Silylene-transfer reactions of cyclic organosilanes induced by phenanthraquinone triplet

Article abstract of DOI:10.1016/S0022-328X(01)01070-1

On irradiation cyclic organosilanes 1a-b, 7 and 8 react with phenanthraquinone (PQ) to afford corresponding dioxasilolenes 2a-b as silylene-transfer products. The quenching experiment by anthracene reveals that PQ should act as triplet (³PQ*) to undergo radical displacement at the silicon atom of 1a-b. In polar solvents, electron-transfer from 1a-b to ³PQ* would generate PQ radical anion and the coupling of the radical ion pair formed would cause the Si-Si bond cleavage followed by intramolecular O-Si bond formation to give silylene transfer product 2a-b. Since the photolysis of PQ with 1a-b in the presence of CCl₄ gives dichlorooligosilanes, intermediacy of oligosilanyl radicals is highly probable. The quenching rate constant k_q of ³PQ* by cyclic organosilane 1b has been determined by laser flash photolysis. In a mixed solvent of CH₃CN and CH₂Cl₂, ³PQ* is readily quenched by the addition of 1b with the formation of PQ radical anion. On the other hand, since PQ radical anion has not been observed on the laser photolysis, the quenching of ³PQ* with 1b in benzene should be less efficient.

Full text of DOI:10.1016/S0022-328X(01)01070-1

Journal of Organometallic Chemistry 636 (2001) 63–68
www.elsevier.com/locate/jorganchem
Silylene-transfer reactions of cyclic organosilanes induced by
phenanthraquinone triplet
a,
a
a
a
Masahiro Kako *, Masumi Ninomiya , Mei Gu , Yasuhiro Nakadaira ,
b
b
c
c
Masanobu Wakasa , Hisaharu Hayashi , Takeshi Akasaka , Takatsugu Wakahara
a
Department of Applied Physics and Chemistry, The Uni6ersity of Electro-Communications, Chofu, Tokyo 182-8585, Japan
b
Molecular Photochemistry Laboratory, The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan
c
Graduate School of Science and Technology, Niigata Uni6ersity, Niigata 950-2181, Japan
Received 23 February 2001; accepted 5 June 2001
Abstract
On irradiation cyclic organosilanes 1a–b, 7 and 8 react with phenanthraquinone (PQ) to afford corresponding dioxasilolenes
3
2
a–b as silylene-transfer products. The quenching experiment by anthracene reveals that PQ should act as triplet ( PQ*) to
3
undergo radical displacement at the silicon atom of 1a–b. In polar solvents, electron-transfer from 1a–b to PQ* would generate
PQ radical anion and the coupling of the radical ion pair formed would cause the SiꢀSi bond cleavage followed by intramolecular
OꢀSi bond formation to give silylene transfer product 2a–b. Since the photolysis of PQ with 1a–b in the presence of CCl gives
dichlorooligosilanes, intermediacy of oligosilanyl radicals is highly probable. The quenching rate constant k of PQ* by cyclic
4
3
q
3
organosilane 1b has been determined by laser flash photolysis. In a mixed solvent of CH CN and CH Cl , PQ* is readily
3
2
2
quenched by the addition of 1b with the formation of PQ radical anion. On the other hand, since PQ radical anion has not been
3
observed on the laser photolysis, the quenching of PQ* with 1b in benzene should be less efficient. © 2001 Elsevier Science B.V.
All rights reserved.
