Flash vacuum pyrolysis silenes

A few routes to new silenes, usually involving flash vacuum pyrolysis at high temperatures, have been reported. Silenes were proposed as the result of the thermal expulsion of trimethylmethoxysilane, or a similar volatile fragment, from the starting material but frequently, proof that the silenes proposed to account for the observed products were in fact formed was not provided.116 119 The other thermal route employed was the retro-Diels-Alder regeneration of a silene from an adduct with an aromatic compound—often a 9,10-anthracene or 1,4-naphthalene adduct or, in some cases, a 1,4-benzene adduct, as illustrated in Eq. (19).120... 

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

An analogous mechanistic scheme (equation 87) has been proposed for the flash vacuum pyrolysis of dimethylsilyl(trimethylsilyl)thioketene148 (256). The pyrolysis of bis(trimethylsilyl)thioketene (257) leads to a more complicated product mixture (equation 88). With 47% conversion, a mixture of trimethylsilylacetylene, 1-trimethylsilyl-1-propyne, bis(trimethylsilyl)acetylene, (trimethylsilyl)thioketene, 2,2,4,4-tetramethyl-2,4-disila-l-thietane and 2,2,4,4-tetramethyl-2,4-disila-l,3-dithietane was obtained. All products can be rationalized, however, by the assumption that carbene 258 undergoes not only a silylcarbene-to-silene rearrangement (as in the preceding two cases) but also isomerization to 2-thiirene and insertion into a methyl-C, H bond. 

Also, the flash vacuum pyrolysis of the phenyl-substituted silanorbornenes 51A and 51B produced the corresponding silenes, which were not configurationally stable at 400-650 °C since identical product mixtures were isolated from trapping with 2,3-dimethylbutadiene210 starting with either isomer (Section III.B.l.a, equation 92). 

Cases in which the attacking nucleophile is present in the silene molecule itself have also been reported. The ring closure of the silicon analogues of an acrolein derivative and thioacrolein to silaoxetenes232 and silathietenes233, which immediately undergo a 2 + 2 cleavage to yield an acetylene and a silanone or silanethione, has been proposed to account for the results of flash vacuum pyrolysis of silylated ketenes and thioketenes (equation 110). 

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