Cu(ii) catalysed chemoselective oxidative transformation of thiourea to thioamidoguanidine/2-aminobenzothiazole

Article abstract of DOI:10.1039/c2ra22240j

2-Haloaryl-sec-alkyl unsymmetrical thioureas (Tu) (halo = -F, -Cl) with a catalytic amount of Cu(ii) salt get oxidised in situ to their disulfide intermediates followed by an imine-disulfide rearrangement to give thioamidoguanidino (Tag) moieties at room temperature. During this process Cu(ii) gets reduced to Cu(i) and forms a complex with the Tag moiety from which Tag moiety can be isolated upon treatment with ammonia. However, when the same reaction was performed at an elevated temperature with a catalytic quantity of Cu(ii) salt, Tu bearing o-halogens (-F, -Cl) gave 2-aminobenzothiazoles via a dehalogenative heteroarylation path and not by the Hugerschoff path involving an electrophilic substitution reaction. For thioureas containing reactive ortho halogens (such as -Br, -I) the reaction proceeds at room temperature giving 2-aminobenzothiazoles via a dehalogenative path requiring a catalytic quantity of Cu(ii). No transformation of thiourea (Tu) to Tag was observed with Cu(i) salts suggesting the requirement of an oxidising Cu(ii) salt for this oxidative transformation. Mild reaction conditions, environmentally benign reagents and solvent, high yields, tolerance of various functional groups are some of the essential features of this methodology.

Full text of DOI:10.1039/c2ra22240j

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Cu(II) catalysed chemoselective oxidative
transformation of thiourea to thioamidoguanidine/2-
Cite this: RSC Advances, 2013, 3, 438
aminobenzothiazole3
Santosh K. Sahoo, Nilufa Khatun, Anupal Gogoi, Arghya Deb and Bhisma K. Patel*
2-Haloaryl-sec-alkyl unsymmetrical thioureas (Tu) (halo = –F, –Cl) with a catalytic amount of Cu(II) salt get
oxidised in situ to their disulfide intermediates followed by an imine-disulfide rearrangement to give
thioamidoguanidino (Tag) moieties at room temperature. During this process Cu(II) gets reduced to Cu(I)
and forms a complex with the Tag moiety from which Tag moiety can be isolated upon treatment with
ammonia. However, when the same reaction was performed at an elevated temperature with a catalytic
quantity of Cu(II) salt, Tu bearing o-halogens (–F, –Cl) gave 2-aminobenzothiazoles via a dehalogenative
heteroarylation path and not by the Hugerschoff path involving an electrophilic substitution reaction. For
thioureas containing reactive ortho halogens (such as –Br, –I) the reaction proceeds at room temperature
giving 2-aminobenzothiazoles via a dehalogenative path requiring a catalytic quantity of Cu(II). No
transformation of thiourea (Tu) to Tag was observed with Cu(I) salts suggesting the requirement of an
oxidising Cu(II) salt for this oxidative transformation. Mild reaction conditions, environmentally benign
reagents and solvent, high yields, tolerance of various functional groups are some of the essential features
of this methodology.
Received 21st September 2012,
Accepted 23rd October 2012
DOI: 10.1039/c2ra22240j
www.rsc.org/advances
zothiazole involves an intramolecular aromatic electrophilic
substitution reaction (Hugerschoff path) facilitated by activa-
Introduction
2-Aryl-sec-alkyl unsymmetrical thioureas (Tu) on treatment
with thiophilic oxidising agents such as bromine (or its
equivalent)1a or iodine1b give thioamidoguanidine products
(Tag) for most thiourea (Tu) substrates instead of giving the
expected 2-aminobenzothiazoles. These results of ours are in
contrast with the classical Hugerschoff reaction known since
19012 and the recent reports by Jordan and Le et al.3 Of course,
unsymmetrical thioureas (Tu) containing activating group(s)
in the aryl rings are susceptible towards intramolecular
reactions in the presence of suitable thiophiles giving
2-aminobenzothiazoles (Hugerschoff product).1a
tion of the sulfur atom of the thiourea with a thiophilic
reagent.1 In order to circumvent the competitive formation of
2-aminobenzothiazole and allow exclusive formation of the
Tag product, the redox active metal Cu(II) is expected to
promote only oxidative dimerisation of thiourea (S–S bond
formation). This is then followed by an intramolecular
iminedisulfide rearrangement leading to the formation of
Tag. Thiophilic redox active metal Cu(II) would not activate the
sulphur atom of thiourea towards an intramolecular electro-
philic substitution reaction (Hugerschoff path). Indeed this
