Chiral Salan Aluminium Ethyl Complexes and Their Application in Lactide Polymerization


Du, Hongzhi and Velders, Aldrik H. and Dijkstra, Pieter J. and Sun, Jingru and Zhong, Zhiyuan and Chen, Xuesi and Feijen, Jan (2009) Chiral Salan Aluminium Ethyl Complexes and Their Application in Lactide Polymerization. Chemistry: a European Journal, 15 (38). pp. 9836-9845. ISSN 0947-6539

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Abstract:Synthetic routes to aluminium ethyl complexes supported by chiral tetradentate phenoxyamine (salan-type) ligands [Al(OC6H2(R-6-R-4)CH2)2{CH3N(C6H10)NCH3}-C2H5] (4, 7: R=H; 5, 8: R=Cl; 6, 9: R=CH3) are reported. Enantiomerically pure salan ligands 1–3 with (R,R) configurations at their cyclohexane rings afforded the complexes 4, 5, and 6 as mixtures of two diastereoisomers (a and b). Each diastereoisomer a was, as determined by X-ray analysis, monomeric with a five-coordinated aluminium central core in the solid state, adopting a cis-(O,O) and cis-(Me,Me) ligand geometry. From the results of variable-temperature (VT) 1H NMR in the temperature range of 220–335 K, 1H–1H NOESY at 220 K, and diffusion-ordered spectroscopy (DOSY), it is concluded that each diastereoisomer b is also monomeric with a five-coordinated aluminium central core. The geometry is intermediate between square pyramidal with a cis-(O,O), trans-(Me,Me) ligand disposition and trigonal bipyramidal with a trans-(O,O) and trans-(Me,Me) disposition. A slow exchange between these two geometries at 220 K was indicated by 1H–1H NOESY NMR. In the presence of propan-2-ol as an initiator, enantiomerically pure (R,R) complexes 4–6 and their racemic mixtures 7–9 were efficient catalysts in the ring-opening polymerization of lactide (LA). Polylactide materials ranging from isotactically biased (Pm up to 0.66) to medium heterotactic (Pr up to 0.73) were obtained from rac-lactide, and syndiotactically biased polylactide (Pr up to 0.70) from meso-lactide. Kinetic studies revealed that the polymerization of (S,S)-LA in the presence of 4/propan-2-ol had a much higher polymerization rate than (R,R)-LA polymerization (kSS/kRR=10.1).
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