A quick Chan-Lam C-N and C-S cross coupling at room temperature in the presence of square pyramidal [Cu(DMAP)4I]I as a catalyst

Article abstract of DOI:10.1039/c5cc04619j

A rapid and efficient protocol for C-N and C-S cross coupling has been developed using a new square pyramidal copper complex, [Cu(DMAP)₄I]I. The complex was successfully synthesized via a disproportionation reaction of CuI and DMAP in DMSO. The catalytic activity of the complex was found to be excellent for Chan-Lam coupling reaction between aryl boronic acid and amine, amide, azide or thiol. The reaction could be carried out in the presence of only 2 mol% of the copper catalyst in methanol at room temperature within a short time.

Full text of DOI:10.1039/c5cc04619j

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A quick Chan–Lam C–N and C–S cross coupling
at room temperature in the presence of square
pyramidal [Cu(DMAP)4I]I as a catalyst†
Cite this:DOI: 10.1039/c5cc04619j
Received 4th June 2015,
Accepted 2nd November 2015
Subhasish Roy, Manas Jyoti Sarma, Bishwapran Kashyap and Prodeep Phukan*
DOI: 10.1039/c5cc04619j
www.rsc.org/chemcomm
A rapid and efficient protocol for C–N and C–S cross coupling has
been developed using a new square pyramidal copper complex,
[Cu(DMAP)4I]I. The complex was successfully synthesized via a
disproportionation reaction of CuI and DMAP in DMSO. The catalytic
activity of the complex was found to be excellent for Chan–Lam
coupling reaction between aryl boronic acid and amine, amide, azide
Scheme 1 Chan–Lam coupling of amines and boronic acid.
or thiol. The reaction could be carried out in the presence of only using a square pyramidal [Cu(DMAP)4I]I complex as a catalyst
2 mol% of the copper catalyst in methanol at room temperature (Scheme 1).
within a short time.
The square pyramidal copper complex, [Cu(DMAP)4I]I, was
synthesized by disproportionation reaction of CuI in the presence of
The formation of carbon–hetero atom bonds is a very important DMAP. Initially, different solvents such as methanol, acetonitrile, a
transformation in organic synthesis. During the past several 1 : 1 mixture of methanol and acetonitrile and DMSO were examined
decades, a variety of methods has been developed for C–N and to find suitable conditions for the formation of the copper complex.
C–S bond forming reactions using different catalysts derived Synthesis was carried out by stirring a mixture of CuI and DMAP
from transition metals such as Pd, Fe, Ni, and Cu.1 In 1998, a in a 1 : 4 molar ratio in a particular solvent at room temperature
mild protocol for Cu-mediated oxidative amination of aryl for 3 hours. The mixture was further allowed to stand at room
boronic acid was developed independently by Chan, Evans temperature. After 2 days, the pure crystal of the [Cu(DMAP)4I]I
and Lam.2 Thereafter, a number of modifications have been made complex was isolated in 85% yield. The complex could be
to improve the yield and efficiency of Chan–Lam reaction using isolated only when, DMSO was used as solvent. It was characterized
different copper salts in the presence of various ligands, which by UV-Vis spectroscopy, FT-IR, ESR analysis, DSC analysis and
include important examples such as Cu(OAc)2,3 CuCl,4 Cu(OTf)2,5 single crystal XRD. The DSC study shows that the compound is
CuF2,6 and the Cu-b-Cyclodextrin complex.7 Besides these examples stable up to 131 1C. The appearance of a strong peak at 644 nm in
on Chan–Lam coupling reaction, there are also other reports in the UV-Vis spectrum indicates the formation of Cu(II) species. The
the literature for C–N and C–S bond formation reactions in the ESR spectrum of the complex shows the presence of an X-band,
presence of various metals such as Cu,8 Pd,9 Ni,10 and Fe.11 which confirms that the copper atom is in +2 oxidation state and
However, long reaction time and high reaction temperature are the complex has C4V symmetry. The structure of the complex was
the most common issues that caused inconveniences in under- determined by single crystal X-ray diffraction studies. CCDC
taking many of these methods for Chan–Lam coupling. Hence, 1040412. The single crystal X-ray structure indicates that the
modifications have been proposed to improve the process complex is square pyramidal in shape, where all the four equatorial
using microwave irradiation which could significantly cut- positions are occupied by DMAP ligands and the axial position
short the reaction time.12 In this communication, we report a by an iodine atom. Another iodine atom was found to be in the
very fast and efficient protocol for the Chan–Lam reaction free-state, which resides outside the coordination sphere (Fig. 1).
Next, we have carried out an investigation to study the catalytic
Department of Chemistry, Gauhati University, Guwahati, Assam, 781014, India.
activity of the synthesized copper complex for Chan–Lam coupling.
An initial experiment was carried out by taking phenyl boronic acid
and N-methyl benzyl amine as model substrates in the presence of
E-mail: pphukan@yahoo.com
† Electronic supplementary information (ESI) available: 1H-NMR, 13C-NMR of
organic compounds and IR, TGA, DSC, EPR, UV and crystallographic data of the
the copper complex (1 mol%). The reaction was performed by
stirring a mixture of phenyl boronic acid (1 mmol), N-methyl benzyl
Cu-complex. CCDC 1040412. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c5cc04619j
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Table 2 Cham–Lan coupling of various amines with different boronic
acidsa
Fig. 1 Ortep diagram of the [Cu(DMAP)4I]I complex with a 50% probability
elipsoid.
Table 1 Optimization experiments for coupling of phenyl boronic acid
