Aluminum chelates supported by β-quinolyl enolate ligands: synthesis and ROP of ε-CL

Abstract

Treatment of tautomers of β-quinolyl ketone-enaminones 1a–6a with AlMe₃ afforded β-quinolyl enolato dialkylaluminium complexes LAlMe₂1b–6b (L = [(2-C₉H₆N)–CHC(R)–O–], R = CH₃ (1b), ^tBu (2b), Ph (3b), o-tolyl (4b), p-tolyl (5b), p-OMePh (6b)), respectively. 2b reacted with benzyl alcohol to generate the corresponding LAl(OBn)₂ complex 2c. Complexes 1b–6b and 2c were characterized by ¹H and ¹³C NMR spectroscopy, elemental analyses and single crystal X-ray diffraction analyses. All complexes were tested as catalyst precursors for ring-opening polymerization of ε-caprolactone (ε-CL). The results indicated that LAlMe₂ (1b–6b) exhibited good activity towards the ROP of ε-CL in the presence of benzyl alcohol at 80 °C, and LAl(OBn)₂2c exhibited higher catalytic activity in the absence of alcohol than 1b–6b for the ROP of ε-CL. However, both polymerizations were less controlled. Kinetic studies showed that the polymerization reaction catalyzed by 1b–6b and 2c proceeded with first-order dependence on the monomer and took place through coordination–insertion.