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P2O5/SiO2 AS ANEFFICIENT REAGENT FORESTERIFICATION OF PHENOLSIN DRY MEDIAHossien Eshghi a , Mohammad Rafei a &Mohammad Hasan Karimi aa Department of Chemistry , Sistan & BaluchestanUniversity , Zahedan, 98135-674, IranPublished online: 16 Aug 2006.

To cite this article: Hossien Eshghi , Mohammad Rafei & Mohammad Hasan Karimi(2001) P2O5/SiO2 AS AN EFFICIENT REAGENT FOR ESTERIFICATION OF PHENOLSIN DRY MEDIA, Synthetic Communications: An International Journal for RapidCommunication of Synthetic Organic Chemistry, 31:5, 771-774, DOI: 10.1081/SCC-100103268

To link to this article: http://dx.doi.org/10.1081/SCC-100103268

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P2O5/SiO2 AS AN EFFICIENT REAGENT

FOR ESTERIFICATION OF PHENOLS

IN DRY MEDIA

Hossien Eshghi,* Mohammad Rafei, and

Mohammad Hasan Karimi

Department of Chemistry, Sistan & BaluchestanUniversity, Zahedan 98135-674, Iran

ABSTRACT

A simple and efficient method for esterification of car-boxylic acid with phenols using P2O5/SiO2 reagent in drymedia is reported.

Esters are common intermediates in natural product chemistry due totheir stability and accessibility for easy interconversion.1 It has long beenknown that the process of esterification may be enormously hastened by theaddition of a strong acid, such as sulfuric acid. The classical methods for thesynthesis of esters are not suitable for acid sensitive compounds and havethe disadvantages of the corrosiveness of strong acid and accompanying sidereactions such as carbonization, oxidation, etc. Further modifications ofmethods have been made with alcohols and carboxylic acids which havesome advantages over the classical acid catalyst.3–5 However acylatedphenols are obtained under standard conditions from the phenol and theappropriate acid chloride or anhydride in the presence of a base.6 The use ofacyl esters as phenol protecting groups has found widespread applicationespecially in preparative chemistry.

771

Copyright & 2001 by Marcel Dekker, Inc. www.dekker.com

*Corresponding author. E-mail: heshghi@hamoon.usb.ac.ir

SYNTHETIC COMMUNICATIONS, 31(5), 771–774 (2001)

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Phosphorous pentoxide has been frequently used in esterification ofcarboxylic acid by excess alcohol.7,8 The nature of the species obtained frominteraction of P2O5 with alcohols is critically dependent upon the stoichio-metry of the components. The reagent formed presumably consists of amixture of alkyl phosphate esters. These organophosphorous reagentshave been used for esterification of several primary aliphatic carboxylicacids in varying yields, but the aromatic acids could be recoveredunchanged.8 Coupled with this fact, this solid acidic oxide possessing anextraordinary dehydrating capacity, made itself a unique material of choicefor the present investigation.

We herein report a very simple and efficient method for the esterifica-tion of carboxylic acids with phenols using the P2O5/SiO2, reagent in drymedia. The reagent was prepared by simple mixing of P2O5 and dried chro-matographic grade silica gel. Stirring of mixture in dry condition for 30minutes gave the P2O5/SiO2 reagent as homogeneous, free flowing, whitepowder. This reagent has the advantage of being easily removed from theorganic product by filtration. A further advantage of this reagent isimproved storage stability in moisture in comparison to P2O5, which isvery sensitive to moisture.

When the P2O5/SiO2 reagent was mixed with equimolar amounts ofthe carboxylic acids and phenols, the corresponding esters were obtained inhigh yields (Table, Scheme). Aliquots of the reagent can be convenientlyused for esterification of aliphatic carboxylic acids with phenols by stirringat room temperature for 6 h. The conversion of aromatic carboxylic acidsunder these conditions, is low (35–60%). The best results were obtained in65oC, but at higher temperature, Fries rearrangement can occur.9

The present method is generally applicable to aromatic and aliphaticcarboxylic acids and provided the corresponding carboxylic esters in highyield with naphthols and phenols. In all examined cases, the esterificationreactions were very clean and led to essentially pure esters. In conclusion,with this readily available reagent, moderate to high yields, mild solvent-freeconditions, as well as easy operation, we think that the present work

772 ESHGHI, RAFEI, AND KARIMI

Scheme

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described herein provides a useful method for esterification of the carboxylicacids with naphthols and phenols. Further studies of the new applications ofP2O5/SiO2 in organic synthesis are under way.

