Catalytic asymmetric 4+2 cycloadditions of unsaturated hydrocarbons by transition metal catalysis and photocatalysis

The enantioselective Diels-Alder reaction is arguably one of the most efficient and straightforward approaches for the construction of chiral six-membered carbocycles. However, the generality of this conventional reaction is limited by restrictive electronic requirements and substitution patterns according to the classic Hoffmann-Woodward rules. To circumvent these limitations, several innovative approaches have emerged in recent years. Using these approaches, various types of electronic mismatched 4+2 cycloadditions of unsaturated carbons, such as alkenes, alkynes, and allenes, have been realized. This review will summarize these advancements in this rapidly growing area within the past three decades, including the transition metal-catalyzed asymmetric 4+2 cycloadditions via the oxidative cyclometallation and reductive elimination pathway, catalytic asymmetric 4+2 cycloadditions via the LUMO activation by π-acids and the HOMO activation by π-bases, as well as catalytic asymmetric radical-mediated 4+2 cycloadditions under light irradiation.