2,2'-BISALLYLMETHANE - THERMAL DEGENERATE REARRANGEMENT OF 1,3-DIMETHYLENECYCLOPENTANE - PARTLY STEREOSPECIFIC GENERATION OF A BIRADICAL

Upon thermolysis 1, 5-diene systems undergo a [3, 3]-sig-matropic shift (Cope rearrangement)1 unless steric constraints prevent the termini of the diene from strongly interacting. Under these circumstances [1, 3] shifts intervene, 2 the stereochemistry of which has been interpreted as resulting from the operation of both Woodward-Hoffmann orbital symmetry conservation and Berson-Salem subjacent orbital control.3 In an effort to delineate the crossover point between concerted [3, 3] shifts and other processes, the pyrolyses of 1, 4-di-methylenecycloalkanes have been investigated. 1, 4-Dimeth-ylenecyclohexane (DMCH) undergoes a [3, 3] shift via a “boat”-like transition state which may resemble [2.2.2] pro-pellane, 4 but 1, 2-dimethylenecyclobutane (DMCB) cleaves to a 2, 2'-bisallyl biradical which recloses to DMCB resulting informal [1, 3]-and [3, 3]-shift products in a 2:1 ratio, kineti-cally.5 Significantly, the latter reaction, though apparently nonconcerted, proceeds with substantial (95%) stereospecificity via a conrotatory C3-C4 bond rupture with simultaneous C1C2 rotation in a fashion that is as meshed-bevel gears at the intersection of the rotation axes. The behavior of the interfacial material, 1, 3-dimethylenecyclopentane (DMCP) is therefore of concern, and we here report that it behaves as DMCB5 does. © 1979, American Chemical Society. All rights reserved.