Chemoselective formation of C–N bond in wet acetonitrile using amberlyst-15(H) as a recyclable catalyst

Article abstract of DOI:10.1080/00397911.2020.1801745

An economically efficient and environmentally benign protocol for the chemoselective one-pot synthesis of diversely N-substituted amides has been developed in good yield through the reaction of benzylic secondary alcohols as well as aliphatic tertiary alcohols and alkyl/aryl nitriles. Commercially available Amberlyst-15(H) has been utilized at 80 °C as an air-stable and reusable heterogeneous inexpensive solid acid catalyst without any anhydrous and inert environment. The attractive features of the present synthetic protocol are mild reaction conditions, short reaction time, excellent chemoselectivity, high atom economy and tolerance of various sensitive moieties.

Full text of DOI:10.1080/00397911.2020.1801745

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Chemoselective formation of C–N bond in wet
acetonitrile using amberlyst®-15(H) as a recyclable
catalyst
Sneha Nandy , Asit Kumar Das & Sanjay Bhar
To cite this article: Sneha Nandy , Asit Kumar Das & Sanjay Bhar (2020): Chemoselective
formation of C–N bond in wet acetonitrile using amberlyst®-15(H) as a recyclable catalyst, Synthetic
Communications, DOI: 10.1080/00397911.2020.1801745
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SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2020.1801745
Chemoselective formation of C–N bond in wet acetonitrile
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using amberlystV-15(H) as a recyclable catalyst
Sneha Nandya, Asit Kumar Dasb, and Sanjay Bhara
b
aDepartment of Chemistry, Organic Chemistry Section, Jadavpur University, Kolkata, India; Department
of Chemistry, Krishnath College, Berhampore, India
ABSTRACT
ARTICLE HISTORY
Received 26 May 2020
An economically efficient and environmentally benign protocol for
the chemoselective one-pot synthesis of diversely N-substituted
amides has been developed in good yield through the reaction of
KEYWORDS
Alcohols; amides;
chemoselectivity; heteroge-
neous catalysis; nitriles
benzylic secondary alcohols as well as aliphatic tertiary alcohols and
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alkyl/aryl nitriles. Commercially available AmberlystV-15(H) has been
utilized at 80 ꢀC as an air-stable and reusable heterogeneous inex-
pensive solid acid catalyst without any anhydrous and inert environ-
ment. The attractive features of the present synthetic protocol are
mild reaction conditions, short reaction time, excellent chemoselec-
tivity, high atom economy and tolerance of various sensitive
moieties.
GRAPHICAL ABSTRACT
R1
R1
H
N
OH
R4
(wet)
R4CN
+
R2
R3
R2
R3
O
R1=R2=R3= Me
R1/R2=Alkyl/Aryl, R3= H
R4= Me/Ph
H3C
NHCOMe
H3C
OH
OH
18 examples
Solvent
Time (4.5-6h), 80oC
(Yield= 82-88%)
OH
(Yield= 83%)
Amberlyst®-15(H)
1. Excellent Chemoselectivity
3. Procedural simplicity
4. Reusability
2. Amides at sterically hindered sites
Formation of C–N bond in aliphatic skeletons, especially from alkanols, is a formidable
synthetic operation. Reaction of nitriles with substituted alkenes or alcohols using con-
centrated sulfuric acid in glacial acetic acid was classically used for this purpose through
the formation of substituted amides. In order to avoid the involvement of such a strong
mineral acid, several modifications have been proposed including other acidic substan-
ces like Tf2O,[1] BF3.OEt2,[2] P2O5-SiO2,[3] Bronsted acid,[4] Fe-montmorillonite K10,[5]
Nafion-H,[6,7] Nafion-H along with microwave irradiation,[8] formic acid under reflux,[9]
CONTACT Sanjay Bhar
sanjay.bhar@jadavpuruniversity.in
Department of Chemistry, Organic Chemistry Section,
Jadavpur University, Kolkata, India.
Supplemental data for this article can be accessed on the publisher’s website
ß 2020 Taylor & Francis Group, LLC

