Interconversion of silylphenyl and phenylsilyl cations in the reaction with benzene

Article abstract of DOI:10.1007/s11176-005-0434-4

The possibility for positive charge migration in SiC₆H ₇ ⁺ cation from carbon to silicon (or vice versa) was studied by the radiochemical method. Silylphenyl cation with initial charge localization on the carbon atom is transformed into phenylsilylium ion where the positive charge is localized on the silicon atom. No migration of positive charge from the silicon atom to carbon occurs. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11176-005-0434-4

Russian Journal of General Chemistry, Vol. 75, No. 9, 2005, pp. 1393 1394. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 9, 2005,
pp. 1465 1466.
Original Russian Text Copyright
2005 by Shishigin, Avrorin, Kochina, Sinotova, Ignat’ev.
Interconversion of Silylphenyl and Phenylsilyl Cations
in the Reaction with Benzene
E. A. Shishigin*, V. V. Avrorin**, T. A. Kochina*, E. N. Sinotova**, and I. S. Ignat’ev**
*
Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, St. Petersburg, Russia
*
* St. Petersburg State University, St. Petersburg, Russia
Received December 21, 2004
+
Abstract The possibility for positive charge migration in SiC H cation from carbon to silicon (or vice
6
7
versa) was studied by the radiochemical method. Silylphenyl cation with initial charge localization on the
carbon atom is transformed into phenylsilylium ion where the positive charge is localized on the silicon atom.
No migration of positive charge from the silicon atom to carbon occurs.
1
Carbenium and silylium ions have long been the
subjects of our studies [1 6]. These reactive species
are formed via radioactive -decay of tritium, fol-
lowed by transformations within hydrocarbon and
substituted silane molecules. Unique capabilities of
this nuclear-chemical method make it possible to
study not only reactions of cations with molecules of
compounds belonging to various classes, but also their
isomerization processes. For example, while studying
ion molecule reactions of ethyl- and diethylsilylium
ions with nucleophiles, we observed isomerization of
ethylsilylium ions [4, 5] into dimethylsilylium, and of
diethylsilylium, first into monoethyl- and then into
dimethylsilylium ions [3]. Here, the transformations
of diethylsilylium ions were accompanied by elimina-
tion of ethylene. Furthermore, taking into account that
the method in hand generates ions in which the
primary charge localization does not correspond to
their most stable isomer, we can monitor migration of
the cationic center from one atom to an atom of a
different element, thus producing a cation of a dif-
ferent nature [7]. For instance, migration of positive
charge from the carbon atom in a silyl-substituted
carbenium ion to the silicon atom could give rise to
silylium ion. The relative stability of isomeric cationic
species may be estimated from the relative yields of
products of the corresponding ion molecule inter-
actions.
8.3 kcal mol , phenylsilylium ion being more stable
1
than its carbon-centered isomer by 43.6 kcal mol .
On the other hand, quantum-chemical estimation of
the energy gain due to geometry relaxation of the
benzene ring upon formation of cation gave a value of
4.5 kcal mol 1 which considerably exceeds the
3
barrier to isomerization, so that the latter becomes
feasible.
In fact, radiochromatographic analysis of the reac-
tion of tritium-labeled silylphenyl cation H SiC H T
with benzene detected diphenylsilane as the only
product. This indicates almost complete transforma-
tion of silylphenyl cation into phenylsilyl cation
+
3
6
3
+
C H TSi H .
6
4
2
+
+
H SiC H T
3
C H TSi H
6 4 2
6
3
benzene
C H TSiH C H .
6
4
2 6 5
The reaction with benzene of phenylsilylium ion
generated from phenylsilane labeled with tritium at
the Si H bond also afforded diphenylsilane as the
only product. No compounds which could result from
reaction of carbon-centered H SiC H ions with
benzene were detected, i.e., migration of the positive
charge from the silicon atom to benzene ring does not
occur.
+
4
3
6
In the present work we examined the relative sta-
+
Thus, both theoretical and experimental data con-
vincingly indicate that the cationic center migrates
from the carbon atom to silicon, which implies higher
thermodynamic stability of phenylsilylium ion C6H5
bility of two isomers of the [SiC H ] ion, silyl-
6
7
+
substituted phenyl cation H SiC H and phenylsily-
3
6
4
+
lium ion C H SiH , in the gas-phase reaction with
6
5
2
benzene. According to the results of B3LYP-6-31G**
quantum-chemical calculations, the barrier to iso-
+
+
SiH relative to its isomer H SiC H with charge
2
3
6
4
+
4
+
2
merization of H SiC H into C H SiH is as low as
localization on the benzene ring.
3
6
6
5
1
070-3632/05/7509-1393 2005 Pleiades Publishing, Inc.

