Advances in catalytic metal-free reductions: from bio-inspired concepts to applications in the organocatalytic synthesis of pharmaceuticals and natural products

This review focuses on recent advances in catalytic metal-free transfer hydrogenations. In recent years dihydropyridines have been widely used as reducing agents in organocatalytic reductions. Analogous to nature's co-factor nicotinamide adenine dinucleotide (NADH), Hantzsch esters serve as efficient hydride donors. In combination with chiral organocatalysts, including chiral secondary amines, hydrogen bond donors or Brønsted acids, efficient catalytic asymmetric reductions have been developed which provide a diverse set of biologically active compounds, synthetic building blocks and natural products. These recent advances in developing green and sustainable reductions employing organocatalytic strategies are promising and important alternatives to conventional metal- and bio-catalyzed reductions.