Photolysis of Bis(organosilyl)imines

Full text of DOI:10.1021/jo991443s

624
J . Org. Chem. 2000, 65, 624-626
without irradiation. Photolysis of some bis(organosilyl)-
P h otolysis of Bis(or ga n osilyl)im in es
imines having various silicon substituents (SiR1 ) and
3
alkyl groups (R2) is summarized in eq 1.
Akira Matsumoto† and Yoshihiko Ito*
Department of Synthetic Chemistry and Biological
Chemistry, Graduate School of Engineering,
Kyoto University, Kyoto 606-8501, J apan
Received September 14, 1999
Photolysis of acylsilanes involves two types of reac-
tions.1 One is a homolytic scission of the silicon-carbonyl
bond in carbon tetrachloride to afford a pair of radicals.2
The organosilyl radical thus generated abstracts a chlo-
rine atom from carbon tetrachloride to yield an organosi-
lyl chloride. Another reaction is a migration of the
organosilicon group to the carbonyl oxygen in methanol,
resulting in the formation of an acetal via the corre-
sponding silyloxycarbene intermediate.3
However, photolysis of bis(organosilyl)ketones is dif-
ferent from that of the acylsilanes. For instance, bis-
(triphenylsilyl)ketone, which is a red-violet solid with UV
absorption around 550 nm, is not stable in solution and
rapidly undergoes photolysis in solution to afford hexa-
phenyldisilane and carbon monoxide.4
On the other hand, N-substituted bis(organosilyl)-
imines, the preparation of which has been found by us,5
are stable not only in the solid state but also in solution.
Bis(organosilyl)imines also exhibit red-shifted UV ab-
sorption around 400 nm with relatively large extinction
coefficients (150-200), which is ascribed to n-π* absorp-
tion.6 Now, we find that bis(organosilyl)imines undergo
photolysis in a different manner from the ketone ana-
logue. Herein, we report new photochemistry of 2,6-xylyl
bis(organosilyl)imines.
A high-pressure mercury lamp was used as a light
source for all the photoreactions. UV irradiation of bis-
(organosilyl)imine (1) for 2 h in benzene-d6 containing
carbon tetrachloride led to a complete conversion, giving
a 2:1 mixture of organosilyl chloride and the correspond-
ing isocyanide together with its derivatives (eq 1). In the
reaction of 1c, the minor byproducts 4c (5.9%),7 5c
(5.6%),7 and 6c8 (6.3%)7 were separated and identified
by GC-MS analysis. The present reaction did not proceed
in refluxing benzene containing carbon tetrachloride
These results might suggest that the photolysis of
N-substituted bis(organosilyl)imines involved a stepwise
elimination of the two organosilyl radicals.9 However, this
hypothesis was ruled out by following results. No reaction
of bis(silyl)imines took place under irradiation for several
hours in benzene and acetonitrile. The photolysis of the
bis(organosilyl)imines in the presence of styrene and
methyl methacrylate in benzene-d6 did not give any
adducts. Radical trapping experiments with 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO) during the pho-
tolysis have also failed.
In contrast, irradiation of 1b with alkyl iodides in
benzene-d6 gave organosilyl iodide, which was readily
hydrolyzed to the corresponding siloxane, along with the
aromatic isocyanide. The photolysis of 1b with ethyl
iodide in the presence of TEMPO afforded dimethyl-
phenylsilyl-TEMPO adduct 7 together with a small
amount of dimethylphenylsilyl iodide 8 and ethyl-
TEMPO adduct 9 (eq 2). The same reaction proceeded
with isopropyl iodide and iodobenzene. No charge-
transfer complex formation in the ground state was
observed between 1b and iodobenzene.10
An exciplex intermediate may be involved prior to the
reaction with carbon tetrachloride or alkyl iodides. An
exciplex formation has also been proposed for the pho-
tolysis of acylsilanes.11 Organic halides were not decom-
posed by irradiation under the same condition, suggesting
that the reaction of 1 was not promoted by the photolysis
of organic halides.
