Pg. Wenthold et Ma. Lipton, 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, J AM CHEM S, 122(38), 2000, pp. 9265-9270
The electronic factors influencing the competition between C-2-C-7 (Myers-S
aito) 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* calcul
ations. The relative energies of the biradical products formed upon cycliza
tion are calculated using a simple bond-additivity estimate and by direct c
alculation. Both approaches predict that benzannulation stabilizes the bis-
dehydromethylfulvene biradical that forms upon C-2-C-6 cyclization with res
pect to the biradical that forms upon C-2-C-7 cyclization by ca. 10 kcal/mo
l. 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.