Borax as an efficient metal-free catalyst for hetero-Michael reactions in an aqueous medium

Article abstract of DOI:10.1002/ejoc.200600691

Borax, a naturally occurring material, very efficiently catalyzed the conjugate addition of thiols, dithiols and amines to α,β-unsaturated ketones, nitriles, amides, aldehydes and esters in an aqueous medium to afford the corresponding Michael adducts in good yields at room temperature. Recycling of the catalyst and scaling up of the reactions are important attributes of this catalysis. The reactions of thiols and dithiols were relatively more facile than those of the corresponding amines. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejoc.200600691

FULL PAPER
DOI: 10.1002/ejoc.200600691
Borax as an Efficient Metal-Free Catalyst for Hetero-Michael Reactions in an
Aqueous Medium
Sahid Hussain,[a] Saitanya K. Bharadwaj,[a] Mihir K. Chaudhuri,*[a] and Harjyoti Kalita[a]
Keywords: Borax / Thiols / Amines / Water / Michael addition
Borax, a naturally occurring material, very efficiently cata-
lyzed the conjugate addition of thiols, dithiols and amines
to α,β-unsaturated ketones, nitriles, amides, aldehydes and
esters in an aqueous medium to afford the corresponding
Michael adducts in good yields at room temperature. Recycl-
ing of the catalyst and scaling up of the reactions are impor-
tant attributes of this catalysis. The reactions of thiols and
dithiols were relatively more facile than those of the corre-
sponding amines.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
micellar solution of sodium dodecyl sulfate (SDS).[19] Each
method has some advantages over the others; however, the
search still continues for improved versions of the Michael
reaction. Notably, most of the methods recently developed
have involved metal-based acidic catalysts, presumably be-
cause the Michael reactions in basic media are rather slug-
gish without providing satisfactory yields even with long
reaction times.[20] However, there is an intrinsic interest in
the Michael addition of thiols to enones in basic aqueous
media under mild conditions[21] and such reactions are
highly relevant to “dynamic combinatorial chemistry”
which offers a conceptually new approach to the investiga-
tion of host–guest interactions.[22] Considering all these and
being intrigued by the fact that the pH of an aqueous solu-
tion of borax is 9.5, we thought it worthwhile to carry out
both thia- and aza-Michael reactions using borax as the
catalyst and water as the reaction medium. Environmental
benignity and appreciable solubility in water are important
attributes of the chosen catalyst. In addition, the reactions
in water not only contribute to “green chemistry” but also
to pseudonatural catalysis chemistry. Reported in this paper
are the borax-catalyzed Michael reactions of thiols and
amines with α,β-unsaturated ketones, nitriles and esters in
water at room temperature (Scheme 1).
Introduction
The construction of C–S and C–N bonds is the key step
in the synthesis of organo-sulfur and -nitrogen compounds,
respectively. While organo-sulfur compounds have multiple
applications[1] in the production of biologically active mole-
cules, including calcium antagonistic dilthiazem,[2] β-amino
ketones exhibit a wide range of biological activities,[3] pos-
sess pharmacological properties and also serve as essential
intermediates in the synthesis of β-amino acids[4] and β-
lactams.[5] Consequently, the development of novel proto-
cols for the conjugate addition of thiols and amines to elec-
tron-deficient olefins leading to the formation of C–S and
C–N bonds, respectively, have attracted a great deal of at-
tention in synthetic organic chemistry. Notably, the success
of conjugate addition reactions lie in the use of either acidic
or basic conditions which, if not selected judiciously, can
be detrimental to the desired synthesis allowing unwanted
side-reactions to contaminate the product. Moreover, the
possibility of poisoning metal-based catalysts with thiols,
alkyl- or arylamines[6] cannot be completely ruled out. In
order to alleviate some of these problems, the Michael reac-
tion has metamorphosed over the years to allow a number
of reagents and catalysts and alternative procedures to be
used, for instance, a variety of inorganic salts,[7] quaternary
ammonium salts,[8] ionic liquids (IL),[9] a combination of IL
and water,[10] palladium,[6] supported CeCl3·7H2O/NaI,[11]
clay,[12] silica gel,[13a] SiO2/HClO4,[13b] solid acid,[13c] KF/
alumina,[14] polyethylene glycol (PEG),[15] Cu(acac)2/IL,[16]
boric acid in water,[17] cyclodextrin in water[18] and even a
[a] Department of Chemistry, Indian Institute of Technology Gu-
wahati,
Guwahati 781 039, India
Fax: +91-361-2582349
E-mail: mkc@iitg.ernet.in
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Scheme 1.
