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

New chiral dithioether compounds (-)-1-benzyl-3,4-bis(methylsulfanyl)pyrrolidine (-)-degusme, (-)-1-benzyl-3,4-bis(isopropylsulfanyl)pyrrolidine (-)-deguspr(i) 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) {(-)-deguspr(i)}]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 H-2 to 1, 2 and 3 at -70 degrees C gave cis-dihydridoiridium(III) complexes [IrH2(cod)L]BF4[L = (-)-degusme 4, (-)-deguspr(i) 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 H-2, and the highest enantiomeric excess obtained was 68%.