Convenient preparation of silylboranes

Article abstract of DOI:10.1021/om000254t

(Triorganosilyl)pinacolboranes were prepared by reaction of triorganosilyllithium reagents with pinacolborane or isopropoxypinacolborane in high yield. The reaction also is applicable to synthesis of 2-(triorganosilyl)-4,4,6-trimethyl-1,3,2-dioxaborinane. A new germylborane derivative was similarly prepared from the corresponding germyllithium.

Full text of DOI:10.1021/om000254t

Organometallics 2000, 19, 4647-4649
4647
Con ven ien t P r ep a r a tion of Silylbor a n es
Michinori Suginome,* Takanori Matsuda, and Yoshihiko Ito*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of
Engineering, Kyoto University, Kyoto 606-8501, J apan
Received March 24, 2000
Summary: (Triorganosilyl)pinacolboranes were prepared
by reaction of triorganosilyllithium reagents with pinacol-
borane or isopropoxypinacolborane in high yield. The
reaction also is applicable to synthesis of 2-(triorgano-
silyl)-4,4,6-trimethyl-1,3,2-dioxaborinane. A new germyl-
borane derivative was similarly prepared from the
corresponding germyllithium.
(Organosilyl)pinacolboranes 2, which have been em-
ployed in the recent silaboration chemistry, were pre-
pared by this methodology.2b However, this method is
not always convenient for practical use, since the
preparation of chlorobis(dialkylamino)boranes can be
difficult, especially due to their high moisture sensitiv-
ity. Therefore, development of a new and convenient
preparation of silylboranes is desirable for further
exploitation of their chemistry.
In tr od u ction
In recent years, additions of silylboranes to unsatur-
ated organic molecules have been developed, leading
to the synthesis of new organometallic compounds
containing both silicon and boron.1 Group 10 metal
complexes have effectively activated the Si-B bond of
the silylboranes, enabling regio- and/or stereoselective
reactions with alkynes,2 alkenes,3 1,3-dienes,4 and 1,2-
dienes.5,6 Synthetic utility of the silylborane derivatives
has been demonstrated by silaborative carbon-carbon
bond forming reactions, which involve successive inser-
tion of unsaturated molecules into the Si-B bond.7
Although compounds containing Si-B bonds have
been known since 1960,8 their reactivity has not been
explored until recently, mainly due to the lack of
synthetic accessibility of silylboranes. Although a series
of silylboranes bearing two heteroatoms such as amino,
alkoxy, and alkylthio groups on the boron atoms have
been synthesized, use of chlorobis(dialkylamino)boranes
is required for their preparation.9 For instance, reaction
of an organosilyllithium with chlorobis(diethylamino)-
borane afforded (organosilyl)bis(diethylamino)borane 1,
which can be converted to the corresponding (organo-
silyl)dialkoxyborane as well as (organosilyl)di(alkylthio)-
borane derivatives via ligand exchange reaction.6a,10
In this paper, we disclose new preparative methods
for the diol-derived silylboranes, in which easily acces-
sible boron precursors are reacted with silyllithium
reagents. The method was applied with good success to
the synthesis of a related germylborane.
Resu lts a n d Discu ssion
R ea ct ion of H yd r ob or a n es w it h Silyllit h iu m s.
Hydroboranes, (R2BH)2, are known to react with Grig-
nard and organolithium reagents (R′M) to generate the
corresponding organoborates (MBR2R′H). Such borates
can afford new organoborane compounds via elimination
of the hydride on the boron atom as a leaving group.11
However, substitution of hydride with silyl anions has
never been achieved in a practical manner.12 Attempted
reaction of silyllithiums with 9-BBN (9-borabicyclo-
[3.3.1]nonane) afforded bishydro- and bis(silyl)borate
derivatives in high yield via disproportionation of the
initially formed (hydro)(silyl)borate.13 Nevertheless, we
examined some reactions of cyclic hydroboranes with
triorganosilyllithiums, since monohydroboranes with
diol ligands such as pinacolborane 3 are easily acces-
sible14 and less susceptible to borate formation than
9-BBN.
