Catalysts hafnium

After the discovery of isotactic polymerisation of propylene using shconocene catalysts, stmcturaHy analogous hafnium catalysts produced from hafnium tetrachloride [13499-05-3] were found to produce high yields of high molecular weight polypropylene (55), but not enough to lead to commercial development. 

The effect of the metals used was then examined (Table 5.4). When the group 4 metals, titanium, zirconium, and hafnium, were screened it was found that a chiral hafnium catalyst gave high yields and enantioselectivity in the model reaction of aldimine lb with 7a, while lower yields and enantiomeric excesses were obtained using a chiral titanium catalyst [17]. 

Tullock C.W. et al.. Polyethylene and elastomeric polypropylene using alumina-supported bis(arene) titanium, zirconium, and hafnium catalysts, J. Polym. Sci, Part A, Polym. Chem., 27, 3063, 1989. Mueller G. and Rieger R., Propene based thermoplastic elastomers by early and late transition metal catalysis. Prog. Polym. Sci., 27, 815, 2002. 

For hafnium this has been reported previously [4], A large excess of aluminium, a hafnium catalyst not undergoing p-hydride termination but smoothly inserting ethene molecules, and a fast exchange between the two metals leads to a desirable Poisson distribution. 

Fig. 12.5 Symyx high-throughput primary screen, in which arrays of metal-ligand combinations are rapidly surveyed for olefin polymerization activity. In this example, a 1-octene primary screen was used to discover a new amide ether-based hafnium catalyst.

Of all the hafnium compounds listed in Figure 2.7, none excels their zirconium counterparts in syndioselectivity. This observation is sometimes attributed to hafnium s decreased rate of bimolecular propagation relative to unimolecular site epimerization. Lower overall activities with hafnium catalysts corroborate this argument. The most syndioselective hafnium catalyst appears to be Cl-symmetric i-92-Hf, which produces syndiotactic polypropylene with [r] = 92.2% and... 

A hafnium catalyst suitable for fluorous biphasic system, which attracts attention from the viewpoint of green chemistry, was reported. In ester condensation reaction shown in Equation 69, hafnium (IV) perfluorooctane sulfonylamide (158) showed high activity to give the corresponding ester (159) in excellent yield from carboxylate (157) and octanol in 1 1 molar ratio [75]. The catalyst is recyclable by simple phase separation in the two phase system after the reaction. 

Ester condensation under mild conditions was realized using 2-thienyl carbonate and then transesterification was carried out using hafnium catalyst as shown in Equation 70 [76]. Carboxylate (160) was converted into the corresponding thienyl ester (162) by the reaction with thienyl carbonate (161) in the presence of DMAP as a catalyst, followed by condensation with allyl alcohol with a catalytic amount of Hf(OTf)4 to produce allyl ester in high yield. Various carboxylates and alcohols are applicable for the reactions, including sterically hindered alcohols, such as i-Pr2CHOH, t-BuMeCHOH, and (-)-menthol. 

Activation of Iridentate amine-diol titanium complexes with MAO also produced poly(l-hexene) with narrow molecular weight distributions [63]. Modification of C,-symmetric hafnium catalysts (Scheme 2.14) [64] resulted in pyridylamido ligands (Scheme 2.15) for the polymerization of isotactic poly(a-olefins) with relatively narrow molecular weight distributions (M 1.20) for molecular masses up to 152,000g mole" [42]. 

A novel chiral hafnium catalyst, which was readily prepared fromHf(OTf)4 and a proline-derived chiral ligand, has been tested in asymmetric Michael reactions of thiols with 3-(2-alkenoyl)-2-oxazolidinones, affording the corresponding adducts in high yield and enantioselectivity. Although chiral Lewis acids are less reactive than the original Lewis acids in many cases, ligand acceleration has been demonstrated in this as)unmetric Michael addition reaction (eq 18). ... 

Figure 93 Titanium, zirconium and hafnium catalyst precursors.

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