Substituent effects in reduction-induced synthesis of ansa-titanocenes

Abstract

Bis{(diphenylvinylsilyl)tetramethylcyclopentadienyl}titanium dichloride [TiCl 2 {g 5 -C 5 Me 4 (SiPh 2 CH= CH 2 )} 2 ] (1) is reduced with a half molar equivalent of magnesium to the monochloride ([TiCl{g 5 -C 5 Me 4 (SiPh 2-CH=CH 2 )} 2 ] (2), whereas one molar equivalent of magnesium affords the titanocene [Ti{g 5 -C 5 Me 4 (SiPh 2 CH= CH 2 )}{g 5 :g 2 -C 5 Me 4 (SiPh 2 CH=CH 2 )}] (3) stabilized by g 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(g 5 :g 5 :g 2 -C 5 Me 4 SiPh 2 CH=CHCH 2 CH 2-SiPh 2 C 5 Me 4 )] (4) possessing the g 2 -coordinated double bond in lateral position of its ansa-chain. The symmetrical ansa-titanocene [Ti(g 5 :g 5 :g 2 -C 5 Me 4 SiPh 2 CH 2 CH=CHCH 2-SiPh 2 C 5 Me 4 )] (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 tarlike by-products which inherently accompany the formation of 4.