Mono- vs. dialkylation of carbanions. Effects of absolute and relative acidity of the conjugate carbon acids in selectivity control

Article abstract of DOI:10.1016/S0040-4020(97)00989-7

The title problem was investigated in the reaction of the dibromide 1 with carbanions 2a-2g covering a range greater than 15 pK units in DMSO. It was found that the bis(monoalkylated) product 3 arises exclusively or predominantly from the carbanions 2d-2g derived from the less acidic carbon acids 7d-7g whereas the cyclic product of dialkylation 4 prevails in the reaction of the carbanions 2a-2c derived from the more acidic carbon acids 7a-7c. The alkylation selectivity thus depends critically on the absolute acidity of the carbon acid participating in the reaction. Rationale for this novel, and on basis of earlier studies unexpected finding is provided in terms of eqs. (1)-(4).