Titanocene-oxo complexes of the type (Cp2Ti)-Ti-x=O(L) (Cp-x = 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 alpha,beta-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 [(Cp2Ti)-Ti-x=O] species. In the case of the retro-[4+2]-cycloaddition from dioxatitanacyclohexene complexes, a Hammett study gives rho 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.
Titanocene-oxo complexes of the type (Cp2Ti)-Ti-x=O(L) (Cp-x = 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 alpha,beta-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 [(Cp2Ti)-Ti-x=O] species. In the case of the retro-[4+2]-cycloaddition from dioxatitanacyclohexene complexes, a Hammett study gives rho 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.