Catalytic Assessment of Copper(I) Complexes and a Polymer Analog towards the One-Pot Synthesis of Imines and Quinoxalines

Article abstract of DOI:10.1002/ejic.201900555

Three copper(I) complexes, [CuCl(L)(PPh₃)₂] [L = FL (1), BL (2) or TL (3)] were prepared from [(PPh₃)₂Cu(μ-Cl)₂Cu(PPh₃)] and N-carbamothioylfuran-2-carboxamide (FL), N-carbamothioylbenzamide (BL) or N-carbamothioylthiophene-2-carboxamide (TL) ligands in benzene and four-coordinated tetrahedral copper complexes were well characterized by various spectroscopic techniques (UV/Vis, FT-IR, ¹H NMR, ¹³C NMR and ³¹P NMR). The molecular structure of the ligands (FL and BL) and complexes was established from single-crystal X-ray diffraction studies. Copper complexes have been shown to catalyse the one-pot synthesis of imines and quinoxalines. Heterogenized catalyst (4) was prepared by reacting more active complex 3 with polystyrene supported triphenylphosphane, and characterized by elemental analyses, and DRS-UV, FT-IR, ICP-OES, and solid-state NMR techniques. Catalytic activity of the complexes (3 and 4) was tested in the formation of imines from alcohols and amines, and quinoxalines from hydroxy ketones and diamines. Heterogeneity and reusability of catalyst 4 were evaluated, and the catalyst can be reused for four runs without any loss in activity.

Full text of DOI:10.1002/ejic.201900555

A Journal of
Accepted Article
Title: Catalytic Assessment of Copper(I) Complexes and its Polymer
Analog Towards One-Pot Synthesis of Imines and Quinoxalines
Authors: Dharmalingam Sindhuja, Punitharaj Vasanthakumar,
Nattamai Bhuvanesh, and R Karvembu
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201900555
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10.1002/ejic.201900555
European Journal of Inorganic Chemistry
Catalytic Assessment of Copper(I) Complexes and its Polymer Analog
Towards One-Pot Synthesis of Imines and Quinoxalines
Dharmalingam Sindhuja,a Punitharaj Vasanthakumar,a Dr. Nattamai Bhuvaneshb and
Prof. Dr. Ramasamy Karvembua,*
a Department of Chemistry, National Institute of Technology, Tiruchirappalli, 620015 India
b Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States
E-mail: kar@nitt.edu
Keywords: Copper(I) complexes, Acylthiourea ligands, Homogeneous and heterogeneous
catalysis, Imines and quinoxalines
Three copper(I) complexes, [CuCl(L)(PPh3)2] [L = FL (1), BL (2) or TL (3)] were prepared
from [(PPh3)2Cu(-Cl)2Cu(PPh3)] and N-carbamothioylfuran-2-carboxamide (FL), N-
carbamothioylbenzamide (BL) or N-carbamothioylfuran-2-carboxamide (TL) ligands in
benzene and four-coordinated tetrahedral copper complexes were well characterized by various
spectroscopic techniques (UV-Vis, FT-IR, 1H NMR, 13C NMR and 31P NMR). The molecular
structure of the ligands (FL and BL) and complexes was established from single crystal X-ray
diffraction studies. Copper complexes have been shown to catalyse one pot synthesis of imines
and quinoxalines. Heterogenized catalyst (4) was prepared by reacting more active complex 3
with polystyrene supported triphenylphosphine, and characterized by elemental analyses, and
DRS-UV, FT-IR, ICP-OES and solid-state NMR techniques. Catalytic activity of the
complexes (3 and 4) was tested in the formation of imines from alcohols and amines, and
quinoxalines from hydroxy ketones and diamines. Heterogeneity and reusability of catalyst 4
were evaluated, and the catalyst can be reused for four runs without any loss in activity.
Introduction
Imines are key intermediates used for the synthesis of various compounds which are useful in
the field of agriculture and pharmaceuticals.[1,2] Imines have been synthesized by the
condensation of carbonyl compounds with amines in the presence of Lewis acid catalyst. There
have been considerable efforts to develop methods for the synthesis of imines, and the
following three non-traditional approaches attracted a great deal of attention. They are (1) the
direct coupling of alcohols and amines under aerobic or anaerobic condition and the by-
products are water and hydrogen, (2) oxidation of primary amines to imines through NH-imine
intermediate and (3) the oxidative dehydrogenation of secondary amines.[3] The direct coupling
of alcohols and amines were catalyzed by various noble (Ru,[4,5] Pd,[6,7] Au[8,9] and Rh[10]) as
well as non-noble metals (Co,[11] Fe[12] and Cu[13–16]) under air or inert atmosphere, and
1
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10.1002/ejic.201900555
European Journal of Inorganic Chemistry
heterogeneous catalysts[17–20] were also reported. The catalysts promote direct dehydrogenation
of alcohols followed by coupling. However, the reports signified the need for additives like
TEMPO or O2 supply for reaction progress. Use of additives, less stability of the catalysts and
harsh reaction conditions hindered the applicability of most of these systems in industries.
