Synthesis of triphenylamines: Via ligand-free selective ring-opening of benzoxazoles or benzothiazoles under superparamagnetic nanoparticle catalysis

Article abstract of DOI:10.1039/c7ra06168d

CuFe₂O₄ superparamagnetic nanoparticles were utilized as an effective recyclable heterogeneous catalyst for the synthesis of triphenylamines via the ligand-free selective ring-opening reaction of benzoxazoles or benzothiazoles with iodoarenes. The nano CuFe₂O₄ demonstrated noticeably higher catalytic efficiency than a series of homogeneous catalysts and heterogeneous catalysts. It was possible to separate the nano CuFe₂O₄ by using a magnet, and the recovered catalyst was reused many times while its activity could be maintained. To the best of our knowledge, this is the first example of heterogeneous catalysis for the transformation of benzoxazoles, and the transformation of benzothiazoles to triphenylamines has not been previously reported in the literature.

Full text of DOI:10.1039/c7ra06168d

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Synthesis of triphenylamines via ligand-free
selective ring-opening of benzoxazoles or
benzothiazoles under superparamagnetic
nanoparticle catalysis†
Cite this: RSC Adv., 2017, 7, 40929
Oanh T. K. Nguyen, Long T. Nguyen, Nhu K. Truong, Viet D. Nguyen, Anh T. Nguyen,
*
*
Nhan T. H. Le, Dung T. Le and Nam T. S. Phan
CuFe2O4 superparamagnetic nanoparticles were utilized as an effective recyclable heterogeneous catalyst
for the synthesis of triphenylamines via the ligand-free selective ring-opening reaction of benzoxazoles or
benzothiazoles with iodoarenes. The nano CuFe2O4 demonstrated noticeably higher catalytic efficiency
than a series of homogeneous catalysts and heterogeneous catalysts. It was possible to separate the
nano CuFe2O4 by using a magnet, and the recovered catalyst was reused many times while its activity
could be maintained. To the best of our knowledge, this is the first example of heterogeneous catalysis
for the transformation of benzoxazoles, and the transformation of benzothiazoles to triphenylamines has
not been previously reported in the literature.
Received 2nd June 2017
Accepted 8th August 2017
DOI: 10.1039/c7ra06168d
rsc.li/rsc-advances
limited in the literature. Although interesting results were
achieved, the homogeneous copper catalyst could not be
1. Introduction
recycled and reused. To develop more environmentally
benign approaches, the synthesis of triphenylamines should
be conducted with solid catalysts.
Triphenylamines and their derivatives represent a notable
family of aromatic compounds that provides a broad range of
practical applications in pharmaceuticals, agrochemicals,
dye-sensitized solar cells, uorescence sensors, photo-
electrochemical sensors, and photofunctional polymers.1–5
These skeletons are conventionally generated by utilizing
Ullmann-type coupling reactions.6–9 Several efforts have been
devoted to improve the formation of triphenylamines, thanks
to their splendid contribution to many elds. Safaei-Ghomi
et al. previously described a CuI-catalyzed synthesis of tri-
phenylamines via the coupling of anilines with iodobenzenes
in the presence of 1,10-phenanthroline as ligand.10 Shaikh
et al. expressed the production of triphenylamine derivatives
by using a Pd2(dba)3-catalyzed C–N bond forming amination
protocol.11 Nevertheless, efficient routes towards the
synthesis of these structures still remain challenging.
Recently, Kim et al. demonstrated for the rst time a CuCN-
catalyzed selective ring-opening of benzoxazoles with iodo-
benzenes to generate triphenylamines in pivalonitrile as
solvent and triphenylphosphine as ligand.12 He et al. subse-
quently modied the method by performing the reaction in
polyethylene glycol under ligand-free conditions (Scheme
1a).13 Indeed, reports on this transformation has been very
Faculty of Chemical Engineering, HCMC University of Technology, VNU-HCM, 268 Ly
Thuong Kiet, District 10, Ho Chi Minh City, Vietnam. E-mail: ltdung@hcmut.edu.vn;
ptsnam@hcmut.edu.vn; Fax: +84 8 38637504; Tel: +84 8 38647256 ext. 5681
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra06168d
Scheme 1
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 40929–40939 | 40929

View Article Online
RSC Advances
Paper
The last decade has witnessed an tremendous breakthrough a permanent magnet. The recovered catalyst was washed thor-
in ‘‘nanocatalysis’’.14–17 Active nanoparticles have attracted oughly with water, ethanol, ethyl acetate, and ether, subse-
considerable attention as they could shorten the gap between quently heated at 150 ꢀC under vacuum on a Shlenk line for 6 h,
heterogeneous and homogeneous catalysts.18–20 At nanometer and then reutilized for new catalytic transformation.
dimension, the external surface of the catalyst could be maxi-
mized, making most of active sites accessible to reactants, and
3. Results and discussion
therefore catalytic activity would be extraordinarily
improved.21–23 Nevertheless, the separation and reusing of the
A series of solid catalysts were screened for the ring-opening
nanoparticles still remains really bothersome, and the issue
reaction of benzoxazole with iodobenzene to generate 2-
should be worked out.24 Superparamagnetic nanoparticles
(diphenylamino)phenol as the major product (Scheme 2),
could incorporate benets of high catalytic activity with
including nano CuFe2O4, nano NiFe2O4, nano CoFe2O4, nano
straightforward
isolation
via
magnetic
separation
Fe2O3, and a number of metal–organic frameworks including
Cu(BDC), Cu(OBA), MOF-199, Cu2(BDC)2(DABCO), Cu2(-
BPDC)2(BPY), Co-ZIF-67, Ni2(BDC)2(DABCO), and MIL-101.
