Visible-light driven regioselective synthesis of 1H-tetrazoles from aldehydes through isocyanide-based [3 + 2] cycloaddition

Article abstract of DOI:10.1039/c8gc01321g

A novel and green Co@g-C₃N₄ catalyzed visible light driven direct regioselective synthesis of 1H-tetrazoles directly from various aldehydes and sodium azide is reported. Herein, NaN₃ not only behaves as a three-nitrogen do

Full text of DOI:10.1039/c8gc01321g

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Visible-light driven regioselective synthesis of 1H-tetrazoles from
aldehydes through isocyanide-based [3+2] cycloaddition
Fooleswar Verma,a Anjumala Sahu,a Puneet K. Singh,b Ankita Rai,b Manorama Singha and Vijai K.
Raia*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A novel and green Co@g-C3N4 catalyzed visible light driven direct regioselective synthesis of 1H-tetrazoles directly from
various aldehydes and sodium azide is reported. Herein, NaN3 not only behaves as three-nitrogen donor of tetrazole ring
but also it converts aldehyde in to isocyanide as one-nitrogen source. Regioselectivity of the product, operational
simplicity, ambient reaction conditions, no use of column chromatography for purification, excellent yield of products (84-
95%) and recyclability of catalyst upto five times without any substantial change in morphology as well as catalytic
www.rsc.org/
efficiency
are
the
salient
features
of
the
envisaged
protocol.
To design and develop new catalytic routes for chemical most stable nitrogen-rich heterocycle.15 Interest in tetrazole
transformations with high activity and selectivity, which is chemistry has increased rapidly over the past few years due to
benign to the environment would be most welcome. Due to its wide application ranging from pharmaceutical, biological,
eco-compatibility, easy availability, safe handling and coordination and material to synthetic organic chemistry for
everlasting abundance as non-conventional energy source for generating
activation of reactions, visible light-induced synthetic tetrazole
a
library of N-heterocycles.15-19 In particular,
derivatives have been used as key
strategies, especially photoredox catalysis, have emerged as pharmacophores,17 potential photosensitive materials,18 and
state-of-art alternative of traditional conventional synthetic non-classical metabolism-resistant isosteres of CO2H group in
methods to advance the green chemistry program.1,2 biologically active molecules.19
Photoredox catalysis triggers the photo excitation of substrate
The classical approach for tetrazole synthesis involves two-
through single-electron transfer (SET) mechanism and component reaction of azide precursors and N-containing
nowadays it has been widely used in organic synthesis.3-6 Thus, compounds via 1,3-dipolar cycloaddition of azide with organic
inexpensive visible light photocatalytic organic transformations nitrile in acidic environment (Scheme 1).20 However, this
have attracted much attention of synthetic chemists in these process is very slow and requires high reaction temperature
environmentally cautious days.
and long reaction time and also strong electron-withdrawing
A catalyst of renewable origin would be in accordance with groups are needed on the nitrile component to expedite the
green chemistry principle. The polymeric graphitic carbon process. Moreover, a large variety of additives like Brønsted
nitride (g-C3N4) has interested earth abundant elements, which and Lewis acids,21 heterogeneous catalysts22 have been
could be obtained by pyrolysis of nitrogen-rich containing employed for fast conversion to tetrazole nucleus. Later on,
precursors, viz., urea, thiourea, cyanamide, dicyandiamide, inorganic azide was
trithiocyanuric acid, melamine, triazine and heptazine
derivatives.7 Though, g-C3N4 finds application as
photocatalyst,8 its photocatalytic behaviour is limited because
of low efficiency, mainly due to rapid charge recombination.9
To overcome this problem, several strategies have been
developed including doping of metal/non-metal in the cavity
of g-C3N4.9-13 Metal-doped g-C3N4, especially Co@g-C3N4 shows
extremely high catalytic activity for organic reactions along
with its reusability.14
Tetrazoles are an elite class of compounds being one of the
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however, after 3 h, only trace of 4a was obtained (Table 1,
entry 1). Then, various metal-doped g-C3N4 such as Ni@g-C3N4
(15 wt%), Cu@g-C3N4 (15 wt%), Fe@g-C3N4 (15 wt%), and
Co@g-C3N4 (15 wt%), were screened as catalyst employing the
same reaction condition (Table 1, entries 1-5). We started with
Ni@g-C3N4 under the same reaction condition, which afforded
4a only in 3h with 34% yield (Table 1, entry 2), however, the
result was exciting due to its selectivity to afford only 4a as
single product. Among all screened catalysts, Co@g-C3N4 was
found the best photocatalyst for the formation of tetrazole 4a
in terms of yield (92%) and conversion rate (3h) at room
temperature in the presence of visible light (Table 1, entry 5).
