion
form, the less electron-donating catecholato group results in a
shortening of the Si–B distance together with an increase of
the sum of bond angles around the silicon atom. The substitution
of oxygen atoms by sulfur atoms (dithiocatecholato group) leads
to further shortening of the Si–B distance and an increase of
the angles. The shortening of the Si–B distances is reflected
in the larger coupling constants (1J(29Si–11B)). More signifi-
cantly, the calculated 29Si NMR chemical shift (–1.0 ppm) of
the dithiocatecholato derivative is significantly downfield shifted
from the other two oxygen derivatives. This may be due to the
larger sp2 character of the silicon atom; that is, dithiocatecholato
derivative has a boratasilene character.
These calculations reveal that the structure of the boron-
stabilized silyl anions is significantly influenced by the substit-
uents on the boron atom. Introduction of less electron-donating
groups on the boron atom may lead to a greater contribution of
the boratasilene form by the delocalization of the anionic charge
to the vacant p orbital of the boron atom.
borylsilane 25 is air-sensitive, silica gel column chromatography
under an argon atmosphere resulted in the successful isolation
of 25 in 58% yield. Its 29Si NMR spectrum showed a signal at
–58.4 ppm, the value of which is comparable to that of
pinacolato derivative 8 (–67.6 ppm).
The reaction of diborylsilane 25 with MeLi followed by
addition of chlorotrimethylsilane afforded a complicated mix-
ture, from which the corresponding Si-trimethylsilylated si-
lylborane 26 was isolated by PTLC (preparative thin-layer
chromatography) in 18% yield (Scheme 22). On the other hand,
when the reaction mixture of 25 and MeLi was quenched with
methanol-d, the corresponding dideuteriosilane 5-d2 was ob-
tained in 75% yield (Scheme 22). These results suggest that
the borylsilyl anion 27, bearing a dithiocatecholato group, had
been successfully generated by the boron–lithium exchange
reaction of 25. However, the yield of silylated product 26 was
much lower than the yields of the corresponding pinacolato
derivative 10 in the trapping reactions of borylsilyl anion 11
with chlorotrimethylsilane (Schemes 6–8). This difference
probably is due to the lower reactivity of anion 27 toward
chlorosilane than that of 11. The low reactivity of anion 27 may
be attributed to its boratasilene character, in which the anionic
charge on the silicon atom is delocalized toward the boron atom.
Experimental Section
General Remarks. All experiments were performed under
anhydrous conditions under an argon atmosphere unless otherwise
1
noted. H NMR (300 MHz), 13C NMR (75 MHz), 11B NMR (96
MHz), and 29Si NMR (59 MHz) spectra were recorded on a JEOL
JNM AL-300 spectrometer. The 1H NMR chemical shifts are
reported in ppm downfield from tetramethylsilane (δ scale) and
referenced to the internal residual CHCl3 (7.25 ppm), C6D5H (7.15
ppm), or sym-C2DHCl4 (5.98 ppm). The 13C NMR chemical shifts
are reported in ppm downfield from tetramethylsilane (δ scale) and
referenced to the carbon-13 signals of CDCl3 (77.0 ppm), C6D6
(128.0 ppm), or sym-C2D2Cl4 (73.8 ppm). Multiplicity of signals
in 13C NMR spectra was determined by DEPT technique. The 11
B
and 29Si NMR chemical shifts are referenced to the external
standards BF3 · OEt2 (0 ppm) and tetramethylsilane (0 ppm),
respectively. IR spectra were recorded on a JASCO FT/IR-5300
or a JASCO FT/IR-460 Plus spectrometer. Mass spectra were
recorded on a JEOL JMS-700 spectrometer. Melting points were
determined on a Yanaco micro melting point apparatus and are
uncorrected. Elemental analyses were carried out at the Microana-
lytical Laboratory of the Institute for Chemical Research, Kyoto
University. GPC-HPLC was performed on an LC-918 or an LC-
908 (Japan Analytical Industry Co., Ltd.) equipped with JAIGEL
1H and 2H columns using chloroform or toluene as an eluent. PTLC
Synthesis of a Borylsilyl Anion Bearing a Dithio-
catecholato Group. Inspired by the results of calculations, we
chose a borylsilyl anion bearing a dithiocatecholato group as a
next target. The thermal reaction of disilene 7 and bis(dithio-
cathecholato)diborane(4)54 afforded diborylsilane 25 containing
a dithiocatecholato group (Scheme 21). The reaction is reason-
ably interpreted in terms of the insertion of silylene 1 into the
B–B bond of bis(dithiocatecholato)diborane(4). Although di-
was performed with Merck Kieselgel 60 PF254
Materials. All reaction solvents were dried and distilled from
CaH2, stored over molecular sieves 4A, and then freshly distilled
.
(54) Lawlor, F. J.; Norman, N. C.; Pickett, N. L.; Robins, E. G.; Nguyen,
P.; Lesley, G.; Marder, T. B.; Ashmore, J. A.; Green, J. C. Inorg. Chem.
1998, 37, 5282–5288.

890 Organometallics, Vol. 27, No. 5, 2008
Kajiwara et al.
from Na/benzophenone ketyl under argon prior to use. Solvents
for reactions to generate a borylsilyl anion were dried over Na and
then a K mirror. Disilene 7,34 Mes*NC,55 silylene–isocyanide
complex 9,36 (hydrosilyl)borane 2,28b (chlorosilyl)boranes 12 and
13,28b bis(pinacolato)diborane(4),35 and bis(dithiocatecholato)d-
iborane(4)54 were prepared according to procedures reported in the
literature.
give hydrosilane 3 (17.1 mg, 22.1 µmol, 26%) together with
recovery of starting material 2 (14.3 mg, 17.3 µmol, 20%).
