Cyclization of 1,2-disilacyclohexa-3,5-dienes initiated by electron transfer reactions with TCNE

Article abstract of DOI:10.1246/cl.2000.1382

Thermal reaction of 1,2-disilacyclohexa-3,5-dienes with tetracyanoethylene (TCNE) gave 8-amino-6,6,11,11-tetramethyl-2,3,4,5-tetraphenyl-7-aza-6,11-disilatricyclo[5,3, 0,1^2,5]undeca-3,8,10(1)-triene-9,10-dicarbonitrile through a charge-transfer complex.

Full text of DOI:10.1246/cl.2000.1382

1382
Chemistry Letters 2000
Cyclization of 1,2-Disilacyclohexa-3,5-dienes Initiated by
Electron Transfer Reactions with TCNE
Kunio Mochida,* Hiromi Shimizu, and Masato Nanjo
Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Tokyo 171-8588
(Received September 18, 2000; CL-000860)
Thermal reaction of 1,2-disilacyclohexa-3,5-dienes with
tetracyanoethylene (TCNE) gave 8-amino-6,6,11,11-tetramethyl-
2,3,4,5-tetraphenyl-7-aza-6,11-disilatricyclo[5,3,0,12,5]undeca-
3,8,10(1)-triene-9,10-dicarbonitrile through a charge-transfer
complex.
Group 14 element (silicon, germanium)–group 14 element
σ bonds with rather low ionization potential are excellent elec-
tron donors. As electron-rich species, these group 14 element
catenates are subject to cleavage by various organic elec-
trophiles as well as transition metal complexes through a
charge-transfer (CT) complex.1 During the past two decades,
the CT interactions between group 14 element–group 14 ele-
ment σ bonds and common electron acceptors such as tetracya-
noethylene (TCNE) have been amply investigated.2–8 In a pre-
vious paper,9 we reported a ring-contraction reaction of 1,2-
digermacyclohexa-3,5-dienes initiated by CT with TCNE. We
describe herein the cyclization reaction of 1,2-disilacyclohexa-
3,5-dienes through a CT complex with TCNE.
A solution of 1,1,2,2-tetramethyl-1,2-disila-3,4,5,6-tetra-
phenylcyclohexa-3,5-diene10 (1) (0.4 mmol) and TCNE (0.4
mmol) in dichloromethane was placed in a Pyrex tube under
argon and sealed. The color of the solution changed from yel-
lowish-green to reddish-brown. The reaction mixture was heat-
ed at 90 °C for 3 h to give 8-amino-6,6,11,11-tetramethyl-
2,3,4,5-tetraphenyl-7-aza-6,11-disilatricyclo[5,3,0,12,5]undeca-
3,8,10(1)-triene-9,10-dicarbonitrile (2, 39%), 1,1,3,3-tetram-
ethyl-4,5,6,7-tetraphenyl-2-oxa-1,3-disilacyclohepta-4,6-diene
(3, 18%), and 5,5,6,6-tetracyano-7,7-dimethyl-1,2,3,4-
tetraphenyl-7-silanorbornene (4, 5%). The rest of 1 remained
unreacted. Concentration of the reaction mixture by evapora-
tion of dichloromethane followed by preparative GPC gave
pure 2 and 3, which showed satisfactory NMR and MS data.11
7-Silanorbornene 4 could not be isolated owing to its small
amount. The silanorbornene 4 was characterized by NMR
spectra in comparison with those of authentic sample.12 Several
unidentified by-products in minor amounts were also formed.
In spite of all efforts to minimize moisture and air, 3 was
obtained in substantial amounts.
The effect of reaction conditions on the yield of 2 in the
present CT reaction was examined. The ratio of 1/TCNE on the
o
yield of 2 at 90 C for 3 h was examined. Increase in the con-
centration of TCNE relative to 1 did not influence the yields of
2. Lowering the reaction temperature from 90 to 70 oC resulted
in lower yields of 2 (39 to 15%). Extending the reaction time
scarcely improved the yield of 2.
The molecular structure of 213 is shown in Figure 1, togeth-
er with selected bond distances and angles. The 1-azacyclo-
pentadiene ring and the C(5)–C(4)–N(1)–Si(2)–C(8) plane are
almost planar. The bond angles of Si(1)–C(8)–C(7) and
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
1383
Si(1)–C(8)–Si(2) are 97.2 (2) and 107.65 (16)o, respectively.
The color of the solution of 1 and TCNE in
dichloromethane strongly suggests the CT interaction between
1 and TCNE. The formation of a CT complex is evidenced by
the ESR spectrum of the TCNE radical anion (g = 2.0031, aN =
1.56 G) at room temperature.3
mechanism.
References and Notes
1
2
3
4