and N-methyl benzyl aminea
Entry [Cu(DMAP)I]I (mol%) Solvent (2 mL) Time (Min) Yieldb (%)
1
2
3
4
5
6
7
1
2
4
2
2
2
2
MeOH
MeOH
MeOH
EtOH
H2O
MeCN
DMSO
15
5
5
69
93
95
85
83
70
17
8
60
20
150
a
Reaction conditions: phenylboronic acid (1 mmol), N-methylbenzyl-
b
amine (1 mmol), solvent (2 mL), rt. Isolated yield.
amine (1 mmol) and the catalyst (0.01 mmol) in methanol at
room temperature (Table 1). The progress of the reaction was
monitored by using TLC. After 15 min of reaction, the reaction
produced the desired coupling product in 69% yield. Thereafter,
the reaction was studied by increasing the catalyst amount to
2 mol%. To our surprise, the reaction was over in just 5 min with
an excellent yield of 93%. A further increase of the catalyst
amount to 4 mol% could not improve the result to a better
extent. The study was further carried out using different solvents
such as ethanol, water, and DMSO. Interestingly, the reaction
also proceeds quite efficiently in water at room temperature for
1 h with 86% yield of the desired product (Table 1, entry 5).
Finally, the use of 2 mol% of the Cu-complex and an equimolar
a
Reaction conditions: amine (1 mmol), boronic acid (1 mmol), MeOH
b
(2 mL) and [Cu(DMAP)4I]I (2 mol%), rt; isolated yield. 1.2 mmol of
boronic acid was used.
Scheme 2
We have also examined the efficacy of the catalyst for alkenyl
amount of the substrates in methanol was considered as the boronic acid. When 1-pentenyl boronic acid is treated with imidazole
optimum condition for the reaction at room temperature. under the same reaction conditions at room temperature, the
After optimizing the reaction conditions, the reaction was corresponding enamine was isolated in 90% yield after 30 min
extended to various primary and secondary amines and different of reaction (Scheme 2).
boronic acids. The results are summarized in Table 2. In all the
Thereafter, the C–N coupling reaction was extended to
cases the reaction is very rapid and the yields of the products are various amides under the same reaction conditions (Table 3).
also very high. It has been observed that the presence of an electron In all the cases, the method produces high yield of the product
withdrawing substituent in boronic acid increases the reaction rate. within a considerably shorter reaction time. However, the rate
On the other hand, enhancement of product yield was observed in of reaction was found to be slower than that for amines with
the case of amines bearing an electron pumping group. The reaction slightly lower yield of the product.
also works well in the case of methyl boronic acid. Reaction of
Next, the scope of the C–N bond forming reactions was
methyl boronic acid with benzyl amine resulted in the formation of examined using tosyl azide under the same reaction conditions.
corresponding N-methyl benzyl amine in 80% yield after 25 min The results are summarized in Table 4. Under the same reaction
of reaction at room temperature in methanol (Table 2, entry 1). conditions, the yield of the products was found to be relatively low.
Secondary amines such as N-methyl benzyl amine and morpholine
After successful application of our catalyst in C–N bond forming
could also be converted to their respective N-arylated products in reactions, the method was further extended to C–S bond forming
high yield. However, in the case of morpholine, 1.2 equivalents of reactions. The process was examined for coupling reaction of
boronic acid was used to improve the yield.
different thiols with boronic acid in methanol in the presence of
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Table 3 Chan–Lam reaction of amide with boronic acidsa
reactions. This new protocol is simple and rapid, which can be
performed at room temperature producing the coupling product
within a short time. The catalyst could be used very effectively for
coupling reaction of amines, amides, azides and thiols with
boronic acids to produce the corresponding coupling products
in high yield.
Financial Support from DST, India (Grant No. SR/S1/OC-43/
2011), is gratefully acknowledged. SR thanks UGC, India, for a
fellowship under the RFMS scheme and BK thanks CSIR for
senior research fellowship.
Notes and references
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Reaction conditions: amide (1 mmol), boronic acid (1 mmol), MeOH
(2 mL) and [Cu(DMAP)4I]I (2 mol%), rt; isolated yield.
Table 4 Chan–Lam reaction of tosylazide with boronic acidsa
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Reaction conditions: azide (1 mmol), boronic acid (1 mmol), MeOH
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The copper complex is also very effective in catalyzing C–S cross
coupling reaction, though the reaction time is somewhat more
compared to that of amines. In this case, thiols with an electron
withdrawing group increase the rate of the reaction as well as
yield of the product.
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square pyramidal in shape with five coordination number
and has a vacant co-ordination site. It could be clearly visualized
from the X-ray structure that one iodine atom is situated away
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40 min, 80%
5d, R1 = H, R2 = 4Br, 5e, R1 = 4MeO,
40 min, 80%
5g, R1 = 4Me, R2 = 4Br, 5h, R1 = 4Br, R2 = 4NO2, 5i, R1 = 4Br, R2 = H,
5b, R1 = H, R2 = 4MeO, 5c, R1 = H, R2 = 4NO2,
50 min, 78%
35 min, 85%
5f, R1 = 4MeO, R2
=
R2 = 4MeO, 55 min, 76% 4NO2, 50 min, 81%
65 min, 80%
50 min, 83%
55 min, 78%
a
Reaction conditions: thiol (1 mmol), boronic acid (1 mmol), MeOH
(2 mL) and [Cu(DMAP)4I]I (2 mol%), rt; isolated yield.
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