Synthesis of Diborylsilane 8. (a) Reaction of Disilene 7 with
Bis(pinacolato)diborane(4). In a 10 mm ø Pyrex tube was placed
a THF solution (2.4 mL) of a mixture of disilene 7 (151 mg, 108
µmol) and bis(pinacolato)diborane(4) (66.7 mg, 263 µmol). After
three freeze–pump–thaw cycles, the tube was evacuated and sealed.
The solution was heated at 60 °C for 122 h, during which time the
original orange color gradually disappeared. The tube was opened
and solvent was removed. Purification by silica gel chromatography
(n-hexane/CHCl3 ) 1:1) afforded diborylsilane 8 (92.4 mg, 96.9
µmol, 45%). 8: colorless crystals, mp 226–229 °C (dec). 1H NMR
(CDCl3): δ –0.09 (br, 18H), 0.02 (s, 18H), 0.06 (br, 18H), 1.15 (s,
12H), 1.19 (s, 12H), 1.23 (s, 1H), 2.00 (br, s, 1H), 2.17 (s, 3H),
2.29 (s, 6H), 3.05 (br, s, 1H), 6.23 (s, 1H), 6.36 (s, 1H), 6.64 (s,
2H). 13C NMR (CDCl3): δ 1.0 (q), 1.5 (q), 1.8 (q), 20.9 (q), 24.8
(q), 25.6 (q), 26.7 (q), 28.0 (d), 28.1 (d), 30.0 (d), 83.6 (s), 122.8
(d), 127.5 (s), 127.8 (d), 128.1 (d), 135.4 (s), 136.7 (s), 142.2 (s),
144.0 (s), 151.8 (s), 152.9 (s). 11B NMR (CDCl3) δ 35.3. 29Si NMR
(CDCl3) δ –67.6, 1.6, 2.2, 2.7. LRMS (FAB): m/z 975 [(M + Na)+],
953 [(M+H)+]. HRMS (FAB): m/z 953.5840, calcd for
C48H95B2O4Si7: 953.5801. Anal. Calcd for C48H94B2O4Si7: C, 60.46;
H, 9.94. Found: C, 59.96; H, 10.13.
Reaction of (Hydrosilyl)borane 2 with LDA. (a) Reaction
time: 1 h. (Hydrosilyl)borane 2 (33.0 mg, 39.9 µmol) was dissolved
in benzene (0.8 mL). To this solution was added a 2.0 M solution
of LDA (lithium diisopropylamide) in heptane/THF/ethylbenzene
(20.0 µL, 40.0 µmol), and the mixture was stirred at room
temperature for 1 h. The reaction was quenched by the addition of
an excess of chlorotrimethylsilane (20 µL). After removal of the
solvent, the residue was separated by HPLC (LC-918, CHCl3) to
give hydrodisilane 3 (4.7 mg, 6.1 µmol, 15%) together with recovery
of some of starting material 2 (28.0 mg, 33.8 µmol, 85%). 3:
colorless crystals. Mp: 185.7–187.4 °C. 1H NMR (CDCl3): δ –0.18
(s, 9H), –0.13 (s, 9H), 0.02 (s, 9H), 0.04 (s, 9H), 0.09 (s, 9H), 0.10
(s, 18H), 1.28 (s, 1H), 1.89 (br, s, 3H), 2.11 (s, 1H), 2.15 (s, 1H),
1
2.20 (s, 3H), 2.51 (br, s, 3H), 5.07 (s, 1H, JSiH ) 176 Hz), 6.28
(s, 1H), 6.41 (s, 1H), 6.61 (br, s, 1H), 6.80 (br, s, 1H). 13C NMR
(CDCl3): δ 0.87 (q), 0.94 (q), 1.06 (q), 1.11 (q), 1.14 (q), 1.57 (q),
1.66 (q), 21.0 (q), 24.9 (q), 26.1 (q), 29.0 (d), 29.3 (d), 30.4 (d),
122.8 (d), 128.10 (s), 128.15 (d), 128.3 (d), 129.1 (d), 132.8 (s),
137.6 (s), 143.0 (s), 144.1 (s × 2), 150.5 (s), 151.0 (s). 29Si NMR
(CDCl3): δ –54.3, –12.2, 1.6, 1.7, 2.0, 2.1. IR (KBr) 2121 [ν (SiH)]
cm–1. LRMS (FAB): m/z 773 [(M + H)+], 757 [(M – Me)+], 699
[(M – SiMe3)+]. HRMS (FAB): m/z 773.4464, calcd for C39H81Si8
773.4492.
(b) Reaction of 7 with an Excess of Bis(pinacolato)dibo-
rane(4). In a 10 mm ø Pyrex tube was placed a THF solution (1.5
mL) of a mixture of disilene 7 (153 mg, 109 µmol) and bis(pina-
colato)diborane(4) (178 mg, 701 µmol). After three freez-
e–pump–thaw cycles, the tube was evacuated and sealed. The
solution was heated at 60 °C for 186 h, during which time the
original orange color gradually disappeared. The tube was opened
and solvent was removed. Purification by silica gel chromatography
(n-hexane/CHCl3 ) 1:1) afforded diborylsilane 8 (111 mg, 116
µmol, 53%).
