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{C ₄H₃N(CHNCH₂Py)Me₂}] (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)AlMe ₂}] (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{C ₄H₃N(CHNCH₂-Py)Me(O-2,6-iPr₂C ₆H₃)}] (3) and [Al{C₄H₃N(CHNCH ₂-Py)}(O-2,6-iPr₂C₆H₃)₂] (4), respectively, as well as an aluminum bis-diketonate compound, [Al{C ₄H₃N(CHNCH₂-Py)}(PhCOCHCOPh)₂] (5). All of the aforementioned derivatives were characterized by ¹H and ¹³C 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 caprolactone in the presence of BnOH and in the catalytic Meerwein-Ponndorf- Verley (MPV) reaction of 1-naphthalenemethanol and 2-naphthalenecarbaldehyde.