Gas-phase reaction of free diethylsilylium ions with hexamethyldisiloxane

Article abstract of DOI:10.1007/s11176-005-0303-1

The gas-phase reaction of diethylsilylium ions with hexamethyldisiloxane was studied by the radiochemical procedure. As in reactions with other nucleophiles, the degree of rearrangement of the diethylsilylium ion in the reaction with hexamethyldisiloxane correlates with the condensation energy. In contrast to the reaction of Et₂SiT⁺ with dibutyl ether, in the reaction with hexamethyldisiloxane the labeled substrate is formed, which is due to isomerization of the trimethylsilyl substituent giving rise to a labile hydrogen atom. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11176-005-0303-1

Russian Journal of General Chemistry, Vol. 75, No. 5, 2005, pp. 708 710. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 5, 2005,
pp. 751 753.
Original Russian Text Copyright
2005 by Kochina, Vrazhnov, Sinotova, Ignat’ev.
Gas-Phase Reaction of Free Diethylsilylium Ions
with Hexamethyldisiloxane
T. A. Kochina*, D. V. Vrazhnov*, 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 October 12, 2004
Abstract The gas-phase reaction of diethylsilylium ions with hexamethyldisiloxane was studied by the
radiochemical procedure. As in reactions with other nucleophiles, the degree of rearrangement of the diethyl-
silylium ion in the reaction with hexamethyldisiloxane correlates with the condensation energy. In contrast to
the reaction of Et2SiT+ with dibutyl ether, in the reaction with hexamethyldisiloxane the labeled substrate is
formed, which is due to isomerization of the trimethylsilyl substituent giving rise to a labile hydrogen atom.
Nuclear-chemical generation and properties of
silylium (silicenium) ions are the subject of our long-
term studies. By the nuclear-chemical method we
generated methyl- [1], ethyl- [2, 3], and phenyl-substi-
tuted [4, 5] silylium ions and studied their reactions
with some n- and -electron-donor compounds.
Labeled product
(C2H5)2HSiOSi(CH3)3
(C2H5)H2SiOSi(CH3)3
(CH3)2HSiOSi(CH3)3
(CH3)3SiOSi(CH3)3
(CH3)(C2H5)HSiOSi(CH3)3
(C2H5)3SiH
Relative yield, %
21 3
17 2
34 3
12 2
7 1
One of specific features of this method is the possi-
bility of generating cations with localization of the
positive charge differing from that in the most stable
isomeric form. This allows the rearrangements of
silylium ions to be studied simultaneously with their
ion molecule reactions. In particular, in the reactions
of the diethylsilylium ion with a series of nucleophilic
compounds [6 10], we observed its transformation
into ethyl- and dimethylsilylium ions:
9 1
siloxane are various disiloxanes. Their total yield is
91%. As in reactions with other nucleophiles [6 10],
the diethylsilyl group is present in the reaction prod-
ucts not only in the initial form but also in rearranged
forms. Taking into account the substrate structure, we
can assume that, as in the reactions of carbenium and
silylium ions with organic ethers, this reaction occurs
via formation of an intermediate oxonium complex
arising from interaction of the vacant p orbital of the
cation with the lone electron pair of the oxygen atom.
(C2H5)2Si+H
(C2H5)Si+H2
(CH3)2Si+H.
C2H4
In [8] we showed that the activity of nucleophilic
compounds inducing this rearrangement varies in the
order Bu2O < BuOH < C6H6, determined by the life-
time of the long-lived complex, which, in turn, de-
pends on the excitation energy of the intermediate
complex and on the possibilities of its transfer.
(C2H5)2Si+T + (CH3)3SiOSi(CH3)3
+
(CH3)3SiOSi(CH3)3
(C2H5)2SiT
Proceeding with studies of ion molecule reactions
of free diethylsilylium ions with nucleophilic com-
pounds, in particular, with oxygen-containing com-
pounds [6 8], we chose hexamethyldisiloxane as
substrate. The labeled products of the reaction of
diethylsilylium ions (C2H5)2Si+T with hexamethyldi-
siloxane in the gas phase are listed below.
The exothermic effect of the condensation in the
model system SiH3+ (SiH3)2O, according to our
B3LYP/6-31G* estimates, is 65.0 kcal mol . The
1
silylium ion itself, generated nuclear-chemically, ap-
pears in a vibrationally excited state [11]; the excitation
energy of the diethylsilylium ion, calculated by the
1
same method, is 21.6 kcal mol . The lifetime of the
9
These results show that the major products of the
reaction of diethylsilylium ions with hexamethyldi-
ion molecule complex is sufficiently long ( 10 s in
the gas phase) for the excitation energy to be redistri-
1070-3632/05/7505-0708 2005 Pleiades Publishing, Inc.

