π‐Participation in Diazoketone Hydrolysis II; Exo ‐ endo Cyclization Ratio in the Hydrolyses of 7‐ syn ‐ and 5‐ endo ‐Diazoacetyl‐2‐norbornene

The rate of the acid‐catalysed hydrosis of 7‐ syn ‐diazoacetyl‐2‐norbornene ( 1a ) is enhanced relative to that of the saturated analogue 5a by a factor of 835. In contrast to the behaviour of other primary diazoketones, the substitution step is no longer rate‐determining (mechanism A ‐ 2 ), but so much accelerated that the preceding proton transfer becomes the slow step (mechanism A ‐ S E 2, demonstrated by a solvent isotope effect k H / k D = 1.76). Product analysis shows 100% cyclization; the product formation is explained in terms of brexyl and brendyl type carbenium ions (or ion pairs). ‐ 5‐ endo ‐Diazoacetyl‐2‐norbornene ( 3a ) shows very slight acceleration, and forms only 27% cyclization products (identical to those formed from 1a ). Thus, in spite of the absence of steric hindrance by hydrogen atoms, the exo ‐ endo rate ratio for anchimeric assistance is ≥ 10 3 .