Diene metallocene

In most cases that were reported in the literature so far, the (s-cis-ri4-diene)metallocene complexes (and their s-trans- counterparts), were synthesized by treatment of the respective bent metallocene dichloride precursor with a suitable conjugated-diene-magnesium reagent. However, in a few cases alternative entries and procedures have been developed and described. None of these has found such a widespread application as the original procedure, but some interesting chemistry has evolved among those methods. 

Even non-conjugated di-olefins may be used in these systems as precursors for the (conjugated diene)metallocene synthesis. They become isomerized under the reaction conditions at the bent metallocene system.81 Even vinylcyclopropanes yield the corresponding (l,3-diene)metallocene complexes when treated with the reagent 71. The three-membered ring is readily opened under the typical reaction conditions.82... 

A rather simple structure/activation energy correlation (AG = — ax4) was found for a series of (s-cA-r 4-l,3-diene)metallocenes whose characteristic structural parameters and ring inversion activation barriers had been reported in the literature.86... 

Other complexes of great interest are the / -conjugated diene metallocenes. They are conveniently prepared by the one-pot reaction of the photochemically induced metallocene with the diene at low temperature. Alternative routes include the treatment of the metallocene dichloride with a conjugated diene dianion equivalent or the coupling of alkenyl ligands in the coordination sphere of the metal. The resulting systems show unusual behavior. For example, buta-1,3-diene directly leads to the s-trans complex (8), which can be thermally equilibrated into the s-cis isomer (9). This form exhibits a substantial metallacyclopentene character and is best described as a d metal (cr, 7r)-type complex (10), as shown by X-ray crystal analysis. On the other hand, the s-trans isomer should be formally regarded as a real d metal -alkene 7T-complex. 

Chemistry of the (i-c /.r-trfltw-t) -Diene)metallocene Isomer Pairs. .. 26... 

In addition, the experimental observation of (s-/rani-ij -diene)metal-locene complexes is kineticcdly facilitated by the special appearance of the energy profile of the dienemetallocene system. For the parent system there is evidence (27-31) that the stable (butadiene)ZrCp2 isomers [about equal amounts of (s-trans- (3a) and (s-cis-T]i -butadiene)zirconocene (5a) are obtained under equilibrium conditions (22, 23)] are connected through a reactive (T -butadiene)zirconocene intermediate (4a) which rapidly equilibrates with 5-tra/u-diene complex 3a, but is separated from the (s-cis-diene)metallocene isomer 5a by a rather substantial activation barrier... 

Many (i7 -diene)metallocenes (5) of conjugated dienes, fixed in their 5-cis conformation by a rigid carbon framework, have been obtained in high yields by most of the synthetic routes used for the preparation of 3... 

Scheme 3. Deflnition of some geometric parameters used for the characterization of (j-cu-diene)metallocene complexes (5). 

Surprisingly, competition of 3a/5a for ethylene under various reaction conditions gave no indication at all of C—C coupling via ethylene insertion into a zirconium-to-carbon o--bond (27). At low temperatures, i.e., under conditions when the (5-tram-/5-cw-butadiene)zirconocene equilibration (3a 5 5a) is sufficiently slow (22, 23), it is only the (s-trans-jf -diene)metallocene isomer (3a) that reacts with ethylene. With increasing temperature, increasing amounts of (s-crs-butadiene)ZrCp2 (5a) are also consumed, but only at a limited rate that allows restoration of the 3 5... 

The (conjugated-diene)zirconocene and -hafnocene systems reviewed above are unique in that three isomeric organometallic species can be distinguished by physical and/or chemical means. Two of these, 3 and 5, represent stable isolable molecules, while the third, (T) -diene)MCp2 (4), functions as a chemically detectable intermediate connecting the (rf-diene)metallocene isomers (see Scheme 1). 

Characteristic chemical shift differences are observed between configurations 3 and 5. Relative to 5, the Cp resonances of (s -rra/u-diene)-metallocene isomers (3) are shifted to high held. As shown for butadiene as an example (Table IV), complexation of the s-cis-diene conformer to the transition metal causes large chemical shift differences of meso, syn, and... 

---