Following this approach, triphenylamines could be formed
from benzoxazoles as alternative starting materials to anilines
under heterogeneous catalysis conditions. The reaction was
approaches.25,26 Certainly, unfunctionalized superparamagnetic
nanoparticles have recently explored as productive heteroge-
neous catalysts for the synthesis of oxindoles and their deriva-
tives,27 the borylation of aryliodides or b-bromostyrenes to
produce arylboronates,28 the synthesis of vinylboronates via
regioselective hydroboration of internal as well as terminal
alkynes,29 the one-pot multicomponent preparation of quinoli-
nones,30 the photocatalytic degradation of organic pollutants,31
the transamidation,32 the synthesis of tetrahydropyridines and
pyrrole derivatives,33 the synthesis of 2,4,5-trisubstituted imid-
azoles,34 the synthesis of azoles,35,36 the oxidation of alcohols to
aldehydes,37 and synthesis of phenols.38 Herein, we would like
to present the synthesis of triphenylamines via ligand-free
selective ring-opening of benzoxazoles or benzothiazoles with
aryl halides under CuFe2O4 superparamagnetic nanoparticle
catalysis (Scheme 1b). To our best knowledge, this is the rst
heterogeneous catalysis for the transformation of benzoxazoles,
and the transformation of benzothiazoles to triphenylamines
was not previously reported. Moreover, using aryl bromides and
aryl chlorides for this reaction was not previously mentioned in
the literature.
ꢀ
conducted in diethylene glycol at 120 C under argon for 2 h,
employing 2.5 equivalents of Cs2CO3, in the presence of
10 mol% catalyst, with 3 equivalents of iodobenzene, at
benzoxazole concentration of 0.15 M. Nano NiFe2O4, nano
CoFe2O4, and nano Fe2O3 were found to be totally inactive for
this transformation, with no trace amount of 2-(diphenylamino)
phenol being recorded aer 2 h. Cu-MOFs offered reasonable
catalytic activity, while cobalt-, nickel-, and iron-based MOFs
were not appropriate for the ring-opening reaction. Among
these catalysts, the CuFe2O4 superparamagnetic nanoparticles
2. Experimental
CuFe2O4 superparamagnetic nanoparticles were supplied by
Sigma-Aldrich. The nanoparticles was consequently character-
ized by utilizing several analysis approaches (see Fig. S1–S6 in
ESI†). In an exemplary experiment, a mixture of benzoxazole
(0.0357 g, 0.3 mmol), iodobenzene (0.1836 g, 0.9 mmol), Cs2CO3
(0.2443 g, 0.75 mmol) and diphenyl ether (0.0511 g, 0.3 mmol)
as an internal standard in diethylene glycol (2 ml) was added
into an 8 ml screw cap vial containing the catalyst. The catalyst
quantity was determined with regard to the copper/benzoxazole
mole fraction. The reaction mixture was stirred at 120 ꢀC for 2 h
under an argon atmosphere. Samples were withdrawn, and
quenched with water (1 ml). Organic constituents were extrac-
ted into ethyl acetate (4 ml), shaken with anhydrous Na2SO4 to
remove any water residue, and analyzed by GC concerning
diphenyl ether. The major product, 2-(diphenylamino)phenol,
was puried on silica gel by column chromatography (petro-
leum ether: ethylacetate ¼ 15 : 1, v/v). The product identity was
Scheme 2 The ring-opening reaction of benzoxazole with iodo-
benzene to generate 2-(diphenylamino)phenol.
1
additionally authenticated by GC-MS, H NMR, and 13C NMR.
For the catalyst reusing investigation, ethanol was added to
reduce the viscosity of the mixture, and the superparamagnetic
nanoparticles were subsequently isolated by decantation using Fig. 1 Screening solid catalysts for the ring-opening reaction.
40930 | RSC Adv., 2017, 7, 40929–40939
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
could be amended considerably to 77% aer 2 h for the case of
diethylene glycol. However, conducting the reaction in tri-
ethylene glycol caused a deterioration in the yield of the ex-
pected product (Fig. 2). This could be explicated based on the
high viscosity of longer polyols, resulting in difficulty for the
dispersion of the solid catalyst in the reaction mixture.
