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43.8.2.3 Base-Catalyzed Rearrangements

DOI: 10.1055/sos-SD-043-00415

Krueger, A.Science of Synthesis, (200843500.

The migration of CC bonds is closely related to allyl rearrangements. In most cases these reactions are prototropic rearrangements of isomeric alkynes and allenes.[‌29‌,‌142‌,‌143‌] They are a versatile method for the synthesis of alkynes. Usually, they are carried out using a base. As early as in 1888, Favorskii had suggested that the triple-bond shift from alk-1-ynes to alk-2-ynes proceeds via an allene (Scheme 44).[‌144‌] This hypothesis was later proven correct and the allenic intermediates can be isolated in many cases as a constituent of the product mixture.[‌29‌,‌142‌,‌143‌,‌145‌‌147‌] However, the formation of equilibrated mixtures is also one of the major drawbacks of the base-catalyzed isomerization of alkynes. The reaction often proceeds under thermodynamic control and leads to the thermodynamically most stable alkyne as the major product, accompanied by a mixture of other alkynes and allenes as isomerization products.[‌29‌,‌142‌‌148‌] Under more drastic conditions the reaction even continues to give conjugated dienes, which are much more stable than their alkynic counterparts.[‌29‌,‌142‌‌148‌]

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