374
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 374–378

Borax as an Efficient Catalyst for Hetero-Michael Reactions in an Aqueous Medium
FULL PAPER
Table 2. Borax-catalyzed Michael addition of thiols to olefins in
water at room temperature.
Results and Discussion
In order to ascertain the efficacy of the catalyst in water,
the conjugate addition reactions of thiophenol to methyl
acrylate were separately conducted in water and four dif-
ferent organic solvents. The results are summarized in
Table 1. It is evident that the reaction takes place in each
case with the best performance being in water containing
10 mol-% of the catalyst. Accordingly, all the reactions dis-
cussed hereafter were conducted with this combination of
solvent and catalyst.
Table 1. Michael addition of thiophenol with methyl acrylate under
different reaction conditions.
Run
Borax [mol-%]
Solvent
Time [min]
Yield [%]
1
2
3
4
5
6
7
8
0
H2O
H2O
H2O
5
5
5
5
5
5
5
5
5
20
10
1
10
10
10
10
95
95
40
30
64
70
70
H2O
MeOH
CH3CN
(CH3)2CO
AcOEt
A variety of electron-deficient olefins such as methyl ac-
rylate, acrylonitrile, acrylamide, cyclohexanone and methyl
methacrylate underwent facile 1,4-addition with a wide
range of thiols catalyzed by borax (10 mol-%) in water at
room temperature to afford the corresponding β-adducts in
high to very high yields (Table 2). Unsaturated ketones, ni-
triles, amides, aldehydes and esters reacted readily with both
aliphatic and aromatic thiols to provide the corresponding
Michael adducts (Table 2, Runs 1–20).
The reactions were clean. The borax/water system also
worked very well with α- and β-substituted Michael ac-
ceptors at ambient temperatures. It was found that with a
methyl group either at the α- or β-position, the reaction
gave good yields (Table 2, Runs 12–15) in 5–10 min, while
with a phenyl group at the β-position longer reaction times
were required (1.5–3 h, Table 2, Runs 23–25). Acceptors like
carvone and pulegone reacted readily with aliphatic thiols
(Table 2, Runs 21 and 22). Notably, this protocol worked
well for the conjugate addition of cysteine to acrylamide
(Table 2, Run 11) giving 88% isolated yield of the adduct.
Incidentally, our experimental conditions, especially the in-
volvement of a weakly alkaline aqueous reaction medium,
mild reaction conditions capable of working efficaciously at
room temperature and with no particular precautions nec-
essary to prevent di- or polymerization, compare very well
with those required for the synthesis of dynamic chemical
libraries (DCLs)[21] by base-mediated Michael addition of
thiols to enones. This reaction is important in the context
of the alkylation of cysteine in proteins with acrylamide un-
[a] Isolated Yield. [b] Yield on a 7 g scale.
The borax-catalyzed Michael addition reaction in water
is also readily applicable to dithiols, as shown in Scheme 2.
Scheme 2.
The reactions proceeded with alacrity giving bis(adducts)
der mild aqueous alkaline conditions and is considered to in very good yields (Table 3). Such reactions are expected
be useful for cysteine identification during protein sequenc- to be useful in the designed synthesis of organo-sulfur poly-
ing.[23]
mers, supramolecular architectures and macromolecules.