Hence, we were interested in developing a stable non-noble metal catalyst for the one pot
synthesis of imines without utilizing harsh conditions. Copper is one of the abundant and cheap
metals known and the activity of copper complexes are well envisioned from the literature
reports,[21–26] and it is well comparable to the activity of platinum group complexes in various
catalytic applications.[27]
Acylthiourea, a simple ligand derived from readily available and inexpensive starting
materials, is well known for its versatile coordination via three donor atoms (O, N and S).
Complexes of platinum,[28,29] ruthenium,[30–33] palladium,[34] cobalt[35] and copper[36,37]
containing acylthiourea are known to have biological and catalytic potentials. In recent years
there is a widespread demand for architecting heterogeneous catalysts because of their inherent
advantages such as easy removal from reaction medium, recyclability, cleaner process and no
metal contamination with the product. Nowadays, researchers adopt different strategies to
overcome the disadvantages of heterogeneous catalysts such as low activity and selectivity.
One of the most viable solutions is to immobilize the metal complexes on inert supports like
alumina, silica, etc.[38–47] Polymer bound triphenylphosphine can be used as a grafting ligand
to make the complex heterogeneous.[48] Resin bound ruthenium and cobalt phosphine
complexes have already been reported as catalysts for transfer hydrogenation, hydrocarbon
oxidation and acid anhydride synthesis.[49,50] So we desired to use the strategy of anchoring the
active Cu-acylthiourea complex to a polymer bound triphenylphosphine.
The scope of the present catalytic system is also extended for the preparation of
quinoxalines, a class of N-heterocyclic compounds which were extensively studied due to their
broad range of biological properties and their applications in pharmaceutical as well as
agrochemical arenas.[51] Jian and co-workers described various routes to access quinoxaline
motif.[52] To overcome the difficulty of handling various starting materials, focus towards
development of simple catalytic system for the preparation of quinoxalines is essential. We
have developed homogenous copper(I) acylthiourea catalysts for imine and quinoxaline
synthesis and the most active complex was heterogenized. The activity of homogeneous and
heterogenized catalysts was compared, and the results are described.
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Results and discussion
Synthesis and characterization of the ligands
The synthetic protocol of acylthiourea ligands (FL, BL and TL) and their Cu(I)
complexes (1, 2 and 3) was reported earlier (Scheme 1).[53] The ligands were prepared by
refluxing corresponding acyl chloride with potassium thiocyanate in acetone for an hour
followed by the addition of liquid ammonia. They were soluble in organic solvents, insoluble
in water and recrystallized from the corresponding acetonitrile solution.
R
Ligand
FL
BL
TL
Scheme 1. Synthesis of the ligands
The FT-IR spectrum of free ligands showed N−H and NH2 peaks at 3207-3225 cm-1
and 3139-3340 cm-1 respectively. The carbonyl and thiocarbonyl stretching frequencies were
1
observed at 1654-1680 and 1237-1264 cm-1 respectively. The H NMR spectra of ligands
recorded in DMSO-d6 displayed signals of N−H and NH2 at 10.9-11.2 and 9.5-9.8 ppm
13
respectively, as singlets. All the aromatic protons reside in the region 6.7-8.0 ppm. The C
NMR spectra of ligands revealed the presence of carbonyl, thiocarbonyl and aromatic carbons
by showing signals at 158-168, 182-183 and 113-149 ppm respectively.
The crystal structures of ligands are provided in Figs. 1 and 2. Good quality crystals of
the ligands were obtained by slow evaporation of their acetonitrile solutions. Ligands FL and
BL crystallized in the monoclinic and triclinic crystal system with the space group P121/c1 and
P-1 respectively. The crystal structure of ligands revealed the existence of strong
intramolecular N−H....O hydrogen bonding with the bond length of 1.94-1.98 Å.
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European Journal of Inorganic Chemistry
Fig. 1 Molecular structure of FL. Selected bond lengths (Å) and angles (): C(5)−O(2) 1.224(2),
C(6)−S(1) 1.6813(17), S(1)−C(6)−N(1) 118.15(12), S(1)−C(6)−N(2) 123.47(13),
N(2)−C(6)−N(1) 118.08(15)
Fig. 2 Molecular structure of BL. Selected bond lengths (Å) and angles (): C(7)−O(1)
1.226(2), C(8)−S(1) 1.6882(19), N(2)−C(8)−S(1) 122.35(15), N(1)−C(8)−S(1) 119.90(14),
N(1)−C(8)−N(2) 117.75(17)
Synthesis and characterization of the complexes
Treatment of the ligands with [(PPh3)2Cu(-Cl)2Cu(PPh3)] (1:1) in benzene in presence
of triphenylphosphine (1 equiv.) afforded yellow colored complexes with an appreciable yield
(Scheme 2).
