A density functional molecular orbital study of the C2-C7 and C2-C6 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.