Reduction of bis[η(5)-(ω-alkenyl)tetramethyl]cyclopentadienyltitanium dichlorides: An efficient synthesis of long-chain ansa-bridged titanocene dichlorides by acidolysis of cyclopentadienyl-ring-tethered titancyclopentanes
Abstract
The reduction of symmetric fully-substituted titanocene dichlorides bearing two pendant tu-alkenyl groups, [TiCl2(η(5)-C5Me4R)(2)], R = CH(Me)CH=CH2, (1a). (CH2)(2)CH=CH2 (1b) and (CH2)(3)CH=CH2 (1c), by magnesium in tetrahydrofuran affords bis(cyclopentadienyl)titanacyclopentanes [Ti-IV{η(1):η(1):η(5):η(5)-C5Me4CH(Me)CH(Ti)CH2CH(CH2- (Ti))CH(Me)(C5Me4] (2a), [Ti-IV{η(1):η(1):η(5): η(5)-C5Me4(CH2)(2)CH(Ti)CH(Me)CH(Me) CH(Ti)(CH2)(2)C5Me4}] (2c), respectively, as the products of oxidative coupling of the 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 titanacyclopentanes with PbCl2 recovers starting materials 1a from 2a and 1b from 2b, but complex 2c affords, under the same conditions, an isomer of 1c with a shifted carbon-carbon double bond, [TiCl2(η5-C5Me4(CH2CH2CH=CHMe)}(2)] (1c'), The titanacycles 2a-c can be opened by HCl to give ansn-titanocene dichlorides ansa-[(η(5):η(5)-C5Me4CH(Me) CH2CH2CH(Me)CH(Me)C5Me4}TiCl2] (3a), ansn-[(η(5):η(5)-C5Me4(CH2)(8)C5Me4}TiCl2] (3b), along with a minor product ansn-[(η(5):η(5)-C5Me4CH2CH=CH(CH2)(5)- C5Me4}TiCl2] (3b'), and ansa-[(η(5):η(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-linking carbon chains in 3a and 3b are fully saturated, compounds 3e and 3b' contain one asymetrically placed carbon-carbon double bond, which evidently arises from the beta-hydrogen elimination that follows the HCl addition.