Next, for optimization of solvent-system, both protic and
aprotic solvents were screened and methanol showed the best
result for the formation of 4a (Table 1, entries 5-13). Regarding
catalyst loading, increase in catalyst amount from 10 to 15 mg
did not increase the conversion rate (Table 1, entries 5 and
15), while decreasing the amount to 5 mg, the yield of product
4a was decreased (Table 1, entries 5 and 14) and hence, 10 mg
of
Scheme 1: Different strategy for synthesis of tetrazoles moiety.
replaced by organic azide viz., trimethylsilyl,23 trialkyltin,24 and
organoaluminium azides.25 Literature records some improved
synthesis of tetrazoles26-29 using Suzuki-Miyaura coupling30 and
Baylis-Hillman reaction.31
Alternatively, three-component strategy utilizes aldehyde,
ammonia and azide via in situ generation of nitrile for 1,3-
cycloaddition for tetrazole synthesis (Scheme 1d,e).20e,f
However, two-component synthesis of tetrazole direct from
aldehyde and azide is not reported so far. Thus, to fill the
literature gap and due to our quest for new organic
transformations,32 we report herein visible-light driven
unprecedented direct synthesis of tetrazole starting from
aldehyde using Co@g-C3N4 as novel photocatalyst (Scheme 1f).
The cornerstone of the envisaged method is application of
NaN3 not only as three-nitrogen donor for 1,3-dipolar addition
but also it converts aldehyde into in situ isocyanide as one-
nitrogen source (Scheme 1f).
Scheme 2: Synthesis of carbon nitride and cobalt-doped carbon nitride.
Result and discussion
For preparation of photocatalyst, urea was calcinated at 500 °C
to get converted as yellowish graphitic carbon nitride (g-C3N4)
and then it was used as support for Co-metal interruption.33
The solution of g-C3N4 (Sol A, 425 mg) and cobalt (II) acetate
Co(OAc)2).4H2O (Sol B, 317 mg, 15 wt % of g-C3N4, Co 75 mg)
was prepared in aqueous methanol (50%) individually through
sonication. Now, Sol B was added in Sol A and stirred for 6h at
rt. Prepared nanoparticle was centrifuged and washed with
water and methanol, centrifuged and dried under vacuum at
50 °C. The prepared Co@g-C3N4 catalyst was then
characterized by UV-Vis, FTIR, X-ray diffraction (XRD),
photoluminescence (PL), scanning electron microscopy (SEM),
energy-dispersive X-ray spectroscopy (EDX), transmission
electron microscopy (TEM) and X-ray photoelectron
spectroscopy (XPS).
Co@g-C3N4 was taken as optimum catalyst loading (Table 1,
entry 5). Next, to ensure the role of visible light, a reaction was
performed employing the same model protocol (i) in absence
of light as well (ii) in dark condition, but we could not isolate
even a trace amount of product 4a (Table 1, entries 16, 17).
Thus, Co@g-C3N4 (10 mg) in MeOH at room temperature in
presence of LED light as visible light source was found the
preeminent condition for transformation of product tetrazole
4a (Table 1, entries 5).
After optimization of the reaction condition (Table 1, entry
5), a variety of structurally different aldehydes have been used
Table 1 Screening of catalyst and solvent system for synthesis of tetrazole 4a.a
In preliminary experimentation, 4-chlorobenzaldehyde 1a
(1.0 mmol), sodium azide
2 (3 mmol), g-C3N4 (10 mg) and
MeOH (5 mL) were taken in a hot air-dried round bottom flask
and stirred under the visible light at room temperature,
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Entry Catalyst (mg)
Solvent
MeOH
Time (h)
03
Yield (%)b,c
afforded satisfactory yield 84-86% of tetrazoles
4 (Scheme 3),
1
@g-C3N4 (10)
Trace
34
56
32
92
35
75
59
76
78
82
84
55
86
92
----
----
however, conversion rate was rather slow in this case and the
reaction was completed in 5h (Scheme 3). As substrate scope,
we took butyraldehyde as aliphatic aldehyde employing the
present optimized reaction conditions, but the yield of
corresponding tetrazole was very poor (31%) presumably due
to poor absorbance of light by aliphatic aldehyde as well as
lower activity of primary radical. We have also performed
experiment using CD3OD as solvent, but the product was same
and no deuterium exchange from methanol was observed in
the product. Structure of all the synthesized products were
confirmed by their 1H and 13C NMR spectra (ESI).
