Innovative approach for the synthesis of N-substituted amides from nitriles and alcohols using propylphosphonic anhydride (T3P) under solvent-free conditions

Article abstract of DOI:10.1080/00397911.2019.1616761

Novel and convenient methodology for the construction of N-substituted amide derivatives have been developed from nitriles and alcohols using propylphosphonic anhydride (T3P). This methodology is an alternate approach to the synthesis of amides via Ritter reaction, which is one of the classical methods for the synthesis of N-substituted amides from nitriles and alcohols. In this approach, first T3P activates the alcohol which is then attacked by nitrile to form N-substituted amides. This methodology can also apply for the synthesis of benzhydryl ether. This developed protocol is one of the novel applications of T3P.

Full text of DOI:10.1080/00397911.2019.1616761

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Innovative approach for the synthesis of N-
substituted amides from nitriles and alcohols
using propylphosphonic anhydride (T3P®) under
solvent-free conditions
Hassan A. Swarup, Nagaraju Chaithra, Nagarakere C. Sandhya, Shobith
Rangappa, Kempegowda Mantelingu & Kanchugarakoppal S. Rangappa
To cite this article: Hassan A. Swarup, Nagaraju Chaithra, Nagarakere C. Sandhya,
Shobith Rangappa, Kempegowda Mantelingu & Kanchugarakoppal S. Rangappa (2019):
Innovative approach for the synthesis of N-substituted amides from nitriles and alcohols using
propylphosphonic anhydride (T3P®) under solvent-free conditions, Synthetic Communications, DOI:
10.1080/00397911.2019.1616761
To link to this article: https://doi.org/10.1080/00397911.2019.1616761
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SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2019.1616761
Innovative approach for the synthesis of N-substituted
amides from nitriles and alcohols using propylphosphonic
R
anhydride (T3PV) under solvent-free conditions
Hassan A. Swarupa, Nagaraju Chaithraa, Nagarakere C. Sandhyab, Shobith
Rangappac, Kempegowda Mantelingua, and Kanchugarakoppal S. Rangappad
b
aSynthetic Laboratory, DOS in Chemistry, University of Mysore, Manasagangotri, Mysore, India; P. G.
c
Department of Chemistry, St. Philomena’s College, Bannimantap, Mysore, India; Adichunchanagiri
d
Cancer Research Centre, Balagangadharanatha Nagara, Nagamangala, Mandya, India; Institution of
Excellence, University of Mysore, Manasagangotri, Mysore, India
ABSTRACT
ARTICLE HISTORY
Received 29 March 2019
Novel and convenient methodology for the construction of N-substi-
tuted amide derivatives have been developed from nitriles and alco-
R
hols using propylphosphonic anhydride (T3PV). This methodology is
KEYWORDS
Nitriles; N-substituted
an alternate approach to the synthesis of amides via Ritter reaction,
amides; propylphosphonic
R
which is one of the classical methods for the synthesis of N-substi-
anhydride (T3PV); solvent-
R
tuted amides from nitriles and alcohols. In this approach, first T3PV
free condition
activates the alcohol which is then attacked by nitrile to form N-sub-
stituted amides. This methodology can also apply for the synthesis
of benzhydryl ether. This developed protocol is one of the novel
R
applications of T3PV.
GRAPHICAL ABSTRACT
Introduction
In nature, both living and non-living organisms, amide bond formation play an import-
ant role for the construction of proteins, peptides, polymers and synthetic molecules.[1]
This amide bonds are not only limited to biological systems, but also it having more
applications in the field of pharmaceutical industry.[2] Some of the top selling drug
composed of amide bonds is Atorvastatin which blocks the production of cholesterol,
Diltiazem using in the treatment of angina and hypertension, Valsartan is a drug which
acts as a blockade of angiotensin-II receptors, Lisinopril acts as an inhibitor of
CONTACT Kempegowda Mantelingu
Mysore, Manasagangotri, Mysore, 570006 Karnataka, India; Kanchugarakoppal S. Rangappa
Institution of Excellence, University of Mysore, Manasagangotri, Mysore, 570006, India.
kmlingu@gmail.com
Synthetic Laboratory, DOS in Chemistry, University of
rangappaks@gmail.com
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
ß 2019 Taylor & Francis Group, LLC