(1) Suginome, M.; Ito, Y. Chem. Rev. 2000, 100, 3221.
(2) (a) Suginome, M.; Nakamura, H.; Ito, Y. Chem. Commun. 1996,
2777. (b) Suginome, M.; Matsuda, T.; Nakamura, H.; Ito, Y. Tetra-
hedron 1999, 55, 8787. (c) Onozawa, S.-y.; Hatanaka, Y.; Tanaka, M.
Chem. Commun. 1997, 1229.
(3) Suginome, M.; Nakamura, H.; Ito, Y. Angew. Chem., Int. Ed.
Engl. 1997, 36, 2516.
(4) Suginome, M.; Matsuda, T.; Yoshimoto, T.; Ito, Y. Org. Lett. 1999,
1, 1567.
(5) (a) Suginome, M.; Ohmori, Y.; Ito, Y. Synlett 1999, 1567. (b)
Onozawa, S.-y.; Hatanaka, Y.; Tanaka, M. Chem. Commun. 1999, 1863.
(6) For the related reactions of silylboranes which do not require
transition-metal catalysts, see: (a) Buynak, J . D.; Geng, B. Organo-
metallics 1995, 14, 3112. (b) Suginome, M.; Fukuda, T.; Nakamura,
H.; Ito, Y. Organometallics 2000, 19, 719.
Pinacolborane 3 was reacted with dimethylphenylsilyl-
lithium in THF/hexanes (1/1) at 0 °C to room temper-
ature. An initial attempt using 1 molar equiv of 3
resulted in the formation of the desired silylborane, but
(7) (a) Suginome, M.; Nakamura, H.; Matsuda, T.; Ito, Y. J . Am.
Chem. Soc. 1998, 120, 4248. (b) Suginome, M.; Matsuda, T.; Ito, Y.
Organometallics 1998, 17, 5233.
(8) Seyferth, D.; Ko¨gler, H. P. J . Inorg. Nucl. Chem. 1960, 15, 99.
Cowley, A. H.; Sisler, H. H.; Ryschkewitsch, G. E. J . Am. Chem. Soc.
1960, 82, 501.
(9) No¨th, H.; Ho¨llerer, G. Angew. Chem. 1962, 74, 718. No¨th, H.;
Ho¨llerer, G. Chem. Ber. 1966, 99, 2197.
(10) Biffar, W.; No¨th, H.; Schwertho¨ffer, R. Liebigs Ann. Chem. 1981,
2067.
(11) Odom, J . D. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Ed.; Pergamon: New York, 1982; Vol. 1, p 253.
(12) Lippert, W.; No¨th, H.; Ponikwar, W.; Seifert, T. Eur. J . Inorg.
Chem. 1999, 817.
(13) Biffer, W.; No¨th, H. Angew. Chem. 1980, 92, 65. Biffer, W.; No¨th,
H. Chem. Ber. 1982, 115, 934.
(14) Pinacolborane is comercially available and easily prepared by
the reported procedure. Tucker, C. E.; Davidson, J .; Knochel, P. J . Org.
Chem. 1992, 57, 3482.
10.1021/om000254t CCC: $19.00 © 2000 American Chemical Society
Publication on Web 09/28/2000

4648 Organometallics, Vol. 19, No. 22, 2000
Notes
only in low yield. However, use of 2 molar equiv of 3
provided silylborane 2a in 73% yield (eq 1).15 The
procedure was applicable also to the six-membered-ring
hydroborane 4 for the synthesis of 5 (eq 2), but use of
catecholborane failed to give the corresponding product
at all. Under the same conditions, reaction of methyl-
diphenylsilyllithium with pinacolborane afforded the
corresponding silylborane, which could not be separated
from a small amount of byproducts.
