Silenes, formation

UV photolysis (Chapman et al., 1976 Chedekel et al., 1976) and vacuum pyrolysis (Mal tsev et al., 1980) of trimethylsilyldiazomethane [122]. The silene formation occurred as a result of fast isomerization of the primary reaction product, excited singlet trimethylsilylcarbene [123] (the ground state of this carbene is triplet). When the gas-phase reaction mixture was diluted with inert gas (helium) singlet-triplet conversion took place due to intermolecular collisions and loss of excitation. As a result the final products [124] of formal dimerization of the triplet carbene [123] were obtained. 

Conlin and co-workers have also studied the fragmentation of a siletane (silacyclobutane). In this case, both the E- and Z-isomers of 1,1,2,3-tetra-methylsilane 45 were prepared and thermolyzed (Scheme 8).144 Both E-and Z-isomers of 45 led to the same products in slightly different ratios the major products were propene with silene 46, and E- and Z-2-butenes with silene 47. Silene formation was inferred from detection of the disila-cyclobutane products. During these processes, the stereochemical integrity of the compounds was largely preserved. 

Analysis of the data in Table XVIII suggests that silene formation is kinetically the most favorable process. However, according to experiment, metallated silenes are formed. This is related to the fact that in polar solvents proton transfer from the carbon atom to silicon is intermolecular, which leads to a considerable decrease in the reaction barrier. We believe that when the migration of substituents from the carbon atom to silicon is suppressed, for example, by the introduction of two alkyl radicals, the elimination of phosphines resulting in silene formation becomes the most probable process. 

The photochemical reactions of 2-silabicyclo[2.2.2]octanes 110 and 112 have been investigated69. Photolysis of 112 in the presence of methanol gives silyl methyl ether 113 as the main product, while 114 is only a minor byproduct (equation 26). This suggests that silene formation is a minor process and the main reaction proceeds via diradical intermediate similar to 115. In agreement, interconvertion of E/Z 110, probably via diradical 115, is found during photolysis along with extensive polymerization (equation 27). 

Photolysis of 1-benzyl-l-methylsilacyclobutane and 1-benzyl-l-phenylsilacyclobutane also leads to formation of isotoluene derivatives as the major primary products (Scheme 5). The primary photolysis in both cases is dominated by reactivity specific to the benzyl chromophore rather than the SCB moiety, unlike the case with other SCBs. Silene formation competes significantly in the case of 1-benzyl-l-phenylsilacyclobutane, when the role of the primary chromophore is shared between the benzyl group and the SCB ring via a second phenyl group attached to silicon. 

In general, the silenes tend to be formed as intermediates in ether solutions, while in THF they are not. In a less nucleophilic solvent such as an alkane, silene formation would be favored even more, but the initial lithiation step then proceeds too slowly. Thus, diethyl ether is the best compromise solvent244. 

Repeated loss of LICI and renewed rearrangement and silene formation... 

While several highly substituted 1,2-disilacyclobutanes are known to revert under very mild conditions to silenes , it is generally believed that 1,3-disilacyclobutanes need more drastic conditions to undergo the cycloreversion yielding silenes . Kinetic data for the pyrolyses of several 1,3-disilacyclobutanes (24, 47, 48) have been reported by Davidson and CO workers and are summarized in Table 2 . Silene formation was inferred from detection of trapping products with TMSOMe and HCl. It was found that methyl substitution at silicon slows down the pyrolysis rate. The initial process for the decomposition of... 

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