A combined experimental and computational study to decipher complexity in the asymmetric hydrogenation of imines with Ru catalysts bearing atropisomerizable ligands. Issue 7 (4th February 2021)

• Record Type: Journal Article
• Authors: León, Félix; Comas-Vives, Aleix; Álvarez, Eleuterio; Pizzano, Antonio
• Is Part Of: Catalysis science & technology. Volume 11:Issue 7(2021)
• Page Start: 2497
• Page End: 2511
• Publication Date: 2021-02-04
• Subjects: Catalysis -- Periodicals; 541.395
• DOI: 10.1039/d0cy02390f
• Languages: English
• ISSNs: 2044-4753

Abstract

Abstract : RuCl2 (P–OP)(N–N) complexes containing an atropisomerizable phosphine–phosphite and a chiral diamine are effective catalyst precursors for the asymmetric hydrogenation of N -aryl imines following an outer-sphere mechanism. Abstract : RuCl2 (P–OP)(N–N) complexes (1 ) containing an atropisomerizable phosphine–phosphite (P–OP) and a chiral C 2 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 t Bu as a base (4 bar H2, room temperature, 5 /1 /KO t Bu = 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.