2
S. NANDY ET AL.
Scheme 1. Reaction of benzhydrol (1a) using different catalysts and reaction medium.
HBF4.Et2O,[10] 2,4-dinitrobenzenesulfonic acid,[11] TfOH,[12] HClO4-functionalized
silica-coated nanoparticles,[13] KI-TBHP[14] and liquid HF.[15] Ritter reaction using tert-
butyl acetate instead of tert-butyl alcohol[16] was also reported to be catalyzed by metal-
triflate[17] and pentafluorophenyl ammonium triflate (PFPAT),[18] Ca(II)-catalyzed
reaction under microwave irradiation,[19] Molecular I2,[20] Bi-salts,[21] FeCl3.6H2O[22]
and ionic liquids[23] have also been employed for this kind of transformation. Recently
a photochemical modification[24] has evolved as one of the successful alternative meth-
ods. However, most of these methods suffer from several disadvantages such as use of
corrosive acid catalysts, toxic and moisture-sensitive reagents, use of expensive com-
pounds and materials, elevated reaction temperatures, prolonged reaction time, suscepti-
bility with acid-labile and bulky functional groups and concomitant formation of several
by-products arising out of different side reactions including rearrangement. But the
main drawback of most of the existing methods is the decomposition of the catalysts
during aqueous work-up leading to tedious protocols for isolation, separation and
recovery of the products.
Recyclable resins[25–28] bearing acidic sites offer the advantages not only due to their
subtle catalytic attributes but also from the standpoint of reusability along with physical
and chemical stability. Further advantages are associated with their heterogeneous
nature in terms of their facile separation from the reaction mixture and easier isolation
of the products. Keeping in mind the aforesaid attributes, we report herein an admirable
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catalytic application of AmberlystV-15(H) for the formation of C–N bond in wet
acetonitrile.
Results and discussion
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To check the applicability of AmberlystV-15(H) the reaction was carried out with benz-
hydrol (1a, 1 mmol), wet MeCN (2a, 1 mmol) in presence of various catalysts along
with different solvent systems at 80 ꢀC to produce N-benzhydrylacetamide (3a)
(Scheme 1). Results are presented in Table 1.
From the results shown in Table 1, we standardized the reaction following the
condition as specified in entry 8 which afforded 88% of the corresponding amide
3a. The reactions did neither occur in anhydrous acetonitrile (entry 9) nor in water
alone; wet acetonitrile came out as the suitable reaction medium where the amount
of water is very crucial for optimum performance (0.1–0.3%). Best result was
obtained with commercially available acetonitrile containing 0.1% of water. Use of
toluene as a solvent in place of acetonitrile led to inferior results (entries 6 and 7).
DMF was found not at all suitable as the solvent in this present protocol (entry 10).

