Internal Pauson-Khand cycloadditions

Simple acyclic olefins are rather poor Pauson-Khand substrates under thermal conditions. Ethylene reacts moderately well with terminal but less well with internal acetylenes [98,107]. Usable reaction rates require forcing conditions which, fortunately, can be optimized for catalytic use of the metal [Eq. (46)] [108]. Substituted olefins give very variable results, but reveal interesting regiochemical aspects of the process. Cycloaddition of vinylcyclohexane with phenylacetylene proceeds to a mixture of 4- and 5-cyclohexyl-2-phenyl-2-cyclopentenones in 45% overall yield. Regioselectivity is total with respect to the acetylene, as expected. However, insertion of the alkene proceeds with little regiochemical preference. Evidently, in going from... 

Finally, Cramer and co-workers described an alternative route to classical Pauson-Khand reaction for the synthesis of cyclopentenones. The proposed procedure involves a reductive Ni -catalyzed [3+2] cycloaddition between aryl enoates and internal alkynes. More interestingly, the use of a chiral NHC led to a highly enantioselective reaction [eqn (10.38)]. Note that with unsymmetric alkynes the reaction is also regioselective. A plausible mechanism was proposed with the hypothesis that facial-selective coordination and incorporation of the enoate are controlled by a single chiral side chain of the carbene. 

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