Cycloadditions microscopic reverse

In the course of our studies on the [6+2] cycloaddition, the formation of products arising from a [6+2-1] path was observed (Scheme 13.20). Conceptually, 249 produces the [6+2] cycloadduct 251, but under certain conditions 49, the [6+2-1] product, is observed, presumably arising through the conversion of 250 to 48. Reflection on this observation suggests that one might start with VCP 41 and an alkyne and trap 48 with CO in the microscopic reverse of the 250-to-48 path to produce 251 by an overall three-component [5+2+1] cycloaddition. This idea worked. 

The first step of the retro-reaction involves loss of silylene 79, which could be trapped with 1-pentyne to give the known silirene 81 (equation 125). In the absence of a trapping agent, 79 recondenses to 77, probably by first dimerizing to the disilene Ar2Si=SiAr2 followed by 2 +1 cycloaddition to give 77 (equation 126). From the principle of microscopic reversibility, the fact that silylene is formed in the retro-reaction leads to the conclusion that 79 must also be an intermediate in the cycloaddition reaction. 

By the principle of microscopic reversibility, the Cj-extrusion or the reversal of a [2 + 1] cycloaddition to give an alkene and a carbene is possible for a cyclopropane derivative. Such reactions have indeed been observed, they can be initiated either photochemically or thermally. " ... 

In summary, the reactivity of strained olefins is different from that of simple alkenes. The wealth of transformations initiated by strained double bonds has recently been reviewed in great detail (276). In this chapter some structural and strain-related features of reactivity observed for olefins with nonplanar double bonds have been presented. It is apparent that in polar reactions as well as cycloadditions of these molecules the release of strain may contribute to enhanced rates, but it is certainly not the controlling factor. In view of the operation of microscopic reversibility, it may be questioned whether the buildup of strain in reverse reactions—eliminations and cycloreversions—is the controlling factor. 

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