Theoretical investigation toward organophosphine‐catalyzed [3 + 3] annulation of Morita–Baylis–Hillman carbonates with azomethine imines: Mechanism, origin of stereoselectivity, and role of catalyst

Abstract

AbstractA computational study on the detailed mechanism and stereoselectivity of the chiral phosphine‐catalyzed C(sp2)H activation/[3 + 3] annulation between Morita–Baylis–Hillman (MBH) carbonates and C,N‐cyclic azomethine imines has been performed. Generally, the catalytic cycle consists of two stages, that is, C(sp2)H activation companied by the dissociation of the t‐BuO group forming phosphonium enolate, and [3 + 3] cycloaddition process followed by regeneration of the catalyst. The calculated results indicate that C(sp2)H activation is rate‐determining while [3 + 3] cycloaddition is stereoselectivity‐determining. Furthermore, the advantageous hydrogen bond interactions and less steric hindrance in the RR configurational CC bond forming transition states should be responsible for the favorability of RR‐configured product among the four possible products. The special role of the organocatalyst was also identified by natural bond orbital (NBO) and global reactivity index (GRI) analyses. The mechanistic insights obtained in the present study should be useful for understanding the novel organocatalytic C(sp2)H activation and cycloaddition cascade reaction of MBH carbonates, and thus provide valuable clues on rational design of efficient organocatalysts for the C(sp2)H activation/functionalizations.