Synthesis of polysilacage compounds containing a trisilane bridge

Article abstract of DOI:10.1055/s-1999-2950

Reductive lithiation of 1,1,5,5-tetrakis(phenylthio)-2,2,3,3,4,4- hexamethyl-2,3,4-trisilapentane or deprotonation of 1,5-bis(phenylthio)- 2,2,3,3,4,4-hexamethyl-2,3,4-trisilapentane gave 1,5-bis(phenylthio)-1,5- dilithio-2,2,3,3,4,4-hexamethyl-2,3,4-tris

Full text of DOI:10.1055/s-1999-2950

1772
LETTER
Synthesis of Polysilacage Compounds Containing a Trisilane Bridge
Masaki Shimizu,* Shuji Ishizaki, Hisashi Nakagawa, Tamejiro Hiyama
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
E-mail: shimizu@npc05.kuic.kyoto-u.ac.jp
Received 27 July 1999
At first, bis(phenylthio)methyllithium was generated
Abstract: Reductive lithiation of 1,1,5,5-tetrakis(phenylthio)-
from bis(phenylthio)methane and silylated with 1,3-
dichloro-1,1,2,2,3,3-hexamethyltrisilane to give trisilane
5 in 71% yield. Treatment of 5 with lithium 4,4’-di-tert-
2,2,3,3,4,4-hexamethyl-2,3,4-trisilapentane or deprotonation of
1,5-bis(phenylthio)-2,2,3,3,4,4-hexamethyl-2,3,4-trisilapentane
gave
1,5-bis(phenylthio)-1,5-dilithio-2,2,3,3,4,4-hexamethyl-
2,3,4-trisilapentane which was silylated with dichlorodimethyl- butylbiphenylide (LDBB) in THF at -78 °C and then with
silane or 1,2-dichloro-1,1,2,2-tetramethyldisilane to give the
corresponding tetrasilacyclohexane or pentasilacycloheptane,
dichlorodimethylsilane gave 1,2,3,5-tetrasilacyclohexane
6a as a diastereomeric mixture (cis : trans = 1 : 1) (run 1-
respectively. The tetrasilacyclohexane was transformed by re-
3).6 When silylation was effected at -98 or -78 °C, 6a was
ductive lithiation and sequential silylation to 2,2,3,3,4,4,6,6,7,7-
produced in 47-50% yield (run 1 and 2); the yield de-
decamethyl-2,3,4,6,7-pentasilabicyclo[3.1.1]heptane
and
creased in the reaction at -30 °C (run 3). Although the
yields are moderate, the results are of synthetic value con-
sidering the steric factors of octamethyltetrasilacyclo-
hexane ring formation. Similarly, silylation of 4 with 1,2-
dichloro-1,1,2,2-tetramethyldisilane at -78 °C yielded the
corresponding 7-membered ring 6b in 69% yield (run 4),
whereas no cyclized product was obtained upon use of
1,3-dichloro-1,1,2,2,3,3-hexamethyltrisilane (run 5).
2,2,3,3,4,4,6,6,7,7,8,8-dodecamethyl-2,3,4,6,7,8-hexasilabicyclo-
[3.2.1]octane.
Key words: silicon, lithium, dianion, trisilane, cyclizations
In view of the fact that s-electrons of such acyclic linkage
as -Si-Si-C- or -Si-Si-Si-Si-C- have recently been sug-
gested to be delocalized along the acyclic framework (s-
conjugation),1,2 cage compounds containing -(Si)n- bridg-
es connected via bridgehead carbons are an attractive sub-
ject of research. We very recently reported that lmax of
1,2,4,5-tetrasilacyclohexane and 2,3,5,6,7,8-hexasilabi-
cyclo[2.2.2]octane exhibited a bathochromic shift as com-
pared with an acyclic standard. This observation suggests
the possibility of three-dimensional s-conjugation in the
cage molecules with disilane moieties.3 To gain further in-
sight into the nature of s-conjugation in polysilacycloal-
kanes, it is intriguing to synthesize bicyclic
polysilacycloalkane 1 containing a trisilane bridge
(Scheme 1). However, efficient synthetic methods for
such compounds are rare,4 and, hence, we felt it necessary
to establish a convenient synthetic method for 1.5 We re-
port here a facile solution for the synthesis of cyclic and
cage compounds having a -Si-Si-Si- unit based on the si-
lylation of 1,5-bis(phenylthio)-1,5-dilithio-2,3,4-trisila-
pentane.
