Phenylacetylene, reaction with iodonium

Several iodonium ylides, thermally or photochemically, transferred their carbene moiety to alkenes which were converted into cyclopropane derivatives. The thermal decomposition of ylides was usually catalysed by copper or rhodium salts and was most efficient in intramolecular cyclopropanation. Reactions of PhI=C(C02Me)2 with styrenes, allylbenzene and phenylacetylene have established the intermediacy of carbenes in the presence of a chiral catalyst, intramolecular cyclopropanation resulted in the preparation of a product in 67% enantiomeric excess [12]. 

Thermal reactions of the iodonium enolate 279 with acetonitrile and carbon disulfide in the presence of Cu(acac)2 lead to the heterobicyclic ketones 285 and 286, respectively (98TL9073), while similar treatment of the hybrid sulfonyl-carbonyl ylide 287 with CS2 or phenylacetylene affords the tricyclic sulfones 288 and 289 (97T9365). 

So far we have understood where the solvolysis produets (and hence iodobenzene) come from. However, the mechanisms discussed above cannot explain how phenylacetylene (the remaining reaction byproduct) is formed. A triple bond can result from an elimination process. Furthermore, when the reaction is carried out with deuterated vinyl iodonium 5, unlabeled phenylacetylene is produced (see Scheme 35.2). Considering these arguments, a reasonable pathway to explain the formation of phenylacetylene could be a solvent-assisted a-elimination (Scheme 35.8). The removal of the H/D by the solvent and simultaneous departure of the iodobenzene would yield a vinylidene carbene 11, which subsequently rearranges to the alkyne by migration of hydrogen. The complete loss of the label would be fully compatible with this route. 

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