Selective Acceptorless Dehydrogenation and Hydrogenation by Iridium Catalysts Enabling Facile Interconversion of Glucocorticoids

Abstract

An iridium(III) pentamethylcyclopentadienyl catalyst supported by 6,6′-dihydroxy-2,2′-bipyridine displays exquisite selectivity in acceptorless alcohol dehydrogenation of cyclic α,β-unsaturated alcohols over benzylic and aliphatic alcohols under mild aqueous reaction conditions. Hydrogenation of aldehydes and ketones occurs indiscriminately using the same catalyst under hydrogen, although chemoselectivity could be achieved when other potentially reactive carbonyl groups present are sterically inaccessible. This chemistry was demonstrated in the reversible hydrogenation and dehydrogenation of the A ring of glucocorticoids, despite the presence of other alcohol/or carbonyl functionalities in rings C and D. NMR studies suggest that an iridium(III) hydride species is a key intermediate in both hydrogenation and dehydrogenation processes.