Carbomercuration

In another example, Hg(OTf)2 has been used for the stereoselective ring expansion of 116 to 117 with excellent stereospecificity for the (Z)-isomer (Equation (43)).156 Carbomercuration reactions have also been observed for derivatives such as 118, which possesses both an allylic silane as well as an alkynyl functionality. For 118, the reaction affords the 5-m -ring compound 119 as the (Z)-isomer. Formation of the (/. )-isomcr is disfavored because the bulk of the TMS group precludes an anti-attack (Equation (44)).157... 

A functionalized mercury(II) compound like ethyl (acetoxymercurio)acetate (136) allows an easy approach to prostaglandin endoperoxide analogs (equation 52).204 Several organomercury(II) compounds, RHgCl (R = Me, aryl, benzylic), are able to add to 1,3-dienes in the presence of a stoichiometric amount of a palladium(II) salt and affonl ir-allylpalladium compounds of type (137) in variable yields (equation 53).20S A related intramolecular carbomercuration has been reported by Snider.206 It allows a stercospe-cific approach to the chloromercury compound (138 equation 54). Similar palladium-mediated reactions... 

Metal-mediated additions of silyl enolates to alkynes also are valuable for intra-and intermolecular vinylation of ketones. The Hg(II)-induced cychzation of alky-nyl-branched SEE provides regio- and stereochemicaUy homogeneous exocychc vinyl mercurial products that can be converted into a variety of functionahzed alkenes [265-267]. Two plausible mechanisms for the intramolecular carbomercura-tion have been proposed, as in the above Pd(II)-mediated cycloaUcenylation-nu-cleophihc enolate addition to the Hg(II)-activated alkyne and concerted addition of an a-keto mercurial species generated by Si-Hg transmetalation. Forsyth et al. have determined that carbomercuration occurs in an anti fashion by the former... 

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