Alkyne zipper reaction

Equilibrium

The alkyne zipper occurs through repeated isomerizations between an alkyne and an allene. First, the base deprotonates the less-substituted methylene adjacent to the alkyne group, to form an allene anion. The anion reprotonates, but at the other end. Then the base attacks the same lesser-substituted carbon on the allene, catalyzing a similar process to form an alkyne: :[[File:Alkyne_zipper_reaction_mechanism.tif|frameless|527x527px|Example mechanism for alkyne zipper reaction.]] Through repetition, the alkyne/allene pseudoparticle can move arbitrarily along an unsubstituted alkane chain. When a terminal alkyne is achieved, the base instead attacks and removes the terminal proton.

A mild acid workup quenches the equilibrium before reprotonating the acetylide anion.

Choice of base

The alkyne zipper reaction requires a base strong enough to deprotonate the final alkyne to form a terminal alkynylide anion salt. Otherwise, the base sets up an isomerization equilibrium, but internal alkynes are thermodynamically favored over terminal alkynes.

Potash amides, from the reaction of potassium hydride and a diamine, are sufficient, and the state of the art in 1975 was potassium 1,3-diaminopropanide, generated in situ from potassium hydride in 1,3-diaminopropane solvent. Ethylenediamine cannot replace 1,3-diaminopropane.

Potassium hydride is expensive and hazardous, and a LiCKOR base is an acceptable substitute. In the synthesis of 9-decyn-1-ol from 2-decyn-1-ol, a mixture of lithium 1,3-diaminopropanide and potassium tert-butoxide affords yields of approximately 85%: :HO–CH2C≡C–(CH2)6CH3 → HO(CH2)8–C≡CH

References

References

1. Favorsky (1887), in ''J. Russ. Phys.-Chem. Soc.'', vol. 19, pp. 414–.
2. C. A. Brown and A. Yamashita. (1975). "Saline hydrides and superbases in organic reactions. IX. Acetylene zipper. Exceptionally facile contrathermodynamic multipositional isomeriazation of alkynes with potassium 3-aminopropylamide". [[J. Am. Chem. Soc.]].
3. (2017). "Alkyne Zipper Reaction".
4. Suzanne R. Abrams and Angela C. Shaw. (1988). "Triple Bond Isomerizations: 2- to 9-decyn-1-ol".