Gold(I)-catalyzed [4 + 1]/[4 + 3] annulations of diazo esters with hexahydro-1,3,5-triazines: Theoretical study of mechanism and regioselectivity

Abstract

Hexahydro-1,3,5-triazines are often used as formaldimine precursors. The recent success of Sun et al. (Angew. Chem. Int. Ed., 2016, 55, 11867) in achieving gold-catalyzed [4 + 1]/[4 + 3] annulations of hexahydro-1,3,5-triazines with diazo esters, in which the hexahydro-1,3,5-triazine is directly used as the 1,4-dipole in the annulation, represents a breakthrough in mechanistic studies. In the present work, density functional theory (DFT) calculations were used for detailed investigation of possible catalytic mechanisms. The proposed catalytic cycle involves five major stages: dinitrogen elimination to form a gold–carbene intermediate; diazoacetate–hexahydro-1,3,5-triazine C–C coupling to give an ammonium ylide; ring opening of the hexahydro-1,3,5-triazine via C–N single-bond cleavage; a second C–N single-bond cleavage; and C–C-bond-forming reductive elimination. The regioselectivities of the cycloaddition of hexahydro-1,3,5-triazines with enol diazoacetates differ from those of cycloadditions with vinyl diazoacetates. The calculations also clarified the origins of the substrate-controlled regioselectivity.