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

✍ Scribed by Roger Malherbe; Hans Dahn

• Publisher: John Wiley and Sons
• Year: 1977
• Tongue: German
• Weight: 699 KB
• Volume: 60
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.19770600807

✦ Synopsis

Abstract

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^.