Chlorine versus bromine: facial selectivity in the Diels–Alder reactions of 5-bromo-1,2,3,4,5-pentachlorocyclopenta-1,3-diene

Abstract

Diels–Alder reactions of diene 11 with both electron-poor and electron-rich dienophiles led to approximately 90% addition to the face of 11 syn to the C-5 chlorine. Addition of 4-phenyl-1,2,4-triazoline-3,5-dione was slightly less facially selective. Adduct ratios were obtained by NMR methods; stereochemistry was determined by X-ray crystallography. The results support the view that facial selectivity in Diels–Alder reactions with heteroatom-substituted dienes is mainly controlled by steric, not stereoelectronic, interactions.