Parallel and cross cycloaddition of triplet penta-1,4-diene. An ab initio MO study

Abstract

ab initio UHF molecular orbital calculations using the 3-21G through 6-31G* basis sets, and including spin projection and electron correlation (APUMP2), have been carried out to investigate the photosensitized cycloaddition reactions of penta-1,4-diene. Two forms of triplet penta-1,4-diene, cis-trans and envelope, with nearly equal energy but with excitation localized on different bonds, are the starting points for the transition states TS1 of cross closure and TS2 of parallel closure, respectively. TS2 has a barrier of 14 kcal mol^–1, about 22 kcal mol^–1 lower than TS1, supporting the experimental dominance of parallel closure. The transition states have structures where a significant ring strain has already developed, and the TS1 – TS2 energy difference is mainly dictated by the strain energy difference of the cyclic triplet product intermediates.