Ring and C-O Bond Fragmentation as Tools for Fingerprinting the Extent of Homolysis during Base-Catalyzed Carbon-Carbon Bond Cleavages of the Haller-Bauer, Cram, and Gilday Types

Article abstract of DOI:10.1021/jo00282a020

The mechanisms of the base-catalyzed cleavage of non-enolizable ketones (Haller-Bauer reaction), fragmentation of the alkali-metal salts of diphenylcarbinols (Cram cleavage), and decarboxylative elimination of methyllithium-carboxylic acid adducts (Gilday process) are probed by attaching a small ring or a carbon-oxygen bond proximal to the ultimate seat of reaction.Particular attention is given to whether product formation in the first case is accompanied by fission of the cyclopropane or cyclobutane subunit.The product distributions constitute a serviceable diagnostic of the relative extent to which carbanion and radical pathways operate concurrently.This distinction is also possible in the oxa analogues since the homolysis/heterolysis dichtomy is matched by retention of an intact C-O bond and the extent of its cleavage, respectively.A key feature of the Haller-Bauer process is its ability to deliver debenzoylated products having intact cyclopropane or cyclobutane rings because of its strong predilection for the generation of carbanions during C-C bond fragmentation.Counterion influences are minimal.The Cram cleavages show a very different product distribution profile.The results can be plausibly fitted to the involvement of free radicals, although the distinction between direct C-C bond hemolysis or heterolysis followed by rapid black-electron transfer cannot be made at this time.Because the Gilday reaction leads directly to styrenes and these products suffer destruction under the reaction conditions, this transformation lacks synthetic value in this particular context.