An efficient synthesis of triazolium ion based NHC precursors using diaryliodonium salts and their photophysical properties

Article abstract of DOI:10.1039/C8OB01818A

Copper-catalysed N-arylation of fused triazoles using diaryliodonium salts as an aryl source is described. This scalable protocol displayed good compatibility towards diverse sensitive functional groups like ester, alkyl and nitro groups and halogens (F, Cl, Br). The synthetic usefulness of the prepared triazolium salts was proved by preparing α-hydroxyketone through benzoin condensation. Photophysical studies of these compounds showed promising Stokes-shifted fluorescence emission in aqueous medium, so this molecular framework could be a proficient probe for biological applications.

Full text of DOI:10.1039/C8OB01818A

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Pilania, M. A. Rohman, V. Arun, M. K. Mehra and S. Mitra, Org. Biomol. Chem., 2018, DOI:
10.1039/C8OB01818A.
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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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An efficient synthesis of triazolium ion based NHC precursor using
diaryliodonium salts and their photophysical properties
Meenakshi Pilania, a,b Mostofa Ataur Rohman,c V. Arun,a Manish Kumar Mehra,a Sivaprasad Mitrac
and Dalip Kumar *a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Copper-catalysed N-aryaltion of fused triazoles using Under basic reaction conditions, thiazolium and triazolium salts are
diaryliodonium salts as an aryl source is described. This scalable known to generate NHCs which facilitates benzoin and stetter
protocol displayed good compatibility towards diverse sensitive condensations.4
functional groups like ester, alkyl, nitro and halogens (F, Cl, Br).
Synthetic usefulness of prepared triazolium salts was proved by thiazolium salts have been demonstrated for their significant and
preparing α-hydroxyketone in benzoin condensation. versatile catalytic activities in wide range of organic
Azolium salts, for example, imidazolium, triazolium and
a
a
Photophysical studies of these compounds showed promising transformations.5 In the recent past, carbene precursors, namely
Stokes-shifted fluorescence emission in aqueous medium, so this 1,2,4-triazol-5-ylidenes, thiazol-2-ylidenes and imidazol-2-ylidenes
molecular framework could be a proficient probe for biological have been reported as stable nucleophilic heterocyclic carbenes.6
applications.
Subsequent efforts resulted in considerable improvement in the
stability of annulated heterocyclic carbenes such as bis(pyrido[a])
annulated imidazol-2-ylidene, pyrido[1,2-a][1,2,4]-triazol-3-ylidenes
N-Heterocyclic carbenes (NHCs) are widely used as umpolung
reagents in unconventional bond forming strategies to prepare
different bioactive heterocycles.1 NHCs are powerful tactic that
convert an electrophilic carbonyl group into a nucleophilic acyl
anion which in turn reacts with electrophilic aldehyde to generate
α-hydroxyketones.2 Initially the reaction of NHCs with aldehyde
generates the “Breslow intermediate” as described by Ronald
and
quinoline[a]-annulated
imidazol-2-ylidenes.
Structural
modification of heterocyclic carbenes may influence the σ-donor/π-
acceptor ligand properties and could be useful in tuning the
electronic properties.7 Owing to the importance of benzannulated
carbenes, various research groups8 explored the properties of
interesting pyrido-annulated NHCs. The combination of triazolium
salts with α, β-unsaturated compounds led to variety of products
Breslow in 1958.3 This Breslow intermediate behaves as
a
such
4-chromanones,11 cyclopentanones,12 dihydropyridinones13 and
pyridazinones.14
Moreover, transition metal-based
as
β-lactone-fused
cyclopentanes,9
δ-lactones,10
nucleophilic synthon and provides the feasibility to react with
another electrophilic substrate to yield benzoin as shown in
figure 1.
Cl
NHCs have been frequently used in important coupling reactions
including Suzuki-Miyaura, Sonogashira, Heck and nickel-catalyzed
Michael reaction15 as illustrated figure 2.
