Reaction of stable silylene-isocyanide complexes with boranes: Synthesis and properties of the first stable silylborane-isocyanide complexes

Article abstract of DOI:10.1246/cl.2001.1076

Stable silylene-isocyanide complexes [Tbt(Mes)SiCNAr (Ar = Tbt, Mes*; Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl; Mes = mesityl; Mes* = 2,4,6-tri-t-butylphenyl)] reacted with BH3·THF to give the first stable silylborane-isocyanide complexes [Tbt(MeS)SiHBH₂CNAr]. The thermal equilibrium between Tbt(Mes)SiHBH₂CNAr and TbtSiH₂BH(Mes)CNAr was observed.

Full text of DOI:10.1246/cl.2001.1076

1076
Chemistry Letters 2001
Reaction of Stable Silylene–Isocyanide Complexes with Boranes:
Synthesis and Properties of the First Stable Silylborane–Isocyanide Complexes
Nobuhiro Takeda, Takashi Kajiwara, and Norihiro Tokitoh*
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011
(Received July 30, 2001; CL-010725)
Stable silylene–isocyanide complexes [Tbt(Mes)SiCNAr (Ar
boranes [PhMe2SiBX2] giving the corresponding (boryl)(silyl)-
iminomethanes [(PhMe2Si)(X2B)C=NR].8
= Tbt, Mes*; Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl;
Mes = mesityl; Mes* = 2,4,6-tri-t-butylphenyl)] reacted with
BH3·THF to give the first stable silylborane–isocyanide complex-
es [Tbt(Mes)SiHBH2CNAr]. The thermal equilibrium between
Tbt(Mes)SiHBH2CNAr and TbtSiH2BH(Mes)CNAr was
observed.
1
The formation of 3a,b was confirmed by H, 13C, and 11B
NMR and IR spectra.9 Although the BH2 protons were not
1
In recent decades, much attention has been paid to the chem-
istry of silylenes (silicon analogues of carbenes) from the stand-
points of not only fundamental chemistry but also applied chem-
istry such as material science and organic syntheses.1 Also, sily-
lene complexes with Lewis bases have been extensively studied
and some complexes have been synthesized in low temperature
matrices so far.2 However, their properties have not been fully
disclosed yet due to their instability at ambient temperature.
Recently, we have succeeded in the synthesis of the first sta-
ble silylene–Lewis base complexes 1a–c3 by the reaction of a
disilene bearing an efficient steric protection group, 2,4,6-tris[bis-
(trimethylsilyl)methyl]phenyl (denoted as Tbt hereafter),4 with
hindered isocyanides. In the reaction with Et3SiH, nitrile oxides,5
and 1,3-dienes,6 complexes 1a–c showed the reactivity as a sily-
lene equivalent, and these results suggest the facile dissociation
of 1a–c into the silylene, Tbt(Mes)Si: (2), and the corresponding
isocyanides at room temperature. Furthermore, we have revealed
that 1c reacted with methanol as a sila-ylide to undergo the 1,2-
addition to the Si–C bond.3 In this paper, we present the reaction
of silylene complexes 1a,b with BH3·THF giving the first stable
silylborane–isocyanide complexes.
observed on the H NMR spectra probably due to the extreme
broadening and the complicated coupling of the signals, the exis-
tence of the BH2 part was evidenced by the observation of absorp-
tions assigned to B–H stretch on the IR spectra (2400 and 2369
cm–1 for 3a and 2400 and 2365 cm–1 for 3b) and signals in the
region of tetracoordinated borons on the 11B NMR spectra (–42.6
ppm for 3a and –43.0 ppm for 3b). The molecular structure of 3a
was finally determined by X-ray structural analysis (Figure 1).10
The ORTEP drawing of 3a shows the tetrahedral geometry of the
silicon atom and the linear B1–C1–N1–C4 structure. The
Si1–B1–C1 angle (105.9(3)°) suggests that the boron atom has
tetrahedral geometry. The Si1–B1 bond length (2.052(4) Å) is
slightly longer than that of a silylborane–phosphine complex
[Me3Si–BH2P(C6H12)3] (2.007(4) Å)11 and lithium silylborates
[Li(Ph2RSi–BH3); R = Ph, t-Bu] (1.984–1.993 Å).12 This value is
close to those of tri- and tetra-silylborates [LiRB(SiMe3)3; R =
SiMe3, Me] (2.017–2.034 Å).12 The B1–C1 bond length (1.538(6)
Å) is shorter than that of o-Me3SiO–C6H4NC–BPh3 (1.616(2) Å)13
and almost equal to those of polyhedral heteroborane–isocyanides
adducts (1.537–1.560 Å).14 The C–N bond length (1.155(5) Å) is
almost the same as that of TbtNC (1.156(7) Å).15
Treatment of silylene complex 1a with BH3·THF in THF at
0 °C led to the gradual disappearance of the blue color of 1a, and
the corresponding silylborane–isocyanide complex 3a was
obtained as a stable compound in 34% yield along with
Tbt(Mes)SiH2 (4) (20%) and TbtSiH3 (5) (5%) (eq 1). Reaction
of complex 1b with BH3·THF also gave the corresponding silyl-
borane–isocyanide complex 3b (44%), 4 (12%), and 5 (8%). By
contrast, the reaction of 1c afforded a complex mixture and no
silylborane complex 3c was obtained, although the formation of 4
and 5 were confirmed. Since silylboranes and silylborates have
attracted much attention especially from a synthetic point of
view,7 it should be very interesting to elucidate the properties of
these novel compounds containing a Si–B bond. Furthermore,
similar type of silylborane–isocyanide complexes have recently
been postulated as reactive intermediates in the insertion reac-
tions of isocyanides [RNC] into the silicon–boron bond of silyl-
The formation of 3a,b may be reasonably interpreted in
terms of the initial insertion of silylene 2 to the B–H bond of
BH3·THF followed by the coordination of the isocyanide to the
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1077
boron atom of the resulting silylborane 6 (Scheme 1).
