Synthesis and reactivity of cationic iridium(I) complexes of cycloocta-1,5-diene and chiral dithioether ligands. Application as catalyst precursors in asymmetric hydrogenation†

Abstract

New chiral dithioether compounds (–)-2,2-dimethyl-4,5-bis(isopropylsulfanylmethyl)-1,3-dioxolane (–)-diospr and (+)-2,2-dimethyl-4,5-bis(phenylsulfanylmethyl)-1,3-dioxolane (+)-diosph were prepared from diethyl (+)-L-tartrate. An alternative synthetic method for preparing the previously described bis(methylsulfanylmethyl) dithioether (–)-diosme was devised. By co-ordinating of the dithioethers to different (cycloocta-1,5-diene)iridium(I) compounds chiral cationic complexes [Ir(cod){(–)-diosme}]BF₄ 1, [Ir(cod){(–)-diospr}]BF₄·CH₂Cl₂ 2 and [Ir(cod){(+)-diosph}]BF₄ 3 were synthesized and then studied by ¹H, ¹³C NMR and FAB mass spectrometry. The complexes reacted with CO to give the corresponding binuclear tetracarbonyls [Ir₂(µ-L)₂(CO)₄][BF]₂ 4-6. The dithioether ligands were replaced by PPh₃ in 1–3 providing [Ir(cod)(PPh₃)₂]BF₄. The addition of H₂ to complexes 1 and 2 at –70 °C gave cis-dihydridoiridium(III) complexes [IrH₂(cod){(–)-L}]BF₄ 7 and 8 which are in equilibrium in solution with the parent complexes, depending on the temperature. Two possible diastereomers were distinguished for 8 at low temperatures. Complexes 1–3 were active precursors in the asymmetric hydrogenation of different prochiral dehydroamino acid derivatives and itaconic acid, at room temperature under an atmospheric pressure of H₂, and the highest enantiomeric excess obtained was 47%.