Reduction of Bis[η5-(ω-alkenyl)tetramethylcyclopentadienyl]titanium Dichlorides: An Efficient Synthesis of Long-Chainansa-Bridged Titanocene Dichlorides by Acidolysis of Cyclopentadienyl-Ring- Tethered Titanacyclopentanes

摘要: The reduction of symmetric, fully-substituted titanocene dichlorides bearing two pendant ω-alkenyl groups, [TiCl2(η5-C5Me4R)2], RCH(Me)CH=CH2 (1 a), (CH2)2CH=CH2 (1 b) and (CH2)3CH=CH2 (1 c), by magnesium in tetrahydrofuran affords bis(cyclopentadienyl)titanacyclopentanes [TiIV{η1:η1:tlsb&endash;3%>η5:η5-C5Me4CH(Me)CH(Ti)CH2CH(CH2(Ti))CH(Me)C5Me4}] (2 a), [TiIV{η1:η1:η5:η5-C5Me4(CH2)2CH(Ti)(CH2)2CH(Ti)(CH2)2C5Me4}] (2 b) [TiIV{η1:η1:η5:η5-C5Me4(CH2)2CH(Ti)CH(Me)CH(Me)CH(Ti)(CH2)2C5Me4}] (2 c), respectively, as the products oxidative coupling double bonds across a intermediate. For case complex 1 c, product bond isomerisation is obtained owing to preferred formation five-membered titanacycles. reaction titanacyclopentanes with PbCl2 recovers starting materials 1 a from 2 a 1 b 2 b, but 2 c affords, under same conditions, an isomer 1 c shifted carbon-carbon bond, [TiCl2{η5-C5Me4(CH2CH2CH=CHMe)}2] (1 c′). titanacycles 2 a-c can be opened HCl give ansa-titanocene ansa-[{η5:η5-C5Me4CH(Me)CH2CH2CH(Me)CH(Me)C5Me4}TiCl2] (3 a), ansa-[{η5:η5-C5Me4(CH2)8C5Me4}TiCl2] (3 b), along minor ansa-[{η5:η5-C5Me4CH2CH=CH(CH2)5C5Me4}TiCl2] (3 b′), ansa-[{η5:η5-C5Me4(CH2)3CH(Me)CH(Me)CH=CHCH2C5Me4}TiCl2] (3 c), bridging aliphatic chain consisting five (3 a) eight (3 b, 3 b′ 3 c) carbon atoms. course acidolysis changes nature group; while cyclopentadienyl ring-linking chains 3 a 3 b are fully saturated, compounds 3 c contain one asymetrically placed which evidently arises β-hydrogen elimination that follows addition.