Unexpected reaction of an overcrowded 9,10-dihydroanthrylchlorosilane leading to the formation of a dibenzo-7-silanorbornadiene derivative

Article abstract of DOI:10.1246/cl.2007.588

A novel and simple synthetic route for a dibenzo-7-silanorbornadiene derivative has been developed. The extremely hindered chlorosilane bearing 9,10-dihydroanthryl group, TbtRSiHCl (Tbt = 2,4,6-tris[bis(trimethylsilyl) methyl]phenyl, R = 9,10-dihydroanthryl) could be quantitatively converted into the corresponding dibenzo-7-silanorbornadiene 1 by the reaction with LDA. The molecular structure of 1 was revealed by the spectroscopic and X-ray crystallographic analyses. Copyright

Full text of DOI:10.1246/cl.2007.588

588
Chemistry Letters Vol.36, No.5 (2007)
Unexpected Reaction of an Overcrowded 9,10-Dihydroanthrylchlorosilane
Leading to the Formation of a Dibenzo-7-silanorbornadiene Derivative
Takahiro Sasamori, Shuhei Ozaki, and Norihiro Tokitohꢀ
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011
(Received January 17, 2007; CL-070060; E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp)
A novel and simple synthetic route for a dibenzo-7-silanor-
pound by the reaction of TbtSiHCl2 with 9,10-dihydroanthryl-
bornadiene derivative has been developed. The extremely
hindered chlorosilane bearing 9,10-dihydroanthryl group,
TbtRSiHCl (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl,
R = 9,10-dihydroanthryl) could be quantitatively converted into
the corresponding dibenzo-7-silanorbornadiene 1 by the reaction
with LDA. The molecular structure of 1 was revealed by the
spectroscopic and X-ray crystallographic analyses.
lithium, which was generated by the reported procedure.8 Treat-
ment of 2 with LDA (1.5 equiv.) in THF at ꢁ40 ꢂC afforded di-
benzo-7-hydro-7-silanorbornadiene 1 quantitatively (Scheme 1).
The structure of 1 was definitely determined based on the spec-
troscopic and X-ray crystallographic analyses.9 In the 29Si and
1H NMR spectra of 1, characteristic signals corresponding to
the central silicon atom and the hydrogen atom at the 7-position
are observed at ꢀSi ¼ 35:6 and ꢀH ¼ 4:83 with the coupling con-
stant 1JSiH ¼ 207 Hz. These spectral features of 1 are similar to
those of previously reported 7-hydro-7-silanorbornadienes.1b,5
The unexpected formation of 1 in this reaction should be
worthy of note as a novel synthetic route for a dibenzo-7-silanor-
bornadiene. It can be considered that 1 is formed by the initial
deprotonation of Ha (Scheme 2) of 2 followed by the simple
SN2 reaction at the central silicon atom (path A). If Hb proton
was abstracted in the initial stage of the reaction of 2 with
LDA, silene 3 would be generated (path B). Theoretical calcula-
tions for the model compounds, 5–8, which have a 2,6-dimethyl-
phenyl (Dmp) group instead of a Tbt group, indicate that the Hb
proton has slightly higher acidity than that of the Ha proton prob-
ably due to the ꢁ effect of a silicon atom (6b is more stable than
6a by ca. 1.3 kcal/mol).10 In addition, theoretical calculations
for the model reactions, i.e., the reaction of 5 with NH2Li leading
to the formation of 7-silanorbornadiene 7 or hydrosilene 8 to-
gether with NH3 and LiCl, indicate that the formation of 7 is
an exothermic reaction of ca. 4 kcal/mol, but that of 8 is an en-
dothermic reaction of 16 kcal/mol. That is, the heat of formation
of 7 should sufficiently make up for the unfavorable deprotona-
tion at 10-position (Ha) of 2. Taking into consideration of
these results, the considerable steric hindrance around the central
silicon atom should be an indispensable qualification for the gen-
eration of a dibenzo-7-silanorbornadiene to prevent deprotona-
tion of the ꢁ proton of the central silicon atom. It can be conclud-
ed that the quantitative formation of dibenzo-7-silanorborna-
diene 1 in the reaction of 2 with LDA is most likely due to the
kinetic effect of the extremely bulky Tbt group.11
The chemistry of 7-silanorbornadienes has attracted much
attention because of their strained structures and unique proper-
ties.1–3 In addition, 7-silanorbornadienes are important species
as a precursor of a silylene, since they are known to undergo
thermal and photochemical dissociation to give the correspond-
ing silylene and aromatic counterpart via retro [1 + 4] peri
cyclic reaction.3 Particularly, a 7-hydro-7-silanorbornadiene de-
rivative has been of great interest as a precursor of not only a hy-
drosilylene,4 which is an important but unprecedented species,
but also a functionalized 7-silanorbornadiene derivative at 7-
position.1b,5 However, the synthesis of stable 7-silanorborna-
dienes are somewhat troublesome, since they should be conven-
tionally achieved by the use of a highly reactive species, such as
a transient silylene or a benzyne, with an excess amount of trap-
ping reagents such as anthracene derivatives or siloles.6 On the
other hand, we have been interested in the synthesis and isolation
of unprecedented low-coordinated species of heavier group 14
elements by taking advantage of kinetic stabilization. Recently,
we have chosen the kinetically stabilized hydrosilene 3 as a
target molecule7 and examined the dehydrochlorination of the
overcrowded chlorosilane 2 substituted by an extremely bulky
Tbt group. Unexpectedly, however, the reaction of 2 with
lithium diisopropylamide (LDA) as a base resulted in the quan-
titative formation of dibenzo-7-silanorbornadiene 1, the structur-
al isomer of 3. We report here the synthesis and structural char-
acterization of the new dibenzo-7-silanorbornadiene derivative 1
together with the mechanistic elucidation using theoretical cal-
culations for the unexpected reaction of 2 with LDA giving 1.
