Diphenylammonium triflate (DPAT): Efficient catalyst for esterification of carboxylic acids and for transesterification of carboxylic esters with nearly equimolar amounts of alcohols

Article abstract of DOI:10.1016/S0040-4039(00)00821-2

Diphenylammonium triflate (DPAT) efficiently catalyzed esterification between equimolar amounts of carboxylic acids and alcohols in good to excellent yields under mild reaction conditions. Transesterification of carboxylic esters with a slight excess of alcohols also proceeded using DPAT as catalyst with TMSCl as a co-catalyst. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00821-2

Tetrahedron Letters 41 (2000) 5249±5252
Diphenylammonium tri¯ate (DPAT): ecient catalyst for
esteri®cation of carboxylic acids and for transesteri®cation of
carboxylic esters with nearly equimolar amounts of alcohols
Kazunori Wakasugi, Tomonori Misaki, Kayo Yamada and Yoo Tanabe*
School of Science, Kwansei Gakuin University, 1-1-155 Uegahara, Nishinomiya, Hyogo 662-8501, Japan
Received 24 April 2000; accepted 19 May 2000
Abstract
Diphenylammonium tri¯ate (DPAT) eciently catalyzed esteri®cation between equimolar amounts of
carboxylic acids and alcohols in good to excellent yields under mild reaction conditions. Transesteri®cation
of carboxylic esters with a slight excess of alcohols also proceeded using DPAT as catalyst with TMSCl as
a co-catalyst. # 2000 Elsevier Science Ltd. All rights reserved.
Esteri®cation (and transesteri®cation) of carboxylic acids (and carboxylic esters) with alcohols
have been recognized as one of the most important unit reactions in organic synthesis.1 Of a
number of well known methods applicable to both reactions, those using catalytic amounts of
promoter and/or dehydrating reagents under mild liquid-phase conditions are quite limited;
representatively, Otera and co-workers found an ecient method using distannoxanes such as
[(SCN)Bu2SnOSnBu2(OH)].2
From a recent atom-economical standpoint, the uses of nearly equimolar amounts of carboxylic
acids (or carboxylic esters) and alcohols are strongly required. Mukaiyama and Shiina reported
such direct esteri®cations mediated by TiCl2(ClO4)2 and TiCl(OTf)3 reagents;3 however, they
necessitated more than equimolar amounts of anhydride and silyl dehydrating additives, respec-
tively. We introduce here diphenylammonium tri¯ate (DPAT),4 which worked as an ecient
catalyst (1±10 mol%) for the esteri®cation between equimolar amounts of carboxylic acids and
alcohols under mild conditions. In addition, the transesteri®cation catalyzed by DPAT (10±20
mol%) was performed between carboxylic esters and slightly excess alcohols, wherein TMSCl (10
mol%) was found to be an ecient co-catalyst (Scheme 1).
PPTS (pyridinium p-toluenesulfonate) is widely used as a mild and ecient catalyst in acetalization
and its related reactions.5 To extend this methodology toward the carboxylic ester syntheses, we
* Corresponding author.
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00821-2

5250
Scheme 1.
examined several ammonium tri¯ates (1.0 mol%) guided by a direct esteri®cation using 3-phenyl-
propionic acid (1.0 mmol) and 1-octanol (1.0 mmol) at 110ꢀC in toluene (2.0 ml). These results
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are as follows: pyridinium tri¯ate (53%, 24 h), Bu3NH OTf (16%, 24 h), and Me2PhNH OTf
(64%, 24 h).6 Compared with these, DPAT showed a remarkable e€ect, namely, 94% within 1 h,
whose result encouraged us to investigate the reactivity of DPAT.7
Table 1 lists those results.8 Only 1 mol% of DPAT was sucient for primary alcohols (entries
1±5, 9 and 10). Relatively sterically crowded secondary alcohols and/or benzoic acid required 5±
10 mol% of DPAT with a little prolonged time (entries 6±8 and 11). Several functionalities such
as a double bond, a halogen, a ketone, and a cyclopropane are tolerated. This method did not
necessitate special use of the dehydrating reagent and/or the technique of azeotropic removal of
water.
Table 1
Esteri®cation between equimolar amounts of carboxylic acids and alcohols using DPAT catalysta)