1394
SHISHIGIN et al.
EXPERIMENTAL
Products of ion molecule reactions between silyl-
REFERENCES
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1
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1
dov, V.D., and Sinotova, E.N., Zh. Obshch. Khim.,
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1
999, vol. 69, no. 6, p. 905.
tium-labeled products were identified by comparing
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4
5
. Kochina, T.A., Shchukin, E.V., Nefedov, V.D., Sino-
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+
3
0
(
1)
C H T SiH
SiH (C H T) + He ,
3 6 3
6
3
2
3
+
3
0
C H SiT
C H Si T + He .
6 5 2
(2)
6
3
3
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no. 2, p. 193.
The procedure for the synthesis of tritium-labeled
phenylsilane H SiC H T was reported in [10], and
phenylsilane labeled with tritium at the Si H bond
was prepared as described in [11].
2
6
3 2
7
. Kochina, T.A., Vrazhnov, D.V., Sinotova, E.N.,
Avrorin, V.V., Katsap, M.Yu., and Mykhov, Yu.V.,
Zh. Obshch. Khim., 2002, vol. 72, no. 8, p. 1303.
Benzene of chemically pure grade was used as
substrate. It was thoroughly dried over metallic
sodium and distilled prior to use; the purity was
checked by gas chromatography. Diphenylsilane used
as reference was prepared by reduction of dichloro-
diphenyl)silane with lithium tetrahydridoaluminate
12]. Biphenylylsilane (as a product expected to be
8. Nefedov, V.D., Sinotova, E.N., Akulov, G.P., and
Syreishchikov, V.A., Radiokhimiya, 1969, vol. 10,
no. 5, p. 600.
9
. Nefedov, V.D., Kochina, T.A., and Sinotova, E.N.,
(
[
Usp. Khim., 1981, vol. 50, no. 5, p. 794.
+
7
formed by reaction of carbon-centered SiC H ion
6
1
1
1
0. Shishigin, E.A., Avrorin, V.V., Kochina, T.A., and
Sinotova, E.N., Zh. Obshch. Khim., 2004, vol. 74,
no. 6, p. 1053.
with benzene) was synthesized from p-bromobiphenyl
and trichlorosilane according to the procedure descri-
bed in [13], followed by reduction of intermediate
chloro derivative with lithium tetrahydridoaluminate
1. Shishigin, E.A., Avrorin, V.V., Kochina, T.A., and
Sinotova, E.N., Zh. Obshch. Khim., 2005, vol. 75,
no. 1, p. 165.
[12].
Gas-phase reactions were carried out in 20-ml
spherical ampules made of molybdenum glass, ben-
zene vapor pressure in each ampule was 10 mm, and
the activity of tritium-labeled phenylsilane was 0.001
Ci. Products were accumulated at room temperature
in the dark over a period of 2 4 months.
2. Westermark, H., Acta Chem. Scand., 1954, vol. 8,
no. 10, p. 1830.
13. Goodman, L. and Konstam, A., J. Am. Chem. Soc.,
1965, vol. 87, no. 5, p. 1012.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 9 2005