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SYNTHETIC COMMUNICATIONSV
3
Table 1. Optimization of the reaction conditionsa using different catalysts and solvents at 80 ꢀC.
Yield of
3a (%)b
Entry
Catalyst
Dowex-50
Dowex-50
Dowex-50
Amberlite
(IR-45)
Amount (mg)
Solvent
Toluene
DMF
Acetonitrile
Toluene
Time (h)
1
2
3
4
40
50
50
40
6
8
8
6
–
–
32
24
5
Amberlite
(IR-45)
Amberlyst -15(H)
50
DMF
8
–
R
V
6
7
8
40
50
50
Toluene
Toluene
Wet Acetonitrile
(0.1% of water)
Acetonitrile
(Anhydrous)
DMF
Toluene
Acetonitrile
Acetonitrile
6
6
6
40
46
88
R
V
Amberlyst -15(H)
R
V
Amberlyst -15(H)
R
V
9
Amberlyst -15(H)
50
8
–
R
V
10
11
12
13
Amberlyst -15(H)
Urea nitrate
Alumina (acidic)
Ni- Alumina
50
80
60
60
8
8
6
6
–
20
–
–
aReaction conditions: 1a (1.0 mmol), 2a (1.0 mmol), catalyst and time (as indicated), solvent (3 mL). byield of iso-
lated product.
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Before going to AmberlystV-15(H), the same reaction was attempted with another
well-known resin Dowex-50 where no conversion took place in toluene and DMF
(entries 1 and 2) and only 32% of the product was obtained in acetonitrile solvent
(entry 3). Similarly Amberlite (IR-45) produced only 24% of the required product in
toluene (entry 4) and no reaction occurred in DMF (entry 5) leading to exclusive
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recovery of the substrate. Looking for another potential alternative of AmberlystV-
15(H), the reaction was carried out with urea nitrate where the product was
obtained only in trace amount (entry 11). Even other acidic supports such as acidic
alumina and Ni-alumina also failed in this case to provide the desired product
(entries 12 and 13). The reaction was also carried out with 1-phenylethanol (1b)
under optimized reaction condition at 80 ꢀC using 1 drop of conc. H2SO4 as the
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catalyst instead of AmberlystV-15(H) where a mixture of 3b and styrene (4a) was
obtained in 1:1.9 ratio (Scheme 2), but 3b was obtained exclusively (88%) from 1b
R
using AmberlystV-15(H).
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In this way the essentiality, efficacy and applicability of AmberlystV-15(H) as the solid
acid resin was firmly established for the conversion of secondary alcohols to their corre-
sponding N-acyl derivatives. Thus the present study led to the advent of a utilitarian
and eco-compatible protocol for C–N bond formation using easily accessible substrates
and catalyst. Moreover the same product 3a was obtained when methyl and ethyl ethers
of 1a were used as the substrates in place of 1a.
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The catalyst i.e., AmberlystV-15(H) was recovered by filtration, repeatedly washed
with ethyl acetate, dried and further reused consecutively with marginal loss of its cata-
lytic activity (Figure 1).
This protocol was further extended to substituted secondary and tertiary alcohols
(Scheme 3 and Scheme 4) where a good number of substituted amides were obtained
with satisfactory yield.

4
S. NANDY ET AL.
List of the products obtained in Scheme 3 using Amberlyst®-15(H)
NHCOMe
Me
NHCOMe
Me
NHCOMe
NHCOMe
Me
Me
3d (5h, 86%)
(5h, 86%)
3c
(6h, 88%)
3a
3b (4.5h, 88%)
Me
NHCOMe
Me
3f
NHCOMe
NHCOMe
NHCOMe
Me
F
COOMe
Br
3e (5h, 87%)
3h
(6h, 83%)
3g(4.5h, 86%)
(6h, 84%)
O
NHCOMe
Me
NHCOMe
NHCOMe
Me
Me
NH
O
3k (5h, 85%)
3i
3j (5.5h, 82%)
(4.5h, 86%)
(5.5h, 82%)
3l
Me
Me
NHCOMe
HN
O
HN
O
OTBDMS
3m(6h, 83%)
3o(5.5h, 82%)
3n(5.5h, 84%)
List of the products formed in Scheme 4 using Amberlyst®-15(H)
Under the optimized condition unsubstituted benzhydrol (1a) produced N-benzhy-
drylacetamide (3a) in satisfactory yield (yield refers to that of the isolated pure product
fully characterized spectroscopically). This condensation reaction also took place very
efficiently with both ring-unsubstituted and alkyl/aryl-substituted secondary benzylic
alcohols to afford 3b, 3c, 3d, 3g, 3i and 3j with good yield. Secondary benzylic carbinols
bearing halogen substituents in the aromatic ring responded smoothly to produce 3e

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SYNTHETIC COMMUNICATIONSV
5
Scheme 2. Reaction of 1-phenylethanol with 1 drop H2SO4 under optimized condition.
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Figure 1. Recycling of AmberlystV-15(H) using 1a in moist MeCN at 80 ꢀC for 6 h; % of yield was yield
of isolated product 3a.
Scheme 3. Reaction of substituted secondary benzylic alcohols with wet acetonitrile under opti-
mized condition.
Scheme 4. Reaction of tert-butanol with alkyl/aryl nitriles.
and 3h with 87% and 83% yields respectively. Hydrolyzable functional group -COOMe
also survived in the present protocol to produce 3f in 84% yield. The reaction gave
quite impressive results with a-tetralol molecule forming 3k with 85% yield.
a-Hydroxyketone also reacted under this protocol and formed 3 l (82%) which is

