Incremental substituent effects leading to steric blockade of the boat-like six-center Cope rearrangement of cis-1,2-dialkenylcyclobutanes

Although the rate of the chair-like, four-center Cope rearrangement of acyclic biallyls is faster than that of the boat-like, six-center one by a factor greater than exp((6000 cal/mol)/RT), there are several cases in the literature in which the order of preference is qualitatively reversed. Among the earliest examples so interpreted are the thermal transformations of cis-1,2-dialkenylcyclobutanes to cis,cis-cycloocta-1,5-dienes. The normal preference for the chair-like pathway in these systems should be diminished because of the high strain energy of the product, a cis,trans-cycloocta-1,5-diene. Although the transient intermediacy of that substance cannot be excluded on purely energetic grounds, the present results strongly support the formulation of the rearrangement of cis-1,2-divinylcyclobutane to cis,cis-cycloocta-1,5-diene as a direct process via a boat-like transition state. Moreover, they show how cis-1,2-dialkenylcyclobutanes, already deprived of the normal chair-like pathway by product strain, can be subjected to incremental steric effects that gradually deny access even to the "second-best" boat-like reaction.