It was previously reported that a base was required for the
ring-opening reaction of benzazoles.39 The inuence of base on
the reaction of benzoxazole with iodobenzene to generate 2-
(diphenylamino)phenol was then explored. The reaction was
ꢀ
performed in diethylene glycol at 120 C under argon for 2 h,
employing 2.5 equivalents of base, in the presence of 10 mol%
catalyst, with 3 equivalents of iodobenzene, at benzoxazole
concentration of 0.15 M. Experimental data revealed that many
carbonates as base could initiate the formation of the triphe-
nylamine. However, Li2CO3 and Na2CO3 offered low reactivity,
affording the desired product in 2% and 19% aer 2 h. The yield
could be boosted to 66% for the case of K2CO3. Among various
bases, Cs2CO3 expressed the best functioning, with 77% yield of
the triphenylamine being recorded aer 2 h. The reaction
Fig. 2 Yields of 2-(diphenylamino)phenol vs. solvent.
exhibited the best performance, affording the desired product
in 77% yield aer 2 h (Fig. 1). It was therefore decided to employ
the nano CuFe2O4 as catalyst for the synthesis of triphenyl-
amines via the ring-opening reaction.
t
utilizing BuOK as base could also provide 60% yield aer 2 h.
K3PO4, KHCO3, and NaHCO3 were not the base of choice for this
reaction, producing the triphenylamine in 29%, 22%, and 20%
yields, respectively, aer 2 h. Organic bases such as pyridine,
morpholine, piperidine, DABCO, and DBU were not suitable for
this reaction (Fig. 3). The quantity of base also expressed
a notable impact on the generation of the triphenylamine. The
reaction could not proceed in the absence of the base, with the
product being recorded in trace amount aer 2 h. The presence
of Cs2CO3 in the reaction mixture considerably accelerated the
conversion. However, utilizing more than 2.5 equivalents of the
base was inessential as the yield of 2-(diphenylamino)phenol
was not ameliorated remarkably (Fig. 4).
As the ring-opening reaction of benzoxazole with iodo-
benzene to generate 2-(diphenylamino)phenol was conducted
in solution phase, the rate of the transformation could be
immensely altered by the reactant concentration as a conse-
quence of the mass transfer issue. It was consequently deter-
mined to probe the inuence of benzoxazole concentration on
the yield of 2-(diphenylamino)phenol. The reaction was imple-
mented in diethylene glycol at 120 ꢀC under argon for 2 h,
Kim,12 He13 et al. previously pointed out that the solvent
displayed a notable impact on the generation of the amine. We
then conducted the transformation in a variety of solvents at
120 ꢀC under argon for 2 h, employing 2.5 equivalents of
Cs2CO3, in the presence of 10 mol% catalyst, with 3 equivalents
of iodobenzene, at benzoxazole concentration of 0.15 M. It was
noted that DMSO, toluene, dichlorobenzene, and diglyme
should not be used for the reaction. DMF, n-butanol, and
dioxane offered better performance, affording 2-(diphenyla-
mino)phenol in 42% yield aer 2 h. Performing the reaction in
DMA, 48% yield of the triphenylamine was obtained aer 2 h.
He et al. previously reported that this transformation using
CuCN catalyst proceeded readily in polyethylene glycol.13
However, in this work, it was observed that only 57% yield of the
triphenylamine was generated in polyethylene glycol aer 2 h.
Ethylene glycol as solvent led to the formation of 2-(dipheny-
lamino)phenol in 58% yield aer 2 h. Interestingly, this value
Fig. 3 Yields of 2-(diphenylamino)phenol vs. base.
Fig. 4 Yields of 2-(diphenylamino)phenol vs. base amount.
RSC Adv., 2017, 7, 40929–40939 | 40931
This journal is © The Royal Society of Chemistry 2017

View Article Online
RSC Advances
Paper
reaction in polyethylene glycol under ligand-free conditions at
10 mol% CuCN catalyst.13 We then probed the inuence of
catalyst quantity on the generation of the triphenylamine. The
reaction was conducted in diethylene glycol at 120 ꢀC under
argon for 2 h, employing 2.5 equivalents of base, with 2 equiv-
alents of iodobenzene, at benzoxazole concentration of 0.15 M,
in the presence of different catalyst amounts. The reaction
could not progress to produce the triphenylamine, with no trace
quantity of 2-(diphenylamino)phenol being noted aer 2 h. This
observation supported the requirement of the catalyst for the
conversion. Utilizing 2 mol% catalyst, the yield of the desired
product was considerably improved to 53% aer 2 h. As ex-
pected, increasing the catalyst quantity facilitated the produc-
tion of the triphenylamine via the ring-opening transformation.
Expanding the catalyst amount to 4 mol% resulted in 61% yield
being probed aer 2 h. The reaction yield could be upgraded to
65% aer 2 h in the presence of 6 mol% catalyst. Raising the
catalyst concentration to 8 mol% offered 68% yield aer 2 h. As
noted earlier, the reaction employing 10 mol% catalyst could
progress to 75% yield aer 2 h. Boosting the catalyst amount to
more than 10 mol% was noticed to be superuous as the yield of
the triphenylamine was not intensied exceptionally (Fig. 7).
