Coupling and fast decarboxylation of aryloxyl radicals of 4-hydroxycinnamic acids with formation of stable p-quinomethanes

The reaction at room temperature of 3,5-di-tert-butyl- and 3,5-di-methoxy-4-hydroxycinnamic acids 1 and 2 with the dpph radical in acetone or other non-hydroxylic polar solvents yields interesting dimeric p-quinomethanes 10-16 characterized by a broad and strong absorption in the visible region. Although the yields appear to be low to moderate (10-40%), this simple synthesis affords quinones not otherwise obtainable, which contain an unsaturated γ-lactone ring (14-16). The structures have been elucidated by interpretation of ESI-MS, FT-IR and NMR spectral data. In particular, FT-IR spectra in a KBr matrix demonstrate the quinone nature of these compounds because of the presence of strong absorption bands at 1604-1640 cm⁻¹ and allows excluding the presence of carboxylic acid groups in the molecules. Kinetic evidence and molecular structures suggest that the formation of these p-quinomethanes is best explained through an 8-8 C-C coupling of the aryloxyl radicals derived from 1 and 2 and a subsequent fast mono- or di-decarboxylation of the initial dimer by an S_E1-type mechanism. Further oxidation of the phenolic intermediates by dpph yields the final quinones.