BF4
O
O
N
Base
NHC
N
n-Bu
N
C6F5
Ar
OH
N
Ar
H
Ar
N
Et
Et
Br
Ar
Besides their usefulness as precursors, NHCs have been
reported for their promising anti-malarial16 and hole blocking
properties.17 Triazolylidene iridium scaffold showed enzymatic
behavior for being a functional analogue of methyl transferases
such as S-adenosyl methionine.18
N
N
S
OH
S
HO
HO
"Breslow intermediate''
.Fig. 1 Azolium salt catalyzed benzoin condensation
aDepartment of Chemistry, Birla Institute of Technology and Science, Pilani-333031,
Rajasthan, India; E-mail: dalipk@pilani.bits-pilani.ac.in.
bDepartment of Chemistry, Manipal University, Jaipur-303007, Rajasthan, India
cDepartment of Chemistry, North-Eastern Hill University, Shillong- 793022, India,
E-mail: smitra@nehu.ac.in
Electronic Supplementary Information (ESI) available.
See DOI: 10.1039/x0xx00000x
Fig. 2 Examples of NHCs catalyzed useful transformations
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Initially, Kunai et al. reported the direct quaternization of azolium salts, whereas, the reaction did not proceed in DMF,
N-alkylimidazoles involving the reaction of imidazoles with in DMSO and acetontirile (entries 8-10). Finally, quaternization of
situ generated arynes.19 However, under these conditions [1,2,4]triazolo pyridines was successfully achieved by using
N-arylimidazoles did not produce the corresponding diaryliodonium salt (1.0 equivalent) in the presence of 5 mol%
imidazolium salts. In 2008, You group reported the synthesis of Cu(OTf)2 in DCE at 110 °C for 3 h.
pyrido-annulated triazolium salts utilizing 1-(pyridin-2-yl)-2-
Table 1 Quaternization of [1,2,4]triazolo[4,3-a]pyridines[a]
phenylhydrazine, ammonium hexafluorophosphate and
triethylorthoformate under refluxing conditions.7 A direct
synthesis of imidazolium salts by Gao and You utilized
diaryliodonium salts and N-substituted imidazoles. This
protocol represents an efficient synthesis of various
bis(imidazolium) salts as pre-catalysts of NHCs.20 Later,
Hollisand
co-workers
described
the
synthesis
of
Entry
Counterion
Catalyst
(5 mol%)
Cu(OAc)2
Solvent
Temp
(° C)
110
Yield[b]
(%)
bis(imidazolium) salts utilizing NHCs based pincer ligand
precursors.21 In 2014, Gao and You prepared unsymmetrical
imidazolium salts by utilizing arylboronic acids as an aryl
source.22 Despite advances and usefulness of azolium salts in
organic synthesis their substrate scope is largely limited to
imidazole moiety and have not been explored for the
quaternization of fused 1,2,4-triazole using diaryliodonium
salts.
(X)
1
OTf
DCE
85
2
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
Cu(OTf)2
CuI
DCE
110
110
110
110
110
80
95
3
DCE
50
4
Pd(OAc)2
Ni(OAc)2
-
DCE
NR
NR
NR
50
5
DCE
6
DCE
7
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
DCE
In recent years, diaryliodonium salts have been utilized as
versatile arylating agents in organic synthesis because of low
toxicity, easy handling, environmentally benign, high reactivity
and selectivity against various nucleophiles.23 These salts are
stable crystalline solids which serve as powerful arylating
agents in various transition metal-catalyzed and metal-free
reactions.24 Besides their usefulness in arylations,
diaryliodonium salts have been employed to construct
different bioactive heterocycles.25 Recently, we reported the
synthesis of different class of bioactive compounds such as
sulfones, arylazoles, biaryls and aryloxyquinolines by
employing diaryliodonium salts.26 In continuation of our
ongoing efforts to discover applications of diaryliodonium
salts, herein we wish to disclose a facile copper-catalyzed
protocol for the construction of useful [1,2,4]triazolo-
pyridinium salts as NHCs precursors.
