Reduction of aliphatic bromoesters: Part II. Electrochemically generated carbanions and derivative carbanions—Their reactivity on electrophilic substrates

Abstract

During the electrochemical reduction of α-bromoesters [CH 2(Br)CO 2Et(I), CH 3CH(Br)CO 2Et(II), (CH 3) 2CBrCO 2Et(III): RX] evidence was gained for the presence both of electrochemically generated, unbrominated carbanions [ECG: (CH 2CO 2Et) −, (CH 3CHCO 2Et) −, ((CH 3) 2CCO 2Et) −] and of derivative brominated carbanions [DC: (CHBrCO 2Et) −, (CH 3CBrCO 2Et) −. In fact, if the reduction was carried out in the presence of substrates containing an activated double bond (CH 2=CHCO 2Et, CH 2=CHCO 2Me), in some cases only cyclic diesters were obtained (RX=I), in other cases only linear diesters (RX=III), and in further cases both cyclic and linear (RX=II). The reactivity of the two kinds of carbanions (ECG and DC) towards electrophilic substrates such as β−, γ− and σ-bromoesters (R 1X) was studied. The yields of mixed diesters (RR 1) and of unsaturated diesters R 1−H were correlated with the concentration ratio of EGC and DC. Electrochemical measurements, such as polarography, coulometry and cyclic voltammetry, both at Hg and vitreous carbon electrodes, as well as gas-chromatographic analyses of the solutions after reduction, were employed to ascertain the reactivity of these carbanions towards the substrates.