(b) Reaction time: 6 h. (Hydrosilyl)borane 2 (75.6 mg, 91.4
µmol) was dissolved in benzene (1.5 mL). To this solution was
added a 2.0 M solution of LDA in heptane/THF/ethylbenzene (48.0
µL, 96.0 µmol), and the mixture was stirred at room temperature
for 6 h. The reaction was quenched by the addition of an excess of
chlorotrimethylsilane (30 µL). After removal of the solvent, the
residue was separated by HPLC (LC-918, CHCl3) to give hydro-
disilane 3 (25.0 mg, 32.3 µmol, 35%), the starting material 2 (8.6
mg, 10 µmol, 11%), and a mixture of dihydrobenzo[b]silete 433 and
dihydrosilane 534 (15.9 mg). The mixture of 4 and 5 could not be
separated, and the yields of these products were estimated as 7
and 18%, respectively, as judged by the ratio of the integrated
(c) Reaction of Silylene–Isocyanide Complex 9 with Bis(pina-
colato)diborane(4). In a 10 mm ø Pyrex tube was placed a THF
solution (2.4 mL) of silylene–isocyanide complex 9, prepared from
disilene 7 (155 mg, 111 µmol) and Mes*NC (63.5 mg, 234 µmol)
in THF (2.4 mL). Bis(pinacolato)diborane(4) (59.1 mg, 233 µmol)
was added to this solution. After three freeze–pump–thaw cycles,
the tube was evacuated and sealed. The solution was heated at 60
°C for 120 h and then at 70 °C for 96 h, during which time the
original deep blue color did not change. Further heating at 80 °C
for 72 h afforded a pale yellow solution. The tube was opened and
solvent was removed. Purification by silica gel chromatography (n-
hexane/CHCl3 ) 1:1) afforded diborylsilane 8 (25.8 mg, 27.1 µmol,
12%) together with Mes*NC (62.1 mg, 229 µmol, 98%).
Reaction of Diborylsilane 8 with n-BuLi at –78 °C. Diboryl-
silane 8 (31.2 mg, 32.7 µmol) was dissolved in hexane (1.0 mL)
and the solution was cooled to –78 °C. To this solution was added
a 0.46 M solution of n-butyllithium in hexane (290 µL, 133 µmol).
The colorless solution turned yellow. The mixture was stirred at
–78 °C for 1 h, and the reaction was quenched by addition of an
excess of chlorotrimethylsilane (100 µL) to afford a colorless
solution. After removal of solvent, the residue was separated by
PTLC (SiO2/n-hexane) to give Si-trimethylsilylated silylborane 10
as the main product (18.0 mg, 20.0 µmol, 61%). 10: colorless
crystals, mp 250–253 °C (dec). 1H NMR (sym-C2D2Cl4, 373 K): δ
–0.28 (s, 9H), 0.10 (br, s, 18H), 0.12 (br, s, 18H), 0.13 (s, 9H),
0.31 (s, 9H), 1.33 (s, 1H), 1.35 (s, 6H), 1.37 (s, 6H), 1.68 (s, 3H),
2.16 (s, 1H), 2.20 (s, 3H), 2.28 (s, 1H), 2.60 (s, 3H), 6.34 (br, s,
1H), 6.47 (br, s, 1H), 6.56 (s, 1H), 6.75 (s, 1H). 13C NMR (sym-
C2D2Cl4, 373 K): δ 0.9 (q), 1.0 (q), 1.7 (q), 1.9 (q), 2.3 (q), 20.6
(q), 25.6 (q), 25.8 (q), 27.3 (q), 27.9 (q), 29.6 (d), 30.4 (d), 30.5
(d), 83.9 (s), 124.6 (d), 128.0 (d), 128.4 (d), 129.2 (d), 131.1 (s),
133.6 (s), 137.0 (s), 142.1 (s), 142.6 (s), 145.9 (s), 150.0 (s), 153.2
(s). 11B NMR (sym-C2D2Cl4): δ 35.8. 29Si NMR (sym-C2D2Cl4): δ
–57.8 (NNE, 296 K), –9.0, 1.6, 1.7 (INEPT, 373 K). LRMS (FAB):
1
intensities of the H NMR signals.
(c) Reaction time: 32 h. (Hydrosilyl)borane 2 (52.6 mg, 63.6
µmol) was dissolved in benzene (1.0 mL). To this solution was
added a 2.0 M solution of LDA in heptane/THF/ethylbenzene (33.0
µL, 66.0 µmol), and the mixture was stirred at room temperature
for 32 h. The reaction was quenched by the addition of an excess
of chlorotrimethylsilane (20 µL). After removal of the solvent,
the residue was separated by HPLC (LC-908, toluene) to give a
mixture of dihydrobenzo[b]silete 4 and dihydrosilane 5 (35.3 mg).
The mixture of 4 and 5 could not be separated, and the yields of
these products were estimated as 65 and 14%, respectively, as
judged by the ratio of the integrated intensities of the 1H NMR
signals.
Reaction of (Hydrosilyl)borane 2 with LiTMP (Lithium
2,2,6,6-tetramethylpiperidide). A 1.37 M solution of n-BuLi (90
µL, 123 µmol) was added to 2,2,6,6-tetramethylpiperidine (20 µL,
118 µmol) at 0 °C, and the mixture was stirred at 0 °C for 1 h. To
this was added (hydrosilyl)borane 2 (71.3 mg, 86.2 µmol) in
benzene (1.0 mL) at 0 °C, and the mixture was stirred at room
temperature for 12 h. The reaction was quenched by addition of an
excess of chlorotrimethylsilane (30 µL). After removal of the
solvent, the residue was separated by HPLC (LC-918, CHCl3) to
(55) Yamamoto, Y.; Aoki, K.; Yamazaki, H. Inorg. Chem. 1979, 18,
1681–1687.

Synthesis and Properties of Borylsilyl Anions
Organometallics, Vol. 27, No. 5, 2008 891
m/z 921 [(M + Na)+], 899 [(M + H)+], 825 [(M – TMS)+], 783
[(M – C6H12O2 + H)+]. HRMS (FAB): m/z 899.5339, calcd for
C45H92BO2Si8 899.5345.
