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 1 equiv of Ti(OR′)4 (R′ = i-Pr or t-Bu) to yield complexes with molecular formula Ti₂(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 η²-coordination to one metal center and η¹-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 Al₂(OArCH₂Ar′(O)CH₂ArO)₂. ¹H NMR spectroscopy and X-ray crystallography reveal that the trisphenolate ligand adopts an asymmetric, C₂ 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.