RESOLUTION OF TERT-BUTYL METHYL SULFOXIDE VIA ORGANOMETALLIC LEWIS-ACIDS AND CHIRAL AMPLIFICATION IN ASYMMETRIC TRANSFORMATIONS OF THE RUTHENIUM CENTER IN [CPRU(CO)(PPH3)]+ ADDUCTS

A method has been developed for the preparation of an optically pure sulfoxide from chiral sulfoxides obtained in moderate enantiomeric excess (ee) by catalytic asymmetric oxidation of dialkyl sulfides. Mutual resolution of the sulfoxide and chiral metal centers allows the preparation of optically pure sulfoxides and metal complexes. Treatment of racemic [CpRu(CO)(PPh)3(acetone)]+ with 32% ee (R)-t-BuMeSO produces an initial mixture with 20% diastereomeric excess (de) of (R(Ru),S(S))- and (S(Ru),S(S))-[CpRu(CO)(PPh3)(t-BuMeSO)]+ diastereomers and their enantiomers. In solution this mixture epimerizes at the metal center to yield 32% ee (S(Ru),S(S))-[CpRu(CO)(PPh3)(t-BuMeSO)]+ with 94% de. The racemate is much less soluble than the pure enantiomer, so that initial precipitation yields a solid with near-zero optical activity. Recrystallization of the complex in the supernatant yields enantiomerically pure (S(Ru),S(S))[CpRu(CO)(PPh3)(t-BuMeSO)]SbF6. This process allows isolation of both the enantiomerically pure sulfur center and metal center starting from a sulfoxide obtained in only modest ee.