A comparative study of amide-bond forming reagents in aqueous media – Substrate scope and reagent compatibility

Article abstract of DOI:10.1016/j.tetlet.2017.10.014

A survey of amidation reagents demonstrating DIC-HOPO, DMT-MM, COMU-collidine, TPTU-NMI, EEDQ, CDI and EDC-Oxyma to be effective for the coupling of carboxylic acids with amines in the presence of water and the absence of problematic dipolar aprotic solvents is reported. DMT-MM was shown to provide the best yields for the coupling of a secondary amine, TPTU-NMI and COMU-collidine for aniline, whilst the combination of DIC with HOPO afforded the broadest substrate scope and the highest yields for a sterically demanding carboxylic acid.

Full text of DOI:10.1016/j.tetlet.2017.10.014

Accepted Manuscript
A Comparative Study of Amide-Bond Forming Reagents in Aqueous Media –
Substrate Scope and Reagent Compatibility
Matthew Badland, Robert Crook, Bastien Delayre, Steven J. Fussell, Iain
Gladwell, Michael Hawksworth, Roger M. Howard, Robert Walton, Gerald A
Weisenburger
PII:
S0040-4039(17)31283-2
https://doi.org/10.1016/j.tetlet.2017.10.014
TETL 49376
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
26 August 2017
3 October 2017
5 October 2017
Please cite this article as: Badland, M., Crook, R., Delayre, B., Fussell, S.J., Gladwell, I., Hawksworth, M., Howard,
R.M., Walton, R., Weisenburger, G.A., A Comparative Study of Amide-Bond Forming Reagents in Aqueous Media
– Substrate Scope and Reagent Compatibility, Tetrahedron Letters (2017), doi: https://doi.org/10.1016/j.tetlet.
2017.10.014
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A Comparative Study of Amide Bond
Forming Reagents in Aqueous Media –
Substrate Scope and Reagent Compatibility
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Matthew Badland, Robert Crook, Bastien Delayre, Steven J. Fussell*, Iain Gladwell, Michael
Hawksworth, Roger M. Howard, Robert Walton and Gerald A Weisenburger

1
Tetrahedron Letters
A Comparative Study of Amide-Bond Forming Reagents in Aqueous Media –
Substrate Scope and Reagent Compatibility
Matthew Badlanda , Robert Crooka, Bastien Delayreb, Steven J. Fussella*, Iain Gladwella, Michael
Hawkswortha, Roger M. Howardc, Robert Waltona and Gerald A Weisenburgerc
aPfizer Worldwide Research and Development, Discovery Park, Sandwich, Kent, CT13 9NJ, United Kingdom
bCPE Lyon, 43 Bd du 11 Novembre 1918, F-69100 Villeurbanne, France
cPfizer Worldwide Research and Development, Eastern Point Road, Groton, CT 06340, United States
* Corresponding author. e-mail: Steven.Fussell@pfizer.com
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A survey of amidation reagents demonstrating DIC-HOPO, DMT-MM, COMU-collidine,
TPTU-NMI, EEDQ, CDI and EDC-Oxyma to be effective for the coupling of carboxylic acids
with amines in the presence of water and the absence of problematic dipolar aprotic solvents is
reported. DMT-MM was shown to provide the best yields for the coupling of a secondary amine,
TPTU-NMI and COMU-collidine for aniline, whilst the combination of DIC with HOPO
afforded the broadest substrate scope and the highest yields for a sterically demanding
carboxylic acid.
Available online
Keywords:
Amide-bond forming
Condensation in water
Coupling reagents
2017 Elsevier Ltd. All rights reserved.
The amide functionality is an important component of many
drug molecules. A recent survey showed that amidation featured
in approximately 50% of all Journal of Medicinal Chemistry
manuscripts examined, making it the most frequently used
synthetic transformation, and importantly with 30% as the final
bond forming step.1 Amide bonds are typically synthesised by
reactions of carboxylic acids and amines with the loss of water,
in most cases aided by a coupling reagent. A plethora of coupling
reagents are commercially available and have been widely used
in large-scale manufacture of drug candidates.2,3 Regrettably,
undesirable dipolar aprotic solvents (particularly the reprotoxic
solvents dimethylacetamide (DMAC), dimethylformamide
(DMF) and N-methyl pyrrolidone (NMP) are commonly used2b,3
due to the poor organic solubility of carboxylic acids,
Herein we describe our efforts to investigate and understand the
performance of new and existing water-compatible amide
coupling systems.
The study was initiated by screening 48 different coupling
conditions for the amidation of benzoic acid with benzylamine in
the presence of water (Fig. 1). The reaction were initially carried
out using NMP as an organic co-solvent to mitigate any solubility
issues and was executed by simultaneous addition of the
carboxylic acid and amine to the amidation reagent in solution. A
range of coupling reagents were shown to afford moderate to
high in situ yields. Lead reagents selected for further study
included a variety of carbodiimides (Table 1, entries 2-6, 8, 9,
11), triazines (Entries 14, 15, 20-22), quinoline based reagents
(Entries 23, 24), COMU (Entry 25), TPTU (Entry 41), pivalic
anhydride (Entry 42) and CDI (Entry 45).
carboxylate salts (including amine / carboxylic acid salt pairs)
and zwitterionic substrates. A number of these solvents are
deemed substances of high concern and are subject to
considerable attention under European REACh regulation.4
Given the importance of amidation reactions, REACh-
unencumbered solvent systems would be highly desirable in
designing new scaleable chemical processes.
It is commonly known that carboxylic acids, carboxylate salts
and α-amino acids can be readily solubilized in aqueous solvent
systems thus providing a potential alternative to dipolar aprotic
solvents. However, for the amide coupling to succeed the rate of
aminolysis must be significantly greater than the rate of
hydrolysis of the activated carboxylic acid intermediate(s) and
the coupling reagent itself. A variety of reagents have been
reported to provide such conditions- most notably EDC5, DPTF,6
DMT-MM,7 CDI,8 COMU9 and N-carboxyanhydrides (NCAs).10
Figure 1. Model reaction for the screening experiments.
Using the lead reagents described above, each system was
optimized for solvent, additive, order of addition and reaction
time in the model reaction (Fig. 1). Problematic dipolar aprotic
co-solvents were omitted at this stage in favor of non-reprotoxic
alternatives. N,N′-Diisopropylcarbodiimide (DIC) in combination
with HOPO as an additive, was shown to be of particular interest