Keywords: Oligosilane; Silylene transfer; Phenanthraquinone; Radical ion; Photo-induced electron-transfer; Radical displacement
1
. Introduction
having disilanyl and digermanyl units resulted in the
insertion of p-benzoquinone into SiꢀSi and GeꢀGe
bonds [3]. Furthermore, benzylsilane has been shown
to react with p-benzoquinone to give the correspond-
ing benzylated quinone [4]. While a lot of examples
of electron-transfer reactions of p-quinones acting as
acceptors have been reported, those of o-quinones
have been rather sparse. In the course of our continu-
ous studies on electron-transfer chemistry of
organometallic compounds, we have demonstrated
that on irradiation cyclic organosilanes undergo novel
silylene-transfer to phenanthraquinone (PQ), for
which we have proposed a mechanism involving PQ
Much attention has been drawn to the electron-
transfer chemistry of organosilicon compounds since
CꢀSi and SiꢀSi s-bonds have much lower ionization
potentials than those of the corresponding s-CꢀC
bonds [1]. It is of particular interest that the electron-
donor property of oligosilanes increases as the chain
length increases as a result of s conjugation of SiꢀSi
bonds [2]. In the meantime, several research groups
investigated electron-transfer reactions of organosi-
lanes with some kinds of electron deficient ketones as
electron acceptors [3,4]. Previously, we reported that
photoreactions of p-benzoquinone with carbocycles
3
triplet ( PQ*) without definite photophysical evidence
[5]. In this paper, coupled with the quenching studies
by laser flash photolysis we describe the novel photo-
induced silylene-transfer reaction of organosilanes 1a–
b, 7 and 8 to PQ* with experimental details.
*
Corresponding author.
3
E-mail address: naka@e-one.uec.ac.jp (M. Kako).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01070-1

64
M. Kako et al. / Journal of Organometallic Chemistry 636 (2001) 63–68
dimethylsilylene, which can be trapped readily by a-
diketones to afford the corresponding dioxasilolene
derivatives [7], 1a is inert to the light source employed
in the present photolyses. Furthermore, the photochem-
ical reaction above does not seem to involve free
dimethylsilylene because the photolysis of 1a and PQ in
the presence of Et SiH, a common silylene trap, did not
3
give expected Et SiSiMe H at all [7b].
3
2
On the other hand, since the silylene-transfer was
efficiently quenched by the addition of anthracene, the
3
triplet PQ ( PQ*) should be the key intermediate as
judged by the comparison of the singlet and the triplet
−
1
energies of PQ (E =ca. 54, E =ca. 48 kcal mol ) [8]
S
T
and those of anthracene (E =ca. 76, E =ca. 43
S
T
−
1
kcal mol ) [9]. Meanwhile, the consumption of 1a was
suppressed by the addition of 1,4-diazabicy-
clo[2.2.2]octane (DABCO), which has a lower oxidation
potential (E = +0.70 V vs. SCE) than that of 1a
ox
(
E = +1.45 V vs. SCE). Table 2 indicates the free
ox
energy changes (DG) calculated by the Rehm–Weller
equation [10] for the electron-transfer from cyclosilanes
3
to PQ*. These data suggest that the reaction may
proceed a partial or complete electron-transfer from the
3
cyclosilanes to PQ* at least in a polar solvent, such as
acetonitrile. In fact, the silylene-transfer reaction pro-
ceeds substantially faster in acetonitrile than in benzene
as shown in Table 1.
