Titanium pyridonate complexes

In 2013, Schafer s group [22b] reported titanium bis(amidate) and bis(pyridonate) complexes for the homopolymerization of rac-lactide and e-caprolactone, and also the formation of a random copolymer of the two. These complexes form pseudo-octahedral six-coordinate species, which were characterized in the solid state. Complexes were synthesized by first installing 2 equiv. of the ligand on homoleptic TifNMe ) followed by protonolysis of dimethylamido ligands with 2 equiv. of alcohol (Figure 19). 

The efficiency that this set of initiators showed for both types of polymerization reactions, and the displayed influence of the bound A,0-chelating ligand, led to testing for copolymerization of the two monomers. Impressively, the titanium pyridonate complexes led to a random copolymer with a nearly 1 1 ratio of monomers. These random copolymers have average molecular weights of 1.8-2.2X 10 g/mol with PDl values of 1.29-1.41. 

To this end, a titanium bis(2-pyridonate) complex (17) has been developed. [22c] These catalytic systems promote catalyst-controlled selectivity for hydroaminoalkylation over hydroamination and for the first time amine-substituted cyclopentanes can be prepared preferentially over piperidine hydroamination products (Scheme 25). 

In addihon to the more generally reachve group 3 elements, examples of group 4 metals with amidate, pyridonate, and sulfonamidate ligands have been reported for ROP of cyclic esters. Such group 4 metals, and in particular titanium, are attractive due to their low cost, low toxicity, and high earth abundance. Furthermore, such complexes are known to be more robust than rare earth element complexes and thus less sensitive to the purity of the monomeric feedstock that is used for ROP. 

Figure 19 Titanium pyridonate and amidate complexes for the copolymerization of e-caprolactone and rac-lactide. Reproduced from [22b] with permission of Royal Society of Chemistry...

The reactions of VKSAs (la-c) with simple imine-TiCl4 (4a) or imine-ZrCl4 (4b) complexes were carried out at -100°C => 25°C for the titanium complexes and at -40°C 25°C for the zirconium complexes. The reactions afforded 5,6-dihydro-2-pyridones... 

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