A General Proline-Catalyzed Synthesis of 4,5-Disubstituted N-Sulfonyl-1,2,3-Triazoles from 1,3-Dicarbonyl Compounds and Sulfonyl Azide

Article abstract of DOI:10.1002/asia.201901015

An efficient proline-catalyzed synthesis of 4,5-disubstituted-N-sulfonyl-1,2,3-triazoles has been accomplished from 1,3-dicarbonyl compounds and sulfonyl azides. The developed reaction is suitable for various symmetrical and unsymmetrical 1,3-dicarbonyl compounds, tolerates various functional groups and affords 4,5-disubstituted-N-sulfonyl-1,2,3-triazoles in good yield with excellent regioselectivity. Rhodium-catalyzed denitrogenative functionalization of 4,5-disubstituted-N-sulfonyl-1,2,3-triazoles further demonstrates their utility in organic synthesis.

Full text of DOI:10.1002/asia.201901015

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: A General Proline Catalyzed Synthesis of 4,5-Disubstituted N-
Sulfonyl-1,2,3-Triazoles from 1,3-Dicarbonyl Compounds and
Sulfonyl Azide
Authors: Shanmugam Rajasekar and Pazhamalai Anbarasan
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10.1002/asia.201901015
Chemistry - An Asian Journal
COMMUNICATION
A General Proline Catalyzed Synthesis of 4,5-Disubstituted N-
Sulfonyl-1,2,3-Triazoles from 1,3-Dicarbonyl Compounds and
Sulfonyl Azide
Shanmugam Rajasekar and Pazhamalai Anbarasan*[a]
Dedication ((optional))
Abstract: An efficient proline catalyzed synthesis of 4,5-
disubstituted-N-sulfonyl-1,2,3-triazoles have been accomplished
from 1,3-dicarbonyl compounds and sulfonyl azides. The developed
reaction is suitable for various symmetrical and unsymmetrical 1,3-
dicarbonyl compounds, tolerates various functional groups and
affords 4,5-disubstituted-N-sulfonyl-1,2,3-triazoles in good yield with
excellent regioselectivity. Rhodium catalyzed denitrogenative
functionalization of 4,5-disubstituted-N-sulfonyl-1,2,3-triazoles further
demonstrates their utility in organic synthesis.
1,2,3-triazoles, viz. N-sulfonyl-1,2,3-triazoles, have emerged as
excellent source for the in-situ generation of α-diazoimines. The
trapping of in-situ formed α-diazoimines with suitable metal
catalyst, such as Rh(II)-catalyst, affords α-iminometallocarbenes,
which are highly electrophilic in nature and undergoes diverse
organic transformations to furnish complex heterocycles.[3] Most
of these transformations utilize 4-substiuted N-sulfonyl-1,2,3-
triazoles as source of α-diazoimines, because they are readily
accessible from Cu(I)/ligand[4] or copper(I) thiophene-2-
carboxylate[5] catalyzed [2+3]-cycloaddition of terminal alkynes
and sulfonyl azides. However, functionalization of 4,5-
disubstituted N-sulfonyl-1,2,3-triazoles have not been studied
extensively, possibly due to their limited access. The only know
method for the synthesis of 4,5-disubstituted N-sulfonyl-1,2,3-
triazoles involves formation of 4-lithiated N-sulfonyl-1,2,3-triazole
from terminal alkyne, sulfonyl azide and nBuLi and subsequent
trapping with suitable electrophile (Scheme 1a).[6] Hence,
development of new, mild and safer method for the synthesis of
4,5-disubstituted N-sulfonyl-1,2,3-triazoles are highly warranted.
1,2,3-Triazoles are important structural moieties found in many
synthetic bioactive compounds and agrochemicals. In particular,
substituted 1,2,3-triazoles and their analogues have been shown
to display potent therapeutic activities, such as, anti-allergic,
anti-bacterial and potassium-channel activator activities.[1]
Representative examples of therapeutically important molecules
containing substituted 1,2,3-triazole is shown in Figure 1. In
addition, 1,4- and 1,5-disubstituted 1,2,3-triazoles have been
used as effective mimics of trans and cis-amide bond,
respectively, well known as amide bioisosteres, because of their
similar size, planarity, dipole moment and hydrogen bonding
capabilities.[2] Although the synthetic molecules that contain
1,2,3-triazole units show diverse biological activities, 1,2,3-
triazole moiety does not occur in nature. Furthermore, 1,2,3-
triazoles are also frequently encountered in organic,
organometallic and material chemistry as a readily accessible
directing group, ligand and stable linker.[3]
a) 4,5-disubstituted N-sulfonyl-1,2,3-triazoles from alkyne: Previous work
E
N
nBuLi, –78 °C
N
+
TsN3
then E+
R
N
R
Ts
b) Substituted 1,2,3-triazoles from carbonyl compounds: Previous work
O
R1
R1
N
N
amines
enamine
catalysis
R2N3
+
N
R
N
or
N
R2
R1
N
H
R
R
O
if R2 = aryl
if R2 = SO2Ar
O
N
N
N
N
c) 4,5-disubstituted N-sulfonyl-1,2,3-triazoles from ketones: This work!