6
S. NANDY ET AL.
Scheme 5. Intermolecular competition experiment (1).
Scheme 6. Reaction of styrene with wet acetonitrile under optimized condition.
otherwise difficult to prepare. TBDMS group also survived under this procedure without
any O-Si bond cleavage and 3 m was obtained in 83% yield. When benzhydrol (1a) was
reacted with benzonitrile and 4-methylbenzonitrile as a reagent in place of acetonitrile
using toluene as the solvent it produced 3n and 3o respectively with satisfactory yield.
No such reaction took place with aromatic or aliphatic primary alcohols where the sub-
strates were recovered unchanged. When a mixture of equimolecular amounts of 1a
and benzyl alcohol was reacted under the optimized condition, only 1a was converted
to 3a selectively leaving benzyl alcohol totally unaffected. Even with dialkyl secondary
alcohol the reaction was inefficient to react under this protocol. Therefore this protocol
is very much selective for only secondary alcohols where the aromatic ring is connected
to the carbinol carbon. tert-Butanol responded efficiently under the present protocol
(Scheme 4) giving N-tert-butyl-amides (6a–c) without any cleavage of any C-C bonds.
To substantiate the selectivity between aromatic and aliphatic alcohols, the competi-
tion reaction was carried out under optimized condition at 80 ꢀC with equimolar mix-
tures of 1-phenylethanol (1b) and 1-heptanol (1p) where the product 3b was obtained
as expected from 1b; leaving 1p intact (Scheme 5). Absence of any signal at d 3.24 ppm
1
in H NMR spectrum of the reaction mixture further confirmed the fact that the ali-
phatic –CH2OH has not been transformed into –CH2NHCOCH3.
As a continuity of this theme, styrene (4a) was subjected to the optimized condition
at 80 ꢀC and produced N-(1-phenylethyl)-acetamide (3b) as the sole product
(Scheme 6).
Equimolecular mixture of 1-phenylethanol (1b) and benzyl alcohol (1q), under opti-
mized condition at 80 ꢀC produced 3b exclusively as the product obtained from 1b leav-
ing behind 1q unaffected (Scheme 7).
Intramolecular completion experiment with the substrate 1r (containing both the aro-
matic primary and secondary carbinol moieties in the same molecule) under this proto-
col, produced 3p exclusively (Scheme 8) with 83% yield. Excellent and subtle
chemoselectivity was thus conclusively substantiated as the most important attribute of
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the present AmberlystV-15(H)-catalyzed protocol.

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SYNTHETIC COMMUNICATIONSV
7
Scheme 7. Intermolecular competition experiment (2).
Scheme 8. Intramolecular competition experiment.
R1
R1
H
N
OH
Amberlyst®-15(H)
RCN
R
O
S
R2
R2
R3
O
S
R3
HO
O
OH
O
Amberlyst®-15(H)
SO3H
O
O
O
O
H2O
S
O
S
O
O
R1
R1
H
OH2
R1
N
C
R
N
R
R2
R2
R3
R3
R2
R3
Carbocation
intermediate
Scheme 9. Proposed mechanism for the reaction.
1
Both H and 13C NMR spectroscopic studies confirmed the occurrence of 3p. A
1
1
doublet at d 1.46 in H NMR (d 21.8 in 13C NMR) and a singlet at d 1.96 in H NMR
(d 23.6 in 13C NMR) established the presence of one alkyl-methyl and one acetamino-
methyl moieties respectively. A singlet at d 4.51 in H NMR (d 71.9 in 13C NMR) indi-
1
cated the unperturbed benzylic primary carbinol moiety.
The aforesaid observations indicate that this subtle chemoselectivity might originate
due to preferential formation of the relatively more stable carbocationic intermediate.
The result obtained in the study delineated in Scheme 6 also indicated towards the
involvement of carbocationic intermediate. Furthermore, when the reaction was carried
out using enantiopure R-1-phenylethanol under the optimized reaction condition, the
product 3b was obtained as a racemic mixture. This observation also supported

