Recent Advances in Transition-Metal-Catalyzed [2+2+2]Cyclo(co)trimerization Reactions

Transition-metal-catalyzed [2+2+2]cyclo(co)trimerization reactions are a powerful methodology to synthesize various complex multi-substituted (poly) cyclic molecules in a single step with optimal atom efficiency. Ever since the discovery, the reaction has been plagued by issues concerning regio- and chemoselectivity. Over the last decades many advances have been made to overcome these issues by, for example, employing regio-directing groups or tethering the reaction partners together in an intramolecular approach. These solutions, however, have certain limitations. Nowadays, it is also possible to synthesize chiral molecules by performing an asymmetric transition-metal-catalyzed [2+2+2]-cyclo(co)trimerization reaction. This review focuses on the recent advances in mechanistic insight, solving the regioselectivity issue, synthesis of chiral molecules and alternative approaches for the synthesis of substituted benzenes, pyridines and 2-pyridones. In addition, recent applications in areas such as total synthesis of natural products are also described, demonstrating that the transition-metal-catalyzed [2+2+2]-cyclo(co)trimerization reaction is a powerful tool and a welcome addition to the chemist's 'synthetic toolbox'. 1 Introduction 2 Synthesis of Benzene Derivatives 2.1 Mechanistic Insight 2.2 Regioselectivity 2.3 Synthesis of Chiral Systems 2.4 Other [2+2+2] Approaches 2.5 Synthesis of (Iso)quinolines 3 Synthesis of Pyridine Derivatives 3.1 Mechanistic Insight 3.2 Regioselectivity 3.3 Synthesis of Chiral Systems 4 Synthesis of Pyridone Derivatives 4.1 Mechanistic Insight 4.2 Regioselectivity 4.3 Synthesis of Chiral Systems 5 Applications 6 Future Directions 7 Conclusion