Homogeneous electron transfer catalysis in the electrochemical carboxylation of arylethyl chlorides

The electrochemical carboxylation of arylethyl chlorides (ArCH(CH3)Cl; Ar = 4-biphenyl (1), 6-methoxy-2-naphthyl (2) and 4-isobutylphenyl (3)) catalysed either by nickel(I) Schiff base complexes (NiI(L)) or by radical anions (D•-) derived from aromatic esters has been investigated in acetonitrile. Controlled-potential electrolyses of CO2-saturated CH3CN solutions containing 1-2 mM catalyst and five- to ten-fold excess of arylethyl chloride (RCl) resulted in the formation of a mixture of the corresponding 2-arylpropanoic acid (RCO2H) and arylethane (RH). The yield of the acid was strongly dependent on the standard potential of the catalyst (E°cat), increasing with decreasing E°cat. The mechanism of the catalytic process has been examined. Both types of catalyst follow a reaction mechanism mainly based on electron transfer reactions. In the first step of the electrocatalytic process, the halide reacts with the reduced form of the catalyst, NiI(L) or D•-, to give an arylethyl radical. Further reduction of the radical to the corresponding carbanion is followed by electrocarboxylation in competition with protonation by residual water. The yield of the carboxylic acid is ruled by the ease of reduction of the arylethyl radical. Analysis of the whole set of data obtained for the chlorides 1-3 under different experimental conditions has shown that the RCO2H yield correlates well with the difference between the standard potentials of the arylethyl radical (R•) and catalyst (EoR•/R – E°cat). It became evident from such a correlation that efficient redox catalysis requires the use of a catalyst couple with an E°cat value about 0.4-0.5 V more negative than E°R•/R-.