Cu(OAc)2-porphyrins as an efficient catalytic system for base-free, nature mimicking Chan–Lam coupling in water

Article abstract of DOI:10.1002/aoc.6223

The use of porphyrins as ligands in organic synthesis reveals the natural process, because these are the constituent motifs of catalysts in many bio-organic reactions. This article presents the synthesis of two N-pincer tetradentate porphyrins; tetrasodium meso-tetra(p-sulfonatophenyl)phorphyrin (H₂TSTpSPP) and meso-tetra(m-carboxyphenyl)porphyrin (H₂TmCPP), and study on their aptness for Cu-catalyzed C–N coupling reactions of arylboronic acids and amines (Chan–Lam coupling reaction) in water under external base free conditions. The porphyrins and Chan–Lam coupling products were well characterized by their spectral analysis. The high product yields, application of nature-inspired conditions, large extent of substrates, ease of making and handling the ligands, avoidance of base, and use of water as reaction media are the attractive attributes of this finding.

Full text of DOI:10.1002/aoc.6223

Received: 28 October 2020
DOI: 10.1002/aoc.6223
Revised: 14 February 2021
Accepted: 19 February 2021
C O M M U N I C A T I O N
Cu(OAc) -porphyrins as an efficient catalytic system for
2
base-free, nature mimicking Chan–Lam coupling in water
Katta Venkateswarlu
| Kanusu Umamaheswara Rao
Laboratory for Synthetic & Natural
Products Chemistry, Department of
Chemistry, Yogi Vemana University,
Kadapa, India
The use of porphyrins as ligands in organic synthesis reveals the natural pro-
cess, because these are the constituent motifs of catalysts in many bio-organic
reactions. This article presents the synthesis of two N-pincer tetradentate por-
Correspondence
phyrins; tetrasodium meso-tetra(p-sulfonatophenyl)phorphyrin (H TSTpSPP)
2
Katta Venkateswarlu, Department of
Chemistry, Yogi Vemana University,
Kadapa 516005, India.
and meso-tetra(m-carboxyphenyl)porphyrin (H TmCPP), and study on their
2
aptness for Cu-catalyzed C–N coupling reactions of arylboronic acids and
amines (Chan–Lam coupling reaction) in water under external base free
conditions. The porphyrins and Chan–Lam coupling products were well char-
acterized by their spectral analysis. The high product yields, application of
nature-inspired conditions, large extent of substrates, ease of making and
handling the ligands, avoidance of base, and use of water as reaction media
are the attractive attributes of this finding.
Email: kvenkat@yogivemanauniversity.
ac.in
Funding information
Science and Engineering Research Board,
Grant/Award Number: SB/FT/CS-
009/2014
K E Y W O R D S
2
aryl(heteroaryl)amines, Chan–Lam coupling, chemoselectivity, Cu(OAc) , porphyrins
[24–27]
1
| INTRODUCTION
aldehydes, and sulfides using H O or O .
2 2 2
Therefore, the use of porphyrin as ligands for transition-
metal-mediated organic synthesis is challenging and
encouraged by mimicking natural phenomenon of bio-
logical process.
Porphyrin and porphyrin-metal complexes are familiar
for their significant biological functions including the
metabolism of living beings and are termed as “pigments
[1,2]
of life.”
The significant biological tasks of porphyrins
The C–N cross-couplings executed by transition-metal
catalysts immersed eloquent part in the construction of a
include the heme and hemato porphyrins controlling
[
3,4]
[28–30]
oxygen transport in animals,
chlorophylls demonstrat-
variety of commercially potent organic molecules.
[5]
ing photosynthesis in plants, and healthy sustainment
of nervous and brain system by vitamin B .
Among the C–N coupled products, N-arylimidazoles and
N-arylbenzimidazoles draw considerable attention due to
their widespread use in organocatalysis, ionic liquids, and
[
6–8]
12
Moreover, the critical role of porphyrins in their metal
complexes is proven in several organic transformations.
The wide choice of metal ions for the complexation with
porphyrins enabled a broad range of applications includ-
ing carbon–carbon bond formations (Suzuki–Miyaura
[29,31–33]
pharmaceutical, biological, and material interest.
Common strategies to access the arylated amines include
(i) the activation of aryl halide/ethers/esters toward aro-
[34–36]
matic nucleophilic substitution (S Ar) using amines,
N
[
9–13]
and Mizoroki–Heck couplings),
borylations of aryl
(ii) Ullmann/Buchwald–Hartwig coupling of aryl halides
[14]
[15,16]
[37–40]
halides,
olefin epoxidations,
alcohol/aldehyde
and amines,
and (iii) Chan–Lam coupling of
[
17–19]
[20,21]
[41–43]
oxidations,
reduction of multiple bonds,
and aziridations of olefins.
arylamines and arylboronic acids.
The first two strat-
[22]
[23]
cycloadditions,
For an
egies often required elevated temperature, stoichiometric
[34–40]
instance, cytochrome P-450 monooxygenase oxidizes sev-
eral substrates including alkanes, olefins, alcohols,
ratios of catalysts and expensive solvents, and bases.
On the other hand, the Chan–Lam coupling enables the
Appl Organomet Chem. 2021;e6223.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
1 of 7
https://doi.org/10.1002/aoc.6223