Photoinduced electron transfer (PET) may be involved
after the exciplex formation.12 Single electron transfer
could occur from bis(organosilyl)imines to organic halide
† Additives, Ciba Specialty Chemicals K. K. J apan; 10-66, Miyuki-
cho, Takarazuka 665-8666, J apan.
(1) Brook, A. G. J . Organomet. Chem. 1986, 300, 21-37.
(2) (a) Brook, A. G.; Duff, J . M. J . Am. Chem. Soc. 1969, 91, 2118-
2119. (b) Brook, A. G.; Dillon, P. J .; Pearce, R. Can. J . Chem. 1971,
49, 133-135.
(3) (a) Brook, A. G.; Duff, J . M. J . Am. Chem. Soc. 1967, 89, 454-
455. (b) Duff, J . M.; Brook, A. G. Can. J . Chem. 1973, 51, 2869-2883.
(c) Bourque, R. A.; Davis, P. D.; Dalton, J . C. J . Am. Chem. Soc. 1981,
103, 697-699.
(4) Brook, A. G.; Peddle, G. J . D. J . Organomet. Chem. 1966, 5, 106-
107.
(5) (a) Ito, Y.; Nishimura, S.; Ishikawa, M. Tetrahedron Lett. 1987,
28, 1293-1294. (b) Ito, Y.; Matsuura, T.; Murakami, M. J . Am. Chem.
Soc. 1988, 110, 3692-3693. (c) Ito, Y.; Suginome, M.; Matsuura, T.;
Murakami, M. J . Am. Chem. Soc. 1991, 113, 8899-8908.
(6) Ramsey, B. G.; Brook, A. G.; Bassindale, A. R.; Bock, H. J .
Organomet. Chem. 1974, 74, C41-C45 and references therein.
(7) The yield is based on GC analysis. The isocyanide 3c was
produced in 82.2% yield.
(8) The formation of aromatic isocyanates by photochemical oxida-
tion with an aromatic isocyanides has been reported. Boyer, J . H.;
Ramakrishnan, V. T.; Srinivasan, K. G.; Spak, A. J . Chem. Lett. 1981,
43-46.
(9) The formation of alkyl radicals and isocyanides from imidoyl
radicals has been reported. (a) Kaba, R. A.; Griller, D.; Ingold, K. U.
J . Am. Chem. Soc. 1974, 96, 6202-6203 (b) Nanni, D.; Pareschi, P.;
Tundo, A. Tetraheadron Lett. 1996, 37, 9337-9340.
(10) UV spectra of 1b did not depend on polarity of solvent used. In
addition, no UV change was observed when iodobenzene was added to
a solution of 1b in acetonitrile and n-hexane. The mathematical sum
of UV spectra of 1b and iodobenzene measured separately was in good
agreement with UV spectra of a mixture of 1b and iodobenzene.
(11) Porter, N. A.; Iloff, P. M., J r. J . Am. Chem. Soc. 1974, 96, 6200-
6202.
10.1021/jo991443s CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/31/1999

Notes
J . Org. Chem., Vol. 65, No. 2, 2000 625
Sch em e 1
Sch em e 2
to yield pairs of an anion radical 10 and a cation radical
11 (Scheme 1). The former would give an alkyl radical
(R•) and a halide anion, which would produce 1 equiv of
organosilyl halide by an attack at the cationic organo-
silicon in 11.13 An imidoyl radical thus generated gives
an isocyanide 3 and an organosilyl radical,9 which
abstracts a halogen atom to afford another 1 equiv of
organosilyl halide.14
The photolysis of 1 proceeds quantitatively in the
presence of benzoquinone in stead of organic halides,
giving a complex mixture containing isocyanide deriva-
tives 3-6 and hydroquinone silyl ethers. However, no
reaction took place when a benzene-d6 solution of ben-
zoquinone in the absence of 1 was irradiated under the
same condition.
The formation of nearly 2 equiv of 7 in the photolysis
of 1b with ethyl iodide in the presence of TEMPO may
suggest that TEMPO is operating as an electron acceptor
in the PET mechanism15 (Scheme 2). Single electron
transfer from 10 to TEMPO could occur to yield 2,2,6,6-
tetramethylpiperidine-1-oxide, which can afford 1 equiv
of 7 by a reaction with a cationic organic silicon. On the
other hand, the imidoyl radical can give the isocyanide
and the organosilyl radical, which affords another 1 equiv
of 7 by trapping the silyl radical with TEMPO.
