Selective Isomerization of 1,5-Cyclooctadiene to 1,4-Cyclooctadiene Catalyzed by Bis(acetylacetonato)nickel-Triethyldialuminum Trichloride-Phosphorus Ligand.

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The isomerization of 1,5-cyclooctadiene (1,5-COD) with Ni(acac)2–Et3Al2Cl3–phosphorus ligand (Ni : Al2 : P=1 : 10 : 3) was first examined with varying P ligands in toluene in order to find suitable conditions for the formation of 1,4-COD. The product distribution depended largely on the P ligands. For several phosphites possessing very strong π-acceptor properties, the main product was 1,4-COD. In particular, the less-bulky bicyclic phosphite, 4-ethyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane (L-3) was very effective and gave 1,4-COD in a high selectivity of 93% at 66% conversion (1,5-COD/ Ni=500, molar ratio) by employing a low temperature of −30°C. However, the reaction stopped before reaching completion because the catalyst was deactivated by the accumulation of 1,4-COD product. The conversion, depending on the 1,5-COD/toluene ratio (volume) rather than the 1,5-COD/Ni ratio, increased with a decrease in the 1,5-COD/ toluene ratio and was 66–69% at a ratio of 0.19. On the other hand, the catalyst (P: L-3) was much less active for 1,4-COD than for 1,5-COD and was deactivated quickly under the same reaction conditions. This appeared to result in a high selectivity of 1,4-COD in the isomerization of 1,5-COD. The mechanistic implications of the experimental results are discussed.
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