With these data on hand, we subsequently tried to improve
the yield of 2-(diphenylamino)phenol by boosting the reaction
temperature. He et al. previously conducted the reaction in
polyethylene glycol with CuCN catalyst under ligand-free
condition at 130 ꢀC for 18 h.13 In the rst demonstration of
this reaction with CuCN catalyst in pivalonitrile as solvent and
triphenylphosphine as ligand, Kim et al. achieved high yields of
triphenylamines at 120 ꢀC aer 24 h.12 The reaction was
implemented in diethylene glycol under argon at benzoxazole
concentration of 0.15 M for 2 h, employing 2.5 equivalents of
Cs2CO3, in the presenceꢀof 10 mꢀol% catalyst, with 2 equivalents
Fig.
5 Yields of 2-(diphenylamino)phenol vs. benzoxazole
concentration.
employing 2.5 equivalents of Cs2CO3, in the presence of
10 mol% catalyst, with 3 equivalents of iodobenzene, at various
benzoxazole concentrations. It was noted that the reaction yield
could be amended by changing the concentration of benzox-
azole. Extending the concentration from 0.075 M to 0.15 M, the
yield of 2-(diphenylamino)phenol was elevated from 68% to
77% aer 2 h. However, increasing the amount of benzoxazole
in the reaction mixture caused a drop in the yield of the tri-
phenylamine (Fig. 5). Furthermore, experimental results dis-
closed that the reactant molar ratio also displayed an impact on
the transformation. The reaction utilizing 1 equivalent of
iodobenzene produced the expected product in 59% yield aer
2 h. This value was boosted to 75% for the reaction with 2
equivalents of benzoxazole. Employing more benzoxazole for
the reaction did not amend the yield substantially. It was also
noted that utilizing a large excess amount of benzoxazole led to
considerable low yield of 2-(diphenylamino)phenol (Fig. 6).
Moreover, 2-(phenylamino)phenol, the diphenylamine product,
was produced noticeably in the presence of excess benzoxazole.
Another feature that must be scrutinized for the ring-
opening reaction of benzoxazole with iodobenzene to generate
2-(diphenylamino)phenol is the required catalyst quantity. In
the rst demonstration of this transformation in pivalonitrile as
solvent and triphenylphosphine as ligand, Kim et al. employed
10 mol% CuCN catalyst.12 He et al. subsequently performed the
ꢀ
ꢀ
ꢀ
of iodobenzene, at 110 C, 120 C, 130 C, 140 C, and 150 C,
respectively. As mentioned earlier, the reaction carried out at
120 ꢀC provided 2-(diphenylamino)phenol with 75% yield aer
2 h. Expanding the reaction time to 5 h at 120 ꢀC did not
intensify the yield of the triphenylamine signicantly.
Decreasing the temperature to 110 ꢀC caused a drop in the yield
of the triphenylamine. It was noticed that higher yields were
achieved when the transformation was conducted at higher
Fig. 6 Yields of 2-(diphenylamino)phenol vs. reactant molar ratios.
40932 | RSC Adv., 2017, 7, 40929–40939
Fig. 7 Yields of 2-(diphenylamino)phenol vs. catalyst concentration.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
Fig. 8 Yields of 2-(diphenylamino)phenol vs. temperature.
Fig. 10 Yields of 2-(diphenylamino)phenol vs. homogeneous catalyst.
temperature. Indeed, boosting the temperature to 130 ꢀC led to
a considerable improvement, with 82% yield of the desired
product being recorded aer 2 h. This value could be enhanced
added to this solution,ꢀ and the mixture was heated with
magnetic stirring at 140 C under argon for 2 h, with samples
being taken during the experiment. It was noticed that no more
2-(diphenylamino)phenol was produced in the absence of the
catalyst (Fig. 9). These observations veried that the ring-
opening of benzoxazole with iodobenzene to generate 2-
(diphenylamino)phenol could only progress in the presence of
the solid CuFe2O4 superparamagnetic nanoparticles.
To spotlight the superiority of utilizing the CuFe2O4 super-
paramagnetic nanoparticles for the ring-opening reaction of
benzoxazole with iodobenzene to generate 2-(diphenylamino)
phenol, the activity of this catalyst was compared with many
popular homogeneous catalysts such as CuCl, CuCl2, CuBr2,
Cu(OAc)2, Cu(NO3)2, FeCl2, Fe2(SO4)3, NiCl2, and CoCl2. The
reaction was carried out in diethylene glycol at 140 ꢀC under
argon for 2 h, employing 2.5 equivalents of Cs2CO3, with 2
equivalents of iodobenzene, at benzoxazole concentration of
0.15 M, in the presence of 10 mol% catalyst. Experimental data
showed that FeCl2, Fe2(SO4)3, NiCl2, and CoCl2 were almost
inactive for this transformation, with less than 5% yield of 2-
ꢀ
to 95% when the reaction was conducted at 140 C. Extending
the reaction temperature to 150 ꢀC was noticed to be worthless,
as the yield of 2-(diphenylamino)phenol was not upgraded
considerably (Fig. 8).
