Dichotomous mechanistic behavior in Narasaka–Heck cyclizations: electron rich Pd-catalysts generate iminyl radicals

Abstract

Pd-catalyzed cyclizations of oxime esters with pendant alkenes are subject to an unusual ligand controlled mechanistic divergence. Pd-systems modified with electron deficient phosphines (<i>e.g.</i> P(3,5-(CF₃)₂C₆H₃)₃) promote efficient aza-Heck cyclization, wherein C-N bond formation occurs <i>via</i> alkene imino-palladation. Conversely, electron rich ligands, such as P(<i>t</i>-Bu)₃, cause deviation to a SET pathway and, in these cases, C-N bond formation occurs <i>via</i> cyclization of an iminyl radical. A series of mechanistic experiments differentiate the two pathways and the scope of the hybrid organometallic radical cyclization is outlined. This study represents a rare example in Pd-catalysis where selection between dichotomous mechanistic manifolds is facilitated solely by choice of phosphine ligand.