Substituent effects in reduction-induced synthesis of ansa-titanocenes

Bis{(diphenylvinylsilyl)tetramethylcyclopentadienyl}titanium dichloride [TiCl2{eta(5)-C5Me4(SiPh2CH=CH2)}(2)] (1) is reduced with a half molar equivalent of magnesium to the monochloride ([TiCl{eta(5)-C5Me4(SiPh2CH=CH2)}(2)] (2), whereas one molar equivalent of magnesium affords the titanocene [Ti{eta(5)-C5Me4(SiPh2CH=CH2)}{eta(5):eta(2)-C5Me4(SiPh2CH=CH2)}] (3) stabilized by eta(2)-coordination of one of the two vinyl groups to titanium(II). In the presence of excess magnesium, the vinyl moieties of 3 undergo intramolecular coupling to afford the ansa-titanocene [Ti(eta(5):eta(5):eta(2)-C5Me4SiPh2CH=CHCH2CH2SiPh2C5Me4)] (4) possessing the eta(2)-coordinated double bond in lateral position of its ansa-chain. The symmetrical ansa-titanocene [Ti(eta(5):eta(5):eta(2)-C5Me4SiPh2CH2CH=CHCH2SiPh2C5Me4)] (5) was not obtained although its DFT-calculated energy is only slightly higher than that of 4. It is considered that transient 5 gives rise to non-identified tar-like by-products which inherently accompany the formation of 4.