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Abstract

1,6-Enynes bearing (electron-rich) aryl substituents on their alkyne moiety rearrange to cyclobutene derivatives in the presence of catalytic amounts of PtCl₂ in toluene. The reaction is significantly accelerated when performed under an atmosphere of CO (1 atm), most likely by increasing the electrophilicity of the metal template by temporary coordination to this π-acidic ligand. This transformation allows the build up of considerable strain in the products as witnessed by the productive formation of tricyclic skeletons, such as 7 or 9. Moreover, these products provide evidence for the mechanistic scenario of platinum-catalyzed cycloisomerization reactions previously proposed, which are thought to proceed via organo−platinum species that mimic the reactivity of metal-complexed “nonclassical” carbocations.