Mechanistic analysis of the four pathways in the 1,3-sigmatropic rearrangements of trans-1,2-trans, trans- and trans-1,2-cis,trans-dipropenylcyclobutane

Abstract

The orbital symmetry allowed and forbidden 1,3-sigmatropic rearrangements of a trans-2-substituted-ltrans-propenylcyclobutane (e.g.. tTT. Scheme I) would give, respectively a trans- and a cis-3,4-disubstituted cyclohexene (e.g., 1 and 2). Thus, an evaluation of the relative importance of the two processes would be available merely from a determination of the trans/cis product ratio. However, a more complete mechanistic analysis would require dissection of the allowed and forbidden reactions into two subreactions each. These can be categorized according to whether the migrating carbon retains or inverts its configuration (r or i) and whether the allylic receptor framework participates in a suprafacial or antarafacial way (s or a). The use of optically active reactants now permits this analysis.

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