Since the ring-opening reaction of benzoxazole with iodo-
benzene to generate 2-(diphenylamino)phenol was performed
in solution phase, it is vital to carry out the leaching test. In
several examples, the reaction might progress under partly
homogeneous catalysis owing to the fact that some active sites
were dissolved into the reaction solvent. In order to work out if
homogeneous catalysis donated to the yield of 2-(diphenyla-
mino)phenol in this transformation, a control experiment was
consequently conducted. The reaction was performed in
ꢀ
diethylene glycol at 140 C under argon for 2 h, employing 2.5
equivalents of Cs2CO3, with 2 equivalents of iodobenzene, at
benzoxazole concentration of 0.15 M, in the presence of
10 mol% catalyst. Aer the experiment, ethyl acetate was added
to reduce the viscosity of the reaction mixture, and the CuFe2O4
superparamagnetic nanoparticles were isolated by magnetic
decantation. The liquid phase was transferred to a new ask,
and ethyl acetate was removed under vacuum, leaving the
diethylene glycol solution. Fresh reagents were subsequently
Fig. 9 Leaching assessment verified that 2-(diphenylamino)phenol
was not generated in the absence of CuFe2O4 superparamagnetic
nanoparticles.
Fig. 11 Yields of 2-(diphenylamino)phenol vs. heterogeneous catalyst.
RSC Adv., 2017, 7, 40929–40939 | 40933
This journal is © The Royal Society of Chemistry 2017

View Article Online
RSC Advances
Paper
Fig. 13 XRD results of the fresh (a) and reutilized (b) catalyst.
As previously mentioned, the ring-opening reaction pro-
ceeded readily in the presence of several copper salts as catalyst.
However, replacing these copper-based catalysts by iron salts,
less than 5% yield of 2-(diphenylamino)phenol was detected.
Moving to heterogeneous catalysts, nano Fe2O3, nano CoFe2O4,
nano NiFe2O4, and Fe-MOFs exhibited no activity for the
transformation, while the reaction using nano CuFe2O4 affor-
ded 95% yield. These data would verify that copper species on
the nano CuFe2O4 were active for the ring-opening reaction. To
receive perception into the pathway of the reaction between
benzoxazole and iodobenzene to generate 2-(diphenylamino)
phenol, some control experiments were then performed. In the
rst experiment, benzoxazole was heated with 2.5 equivalents of
Cs2CO3 in diethylene glycol at 140 ꢀC under argon for 2 h, in the
presence of 10 mol% catalyst. It was noted that 95% of ben-
zoxazole was converted to 2-aminophenol under these condi-
tions. Aer that, 2 equivalents of iodobenzene were added, and
the reaction mixture was heated at 140 ꢀC under argon for
additional 2 h. The experiment afforded 39% yield of 2-
(diphenylamino)phenol. In the second experiment, the reaction
Scheme 3 Plausible pathway for the transformation.
(diphenylamino)phenol being recorded aer 2 h. Several
common copper salts expressed high catalytic activity for the
generation of 2-(diphenylamino)phenol via the ring-opening
reaction. The reaction utilizing CuCl catalyst progressed to
84% yield aer 2 h, while 70% yield was observed for that using
CuCl2 catalyst. This value could be upgraded to 78% in the
presence of CuBr2 catalyst. Similarly, the Cu(NO3)2-catalyzed
ring-opening reaction of benzoxazole with iodobenzene affor-
ded 78% yield of 2-(diphenylamino)phenol aer 2 h. Cu(OAc)2
emerged to be less active for the transformation than Cu(NO3)2,
though 73% yield of the expected product was detected aer 2 h
(Fig. 10). It should be noted that the reaction utilizing CuFe2O4
superparamagnetic nanoparticles as catalyst proceeded to 95%
yield aer 2 h. Once again, under the optimized reaction
conditions, the CuFe2O4 superparamagnetic nanoparticles
offered better performance than other solid catalysts, with 95%
yield of 2-(diphenylamino)phenol being recorded aer 2 h
(Fig. 11).
Fig. 12 Catalyst reutilizing investigation.
40934 | RSC Adv., 2017, 7, 40929–40939
Fig. 14 TEM micrograph of the reutilized catalyst.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
Table 1 Synthesis of different triphenylamines via ligand-free selective ring-opening reaction utilizing CuFe2O4 catalyst
Isolated yield
(%)
Entry
Reactant 1
Reactant 2
Products
1
92
87
2
3
91
4
65a
5
63a
6
68
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 40929–40939 | 40935

View Article Online
RSC Advances
Paper
Table 1 (Contd.)
Isolated yield
(%)
Entry
Reactant 1
Reactant 2
Products
7
86
8
88
9
72
10
79a
11
85a
12
84a
40936 | RSC Adv., 2017, 7, 40929–40939
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
Table 1 (Contd.)