The resulting deep red mixture was stirred at 0 °C for 30 min, and
the reaction was quenched by the addition of an excess of
methanol-d (100 µL) to afford a colorless solution. After removal
of solvent, the residue was separated by PTLC (SiO2/n-hexane) to
give dideuteriosilane 5-d2 as the main product (10.4 mg, 14.8 µmol,
65%).
Reaction of (Chlorosilyl)pinacolborane 13 with Na/K Alloy
at –78 °C. To a mixture of (chlorosilyl)borane 13 (56.8 mg, 65.9
µmol) and Na/K alloy (14.2 mg) was added ether (2.5 mL) at –78
°C. The resulting suspension was stirred at –78 °C for 12 h and
then quenched by the addition of an excess of chlorotrimethylsilane
(200 µL). After removal of solvent, the residue was separated by
PTLC (SiO2/n-hexane) to give Si-trimethylsilylated silylborane 10
as the main product (34.9 mg, 38.8 µmol, 59%).
Reaction of Diborylsilane 8 with an Excess of n-BuLi.
Diborylsilane 8 (44.0 mg, 46.1 µmol) was dissolved in hexane (2.0
mL) and the solution was cooled to –78 °C. To this solution was
added a 1.50 M solution of n-butyllithium in hexane (310 µL, 465
µmol). The resulting yellow mixture was stirred at –78 °C for 30
min, and the reaction was quenched by the addition of an excess
of dimethyl sulfate (100 µL) to afford a colorless solution. After
removal of solvent, the residue was separated by PTLC (SiO2/n-
hexane/CHCl3 ) 2:1) to give Si-methylated silylborane 14 as the
main product (32.2 mg, 38.3 µmol, 83%).
Reaction of (Chlorosilyl)borane 13 with Na/K Alloy at –40
°C. To a suspension of (chlorosilyl)borane 13 (46.9 mg, 54.4 µmol)
in diethyl ether (2.0 mL) was added Na/K alloy (ca. 10 mg) at –40
°C, and the mixture was stirred at –40 °C for 4.5 h. Chlorotri-
methylsilane (200 µL) was added, and the mixture was stirred
overnight, during which time it was warmed to room temperature.
The solvent was evaporated and hexane was added to the residue.
After filtration through Celite, the solvent was removed and the
residue was separated by PTLC (SiO2/n-hexane) to give Si-
trimethylsilylated silylborane 10 (35.2 mg, 39.1 µmol, 72%).
Reaction of Diborylsilane 8 with n-BuLi at 0 °C. Diborylsilane
8 (44.4 mg, 46.6 µmol) was dissolved in hexane (1.5 mL), and the
solution was cooled to 0 °C. To this solution was added a 0.43 M
solution of n-butyllithium in hexane (220 µL, 94.6 µmol). The
initially colorless solution turned deep red. The mixture was stirred
at 0 °C for 1 h, and the reaction was quenched by addition of an
excess of chlorotrimethylsilane (200 µL) to afford a colorless
solution. After removal of solvent, the residue was separated by
PTLC (SiO2/n-hexane) to give Si-trimethylsilylated silylborane 10
as the main product (21.8 mg, 24.2 µmol, 52%).
Reaction of (Hydrosilyl)borane 2 with Sodium Methoxide.
(Hydrosilyl)borane 2 (57.9 mg, 70.0 µmol) was dissolved in THF
(1.5 mL). To this solution was added a solution of sodium
methoxide (66.2 mg) in methanol (1.0 mL), and the mixture was
stirred overnight at room temperature. The resulting mixture was
poured into water, and the organic layer was extracted with hexane
and dried over Na2SO4. After removal of solvent, the residue was
separated by PTLC (SiO2/n-hexane) to give dihydrosilane 5 as the
main product (36.1 mg, 51.5 µmol, 74%).
Reaction of Borylsilyl Anion 11 with Pivaloyl Chloride.
Diborylsilane 8 (51.3 mg, 53.8 µmol) was dissolved in hexane (2.0
mL) and the solution was cooled to 0 °C. To this solution was
added a 0.43 M solution of n-butyllithium in hexane (260 µL, 112
µmol). The resulting deep red mixture was stirred at 0 °C for 30
min, and pivaloyl chloride (100 µL) was added at 0 °C. The mixture
was stirred at 0 °C for 30 min and overnight, during which time it
was warmed to room temperature. After removal of solvent, the
residue was separated by PTLC (SiO2/n-hexane/CHCl3 ) 3:2) to
give (chlorosilyl)borane 13 (14.4 mg, 16.7 µmol, 31%) together
with (hydrosilyl)borane 2 (11.1 mg, 13.4 µmol, 25%).
Reaction of Borylsilyl Anion 11 with Iodomethane. Diboryl-
silane 8 (59.0 mg, 61.9 µmol) was dissolved in hexane (2.0 mL),
and the solution was cooled to 0 °C. To this solution was added a
0.43 M solution of n-butyllithium in hexane (290 µL, 125 µmol).
The colorless solution turned deep red. The mixture was stirred at
0 °C for 30 min., and the reaction was quenched by the addition of
an excess of iodomethane (100 µL) to afford a colorless solution.
After removal of solvent, the residue was separated by PTLC (SiO2/
n-hexane/CHCl3 ) 2:1) to give Si-methylated silylborane 14 as the
main product (30.5 mg, 36.2 µmol, 58%). 14: colorless crystals.
