Isobenzofulvenes. VIII. The structure of the dimers derived from 8,8-dimethylisobenzofulvene. A PMO treatment of the perispecificity in this dimerization and related cycloadditions involving isobenzofulvenes

8,8-Dimethylisobenzofulvene, generated from the reaction of ll-isopropylidenetricyclo[6,2,1,02.7]- undeca-2,4,6,9-tetraene (23) and 3,6-di(pyridin-2-yl)-s-tetrazine, undergoes spontaneous dimerization to form a mixture of two isomers. This is shown to be a mixture of endo (33) and exo isomers (37) resulting from [π10s+π8s] cyclodimerization, with the former isomer predominating. 1H and 13C n.m.r. spectroscopy, in conjunction with lanthanide shift reagents and spin decoupling, was used, together with chemical degradation, to determine the structure and stereochemistry of these products. Chemical interconversion has placed these structures on a firm basis since the structure of the minor isomer has been determined by X-ray structure analysis. The constancy of this dimer ratio in solvents of widely differing polarity, and in the absence of solvent, supports an isopolar transition state which is typical of many concerted electrocyclic processes. A complete PMO treatment of all possible modes of isobenzofulvene dimerization is presented and the results of these calculations are discussed in terms of the observed peri- and stereospecificities. In a similar way, the PMO model is successfully employed to explain the relative efficiencies of various polyenophiles in trapping the isobenzofulvene nucleus.