Synthesis and Characterization of a New Chiral Phosphinothiol Ligand Derived from (−)-Menthone and Its Palladium(II) and Platinum(II) Complexes

Abstract

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 [Pd0(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 λ 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 5·1/2CH2Cl2 and 6·1/2CH2Cl2 are isomorphic, belonging to the monoclinic crystal system C2. Both chiral structures show mononuclear square-planar trans complexes with locked λ 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.

Tuning diastereoisomerism in platinum(ii) phosphino- and aminothiolato hydrido complexes

“…In this way, the axial position of the chelate ring substituent is avoided, and two enantiomers appear for each geometric isomer. [10][11][12] Obviously, the energy of each pair of enantiomers is calculated to be the same and the trans isomer is slightly more stable than the cis one by a difference of only 0.2 kcal mol À1 , which confirms the more displaced equilibrium to the trans isomer, observed experimentally. For complex 10, only an enantiomer of the ligand was used for the optimization of the geometric isomers, resulting in a more stable cis form in the gas phase by only 0.3 kcal mol À1 .…”

“…In solid state, the phenyl groups of the triphenylphosphine ligand appear disordered in two positions each and the square-planar geometry of the complex is somewhat distorted with a chelate angle slightly reduced from the idealized value of 901. Quite similar distortions from the idealized geometry are also observed in other platinum(II) complexes with bidentate 2-phosphinoethanethiolato 10,16 and 2-phosphinophenolato 5 ligands (Table 2, entries [16][17][18]. The Pt-S distance is comparable to the related 2-phosphinophenolato complex 5 but appreciably longer than those observed for complexes with cyclic alkanethiolate ligands trans to triphenylphosphine 16,17 (Table 2, entries 16 and 18-23).…”

“…It is well-known that modifications in the chelate ring of the phosphinothiolate ligand can produce significant changes in the coordination conformation around the metal and in the nuclearity of the complexes. [10][11][12]22 Accordingly, three modifications were introduced in the former phosphinothiol ligand intending to explore the parameters that affect the cis/trans equilibrium observed for 8: introduction of a substituent in the chelate ring by using 1-(benzyloxy)-2-(diphenylphosphino)ethanethiol (Hbdppet, 2), inclusion of the chelate ring carbons in a cyclic structure by means of 2-(diphenylphosphino)cyclohexanethiol (Hdppcyt, 3) and enlargement of the chelate ring by using 3-(diphenylphosphino)propanethiol (Hdpppt, 4). Phosphinothiols 2 11 and 4 41 were synthetized as previously reported and racemic 3 was prepared from cyclohexene oxide in a low yield two-step synthesis 42,43 and has been structurally characterized in solution by NMR spectroscopy (see the Experimental section and the ESI †).…”

“…A new family of potentially catalytic platinum(II) hydrides bearing phosphinothiolato ligands (8)(9)(10)(11) were synthetized. 2-Phosphinothiolato complexes (8-10) display a chemical equilibrium between the cis-P,P and trans-P,P geometries, strongly displaced towards the trans-P,P configuration, whereas the corresponding complexes with aminothiolato ligands (5)(6)(7) showed the inverse stability trend, with the absence of trans isomers.…”

“…In this context, the diastereochemistry of these complexes has particular importance, as different diastereomers should present different activity, regioselectivity, and enantioselectivity in a given catalytic process. 7 As part of a project on the synthesis of new amino-and phosphinothiolato complexes of the group 10 metals and on the analysis of the ligand-based stereoelectronic effects that are determinants in the coordination conformations around the metal, 3,[8][9][10][11][12][13] we now report the preparation of new platinum(II) hydride complexes (8)(9)(10)(11) with 2-phosphinoethanethiolate ligands (2-(diphenylphosphino)ethanethiolate dppet 8, 1-(benzyloxy)-2-(diphenylphosphino)ethanethiolate bdppet 9, and 2-(diphenylphosphino)cyclohexanethiolate dppcyt 10) and a 3-phosphinothiolate ligand (3-(diphenylphosphino)propanethiolate dpppt 11). We also present, for the first time to our knowledge, the crystal structure determined by the X-ray analysis of a platinum(II) hydride complex with 2-phosphinoalkylthiolate ligand (8) and with 2-aminothiolate ligands: cysteamine (5) and natural chiral ligands methyl (cysMe) and ethyl (cysEt) ester-protected natural L-cysteine derivatives (6)(7).…”