Si
Si
1) LDBB, THF
-78 °C
Si
Si
Si
Si
PhS
PhS
SPh
SPh
(Si) m
PhS
SPh
2) Cl(SiMe2)mCl
Temp.
5
6
Table 1 Synthesis of 6 via reductive lithiation of 5
m
Temp/°C
Run
6
Yield/%
1
1
1
2
3
-98
-78
-30
-78
-78
1
2
3
4
5
47
50
27
69
0
6a
6a
6a
6b
6c
For the dianion reagent with a trisilane unit, we designed
Since separation of the polysilacycloalkanes from the
contaminants like benzenethiol entailed a tedious proce-
dure, 4 was alternatively generated by deprotonation of 7
with a base. Trisilane 7 was prepared by treatment of phe-
nylthiomethyllithium with 1,3-dichloro-1,1,2,2,3,3-hex-
amethyltrisilane in THF at -78 °C in 65% yield. Treatment
of 7 (1 mol) with a base (2.2 mol) under various condi-
tions was followed by silylation with dichlorodimethylsi-
lane as shown in eq. 1.7 The most effective was the use of
s-BuLi at -30 °C, giving 6a in 51% yield, comparable to
the yield via the reductive lithiation protocol.
1,5-bis(phenylthio)-1,5-dilithio-2,3,4-trisilapentane
4,
wherein a phenylthio group could facilitate its generation,
stabilize 4, and be easily reduced by lithium radical anions
to afford requisite dianion reagent 2 via initial polysila-
carbocycle 3.
Si
Si
Si
Si
Si
Si
Si
Si
Si
Cl(Si)nCl
Cl(Si)mCl
(Si)n
(Si)m
(Si)m
R
R
PhS
SPh
Li Li
1
2 (R = Li)
4
3 (R = SPh)
(Si = SiMe2)
Scheme 1
Synlett 1999, No. 11, 1772–1774 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Synthesis of Polysilacage Compounds Containing a Trisilane Bridge
1773
Si
Si
Si
1) base (2.2 mol)
THF, Temp.
Si
Si
Si
Si
PhS
SPh
PhS
SPh
2) Me2SiCl2
-78 °C
7
6a
s-BuLi, -50 °C (38%); s-BuLi, -30 °C (51%); BuLi/t-BuOK, -78 °C (32%)
Equation 1
With 6a in hand, we next studied the generation of a cyclic
dianion reagent and its cyclization toward the cage com-
pounds (Scheme 2). Treatment of 6a with LDBB in THF
at -78 °C effectively produced cyclic 1,3-dianion reagent
2 (m = 1) which, upon quenching with aq. NH4Cl, gave
1,2,3,5-tetrasilacyclohexane 8 in 82% yield. Silylation of
2 (m = 1) with dichlorodimethylsilane or 1,2-dichloro-
1,1,2,2-tetramethyldisilane at -40 °C proceeded success-
fully, giving rise to 2,3,4,6,7-pentasilabicyclo[3.1.1]hep-
tane 98 or 2,3,4,6,7,8-hexasilabicyclo[3.2.1]octane 109 in
62% or 63% yields, respectively. Since 6a was proved to
be a 1 : 1 diastereomeric mixture, the fact that the yields
of 9 and 10 were over 50% indicates that epimerization at
the lithiated carbons was faster than the cyclization at -40
°C.
UV spectra of octamethyltrisilane, 8, 9, and 10
Figure 1
In summary, we have demonstrated that 1,5-bis(phenyl-
thio)-1,5-dilithio-2,3,4-trisilapentane 4 is effective for the
synthesis of such cyclic compounds as 1,2,3,5-tetrasilacy-
clohexane 6a and 1,2,3,5,6-pentasilacycloheptane 6b. In
addition, the synthesis of the cage compounds 2,3,4,6,7-
pentasilabicyclo[3.1.1]heptane 9 and 2,3,4,6,7,8-hexasi-
labicyclo[3.2.1]octane 10 was accomplished using 6a as a
common precursor. Further synthetic and spectroscopic
studies on polysilacycloalkanes are in progress.
