of the Lewis acidity of the endocyclic boron atom for coordination to the ketone. Scope and limitations Stereo and chemoselectivity The CBS reduction has proven to be an effective and powerful method to reduce a wide range of different types of ketones in both a stereoselective and chemoselective manner. Substrates include a large variety of aryl-aliphatic, di-aliphatic, di-aryl, α,β unsaturated enone and ynone systems, as well as ketones containing heteroatoms. Combinations of different derivatives of the CBS catalyst and borane reducing agents have been employed to optimize enantioselectivity. Several interesting cases are worth noting in this selection of substrates. First, in the case of the diaryl system 9, relatively high stereoselectively is achieved despite the isosteric nature of the ketone substituents, suggesting that electronics in addition to sterics may play a role in the stereoselectivity of the CBS reduction. Differences in the substitution of the alkyne moieties in ynones 11 and 12 result in a change of selectivity for the alkyne to function as the more sterically bulky substituent rather than the smaller one. For the α,β unsaturated systems 10-12, efficient reduction of the ketone occurs despite the possible side reaction of hydroboration of the C-C unsaturated bond. The CBS reduction has also been shown to tolerate the presence of heteroatoms as in ketone 13, which is capable of coordinating to the borane. Experimental considerations and limitations The presence of water in the reaction mixture has been shown to have a significant effect on enantiomeric excesses, and thus the CBS reduction must be conducted under anhydrous conditions. Temperature also plays a critical role in the observed stereoselectivity. In general, at lower temperatures enantiomeric excesses (ee's) are obtained. However, when the temperature is increased, the ee values reach a maximum value that is dependent on the catalyst structure and borane reducing agent used. The use of the borane reagent catecholborane, which has been shown to participate in CBS reductions carried out at temperatures as low as -126 °C with marked enantioselectivity, offers a potential solution to improving the diminished ee values obtained at lower temperatures. Enantioselectivity issues associated with the use of BH3 as the reducing agent for the CBS reduction have been reported. Commercially available solutions of BH3•THF evaluated by Nettles et al. were shown to contain trace amounts of borohydride species, which participate in nonselective reductions that led to the diminished enantioselectivity. Though the borohydride catalyzed reduction pathway is