Metal-Free Reagents and Catalysts for C–C Multiple Bond Reduction

Organic reductants are increasingly important for the reduction of C–C multiple bonds, as they avoid scarce platinum-group metals and broadly tolerate Lewis-basic motifs common in medicinal chemistry. Diverse reduction mechanism via hydride transfer, protonation, hydroboration, frustrated Lewis pair-mediated H₂ activation, diimide-mediated hydrogen transfer, and photoinduced electron, energy, or hydrogen atom transfer provide distinct and often complementary reactivity orders enabling predictable reductions even in complex molecules. This review discusses the main mechanisms and categorizes them based on the reducible substrate classes and the tolerance of Lewis-basic motifs, highlighting methods with particular relevance to medicinal chemistry applications.