A Density Functional Molecular Orbital Study of the C 2 −C 7 and C 2 −C 6 Cyclization Pathways of 1,2,4-Heptatrien-6-ynes. The Role of Benzannulation

The electronic factors influencing the competition between C 2 −C 7 (Myers−Saito) and C 2 −C 6 (Schmittel) cyclizations of 1,2,4-heptatrien-6-yne and 1-ethynyl-2-propadienylbenzene are investigated using Becke3LYP/6-31G* calculations. The relative energies of the biradical products formed upon cyclization are calculated using a simple bond additivity estimate and by direct calculation. Both approaches predict that benzannulation stabilizes the bis-dehydromethylfulvene biradical that forms upon C 2 −C 6 cyclization with respect to the biradical that forms upon C 2 −C 7 cyclization by ca. 10 kcal/mol. Transition state calculations predict that benzannulation decreases the barrier for C 2 −C 6 cyclization, relative to that for C 2 −C 7 cyclization, by 2.3 kcal/mol. The results suggest that benzannulation plays an important role in promoting C 2 −C 6 cyclization.