Synergistic Stereocontrol in the Enantioselective Ruthenium-Catalyzed Sulfoxidation of Spirodithiolane-Indolones

A chiral ruthenium catalyst was developed for the enantioselective sulfoxidation of the title compounds. The catalyst combines two elements of chirality, a chiral pybox ligand and a chiral bicylic lactam unit, to which the ligand is attached. The latter unit was shown to improve significantly the performance of the catalyst by exposing one of the two enantiotopic sulfur atoms to the active site via hydrogen-bond mediated coordination. Ten differently substituted substrates were converted into the respective sulfoxides in yields of 52-71% and with 90% ee.