Catalysis by ionic liquid. A green protocol for the stereoselective debromination of vicinal-dibromides by [pmIm]BF4 under microwave irradiation

Article abstract of DOI:10.1021/jo051373r

An easily accessible ionic liquid, 1-methyl-3-pentylimidazolium fluoroborate, [pmIm]BF₄, has been demonstrated to be an efficient catalyst as well as reaction medium for the stereoselective debromination of a variety of structurally diverse vicinal-dibromides to the corresponding (E)-alkenes in high yields under microwave irradiation. This reaction does not require any organic solvent and any metal or any conventional reducing agent, and the ionic liquid is recycled without any appreciable loss of its catalytic efficiency.

Full text of DOI:10.1021/jo051373r

SCHEME 1. Debromination of vic-Dibromides
Catalysis by Ionic Liquid. A Green Protocol
for the Stereoselective Debromination of
vicinal-Dibromides by [pmIm]BF4 under
Microwave Irradiation
Brindaban C. Ranu* and Ranjan Jana
Department of Organic Chemistry, Indian Association
for the Cultivation of Science, Jadavpur,
Kolkata 700 032, India
selective debromination of vic-dibromides to the corre-
sponding (E)-alkenes (Scheme 1).
Usually the debromination reaction is carried out using
a metal such as Sm,4a In,4b Mg,4c or Zn4d in a refluxing
organic solvent such as THF and MeOH. Several other
reducing agents (e.g., sodium borohydride,5a sodium
sulfidewithphasetransferagent,5b,c sodiumnaphthalenide,5d
and dichloroindium hydride5e) have also been used for
the reductive debromination. Thus, our approach using
a nonreducing reagent such as an ionic liquid [pmIm]-
BF4 is novel.
ocbcr@iacs.res.in
Received July 2, 2005
The experimental procedure is very simple. A mixture
of vic-dibromide and [pmIm]BF4 was heated by micro-
wave irradiation for 2-5 min (TLC) in a domestic
microwave oven. The product was isolated by extraction
with ether followed by purification through column
chromatography.
A wide range of structurally varied vic-dibromides
underwent debrominations by this procedure to provide
the corresponding alkenes. The results are summarized
in Table 1. Very significantly, only trans olefins are
obtained from all the open chain substrates irrespective
of whether the dibromides are meso/erythro or dl/threo.
Thus, a (Z)-alkene such as diethyl maleate or cis-methyl
cinnamate is easily converted to the corresponding (E)-
isomer diethyl furmarate or trans-methyl cinnamate
through their dibromides (entries 3 and 10). This proce-
dure is equally effective for both aryl- or alkyl-substituted
An easily accessible ionic liquid, 1-methyl-3-pentylimidazo-
lium fluoroborate, [pmIm]BF4, has been demonstrated to be
an efficient catalyst as well as reaction medium for the
stereoselective debromination of a variety of structurally
diverse vicinal-dibromides to the corresponding (E)-alkenes
in high yields under microwave irradiation. This reaction
does not require any organic solvent and any metal or any
conventional reducing agent, and the ionic liquid is recycled
without any appreciable loss of its catalytic efficiency.
Since the introduction of ionic liquids in organic
synthesis, these compounds have attracted increasing
interest in the context of green chemistry because of their
great potential as environmentally benign media.1 Now
they have crossed the barrier of solvent and entered very
successfully into the area of catalysis.2 Although one of
the first successful uses of an ionic liquid, namely
dialkylimidazolium chloroaluminate, as a catalyst in
