SOLVENT AND LEAVING GROUP EFFECTS ON THE MONOALKYLATION VS DIALKYLATION OF ALKALI SALTS OF DIETHYL MALONATE WITH 1,2-BIS(HALOMETHYL)BENZENES, 1,2,4,5-TETRAKIS(HALOMETHYL)BENZENES AND 1,2,3,4,5,6-HEXAKIS(HALOMETHYL)BENZENES - A NEW INSIGHT INTO SELECTIVITY CONTROL OF MALONESTER SYNTHESIS

Contrary to the widely held opinion that protic (''acidic'') solvents favor monoalkylation whereas aprotic (''inert'') solvents support dial kylation of diethyl malonate carbanion, exactly opposite results have been obtained in the reaction of the dibromide 7, tetrabromide 4 and h exabromide 1 in ethanol and dimethyl sulfoxide, the former solvent pre ferring strongly dialkylation (cyclization) and the latter monoalkylat ion. Investigation in a broader spectrum of solvents demonstrated that hydrogen bonding as well as ion-pairing may play an important role in the selectivity control, both strongly supporting dialkylation. When a separation of ion-pairs is induced with 18-crown-6, monoalkylation p revails in the reaction. The solvent and the leaving group employed ha ve been found to participate in the selectivity control. In DMSO, prop ensity to dialkylation increases strongly in the order I < Br << Cl, a gain in discord with earlier predictions. Rationale for the novel find ings is provided on the basis of kinetic analysis of the overall react ion and is expressed by the limiting equations (5) and (7). (C) 1997 E lsevier Science Ltd.