Facial selectivity in the Diels-Alder reaction of plane-nonsymmetric cyclopentadiene derivatives

Abstract

π-Facial selectivity in the Diels-Alder reaction of 1,3-cyclopentadienes substituted at C-5 with various halogens (C1, Br, I) and alkyl groups (Me, Et, π-Bu, CH₂OCH₃) is detailed. It was found that N-phenylmaleimide, 4-phenyl-1,2,4-triazoline-3,5-dione and tetra- cyanoethylene gave markedly different diastereofacial outcomes with a given diene thus proving that the nature of the dienophile as well as the C-5 substituent on the diene imparts a significant influence on the reaction. -- The results obtained were investigated by collaborators using high level 6-31G* theoretical calculations. The mechanism of facial selectivity is explained on the basis of a steric interaction between the diene and the dienophile that induces bending mainly in the diene. This bending translates into torsional strain in the diene in its syn transition state and the amount of distortion that occurs determines the diastereofacial outcome of the reaction. It was also found that the dienophiles studied imparted different steric effects on the substituted dienes thereby each affording different facial outcomes with a given diene. These results also disprove many popular theories that have recently been proposed involving the mechanism of π-facial selectivity in Diels-Alder reactions.