R
Complex
1
2
3
Scheme 2. Synthesis of the complexes
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European Journal of Inorganic Chemistry
FT-IR spectral data of the acylthiourea ligands and their complexes were compared to
confirm the coordination of the ligands to copper ion. In the spectra of complexes 1-3, sharp
peaks due to N−H and NH2 were observed at 3216-3222 and 3153-3323 cm-1 respectively,
along with carbonyl (C=O) and thiocarbonyl (C=S) bands at 1663-1687 and 1182-1249 cm-1
respectively. The stretching frequency of C=S in the complexes was slightly reduced from the
value found in the spectra of the ligands, while there was no frequency alteration with respect
to C=O, N−H and NH2 stretching. This fact signified the coordination of acylthiourea ligands
to copper ion via S atom only (neutral monodentate fashion). Complexation was further
supported by the occurrence of three new sharp bands of triphenylphosphine ligands at 1423-
1
1430, 1091-1099 and 690-694 cm-1. In the H NMR spectra of complexes, signals of amidic
and thioamidic protons were appeared in the deshielded regions at 10.3-11.5 and 8.6-10.2 ppm
respectively compared to the ligands. The signals due to aromatic protons were observed as a
multiplet at 6.7-8.5 ppm. The 13C NMR spectra of complexes showed signals due to the
presence of carbonyl (157-168 ppm), thiocarbonyl (180-181 ppm) and aromatic (113-145 ppm)
carbons. Complexes 1-3 showed only a signal at 29.3, 29.2 and 25.5 ppm respectively in their
31P NMR spectra.
Yellow crystals of the complexes were grown by evaporation of their chloroform or
dichloromethane solutions at room temperature. Complexes 1 and 3 crystallized in monoclinic
fashion with P121/n1 space group, whereas 2 crystallized in triclinic fashion with P-1 space
group. The molecular structures of complexes are shown in Figs. 3-5. Ligands were
coordinated via only S atom as expected from the other spectroscopic data. The coordination
of ligand to metal resulted in the slight elongation of C−S bond length. The other notable bond
lengths such as Cu−Cl, Cu−S and Cu−P were in the expected ranges of 2.316-2.335, 2.363-
2.382 and 2.481-2.632 Å respectively.[54–56] The bond angle (100-128) around copper
indicated the distorted tetrahedral geometry in all the complexes. Two types of intramolecular
hydrogen bonding (N−H....O, 1.94-1.96 Å; N−H....Cl, 2.32-2.52 Å) were observed in the
complexes.
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European Journal of Inorganic Chemistry
Fig. 3 Molecular structure of 1. Selected bond lengths (Å) and angles (): Cu(1)−Cl(1)
2.316(6), Cu(1)−S(1) 2.3635(6), Cu(1)−P(1) 2.2632(6), Cu(1)−P(2) 2.2481(6), C(1)−S(1)
1.690(2), C(2)−O(1) 1.222(3), N(1)−H(1) 0.8800, N(2)−H(2A) 0.8800, N(2)−H(2B) 0.8800,
Cl(1)−Cu(1)−S(1) 109.93(2), Cl(1)−Cu(1)−P(1) 101.45(2), S(1)−Cu(1)−P(1) 103.56(2),
S(1)−Cu(1)−P(2) 108.40(2), Cl(1)−Cu(1)−P(2) 104.14(2), P(1)−Cu(1)−P(2) 128.49(2),
N(1)−C(1)−S(1) 120.91(16), N(1)−C(1)−N(2) 118.0(2), N(2)−C(1)−S(1) 121.03(17)
Fig. 4 Molecular structure of 2. Selected bond lengths (Å) and angles (): Cu(1)−Cl(1)
2.3244(5), Cu(1)−S(1) 2.3827(5), Cu(1)−P(1) 2.2890(5), Cu(1)−P(2) 2.2770(5), C(1)−S(1)
1.6813(17), C(2)−O(1) 1.220(2), N(1)−H(1) 0.8800, N(2)−H(2A) 0.8800, N(2)−H(2B) 0.8800,
Cl(1)−Cu(1)−S(1) 103.93(18), Cl(1)−Cu(1)−P(1) 114.23(19), S(1)−Cu(1)−P(1) 103.64(19),
S(1)−Cu(1)−P(2) 105.80(19), Cl(1)−Cu(1)−P(2) 105.33(19), P(1)−Cu(1)−P(2) 122.13(19),
N(1)−C(1)−S(1) 120.40(15), N(1)−C(1)−N(2) 118.85(17), N(2)−C(1)−S(1) 120.74(17)
6
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European Journal of Inorganic Chemistry
Fig. 5 Molecular structure of 3. Selected bond lengths (Å) and angles (): Cu(1)−Cl(1)
2.3354(6), Cu(1)−S(1) 2.3636(6), Cu(1)−P(1) 2.2528(6), Cu(1)−P(2) 2.2643(6), C(1)−S(1)
1.697(2), C(2)−O(1) 1.222(3), N(1)−H(1) 0.8800, N(2)−H(2A) 0.8800, N(2)−H(2B) 0.8800,
Cl(1)−Cu(1)−S(1) 110.72(2), Cl(1)−Cu(1)−P(1) 102.70(2), S(1)−Cu(1)−P(1) 109.58(2),
S(1)−Cu(1)−P(2) 104.08(2), Cl(1)−Cu(1)−P(2) 100.12(2), P(1)−Cu(1)−P(2) 128.69(2),
N(1)−C(1)−S(1) 120.48(17), N(1)−C(1)−N(2) 118.5(2), N(2)−C(1)−S(1) 120.98(18)
Heterogenization of an active complex
