Novel CuCl2-cryptand-[2.2.Benzo] complex: A base free and oxidant free catalyst for Ipso-Hydroxylation of aryl/heteroaryl-boronic acids in water at room temperature

Article abstract of DOI:10.1016/j.jorganchem.2017.09.011

A novel cryptand and its copper complex was synthesised, which found to have enormous catalytic activity towards ipso-hydroxylation of aryl or heteroarylboronic acids and esters in water without using H₂O₂ or other oxidising agent and base at room temperature. This newly developed method efficiently converts aryl boronic acids and esters as well as heteroaryl boronic acids to their corresponding phenols with high yields within a very short reaction time. This protocol has found to be well-matched with a wide variety of functional groups. High yields, very short reaction time, easy separation, recyclability up to 6th time are the advantages of this method.

Full text of DOI:10.1016/j.jorganchem.2017.09.011

Journal of Organometallic Chemistry 851 (2017) 52e56
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Novel CuCl2-cryptand-[2.2.Benzo] complex: A base free and oxidant
free catalyst for Ipso-Hydroxylation of aryl/heteroaryl-boronic acids in
water at room temperature
Sankar Jyoti Bora, Bolin Chetia*
Department of Chemistry, Dibrugarh University, Dibrugarh, 786004, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel cryptand and its copper complex was synthesised, which found to have enormous catalytic
activity towards ipso-hydroxylation of aryl or heteroarylboronic acids and esters in water without using
H2O2 or other oxidising agent and base at room temperature. This newly developed method efficiently
converts aryl boronic acids and esters as well as heteroaryl boronic acids to their corresponding phenols
with high yields within a very short reaction time. This protocol has found to be well-matched with a
wide variety of functional groups. High yields, very short reaction time, easy separation, recyclability up
to 6th time are the advantages of this method.
Received 20 May 2017
Received in revised form
9 August 2017
Accepted 5 September 2017
Available online 11 September 2017
Keywords:
Cryptand
© 2017 Elsevier B.V. All rights reserved.
Phenols
Catalysis
Heterogeneous
Ipso-HydroxyLation
Water
1. Introduction
natural antioxidants [16]. There are various traditional methods for
the synthesis of phenols which have some limitations such as harsh
Supramolecules gets tremendous importance for their signifi-
cant ability to form complexes with cations, particularly with alkali
and alkaline earth metal families [1]. The binding of the metals with
these macromolecules helps the catalyst to remain stable in the
reaction. There are many advantages in design and synthesis of
novel macromolecules to accelerate the development of supra-
molecular chemistry [2]. Cryptands which was first reported by
Lehn [3] has got tremendous applications in various fields like
sensing [4], biological activity [5], catalytic activity [6] etc. Cryptand
catalyzed reactions have many advantages because of their versa-
tile processing capabilities, separation and recycling. In recent years
several cryptand catalysts have already been developed for various
reactions like Suzuki cross-coupling [7], synthesis of ynones [8a],
formation of aryl-aryl and aryl-sulfur bonds [8b], n-boc protection
of amine [9], coupling of diorgano dichalcogenides with terminal
alkynes [10], MizorokieHeck cross-coupling [11], asymmetric hy-
drogenation of dehydroamino acid esters etc [12].
reaction conditions, low yield, more reaction time etc. [17] But ipso-
hydroxylation of boronic acids have enormous advantages over
conventional methods due to availability and wide range of func-
tionality of boronic acids, effortless feasibility of the reaction. There
are a number of existing protocols for ipso-hydroxylation of aryl-
boronic acids using H2O2 or other oxidants such as biosilica-H2O2,
I2eH2O2, Al2O3eH2O2, PEG400eH2O2, H3BO4eH2O2, organic
hypervalent iodine(III)-NaIO4, CuCl2-Brij S-100-O2, Amberlite IR-
120 resin-H2O2, supported silver nano particle-H2O2, KOHeTBHP,
WERSA-H2O2 etc. [18] But, there are only a few reports without
using H2O2 which includes Ru(byp)3Cl2$6H2O, NH2OH, CuSO4e-
phenanthroline, Cu2O nano particle, MeNHNH2 etc [19]. The ma-
jority of above mentioned protocols have many disadvantages like
requirement of oxidising agent, base, supports and additives, high
temperature, oxygen atmosphere, long reaction time, lack of recy-
clability etc.