Eur. J. Org. Chem. 2007, 374–378
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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375

S. Hussain, S. K. Bharadwaj, M. K. Chaudhuri, H. Kalita
FULL PAPER
Incidentally, H2N–CO–CH2–CH2–S–CH2–CH2–CH2–S– tions[17] (Table 4) suggests that under similar experimental
CH2–CH2–CO–NH2 obtained from the reaction of 1,3-pro- conditions borax is either as good as or slightly better than
panedithiol and 2 equiv. of acrylamide (Table 3, Run 2) is boric acid in catalyzing the chosen reactions.
an interesting compound,[24] the preparation of which does
Finally, upon completion of the reaction, recyclability of
not seem to be available in the open literature. The use of the catalyst was examined through a series of reactions with
the compound as a photographic development accelerator thiophenol and methyl acrylate using an aqueous phase
and as a constituent of colour photographic developer has, containing borax. The reaction continued to give good re-
however, been reported in two Japanese patents.[24] It has sults from the second through to the fifth cycle with the
also been used in lithographic plate processing solutions.
yields being 95, 94, 92 and 90%, respectively. However, the
yield was reduced to 80% in the sixth cycle. This is ex-
plained by attrition and leaching of the catalyst. It is also
important to note that the borax/water protocol can be ap-
plied to a relatively larger scale (7 g) of operation, giving
very good yields (Table 2, Runs 1 and 9).
Table 3. Borax-catalyzed Michael addition of dithiols to olefins in
water at room temperature.
Conclusions
Borax has emerged as an efficacious, environmentally ac-
ceptable and highly cost-effective novel catalyst for thia-
and aza-Michael reactions thereby making an important
addition to the existing toolbox of the Michael reaction.
The fact that the reactions work very well in water is an
important attribute of this protocol. The controlled basicity
of the catalyst causes thiol additions to α,β-unsaturated
ketones, nitriles, amides, aldehyde, and esters to be relatively
more facile than the corresponding amine addition reac-
tions. Moreover, borax, being a naturally occurring mate-
rial, soluble and capable of functioning efficiently in water,
satisfies some tenets of “green chemistry.” The reactions are
thus more conducive to the environment and provide scope
for further studies. The thia-Michael reaction between cys-
teine and acrylamide (Table 2, Run 11) may be relevant to
the understanding of the alkylation of cysteine in proteins
with acrylamide under mildly alkaline conditions.
[a] Isolated yields.
The borax/water protocol is applicable to aza-Michael
reactions as well. A variety of α,β-unsaturated compounds
underwent 1,4-addition with a wide range of aliphatic
amines in the presence of 10 mol-% of borax at ambient
temperature to afford the corresponding β-amino com-
pounds in high yields. Some representative examples are
listed in Table 4. A comparison of the results of the thiol
and amine addition reactions under similar experimental
conditions shows that the former are more facile than the
latter. Indeed, this observation is in agreement with the re-
sults of an earlier kinetic study.[20a] Based on kinetic data,
it was predicted that –SH groups are many times more reac-
tive than amines in aqueous alkaline solution. A compari-
son of the results obtained in this work with those of the
corresponding boric acid catalyzed aza-Michael reac-
Experimental Section
Experimental Procedure: Borax (0.2 mmol, 0.076 g) was dissolved
in water (2 mL) followed by the addition of a thiol or an amine
(2 mmol) or a dithiol (1 mmol) and an α,β-unsaturated compound
(2.2 mmol) and the mixture was stirred at room temperature. The
reaction was monitored by TLC. After completion of the reaction,
the mixture was extracted with ethyl acetate (3ϫ5 mL). The com-
bined extracts (dried with Na2SO4) were concentrated in vacuo and
the resulting product was purified on silica gel with ethyl acetate
and n-hexene (1:9 for thia and 3:7 for aza adduct) as eluent to
afford the pure adduct. The aqueous layer containing borax can be
reused in the next run. A 25% solution of methanol in water was
used as the reaction medium for Runs 23–25 of Table 2. Precipi-
tation occurred in the reactions of amide with thiols (Table 2,
Runs 9–11, 21 and 22, and Table 3, Runs 3 and 4). The precipitates
were filtered and washed with water and then recrystallized from
MeOH or hot water. 1H and 13C NMR spectra were recorded with
a Bruker 200 MHz and a Varian 400 MHz spectrometer. IR spectra
were recorded either in KBr or neat with a Nicolet Impact 410
spectrophotometer. GC-MS data were recorded with a Perkin-El-
mer Precisely Clarus 500 instrument using a capillary column
(30ϫ0.25ϫ0.25 mµ). Elemental analyses were carried out with a
Perkin-Elmer 2400 automatic C,H,N,S analyzer.
