Pauson-Khand reaction electronic effects

To explain this observation, it was suggested that electron density differences at the acetylenic carbons can combine with steric effects to determine the regiochemical outcome of the reaction. Krafft concludes electronic effects seem to play a contributing role in determining the regiochemical outcome of the Pauson-Khand reaction, but, steric influences over the transition state apparently exert a more powerful directing effect. ... 

Electronic effects on alkene regioselectivity in the Pauson-Khand reaction have also been observed. The regioselectivity observed in cycloadditions of norbomen-2-ones has been interpreted as arising from an electronic preference for attachment of the 5 C-S of the alkene to an alkyne carbon rather than cobalt in the bond-forming insertion step (equation 13). In these systems electronic and steric effects have been separated by carrying out identical reactions with the corresponding norbomen-2-ols, in which the... 

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