Low-temperature sonoelectrochemical processes: Part 3. Electrodimerisation of 2-nitrobenzylchloride in liquid ammonia

Dinitrobibenzyls are key intermediate species in certain drug syntheses. They can be formed by the electrochemical reduction of nitrobenzyl halides (in this work, 2-nitrobenzylchloride) in various solvents. In liquid ammonia at −60°C, the mechanism involves a one-electron reduction and de-chlorination followed by the coupling of the neutral radical intermediate species. Exhaustive voltammetric studies, including fast scan cyclic voltammetry, of the starting material are presented prior to preparative electrolysis experiments. Electrolysis under both potentiostatic and galvanostatic conditions are compared. Under conditions of severe dryness, potentiostatic reduction at platinum gauze set at a voltage of −0.30 V (vs. Ag wire) in the presence of ultrasound yields the dimer 2,2-dinitrobibenzyl (>95%) and no detectable side products. Water and oxygen have been found to decrease both the current efficiency and product yield of the process. Ultrasound is beneficial by: (i) enhancing the dissolution kinetics of the starting material; (ii) mass transport from the bulk towards the electrode, and vice versa, is greatly enhanced, thus considerably reducing the reaction times and optimising the current efficiency and product yields.