Face Selectivity of the Diels‐Alder Reaction of 5,6‐Bis((D)methylidene)‐2‐bicyclo[2.2.2]octene

The face selectivity (endo‐face vs. exo‐face attack onto the exocyclic s‐cis‐butadiene moiety) of the [4+2]cycloadditions of 5,6‐bis((D)methylidene)‐2‐bicyclo‐[2.2.2]octene (11) to strong dienophiles has been determined in benzene at 25°. It is ca. 95/5, 75/5, 70/30, 60/40 and 50/50 for N‐phenyltriazolinedione (NPTAD), tetracyanoethylene (TCE), dimethyl acetylenedicarboxylate (DMAD), maleic anhydride (MA) and singlet oxygen (1O2), respectively. The endo‐face preference is probably due to a participation of the homoconjugated double bond at C(2), C(3) which makes the etheno bridge more polarizable than the ethano bridge in 11. The absence of face selectivity with 1O2 is consistent with an entropy‐controlled mechanism involving the intermediacy of an exciplex.