Oxidation of Thioether Ligands in Pseudotetrahedral Cyclopentadienylruthenium Complexes: Toward a New Stereoselective Synthesis of Chiral Sulfoxides(1)

Ionic ruthenium thioether complexes [Cp(LL')Ru(SRR')]PF(6) (LL' = Ph(2)PCH(2)PPh(2) (1), Ph(2)PC(2)H(4)PPh(2) (2), (Ph(3)P, CO) (3), Me(2)PC(2)H(4)PPh(2) (4), (S,S)-Ph(2)PCHMeCHMePPh(2) (5), SRR' = MeSPh (a), MeS-i-Pr (b), MeSBz (c), i-PrSBz (d), EtSBz (e), MeSCy (f), SC(4)H(8) (g)) were synthesized from the corresponding chloro complexes [Cp(LL')RuCl] and thioethers. 5a crystallized in the orthorhombic system, space group P2(1)2(1)2(1) (No. 19), with a = 11.269(3) Å, b = 15.104(2) Å, c = 23.177(4) Å, and Z = 4. 5b crystallized in the monoclinic system, space group P2(1) (No. 4), with a = 10.539(5) Å, b = 16.216(9) Å, c = 11.011(8) Å, beta = 106.04(2) degrees, and Z = 2. A similar ligand exchange reaction yielded the analogous sulfoxide complexes [Cp(LL')Ru(S(O)RR')]PF(6) (6-10). 10a crystallized in the orthorhombic system, space group P2(1)2(1)2(1) (No. 19), with a = 14.1664(13) Å, b = 15.792(2) Å, c = 17.641(2) Å, and Z = 4. 10b.0.93CH(2)Cl(2) crystallized in the orthorhombic system, space group P2(1)2(1)2(1) (No. 19), with a = 12.069(2) Å, b = 17.379(2) Å, c = 19.760(5) Å, and Z = 4. The thioether complexes can also be directly converted to sulfoxide complexes with the strong oxygen transfer reagent dimethyldioxirane (DMD). No crossover products are formed when mixtures of two thioether complexes (e.g., 1a/2c or 1c/2a) are treated with DMD, demonstrating that no Ru-S bond cleavage is involved. Moderate diastereoselectivities are observed for the oxygen transfer to chiral, racemic thioether complexes 3 (8-28%) and 4 (34-60%). Oxidation of the (S,S)-CHIRAPHOS complexes 5, however, is highly stereoselective (de = 46-98%). Treatment of the sulfoxide complexes 10 with sodium iodide removes the chiral, nonracemic sulfoxides from the metal with retention of the configuration at sulfur.