Countercation- and solvent-controlled selective borohydride hydrogenation of alkenes in diaryl enones

Abstract

Borohydrides are considered benchmark reagents for the selective hydrogenation of ketones in the presence of alkenes, a reaction described in organic textbooks. However, the opposite, i.e. the borohydride-promoted hydrogenation of an alkene in the presence of a ketone, is barely described. Here we show that the alkene functionality in diaryl enones is preferentially hydrogenated to the ketone under standard uncatalyzed reaction conditions, after using a stoichiometric amount of a metal borohydride (i.e. NaBH₄). For gem-diaryl enones, mechanistic studies indicate that the combination of a suitably cation-substituted borohydride (from Li⁺ to K⁺) and the particular disposition of the highly-conjugated terminal alkene favors a highly selective 1,4-hydride addition, giving access to α-benzyl-substituted propiophenones in high yields, at room temperature and after just 30 min reaction time, without the assistance of any catalyst or additive. For trans-diaryl enones (chalcones), the simple change of the protic co-solvent from MeOH to electron-deficient and sterically-hindered alcohols triggers the selective hydrogenation of the alkene group. These results defy the established reactivity of borohydrides for enones and open a way to employ common borohydride reagents for selective alkene hydrogenation reactions, with potential application in synthetic chemistry.