Janus-faced 5-methyl group in 2-hydroxy-5-methyl-[1,4]-benzoquinone

Abstract

The 5-methyl group in 2-hydroxy-[1,4]-benzoquinone shows a peculiar reactivity that depends on the pH of the reaction medium. Under acidic conditions, ortho-quinone methide tautomers of the parent para-quinones are present in equilibrium. The 5-methylene group is stabilized by resonance, incorporating canonic structures with a positive charge at this position and now acting as carbenium equivalent. These ortho-quinone methide intermediates can be trapped in hetero-Diels-Alder reactions with inverse electron demand or by reaction with trimethylsilyl chloride. Under alkaline/basic conditions, the 5-methyl group is deprotonated, this time performing as carbanion equivalent being once more stabilized by resonance. This facet of reactivity was used for instance in regioselective perdeuteration of the 5-methyl position or in Michael additions. The reaction medium is thus able to govern the chemical behavior of the 5-methyl group, to effect its umpoling and to switch the reactivity to the opposite. Like the Roman god Janus, these methyl groups have two faces, and the reaction medium decides which one is presented.