Reduction-Induced Cyclization and Redox Reactions of Fully Methylated Titanocene Dichlorides Bearing Pendant Alkenyldimethylsilyl Groups, [TiCl₂{η⁵-C₅Me₄(SiMe₂R)}₂] (R = Vinyl and Allyl)

Reduction of titanocene dichlorides bearing (ω-alkenyl)dimethylsilyl substituents [TiCl 2 {η 5 -C 5 Me 5 (SiMe 2 R)} 2 ], where R = CH CH 2 ( 3 ) and CH 2 CH CH 2 ( 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 °C affords cleanly the cyclopentadienyl-ring-tethered titanacyclopentane, [Ti(η 1:η 1:η: 5 η 5 -C 5 Me 4 SiMe 2 CH (Ti) CH(Me)CH(Me)CH (Ti) SiMe 2 C 5 Me 4 ] ( 5 ), which can be opened by HCl to an ansa -titanocene dichloride with a six-membered saturated bridging chain, [TiCl 2 {η: 5 η 5 -C 5 Me 4 SiMe 2 CH 2 CH(Me)CH(Me)CH 2 SiMe 2 C 5 Me 4 ) 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(η 1:η 1:η: 5 η 5 -C 5 Me 4 SiMe 2 CH (Ti) CH 2 CH 2 CH (Ti) SiMe 2 C 5 Me 4 ] ( 10 ) and the fluxional η 2 -alkene complex [Ti(η 5 -C 5 Me 4 SiMe 2 CH CH 2 )(η 2:η 5 -C 5 Me 4 SiMe 2 CH CH 2 )] ( 9 ). At 60 °C, the reduction of 3 yields a mixture of ansa -η 2 -alkene complex [Ti{η 2:η 5:η 5 -C 5 Me 4 SiMe 2 CH 2 CH CHCH 2 SiMe 2 C 5 Me 4 }] ( 7 ) and the product of a hydrogen loss, the doubly tethered, paramagnetic η 3 -allyl Ti(III) complex [Ti{η 3:η 5:η 5 -C 5 Me 4 SiMe 2 CHCH CHCH 2 SiMe 2 C 5 Me 4 }] ( 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 [TiX{η: 5 η 5 -C 5 Me 4 SiMe 2 CH 2 CH CHCH 2 SiMe 2 C 5 Me 4 }] (X = Cl, 12; OMe, 13 ). Oxidation of 8 with PbCl 2 yields η 1 -alkenyl complex [TiCl{η 1:η: 5 η 5 -C 5 Me 4 SiMe 2 CH (Ti) CH CHCH 2 SiMe 2 C 5 Me 4 }] ( 11 ). A similar oxidation of 12 to ansa -titanocene dichloride [TiCl 2 {η: 5 η 5 -C 5 Me 4 SiMe 2 CHCH CHCH 2 SiMe 2 C 5 Me 4 }] ( 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.