Synthesis and structure of azobenzenes bearing silyl, germyl, and stannyl groups at 2-position

Article abstract of DOI:10.1246/cl.2001.338

Azobenzenes, 2-R¹₂R²MC₆H₄N=NPh (M = Si (1a-c), Ge (7), and Sn (8)), were synthesized by reactions of 2-lithioazobenzene (6) with the corresponding chloro compounds, R¹₂R²MCl, respectively, in good to moderate yields. The crystal structure of 1c was established by the X-ray crystallographic analysis.

Full text of DOI:10.1246/cl.2001.338

338
Chemistry Letters 2001
Synthesis and Structure of Azobenzenes Bearing Silyl, Germyl, and Stannyl Groups at 2-Position
Naokazu Kano, Fuminori Komatsu, and Takayuki Kawashima*
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Received January 23, 2001; CL-010070)
Azobenzenes, 2-R1 R2MC6H4N=NPh (M = Si (1a–c), Ge
2
(7), and Sn (8)), were synthesized by reactions of 2-lithioazoben-
zene (6) with the corresponding chloro compounds, R1 R2MCl,
2
respectively, in good to moderate yields. The crystal structure of
1c was established by the X-ray crystallographic analysis.
The chemistry of azobenzenes is well studied because of their
easiness of cis-trans photoisomerization1 and their application to
We next tried mono-desilylation of 2,2'-bis(trimethylsilyl)-
the liquid crystals.2 The properties of the azobenzene such as
azobenzene (4) (22%) prepared from 2a with trifluoromethanesul-
fonic acid as shown in Scheme 2.10 Compound 1a was obtained
in 32% yield with recovery of 4 (33%) after chromatographic sep-
aration.
absorption maxima in the UV/vis spectra are effected by the elec-
tron-withdrawing and electron-donating groups at 2-position of an
azobenzene as well as those at 4-position.3 These results prompted
us to investigate on azobenzenes with heavier group 14 element
substituents at 2-position, which can act only inductively. On the
other hand, a 2-(phenylazo)phenyl group is expected to be useful in
stabilization of pentacoordinate organosilicon compounds judging
from the structural resemblance with the van Koten ligand4 and
many examples of transition metal complexes which are intramole-
cularly coordinated by an azo group.5 Although few 2-silyl-
azobenzenes were previously reported, the conventional synthetic
methods for them have been restricted to azo-coupling reaction of
diazonium salt with [3-(dimethylamino)phenyl]silanes.6 This
method is not effective for the synthesis of 2-silylazobenzenes
without electron-donating substituents. Neither useful synthetic
methods for the naked 2-silyl-azobenzenes, nor the crystal struc-
ture of the 2-silylazobenzenes has been reported yet. We report
here an effective synthetic method for 2-silylazobenzenes and the
synthesis of germyl and stannyl derivatives. We have also estab-
lished the molecular structure of one of 2-silylazobenezenes by X-
ray crystallographic analysis.
More effective synthetic methods are desirable for the
introduction of various silyl groups at 2-position of azobenzene
because yields by these two methods were quite low. We final-
ly attempted the lithium–halogen transmetallation of 2-halo-
azobenzene followed by the reaction with a variety of elec-
trophiles. 2-Bromoazobenzene (5a)8 (0.36 g, 1.4 mmol) was
treated with n-BuLi (1.05 equiv) and chlorotrimethylsilane
(1.05 equiv) successively in THF (8 mL) at –105 °C to give 1a
in 25% yield (Scheme 3, Table 1).11 Lithiation of 2-iodo-
azobenzene (5b) instead of 5a afforded 2-lithioazobenzene (6)
First trial for the synthesis of 1a by dehydrative coupling reac-
tion of 2-anilinotrimethylsilane 2a7 and nitrosobenzene 3 in acetic
acid, which was used for the synthesis of unsymmetrically substitut-
ed azobenzenes,8 was failed because of instability of both 1a and 2a
under acidic conditions. We used basic conditions for the coupling
reaction of 2a (83 mg, 0.5 mmol) and 3 (2 equiv) with KOH (1
equiv) and K2CO3 (1.4 equiv), in pyridine (5 mL) at 70 °C (Scheme
1). Although 1a was obtained in 7% yield after stirring for 2 days
and usual work up, 2a was mainly recovered.9 Furthermore, use of
anilines with bulkier silyl groups, 2-anilino(dimethyl)phenylsilane
2b, instead of 2a in the above coupling reaction was unsuccessful
