Asymmetric hydrogenation of prochiral olefins catalysed by furanoside thioether–phosphinite Rh(i) and Ir(i) complexes

Abstract

Thioether–phosphinite ligands (P–SR, R = Ph, Prⁱ and Me) bearing substituents with different steric demands on the sulfur centre were tested in the rhodium- and iridium-catalysed asymmetric hydrogenation of prochiral olefins. High enantiomeric excesses (up to 96%) and good activities (TOF up to 860 mol product × (mol catalyst precursor × h)⁻¹) were obtained for α-acylaminoacrylates derivatives. Our results show that enantiomeric excesses depended strongly on the steric properties of the substituent in the thioether moiety, the metal source and the substrate structure. A bulky group in the thioether moiety along with the metal Rh had a positive effect on enantioselectivity. Reaction of these chiral ligands with [M(cod)₂]BF₄ (M = Ir, Rh; cod = 1,5-cyclooctadiene) yielded complexes [M(cod)(P–SR)]BF₄, which were present in only one diastereomeric form having the sulfur substituent in a pseudoaxial disposition. The addition of H₂ to iridium complexes gave the cis-dihydridoiridium(III) complexes [IrH₂(cod)(P–SR)]BF₄. For complexes [IrH₂(cod)(P–SPh)]BF₄ and [IrH₂(cod)(P–SMe)] only one isomer was present in solution. However, for the complex [IrH₂(cod)(P–Si–Pr)]BF₄, which contained the more hindered substituent on sulfur, two isomers were detected. In all cases there was a pseudoaxial disposition of the sulfur substituents.