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).

Ring-Opening Polymerization of rac-Lactide with Aluminum Chiral Anilido-Oxazolinate Complexes

“…Although the stereoselectivity observed was not particularly high, the findings here are interesting because the change of selectivity was due to the variation of substituents on R 1 , R 3 , and R 4 in the similar initiator structures. Such a strategy of modulation of tacticity by changing substituents has been utilized in other aluminum 13 and zirconium 24 systems.…”

“…9 The pioneering works by Spassky 10 in the control of PLA microstructure showed that a chiral salen aluminum complex (Scheme 1; I , where R = ( R )-2,2′-binaphthyl, R 1 = R 2 = H, X = OMe) was highly isoselective for the ROP of rac -lactide. This system was further exploited 11 and expanded to other salen species, 12 their reduced derivatives such as salan ( II ; Scheme 1) 13 and salalen, 14 and related N 2 O 2 ( III ; Scheme 1) and N 4 type ligands. 15 These initiators typically feature a five-coordinate aluminum center supported by dianionic, tetradentate ligands.…”

Aluminum Salen and Salan Polymerization Catalysts: From Monomer Scope to Macrostructure Control

“…The two chloro-substituted complexes, 43 and 44, gave the highest level of heterotacticity with P r = 0.96 and 0.88, respectively. Feijen developed a series of aluminum salan complexes with a cyclohexyl diamine bridge, with both racemic and (R,R) analogues (47). Racemic and enantiopure complexes with either no phenoxy substitution or methyl phenoxy substitution gave isotactic enriched PLA but with only moderate isoselectivity (P m up to 0.66), while chloro phenoxy substitution yielded heterotactic biased PLA (P r up to 0.73).…”

Aluminium coordination complexes in copolymerization reactions of carbon dioxide and epoxides

“…Historically however, of the main group metals, aluminium complexes have played an important role in the development of copolymerization reactions of carbon dioxide and should be investigated further. Importantly, aluminium complexes are known to catalyse a wide range of other polymerization reactions including ring-opening polymerization (ROP) of cyclic esters [14][15][16][17][18][19][20][21][22][23][24][25][26] and trimethylene carbonate, 18,27,28 and cationic polymerization of alkenes (incl. isoprene, benzofuran and styrenes).…”