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

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 Pd⁰(dba)₂ is not stable in most solvents (acetone, DMF, CH₂Cl₂). Indeed, Pd⁰(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 Pd⁰ centre. The formation of the Pd^II complex competes with the reaction of Pd⁰(4) with (E)-PhCHCH-CH(OAc)-Ph, excluding any investigation of the kinetics of the latter reaction. The ionization steps from intermediate (η²-PhCHCH-CH(OAc)-Ph)Pd⁰(4) were found to be very slow. The cationic P,P complex (η³-Ph-CH-CH-CH-Ph)Pd(4)]⁺, expected to be generated by addition of 2 equiv. of 4 to the precursor (η³-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 {(η³-Ph-CH-CH-CH-Ph)Pd]₂(4)]}²⁺, (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)-PhCHCH-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.