Plasma Electrochemistry for Carbon-Carbon Bond Formation Via Pinacol Coupling Reaction

Abstract

Low-temperature, atmospheric-pressure plasmas in contact with liquids have attracted interest for various chemical applications including the degradation of organic pollutants,1 conversion of abundant feedstocks,2,3 and more recently, organic chemistry.4 Compared to other chemical approaches, plasma-liquid chemistry does not require a catalyst material, is electrified, and produces unique reactive species such as solvated electrons, one of the strongest chemical reducing species.5 Here, we present an application of plasma-liquid chemistry to the formation of carbon-carbon bonds via the well-known pinacol coupling reaction. Pinacol coupling is conventionally carried out by using an electron donating reagent such as magnesium to initially reduce a carbonyl group forming a ketyl radical anion species. A pair of these ketyl groups then react to form a vicinal diol, which in the presence of a proton donor such as water, leads to the final diol product. We show that the pinacol coupling reaction can instead be driven at a plasma-liquid interface without any catalyst. Our study was performed using a direct-current (DC) powered plasma formed between a metal electrode and liquid water surface similar to previous reports.2 Parametric studies were focused on methyl-4-formylbenzoate (MFB) as the substrate. We find that the faradaic efficiency of the pinacol product depends on the initial concentration, increasing from 20% to 80% as the concentration increased from 0.06 M to 0.18 M. For an initial concentration of 0.12 M and a constant operating current of 3 mA, a yield of 46% can be reached after 12 h. The experimental results were supported by a reaction-diffusion model, and a rate constant of 10^10 L-1 mol-1 s-1 was extracted from kinetic studies. In addition to the pinacol product, nuclear magnetic resonance (NMR) spectroscopy also indicated methyl 4-(dimethoxymethyl)benzoate, methyl 4-(hydroxymethyl)benzoate, and 4-(methoxycarbonyl)benzoic acid as side products. By carrying out scavenger control experiments, we show that the vicinal diol is produced by solvated electron reduction while these other side products are formed by reactions with other radicals or the solvent. The generality of the approach is demonstrated by extending to several other aromatic aldehydes and ketones.