To overcome all these limitations we build up an efficient, mild,
environment and user-friendly complex of CuCl2 with newly syn-
thesised novel cryptand [2.2.Benzo], which found to be an excellent
heterogeneous catalyst for ipso-hydroxylation of aryl/hetero-
arylboronic acids and esters to corresponding phenols in water
Phenols and their derivatives have numerous applications in
pharmaceuticals [13], agrochemicals [14], polymers [15] and as
* Corresponding author.
E-mail address: bolinchetia@dibru.ac.in (B. Chetia).
http://dx.doi.org/10.1016/j.jorganchem.2017.09.011
0022-328X/© 2017 Elsevier B.V. All rights reserved.

S.J. Bora, B. Chetia / Journal of Organometallic Chemistry 851 (2017) 52e56
53
without using H2O2, base, other oxidising agents, support and ad-
ditives at room temperature under air (Scheme 1). This catalyst is
air stable and remains effective for a long period without losing its
catalytic activity. The chelating effect of nitrogen and oxygen atoms
present in macrocycles along with its flexibility furnishes supports
in stabilizing the complex. This protocol gives high yield at very low
reaction time and to the best of our knowledge; this catalyst is used
for the synthesis of phenols for the first time.
Scheme 3. (i) Na2CO3, CH3CN (dry), reflux, 48 h. 42% yield. (ii) CuCl2$2H2O, ethanol,
reflux, and 7h.
2. Materials and methods
general existing procedure [21,22] (Scheme 3). In a 500 ml round
bottom flask 6.842g (18.494 mmol) of 1c was dissolved in 50 ml
acetonitrile. The mixture was boiled and 5.825 g (54.958 mmol) of
sodium carbonate was added to this boiling solution. Then 1.0 g
(9.247 mmol) of m-phenylenediamine (1d) previously dissolved in
acetonitrile (50 ml) was added slowly for a period of 5 h. The re-
action mixture was refluxed at 90 ꢁC for 48 h. After completion of
the reaction, the solvent was evaporated by rotary evaporator un-
der reduced pressure and product was isolated with chloroform.
The crude product obtained was purified by using column
chromatography with ethyl acetate-hexane mixture as mobile
phase to give the desired product L1 (1.305 g, yield 42%). Melting
point 165-170 ꢁC. Mass calculated m/z: 336.205, found m/z:
336.3098. IR (KBr, ʋ cmꢀ1): aromatic C-H (stretch) 3136, aliphatic C-
H of CH2 2939, C-H (Stretch) of CH2-N- 2771, aromatic C¼C 1662 to
1489, C-H (bend) of CH2 1440, C-H (wag) of CH2-1363, aromatic C-N
(stretch) 1263, N-C (stretch)-CH2-1180, C-N-C 1028, C-O-C 977-950,
-CH2-(roc) 779, aromatic C-H (bending) 659, C-N-C (deforming)
2.1. General information
2.1.1. Materials and physical measurements
All the reagents were purchased from Sigma-Aldrich chemical
co. and used without further purification. The solvents were pur-
chased from Merck Co. and used after distillation. All reactions
were monitored by thin layer chromatography (TLC) on silica gel
plates (Merck Kieselgel 60 F254) which were visualized with ultra-
violet light. Melting points were determined on a Thomas Hoover
capillary apparatus. 1H-NMR and 13C-NMR spectra were recorded at
400 MHz and 100 MHz respectively on FTeNMR Brukar Avance II,
400 MHz instrument in either CDCl3 or DMSO-d6 and tetrame-
thylsilane (TMS,
d
¼ 0.00) as an internal standard. Multiplicities
were designated as broad (br), singlet (s), doublet (d), and multiplet
(m). Infrared spectra were recorded in the range 400e4000 cmꢀ1
on SHIMADZU FT-IR spectrophotometer (model no. IR prestige-21)
using KBr pellets. Band intensities were assigned like weak (w),
medium (m), shoulder (sh), strong (s), and broad (br). Mass spectra
of cryptand [2.2.Benzo] and its complex were recorded on JEOL
GCMATE II GC-MS and GC-MS analysis of the products were per-
formed on Agilent technologies 7820A GC System with electron
impact ionization (70 eV) using HP-5MS column (30 m length and
0.25 mm diameter and 0.25 micron film thickness). High purity
helium was used as a carrier gas with the flow rate of 1ml/min. The
oven temperature was maintained at 50-250 ꢁC at a rate of 10ꢁC/
597-576. 1H NMR (400 MHz, CDCl3,
d
ppm): 7.73 (t, J ¼ 7.8 Hz, 1H),
7.42 (d, J ¼ 7.8 Hz, 2H), 6.09 (s, 1H), 3.68 (s, 8H), 3.61 (t, J¼6.8, 8H),
3.52 (t, J¼7.3, 8H). 13C NMR (100 MHz, CDCl3,
d ppm): 150.0, 130.6,
125.0, 103.9, 70.0, 69.0, 59.1.
