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π‐Participation in Diazoketone Hydrolysis II; Exo ‐ endo Cyclization Ratio in the Hydrolyses of 7‐ syn ‐ and 5‐ endo ‐Diazoacetyl‐2‐norbornene
Author(s) -
Malherbe Roger,
Dahn Hans
Publication year - 1977
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19770600807
Subject(s) - chemistry , norbornene , steric effects , kinetic isotope effect , ion , hydrolysis , stereochemistry , medicinal chemistry , photochemistry , deuterium , organic chemistry , physics , monomer , quantum mechanics , polymer
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 .