Zirconocene complexes automerization

The activation barrier of this characteristic automerization process of (5-c -conjugated diene)zirconocene and -hafnocene complexes has proved to be very dependent on structure and substituents of the diene ligand (22, 45). The highest known activation energy was observed for compound 51, the zirconocene complex of Hoogeveen s diene ... 

The [(s-trans-diene)ZrCp2] complex (s-trans-1) equilibrates with the [(s-cA-diene)ZrCp2] isomer (x-cA-l) via a reactive high lying (r 2-butadiene) metallocene intermediate (2) [A(s-trans-1 s-cis-l, 283 K) = 22.7 0.3 kcal mol-1]. Syntheses of the (butadiene)zirconocene system carried out under kinetic control invariably led to pure s-trans-1, whereas a ca. 1 1 equilibrium of s-trans-1 and. v-ci.v-l was obtained under conditions of thermodynamic control.5,6 The cr,7i-structured s-cis-l isomer undergoes a dynamic ring-flip automerization process (see Scheme 2) that is rapid on the NMR time scale [AG futom = 12.6 0.5 kcal mol ].5... 

Automerization Activation Barriers of Selected (s-cu-Diene)zirconocene And -HAFNOCENE Complexes (5)... 

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