Polymer anchored complex (4) was prepared from 3 according to the procedure
reported earlier.[49] (Scheme 3) Since the heterogenized complex was insoluble, its
characterization was limited to elemental analyses, DRS-UV, FT-IR, ICP-OES and solid-state
NMR (31P and 13C) techniques. The elemental analyses results indicated the presence of carbon
(67 %), hydrogen (5.08 %), nitrogen (1.3 %) and sulphur (2.32 %) in 4, which confirmed the
substitution of triphenylphosphine ligands by polystyrene bound triphenylphosphine. DRS-UV
spectrum of 4 showed two transitions at 267 and 389 nm corresponding to →* and metal to
ligand charge transfer respectively.[57] FT-IR spectrum of 4 and polystyrene bound
triphenylphosphine are shown in Fig. S15. The spectrum of polystyrene bound
triphenylphosphine showed aromatic and aliphatic C−H stretching frequencies at 3062 and
2925 cm-1 respectively. In the spectrum of 4, aromatic and aliphatic C−H stretching frequencies
were observed at 3048 and 2913 cm-1 respectively. In addition, characteristic peaks of carbonyl
and thiocarbonyl stretching were seen at 1665 and 1243 cm-1 respectively. Further, bands due
to polymer bound triphenylphosphine were observed at 1439, 1090 and 692 cm-1. ICP-OES
analysis was done to find the copper amount in 4. It showed that 0.18206 mmol/g of copper
was present after anchoring. 31P and 13C solid-state NMR spectra were recorded to confirm the
31
incorporation of polymer backbone in 4. P NMR spectrum showed a signal at 29.17 ppm
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European Journal of Inorganic Chemistry
which indicated replacement of triphenylphosphine from the complex and anchoring of the
complex on polystyrene backbone. During the anchoring process, replacement of only one
triphenylphosphine by polystyrene supported triphenylphosphine cannot be ruled out. Fig. S17
13
displays C MAS NMR spectrum of 4 in which C=S and C=O carbons resonated at 208.2-
195.0 ppm, and aromatic carbons resonated in the region 119.6-163.8 ppm.[58,59] In addition,
aliphatic carbons present in the polymeric chain were elucidated by the presence of signals at
11.1-66.8 ppm.
Scheme 3. Synthesis of polystyrene anchored copper(I) complex (4)
Catalytic activities
Optimization of reaction conditions
The optimum conditions were arrived by reacting benchmark substrates benzyl alcohol
and aniline. The conversions were monitored by GC or GC-MS. Initially, screening of the
catalysts (1-3) was done, and good conversion of alcohol was achieved in the case of complex
3. Similarly, the catalyst amount was varied from 0.0-0.5 mol %, and complete conversion of
alcohol and better selectivity of imine were achieved in the case of 0.5 mol % of the catalyst.
During the course of finding suitable base, it was found that the carbonate bases gave poor
selectivity (<4 %). Hence, hydroxide bases and KOtBu were examined, in which KOtBu
offered excellent conversion in 3 h. Effect of various solvents was studied. Among the solvents
tested, dioxane and toluene were found to be good. To predict the optimum temperature, the
reaction temperature was varied from ambient to 90 C, the satisfied conversion was observed
at 90 C (Table S4). With the optimum reaction conditions in hand, the scope of the present
catalytic system was extended to various alcohols and amines. Complexes 3 and 4 were
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European Journal of Inorganic Chemistry
employed for imine synthesis and the activity of the catalysts was compared in terms of
conversion and selectivity (Table 1).
Synthesis of imines
Catalytic activity is tested for a variety of benzyl alcohols with electron withdrawing or
releasing substituents. All alcoholic substrates undergo the dehydrogenative coupling with
amines smoothly regardless of the substituents in the aromatic ring. N-benzylideneaniline (7a)
was obtained by reacting benzyl alcohol with aniline in good selectivity (97 %) in 3 h. N-(4-
methoxy)benzylideneaniline (7b) and N-(4-methyl)benzylideneaniline (7c) were obtained with
the selectivity of 92 and 96 % respectively, from the corresponding benzyl alcohol and aniline.