Reaction of Silylborane 14 with Potassium tert-Butoxide. To
a mixture of silylborane 14 (36.4 mg, 43.3 µmol) and potassium
tert-butoxide (20.2 mg, 180 µmol) was added THF (1.5 mL) at
–78 °C, and the mixture was stirred at –78 °C for 1 h. The reaction
was quenched by the addition of an excess of dimethyl sulfate (50
µL). After removal of solvent, the residue was separated by PTLC
(SiO2/n-hexane) to give hydrosilane 17 (12.7 mg, 17.7 µmol, 41%)
together with recovery of 14 (21.5 mg, 25.5 µmol, 59%). 17:
colorless crystals, mp 198.6–200.4 °C. 1H NMR (CDCl3): δ –0.09
(s, 9H), –0.06 (s, 9H), –0.03 (s, 18H), 0.03 (s, 18H), 0.69 (d, 3H,
3JHH ) 4.6 Hz), 1.29 (s, 1H), 2.08 (s, 1H), 2.16 (s, 1H), 2.23 (s,
1
Mp: 205–208 °C (dec). H NMR (CDCl3): δ –0.17 (br, s, 18H),
0.02 (s, 9H), 0.03 (s, 9H), 0.06 (s, 9H), 0.09 (s, 9H), 0.81 (s, 3H),
1.02 (s, 6H), 1.07 (s, 6H), 1.26 (s, 1H), 2.16 (s, 3H), 2.36 (br, 6 +
2H), 6.22 (s, 1H), 6.35 (s, 1H), 6.64 (s, 2H). 13C NMR (CDCl3):
δ 0.85 (q), 0.91 (q), 1.0 (q), 1.3 (q), 1.8 (q), 2.1 (q), 5.6 (q), 20.9
(q), 24.1 (q), 25.0 (q), 25.5 (q), 27.4 (d), 27.6 (d), 30.1 (d), 83.5
(s), 122.7 (d), 127.8 (d), 128.9 (d), 130.0 (s), 133.5 (s), 137.4 (s),
142.8 (s), 144.1 (s), 151.3 (s × 2). 11B NMR (CDCl3): δ 35.7.
29Si NMR (CDCl3): δ –37.5, 1.6, 1.7, 2.1, 2.4, 2.6, 2.7. LRMS
(FAB): m/z 841 [(M + H)+], 825 [(M – Me)+], 756 [(M – C6H12)+],
725 [(M – C6H12O2 + H)+]. HRMS (FAB): m/z 841.5137, calcd
for C43H86BO2Si7 841.5106.
3
3H), 2.37 (s, 6H), 5.20 (q, 1H, JHH ) 4.6 Hz), 6.20 (s, 1H), 6.35
(s, 1H), 6.78 (s, 2H). 13C NMR (CDCl3): δ 0.71 (q), 0.79 (q), 0.85
(q), 1.0 (q), 1.1 (q), 1.4 (q), 3.2 (q), 21.0 (q), 24.9 (q), 27.1 (d),
27.3 (d), 30.3 (d), 122.5 (d), 126.0 (s), 127.5 (d), 128.9 (d), 133.5
(s), 138.8 (s), 143.7 (s), 143.9 (s), 151.5 (s), 151.7 (s). 29Si NMR
(CDCl3): δ –34.2, 1.7, 2.1, 2.4. IR (KBr) 2173 [ν(SiH)] cm–1.
LRMS (FAB): m/z 714 [(M)+], 699 [(M – Me)+], 595 [(M –
Mes)+]. HRMS (FAB): m/z 714.4178, calcd for C37H74Si7: 714.4175.
Anal. Calcd for C37H74Si7: C, 62.10; H, 10.42. Found: C, 61.98;
H, 10.52.
Reaction of Borylsilyl Anion 11 with Dimethyl Sulfate.
Diborylsilane 8 (48.4 mg, 50.8 µmol) was dissolved in hexane (2.0
mL), and the solution was cooled to 0 °C. To this solution was
added a 0.43 M solution of n-butyllithium in hexane (240 µL, 103
µmol). The resulting red mixture was stirred at 0 °C for 30 min,
and the reaction was quenched by the addition of an excess of
dimethyl sulfate (100 µL) to afford a colorless solution. After
removal of solvent, the residue was separated by PTLC (SiO2/n-
hexane/CHCl3 ) 2:1) to give Si-methylated silylborane 14 as the
main product (32.8 mg, 39.0 µmol, 77%).
Reaction of Borylsilyl Anion 11 with Methanol-d. Diborylsi-
lane 8 (21.6 mg, 22.7 µmol) was dissolved in hexane (1.0 mL),
and the solution was cooled to 0 °C. To this solution was added a
0.43 M solution of n-butyllithium in hexane (105 µL, 45.2 µmol).
Synthesis of Hydrosilane 17. (a) Reaction of (Methylsilyl)bo-
rane 14 with Sodium Methoxide. (Methylsilyl)borane 14 (20.9
mg, 24.8 µmol) was dissolved in THF (1.0 mL). To this solution
was added a solution of sodium methoxide (29.7 mg) in methanol-d
(1.0 mL), and the mixture was stirred overnight at room temper-
ature. The resulting mixture was poured into water, and the organic
layer was extracted with hexane and dried over Na2SO4. After
removal of solvent, the residue was separated with PTLC (SiO2/
n-hexane) to give hydrosilane 17-d as the main product (14.4 mg,
20.1 µmol, 81%).

892 Organometallics, Vol. 27, No. 5, 2008
Kajiwara et al.