2
Tetrahedron
Table 1. Coupling reagent (1.0 eq.) screen for the amidation of benzoic acid with benzylamine (1.0 eq.) in NMP (21 mL/g) / water (9 mL/g) at 20 oC.
Entry
1
Type
Reagent
Yield (%)a Entry Type
Reagent
COMU
Yield (%)a
DCC
13
65
63
65
86
81
4
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
40
32
52
52
50
49
50
46
46
49
11
12
35
32
12
23
20
2
DCC-HOBt•H2O
DCC-Oxyma
DIC
HOTU
3
HATU
4
HBTU
5
DIC-HOBt•H2O
DIC-Oxyma
EDC
HCTU
6
HATU-HOBt•H2O
HBTU-HOBt•H2O
HCTU- HOBt•H2O
HBTU- Oxyma
TPTU
7
8
EDC-HOBt•H2O
EDC-Oxyma
EDC methiodide
EDC methiodide-HOBt•H2O
CMC
82
83
3
9
10
11
12
13
14
15
16
17
79
1
HSTU
TSTU
EDC.HCl
42
59
94
15
30
TOTU
DMT-MM•Cl
DMT-MM•BF4
Cyanuric chloride
Cyanuric chloride-QDb
PyAOP
Phosphonium
Imidazolium
PyBrOP
CIP
DMC
18
DCMT
13
42
Pivalic anhydride
59
19
20
21
22
23
24
DCMT-QDb
41
62
83
68
54
42
43
44
45
46
47
48
Benzyl chloroformate
28
20
36
0
CDMT
TFFH
CDI
CDMT-QDb
CDMT-DABCO
EEDQ
DPP
DTPC
TODT
2
Quinoline
IIDQ
25
a In-situ yield by HPLC analysis with 1,3-benzodoxole as an internal standard. b Quinuclidine abbreviated to QD.
affording the highest in situ yields of product 3 (Table 2, entry 1).
An EDC-Oxyma cocktail5b also gave promising results, as did
DMT-MM•BF4 which significantly outperformed the
Table 2. Optimized yields for the coupling of benzoic acid with
benzylamine in MeCN/Water. See ESI for conditions.
Entry Reagent
Additive
HOPO
Yield (%)a
corresponding chloro analogue (Entries 4-6). Improvement of
the in situ yields for the TPTU and COMU systems required the
addition of an organic base. N-Methylimidazole (NMI) in
combination with TPTU (Entry 8) and collidine with COMU
(Entry 7) proved to be the two best combinations of those tested,
mirroring the system reported by Lipshutz and co-workers who
employed a surfactant with a COMU and collidine reagent
cocktail.9 In contrast, optimization of the quinoline-based
coupling reagents indicated the requirement for sub-
stoichiometric amounts of hydrochloric acid and longer reaction
times (Entries 9 and 10). In the case of CDI and pivalic
anhydride, a yield enhancement was achieved by sequential
activation of benzoic acid in acetonitrile followed by the addition
of benzylamine as an aqueous solution (Entries 11 and 12).
Additional co-solvent screening demonstrated that NMP could be
readily replaced by non-reprotoxic water-miscible alternatives
such as acetonitrile and tetrahydrofuran (see ESI, section 3.1).
1
DIC
93
2
DIC
Oxyma
80
3
DIC
HOBt.H2O
Oxyma
81
4
EDC
82
5
DMT-MM•BF4
DMT-MM•Cl
COMU
TPTU
90
6
58
7
Collidine
NMI
87
8
90
9
EEDQ
63b
79b
58c
85c
10
11
12
IIDQ
Pivalic anhydride
CDI
a In situ yield by comparison to 1,3-benzodoxole as an internal standard. b
Addition of 4M HCl (0.1 eq.) in dioxane, 72 h reaction time. c Sequential
activation of the carboxylic acid in acetonitrile followed by addition of the
amine coupling partner as an aqueous solution.
Following these initial studies, the substrate scope of each of
the preferred amidation conditions was investigated (Table 3).
The results, as expected, confirmed that the performance of each