8
DMF
DMSO
CH3CN
110
110
110
NR
NR
NR
9
10
[a]Reaction conditions:
1
(0.42 mmol), 2 (0.42 mmol) and catalyst (5 mol%)
were stirred in solvent (2 mL) for 3 h. [b]Isolated yield. NR= No reaction
With optimal reaction conditions, we investigated the
scope of developed methodology by using different
diaryliodonium salts
(2b-d). As shown in scheme 1,
diphenyliodonium salts with PF6 and BF4 counter ions rapidly
furnished the desired products (3b-c) in good yields (78-86%).
However, 2d with bromide counter ion did not arylate 1a
under the similar conditions. Next, we examined the scope of
developed methodology by using variety of diaryliodonium
salts (2e-g and 2n) and fused triazole 1a to prepare a series of
diverse triazolium salts 3a-g’ as illustrated in scheme 1. The
triazolium salt 3e was obtained in 89% yield by using
diaryliodonium salt 2e with an electron-donating methyl
group. Similarly, product 3f, was prepared in 78% yield from
[1,2,4]Triazolo[4,3-a]pyridines are prepared via iodoben-
zene diacetate-mediated oxidative cyclization of in situ
generated hydrazones from 2-hydrazinopyridine and
corresponding aldehyde.27 In a model reaction [1,2,4]triazolo-
[4,3-a]pyridine (1a) and diaryliodonium salt (2a) were used as
substrates to identify the optimized reaction conditions. The
reaction of 1a with 2a occurred in the presence of Cu(OAc)2 in
DCE to afford triazolium salt with 85% yield (Table 1, entry 1).
In successive efforts we found Cu(OTf)2 showed the highest
reactivity towards this reaction in DCE at 110 °C to furnish 3a
in 95% yield (Table 1, entry 2). A subsequent survey of reaction
conditions varying the Cu(II) to Cu(I) catalyst did not improve
the reaction efficiency and yielded 3a in 50% yield (Table 1,
entry 3). In presence of Pd(OAc)2 and Ni(OAc)2 the reaction
failed to deliver 3a (Table 1, entries 4-5). Also, no product was
formed (Table 1, entry 6) in absence of copper catalyst. Next,
some other parameters like temperature and solvent were
investigated. A sharp decrease in product yield was observed
by decreasing the temperature from 110 °C to 80 °C (Table 1,
entry 7). DCE was found to solvent of choice to prepare
5 mol% Cu(OTf)2
N
DCE, 110 °
N
N
N
Ar1
I
X
N
X
C
Ár2
2
N
1a
H
H
Ar1
3
X
OTf
OTf
I
I
I
R
R
2e
, R = CH3
2a
2b
, X = PF6
, X = OTf
COOMe
BF4
2g
2f, R = Cl
2c, X= BF4 2d, X = Br
Br
N
PF6
OTf
N
N
N
N
N
N
N
N
N
N
N
3a, 95%
OTf
3d, 0%
3b, 78%
OTf
3c, 86%
N
N
OTf
OTf
N
N
N
N
N
N
N
N
N
N
CH3
Cl
H
3e, 89%
3f, 78%
3g, 80%
3g', 75%
COOMe
Scheme 1 Preparation of triazolium salts 3a-3g′
2 | J. Name., 2012, 00, 1-3
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diaryliodonium salt 2f with an electron-deficient chloro group. This condensation reaction. Initially, triazolium salt 3a was treated
difference in product yield could be explained by the higher with t-BuOK for the generation of in situ carbene, which
reactivity of diaryliodonium salt with electron-donating group over converted benzaldehyde into benzoin
electron-deficient as described in previous reports.28 Unsymmetrical scheme 3.
