Aluminum Compounds Containing Pyrrole-Imine Ligand Systems - Synthesis, Characterization, Structure Elucidation, Ring-Opening Polymerization, and Catalytic Meerwein-Ponndorf-Verley Reaction

The reactions of AlMe3 with the tridentate pyrrole ligand precursor C4H3NH(2-CH=NCH2Py) and subsequent treatment of the derivatives with small organic molecules such as 2,6-diisopropylphenol or dibenzoylmethane were conveniently performed, and the products were characterized. The reaction between 1 equiv. of AlMe3 and 1 equiv. of a pyrrole-imine-pyridine ligand in toluene affords [Al{C4H3N(CHNCH2Py)Me-2}] (1) in high yield. In addition, the use of 2 equiv. of AlMe3 with the same tridentate precursor results in the formation of a dialuminum compound [AlMe3{C4H3N(CHNCH2-Py)AlMe2}] (2) in moderate yield. Furthermore, the combination of 1 with either 1 or 2 equiv. of 2,6-diisopropylphenol or 2 equiv. of dibenzoylmethane in toluene yields aluminum monophenoxide or diphenoxide compounds [Al{C4H3N(CHNCH2-Py)Me(O-2,6-iPr(2)C(6)H(3))}] (3) and [Al{C4H3N(CHNCH2-Py)}(O-2,6-iPr(2)C(6)H(3))(2)] (4), respectively, as well as an aluminum bis-diketonate compound, [Al{C4H3N(CHNCH2-Py)}(PhCOCHCOPh)(2)] (5). All of the aforementioned derivatives were characterized by H-1 and C-13 NMR spectroscopy, and their solid-state molecular structures were determined by single-crystal X-ray diffraction. The geometries of 1-5 show that the pyrrole ligand exists in the pyrrolyl-imine and azafulvene-amido resonance forms. Compounds 1, 3, 4 and 5 were used in the ring-opening polymerization of E-caprolactone in the presence of BnOH and in the catalytic Meerwein-Ponndorf-Verley (MPV) reaction of 1-naphthalenemethanol and 2-naphthalenecarbaldehyde.