Chlorosilane 2 was prepared as a stable crystalline com-
H
H
Ar
Si
path A
−Ha
Ar Si
Ha
−Cl
−Cl
Hb
Cl
H
H
Ha
4a: Ar =Tbt
6a: Ar = Dmp
1: Ar =Tbt
7: Ar = Dmp
Me3Si
Me3Si
SiMe3
SiMe3
Si
Ar Si
Tbt
Ha
H
Tbt Si
Cl
Hb
Cl
H
−Hb
3
H
Ha
Ha
SiMe3
SiMe3
Tbt group
H
Tbt
Ar Si
Cl
Si
LDA
THF
−40 °C
2: Ar =Tbt
5: Ar = Dmp
H
path B
Ar
Si
2
4b: Ar =Tbt
6b: Ar = Dmp
3: Ar =Tbt
8: Ar = Dmp
1
Scheme 1.
Scheme 2.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.5 (2007)
589
References and Notes
Ar
φ1
H
(a)
(b)
1
For recent topics on 7-silanorbornadienes, see: a) O. S. Volkova,
M. B. Taraban, V. F. Plyusnin, T. V. Leshina, M. P. Egorov,
O. M. Nefedov, J. Phys. Chem. A 2003, 107, 4001. b) J. Schuppan,
Si
φ2
θ2
θ1
B. Herrschaft, T. Muller, Organometallics 2001, 20, 4584. c)
¨
θ3
M. B. Taraban, A. I. Kruppa, O. S. Volkova, I. V. Ovcharenko,
R. N. Musin, T. V. Leshina, E. C. Korolenko, K. Kitahara, J. Phys.
Chem. A 2000, 104, 1811. d) M. B. Taraban, O. S. Volkova, A. I.
Kruppa, V. F. Plyusnin, V. P. Grivin, Y. V. Ivanov, T. V. Leshina,
M. P. Egorov, O. M. Nefedov, J. Organomet. Chem. 1998, 566, 73.
e) I. M. Magin, P. A. Purtov, A. I. Kruppa, T. V. Leshina, J. Phys.
Chem. A 2005, 109, 7396.
a) M. N. Paddon-Row, S. S. Wong, K. D. Jordan, J. Am. Chem.
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a) T. Sanji, T. Mori, H. Sakurai, J. Organomet. Chem. 2006, 691,
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Chem. Soc. 1986, 108, 4241. c) M. B. Taraban, O. S. Volkova,
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Nefedov, J. Phys. Chem. A 2003, 107, 4096. d) P. P. Gasper,
R. West, in The Chemistry of Organic Silicon Compounds, ed.
by Z. Rappoport, Y. Apeloig, Wiley, Chichester, 1998, Vol. 2,
pp. 2463–2568. e) M. Kako, S. Oba, R. Uesugi, S. Sumiishi, Y.
Nakadaira, K. Tanaka, T. Takada, J. Chem. Soc., Perkin Trans. 2
1997, 1251.
Transition-metal complexes of a hydrosilylene have been reported,
see: a) B. V. Mork, T. D. Tilley, A. J. Schultz, J. A. Cowan, J. Am.
Chem. Soc. 2004, 126, 10428. b) T. Watanabe, H. Hashimoto, H.
Tobita, Angew. Chem., Int. Ed. 2004, 43, 218. c) T. Watanabe, H.
Hashimoto, H. Tobita, J. Am. Chem. Soc. 2006, 128, 2176.
A. Kawachi, M. Okimoto, Y. Yamamoto, Chem. Lett. 2005, 34,
960.
For examples, see: a) W. C. Joo, J. H. Hong, S. B. Choi, H. E. Son,
C. H. Kim, J. Organomet. Chem. 1990, 391, 27. b) Y. L. Pan,
J. H. Hong, S. B. Choi, P. Boudjouk, Organometallics 1997, 16,
1445. c) H. Sakurai, H. Sakaba, Y. Nakadaira, J. Am. Chem. Soc.
1982, 104, 6156. d) H. Sakurai, K. Oharu, Y. Nakadaira, Chem.
Lett. 1986, 1797.