Similarly, halide (Br and F) substituted benzyl alcohols reacted with aniline to give
corresponding imines (7d and 7e) with an excellent selectivity. Complexes 3 and 4 catalyzed
the coupling of benzyl alcohol with 4-trifluromethyl aniline to give selectively N-
(benzylidene)-4-trifluromethylaniline. Benzyl alcohol underwent coupling with 4-
ethoxyaniline to yield N-(benzylidene)-4-ethoxyaniline with 100 % selectivity. Notably,
biomass derived furfuryl alcohol also coupled with aniline to give N-(furan-2yl-
methylene)aniline. Heterogenized complex 4 gave N-(furan-2yl-methylene)aniline with an
excellent selectivity of 100 %, but complex 3 showed only 69 % selectivity.
Further, benzyl alcohol combined with substituted benzyl amines and cyclohexyl
amine. The reaction between 4-methylbenzyl alcohol and 4-methylbenzyl amine in presence
of 3 or 4 gave corresponding imine (7i) with 100 % selectivity. Benzyl alcohol also coupled
with benzyl amine to give corresponding imine (7j) with 96 % selectivity in presence of 3,
whereas 100 % selectivity was observed with 4. Complexes 3 and 4 also efficiently catalysed
the dehydrogenative coupling of 4-bromobenzyl alcohol with benzyl amine. The selectivity of
imine (7k) was excellent (100 %) with 4, whereas complex 3-catalysed reaction yielded the
secondary amine along with the desired imine (36 %). Coupling of benzyl alcohol with
cyclohexyl amine was carried out, N-benzylidenecyclohexylamine (7l) was obtained with the
selectivity of 100 and 74 % while using 3 and 4 respectively. The conversion during the
formation of 7l in presence of 3 was only 89 %. There was no further increase in conversion
even after extending the time. Further, aliphatic and unsaturated alcohols (propargyl alcohol
and cinnamyl alcohol) were tested, and no conversion was seen. Comparison of catalytic
activity of the present catalysts with that of the reported catalysts is given in table 4.
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European Journal of Inorganic Chemistry
Table 1 Copper complex-catalyzed imine synthesisa
3
4
Entry
Imine
Time Conversion/ Time Conversion/
Selectivityb
(%)
(h)
Selectivityb
(%)
(h)
1
3
99/100
99/92
3
99/100
99/100
7a
2
8
10
7b
7c
3
4
8
6
99/96
99/99
9
9
99/89
99/100
7d
7e
7f
5
6
4
99/100
99/99
6
99/100
99/100
12
12
10
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7
12
12
99/100
99/69
12
12
99/100
7g
8
99/100
7h
7i
9
12
6
99/100
99/96
99/36
12
4
99/100
99/99
10
11
7j
12
12
99/100
7k
12
6
89/100
9
99/74
7l
aReaction conditions: 5 (1 mmol), 6 (1.1 mmol), 3 (0.5 mol %) or 4 (0.1 mmol of Cu), KOtBu (1 mmol),
Toluene (3 mL) bGC conversion and selectivity
Synthesis of quinoxalines
Quinoxalines were synthesized by the treatment of aromatic diamines with hydroxy
ketones (Table 2). Various diamines with electron withdrawing (Cl or NO2) or donating (OMe)
substituents were studied. Complexes 3 and 4 yielded 2, 3-diphenylquinoxaline (10a) with 95
and 100 % selectivity respectively. Similarly, 4-chlorophenylenediamine reacted with benzoin
and furoin to yield corresponding quinoxaline (10b and 10e) with an excellent selectivity in
presence of complex 3 or 4. In the case of 4-nitrophenylene diamine substrate, complex 3
yielded 10c with 100 % selectivity. While using 4, further reduction of nitro group was
observed; 2,3-diphenylquinoxaline-6-amine was formed with 100 % selectivity. Both the
catalysts gave 10d and 10f with good selectivity. The isolated yield of the product (10g and
10h) formed from the reactions between 4-methoxy-o-phenylenediamine and benzoin or furoin
was 68 or 77 % respectively. 6-chloro-2-phenylquinoxaline was obtained from 4-chloro-o-
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European Journal of Inorganic Chemistry
phenylenediamine and 2-hydroxyacetophenone with 66 and 72 % selectivity respectively in
presence of 3 and 4. Comparison was made between the present catalysts and few reported
catalysts to show their efficiency in quinoxaline synthesis (Table 5).
Table 2 Copper complex-catalyzed quinoxaline synthesisa
3
4
Time Conversion/ Time Conversion/
Entry
Quinoxaline
(h)
Selectivityb
(%)
(h)
Selectivityb
(%)
1
2
99/95
2
99/100
10a
10b
2
4
4
1
99/95
99/100
4
4
1
99/100
99/100c
99/100
3
4
10c
10d
99/100, 82d
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5
3
4
99/100
99/100
3
4
99/100
10e
10f
6
7
99/100
99/100
5
99/100,68d
5
10g
8
9
2
5
99/100,77d
2
5
99/100
99/72
10h
10i
99/66
aReaction conditions: 8 (1 mmol), 9 (1.1 mmol), 3 (0.5 mol %) or 4 (0.1 mmol of Cu), KOtBu (1 mmol),
Toluene (3 mL) bGC conversion and selectivity c2,3-diphenylquinoxaline-6-amine dIsolated yield
Hot filtration test and reusability of the heterogenized complex
To check the heterogeneity, 4 was used as a catalyst for the imine formation from benzyl
alcohol and aniline. After 1 h, the catalyst was removed from the reaction medium by
centrifugation. Centrifugate was separated into two portions, first was subjected to AAS
analysis to know copper content and leaching of only 0.5 % of total copper was observed. The
other portion was heated further for 2 h. No further conversion showed that the reaction
proceeded through heterogeneous pathway.
