Photo-induced silylene-transfer reactions of cyclic organosilanes phenanthraquinone

Article abstract of DOI:10.1039/a702161e

Cophotolysis of cyclic organosilanes 1-4 with phenanthraquinone gave silylene insertion products via radical displacement at the silicon atoms of organosilanes by the photochemically excited phenanthraquinone.

Full text of DOI:10.1039/a702161e

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Photo-induced silylene-transfer reactions of cyclic organosilanes to
phenanthraquinone
Masahiro Kako, Masumi Ninomiya and Yasuhiro Nakadaira*
Department of Chemistry, The University of Electro-Communications, Chofu, Tokyo, 182, Japan
Cophotolysis of cyclic organosilanes 1–4 with phen-
anthraquinone gave silylene insertion products via radical
displacement at the silicon atoms of organosilanes by the
photochemically excited phenanthraquinone.
from PQ while substantial amounts of PQ were recovered after
the photolyses.
It is well-known that ultraviolet photolysis of 1 produces free
dimethylsilylene, which can be trapped readily by a-diketones
to afford trioxadisilepine derivatives.4 However, 1 is entirely
inert to the visible light employed in the present photolyses.
Furthermore, the present reaction does not seem to involve free
dimethylsilylene because the cophotolysis of 1 and PQ in the
presence of Et3SiH, a common silylene trap, did not give any
Et3SiMe2SiH.4b
Electron-transfer reactions of heavier group 14 organometallic
compounds have been studied extensively from the mechanistic
and synthetic viewpoints.1 For these reactions, electron-
deficient carbonyl compounds, especially p-benzoquinones, are
frequently used as electron acceptors.2,3 We and the other
groups found that the photoreactions of some disilane and
digermane derivatives with p-benzoquinones underwent elec-
tron-transfer processes to produce O-silylated and
O-germylated p-benzoquinones.2 On the other hand, it was
reported that the photolysis of a-diketones with alkyl and allylic
stannanes afforded exclusively the alkylated and allylated
a-ketols without giving any stannylated products.3 These
previous results prompted us to investigate the photoreaction of
organosilanes and o-benzoquinones, which serve as good
silylene traps due to their s-cis a-diketone structures.4 We now
report the photoreaction of cyclic organosilanes 1–4 with
phenanthraquinone (PQ), providing the first example of silylene
transfer reactions induced by photoexcited triplet
o-benzoquinones.
Since the reaction was efficiently quenched by addition of
anthracene, the triplet PQ is suggested to be the key inter-
mediate as judged by the comparison of the singlet and the
triplet energies of PQ (ES = ca. 54, ET = ca. 48 kcal mol21 5
)
and those of anthracene (ES = ca. 76, ET = ca. 43 kcal mol21).6
Meanwhile, the consumption of 1 was suppressed by addition of
1,4-diazabicyclo[2.2.2]octane (DABCO), which has a lower
oxidation potential (Eox = +0.70 V vs. SCE) than that of 1 (Eox
= +1.45 V vs. SCE). The free energy changes (DG) for the
electron-transfer from cyclosilanes to the triplet PQ are given in
Table 2.‡ These data suggest that the reaction may proceed via
a partial or complete electron-transfer from the cyclosilanes to
the photoexcited PQ at least in a polar solvent, such as MeCN.
In fact, the silylene-transfer reactions were found to proceed
substantially faster in MeCN than in benzene, as shown in Table
1.
Me Me
Pri Pri
Si Si
Si Si
Me
Me
Me
Me
Si
We propose the following mechanism on the basis of
Pedulli’s result that photoexcited ketones are capable of
cleaving Si–Si bonds of polysilanes via SH2 type radical
displacement at the silicon atoms.8 Thus, the photochemically
Si
Pri
Pri
Pri
Pri
Me
Si
Si
Si
Me
Me
Me
Me
Me
Me
Me
Si
Si
Si
Pri Pri
Ph
Me Me
1
2
3
Table 1 Photo-induced silylene-transfer reactions of cyclosilanes to PQ
R
Pri
Pri
Pri
Pri
Conversion
(%)
R
R
O
Si
Si
Si
O
Product (% yield)a
R
R
Silane Conditions
t/h
Si
O
O
Si
1
hn/PQ/CH3CN/ 4.5 61
CH2Cl2
5 (138), 6 (33)
O
R
1
2
2
3
hn/PQ/CH6H6 10.5 49
5 (161), 6 (31)
7 (82)
7 (57)
5 (28), 6 (11), biphenyl
(100)
5 (62), 6 (13), biphenyl (94)
7 (85), anthracene (97)
7 (69), anthracene (100)
4
5 R = Me
7 R = Pri
6 R = Me
8 R = Pri
hn/PQ/CH2Cl2l 0.5 70
hn/PQ/CH6H6
1.5 70
hn/PQ/CH3CN/ 0.5 89
CH2Cl2
Me
O
3
4
4
hn/PQ/CH6H6
hn/PQ/CH3CN
1.5 65
3.0 100
hn/PQ/CH6H6 18.0 70
Si
Me
Si
O
Me
Me
a Based on the consumption of silanes.
10
Table 2 The free energy changes (DG) for the electron-transfer from
cyclosilanes to PQ
Irradiation of a solution (MeCN–CH2Cl2 = 4:1) of dodeca-
methylcyclohexasilane 1 (1.1 3 1022 m) and PQ (3.5 3 1022 m)
in a sealed tube with two 500 W tungsten-halogen lamps
(passing through an aqueous NaNO2 filter, cutoff < 400 nm)
afforded dioxasilolene 5,† as a silylene adduct of PQ, along with
a minor amount of trioxadisilepine 6. In the case of photolysis
of cyclotetrasilane 2, dioxasilolene 7 was produced, whereas the
corresponding trioxadisilepine 8 was not formed at all, as
summarized in Table 1. We detected no other product derived
DG/kcal mol21
in MeCN
Eox/V vs. SCE
in C6H6
1
2
3
4
+1.45
+1.00
+0.42
+1.17
21.41
211.79
225.16
27.9
+17.84
+7.47
26.0
+11.4
Chem. Commun., 1997
1373

