Titanocene-oxo complexes of the type Cpx2Ti=O(L) (Cpx = pentamethylcyclopentadienyl; tetramethylcyclopentadienyl; L = pyridine or derivatives) are synthesized from the corresponding titanocene-ethylene complexes via oxidation with pyridine N-oxides or styrene oxide. These oxo complexes react with alkynes, nitriles, and α,β-unsaturated carbonyls to form titanacycles, which undergo exchange reactions with organic substrates or react with 4-dimethylaminopyridine to regenerate the titanocene-oxo. Mechanistic experiments support a dissociative mechanism in which the first step is rate-determining retrocycloaddition followed by trapping of the reactive [Cpx2Ti=O] species. In the case of the retro-[4+2]-cycloaddition from dioxatitanacyclohexene complexes, a Hammett study gives ρ values of -1.18 and -1.04 for substituents on two different phenyl rings on the metallacycles, suggesting positive charge buildup and a slightly asynchronous cycloreversion in the rate-determining step.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry