Palladium(II) Complexes of Chiral 1,2-Diiminophosphoranes: Synthesis, Structural Characterization, and Catalytic Activity for the Allylic Alkylation

Abstract

Chiral 1,2-bis[tris(dimethylamino)phosphinimino]cyclohexane (7a, 80% yield), 1,2-bis[tris(dimethylamino)phosphinimino]-1,2-diphenylethane (7b, 74% yield), 1,2-bis[triphenoxyphosphinimino]cyclohexane (9a, 85% yield), and 1,2-bis[triphenoxyphosphinimino]-1,2-diphenylethane (9b, 70% yield) have been prepared using (1R,2R)-1,2-diaminocyclohexane or (1R,2R)-1,2-diphenylethylenediamine and the corresponding phosphine dibromide derivatives (the Kirsanov route). 1,2-Bis[triphenylphosphinimino]cyclohexane (2), 1,2-bis[triphenylphosphinimino]-1,2-diphenylethane (3), and 1,2-diiminophosphoranes 7b and 9b reacted with [Pd(η3-C3H5)Cl]2 in the presence of a silver salt in CH2Cl2 at room temperature to afford the cationic complexes 10 (81% yield), 11 (84% yields), 15 (78% yield), and 16 (74% yield), respectively. According to the same general procedure, palladium complexes [(η3-PhCHCHCHPh)(2)Pd]TfO (13, 88% yield) and [(η3-PhCHCHCHPh)(3)Pd]TfO (14, 86% yield) have been prepared. Single-crystal X-ray diffraction studies of derivatives 10, 11, and 13 have been carried out. They revealed that C2 symmetry was retained for derivative 3 upon coordination, but lost for 2 in the coordination sphere of the metal. 13C NMR chemical shifts for the terminal C atoms of the allyl moiety of these complexes indicate that the donor ability of 1,2-diiminophosphoranes varies with the nature of the P-substituents and is comparable to that of other sp2-hybridized nitrogen ligands. 1,2-Diiminophosphoranes were evaluated as ligands for the Pd-catalyzed enantioselective allylic substitution reaction of rac-1,3-diphenylprop-2-enyl acetate with the anion of dimethyl malonate. Ligands 3 and 7a,b induce good catalytic activities compared with other N,N-ligands but moderate ee's (10−77%) at 36 °C. Higher ee's (85%) were obtained at room temperature with ligand 3 but at the expense of the catalytic activity. This study revealed that 1,2-diimonophosphoranes are able to stabilize Pd(0) species during a catalytic process and to induce notable levels of enantioselectivity.