Photoswitching of the reactivity involving hydrosilylation of a 1,1,3,3-tetrahydrodisiloxane bearing two azo groups

Article abstract of DOI:10.1016/j.tetlet.2007.04.020

A pentacoordinate (E,E)-1,1,3,3-tetrahydrodisiloxane bearing two azo groups gave hydrazobenzene in the fluoride ion-induced hydrolysis involving hydrosilylation of the azo groups followed by cleavage of the Si-C bonds, while a tetracoordinate (E,E)-forms formed by its photoirradiation gave an octaarylsilsesquioxane without cleavage of the Si-C bonds.

Full text of DOI:10.1016/j.tetlet.2007.04.020

Tetrahedron Letters 48 (2007) 4033–4036
Photoswitching of the reactivity involving hydrosilylation
of a 1,1,3,3-tetrahydrodisiloxane bearing two azo groups
Masaki Yamamura, Naokazu Kano and Takayuki Kawashima*
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan
Received 26 February 2007; revised 29 March 2007; accepted 5 April 2007
Available online 11 April 2007
Abstract—A pentacoordinate (E,E)-1,1,3,3-tetrahydrodisiloxane bearing two azo groups gave hydrazobenzene in the fluoride ion-
induced hydrolysis involving hydrosilylation of the azo groups followed by cleavage of the Si–C bonds, while a tetracoordinate
(E,E)-forms formed by its photoirradiation gave an octaarylsilsesquioxane without cleavage of the Si–C bonds.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Highly coordinate hydrosilanes have been known to
reduce carbonyl compounds without the intervention
of a transition metal catalyst because the nucleophilicity
of the hydride is increased from that of tetracoordinate
hydrosilanes.1 Such a hydrosilane is expected to be effec-
tive for reduction of various doubly bonded compounds,
but hydrosilylation of azo compounds to produce the
corresponding N-silylhydrazines has not been reported
to date. Azobenzene, a most popular example of the
azo compounds, has been utilized for optical molecular
switches.2 We have reported the synthesis and photo-
switching of the properties and reactivity of azobenzenes
bearing a heteroatom such as boron,3 silicon,4 and phos-
phorus5 at the 2 position. Reactivity of a hydrosilane
bearing a 2-(phenylazo)phenyl group is also expected
to be changed by photoirradiation. We report here the
synthesis of a pentacoordinate 1,1,3,3-tetrahydrodisilox-
ane bearing azobenzene moieties and the reduction of
the azo groups by intramolecular hydrosilylation.
In the 29Si NMR spectra, the signal of (E,E)-4 (dSi
À58.9) was observed at a higher field than that of
1,1,3,3-tetrahydro-1,3-diphenyldisiloxane9
(5)
(dSi
À25.4), indicating a pentacoordinate state of silicon
atoms in the solution state. The coupling constant of
(E,E)-4 (1JSiH = 248 Hz) is larger than that of 5
(1JSiH = 221 Hz), revealing the change of the s-character
of an orbital on the silicon atoms used for the equatorial
Si–H bonds. In the UV/vis spectra, (E,E)-4 showed a red
shift of the absorption maximum (k = 335 nm) assigned
to a p–p* transition compared with that of unsubstituted
azobenzene (k = 320 nm), similarly to all other azobenz-
enes coordinating to silicon atoms, suggesting the per-
turbation of the electronic structure of the azo moiety.4
Tetrahydrodisiloxane (E,E)-4 was successfully isomer-
ized by photoirradiation with high-pressure mercury
Reaction of triethoxysilane (E)-14e with LiAlH4 in ether
at room temperature gave trihydrosilane (E)-2 (Scheme
1).6 Chlorination of (E)-2 with CuCl2 (2 equiv) and
CuI (cat.)7 at room temperature for 2 h gave chloro-
dihydrosilane (E)-3 quantitatively and hydrolysis of
(E)-3 with aq NaHCO3 gave 1,1,3,3-tetrahydrodisilox-
ane (E,E)-4 (85%).8
Keywords: Hydrosilylation; Pentacoordinate silicon compound;
Azobenzene.
*
Corresponding author. Tel./fax: +81 3 5800 6899; e-mail: takayuki@
chem.s.u-tokyo.ac.jp
Scheme 1. Synthesis of pentacoordinate tetrahydrodisiloxane (E,E)-4.
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.04.020

