Electroorganic synthesis in aqueous solution via generation of strongly oxidizing and reducing intermediates

Abstract

Water is the ideal green solvent for organic electrosynthesis. However, a majority of electroorganic processes require potentials that lie beyond the electrochemical window for water. In general, water oxidation and reduction lead to poor synthetic yields and selectivity or altogether prohibit carrying out a desired reaction. Herein, we report several electroorganic reactions in water using synthetic strategies referred to as reductive oxidation and oxidative reduction. Reductive oxidation involves the homogeneous reduction of peroxydisulfate (S₂O₈²⁻) via electrogenerated Ru(NH₃)₆²⁺ at potential of −0.2 V vs. Ag/AgCl (3.5 M KCl) to form the highly oxidizing sulfate radical anion (E⁰′ (SO₄˙⁻/SO₄²⁻) = 2.21 V vs. Ag/AgCl), which is capable of oxidizing species beyond the water oxidation potential. Electrochemically generated SO₄˙⁻ then efficiently abstracts a hydrogen atom from a variety of organic compounds such as benzyl alcohol and toluene to yield product in water. The reverse analogue of reductive oxidation is oxidative reduction. In this case, the homogeneous oxidation of oxalate (C₂O₄²⁻) by electrochemically generated Ru(bpy)₃³⁺ produces the strongly reducing carbon dioxide radical anion (E⁰′ (CO₂˙⁻/CO₂) = −2.1 V vs. Ag/AgCl), which can reduce species at potential beyond the water or proton reduction potential. In preliminary studies, the CO₂˙⁻ has been used to homogeneously reduce the C–Br moiety belonging to benzyl bromide at an oxidizing potential in aqueous solution.