diaryliodonium salt 2g bearing ester group at meta-position
(5) as shown in
afforded the desired 3g in 80% yield. Sterically demanding
dimesityliodonium triflate 2n also smoothly afforded arylated
products 3g’ in good yield (75%). Next, we arylated 3-phenyl-
[1,2,4]triazolo[4,3-a]pyridine (1b) using diverse diaryliodonium salts Scheme 3 Triazolium salt 3a catalyzed Benzoin condensation
bearing different functionalities (Scheme 2). In the case of
unsymmetrical diaryliodonium salts 2h-m less sterically hindered
In view of literature findings29a and results of the present
aryl ring always transferred to the substrate 1b while mesityl moiety work, a plausible reaction mechanism for the formation of
acted as a dummy group. o-Nitro, ester, meta-methyl, dimethyl and triazolium salts 3a-t is depicted in Scheme 4. Initially either by
di-fluorophenyl units transferred smoothly to fused triazoles 3- the substrate
phenyl-[1,2,4]triazolo[4,3-a]pyridine (1b) to prepare a variety of Cu(II) to reactive Cu(I) species.29b Then oxidative addition of
triazolium salts 3h-p in 80-95% yields. Further, symmetrical [Mes-I- diaryliodonium salt to Cu(I) species generate highly reactive
Mes] iodonium salt 2n also afforded the corresponding mesityl Cu(III)-aryl intermediate with the release of iodobenzene.
triazolium salt (3q) in 84% yield. N-Heteroaryltriazoles bearing Further coordination of fused triazoles to intermediate
thienyl, furyl and pyridyl moieties 1c-e successfully arylated to facilitates the formation of intermediate II. Finally, reductive
1 or disproportionation of Cu(II) may reduce
2
I
1
I
afford the desired products 3r-t in excellent yields (90-91%). In case elimination of II may deliver the product
3
with reforming the
of 3-(pyridin-2-yl)-[1,2,4]triazolo[4,3-a]pyridine (1e), diaryliodonium Cu(I) species for the cycle.
Cu(OTf)2
salt 2a selectively quaternized the triazoles nitrogen instead of
pyridine nitrogen to afford triazolium salt 3t in 91% yield. It is worth
Cul(OTf)
mentioning that this protocol showed good compatibility towards
N
Ar1
I
OTf
N
OTf
diverse functional groups namely ester, methyl, methoxy, nitro and
especially halogens (F, Cl, Br) which could be used in several organic
synthetic transformations.
N
Ár2
2
H
Ar1
3
Ar2I
N
N
OTf
Culll
Ar1
N
[Ar1CulllOTf]OTf
H
II
I
N
N
N
H
1
Scheme 4 Plausible pathway for the formation of
3
The heteroleptic bis(tridentate) ruthenium(II) and other
metal complexes comprised of a C,N,C-tridentate coordinating
bis(triazolylidene)pyridine
pyridine ligands are known to display interesting
photochemical However, photophysical
properties.30
and
2,6-bis(1,2,3-triazol-4-yl)
behaviour of this class of compounds is scantly discussed. In
view of the interesting photochemical properties of triazolium
salts, herein we report the basic photophysical behavior of
some selected compounds, viz. 3a, 3h, 3o, 3p and 3r in
representative homogeneous solvent systems like CHCl3,
acetonitrile, DMSO, methyl alcohol and water. The selected
classes of compounds represent varying electron density on
the triazolium moiety due to different substitutions on the
peripheral rings and the solvents were chosen to represent the
variation in polarity, viscosity as well as hydrogen bond
donation and acceptance ability. The results are summarized in
table 2 and compared with 3a as a model system.
