Iridium complexes containing the first sugar dithioether ligands. Application as catalyst precursors in asymmetric hydrogenation

Abstract

The first family of sugar derivative dithioethers 1,2-O-isopropylidene-3,5-bis(methylsulfanyl)-, (+)-RiSSMe₂, 1,2-O-isopropylidene-3,5-bis(isopropylsulfanyl)-, (+)-RiSSPrⁱ₂, and 1,2-O-isopropylidene-3,5-bis(phenylsulfanyl)-α-D-(+)-ribofuranose, (+)-RiSSPh₂, was prepared from 1,2-O-isopropylidene-3,5-di-O-trifluoromethanesulfonyl-D-xylofuranose. Reaction of these chiral C₁ symmetrical dithioether ligands with [Ir(cod)₂]BF₄ (cod = 1,5-cyclooctadiene) yielded the iridium complexes [Ir(cod){(+)-RiSSR₂}]BF₄ 1–3. Their reaction with H₂ at 0 °C gave the cis-dihydridoiridium(III) complexes [IrH₂(cod){(+)-RiSSR₂}]BF₄ 4–6. Complexes 1–3 were tested in the asymmetric hydrogenation of acrylic acid derivatives at 1 bar of H₂ and room temperature, providing enantioselectivities of up to 62%.