probably due to the steric hindrance.
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
339
change from tetrahedral structure, however, was not observed for
the silicon atom, indicating the lack of intramolecular coordina-
tion of N1 to Si1.15 This is the first example of X-ray crystallo-
graphic analysis of a 2-silylazobenzene. The chemical shift of
29Si NMR (δSi –19.90 ppm in CDCl3) also supports the tetracoor-
dinate structure of the Si1 atom in solution state. Other 2-silyl-
azobenzenes 1a and 1b showed the chemical shifts at –3.98 and
–7.53 ppm, respectively, suggesting their tetracoordinate struc-
tures similar to 1c in solution state.
In summary, we have developed a novel synthetic method
for 2-silylazobenzenes whose absorption maxima are red shifted.
The first X-ray crystallographic analysis of 2-silylazobenzene 1c
revealed tetrahedral structure of the silicon atom. 2-
Lithioazobenzene was found to be a useful synthetic intermediate
for group 14 element derivatives and its reaction would be versa-
tile for the synthesis of other main group element derivatives.
efficiently, and successive treatment of chlorotrimethylsilane
afforded 1a in satisfactory yield (80%). This method is effective
for the synthesis of other silyl derivatives such as dimethylphenyl
derivative 1b (92%), and hydrodiphenylsilyl derivative 1c (79%).
Furthermore, treatment of 6 with the chlorotrimethylgermane and
tri-n-butylchlorostannane afforded the corresponding germanium
and tin derivatives 7 (98%) and 8 (65%), respectively. In elec-
tronic spectra, the absorption maxima of these 2-silyl, germyl,
and stannyl substituted azobenzenes in CH2Cl2 are red shifted
compared with those of unsubstituted azobenzene (232, 318, 441
nm) (Table 1). These red-shifts are explained by strong elec-
tropositiveness of silyl, germyl, and stannyl groups.
The crystal structure of 2-silylazobenzene 1c was deter-
mined by X-ray structural analysis (Figure 1).12 Trans conforma-
tion was confirmed for 2-silylazobenzene 1c where azobenzene
unit is almost coplanar judging from the dihedral angle between
two phenyl groups attached to the azo group (8.37°). The N1–N2
bond length (1.255(2) Å) exhibited little structural difference
compared to the average dimensions (1.25 Å) of previously
reported trans-azobenzenes.13 Silyl groups and N2 atom are
arranged in a trans fashion with regard to the C1–N1 axis unlike
the many transition metal complexes bearing an azobenzene unit
where the nitrogen atom coordinates to the central metal atom.5
These results apparently indicate the absence of Si1–N2
intramolecular interaction. A steric repulsion overcomes the
attractive electrostatic interaction which provides the intramolec-
ular coordination. The distance between Si1 and N1 [3.010(5) Å]
is necessarily in the range of sum of the van der Waals radii of
silicon and nitrogen, 3.65 Å, although it is much longer than the
sum of the covalent bond radii, 1.92 Å.14 Any conformational
This work was partially supported by a Grant-in-Aid from
the Ministry of Education, Science, Sports and Culture of
Japan. We are also grateful to Shin-etsu Chemical Co., Ltd.,
Central Glass, and TOSOH FINECHEM CORPORATION for
the generous gift of chlorosilanes, trifluoromethanesulfonic
acid, and alkyllithiums, respectively.
References and Notes
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All new compounds gave satisfactory spectral and analytical data.
2
3
4
5
6
7
8
9
10 N. A. Noureldin and J. W. Bellegarde, Synthesis, 1999, 939.
11 For a similar halogen–lithium transmetallation using 4-bromo-4'-
ethylazobenzene, see: T. Kozlecki, L. Syper, and K. A. Wilk,
Synthesis, 1997, 681.
12 Crystal data of 1c: C24H20N2Si, fw = 364.52, triclinic, space group
–
P1, a = 10.5060(6) Å, b = 10.8170(6) Å, c = 11.1600(5) Å, α =
91.770(3)°, β = 114.618(3)°, γ = 117.163(3)° V = 986.8(1) Å3, Z =
2, Dcalc = 1.227 g·cm–3, R1 = 0.071, wR2 = 0.128.
13 R. Allmann, in “The Chemistry of the Hydrazo, Azo and Azoxy
Groups,” ed. by S. Patai, John Wiley & Sons, London (1975),
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15 W. S. Sheldrick, in “The Chemistry of Organic Silicon
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