Cyclic voltammetry (CV) and controlled-potential electrolysis (CPE) were employed to examine the reaction between electrogenerated ligand-reduced nickel(II) salen and (bromomethyl)cyclopropane. Cyclic voltammograms for nickel(II) salen in the presence of (bromomethyl)cyclopropane exhibit characteristic features for the catalytic reduction of the substrate. Bulk electrolyses of (bromomethyl)cyclopropane at carbon cathodes in dimethylformamide catalyzed by nickel(II) salen were carried out to investigate the mechanism for the formation of dialkylated nickel(II) salen, which was analyzed and identified by high-performance liquid chromatography (HPLC). The corresponding dialkylated nickel(II) salen was further purified and collected by preparative-scale HPLC. Its complete structure was revealed by electrospray-ionization mass spectrometry (ESI-MS), 1 H NMR, COSY, and HECTOR NMR spectrometry. The clear-cut reaction mechanism for its formation was proposed on the basis of current and previous studies.

Nickel(II) salen has been widely used as the catalyst for the electrochemical reduction of organic halides (RX). The corresponding catalytic reaction mechanism were examined and proposed by various research groups.1 -5 Generally, nickel(II) salen (1) would undergo a one-electron reversible reduction to generate either the metal-reduced nickel(I) salen (2) or the ligand-reduced radical-anion (3, Scheme 1), which can subsequently transfer an electron to the organic halide substrate to produce a radical and a halide ion. Afterward, the substrate radicals can undergo different reactions such as coupling,6,7 disproportionation,6 intramolecular cyclization,8,9 abstraction of hydrogen atom from solvent, etc to afford a series of products. However, side reactions may also take place to cause the alkylation of nickel(II) salen. As the result, a significant amount of substrates could be lost9 and nickel(II) salen would be deactivated.10,11 Peters and his colleagues proposed two possible routes (Route 1 or 2, Scheme 2) involving the SN2 nucleophilic substitution and radical coupling reactions between catalyst 3 and substrates for the formation of dialkylated nickel(II) salen.3,4 Alternatively, a derivative pathway (Route 3, Scheme 2) as well as the direct radical addition to the imino bond of nickel(II) salen (Route 4, Scheme 2)12cannot be ruled out. Nevertheless, a definite reaction mechanism still awaits further research. In this study, we employed (bromomethyl)cyclopropane as the substrate for the electrochemical reduction catalyzed by nickel(II) salen. The catalytic process should lead to the formation of cyclopropylmethyl radicals, which undergo an extremely fast ring opening rearrangement to give 3-butenyl radicals at a rate constant of 8.6×107 s-1 (298 K).13 The cyclopropyl ring could be retained in SN2 nucleophilic substitution while the radical coupling would involve 3-butenyl radicals. Consequently, the dialkylation of nickel(II) salen will render different products (4-7, Scheme 3), depending upon which reaction route it takes. We carried out cyclic voltammetry (CV) and controlled-potential electrolysis (CPE) for the initial investigations and the electrolyzed solution was subject to HPLC analysis. After purified by preparative-scale HPLC, the dialkylated nickel(II) salen was examined by ESI-MS, 1H NMR, COSY, and HECTOR NMR spectrometry. Its complete structure was resolved and found to be species 4. Thus, we concluded that Route 1 (Scheme 2) should be the plausible reaction mechanism for the dialkylation of nickel(II) salen in the catalytic reduction of organic halides. References D. M. Goken, D. G. Peters, J. A. Karty, and J. P. Reilly, J. Electroanal. Chem., 564, 123 (2004). J. A. Miranda, C. J. Wade, and R. D. Little, J. Org. Chem., 70, 8017 (2005). D. M. Goken, M. A. Ischay, D. G. Peters, J. W. Tomaszewski, J. A. Karty, J. P. Reilly, and M. S. Mubarak, J. Electrochem. Soc., 153, E71 (2006). P. W. Raess, M. S. Mubarak, M. A. Ischay, M. P. Foley, T. B. Jennermann, K. Raghavachari, and D. G. Peters, J. Electroanal. Chem., 603, 124 (2007). M. N. Nguyen, M. E. Tomasso, D. C. Easter, and C. Ji, J. Electrochem. Soc., 163, G1 (2016). M. S. Mubarak and D. G. Peters, J. Electroanal. Chem., 388, 195 (1995). A. L. Bulter and D. G. Peters, J. Electrochem. Soc., 144, 4212 (1997). M. S. Mubarak and D. G. Peters, J. Electroanal. Chem., 332, 127 (1992). D. M. Fang, D. G. Peters, and M. S. Mubarak, J. Electrochem. Soc., 148, E464 (2001). A. Gennaro, A. A. Isse, and F. Maran, J. Electroanal. Chem., 507, 124 (2001). A. A. Isse and A. Gennaro, J. Electrochem. Soc., 149, D113 (2002). H. Miyabe, Y. Yamaoka, and Y. Takemoto, J. Org. Chem., 71, 2099 (2006). V. W. Bowry, J. Lusztyk, and K. U. Ingold, J. Am. Chem. Soc., 113, 5687 (1991). Figure 1