N
N
N
OH
N
O
N
O
F
N
N
Me
MugluR1 antagonist
Me
R
F
Cat. Proline
R2SO2N3
+
R
N
Pottassium channel
activator
N
R1
N
R1
1
O
2
SO2R2
F
3
MugluR1 antagonist
Scheme 1. Synthetic approaches to substituted 1,2,3-triazoles.
Figure 1. 1,2,3-Triazole containing therapeutically important molecules.
Recently, the groups of Ramachary,[7] Bressy[8], Wang[9] and
Alves[10] have developed a method for construction of 1,2,3-
triazoles from enolizable carbonyl compounds and aryl azides
employing enamine catalysis (Scheme 1b). In particular, reaction
of enolizable carbonyl compounds and sulfonyl azides tend to
give NH-triazole or diazo compound along with small amount of
N-sulfonyl-1,2,3-triazoles.[11] Inspired by these studies and our
continuing interest in the synthesis and functionalization of N-
sulfonyl-1,2,3-triazoles,[12] we envisioned a general and efficient
Besides these application, most recently, suitably substituted
[a]
Mr. S. Rajasekar and Prof. Dr. P. Anbarasan
Department of Chemistry
Indian Institute of Technology Madras
Chennai – 600036, India.
E-mail: anbarasansp@iitm.ac.in
Webpage: https://home.iitm.ac.in/anbarasansp/
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/asia.201901015
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COMMUNICATION
synthesis of 4,5-disubstituted N-sulfonyl-1,2,3-triazoles from
enolizable carbonyl compound and sulfonyl azide employing
organocatalyst (Scheme 1c).
pyrrolidine, piperidine and morpholine led to the formation of 3a
in only ~45% yield (Table 1, entries 10-12). Similar result was
observed with acyclic secondary amine, like diethylamine (Table
1, entry 13). These observations suggested that presence of
carboxylic acid functionality in secondary amine is highly
essential for the successful annulation. Decreasing the loading
of proline to 10 mol% also let to the product 3a in relatively low
yield. Thus, 20 mol% of proline in BuOH at rt were chosen as
optimized reaction conditions.
After identifying the suitable reaction conditions for the synthesis
of 4,5-disubstituted N-sulfonyl-1,2,3-triazoles 3, the scope and
generality of the method was examined with various substituted
1,3-diketones and sulfonyl azides. Thus, diverse alkyl and
To start the investigation, 2,4-pentanedione 1a and p-
toluenesulfonylazide 2a were chosen as a model substrate.
Initially, two equivalents of 1a and one equivalent of 2a were
treated in the presence of 20 mol% of L-proline in methanol.
Gratifyingly, the formation of expected product 3a was observed
in 56% yield after 12 h as single regioisomer, estimated by 1H
NMR. (Table 7.1, entry 1). Subsequently, the investigation was
directed to improve the yield of 3a. Thus, various polar protic
and aprotic solvent were screened (Table 1, entries 2-7). Among
them polar protic solvents (isopropanol, tert-butanol, tert-amyl
alcohol) were found to be superior to the polar aprotic solvents
(DMF and DMSO). The best yield of 81% was observed when
tert-butanol was employed as solvent (Table 1, entry 4).
Unfortunately, formation of product 3a was not observed when
water was used as a solvent (Table 1, entry 8). Similarly,
absence of proline also led to no reaction, which indicates the
importance of proline for the present annulation reaction (Table
1, entry 9).
t
arylsulfonyl azides
2 were examined under the optimized
conditions to afford the corresponding N-sulfonyl-1,2,3-triazoles
3 (Scheme 2). Reaction of benzenesulfonyl azides with 1a in the
presence of 20 mol% of proline gave the triazole 3b in 73% yield
as single regioisomer. Halogen containing arylsulfonyl azides
such as 4-fluorophenyl and 4-chlorophenyl derivatives
underwent smooth reaction to afford the triazoles 3c and 3d in
86% and 74% yield, respectively. Similarly, formation of electron
donating methoxyaryl substituted triazole 3e was observed in
87% yield. Interestingly, alkylsulfonyl azide, like methane- and
ethanesulfonyl azide also gave the corresponding triazoles 3f
and 3g in excellent yield. Unfortunately, reaction of 1,3-diphenyl
propan-1,3-dione did not furnish the expected product 3h under
the optimized conditions, possibly due to the steric reasons.
