Insights into the radical-radical and radical-substrate dimerization processes for substituted phenylmethylenepyrans

The electrochemically-induced C-C bond making/breaking for six differently R-substituted phenylmethylenepyrans has been investigated by voltammetry in organic media. All compounds display an irreversible oxidation peak whose potential is fully dependent on the electrophilic property of the substituent R. The electrochemical oxidation yields bis-pyrylium compounds by sigma-sigma C-C bond formation. The initial methylenepyrans are recovered by cleavage of the C-C bond through electrochemical reduction of the bis-pyrylium species. According to the voltammetric analysis, the mechanistic pathway, radical-radical or radical-substrate, for the intermolecular dimerization is fully R-dependent. Electronic structure calculations show that the spin population in the radical cation and the strength of the sigma-sigma C-C bond in the dimer strongly depend on the nature of R. In addition, low-temperature electrochemical voltammetry (175 K), and room-temperature high scan rate cyclic voltammetry have been used to unravel the kinetics of the C-C bond formation. (C) 2019 Elsevier Ltd. All rights reserved.