borane 8. Thus, reaction of isopropoxyborane 6a with
dimethylphenylgermyllithium provided (dimethylphen-
ylgermyl)pinacolborane 8 in 83% yield, whereas use of
the hydroborane 3 gave 8 in lower yield (eq 5).17
A reaction of trimethylstannyllithium with 6a (2
equiv) was also attempted under the same reaction
conditions as those for the preparation of the germyl-
borane. However, no formation of the desired stannyl-
1
borane was indicated by H, 11B, and 119Sn NMR anal-
yses of the reaction mixture. The analyses suggested
that hexamethyldistannane with unidentifiable boron
compound(s), which may have three oxygen substituents
on the boron atom, was formed in the reaction.18
Rea ction of Alk oxybor a n es w ith Silyllith iu m s.
The next candidates selected as potential starting
materials in the silylborane synthesis were (alkoxy)-
pinacolboranes 6, which are easily prepared by reaction
of commercially available trialkoxyboranes with pinacol
by heating.16 In a manner similar to the reactions with
hydroboranes, use of 2 molar equiv (rather than 1) of
isopropoxypinacolborane (6a ) with dimethylphenyl-
silyllithium gave a 74% yield of (dimethylphenylsilyl)-
pinacolborane 2a (eq 3). Methoxypinacolborane (6b)
Con clu sion
New synthetic routes to the silylboranes having diol
ligands were established. The reaction of readily acces-
sible boron sources, e.g., hydroboranes or, preferably,
alkoxyboranes having diol substituents on the boron
atoms, with silyllithium reagents gave synthetically
useful silylboranes without difficulty. Application of this
method to the synthesis of a germylborane was also
successful.
The new method makes possible not only the practical
utilization of the silylboranes but also their further
development as powerful tools in organic synthesis.
Exp er im en ta l Section
Gen er a l Com m en ts. All reactions were carried out under
a nitrogen or argon atmosphere. 1H and 13C NMR spectra were
recorded on a Varian VXR-200 and a Varian Gemini 2000
equipped with 4.7 and 7.0 T magnets, respectively. Proton
chemical shifts were reported in ppm downfield from tetram-
ethylsilane (δ scale) and referenced to internal residual CHCl3
(7.26 ppm). Carbon chemical shifts were reported in ppm
downfield from tetramethylsilane (δ scale) and referenced to
the carbon signal of CDCl3 (77.0 ppm). 11B NMR spectra were
recorded on a J EOL J NM-A400 equipped with 9.4 T magnet.
The boron chemical shifts were referenced to the external
standards BF3‚OEt2 (0 ppm). Mass spectra were recorded on
a J EOL J MS-HX100A or a J EOL J MS-SX102A.
gave a lower yield (55%) for 2a in the reaction with
dimethylphenylsilyllithium under the same reaction
conditions. In contrast with the silylborane synthesis
with hydroboranes, the use of alkoxypinacolboranes was
applicable to the synthesis of other silylborane deriva-
tives. Indeed, methyldiphenylsilyl derivative 2b and
triphenylsilyl derivative 2c were prepared by the reac-
tions of 6a with the corresponding silyllithiums in good
yield (eq 3). Furthermore, six-membered ring derivative
5 was successfully prepared from the silyllithium and
the corresponding isopropoxyborane 7 (eq 4).
THF and hexanes were distilled from Na/benzophenone and
Na, respectively, under an atmosphere of nitrogen.
Dimethylphenylsilyllithium, methyldiphenylsilyllithium, and
triphenylsilyllithium (ca. 1 mol/L in THF) were freshly pre-
pared from the corresponding chlorosilanes with lithium (4
equiv) in THF. Dimethylphenylgermyllithium (0.75 mol/L in
THF) was freshly prepared from chlorodimethylphenylger-
mane with lithium (4 equiv) in THF.19 Pinacolborane (3),14
4,4,6-trimethyl-1,3,2-dioxaborinane (4),20 isopropoxypinacolbo-
rane (6a ),16 methoxypinacolborane (6b),16 and 2-isopropoxy-
Syn th esis of Ger m ylbor a n e. Pinacolborane 3 as
well as isopropoxypinacolborane 6a served well as the
boron sources in the preparation of the new germyl-
(17) For examples of germylboranes, see: Mayer, E. P.; No¨th, H.;
Rattay, W.; Wietelmann, U. Chem. Ber. 1992, 125, 401.