Next, 1,3-diketone was replacement with β-ketoester derivatives.
Gratifyingly, ethyl acetoacetate and tert-butyl acetoacetate were
also found to be reactive under the optimization with p-
toluenesulfonyl azide 2a and led to the formation of products 3i
and 3j in good yield.
Table 1. Secondary amine catalysed synthesis of 3a: Optimization.[a]
O
O
N
N
Me
Me
amine (20 mol%)
solvent, rt, time
+
Me
TsN3
N
Me
O
2a
Ts
1a
3a
entry
1
amine
solvent
Methanol
Ethanol
iPrOH
time (h)
yield (%)[b]
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
-
12
12
24
24
12
24
24
24
24
24
24
24
24
56
2
59
O
3
78
O
O
R
N
N
L-Proline
tBuOH, rt, 24 h
+
R1SO2N3
N
tBuOH
tAmOH
DMF
81 (67)[c]
R
Me
Me
4
1
Me
2
SO2R1
3
5
71
51
28
-
O
O
O
O
Me
Me
6
Me
Me
N
N
N
Me
Me
N
N
N
7
DMSO
H2O
N
N
N
S
N
S
Me
N
N
O
O
8
Ts
SO2Ph
3b: 73%
O
O
3a: 81%
Cl
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
-
F
3d: 74%
3c:86%
9
O
O
O
Me
Me
10
11
12
13
Pyrrolidine
Piperidine
Morpholine
41
49
42
46
N
Me
Me
Me
N
N
N
N
S
N
N
Me
N
N
O
SO2Me
3f: 79%
SO2Et
3g: 84%
O
3e: 87%
OMe
O
Diethylamine
O
O
tBuO
Me
EtO
Ph
Ph
N
N
N
N
N
[a] Reaction conditions: 2,4-petanedione 1a (0.2 mL, 1.95 mmol, 2 equiv.),
TsN3 2a (193 mg, 0.98 mmol, 1 equiv.), amine (cat.) solvent (10 mL), rt, time.
[b] Isolated yields. [c] With 10 mol% of L-proline.
N
N
Me
N
N
S
S
S
O
O
O
O
O
O
3j: 51%
OMe
Me
3h: 0%
Me
3i: 56%
On the other hand, other cyclic and acyclic secondary amines
were also examined in place of proline. Cyclic amines, such as
Scheme 2. Scope of sulfonyl azide 2.
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COMMUNICATION
Subsequently, the generality of substituted 1,3-diketone was
investigated with various aroylacetones under the optimized
conditions (Scheme 3). Reaction of benzoylacetone with 2a in
the presence of 20 mol% of proline furnished the triazole 3k in in
51% yield. Interestingly, increasing the loading of proline to 40
mol% gave the product 3k in 66% yield with complete
regioselectivity. The structure and regioselectivity of 3k was
unambiguously confirmed by single crystal X-ray analysis
(Figure 1).[13]
After a successful study of the scope of the various 1,3-diketone
and sulfonyl azides, utility of synthesized 4,5-disubstituted N-
sulfonyl-1,2,3-triazoles were studied through rhodium catalyzed
denitrogentaive functionalization.[14] Initially, dentriogenative
rearrangement of 3a with 4-nitrophenyl allyl sulfide 4 and allyl
alcohol 6 was investigated (Scheme 4a). Thus, reaction of
triazole 3a and 4-nitrophenyl allyl sulfide 4 in presence of 4
mol% of Rh(II) catalyst in toluene at 110 °C furnished the α-
thioimine 5 in 59% yield, via formation of sulfur ylide and
subsequent [2,3]-sigmatropic rearrangement.[12g, 15] On the other
hand, synthesis of 7 was achieved in 61% yield from 3a and allyl
alcohol
6
under
similar
conditions.[16]
Interestingly,
denitrogentaive transannulation of 3h with ethyl vinyl ether 8 in
the presence of 2 mol% of Rh2(Oct)4 in DCE at 100 °C gave the
1,2,3-trisubstituted pyrrole 9 in 46% yield (Scheme 4b).[12h]
a) Denitrogenative rearrangement
S
O
Figure 1. ORTEP diagram of 3k.
