Ethynyliodonium salts

Alkynyl(phenyl)iodonium salts can be used for the preparation of substituted alkynes by the reaction with carbon nucleophiles. The parent ethynyliodonium tetrafluoroborate 124 reacts with various enolates of /J-dicarbonyl compounds 123 to give the respective alkynylated products 125 in a high yield (Scheme 51) [109]. The anion of nitrocyclohexane can also be ethynylated under these conditions. A similar alkynylation of 2-methyl-1,3-cyclopentanedione by ethynyliodonium salt 124 was applied in the key step of the synthesis of chiral methylene lactones [110]. 

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. 

All pure alkynyliodonium species are microcrystalline solids. Their solubility is limited to non-nucleophilic polar solvents. They are insoluble in hydrocarbon, aromatic, ether, and other nonpolar solvents. They tend to decompose in methanol and ethanol as well as in dimethyl sulfoxide (DMSO), dimethylformamide (DMF), etc. With the exception of the parent ethynyliodonium salt they are also insoluble in water. The best solvent seems to be acetonitrile. Reactions may also be carried out under initially heterogeneous conditions in solvents like CH2CI2, CHCI3 where the mixture becomes homogeneous as the reaction proceeds. 

In general, the parent ethynyliodonium salts and simple alkyl-substituted compounds ate the most stable and may be indefinitely stored as pure solids in a refrigerator. The P-func-tionalized (11) and bis-iodonium (34-36) compounds are somewhat less stable but may be isolated, characterized, and stored cold for several days in pure form. The stability of both the diynyliodoniums 26 and the dialkynyl compounds 28 are greatly dependent upon the substituents R. The greater the steric bulk of R, the more stable the compound. Perhaps, the least stable, fully characterized species is the bis-iodonium diyne 32, which decomposes rapidly above — 20°C and hence is best made fresh and used in situ. 

The infrared spectra have a weak, but clearly discernible, C s C absorption between 2120 and 2190 cm . Strong absorption, highly characteristic of the anions is also present intense, broad bands between 1000 and 1100 cm for BF " and two intense signals around 1000 and 1270 cm for CF3SOf. Moreover, the P-functionalized ethynyliodonium salts 11 display useful, characteristic absorptions for the specific functionality such as CzN, carbonyl, etc. 

The most investigated ctirbon nucleophiles are p-dicarbonyl or related enolates, generally cyclic in structure and fully substituted (i. e., tertiary) at the nucleophilic carbon. The products of reaction of these enolates with alkynyliodonium salts are p-dicarbonyl compounds, and their formation is mostly dependent upon the migratory aptitude of the substituent on the ethynyliodonium salt. 

As a consequence of the superior migratory aptitude of hydrogen, the parent ethynyliodonium salt gives exclusively rearrangement (alkynylation) products with diverse P-dicarbonyl nucleophiles in 63-78<7o yield (Scheme 3-4) [28]. This reaction represents a very convenient way of introducing the HC = C — functionality into keto and ester P-dicarbonyl compounds. 

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