Mono-: N -protected amino acid ligands stabilize dimeric palladium(II) complexes of importance to C-H functionalization

Abstract

Mono-protected amino acid (MPAA) ligands are used in a number of Pd-catalyzed C-H functionalization reactions. MPAAs have been proposed to bind to Pd(ii) via κ2-(N,O) coordination, but such binding has not yet been experimentally validated. Herein, we report the synthesis and detailed characterization of a series of MPAA complexes prepared via cyclopalladation of dimethylbenzylamine in the presence of MPAAs. The isolated complexes exist as μ-carboxylato (MPAA) bridged dimers and feature potential M-M cooperativity and secondary sphere hydrogen bonding. Selective MPAA coordination and relay of stereochemistry, previously suggested to uniquely result from κ2-(N,O) MPAA coordination, are both observed. The isolated MPAA complexes undergo C-C and C-X (X = Cl, Br, I) bond formation when treated with electrophiles used for catalytic C-H functionalization. Stoichiometric iodination of MPAA palladacycles was found to proceed via a dinuclear palladium species with one equivalent of iodine in the rate limiting transition structure, and the isolated complexes also served as viable precatalysts for catalytic C-H functionalization. Together, these results provide a number of insights into the reactivity of Pd-MPAA complexes relevant to C-H bond functionalization.