Stoichiometric reduction of primary alkyl monohalides with electrogenerated nickel(I) salen: Formation of aldehydes

Addition of a stoichiometric amount of a primary alkyl monohalide (1-bromoalkane or 1-iodoalkane) to a solution of nickel(I) salen electrogenerated at a reticulated vitreous carbon cathode in dimethylformamide (DMF) containing 0.10 M tetramethylammonium tetrafluoroborate (TMABF4) in the presence of water, followed by irradiation with a xenon arc lamp and exposure to oxygen (O2), results in the formation of an aldehyde in substantial yield. Using cyclic voltammetry (CV) and controlled-potential electrolysis (CPE), we have explored the effects of light, temperature, water, and a proton donor on the process. Reduction of an alkyl monohalide in the presence of isotopically labeled reagents, along with CV-CPE experiments (in combination with ultraviolet-visible spectroscopy) and high-performance liquid chromatography (HPLC), suggests the possibility that O2 interacts with an alkylnickel(II) species (which could arise from a two-to-one nickel(I) salen-to-alkyl monohalide reaction in the presence of water) to produce an aldehyde, which has a tendency to undergo a base-catalyzed aldol condensation. In addition to the aldehyde, other products (dimers, alkanes, and alkenes) are formed via classic radical reactions.