(18) For the disproportionation of stannylboranes, see: No¨th, H.;
Schwertho¨ffer, R. Chem. Ber. 1981, 114, 3056.
(15) Lithium dihydro(pinacol)borate may inevitably be formed as a
byproduct, although we have no evidence for it.
(16) Hoffmann, R. W.; Metternich, R.; Lanz, J . W. Liebigs Ann.
Chem. 1987, 881.
(19) Tamao, K.; Kawachi, A. Adv. Organomet. Chem. 1995, 38, 1.
(20) Woods, W. G.; Strong, P. L. J . Am. Chem. Soc. 1966, 88, 4667.
We found that the method reported by Knochel (ref 14) was also
applicable to the synthesis of this compound.

Notes
Organometallics, Vol. 19, No. 22, 2000 4649
4,4,6-trimethyl-1,3,2-dioxaborinane (7)16 were prepared ac-
cording to the literature methods.
11.7 Hz, 1H), 1.81 (dd, J ) 13.7, 3.1 Hz, 1H), 4.06-4.25 (m,
1H), 7.26-7.36 (m, 3H), 7.55-7.63 (m, 2H); 13C NMR (CDCl3)
δ -3.2, 23.1, 28.4, 31.2, 46.4, 64.2, 70.4, 127.5, 128.2, 134.3,
140.8; 11B NMR (C6D6) δ 31.7; HRMS calcd for C13H20BO2Si
Gen er a l P r oced u r e for t h e Syn t h esis of Silyl-
bor a n es. To a stirred solution of the hydroborane or alkoxy-
borane 3, 4, 6a , 6b, or 7 (128 mmol) in hexane (60 mL) was
added dimethylphenylsilyllithium (ca. 1 mol/L in THF, 62 mL,
62 mmol) dropwise at 0 °C over 30 min. After the addition,
the cooling bath was removed. The mixture was stirred
overnight at room temperature. Evaporation of the volatile
materials left white residual solid, which was taken up to
remove material insoluble in hexane. After suction filtration
under nitrogen, the filtrate was concentrated in vacuo. Distil-
lation of the residue gave the silylborane as colorless liquid.
2-(Dim eth ylp h en ylsilyl)-4,4,5,5-tetr a m eth yl-1,3,2-d iox-
a bor ola n e (2a ). The title compound was prepared from 3, 6a ,
or 6b and dimethylphenylsilyllithium according to the general
procedure and isolated by distillation (97-99 °C/0.1 mmHg).
For the title compound, only 11B NMR resonance and the
elemental analysis are given, since the same compound was
synthesized previously and characterized, but without those
descriptions.2b 2a : 11B NMR (C6D6) δ 34.3. Anal. Calcd for
[M+ - CH3] 247.1324, found 247.1313. Anal. Calcd for C14H23
-
BO2Si: C, 64.12; H, 8.84. Found: C, 64.06; H, 8.91.
Syn th esis of 2-(Dim eth ylp h en ylger m yl)-4,4,5,5-tetr a -
m eth yl-1,3,2-d ioxa bor ola n e (8). To a stirred solution of 6a
(17.7 g, 95 mmol) in hexane (60 mL) was added dimethylphe-
nylgermyllithium (0.75 mol/L in THF, 60 mL, 45 mmol)
dropwise at 0 °C over 30 min. After the addition, the cooling
bath was removed. The mixture was stirred overnight at room
temperature. Evaporation of the volatile material left a white
residue, which was taken up in hexane. After suction filtration
under nitrogen, the filtrate was concentrated in vacuo. Distil-
lation of the residue (92-94 °C/0.6 mmHg) gave the germylbo-
rane 8 as colorless liquid (9.8 g, 83%). 8: 1H NMR (CDCl3) δ
0.42 (s, 6H), 1.27 (s, 12H), 7.25-7.38 (m, 3H), 7.48-7.58 (m,
2H); 13C NMR (CDCl3) δ -3.9, 24.9, 83.8, 127.86, 127.89, 133.9,
141.6; 11B NMR (C6D6) δ 35.4; FABHRMS calcd for C13H20
BGeO2 [M+ - CH3] 293.0767, found 293.0763.
