Chiral Salan Aluminium Ethyl Complexes and Their Application in Lactide Polymerization

Abstract Synthetic routes to aluminium ethyl complexes supported by chiral tetradentate phenoxyamine (salan‐type) ligands [Al(OC 6 H 2 (R‐6‐R‐4)CH 2 ) 2 {CH 3 N(C 6 H 10 )NCH 3 }‐C 2 H 5 ] ( 4 , 7 : R=H; 5 , 8 : R=Cl; 6 , 9 : R=CH 3 ) 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) 1 H NMR in the temperature range of 220–335 K, 1 H– 1 H 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 1 H– 1 H 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 ( P m up to 0.66) to medium heterotactic ( P r up to 0.73) were obtained from rac ‐lactide, and syndiotactically biased polylactide ( P r 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 ( k SS / k RR =10.1).