strategy was quite successful and the unsymmetrical thiourea
(Tu) was transformed into a thioamidoguanidine (Tag) moiety
with concomitant reduction of Cu(II) to Cu(I) which however
A
competitive formation of 2-aminobenzothiazole
(Hugerschoff product) and thioamidoguanidine (anti-
Hugerschoff product) was observed for the moderately
activated substrates.1 The formation of the thioamidoguani-
dine (Tag) product from 2-aryl-sec-alkyl unsymmetrical thiour-
eas goes via an oxidative dimerization (S–S bond formation)
followed by an intramolecular imine-disulfide rearrange-
ment.1a On the other hand the formation of 2-aminoben-
formed a [Cu2 (m2-Br)2Tag2] complex.4 Furthermore, we know
I
that 2-halothioureas prefer a dehalogenative path during the
formation of an intramolecular C–S bond formation giving
2-aminobenzothiazoles for the entire range of halogens where
as Pd favors a C–H activation path.5 Recently, we have
engineered a greener strategy for the synthesis of 2-amino-
benzothiazole from ortho-halo (–F, –Cl, –Br and –I) substituted
unsymmetrical thioureas using CuO nanoparticles.6 In this
strategy for ortho –I and –Br substituted thioureas the reaction
affords 2-aminobenzothiazoles under metal free conditions via
a base promoted intramolecular nucleophilic aromatic sub-
Department of Chemistry, Indian Institute of Technology Guwahati, 781 039, Assam,
India. E-mail: patel@iitg.ernet.in; Fax: +91-3612690762
Electronic supplementary information (ESI) available: 1H and 13C NMR spectra
3
and HRMS spectra. CCDC reference numbers 894067 and 894068. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c2ra22240j
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Scheme 2 Formation of thioamidoguanidino moiety from thiourea.
Scheme 1 Various possible reaction pathways of 2-halo thiourea.
solvent system was switched to EtOAc : H2O (3 : 1) instead of
ethanol because of the convenience in work up at a later stage.
The in situ generated unsymmetrical thiourea (Tu) obtained by
reacting phenylisothiocyanate (1) with morpholine (a)
(Scheme 2) in EtOAc : H2O (3 : 1) medium was treated with
an aqueous solution of CuBr2 (1 equiv.). The green colour of
CuBr2 disappeared immediately giving a yellow solution along
with yellow precipitate. From our earlier report we know that
stitution. However, base and Cu catalyst were essential for the
relatively inert ortho –Cl and –F substrates.6
2-Haloaryl-sec-alkyl unsymmetrical thioureas (Tu) on treat-
ment with Cu(II) salts there exists several possible reaction
pathways as shown in Scheme 1. In path-a, it can undergo an
intramolecular C–S bond formation via a dehelogenative path
giving 2-aminobenzothiazole. Depending on the nature of the
2-halo substituents, particularly inert halogens (–Cl, –F) or in
the absence of any halo substituents it may furnish 2-amino-
benzothiazole via a C–H activation strategy (path-b). The
possibility of an oxidative dimerisation (S–S bond formation)
of a thiourea (Tu) in the presence of a redox active metal Cu(II)
cannot be ruled out as shown in path-c which would eventually
lead to the formation of a Tag moiety after an intramolecular
imine-disulfide rearrangement. Thus we wish to investigate
which of the above paths would operate when 2-haloaryl-sec-
alkyl unsymmetrical thioureas (Tu) are treated with Cu(II) salt
and also whether o-halogens would have any affect on the
outcome of the product. For substrates without o-halo groups,
would path-b (C–H activation) or path-c (S–S bond formation)
be followed when treated with a redox active Cu(II) salt? If
path-c operates, then a library of guanidine class of molecules
can be generated. The molecules containing guanidine
moieties have shown number of biological and pharmaceu-
tical applications. Furthermore, guanidine possessing mole-
cules are also capable of catalysing organic reactions, can be
used as a super bases and they exhibit a variety of co-
ordination modes leading to compatibility with a wide range
of metal ions.7
I
the yellow compound formed is a bimetallic [Cu2 (m2-Br)2Tag2]
complex.4 The co-ordinated copper was removed from the
complex by treating the crude reaction mixture with an
aqueous ammonia solution. The isolated ligand was found
to be the expected thioamidoguanidino moiety (Tag) (1a) and
no traces of 2-aminobenzothiazole (119a) formation (Table 4)
was observed. It may be mentioned here that the use of
bromine equivalent, 1,19-(ethane-1,2-diyl)dipyridinium bistri-
bromide (EDPBT) gave a mixture of thioamidoguanidino
