Hydrogen-Mediated C−C Bond Formation: Catalytic Regio- and Stereoselective Reductive Condensation of α-Keto Aldehydes and 1,3-Enynes

Hydrogenation of 1,3-enynes in the presence of α-keto aldehydes using cationic Rh(I) catalysts
enables regio- and stereoselective reductive coupling to the acetylenic terminus of the enyne to afford
(E)-2-hydroxy-3,5-dien-1-one products. Reductive condensation of 1-phenyl but-3-en-1-yne 1a with phenyl
glyoxal 2a performed under an atmosphere of D2 provides the product of mono-deuteration, (E)-2-hydroxy-3-deuterio-3,5-dien-1-one deuterio-3a, in 85% yield. Competition experiments involving catalytic hydrogenation of phenyl glyoxal in the presence of equimolar quantities of 1,4-diphenylbutadiene and 1,4-diphenylbut-3-en-1-yne 10a, as well as 1,4-diphenylbut-3-en-1-yne 10a and 1,4-diphenylbutadiyne, are chemoselective
for coupling to the more highly unsaturated partner, suggesting a preequilibrium involving precoordination
and exchange of the π-unsaturated pronucleophiles with the catalyst prior to C−C bond formation, as well
as a preference for coordination of the most π-acidic reacting partner, as explained by the Dewar−Chatt−Duncanson model for alkyne coordination.