Gas-phase ion-molecular reactions of free ethylsilylium ions with ethylene

Article abstract of DOI:10.1023/A:1012339103054

Gas-phase reaction of ethylsilylium ions with ethylene was studied by the radiochemical method. The reaction occurs via excited adduct C₄H₁₁Si⁺ which structurally is a complex of ethylsilylium cation with ethylene. The lifetime of this complex is long enough for the isotope exchange between tritium atoms of the cation and a proton of ethylene, as well as the isomerization of ethylsilylium cation to dimethylsilylium to occur. Decomposition of the complex results in predominant formation of labeled ethylene.

Full text of DOI:10.1023/A:1012339103054

Russian Journal of General Chemistry, Vol. 71, No. 2, 2001, pp. 206 209. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 2, 2001,
pp. 232 236.
Original Russian Text Copyright
2001 by Shchukin, Kochina, Sinotova, Ignat’ev.
Gas-Phase Ion-Molecular Reactions of Free Ethylsilylium Ions
with Ethylene
E. V. Shchukin, T. A. Kochina, E. N. Sinotova, and I. S. Ignat’ev
Institute of Silicate Chemistry, Russian Academy of Sciences, St. Petersburg, Russia
St. Petersburg State University, St. Petersburg, Russia
Received November 11, 1999
Abstract Gas-phase reaction of ethylsilylium ions with ethylene was studied by the radiochemical method.
The reaction occurs via excited adduct C4H11Si+ which structurally is a complex of ethylsilylium cation with
ethylene. The lifetime of this complex is long enough for the isotope exchange between tritium atoms of the
cation and a proton of ethylene, as well as the isomerization of ethylsilylium cation to dimethylsilylium to
occur. Decomposition of the complex results in predominant formation of labeled ethylene.
Earlier we performed a radiochemical study of ion-
molecular reactions of ethylsilylium cations with re-
presentatives of various classes of compounds (al-
cohols, benzene, silylamine) [1 3]. These studies
showed that the reactions involve a highly excited
ion-neutral complex in which isotope-exchange and
isomerization reactions occur.
exclusive formation of -silyl-substituted carbenium
ions, even though -silyl-substituted carbenium ions
are also formally not improbable to form.
Me3SiCH=CH2 + BH+
Me3Si(CH2CH2)+ + B, (1)
BH+ = C2H5 , H3O+, Me3C+.
+
To extend the range of objects studied, we turned
The high stability of -silyl-substituted carbenium
to ethylene as the simplest unsaturated compound. ions (and, consequently, their preferred formation) can
The choice of this compound was additionally motiv-
ated by the expectation to observe in the reaction of
be explained by delocalization of the positive charge
on the carbenoid center due to p
hyperconjuga-
ethylsilylium cation with ethylene the so-called
-
tion. This brings up the question oCfStihe structure of
the -silyl-substituted carbenium ion. It can have
either a cyclic (bridged) structure I, where silicon
makes use of its ability to extend coordination, or an
acyclic structure II [9 11].
effect, one of the most remarkable phenomena in
organosilicon chemistry. As known, this effect con-
sists in strong stabilization of carbenium ions by silyl
substituents to the carbenoid center [4 12]. It will
be remembered that -silyl substituents strongly de-
stabilize carbenium ions [6, 9, 10]. These problems
attract much researcher’s attention, since silyl-sub-
stituted carbenium ions are common reaction inter-
mediates in organosilicon chemistry.
+
SiR3
SiR3
+
C C
C
C
I
II
The
and
effects have first been revealed in
To decide between these structures is very difficult.
It, however, should be noted that the effect is the
stronger the stroger is p-
condensed phase by increased or decreased rate con-
stants of organosilicon compounds compared with
their hydrocarbon analogs [4 7]. But the strongest
evidence for these effects has been obtained in
quantum-chemical studies on - and -silyl-substi-
hyperconjugation. The
CSi
latter effect is a maximum when the vacant 2p orbital
of the carbenoid atom and the bonding orbital
CSi
tuted carbenium ions, as well as by mass spectrometry. are coplanar. The enthalpy of association of Me3Si+
Apeloig et al. [9] in their study on rearrangements of
isomeric C5H13Si+ cations showed that an -silyl-
substituted carbenium ion fully isomerizes into a
silylium cation (80%) and a -substituted carbenium
ion (20%). Hajdasz and Scuires [10] found that proto-
nation of trimethylvinylsilane [reaction (1)] results in
with ethylene is 23.6 kcal/mol [13], which cor-
responds to the estimate in [9] for the enthalpy of
formation of the bridged form of the -silyl-substi-
tuted carbenium ion. According to [9, 14], bridged
H3Si(CH2CH2)+ and Me3Si(CH2CH2)+ cations are
lower on the potential energy surface than their open-
1070-3632/01/7102-0206$25.00 2001 MAIK Nauka/Interperiodica

