Silacyclobutanes thermolysis

The first evidence for the formation of silenes came from the thermolysis of silacyclobutanes, which resulted in a retro-[2+2] process leading to the silene Me2Si=CH2 and ethylene1 ... 

Generally, only simple silenes having small groups (H, Me, CH2=CH) are obtained as transient species from the thermolysis of silacyclobutanes. In part this is due to the high temperatures (usually above 450°C) required for the ring cleavage. Substitution on the carbon atom adjacent to silicon in the ring can lead to carbon-substituted silenes. 1,3-Disilacyclobutanes do not readily revert to silenes under thermal conditions, but examples... 

A much explored pathway to simple silenes involves the thermolysis of silacyclobutanes at 400-700°C, the original Gusel nikov-Flowers (155) route. Such temperatures are not readily conducive to the isolation and study of reactive species such as silenes except under special conditions, and flash thermolysis, or low pressure thermolysis, coupled with use of liquid nitrogen or argon traps has frequently been employed if study of the physical properties is desired. Under these high temperature conditions rearrangements of simple silenes to the isomeric silylenes have been observed which can lead to complications in the interpretation of results (53,65). Occasionally phenyl-substituted silacyclobutanes have been photolyzed at 254 nm to yield silenes (113) as has dimethylsilacyclobutane in the vapor phase (147 nm) (162). 

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... 

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. 

Very low pressure thermolysis of (2-silapropen-2-yl)benzene (CH2 Si(Me)C6H5) causes a 1,3-hydrogen shift from the aryl carbon to the sp2 silicon with the formation of (53) (940M4661). In a related reaction, thermolysis of the silacyclobutane (54) yields (55) by a 1,4-hydrogen shift after the four-membered ring has decomposed by loss of ethene (95JOMC4). 

Photoelectron (PE) spectroscopy has been used to follow the flash vacuum thermolysis (FVT) of 1-phenyl-2-dimethyl-l-phospha-2-silacyclobutane (Section 2.21.6.1, Scheme 10) <19970M1635>. 

Because of the close relationship between silicon and carbon, many attempts have been made to try to synthesize species containing multiple bonds to silicon (Si=C, Si=0, Si=Si, etc.). However, it was not until 1967 that compelling evidence was presented that Si=C might exist in the thermal reaction of 1,1-dimethyl-1-silacyclobutane (equation 90). The first evidence for the existence of Si=Si as transient intermediate was provided in the thermolysis of bridged disilane derivatives (equation 91). Since then, many studies have been published on these unsaturated species, but it was in 1981 that synthesis and characterization of relatively stable crystalline compounds containing Si=C (silene) (equation 92) and Si=Si (disilene) (equation 85) were reported (equations 90-92). 

A direct approach to silacyclobutane (siletane) derivatives is the reaction of meth-ylsilenes with other unsaturated compounds. Thus, the silaethylene (silene) (326), formed from pivaloyl-tris(TMS)-silane (325) via thermolysis (10h/140°C), reacts with 1-phenylprop-l-yne to yield the silacyclobutene (silete) derivative 327 (equation 148)172. 

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. 

In order to obtain more reliable structural data on silaethenes with unstabilized Si=C double bonds, we tried to obtain the mmw spectrum of the parent species 3a. Contrary to the synthesis of substituted silaethenes such as 3b, the thermolysis of the corresponding silacyclobutane (4) proved to be less useful for obtaining the parent species 3a (route A in Scheme 6.1 1 ). 

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