J. K. Kochi, “Organometallic Mechanisms and Catalysis,”
Academic Press, New York (1978).
V. F. Traven and R. West, J. Am. Chem. Soc., 95, 6824
(1973).
H. Sakurai, M. Kira, and T. Uchida, J. Am. Chem. Soc., 95,
6826 (1973).
M. Kira, K. Sakamoto, and H. Sakurai, J. Am. Chem. Soc.,
105, 7496 (1983); H. Sakurai, K. Sakamoto, and M. Kira,
Chem. Lett., 1984, 1213; M. Kira, K. Takeuchi, and H.
Sakurai, “Studies in Organic Chemistry,” ed. by M.
Kobayashi, Elsevier, Amsterdam (1987), Vol. 31, p. 407;
M. Kira, K. Takeuchi, C. Kabuto, and H. Sakurai, Chem.
Lett., 1988, 353.
From these results, we propose a possible mechanism for the
formation of products 2–4 by CT of 1 with TCNE as depicted in
Scheme 1. Initially, 1 is readily oxidized by TCNE to give radi-
+
•
cal cation 1 , in which the Si–Si bond possibly gives an open
+
•
intermediate 5 . Then, subsequent intramolecular addition of
+
•
one of the silyl groups to the diene moiety gives to 6 . The
intermediate 6+ reacts with TCNE– to give biradical 7.
•
•
Compound 2 is derived from 7 after several steps. On the other
+
•
hand, the intermediate 6 undergoes spontaneous scission to a
radical cation of 1-silacyclopenta-2,4-diene (silole) and
5
6
Y. Nakadaira, N. Komatsu, and H. Sakurai, Chem. Lett.,
1985, 1781; H. Watanabe, M. Kato, E. Tabei, H.
Kawabara, N. Hirai, T. Sato, and Y. Nagai, J. Chem. Soc.,
Chem. Commun., 1986, 1662.
S. Kyushin, Y. Ehara, Y. Nakadaira, and M. Ohashi, J.
Chem. Soc., Chem. Commun., 1989, 279; Y. Nakadaira, A.
Sekiguchi, Y. Funada, and H. Sakurai, Chem. Lett., 1991,
327; Y. Nakadaira, S. Otani, S. Kyushin, M. Ohashi, H.
Sakurai, Y. Funada, K. Sakamoto, and A. Sekiguchi,
Chem. Lett., 1991, 601.
dimethylsilyene (Me2Si:). The radical cation of silole is reduced
–
•
with TCNE to afford silole 8 and TCNE. The silole 8 thus
formed reacts with TCNE to give 7-silanorbornene 4. Some
+
•
parts of 5 react with oxygen to give 2,3-disila-1-oxacyclohepta-
4,6-diene 3. None of the eliminated Me2Si: part could be detect-
ed. Further mechanistic studies of 2 are in progress to verify
these speculations.
The present results are in marked difference to those of the
formation of siloles in photo-induced electron transfer of 1,2-
disilacyclohexadienes with methylene blue14 and dibenzosilole
in the photoexcitation of the CT complex between dibenzodisila-
7
Y. Nakadaira, S. Kyushin, and M. Ohashi, Yuki Gosei
Kagaku Kyokai Shi, 48, 331 (1990) and references cited
therein.
8
9
K. Mochida, C. Hodota, R. Hata, and S. Fukuzumi,
Organometallics, 12, 586 (1993).
K. Mochida, H. Shimizu, and M. Nanjo, Chem. Lett., 2000,
1226.
10 Y. Nakadaira, S. Kanouchi, and H. Sakurai, J. Am. Chem.
Soc., 96, 5623 (1974).
11 2: 1H NMR (δ in CDCl3) 0.10 (s, 3H), 0.17 (s, 3H), 0.61 (s,
3H), 4.03 (s, 2H), 6.45–7.16 (m, 20H). 13C NMR (δ in
CDCl3) –6.1, –2.7, –1.9, –1.4, 39.4, 46.4, 93.1, 112.9,
113.7, 125.0, 125.7, 125.9, 126.2, 126.7, 127.2, 127.6,
127.7, 129.0, 130.2, 131.0, 136.1, 137.1, 137.3, 137.9,
138.6, 145.0, 145.9, 148.4. MS m/z 602 (M+). 3: 1H
NMR (δ in CDCl3) –0.04 (s, 6H), 0.47 (s, 6H), 6.78–7.11
(m, 20H). 13C NMR (δ in CDCl3) 11.13, 105.8, 124.6,
125.4, 126.4, 127.1, 129.2, 140.1, 142.6, 144.4, 153.9; MS
m/z 488 (M+–Me).
1
12 4: H NMR (δ in CDCl3) 1.00 (s, 3H), 1.10 (s, 3H),
6.78–7.30 (m, 20H). 13C NMR (δ in CDCl3) –3.03, 3.87,
54.4, 59.3, 111.3, 113.5, 127.4, 127.5, 128.6, 128.9, 129.7,
130.0, 132.4, 134.2 ; MS m/z 413 (M+–TCNE).
13 Crystal data for 2: C38H32N4Si2; fw = 600.87; crystal size
0.2 × 0.2 × 0.2 mm; monoclinic, space group P21/c, Z = 4;
a = 9.7060(6) Å, b = 15.7580(10) Å, c = 23.6360(8) Å, β =
98.928(4)o; V = 3571.3(3) Å3, D = 1.275 g/cm3; Goodness
of fit = 1.831, R = 0.0691, Rw = 0.2192, 5114 reflections
measured.
14 M. Kako, H. Takada, and Y. Nakadaira, Tetrahedron Lett.,
38, 3525 (1997).
15 H. Sakurai, K. Sakamoto, and M. Kira, Chem. Lett., 1984,
1213.
cyclohexadiene and TCNE.15
We thank Prof. Yasuhiro Nakadaira of The University of
Electro-Communications for valuable discussion of reaction