2. Result and discussion
Based on previous observation that the ketone triplet
cleaves a SiꢀSi bond of oligosilanes via S 2 type radical
H
2.1. Photo-induced silylene-transfer reaction of
displacement at the silicon atom [11], we depicted the
cyclosilanes
plausible reaction scheme as exemplified by the case of
3
1
a (Scheme 1). Thus, PQ* attacks one of the silicon
−
2
Irradiation of a solution of 1a [6a] (1.1×10
and PQ (3.5×10
M)
atoms of 1a to afford diradical 4. Prior to the forma-
tion of 4, a partial or complete electron-transfer from
−
2
M) in a mixed solvent (CH CN–
3
3
CH Cl =4/1) with two 500 W tungsten–halogen lamps
1a to PQ* is likely to occur, at least in polar solvents,
2
2
(
passing through an aqueous NaNO2 solution filter,
as mentioned above. Subsequently, intramolecular OꢀSi
bond formation in 4 would lead to 2a. The yields of 2a
indicate that a molecule of 1a donates more than one
silylene unit to PQ. After careful separation from the
photolysate by HPLC, we obtained polymeric products
containing silylene units, which might be arise from the
fragmentation of the starting materials. In addition, the
cutoffB400 nm) produced 2a and 3a, as silylene-trans-
fer products. Under similar reaction conditions 1b [6b]
afforded silylene-transfer product 2b whereas it is note-
worthy that another type of product 3b was not ob-
tained as shown in Table 1. Although ultraviolet
photolysis of 1a is well known to produce free
Table 1
Phtoto-induced silylene-transfer reactions of cyclosilanes to PQ
Silane
Condition
Time (h)
Conv. (%)
Product (yield (%)) a
1a
1a
1b
1b
7
7
8
8
hw/PQ–CH CN–CH Cl
4.5
10.5
0.5
1.5
0.5
1.5
3.0
18.0
61
49
70
70
89
65
100
70
2a (138), 3a (33)
2a (161), 3a (31)
2b (82)
2b (57)
2a (28), 3a (11), biphenyl (100)
2a (62), 3a (13), biphenyl (94)
2b (85), anthracene (97)
2b (69), anthracene (100)
3
2
2
hw/PQ–C H
6
6
hw/PQ–CH Cl
2
2
hw/PQ–C H
6
6
hw/PQ–CH CN–CH Cl
3
2
2
hw/PQ–C H
6
6
hw/PQ–CH CN
3
hw/PQ–C H
6
6
a
Yields were determined based on the amounts of consumed silanes.

M. Kako et al. / Journal of Organometallic Chemistry 636 (2001) 63–68
65
Table 2
molecular oxygen, the yield of 3a increased while that
of 2a decreased somewhat. Since 2a is stable in the air,
it is suggested molecular oxygen should be incorporated
probably by way of some radical intermediates to af-
ford 3a during the photoreactions. Meanwhile, photoly-
sis of 7-silanorbornadiene 6 [14] with PQ gave 2a and
The free energy changes (DG) for the electron-transfer from cyclosi-
lanes to PQ
DG (kcal mol−1)
Eox (V vs. SCE)
in CH CN
in C H
3
6
6
3
a under similar reaction conditions. This result indi-
1
1
7
8
a
b
+1.45
+1.00
+0.42
+1.17
−1.4
−11.8
−25.2
−7.9
+17.8
+7.5
−6.0
cates that transfer of silylene units proceeds stepwise to
produce 3a, possibly by way of 2a. Since the photoreac-
tion of 1b having more bulky isopropyl groups did not
afford 3b as mentioned above, conversion of 2b to 3b
may be prevented by the steric restriction caused by
isopropyl groups.
+11.4
Recently, it has been demonstrated that 7,8-disilabi-
cyclo[2.2.2]octa-2,5-dienes such as 7 [15a,b] and 8 [15c],
which are known as good disilene precursors [13], also
serve as efficient electron donors [16]. Therefore, it was
expected that selective disilanylene not silylene transfer
to PQ might take place when 7 and 8 were employed as
electron donors. However, the photolysis of 7 in the
presence of PQ afforded silylene addition product 2a
together with 3a, but expected 5a was not detected in
the photolysate. Similarly, under the same conditions 8
was photolyzed to yield only 2b, 3b was not detected in
the reaction mixture as the case of 1b. As proposed for
the case of 1a–b, this result should be interpreted
reasonably in terms of the reaction scheme involving a
diradical intermediate 9 (Scheme 2). Then, 9 might
collapse to give biphenyl and free dimethylsilylene,
which could contribute to the formation of 2a and 3a.
Participation of free dimethylsilylene was suggested by
the fact that Et SiSiMe H was obtained in the photoly-
Scheme 1.