The principle absorption band of all the investigated compounds
Scheme 2 Preparation of triazolium salts 3h-3t
appear in the range of 290 – 330 nm in all the solvents (ε = 0.6 – 1.0
×
104 M-1 cm-1). Although the absorption spectral profile
To investigate the synthetic utility of this protocol, triazolium
of a particular compound is insensitive to solvent properties, the
salt 3a was further used as a precursor of NHC in a benzoin
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Table 2. Summary of photophysical parameters of some selected triazolium salts (3a, 3h, 3o, 3p and 3r) in different solvents[a]
Solvents
Acetonitrile
CHCl3
λabs/nm
290
294
290
292
290
300
299
304
300
297
300
300
300
300
300
295
295
295
295
295
293
295
295
294
293
λem/nm
424
413
404
427
427
426
426
430
424
426
452
452
452
452
452
421
421
396
421
421
428
428
428
428
428
∆νSS/cmꢀ1
10898
9801
ɸf
<τ>/ns
1.9
1.9
5.6
2.2
1.6
2.4
1.7
1.4
1.1
2.0
3.8
2.9
2.5
3.4
1.3
1.4
1.5
4.0
1.6
1.9
1.0
1.2
1.0
1.2
2.0
κr/109sꢀ1
Σκnr/109sꢀ1
0.42
0.39
0.18
0.38
0.54
0.32
0.47
0.71
0.69
0.42
0.17
0.23
0.40
0.21
0.67
0.66
0.54
0.24
0.51
0.47
0.95
0.74
1.01
0.79
0.46
N
0.19
0.10
N
N
OTf
H
0.24
0.13
3a
DMSO
9730
1.2×10-3
0.16
2.1×10-4
0.07
Methyl alcohol
Water
10827
11064
9859
0.12
0.07
Acetonitrile
CHCl3
0.25
0.10
N
N
N
9971
0.19
0.11
OTf
DMSO
9639
0.7×10-2
0.22
5.0×10-3
0.20
3h
Methyl alcohol
Water
9748
10196
11209
11209
11209
11209
11209
10145
10145
8646
0.17
0.08
Acetonitrile
CHCl3
0.34
0.09
N
N
N
OTf
0.32
0.11
CH3
CH3
1.9×10-2
0.27
5.0×10-3
0.08
3o
DMSO
Methyl alcohol
Water
0.14
0.11
Acetonitrile
CHCl3
0.07
0.05
N
N
N
0.19
0.13
OTf
F
F
DMSO
0.04
0.01
3p
Methyl alcohol
Water
10145
10145
10765
10534
10534
10649
10765
0.17
0.11
0.11
0.06
Acetonitrile
CHCl3
0.08
0.08
N
N
0.08
0.07
N
OTf
S
DMSO
0.4×10-2
0.07
5.0×10-3
0.06
3r
Methyl alcohol
Water
0.06
0.03
[a]Absorption (λabs) and fluorescence emission (λem) maximum; Stokes shift (ΔνSS); Average fluorescence decay time (<τ> =
Σai×τi2/Σai×τi, where ai and τi represents pre-exponential factor and lifetime of i-th component, Fluorescence yield (φf); Radiative (κr)
and total non-radiative (Σκnr) decay rate constant
4 | J. Name., 2012, 00, 1-3
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Fig. 3b Steady state fluorescence emission spectra of the
investigated systems in acetonitrile. The number designation of the
investigated compounds is as follows: 3a (1), 3h (2), 3o (3), 3p (4)
and 3r (5)
Fig. 3a Steady state absorption spectra of the investigated systems
in methanol. The number designation of the investigated
compounds is as follows: 3a (1), 3h (2), 3o (3), 3p (4) and 3r (5).
electron donation and withdrawing nature of substituents in
different derivatives has marked effect. The model compound 3a
shows well-shaped absorption peaking at ca. 290 nm; however, for
the substituted derivatives (3h, 3o and 3p) the absorption is
relatively broad with loss in spectral shape and red-shifted by about
10-15 nm in different cases. Interestingly, 3r shows structured
absorption with peaks at 260 and 290 nm (Figure 3a). Excitation at
the respective absorption band, all the compounds showed clean
and unstructured fluorescence emission with 350-550 nm region
(Figure 3b). Substitution has very strong effect on emission spectral
Interestingly all the compounds showed very large Stokes’ shifted
fluorescence emission. Particularly, the highest Stokes shift for all
the investigated systems in aqueous medium (ca. 11000 cm-1),
which eliminates the possibility of reabsorption, is promising for
these class of compounds to be a precursor for biological as well as
technological applications.31
Fluorescence quantum yields (φf) of all the compounds were
measured with respect to quinine bisulfate in 0.5 M H2SO4 solution
as standard by following the procedure mentioned in the
literature.32 For all the systems, the quantum yield is found to be
the lowest in DMSO, whereas, in aqueous medium the yield is
moderate to high. The high value of molar absorptivity along with
good fluorescence yield for these systems are indicative of π→π*
transition in molecular excitation process.33 The extremely low
fluorescence quantum yield in DMSO is rationalised on the basis of
very inefficient radiative decay channel (see below).
peak position
.