2.4. Synthesis of CuCl2-cryptand [2.2.Benzo] complex (C1)
min. The split ratio was 1:5 and the injector volume was 1 ml. The
UVevisible spectra were recorded on JASCO V-750 and the wave
length ranging from 900 to 200 nm.
CuCl2$2H2O, 0.151 g (0.891 mmol) was dissolved in 10 ml
ethanol and added slowly to a solution of 0.3 g (0.8910 mmol)
cryptand [2.2.Benzo] in 15 ml ethanol (Scheme 3). The reaction
mixture became sky blue as soon as the cryptand was added and it
was refluxed at 80 ꢁC for 7 h. Finally, the colour of the product of the
reaction mixture was changed to dark green. The product was
filtered and washed with ethanol for several times. The powdered
CuCl2-cryptand [2.2.Benzo] complex was dried in a desiccator and
used for further analysis and catalysis. Melting point: 210-215 ꢁC.
Mass calculated m/z: 469.027, m/z found: 469.0498. IR (KBr, ʋ
cmꢀ1): Aromatic C-H 3343, C-H (Stretch) of CH2-N- 2860, C-N of
CH2-N- 1205, C-N-C 999, C-O-C 840. 1H NMR (400 MHz, DMSO-d6,
2.2. Preparation of 1,2-bis(2-iodoethoxy)ethane (1c)
To synthesise the cryptand, first triethylene glycol (1a) was
converted to (ethane-1,2-diylbis(oxy))bis(ethane-2,1-diyl)bis(4-
methylbenzenesulfonate) (1b). The ditosylate (1b) thus obtained
was treated with excess NaI in acetone for 48 h to give the corre-
sponding diiodide (1c) with 94% yield [20]. The spectroscopic data
of the desired products were satisfied with the literature [21].
(Scheme 2).
d
ppm): 7.9 (br, 1H), 7.6 (br, 2H), 6.2 (s, 1H), 3.5 (br, 24H).
2.3. Synthesis of cryptand [2.2.Benzo] (L1)
2.5. General procedure for synthesis of phenols by CuCl2-cryptand
[2.2.Benzo] catalyst
The cryptand [2.2.Benzo] (L1) was synthesised by modifying the
For ipso-hydroxylation of aryl/heretoaryboronic acids and esters
(Scheme 1), 1 mmol aryl or heteroaryl boronic acid and esters in
2 ml distilled water was taken in a 50 ml RB and 5 mol% of complex
C1 was added. The mixture was stirred at room temperature for
20 min. After completion of the reaction (as monitored by TLC), the
reaction mixture was centrifuged to separate the catalyst and
reused in further reactions. The product obtained was extracted
with diethyl ether from the reaction mixture for two to three times.
All the synthesised products of phenols were purified by column
chromatography and characterized by GC-MS, FT-IR, 1H and 13C
NMR spectroscopy.
Boronic
acids
Ar/(Het)Ar
Ar
CuCl2-cryptand[2.2.Benzo]
Ar/(Het)Ar
OH
or
Boronic
esters
Water, r.t., Air
Scheme 1. Synthesis of phenols using CuCl2-cryptand[2.2.Benzo] complex.
Scheme 2. (i) TsCl, KOH, DCM, 0 ꢁC, 3hrs. (ii) NaI, acetone, 48h.

