Rh2(OAc)4-catalyzed reaction of 2-(2-carbonylvinyl)-3-phenyl-2H-azirines with diazo esters

Abstract

Rh₂(OAc)₄-catalyzed reaction of 2-(2-carbonylvinyl)-3-phenyl-2H-azirines with diazo esters proceeds through an intermediate generation of azirinium ylide suffering a nonstereoselective ring opening to form (3Z)- and (3E)-2-azahexa-1,3,5-trienes. The former depending on configuration of the C⁵=C⁶ bond may undergo cyclization either in derivative of 2,3-dihydropyridine, or in pyrrolium ylide that isomerizes into a derivative of 1H-pyrrole. According to DFT calculation, the preferred formation of pyrroles at increasing volume of Z-substituent at the atom C⁶ and of substituents at the atom C¹ of 2-azahexatriene occurs due to the destabilization of more sterically loaded transition states of 1,6-cyclization.