The stereochemistry and mechanism of the [σ2 + σ2 +σ2] cycloreversion

Abstract

The search for concert in the cycloreversion of a 6-membered ring to three pi-bonded fragments uses three mechanistic criteria: an enhanced rate as compared to model compounds, the absence of ring-closure products, and stereospecificity. By these criteria, the decompositions of 1,2- diazacyclohexa-1,4-dienes ( 57) and 2,3-diazabicyclo [4.1.0] hept-2-enes ( 58) are clearly concerted. The decomposition of 2,3-diazabicyclo [4.2.0] oct-2-enes ( 59), although it shows only a small rate enhancement, nevertheless meets the product and stereospecificity criteria and probably is concerted. The data suggest that the degree of concert gradually declines as the pi-character of the participating ring bond decreases. The cycloreversions of both sets of bicyclic compounds 58 and 59 follow an “inward-disrotatory” path undoubtedly because their transition states have the most efficient orbital overlap. An analysis of the [2 + 2 + 2] cycloreversion reveals an ambiguity in nomenclature that is a general feature of the Woodward-Hoffmann Rules in their 2-electron-component form. For a reaction involving any given number of such components, all allowed stereochemical pathways have the same set of descriptions.