Norbomenes Pauson-Khand reaction

Co2(CO)6 fragments at the C=C triple bonds further from the Pt center. The Pauson-Khand reaction between tra 5-Pt C=CC2H[Co2(CO)6] (PR3)2 (R = Et, Bu) (197) and norbomene or cyclopentene gives the corresponding cyclopen-tenones 198 (Scheme 44). ... 

Ethynylcyclopropanes, like normal acetylenes, react with dicobalt octacarbonyl in ether to form stable dinuclear cluster-like hexacarbonyl complexes (equation 170)236. The complex with l-chIoro-2,2,3,3-tetramethylethynylcyclopropane reacts stereo- and regioselec-tively with norbomene in a typical Pauson-Khand reaction to give the exn-2-cyclopropyl substituted cyclopentenone (equation 171). Similarly, the reaction of 2-ethoxycyclo-propylacetylene with cyclopentene in the presence of Co2(CO)8 under CO gave 3-(2-ethoxycyclopropyl)-cw-bicyclo[3.3.0]oct-3-en-2-one (equation 172)242. 

There is only one example of a catalytic Pauson-Khand reaction in an ionic liquid1471 although the reaction has also been conducted in ionic liquids using stoichiometric amounts of Co2(CO)8/481 In the catalysed reaction 10 mol% of Co2(CO)8 was used in [C4Ciim][PF6] under 10 bar of CO. Under these conditions, diethylallyl malonates could be obtained in 90-99% yield within 90 minutes at 80°C, as shown in Scheme 9.14. However, with hetero-bridged enynes, as well as in the reaction between norbomene and phenylacetylene, only poor to moderate yields were achieved. A slight increase in activity was observed with the analogous tetrafluoroborate ionic liquid. 

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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