3
2
sis of 2a and PQ in the presence of Et SiH. In contrast
3
no concrete evidence for free diisopropylsilylene was
obtained, and the co-photolysis of 8 and PQ in the
Scheme 2.
presence of Et SiH did not give the trapped product
3
i
Et SiSi Pr H.
photolysis of 1a and PQ in the presence of CCl gave
3
2
4
Cl(Me Si) Cl (n=4–6), which would substantiate the
2
n
2
.2. Laser flash photolysis
mechanism above involving silyl radical intermediates
such as 4a [12]. Under the reaction condition, the
formation of Cl(Me Si) Cl may indicate the electron-
In order to gain more insight into the reaction mech-
2
6
3
anism discussed above, we investigated quenching of
transfer from 1a to PQ*, and this affords the corre-
3
PQ* by oligosilane 1b by means of laser flash photoly-
+
’
sponding radical cation 1a which readily reacts with
sis. Fig. 1a shows the transient absorption spectra
obtained after laser excitation of a solution (CH CN–
CCl to give Cl(Me Si) Cl [12b].
4
2
6
3
While the pathway leading to 3a has not been
clarified, it was proposed that an intermediate 5a,
which may arise from intramolecular cyclization of 4,
would undergo atmospheric oxidation to give 3a. How-
ever, the intermediate 5a was never detected in the
photolysates. Further, several trapping experiments for
free dimethylsilylene and dimethylsilanone [13], which
could react with 2a to afford 5a and 3a, respectively,
have been unsuccessful. The residual molecular oxygen
would be responsible for the formation of 3a. In fact,
when the photolysis was carried out after saturation of
CH Cl =4/1) of PQ at room temperature, in which
2
2
triplet–triplet (T–T) absorption appeared around 450
nm [17]. In the presence of cyclotetrasilane 1b, the T–T
3
absorption of PQ* decayed faster due to quenching
with 1b and at the same time a new band around 520
nm assignable to PQ radical anion [17b] became intense
as shown in Fig. 1b. From the dependence of lifetime of
3
PQ* upon the concentration of 1b, the quenching rate
9
constant (kq) was calculated to be 4.35×10
(M
−
1
−1
s
). These results indicate that the photo-in-
duced electron-transfer reaction takes place from 1b to

6
3
6
M. Kako et al. / Journal of Organometallic Chemistry 636 (2001) 63–68
PQ* to give the corresponding radical ion pair. Same
types of electron transfer should take place on irradia-
tion of 1a, 7, and 8 as suggested by DG values in Table
as 11 and finally afford the product 2b as in the case of
2a (Scheme 3).
On the other hand, electron-transfer does not seem to
operate efficiently in benzene, a less polar solvent. As
shown in Fig. 1c, PQ radical anion was not observed on
laser irradiation of PQ and 1b in benzene. Furthermore,
2
. Coupling of the radical ion pair thus formed would
result in the formation of a diradical intermediate such
8
−1 −1
the k value was found to be 8.17×10 (M
s
),
much smaller than that obtained for CH CN–CH Cl
2
q
3
2
solution. This is consistent with the experimental result
that the present photoreaction proceeds faster in
CH CN–CH Cl than in benzene (Table 2). These sol-
3
2
2
vent effects clearly indicate that some alternative pro-
cess other than electron-transfer may be responsible for
3
the quenching of PQ*, at least in benzene. As dis-
cussed above, S 2-type radical substitution is a plausi-
H
ble rationale for the quenching process in non-polar
3
media. Formation of an exciplex from PQ* and 2b
should be also considered, whereas no exciplex emission
has been observed on irradiation of PQ and 2b in
benzene.
3
. Experimental
3.1. General procedure
NMR spectra were recorded with a Varian Unity-
plus 500 spectrometer. Deuterated CHCl and C H
3
6
6
were used as the solvents. Mass spectral data were
obtained on a Shimadzu QP-1000 mass spectrometer.
High-resolution mass spectral data were obtained on a
JEOL SX-102 mass spectrometer. UV–vis spectra were
obtained with a Hitachi U-3300 spectrometer. Fluores-
cence spectra were obtained with a Hitachi F-4500
spectrometer. GLC analyses were carried out on a
Shimadzu GC-14A equipped with an FID detector and
a 0.25 mm×25 m CBP1 capillary column. Preparative
GLC was performed on an Ohkura Riken 802 gas
chromatograph with a TCD detector and a 3 mm×1.7
m SE-30 (10%) packed column. Gel permeation chro-
matography was also used for separation of reaction
products using a series of JAIgel 1H and 2H columns
with a flow of toluene on an LC-908 liquid chro-
matograph of Japan Analytical Industry Co. Ltd.