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Separated catalyst was washed with diethyl ether and ethanol, and then dried.
Recovered catalyst was used for imine synthesis from benzyl alcohol and aniline. Catalyst can
be used for three cycles without loss of activity and there was a little decrease in conversion
after fourth cycle (Table 3). FT-IR spectrum of the recovered catalyst was recorded and
compared with the fresh catalyst (Fig. S60). There was no significant change in the catalyst
structure.
Table 3 Reusability of 4a
Cycle
Conversion/
Selectivityb (%)
99/100
Fresh
1
2
3
4
99/100
99/100
99/100
86/100
aReaction conditions: Benzyl alcohol (1 mmol), aniline (1.1 mmol), 4 (0.1 mmol of Cu), KOtBu (1
mmol), Toluene (3 mL), 3 h bGC conversion and selectivity
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Table 4 Comparison of the catalytic activity with literature reports for the coupling of benzyl
alcohol with aniline
Entry
Catalyst
(mol %)
Solvent
Temperature Time Yield Ref.
(h)
(%)
(C)
[12]
[60]
1a
2b
Toluene
Water
80
24
96
Fe(NO3)39H2O (10)
Pd/DNA (2.9)
50
12
96
[16]
[15]
[2]
3c
4d
5e
Toluene
Acetonitrile
THF
70
RT
RT
19
12
24
96
99
81
Cu(ClO4)26H2O (5)
CuI (1)
[(η6-p-cy)2Ru2Cl2(µ-
L1)] (1)
6
7
3 (0.5)
Toluene
Toluene
90
90
3
3
96
This
work
4 (0.1 mmol)
100
This
work
c
d
aTEMPO (10 mol %), 0.5 mmol KOH, air bLiOHH2O (0.4 mmol), N2 KOH (1.5 mmol), O2 TEMPO (2 mol %),
air etBuOK (10 mol %), 4Å molecular sieves, air
Table 5 Comparison of the catalytic activity with literature reports for the coupling of
phenylene-1,2-diamine with benzoin
Entry
Catalyst
(mg/mol %)
Solvent
Temperature Time
Yield
(%)
Ref.
(h)
(C)
[61]
[62]
1a
2b
AuCNT (0.2)
Toluene/Water
Neat
RT
26
5
96
95
Amberlite IR-120H
(200 mg)
FeMPA (50 mg)
110
[63]
3c
Acetonitrile
80
3
95
4
3 (0.5)
Toluene
90
2
95
This
work
This
5
4 (0.1 mmol)
Toluene
90
2
100
work
b
aPhenylene-1,2-diamine and benzoin (0.1 mmol), NaOH (3 equiv), open flask Phenylene-1,2-diamine and
benzoin (1 mmol) cPhenylene-1,2-diamine (2 mmol) and benzoin (1 mmol), O2
15
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10.1002/ejic.201900555
European Journal of Inorganic Chemistry
Conclusions
Three tetrahedral copper(I) complexes containing acylthiourea ligand were synthesized.
Complexes were well characterized by means of spectroscopic and crystallographic
techniques. The catalytic activity of these complexes in imine and quinoxaline synthesis was
explored. Active complex was heterogenized by anchoring it over polystyrene bound
triphenylphosphine. Various analytical techniques confirmed the heterogenization of the
complex. Though the activity of both the complexes was same, heterogeneous system has the
advantage of reusability at least for three cycles. The catalytic activity was better than many
systems reported earlier.
Experimental section
Materials and physical measurements: Reagent grade chemicals were used without any
further purification. The solvents were purified by standard procedures. [(PPh3)2Cu(-
Cl)2Cu(PPh3)] was synthesized by following a literature procedure.[64] Melting points were
measured in open capillary tubes in a Sigma melting point apparatus. UV-visible spectra were
recorded using a Shimadzu UV-2600 instrument. Fourier transform infrared (FT-IR) spectra
were recorded as KBr pellets on Thermo Scientific Nicolet iS5 FT-IR spectrometer in the range
of 550-4000 cm-1. 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a
Bruker 500 MHz spectrometer in DMSO-d6 or CDCl3 with tetramethyl silane as an internal
reference. Elemental analyses were done in an Elemental Vario EL-III analyser. Copper was
quantified by using PerkinElmer Optima 5300 DV inductively coupled optical emission
spectrometer. Solid state 13C cross-polarization magic-angle spinning (CPMAS) and 31P NMR
spectra were recorded using JEOL Resonance 400 MHz spectrometer. The catalytic products
were analyzed using Shimadzu GC 2010 and Shimadzu GCMS-QP2010 Ultra instruments.