3
Table 3. Substrate scope for water-compatible coupling reagents. MeCN/water (1:1), coupling reagent (1.05 eq.), additive (1.0 eq.) at
20 °C.
Entry
Carboxylic Acid
Amine
Yield (%)b
1
2
3
15
82
71
71
85
76
79
80
71
19
10
7
14
85
78
39
65
57
69
89
79
63
66
56
4
85
28
69
26
1
28
68
65
5
6
7
8
57
62
46
89
84
69
63
76
81
47
21
86
2
71
74
55
25
76
57
16
76
78
74
72
71
56
52
56
70
3
37
5
9
37
76
73
89
85
88
74
61
89
87
91
75
41
39
41
53
53
86
86
5
74
73
61
14
83
87
81
98
54
60
71
67
10
11
12
66
13 c
14 c
3
0
0
1
5
0
0
1
0
0
0
(60)d
0
1
64
0
a Sequential activation of the carboxylic acid in MeCN for DMT-MM, CDI, and Pivalic anhydride. b In situ yield by comparison to 1,3-benzodioxole as an
internal standard. c Reactions with 2,6-dimethylbenzoic acid were conducted at 70°C, 48 h. d Isolated yield.
coupling reagent is substrate-dependent, underlining the
and coupled aniline effectively. However, both COMU-collidine
importance of reagent screening when developing an amide bond
forming process. COMU-collidine demonstrated one of the
broadest substrate scopes, affording moderate to high yields for
the coupling of both benzoic acid and 3-phenylpropanoic acid
with primary and secondary amines including aniline (Table 3,
entries 1-4 and 9-12). Similarly, TPTU-NMI showed broad scope
and TPTU-NMI were unable to couple the sterically hindered
2,6-dimethylbenzoic acid resulting in little or no product being
formed with either amine partner tested (Entries 13 and 14).
DMT-MM performed well for the coupling of dibenzylamine,
generally outperforming all other coupling reagents for this
amine (Entries 4, 8 and 12), yet it did not readily accept aniline or