-
C
14H23BO2Si: C, 64.12; H, 8.84. Found: C, 63.83; H, 8.94.
Attem p ted Syn th esis of a Sta n n ylbor a n e. To a stirred
solution of 6a (7.4 g, 40 mmol) in hexane (35 mL) was added
a solution of trimethylstannyllithium (0.6 mol/L in THF, 33
mL, 20 mmol) dropwise at 0 °C over 30 min. The mixture was
stirred overnight at room temperature. Evaporation of the
volatile material left a residue, which was taken up in hexane.
The suspension was filtered through a pad of Celite under
nitrogen. The resultant filtrate was concentrated in vacuo. The
2-(Meth yld ip h en ylsilyl)-4,4,5,5-tetr a m eth yl-1,3,2-d iox-
a bor ola n e (2b). The title compound was prepared from 6a
and methyldiphenylsilyllithium according to the general pro-
cedure and isolated by distillation (132-142 °C/0.2 mmHg).
2b: 1H NMR (CDCl3) δ 0.61 (s, 3H), 1.28 (s, 12H), 7.30-7.36
(m, 6H), 7.56-7.65 (m, 4H); 13C NMR (CDCl3) δ -4.5, 24.9,
83.6, 127.8, 128.9, 135.0, 137.2; 11B NMR (C6D6) δ 34.2;
FABHRMS calcd for C18H22BO2Si [M+ - CH3] 309.1481, found
309.1474.
2-(Triphenylsilyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(2c). A modified procedure was used for the synthesis of the
title compound by reaction of 6a with triphenylsilyllithium.
After stirring overnight, the precipitate formed was filtered.
Concentration of the filtrate gave a viscous oil, which gradually
solidified on standing. The solid was washed with hexane and
dried in vacuo to give 2c. 2c: mp 132.0-133.2 °C (sealed tube);
1H NMR (CDCl3) δ 1.30 (s, 12H), 7.30-7.38 (m, 9H), 7.53-
7.58 (m, 6H); 13C NMR (CDCl3) δ 24.9, 83.9, 127.9, 129.1, 135.2,
136.2; 11B (C6D6) δ 34.1; FABHRMS calcd for C18H22BO2Si [M+
- C6H5] 309.1481, found 309.1494.
1
presence of hexamethyldistannane was indicated by H NMR
(singlet at 0.20 ppm with satellites due to Sn-H couplings of
1
48.8, 46.4, and 15.8 Hz) and 119Sn NMR (-109.6 ppm, J Sn-Sn
) 4231 Hz). No bis(pinacolato)diboron (30.3 ppm), which may
be formed in association with the formation of the distannane,
was detected by 11B NMR analysis (br s, 21.8 ppm).
Ack n ow led gm en t. We thank Mr. Akira Matsumoto
for his assistance in the preparation of 2c. T.M. thanks
fellowship support from the J apan Society for the
Promotion of Science for Young Scientists.
2-(Dim eth ylph en ylsilyl)-4,4,6-tr im eth yl-1,3,2-dioxabor i-
n a n e (5). The title compound was prepared from 4 or 7 and
dimethylphenylsilyllithium according to the general procedure
and isolated by distillation (88-92 °C/0.6 mmHg). 5: 1H NMR
(CDCl3) δ 0.25 (s, 6H), 1.19-1.30 (m, 9H), 1.51 (dd, J ) 13.7,
Su p p or tin g In for m a tion Ava ila ble: 1H and 13C NMR
charts for compounds 2a -c, 5, and 8. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM000254T