8
S. NANDY ET AL.
the formation of planar carbocation during the course of the reaction. On the basis of
the above-mentioned outcomes, a plausible mechanistic scheme has been put forward
(Scheme 9) involving the formation of carbocation through acid-catalyzed dehydration
of alcohol with concomitant nucleophilic attack by nitrile followed by another nucleo-
philic attack by water to end up with the product amide. This mechanism also accounts
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for the catalytic role of AmberlystV-15(H) in terms of Bronsted acidity and recyclability.
No reaction took place with highly substituted triphenylmethanol due to lower stability
of propeller-shaped triphenylmethyl carbocation as well as increased steric crowding.
Conclusion
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In conclusion, commercially available AmberlystV-15(H) has been effectively utilized as
an air stable and recyclable heterogeneous inexpensive solid acid catalyst for the con-
struction of C–N bond through chemoselective formation of diversely N-substituted
amides using benzylic secondary alcohols as well as aliphatic tertiary alcohols and alkyl/
aryl nitriles under environmentally acceptable conditions without any necessity of
anhydrous and inert environment. Use of reagents, solvents and catalyst of negligible
toxicity, mild reaction conditions, tolerance of various sensitive moieties, excellent che-
moselectivity, wide substrate scope, high atom economy, formation of the most innocu-
ous by-product (namely water), procedural simplicity, good yields and recyclability of
the catalyst are the outstanding features of the present method with greater applicability
compared to many existing protocols.
Experimental
A. To a mixture of benzhydrol 1a (184 mg, 1.0 mmol) and wet MeCN 2a (4 mL) the catalyst
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AmberlystV-15(H) (50mg) was added. The reaction mixture was stirred for 6 hours at 80ꢀC
till the reaction was complete (monitored with TLC). Then the reaction mixture was cooled
to room temperature, excess MeCN was removed under reduced pressure, the catalyst was
filtered out keeping a cotton plug on a funnel and washed repeatedly by ethyl acetate
(3 ꢁ 10mL) to dissolve and collect the product. The product was thoroughly washed with
water (2 ꢁ 10mL) to remove any unused MeCN. The aqueous reaction mixture was then
repeatedly extracted with ethyl acetate (3 ꢁ 5 mL). The combined organic extracts were
washed with water (3ꢁ 10mL) and dried over anhydrous Na2SO4. The crude product was
obtained by removal of the solvent under reduced pressure and purified by filtration chro-
matography on a short column of silica gel using 1–4% ethyl acetate-hexane as eluent to
afford 3a[21] (198 mg, Yield 88%, mp143–144 ꢀC).
B. To a mixture of tert-butanol 5 (74 mg, 1.0 mmol), and PhCN 2b (103 mg,
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1.0 mmol) in 4 mL toluene, the catalyst AmberlystV-15(H) (50 mg) was added. The reac-
tion mixture was stirred for 6 hours at 80 ꢀC till the reaction was complete (monitored
with TLC). Then the reaction mixture was cooled to room temperature, excess toluene
was removed under reduced pressure, keeping a cotton plug on a funnel the catalyst
was filtered out and washed repeatedly by ethyl acetate (3 ꢁ 10 mL) to dissolve and col-
lect the product. The product was thoroughly washed with water (2 ꢁ 10 mL). The aque-
ous reaction mixture was then repeatedly extracted with ethyl acetate (3 ꢁ 5 mL).

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SYNTHETIC COMMUNICATIONSV
9
The combined organic extracts were washed with water (3 ꢁ 10 mL) and dried over
anhydrous Na2SO4. Removal of the solvent under reduced pressure furnished the prod-
uct 6b[29] (152 mg, Yield 86%, mp132–133 ꢀC) without any necessity of further
purification.
Supporting information
This part contains experimental procedure, physical characteristics as well as spectral
(1H and 13C NMR) and HRMS/analytical data (wherever applicable) for the products.
This material can be found via the “Supplementary Information” section of this
article’s webpage.
Acknowledgements
S. N. thanks DST-INSPIRE, India for senior research fellowship. Financial support from UGC-
CAS-II program in Chemistry and RUSA program 2.0 at Jadavpur University as well as
Infrastructural support from DST-FIST program are gratefully acknowledged. The authors
express sincere gratitude to Ms. P. Das of Indian Institute of Chemical Biology for neces-
sary assistance.
Disclosure statement
No potential conflict of interest was reported by the author(s).
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