moiety (Tag) and 2-aminobenzothiazole (119a). Thus our
envisioned strategy was indeed successful in suppressing the
formation of 2-aminobenzothiazole (119a). Our group has been
actively involved in the synthesis of various N, O, S hetero-
cycles using Cu catalyst.8 From a green chemistry perspective,
it is desirable to have a catalytic quantity of Cu instead of
stoichiometric amount for any transformations. The reaction
when carried out with 10, 20, 30 and 40 mol% of CuBr2 it was
found that 30 mol% of CuBr2 was optimum in converting the
starting material into the product. As evident from our recent
result when thiourea is treated with Cu(II) salt, it is first
oxidised to its disulfide intermediate which then undergo an
intramolecular rearrangement giving Tag and during the
process the Cu(II) gets reduced to Cu(I). The in situ generated
Cu(I) is reoxidised to Cu(II) by the atmospheric oxygen and the
catalytic cycle continues. The amount of catalyst required is
more (30 mol%) compared to any typical catalytic reactions
because of the propensity of the in situ generated Cu(I) species
to form a complex with the in situ generated Tag unit thereby
making part of the catalyst unavailable.4 Other divalent Cu(II)
salts like CuSO4?5H2O, Cu(OAc)2?2H2O, Cu(NO3)2?3H2O,
Results and discussion
We directly adopted our recently established procedure4 for
the preparation of the Tag moiety from thiourea but the
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Table 1 Synthesis of thioamidoguanidino products from arylisothiocyanates and sec aminesa
Substrate
Product
Yieldb
Substrate
Product
Yieldb
86%
71%
89%
80%
83%
73%
88%
85%
86%
73%
74%
78%
83%
85%
a
b
Reaction monitored by TLC. Confirmed by IR, 1H, 13C NMR spectra.
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from aromatic isothiocyanate containing two weakly activating
(methyl) substituents such as 3,4-dimethyl phenylisothiocya-
nate (3) and various secondary aliphatic amines like morpho-
line (a), thiomorpholine (b), N-phenyl piperazine (c) and
piperidine (d) all afforded Tag products (3a), (3b), (3c), and
(3d) respectively under the present reaction conditions.
Structure of the product (3d) has been confirmed by single
crystal X-ray crystallography as shown in Fig. 1. It may be
worth mentioning here that the same thiourea derived from
3,4-dimethylphenylisothiocyanate (3) and morpholine (a) gave
a regioisomeric mixture of two 2-aminobenzothiazoles and no
traces of Tag product (3a) was observed when bromine
equivalent (EDPBT) was used as the thiophilic reagent.1
Thus, the oxidising ability (S–S bond formation) of Cu(II)
predominates over its thiophilicity. Furthermore, the absence
of any intramolecular aromatic electrophilic substitution
product (2-aminobenzothiazole) using Cu(II) salt confirms its
lower thiophilicity as compared to EDPBT. Thus, the use of
Cu(II) as an oxidising agent is advantageous over EDPBT in
giving Tag products. Not surprisingly, thiourea derived from
p-butyl phenylisothiocyante (4) and morpholine (a) yielded the
corresponding Tag product (4a).
Unsymmetrical thioureas (Tu) possessing activating groups
in the aryl rings are prone towards intramolecular aromatic
electrophilic substitution reaction in the presence of a
thiophilic reagent thereby enhancing the chances of forming
2-aminobenzothiazoles via a Hugerschoff path. However, the
use of Cu(II) salt gave exclusively Tag products for substrates
containing not only for deactivating substrates but also for
activating substrates. However, the presence of weakly
deactivating substituents in the aryl ring of the thiourea is
more likely to give Tag products only. To verify this fact and
demonstrate the versatility of the method thioureas derived
from 3-chloro phenylisothiocyanate (5) and aliphatic second-
ary amines such as morpholine (a) and piperidine (d) on
treatment with CuBr2 gave exclusively Tag products (5a) and
(5d) respectively in excellent yields (Table 2). The treatment of
the thiourea derived from 3-bromo phenylisothiocyanate (6)
and thiomorpholine (b) with Cu(II) gave Tag product (6b) in a
modest yield. Thioureas derived from sets of arylisothioca-
nates such as 3-nitro phenylisothiocyanate (7), 4-chloro
phenylisothiocyanate (8), 4-bromo phenylisothiocyanate (9)
and 4-trifluoromethyl phenylsiothiocyanate (10) and sets of
aliphatic secondary amines (a–d) all gave their expected Tag
products in good to excellent yield under the present reaction
conditions as shown in Table 2.