2
Ni@g-C3N4 (10) MeOH
Cu@g-C3N4 (10) MeOH
Fe@g-C3N4 (10) MeOH
Co@g-C3N4 (10) MeOH
Co@g-C3N4 (10) H2O
Co@g-C3N4 (10) EtOH
Co@g-C3N4 (10) DMF
Co@g-C3N4 (10) CH3CN
03
3
03
4
03
5
03
6
03
7
03
8
03
9
03
10
11
12
13
14
15
16d
17e
Co@g-C3N4 (10) H2O/MeOH (3:2) 03
Co@g-C3N4 (10) H2O/MeOH (2:3) 03
Co@g-C3N4 (10) H2O/MeOH (1:4) 03
Co@g-C3N4 (10) H2O/MeOH (4:1) 03
We anticipated to obtain 1-aryl-5H-tetrazole 3a via cyanide
intermediate (path a, Scheme 4), but we did not isolate either
cyanide or 3a. However, regioselective construction of 1H-
Co@g-C3N4 (05) MeOH
Co@g-C3N4 (15) MeOH
Co@g-C3N4 (10) MeOH
Co@g-C3N4 (10) MeOH
03
03
03
03
tetrazole
4 may be explained through isocyanide 8
intermediate (path b), which was further confirmed by a
singlet at δ 8.83 for one 5-H proton and also supported by
literature report.20 Plausible mechanism for the formation of
aReaction conditions: 4-chlorobenzaldehyde 1a (1 mmol), sodium azide 2a (3
mmol), 7-volt LED light, r.t., in air. b Yield of isolated and purified products. cAll
compounds gave C, H and N analyses within ±0.35 %, and satisfactory spectral (1H
NMR, 13C NMR, IR and EIMS) data. dReaction in absence of visible light. eReaction
in dark.
1H-tetrazole
4
involves the Co@g-C3N4 promoted initial attack
to generate intermediate
of azide 2 on aldehyde
1
7.
Scheme 4: Plausible mechanism for regioselective construction of 1H-tetrazole 4.
The XPS analysis of Co@g-C3N4 shows that cobalt is present in
Co(II) state, which can accommodate easily the lone pair
electron of oxygen and thereby increasing the electrophilicity
Scheme 3. Substrate scope of 1H-tetrazoles 4.
of carbonyl carbon of aldehyde to afford intermediate
next step, Co@g-C3N4 induced photocatalytic excitation of
intermediate presumably results 1,2-phenyl radical migration
from C to N, followed by removal of N2 to afford the isocyanide
intermediate 8. Finally, the isocyanide intermediate
undergoes [3+2] cycloaddition with azide to afford target
compound (Scheme 4).
7. In the
to check the general validity of the envisaged method. The
7
reaction afforded good to excellent yield of tetrazoles
4 (84-
95%) with the highest yield of 4c (Scheme 3). However, there
8
is no specific effect of substituents at benzene ring and
2
aldehyde
1 tolerated both the electron withdrawing and
4
electron releasing groups smoothly for the formation of
4
(Scheme 3). Heterocyclic aldehydes were also investigated and
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Furthermore, experiment for reusability of the catalyst was image of Co@g-C3N4 has also confirmed the graphitic- like
also performed. After completion of the reaction (monitored structure and it supported the XRD analysis very well (Figure
by TLC), water was added to the reaction mixture to take the 1c, d).
remaining sodium azide in aqueous layer separated from
The Fourier transform infrared (FTIR) spectrum exhibits
organic part. Then, the catalyst was simply filtered from evidently exposed broad bands in the 3000−3500 cm−1 region
reaction mixture and washed with water and ethanol, dried in that could be recognized for O-H stretching and N-H stretching
under vacuum and was reused for further experiment without (Figure 2b).11a The absorption in the range of 1200−1700 cm−1
any substantial change in its morphology as well as in catalytic are consigned to the characteristic stretching modes of g-C3N4
efficiency up to five cycles (Figure 4).
