A combined experimental and computational study to decipher complexity in the asymmetric hydrogenation of imines with Ru catalysts bearing atropisomerizable ligands

Abstract

RuCl₂(P–OP)(N–N) complexes (1) containing an atropisomerizable phosphine–phosphite (P–OP) and a chiral C₂ symmetric diamine (N–N) are readily prepared as trans isomers by successive addition of P–OP and N–N ligands to RuCl₂(PPh₃)₃. For these complexes, fast atropisomerization of the biaryl fragment at room temperature has been observed. Compound trans-1a cleanly isomerizes into a mixture of cis isomers in EtOH upon heating. DFT calculations reproduce accurately the ratio of isomers observed as well as the greater thermodynamic stability of the cis isomers of 1a. Complexes 1 are efficient catalyst precursors for the asymmetric hydrogenation of N-aryl imines 5 in toluene under very mild conditions using KO^tBu as a base (4 bar H₂, room temperature, 5/1/KO^tBu = 500/1/10). Among the catalyst precursors, 1f provides good enantioselectivities in the hydrogenation of a wide range of N-aryl imines (84–96% ee, 16 examples). From DFT calculations, a mechanism consisting in stepwise transfer of a hydride and a proton from the dihydride to the imine has been proposed, with the most favourable paths for R and S products involving cis-dihydrides d1R and d3S, respectively. Among several hydrogen activation pathways examined in the pro-R route, the most favorable one consists of hydrogen coordination to a Ru–amido/amine adduct, followed by amine assisted activation of dihydrogen.