Synthesis, structure, and catalytic activity of binuclear aluminum chloride complexes with chiral biaryl-based ligands

Four new binuclear aluminum chloride complexes {(1)AlCl}2 (5), {(2)AlCl}2 (6), {(3)AlCl}2 (7) and {(4)AlCl}2 (8) have been prepared in good yields by alkane elimination from the reaction of AlEt2Cl and chiral biaryl-based N2O ligands, (S)-2-(pyridine-2-ylmethylamino)-2′-hydroxy-1,1′-binaphthyl (1H2), (S)-5,5′,6,6′,7,7′,8,8′-octahydro-2-(pyrrol-2-methyleneamino)-2′-hydroxy-1,1′-binaphthyl (2H2), (S)-5,5′,6,6′,7,7′,8,8′-octahydro-2-(pyridine-2-ylmethylamino)-2′-hydroxy-1,1′-binaphthyl (3H2) and (S)-2-(pyrrol-2-methyleneamino)-2′-hydroxy-6,6′-dimethyl-1,1′-biphenyl (4H2), which are derived from (S)-2-amino-2′-hydroxy-1,1′-binaphthyl or (S)-2-amino-2′-hydroxy-6,6′-dimethyl-1,1′-biphenyl, respectively. Complexes 5–8 have been characterized by various spectroscopic techniques, elemental analyses, and X-ray diffraction analyses. The complexes 5–8 have a similar C2 quasi-symmetric ClN2AlO2AlN2Cl core structure, and they are active catalysts for the polymerization of rac-lactide in the presence of propylene oxide (PO), leading to the heterotactic-rich polylactides.