Three aluminium alkoxide complexes containing N(2)O(2) bis(phenoxy)-amine ligands were synthesized from reactions of the corresponding N(2)O(2) ligands with Al(O(i)Pr)(3) in toluene. Different amine side chains of the ligands included pyridine (1), CH(2)NMe(2) (2), and CH(2)NEt(2) (3). The related chloro aluminium analog (4) was prepared from a reaction between AlCl(3) and the potassium salt of the N(2)O(2) ligand having CH(2)NMe(2) side chain. X-Ray crystallography reveals that complexes 3 and 4 have a monomeric five-coordinate aluminium center. Complexes 1-3 catalyzed the polymerization of epsilon-caprolactone (epsilon-CL) at 70 degrees C in toluene with the relative reactivities of 1<2< 3. In contrary, only complex 1 was active for the polymerization of lactide under the same polymerization conditions. (1)H NMR spectroscopy shows that treatment of 2 with 1 equivalent of lactide afforded the ring-opened product L(2)Al-OCH(Me)C(O)OCH(Me)C(O)O(i)Pr. Electronic effects are believed to be responsible for the observed trend in the epsilon-CL polymerization rates. On the other hand, steric hindrance at the amine side chain is the main contributor to the observed rates of lactide polymerization.
Polymerizations of biodegradable lactide and lactones have been the subjects of intense research during the past decade. They can be polymerized/copolymerized effectively by several catalyst systems. With bis(phenolate)-amine aluminum complex, we have shown for the first time that lactide monomer can deactivate the aluminum complex during the ongoing polymerization of ε-caprolactone to a complete stop. After hours of dormant state, the aluminum complex can be reactivated again by heating at 100 °C without the addition of any external chemicals still giving polymer with narrow dispersity. Studies using NMR, in situ FTIR, and single-crystal X-ray crystallography indicated that the coordination of the carbonyl group in lactyl unit was responsible for the unusual behavior of lactide. In addition, the unusual methyl-migration from methyl lactate ligand to the amine side chain of the aluminum complex was observed through intermolecular nucleophilic-attack mechanism.
The effects of appended hydroxyl groups and the chain length of a series of (imino)pyridine ligands on copper coordination and copper-catalyzed aerobic oxidation of alcohols were investigated.
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