Rhodium(I)- or ruthenium(II)-catalyzed direct coupling of vinyl ketones with aldehydes and the subsequent reduction to give aldol derivatives anti-selectively

Add time:08/25/2019    Source:sciencedirect.com

A vinyl ketone reacts with an aldehyde to give an α-methylene-β-hydroxyalkanone with the concomitant formation of the vinyl ketone dimer in the presence of catalytic amount of RhH(PPh3)4 or RuH2(PPh3)4 under almost neutral conditions. The selectivity of the cross-coupling product is remarkably improved in the presence of an extra mole of aldehyde. This type of cross-coupling is explained by the intermediacy of the transition metal enolate which is formed by the Michael-type addition of MH to a vinyl ketone. The subsequent hydrogenation of the carbon—carbon double bond of α-methylene-β-hydroxyalkanone proceeds readily to give aldol derivatives in the presence of the catalyst. [Rh(COD)(DPPB)]PF6, [COD = 1,5-cyclooctadiene, DPPB = 1,4-bis(diphenylphosphino)butane] is the best choice of catalyst and gives aldol derivatives anti-selectively. Thus, the two-step operation described provides aldol derivatives by an anti-selective route under almost neutral conditions.

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