(b) Reaction of (Hydrosilyl)borane 2 with n-BuLi. (Hydrosi-
lyl)borane 2 (71.4 mg, 86.3 µmol) was dissolved in hexane (2.5
mL) and the solution was cooled to 0 °C. To this solution was
added a 0.43 M solution of n-butyllithium in hexane (420 µL, 181
µmol). The resulting pale yellow mixture was stirred at 0 °C for
6 h, and the reaction was quenched by the addition of an excess of
dimethyl sulfate (50 µL) to afford a colorless solution. After removal
of solvent, the residue was separated by PTLC (SiO2/n-hexane) to
give a mixture of hydrosilane 17 and dihydrosilane 5 (36.5 mg)
together with recovery of 2 (20.6 mg, 24.9 µmol, 29%). The mixture
of 17 and 5 could not be separated, and the yields of these products
were estimated as 30 and 30%, respectively, as judged by the ratio
Reaction of Diborylsilane 8 with MeLi at Room Temper-
ature. Diborylsilane 8 (127.9 mg, 134.1 µmol) was dissolved in
THF (3.0 mL). To this solution was added a 1.20 M solution of
methyllithium in diethyl ether (240 µL, 288 µmol). The resulting
deep red mixture was stirred at room temperature for 19.5 h, during
which time the color of the solution disappeared and a white
precipitate formed. The reaction was quenched by the addition of
an excess of methanol-d (100 µL) to afford a colorless solution.
After removal of solvent, the residue was separated by PTLC (SiO2/
n-hexane) to give dihydrosilane 20-d as the main product (68.6
mg, 82.8 µmol, 62%).
Reaction of 2,3-Dihydro-1H-benzo[d][1,2]silaborole 19 with
Potassium Hydride. To a mixture of 2,3-dihydro-1H-benzo[d]-
[1,2]silaborole 19 (52.4 mg, 63.3 µmol) and potassium hydride (6.5
mg, 160 µmol) was added THF (1.5 mL), and the mixture was
stirred at room temperature 18.5 h. To this mixture was added an
excess of methanol-d (150 µL), and the resulting mixture was stirred
overnight. After removal of solvent, the residue was separated by
PTLC (SiO2/n-hexane) to give dihydrosilane 20-d (19.6 mg, 23.7
µmol, 37%).
1
of the integrated intensities of the H NMR signals.
Formation of 2,3-Dihydro-1H-benzo[d][1,2]silaborole 19. To
diborylsilane 8 (122.7 mg, 128.7 µmol) was added THF (0.2 mL),
and the suspension was cooled to –78 °C. To this suspension was
added a 1.20 M solution of methyllithium in diethyl ether (430
µL, 516 µmol). The resulting deep red mixture was stirred at –78
°C for 30 min and overnight, during which time it was warmed to
room temperature. To the resulting yellow suspension was added
dimethyl sulfate (200 µL). After removal of solvent, the residue
was separated by PTLC (SiO2/n-hexane/CHCl3 ) 1:1) to give 2,3-
dihydro-1H-benzo[d][1,2]silaborole 19 as the main product (78.9
mg, 95.3 µmol, 74%). 19: colorless crystals, mp 195–197 °C (dec).
1H NMR (CDCl3, 333 K): δ –0.35 (s, 9H), 0.04 (s, 9H), 0.05 (s,
9H), 0.06 (s, 9H), 0.12 (s, 9H), 0.25 (s, 9H), 1.07 (s, 3H), 1.20 (s,
Reaction of Disilene 7 with Bis(dithiocatecholato)dibo-
rane(4). In a 10 mm ø Pyrex tube was placed a benzene solution
(0.6 mL) of a mixture of disilene 7 (53.1 mg, 38.0 µmol) and
bis(dithiocatecholato)diborane(4) (27.8 mg, 92.0 µmol). After three
freeze–pump–thaw cycles, the tube was evacuated and sealed. The
solution was heated at 60 °C for 20 h, during which time the original
orange color gradually disappeared. The tube was opened in a
glovebox and solvent was removed. Purification by silica gel
chromatography (n-hexane/toluene ) 2:1) under argon afforded
diborylsilane 25 (44.0 mg, 43.9 µmol, 58%). 25: colorless crystals,
mp 167–171 °C (dec). 1H NMR (C6D6): δ 0.21 (s, 18H), 0.246 (s,
18H), 0.250 (br, 18H), 1.56 (s, 1H), 2.14 (s, 3H), 2.43 (s, 6H),
2.95 (s, 1H), 2.99 (s, 1H), 6.62 (s, 1H), 6.76 (s, 1H), 6.79 (s, 2H),
6.80–6.85 (m, 4H), 7.42–7.48 (m, 4H). 13C NMR (C6D6): δ 1.3
(q), 2.4 (q), 2.5 (q), 21.0 (q), 29.6 (q), 30.5 (d), 30.8 (d), 31.8 (d),
124.4 (d), 125.5 (d), 126.0 (d), 128.7 (s), 129.8 (d), 130.3 (d), 135.4
(s), 138.9 (s), 144.1 (s), 144.5 (s), 145.2 (s), 153.3 (s), 153.6 (s).
11B NMR (C6D6): δ 66.9. 29Si NMR (C6D6): δ –58.4, 2.1, 2.7,
3.0. LRMS (FAB): m/z 1001 [(M + H)+], 985 [(M – Me)+], 881
[(M – Mes)+], 849 [(M – B(Scat))+], 793 [(M – Mes – TMS –
Me)+], 761 [(M – B(Scat) – TMS – Me)+], 731 [(M – Mes –
B(Scat) + H)+], 715 [(M – Mes – B(Scat) – Me)+], 699 [(M –
B2(Scat)2 + H)+]. HRMS (FAB): m/z 1001.3616, calcd for
C48H79B2S4Si7 1001.3636. Anal. Calcd for C48H78B2S4Si7: C, 57.56;
H, 7.85. Found: C, 56.86; H, 7.80.
Reaction of Diborylsilane 25 with MeLi. (a) Quench by
TMSCl. Diborylsilane 25 (72.5 mg, 72.4 µmol) was dissolved in
THF (2.0 mL) and the solution was cooled to 0 °C. To this solution
was added a 1.20 M solution of methyllithium in diethyl ether (125
µL, 150 µmol). The resulting deep red mixture was stirred at 0 °C
for 40 min, and the reaction was quenched by addition of an excess
of chlorotrimethylsilane (200 µL) to afford a colorless solution.
