Synthesis and characterization of a new chiral phosphinothiol ligand derived from (-)-menthone and its palladium(II) and platinum(II) complexes

The chiral ligand (1R,2S,5R)-1-((diphenylphosphino)methyl)-2-isopropyl-5-methylcyclohexanethiol (4) has been prepared from low-cost commercial (-)-menthone in a three-step enantioselective synthesis. Oxidative addition of 2 equiv of this phosphinothiol ligand to [Pd-0(PPh3)(4)] gave the enantiopure bis(phosphinothiolate)palladium(II) complex 5, which only exists as the trans-P,P geometrical isomer, in both the solid state and solution, owing to the preference of the chelate rings to adopt the lambda conformation in which the position of the menthane ring does not allow a conformation of the phenyl groups compatible with the more sterically hindered cis geometry. These stereoelectronic coordination preferences of the chiral phosphinothiolate ligand have been confirmed by the structure of the analogous Pt(II) complex 6, which also exhibits the trans-P,P geometry exclusively, both in the solid state and in solution. Crystals of 5.1/2CH(2)Cl(2) and 6.1/2CH(2)Cl(2) are isomorphic, belonging to the monoclinic crystal system C2. Both chiral structures show mononuclear square-planar trans complexes with locked lambda chelate ring conformations. Compound 6 represents the first example of a structurally characterized mononuclear bis(phosphinothiolate)platinum(II) complex. Addition of 1 equiv of ligand 4 to a solution of [PdCl2(PPh3)(2)] gave the less sterically hindered complex chloro(phosphinothiolate)(triphenylphosphine)palladium(II) (7), which exhibits the expected cis-trans equilibrium in solution, but strongly displaced to the more sterically stable trans isomer.