Synthesis, Cycloaddition, and Cycloreversion Reactions of Mononuclear Titanocene–oxo Complexes

Abstract

Titanocene–oxo complexes of the type Cp x 2 Ti═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 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 [Cp x 2 Ti═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.