Ta ble 1. P h otoin d u ced P olym er iza tion In itia ted by
1b a n d P h I
entry R3
R4
Me Me
Me Me
Me Me
Me
additive yield (%) Mn (×104) Mw/Mn
1
2
3
4
5
none
9
20
npa
50
28
0.95
2.31
1.55
1.33
CuBr
TEMPO
CuBr
H
2.87
1.00
1.82
2.62
Me CH2Ph CuBr
a
No polymerization.
in MMA (1.0 mL) and benzene (2.0 mL) for 1.5 h produced
poly(methyl methacrylate) (PMMA) in 9% yield (Table
1, entry 1). The low yield may be due to inhibition of the
polymerization by the isocyanide generated during ir-
radiation. The inhibition by 2,6-xylyl isocyanide was also
observed in the polymerization of MMA initiated by
AIBN. The radical polymerization of MMA (20 mmol)
with 0.08 mmol of AIBN in the presence of 4 mmol of
2,6-xylyl isocyanide was carried out at 65 °C for 15 min,
affording only trace amounts of PMMA (0.2%, Mn
)
2.17 × 104, Mw/Mn ) 1.25).16
Irradiation of MMA in the presence of TEMPO did not
give PMMA (Table 1, entry 3), indicating that this
polymerization reaction proceeded in a radical fashion.
Irradiation of MMA in the presence of iodobenzene
without bis(organosilyl)imines did not induce the polym-
erization either, suggesting that the photolysis of bis-
(organosilyl)imines was crucial for an initiation of the
polymerization reaction. Intervention of organosilyl radi-
cal as well as phenyl radical, which worked as an
initiating species, was confirmed by 1H NMR analysis of
PMMA indicating that both of the dimethylphenylsilyl
The organosilyl radical as well as the alkyl radical thus
generated photochemically promoted radical polymeri-
zation of methyl methacrylate (MMA) (eq 3). Irradiation
of a mixture of 1b (0.1 mmol) and iodobenzene (0.4 mmol)
(12) PET reactions of organosilanes are well-known. For example,
see Steinmetz, M. G. Chem. Rev. 1995, 95, 1527-1588 and references
therein.
(13) Dinnocenzo, J . P.; Farid, S.; Goodman, J . L.; Gould, I. R.; Todd,
W. P.; Mattes, S. L. J . Am. Chem. Soc. 1989, 111, 8973-8975.
(14) Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J . C. J . Am. Chem.
Soc. 1982, 104, 5123-5127.
(15) (a) Belonogova, O. V.; Likhtenstein, G. I.; Krinichnyi, V. I. J .
Photochem. Photobiol. A: Chem. 1997, 107, 35-42. (b) Vogel, V. R.;
Rubtsova, E. T.; Likhtenstein, G. I.; Hideg, K. J . Photochem. Photobiol.
A: Chem. 1994, 83, 229-236 and references therein.
(16) In the absence of 2,6-xylyl isocyanide, PMMA was obtained in
7% yield (Mn ) 4.75 × 104, Mw/Mn ) 1.08) under the same condition.

626 J . Org. Chem., Vol. 65, No. 2, 2000
Notes
1
observed by H NMR and GC-MS. Photolysis of 1a , 1b, and 1d
were carried out according to the same procedure.
group and the phenyl group were included in the ratio
of 60:40, respectively, as terminal groups of the PMMA
polymer chains.
P h otolysis of 1b w ith Alk yl Iod id e in th e P r esen ce of
TEMP O. In benzene-d6 (0.8 mL) were dissolved 1b (0.0358
mmol), ethyl iodide (0.143 mmol), and TEMPO (0.0716 mmol).
The solution was irradiated under nitrogen atmosphere for 2 h
in a water bath at ambient temperature. Structures of the
products were identified by 1H NMR and GC-MS. Structural
data of 7 was identical in all respect with an authentic sample
prepared by the reported procedure.17 Photolysis of 1b with other
alkyl iodides was carried out according to the same procedure.