Cyclic voltammograms of cyclosilanes were obtained in
Fig. 1. Transient Absorption Spectra Obtained by Laser Excitation of
PQ. (a) in CH CN/CH Cl . (b) in the presence of 1b in CH CN/
3
2
2
3
CH Cl . (c) in the presence of 1b in C H .
2
2
6
6
0
.1 M n-Bu NClO –CH Cl (vs. SCE; scan rate, 200
4 4 2 2
−
1
mV s ; Hokuto Denko Ltd, a potentiostat/galvanos-
tat HA-501 and a function generator HB-104). The DG
values were calculated according to the Rehm–Weller
−
1
+
equation [10] (DG (kcal mol )=23.06[E(D/D )−
−
E(A /A)−DEexcit. +DEcoul.)]. The reduction potential
of PQ is −0.66 V (vs. SCE), and the triplet energy of
−
1
PQ (48 kcal mol ) was used as the excitation energy
DEexcit.) [8]. The Coulomb interaction energies (DEcoul.
are −0.06 and +0.78 eV for MeCN and C H , respec-
(
)
6
6
Scheme 3.
tively [10b]. PQ (Wako Pure Chemical) was commer-

M. Kako et al. / Journal of Organometallic Chemistry 636 (2001) 63–68
67
cially available and used as received. Compounds 1a [6a],
b [6b], 6 [14], 7 [15a,b], and 8 [15c] were prepared
according to the literatures.
from the Ministry of Education, Science, Sports and
Culture in Japan. We also thank Toshiba Silicone Co.
Ltd for a gift of organosilicon reagents.
1
3.2. Photoreactions of cyclosilanes
References
A typical procedure is exemplified as follows. A
−
2
[1] M. Kako, Y. Nakadaira, Coord. Chem. Rev. 176 (1998) 87 (and
references therein).
degassed solution of 1a (1.1×10
M) and PQ (3.5×
−
2
1
0
M) in a mixed solvent (CH CN–CH Cl =4/1) was
3 2 2
[
2] (a) Y.F. Traven, R. West, J. Am. Chem. Soc. 95 (1973) 6824;
b) H. Sakurai, M. Kira, T. Uchida, J. Am. Chem. Soc. 95
(1973) 6826;
c) H. Sakurai, Y. Kamiyama, J. Am. Chem. Soc. 96 (1974)
192.
irradiated with two 500 W tungsten–halogen lamps
passing through an aqueous NaNO2 solution filter,
(
(
(
6
cutoffB400 nm) at room temperature (r.t.). The silylene
adducts 2a and 3a were obtained by preparative GLC
and HPLC. The yields of 2a and 3a were determined by
analytical GLC. Photolyses of 1b, 6, 7, and 8 were
conducted in the same procedure and the results were
[
3] (a) M. Igarashi, T. Ueda, M. Wakasa, Y. Sakaguchi, H.
Hayashi, J. Organomet. Chem. 421 (1991) 9;
(
b) M. Wakasa, M. Igarashi, Y. Sakaguchi, H. Hayashi, Ni-
honkagakukaishi (1994) 267;
c) H. Komoriya, M. Kako, Y. Nakadaira, K. Mochida, M.
Tonogaki-Kubota, T. Kobayashi, Chem. Lett. (1984) 1213;
d) H. Komoriya, M. Kako, Y. Nakadaira, K. Mochida, M.