A number of theoretical studies and trapping reactions of transient
hydrosilenes have been reported. For examples, see: a) H. Ottosson,
Chem.—Eur. J. 2003, 9, 4144. b) W. J. Leigh, R. Boukherroub,
C. Kerst, J. Am. Chem. Soc. 1998, 120, 9504. c) C. J. Bradaric,
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a) P. W. Rabideau, D. M. Wetzel, C. A. Husted, J. R. Lawrence,
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All the new compounds here obtained showed satisfactory spectral
data. Experimental procedures, compound data, and crystal data of
1 (CCDC 620514) are described in Supporting Information, which
is available electronically on the CSJ Journal web site; http://
www.csj.jp/journals/chem-lett/.
Ar = Tbt
(obsd.)
Ar = Dmp
(calcd.)
C15
Si1
C8
C9
H1
θ
θ
θ
φ
φ
1 = 55.6°
2 = 64.7°
3 = 59.8°
1 = 139.0°
2 = 114.9°
θ
1 = 58.7°
2 = 61.6°
3 = 59.6°
1 = 131.6°
2 = 119.0°
θ
θ
φ
φ
C7
C1
C2
C10
2
3
Figure 1. (a) ORTEP drawing of 1 (50% probability). The
hydrogen atoms of the Tbt group were omitted for clarity. (b)
Depiction of dibenzo-7-silanorbornadienes 1 (observed) and 7
(calculated) from C1–C8 axis.
H
Tbt
h
ν,
Si
Tbt
(excess)
C6D6
Si
+
or
H
1
11
Tbt
h
ν
Si
:
[1+4]
cycloaddition
4
H
10
Scheme 3.
5
6
Although a 7-hydro-7-silanorbornadiene skeleton has been
already structurally characterized so far,6 we found unique
structural features of 1. The structural parameters of 1 revealed
by X-ray crystallographic (Figure 1a) analysis are also similar to
those of the previously reported dibenzo-7-silanorbornadiene
(9),6d,12 which has two Dmp groups on the silicon atom at 7-
position. Interestingly, dihedral angles ꢂ1 (Figure 1b) of the di-
benzo-7-silanorbornadiene moiety of 1 is smaller than ꢂ2 despite
of the steric repulsion due to the very bulky Tbt group, while 9
shows a symmetric structure as expected (the corresponding ꢂ1
and ꢂ2 are almost the same values in 9). Although the reason
is unclear at present, similar tendencies are found in the theoret-
ically optimized structural parameters of the less hindered model
compound 7,10 indicating that the smaller ꢂ1 values and the larg-
er ꢂ2 values observed in 1 may not be due to the steric reason but
to somewhat attractive interaction between the Tbt group and the
aromatic moiety. On the other hand, ꢃ1 is larger than ꢃ2 in 1
(Figure 1b) most likely due to the steric repulsion between the
bulky Tbt group and the dibenzo-7-silanorbornadiene moiety.
When the C6D6 solution of 1 was irradiated by the medium-
or low-pressure Hg lamp in the presence of 2,3-dimethyl-1,3-
butadiene (DMB) at room temperature, [1 + 4]-cycloadduct
11 was obtained in 36 or 50% yield, respectively, together with
anthracene and/or anthracene-photodimer, indicating the photo-
chemical generation of hydrosilylene 10 (Scheme 3).9,13 Thus,
7-hydro-dibenzo-7-silanorbornadiene 1 was proved to be a good
precursor of the corresponding hydrosilylene 10.
7
8
9
10 Theoretical calculations were performed at the B3PW91/6-
31+G(d) level without ZPE corrections. It is difficult to take
the solvent effect into the calculations, since the calculations
containing solvent molecules should be highly complicated.
11 Reaction of a less hindered chlorosilane, (9,10-dihydroanthryl)-
MesSiHCl, with LDA under the same conditions afforded a
complicated mixture containing no dibenzo-7-silanorbornadiene
derivative.
12 M. Kako, M. Mori, K. Hatakenaka, S. Kakuma, Y. Nakadaira, M.
Yasui, F. Iwasaki, Tetrahedron 1997, 53, 1265.
13 a) A. Kawachi and Y. Yamamoto have recently reported the trap-
ping reaction of the hydrosilylene, Mes(H)Si:, generated by the
photoirradiation of the corresponding 7-hydro-dibenzo-7-silanor-
bornadiene, see: The 86th Annual Meeting of the Chemical Society
of Japan, Abstr., No. 3F4-12, 2006. b) Heating of the toluene-d8 so-
lution of 1 at 150 ꢂC in the presence of DMB resulted in no reaction.
c) Relatively low yields of 11 under these conditions are probably
due to the over-photoreaction of resulting 11.
This work was supported by Grant-in-Aid for Scientific
Research (Nos. 17GS0207 and 18750030) and the 21st Century
COE Program, Kyoto University Alliance for Chemistry, from
Ministry of Education, Culture, Sports, Science and Technology,
Japan.
Published on the web (Advance View) March 29, 2007; doi:10.1246/cl.2007.588