Conclusions
In conclusion, we have disclosed the regioselective synthesis of
1H-tetrazoles directly from aldehydes in the presence of visible
light at room temperature. The envisaged protocol involves
the novel application of Co@g-C3N4 as photocatalyst.
Operational simplicity, atom economy, ambient reaction
conditions, no use of column chromatography for purification,
avoiding hazardous chemicals, reusability of the photocatalyst
up to five times without significant loss of its activity are the
silent features and warrant this protocol a highly green
alternative method for tetrazole synthesis directly from
aldehydes.
Fig. 1. (a) SEM image of g-C3N4, inset: EDX, and SEM image of Co@g-C3N4, inset: EDX, (c)
Experimental
& (d) HR-TEM images of Co@g-C3N4; inset: SAED pattern.
General Information
heterocycles. At
be the out-of-plane skeletal bending modes of triazine and the
band at
1640 cm−1 is recognized to the C=N stretching
vibration mode.11b The UV- visible spectra of g-C3N4 and Co@g-
C3N4 shows the absorbance peaks in the UV ( 235 nm) regions
(Figure 2c).34 Blue luminescence was observed in
photoluminescence spectra of g-C3N4 and Co@g-C3N4 because
of transition among lone pair (LP) states in the valence band
and the π* antibonding states in the conduction band. The PL
spectrum of g-C3N4 and Co@g-C3N4 shows strong blue
∼
806 cm−1 absorption band is considered to
All chemicals were purchased from Aldrich, Sd-Fine and HI-
MEDIA (India) and used as received, except all solvents which
were used after distillation. All reactions were carried out with
oven-dried glassware under air. Analytical TLC was performed
on Merck 60F254 silica gel plates (0.25 mm thickness). 1H NMR
∼
∼
1
spectra were recorded on Bruker AV 400 and 500 MHz. The H
NMR chemical shifts are reported relative to the centre of
solvent resonance (CDCl3: 7.26 (1H). Chemical shifts are
expressed in parts per million (δ) and the signals were
reported as s (singlet), d (doublet), dd (double doublet), t
(triplet), dt (doublet triplet), q (quartet), m (multiplet) and
coupling constants J were given in Hz. 13C NMR spectra were
recorded at 100 and 125 MHz in CDCl3 and DMSO-d6 solution.
Chemical shifts are expressed in parts per million (δ) and are
referenced to CDCl3 (δ = 77.16) as internal standard.
luminescence at
∼465 nm and ∼460 nm, respectively. The
states consisting of the sp3 C−N σ band, the sp2 C−N π band,
and the LP state of the bridge nitride atom are accountable for
the luminescence from g-C3N4 (Figure 2d).35 The absorption in
the visible region for g-C3N4 helps in investigation of its visible
light photocatalysis.
Moreover, the X-ray photoelectron spectroscopy (XPS)
measurements were performed to observe the chemical
composition and chemical position of the constituent elements
in the Co@g-C3N4 sample (Figure 3). The XPS survey spectra
depicts that C, N, and Co elements were detected on the
Co@g- C3N4 sample (Figure 3a), indicating the loading of Co on
g-C3N4. The peaks of Co 2p3/2 and Co 2p1/2 were located at
769.3 and 789.2 eV, respectively, along with the presence of
the two strong satellite peaks of cobalt at 773.3 and 792.3 eV,
indicating the presence of Co(II) in Co@g-C3N4, without the
formation of any other oxidation state (Figure 3b). The C 1s
spectra peaks at 285.7 and 287.1 eV were assigned to the C=N
coordination and sp2-hybridized carbon in N-containing
Characterization of Co@g-C3N4
SEM images exposed the two-dimensional arrangement of g-
C3N4 (Figure 1a), and Co@g-C3N4 (Figure 1b) sheets along with a
fine agglomeration. Carbon and nitrogen in g-C3N4 carbon,
nitrogen and a small amount of Co metal in Co@g-C3N4 were
studied by EDX spectra (Figure 1a, b; inset). The X-ray
diffraction (XRD) pattern has shown the graphitic-like phase of
C3N4 in g-C3N4 and Co@g-C3N4. The strong peaks at 27.6°
communicated to typical (002) interlayer stacking (JCPDS No.
