On the formation of Pd(II) complexes of Trost modular ligand involving N–H activation or P,O coordination in Pd-catalyzed allylic alkylations

Abstract

Specific chiral ligands have been designed by Trost et al. to perform enantioselective Pd-catalyzed allylic alkylations. It is shown that the Pd(0) complex formed by addition of the Trost ligand ( 4) to Pd 0(dba) 2 is not stable in most solvents (acetone, DMF, CH 2Cl 2). Indeed, Pd 0(dba)( 4) leads to the formation of a stable Pd II complex 5 (X-ray structure), likely by activation of the two N–H bonds of the ligand by the Pd 0 centre. The formation of the Pd II complex competes with the reaction of Pd 0( 4) with ( E)-PhCH CH–CH(OAc)–Ph, excluding any investigation of the kinetics of the latter reaction. The ionization steps from intermediate (η 2-PhCH CH–CH(OAc)–Ph)Pd 0( 4) were found to be very slow. The cationic P,P complex [(η 3-Ph–CH–CH–CH–Ph)Pd( 4)] +, expected to be generated by addition of 2 equiv. of 4 to the precursor [(η 3-Ph–CH–CH–CH–Ph)Pd(μ-Cl)] 2, in the presence of a chloride scavenger, leads to a complex mixture whereas addition of 1 equiv. of 4 affords a stable bis-cationic Pd II complex {[(η 3-Ph–CH–CH–CH–Ph)Pd] 2( 4)]} 2+, 2 ( BF 4 - ) (X-ray structure) via a P,O complexation of each allyl-Pd moieties. This dissymmetric P,O coordination will favour the enantioselectivity of Pd-catalyzed allylic alkylation of (E)-PhCH CH–CH(OAc)–Ph by the control of the regioselectivity of the nucleophilic attack onto the allylic ligand which is responsible of the enantioselectivity of the overall catalytic reaction.