Physical and spectral data for 1-(dimethylphenylsilyloxy)-
2,2,6,6-tetramethylpiperidine 7: bp 100-105 °C (0.5 mmHg); 1H
NMR (C6D6) δ 0.51 (s, 6H), 1.08 (s, 6H), 1.16 (s, 6H), 1.00-1.22
(m, 2H), 1.26-1.47 (m, 4H), 7.17-7.27 (m, 3H), 7.64-7.69 (m,
2H); 13C NMR (C6D6) δ 0.2, 17.2, 19.4, 33.9, 40.0, 59.8, 129.3,
133.7, 140.5; HRMS calcd for C17H29ONSi 291.2017, found
291.2011.
P h otoin d u ced P olym er iza tion w ith 1b a n d Iod oben -
zen e. A mixture of 1b (0.1 mmol) and iodobenzene (0.4 mmol)
in 1.0 mL of MMA and 2.0 mL of benzene was irradiated under
nitrogen atmosphere for 1.5 h in a water bath at ambient
temperature. After volatile compounds were removed under
reduced pressure, the reaction mixture was poured into 50 mL
of methanol. PMMA precipitated was collected by filtration and
dried. The molecular weight was determined by GPC analysis
with polystyrene standard. The ratio of the terminal groups
(PhMe2Si vs Ph) was calculated on the basis of the integration
ratio of the peaks at δ 0.02 (bs, 6H [PhMe2Si]) and at δ 7.18-
7.57 (m, 5H [PhMe2Si] + 5H [Ph]) in 1H NMR (DMSO-d6). A
similar polymerization of MMA was carried out in the presence
of either TEMPO (0.4 mmol) or CuBr (0.4 mmol) under the same
condition. Copper compounds precipitated were removed by
filtration using Celite after the irradiation. Polymerization of
methyl acrylate and benzyl methacrylate was conducted accord-
ing to the similar procedure.
It is worthwhile to note that the yield of PMMA
produced in the photochemically promoted radical po-
lymerization was increased up to 20% in the presence of
copper(I) salts (Table 1, entry 1 and 2). Probably, isocy-
anide generated was trapped and deactivated by com-
plexation with Cu(I). No polymerization of MMA occurred
in the presence of 1b and iodobenzene together with
CuBr at ambient temperature without irradiation. Simi-
larly, the photochemically induced radical polymerization
of methyl acrylate and benzyl methacrylate proceeded in
the presence of 1b, iodobenzene, and CuBr in benzene,
giving poly(methyl acrylate) and poly(benzyl methacry-
late) in 50% and 28% yield, respectively (Table 1, entry
4 and 5).
In summary, bis(organosilyl)imines, which are stable
toward UV light unlike the ketone analogue, undergo
photolysis in the presence of carbon tetrachloride and
organic iodides to afford organosilyl halides and isocy-
anides. PET mechanism is proposed for the reaction.
Generation of the organosilyl radical as well as the alkyl
radical was demonstrated by polymerization of acrylates
and methacrylates.
Exp er im en ta l Section
Gen er a l Meth od s. All photochemical reactions were carried
out with RIKO UVL-100HA (100W) and Pyrex glassware.
Reagents and solvents were handled with standard syringe
techniques. Solvents were distilled in the presence of appropriate
drying agents before use. Methyl methacrylate, methyl acrylate,
and benzyl methacrylate were distilled prior to use. Other
commercially available reagents were used without further
purification.
P h otolysis of 1 w ith Ca r bon Tetr a ch lor id e in Ben zen e-
d 6. In benzene-d6 (0.8 mL) were dissolved 1c (0.0360 mmol) and
carbon tetrachloride (0.144 mmol). The solution was irradiated
under air for 2 h in a water bath at ambient temperature.
Trimethylsilyl chloride and isocyanide derivatives 3c-6c were
Ack n ow led gm en t. A.M. is sincerely grateful to Ciba
Specialty Chemicals K.K. for general support to this
research.
J O991443S
(17) Armbrecht, M.; Maringgele, W.; Meller, A.; Noltemeyer, M.;
Sheldrick, G. M. Z. Naturforsch., Teil B 1985, 40b, 1113-1122.