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VENKATESWARLU AND RAO
preparation of arylamines at mild conditions using highly
reactive substrates such as organoboronic acids and
2.2 | Preparation of meso-tetra(m-
carboxyphenyl)porphyrin (H TmCPP)
2
[41–43]
amines.
Recently, several copper-based catalytic sys-
tems were developed for the Chan–Lam coupling,
Porphyrin, H TmCPP was synthesized according to the
2
0
[44]
[12,58]
which
include
Cu(OAc) /2,2 -bipyridine/Cs CO ,
reported procedures
has been provided in the
2
2
3
[
45]
Cu-MOF/K CO ,
lutidine,
Cu(OAc) /myristic acid/fac-[Ir(ppy) ]/
Supporting Information.
2
3
2
3
[47]
[46]
II
Cu @montmorillonite
CuNPs@nitrogen enriched GO,
K10/K CO ,
2 3
Cu@Ag-TiO -NGO,
[48]
[49]
2
II
[50]
Cu -N-(2-pyridylmethyl)morpholine-N-oxide complex,
2.3 | Preparation of meso-tetra(p-
0
[51]
and Cu(OAc) /2,2 -bipyridine.
However, these methods
sulfonatophenyl)phorphyrin (H TSTpSPP)
2
2
associate with certain drawback like the necessity of base,
organic solvent, or expensive catalyst. Therefore, a new
development in Chan–Lam coupling to access arylated
amines at mild conditions using environmentally benign
solvents and ligand systems is highly demanding.
[
12,58]
The synthesis of H TSTpSPP
the Supporting Information.
has been discussed in
2
In continuing our research on carbon–carbon and
2.4 | General experimental procedure for
Chan–Lam coupling
[12–14,52–54]
carbon–heteroatom couplings,
we divulged in
this article our recent research on the development of
efficient and nature mimicking protocol for the C–N
coupling of arylboronic acids and aryl(heteroaryl)amines
To a mixture of arylboronic acid (1) (1.0 mmol), amine
(2) (1.10 mmol), and H TSTpSPP (3 mol%) was added
2
(
Chan–Lam coupling) using Cu(OAc) and porphyrins in
5 ml of water and Cu(OAc) (2 mol%). The reaction mix-
ture was stirred at 50 C, and the reaction progress was
2
2
ꢀ
water. To the best of our knowledge, this is the first report
on the application of porphyrins as ligands for transition
metal catalyzed Chan–Lam coupling. Moreover, the appli-
cation of water/aqueous solvents is the challenging and
urgently needed task in current state of organic synthesis.
When water is exploited as solvent, it palliates the pollu-
tion caused by the volatile organic solvents, which are the
ensured using TLC. After completion of the reaction, as
indicated by TLC, 5 ml of EtOAc was added. EtOAc was
separated, and the reaction mixture was again extracted
with 2 × 5 ml of EtOAc, and the combined portions of
EtOAc was subjected for evaporation to obtain crude
aryl(heteroaryl)amines. The crude amines were purified
using silica packed columns using varying ratios of
EtOAc and hexanes as eluent. Pure aryl(heteroaryl)
[54–57]
basis for the organic synthesis.
Water evidences the
economic and environmental benefits; thus, it is highly
desirable to develop the synthetic methods using water/
aqueous solvents as reaction media.
1
amines (3) are used to record the spectral ( H NMR,
13
C NMR, and mass) data for the identification of their
structures (please find at the Supporting Information),
and these reveals the reported data.
2
2
| EXPERIMENTAL
.1 | Materials and methods
3 | RESULTS AND DISCUSSION
The arylboronic acids, amines, and other chemicals
employed in the present methodology, are purchased
from Merck, Spectrochem, Alfa Aesar, TCI Chemicals,
AVRA, and Sigma-Aldrich. The porphyrins were pre-
Initially, we have synthesized two porphyrins,
H TSTpSPP and H TmCPP (Figure 1), and are screened
2
2
for the Cu-catalyzed Chan–Lam coupling reaction of phe-
nylboronic acid (1a) and imidazole (2a). Table 1 repre-
sents the screened parameters. The reaction of 1a and 2a
at room temperature (rt), using 1 mol% of Cu(OAc)2
[12]
pared based on our recent reports.
Pyrrole was puri-
fied by distillation prior to its use. The progress of the
reactions was ensured using thin layer chromatography
and H TSTpSPP in water gave 15% yield of
2
(
TLC) (Merck silica gel 60 F254). Silica gel-based columns
N-phenylimidazole (3a) in 12 h (Table 1, entry 1). Then
ꢀ
ꢀ
ꢀ
were utilized for obtaining pure arylamines using a mix-
ture of hexane and EtOAc as eluent. The H and
NMR spectra were recorded using Avance 400/100-MHz
spectrometer employing tetramethylsilane (TMS) as ref-
erence standard. High-resolution mass spectra (HRMS)
was obtained using triple quadrupole mass spectrometer
the reaction was conducted at 40 C, 50 C, and 60 C and
observed the formation of 3a in 46%, 63%, and 63%
(Table 1, entries 2–4) in 12 h indicating the reaction can
1
13
C
ꢀ
conveniently be carried out at 50 C. The absence of
Cu(OAc)2 or H TSTpSPP resulted no yield of 3a
2
(Table 1, entries 5 and 6), and thus, Cu(OAc)2 and
(Quattro Micro, Waters, Manchester, UK) for ESIMS.
H TSTpSPP are critical for the present investigation.
2

