Zinc and manganese redox potentials in organic solvents and their influence on nickel-catalysed cross-electrophile coupling

Abstract

Zinc and manganese are widely used as reductants in synthetic methods, such as nickel-catalysed cross-electrophile coupling (XEC) reactions, but their redox potentials are unknown in organic solvents. Here we show how open-circuit potential measurements may be used to determine the thermodynamic potentials of Zn and Mn in different organic solvents and in the presence of common reaction additives. The impact of these Zn and Mn potentials is analysed for a pair of Ni-catalysed reactions, each showing a preference for one of the two reductants. Ni-catalysed coupling of N-alkyl-2,4,6-triphenylpyridinium reagents (Katritzky salts) with aryl halides are then compared under chemical reaction conditions, using Zn or Mn reductants, and under electrochemical conditions performed at applied potentials corresponding to the Zn and Mn reduction potentials and at potentials optimized to achieve the maximum yield. The collective results illuminate the important role of reductant redox potential in Ni-catalysed XEC reactions.