Two-faced reactivity of alkenes: cis- versus trans-aminopalladation in aerobic Pd-catalyzed intramolecular aza-Wacker reactions

Abstract

A number of different PdII catalyst systems have been reported recently for the Wacker-type aerobic oxidative cyclization of alkenes bearing tethered nitrogen nucleophiles. This study examines the stereochemistry of the aminopalladation step with five different catalyst systems: Pd(OAc)2/DMSO (A), PdX2/pyridine [X = OAc (B), O2CCF3 (C)], Pd(IMes)(O2CCF3)2(OH2) (D), and Pd(O2CCF3)2/(-)-sparteine (E). Use of a stereospecifically deuterated cyclopentene substrate reveals that four of the five catalyst systems (A, B, C, and E) promote exclusive cis-aminopalladation of the alkene, whereas both cis- and trans-aminopalladation occur with the N-heterocyclic-carbene (NHC) catalyst system. If stoichiometric Brønsted base (NaOAc, Na2CO3) is added to the latter reaction conditions, however, only cis-aminopalladation is observed. The identity of the nitrogen nucleophile also affects the aminopalladation pathway, with results ranging from exclusively cis- to exclusively trans-aminopalladation. These results have important implications for ongoing efforts to develop enantioselective methods for Pd-catalyzed oxidative amination of alkenes.