Iridium complexes with new 1,2-dithioether chiral ligands containing a rigid cyclic backbone. Application in homogeneous catalytic asymmetric hydrogenation†

Abstract

New chiral dithioether compounds (–)-1-benzyl-3,4-bis(methylsulfanyl)pyrrolidine (–)-degusme, (–)-1-benzyl-3,4-bis(isopropylsulfanyl)pyrrolidine (–)-degusprⁱ and (+)-1-benzyl-3,4-bis(phenylsulfanyl)pyrrolidine (+)-degusph were prepared from (+)-L-tartaric acid. The addition of the dithioether compounds to a dichloromethane solution of [Ir(cod)₂]BF₄ afforded the chiral cationic complexes [Ir(cod){(–)-degusme}]BF₄ 1 [Ir(cod){(–)-degusprⁱ}]BF₄·CH₂Cl₂ 2 and [Ir(cod){(+)-degusph}]BF₄ 3. The dithioether ligands were replaced by PPh₃ in complexes 1, 2 and 3 providing the [Ir(cod)(PPh₃)₂]BF₄ complex. The addition of H₂ to 1, 2 and 3 at –70 °C gave cis-dihydridoiridium(III) complexes [IrH₂(cod)L]BF₄ [L = (–)-degusme 4, (–)-degusprⁱ 5 or (+)-degusph 6]. The relative stability of possible isomers for complexes 1–6 was studied by molecular mechanics calculations. Complexes 1–3 were active precursors in the asymmetric hydrogenation of different prochiral dehydroamino acid derivatives and itaconic acid, at room temperature under atmospheric pressure of H₂, and the highest enantiomeric excess obtained was 68%.