Zirconium complexes amides

Zirconium bis(amides) such as (35) and (36) display moderate ethylene polymerization activities.133,134 Complex (37) containing a chelating diamide ligand has been shown to initiate the living polymerization of a-olefins such as 1-hexene (Mw/Mn= 1.05-1.08) with activities up to 750gmmol-1 h-1.135-137 The living polymerization of propylene using this system activated with... 

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... 

The diastereomers of EBTHI zirconaaziridines are formed in comparable amounts via C-H activation, but equilibrate quickly (within an hour) to yield a thermodynamic mixture of diastereomers. Grossman observed little difference in the loss of MeH vs MeD from deuterium-labeled (EBTHI)zirconium methyl amide complexes 161 (Scheme 2). Loss of MeH and of MeD lead to different diastereomers, although their relative rate will also reflect the primary kinetic isotope effect for C-H activation. Neither kxlk2 nor k3lk4 is large, and both are largely the result of isotope effects rather than diastereoselectivity [42]. 

Norton and coworkers noticed significant differences in diastereoselectivity for formation of EBTHI zirconaaziridines 17m under kinetic vs thermodynamic conditions. In general, 17m was prepared by heating the (EBTHI)zirconium methyl amide complex at 70 °C (Scheme 3). Use of a 20-fold excess of carbonate, by accelerating insertion relative to diastereomer equilibration, permitted... 

Zirconium imido complexes, generated by thermolysis of zirconium alkyl amides and bis(amides)104 or tetrazenes105 in the presence of excess norbornene, gave the [2 + 2] cycloaddition products with exo configuration, e.g., 11 (X-ray)104. 

Enantioselectivities of up to 93% ee may be achieved using chiral bis (amidate) zirconium complex (S) SS (Scheme 11.16), first introduced by Schafer [89] and soon after by Scott [90]. 

Scheme 12 Intramolecular hydroamination of primary aminoalkenes with neutral bis(amidate) zirconium complexes. Reproduced from [64e] with permisssion of American Chemical Society and from [65 a] with permission of John Wiley Sons, Ltd...

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°. 

Previously reported bis(amidate)- and tethered-amidate-supported zirconium complexes can be used for alkene hydroamination catalysis, and all substrate scope and mechanistic investigations of these systems are consistent with the [2+2] cycloaddition mechanistic profile [61, 62). However, more recent catalyst systems that can be used with secondary amines show broader substrate scope, similar to that attained by rare earth elements and suggest a mechanistic similarity to that observed for previously intensely investigated rare earth hydroamination catalyst systems [7j. Such complexes are proposed to achieve ring closure via o-bond insertion, and thus, consideration of such a mechanistic profile in this case demanded further investigation. 

Higher enantioselectivities of up to 93% ee were achieved using the chiral bis(amidate) zirconium complex (5)-77 (Mes = 2,4,6-Me3CgH2), [250-253, 256], but again the high selectivities are limited to the formation of pyrrolidines, and unlike 75 and 76, only gcw-disubstituted substrates were reactive. 

The synthesis, structures, and reactivity of neutral and cationic mono- and bis(guanidinato)zirconium(rV) complexes have been studied in detail. Either salt-metathesis using preformed lithium guanidinates or carbodiimide insertion of zirconium amides can be employed. Typical examples for these two main synthetic routes are illustrated in Schemes 73 and 74. Various cr-alkyl complexes and cationic species derived from these precursors have been prepared and structurally characterized. 

Other examples of this synthetic strategy are known for example, a recent zirconium polymer by Illingsworth and Burke (8), who joined amine side groups of a zirconium bis(quadridentate Schiff-base) with an acid dianhydride to give amide linkages. Once again, caution is necesary, as Jones and Power (2) learned when they attempted to link metal bisO-diketonates) with sulfur halides that is, they obtained insoluble metal sulfides because the p-diketone complexes which they used were fairly labile and the insolubility drove the reactions to completion in the wrong direction. 

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