Intermetallic Cooperation in C–H Activation Involving Transient Titanium-Alkylidene Species: A Synthetic and Mechanistic Study

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Remote carbon–hydrogen activation on titanium dinuclear complexes [{Ti(η 5 -C 5 Me 5 )R 2 } 2 (μ-O)] [R = CH 2 SiMe 3 2, CH 2 CMe 3 3, and CH 2 Ph 5 ) have been examined both synthetically and theoretically. While the thermal treatment of the oxoderivative [{Ti(η 5 -C 5 Me 5 )(CH 2 SiMe 3 ) 2 } 2 (μ-O)] ( 2 ) led to a series of metallacycle complexes ( 2a – c ) by sequential carbon–hydrogen activation processes, [{Ti(η 5 -C 5 Me 5 )(CH 2 CMe 3 ) 2 } 2 (μ-O)] ( 3 ) gave rise to the formation of the metallacycle tuck-over species [Ti 2 (η 5 -C 5 Me 5 )(μ-η 5 -C 5 Me 4 CH 2 -κC)(CH 2 CMe 3 )(μ-CH 2 CMe 2 CH 2 )(μ-O)] ( 4 ), as result of hydrogen abstraction from a η 5 -C 5 Me 5 ligand. However, the thermolysis of the tetrabenzyl complex [{Ti(η 5 -C 5 Me 5 )(CH 2 Ph) 2 } 2 (μ-O)] ( 5 ) yielded the derivative [Ti 2 (η 5 -C 5 Me 5 )(μ-η 5 -C 5 Me 4 CH 2 -κC)(CH 2 Ph) 3 (μ-O)] ( 6 ) that only exhibits tuck-over η 5 -C 5 Me 5 metalation. DFT calculations show that the mechanism involves a first α-hydrogen abstraction to generate a transient titanium alkylidene, which enables it to activate β- and γ-C(sp 3 )-H bonds on the adjacent titanium center. The calculations also establish a reactivity order for the different type of γ-H abstractions, trimethylsilyl > neopentyl ≌ benzyl, allowing us to explain the observed selectivity.

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