Titanocenes and zirconocenes with (chiral) O-functionalized side chains on the Cp ring

Abstract

A series of titanocene and zirconocene dichlorides was synthesized bearing ligands that contain a (chiral) ether group linked by an ethylene spacer group to the cyclopentadienyl ring. Surprisingly, the chiral zirconocenes ( η 5-C 5H 4CH 2CH 2OR) 2ZrCl 2 with R=isobornyl and menthyl do not show Ziegler–Natta activity. This is probably caused by attack of the cocatalyst (MeAlO) n on the side chains, causing immobilization of the catalyst. Intramolecular coordination of the side chains is provoked by converting the dichlorides to the more Lewis-acidic mono and bis triflate complexes. The coordination of the ether chain depends on the size of the terminal group in the chain. In the achiral derivative ( η 5-C 5H 4CH 2CH 2OMe) 2Zr(O 3SCF 3) 2 there is a rapid equilibrium between species with one and two intramolecularly-coordinated ether side chains. The monotriflate complex ( η 5-C 5H 4CH 2CH 2O isobornyl) 2ZrCl(O 3SCF 3) decomposes rapidly, because ether coordination results in facile C–O bond cleavage and liberation of camphene. Accordingly, the bistriflate ( η 5-C 5H 4CH 2CH 2O isobornyl) 2Zr(O 3SCF 3) 2 could not be synthesized. The chiral triflate complexes ( η 5-C 5H 4CH 2CH 2OR) 2Zr(O 3SCF 3) 2 with R=menthyl and fenchyl are active Diels–Alder catalysts, although no optical induction was observed, probably because the chiral ether handles are not sufficiently coordinated during catalysis.