EXPERIMENTAL SECTION

All melting points recorded are uncorrected open capillary measure-ments. Infrared spectra were recorded on a Shimadzu-IR 470 spectrophoto-meter. 1H-NMR spectra were recorded on a Bruker-80 MHz instrumentusing tetramethylsilane (TMS) as internal standard. The silica gel 60 wasobtained from E Merck and dried in an oven at 120oC 2 h.

General Esterification Procedures: A mixture of 1.42 g (0.01 mol)phosphorus pentoxide and 2.5 g of chromatography grade silica gel wasplaced in a flask and stirred for 30 minutes. A mixture of equimolaramounts (5mmol) of the carboxylic acid and phenol was added. Usuallyan immediate color change was observed. After stirring for 6 h at the tem-perature which indicated in the Table, methylene chloride (50 ml) wasadded. The mixture was stirred for 1 minute and then filtered. The spent

ESTERIFICATION OF PHENOLS 773

Table. Esterification of the Carboxylic Acids with Phenols by Using P2O5/SiO2 inDry Media

Mp or bp (�C/torr)

R X T/�C Yield %a,b Foundc Reportedref

CH3 H 25 68 195/760 196/76010

CH3 p-CH3 25 84 210/760 212-2l3/76010

CH3 m-CH3 25 72 211/760 212/76010

CH3 p-NO2 25 65 75–76 76–77.511

CH3 1-Naphthyl 25 75 46–48 4510

CH3 2-Naphthyl 25 70 69–70 69–7010

Ph H 65 75 69 7110

Ph p-CH3 65 80 70 71.510

Ph m-CH3 65 82 54 55–5610

Ph o-CH3 65 70 310/760 307/72812

Ph p-NO2 65 65 142 140–141.513

Ph 1-Naphthyl 65 72 54 5610

aIsolated yields by column chromatography.bThe conversion yields are 70–90%.cThe structure of the obtained esters, was confirmed through the 1H-NMR and IRspectra and also by comparison with authentic samples.

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reagent was washed twice with methylene chloride (10 ml). The combinedorganics were washed with aqueous NaOH solution, water, and dried oversodium sulphate, and the solvent was removed under reduced pressure.

This crude reaction mixture was analysed and purified in a conven-tional manner. The esters (Table) were identified by comparison of the1H-NMR and IR spectra with authentic samples.

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2. (a) Fischer, E. Ber. 1895, 28, 3254. (b) Locgccion, R.; Enghozy, F. Bull.Soc. Chem. 1927. 41, 445. (c) Butts, J. J. Am. Chem. Soc. 1931, 53,3560. (d) Frnst Thielepape, Ber. 1933, 66, 1454.

3. Olah, G.A.; Keumi, T.; Meldar, D. Synthesis, 1978, 929.4. Choarat, T.S.; Yadav, V.M.; Yadav, G.D. Appl. catal. 1992, A90, 73.5. Zhang, G.S.S. Synth. Commun. 1998, 28, 1159.6. McOmie, J.F.W. Protective Groups in Organic Chemistry; Plenum

Press: New York, 1982.7. Balasubramaniyan, V.; Bhatia, V.G.; Wagh, S.B. Tetrahedron 1983, 39,

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Compounds; Chapman and Hall: New York, 5th edn. 1982.11. Li, T.S. and Li, A.X. J. Chem. Soc. Perkin Trans. 1 1998, 1913.12. Weast, R.C.; Graffelli, J.G. Handbook of Data on Organic Compounds;

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Received in the U.K. December 20, 1999

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