Isolated yield
(%)
Entry
Reactant 1
Reactant 2
Products
13
58a
14
68a
15
77a
16
61a
a
The reaction was carried out for 16 h.
between 2-aminophenol and iodobenzene was conducted under ring-opening reaction of benzoxazole with iodobenzene to
similar conditions. Interestingly, 41% yield of 2-(diphenyla- generate 2-(diphenylamino)phenol, the straightforwardness of
mino)phenol was recorded for this transformation. These data catalyst recycling was then studied. The superparamagnetic
suggested that the nano CuFe2O4-catalyzed reaction between nanoparticles were consequently investigated for reusability for
benzoxazole and iodobenzene to generate 2-(diphenylamino) the transformation over 5 consecutive runs. The reaction was
ꢀ
phenol would proceed via both pathways: (1) the ring-opening conducted in diethylene glycol at 140 C under argon for 2 h,
of benzoxazole to form 2-aminophenol, following by Ullmann- employing 2.5 equivalents of Cs2CO3, with 2 equivalents of
type coupling with iodobenzene, and (2) concerted ring- iodobenzene, at benzoxazole concentration of 0.15 M, in the
opening of benzoxazole/Ullmann-type coupling in one step presence of 10 mol% catalyst. Aer the rst run, ethanol was
(Scheme 3). However, further studies would be necessary to added to reduce the viscosity of the mixture, and the catalyst
elucidate the mechanism of this transformation.
was subsequently isolated by decantation utilizing a permanent
As previously mentioned, the CuFe2O4 superparamagnetic magnet. The recovered catalyst was washed thoroughly with
nanoparticles displayed better catalytic efficiency than a series water, ethanol, ethyl acetate, and ether to eliminate any phys-
ꢀ
of homogeneous and heterogeneous catalysts. To spotlight the isorbed compounds, and then heated at 150 C under vacuum
superiority of utilizing these nanoparticles as catalyst for the on a Shlenk line for 6 h. New catalytic transformation was
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 40929–40939 | 40937

View Article Online
RSC Advances
Paper
aerwards conducted utilizing the recovered catalyst. Experi- benzenethiol (Entry 12) was formed in 84% yield. The research
mental data revealed that it was possible to reutilize the catalyst scope was also expanded to the ring-opening reaction using aryl
for the ring-opening reaction of benzoxazole with iodobenzene bromides and aryl chlorides. Certainly, these halides were not
without a remarkable decline in catalytic activity. Certainly, previously employed for this transformation. Aryl bromides were
89% yield of 2-(diphenylamino)phenol was still obtained in the found to be reactive towards the ring-opening transformation.
5th run (Fig. 12). In addition, XRD analysis of the recovered Utilizing the CuFe2O4 catalyst, 1,10-(3,30-(2-hydroxyphenyl-
CuFe2O4 superparamagnetic nanoparticles displayed that the azanediyl)bis(3,1-phenylene))diethanone (Entry 13) was formed
catalyst structure was preserved during the course of the in 58% yield from the reaction of benzoxazole and 1-(3-bromo-
experiment (Fig. 13). TEM (Fig. 14) micrograph of the recovered phenyl)ethanone. Similarly, the reaction between benzothiazole
catalyst indicated that the particles were still in nanosize range, and
1-bromo-3-methoxybenzene
produced
2-(bis(3-
though a slight agglomeration was recorded. Elemental analysis methoxyphenyl)amino)benzenethiol (Entry 14) in 68% yield,
with AAS veried that the Cu : Fe molar ratio in the recovered while 77% yield of 2-(bis(2-methoxyphenyl)amino)benzenethiol
catalyst was approximately 1 : 2, corresponding to CuFe2O4 (Entry 15) was achieved for the case of 1-bromo-2-
structure.
methoxybenzene. Interestingly, 1-chloro-4-methoxybenzene was
The research scope was aerwards expanded to the synthesis reactive towards the reaction, forming 2-(bis(4-methoxyphenyl)
of different triphenylamines via ligand-free selective ring- amino)benzenethiol (Entry 16) in 61% yield.