GAS-PHASE REACTION OF FREE DIETHYLSILYLIUM IONS
709
buted over all the possible degrees of freedom in this
complex, including the rearrangement of structural
units, i.e., isomerization.
ties for isomerization of structural units and hence for
consumption of the excitation energy. Both factors
increase the lifetime of the intermediate oxonium
complex with hexamethyldisiloxane, compared to
dibutyl ether.
It is interesting that the products of the reaction of
diethylsilylium ions with hexamethyldisiloxane con-
tain the labeled substrate; furthermore, it is present not
only in the initial but also in the isomerized chemical
form (total yield 19%).
The observed correlation between the degree of
rearrangement of the diethylsilylium ion and the con-
densation energy calculated for the model systems [8]
appears to be valid for hexamethyldisiloxane also.
In particular, the energy of condensation of the H3Si+
cation with siloxane (H3Si)2O is intermediate between
the energies of its condensation with benzene and
According to our previous study, the reaction of
diethylsilylium ions with dibutyl ether [8] yields no
labeled substrate, in contrast, e.g., to the reactions
with alcohols [6, 7], apparently because of the absence
of labile hydrogen atoms in the ether. Hexamethyldi-
siloxane, being an organosilicon ether, contains no
labile hydrogen atoms either; therefore, at first glance,
formation of the labeled substrate in this case is as
improbable as with dibutyl ether.
1
methanol (kcal mol ):
C6H6 (47.6) < (H3Si)2O (65.0) < MeOH (69.7)
< Me2O (72.7).
Hexamethyldisiloxane also occupies a consistent
place in the series of nucleophilic compounds induc-
ing the rearrangement of diethylsilicenium ions (in
parentheses is the content of rearranged forms of the
diethylsilylium ion):
To understand why we did detect labeled substrate
with hexamethyldisiloxane, let us consider the calcula-
tion results for model oxonium complexes formed by
association of the methyl cation with dimethyl ether
and siloxane [12]. According to these calculations, the
ether methyl group in the complex (CH3)3O+ retains
the tetrahedral structure, whereas the silyl group in the
complex CH3(SiH3)2O+ is virtually planar as in the
cation. This allows the silyl group in the intermediate
oxonium complex to be considered as a cation. Appar-
ently, the trimethylsilyl substituent in hexamethyldi-
siloxane and oxonium complex is sufficiently ionic
to isomerize into (C2H5)(CH3)SiH provided that its
energy is sufficient. We observed such processes in
the reaction of methyl cation CH+3 with hexamethyl-
disilazane [13]. The possible isomerization of the
(CH3)3Si+ ion into the (CH3)(C2H5)SiH+ ion contain-
ing a labile hydrogen atom at the silicon atom makes
possible the isotope exchange between the hydrogen
atom of the methylethylsilyl substituent in the sub-
strate and the tritium atom of the diethylsilylium ion,
with the formation of the labeled substrate.
Bu2O (10%) < BuOH (38%) < (Me3Si)2O (72%)
< C6H6 (87%).
EXPERIMENTAL
The reaction products were analyzed radiochro-
matographically with a Tsvet-110 chromatograph
equipped with a tritium -radiation detector; a 3000
2-mm column was packed with 15% SE-30/Chezasorb
N-AW. The flow rates of the carrier gas (He) and
1
quenching gas (CH4/Ar) were 15 and 10 ml min ,
respectively. The column temperature was 80 C. The
labeled reaction products were identified by compar-
ing their retention times with those of reference sub-
stances under identical chromatographic conditions.
The relative yield of the detected labeled products was
determined as the ratio of the activity of a given com-
pound to the total activity of all the detected products.
Free diethylsilylium ions were generated nuclear-
chemically [14]:
Thus, although hexamethyldisiloxane, like dibutyl
ether, contains no labile hydrogen atoms, such an
atom appears upon isomerization of the trimethylsilyl
group.
(C2H5)2SiT2
(C2H5)2TSi+ + He.
It should be noted that, in the case of hexamethyl-
disiloxane, isomerization occurs to a much greater
extent ( 70%) than in the case of dibutyl ether (10%)
[8]. This is due, on the one hand, to different conden-
The procedure for preparing diethylsilane doubly
labeled with tritium was described in [3]. 1,1,1-Tri-
methyl-3-ethyl, 1,1,1,3,3-pentamethyl-, 1,1-diethyl-
3,3,3-trimethyl, 1,3,3,3-tetramethyl-1-ethyl-, and hex-
amethyldisiloxane used as references were prepared
by cohydrolysis of appropriate chlorosilanes in the
presence of NaOH [15].
1
sation energies: 72.7 kcal mol with dibutyl ether [8]
1
and 65.0 kcal mol with hexamethyldisiloxane. On
the other hand, as noted above, in the oxonium com-
plex formed by the reaction of the diethylsilylium ion
with hexamethyldisiloxane, there are more possibili-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 5 2005

