Chiral hafnium catalyst

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]. 

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). ... 

Ewen, J. A. Crystal structures and stereospecific propylene polymerizations with chiral hafnium metallocene catalysts. J. Am. Chem. Soc. 1987,100, 6544—6545. 

Titanium, Zirconium, Hafnium - The catalytic asymmetric hydrogenation of imines has been reported using a chiral titanocene catalyst. " An enantiopure titanocene catalyst has been used in the catalytic asymmetric hydrogaution of disubstituted enamines. Kinetic resolution of a racemic disubstituted pyrroline has been effected by asymmetric reduction with a chiral titanocene catalyst. Poly(methylhydrosiloxane) has been used as a stoichiometric... 

As exemplified by the reactions of Schemes 1 and 4, fluorotitanium compounds could open new possibilities for metal-catalyzed processes. Their fascinating structural diversity [7] as well as further catalytic possibilities in the field of olefin polymerizations [7i, 16] have been put forward by the pioneering work of Roesky, Noltemeyer and co-workers. Similar properties were also exhibited by the analogous zirconium and hafnium compounds [7b,i]. A Zr binaphtholate has already been successfully applied for the enantioselective allylstannylation of aldehydes [2f], Buch-wald and co-workers successfully used a chiral titanocene difluoride as precursor for the corresponding Ti(lII) hydride, a very efficient catalyst for the enantioselective hydrosilylation of imines [17]. 

The in situ catalysts formed from diethyl tartarate (DET) and titanium(IV) isopropoxide catalyze the asymmetric epoxidation reactions (72). The seminal work of Sharpless has been applied for a great variety of synthetic reactions. (The replacement of titanium with zirconium, hafnium, or thallium does not improve the chiral efficiency)... 

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