Ketones, catalytic chemoselective reducing

Catalytic hydrogenation reduces the imine (as the protonated iminium ion) but not the ketone from which it is formed. This chemoselectivity (reduction of iminium ions but not ketones) is also displayed by sodium cyanoborohydride and we can add NaCNBH3 to complete our table of reactivity, if we insert imines at the left-hand end. 

Chemoselective catalytic reduction of a,/3 unsaturated ketones to allylic alcohols is a challenging problem since, but a few exceptions [1-3], this reaction generally proceeds with formation of saturated ketones or saturated alcohols [4]. This reduction indeed is best carried out with stoicheiometric hydrides [4] but even in this case overreduction products are often obtained [5]. Recently, we reported in a preliminary communication [6] the unprecedented observation that a,/3 unsaturated ketones are reduced to the corresponding allylic alcohols by hydrogen transfer from propan-2-ol over MgO as catalyst according to the following scheme ... 

Conjugate Reduction. The carbon-carbon double bonds of various a, 8-unsaturated ketones and related compounds can be selectively reduced with a heterogeneous palladium catalyst. When nonpolar solvents such as toluene were employed, 1,4-reduction products of a,/8-unsaturated ketones, esters, amides, and nitriles were obtained in high yields (87-100%) with high chemoselectivities in the presence of a catalytic amount of Pd/C(2.5-5 mol %) and acetic acid. Highly-polar solvents enhanced 1,2-reduction to afford alcohols as a result of overreduction. 

The hydrogenation of a,j0-unsaturated aldehydes to allylic alcohols in quantitative yields in j -PrOH is catalysed by [FeF(P(PhPPh2)3)][BF4] in the presence of trifluo-roacetic acid. The catalyst could reduce a broad range of aromatic and aliphatic aldehydes in the presence of other reducible moieties (ketones, olefins, esters, etc.) with high chemoselectivity. Computational calculations and detection of the hydride species [FeH(H2)(P(PhPPh2)3)]", in situ, suggest a catalytic cycle. ... 

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