Alkyne-allene interconversions

II. REARRANGEMENTS INVOLVING ACETYLENES A. Alkyne-Allene Interconversions... 

Cyanides which possess one or more hydrogen atoms attached to the a-carbon atom are potentially capable of tautomeric equilibrium between the cyanide (97) and the ketenimine (98), a reaction similar to the alkyne-allene interconversion discussed earlier. 

All the organometallic alkynes rearrange along similar lines, both during reaction and, in some cases, spontaneously. The alkyne-to-allene interconversion is a consistent reaction of alkynes over a very wide range of substituents. 

Cyclic alkynes (C9, Ci0, or Cn) also rearrange in the presence of bases to an equilibrium mixture containing cyclic allene [43]. The bases used were NaNH2-liq. NH3 at -33.4°C, KOH-C2H5OH at 131°-134°C (sealed tubes), KO-f-Bu-f-BuOH at 79.4°-l 20.0°C (sealed tubes) [44]. A solution of sodamide in liquid ammonia gave the most rapid ( to 3 hr) allene-acetylene interconversions of all systems examined. 

The process may be reversed by treating 2-alkynes with sodium to get 1-alkynes after acidification of the product. The presence of an alkyl substituent at C(3) of a 1-alkyne prevents its isomerization to the corresponding 2-alkyne. Instead, an allene is formed [Eq. (4.27)]. This observation led to the suggestion of the involvement of allene intermediates to interpret the shift of the triple bond in the interconversion of acetylenes [Eq. (4.28)] ... 

Interconversion of alkynes and allenes can be induced electrochemically it probably involves deprotonation of the parent compound by the electrogenerated radical anion of the parent species. 

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