54
S.J. Bora, B. Chetia / Journal of Organometallic Chemistry 851 (2017) 52e56
Table 2
3. Result and discussion
Recyclability test of the catalytic system.a
The cryptand [2.2.Benzo] L1 and its CuCl2 complex C1 was
prepared by following Scheme 3. On comparison of the FT-IR
spectra, C-H stretch and C-N stretch bands of CH2-N of L1 were
shifted from 2771 to 2860 and 1180 cmꢀ1 to 1205 cmꢀ1 in the
complex C1 respectively and the band at 1028 cmꢀ1 assigned to C-
N-C was shifted to 999 cmꢀ1 confirming the complexation of CuCl2
through N atom of the cryptand. Shifting of aromatic C-H from
3136 cmꢀ1 to 3343 cmꢀ1 and C-O-C from 977 cmꢀ1 to 840 cmꢀ1
reveals that the CuCl2 was encapsulated in the cavity of the
cryptand. Formation of complex was also confirmed by UV-Vis
spectra (see SI Fig. S8) where the L1 in ethanol shows two intense
absorption bands at 254 nm and 265 nm. The absorption band of
CuCl2 in ethanol was at 204 nm and 270 nm. When we mixed the
solutions of L1 and CuCl2 in 1:1 ratio a new intense absorption band
was observed at 254 nm and a low intense band obtained at
208 nm.
Entry
Run
Time(min)
Yields(%)b
1
2
3
4
5
6
1st
10
10
10
10
10
10
96
94
94
89
88
85
2nd
3rd
4th
5th
6th
a
Reaction condition: phenylboronic acid (1 mmol), water 2 ml, catalyst C1 (5 mol
based on isolated yield.
%).
b
Table 3
Our first aim is to optimize the reaction condition so that we can
apply it to a wide variety of substrates. We took the hydroxylation
of phenylboronic acid to phenol as a model reaction for optimiza-
tion. As shown in Table 1, we first carried out the reaction only with
CuCl2$2H2O (30 mol %) for 24 h (Entry 1), after that we increased
the amount of CuCl2$2H2O to 50 mol % (Entry 2) and stirred the
same reaction for 24 h at 60 ꢁC (Entry 3) but no yield was obtained.
Similarly, we run the reaction only with different mol % of the
cryptand [2.2.Benzo] (L1) (entries 4-5), again no formation of
phenol was observed. Next, we carried out the reaction with CuCl2-
cryptand [2.2.Benzo] (C1) catalyst at various mol % (Table 1, entries
6-8). The reaction gave good yield at 5 mol % of the catalyst at
10 min but the yield was excellent when we lengthened the time to
20 min (Table 1, Entry 8-11). From the entire investigation, it is
evident that 5 mol% (Entry 10) of the catalyst was sufficient to
complete the reaction in 20 min at room temperature.
To investigate the recyclability of the catalyst we considered the
optimized reaction of phenyl boronic acid to phenol. After
completion of the reaction, the reaction mixture was centrifuged to
separate the catalyst and desired products were extracted with
diethyl ether from the reaction mixture. The catalyst collected from
the mixture was washed with ethanol followed by diethyl ether for
several times and allowed to air-dry.
Advantages over the reported protocol without H2O2.
Entry Catalyst
Base/Recyclability Time (min/h) Yields
(%)b
1
CuSO4-Phenanthroline
[19c]
[Ru(byp)3Cl2]$6H2O [19a] Et3N/No
KOH/No
3 h
90
2
3
19 h
26 h
94
68
NH2OH
[19b]
Cu2O NPs
[19d]
MeNHNH2
[19e]
NaOH/No
No/Yes
No/No
4
5
6
5 h
95a
85
6 h
C1 (this work)
No/Yes
20 min
96
a
All reactions were performed at room temperature except Entry 4 (60 ꢁC).
based on isolated yield.
b
loss of catalyst during handling error as we weighed out the catalyst
after each cycle.
In Table 3 we tried to compare the catalytic activity of CuCl2-
cryptand [2.2.Benzo] complex with a few existing protocols re-
ported without using H2O2 or other oxidants and found many ad-
vantages over the existing methods.
