Reduction-induced double bond coordination and multiple C-H activation in fully-substituted titanocenes bearing a pendant double bond or an eight-membered hydrocarbyl ansa-chain
Abstract
Reduction of isomeric, fully-substituted titanocene dichlorides having a pendant double bond, [TiCl2(η5-C5Me4R)(η5-C5Me5)] (R = CH2CH2CH=CH2, 1; R = CH(Me)CH=CH2, 2), with magnesium in THF in the presence of bis(trimethylsilyl)ethyne (btmse) affords different products depending on the alkenyl chain length. Whereas the reduction of 1 yields a compound with intramolecularly η2-coordinated double bond, [Ti(η5-C5Me5)(η2:η5-C5Me4(CH2CH2CH=CH2)] (3), compound 2 affords under identical conditions a product with two new Ti-C bonds, [Ti(η1:η1:η5-C5Me3(CH2)(CH(Me)CH2CH2)}(η5-C5Me5)] (4).
The formation of 3 can be rationalized as a simple intramolecular stabilization of a titanocene intermediate formed by reductive removal of chloride ligands. The formation of 4 from the respective titanocene, however, requires formally an activation (oxidative addition) of one C-H bond at the methyl group adjacent to the unsaturated chain followed by a hydrogen shift.