Rationally tuning the electronic effect of catalysts is one of the most important strategies to improve stereoselectivity in asymmetric catalysis. (S)-2-aryl-3,1,2-oxazaborobicyclo[3.3.0] octanes, which can be considered as electronically tuned (S)-2-phenyl-3,1,2-oxazaborobicyclo[3.3.0] octane, were prepared and evaluated in the asymmetric borane reduction of ketones. An unexpected influence of the electronic effect of catalysts on the enantioselectivity was observed and attributed to the catalyst dimerization that was further confirmed experimentally. The unsuccessful tuning is accounted for by assuming that hydride transfer in the catalytic cycle is the rate-determining step in the reduction. catalyzed by B-aryl catalysts. (C) 2008 Published by Elsevier B.V.