INTERACTION OF α-CARBANIONS OF LITHIUM ACYLATES WITH 1,2-DIBROMOALKANES

We have studied the interaction of α-carbanions of lithium acylates with 1,2-dibromoalkanes with different length of the carbon chain in order to establish the direction of the reaction. Interaction of α-carbanions of lithium acylates obtained by metallation of acetic, butyric and isobutyric acids with lithium diisopropylamide and 1,2-dibromoalkanes (1,2-dibromohexane, 1,2-dibromoheptane or 1,2-dibromononane) with a molar ratio of reactants equal to 1: 2: 1 in argon atmosphere in tetrahydrofuran under normal conditions (20-25 °C) for 2 h leads to the products of the oxidative coupling of enolate anions: succinic acid, 2,3-diethylsuccinic acid (mixture of diastereomers of meso- and (±) -form in a ratio of ~ 2: 1) and 2,2,3,3-tetramethylsuccinic acid with 10-70% yields. We didn’t observe any products of sequential nucleophilic substitution in reaction. The yields of dicarboxylic acids depend on the structure of the initial α-carbanion of lithium acylate and on the carbon chain length of 1,2-dibromoalkane involved in the reaction. We established that the oxidative coupling proceeds most effectively in the interaction of 1,2-dibromoalkanes with an α-carbanion containing an anionoid center at the secondary α-carbon atom - α-carbanion of lithium butyrate (2,3-diethylsuccinic acid yields are 31-70%). We suggested the most probable anion-radical scheme of the formation of the products of the oxidative coupling of α-carbanions of lithium acylates through electron transfer from the enolate anion to 1,2-dibromoalkane with the generation of its radical anion. The pathways for the formation of primary bromoalkanes (1-bromohexane, 1-bromoheptane, 1-bromononane) and terminal olefins (hexene-1, heptene-1, nonene-1) that are generated from the corresponding 1,2-dibromoalkanes during the elimination of their radical anions with the loss of the bromine anion and the subsequent stabilization of 1-bromoalkyl radicals by abstraction of the hydrogen atom from the solvent, or by bromine elimination, are shown.