3,6-Di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz) catalysed metal-free amide bond formation from thioacids and amines at room temperature

Article abstract of DOI:10.1016/j.tetlet.2020.152272

A 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz) catalysed efficient, mild and metal-free method has been developed for direct amide bond synthesis from simple thioacids and amines as starting materials. This methodology is useful for aromatic, aliphatic, and heteroaromatic thioacids as well as primary, secondary, heterocyclic, and even functionalized amines. A wide substrates scope, operationally mild conditions, and acylation of amines without affecting other functional groups such as alcohols, esters, carbodithioates, among others make this strategy very attractive and practical.

Full text of DOI:10.1016/j.tetlet.2020.152272

Journal Pre-proofs
3,6-Di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz) Catalysed Metal-Free Amide
Bond Formation from Thioacids and Amines at Room Temperature
Suvendu Samanta, Shounak Ray, Samanka Narayan Bhaduri, Partha Kumar
Samanta, Papu Biswas
PII:
S0040-4039(20)30739-5
DOI:
Reference:
https://doi.org/10.1016/j.tetlet.2020.152272
TETL 152272
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Tetrahedron Letters
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Revised Date:
Accepted Date:
17 March 2020
13 July 2020
18 July 2020
Please cite this article as: Samanta, S., Ray, S., Bhaduri, S.N., Samanta, P.K., Biswas, P., 3,6-Di(pyridin-2-
yl)-1,2,4,5-tetrazine (pytz) Catalysed Metal-Free Amide Bond Formation from Thioacids and Amines at Room
Temperature, Tetrahedron Letters (2020), doi: https://doi.org/10.1016/j.tetlet.2020.152272
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3,6-Di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz) Catalysed Metal-Free Amide Bond
Formation from Thioacids and Amines at Room Temperature
Suvendu Samanta, Shounak Ray, Samanka Narayan Bhaduri, Partha Kumar Samanta and Papu
Biswas
Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur,
Howrah 711 103, West Bengal, India. E-mail: papubiswas_besus@yahoo.com

Abstract
A 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz) catalysed efficient, mild and metal-free
method has been developed for direct amide bond synthesis from simple thioacids and amines as
starting materials. This methodology is useful for aromatic, aliphatic, and heteroaromatic
thioacids as well as primary, secondary, heterocyclic, and even functionalized amines. A wide
substrates scope, operationally mild conditions, and acylation of amines without affecting other
functional groups such as alcohols, esters, carbodithioates, among others make this strategy very
attractive and practical.
Keywords: Amide bond; thioacid; metal-free reaction; room temperature; organic oxidative
mediator; tetrazine

Amides are ubiquitous in nature and synthetic chemicals. Through the amide bond
formation amino acids are covalently linked to determine the primary structure of proteins.1-4
Amide bonds are also foundation for some of the most versatile and widely used synthetic
polymers, insecticides, agrochemicals, nutraceuticals, and pharmaceutical drugs including
lactams (antibiotic).1-9 It has been estimated that almost 25% of all synthetic pharmaceutical
drugs contain amide functionality.5-9 Therefore, efficient synthesis of amide bonds is an
important topic in synthetic organic chemistry.1-4 At present, most common methods for
synthesizing amide bonds rely on activation of carboxylic acid using carbodiimide, uronium, or
phosphonium reagents and subsequent ("in situ") coupling of the activated species with an
amine.1-4 These reagents are expensive, often-toxic, and produce stoichiometric amount of
unwanted co-products.1-4 Moreover, these processes often involve co-reagents including bases,
super-nucleophiles and other additives in stoichiometric excess. To overcome these challenges,
thioacid-mediated amide bond formation has attracted much attention as alternative strategy.
Recent studies also demonstrate some unique reactivity and selectivity of thioacids compared to
those of carboxylic acids. The reactivity of thioacids with organo-azides,10,11 isonitriles,12
thioisocyanates,13,14 isocyanates,13,14 aziridines,15 electron deficient sulphonamides,16 nitroso
derivatives17 and dinitrofluorobenzene18 have been efficiently exploited in the direct peptide
bond formation or native chemical ligation. To date, very few methods have been developed
which utilize direct coupling of thioacids and amines. The synthesis of amide bonds via
formation of S-nitrosothioacids (NTA) at 0°C using thioacids and amines was reported by Xian
et al.19 The peptide bond synthesis through in situ activation of thioacids with
bistrimethylsilylacetamide (BSA) was reported by Liebeskind and co-worker.20 Copper(II)
mediated ultrafast peptide synthesis using amines and thioacids was reported by Gopi and his
group.21,22 Very recently, visible-light-promoted amide bond synthesis from thioacids and amines
has also been reported.23-25
The catalysis or organic reaction catalysed with small organic molecules, where a metal
is not part of the active principle, has become a highly active area in chemical research in recent
past.26,27 In recent past, 1,2,4,5-tetrazine based molecules have been successfully utilized as
catalysts for various organic transformations. Our group previously reported 3,6-di(pyridin-2-yl)-
1,2,4,5-tetrazine (pytz) as visible light photoredox catalyst for synthesis of 2-substituted
benzimidazoles and oxidation of alcohols to carbonyls.28,29 The oxidation of thiols or thioacids to