Friedel-Crafts acylation was reported in 1986,2a the
moisture sensitivity and decomposition of this chloroalu-
minate salt under normal atmospheric conditions has led
to the development of a second generation of more stable
and user-friendly ionic liquids by replacement of chloro-
aluminate with anionic species such as tetrafluoroborate
and hexafluorophosphate. As a part of our continuing
program to explore novel uses of ionic liquids as catalysts
as well as reaction media for useful transformations3
avoiding hazardous organic solvents and toxic catalysts,
we report here the application of an acidic, inexpensive,
and easily accessible ionic liquid, [pmIm]BF4, for stereo-
(3) (a) Ranu, B. C.; Das, A.; Samanta, S. J. Chem. Soc., Perkin Trans.
1 2002, 1520. (b) Ranu, B. C.; Dey, S. S. Tetrahedron Lett. 2003, 44,
2865. (c) Ranu, B. C.; Dey, S. S.; Hajra, A. Tetrahedron 2003, 59, 2417.
(d) Ranu, B. C.; Dey, S. S. Tetrahedron 2004, 60, 4183. (e) Ranu, B.
C.; Das, A. Aust. J. Chem. 2004, 57, 605. (f) Ranu, B. C.; Jana, R.;
Dey, S. S. Chem. Lett. 2004, 33, 274. (g) Ranu, B. C.; Jana, R. Eur. J.
Org. Chem. 2005, 755. (h) Ranu, B. C.; Banerjee, S. Org. Lett. 2005, 7,
3049. (i) Ranu, B. C.; Jana, R. Adv. Synth. Catal., in press.
(4) (a) Yanada, R.; Negoro, N.; Yanada, K.; Fujita, T. Tetrahedron
Lett. 1996, 37, 9313. (b) Ranu, B. C.; Guchhait, S. K.; Sarkar, A. Chem.
Commun. 1998, 2113. (c) Hudlicky, T.; Sinai-Zingde, G.; Natchus, M.
G. Tetrahedron Lett. 1987, 28, 5287. (d) House, H. O.; Ro, R. S. J. Am.
Chem. Soc. 1958, 80, 182.
(5) (a) King, J. F.; Allbutt, A. D.; Pews, R. G. Can. J. Chem. 1968,
46, 805. (b) Nakayama, J.; Machida, H.; Hoshino, M. Tetrahedron Lett.
1983, 24, 3001. (c) Landini, D.; Milesi, L.; Quadri, M. L.; Rolla, F. J.
Org. Chem. 1984, 49, 152. (d) Adam, W.; Arce, J. J. Org. Chem. 1972,
37, 507. (e) Ranu, B. C.; Das, A.; Hajra, A. Synthesis 2003, 1012.
(6) Pouchert, C. J. The Aldrich Library of NMR Spectra, 2nd ed.;
Aldrich Chemical Co.: Milwaukee, WI, 1983; Vols. 1 and 2.
(7) Zhang, Z.; Weidong, Y.; Wuwang, G. Chem. Lett. 2003, 32, 966.
(8) Zhu, Y.; Pan, Y. Chem. Lett. 2004, 33, 668.
(1) (a) Welton, T. Chem. Rev. 1999, 99, 2071. (b) Wasserscheid, P.;
Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3773. (c) Wilkes, J. S. Green
Chem. 2002, 4, 73. (d) Zerth, H. M.; Leonard, N. M.; Mohan, R. S. Org.
Lett. 2003, 5, 55. (e) Kumar, A.; Pawar, S. S. J. Org. Chem. 2004, 69,
1419. (f) Gu, D.-G.; Ji, S.-J.; Jiang, Z.-Q.; Zhou, M.-F.; Loh, T.-P. Synlett
2005, 959.
(2) (a) Boon, J. A.; Levinsky, J. A.; Pflug, J. L.; Wilkes, J. S. J. Org.
Chem. 1986, 51, 480. (b) Sheldon, R. Chem. Commun. 2001, 2399. (c)
Harjani, J. R.; Nara, S. J.; Salunkhe, M. M. Tetrahedron Lett. 2002,
43, 1127. (d) Namboodiri, V. V.; Varma, R. S. Chem. Commun. 2002,
342. (e) Sun, W.; Xia, C.-G.; Wang, H.-W. Tetrahedron Lett. 2003, 44,
2409. (f) Akaiyama, T.; Suzuki, A.; Fuchibe, K. Synlett 2005, 1024.
(9) Westman, J. Org. Lett. 2001, 3, 3745.
(10) Nagesam, M.; Mohomo, R. K.; Subramanyam, R. Acta Cienc.
Indica, Chem. 1984, 10, 165.
(11) Ariza, X.; Bach, J.; Berengner, R.; Farras, J.; Fontes, M.; Garcia,
J.; Lopez, M.; Ortiz, J. J. Org. Chem. 2004, 69, 5307.
(12) Xu, Q.; Borremanas, F.; Devreese, B. Tetrahedron Lett. 2001,
42, 7261.
(13) Vereshchagim, L. I.; Kirillova, L. P.; Buzilova, S. R. Zh. Org.
Khim. 1975, 11, 292.
(14) Bates, D. K.; Jones, M. C. J. Org. Chem. 1978, 43, 3856.
10.1021/jo051373r CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/10/2005
J. Org. Chem. 2005, 70, 8621-8624
8621