S
O2N
4
Me
Me
O
Rh2(Oct)4 (4 mol%)
toluene, 110 °C
4 h
N
NO2
Similarly,
naphthyl
derived
aroylacetone
with
p-
N
N
Me
Ts
methoxybenzenesulfonyl azide gave the product 3l in 65%
yield. Fluoro and bromo substitution on the aroylacetone was
well tolerated under the optimized conditions and led to the
formation of the corresponding triazoles 3m, 3n and 3p in good
yield. On the other hand, formation of alkoxy (methoxy and
ethoxy) substituted aroyl containing triazoles 3o, 3q and 3r were
achieved in moderated to good yield. A sterically demanding 2-
ethoxyphenyl substitution containing 1,3-diketone also
underwent smooth reaction to give the product 3s in 44% yield.
Heterocycle containing 1,3-diketone, 2-furoylacetone furnished
the corresponding triazole 3t in 43% yield. It is important to note
that the regioselectivity of the reaction remained same for all the
substrates.
N
5: 59%
Me
O
OH
6
Ts
3a
O
Rh2(Oct)4 (4 mol%)
toluene, 110 °C
3 h
Me
Me
NHTs
O
7: 61%
b) Denitrogenative transannulation
O
8
Rh2(Oct)4 (2 mol%)
DCE, 100 °C
16 h
OEt
EtO
Me
N
EtO
N
N
N
Me
3h
Ts
9: 46%
Ts
c) Denitrogenative C-H/X-H insertion
H
N
Et
Me
Et
N
10
R1
O
O
O
N
N
L-Proline
tBuOH, rt, 24 h
Rh2(Oct)4 (4 mol%)
toluene, 110 °C, 3 h
Et
N
R2SO2N3
NH
Ts
Me
+
R1
Ph
Me
Me
11: 55%
SO2R2
1
2
3
Et
O
N
N
O
Et
O
O
Me
Et
N
N
Me
12
N
F
N
N
N
N
O
Rh2(Oct)4 (4 mol%)
toluene, 100 °C, 3 h
Me
N
N
Ts
N
Me
13: 59%
Me
Me
Me
Ts
N
N
HN
Me
3a
SO2R2
SO2R2
O
3m: 71%†
3l: 65%†
Ts
3k: 66%†
R
O
O
N
Me
Rh2(Oct)4 (4 mol%)
DCE, 100 °C, 3 h
Br
N
EtO
N
N
N
O
N
N
Me
Me
N
Me
N
N
HN
Ts
Me
SO2R2
O
14
SO2R2
15: 76%
N
Me
SO2R2
R= Br; 3p: 57%†
R = OMe; 3q: 67%†
3o: 69%
OEt
3n: 42%
MeO
MeO
O
O
Scheme 4. Synthetic utility of 4,5-disubstituted N-sulfonyl-1,2,3-triazoles 3.
N
N
N
O
N
N
N
Me
3r: 53%†
Me
3s: 44%
R2 = p-(MeO)C6H4-
N
N
Me
N
After the successful demonstration of denitrogentaive
rearrangement and transannulation, rhodium catalyzed
denitrogative N-H and C-H insertion was studied (Scheme 4c).
Thus, the rhodium-catalyzed reaction of 3a with N-ethyl aniline
SO2R2
SO2R2
Ts
3t: 43%
Scheme 3. Scope of 1,3-dicarbonyl compounds 1. † with 40 mol% of proline.
