Selective hydrogenation of carvone

(R)-Carvone, responsible for the smell of spearmint, has two C=C double bonds: a trisubstituted one conjugated with the carbonyl, in the ring, and a 1,1-disubstituted one in the isopropenyl group.

With Wilkinson’s catalyst under a hydrogen atmosphere, in benzene at room temperature, only the double bond of the isopropenyl group, less substituted and more accessible to the bulky rhodium complex, is reduced, and (R)-carvotanacetone is obtained. The stereocentre is untouched, and neither the enone nor the carbonyl is reduced.