TABLE 1. Stereoselective Debromination of vic-Dibromides Catalyzed by [pmIm]BF4
8622 J. Org. Chem., Vol. 70, No. 21, 2005

Table 1. (Continued)
a Yields refer to those of pure isolated products characterized by IR and 1H and 13C NMR spectroscopic data.
dibromides. The cyclic dibromides also underwent de-
brominations by this reagent. The sensitive molecules
such as furan and thiophene derivatives are acceptable
under the reaction conditions (entries 16 and 17). Several
structurally diverse vic-dibromoalkenes were also sub-
jected to this procedure to provide the corresponding
alkynes (entries 20-25). The formation of one CdC and
one CtC bond in one stroke is successfully achieved from
the corresponding tetrabromide substrate (entry 24)
under the present reaction conditions.
It has been observed that this reaction does not proceed
at all at room temperature or under conventional heating
(90 °C for 12 h). Without ionic liquid also the reaction
does not proceed by microwave irradiation only. The other
ionic liquid, [pmIm]Br, is also not very successful in
catalyzing the reaction effectively. Thus, this particular
ionic liquid, [pmIm]BF4, plays a vital role in catalyzing
this reaction under microwave irradiation. It has been
observed that, with the progress of the reaction bromine,
gas that has been trapped by cyclooctene in CCl4 is
liberated. The 1,2-dibromocyclooctane has subsequently
been isolated and characterized. Thus, it may be specu-
lated that the reaction proceeds through an ionic mech-
anism as outlined in Scheme 2. The ionic liquid remained
intact (1H NMR) and was used for subsequent runs
without any difficulty. This also supports the depicted
mechanism. Regarding formation of (E)-alkenes from all
acyclic substrates, it may be assumed that the anion
formed from the dl/threo-dibromides underwent rapid
inversions to the thermodynamically more stable one
leading to trans products. This hypothesis gained support
when it was observed that the formation of cis product
was not detected at any stage of the debromination
reaction. The cis alkene also remained unchanged when
treated under similar reaction conditions, indicating no
isomerization of pure cis alkenes during the reaction.
In general, debrominations by this procedure are very
clean, high-yielding, and very fast. This reaction is also
very general, being applicable to aryl- as well as alkyl-
substituted dibromides. There is no chance of over
reduction of the CdC and CtC bonds formed in this
process, unlike in procedures using metals such as Sm4a
and Mg.4c
In conclusion, this procedure using an ionic liquid
demonstrates a novel and efficient protocol for debromi-
nation of vicinal dibromides to (E)-alkenes and alkynes.
To the best of our knowledge, we are not aware of any
such use of ionic liquid. The significant advantages
J. Org. Chem, Vol. 70, No. 21, 2005 8623

SCHEME 2. Plausible Mechanism for Stereoselective Debromination
1H), 7.63-7.66 (m, 2H), 7.50-7.59 (m, 4H), 7.41-7.43 (m, 3H);
offered by this method are operational simplicity, very
fast reaction, high yields of products, excellent stereose-
lectivity, general applicability to a wide variety of sub-
strates, no over reduction of CdC and CtC bonds,
greenness of procedure by avoiding toxic catalysts and
hazardous organic solvent during the reaction, and
recyclability of the catalyst. Thus, it provides a better
and practical alternative to the existing procedures4,5 and
provides great promise toward further useful applica-
tions.
13C NMR (75 MHz, CDCl3) δ 190.6, 144.9, 138.2, 134.8, 132.8,
130.5, 129.0 (2C), 128.6 (2C), 128.5 (2C), 128.4 (2C), 122.0. These
values were in good agreement with the reported ones.4b
This procedure was followed for all the reactions listed in
Table 1. The products obtained were all known compounds and
were identified by comparison of their spectroscopic data (IR,
1H and 13C NMR) with those reported (references in Table 1).
The ionic liquid remaining in the flask was rinsed with ether
and dried under vacuum at 80 °C to be used for subsequent
reactions. This can be used for reactions up to five runs without
any appreciable loss of efficiency. After five runs, about 50%
fresh ionic liquid was added to it, and the mixture was found to
be good for several reactions.
Experimental Section
Although the representative procedure is based on a milligram
scale, it has been scaled up to multigram level without any
difficulty.
General Experimental Procedure for the Debromina-
tion of vicinal Dibromides to (E)-Alkenes. Representative
Procedure for Debromination of erythro-1-Benzoyl-2-
Phenyl-1,2-Dibromoethane (Entry 13). A mixture of erythro-
1-benzoyl-2-phenyl-1,2-dibromoethane (368 mg, 1 mmol) and
[pmIm]BF4 (400 mg, 1.6 mmol) was subjected to heating by
microwave irradiation (20% power, 240 W, 130-135 °C as
measured by a thermometer placed inside) in a domestic
microwave oven (manufactured by BPL, India) for 2 min (TLC).
The reaction mixture was allowed to cool and extracted with
ether (3 × 5 mL). The ether extract was washed with brine and
dried (Na2SO4). Evaporation of solvent left the crude product,
which was purified by column chromatography to provide the
pure product, trans-1-benzoyl-2-phenylethene (191 mg, 92%), mp
Acknowledgment. This investigation has enjoyed
financial support from CSIR [Grant No. 01(1936)/04],
New Delhi. R.J. is also thankful to CSIR for his
fellowship.
Supporting Information Available: Spectroscopic (IR,
1H and 13C NMR) data of products listed in entries 14-25 and
13C NMR spectra of all the products listed in Table 1. This
material is available free of charge via the Internet at
http://pubs.acs.org.
56 °C; IR(KBr) 1664, 1606, 1573, 1448, 1336 cm-1
;
1H NMR
(CDCl3, 300 MHz) δ 8.01-8.04 (m, 2H), 7.82 (d, J ) 15.6 Hz,
JO051373R
8624 J. Org. Chem., Vol. 70, No. 21, 2005