Table 4. Borax-catalyzed Michael addition of amines to olefins in
water at room temperature.
[a] Isolated yields.
376
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Eur. J. Org. Chem. 2007, 374–378

Borax as an Efficient Catalyst for Hetero-Michael Reactions in an Aqueous Medium
FULL PAPER
Spectral Data of Some Selected Compounds
DMSO): δ = 26.87 (2 C), 31.27 (2 C), 35.65 (2 C), 172.22 (2 C)
ppm. C8H16N2O2S2 (236.36): calcd. C 40.65, H 6.82, N 11.85, S
27.13; found C 40.54, H 6.88, N 11.90, S 27.10.
3a: Yield: 0.443 g, 92%, light yellow solid (m.p. 56–57 °C). IR
(KBr): ν = 1340 and 1511 (NO ), 1733 (C=O) cm–1. 1H NMR
˜
2
27a: Yield: 0.235 g, 94%, white solid (m.p. 147–149 °C). IR (KBr):
(400 MHz, CDCl3): δ = 2.71 (t, J = 7.6 Hz, 2 H), 3.30 (t, J =
7.2 Hz, 2 H), 3.71 (s, 3 H), 7.34 (d, J = 8.8 Hz, 2 H), 8.12 (d, J =
8.8 Hz, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 27.39, 33.73,
52.37, 124.20 (2 C), 126.76 (2 C), 145.21, 146.27, 171.89 ppm.
C10H11NO4S (241.27): calcd. C 49.78, H 4.60, N 5.81, S 13.29;
found C 49.62, H 4.65, N 5.85, S 13.25.
ν = 1648 (C=O), 3200 and 3397 (NH ) cm–1. 1H NMR (400 MHz,
˜
2
[D6]DMSO): δ = 1.74 (quint, J = 7.2 Hz, 2 H), 2.30 (t, J = 7.2 Hz,
4 H), 2.55 (t, J = 7.2 Hz, 4 H), 2.64 (t, J = 7.6 Hz, 4 H), 6.81 (br.
s, NH), 7.31 (br. s, NH) ppm. 13C NMR (100 MHz, [D6]DMSO):
δ = 26.88 (2 C), 29.03, 29.89 (2 C), 35.54 (2 C), 172.21 (2 C) ppm.
C9H18N2O2S2 (250.38): calcd. C 43.17, H 7.25, N 11.19, S 25.61;
found C 43.05, H 7.29, N 11.17, S 25.53.
4a: Yield: 0.264 g, 89%, pale yellow liquid. IR (neat): ν = 1744
˜
(C=O) cm–1. 1H NMR (400 MHz, CDCl3): δ = 1.24 (t, J = 7.2 Hz,
3 H), 2.54 (q, J = 7.2 Hz, 2 H), 2.59 (t, J = 7.6 Hz, 2 H), 2.77 (t,
J = 7.6 Hz, 2 H), 3.67 (s, 3 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 14.87, 26.11, 26.67, 34.83, 51.84, 172.24 ppm. C6H12O2S
(148.23): calcd. C 48.62 H 8.16, S 21.63; found C 48.47, H 8.20, S
21.55.
30a: Yield: 0.229 g, 86%, colourless oil. IR (neat): ν = 1732 (C=O)
˜
1
cm–1. H NMR (400 MHz, CDCl3): δ = 2.62 (t, J = 7.2 Hz, 4 H),
2.74 (s, 4 H), 2.82 (t, J = 7.2 Hz 4 H), 3.69 (s, 6 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 27.33 (2 C), 32.39 (2 C), 34.96 (2 C), 52.12
(2 C), 171.22 (2 C) ppm. C10H18O4S2 (266.38): calcd. C 45.09, H
6.81, S 24.07; found C 44.94, H 6.85, S 24.11.