87-1526) and confirms the dissimilarity in g-C3N4 and Co@g-
C3N4 (Figure 2a). A transmission electron microscopy (TEM)
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aromatic rings (N-C=N), respectively (Figure 3c). The N 1s remaining sodium azide in aqueous layer leaving target
spectra shown can be deconvoluted into four peaks. The peaks compound in organic residue. The catalyst was then simply
centred at 398.1 and 398.7 eV are derived from the sp2- filtered from reaction mixture and washed with water and
hybridized nitrogen atoms in C=N-C groups and the amino ethanol, dried under vacuum and was reused for further
function groups, respectively. The peak at around 399.4 eV experiment. Then, the organic layer was washed with brine (2
was attributed to the charging effects or positive charge x 10 mL) and water (2 x 10 mL), dried under vacuum
localization in heterocycles, while that at 400.2 eV is attributed evaporator to obtain the crude isolates as colourless solid.
to the tertiary nitrogen in N-C3 groups or in H-N-C2 (Figure Finally, recrystalized of crude isolate in ethanol afforded the
3d).36
analytically pure sample of tetrazole 4 without any column
chromatography. Structure of all the synthesized compounds
were confirmed by their 1H and 13C NMR spectral analysis (ESI).
Synthesis of tetrazole 4g directly from phenyl isocyanide
A mixture of phenyl isocyanide (1 mmol), sodium azide
2 (1.5
mmol), Co@g-C3N4 (10 mg) and MeOH (5 mL) in a hot air-dried
round bottom flask was stirred at room temperature under the
visible light. After completion of the reaction (as monitored by
TLC), water was added to the reaction mixture to separate the
remaining sodium azide in aqueous layer leaving target
compound in organic residue. The catalyst was then simply
filtered from reaction mixture and washed with water and
ethanol, dried in under vacuum and was reused for further
experiment. Then, the organic layer was washed with brine (2
× 10 mL) and water (2 × 10 mL), dried under vacuum
evaporator to obtain the crude isolates as pale yellow solid.
Finally, recrystallization of crude isolate in ethanol afforded
the analytically pure sample of tetrazole 4g without any
column chromatography. This was further confirmed by a
singlet at δ 9.06 for one 5-H proton.
Fig. 2. Characterisation of g-C3N4 and Co@g-C3N4; (a) XRD spectra, (b) FTIR, (c) UV-
Visible and (d) Photoluminescence spectra.
Fig. 3. XPS data of Co@g-C3N4; (a) Scan survey, (b) Co 2p, (c) C 1s and (d) N 1s data.
Fig. 4. Recyclability of Co@g-C3N4 for the synthesis of tetrazole 4a.
General procedure for synthesis of tetrazole 4
Acknowledgements
A mixture of aldehyde
1 (1 mmol), sodium azide 2 (3 mmol),
Co@g-C3N4 (10 mg) and MeOH (5 mL) in a hot air-dried round
bottom flask was stirred at room temperature under the
visible light. After completion of the reaction (as monitored by
TLC), water was added to the reaction mixture to separate the
We sincerely thank the SERB, New Delhi, India (File No.
SR/FT/CS-99/2010) for partial financial support. We are also
thankful to AIRF, JNU, New Delhi, NIT Raipur and IIT Kanpur for
providing microanalyses and spectra.
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Y. Su, X. Zhang, Y. Liu, X. Wu and B. Li, Langmuir, 2016, 32
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TABLE OF CONTENT
Visible-light driven regioselective synthesis of 1H-tetrazoles from
aldehydes through isocyanide-based [3+2] cycloaddition
Fooleswar Verma, Anjumala Sahu, Puneet K. Singh, Ankita Rai, Manorama Singh and Vijai K. Rai
A novel and green Co@g-C3N4 catalyzed visible light driven direct regioselective synthesis of 1H-tetrazoles
directly from various aldehydes and sodium azide is reported. Herein, NaN3 not only behaves as three-
nitrogen donor of tetrazole ring but also it converts aldehyde in to isocyanide as one-nitrogen source.
Regioselectivity of the product, operational simplicity, ambient reaction conditions, no use of column
chromatography for purification, excellent yield of products (84-95%) and recyclability of catalyst upto five
times without any substantial change in morphology as well as catalytic efficiency are the salient features of
the envisaged protocol.