corresponding disulfides using pytz as oxidant at ambient temperature has also been reported by
us.30 A simple one-pot synthesis of 2-substituted-1H-4-carboxamide benzimidazole using pytz as
photocatalyst was reported by Zhang et al.31 Masson and her group reported several s-tetrazine
based photocatalyst for oxidative nitroso-Diels-Alder reaction of N-arylhydroxylamines with
dienecarbamates32 and oxidative C–S bond cleavage of thioethers.33 In our continuous efforts to
study new chemistry and synthetic application of 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz),28-
30 we envisioned that if thioacids react with pytz (Scheme 1) the corresponding disulfides would
be generated in situ as reported by us earlier.30 Subsequent reaction between disulfide and free
amine leads to the formation of an amide bond.23,25,34,35 Herein, we report a very efficient metal-
free amide bond formation from thioacids and amines catalyzed by pytz at mild conditions.
Scheme 1. Synthesis of amide formation from thioacids and amines
Initially, to optimize the reaction conditions, we selected the reaction of thiobenzoic acid
1a with aniline 2a as the model reaction as shown in Table 1. First, the efficiency of different
1,2,4,5-tetrazines (tz) were screened in tetrahydrofuran (THF) at room temperature with 1
equivalent of tetrazine. The results showed that pytz was the most efficient for this reaction
(entry 1), while other tetrazines resulted in a significant drop in yields (entries 2 and 3). In the
absence of pytz, no desired product was obtained (entry 4). Further assessment of the reaction
conditions indicated that THF was the optimal solvent, while other commonly used solvents such
as chloroform, dichloromethane (DCM), ethanol (EtOH), methanol (MeOH), dimethyl
formamide (DMF), acetonitrile and water gave lower yields (entries 1 and 5−11). Notably,
gradual decrease in the amount of pytz to 0.1 equiv (entries 12-14) gave the identical result to
those of 1.0 equiv pytz (entry 1). By increasing or decreasing the reaction temperature, no
improvements in either yield or reaction time were observed (entries 15 and 16). To summarize