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COMMUNICATION
Synthesis of 4,5-disubstituted N-sulfonyl-1,2,3-triazoles 3a: In an
oven dried reaction tube equipped with stir bar was charged with acetyl
acetone 1a (0.2 mL, 1.96 mmol, 2.0 equiv.) and tBuOH (10 mL). Next, L-
proline (22 mg, 0.1 mmol, 10 mol %) and p-toluenesulfonyl azide 2a (193
mg, 0.98 mmol, 1.0 equiv.) were added in to the reaction mixture and the
resultant reaction mixture was stirred at room temperature up to
consumption of sulfonyl azide. After the completion of the reaction,
solvent was evaporated under reduced pressure and the obtained crude
was further purified through column chromatography using ethyl
acetate/hexane as an eluent to afford the 3a in 81% yield (221 mg) as
colorless solid. Mp: 98-100 °C; FTIR (KBr): 2923, 1688, 1638, 1594,
10 gave the tetra-substituted enamide product 11 in 55%
yield.[17] Next, denitrogentaive C-H bond insertion of 3a with N,N-
diethyl aniline 6 in the presence of 4 mol% of Rh2(Oct)4
furnished the tetra-substituted enamide derivative 13 in 59%
yield.[12i] Likewise, replacement of N,N-diethyl aniline with N-
methylindole 14 under the similar conditions afforded the
corresponding tetra-substituted enamides 15 in 76% yield.[12f]
After establishing the generality of the proline catalyzed
synthesis of 4,5-disubstituted N-sulfonyl-1,2,3-triazoles 3, the
plausible mechanism of the reaction was proposed based on the
1397, 1193, 1123, 1029, 951, 814 cm–1 1H NMR (400 MHz, CDCl3,
;
literature
precedence.[7-10]
Treatment
of
1,3-dicarbonyl
24 °C): δ 7.97 (d, 2H, J = 8.4 Hz), 7.40 (d, 2H, J = 7.9 Hz), 2.83 (s, 3H),
2.65 (s, 3H), 2.45 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 24 °C): δ 194.0,
147.8, 142.8, 139.2, 133.3, 130.7, 128.9, 28.4, 22.0, 10.0; HRMS: calcd.
for C12H13N3O3S+H: 280.0756; found: 280.0759.
compound with proline would generate the enamine
intermediate A. The selective formation of enamine intermediate
A determines the regioselectivity in the 1,3-diketone. Formation
of cyclic intermediate C could be rationalized through the
enamine nitrogen triggered addition of electron rich β-carbon of
A to terminal nitrogen of sulfonyl azide 2 and consequent ring
closer, a formal [3+2]-annulation. This regioselective addition
further favored by the transition state B involving hydrogen-
bonding interactions. Later, elimination of proline from C would
generate the 4,5-disubstituted N-sulfonyl-1,2,3-triazoles 3 in an
highly regioselective manner and regenerate the proline to
continue the catalytic cycle (Scheme 5).
Acknowledgements
We thank the DST-SERB, New Delhi, India (EMR/2016/
003677/OC) for funding this work. S.R. thanks CSIR for
fellowships. We also thank Ramkumar, IITM for single-crystal
analysis support.
Keywords: N-sulfonyl-1,2,3-Triazoles • organocatalyst • proline
R
• 1,3-dicarbonyl compound • annulation
R1
N
O
O
OH
O
R1
R
N
R2O2S
1
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N
H
O
N
3
CO2H
R1
O
R
N
O
A
[2]
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N
HO
O
R1
R
R2O2S
N
N
N
C
O
R2SO2N3
N
O
N
O
2
R1
R
H
N
N
R2O2S
B
Scheme 5. Plausible mechanism.
[4]
a) E. J. Yoo, M. Ahlquist, S. H. Kim, I. Bae, V. V. Fokin, K. B. Sharpless,
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In conclusion, an efficient synthesis of 4,5-disubstituted N-
sulfonyl-1,2,3-triazoles have been achieved from substituted 1,3-
dicarbonyl compounds and sulfonyl azides employing proline as
a catalyst. The present method tolerates various functional
groups and allowed the access to various 4,5-disubstituted N-
[5]
[6]
J. Raushel, V. V. Fokin, Org. Lett. 2010, 12, 4952-4955.
M. E. Meza-Avina, M. K. Patel, C. B. Lee, T. J. Dietz, M. P. Croatt, Org.
Lett. 2011, 13, 2984-2987.
sulfonyl-1,2,3-triazoles
in
good
yield
with
excellent
[7]
a) D. B. Ramachary, A. B. Shashank, Chem. –Eur. J. 2013, 19, 13175-
13181; b) D. B. Ramachary, A. B. Shashank, S. Karthik, Angew. Chem.,
Int. Ed. 2014, 53, 10420-10424; c) A. B. Shashank, S. Karthik, R.
Madhavachary, D. B. Ramachary, Chem. –Eur. J. 2014, 20, 16877-
16881.
regioselectivity. The utility of the developed methodology was
further demonstrated through the rhodium catalyzed
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Shanmugam Rajasekar and Pazhamalai
Anbarasan*
Page No. – Page No.
A General Proline Catalyzed
Synthesis of 4,5-Disubstituted N-
Sulfonyl-1,2,3-Triazoles from 1,3-
Dicarbonyl Compounds and Sulfonyl
Azide
A general and efficient synthesis of 4,5-disubstituted N-sulfonyl-1,2,3-triazoles have
been achieved from substituted 1,3-dicarbonyl compounds and sulfonyl azides
employing proline as a catalyst.
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