Reactivity of a terminal Ti(IV) imido complex toward alkenes and alkynes: Cycloaddition vs C-H activation

This paper describes the reactivity of the base-free titanium imido complex Cp*Ti-2=NPh (1) (Cp* pentamethylcyclopentadienyl) toward alkenes and alkynes. Complex 1 reacts with ethylene and acetylene to generate the azametallacycles Cp*Ti-2(N(Ph)CH2CH2) (2) and Cp*Ti-2(N(Ph)CH=CH) (3), respectively. In the case of ethylene, the cycloaddition is readily reversible, and 2 was characterized spectroscopically under an ethylene atmosphere. We have also examined the reactivity of 1 toward phenyl- and trimethylsilylacetylene and have found that 1 activates the alkynyl C-H bond to give anilido-acetylide complexes Cp*Ti-2(N(Ph)H)C=CR (6, R = Ph; 7, R = SiMe3). This reaction proceeds without observable intermediate metallacyclobutene complexes such as those observed previously for the reaction of the related oxo complex Cp*Ti-2(O)pyr (pyr pyridine) with terminal alkynes. Thermolysis of azametallacyclobutene 3 results in formation of the novel ring-activated complex Cp*(eta(5),eta(1)-C5Me4CH2CH=CH)TiN(Ph)H (5).