Ruthenium complexes of new chiral phosphine‐amine‐ether ligands (Ru‐PNO) for asymmetric hydrogenation – the role of backbone chirality in pincer ligand design

Abstract

New chiral phosphine‐amine‐ether (PNO) ligands of the general formula Ph2PCH(R1)(CH2)nCH(R1)N(R2)CH(R3)CH2OMe, where R1, R2, and R3 = H or Me, n = 0 or 1, and their ruthenium complexes of the type [RuCl2(PPh3)(PNO)] have been synthesized. The coordination compounds were characterized by 1D and 2D NMR spectroscopy, modeled by DFT calculations, and in one case analyzed by X‐ray crystallography. The combined spectroscopic and theoretical investigations revealed that the relative configuration of the stereogenic elements in the P–N and N–O backbone represents a crucial factor in determining the conformation of the pincer‐type chelates and may also affect the configuration of the coordinated stereogenic nitrogen in the NH subunit, an essential element of stereochemical communication in outer sphere bifunctional catalysis. The new complexes were applied in the asymmetric hydrogenation of fused ring bicyclic ketones (i.e., 1‐tetralone and 4‐chromanone derivatives), a challenging substrate class, where enantioselectivities up to 97% could be obtained. Based on the spectroscopic and theoretical studies and catalytic experiments, structural features affecting the stereochemistry of the coordination could be identified and a qualitative enantioinduction model has been proposed.