Rapid Aerial Oxidation of Ruthenium-Dithiocatecholate Catalysts: A Challenge to Stereoretentive Olefin Metathesis

Ruthenium dithiolene complexes, particularly the dithiocatecholates
Ru(S2C6H2-3,6-Cl2)(NHC)(=CHC6H4-2-OiPr) (NHC = H2IPr, H2IMes), are an important class of olefin
metathesis catalysts employed for the stereocontrolled assembly of
C=C bonds. The poorly understood sensitivity of these catalysts results
in unpredictable performance. Here they are shown to resist degradation
by amine nucleophiles, in contrast with their dichloride parents;
however, they are significantly more oxygen-sensitive. Even as the
precatalysts, they decompose within minutes in solution under O2, eliminating the alkylidene ligand as O=CHC6H4-2-OiPr and forming unusual, four-coordinate
Ru(IV)-oxo products, Ru(O)(S2C6H2-3,6-Cl2)(NHC). Participation of the latter in O-transfer
with PPh3 suggests potential opportunities in catalytic
oxidation.