Titanium complexes amide ligands

Dianionic bis(amide) ligands bearing additional donor atoms have been described by several researchers. High activities for ethylene polymerization are observed for pyridyldiamido zirconium complexes such as (42) (1,500gmmol-1 bar-1 h-1),145 although the corresponding titanium complex is much less active.146... 

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). 

Scheme 7 Modification of mixed Cp/Cp titanium amidate complexes to probe ligand effects upon alkene polymerization [1 Ob]...

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. 

A suitable entry into titanium and zirconium complex chemistry is the use of group IV amides and alkoxides. For example, when Ti(OEt)4 reacted with 2 equiv. of (roc)-2 in heptane, the titanium bis(disiloxide) 3 could be isolated in 91 % yield as a yellow microcrystalline material. Tbe results of the X-ray analysis (Fig. 1) of 3 confirm the expected extensive shielding of the titanium atom by the two sterically demanding disiloxide ligands. The geometry around the titanium atom is described best as distorted tetrahedral, with an 02-Til-02 chelate angle of 99° and an 02-T11-01 angle of 115.5°. 

Combination of achiral enolates vith achiral aldehydes mediated by chiral ligands at the enolate counter-ion opens another route to non-racemic aldol adducts. Again, this concept has been extremely fruitful for boron, tin, titanium, zirconium and other metal enolates. It has, ho vever not been applied very frequently to alkaline and earth alkaline metals. The main, inherent, dra vback in the use of these metals is that the reaction of the corresponding enolate, vhich is not complexed by the chiral ligand, competes vith that of the complexed enolate. Because the former reaction path vay inevitably leads to formation of the racemic product, the chiral ligand must be applied in at least stoichiometric amounts. Thus, any catalytic variant is excluded per se. Among the few approaches based on lithium enolates, early vork revealed that the aldol addition of a variety of lithium enolates in the presence of (S,S)-l,4-(bisdimethylamino)-2,3-dimethoxy butane or (S,S)-1,2,3,4-tetramethoxybutane provides only moderate induced stereoselectivity, typical ee values being 20% [177]. Chelation of the ketone enolate 104 by the chiral lithium amide 103 is more efficient - the j5-hydroxyl ketone syn-105 is obtained in 68% ee and no anti adduct is formed (Eq. (47)) [178]. 

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