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DOI: 10.1055/sos-SD-001-00322

Malacria, M.; Aubert, C.; Renaud, J.-L.Science of Synthesis, (20011475.

Although π-allylcobalt complexes may be prepared from allylic substrates (see Section, most synthetic applications have utilized species generated in situ from dienes. Indeed, the reaction of buta-1,3-dienes such as 2-methylbuta-1,3-diene, 2,3-dimethylbuta-1,3-diene, and penta-1,3-diene, and penta-1,4-diene with tetracarbonylhydridocobalt(I) (64) can be used to synthesize π-allylcobalt complexes (Scheme 44).[‌159‌] Compound 64 can be generated in situ using potassium tetracarbonylcobaltate(I) or octacarbonyldicobalt(0) in glacial acetic acid. These reactions proceed via addition to the diene to produce but-2-enyltetracarbonyl intermediates which are not very stable. At 0°C or at higher temperature, they lose carbon monoxide to form the much more stable η3-allyl derivatives. The η3-allyl compound obtained from buta-1,3-diene is produced as a mixture of the geometrical isomers syn-65 and anti-65 in a ratio of circa 3:2. The syn-isomer is thermally the more stable and on heating the mixture the anti-isomer is almost completely converted into the syn form. The reactions of penta-1,3-diene and penta-1,4-diene with complex 64 also afford mixtures of isomers of the 1,3-dimethyl- and 1-ethyl-η3-allyl compounds, reflecting the different mechanisms available for diene insertion into the CoH bond.

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