Oxidation of mono-phenols to para-benzoquinones: a comparative study

the oxidation of mono-phenols to para-benzoquinones is of continuing interest due to the existence of numerous natural products containing this structural unit. the chemical reactivity of para-benzoquinones is also noteworthy, as oxidants and dienophiles in the diels-alder reaction. we have used for quite some time now, molecular oxygen and catalysis with [coii(salen)] as the oxidation procedure, but felt the need for other oxidants and conditions to be of use with different phenol substrates. we now present our results on this important oxidation with a variety of oxidants, using eleven mono-phenols as substrates. the oxidants tested are cobalt, nickel, copper and vanadyl metals, with a selection of different salen type ligands. completing this study we also investigated the use of hydrogen peroxide, oxone？, dimethyl dioxirane and iodoxybenzoic acid.