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excited PQ attacks one of the silicon atoms of 1 to afford a
biradical intermediate 9. At the initial interaction, a partial or
complete electron-transfer from 1 to the triplet PQ may take
place in polar solvents as mentioned above. Subsequent
intramolecular O–Si bond formation in 9 would provide 5 as
illustrated in Scheme 1. The yields of 5 indicate that a molecule
of 1 donates more than an equimolar amount of silylene units to
PQ. We obtained polymeric products containing silane frag-
ments in these reactions, which might have arisen from
framentation of the starting materials. In addition, the photo-
reaction of 1 and PQ in the presence of CCl4 gave Cl(Me2Si)nCl
(n = 4–6), which would substantiate the present mechanism
involving the silyl radical intermediates.9
There seemed to be three possible pathways leading to 6: (i)
intramolecular cyclization in 9 could afford 10, which would be
susceptible to atmospheric oxidation to give 6, (ii) addition of
free dimethylsilylene to 5 could give rise to 6 via 10, and (iii)
dimethylsilanone,10 if formed in the course of reaction, could
insert into 5 to produce 6. However, the intermediate 10 was
never detected in the photolysates while all trapping experi-
ments for free dimethylsilylene and dimethylsilanone were not
successful. When the reaction was carried out under an aerobic
atmosphere, the yields of 6 increased while those of 5
decreased. The residual molecular oxygen plays an important
role in the formation of 6. Since 5 is stable in the air, it is
suggested that molecular oxygen should be incorporated,
probably as some radical species, to afford 6 during the
photoreactions. Further investigations are in progress to clarify
the mechanism.
Et3SiH. Based on the mechanism proposed for 1, the reaction of
3 is interpreted as shown in Scheme 2. The initially formed
adduct 11 would collapse to afford 5, 12, free dimethylsilylene
and biphenyl successivley. It should be noted that free
dimethylsilylene could contribute to the formation of 5 and 6.
Compound 4 gave a similar result, although no trioxadisilepine
8 was obtained as the case of 2.
In summary, we have indicated that silylene-transfer reac-
tions take place between cyclic organosilanes 1–4 and the
photochemically excited triplet PQ. The formation of free
dimethylsilylene is noteworthy since there has been no
precedent of generation of free silylenes induced by photo-
excited carbonyl compounds as far as we know.
This work was supported in part by Grants-in-Aid from the
Ministry of Education, Science, and Culture in Japan. We thank
Toshiba Silicon Co. Ltd. for a gift of organosilicon reagents.
Footnotes
1
† Selected spectral data for 5: H NMR (C6D6): d 8.54 (d, 2H, J 8.0 Hz),
8.46 (6, 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); 13C NMR (CDCl3): d 139.35 (s), 127.38 (s), 126.97 (d), 125.39 (s),
124.81 (d), 123.50 (d), 121.06 (d), 20.80 (q); m/z 266 (M+, 6%), 252 (21),
210 (100), 181 (34), 152 (35), 43 (20). For 6: 1H NMR (CDCl3): d 8.62 (d,
2H, J 8.0 Hz), 8.16 (d, 2H, J 8.0 Hz), 7.62–7.58(m, 4H), 0.36 (s, 12H); 13
C
NMR(CDCl3): d 135.43 (s), 129.00 (s), 127.73 (s), 126.65 (d), 125.20 (d),
122.39 (d), 122.14 (d), 20.90 (q); m/z 340 (M+, 100%), 325 (30), 266 (82),
