Reduction-Induced Cyclization and Redox Reactions of Fully Methylated Titanocene Dichlorides Bearing Pendant Alkenyldimethylsilyl Groups, [TiCl<sub>2</sub>{η<sup>5</sup>-C<sub>5</sub>Me<sub>4</sub>(SiMe<sub>2</sub>R)}<sub>2</sub>] (R = Vinyl and Allyl)
作者:Lenka Lukešová、Petr Štěpnička、Karla Fejfarová、Róbert Gyepes、Ivana Císařová、Michal Horáček、Jiří Kubišta、Karel Mach
DOI:10.1021/om020150d
日期:2002.6.1
Reduction of titanocene dichlorides bearing (omega-alkenyl)dimethylsilyl substituents [TiCl2eta(5)-C5Me5(SiMe2R)}(2)], where R = CH=CH2 (3) and CH2CH=CH2 (4), affords highly reactive Ti(II) intermediates, which immediately undergo intramolecular reactions with the pendant double bonds in a way dependent on the length of the alkenyl chain. The reduction of 4 with magnesium at 60 degreesC affords cleanly the cyclopentadienyl-ring-tethered titanacyclopentane, [Ti(eta(1):eta(1):eta(5):eta(5)-C5Me4SiMe2CH(Ti)CH(Me)CH(Me)CH(Ti)SiMe2C5Me4](5), which can be opened by HCl to an ansa-titanocene dichloride with a six-membered saturated bridging chain, [TiCl2-eta:(5)eta(5)-C5Me4SiMe2CH2CH(Me)CH(Me)CH2SiMe2C5Me4)(2)}] (6). An analogous reaction of 3 can be accomplished at low temperatures and only to some extent, providing a mixture of titanacyclopentane complex [Ti(eta(1):eta(1):eta(5):eta(5)-C5Me4SiMe2CH(Ti)CH2CH2CH(Ti)SiMe2C5Me4] (10) and the fluxional eta(2)-alkene complex [Ti(eta(5)-C5Me4SiMe2CH=CH2)(eta(2):eta(5)-C5Me4SiMe2CH=CH2)1 (9). At 60 degreesC, the reduction of 3 yields a mixture of ansa-eta(2)-alkene complex [Tieta(2):eta(5)-C5Me4SiMe2CH2CH=CHCH2SiMe2C5Me4}] (7) and the product of a hydrogen loss, the doubly tethered; paramagnetic eta(3)-allyl Ti(III) complex [Tieta(3):eta(5):eta(5)-C5Me4SiMe2CHCH=CHCH2SiMe2C5Me4}] (8). When the reduction of 3 with magnesium is carried out in the presence of bis(trimethylsilyl)ethyne as the proton acceptor, 8 is obtained as the sole isolated product. Complex 8 reacts with protic reagents such as HCl and methanol under protonation/decomplexation of the allyl system to give the d(1) paramagnetic complexes [TiXeta:(5)eta(5)-C5Me4SiMe2CH2CH=CHCH2SiMe2C5Me4}] (X = Cl, 12; OMe, 13). Oxidation of 8 with PbCl2 yields eta(1)-alkenyl complex [TiCleta(1):eta(5):eta5-C5Me4SiMe2CH(Ti)CH=CHCH2SiMe2C5Me4}] (11). A similar oxidation of 12 to ansa-titanocene dichloride [TiCl2eta:(5)eta-C5Me4SiMe2CHCH=CHCH2SiMe2C5Me4}] (14) containing a symmetrical unsaturated bridging chain requires a stronger oxidizing agent, AgCl. Compounds 5, 7, 8, and 10-14 have been characterized by X-ray crystallography.