Expression of chirality in salicyloxazolinate complexes of zirconium

Treatment of chiral non-racemic salicyloxazoline proligands HL with tetrabenzylzirconium(IV) gave species [L2Zr(CH2Ph)2]. Reactions of KL with ZrCl4(THF)2 gave similar chloro complexes. One example of a benzyl complex was shown to exist as the Λ-trans,cis,cis diastereomer by X-ray crystallography. DFT calculations showed the observed isomer to be the most stable by 35 kJ mol−1, indicating that thermodynamic diastereoselection for this species is excellent. Examination of the chiral environment about the benzyl co-ligands indicates however that the degree of expression of the chirality of the structure in what would be the site of metal-based reactions is poor in comparison to related systems. Variable temperature 1H NMR data are consistent with this in that the low temperature spectrum exhibits a very small chemical shift difference between the chemically inequivalent benzylic CH atoms, and at higher temperatures dissociation of the portion of the ligand that contains the chiral information, i.e., the oxazoline unit, leads to apparent equivalence.

DFT calculations and dynamic NMR spectra indicate that [L2ZrX2] (L = salicyloxazolinate) exhibits excellent thermodynamic diastereoselection for metal-centred chirality, although it is argued that the chirality of the system is not well expressed in the environment of the co-ligands.Figure optionsDownload as PowerPoint slide