Pauson-Khand Reaction

The Pauson-Khand Reaction is the transition metal catalyzed formal [2+2+1] cycloaddition of alkynes, alkenes, and carbon-monoxide to form substituted cyclopentenones.
General features:
1. The reaction can be inter- and intramolecular.
2. Internal alkynes tend to give lower yields than terminal alkynes.
3. Strained cyclic alkenes react faster, followed by terminal alkenes, disubstituted alkenes, and trisubstituted alkenes. Alkenes with strongly electron-withdrawing groups are less reactive.
4. The reaction is highly regioselective: the larger alkyne substituent ends up next to the carbonyl group in the product.
5. Intramolecular reactions proceed with excellent regio- and stereoselectivity.
6. General reaction conditions: elevated temperatures and pressures in benzene, toluene, tetrahydrofuran, or 1,2-dichloroethane as a solvent.
Reaction mechanism:
1. Formation of an alkyne-dicobalt intermediate.
2. Loss of CO and alkene coordination to the 16 electron complex.
3. Alkene insertion followed by CO insertion.
4. Final reductive elimination delivers the cyclopentenone and regenerates the catalyst.
Seminal publications:
1. Khand, I. U., Knox, G. R., Pauson, P. L., Watts, W. E., Foreman, M. I. J. Chem. Soc., Perkin Trans. 1 1973, 975. doi.org/10.103...
2. Khand, I. U., Knox, G. R., Pauson, P. L., Watts, W. E., Foreman, M. I. J. Chem. Soc., Perkin Trans. 1 1973, 977. doi.org/10.103...
References:
NROChemistry: nrochemistry.c...
Additional references:
1.en.wikipedia.o...
2. www.organic-ch...

[@NROChemistry]

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