4
Tetrahedron
2,6-dimethylbenzoic acid coupling partners. EEDQ and EDC-
Oxyma achieved modest yields with no clear trends for substrate
scope shown. The traditional reagent, CDI, was effective for the
coupling of aliphatic primary amines with sterically unhindered
acids (Entries 2, 3, 6, 7, 10 and 11) but only moderately so for
aniline and poorly so for dibenzylamine (Entries 1, 4, 5, 8, 9 and
12). In contrast to all other reagents, the combination of DIC-
HOPO was shown to perform well with all carboxylic acid and
amine partners tested. Pleasingly, under forcing conditions (70 oC
2 d), DIC-HOPO even accepted the sterically demanding 2,6-
dimethylbenzoic acid with both benzylamine and pyridin-2-
ylmethanamine (Entries 13 and 14). However, dimethylbenzoic
acid was not as well coupled to aniline and dibenzylamine,
giving negligible conversions at 20 oC and thus was not
optimised (results not presented).
aprotic solvents. The combination of DIC-HOPO provided the
most impressive substrate scope and activated intermediate
stability. It was also found that DMT-MM was preferred for the
conversion of dibenzylamine (the only secondary amine tested)
and the uronium based TPTU-NMI and COMU-collidine reagent
cocktails provided the highest conversions for aniline. It is
hoped that these results will encourage greater utilization of
aqueous media in amide coupling and that the reliance on
problematic dipolar aprotic solvents may be overcome in the
pursuit of sustainable and greener chemistry.
Table 4. Stability of activated benzoic and hydrocinnammic acid
in aqueous media (held at 30 oC for 15 minutes in MeCN/water).
Activating
Entry agent
Retained activity of
activated intermediate (%)a
Reaction profiling of the amidation of 2,6-dimethylbenzoic
acid with benzylamine showed that the addition of HOPO with
DIC completely suppressed formation of the unwanted N-
acylurea 6, allowing rapid conversion to the activated HOPO
intermediate 7 via the diisopropylcarbamimidic anhydride 5 (Fig.
2).11 The formation of intermediate 7 at 20 oC in acetonitrile was
complete within 5 minutes and was stable to the addition of water
and heating to 70 oC. Subsequent addition of benzylamine
afforded rapid conversion to the desired product in moderate in
situ yield with significant levels of residual activated HOPO
intermediate 7 remaining. Continued heating of the reaction
mixture at 70 oC for one hour confirmed that intermediate 7 was
stable to the aqueous reaction medium with no regeneration of
the 2,6-dimethylbenzoic acid.
PhCOOH
Ph(CH2)2COOH
1
2
3
4
5
6
7
DIC-HOPO
DIC
99
89
16
10
85
49
88
94
83
8
COMU
TPTU
23
99
67
95
CDI
DMT-MM•BF4
EEDQ
a The reaction mixture was quenched into n-butylamine and conversion to the
corresponding amide was calculated.
Acknowledgments
The authors would like to thank David Daniels and Olivier
Dirat for their helpful suggestions and David Mason for his
analytical support.
References
Figure 2. Reaction pathway for the amidation using DIC-HOPO.
1.
2.
Brown, D. G.; Boström, J. J. Med. Chem. 2016, 59, 4443-4458.
(a) Dunetz, J. R.; Magano, J.; Weisenburger, G. A. Org. Process
Res. Dev. 2016, 20, 140-177. (b) Montalbetti, C. A. G. N.; Falque,
V. Tetrahedron. 2005, 61, 10827- 10852.
To further compare the performance of the leading coupling
reagents, the relative aqueous stabilities of their corresponding
activated acids were investigated. Both benzoic and
hydrocinnamic acid were selected as test substrates and were
activated with DIC-HOPO, DIC, COMU (without additive),
TPTU (without additive), CDI, DMT-MM•BF4 and EEDQ in
MeCN. Water was charged to each activated acid solution and
their retained activity measured by quenching the reaction with n-
butylamine after 15 minutes (Table 4). When the DIC-HOPO-
activated intermediates were studied as such we were pleased to
discover that only 1% of the original activity was lost by
hydrolysis back to benzoic acid and only 6% lost by hydrolysis to
hydrocinnamic acid (Entry 1). DIC (without additive), CDI and
EEDQ activated intermediates also retained the majority of their
original activity under the aqueous conditions tested (Entries 2, 5
and 7).
3.
(a) MacMillan, D. S.; Murray, J.; Sneddon, H. F.; Jamieson, C.;
Watson, A. J. B. Green Chem. 2013, 15, 596-600. (b) El-Faham,
A.; Albericio, F. Chem. Rev. 2011, 111, 6557-6602. (c) Sperry, J.
B.; Farr, R. M.; Levent, M.; Ghosh, M.; Hoagland, S. M.;
Varsolona, R. J.; Sutherland, K. Org. Process Res. Dev.
2012, 16, 1854-1860. (d) Ormerod, D.; Willemsens, B.; Mermans,
R.; Langens, J.; Winderickx, G.; Kalindjian, S. B.; Buck, I. M.;
McDonald, I. M. Org. Process Res. Dev. 2005, 9, 499-507. (e)
Shendage, D. M.; Fröhlich, R.; Haufe, G. Org. Lett. 2004, 6, 3675-
3678.
European Chemicals Agency (ECHA), Candidate list of
substances of very high concern for authorisation, available online
at http://echa.europa.eu/candidate-list-table. 1-Methyl-2-
pyrrolidone (NMP), Toxic for reproduction (Article 57c) sampled
on the 30th June 2017.
4.
a) Konwar, M.; Ali, A. A.; Sarma, D. Tetrahedron Lett.
2016, 57, 2283-2285. (b) Wang, Q.; Wang, Y.; Kurosu, M. Org.
Lett. 2012, 14, 3372-3375. (c) Galanis, A. S.; Albericio, F.;
Grotli, M. Org. Lett. 2009, 11, 4488-4491. (d) Hojo, K.; Ichikawa,
H.; Fukumori, Y.; Kawasaki, K. Int. J. Pept. Res. Ther. 2008, 14,
373-380. (e) Shi, Y.-J.; Cameron, M.; Dolling, U. H.; Lieberman,
In conclusion, a variety of amidation reagents have been
shown to promote the coupling of carboxylic acids with amines
in aqueous reaction media, thus avoiding problematic dipolar

