Stereochemistry of the Diels–Alder reaction: reaction of dienophiles with cyclopentadiene and methylcyclopentadiene

Abstract

Structures are assigned to adducts of 1-methylcyclopentadiene and 2-methylcyclopentadiene with acrylonitrile and methyl acrylate. Reactions with 1-methylcyclopentadiene are highly regiospecific but 2-methylcyclopentadiene has lower regiospecificity. Methyl acrylate shows similar endo-selectivity with cyclopentadiene and methylcyclopentadienes but acrylonitrile shows little selectivity and in one case preferentially gives an exo-adduct. Effects of solvent on regiospecificity and endo-selectivity are examined. The relative importance of endo-selectivity of steric effects, secondary orbital overlap, and electronic effects are discussed. The importance of electronic effects are illustrated in additions of cyclopentadiene to 2-arylacrylic acids.