2
H. A. SWARUP ET AL.
angiotensin converting enzyme.[3] Amide bonds are also playing very important role in
the preparation of biodegradable polymers which is called as Poly ester amides
(PEAs).[4] PEAs are interesting polymers, which have gained interest in medical applica-
tions in tissue engineering, drug delivery systems, hydrogels, non-viral gene carriers,
smart materials, composites and adhesives. These are synthetic polymers contain amide
and ester groups in which its having degradable character and provide good thermal
and mechanical properties.[5] Due to its wide applications in the various fields, many
synthetic approaches have been developed for the amide bond formation from nitriles
and alcohols[6] among them ‘Ritter reaction’ is a classical method for amide bond for-
mation, which was first published by Ritter.[7] This method involved the condensation
of the alcohols in the presence of stoichiometric amount of sulfuric acid.
The Ritter reaction has application in the preparation of bulky amides like indus-
trial-scale production of anti-HIV drug indinavir, which is useful in the formation
of amides in which the nitrogen has a tertiary alkyl group. Falcipain-2 inhibitor (PK
11195) and production of dopamine receptor ligands (amphetamine) were also syn-
thesized via Ritter reaction. Milne and group have been developed the scalable syn-
thesis of tert-butyl amides via Ritter reaction using nitrile, tert-butyl acetate, acetic
acid and H2SO4.[8]
These methodologies having many drawbacks like, it requires a high temperature,
strong acid catalyst for the production of carbocation which is toxic, corrosive and
environmental hazard. Due to its major limitations, a number of important improved
and advanced methodologies have been achieved in recent years with respect to reaction
conditions, substrates, catalysts and diversity of products.[9] Yaragorla and group[10]
have reported the synthesis of N-substituted amides from alcohols and nitriles using
Ca(OTf)2 (calcium trifluoromethanesulfonate) as a catalyst and Bu4NPF6 (tetrabutylam-
moniumhexafluorophosphate) as an additive under microwave condition. Anxionnat
et al proposed an approach for the synthesis of N-substituted amides from different
nitriles and alcohols using FeCl3.6H2O via Ritter type reaction.[11] Here FeCl3.6H2O is
able to activate benzylic alcohols to produce carbocation intermediates, which is then
attacked by nitriles to form amides. Gold (I) catalyzed reaction for the synthesis of
amides from benzohydric alcohols and nitriles was discovered by Ibrahim et al.[12] using
(Mes)3PAuCl and AgNTf2 in acetonitrile (Scheme 1). Tamaddon and group[13] reported
the synthesis of N-substituted carboxylic amides from alcohols and nitriles using P2O5/
SiO2 (60% w/w) under microwave condition. Previously reported methods have limita-
tions like use of transition-metal catalyst, expensive reagents, tedious work-up procedure
and longer reaction times with a poor yield.[14–18]
In continuation of our interest in applications of T3P,[19–22] cycloaddition reac-
tions,[23–26] applications of dithioesters,[27–32] and synthesis of heterocyclic com-
pounds,[33,34] we also explored a method to access N-substituted amides (Scheme 2).
R
T3PV (Propylphosphonic anhydride) having wide application in the field of synthetic
organic chemistry because its having excellent reaction selectivity, less toxicity, low epi-
merization character, less allergenic and broad functional group tolerance.[35,36] In ini-
R
tial days, T3PV was used as coupling reagent in acid-amine coupling reactions[37] but in
R
recent days, T3PV found a wide range of applications like cyclization reaction,[38] con-
densation reaction,[39] reduction of carboxylic acids into alcohols,[40] chemoselective

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SYNTHETIC COMMUNICATIONSV
3
Scheme 1. Methods for the construction of N-substituted amides.
Scheme 2. Synthesis of N-substituted amides.
acetalization, thioacetalization of aldehydes,[41] and also used for the synthesis of
amides, nitriles, isothiocyanates, chromones, and so on.[42–48]
Results and discussion
In initial experiments, feasibility of the reaction was checked by using benzhydrol 1a
R
(1 eq) with benzonitrile 2a (1eq) using T3PV (25 mol%) in presence of toluene at 90 ꢀC.
Gratifyingly, required product 3a obtained with 40% yield after 16 h (Table 1, entry 1).
Inspired by this result, the amount of T3P was increased from 25 mol% to 50 mol% and
there was an improvement in the yield was observed (Table 1, entry 2). Further increas-
ing in the amount of T3P from 50 mol% to 100 mol% (Table 1, entry 3), there was no
significant improvement in the yield was observed. For getting a better result, solvent

