Hydrogen bond-directed coordination of phosphine-amino-alcohol (P, N, OH) ligands: Stereochemical considerations and catalytic studies

Novel phosphine-aminoalcohol type chiral ligands of the chemical formula Ph2PCH(CH3)CH2CH(CH3)NHCH(R)CH2OH (1a: (R, R)-(S), R = CH3 , 1b: (S, S)-(S), R = CH3 , 1c: (S, S), R = H) have been synthesized in two simple steps using cyclic sulfates. The coordination behavior of 1a-c having stereochemically labile nitrogen donor to square planar Pd(II) center was investigated by X-ray crystallography, 1D and 2D NMR methods and by DFT calculations. In the solid state of complex [Pd(1a)Cl-2] an intramolecular hydrogen bond could be observed between the OH-moiety and one of the Cl co-ligands, while intermolecular hydrogen bonds were detected in the case of [Pd(1b)Cl-2] between the same functionalities. In the dichloromethane solution of the complexes the hydrogen bond was identified as a crucial factor in determining ring conformation and nitrogen configuration. Ligand 1a coordinated stereoselectively to the metal in [Pd(1a)Cl-2] leading to a complex having a single conformationally rigid six-membered chelate and a configurationally fixed N-donor. In contrast, coordination of ligands 1b-c resulted in the formation of a mixture of isomers with different chelate conformation and nitrogen configuration. The ligands were utilized in Pd-catalyzed asymmetric allylic alkylation where high enantioselectivities (ees up to 96 %) and activities could be obtained.

Automated summary

Novel phosphine-aminoalcohol chiral ligands were synthesized and their coordination behavior was studied. The hydrogen bond was found to play a crucial role in determining the conformation and configuration of the ligands. These ligands exhibited excellent catalytic activity in asymmetric allylic alkylation reactions.