Multidentate Lewis acids. Synthesis of an organic framework designed to hold two sites of Lewis acidity in a convergent orientation

Treatment of 2,2'-(1,2-ethynediyl)bis[6-(1,1-dimethylethyl)-4-methylphenol] (2) with 2 equiv of Al(i-Bu)(3) converts the hydroxyl groups into potentially Lewis acidic OAl(i-Bu)(2) groups with a convergent orientation. Their intramolecular interaction produces the novel structure 11, which incorporates an Al2O2 ring that is markedly distorted by constraints imposed by the diphenylacetylene framework. Unlike normal Al2O2 rings, which are planar and have trigonal planar oxygen atoms, the Al2O2 ring of compound 11 is puckered, and its oxygen atoms are distinctly pyramidalized. In addition, the average Al-O bond length in compound 11 (1.913(2) Angstrom) is significantly longer than these found in related dimers of alkylaluminum alkoxides and aryloxides (1.840-1.895 Angstrom).