Steric Effects in the Reaction of Electrogenerated Ligand-Reduced Nickel Salen with Organic Halides

Nickel salen catalysts have been extensively used for the electrochemical reduction of various organic halides. In general, nickel(II) salen (1) would undergo one-electron reduction to generate either the metal-reduced nickel(I) salen (2) or the ligand-centered radical-anion (3, Scheme 1), which can subsequently transfer an electron to the organic halide substrate to produce a radical and a halide ion (1-3). In this catalytic process, the ligand-reduced nickel salen (3) may also react with alkyl halide to form a nickel(II) salen that is alkylated at the imino bonds of the ligand (2,3). Nevertheless, when the sterically hindered dimethylated nickel(II) salen is employed as the catalyst, the corresponding alkylation reaction could be minimized (4).

In this study, nickel salen-catalyzed reduction of benzyl bromide and 1-bromomethylnaphthalene was examined by cyclic voltammetry (CV) and controlled-potential electrolysis (CPE) for comparison. Furthermore, 1-bromooctane and bromodiphenylmethane were also used as the substrates for the reaction with electrochemically reduced nickel salen species (2 and 3). The results show that the reaction of ligand-reduced nickel salen (3) with organic halides can be significantly affected by the substrate structure due to steric effects.

Fig. 1 depicts the CVs recorded at a scan rate of 100 mV s^–1 for the reduction of nickel(II) salen in the presence of four different concentrations of benzyl bromide in dimethylformamide (DMF) containing 0.050 M tetramethylammonium tetrafluoroborate (TMABF₄). As shown by curves B-E, the first cathodic peak is approximately 100-150 mV more positive than that for reduction of nickel(II) salen by itself (curve A), attributed to the catalytic cleavage of carbon-bromine bond of the substrate. The magnitude of this peak increases with the substrate concentration and eventually it merges with the reduction peak of nickel salen (second cathodic wave at -1.70 V). Moreover, the anodic peak due to oxidation of nickel(I) salen disappears. These features are generally characteristic of the catalytic reduction of organic halides (3,4). Large scale electrolysis of 20 mM benzyl bromide with 2.0 mM nickel salen at reticulated vitreous carbon electrodes held at -1.55 V gives toluene, bibenzyl, and benzyl ether as the major products in a total yield of 94%, which is higher than that for the catalytic reduction of 1-bromooctane (86%) (5), suggesting the reaction of 3 with benzyl bromide should be less prominent than with 1-bromooctane to form the alkylated nickel salen. Similar CV and CPE results were obtained for the catalytic reduction of 1-bromomethylnaphthalene, mostly due to steric effect caused by the bulky substrate.

Additionally, the reactivity of electrogenerated ligand-reduced nickel salen (3) towards the four organic halides follows the sequence of 1-bromooctance > benzyl bromide > 1-bromomethylnaphthalene > bromodiphenylmethane, as revealed by Fig. 2. The alkylation of 3 diminishes as the substrate molecules turn to be more bulky and therefore sterically hinder the corresponding nucleophilic reactions (4,5).

 References

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