P2O5/SiO2 as an efficient reagent for esterification of phenols in dry media

Article abstract of DOI:10.1081/SCC-100103268

A simple and efficient method for esterification of carboxylic acid with phenols using P₂O₅/SiO₂ reagent in dry media is reported.

Full text of DOI:10.1081/SCC-100103268

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P2O5/SiO2 AS AN
EFFICIENT REAGENT FOR
ESTERIFICATION OF PHENOLS
IN DRY MEDIA
Hossien Eshghi a , Mohammad Rafei a
Mohammad Hasan Karimi a
&
a Department of Chemistry , Sistan & Baluchestan
University , Zahedan, 98135-674, Iran
Published 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 PHENOLS
IN DRY MEDIA, Synthetic Communications: An International Journal for Rapid
Communication 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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SYNTHETIC COMMUNICATIONS, 31(5), 771–774 (2001)
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 & Baluchestan
University, Zahedan 98135-674, Iran
ABSTRACT
A simple and efficient method for esterification of car-
boxylic acid with phenols using P2O5/SiO2 reagent in dry
media is reported.
Esters are common intermediates in natural product chemistry due to
their stability and accessibility for easy interconversion.1 It has long been
known that the process of esterification may be enormously hastened by the
addition of a strong acid, such as sulfuric acid. The classical methods for the
synthesis of esters are not suitable for acid sensitive compounds and have
the disadvantages of the corrosiveness of strong acid and accompanying side
reactions such as carbonization, oxidation, etc. Further modifications of
methods have been made with alcohols and carboxylic acids which have
some advantages over the classical acid catalyst.3–5 However acylated
phenols are obtained under standard conditions from the phenol and the
appropriate acid chloride or anhydride in the presence of a base.6 The use of
acyl esters as phenol protecting groups has found widespread application
especially in preparative chemistry.
* Corresponding author. E-mail: heshghi@hamoon.usb.ac.ir
771
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

772
ESHGHI, RAFEI, AND KARIMI
Phosphorous pentoxide has been frequently used in esterification of
carboxylic acid by excess alcohol.7,8 The nature of the species obtained from
interaction of P2O5 with alcohols is critically dependent upon the stoichio-
metry of the components. The reagent formed presumably consists of a
mixture of alkyl phosphate esters. These organophosphorous reagents
have been used for esterification of several primary aliphatic carboxylic
acids in varying yields, but the aromatic acids could be recovered
unchanged.8 Coupled with this fact, this solid acidic oxide possessing an
extraordinary dehydrating capacity, made itself a unique material of choice
for 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 dry
media. The reagent was prepared by simple mixing of P2O5 and dried chro-
matographic grade silica gel. Stirring of mixture in dry condition for 30
minutes gave the P2O5/SiO2 reagent as homogeneous, free flowing, white
powder. This reagent has the advantage of being easily removed from the
organic product by filtration. A further advantage of this reagent is
improved storage stability in moisture in comparison to P2O5, which is
very sensitive to moisture.
When the P2O5/SiO2 reagent was mixed with equimolar amounts of
the carboxylic acids and phenols, the corresponding esters were obtained in
high yields (Table, Scheme). Aliquots of the reagent can be conveniently
used for esterification of aliphatic carboxylic acids with phenols by stirring
at room temperature for 6 h. The conversion of aromatic carboxylic acids
under these conditions, is low (35–60%). The best results were obtained in
65oC, but at higher temperature, Fries rearrangement can occur.9
Scheme
The present method is generally applicable to aromatic and aliphatic
carboxylic acids and provided the corresponding carboxylic esters in high
yield with naphthols and phenols. In all examined cases, the esterification
reactions were very clean and led to essentially pure esters. In conclusion,
with this readily available reagent, moderate to high yields, mild solvent-free
conditions, as well as easy operation, we think that the present work

ESTERIFICATION OF PHENOLS
773
Table. Esterification of the Carboxylic Acids with Phenols by Using P2O5/SiO2 in
Dry Media
Mp or bp (ꢀC/torr)
R
X
T/ꢀC
Yield %a,b
195 196/760
Foundc
Reportedref
10
CH3
CH3
CH3
CH3
CH3
CH3
Ph
Ph
Ph
Ph
Ph
H
p-CH3
m-CH3
p-NO2
1-Naphthyl
2-Naphthyl
H
2568 /760
2584
2572
10
210/760
211/760
212-2l3/760
212/760
10
11
–76 256575 76–77.5
257546–48
2570
10
10
45
10
69–70
70
69–70
71.5
54
307/728
6575 69
6580
65
6570
6565 142
65
71
10
p-CH3
m-CH3
o-CH3
p-NO2
82
310/760
140–141.5
55–5610
12
13
Ph
1-Naphthyl
72
54
5610
aIsolated yields by column chromatography.
bThe conversion yields are 70–90%.
1
cThe structure of the obtained esters, was confirmed through the H-NMR and IR
spectra and also by comparison with authentic samples.
described herein provides a useful method for esterification of the carboxylic
acids with naphthols and phenols. Further studies of the new applications of
P2O5/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-
1
meter. H-NMR spectra were recorded on a Bruker-80 MHz instrument
using tetramethylsilane (TMS) as internal standard. The silica gel 60 was
obtained 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 2.5g of chromatography grade silica gel was
placed in a flask and stirred for 30 minutes. A mixture of equimolar
amounts (5mmol) of the carboxylic acid and phenol was added. Usually
an immediate color change was observed. After stirring for 6 h at the tem-
perature which indicated in the Table, methylene chloride (50 ml) was
added. The mixture was stirred for 1 minute and then filtered. The spent

774
ESHGHI, RAFEI, AND KARIMI
reagent was washed twice with methylene chloride (10 ml). The combined
organics were washed with aqueous NaOH solution, water, and dried over
sodium 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 the
1H-NMR and IR spectra with authentic samples.
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