We extended our studies to different substrates of aryl or het-
eroaryl boronic acids under optimized conditions to expand the
scope of ipso-hydroxylation reaction for various substrates. The
results of Table 4 demonstrated that the CuCl2-cryptand [2.2.benzo]
complex catalyzed ipso-hydroxylation of aryl boronic acids have
great tolerance to various functional groups with very good yield in
a very short reaction time. This protocol shows an unexpected
result for all the reactions irrespective of electron withdrawing or
donation group present in the substrates. Moreover, the hetero-
arylboronic acids also proceed smoothly with very good yield.
Therefore this mild and efficient protocol without H2O2 has
extensive choice of various substrates.
This catalyst can be reused directly without further purification
and it was found that it can be reused up to 6th cycle (Table 2). A
small decrease of yield after 4th cycle (Entry 4) was observed due to
Table 1
Search for optimum reaction conditions.a
After establishing the wide range substrate scope of this pro-
tocol with various aryl or heteroaryl boronic acids, we extended our
study to ipso-hydroxylation of boronic esters (Table 5). Phenyl
boronic esters were efficiently converted to their corresponding
phenols within a short reaction time.
To study the source of OH, three separate reactions were per-
formed with the standard optimized reaction condition, first in N2-
atmosphere, second in O2-atmosphere, and third in air to establish
the mechanistic pathway. As observed in Scheme 4, the reaction did
not proceed in nitrogen atmosphere whereas ipso-hydroxylation
took place in oxygen or air atmosphere [18f,19a,19e]. From this it is
clear that oxygen is essential for the reaction and it may come from
aerial oxygen.
Entry
Catalyst (mol %)
Temp. (ꢁC)
Time (min/h)
Yields(%)b
1
2
3
4
5
6
7
8
CuCl2$2H2O (30)
CuCl2$2H2O (50)
CuCl2$2H2O (50)
L1 (30)
L1 (50)
C1 (01)
C1 (02)
C1 (05)
C1 (05)
C1 (05)
rt
rt
60
rt
60
rt
rt
rt
rt
rt
rt
24h
24h
24h
24h
0
0
0
0
24h
0
10 min
10 min
10 min
15 min
20 min
25 min
25
50
80
93
96
96
9
10
11
C1 (05)
a
Reaction condition: phenylboronic acid (1 mmol), water 2 ml, catalyst C1 (mol%)
at rt (26 ꢁC).
b
based on isolated yield.

S.J. Bora, B. Chetia / Journal of Organometallic Chemistry 851 (2017) 52e56
55
Table 4
Conversion of different substrates of aryl or heteroarylboronic acids to corresponding phenols.a,b,c
a
Reaction conditions: aryl/heteroarylboronic acid (1 mmol), water 2 ml, catalyst C1 (5 mol%) at room temperature.
Isolated yields.
All reactions are carried out for 20 min and products were isolated.
b
c
Table 5
4. Conclusion
Ipso-hydroxylation of boronic ester derivatives.a
In conclusion,
a
wide-ranging and efficient protocol was
Entry
1
Substrate
Product
Time (min)
30min
Yieldb (%)
85
developed for ipso-hydroxylation of Aryl/Heteroaryl boronic acids
and esters by a CuCl2-cryptand [2.2.Benzo] catalyst at room tem-
perature in water without using H2O2, base, additives and other
oxidising agents. The present method has the characteristics of air
stable catalyst, mild conditions, short reaction time, and high
yields. This method can equally catalyze the substrates having
electron withdrawing or donating group. To the best of our
knowledge, this is the first cryptand complex catalyzed ipso-hy-
droxylation of aryl/heteroarylboronic acids and esters in water at
room temperature.
2
3
30min
30min
87
89
a
Reaction conditions: boronic ester (1 mmol), water 2 ml, catalyst C1 (5 mol%) at
room temperature.
b
Isolated yields.
Acknowledgement
The authors acknowledge the Department of Science and
Technology for financial assistance under DST-SERB (No. SB/FT/
CS-161/2012), UGC-SAP (No. 540/14/DRS-I/2016(SAP-I) and also
thankful to SAIF, NEHU-Shillong, SAIF-Cochin-Kerala, SAIF-IITM,
SAIC-TU for spectral data.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2017.09.011.
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Scheme 4. Study of preliminary mechanistic pathway, Reaction conditions: phenyl-
boronic acid (1 mmol), water 2 ml, catalyst C1 (5mol%).
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