ALKYNES, ALLENES, AND ALKENES IN [3,3]-SIGMATROPY - FUNCTIONAL DIVERSITY AND KINETIC MONOTONY - A THEORETICAL-ANALYSIS

The [3,3]-sigmatropic shifts of 1,5-hexadiyne, 1,2-hexadien-5-yne, 1,2 ,5-hexatriene, 1,2,4,5-hexatetraene, and 1-hexen-5-yne were studied, u sing a hierarchy of methods (RHF/6-31G, MP2//RHF/6-31G*, BLYP/6-31G*, Becke3LYP/6-31G, and CASSCF/6-31G*). Hartree-Fock, MP2, and density functional methods predict the mechanisms to be concerted, involving a romatic transition structures, whereas CASSCF/6-31G favors the stepwi se pathways via diradical intermediates. Both MP2//RHF/6-31G and Beck e3LYP/6-31G give activation energies in good agreement with the exper imental values. Becke3LYP is a reliable tool for predicting the activa tion energies of such reactions, although it has a tendency to oversta bilize allene systems. The activation energies for the series of react ions are remarkably constant at 31-35 kcal/mol, consistent with the ar omatic character of their transition states. The origin of this kineti c monotony even in the face of structural diversity is analyzed.