Alkynyl cations

Alkynyl cations are so unstable that they cannot be generated even with very good leaving groups. However, one way in which they have been generated was by formation of a tritiated substrate. 

When the tritium (half-life 12.26 years) decays it is converted to the helium-3 isotope, which, of course, does not form covalent bonds, and so immediately departs, leaving behind the alkynyl cation. When this was done in the presence of benzene, RC CCgHs was isolated. The tritium-decay technique has also been used to generate vinylic and aryl cations. 

Allenyl cations 1 are a stabilized form of vinyl cations1-3 in which the /1-carbon atom of the vinylic structure is part of the substituent which effects the stabilization of the ion via its electron-donating ability. This leads to a resonance hybrid having formally the alkynyl cation structure 2. Allenyl cations should be distinguished from the allenyl substituted carbenium ions 3 formulated as the mesomeric structures of the vinyl cations 4 (dienyl cations) stabilized by an w-vinyl group (equation 1). 

These highly reactive yet stable species are strong electrophiles of tetraphilic character, since nucleophiles may attack three different carbon atoms (a,/ ,a ) and iodine. In most reactions the first step is a Michael addition at fi-C with formation of an alkenyl zwitterionic intermediate (ylide) which normally eliminates iodoben-zene, generating an alkylidene carbene then, a 1,2-shift of the nucleophile ensues. The final result is its combination with the alkynyl moiety which behaves formally as an alkynyl cation. The initial adduct may react with an electrophile, notably a proton, in which case alkenyl iodonium salts are obtained also, cyclopentenes may be formed by intramolecular C-H 1,5-insertion from the alkylidenecarbenes ... 

Aryllead, vinyllead, and alk-l-ynyllead tricarboxylates behave as aryl, vinyl and alkynyl cation equivalents to react with a variety of nucleophiles, especially soft carbon nucleophiles such as T -dicarbonyl compounds, phenols, and nitroalkanes. In these reactions, unique regioselectivity is obtained in which there is a preference for the generation of quaternary carbon centers. This aspect of reactivity has been put to use in a number of natural product syntheses and can result in the formation of highly hindered structures. 

The particular reactivity of this type of compounds was underlying already in this first report by Beringer. If the alkynyl cation equivalent behaviour was well demonstrated, the Michael addition behaviour was as well already observed during the attempted anion exchange with sodium tetrafluoroborate. 

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