For example, electron donating substituents in 3o caused the
fluorescence emission to appear at ca. 454 nm in contrast to the
other investigated systems showing fluorescence at 426 nm in
methanol. Similar observation is also seen in other solvents (table -
2).
Fluorescence decay behaviour of all these systems needs at
least two exponential decay functions to fit the experimental data
adequately. A representative decay data along with the fitted
parameter is shown in Figure 4.
The detailed result of the fluorescence decay data analysis is
presented in Table 2. Interestingly, the contribution of the short
decay component (τ1 = 0.5 – 1.0 ns) is always very high and mostly
contributes in the total fluorescence decay in comparison with the
longer decay component (τ2>2.0 ns). Nevertheless, the average
fluorescence decay time (<τ>) and the kinetic parameters
characterizing the excited state deactivation process, like radiative
(κr) and total non-radiative (Σκnr) decay rate constants were
calculated with the following relations and listed in table 2. The
non-radiative rate is very high, particularly for the substituted
derivatives in comparison with the model system and can be
rationalized on the basis of free rotation of N – C single bond.
Fig.
4 Time resolved fluorescence decay behaviour of 3r in
chloroform along with the instrument response function (IRF). The
fitted line is a deconvolution curve with 2-exp decay function. The
comparative statistical parameters like reduced chi-square (χR2),
Durbin
– Watson (DW) parameter, distribution of weighted
residuals (upper panels) and autocorrelation (Ac) function (inset)
justifies the 2-exponential fitting model.
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13 B. Wanner, J. Mahatthananchai, J. W. Bode, Org. Lett. 2011
13, 5378.
14 A. Chan, K. A. Scheidt, J. Am. Chem. Soc. 2007, 129, 5334.
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Med. Chem. 2010, 18, 6184.
17 J. Wu, Q. You, J. Lan, Q. Guo, X. Li, Y. Xue, J. You, Org. Biomol.
Chem. 2015, 13, 5372.
,
In conclusion,
a
rapid and efficient copper-catalyzed
methodology has been developed to prepare a variety of triazolium
salts from the reaction of fused triazoles and readily available
diaryliodonium salts. In unsymmetrical diaryliodonium salts mesityl
always act as a dummy group and less sterically hindered aryl ring
transferred to the fused triazoles with concomitant release of
iodomesitylene. In addition, the synthesized triazolium salts served
18 K. F. Donnelly, A. Petronilho, M. Albrecht, Chem. Commun.
2013, 49, 1145.
as
a precursor of NHCs and successfully utilized in benzoin
condensation to prepare α-hydroxyketone. After the reaction,
released iodoarenes were recovered and reused in the preparation
of diaryliodonium salts. This high-yielding, scalable, ligand and base-
free protocol shows good compatibility towards various functional
groups and complexity can be introduced to the molecule by further
functionalization. Next, the photophysical properties and
solvatochromism for selected group of compounds have been
studied, which predicts the utility of this novel group of systems as
biological stains and/or in technological applications.
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Conflicts of interest
There are no conflicts to declare
Acknowledgements
We gratefully acknowledge CSIR, New Delhi for financial support
(CSIR, No. 02(02399)/17/EMR-II) and awarding Senior Research
Fellowship (MP, VA & MKM). We are also thankful to DST-FIST and
BITS Pilani for providing NMR and HRMS facilities.
Tolnai, U. J. Nilsson, B. Olofsson, Angew. Chem. Int. Ed. 2016
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