A novel enamination of β-dicarbonyl compounds catalyzed by Bi(TFA)3 immobilized on molten TBAB

Article abstract of DOI:10.1139/v05-021

Enamination of a wide variety of primary amines was successfully carried out in the presence of catalytic amounts of bismuth(III) trifluoroacetate immobilized on molten tetrabutylammonium bromide as "green" media under mild conditions. This new system of the catalyst is recyclable and reusable. Generally, the results of the reaction in tetrabutylammonium bromide is better than the previously obtained results in water because of their yields and reaction times.

Full text of DOI:10.1139/v05-021

209
A novel enamination of ␤-dicarbonyl compounds
catalyzed by Bi(TFA)3 immobilized on molten TBAB
Mohammad M. Khodaei, Ahmad R. Khosropour, and Mehdi Kookhazadeh
Abstract: Enamination of a wide variety of primary amines was successfully carried out in the presence of catalytic
amounts of bismuth(III) trifluoroacetate immobilized on molten tetrabutylammonium bromide as “green” media under
mild conditions. This new system of the catalyst is recyclable and reusable. Generally, the results of the reaction in tetra-
butylammonium bromide is better than the previously obtained results in water because of their yields and reaction times.
Key words: amine, bismuth compounds, β-dicarbonyl compounds, enamination, ionic liquid.
Résumé : On a procédé avec succès à une évaluation d’une grande variété d’amines primaires, en présence de quanti-
tés catalytiques de trifluoroacétate de bismuth(III) immobilisé sur de bromure de tétrabutylammonium fondu comme
milieu « vert », dans des conditions douces. Ce nouveau système de catalyseur est recyclable et réutilisable. En géné-
ral, les résultats de la réaction dans le bromure de tétrabutylammonium sont meilleurs que ceux obtenus antérieurement
dans l’eau en raison de leurs rendements et de leurs temps de réaction.
Mots clés : amine, composés du bismuth, composés β-dicarbonylés, énamination, liquide ionique.
[Traduit par la Rédaction] Khodaei et al. 212
Introduction
Scheme 1.
The increasing attention during the last decades for envi-
ronmental protection has influenced both modern academic
and industrial groups to develop chemical processes with
maximum yield under minimum amounts of harmful re-
agents. Therefore, much attention has been devoted to the
use of alternative reaction media (1, 2). Besides the use of
water or supercritical fluids, organic reaction in ionic liquids
(ILs) as new reaction media, are aimed toward green chem-
istry (3). The use of these liquids in place of conventional
organic solvents can lead to significant improvements in the
rates and yields of reactions. In addition, they possess a
number of interesting properties, especially their lack of va-
por pressure, a widely accessible temperature range with
lack of flammability, and ease of recycling that facilitates
product recovery and may reduce environmental emissions
(4–8). Also, ILs often contain poorly coordinating anions
and thus form a new class of polar, noncoordinating solvents
(9). Consequently, ionic liquids have been used in several
catalytic reactions involving many different catalysts. Sev-
eral catalysts have been immobilized in ionic liquids and
successfully recycled as a result of the involatile nature of
these media. Some reactions that have been investigated in
these catalytic media are the use of palladium catalysts for
Suzuki (10), Stille (11), and Heck coupling reactions (12,
13), a chromium catalyst for asymmetric epoxide openings
(14), a ruthenium catalyst for asymmetric alkenes hydro-
genations (15), and etc. (16–19). Recently, molten tetra-
butylammonium bromide (TBAB) as an ionic liquid for
immobilization of some catalysts has been used successfully
(20–25).
Results and discussion
As part of our program aimed at developing new selective
and environmentally friendly methodologies for the prepara-
tion of fine chemicals (26–27), we were interested in investi-
gating another method (28–31) for the enamination reaction.
Very recently, we have reported that Bi(TFA)3, as an effi-
cient and water-tolerant Lewis acid, catalyzes the enamina-
tion of β-dicarbonyl compounds in aqueous media (32).
Herein, we would like to report another modified procedure
on the enamination reaction of various primary amines with
β-dicarbonyl compounds in the presence of catalytic
amounts of Bi(TFA)3 immobilized on molten TBAB as inex-
pensive ionic liquid at room temperature (Scheme 1).
Received 12 July 2004. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 16 March 2005.
M.M. Khodaei,1 A.R. Khosropour,2 and M. Kookhazadeh.
Department of Chemistry, Faculty of Science, Razi
University, Kermanshah 67149, Iran.
1Corresponding author (e-mail: mmkhoda@razi.ac.ir).
2Corresponding author (e-mail: arkhosropour@razi.ac.ir).
Can. J. Chem. 83: 209–212 (2005)
doi: 10.1139/V05-021
© 2005 NRC Canada

