π-Facial Selectivity in the Diels–Alder Reaction of 5-Substituted 1,3-Cyclopentadienes

Abstract

Abstract Theoretical and experimental studies are presented about π-facial selectivities in Diels–Alder reactions of 5-substituted cyclopentadienes (1). The HOMO and the NHOMO of 1 were readily predicted from the orbital mixing rule to be distorted to favor the syn- and anti-attack, respectively. The frontier orbital is dependent on the n-orbital energy (εn) of the substituent relative to the π-HOMO energy (επ) of the diene. For επ > εn, the syn π-facial selectivity is predicted since the HOMO contains the diene π-HOMO as the main component. For επ < εn, the anti π-facial selectivity is predicted since the π-HOMO most contributes to the NHOMO. For ε π εn, the loss of π-facial selectivity is predicted since the HOMO and the NHOMO both contribute to the reaction. The qualitative theory was examined by ab initio molecular orbital calculation on 1 (X = NH2, PH2, AsH2, OH, SH, SeH, F, Cl, and Br) and PM3 calculation of the activation energies on Diels-Alder reactions of 1 (X = NH2, PH2, AsH2, SbH2, OH, SH, SeH, TeH, F, Cl, Br, and I) with maleic anhydride. The observed selectivities of chalcogen-substituted cyclopentadienes (X = SPh and SePh) were in agreement with the theoretical prediction.