this study, the optimized reaction conditions for the amide 3a included reaction of 1a with 2a in
presence of 0.1 equiv of pytz in THF at room temperature under molecular oxygen.
Table 1
Optimization of the reaction conditions.a
Entry Tetrazine
Solvent
Time (h) Amount of Tetrazine (equiv)
Yieldb (%)
1
2
pytz
phtzc
4,4'-bptzd
–
THF
THF
5.0
6.0
6.0
6.0
7.0
7.0
8.0
8.0
8.0
7.0
8.0
5.0
5.0
5.0
5.0
6.0
1.0
1.0
1.0
–
98
52
70
trace
50
60
13
17
23
40
15
98
97
97
97
95
3
THF
4
THF
5
pytz
pytz
pytz
pytz
pytz
pytz
pytz
pytz
pytz
pytz
pytz
pytz
CHCl3
DCM
EtOH
MeOH
DMF
CH3CN
water
THF
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.3
0.1
0.1
0.1
6
7
8
9
10
11
12
13
14
15e
16f
THF
THF
THF
THF
a
Reaction conditions: 1a (1.0 mmol), 2a (1.0 mmol), tetrazine in solvent (10 mL), room
temperature, molecular oxygen.
b Isolated yield.
c PhTz = 3,6-diphenyl-1,2,4,5-tetrazine.
d 4,4´-bptz = 3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine.
e The reaction temperature was 50 °C.
f The reaction temperature was 10 °C.
Having identified the optimal reaction conditions, we next examined the substrate scope
and versatility of this methodology. As shown in Table 2, a wide range of aniline derivatives 2

upon reaction with thiobenzoic acid 1a afforded corresponding products 3aa–3ah with the yield
ranging from 75% to 97%. The results indicate that aniline, substituted anilines with electron-
donating group (OMe), electron withdrawing group (COOEt) at para-positions, weak σ-electron
donating group (CH3) at ortho and meta-position and halogen group (Br) at para-position
smoothly transformed into the desired products in good to excellent yields (69-85%; Table 2,
3ab–3af) at room temperature. Steric effects had a little influence on the reaction since 2,5-
dimethylaniline, 2,6-dimethylaniline, and N-methylaniline gave moderate to good yields (Table
2, 3ae–3ag). Moreover, 2-amino pyridine also furnished very good yield (Table 2, 3ah).
To further extend the utility of this methodology, a series of aliphatic primary and
secondary amines 2 were employed to react with thiobenzoic acid 1a under the optimized
reaction conditions, all effectively affording the corresponding amide products 3ai−3ap in
moderate to excellent yields (65%−92%, Table 2). A collection of formidable substrates, amino
esters 2l, 2m, amino amide 2n, amino carbodithioates 2o, 2p and amino alcohol 2r were found to
be well tolerated under the reaction conditions. Benzyl amine 2q, 2-amino-2-phenylethanol 2r,
and pyridine 2-methyl amine 2s were also found to be suitable and afforded the desired amides
(3aq−3as) with thiobenzoic acid.
Next we examined reactivity of thioacetic acid 1b with various amines (Table 2).
Thioacetic acid was found to react smoothly with aniline amines 2a–2c and quinolin-8-amine 2t
under optimal conditions to provide the corresponding amide products 3ba−3bc and 3bd in very
good to excellent yields (Table 2, 91–97%). Moreover, heterocyclic thioacid, pyridine-3-
carbothioic S-acid 1c was also found to be reactive and furnished 82% yield with aniline (Table
2, 3ca).
Furthermore, the structure of methyl 2-benzamidocyclopent-1-enecarbodithioate 3ao and
methyl 2-(2-benzamidoethylamino)cyclopent-1-enecarbodithioate 3ap were confirmed and
analyzed by X-ray crystallographic analysis (For details see Table S1 and X-ray crystallography
section in the Supplementary Data) of their single crystals, which were obtained by re-
crystallization using methanol-hexane (1:1) mixture (Fig. 1a and b).

