Optically active bis(β-diketonate) complexes of titanium

Abstract

The 2,2′-bis(methylene)biphenyl bridged bis(diketonate) (Tol₂Bob)Ti^IV fragment is resolved by reaction of the isopropoxide complex with (R)-1,1′-bi-2-naphthol, which selectively forms (S,Δ)-(Tol₂Bob)Ti(R-BINOL). The binaphtholate ligand can be cleaved from titanium by careful treatment with trifluoromethanesulfonic acid to give (S,Δ)-(Tol₂Bob)Ti(OTf)₂, which has a half-life toward racemization of at least 34 h at 51 °C. Racemization of the (Tol₂Bob)Ti^IV fragment is strongly accelerated under protic conditions, probably due to protonolysis of one of the diketonate ligands. Analogous optically active titanium bis(diketonates) can also be prepared by using an optically active 2,2′-bis(methylene)-1,1′-binaphthyl bridged bis(diketonate) ligand, Tol₂Bobbinap, prepared from (R)-BINOLH2. Complexation of (R)-Tol₂BobbinapH₂ with Ti(OiPr)₄ gives only a single diastereomer with the Λ configuration at titanium. The bis(diketonate)titanium(IV) fragment gives rise to characteristic signals in circular dichroism spectra which can be used to identify the configuration at the metal centre.