Migration ethynyl groups

The observed product distributions reflect the influence of both factors. In the parent compound 102 and in the silyl substituted 104, the initially formed open-chain silylenes 103 and 146 are relatively sensitive and the migration tendency of the group R is quite large, and therefore the silacyclopropenylidenes 104 and 145 are found. In contrast, the methyl and the ethynyl compound 138 and 150 are stable enough to pass the pyrolysis tube and both can be matrix-isolated upon pyrolysis of the appropriate precursors 137 and 149. Due to the small migration tendencies of the methyl and the ethynyl group, the rearrangement product 135 is not found at all and 151 is detected only in relatively minor amounts. 

Typical substrates for Fritsch-Buttenberg-Wiechell rearrangements are chloro-ethenes and ge/w-dibromoethenes. They are readily prepared by Wittig reaction from the corresponding ketones. The method offers a versatile entry to aromatic or heterocyclic acetylenes. Moreover, ethynyl groups can embark also on 1,2-migrations, thus, making simple or functionalized diynes and triynes (e.g., 321) accessible (Scheme 1-250). ... 

Because the hydrogen atom and phenyl group migrate so readily, the reactions of / -dicarbonyl enolates with ethynyl- and (phenylethynyl)iodonium salts can be expected to result in alkynylation. It has already been noted that the 2- -hexyl-l,3-indandionate ion undergoes alkynylation with (phenylethynyl)phenyliodonium tetrafluoroborate (equation 43), despite the availability of the -hexyl group for [2 + 3] annulation. Ethynylations of six / -dicarbonyl enolates and the anion of 2-nitrocyclohexane with ethynyl(phenyl)-iodonium tetrafluoroborate in THF have also been reported27. For example, admixture of the ethynyliodonium salt and the anion of ethyl 2-cyclopentanone-l-carboxylate in THF affords the 1-ethynyl derivative in 71% isolated yield (equation 124)27. 

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