Alkyne zipper reaction
Chemical reaction
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Through Organic reaction, Saturated and unsaturated compounds and Base (chemistry)
The reaction is an equilibrium reaction, and is driven by formation (and possibly precipitation) of the terminal anion. The conversion proceeds for straight-chain alkynes and acetylinic ethers, and provides remote functionalization in long chains.
Zipper isomerization was first reported by Alexey Favorsky in 1887.
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:
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.
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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.
As potassium hydride is expensive and hazardous, 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