VENKATESWARLU AND RAO
3 of 7
The increase of Cu(OAc) loading to 2 mol% and 3 mol%
2
provided 3a in 91% and 90% (Table 1, entries 7 and 8)
shows the necessity of 2 mol% of Cu(OAc) . The aptness
2
of the solvents such as DMF, DMSO, methanol, ethanol,
THF, and DCE was studied and found to form 3a in 20%,
SCHEME 1 Proposed mechanism of Cu(OAc)
catalyzed Chan–Lam coupling
2
2
–H TSTpSPP
FIGURE 1 Structures of porphyrins
TABLE 1 Optimization of reaction conditions
ꢀ
Entry
Catalyst (mol%)
Porphyrin
Solvent
Temperature ( C)
Time (h)
Isolated yield (%)
1
2
3
4
5
6
7
8
9
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
-
2
2
2
2
2
(1)
(1)
(1)
(1)
(1)
H
H
H
H
-
2
2
2
2
TSTpSPP
TSTpSPP
TSTpSPP
TSTpSPP
Water
Water
Water
Water
Water
Water
Water
Water
DMF
rt
12
12
12
12
12
12
2
15
46
63
63
-
40
50
60
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
TSTpSPP
TSTpSPP
TSTpSPP
TSTpSPP
TSTpSPP
TSTpSPP
TSTpSPP
TSTpSPP
TSTpSPP
TmCPP
-
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
2
2
2
2
2
2
2
2
2
(2)
(3)
(2)
(2)
(2)
(2)
(2)
(2)
(2)
91
90
20
32
18
<1
<1
-
2
12
12
12
12
12
12
12
6
10
11
12
13
14
15
16
17
18
19
20
21
DMSO
MeOH
EtOH
THF
DCE
Water
Water
Water
Water
Water
Water
Water
15
42
18
33
Trace
12
-
CuCl
CuCl (2)
CuBr (2)
CuSO (2)
CuI (2)
CuF (2)
2
(2)
TSTpSPP
TSTpSPP
TSTpSPP
TSTpSPP
TSTpSPP
TSTpSPP
12
6
2
4
12
12
12
Note: Reaction conditions: 1a (1.0 mmol), 2a (1.10 mmol), solvent (5 ml), and porphyrin (3 mol%).

4
of 7
VENKATESWARLU AND RAO
3
2%, 18%, <1%, <1%, and 0% (Table 1, entries 9–14). This
current conversion (Table 1, entries 16–21) and found
study indicated the necessity of water as media for the
current conversion. The partial solubility or insolubility
of the ligand may evidence the trace or no yield of 3a in
ethanol, THF and DCE. Further investigation using the
that Cu(OAc) is the best choice among these catalysts.
2
This trend may be due to the easy ionization of Cu(OAc)2
under the developed conditions, which enables the quick
II
formation of Cu −porphyrin intermediary complex
ligand, H TmCPP, the yield of 3a was formed in 15%
(Scheme 1).
2
(
Table 1, entry 15), which suggests that H TmCPP is not
Other boron nucleophiles such as potassium phe-
nyltrifluoroborate (4), phenylboronic acid pinacol ester
(5), phenylboronic acid neopentylglycol ester (6),
phenylboronic acid diethanolamine derivative (7), and
2
an efficient ligand under the present aqueous conditions.
The catalysts such as CuCl , CuCl, CuBr , CuSO , CuI,
and CuF are also screened for their suitability under
2
2
4
TABLE 2 Screening for boron
nucleophile
Entry
Boron nucleophile
Time (h)
Isolated yield of 3a (%)
1
.
3
59
2
.
16
16
-
-
3
.
4
5
.
.
6
4
39
52
Note: Reaction conditions: Boron nucleophile (1.0 mmol), 2a (1.10 mmol), Cu(OAc)
2
(2 mol%), H
2
TSTpSPP
ꢀ
(
3 mol%), and water (5 ml) at 50 C.