Table 2
Substrate scope in 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz) catalysed amide bond
formation.a,b
aReaction conditions: 1 (1.0 mmol), 2 (1.0 mmol), pytz (0.1 mmol), THF (10 mL),
room temperature, molecular oxygen.
bIsolated yield, purity checked by 1H NMR.

a
b
Fig. 1. ORTEP diagram of 3ao (a) and 3ap (b). Thermal ellipsoids are drawn at 50% probability
level. Hydrogen atoms are omitted for clarity.
To obtain more insight of the reaction mechanism, a series of additional experiments
were conducted (Scheme 2). The reaction between thiobenzoic acid 1a and aniline 2a in absence
of pytz led to the formation of amide 3aa in trace amounts under the optimized reaction
conditions. Thiobenzoic acid 1a upon reaction with pytz provided corresponding disulfide 4 as
reported by us earlier.30 Subsequently, treatment of the freshly prepared disulfide 4 with aniline
2a effectively afforded the corresponding amide 3aa which was demonstrated earlier. 23,25,34,35 To
ascertain radical intermediate, the reaction was conducted in presence of excess 2,2,6,6-
tetramethylpiperidin-1-oxyl (TEMPO) and a yield of <10% for 3a was observed.
Scheme 2. Control experiments.

On the basis of the control experiment and previous reports,23-25,34,35 a plausible
mechanism is proposed, as shown in Scheme 3. First, two molecule thioacid 1 react with a pytz
molecule to produce disulfide intermediate 4 which then simultaneously converted into amide in
presence of amine23,25 and an acyldisulfide intermediate 5. Due the substantially high reduction
potential28,29 of pytz than thioacids,36,37 pytz easily oxidizes thioacids to corresponding disulfide
and itself converted into H2pytz. The acyldisulfide 5 could further produce amide through
aminolysis as reported previously34,35 and H2S2 formed as only by-product which readily
decomposes to H2S and elemental sulfur.36 The lead acetate paper test confirmed the presence of
H2S and elemental sulfur was isolated from the reaction mixture after the completion of the
reaction. On the other hand, the acyldisulfide 5 could also react with another molecule of
thioacid acid to afford corresponding disulfide 4 as suggested previously by Danishefsky et al.35
3,6-di(pyridin-2-yl)-1,4-dihydro-1,2,4,5-tetrazine (H2pytz) molecule formed during the reaction
is oxidised by molecular oxygen to produce pytz again.29
Scheme 3. Proposed reaction mechanism.
In conclusion, we have developed here a mild and efficient amide bond formation
strategy from simple thioacids and amines in THF at room temperature. The reactions were
catalyzed by 10 mol% 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz). Compatibility of the reaction

was explored with a wide range of aliphatic and aromatic amines containing various other
functional groups. All amides were isolated in moderate to high yields. The simple workup, mild
reaction conditions and high yields of this reaction may serve as an attractive alternative to those
of the existing methods. In our opinion, this method should also be promising for peptide
coupling/ligation and selective N-acylation.
Experimental
General Experimental Procedure for the Synthesis of Amide
Thioacids (1 mmol, 1.0 equiv.), pytz (0.1 mmol, 0.1 equiv.) in THF (5 mL) were placed
in a 50 mL round bottom flask and amines (1 mmol, 1.0 equiv.) dissolved in 5 ml THF was then
added to it slowly under oxygen. The reaction mixture was stirred at room temperature for 4-7 h.
When the reaction was completed (monitored by TLC), reaction mixture was evaporate to
dryness at room temperature under vacuum. The residue was re-dissolved in chloroform (5 mL),
washed with water and brine and dried over anhydrous Na2SO4. After filtration and removal of
solvent, the residue was purified by flash column chromatography (hexane/ethyl acetate, 10:1) to
afford the desired products 3aa−3as, 3ba−3bc, 3bt.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We acknowledge SAIF-IIEST, Shibpur for providing Single Crystal X-ray Diffractometer
facility and SAIF-NEHU for providing NMR facility.
Appendix A. Supplementary data
Supplementary data (details of X-ray Crystallography and characterization details of amides by
1H, 13C NMR, LC-MS and HRMS) to this article can be found online at
https://doi.org/10.1016/j.tetlet.2020.xxxxxx.

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Highlights
Organocatalysis by 3,6-Di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz)
Amide bond formation from thiacids and amines
Metal-free mild reaction conditions
Wide range of aliphatic and aromatic amines as substrate
Graphical Abstract