The Electron-Way: Metal-Catalyzed Reductive Umpolung Reactions of Saturated and α,β-Unsaturated Carbonyl Derivatives

Reductive umpolung reactions of saturated and unsaturated carbonyl compounds enable the direct synthesis of 1,2-, 1,4-, 1,6-, etc. substituted carbon frameworks that are difficult to access by other methodologies. Herein, the evolution from stoichiometric to catalytic processes with high chemo-, regio- and stereoselectivity is discussed for each carbon–carbon bond connection type. At certain points, summaries of the known reaction conditions and discussions of the underlying mechanisms are included. 1 Introduction 1.1 Redox Umpolung 2 Catalyzed Pinacol Couplings and Related Reductive Homo-Dimerization Reactions 2.1 Evolution of Catalytic Systems 2.2 Diastereoselective Pinacol Coupling Reactions 2.3 Enantioselective Pinacol Coupling Reactions 2.4 Pinacol-Type Homo-Dimerization of Imines 2.5 Reductive Homo-Dimerization of Carboxylic Acid Derivatives 3 Cross-Coupling Reactions of Aldehydes, Ketones or Imines by Catalyzed Reductive Umpolung 3.1 Cross-Pinacol Coupling of Aldehydes and Ketones 3.2 Cross-Coupling of Carbonyl Derivatives with Michael Acceptors 3.3 Cross-Coupling of Carbonyls with a Carboxylic Acid Derivative or Nitrile 4 Reductive Coupling between α,β-Unsaturated Fragments 4.1 Reductive Cyclization Reactions 4.2 Intermolecular Reductive Homo-Coupling 4.3 Intermolecular Reductive Cross-Coupling 5 Conclusion