Intramolecular Nucleophilic Addition to Unsaturated Carbon. Dependence of Cyclization Efficiency on the Method of Carbon-Carbon Bond Cleavage Utilized To Generate the Reactive Species

Article abstract of DOI:10.1021/jo00282a019

The following three reactions have been studied for the purpose of comparing their intrinsic ability to generate carbanionic intermediates capable of intramolecular cyclization: (a) the Haller-Bauer cleavage of ketones 15a and 15b, as well as the (S)-(+)-antipode of 15a (viz. 47); (b) the base-promoted cleavage of 1,1-diarylcarbinols 16a and 16b; (c) decarboxylative elimination within the methyllithium adducts of carboxylic acids 17a and 17b.The Haller-Bauer process proceeds predominantly via carbanion intermediates, which most often experience protonation to give 18 an d 21.Cyclization becomes possible, however, under certain circumstances and reaches a maximum of 33percent with NaNH2 in benzene.Using (+)-47 as a probe, it has been possible to ascertain that 56percent of the reactive intermediate molecules racemize and that only the racemic species generates cyclic product.On the other hand, the Cram-type cleavages of 16a and 16b proceed mainly by homolytic cleavage to generate the benzophenone radical anion and free-radical intermediate.The latter dimerize, capture solvent, and abstract hydrogen to varying degrees depending upon counterion and solvent.Finally, reactions of type c are the most efficient at effecting intramolecular ring closure.