However, the mechanism via the initial coordination of BH3 on
the silicon atom of the silylene complex 1a,b cannot be ruled
out at present. In order to elucidate the mechanism for the for-
mation of 4 and 5, the reaction of 1a with an excess (5.5 molar
equivalents) of BH3·THF was examined. This reaction gave 4
(59%) as a main product along with 3a (4%) and 5 (15%).
Since 3a did not react with BH3·THF, it was considered that
these hydrosilanes 4 and 5 were generated by the reaction of the
intermediary 6 and/or 7 with BH3·THF.
This work was partially supported by Grants-in-Aid for COE
Research on Elements Science (No. 12CE2005) and Scientific
Research (Nos. 11133247 and 13740353) from Ministry of
Education, Culture, Sports, Science, and Technology of Japan.
We thank Central Glass and Shin-Etsu Chemical Co., Ltds. for the
generous gifts of tetrafluorosilane and chlorosilanes, respectively.
Dedicated to Professor Hideki Sakurai on the occasion of
his 70th birthday.
References and Notes
1
For a recent review on silylenes, see: P. P. Gaspar and R. West, in “The
Chemistry of Organic Silicon Compounds,” ed. by Z. Rappoport and Y.
Apeloig, John Wiley & Sons, New York (1998), Vol. 2, Part 3, pp
2463–2568.
2
3
4
For a recent review on silylene–Lewis base complexes, see: J. Belzner
and H. Ihmels, Adv. Organomet. Chem., 43, 1 (1999).
N. Takeda, H. Suzuki, N. Tokitoh, R. Okazaki, and S. Nagase, J. Am.
Chem. Soc., 119, 1456 (1997).
R. Okazaki, M. Unno, and N. Inamoto, Chem. Lett., 1987, 2293; R.
Okazaki, N. Tokitoh, and T. Matsumoto, in “Synthetic Methods of
Organometallic and Inorganic Chemistry,” ed. by W. A. Herrmann, Vol.
ed. by N. Auner and U. Klingebiel, Thieme, New York (1996), Vol. 2,
pp 260–269.
Although silylborane–isocyanide complex 3a are stable
towards moisture and air, thermolysis of 3a at 120 °C gave a 1 : 5
mixture of 3a and the corresponding migration product 8a16 (eq 2)
in contrast to the previously reported intermediary
silylborane–isocyanide complexes which give the corresponding
(boryl)(silyl)iminomethanes as final products.8 The structure of
5
6
7
8
N. Takeda, N. Tokitoh, and R. Okazaki, Chem. Lett., 2000, 244.
N. Takeda, N. Tokitoh, and R. Okazaki, Chem. Lett., 2000, 622.
M. Suginome and Y. Ito, Chem. Rev., 100, 3221 (2000).
M. Suginome, T. Fukuda, H. Nakamura, and Y. Ito, Organometallics,
19, 719 (2000).
1
9
Spectral data for 3a: colorless crystals, mp 173–174 °C (decomp); H
NMR (CDCl3, 300 MHz) δ –0.14 (s, 9H), –0.08 (s, 9H), 0.02 (s, 18H),
0.04 (s, 18H), 1.26 (s, 1H), 1.29 (s, 18H), 1.30 (s, 9H), 2.14 (s, 3H), 2.37
(s, 6H), 2.49 (s, 1H), 2.61 (s, 1H), 5.21 (d, 1H, J = 7 Hz), 6.19 (s, 1H),
6.33 (s, 1H), 6.61 (s, 2H), 7.31 (s, 2H), the signals of BH2 were not
observed; 13C NMR (CDCl3, 75 MHz) δ 0.6 (q), 0.8 (q), 0.90 (q), 0.94
(q), 1.2 (q), 1.5 (q), 20.9 (q), 25.3 (q), 26.8 (d), 27.0 (d), 29.7 (q), 30.0
(d), 31.2 (q), 35.4 (s), 35.5 (s), 120.3 (s), 122.1 (d), 122.4 (d), 127.5 (d),
128.5 (d), 129.3 (s), 136.8 (s), 137.2 (s), 142.2 (s), 143.4 (s), 147.5 (s),
151.2 (s), 151.7 (s), 151.8 (s), 152.3 (s); 11B NMR (CDCl3, 96 MHz) δ
–42.6; IR (KBr) 2400 [ν(BH)], 2369 [ν(BH)], 2213 [ν(CN)], 2139
[ν(SiH)] cm–1; FABMS m/z 1006 [(M+Na)+], 983 [M+], 968 [(M –
Me)+], 864 [(M – Mes)+], 699 [Tbt(Mes)SiH+]. Anal. Calcd for
C55H102BNSi7: C, 67.08; H, 10.44; N, 1.42%. Found: C, 66.94; H,
10.69; N, 1.50%.
