Chiral salan aluminium ethyl complexes and their application in lactide polymerization

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).