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.