210
Can. J. Chem. Vol. 83, 2005
Table 1. Enamination reaction catalyzed with Bi(TFA)3 immoblized on TBAB.
This condensation reaction with a wide range of aromatic
and aliphatic primary amines and β-dicarbonyl compounds
proceeded effectively in these conditions. The results are
summarized in Table 1. The reaction is performed at ambi-
ent temperature with very short reaction times and does not
require the use of an inert atmosphere (16).
In all cases, in comparison to the reaction under aqueous
media, the reactions were very rapid and in excellent yields.
© 2005 NRC Canada

Khodaei et al.
211
Table 1 (concluded).
aAll products were identified by comparison of their physical and spectral data with those of authentic samples (17).
bIsolated yields.
For example, the yield from the enamination of 1-butyl
amine with acetyl acetone in the presence of Bi(TFA)3 under
aqueous media was only 63% for 3 h while, the reaction
yield and rate were improved significantly using 50 mol %
of molten TBAB instead of water (entry 1, Table 1). It was
also observed that these reactions could not be performed in
the absence of the Lewis acid.
This method was successfully applied to enamination of
diketones (entries 1–11, Table 1), linear (entries 12–19, Ta-
ble 1), and cyclic β-ketoesters (entries 20–24, Table 1). The
regiochemistry observed from these β-dicarbonyls is similar
to that in aqueous media, which was limited to preferential
amines’ attack at the acetyl position and no by-product was
observed.
Interestingly, when Bi(TFA)3 was added to the molten
TBAB, the mixture did not result in solidification after cool-
ing to room temperature and remained a liquid paste in such
a way that the magnetic bar stirred well.
Another advantage of this method for the transformation
is recyclability of this catalyst. Since Bi(TFA)3–TBAB was
weakly soluble in Et2O, it was separated by washing with
Et2O and dried at 60 °C under reduced pressure and reused
in five runs without any loss of activity.
compounds. Mild reaction conditions, short reaction times,
excellent yields, and high chemoselectivity are noteworthy
advantages of this procedure. Moreover, stability and low
toxicity of the Lewis acid and the ionic liquid are the other
merits of this method.
Experimental
General experimental procedure
To a solution of the amine (1 mmol) and β-dicarbonyl
compound (1 mmol) in molten TBAB (0.5 mmol), Bi(TFA)3
(0.05 mmol) was added and the reaction mixture stirred at
room temperature for the appropriate time according to Ta-
ble 1. After completion of the reaction as indicated by GLC
or TLC, the mixture was washed with Et2O (3 × 10 mL).
Crude products were purified by preparative chromatogra-
phy on silica gel using n-heptane–AcOEt (10:1) as the
eluent. The β-enaminones were thus prepared in 60%–99%
yields.
Spectroscopic data
Compound 5: mp 49 to 50 °C. IR (KBr, cm–1) νmax: 3290,
3162, 2906 1604, 1573, 1475, 1360, 903, 839. 1H NMR
(200 MHz, CDCl3) δH: 12.4 (br, 1H, NH), 7.55–6.9 (m, 4H,
In conclusion, Bi(TFA)3–TBAB is a new, highly efficient,
and reusable catalyst for the enamination of 1,3-dicarbonyl
© 2005 NRC Canada

212
Can. J. Chem. Vol. 83, 2005
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Acknowledgments
We are thankful to the Razi University Research Council
for partial support of this work.
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