The reaction of 9-aryl-9-borabicyclo[3.3.1]nonanes (B-Ar-9BBN) with alpha,beta-unsaturatedketones and aldehydes in the presence of 3 mol % [Rh(OMe)(cod)](2) in toluene at 20 degrees C for 2 h gave high yields of the tandem 1,4-addition-aldol reactionproducts with high syn selectivity. The reaction proceeds through the catalytic cycle consisting of 1,4-addition of an organorhodium species to an alpha