MONOALKYLATION VS. DIALKYLATION OF CARBANIONS - EFFECTS OF ABSOLUTE AND RELATIVE ACIDITY OF THE CONJUGATE CARBON ACIDS IN SELECTIVITY CONTROL

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 D MSO. It was found that the bis(monoalkylated) product 3 arises exclusi vely or predominantly from the carbanions 2d - 2g derived from the les s acidic carbon acids 7d - 7g whereas the cyclic product of dialkylati on 4 prevails in the reaction of the carbanions 2a - 2c derived from t he more acidic carbon acids 7a - 7c. The alkylation selectivity thus d epends critically on the absolute acidity of the carbon acid participa ting in the reaction. Rationale for this novel, and on basis of earlie r studies unexpected finding is provided in terms of eqs. (1) -(4). (C ) 1997 Elsevier Science Ltd.