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Vinylogy has been defined as the transmission of electronic effects through a conjugated organic bonding system. The concept was introduced in 1926 by Ludwig Claisen to explain the acidic properties of formylacetone and related ketoaldehydes. Its adjectival form, vinylogous, is used to apply the concept of vinylogy taught in intermediate undergraduate through graduate/research organic chemistry.[clarification needed]
To be specific, the concept describes structures and reactivity that involve an atom (or group of atoms) attached via a carbon-carbon double bond (>C=C<, a "vinyl" moiety) to a further electropositive (electron deficient) atom/group (e.g., an electron-withdrawing group, EWG, such as a carbonyl). Vinylogous reactivity is the modified behavior observed of the atom/group that is attached via the double bond, which is therefore in conjugation with the electropositive center (EWG) — where the attached group's reactivity is observed to be analogous to reactions with the EWG itself. For instance, an hydroxyl (-OH) group attached directly to a carbonyl is by definition a carboxylic acid that can ionize by loss of the proton (H+) from the hydroxyl group with an equilibrium constant termed the acid's pKa, a numerical value that reflects the acid's strength. Likewise, when a hydroxyl group is "indirectly" attached to a carbonyl via an intervening vinyl (>C=C<) moiety, it is termed a vinylogous carboxylic acid, and it also ionizes by loss of a proton, and with an equilibrium constant very similar to that of the analogous parent carboxylic acid (see image).
Vinylogous reactions are believed to occur when orbitals of the double bonds of the vinyl group and of the EWG group (the π orbitals) are aligned and so can overlap and mix (i.e., are conjugated). This enables the EWG to receive electron density (experience a higher probability of electron localization) through participation of the conjugated system. As noted, this reactivity is seen in vinylogous carboxylic acids, which have the >C=C(OH)- (enol) adjacent and in conjugation with the carbonyl group (akin to the keto-enol tautomerization observed in acetylacetone). Vinylogous reactions also include conjugate additions, where an additional electron-rich moiety (nucleophile) reacts at the terminous of the vinyl group, as well as the vinylogous variation of the aldol reaction, where an additional electron-deficient moiety (electrophile) is attacked by the nucleophile provided by a vinylogous enolate (see first and following image). The vinylogous enolate attacks with the terminal carbon of its double-bond system (the γ-carbon/position) rather than the carbon immediately adjacent to the carbonyl (the α-carbon/position, as in a simple enolate). Allylic nucleophiles often react by vinylogous nucleophilic addition instead of direct addition; these are termed allylic rearrangements.
As noted, behaviors in organic reactivity that are consistent with the vinylogy concept are typically explained by delocalization of the electrons of the conjugated system, where the π electrons of the double bond are shared with the π or lone pair electrons in the EWG. A further acid-base example: Ascorbic acid (vitamin C) behaves as a vinylogous carboxylic acid with involvement of its carbonyl double bond, a double bond within its ring, and the lone pair on the hydroxyl group acting as the conjugated system. The hydroxyl proton at the terminus of the vinyl group in ascorbic acid is unusually acidic compared to a typical alcohol-type hydroxyl group because two major resonance structures can stabilize the negative charge on the resulting anion (conjugate base) of ascorbic acid (center and right structures in last image), analogous to the two resonance structures that stabilize the negative charge on the anion that results from removal of a proton from a simple carboxylic acid (cf. first image).
- 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
- 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