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 PdII 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 PdII 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 PdII complex {[(η3-Ph–CH–CH–CH–Ph)Pd]2(4)]}2+, 2(BF4-) (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.

Side Pd(II) complex 5 formed together with the Pd(0) ligated by the Trost ligand 4 P,O coordination of the Trost ligand 4 in stable {[(η3-allyl)Pd]2(4)}2+.Figure optionsDownload as PowerPoint slide