Science of Synthesis

Shibata, T., Science of Synthesis: Stereoselective Synthesis, (2011) 3, 125.

In 1973, Khand and Pauson reported the intermolecular coupling of alkyne–hexacarbonyldicobalt(0) complexes with alkenes for the construction of cyclopentenones.[‌2‌] In early work on the Pauson–Khand reaction only symmetrical alkenes, such as ethene and norbornene, were used because of the problems associated with the formation of regioisomers with unsymmetrical alkynes and alkenes. The intramolecular reaction of enynes overcomes this problem, and it has been employed extensively in natural product syntheses.[‌3‌] A catalytic Pauson–Khand reaction of enynes has been achieved using the hexacarbonyldicobalt(0)–triphenyl phosphite [Co2(CO)6–P(OPh)3] complex under a slightly pressurized atmosphere of carbon monoxide.[‌4‌] Other significant improvements in the Pauson–Khand reaction have been achieved using transition metals other than cobalt as the catalyst. For example, a low-valent titanium complex is the first complex able to catalyze the carbonylative coupling under an almost atmospheric pressure of carbon monoxide.[‌5‌]

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