1
(
summarized in Table 1. 2a. H-NMR (C D ): l 8.54 (d,
6
6
2
H, J=8.0 Hz), 8.46 (d, 2H, J=8.0 Hz), 7.53 (t, 2H,
J=8.0 Hz), 7.40 (t, 2H, J=8.0 Hz), 0.20 (s, 6H);
(
Tonogaki-Kubota, T. Kobayashi, J. Organomet. Chem. 499
(1995) 123;
1
3
C-NMR (CDCl ): l 139.35 (s), 127.38 (s), 126.97 (d),
3
1
25.39 (s), 124.81 (d), 123.50 (d), 121.06 (d), −0.80 (q).
(e) E. Baciocchi, T.D. Giacco, F. Elisei, M. Ioele, J. Org. Chem.
+
6
0 (1995) 7974.
MS (70 eV); m/z (%): 266 [M , 6], 252 (21), 210 (100),
[
4] (a) K. Maruyama, H. Imahori, A. Osuka, A. Takuwa, H.
Tagawa, Chem. Lett. (1986) 1719;
181 (34), 152 (35), 43 (20). Found: 266.0780. Exact mass
1
Calc. for C H O Si: 266.0763. 3a. H-NMR (CDCl ):
l 8.62 (d, 2H, J=8.0 Hz), 8.16 (d, 2H, J=8.0 Hz),
1
6
14
2
3
(
b) A. Takuwa, H. Tagawa, H. Iwamoto,
O Soga, K.
Maruyama, Chem. Lett. (1987) 1091;
(c) A. Takuwa, Y. Nishigaichi, K. Yamashita, H. Iwamoto,
Chem. Lett. (1990) 639;
1
3
7
1
1
3
.62–7.58 (m, 4H), 0.36 (s, 12H); C-NMR (CDCl ): l
35.43 (s), 129.00 (s), 127.73 (s), 126.65 (d), 125.20 (d),
22.39 (d), 122.14 (d), −0.90 (q). MS (70 eV); m/z (%):
40 [M , 100], 325 (30), 266 (82), 236 (44), 133 (58), 73
3
(
d) A. Takuwa, Y. Nishigaichi, K. Yamashita, H. Iwamoto,
Chem. Lett. (1990) 1761;
e) A. Takuwa, Y. Nishigaichi, T. Yamaoka, K. Iihama, J.
+
(
(
21). Found: 340.0927. Exact mass Calc. for C H O Si :
18 20 3 2
Chem. Soc. Chem. Commun. (1991) 1359;
(f) L. Cermenati, M. Freccero, P. Venturello, A. Albini, J. Am.
Chem. Soc. 117 (1995) 7869.
1
3
8
40.0951. 2b. H-NMR (CDCl ): l 8.67 (d, 2H, J=
.0Hz), 8.15 (d, 2H, J=8.0 Hz), 7.65–7.53 (m, 4H), 1.43
3
13
[5] M. Kako, M. Ninomiya, Y. Nakadaira, Chem. Commun. (1997)
373.
6] (a) H. Gilman, R.A. Tomasi, J. Org. Chem. 28 (1963) 1651;
b) H. Watanabe, T. Maruoka, M. Kageyama, K. Yoshizumi, Y.
(
sept, 2H, J=7.5 Hz), 1.16 (d, 12H, J=7.5 Hz); C-
1
NMR (CDCl ): l 139.66 (s), 126.55 (s), 126.46 (d), 124.63
3
[
[
(
s), 124.25 (d), 120.88 (d), 120.71 (d), 15.80 (q), 13.04 (d).
(
+
MS (70 eV); m/z (%): 322 [M , 100], 236 (28), 208 (10),
Nagai, Organometallics 3 (1984) 141.
7] (a) W. Ando, M. Ikeno, J. Chem. Soc. Chem. Commun. (1979)
43 (20). Exact mass Calc. for C H O Si: 322.1389.
20 22 2
6
(
55;
b) P.P. Gasper, in: M. Jones Jr., R.A. Moss (Eds.), Reactive
Intermediates, vol. 1, Wiley, New York, 1978, p. 229 (vol. 2,
981, p. 335; vol. 3, 1985, p. 333).
Found: 322.1388.
