Vinylogous

Vinylogous

Vinylogous 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. Vinylogous reactions are found when the pi orbitals of the vinyl group and the electron-withdrawing group (EWG) are aligned and conjugated, enabling the EWG to receive electrons through the conjugated system. 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 (a variation of the 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 .

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Allylic nucleophiles often react by vinylogous nucleophilic addition instead of direct addition; this is called allylic rearrangement.

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.)
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The related term vinylogy has been defined as the transmission of electronic effects through a conjugate system ["The Vinylogous Aldol Reaction: A Valuable, Yet Understated Carbon-Carbon Bond-Forming Maneuver" Giovanni Casiraghi, Franca Zanardi, Giovanni Appendino, and Gloria Rassu Chem. Rev. 2000; 100(6) pp 1929 - 1972; (Review) DOI|10.1021/cr990247i] . This concept was introduced in 1926 by Ludwig Claisen when he explained acidic properties of formylacetone and related beta-keto aldehydes ["Zu den O-Alkylderivaten des Benzoyl-acetons und den aus ihnen entstehenden Isoxazolen." (Entgegnung an Hrn. O. Weygand.) Berichte der deutschen chemischen Gesellschaft (A and B Series) Volume 59, Issue 2, Date: 10. Februar 1926, Pages: 144-153 L. Claisen. DOI|10.1002/cber.19260590206] .

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