PI-FACIAL SELECTIVITY IN THE DIELS-ALDER REACTION OF 5-SUBSTITUTED 1,3-CYCLOPENTADIENES

Theoretical and experimental studies are presented about pi-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 (En) Of the substituent relative to the pi-HOMO energy (epsilon(pi)) of the diene. For epsilon(pi) > epsilon(pi), the syn pi-facial selectivity is predicted since the HOMO contains the diene pi-HOMO as the main component. For epsilon(pi) < epsilon(n), the anti pi-facial selectivity is predicted since the pi-HOMO most contributes to the NHOMO. For epsilon(pi) congruent-to epsilon(n), the loss of pi-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.