4
H. A. SWARUP ET AL.
Table 1. Optimization of the reaction conditions.
Entry
Solvent
T3P (mol%)
Temp (ꢀC)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
Toluene
Toluene
Toluene
Xylene
1,4, dioxane
THF
DMF
Water
No solvent
No solvent
No solvent
No solvent
No solvent
No solvent
No solvent
No solvent
25%
50%
90
90
90
90
90
80
90
90
90
90
90
90
90
90
80
100
16
16
16
16
16
16
16
16
16
16
16
12
8
40
43
46
45
57
Trace
Trace
NR
89
85
67
100%
100%
100%
100%
100%
100%
50%
100%
25%
50%
9
10
11
12
13
14a
15
16
85
87
95
87
50%
50%
50%
50%
3
8
1
90
R
aReaction condition: Reactions were carried out with 1a (1.0 mmol) and 2a (1.0 mmol) in presence of 50 mol% T3PV
(50% in ethyl acetate) under solvent free condition at 90 ꢀC for 3 h.
was changed from toluene to xylene and 1,4-dioxane (Table 1, entries 4 and 5) and
again there was no improvement in the yield was observed.
We have screened for other solvents like THF and DMF but trace amount of product
was observed (Table 1, entries 6 and 7). Further reaction was performed by using water
as a solvent but we didn’t observe any reaction (Table 1, entry 8). Later, the reaction
was carried out under solvent-free condition using T3P (50 mol%) at 90 ꢀC for 16 h and
surprisingly, complete consumption of starting materials and formation of product 1a
was isolated in 89% of yield (Table 1, entry 9).
With the success of this reaction, the reaction was performed by increasing the
amount of T3P from 50 mol% to 100 mol % and product 1a yield was not increased
significantly (Table 1, entry 10) and the reaction was also performed by decreasing
the amount of T3P from 50 mol% to 25 mol% and the percentage of yield was grad-
ually reduced from 89% to 67% (Table 1, entry 11). Thus T3P loading was opti-
mized at 50 mol%. With continued our interest, the effect of reaction time on yield
was also studied, the reaction time was reduced from 16 h to 12 h and 8 h and there
were no much improvement in the yield was observed (Table 1, entries 12 and 13).
But, when the reaction time was reduced to 3 h, the yield of the product 3a was
increased gradually from 89% to 95% (Table 1, entry 14). Furthermore, the effect of
temperature was also tested by a decrease or increase of temperature from 90 ꢀC to
80 ꢀC and 100 ꢀC (Table 1, entries 15 and 16) and could not find better results.

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Figure 1. Substrate scope of reactions.
With the above optimized reaction conditions, the efficiency and versatility of this
newly developed approach was generalized using a variety of substituted nitriles and
substituted alcohols, the reaction proceeded smoothly in a few hours with good to
excellent yields and the results are summarized (Figure 1).
Thus developed methodology proceeded smoothly using 50 mol% T3P with a variety
of alcohols like diphenylmethanol, methylbenzyl alcohol, substituted methylbenzyl alco-
hol, 1-(1-naphthyl)ethanol, tert-butyl alcohol were used and the variety of nitriles like
benzonitrile, substitute benzonitrile having electron donating and electron withdrawing
substituents such as m-methyl, p-methyl, dimethoxy, o-bromo, p-bromo, o-chloro,
o-nitro and p-trifluoro were employed and the desired products were obtained in

6
H. A. SWARUP ET AL.
Figure 2. Substrate scope of reactions.
Scheme 3. Plausible reaction mechanism for the formation of 3a.