VENKATESWARLU AND RAO
5 of 7
phenylboronic acid N-methyldiehtanolamine derivative
Pyrazole on reaction with 1a gave 82% of its N-phenyl
derivative in 2 h (Table 3, Compound 3m). The reactions
of 5-aminopyrazole and 5-chlorotetrazole with 1a dis-
plays high chemoselectivity in the arylation of heteroaryl
(
8) are examined for the Cu(OAc) -catalyzed N-arylation
2
of 2a in water (Table 2). As can be seen, the pinacol and
neopentylglycol based phenylboron nucleophiles such as
5
and 6 are not suitable for the present Chan–Lam cou-
“–NH–” group over “–NH ” or “–Cl” groups (Table 3,
2
pling (Table 2, entries 2 and 3). The potassium phe-
nyltrifluoroborate, 4, shows good yields (59% in 3 h) than
the diethanolamine analogs of phenylboronic acid (such
as 7 and 8) (Table 2, entries 1, 4, and 5). The nucleophile,
Compounds 3n and 3o). Finally, arylamines such as ani-
line, 1,4-phenylenediamine, and 4-nitroaniline were
investigated for their mono-N-arylation using 1a and
found to form the products with yields 62%, 59%, and
75% (Table 3, Compounds 3p–3r). The selective arylation
of one amine group of 1,4-phenylenediamine in the pres-
ence of other identical amine group is further example of
high chemoselectivity using this protocol. The formyl,
chloro, keto, nitrile, and amine groups present on the
arylboronic acids or amines got tolerated under present
Chan–Lam reaction conditions.
4, shows lesser yield of 3a than 1a (Table 1, entry 7), and
hence, 1a seems to be the better nucleophile than other
boron nucleophiles under this study.
The protocol was extended due to its applicability to a
variety of aryl(heteroaryl)amines using a wide range of
arylboronic acids (Table 3). Imidazole is effectively
arylated at the –NH– group using a variety of arylboronic
acids with electron donating (ED) (such as –OMe) and
electron withdrawing (EW) (such as –CHO, –COCH3,
The plausible mechanism of Cu(OAc) –H TSTpSPP
2
2
catalyzed Chan–Lam coupling reaction is represented in
–
Cl, –S(O) Me, –NO and –CN) groups using Cu(OAc)
Scheme 1, which has been derived from a review
2
2
2
[
41]
and H TSTpSPP in water (Table 3, Compounds 3b–3h).
reported by West et al.
acetate forms mononuclear Cu -complex (I)
(denucleation) by reacting with H TSTpSPP.
First, the dimeric copper
2
II
Relatively, the arylboronic acid with EW groups shows
quick reactivity than with ED groups. Similar trend in
the reactivity of arylboronic acids was observed with
benzimidazole (Table 3, Compounds 3i–3l).
a
[
41]
The for-
2
mation of I has been indicated by the Fourier-transform
infrared (FT-IR) and ultraviolet (UV)–visible spectral
TABLE 3 Feasibility study of substrates
ꢀ
Note: Reaction conditions: 1 (1.0 mmol), 2 (1.10 mmol), Cu(OAc)
2
(2 mol%), H
2
TSTpSPP (3 mol%), and water (5 ml) at 50 C. Isolated yields are presented with
superscript b.

6
of 7
VENKATESWARLU AND RAO
data (see the Supporting Information). A new absorption
ORCID
995 cm− in the FT-IR spectra of the solid obtained by
1
Katta Venkateswarlu https://orcid.org/0000-0002-1475-
1126
ꢁ
the evaporation of aqueous portion after the separation
of the products using solvent extraction represents Cu–N
Kanusu Umamaheswara Rao https://orcid.org/0000-
0002-4852-3404
[59,60]
bond formation.
A clear difference between the
absorption bands of H TSTpSPP and the solid formed
2
after the evaporation of water by the separation of
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II
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arylboronic acids and aryl(heteroaryl)amines in water.
The present investigation shows several advantages,
such as the use of aqueous media, avoidance of base,
high product yields, use of nature inspired ligands,
low catalyst load, moderate reaction temperature,
avoidance of inert and moisture-free conditions, and
vide applicability to a variety of substrates. This
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Transition Metal Complexes, Imperial College Press, London
2000.
project
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Umamaheswara Rao: Conceptualization; data curation;
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