5251
The transesteri®cation9 was also performed using DPAT as catalyst (10 mol%), although the
rate of conversion was somewhat slower than that of esteri®cation mentioned above: A mixture
of methyl 3-phenylpropionate (164 mg, 1.0 m mol), 1-octanol (195 mg, 1.5 mmol), and DPAT (32
mg, 0.1 mmol) were heated (80ꢀC, 24 h) in toluene to give desired 1-octyl ester (197 mg, 75%).
When 10 mol% TMSCl (11 mg, 0.1 mmol) was employed as co-catalyst, the yield was improved
(83%, 80ꢀC, 24 h; 90%, 80ꢀC, 20 h and 110ꢀC, 4 h).
Table 2 lists the results of the transesteri®cation. DPAT and TMSCl system could catalyze the
transesteri®cation of primary alcohols eciently (entries 1±3 and 5). Unfortunately, 2-octanol,
tert-Bu ester, and a-branched esters resisted the esteri®cation (entries 4, 7 and 8). It should be
noted a b-keto ester, which is prone to decarboxylation, was employable (entry 9).
Table 2
Transesteri®cation between nearly equimolar amounts of carboxylic esters and
alcohols using DPAT catalyst and TMSCl co-catalysta)
In conclusion, we achieved both the ecient esteri®cation and transesteri®cation catalyzed by
DPAT whose reactivities are comparable to the methods using organotin2,9 and organotitanium3
catalysts. The use of DPAT includes a lot of merits from the viewpoint of green chemistry, for
example DPAT is air-stable, cheap, low toxic, and easy to handle. The operation is quite simple,
because the dehydrating system such as a Dean±Stark apparatus is not necessary.
Acknowledgements
This work was partially supported by a Grant-in-Aid for Scienti®c Research from the Ministry
of Education, Science, Sports and Culture (Japan).

5252
References
1. (a) Beaz, G. In Comprehensive Organic Synthesis; Trost, B. M.; Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 6,
p. 323. (b) Otera, J. Chem. Rev. 1993, 93, 1449. (c) Franklin, A. S. J. Chem. Soc., Perkin Trans. 1 1998, 2451 and
1999, 3537.
2. Otera, J.; Dan-oh, N.; Nozaki, H. J. Org. Chem. 1991, 56, 5307.
3. (a) Shiina, I.; Mukaiyama, T.; Miyoshi, S.; Miyashita, M. Chem. Lett. 1994, 515. (b) Mukaiyama, T.; Shiina, I.
Chem. Lett. 1995, 141.
4. DPAT was very easily prepared in a high yield as follows: CF3SO3H (0.75 g, 5.0 mmol) was added to a solution of
Ph2NH (0.85 g, 5.0 mmol) in toluene (10.0 ml) at 20±25ꢀC, and the mixture was stirred for 15 min. Evaporation of
the solvent gave the crude product, which was washed with hexane (ca. 50 ml) to give a pure DPAT (1.55 g, 97%)
as colorless crystals (mp 172±173ꢀC).
5. Miyashita, M.; Yoshikoshi, A.; Grieco, P. A. J. Org. Chem. 1977, 42, 3772.
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6. Et3NH OTf and Ph3NH OTf did not crystallize, so they were excluded from this investigation.
7. With decreasing the basicity of the counter amine part, the ability of the esteri®cation seem to be higher. Solvent
e€ect (DPAT, 1 mol%, 80ꢀC, 15 h): toluene (93%), 1,2-dichloroethane (90%), 1,4-dioxane (54%), CH3CN (46%),
DME (27%), DMF (trace).
8. Typical procedure: 3-Phenylpropionic acid (150 mg, 1.0 mmol), 1-octanol (130 mg, 1.0 mmol), and DPAT (3.2 mg,
0.01 mmol) were heated (80ꢀC) in toluene for 4 h. Evaporation of toluene (ca. 40ꢀC) under reduced pressure gave
the crude material, which was puri®ed by column chromatography (hexane:ether=10:1) to give the desired
carboxylic ester (244 mg, 93%). For recent representative works, see: (a) Masaki, Y.; Tanaka, N.; Miura, T.
Chem. Lett. 1997, 55. (b) Ponde, D. E.; Deshpande, V. H.; Bulbule, V. J.; Sudalai, A.; Gajare, A. S. J. Org. Chem.
1998, 63, 1058. (c) Furlan, R. L. E.; Mata, E. G.; Mascaretti, O. A. Tetrahedron Lett. 1998, 39, 2257. (d) Krasik, P.
Tetrahedron Lett. 1998, 39, 4223.
9. In the case of an organotin-catalyzed method,2 the transesteri®cation is smoother than the esteri®cation. In
contrast, the present method shows a reverse relationship.