Origin of regioselectivity in photocycloaddition reactions of 2-cyclohexenone with cycloalkenecarboxylates

The regiochemical results in [2 + 2]photocycloaddition of 2-cyclohexenone (1) with 1-cycloalkenecarboxylates (24) significantly changed from head-to-head (hh) to head-to-tail (ht) adducts depending on increments of the ring-size, from four to six, of the carboxylates. The root of the drastic regioselective change was inferred from the transition state (TS) analysis of the triplet reactions, including the back-reaction at the PM5 and B3LYP levels. The TS energy of the first step (TS1) is the most important factor for this reaction. The change in TS energies at hh adducts was larger than that at ht adducts. For more detailed analysis, the TS energies of the hh and ht reactions (at the three reactions) were partitioned to the two reactant deformation (E-df) and the interaction (or repulsion, E-int) energies. The E-df and E-int energy changes were found to be dependent on the ring-size of the 1-cycloalkenecarboxylates 2-4. Thus, since the larger ring of 4 has relatively higher TS I and deformation energies for giving the hh adduct, they may cause the preferential formation of the ht adduct, and result in the significant changes in the ht/hh ratio owing to the ring-size. The root of the significant hh diminution may be the dipole-dipole repulsion between the enone carbonyl and larger carboxylate (4) moieties. The improved PM5 and B3LYP/6-31+G(d) calculations are effective and economical for disclosing the origin of regioselectivity in [2 + 2]photocycloadditions.