After removal of solvent, the residue was separated by PTLC (SiO2/
n-hexane) to give Si-trimethylsilylated silylborane 26 (12.1 mg, 13.1
µmol, 18%). 26: colorless crystals. 1H NMR (C6D6): δ 0.11 (s,
9H), 0.15 (s, 9H), 0.23 (s, 9H), 0.24 (s, 9H), 0.34 (s, 9H), 0.35 (s,
9H), 0.47 (s, 9H), 1.54 (s, 1H), 2.07 (s, 3H), 2.32 (s, 3H), 2.44 (s,
3H), 2.62 (s, 1H), 2.69 (s, 1H), 6.63 (s, 1H), 6.70 (s, 1 + 1H),
6.74 (s, 1H), 6.83–6.88 (m, 2H), 7.51–7.56 (m, 2H). 13C NMR
(C6D6): δ 1.3 (q), 1.4 (q), 2.57 (q), 2.64 (q), 3.1 (q), 3.4 (q), 20.9
(q), 29.3 (q), 29.8 (q), 30.3 (d), 30.6 (d), 30.7 (d), 123.9 (d), 125.5
(d), 125.8 (d), 129.3 (d), 129.9 (d), 130.1 (d), 131.4 (s), 134.1 (s),
138.4 (s), 143.3 (s), 144.2 (s), 144.6 (s), 145.4 (s), 152.8 (s), 153.2
(s). 11B NMR (C6D6): δ 65.7. LRMS (FAB): m/z 923 [(M + H)+],
907 [(M – Me)+], 849 [(M – SiMe3)+]. HRMS (FAB): m/z
923.4250, calcd for C45H84BS2Si8: 923.4262. Anal. Calcd for
C45H83BS2Si8: C, 58.51; H, 9.06. Found: C, 58.30; H, 9.24.
3H), 1.27 (s, 3H), 1.38 (s, 1H), 1.39 (s, 3H), 1.56 (s, 1H), 1.94 (s,
1
3H), 2.22 (s, 3H), 2.57 (s, 1H), 2.62 (s, 3H), 4.97 (s, 1H, JSiH
)
179 Hz), 6.40 (br, s, 1H), 6.60 (br, s, 1H), 6.67 (s, 1H), 6.87 (s,
1H). 13C NMR (CDCl3, 333 K): δ 0.5 (q), 0.9 (q × 2), 1.1 (q), 2.1
(q), 4.0 (q), 21.0 (q), 23.5 (q), 24.3 (q), 24.9 (q), 25.3 (q), 25.6 (q),
26.2 (q), 31.0 (d), 31.6 (d), 75.3 (s), 86.8 (s), 122.1 (d), 128.2 (d
× 2), 128.9 (s), 129.2 (d), 131.4 (s), 131.6 (s), 138.9 (s), 144.4 (s),
144.5 (s), 150.2 (s), 155.8 (s), the signal of BC was not observed.
11B NMR (CDCl3): δ 65.4. 29Si NMR (CDCl3): δ –55.3, –2.5, –0.3,
1.4, 1.8, 1.9, 2.5. IR (KBr): 3588 [ν(OH)], 2128 [ν(SiH)] cm–1.
LRMS (FAB): m/z 849 [(M + Na)+], 826 [(M)+]. HRMS (FAB):
m/z 826.4877, calcd for C42H83BO2Si7 826.4871. Anal. Calcd for
C42H83BO2Si7: C, 60.96; H, 10.11. Found: C, 60.74; H, 10.13.
Reaction of (Chlorosilyl)pinacolborane 13 with Na/K Alloy
at Room Temperature. (a) Quenched by MeOH. To a suspension
of (chlorosilyl)borane 13 (65.9 mg, 76.5 µmol) in diethyl ether (2.0
mL) was added an excess of Na/K alloy (18.5 mg). The mixture
was stirred at room temperature for 27.5 h and then quenched by
the addition of methanol (100 µL). After removal of solvent, the
residue was separated by PTLC (SiO2/n-hexane) to give dihydrosi-
lane 20 as the main product (51.5 mg, 62.2 µmol, 81%). 20:
colorless crystals, mp 219–222 °C (dec). 1H NMR (CDCl3): δ –0.13
(s, 18H), 0.05 (s, 18H), 0.21 (s, 18H), 1.31 (s, 12 + 1H), 1.69 (s,
1
1H), 2.17 (br, s, 6H), 2.20 (s, 3H), 5.06 (s, 2H, JSiH ) 201 Hz),
6.37 (d, 1H, 4JHH ) 1.5 Hz), 6.72 (s, 2H), 6.80 (d, 1H, 4JHH ) 1.5
Hz). 13C NMR (CDCl3): δ 0.3 (q), 1.2 (q), 3.8 (q), 21.0 (q), 24.5
(q), 26.0 (q), 29.6 (d), 30.2 (d), 83.2 (s), 122.5 (d), 128.6 (s), 128.8
(d), 130.6 (s), 131.9 (d), 138.7 (s), 142.6 (s), 144.7 (s), 150.1 (s),
152.6 (s), the signal of BC was not observed. 11B NMR (CDCl3):
δ 33.5. 29Si NMR (CDCl3): δ –53.8, 2.0, 2.1, 2.3. IR (KBr): 2218
[ν(SiH)], 2175 [ν(SiH)] cm–1. LRMS (FAB): m/z 849 [(M + Na)+],
826 [(M)+], 742 [(M – C6H12)+], 726 [(M – C6H12O)+], 707 [(M
– Mes)+]. HRMS (FAB): m/z 826.4863, calcd for C42H83BO2Si7
826.4871. Anal. Calcd for C42H83BO2Si7: C, 60.96; H, 10.11. Found:
C, 61.01; H, 10.16.
