Rhodium-catalyzed 1,4-addition of arylboronic acids to α,β-unsaturated carbonyl compounds:: Large accelerating effects of bases and ligands

The effects of ligands and bases in the rhodium(I)-catalyzed 1,4-addition of arylboronic acids to alpha,beta-unsaturated carbonyl compounds were reinvestigated to carry out the reaction under mild conditions. Rhodium(l) complexes possessing a 1,5-cyclooctadiene (cod) and a hydroxo ligand such as [RhOH(cod)](2) exhibited excellent catalyst activities compared to those of the corresponding rhodium-acac or -chloro complexes and their phosphine derivatives. The reaction was further accelerated in the presence of KOH, thus allowing the 1,4-addition even at 0 degreesC. A cationic rhodium(I)-(R)-binap complex, [Rh(R-binap)(nbd)]BF4, catalyzed the reaction at 25-50 degreesC in the presence of Et3N with high enantioselectivities of up to 99% ee for alpha,beta unsaturated ketones, 92% for aldehydes, 94% for esters, and 92% for amides.