opening reaction utilizing the CuFe2O4 catalyst (Table 1). The
reaction was performed in diethylene glycol at 140 ꢀC under
argon for 2 h, employing 2.5 equivalents of Cs2CO3, with 2
4. Conclusions
equivalents of iodobenzene, at benzoxazole concentration of
CuFe2O4 superparamagnetic nanoparticles were utilized as an
0.15 M, in the presence of 10 mol% catalyst. Aer the experi-
effective recyclable heterogeneous catalyst for the synthesis of
ment, triphenylamines were puried by column chromatog-
triphenylamines via the ligand-free selective ring-opening of
raphy utilizing silica gel. Following this procedure, 2-
benzoxazoles or benzothiazoles with iodoarenes. The nature of
(diphenylamino)phenol was achieved in 92% isolated yield via
the base and the solvent expressed a notable impact on the
the ring-opening reaction of benzoxazole with iodobenzene
transformation, and the combination of Cs2CO3 as the base and
(Entry 1). Moreover, to demonstrate the potential of industrial
diethylene glycol as the solvent should be the best option. The
application, a gram-scale reaction was conducted with 0.71 g of
CuFe2O4 nano catalyst was more active towards the ring-
benzoxazole and 3.67 g of iodobenzene. Under optimized
opening reaction than nano NiFe2O4, nano CoFe2O4, and
reaction conditions, 1.36 g of 2-(diphenylamino)phenol was
nano Fe2O3, as well as several solid catalysts such as Cu(BDC),
obtained, affording 87% isolated yield. Iodoarenes bearing
Cu(OBA), MOF-199, Cu2(BDC)2(DABCO), Cu2(BPDC)2(BPY), Co-
a substituent were noticed to be less reactive in the reaction
ZIF-67, Ni2(BDC)2(DABCO), and MIL-101. The CuFe2O4 nano
with benzoxazole. The ring-opening of benzoxazole with 1-iodo-
catalyst also displayed better performance than numerous
2-methylbenzene progressed to 87% yield of 2-(dio-tolylamino)
homogeneous catalysts. Aer each catalytic run, it was possible
phenol (Entry 2), while 91% of 2-(dip-tolylamino)phenol yield
to separate the nano CuFe2O4 by utilizing a magnet, and the
was obtained for the case of 1-iodo-4-methylbenzene (Entry 3).
recovered catalyst was reutilized many times while its efficiency
1-Iodo-4-methoxybenzene exhibited lower reactivity than 1-
could be maintained. The fact that the synthesis of triphenyl-
iodo-4-methylbenzene, though 65% yield of 2-(bis(4-
amines has been very limited in the literature, and that these
methoxyphenyl)amino)phenol (Entry 4) was still obtained.
structures could be generated using a recyclable heterogeneous
Similarly, 2-(bis(2-methoxyphenyl)amino)phenol (Entry 5) was
catalyst would attract attention from the chemical industry.
produced in 63% yield, while 68% yield of 2-(bis(3-
methoxyphenyl)amino)phenol (Entry 6) was achieved via the
reaction between benzoxazole and 1-iodo-3-methoxybenzene.
2,5-Dimethylbenzo[d]oxazole was also reactive towards this
Conflicts of interest
transformation, producing 2-(diphenylamino)-4-methylphenol There are no conicts to declare.
(Entry 7), 2-(dip-tolylamino)-4-methylphenol (Entry 8), and 2-
(bis(3-methoxyphenyl)amino)-4-methylphenol (Entry 9) in 86%,
88%, and 72% yields, respectively.
Acknowledgements
Inspired by these results, we then investigated the ring-opening
We would like to thank the Viet Nam National Foundation for
reaction of benzothiazole with iodobenzene in the presence of the
Science and Technology Development (NAFOSTED) for nan-
CuFe2O4 catalyst. Indeed, the conversion of benzothiazole to cor-
cial support under Project code 104.01-2015.35 (Contract
responding triphenylamines was not previously reported in the
number 14/2016/104/HĐTN).
literature. It was noticed that benzothiazole was less reactive than
benzoxazole, and the reaction time had to be extended to 16 h. The
reaction between benzothiazole and iodobenzene produced 2-
(diphenylamino)benzenethiol in 79% yield (Entry 10). Similarly, 2-
References
(bis(2-methoxyphenyl)amino)benzenethiol (Entry 11) was gener-
ated in 85% yield, while 2-(bis(4-methoxyphenyl)amino)
1 A. Chowdhury and P. S. Mukherjee, J. Org. Chem., 2015, 80,
4064–4075.
40938 | RSC Adv., 2017, 7, 40929–40939
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
2 G. Wu, F. Kong, Y. Zhang, X. Zhang, J. Li, W. Chen, W. Liu, 21 R. K. Rai, K. Gupta, D. Tyagi, A. Mahata, S. Behrens, X. Yang,
Y. Ding, C. Zhang, B. Zhang, J. Yao and S. Dai, J. Org.
Chem., 2014, 118, 8756–8765.
Q. Xu, B. Pathak and S. K. Singh, Catal. Sci. Technol., 2016, 6,
5567–5579.
3 C. Fan, C. Ye, X. Wang, Z. Chen, Y. Zhou, Z. Liang and X. Tao, 22 S. Cao, F. F. Tao, Y. Tang, Y. Li and J. Yu, Chem. Soc. Rev.,
Macromolecules, 2015, 48, 6465–6473. 2016, 45, 4747–4765.
4 Z. Fan, H. Zhao, N. Li, Y. Quan, Q. Chen, S. Ye, S. Li, Y. Wang, 23 J. Lemus, J. Bedia, L. Calvo, I. L. Simakova, D. Y. Murzin,
Q. Fan and W. Huang, ACS Appl. Mater. Interfaces, 2015, 7,
9445–9452.
B. J. M. Etzold, J. J. Rodriguez and M. A. Gilarranz, Catal.
Sci. Technol., 2016, 6, 5196–5206.