3.3. Laser flash photolysis
1
[
[
8] R. Foster, M.I. Foreman, in: S. Patai (Ed.), The Chemistry of
the Quinonoid Compounds, Wiley, New York, 1974, p. 257
Laser flash photolysis experiments were performed on
(
chap. 6).
nitrogen-bubbled solutions at r.t. by using the third
harmonic (355 nm) of a Nd:YAG laser as an exciting
light source. The concentrations of substrates employed
9] S.L. Murov, Handbook of Photochemistry, Marcel Dekker, New
York, 1973.
[
10] (a) D. Rehm, A. Weller, Isr. J. Chem. 8 (1970) 259;
(b) J. Mattay, J. Runsink, J. Gersdorf, T. Rumbach, C. Ly,
Helv. Chim. Acta 69 (1986) 442.
11] A. Alberti, S. Dellonte, C. Paradisi, S. Roffia, G.F. Pedulli, J.
Am. Chem. Soc. 112 (1990) 1123.
−
4
−3
−3
were 4.80×10
M for PQ, 4.40×10 , 2.20×10
,
−
3
−4
−4
1
.10×10 , 4.40×10 , and 2.20×10
M for 1b,
[
[
respectively, in a mixed solvent (CH CN–CH Cl =4/1).
3
2
2
−
4
In C H , the concentrations were 4.80×10 M for PQ,
.30×10 , 1.00×10 , and 3.00×10
6
6
12] (a) H. Sakurai, K. Sakamoto, M. Kira, Chem. Lett. (1984) 1213;
−
3
−3
−4
3
M for 1b,
(
b) Y. Nakadaira, N. Komatsu, H. Sakurai, Chem. Lett. (1985)
respectively.
1781.
[
13] (a) G. Raabe, J. Michl, Chem. Rev. 85 (1985) 419;
(
b) G. Raabe, in: S. Patai, Z. Rappoport (Eds.), The Chemistry
of Organosilicon Compounds, Wiley, New York, 1989, p. 1015
Acknowledgements
(
(
chap. 17);
c) Y. Nakadaira, T. Otsuka, H. Sakurai, Tetrahedron Lett. 22
This work was supported in part by Grants-in-Aid
(1981) 2417;

68
M. Kako et al. / Journal of Organometallic Chemistry 636 (2001) 63–68
d) J.D. Rich, T.J. Drahnak, R. West, J. Michl, J. Organomet. Hatakenaka, M. Ohashi, Bull. Chem. Soc. Jpn. 66 (1993)
(
Chem. 212 (1981) C1.
344;
[
[
14] H. Alper, L.W. Gross, B. Mayer, W.P. Neumann, J. Organomet.
Chem. 291 (1985) 9.
15] (a) D.N. Roark, G.J.D. Peddle, J. Am. Chem. Soc. 94 (1972)
(b) S. Kyushin, M. Ikarugi, K. Takatsuna, M. Goto, H. Mat-
sumoto, J. Organomet. Chem. 510 (1996) 121;
(c) M. Kako, K. Hatakenaka, S. Kakuma, M. Ninomiya, Y.
Nakadaira, M. Yasui, F. Iwasaki, M. Wakasa, H. Hayashi,
Tetrahedron Lett. 40 (1999) 1133.
5
(
837;
b) K. Sakamoto, K. Obata, H. Hirata, M. Nakajima, H.
Sakurai, J. Am. Chem. Soc. 111 (1989) 7641;
c) H. Matsumoto, T. Arai, H. Watanabe, Y. Nagai, J. Chem.
[17] (a) J.C. Scaiano, Handbook of Organic Photochemistry, CRC
Press, Boca Raton, FL, 1989;
(
Soc. Chem. Commun. (1984) 724.
16] (a) Y. Nakadaira, Y. Gomi, H. Hosoe, S. Kyushin, M. Kako, K.
(b) T. Shida, Electronic Absorption Spectra of Radical Ions. In:
Physical Science Data, vol. 34, Elsevier, Amsterdam, 1988.
[