Dramatic Effect of Ancillary NHC Ligand in the Highly Selective Catalytic Oxidative Carbon–Carbon Multiple Bond Cleavage

Selective
and controlled oxidation of olefins to aldehydes is a commonly used
important transformation in chemistry. However, chemists still use
the dangerous and inconvenient ozonolysis method or the less selective,
low-yielding Lemieux–Johnson protocol. In a program of developing
effective catalysts for this important reaction, we disclose here
that an ancillary ligand can play a dramatic role in the above catalytic
phenomenon, depending on the design of the ligand precursor chosen.
Proof-of-principle is demonstrated with the help of two newly designed
[LnRuII-NHC] precatalysts (NHC = an imidazolydene-based
NHC, Im-NHC, or a triazolydene-based NHC, Trz-NHC; Ln = para-cymene) for catalytic selective
oxidation of olefins/alkynes to carbonyl compounds. With the electron-deficient Trz-NHC ligand, [(para-cymene)­RuII(Im-Trz)]+ precatalyst was found to be an
order of magnitude more efficient than the [(para-cymene)­RuII(Im-NHC)]+ precatalyst.