General procedure for the synthesis of the ligands: Ligands were prepared from acyl
chloride (furan-2-carbonyl chloride, benzoyl chloride or thiophene-2-carbonyl chloride),
potassium thiocyanate and ammonia according to the published procedure.[53] TL was prepared
and already reported by us.[65]
N-carbamothioylfuran-2-carboxamide (FL): Yield: 82 %. White solid. Mp.: 173 °C. UV
(CHCl3) λmax, nm: 233, 273, 294. FT-IR (KBr, cm-1): 3317, 3139 (s; (NH2)), 3216 (s; (N−H)),
1678 (s; (C=O)), 1264 (s, (C=S)). 1H NMR (500 MHz, DMSO-d6) δ, ppm: 10.9 (s, 1H), 9.6
(s, 1H), 9.5 (s, 1H), 8.0 (d, J = 0.8 Hz, 1H), 7.8 (d, J = 3.0 Hz, 1H), 6.7 (dd, J = 3.6, 1.6 Hz,
16
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10.1002/ejic.201900555
European Journal of Inorganic Chemistry
13
1H). C NMR (125 MHz, DMSO-d6) δ, ppm: 182 (C=S), 158 (C=O), 149, 145, 119, 113
(carbons of furan).
N-carbamothioylbenzamide (BL): Yield: 79 %. White solid. Mp.: 165 °C. UV (CHCl3) λmax
,
nm: 202, 238, 279. FT-IR (KBr, cm-1): 3306, 3153 (s; (NH2)), 3225 (s; (N−H)), 1680 (s;
(C=O)), 1237 (s; (C=S)). 1H NMR (500 MHz, DMSO-d6) δ, ppm: 11.2 (s, 1H), 9.8 (s, 1H),
13
9.5 (s, 1H), 7.9 (d, J = 7.3 Hz, 2H), 7.6 (t, J = 7.4 Hz, 1H), 7.5 (t, J = 7.8 Hz, 2H). C NMR
(125 MHz, DMSO-d6) δ, ppm: 183 (C=S), 168 (C=O), 133, 133, 129, 129 (aromatic carbons).
General procedure for the synthesis of the complexes: Ligand (1.5 mmol) and
triphenylphosphine (0.75 mmol) were dissolved in benzene (15 mL), and added to
[(PPh3)2Cu(-Cl)2Cu(PPh3)] (0.75 mmol) in benzene (15 mL), and stirred for 30-36 h. After
completion of the reaction, solvent was removed under reduced pressure. To the formed oily
precipitate, 25 mL of n-hexane-ethyl acetate (9:1) mixture was added and stirred well. The
product was filtered off and washed with petroleum ether to remove the traces of
triphenylphosphine. Synthesis of the complexes can also be carried out in dichloromethane or
tetrahydrofuran. Same procedure was followed to recover the complexes in pure form.
[CuCl(FL)(PPh3)2] (1): Yield: 70 %. Yellow solid. Mp.: 232 °C. Anal. Calc.
C42H36ClCuN2O2P2S (%): Calcd. C, 63.55; H, 4.57; N, 3.53; S, 4.04, Found. C, 63.59; H, 4.77;
N, 3.77; S, 4.16. UV (C2H5OH) λmax, nm (ε, dm3mol-1cm-1): 239 (30303), 265 (32700), 293
(21100). FT-IR (KBr, cm-1): 3323, 3168 (s; (NH2)), 3222 (s; (N−H)), 1687 (s; (C=O)), 1249
(s; (C=S)), 1430, 1099 and 690 (s; (PPh3)). 1H NMR (500 MHz, DMSO-d6) δ, ppm: 11.2 (s,
1H), 9.9 (s, 1H), 9.8 (s, 1H), 8.0 (s, 1H), 7.9 (d, J = 3.5 Hz, 1H), 7.6 (dd, J = 12.0, 7.5 Hz, 1H),
7.5-7.5 (m, 1H), 7.4 (d, J = 6.5 Hz, 6H), 7.4-7.3 (m, 22H), 6.7 (d, J = 2.2 Hz, 1H). 13C NMR
(125 MHz, CDCl3) δ, ppm: 180 (C=S), 157 (C=O), 147, 145, 134, 134, 133, 133, 130, 129,
128, 128, 120, 113 (aromatic carbons). 31P NMR (202 MHz, DMSO-d6) δ, ppm: 29.3 (PPh3).
[CuCl(BL)(PPh3)2] (2): Yield: 69 %. Yellow solid. Mp.: 202 °C. Anal. Calc.