Fig. 1 ORTEP view (30% probability ellipsoids) of compound 3d.
CuCl2?2H2O, CuI2 were tested and gave the desired transfor-
mations, but were found to be inferior to CuBr2.
Despite their use as efficient vulcanising and herbicide
agents, prior to our greener strategies there is only one
synthetic method reported using 2-aryl-sec-alkyl unsymmetri-
cal thioureas (Tu) and iodine in chloroform.9a Another report
is on the crystal structure determination.9b Beside this report
there are two more methods reported for the synthesis of
thioamido guanidine in the literature. Both methods are from
our group only, starting from aryl-sec-alkyl unsymmetrical
thiourea using thiophilic reagents such as bromine or iodine.1
Thus we wished to synthesize a series of thioamidoguanidine
(Tag) moieties by varying the secondary aliphatic amines in
thioureas keeping the aryl part constant. Thiomorpholine (b),
N-phenyl piperazine (c), piperidine (d), 4-benzylpiperidine (e)
and pyrrolydine (f) derived thioureas from phenyl isothiocya-
nate (1) all gave good yields of their corresponding Tag
products (1b), (1c), (1d), (1e), and (1f), respectively as shown in
Table 1. From the present study it was found that the aliphatic
secondary amines in thioureas seem to have little or no effect
on the outcome of the product yields. Thus we planned to
investigate whether the substituents present in the aryl rings
would influence the reaction path. Besides being an oxidising
agent, copper(II) salts are also thiophilic in nature, so the
presence of activated substituents in the aryl rings of thioureas
(Tu) might promote an intramolecular aromatic electrophilic
substitution (Hugerschoff path), path-b as was observed using
bromine equivalent or iodine.1 Alternatively, some of the
substrates might exert C–H activation (path-b) giving 2-ami-
nobenzothiazole. With this objective thioureas derived from
p-methyl phenylisothiocyanate (2) and morpholine (a) was
treated with CuBr2 and the product isolated after an aqueous
ammonia treatment was found to be the Tag product (2a)
(Table 1) exclusively and no traces of corresponding 2-amino-
benzothiazole was observed. Similarly thioureas derived from
p-methyl phenylisothiocyanate (2) and other secondary amines
such as thiomorpholine (b), N-phenyl piperazine (c) gave only
their respective Tag products (2b) and (2c). Thioureas derived
2-Halo substituted thioureas undergoing copper catalysis
are prone to intramolecular heteroarylation via a dehalogena-
tive path.5,6,8b,10 All the substrates examined in Table 1 and 2
are devoid of 2-halosubstituents thus we wish to examine the
effect of 2-halo substituents on the outcome of the products
where all the three possibilities (path-a, path-b and path-c)
exist equally as shown in Scheme 1. Recently, we have
demonstrated that the use of Cu(I) gave 2-aminobenzothia-
zoles via a dehalogenative path for the entire range of 2-halo
(–F, –Cl, –Br, and –I) substituted thioureas.5 It is further
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Table 2 Synthesis of thioamidoguanidino products from arylisothiocyanates and sec aminesa
Substrate
Product
Yieldb Substrate
Product
Yieldb
92%
91%
79%
78%
82%
71%
85%
70%
79%
93%
74%
80%
a
b
Reaction monitored by TLC. Confirmed by IR, 1H, 13C NMR spectra.
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Table 3 Synthesis of thioamidoguanidino products from arylisothiocyanates and sec aminesa
Substrate
Product
Yieldb
Substrate
Product
Yieldb
86%
73%
92%
87%
74%
83%
81%
85%
a
b
Reaction monitored by TLC. Confirmed by IR, 1H, 13C NMR spectra.
