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 S_N2 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 S_N2 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

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