Chapter 20: Formation and Reactions of Enols and Enolates

The chapter introduces hydride donor reagents as the primary tools for carbonyl reduction, contrasting lithium aluminium hydride with sodium borohydride in terms of reactivity and selectivity profiles. Lithium aluminium hydride demonstrates broader reactivity across multiple functional group classes but shows lower selectivity, while sodium borohydride provides milder conditions suitable for selective transformations in complex molecules. The text extends these concepts to more challenging substrates including esters, carboxylic acids, and nitrogen-containing compounds, explaining how different reagent choices and reaction conditions enable partial or complete reduction. Diisobutylaluminium hydride appears as a specialized reagent for controlled partial reductions, exemplified by the conversion of esters to aldehydes without over-reduction. Catalytic hydrogenation using transition metal catalysts represents an alternative reduction strategy applicable to unsaturated carbon frameworks and aromatic systems, with detailed analysis of stereochemical outcomes including syn and anti addition patterns. The chapter emphasizes chemoselectivity and stereoselectivity as critical concepts, demonstrating how synthetic chemists select appropriate reagents and conditions to reduce targeted functional groups while preserving others in polyunctional molecules. Advanced treatment includes enzymatic reduction pathways utilizing biological cofactors such as NADH and NADPH, which illustrate how reduction chemistry operates in living systems and informs drug design strategies. Throughout the chapter, mechanistic understanding combines with practical synthetic application, enabling students to approach multi-step synthesis problems with precision in controlling reactivity, selectivity, and molecular stereochemistry through systematic reduction strategies.