Diels-Alder reactivity of benzannulated isobenzofurans as assessed by density functional theory.

Density functional theory (DFT) calculations at the B3LYP/6-31G* level for isobenzofuran1 and eleven benzannulated derivatives of types2 and3 have been performed in order to compare their relative reactivities as dienes in Diels-Alder reactions. The transition state (TS) energies for their reactions with ethylene have been determined and shown to form a linear correlation between activation energies and structure count (SC) ratios. TS energies as a method for comparison of diene reactivities can be applied to IBFs bearing substituents on the ring as well as those containing heteroatoms, for which the SC ratio method failed. Different measures of aromaticity of benzannulated IBFs indicated a decrease in aromaticity going from 4 to 14, which is also reflected in their reactivity as a dienes in Diels-Alder reaction.