Linear trimer analogues of calixarene as chiral coordinating ligands: X-ray crystallographic and NMR spectroscopic characterization of chiral and achiral trisphenolates complexed to titanium(IV) and aluminum(III)

Achiral and chiral linear trisphenol analogues of calixarene (HOArCH2Ar'(OH)C(R)HArOH, Ar = 4,6-di-tert-butylphenyl; Ar' = 4-tert-butylphenyl; R = H (achiral), Me (chiral)) were prepared in anticipation of their adoption of a chiral conformation upon coordination to Lewis acidic metal centers. The trisphenols react with I equiv of Ti(OR')(4) (R' = i-Pr or t-Bu) to yield complexes with molecular formula Ti-2(OArCH2Ar'(O)C(R)HArO)(2)(OR')(2) (R = H, Me; R' = i-Pr or t-Bu). An X-ray crystal structure of the titanium complex of the achiral trisphenol (R = H; R' = t-Bu) reveals that the trisphenolate ligand adopts an unsymmetrical (and therefore chiral) conformation, with eta(2)-coordination to one metal center and eta(1)-coordination to the second metal center. The chiral trisphenol, which contains a stereogenic center (indicated as C in the shorthand notation used above), coordinates titanium in an analogous fashion to produce only one diastereomer (out of four possible); therefore, the configuration of the stereogenic center controls the conformation adopted by the bound ligand. The reaction of achiral trisphenol with AlMe3 produces a compound with molecular formula Al-2(OArCH2Ar'(O)CH2ArO)(2). H-1 NMR spectroscopy and X-ray crystallography reveal that the trisphenolate ligand adopts an asymmetric, C-2 conformation in this complex, where the central phenolate oxygen bridges the aluminum centers and the terminal phenolate oxygens each coordinate a separate aluminum center. Because these trisphenolate ligands adopt chiral conformations when coordinated to metal centers, they may be useful for developing diastereo- or enantioselective catalysts and reagents.