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M. Yamamura et al. / Tetrahedron Letters 48 (2007) 4033–4036
Scheme 2. Photoisomerization of (E,E)-4 to (Z,Z)-4 in CDCl3.
lamp through a colored-glass filter (k = 360 nm) to give
(Z,Z)-4 quantitatively as demonstrated by the 1H NMR
spectra of the reaction solution (Scheme 2).10 The quan-
titative photoisomerization of the azobenzene moieties
is remarkable. In the 29Si NMR spectrum, (Z,Z)-4
showed one triplet (dSi À28.3 (1JSiH = 230 Hz)) whose
chemical shift and coupling constant are similar to those
of 5, respectively, suggesting a tetracoordinate state of
the silicon atoms of (Z,Z)-4.
Scheme 4. Reaction of (E,E)-4 with fluoride ion.
induced by a fluoride ion. The reaction at room temper-
ature for 5 min in the presence of water or ethanol did
not give 6, but hydrazobenzene (7).
Photoirradiation (k = 445 nm) of (Z,Z)-4 for 2 h gave a
4:26:70 mixture of (Z,Z)-4, (E,Z)-4,11 and (E,E)-4
(Scheme 3). The CDCl3 solution of (Z,Z)-4 was ther-
mally isomerized to (E,E)-4 quantitatively upon stand-
ing at room temperature for 36 h. Therefore, (Z,Z)-4
was not isolated. These results indicate that coordina-
tion number change of the silicon atoms based on the
isomerization between (E,E)-4 and (Z,Z)-4 is reversible
and quantitative.
Even in the presence of D2O, product 7 contained no
deuterium at the benzene ring. Conversely, when
1,1,3,3-tetradeuteriodisiloxane (E,E)-4-d4 was used, the
D-content of 7-d was 100% (Scheme 5). These results
indicate that the desilylation of (E,E)-4 was not caused
by protonation of an intermediary fluorosilicate with
water or ethanol as is observed in deprotective reactions
of silyl groups.12
In the reaction of (E,E)-4 with tetrabutylammonium
fluoride (TBAF) under anhydrous conditions, the yellow
solution slowly became colorless, which suggested the
conversion of the azo groups to hydrazo groups
(Scheme 4). In the 29Si NMR spectra, two doublet sig-
nals were observed. The chemical shifts and coupling
The formation of 6 suggests that the hydrosilylation
intramolecularly proceeds via fluorosilicate 8 to give
aminosilicate 9 in equilibrium with 6. Considering the
deuterium experiment, there are two plausible mecha-
nisms for the cleavage of the Si–C bond as follows
(Scheme 6): one is water-assisted ligand coupling reac-
tion between hydrogen and aryl group in fluorosilicate
9 derived from 6 (mechanism A), and the other is 1,3-
hydrogen shift in fluorosilicate 9 from nitrogen to the
ipso carbon attached to the silicon atom followed by
bond cleavage of the Si–C bond (mechanism B). The
latter mechanism must involve the fast 1,3-shift from
the NH(D) group formed by hydrosilylation of the azo
group without proton exchange of NH(D) with water
or ethanol, which seems very difficult under the reaction
condition. The former mechanism is suggested to be
more operative in this reaction, though reports on
ligand coupling reactions in highly coordinate silicon
compounds are so limited.13 In the absence of water,
the cleavage of the Si–C bond did not occur, indicating
that water took part in the bond cleavage to result in
the protonation of the silicon atom. Protonation,
constants of the two signals (dSi À34.3 (1JSi–H
=
281 Hz), À34.0 (1JSi–H = 279 Hz)) are almost the same.
The products are identified as two diastereomers of
cyclic aminosilane 6, which should be colorless. In the
1H NMR spectra, two peaks (dH 4.78, 5.05) due to
H–Si protons of diastereomers of 6 are clearly distin-
guished and the ratio of diastereomers is estimated to
be 1:1 by integration. Compounds 6 seem to be formed
by intramolecular hydrosilylation of the azo groups
Scheme 3. Photoisomerization and thermal isomerization of (Z,Z)-4
to (E,E)-4 in CDCl3.
Scheme 5. Reaction of (E,E)-4-d4 with fluoride ion.