5a: Yield: 0.433 g, 75%, colorless oil. IR (neat): ν = 1747 (C=O)
˜
1
31a: Yield: 0.238 g, 85%, colourless oil. IR (neat): ν = 1735 (C=O)
cm–1. H NMR (400 MHz, CDCl3): δ = 0.87 (t, J = 6.4 Hz, 3 H),
˜
1
cm–1. H NMR (400 MHz, CDCl3): δ = 1.85 (quint, J = 7.2 Hz, 2
1.25–1.59 (m, 20 H), 2.51 (t, J = 7.6 Hz, 2 H), 2.60 (t, J = 7.2 Hz,
2 H), 2.77 (t, J = 7.2 Hz, 2 H), 3.69 (s, 3 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 14.80, 23.36, 27.66, 29.53, 29.88, 29.99 (2
C), 30.18 (2 C), 30.25 (2 C), 32.56, 32.86, 35.40, 52.37, 172.38 ppm.
C16H32O2S (288.50): calcd. C 66.61, H 11.18, S 11.11; found C
66.52, H 11.25, S 11.07.
H), 2.60 (t, J = 7.6 Hz, 4 H), 2.62 (t, J = 7.6 Hz, 4 H), 2.76 (t, J =
7.2 Hz, 4 H), 3.68 (s, 6 H) ppm. 13C NMR (100 MHz, CDCl3): δ
= 27.26 (2 C), 29.32, 31.14 (2 C), 34.93 (2 C), 52.04 (2 C), 172.28
(2 C) ppm. C11H20O4S2 (280.41): calcd. C 47.12, H 7.19, S 22.87;
found C 47.02, H 7.25, S 22.79.
3b: Yield: 0.344 g, 89%, colourless liquid. IR (neat): ν = 3467 (N–
˜
8a: Yield: 0.368 g, 72%, colorless oil. IR (neat): ν = 2340 (CN)
˜
H) 1742(C=O) cm–1. 1H NMR (CDCl3, 400 MHz): δ = 1.87 (s,
1NH), 2.52 (t, J = 6.8 Hz, 2 H), 2.88 (t, J = 6 Hz, 2 H), 3.66 (s, 2
H), 3.79 (s, 3 H), 7.20–7.30 (m, 5 H) ppm. C11H15NO2 (193.25): C
68.37, H 7.28, N 7.25; found C 68.32, H 7.33, N 7.29.
1
cm–1. H NMR (400 MHz, CDCl3): δ = 0.87 (t, J = 6.8 Hz, 3 H),
1.25–1.60 (m, 20 H), 2.58 (t, J = 8 Hz, 2 H), 2.62 (t, J = 7.2 Hz, 2
H), 2.77 (t, J = 7.2 Hz, 2 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 14.24, 19.05, 22.79, 27.76, 28.87, 29.28, 29.43, 29.57 (2 C), 29.72
(3C), 32.00, 32.44, 118.25 ppm. C15H29NS (255.47): calcd. C 70.52,
H 11.44, N 5.48, S 12.55; found C 70.32, H 11.49, N 5.49, S 12.48.
4b: Yield: 0.396 g, 92%, colourless liquid. IR (neat): ν = 1743
˜
(C=O) cm–1. 1H NMR: (CDCl3, 400 MHz): δ = 0.90 (t, J = 6.8 Hz,
6 H), 1.26–1.44 (m, 8 H), 2.39 (t, J = 7.2 Hz, 2 H), 2.60 (t, J =
7.2 Hz, 2 H), 2.77 (t, J = 7.6 Hz, 2 H), 3.66 (s, 3 H) ppm. 13C
NMR (CDCl3, 100 MHz): δ = 14.50, 21.04, 29.70, 32.68, 47.18,
49.70, 50.17, 51.82, 54.00, 173.42 ppm. MS (EI): m/z (%) = 215
(33) [M]+, 172 (100). C12H25NO2 (215.34): calcd. C 66.93, H 11.70,
N 6.50; found C 66.78, H 11.69, N 6.52.
10a: Yield: 0.280 g, 87%, white solid (m.p. 78 °C). IR (KBr): ν =
˜
1657 (C=O), 3197 and 3374 (NH2) cm–1. 1H NMR (400 MHz,
CDCl3): δ = 0.91 (t, J = 7.6 Hz, 3 H), 1.41 (sext., J = 7.6 Hz, 2 H),
1.57 (quint., J = 7.6 Hz, 2 H), 2.48–2.56 (m, 4 H), 2.79 (t, J =
7.2 Hz, 2 H), 6.02 (br. s, NH), 6.02 (br. s, NH) ppm. 13C NMR
(100 MHz, CDCl3): δ = 13.95, 22.21, 27.71, 31.85, 32.24, 36.23,
174.14 ppm. C7H15NOS (161.27): calcd. C 54.14, H 9.38, N 8.69,
S 19.88; found C 52.09, H 9.39, N 8.72, S 19.79.
