Stoichiometric reduction of secondary alkyl monohalides by electrogenerated nickel(I) salen in the presence of oxygen and water: Prospects for the formation of ketones

A one-to-one reaction between electrogenerated nickel(I) salen and a secondary alkyl monohalide (2-bromo- or 2-iodohexane, 2-bromo- or 2-iodooctane, or 3-bromooctane) in dimethylformamide (DMF) containing 0.10 M tetramethylammonium tetrafluoroborate (TMABF4), and in the presence of atmospheric oxygen (O2), results in the formation of a ketone in up to 24% yield, depending upon the identity of the alkyl halide and other experimental conditions. However, for a two-to-one nickel(I) salen-secondary alkyl monohalide ratio, the ketone is obtained in 35% yield. In a fashion similar to production of an aldehyde via the interaction of nickel(I) salen with a primary alkyl monohalide, water that is deliberately added to the DMF–0.10 M TMABF4 medium noticeably improves the yield of ketone, but the optimal water concentration required for the ketone tends to be higher than that for an aldehyde. Other products are alkanes, alkenes, and dimeric species that are derived from classic radical pathways, and the presence of a proton donor (1,1,1,3,3,3-hexafluoro-2-propanol or acetic acid) greatly improves the total product yield, especially the amount of an alkane.