C44H38ClCuN2OP2S (%): Calcd. C, 65.75; H, 4.77; N, 3.49; S, 3.99, C, 65.86; H, 4.93; N, 3.53;
S, 4.04. UV (C2H5OH) λmax, nm (ε, dm3mol-1cm-1): 241 (57800), 277 (32800). FT-IR (KBr,
cm-1): 3315, 3153 (s; (NH2)), 3219 (s; (N−H)), 1680 (s; (C=O)), 1182 (s; (C=S)), 1430,
1092, 694 (s; ʋ(PPh3)). 1H NMR (500 MHz, CDCl3) δ, ppm: 10.3 (s, 1H), 10.2 (s, 1H), 8.6 (s,
1H), 8.0 (d, J = 7.4 Hz, 2H), 7.8-7.6 (m, 4H), 7.6-7.4 (m, 6H), 7.4 (dd, J = 16.6, 7.7 Hz, 9H),
7.3 (t, J = 7.3 Hz, 5H), 7.3 (t, J = 7.5 Hz, 9H). 13C NMR (125 MHz, DMSO-d6) δ, ppm: 181
17
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10.1002/ejic.201900555
European Journal of Inorganic Chemistry
(C=S), 168 (C=O), 134, 134, 134, 134, 134, 132, 130, 130, 129, 129, 129 (aromatic carbons).
31P NMR (202 MHz, DMSO-d6) δ, ppm: 29.2 (PPh3).
[CuCl(TL)(PPh3)2] (3): Yield:76 %. Yellow solid. Mp.: 210 °C. Anal. Calc.
C42H36ClCuN2OP2S2 (%): Calcd. C, 62.29; H, 4.48; N, 3.46; S, 7.92, Found: C, 62.40; H, 4.42;
N, 3.60; S. 7.91. UV (C2H5OH) λmax, nm (ε, dm3mol-1cm-1): 240 (44,000), 257 (43,900), 293
(31,900). FT-IR (KBr, cm-1): 3315, 3143 (s; (NH2)), 3216 (s; (N−H)), 1663 (s; (C=O)),
1196 (s; (C=S)), 1423, 1091, 693 (s; (PPh3)). 1H NMR (500 MHz, DMSO-d6) δ, ppm: 11.5
(s, 1H), 10.0 (s, 1H), 9.8 (s, 1H), 8.5 (s, 1H), 8.0 (d, J = 4.6 Hz, 1H), 7.3 (dt, J = 16.2, 7.3 Hz,
18H), 7.2 (d, J = 6.7 Hz, 12H), 7.2 (s, 1H). 13C NMR (125 MHz, DMSO-d6) δ, ppm: 181 (C=S),
162 (C=O), 137, 136, 134, 134, 134, 134, 133, 130, 129, 129, 129 (aromatic carbons). 31P NMR
(202 MHz, DMSO-d6) δ, ppm: 25.5 (PPh3).
General procedure for the synthesis of the polymer anchored copper(I) complex
100 mg of polystyrene bound triphenylphosphine was added to the dichloromethane
solution of 0.175 mmol of 3. The resultant mixture was stirred for 12 h at 27 C. The
heterogeneous complex was obtained by filtration and the residue was washed with hexane
(3×10 mL) and dichloromethane (2×10 mL). Pale brown colour of the polymer was changed
to yellow during heterogenization of the complex.
General procedure for the synthesis of imines
The mixture of alcohol (1 mmol) and amine (1.1 mmol) was stirred in an open air at 90
°C. To the above mixture, base (1 mmol) and the catalyst (0.5 mol% of 3 or 0.1 mmol of Cu in
the form of 4) were added and stirring continued for the requisite time. The progress of the
reaction was monitored by thin layer chromatography (TLC). After completion of the reaction,
the crude was passed through pre-neutralized silica column (petroleum ether/ethyl acetate) to
obtain desirable product and it was then analyzed by GC-MS. In heterogeneous catalytic
system, the catalyst was separated by centrifugation and the centrifugate was passed through
pre-neutralized silica (petroleum ether/ethyl acetate) to get the corresponding imine.
General procedure for the synthesis of quinoxalines
Complex 3 (0.5 mol%) or 4 (0.1 mmol of Cu), base (1 mmol) and hydroxyketone (1
mmol) in toluene (3 mL) were taken in a round bottom flask. The mixture was stirred at 90 C
and diamine (1.1 mmol) was added. The stirring was continued, and reaction progress was
monitored by TLC. After completion of the reaction, reaction mixture was extracted with ethyl
18
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10.1002/ejic.201900555
European Journal of Inorganic Chemistry
acetate. Organic layers were dried over anhydrous sodium sulphate and concentrated under
vacuum. The residue was purified by column chromatography to afford quinoxaline product.
Acknowledgement
We thank BRNS (34/14/52/2014-BRNS/2069) for the financial support.
Supplementary material
Crystallographic data for the structures reported in this paper have been deposited in
Cambridge crystallographic data center (CCDC 1915328-1915332).
19
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10.1002/ejic.201900555
European Journal of Inorganic Chemistry
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For table of contents only
or
9 examples
12 examples
Copper(I) complex containing acylthiourea and its heterogeneous counterpart catalyse the
synthesis of imines and quinoxalines. Heterogeneous catalyst has the advantage of reusability.
Key topic: Catalysis by Heterogenized Copper(I) Complex.
23
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