Silacyclobutanes pyrolysis

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... 

Keywords silacyclobutanes, pyrolysis, silenes, rearrangement, 3,4-benzo-1-silacyclobutenes, 2-silaindanes, B3LYP/6-31G calculations... 

While the decomposition of silacyclobutanes as a source of silenes has continued to be studied in the last two decades, the interest has largely focused on mechanisms and kinetic parameters. However, a few reports are listed in Table I of the presumed formation of silenes having previously unpublished substitution patterns, prepared either thermally or photo-chemically from four-membered ring compounds containing silicon. Two cases of particular interest involve the apparent formation of bis-silenes. Very low-pressure pyrolysis of l,4-bis(l-methyl-l-silacyclobutyl)ben-zene94 apparently formed the bis-silene 1, as shown in Eq. (2), which formed a high-molecular-weight polymer under conditions of chemical vapor deposition. 

Conlin148 also studied the pyrolysis of 1-methyl-1-silacyclobutane in the presence of excess butadiene at various temperatures where the decomposition followed first-order kinetics and where the silene isomerized to the isomeric silylene prior to reacting with the butadiene. The value for the preexponential factor A for the silene-to-silylene isomerization was found to be 9.6 0.2 s-1 and the Ewl for the isomerization was 30.4 kcal mol-1 with A// = 28.9 0.7 kcal mol-1 and AS = -18.5 0.9 cal mol-1 deg. More recently, the photochemical ring opening of l,l-dimethyl-2-phenylcyclobut-3-ene and its recyclization was studied. The Eact for cycli-zation was 9.4 kcal mol-1.113... 

Grobe15 has described the pyrolysis of 1 -methyl-1 -vinyl- and 1,1 -diviny 1-1-silacyclobutanes 166 which led to the formation of methylvinylsilene and divinylsilene, respectively. Under the experimental conditions used, it was suggested that the silenes rearrange to exo-methylene- 1-silacyclo-propanes 167 which extrude methylsilylene or vinylsilylene, respectively. In support of this proposal, when the reactions were carried out in the presence of 2,3-dimethylbutadiene, the anticipated silylenes were trapped as their respective l-silacyclopent-3-enes 168. 

The extrusion of silylenes by pyrolyzing silacyclopropanes provides one of their more interesting reactions. Silacyclobutanes generate equally novel pyrolysis products. 

This fragmentation mode is not altered for silacyclobutanes bearing a vinyl group at the silicon17, as the same Arrhenius parameters are found for the decomposition of 1 and of 1 -methyl-1-vinylsilacyclobutane 3 (logA = 15.64 s 1, E = 62.6 kcalmol-1), in sharp contrast to the pyrolysis of cyclobutanes where a vinyl group accelerates the pyrolysis by a factor of nearly 60018. 2-Silabuta-l,3-diene 4 was produced in a laser-photosensitized (SFg) decomposition (LPD) of 1-methyl-1-vinylsilacyclobutane 3... 

In contrast, in the excited state the primary cleavage mechanism in silacyclobutanes like 5 involves the breaking of a silicon-carbon bond23. The initially formed silyl radicals 15 and 16 are stabilized by an intramolecular disproportionation reaction giving the silenes 17 and 18 and the homoallylsilane 19.17 and 18 were identified by their trapping products (20, 21) with methanol (equation 5)23. From pyrolysis of Z-5 a different set of products from 1,4-diradical disproportionation is obtained, which can be attributed to predominant cleavage of the carbon-carbon bond23. 

Pyrolysis of the bis-silacyclobutane 34 yields under chemical vapour deposition conditions polymeric material 37 (equation 9)38. 37 is the mixed product of the... 

In another study several simple silenes RR Si=CH2 (R, R = Me, Vinyl etc.) were formed by laser-powered pyrolysis and were found to form linear polymers, in contrast to the usual behavior of silenes which yield cyclodimers when formed by conventional thermolysis techniques16. Reactions of the silenes in the presence of several monomers such as vinyl acetate, allyl methyl ether and methyl acrylate were also studied. Laser-induced decomposition of silacyclobutane and 1,3-disilacyclobutane gave rise to silenes and other oxygen-sensitive deposits17,18. 

Scheme 2. Very low pressure pyrolysis products of 1 -tolyl-1 -methyl-1 -silacyclobutanes.

The pyrolysis of 1,1-disubstituted silacyclobutanes has been used as a method for the preparation of 1,1,3,3-tetrasubstituted 1,3-disilacyclobutanes183. Copyrolysis of two different 1,1-disubstituted silacyclobutanes yielded mixtures containing 1,3-disilacyclobutanes arising from all possible combinations of the two silenes present (equation 51). The preparative method failed in the case of cyclopentadienyl-substituted silacyclobutanes183, 184, presumably due to competing intramolecular reactions of the silene. The thermolysis of trimethylsilylcyclopentadiene may also proceed via a silene184. 

The pyrolysis of 1,1-dihalogenated silacyclobutanes yielded the head-to-tail dimerization product as well as the hydrogen halide adducts of the presumably formed transient 1,1-dihalosilenes185-187. The head-to-head dimer was also formed, but the formation of several of the other products from 1,1-dichlorosilacyclobutane was interpreted as due to the intermediacy of dichlorosilylene185,187, which could also be trapped in an argon matrix185. 

The pyrolysis of 1-halo-l-methylsilacyclobutanes yielded primarily the head-to-tail dimers of the expected 1-halosilenes as well as significant amounts of their hydrogen halide adducts. In several of these reactions polymers were obtained as well159,186. The pyrolytic behavior of halogenated silacyclobutanes appears to be quite complicated, and although it is certainly possible that the initial products are ethylene and 1,1-dihalosilene almost exclusively, this has not been established beyond reasonable doubt. 

The pyrolysis of 1-methylsilacyclobutane appears to be similarly complicated as the pyrolysis of silacyclobutane itself. In the most detailed investigation available to date, the results were rationalized assuming that 1-methylsilene is formed cleanly in the initial pyrolysis step and is capable of subsequent isomerization to dimethylsilylene, a step to be discussed in more detail below75, 205. It is of interest to note that under these high-... 

The retro-ene mechanism was considered unlikely225 since the dimer of 1-methoxy-1-methylsilene, l,3-dimethoxy-l,3-dimethyl-l,3-disilacyclobutane, was not observed. However, since little is known about the effect of the methoxy substituent on the dimerization and cross-dimerization rates of silenes, we consider this argument somewhat inconclusive. It is supported, however, by the observation225 that 1-methyl-1-propylsilene, formed by the pyrolysis of 1 -propyl-1 -methyl- 1-silacyclobutane under similar conditions, fails to undergo a retro-ene-type fragmentation, although such a process should be more exothermic, as it would produce a silene and an olefin instead of two silenes. 

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