Vinylogous

Vinylogous (portmaneau of vinyl and analogous) reactivity is the behavior of a vinyl group in conjugation with an electron-withdrawing group analogous to the reactions of the electron-withdrawing group itself; typically, this is a carbonyl group. Vinyls attached to the same carbon as the electron-withdrawing group are conjugated. This reactivity is seen in in vinylogous carboxylic acids, which have an enol right next to and in conjugation with the carbonyl group, for example in the enol form of acetylacetone. Vinylogous reactions are for example conjugate addition, where the electrophile reacts at the vinyl, and the vinylogous aldol reaction, where the attacking nucleophile is a vinylogous enolate, attacking with its vinylic γ position rather than its α position like a normal enolate. Allylic nucleophiles often react by vinylogous nucleophilic addition instead of direct addition; this is called allylic rearrangement.

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A classical explanation is resonance, where the electrons of the double bond move to connect to the carbon geminal to the electron-withdrawing group. A common example of a vinylogous carboxylic acid is ascorbic acid (vitamin C). The enol double bond can resonate between the two possible positions. (In exact terms, it is delocalized between the two positions.)