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

Remote carbon–hydrogen activation
on titanium dinuclear
complexes [{Ti­(η5-C5Me5)­R2}2(μ-O)] [R = CH2SiMe3 2, CH2CMe3 3, and
CH2Ph 5) have been examined both synthetically
and theoretically. While the thermal treatment of the oxoderivative
[{Ti­(η5-C5Me5)­(CH2SiMe3)2}2(μ-O)] (2) led to a series of metallacycle complexes (2a–c) by sequential carbon–hydrogen activation processes,
[{Ti­(η5-C5Me5)­(CH2CMe3)2}2(μ-O)] (3) gave rise to the formation of the metallacycle tuck-over species
[Ti2(η5-C5Me5)­(μ-η5-C5Me4CH2-κC)­(CH2CMe3)­(μ-CH2CMe2CH2)­(μ-O)] (4), as result of hydrogen abstraction
from a η5-C5Me5 ligand. However,
the thermolysis of the tetrabenzyl complex [{Ti­(η5-C5Me5)­(CH2Ph)2}2(μ-O)] (5) yielded the derivative [Ti2(η5-C5Me5)­(μ-η5-C5Me4CH2-κC)­(CH2Ph)3(μ-O)] (6) that only exhibits
tuck-over η5-C5Me5 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(sp3)-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.