demonstrated that the dehaloganative path is preferred even
for less reactive halogens (–F, –Cl) using CuO nanoparticles in
an aqueous medium at an elevated temperature.6 In the
present case thioureas derived from 2-fluoro phenylisothio-
cyanate with morpholine (a) and piperidine (d) when treated
with Cu(II) salt gave Tag products (11a) and (11d) and not the
expected 2-aminobenzothiazoles. Thus with lesser reactive
halogens (–F, –Cl) Cu(II) behaves better as an oxidising (S–S
bond forming) agent at room temperature resulting in the
formation of Tag. Formation of 2-aminobenzothiazoles were
not observed either via an intramolecular C–S bond forming
path (dehalogenation path-a, Scheme 1) or via a C–H activation
path (path-b, Scheme 1). Thus, the thioureas derived from
isothiocyanates (12), (13) and (14) and aliphatic secondary
amines such as morpholine (a), thiomorpholine (b), N-phenyl
piperazine (c) and piperidine (d) all gave only their corre-
sponding Tag products as shown in Table 3 confirming the
preferential oxidative path for 2-halo (–F, –Cl) possessing
substrates.
The results in Table 3 are in contrast to our recent reports
where Cu as a catalyst has more propensities towards
intramolecular C–S bond formation via a dehalogenative path
even for lesser activated halogens (such as –F, –Cl).5 A reaction
temperature of 80 uC was used in our previous investigation
with a catalyst loading of 5 mol%, but in the present case the
reaction is carried out at room temperature requiring 30 mol%
of the catalyst. Thus an increase in the temperature to 80 uC
might result in the formation of 2-aminobenzothiazoles from
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Table 4 Synthesis of 2-aminobenzothiazoles from 2-halo arylisothiocyanates and sec aminesa
Substrate
Productb,c,d
Substrate
Productb,c,d
a
b
c
d
Isolated yield. Confirmed by IR, 1H, 13C NMR spectra. CuBr2 (5 mol%), Na2CO3 (1 equiv.), DMSO, 85 uC, 22 h, (when X = F, Cl). CuBr2
(2–5 mol%), 0.5–2 h, room temperature (when X = Br, I).
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dehalogenative path is preferred giving 2-aminobenzothiazole,
while a lower temperature favors an oxidative dimerisation
path giving Tag products. It is well documented that thioureas
derived from more activated halogens such as 2–Br or 2–I
undergo intramolecular C–S bond formation under ligand and
catalyst free conditions at 130 uC in the presence of Cs2CO3.11
For 2-bromo thiourea derived from 2-bromo phenylisothiocya-
nate (17) and morpholine (a) the reaction proceeded at room
temperature with just 5 mol% of the catalyst. It may be
mentioned here that the use of EDPBT gave exclusively the
anti-Hugerschoff product (Tag) for the same substrate. Other
2-bromo substituted thioureas derived from respective iso-
thiocyanates (18), (19) and (20) and secondary amines such as,
morpholine (a) and piperidine (d) gave the respective 2-amino
benzothiazoles via a dehalogenetive path as shown in Table 4.
2-Iodo thioureas derived from respective arylisothiocyanates
(21), (22) and (23) and secondary amines morpholine (a) and
piperidine (d) were found to be much more reactive than their
bromo analogues and the reaction goes at room temperature
with just 2 mol% of the catalyst.
A proposed catalytic cycle for the synthesis of 2-amino
benzothiazole using Cu(II) can be envisaged taking cues from
the literature as shown in Scheme 3. The copper(II) salt is
initially reduced in situ to a copper(I) species by thiourea.12 Co-
ordination of thioureas with thiophilic copper(I) gives inter-
mediate (A) which is then followed by an oxidative addition
giving a copper(III) intermediate (B). Subsequent reductive
elimination provides benzothiazole with concomitant regen-
eration of catalytic copper species for the next cycle.
Fig. 2 ORTEP view (30% probability ellipsoids) of compound 159a.
