Hafnium complex hafnocenes

The actual reaction path followed becomes more evident if the corresponding hafnium complexes are used as substrates. In this case the crucial intermediate 29, which can react to form either product 30 or 17, can be isolated. The thermodynamically favored ( -m-butadiene)hafnocene (5b) (32) turns out to be inert toward ethylene under the conditions applied. Even heating to 120°C in an ethylene atmosphere (1 bar) for several hours does not result in consumption of the metal complex. In contrast, s-trans-rf-butadiene)hafnocene (3b) rapidly takes up 1 molar equivalent of C2H4 even at -10°C to yield a C—C coupling product, i.e., the five-membered metallacyclic cr-allylhafnocene complex 29b. Above 0°C, vinylhafnacy-clopentane reacts with additional ethylene to form bis(cyclo-pentadienyl)hafnacyclopentane (30b) and free butadiene. In the absence... 

Treatment of 138-2Li" with zirconocene dichloride, hafnocene dichloride, and uranium tetrachloride gave acepentalene complexes 149-151 (Cp = cyclopentadienyl), as indicated by NMR spectroscopy. Interestingly, the hafnium complex was described as a tris-etheno-bridged trimethylenemethane complex (Scheme 10.53) [182,183]. 

Zrrconium(IV) and hafnium(IV) complexes have also been employed as catalysts for the epoxidation of olefins. The general trend is that with TBHP as oxidant, lower yields of the epoxides are obtained compared to titanium(IV) catalyst and therefore these catalysts will not be discussed iu detail. For example, zirconium(IV) alkoxide catalyzes the epoxidation of cyclohexene with TBHP yielding less than 10% of cyclohexene oxide but 60% of (fert-butylperoxo)cyclohexene °. The zirconium and hafnium alkoxides iu combiuatiou with dicyclohexyltartramide and TBHP have been reported by Yamaguchi and coworkers to catalyze the asymmetric epoxidation of homoallylic alcohols . The most active one was the zirconium catalyst (equation 43), giving the corresponding epoxides in yields of 4-38% and enantiomeric excesses of <5-77%. This catalyst showed the same sense of asymmetric induction as titanium. Also, polymer-attached zirconocene and hafnocene chlorides (polymer-Cp2MCl2, polymer-CpMCls M = Zr, Hf) have been developed and investigated for their catalytic activity in the epoxidation of cyclohexene with TBHP as oxidant, which turned out to be lower than that of the immobilized titanocene chlorides . ... 

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