(b) Quenched by MeOD. The same procedure was used in a
reaction of (chlorosilyl)borane 13 (67.7 mg, 78.5 µmol) in diethyl
ether (2.0 mL) with an excess of Na/K alloy (18.4 mg) except that
methanol-d (100 µL) was added. Dihydrosilane 20-d was isolated
(52.8 mg, 63.7 µmol, 81%).

Synthesis and Properties of Borylsilyl Anions
Organometallics, Vol. 27, No. 5, 2008 893
Table 4. Crystallographic Data for 3, 8, 10, 14, 17, and 19
3
8
10
14
17
C37H74Si7
715.59
19
formula
fw
C39H80Si8
773.75
C48H94B2O4Si7
953.48
C45H91BO2Si8
899.71
C43H85BO2Si7
841.55
C42H83BO2Si7
827.52
cryst size (mm)
cryst syst
space group
a (Å)
b (Å)
c (Å)
0.25 × 0.25 × 0.25 0.35 × 0.25 × 0.15 0.40 × 0.35 × 0.20 0.30 × 0.15 × 0.05 0.40 × 0.25 × 0.20 0.35 × 0.20 × 0.10
triclinic
triclinic
triclinic
triclinic
triclinic
monoclinic
C2/c (#15)
35.738(19)
13.262(6)
j
j
j
j
j
P1 (#2)
P1 (#2)
P1 (#2)
P1 (#2)
P1 (#2)
9.708(3)
14.996(4)
17.847(5)
79.350(7)
87.704(10)
81.536(7)
2525.4(12)
2
12.189(4)
13.204(5)
19.744(7)
79.858(14)
72.051(13)
78.938(16)
2943.3(17)
2
13.100(7)
20.193(10)
24.014(11)
65.154(12)
78.169(12)
88.869(18)
5626(5)
4
9.220(6)
12.874(8)
23.730(14)
100.332(6)
92.284(9)
108.119(12)
2620(3)
2
11.384(4)
13.277(5)
15.422(5)
79.505(9)
80.473(9)
87.062(9)
2259.9(14)
2
22.777(12)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
110.042(8)
10141(9)
8
Dcalc (g cm–3
)
1.018
2.36
1.076
1.99
1.062
2.22
1.067
2.13
1.052
2.34
1.084
2.19
µ (cm–1
)
no. of indep reflns/params 8683/449
10 158/579
0.0523
0.1221
18 704/1065
0.0605
0.1481
9040/505
0.0790
0.2138
7801/420
0.0854
0.2193
8933/495
0.0859
0.1804
1.149
R1 (I > 2σ(I))
wR2 (all data)
goodness of fit
0.0948
0.2775
1.065
1.075
1.079
1.071
1.065
(b) Quench by MeOD. The same procedure was used in the
reaction of diborylsilane 25 (54.2 mg, 54.1 µmol) with methyl-
lithium in diethyl ether (90 µL, 108 µmol), with subsequent quench
by an excess of methanol-d (50 µL). Dideuteriosilane 5-d2 (28.7
mg, 40.8 µmol, 75%) was obtained.
refined by full-matrix least-squares procedures on F2 for all
reflections (SHELXL-97).59 All the non-hydrogen atoms were
refined with anisotropic thermal parameters. All hydrogen atoms
were placed in calculated positions. Crystallographic data for 3, 8,
10, 14, 17, and 19 are given in Table 4.
Theoretical Calculations. All theoretical calculations were
carried out using the Gaussian 98 M or Gaussian 03 M programs49
with density functional theory at the B3LYP level.56 The structural
optimization was performed at the B3LYP/6-31+G(d) level. The
NMR chemical shifts were calculated at the GIAO-B3LYP/6-
311+G(2d,p) level. Reference molecules for the chemical shifts
were also calculated at the same level: Si(CH3)4 for Si (0 ppm)
and B2H6 for B (16.6 ppm).57
X-Ray Crystallographic Analysis. Single crystals of 3, 8, 10,
14, 17, and 19 suitable for X-ray analysis were obtained by
recrystallization from dichloromethane/methanol at room temper-
ature (for 3, 8, 14, 17, and 19) and from hexane at room temperature
(for 10). A colorless crystal was mounted on a loop. The intensity
data were collected on a Rigaku/MSC Mercury CCD diffractometer
with graphite-monochromated Mo KR radiation (λ ) 0.71070 Å).
The structures were solved by a direct method (SIR-97)58 and
Acknowledgment. This work was partially supported by
Grants-in-Aid for COE Research on “Elements Science” (No.
12CE2005), Scientific Research on Priority Area (No.
14078213, “Reaction Control of Dynamic Complexes”), and
the 21st Century COE Program on “Kyoto University
Alliance for Chemistry” from MEXT, Japan. We thank
Central Glass Co., Ltd. for the generous gift of tetrafluo-
rosilane.
Supporting Information Available: Calculated atomic charges
for R2SiH2 molecules,32 calculated molecular coordinates for boron-
stabilized silyl anions, PES scans for boron-stabilized silyl anions
and related molecules, 1H and 13C NMR spectra for new com-
pounds, and full citation of ref 49. X-ray crystallographic data of
3, 8, 10, 14, 17, and 19 in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
(56) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(57) Onak, T. P.; Landesman, H.; Williams, R. E.; Shapiro, I. J. Phys.
Chem. 1959, 63, 1533–1535.
(58) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115–119.
OM7008974
(59) Sheldrick, G. M. SHELX-97, Program for the Refinement of Crystal
Structures; University of Göttingen: Göttingen, Germany, 1997.