Single diastereomer half-sandwich salicyloxazoline complexes of titanium and zirconium

A series of half-sandwich complexes [Cp*MLX2] (L = four optically pure kappa(2)-salicyloxazolines, M = Ti, Zr; X = Cl, Me) have been prepared via salt elimination and protonolysis routes. X-ray crystallographic analyses demonstrate that the complexes contain stereogenic metal centers as a result of diastereoselective coordination of the N-O ligand. This chirality persists in solution; NMR spectroscopic investigations revealed that for most members of both the Ti and Zr series a single diastereomeric species is present at all accessible temperatures. Systems with incomplete diastereoselection are also accessible by notionally moving the C-stereogenic center on the oxazoline ring to a position where its chirality is less well expressed in the structure of the complex. In one such case (for M = Zr) a lower limit barrier to epimerization of 80 kJ mol(-1) is estimated at high temperature, while for an analogous Ti complex no exchange between epimers could be observed on the NMR chemical shift time scale. In contrast, unsubstituted cyclopentadienyl series CpTiLX2 gives diastereomeric mixtures in most cases and undergoes thermally accessible exchange between epimers.