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

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 the 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 (hydrometallation). Under similar conditions, the complex with asymmetric unsaturated ansa-bridge [TiCl2{η5:η5-C5Me4CH2CHCH(CH2)5C5Me4}] (5) is reduced to a titanocene–η2-alkene complex with the double bond shifted to the central position, ansa-[Ti{η2:η5:η5-C5Me4(CH2)3CHCH(CH2)3C5Me4}] (7). In the absence of btmse, analogous complex with saturated bridge ansa-[TiCl2{η5:η5-C5Me4(CH2)8C5Me4}] (6) undergoes a hydrogen abstraction to give 7 as well albeit in a lower yield. The reduction of 6 in the presence of btmse gives η2-alkyne complex ansa-[Ti{η5:η5-C5Me4(CH2)8C5Me4}(η2-Me3SiCCSiMe3)] (8) which is converted upon thermolysis at 150 °C to a mixture of asym- and sym-isomers of the doubly tucked-in complex ansa-[Ti{(η3:η4-C5Me2(CH2)2)(CH2)8(η5-C5Me4)}] (9) in 2:1 ratio; no CH activation involving methylene groups of the ansa tether was observed. The crystal structures of 4, 7, and 8 have been determined by X-ray crystallography.