5b: Yield: 0.248 g, 90%, colourless liquid. IR (KBr): ν = 2259 (CN)
˜
cm–1. 1H NMR: (CDCl3, 200 MHz): δ = 1.44–1.49 (m, 2 H), 1.57–
1.65 (m, 4 H), 2.40–2.50 (m, 6 H), 2.62–2.69 (m, 2 H) ppm. MS
(EI): m/z (%) = 138 (23) [M]+, 98 (100). C8H14N2 (138.21): C 69.52,
H 10.21, N 20.27; found C 69.49, H 10.25, N 20.29.
18a: Yield: 0.442 g, 88%, yellow solid (m.p. 70 °C). IR (KBr): ν =
˜
1340 and 1508 (NO2), 1717 (C=O) cm–1. 1H NMR (400 MHz,
CDCl3): δ = 1.81–1.89 (m, 2 H), 2.15–2.26 (m, 2 H), 2.39–2.49 (m,
3 H), 2.76–2.80 (dd, J1 = 4.4 Hz, J2 = 14 Hz, 1 H), 3.68–3.75 (m,
1 H), 7.40 (d, J = 8.4 Hz, 2 H), 8.12 (d, J = 8.4 Hz, 2 H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 24.25, 31.22, 41.09, 44.73, 47.50,
124.22 (2 C), 129.21 (2 C), 144.28, 146.04, 207.27 ppm.
C12H13NO3S (251.31): calcd. C 57.35, H 5.21, N 5.57, 12.76; found
C 57.17, H 5.27, N 5.60, S 12.73.
6b: Yield: 0.275 g, 86%, colorless liquid. IR (neat): ν = 3334 (N–
˜
1
H), 2254 (CN) cm–1. H NMR: (CDCl3, 200 MHz): δ = 1.50 (s, 1
H), 2.47 (t, J = 4.4 Hz, 2 H), 2.90 (t, J = 4.6 Hz, 2 H), 3.81 (s, 2
H), 7.24–7.28 (m, 5 H) ppm. MS (EI): m/z (%) = 160 (10) [M]+, 91
(100). C10H12N2 (160.22): C 74.97, H 7.55, N 17.48; found C 74.77
H 7.61, N 17.52.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details, analytical data and the 1H and 13C
NMR spectra of selected compounds.
20a: Yield: 0.417 g, 70%, colorless oil. IR (neat): ν = 1717 (C=O)
˜
1
cm–1. H NMR (400 MHz, CDCl3): δ = 0.87 (t, J = 6.4 Hz, 3 H),
1.25–1.59 (m, 20 H), 1.67–1.73 (m, 2 H), 2.10–2.15 (m, 2 H), 2.32–
2.39 (m, 3 H), 2.53 (t, J = 7.6 Hz, 2 H), 2.67–2.72 (m, 1 H), 3.10–
3.11 (m, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 14.80, 23.35,
24.96, 29.61, 29.86, 29.99, 30.15, 30.27 (2 C), 30.39 (2 C), 31.24,
32.36, 32.55, 41.59, 43.41, 48.89, 209.22 ppm. C18H34OS (298.54):
calcd. C 72.42, H 11.48, S 10.74; found C 72.20, H 11.54, S 10.71.
Acknowledgments
S. H. and S. K. B. are grateful to the CSIR, New Delhi, for research
fellowships.
26a: Yield: 0.217 g, 92%, white solid (m.p. 179–181 °C). IR(KBr):
ν = 1646 (C=O), 3200 and 3395 (NH ) cm–1. 1H NMR (400 MHz,
˜
2
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(br. s, NH), 7.33 (br. s, NH) ppm. 13C NMR (100 MHz, [D6]-
8.
Eur. J. Org. Chem. 2007, 374–378
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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377

S. Hussain, S. K. Bharadwaj, M. K. Chaudhuri, H. Kalita
FULL PAPER
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