1
8a was determined by H, 13C, and 11B NMR spectra, difference
NOE experiments, and IR spectrum. The difference NOE experi-
ments on 8a showed the NOEs of the SiH proton only with the o-
methine protons of the Tbt group. On the other hand, in the case
of 3a, the irradiation of the SiH protons resulted in the enhance-
ment of both peaks for the o-methine protons of the Tbt group and
the o-methyl protons of the mesityl group. These NOE experi-
ments indicate the absence of the mesityl group on the silicon
atom in 8a. Since the 1:5 mixture of 3a and 8a was also obtained
from the isolated compound 8a on heating at 120 °C, the exis-
tence of the equilibrium between 3a and 8a is strongly suggested.
Also, thermolysis of 3b at 120 °C resulted in the formation of the
mixture of 3b and 8b with the ratio of 1:10.
10 Crystal data for 3a: Formula C55H102BNSi7, fw = 984.83, triclinic,
–
space group P1 (#2) Z = 2, a = 9.709(5) Å, b = 16.612(7) Å, c =
20.306(10) Å, α = 83.42(1)°, β = 89.18(2)°, γ = 78.73(1)°, V =
3190.8(24) Å3, Dcalcd = 1.025 g/cm3, µ = 1.81 cm–1, 2θmax = 55.0°, T =
93 K, R1 (I > 2σ(I)) = 0.068, wR2 (all data) = 0.155, GOF = 1.07 for
14013 reflections and 688 parameters.
11 A. Blumenthal, P. Bissinger, and H. Schmidbaur, J. Organomet.
Chem., 462, 107 (1993).
12 W. Lippert, H. Nöth, W. Ponikwar, and T. Seifert, Eur. J. Inorg.
Chem., 1999, 817.
13 M. Tamm, T. Lügger, and F. E. Hahn, Organometallics, 15, 1251
(1996).
Reaction of 1a with BF3·Et2O gave the fluorosilane 9 in
47% yield (eq 3). The formation of 9 can be explained by the
hydrolysis of intermediary silylborane 10 during separation.
The lack of the stable silylborane–isocyanide complex in this
reaction might be due to the low Lewis acidity of fluoroboranes
compared to hydroboranes. Reactions of 1a with BPh3,
B(NMe2)3, and B(OMe)3 did not proceed.17
14 B. Stíbr, J. Holub, T. Jelínek, X. L. R. Fontaine, J. Fusek, J. D. Kennedy,
and M. Thornton-Pett, J. Chem. Soc., Dalton Trans., 1996, 1741.
15 T. Kajiwara, N. Takeda, and N. Tokitoh, unpublished results.
1
16 Spectral data for 8a: colorless crystals, H NMR (CDCl3, 300 MHz) δ
–0.18 (s, 18H), 0.01 (s, 18H), 0.02 (s, 18H), 1.25 (s, 1H), 1.31 (s, 9H),
1.41 (s, 18H), 1.92 (br s, 2H), 2.17 (s, 3H), 2.42 (s, 6H), 4.54–4.56 (br
m, 2H), 6.18 (s, 1H), 6.30 (s, 1H), 6.68 (s, 2H), 7.36 (s, 2H), the signal
of BH was not observed; 13C NMR (C6D6, 75 MHz) δ 0.9 (q), 1.0 (q),
1.1 (q), 1.3 (q), 1.5 (q), 1.7 (q), 21.1 (q), 26.0 (d), 30.0 (q), 30.6 (d),
31.0 (q), 31.8 (q), 35.4 (s), 35.8 (s), 120.9 (s), 121.7 (d), 122.5 (d),
126.4 (d), 128.7 (s), 128.9 (d), 129.1 (s), 134.7 (s), 142.9 (s), 143.2 (s),
149.0 (s), 152.0 (s), 152.1 (s), 153.2 (s); 11B NMR (CDCl3, 96 MHz) δ
–37.0; IR (KBr) 2348 [ν(BH)], 2199 [ν(CN)], 2110 [ν(SiH)] cm–1
;
FABMS m/z 983 [M+].
17 Metzler et al. have reported that the reaction of a bis(amino)silylene
with B(C6F5)3 initially affords the corresponding silylene–borane
adduct and then the migration of C6F5 group from the boron atom to the
silicon atom occurs to give the corresponding silylborane. N. Metzler
and M. Denk, Chem. Commun., 1996, 2657.
Further investigations on the properties of silylborane–iso-
cyanide complexes 3a,b, the equilibrium between 3a,b and
8a,b, and reactions of silylene–isocyanide complexes 1a,b with
other boranes are currently in progress.