236 (44), 133 (58), 73 (21). For 7: 1H NMR (CDCl3): d 8.67 (d, 2H, J 8.0
Hz), 8.15 (d, 2H, J 8.0 Hz), 7.65–7.53 (m, 4H), 1.43 (sept, 2H, J 7.5 Hz),
1.16 (d, 12H, J 7.5 Hz); 13C NMR (CDCl3): d 139.66 (s), 126.55(s), 126.46
(d), 124.63 (s), 124.25 (d), 120.88 (d), 120.71 (d), 15.80 (q), 13.04 (d); m/z
322(M+, 100%), 236 (28), 208 (10), 43 (20).
‡ The DG values were calculated according to the Rehm–Weller equation
(ref. 7) {DG/kcal mol21 = 23.06[E(D/D+)2E(A2/A)2DEexcit. + DE-
coul.)]}. The reduction potential of PQ is 20.66 V (vs. SCE) and the triplet
energy of PQ was used as the excitation energy (DEexcit.) (ref. 5). The
Coulomb interaction energies (DEcoul.) are 20.06 and +0.78 eV for MeCN
and benzene, respectively [ref. 7(b)].
Recently, it has been demonstrated that 7,8-disilabi-
cyclo[2.2.2]octa-2,5-diene 3, which is known as a disilene
precursor,10 also acts as a good electron donor.11 Therefore, we
anticipated that cophotolysis of 3 with PQ would produce
selectively the corresponding disilanylene adduct 10. Inter-
estingly, the silylene adduct 5 was obtained along with 6 instead
of 10. Moreover, in contrast to the case of 1, the intermediacy of
free dimethylsilylene was evidenced by the formation of
Et3SiMe2SiH in the cophotolysis of 3 with PQ in the presence of
References
1 M. Kako, M. Mori, K. Hatakenaka, S. Kakuma, Y. Nakadaira, M. Yasui
and P. Iwasaki, Tetrahedron, 1997, 53, 1265 and references cited
therein.
hn
ISC
PQ (S1)
PQ (T1)
1
PQ
2 M. Igarashi, T. Ueda, M. Wakasa and Y. Sakaguchi, J. Organomet.
Chem., 1991, 421, 9; H. Komoriya, M. Kako, Y. Nakadaira, K.
Mochida, M. Tonogaki-Kubota and T. Kobayashi, Chem. Lett., 1984,
1213; J. Organomet. Chem., 1995, 499, 123; E. Baciocchi, T. D. Giacco,
F. Elisei and M. Ioele, J. Org. Chem., 1995, 60, 7974.
3 A. Takuwa, Y. Nishigaichi, K. Yamashita and H. Iwamoto, Chem. Lett.,
1990, 639; 1990, 1761; A. Takuwa, Y. Nishigaichi, T. Yamaoka and K.
Iihama, J. Chem. Soc., Chem. Commun., 1991, 1359.
Me2 Me2
Si
Si
•
SiMe2
O
Si
Si
Si
5
Me2 Me2 Me2
O
•
4 (a) W. Ando and M. Ikeno, J. Chem. Soc., Chem. Commun., 1979, 655;
(b) P. P. Gasper, in Reactive Intermediates, ed. M. Jones, Jr. and R. A.
Moss, Wiley, New York, 1978, vol. 1, p. 229; 1981, vol. 2, p. 335; 1985,
vol. 3, p. 333.
9
Scheme 1
5 R. Foster and M. I. Foreman, in The Chemistry of the Quinonoid
Compounds, ed. S. Patai, Wiley, New York, 1974, ch. 6, p. 257.
6 S. L. Murov, Handbook of Photochemistry, Marcel Dekker, New York,
1973.
7 (a) D. Rehm and A. Weller, Isr. J. Chem., 1970, 8, 259; (b) J. Mattay,
J. Runsink, J. Gersdorf, T. Rumbach and C. Ly, Helv. Chem. Acta, 1986,
69, 442.
Me2
Si
O
Si
•
Me2
O
•
Ph
8 A. Alberti, S. Dellonte, C. Paradisi, S. Roffia and G. F. Pedulli, J. Am.
Chem. Soc., 1990, 112, 1123.
11
9 H. Sakurai, K. Sakamoto and M. Kira, Chem., Lett., 1984, 1213; Y.
Nakadaira, N. Komatsu and H. Sakurai, Chem. Lett., 1985, 1781.
10 G. Raabe and J. Michl, Chem. Rev., 1985, 85, 419; in The Chemistry of
Organosilicon Compounds, ed. S. Patai and Z. Rappoport, Wiley, New
York, 1989, ch. 17, p. 1015.
5
PQ
•
SiMe
Ph
•
•
Me2Si
2
•
11 Y. Nakadaira, Y. Gomi, H. Hosoe, S. Kyushin, M. Kako, K. Hatakenaka
and M. Ohashi, Bull. Chem. Soc. Jpn., 1993, 66, 344.
biphenyl
12
Scheme 2
Received in Cambridge, UK, 1st April 1997; 7/02161E
1374
Chem. Commun., 1997