The Reversible Acetylene-Allene Transformation

The first rearrangement involving an acetylenic compound was reported by Favorskii133 as early as 1887. He found that when 1-butyne and higher-molecular-weight straight-chain 1-alkynes are heated with an alcoholic solution of potassium hydroxide, they are isomerized to 2-alkynes  

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)]  

The transformation termed propargylic rearrangement attracted much attention, and detailed discussions are available in review papers.134-141 Suitable reagents to bring about this rearrangement are metal alkoxides and metal amides in alcohols and dipolar aprotic solvents (DMSO, HMPT), and metal amides in ammonia. Reactivities are strongly dependent on the base employed, the solvent, and the reaction conditions. 

Considering the thermodynamic stability of isomeric acetylenes and dienes 1,3-dienes, by far the most stable compounds involved in propargylic rearrangements should be formed in the highest amount. They are, however, rarely observed since they are formed in a slow reaction. The product distribution, consequently, is kinetically controlled. 

The order of stability of other isomers is 2-alkyne 2,3-diene 1,2-diene 1-alkyne.142 The equilibrium product distribution in the C5 system is 1.3% 

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