GAS-PHASE ION-MOLECULAR REACTIONS OF FREE ETHYLSILYLIUM IONS
207
acyclic isomers. By contrast, the calculations in [15]
for [H3SiCH2CHCH3]+ resulted in a 4 kcal/mol
preference of the acyclic form over bridged.
Relative yields of tritiated products of the gas-phase reac-
tion of the ethylsilylium ions (C2H5)Si+T2 with ethylene
Tritiated product
Relative yield, %
Chiavarino et al. [13] showed that ion-molecular
reactions of R3Si+ ions generated by -radiolysis [16]
with various alkenes yield a fairly stable adduct
[R3Si alkene]+ which decomposes only in the pre-
sence of strong bases (Et3N, etc.) or oxygen-contain-
ing nucleophiles (H2O, MeOH, etc.). In the fist case,
silylation products are formed, while in the second,
the starting alkene.
Ethylene
(CH3)2SiCH=CH
(CH3)2SiH2
(C2H5)(CH3)SiH2
(CH3)2C2H5SiH
(C2H5)2SiH2
74 4
10 1
2 0.5
2 0.5
6 1
2
6 1
B
BH + silylation product,
certain excitation energy which it acquires upon the
relaxation of the parent tetrahedral structure into
planar [21] and which can be contributed in the
energy of the complex. According to estimates in [22],
this contrubution is at least 24 kcal/mol. Thus, the
total excitation energy of the adduct in the reaction of
ethylsilylium ion and ethylene is no less than
60 kcal/mol.
[R3Si-alkene]+
Nu
R3SiNu+ + alkene.
(2)
The retention of stereochemistry in the reactions of
R3Si+ ions with cis- and trans-2-butenes is more
likely to agrue in favor of a cyclic rather than an
acyclic adduct.
Our previous quantum-chemical calculations [20]
showed that the complex C2H5SiH+2 C2H4 resides in a
local minimum on the potential energy surface of the
system C4H11Si+. This minimum is higher by
28.6 kcal/mol of the global minimum correspoding to
(CH3)2C2H5Si+ and by 8.3 kcal/mol of the local
minimum corresponding to diethylsilylium cation.
The low barrier between the complex C2H5SiH2+
C2H4 and the ion (C2H5)2SiH+ (7.8 kcal/mol) allows
the complex, even if its energy in not very high, to
pass into the complex and back. This, in turn, makes
possible T/H exchange between (C2H5)Si+T2 and
ethylene. The H/D exchange in the reaction of ethyl-
silylium ion and ethylene-d4 has been reported [23
25]. Therewith, the exchange involves all the seven
hydrogen atoms of ethylsilylium cation, unlike di-
methylsilylium, where the exchange involves only one
hydrogen atom. In the reaction of Me3Si+ and C2D4
no isotope exchange has been observed at all [10].
These results allowed a possible mechanism of the
exchange to be proposed and reasons for the dirrefent
behavior of the cations to be revealed.
As seen, the evidence in hand is insufficient to
between the cyclic or acyclic structure of the
-
silyl-substituted carbenium ion (adduct). It, however,
can be suggested that the list of possible structures is
not exhausted by the two mentioned ones. According
to data in [8, 11, 12],
conjugation can give rise
to a structure where silylium cation is conjugated with
multiple bond.
Si
Si
C C+
C=C
Turning to a radiochemical study of the gas-phase
reaction of ethylsilylium cation with ethylene, we set
ourselves the task to find out how alkenes would react
with a monosubstituted silylium cation. We also
intended to establish the structure of the resulting
adduct and its fate in the absence of any additive
(Lewis acids, oxygen-containing nucleophiles). The
results of the radiochromatographic analysis of
the reaction products of ethylsilylium cation with
ethylene are listed in the table.
The excitation energy ( 60 kcal/mol) and the suf-
9
As seen from the table, the major products of the
ion-molecular reaction of ethylsilylium cation with
ethylene are labeled ethylene (74%) and dimethylvinyl-
silane (10%). In terms of the commonly accepted
mechanism of ion-molecular reactions [17 19], the
reaction can be described as follows. The ion ap-
ficiently long lifetime ( 10 at a substrate vapor
pressure of 10 mm [26]) of the complex allow the
ethylsilylium cation to isomerize into the thermo-
dynamically more stable dimethylsilylium cation. It
is known [27] that ethylsilylium cation activated by
alternating magnetic field almost completely (by 90%)
proaches ethylene molecule to form a rather excited isomerizes into dimethylsilylium, and the barrier to
ion-neutral complex [18, 19]. The heat effect of the
condesation is, according to our estimates [20],
34.2 kcal/mol. The ethylsilylium ion, too, possess a
the isomerization is 51 kcal/mol. The potential
energy surface of the system C4H11Si+ [20] shows a
local minimum corresponding to the complex (CH3)2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 2 2001

