Synthesis of [ethylene-1-(eta(5)-4,5,6,7-tetrahydro-1-indenyl)-2- (eta(5)-4',5',6',7'-tetrahydro-2'-indenyl)]titanium dichloride, the elusive isomer of the Brintzinger-type ansa-titanocenes

We present a short synthesis of 1-(2-indenyl)-2-(3-indenyl)ethane (5) and a method for its conversion to the ansa-metallocene ethylene(eta5-inden-1-yl)(eta5-inden-2-yl)]titanium dichloride (13). The synthetic strategy applied to prepare bisindene 5 relies on the efficient alkylation of 2-(phenylsulfonyl)indane followed by HMPA-assisted E1cB-elimination of phenylsulfinate. This tandem sequence circumvents the predisposition of indene to undergo C(1)-alkylation and enables access to C(2)-substituted indenes. The key step in the synthesis of the title ansa-titanocene (4) features a previously unreported equilibration step to generate the bis(indenide anion) of 5. Complexation with TiCl4.(THF)2 followed by hydrogenation of the product metallocene furnishes ansa-titanocene 4.