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Condensation of Pyrrole with Benzaldehyde Under Adler Conditions; Tetraphenylporphyrins

SyntheticPage 316
DOI: 10.1039/SP316
Submitted Oct 21, 2010, published Oct 21, 2010
Dan Carney (daniel_carney@brown.edu)
A contribution from Connors Group, WPI


			Reaction Scheme: Condensation of Pyrrole with Benzaldehyde Under Adler Conditions

Chemicals Used

Propionic Acid
Pyrrole
Benzaldehyde
Methanol
Methylene Chloride

Procedure

Pyrrole was freshly distilled. benzaldehyde and propionic acid, purchased recently, were used as is. Pyrrole (1.9mL, 2.69E-2mol) and benzaldehyde (3.0mL, 2.69E-2mol) were combined in a graduated cylinder then agitated until thoroughly mixed.  Propionic acid (100mL) was added to a 250 mL round bottom flask fixed with a cold water condenser and heated to reflux.   The benzaldehyde and pyrrole mixture was added to the refluxing propionic acid and allowed to react for 30 minutes.  Color change to red/brown occurred rapidly and by the end of the reaction the crude solution was opaque.  The crude solution was cooled to room temperature then filtered using a Buchner funnel.
The reaction flask and filtered solid were washed with methanol leaving behind shiny purple crystals.  The filtered solid as well as the solid remaining in the reaction flask were then washed with methylene chloride into an oven dried and pre-weighed round bottom flask. The solution was concentrated to dryness then weighed for yield determination.
Mass Collected: 716mg
%Yield: 17.3%

Author's Comments

The reaction is simple and reliable.  The challenge is achieving optimum yield.  Reactant concentrations and reaction time are very important.  For optimal yield, reactant concentration should be approximately 0.27M.   Although the Adler synthesis of TPP constitutes a quick and reliable method for synthesizing tetraarylporphyrins, it is by nature a very low yielding reaction.  Inspection of the mechanism for porphyrin formation reveals why. 
If available, dry propionic acid gives greater yields.  Make sure both pyrrole and benzaldehyde are fresh and in high purity.  I recommend storing both in sealed containers under nitrogen and in a refrigerator.  Chromatography on the product is not necessary.  Chlorin side product can be oxidized to porphyrin by refluxing the reaction product with DDQ in toluene.

Data

HNMR data in chloroform-d:
2H singlet -2.8ppm
12H multiplet 7.7ppm
8H doublet 8.1ppm
8H singlet 8.8ppm

Lead Reference

Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour, J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476-476.

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Keywords: Adler, aldehydes, aromatics/arenes, benzaldehyde, condensation, heterocyclic compounds, oxidation, pyrrole

Post new comment
  • Guest Robert LancashireNov 4 2010 10:35AMMicrowave Oven Reaction

    Some time ago we discovered that heating in a sealed Teflon container in a microwave oven could reduce the reaction time considerably. Yields were never higher than 20% though and the work was never followed up or published.

    • Registered user Leandro PedrosaApr 1 2011 8:27PMReference

      This paper reports a typical synthesis of meso-tetraphenylporphyrin with 35% yield. De Paula, R., Faustino, M. A. F., Pinto, D. C., Neves, M. G. and Cavaleiro, J. A. S., "Kinetic study of meso-tetraphenylporphyrin synthesis under microwave irradiation." Journal of Heterocyclic Chemistry, (2008), 45, 453–459. doi:10.1002/jhet.5570450224

  • Guest Luis MaqueiraJun 4 2013 2:45PMreference

    In this paper: "Preparation and characterization of SDS-stabilized hydrophobic porphyrinic nanoaggregates in water". Journal of Porphyrins and Phthalocyanines. 16(3) 267-272, 2012.DOI: 10.1142/s1088424612500320 was reported, a modification of Adler synthesis method and was obtained 78% yield

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