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26.9.1.1.11.1 Variation 1: Using Hexacarbonyldicobalt–Alkyne Complexes (Pauson–Khand Reaction)

DOI: 10.1055/sos-SD-026-00987

Marsden, S. P.Science of Synthesis, (2005261087.

The formation of cyclopentenones by the coupling of hexacarbonyldicobaltalkyne complexes such as 235 with alkenes and carbon monoxide is known as the PausonKhand reaction.[‌196‌,‌197‌] The reactions can be carried out in both inter- and intramolecular manifolds. In intermolecular cases the reaction works best with strained alkenes such as norbornene 236, giving reasonable yields of the tricyclic cyclopentenones 237 (Scheme 43).[‌198‌] Yields are often poorer with simple alkenes and electron-deficient alkenes do not participate.[‌197‌] Vinyl ethers and esters such as 238 can be used as convenient alternatives to ethene, giving high yields of cyclopentenones (e.g., 239) and avoiding the need for high-pressure conditions.[‌199‌] In terms of the regioselectivity of intermolecular reactions, the reactions of unsymmetrical alkyne complexes show generally good regioselectivity, with the larger group positioning itself next to the carbonyl, as seen in the formation of 237.[‌197‌,‌198‌] Regiocontrol in the reactions of unsymmetrical alkenes is generally much poorer,[‌200‌] and is also dependent upon the substitution pattern of the alkyne employed. Thus, reaction of oct-1-ene with complex 240 (R1=H) gives a 1:1 mixture of isomeric cyclopentenones 241 (R1=H) and 242 (R1=H) whereas the use of the disubstituted cobalt complex 240 (R1=Me) gives complete control of alkyne regioselectivity and excellent control of alkene regioselectivity in the formation of 241 (R1=Me).[‌201‌] PausonKhand reactions frequently require elevated temperatures and are often carried out under an atmosphere of carbon monoxide, but the reactions can be greatly accelerated by the addition of promoters such as amine N-oxides[‌202‌] or alkyl methyl sulfides,[‌203‌] allowing reactions to take place at room temperature. Asymmetric variants of the reaction have been reported, using chiral amine N-oxides as promoters. Thus, exposure of cobalt complex 243 to norbornene 236 and brucine N-oxide gives the cyclopentenone 244 in good yield and enantioselectivity.[‌204‌]

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