208
SHCHUKIN et al.
SiH+ C2H4, which is even lower that the local
minimum of diethylsilylium cation. In essence, the
complex (CH3)2SiH+ C2H4 is the second most stable
isomer of the system C4H11Si+. It should also be
noted that no minimum for the acyclic adduct
C2H5SiH2CH2CH+2 could be found [20]. The above
evidence suggests that the adduct C4H11Si+ is most
likely a complex between ethylsilylium cation and
ethylene, which, if has enough energy, can transform
into the complex (CH3)2SiH+ C2H4, rather than into
an acyclic -silyl-substituted carbenium ion.
Free ethylsilylium ions were obtained nuclear-
chemically [29].
(C2H5)SiT3
(C2H5)SiT2+ + He0.
The procedure for synthesis of [3H3]ethylsilane has
been described in [30]. Dimethylvinylsilane was
synthesized by the Grignard reaction between methyl
iodide and dichloromethylvinylsilane, followed by
reduction of the resulting chlorodimethylvinylsilane
with lithium hydride in dioxane [31].
The adduct can decompose either unimolecularly
to give ethylene and ethylsilylium cation (probably,
already isomerized) or bimolecularly with transfer
on ethylene molecules of protons present in excess in
the reaction mixture (note that in accordance with the
experimental conditions no strong bases and oxygen-
containing nucleophiles were added).
The alkyl-substituted silanes (C2H5)(CH3)SiH2 and
(C2H5)(CH3)2SiH were prepared by the Grignard re-
actions between the corresponding alkyl halides and
chlorosilanes, followed by reduction of the resulting
chlorosilanes with lithium hydride, and the silanes
(CH3)2SiH2 and (C2H5)2SiH2, by reduction of chloro-
dimethyl- and chlorodiethylsilanes with lithium
hydride in dioxane.
We failed to estimate the enthalpy of proton trans-
fer from the adduct SiC4H+11 to ethylene because of
the lack of published proton affinity data for EtH2
SiCH=CH2 and Me2HSiCH=CH2. With the proposed
proton affinity of Me3SiCH=CH2 (199 kcal/mol) [10],
the process will most likely be endothermic (the
proton affinity of C2H4 is 162.6 kcal/mol [28]).
However, presumably, at least part of the adduct
SiC4H+11 will have an energy sufficient for proton
transfer to ethylene molecules. The formation of di-
methylvinylsilane (10%) testifies that this is indeed
the case.
Gas-phase reactions of ethylsilylium ions with
ethylene were studied using 20-ml molybdate ampules
degassed by heating and pumping before use. The
activity of ethylsilane in ampule was 0.001 Ci, and
the pressure of ethylene, 10 mm. The products were
accumulated in the dark at room temperature for
4 months.
ACKNOWLEDGMENTS
The work was financially supported by the Russian
Foundation for Basic Research (project no. 99-03-
33041).
The reaction products also include alkyl-substitued
silanes which may be formed by reaction of ethyl-
silylium ion or its isomer with a source of protons
(ethylsilane) via abstraction from the latter of hydride,
methyde, or ethyde ions [1, 2, 3]. It, however, not
inconceivable that the silanes (CH3)2C2H5SiH and
(C2H5)2SiH2 result from reaction with the ion source
of such cations as (CH3)2C2H5Si+ or (C2H5)2SiH+,
which are isomers of the adduct C4H11Si+.
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