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Reduction of Bis[ η 5 ‐( ω ‐alkenyl)tetramethylcyclopentadienyl]titanium Dichlorides: An Efficient Synthesis of Long‐Chain ansa ‐Bridged Titanocene Dichlorides by Acidolysis of Cyclopentadienyl‐Ring‐ Tethered Titanacyclopentanes
Author(s) -
Horáček Michal,
Štěpnička Petr,
Gyepes Róbert,
Císařová Ivana,
Tišlerová Iva,
Zemánek Jaroslav,
Kubišta Jiří,
Mach Karel
Publication year - 2000
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20000703)6:13<2397::aid-chem2397>3.0.co;2-h
Subject(s) - chemistry , cyclopentadienyl complex , double bond , medicinal chemistry , tetrahydrofuran , titanium , magnesium , stereochemistry , polymer chemistry , catalysis , organic chemistry , solvent
The reduction of symmetric, fully‐substituted titanocene dichlorides bearing two pendant ω ‐alkenyl groups, [TiCl 2 ( η 5 ‐C 5 Me 4 R) 2 ], RCH(Me)CH=CH 2 ( 1 a ), (CH 2 ) 2 CH=CH 2 ( 1 b ) and (CH 2 ) 3 CH=CH 2 ( 1 c ), by magnesium in tetrahydrofuran affords bis(cyclopentadienyl)titanacyclopentanes [Ti IV { η 1 : η 1 :tlsb&=ndash;3%> η 5 : η 5 ‐C 5 Me 4 CH(Me)CH( Ti )CH 2 CH(CH 2 ( Ti ))CH(Me)C 5 Me 4 }] ( 2 a ), [Ti IV { η 1 : η 1 : η 5 : η 5 ‐C 5 Me 4 (CH 2 ) 2 CH( Ti )(CH 2 ) 2 CH( Ti )(CH 2 ) 2 C 5 Me 4 }] ( 2 b ) and [Ti IV { η 1 : η 1 : η 5 : η 5 ‐C 5 Me 4 (CH 2 ) 2 CH( Ti )CH(Me)CH(Me)CH( Ti )(CH 2 ) 2 C 5 Me 4 }] ( 2 c ), respectively, as the products of oxidative coupling of the double bonds across a titanocene intermediate. For the case of complex 1 c , a product of a double bond isomerisation is obtained owing to a preferred formation of five‐membered titanacycles. The reaction of the titanacyclopentanes with PbCl 2 recovers starting materials 1 a from 2 a and 1 b from 2 b , but complex 2 c affords, under the same conditions, an isomer of 1 c with a shifted carbon‐carbon double bond, [TiCl 2 { η 5 ‐C 5 Me 4 (CH 2 CH 2 CH=CHMe)} 2 ] ( 1 c′ ). The titanacycles 2 a ‐ c can be opened by HCl to give ansa ‐titanocene dichlorides ansa ‐[{ η 5 : η 5 ‐C 5 Me 4 CH(Me)CH 2 CH 2 CH(Me)CH(Me)C 5 Me 4 }TiCl 2 ] ( 3 a ), ansa ‐[{ η 5 : η 5 ‐C 5 Me 4 (CH 2 ) 8 C 5 Me 4 }TiCl 2 ] ( 3 b ), along with a minor product ansa ‐[{ η 5 : η 5 ‐C 5 Me 4 CH 2 CH=CH(CH 2 ) 5 C 5 Me 4 }TiCl 2 ] ( 3 b′ ), and ansa ‐[{ η 5 : η 5 ‐C 5 Me 4 (CH 2 ) 3 CH(Me)CH(Me)CH=CHCH 2 C 5 Me 4 }TiCl 2 ] ( 3 c ), respectively, with the bridging aliphatic chain consisting of five ( 3 a ) and eight ( 3 b, 3 b′ and 3 c ) carbon atoms. The course of the acidolysis changes with the nature of the pendant group; while the cyclopentadienyl ring‐linking carbon chains in 3 a and 3 b are fully saturated, compounds 3 c and 3 b ′ contain one asymetrically placed carbon‐carbon double bond, which evidently arises from the β ‐hydrogen elimination that follows the HCl addition.