Si
Si
Si
Acknowledgement
Si
NH4Cl
Si
Si
Si
PhS
SPh
The present work was partially supported by a Grant-in Aid for Sci-
entific Research on Priority Areas, “The Chemistry of Inter-element
Linkage” (No. 09239102) from the Ministry of Education, Science,
Sports, and Culture, Japan. M. S. acknowledges financial support
from the Nissan Science Foundation and Arakawa Chemical Indu-
stries, Ltd. We thank Shin-Etsu Chemical Co., Ltd., for the ge-
nerous gifts of organosilicon compounds.
-78 °C
Si
6a
82%
LDBB, THF
-78 °C
8
Si
Si
Si
Me2SiCl2
Si
Si
Li
H
Si
Si
Si
Si
-40 °C
References and Notes
Li
H
63%
(1) Reviews of organopolysilanes: a) Kumada, M.; Tamao, K.
Adv. Organomet. Chem. 1968, 6, 19; b) Sakurai, H. J.
Organomet. Chem. 1980, 200, 261; c) Miller, R. D.; Michl, J.
Chem. Rev. 1989, 89, 1359; d) West, R. In Comprehensive
Organometallic Chemistry II; ed.; E. W. Abel; F. G. A. Stone
and G. Wilkinson, Ed.; Pergamon: Oxford, 1995; Vol. 2; p 77.
Reviews of cyclic and cage organopolysilanes: e) Hengge, E.
F. J. Organomet. Chem. Lib. 1979, 9, 261; f) West, R. Pure &
Appl. Chem. 1982, 54, 1041; g) Watanabe, H.; Nagai, Y. In
Organosilicon and Bioorganosilicon Chemistry: Structure,
Bonding, Reactivity and Synthetic Application; Sakurai, H.,
Ed.; Ellis Horwood: Chichester, 1985; p 107; h) Tsumuraya,
T.; Batcheller, S. A.; Masamune, S. Angew. Chem. Int. Ed.
Engl. 1991, 30, 902; i) Sekiguchi, A.; Sakurai, H. Adv.
Organomet. Chem. 1995, 37, 1.
(2) a) Isaka, H.; Teramae, H.; Fujiki, M.; Matsumoto, N.
Macromolecules 1995, 28, 4733; b) Sanji, T.; Hanao, H.;
Sakurai, H. Chem. Lett. 1997, 1121.
(3) Shimizu, M.; Inamasu, N.; Nishihara, Y.; Hiyama, T. Chem.
Lett. 1998, 1145.
(4) a) Sakurai, H.; Kobayashi, Y.; Nakadaira, Y. J. Am. Chem.
Soc. 1971, 93, 5272; b) Sakurai, H.; Kobayashi, Y.;
Nakadaira, Y. J. Am. Chem. Soc. 1974, 96, 2656; c) Ishikawa,
9
2 (m = 1)
Si
H
Cl(SiMe2)2Cl
Si
Si
Si
H
Si
-40 °C
Si
62%
10
Scheme 2
The UV absorption spectra of octamethyltrisilane, 8, 9,
and 10 were measured in cyclohexane (1x10-4 M) at room
temperature. As shown in Figure 1, lmax originated from
a trisilane linkage of octamethyltrisilane (217 nm, e =
7590), 8 (223 nm, e = 6130), 9 (225 nm, e = 5720), and 10
(223 nm, e = 7780) exhibited a bathochromic shift when
the dimensions of the molecular structure increased.
Synlett 1999, No. 11, 1772–1774 ISSN 0936-5214 © Thieme Stuttgart · New York

1774
M. Shimizu et al.
LETTER
M.; Yamanaka, T.; Kumada, M. J. Organomet. Chem. 1985,
292, 167; d) Ando, W.; Sugiyama, M.; Suzuki, T.; Kato, C.;
Arakawa, Y.; Kabe, Y. J. Organomet. Chem. 1995, 499, 99;
e) Giraud, L.; Jenny, T. Organometallics 1998, 17, 4267;
f) Shimizu, M.; Iwakubo, M.; Nishihara, Y.; Hiyama, T.
