Electrocarboxylation of acetophenone in ionic liquids: the influence of proton availability on product distribution

Abstract

The electroreduction of acetophenone has been investigated in two dry ionic liquids (1-butyl-2,3-dimethylimidazolium tetrafluoroborate ([BMMIM][BF₄]; [H₂O] = 9.2 mM) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMPyrd][TFSI]; [H₂O] = 1.0 mM)) under both N₂ and CO₂ atmospheres using transient cyclic voltammetry, near steady-state voltammetry, the bulk electrolysis technique and numerical simulations. The proton availability in both solvents is low. In these dry ionic liquids under a N₂ atmosphere, the sole reduction product detected is a dimer. The rate constants for dimer formation determined by comparison of experiment and simulation are 5.0 × 10⁴ M⁻¹ s⁻¹ and 4.0 × 10³ M⁻¹ s⁻¹, in [BMMIM][BF₄] and [BMPyrd][TFSI], respectively. In dry [BMMIM][BF₄] under a CO₂ atmosphere, the products of the electroreduction of acetophenone are mixtures of 2-hydroxy-2-phenylpropionic acid, 1-phenylethanol and dimers. By contrast, the major reduction product in dry [BMPyrd][TFSI] is 2-hydroxy-2-phenylpropionic acid, suggesting that this ionic liquid is a suitable medium for electrocarboxylation. In water saturated [BMPyrd][TFSI] ([H₂O] = 0.63 M), dimers are the major products under both N₂ and CO₂ atmospheres. The dimerization rate constant determined for this reaction under a N₂ atmosphere was 1.0 × 10⁷ M⁻¹ s⁻¹; more than three orders of magnitude higher than that found in dry [BMPyrd][TFSI]. Presumably strong interactions between the acetophenone radical anions and water through an extensive hydrogen bonding network lead to a larger degree of charge delocalization and thus favour dimer formation.