5
D. R.; Lynch, J. E.; Reamer, R. A.; Robbins, M. A.; Volante, R.
P.; Reider, P. J. Synlett. 2003, 5, 647–650. (f) Nakajima, N.; Ikada,
Y. Bioconjugate Chem. 1995, 6, 123-130. (g) Ho, G. J.; Emerson,
K. M.; Mathre, D. J., Shuman, R. F.; Grabowski, E. J. J. J. Org.
Chem. 1995, 60, 3569-3570. (h) Sheehan, J. C.; Hlavka, J. J. Org.
Chem. 1956, 21, 439-441.
5.
6.
Murakami, M.; Hayashi, M.; Tamura, N.; Hoshino, Y.; Ito, Y.
Tetrahedron Lett. 1996, 37, 7541-7544.
(a) Mizuhara, T.; Hioki, K.; Yamada, M.; Sasaki, H.; Morisaki,
D.; Kunishima, M. Chem. Lett. 2008, 37, 1190-1191. (b) Hojo, K.;
Maeda, M.; Tanakamaru, N.; Mochida, K.; Kawasaki, K. Protein
Pept. Lett. 2006, 13, 189-192. (c) Kunishima, M.; Kawachi, C.;
Hioki, K.; Terao, K.; Tani, S. Tetrahedron. 2001, 57, 1551-1558.
(a) Verma, S. K.; Ghorpade, R.; Pratap, A.; Kaushik, M. P.
Tetrahedron Lett. 2012, 53, 2373-2376. (b) Padiya, K. J.; Gavade,
S.; Kardile, B.; Tiwari, M.; Bajare, S.; Mane, M.; Gaware, V.;
Varghese, S.; Harel, D.; Kurhade, S. Org. Lett. 2012, 14, 2814-
2817.
7.
8.
9.
Gabriel, C. M.; Keener, M.; Gallou, F.; Lipshutz, B. H. Org. Lett.
2015, 17, 3968-3971.
De Marco, R.; Tolomelli, A.; Greco, A.; Gentilucci, L. ACS
Sustainable Chem. Eng. 2013, 1, 566-569.
10. (a) Iwasawa, T.; Wash, P.; Gibson, Ch.; Rebek, J.
Tetrahedron. 2007, 63, 6506-6511. (b) Mikolajczyk, M.;
Kielbasinski, P. Tetrahedron. 1981, 37, 233-284.
Supplementary Data
Document contains general information, experimental
procedures, additional screening results and product
characterization data.
Highlights





The performance of 48 different coupling reagents in
aqueous media is reported.
DIC-HOPO provided the most impressive substrate scope
and stability.
DMT-MM was preferred for the conversion of secondary
amines.
TPTU-NMI and COMU-collidine provided the highest
conversions for aniline.
The use of water as the solvent avoids the use of REACh
endangered solvents.