Trimethylsilyl group migration in the Criegee intermediate of gas-phase ozonolysis of trimethylsilylethenes

Article abstract of DOI:10.1016/S0040-4039(00)00149-0

GC, GC/MIS and GC/FTIR analyses of the final products of ozonolysis of trimethylsilylethene and trans-1,3-bis(trimethylsilyl)ethene reveal that the ester channel of decomposition of the Criegee intermediate takes place exclusively via migration of Me

Full text of DOI:10.1016/S0040-4039(00)00149-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 2435–2438
Trimethylsilyl group migration in the Criegee intermediate of
gas-phase ozonolysis of trimethylsilylethenes
∗
Radek Fajgar and Josef Pola
Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 165 02 Prague, Czech Republic
Received 10 November 1999; accepted 18 January 2000
Abstract
GC, GC/MS and GC/FTIR analyses of the final products of ozonolysis of trimethylsilylethene and trans-1,3-
bis(trimethylsilyl)ethene reveal that the ester channel of decomposition of the Criegee intermediate takes place
exclusively via migration of Me3Si group. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: ozonolysis; rearrangement; alkenes; silicon and compounds.
Much attention has been paid for over 40 years to the mechanism of the gas-phase ozonolysis of
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alkenes and this topic was recently reviewed. The ozonolysis of alkenes is a multi-step reaction and it
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proceeds via an unstable primary ozonide (POZ) which decomposes to a carbonyl compound (CC) and
a carbonyl oxide diradical (referred to as the Criegee intermediate, CI). The latter species plays a pivotal
role in the ensuing steps. Apart from its recombination with the geminal CC to a secondary ozonide
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(
SOZ), it undergoes bimolecular reactions and decomposes via at least three different routes. These
are O-atom elimination (a), the ester (b) and the hydroperoxide (c) channels. All these ozonolysis steps
are illustrated in Scheme 1.
The decomposition channels afford a multitude of mostly lower molecular weight species. Their
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relevance is ascribed to the occurrence of unique indicator products and is dependent on the CI structure
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and ozonolysis conditions. Thus, the O( P)-atom elimination channel is significant at low but not at
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a
atmospheric pressure and the hydroperoxide channel, an important source of OH radicals, is favoured
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a
with syn-positioned methyl (alkyl) groups and feasible only with H atom(s) at the carbon bonded
directly to the C radical centre. As to the ester channel, it affords in principle two different rearranged
products, but the present state of knowledge leaves considerable uncertainty on which group migration
is preferred.
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a,c
In conjunction with our previous studies on the gas-phase ozonolysis of alkenes
and the gas-phase
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decomposition of secondary ozonides, we address in this note the problem of the selectivity in the CI
∗
Corresponding author.
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040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00149-0
tetl 16429

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Scheme 1.
ester decomposition channel in the gas-phase ozonolysis of trimethylsilylethene (TMSE) and trans-1,3-
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bis(trimethylsilyl)ethene (BTMSE). With these compounds, the O( P)-atom elimination channel will not
be important at atmospheric pressure and the CI hydroperoxide channel will not be operative due to
the absence of H atoms β to the C_C bond. This leaves the ester channel as the only route of the CI
decomposition and trimethylsilyl formate and trimethylsilanecarboxylic acid as the respective indicator
products of migration of the trimethylsilyl group and hydrogen. The analysis of the final products in the
ozonolysis of TMSE and BTMSE can thus reveal the relative rate of migration of the H and the Me Si
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group in the CI diradical.
The ozonolysis was carried out by mixing ozone and the alkene (both in an excess of nitrogen).8
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The progress of the ozonolysis was monitored by FTIR spectroscopy and the reaction mixture was
analysed 30 min after mixing by gas chromatography, gas chromatography–mass spectrometry and gas
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0
chromatography–FTIR spectroscopy.
The ozonolysis of BTMSE resulted in a 54% depletion of BTMSE and the formation (yield in mol/mol
of BTMSE reacted) of trimethylsilyl formate (0.88), carbon monoxide (0.75), trimethylsilanol (0.74),
formic acid (0.45), carbon dioxide (0.008) and hexamethyldisiloxane (0.003). The major products can
be rationalised in terms of Scheme 2, where the POZ decomposes into the CI and the CC and where the
tentatively assumed yields are given in brackets.
Scheme 2.
While the decomposition modes of the unstable12 trimethylsilylformaldehyde (CC) are uncertain and
may involve Si–C bond homolysis assisted with traces of water or the cell surface, the decomposition
of trimethylsilylcarbonyl oxide diradical (CI) is unequivocally shown, through the high yield of trime-

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thylsilyl formate and the absence of stable13 trimethylsilanecarboxylic acid, to take place via exclusive
migration of the Me Si group and not of the hydrogen. Some deficiency for silicon cannot be, however,
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accounted for at present.
The ozonolysis of TMSE resulted in a 64% depletion of TMSE and yielded (in mol/mol of TMSE
reacted) formaldehyde (0.40), carbon monoxide (0.39), trimethylsilanol (0.38), trimethylsilyl formate
(0.25), hexamethyldisiloxane (0.16), formic acid (0.024), (hydroxy)pentamethyldisiloxane (0.01), hexa-
methylcyclotrisiloxane (0.003) and octamethyltrisiloxane (0.03). This ozonolysis is in keeping with
the formation of the two pairs of CC and CI products (Scheme 3). Among the major final products,
formaldehyde and trimethylsilyl formate can be identified as the primary CC product and the product of
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,3-Me Si group migration in the CI, respectively.
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Scheme 3.
The absence of trimethylsilanecarboxylic acid is again in line with the absence of a 1,3-H shift in the
CI. The smaller yield of trimethylsilyl formate and the higher yield of hexamethyldisiloxane, compared
to those in the ozonolysis of BTMSE, are consistent with formation of water in this ozonolysis. The
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a,6c,14
H COO diradical is known
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to decompose, aside from other routes, to H O+CO and HCO+OH. It
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is thus conceivable that the radicals so produced abstract H from the Me Si group and induce formation of
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15
dimethylsilene Me Si_CH which adds H CO and affords dimethylsilanone Me Si_O. Only the oc-
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2
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2
currence of the latter transient can explain the formation of the observed (hydroxy)pentamethyldisiloxane
and trisiloxanes, since the insertion of Me Si_O into the Si–O bond of trimethylsilanol and hexame-
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15
thyldisiloxane, or its cyclotrimerisation affords these compounds. It is thus evident that although the
terminal CH group of the alkene causes the formation of more products, the ester channel of the CI
2
decomposition is controlled by the migration of the Me Si group even in this case.
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The 1,3-migration of the Me Si group in the Criegee intermediate adds to the class of rearrangements
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1
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17
of diradicals and to the family of organosilicon carbon-functional species.
Acknowledgements
We thank the Grant Agency of the Czech Republic (Grant No. 104/96/0472).
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