- Povarov reaction
[
frame|Scheme 3 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 atroom temperature ]The Povarov reaction is a
chemical reaction described as a formalcycloaddition between anaromatic imine and analkene . Theimine in thisorganic reaction is acondensation reaction product from ananiline type compound and abenzaldehyde type compound Ref|1. Thealkene must be electron rich which means thatfunctional group s attached to the alkene must be able to donate electrons. Such alkenes areenol ether s andenamine s. The reaction product in the original Povarov reaction is aquinoline . Because the reactions can be carried out with the three components premixed in one reactor it is an example of amulti-component reaction .Reaction mechanism
The
reaction mechanism for the Povarov reaction to thequinoline is outlined in "scheme 1". In step oneaniline andbenzaldehyde react to theSchiff base in acondensation reaction . The Povarov reaction requires alewis acid such asboron trifluoride to activate theimine for anelectrophilic addition of the activatedalkene . This reaction step forms anoxonium ion which then reacts with thearomatic ring in a classicalelectrophilic aromatic substitution . Two additionalelimination reaction s create the quinoline ring structure.Examples
The reaction depicted in "scheme 2" illustrates the Povarov reaction with an
imine and anenamine in the presence ofyttrium triflate as thelewis acid Ref|2. This reaction isregioselective because theiminium ion preverentially attacks thenitro ortho position and not the para position. The nitro group is a meta directing substituent but since this position is blocked, the most electron rich ring position is now ortho and not para. The reaction is also diastereoselective because the enamine addition occurs with a preference for trans addition without formation of the cis isomer.Variations
One variation of the Povarov reaction is a four component reaction Ref|3. Whereas in the traditional Povarov reaction the intermediate
carbocation gives anintramolecular reaction with the aromat, this intermediate can also be terminated by an additionalnucleophile such as analcohol . "Scheme 3" depicts this 4 component reaction with theethyl ester ofglyoxylic acid , 3,4-dihydro-2H-pyran,aniline andethanol withlewis acid scandium(III) triflate andmolecular sieve s.References
# Povarov, L. S. "
Russian Chem. Rev. " 1967, 36, 656.
# "Unprecedented regio and stereocontrol in Povarov reaction of benzylidene-(3-nitrophenyl)amine" Paul J. Stevenson and Isla GrahamArkivoc AM-717D 2003. ( [http://www.arkat-usa.org/ark/journal/2003/I07_McKervey/AM-717D/717D.asp Article] )
# "Straightforward Access to a Structurally Diverse Set of Oxacyclic Scaffolds through a Four-Component Reaction " Oscar Jiménez, Guillermo de la Rosa, Rodolfo LavillaAngewandte Chemie International Edition Volume 44, Issue 40 , Pages 6521 - 6525 2005 [http://www3.interscience.wiley.com/cgi-bin/abstract/112092897/ABSTRACT Abstract]ee also
*
Doebner reaction
*Doebner-Miller reaction
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