Tetrahedron Lett. 1998, 39, 3193.
(7) All attempts at treating 5 with t-BuLi (THF, -78 °C), s-BuLi
(THF, -78 °C), s-BuLi (Et2O, -30 °C), BuLi (THF, 0 °C), or
BuLi/t-BuOK (THF, -78 °C) followed by treatment with
Me2SiCl2 turned out futile.
(8) 9: mp 121-122 °C; 1H NMR (200 MHz, CDCl3) d 0.03 (s, 2H),
0.10 (s, 12H), 0.21 (s, 6H), 0.27 (s, 6H), 0.28 (s, 6H); 13
C
(5) Reviews for the synthesis of polysilacarbocycles: a) Barton,
T. J. In Comprehensive Organometallic Chemistry. The
Synthesis, Reactions and Structures of Organometallic
Compounds; Wilkinson, G.; Stone, F. G. A. and Abel, E. W.,
Ed.; Pergamon Press: Oxford, 1982; Vol. 2; p 205; b) Fritz, G.
Angew. Chem. Int. Ed. Engl. 1987, 26, 1111; c) Aylett, B. J.;
Sullivan, A. C. In Comprehensive Organometallic Chemistry
II; Abel, E. W.; Stone, F. G. A. and Wilkinson, G., Ed.;
Pergamon Press: Oxford, 1995; Vol. 2; p 45.
(6) One isomer of the diastereomeric mixture was separable by
recrystallization from hexane. 6a (one isomer): mp 121-122
°C; 1H NMR (200 MHz, CDCl3) d 0.06 (s, 3H), 0.19 (s, 6H),
0.20 (s, 3H), 0.21 (s, 3H), 0.23 (s, 3H), 0.25 (s, 3H), 1.62 (s,
6H), 7.10-7.38 (10H);13C NMR (50 MHz, CDCl3) d -6.9, -6.6,
-3.5, -3.4, -2.5, -1.0, 19.6, 125.2, 128.0, 128.6, 140.1; MS (EI,
70 eV) m/z 481 (M++5, 0.1), 480 (M++4, 0.6), 479 (M++3, 1),
478 (M++2, 3), 477 (M++1, 4), 476 (M+, 8), 399 (8), 367 (93),
309 (5), 257 (100), 73 (29); HRMS calcd for C22H36S2Si4
476.1335; found 476.1322.
NMR (50 MHz, CDCl3) d -5.5, 1.7, 3.6, 5.1, 9.9; MS (EI, 70
eV) m/z 321 (M++5, 0.5), 320 (M++4, 2), 319 (M++3, 5), 318
(M++2, 16), 317 (M++1, 29), 316 (M+, 76), 301 (100), 243
(85), 227 (38), 185 (17), 73 (36); HRMS calcd for C12H32Si5
316.1350; found 316.1334.
(9) 10: mp 152-153°C; 1H NMR (200 MHz, CDCl3) d -0.45 (s,
2H), 0.04 (s, 3H), 0.09 (s, 3H), 0.15 (s, 12H), 0.168 (s, 3H),
0.173 (s, 12H), 0.20 (s, 3H); 13C NMR (50 MHz, CDCl3) d -
6.0, -5.1, 1.3, 1.9, 2.7, 3.5, 4.6, 5.3, 6.2; MS (EI, 70 eV) m/z
379 (M++5, 1), 378 (M++4, 3), 377 (M++3, 10), 376 (M++2,
30), 375 (M++1, 45), 374 (M+, 100), 301 (89), 243 (20), 227
(30), 73 (38); HRMS calcd for C14H38Si6 374.1589; found
374.1587.
Article Identifier:
1437-2096,E;1999,0,11,1772,1774,ftx,en;Y16299ST.pdf
Synlett 1999, No. 11, 1772–1774 ISSN 0936-5214 © Thieme Stuttgart · New York