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* 2 Ti NPh ( 1 ) (Cp* = pentamethylcyclopentadienyl) toward alkenes and alkynes. Complex 1 reacts with ethylene and acetylene to generate the azametallacycles Cp* 2 Ti(N(Ph)CH 2 CH 2 ) ( 2 ) and Cp* 2 Ti(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* 2 Ti(N(Ph)H)C⋮CR ( 6, R = Ph; 7, R = SiMe 3 ). This reaction proceeds without observable intermediate metallacyclobutene complexes such as those observed previously for the reaction of the related oxo complex Cp* 2 Ti(O)pyr (pyr = pyridine) with terminal alkynes. Thermolysis of azametallacyclobutene 3 results in formation of the novel ring-activated complex Cp*(η 5,η 1 -C 5 Me 4 CH 2 CH CH)TiN(Ph)H ( 5 ).