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 chiralC2symmetric diamine (N-N) are readily prepared astransisomers 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. Compoundtrans-1acleanly isomerizes into a mixture ofcisisomers in EtOH upon heating. DFT calculations reproduce accurately the ratio of isomers observed as well as the greater thermodynamic stability of thecisisomers of1a. Complexes1are efficient catalyst precursors for the asymmetric hydrogenation ofN-aryl imines5in toluene under very mild conditions using KOtBu as a base (4 bar H2, room temperature,5/1/KOtBu = 500/1/10). Among the catalyst precursors,1fprovides good enantioselectivities in the hydrogenation of a wide range ofN-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 forRandSproducts involvingcis-dihydridesd1Randd3S, respectively. Among several hydrogen activation pathways examined in thepro-Rroute, the most favorable one consists of hydrogen coordination to a Ru-amido/amine adduct, followed by amine assisted activation of dihydrogen.