Reduction of bis[eta(5)-(omega-alkenyl)tetramethylcyclopentadienyl]titanium dichlorides: An efficient synthesis of long-chain ansa-bridged titanocenedichlorides by acidolysis of cyclopentadienyl-ring-tethered titanacyclopentanes

The reduction of symmetric fully-substituted titanocene dichlorides bearing two pendant tu-alkenyl groups, [TiCl2(eta(5)-C5Me4R)(2)], R = CH(Me)CH=CH2 , (1a). (CH2)(2)CH=CH2 (1b) and (CH2)(3)CH=CH2 (1c), by magnesium in tetrah ydrofuran affords bis(cyclopentadienyl)titanacyclopentanes [Ti-IV{eta(1):et a(1):eta(5):eta(5)-C5Me4CH(Me)CH(Ti)CH2CH(CH2- (Ti))CH(Me)(C5Me4] (2a), [Ti -IV{eta(1):eta(1):eta(5): eta(5)-C5Me4(CH2)(2)CH(Ti)CH(Me)CH(Me) CH(Ti)(CH2 )(2)C5Me4}] (2c), respectively, as the products of oxidative coupling of th e double bends across a titanocene intermediate, For the case of complex 1c , a product of a double bond isomerisation is obtained owing to a preferred formation of five-membered tilanacycles, The reaction of the titanacyclope ntanes with PbCl2 recovers starting materials 1a from 2a and 1b from 2b, bu t complex 2c affords, under the same conditions, an isomer of 1c with a shi fted carbon-carbon double bond, [TiCl2(eta 5-C5Me4(CH2CH2CH=CHMe)}(2)] (1c' ), The titanacycles 2a-c can be opened by HCl to give ansn-titanocene dichl orides ansa-[(eta(5):eta(5)-C5Me4CH(Me) CH2CH2CH(Me)CH(Me)C5Me4}TiCl2] (3a) , ansn-[(eta(5):eta(5)-C5Me4(CH2)(8)C5Me4}TiCl2] (3b), along with a minor p roduct ansn-[(eta(5):eta(5)-C5Me4CH2CH=CH(CH2)(5)- C5Me4}TiCl2] (3b'), and ansa-[(eta(5):eta(5)-C5Me4(CH2)(3)CH(Me)CH(Me)CH=CH- CH2C5Me4}TiCl2] (3c), respectively, with the bridging aliphatic chain consisting of five (3a) and eight (3b, 3b' and 3c) carbon atoms. The course of the acidolysis changes with the nature of the pendant group, while the cvclopentadienyl ring-linki ng carbon chains in 3a and 3b are fully saturated, compounds 3e and 3b' con tain one asymetrically placed carbon-carbon double bond, which evidently ar ises from the beta-hydrogen elimination that follows the HCl addition.