5 N. Kaneza, J. Zhang, H. Liu, P. S. Archana, Z. Shan, 24 L. Hu, R. Zhang and Q. Chen, Nanoscale, 2014, 6, 14064–
M. Vasiliu, S. H. Polansky, D. A. Dixon, R. E. Adams, 14105.
R. H. Schmehl, A. Gupta and S. Pan, J. Phys. Chem. C, 2016, 25 N. T. S. Phan, C. S. Gill, J. V. Nguyen, Z. J. Zhang and
120, 9068–9080. C. W. Jones, Angew. Chem., Int. Ed., 2006, 45, 2209–2212.
6 H. B. Goodbrand and N.-X. Hu, J. Org. Chem., 1999, 64, 670– 26 K. V. S. Ranganath and F. Glorius, Catal. Sci. Technol., 2011,
674. 1, 13–22.
7 J. P. Wolfe, H. Tomori, J. P. Sadighi, J. J. Yin and 27 F. M. Moghaddam, G. Tavakoli, F. Lati and B. Saeednia,
S. L. Buchwald, J. Org. Chem., 2000, 65, 1158–1174. Catal. Commun., 2016, 75, 37–41.
8 J. Hassan, M. Sevignon, C. Gozzi, E. Schulz and M. Lemaire, 28 B. Mohan, H. Kang and K. H. Park, Catal. Commun., 2016, 85,
Chem. Rev., 2002, 102, 1359–1469. 61–65.
9 I. P. Beletskaya and A. V. Cheprakov, Coord. Chem. Rev., 2004, 29 B. Mohan and K. H. Park, Appl. Catal., A, 2016, 519, 78–84.
248, 2337–2364.
10 J. Safaei-Ghomi, Z. Akbarzadeh and A. Ziarati, RSC Adv.,
2014, 4, 16385–16390.
11 A. M. Shaikh, B. K. Sharma, S. Chacko and R. M. Kamble,
RSC Adv., 2016, 6, 60084–60093.
12 D. Kim, K. Yoo, S. E. Kim, H. J. Cho, J. Lee, Y. Kim and
M. Kim, J. Org. Chem., 2015, 80, 3670–3676.
13 Y. He, J. Mao, G. Rong, H. Yan and G. Zhang, Chem.–Asian J.,
2016, 11, 1672–1676.
30 M. A. E. A. A. A. El-Remaily and H. A. Hamad, J. Mol. Catal. A:
Chem., 2015, 404–405, 148–155.
31 R. Rameshbabu, N. Kumar, A. Karthigeyan and
B. Neppolian, Mater. Chem. Phys., 2016, 181, 106–115.
32 P. B. Thale and P. N. B. G. S. Shankarling, RSC Adv., 2016, 6,
52724–52728.
33 M. A. E. A. A. El-Remaily, A. M. Abu-Dief and R. M. El-Khatib,
Appl. Organomet. Chem., 2016, 30, 1022–1029.
34 E. Eidi, M. Z. Kassaee and Z. Nasresfahani, Appl. Organomet.
Chem., 2016, 30, 561–565.
35 D. Yang, X. Zhu, W. Wei, N. Sun, L. Yuan, M. Jiang, J. You
and H. Wang, RSC Adv., 2014, 4, 17832–17839.
14 W. J. Stark, P. R. Stoessel, W. Wohlleben and A. Hafner,
Chem. Soc. Rev., 2015, 44, 5793–5805.
15 R. Purbia and S. Paria, Nanoscale, 2015, 7, 19789–19873.
16 R. K. Sharma, S. Sharma, S. Dutta, R. Zboril and 36 D. Yang, X. Zhu, W. Wei, M. Jiang, N. Zhang, D. Ren, J. You
M. B. Gawande, Green Chem., 2015, 17, 3207–3230. and H. Wang, Synlett, 2014, 25, 0729–0735.
17 D. Wang, Y. Sun, Y. Sun, J. Huang, Z. Liang, S. Li and 37 X. Zhu, D. Yang, W. Wei, M. Jiang, L. Li, X. Zhu, J. You and
L. Jiang, Nanoscale, 2017, 9, 7727–7733.
H. Wang, RSC Adv., 2014, 4, 64930–64935.
38 D. Yang, B. An, W. Wei, M. Jiang, J. You and H. Wang,
Tetrahedron, 2014, 70, 3630–3634.
39 L. Yao, Q. Zhou, W. Han and S. Wei, Eur. J. Org. Chem., 2012,
2012, 6856–6860.
18 C. Ray and T. Pal, J. Mater. Chem. A, 2017, 5, 9465–9487.
19 A. Ghorbani-Choghamarani, A. A. Derakhshan, M. Hajjami
and L. Rajabi, RSC Adv., 2016, 6, 94314–94324.
20 W. Yang, L. Wei, F. Yi and M. Cai, Catal. Sci. Technol., 2016,
6, 4554–4564.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 40929–40939 | 40939