710
KOCHINA et al.
Gas-phase reactions of diethylsilylium ions with
6. Kochina, T.A., Vrazhnov, D.V., Ignat’ev, I.S., and
Sinotova, E.N., Zh. Obshch. Khim., 2003, vol. 73,
no. 1, p. 65.
hexamethyldisiloxane were studied in 20-ml molyb-
denum glass ampules. Prior to filling, the ampules
were degassed with heating and evacuated. The dieth-
ylsilane activity in each ampule was 0.001 Ci, and the
hexamethyldisiloxane vapor pressure, 10 mm Hg.
7. Kochina, T.A., Vrazhnov, D.V., and Ignatyev, I.S.,
J. Organomet. Chem., 2002, vol. 656, nos. 1 2, p. 258.
8. Kochina, T.A., Vrazhnov, D.V., Sinotova, E.N., and
Shishigin, E.A., Zh. Obshch. Khim., 2004, vol. 74,
no. 2, p. 219.
The products were accumulated at room tempera-
ture in the dark for 2 months.
9. Kochina, T.A., Vrazhnov, D.V., Ignat’ev, I.S., Ne-
fedov, V.D., and Sinotova, E.N., Zh. Obshch. Khim.,
1999, vol. 69, no. 6, p. 942.
REFERENCES
1. Nefedov, V.D., Kharitonov, N.P., Sinotova, E.N.,
Kochina, T.A., and Balakin, I.M., Zh. Obshch. Khim.,
1980, vol. 50, no. 11, p. 2499.
10. Kochina, T.A., Vrazhnov, D.V., and Ignatyev, I.S.,
J. Organomet. Chem., 1999, vol. 586, nos. 1 2, p. 241.
2. Nefedov, V.D., Kochina, T.A., Shchukin, E.A., and
Sinotova, E.N., Radiokhimiya, 1998, vol. 40, no. 5,
p. 445.
11. Shchegolev, B.F. and Kochina, T.A., Main Group
Chem., 1998, vol. 2, no. 3, p. 203.
12. Ignatyev, I.S. and Kochina, T.A., J. Mol. Struct.
(THEOCHEM), 1991, vol. 236, p. 249.
3. Kochina, T.A., Vrazhnov, D.V., Nefedov, V.D., and
Sinotova, E.N., Zh. Obshch. Khim., 1998, vol. 68,
no. 6, p. 967.
13. Ignat’ev, I.S., Kochina, T.A., Nefedov, V.D., Sinoto-
va, E.N., and Vrazhnov, D.V., Zh. Obshch. Khim.,
1997, vol. 67, no. 6, p. 962.
4. Shishigin, E.A., Avrorin, V.V., Kochina, T.A., and
Sinotova, E.N., Zh. Obshch. Khim., 2005, vol. 75,
no. 1, p. 165.
14. Nefedov, V.D., Kochina, T.A., and Sinotova, E.N.,
Usp. Khim., 1986, vol. 55, no. 5, p. 794.
5. Kochina, T.A., Vrazhnov, D.V., Sinotova, E.N., Av-
rorin, V.V., Katsap, M.Yu., and Mykhov, Yu.V.,
Zh. Obshch. Khim., 2002, vol. 72, no. 8, p. 1303.
15. Okawara, R. and Sakijama, M., Bull. Chem. Soc. Jpn.,
1956, vol. 29, no. 5, p. 547.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 5 2005