M. Yamamura et al. / Tetrahedron Letters 48 (2007) 4033–4036
4035
Scheme 7. Reaction of tetracoordinate hydrosilane (Z,Z)-4 with
fluoride ion in the presence of water.
dehydrative condensation. Considering that octa-
phenylsilsesquioxane was a main product in hydrol-
ysis of phenyltrichlorosilane, the formation of 13 is
reasonable.15 Thus, it is concluded that the reaction of
(Z,Z)-4 with fluoride ion resulted in intermolecular
hydrosilylation of the azo groups without cleavage of
Si–C bonds. The difference in reactivity between (E,E)-
4 and (Z,Z)-4 is explained as follows: the reaction of
(E,E)-4 proceeded via cyclic aminosilane 6 that was elec-
trophilic enough to react with fluoride ion for desilyl-
ation, whereas intermediate 6 was not formed in the
reaction of (Z,Z)-4 because the azo groups of (Z,Z)-4
could not coordinate intramolecularly to the silicon
atoms.
Scheme 6. Plausible mechanisms for formation of hydrazobenzene (7)
from (E,E)-4.
In conclusion, the reaction of pentacoordinate tetra-
hydrodisiloxane (E,E)-4 with TBAF in the presence of
water resulted in desilylation to give 7. Conversely, the
reaction of tetracoordinate tetrahydrodisiloxane (Z,Z)-
4, prepared by photoirradiation of (E,E)-4, caused inter-
molecular hydrosilylation and the final product main-
tained the Si–C bonds. Although the reaction
intermediates were not clear, both reactions of (E,E)-4
and (Z,Z)-4 gave apparently different final products.
The reactivity of the tetrahydrodisiloxane has been suc-
cessfully changed by photoswitching of the coordination
number of silicon atoms.
which is an exothermic reaction, seems to be the driv-
ing force of the reaction. After the cleavage of the
Si–C bond, 7 was formed by hydrolysis of the Si–N
bonds.
The reaction of tetracoordinate hydrosilane (Z,Z)-4
with TBAF in the presence of water for 5 min did
not give 7, but a colorless unidentified compound that
showed broad peaks in the 1H NMR spectra, indicating
the formation of polymeric compound 12. Intermediate
12 disappeared after standing at room temperature for
2 h and one main product that showed sharp peaks in
1
the H NMR spectra was formed finally (Scheme 7).
Acknowledgments
This compound showed one singlet (dSi À71.3) in the
29Si NMR spectra, whose chemical shift was similar to
those (dSi À73.3, À77.4) of octa(2- or 4-aminophen-
yl)silsesquioxane.14 In MALDI-TOF MS of the prod-
uct, a molecular ion peak was detected at m/z 1880.
Thus, the product was identified as octa[2-(1-phenylhyd-
razino)phenyl]silsesquioxane 13. Compound 12 seems to
be formed via intermolecular hydrosilylation of the azo
groups, contrary to (E,E)-4 which underwent intramo-
lecular hydrosilylation. Compound 13 is considered to
be obtained by hydrolysis of the polymer and successive
We thank Shin-etsu Chemical Co., Ltd, and Tosoh
Finechem Corp. for gifts of organosilicon compounds
and alkyllithiums, respectively. This work was partially
supported by Grants-in-Aid for 21st Century COE Pro-
gram for Frontiers in Fundamental Chemistry and for
Scientific Research Nos. 14740395 (N.K.), 15105001
(T.K.), 16033215 (T.K.), and 1611512 (M.Y.) from
Ministry of Education, Culture, Sports, Science and
Technology, Japan, and Japan Society for Promotion
of Science, and Yamada Science Foundation.

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M. Yamamura et al. / Tetrahedron Letters 48 (2007) 4033–4036
130.97 (CH), 131.34 (CH), 131.52 (CH), 137.09 (CH),
149.98 (CN), 157.28 (CN); 29Si NMR (99 MHz, CDCl3) d
À58.9 (td, JSiH = 247.5 Hz, JSiH = 5.7 Hz); UV/vis
(CHCl3) kmax (e) 335 nm (3.3 · 104). Anal. Calcd for
C24H22N4OSi2: C, 65.72; H, 5.06; N, 12.77. Found: C,
65.49; H, 5.17; N, 12.53.
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10. Compound data for (Z,Z)-4: H NMR (500 MHz, CDCl3)
d 5.32 (s, 1JSiH = 230.1 Hz, 4H), 6.15 (d, 3J = 8.0 Hz, 2H),
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3
3
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