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7
Scheme 4. Plausible reaction mechanism for the formation of 4a.
excellent yields.Addition to these, phenylacetonitrile substituents like p-methoxy,
p-bromo, p-fluoro, o-nitro and biphenyl acetonitrile were also compatible in this meth-
odology to give the desired products in excellent yields (Figure 1).
Importantly, other nitriles like pyridine carbonitrile, indole-3-carbonitrile and cyclohex-
ane carbonitrile were also tested and reaction proceeded smoothly to get good yields
(Figure 2).
The mechanism for the amidation reaction was proposed and shown in Scheme 3. In
first step, activation of 1a takes place when lone pair of electrons on the oxygen atom
of 1a attacks on A (T3P) to form an intermediate B and secondly, intermediate B
attacked by nitrile 2a to form nitrilium ion intermediate D by eliminating the by-prod-
uct open hydrated T3P C and later nitrilium ion intermediate D was attacked by open
hydrated T3P C to form intermediate E. Further, the intermediate E undergoes cycliza-
tion to regenerate T3P with the expulsion of amide 3a (Scheme 3).
With the continuation of our interest, this reaction was performed with benzhydrol
1a in the absence of nitrile and surprisingly, benzhydryl ether 4a was found using
50 mol% T3P under solvent free condition.
The mechanism for the etherification reaction was proposed as shown in Scheme 4.
In first step activation of benzhydrol1a takes place, when lone pair of electrons on the
oxygen atom 1a attacks on A (T3P) to form an intermediate B and in second step,
again 1a react with intermediate B and undergoes self-coupling to form benzhydryl
ether 4a by eliminating the by-product C and this by-product C undergoes dehydration
to regenerate the A (T3P) (Scheme 4).

8
H. A. SWARUP ET AL.
Conclusion
In conclusion, we have developed an efficient and green synthetic approach for the syn-
thesis of amides using T3P under solvent free condition. Addition to this, benzhydryl
ether was also successfully synthesized by using this methodology. This optimized reac-
tion conditions are compatible for larger-scale synthesis and also for the preparation of
bulky amides. These reactions were carried out using only 50 mol% of T3P and synthe-
sized compounds were purified without column chromatography. The neutral and mild
character of the applied conditions for this transformation makes it a valuable alterna-
tive to those previously described in the literature. The developed protocol will broaden
the strategies for the amide bond formation in the field of synthetic organic chemistry.
Further exploration of the application of this method is in progress in our laboratory.
Experimental
Materials and methods: All work relating to analytical thin layer chromatography were
performed with E. Merck silica gel 60 F254 aluminum plates and were visualized with
UV light. The following mobile phases were employed for TLC: chloroform, methanol,
hexane, and ethyl acetate in different ratios. The instrumental techniques employed for
1
the characterization of the newly synthesized compounds include H and 13 C NMR and
1
mass spectroscopy. H and 13 C NMR spectra were recorded on a Bruker-AV (500, 400
and 126, 101 MHz, respectively) and Agilent WM (400 and 100 MHz) Fourier trans-
forms spectrophotometer in CDCl3 or DMSO-d6 solution using tetramethylsilane (TMS)
as internal standard. Chemical shifts were recorded in ppm relative to TMS. Mass and
purity were recorded on an LC–MSD-Trap-XCT (Agilent Technologies Inc). All the
reagents and chemicals used were from Sigma Aldrich chemicals.
General procedure for the synthesis of N-substituted amides 3
To a mixture of substituted alcohol 1 (1 mmol) and substituted nitrile 2 (1 mmol), T3P
50 wt% in ethyl acetate (50 mol%) was added and stirred at 90 ꢀC. After the reactant dis-
appeared (monitored by TLC), the reaction mixture was cooled to room temperature,
excess of water was added and extracted with ethyl acetate and combined organic layer
was washed with brine, dried over Na2SO4 and concentrated under reduced pressure.
The crude product was purified by recrystallization using DCM: pet ether (1:9) mixture
to get pure compound 3.
General procedure for the synthesis of benzhydryl ether 4
To a substituted alcohol 1 (1 mmol,), T3P 50 wt% in ethyl acetate (50 mol%) was added
and stirred at 90 ꢀC. After the reactant disappeared (monitored by TLC), the reaction
mixture was cooled to room temperature, excess of water was added and extracted with
ethyl acetate and combined organic layer was washed with brine, dried over Na2SO4
and concentrated under reduced pressure. The crude product was purified by recrystal-
lization using di-ethyl ether: pet ether (1:9) mixture to get pure compound 4.

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9
Acknowledgements
We thank NMR Facility, Institute of Excellence, University of Mysore, Manasagangotri, Mysuru,
India for spectral data.
Funding
Financial support from the DBT Glue grant No.BT/PR23078/MED/29/1253/2017 dated-22/03/
2018 and VGST [K-FIST (L1)] grant No.KSTePS/VGST/K-FIST (L1)/2017-18/GRD-681/79/2018-
2019 dated-17/09/2018 is gratefully acknowledged.
ORCID
Kanchugarakoppal S. Rangappa
http://orcid.org/0000-0003-0572-6305
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