2-halo (–F, –Cl) thioureas via a dehalogenative path as was
observed earlier.5 With this objective the in situ generated
thiourea obtained by reacting 2-fluoro phenylisothiocyanate
(11) and morpholine (a) when treated with a catalytic quantity
(5 mol%) of CuBr2 at 80 uC gave 2-aminobenzothiazole (119a)
in a poor yield of 40% when EtOAc : H2O was used as the
solvent. Thus differential reactivity of 2-fluoro thiourea was
observed at two different temperatures, at 80 uC path-a is
followed and at room temperature path-c is followed
(Scheme 1). Further experiments revealed that the use of
DMSO as the solvent gave superior yield 75% of (119a)
compared to other solvents such as CH3CN, DMF, toluene,
dioxane, EtOH and DMA tested. Similarly the thiourea
generated from 2-fluoro phenylisothiocyanate (11) and piper-
idine (d) gave corresponding 2-aminobenzothiazole (119d) in
good yield. This dehalogenative strategy at higher tempera-
tures was also equally successful for other in situ generated
2-fluoro thioureas as shown in Table 4. The structure of the
product (159a) has been further confirmed by crystal X-ray
crystallography (Fig. 2). Not surprisingly the in situ generated
2-chloro thioureas underwent similar dehalogenative path
giving the corresponding 2-aminobenzothiazoles as shown in
Table 4. From this study it is clear that using a catalytic
Conclusion
In conclusion 2-haloaryl-sec-alkyl unsymmetrical thioureas (Tu)
(halo = –F, –Cl) with a catalytic amount of Cu(II) salt at room
temperature gives thioamidoguanidino (Tag) which is obtained
via a oxidative dimerisation followed by an imine-disulfide
rearrangement. Changing the reaction temperature to 80 uC
gives 2-aminobenzothiazole via a dehelogenative path and not
by the Hugerschoff path involving an electrophilic substitution
reaction. For thioureas containing reactive ortho halogens such
as (–Br, –I) the reaction proceeds at room temperature giving
2-aminobenzothiazoles via a dehalogenative path. Failure to
transform thiourea (Tu) to Tag with Cu(I) salts suggests the
requirement of oxidising Cu(II) salts for this oxidative transfor-
mation. Thus this method gives an easy access to a variety of
Tag moieties using environmentally benign reagents. These Tag
moieties might find applications as metal scavengers and may
be used as vulcanising agents. Mild reaction conditions, high
yields and tolerance of various functional groups are some of
the main attributes of this methodology.
amount of CuBr2 (5 mol%) at higher temperatures
a
General experimental procedure
Phenylisothiocyanates were prepared using our greener
procedures.13
Scheme 3 Proposed mechanism for the formation of 2-aminobenzothiazole.
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Experimental procedure for the synthesis of anti-Hugerschoff
product (1a)
References
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(20%, 10 mL), was added to the above ethyl acetate suspended
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room temperature. During this time (10 min) the suspended
insoluble yellow solid got dissolved into the ethyl acetate layer
leaving the ammonia layer blue in color. The ethyl acetate layer
was separated and dried over anhydrous Na2SO4 and concen-
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by recrystallisation using ethyl acetate and pentane (9 : 1) (551
mg, 89%). Alternatively, if desired the product can be purified
by passing through silica gel column and eluted with
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General procedure for preparation of 2-morpholin benzo[d]-
thiazole (119a) from N-(2-fluoro phenyl)morpholine-4-
carbothiamide (5a) using CuBr2.
2-Fluoro phenylisothiocyanate (11) (2 mmol) in DMSO (2 mL)
was added morpholine (2 mmol) and stirred at room
temperature complete formation of N-(2-fluoro phenyl) mor-
pholine-4-carbothiamide 119a was observed within 15 min. To
this was added Na2CO3 (2 mmol), CuBr2 (0.01 mmol, 5 mol%)
and the reaction mixture was heated in an oil bath at 80 uC.
The progress of the reaction was monitored by TLC. After 22 h
the reaction mixture was cooled to room temperature and
diluted with ethyl acetate (10 mL). The ethyl acetate layer was
dried over anhydrous Na2SO4 and the solvent was removed
under reduced pressure. The crude product was purified over a
column of silica gel with EtOAc : pentane (2 : 8) as the eluents
to give the product 1a in 0.330 g 75% isolated yield.
Acknowledgements
11 E. Feng, H. Huang, Y. Zhou, D. Ye, H. Jiang and H. Liu, J.
Comb. Chem., 2010, 12, 422.
B. K. P acknowledges the support of this research by the
Department of Science and Technology (DST) (SR/S1/OC-79/
2009), New Delhi, and the Council of Scientific and Industrial
Research (CSIR) (01(2270)/08/EMR-II). SKS and NK thank CSIR
for fellowships. Thanks are due to Central Instruments Facility
(CIF) IIT Guwahati for NMR spectra and DST-FIST for XRD
facility.
12 For the reduction of copper(II) salts to copper(I) species
using thiourea, see: (a) G. A. Bowmaker, J. V. Hanna,
C. Pakawatchai, B. W. Skelton, Y. Thanyasirikul and A.
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446 | RSC Adv., 2013, 3, 438–446
This journal is ß The Royal Society of Chemistry 2013