carbonyls are more readily reduced to their respective alkanes than aliphatic compounds. For example, ketones are reduced to their respective alkyl benzenes by catalytic hydrogenation or by Birch reduction under mild conditions. α,β-unsaturated carbonyls In α,β-reduction (also called conjugate reduction), the substrate is an α,β-unsaturated carbonyl, an enone or enal. When these substrates are reduced, 1,2-reduction - which produces an allyl alcohol - is in competition with the 1,4-reduction - which forms the saturated ketone or aldehyde. The following NaBH4 reduction of an enone shows two possible products: the first from 1,4-reduction and the second from 1,2-reduction. The more sterically hindered the enone substrate, the more likely 1,2 reduction becomes. Additionally, to selectively form the alcohol and avoid the 1,4 product, the Luche reduction uses the bigger molecule Ce(BH4)3 (derived from NaBH4 and CeCl3 combined in situ) as the hydride source. The hydride source Zn(BH4)2 also shows 1,2 selectivity, as well as greater diastereoselectivity; it does so by coordinating not only to the carbonyl oxygen but also to adjacent atoms: Stereoselectivity Diastereoselective reduction In the reduction of cyclohexanones, the hydride source can attack axially to produce an equatorial alcohol, or equatorially to produce an axial alcohol. In axial attack (shown in red), the hydride encounters 1,3-diaxial strain. In equatorial attack (shown in blue), the hydride avoids the 1,3-diaxial interaction, but the substrate undergoes unfavorable torsional strain when the newly formed alcohol and added hydrogen atom eclipse each other in the reaction intermediate (as shown in the Newman projection for the axial alcohol). Large reducing agents, such as LiBH(Me2CHCHMe)3, are hindered by the 1,3-axial interactions and therefore attack equatorially. Small reducing agents, such as NaBH4, preferentially attack axially in order to avoid the eclipsing interactions, because the 1,3-diaxial interaction for small molecules is minimal; stereoelectronic reasons have also been cited for small reducing agents' axial preference. Making the substrate bulkier (and increasing 1,3-axial interactions), however, decreases the prevalence of axial attacks, even for small hydride donors. Enantioselective reduction When asymmetrical ketones are reduced, the resulting secondary alcohol has a chiral center whose can be controlled using chiral catalysts. Well-known carbonyl reductions in asymmetric synthesis are the Noyori asymmetric hydrogenation (beta-ketoester reduction/Ru/BINAP) and the CBS reduction (BH3, proline derived chiral catalyst). See also Baker's yeast, a biotransformation route for carbonyl reductions. References Organic redox reactions Nucleophilic addition reactions