Povarov reaction

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The Povarov reaction is an

aromatic imine and an alkene. The imine in this organic reaction is a condensation reaction product from an aniline type compound and a benzaldehyde type compound.[1][2][3] The alkene must be electron rich which means that functional groups attached to the alkene must be able to donate electrons. Such alkenes are enol ethers and enamines. The reaction product in the original Povarov reaction is a quinoline. Because the reactions can be carried out with the three components premixed in one reactor it is an example of a multi-component reaction
.

Povarov reaction

Reaction mechanism

The

aromatic ring in a classical electrophilic aromatic substitution. Two additional elimination reactions
create the quinoline ring structure.

Scheme 1. The Povarov reaction

The reaction is also classified as a subset of

aza Diels-Alder reactions;[4]
however, it occurs by a step-wise rather than concerted mechanism.

Examples

The reaction depicted in Scheme 2 illustrates the Povarov reaction with an

stereospecific
based on the alkene geometry.

Scheme 2. regio- and diastereoselective Povarov reaction

In 2013, Doyle and coworkers reported a Povarov-type, formal [4+2]-cycloaddition reaction between donor-acceptor cyclopropenes and imines (Scheme 3). In the first step, a dirhodium catalyst effects diazo decomposition from silyl enol ether diazo compound to yield a donor/acceptor cyclopropene. The donor/acceptor cyclopropene is then reacted with an aryl imine under scandium(III) triflate catalyzed conditions to yield cyclopropane-fused tetrahydroquinolines in good yields and diastereoselectivities. Treatment of these compounds with TBAF invokes a ring-expansion that provides the corresponding benzazepines.[6]

Donor-acceptor cyclopropene formation and subsequent [4+2] cycloaddition to yield cyclopropane-fused tetrahydroquinolines. Subsequent treatment with TBAF opens the cyclopropane ring to give benzazepines.

Variations

One variation of the Povarov reaction is a four component reaction.

scandium(III) triflate and molecular sieves
.

Scheme 4 Four component Povarov reaction. In order to clarify the role of the lewis acid, a solid scandium nitrogen bond is drawn. Reaction conditions 2 days in acetonitrile at room temperature

References

  1. ^ Povarov, L. S.; Mikhailov, B. M. Izv. Akad. Nauk SSR, Ser. Khim. 1963, 953–956.
  2. ^ Povarov, L. S.; Grigos, V. I.; Mikhailov, B. M. Izv. Akad. Nauk SSR, Ser. Khim. 1963, 2039–2041.
  3. S2CID 250825235
    .
  4. doi:10.1016/j.tet.2008.12.059
  5. ^ Unprecedented regio and stereocontrol in Povarov reaction of benzylidene-(3-nitrophenyl)amine Paul J. Stevenson and Isla Graham Arkivoc AM-717D 2003. (Article)
  6. PMID 23777207
    .
  7. Angewandte Chemie International Edition Volume 44, Issue 40 , Pages 6521 - 6525 2005 Abstract

See also
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