Preparation of different amides via Ritter reaction from alcohols and nitriles in the presence of silica-bonded N-propyl sulphamic acid (SBNPSA) under solvent-free conditions

Article abstract of DOI:10.1007/s12039-012-0309-2

A number of methods have been proposed for the modification of the Ritter reaction. However, many of these methods involve the use of strongly acidic conditions, stoichiometric amounts of reagents, harsh reaction conditions and extended reaction times. Th

Full text of DOI:10.1007/s12039-012-0309-2

c
J. Chem. Sci. Vol. 124, No. 5, September 2012, pp. 1025–1032. ꢀ Indian Academy of Sciences.
Preparation of different amides via Ritter reaction from alcohols
and nitriles in the presence of silica-bonded N- propyl sulphamic
acid (SBNPSA) under solvent-free conditions
MARYAM-SADAT SHAKERIa,∗, HASSAN TAJIKa,b and KHODABAKHSH NIKNAMa
aDepartment of Chemistry, Faculty of Sciences, Persian Gulf University, Bushehr 75169, Iran
bDepartment of Chemistry, Faculty of Sciences, Guilan University, Rasht, Iran
e-mail: shakery.maryam62@gmail.com
MS received 29 November 2011; revised 22 May 2012; accepted 18 June 2012
Abstract. A number of methods have been proposed for the modification of the Ritter reaction. However,
many of these methods involve the use of strongly acidic conditions, stoichiometric amounts of reagents, harsh
reaction conditions and extended reaction times. Therefore, the development of mild, efficient, convenient and
benign reagents for the Ritter reaction is desirable. In this research, we have developed a clean and environmen-
tally friendly protocol for the synthesis of amides by using different benzylic or tertiary alcohols and different
nitriles in the presence of silica-bonded N- propyl sulphamic acid (SBNPSA) as catalyst under solvent-free
conditions in high yields.
Keywords. Ritter reaction; SBNPSA; alcohol; nitrile; amide.
1. Introduction
be reused. In recent years, the search for environmen-
tally benign chemical processes or methodologies has
received much attention. Heterogenization of homoge-
neous catalysts has been an interesting area of research
from an industrial point of view; this combines the
advantages of homogeneous catalysts (high activity,
selectivity, etc.) with the engineering advantages of
heterogeneous catalysts (easy catalyst separation, long
catalytic life, easy catalyst regeneration, thermal sta-
bility, and recyclability). Application of solid acids in
organic transformations is important because the solid
acids have many advantages such as simplicity in hand-
ling, decreased reactor and plant corrosion problems,
and more environmentally safe disposal.7–30 Green
chemistry not only requires the use of environmentally
benign reagents and solvents, but also requires the
use of recyclable catalysts. One way to overcome the
problem of recyclability of the traditional acid catalysts
is to chemically anchor their reactive centre onto an
inorganic solid carrier with a large surface area to cre-
ate a new organic–inorganic hybrid catalyst.11–16 The
reactive centres in these solid-supported catalysts are
highly mobile similar to homogeneous catalysts and at
the same time have the advantage of recyclability like
heterogeneous catalysts. In this regard, various organic
and inorganic materials have been investigated; one of
the most popular of these materials is silica gel with a
surface area equal to 200 m2/g that has high thermal
and chemical stability (with the exception of a few
The conversion of nitriles to amides by reaction with
alcohols or alkenes in the presence of sulphuric acid
is named Ritter reaction. Acidification of the appro-
priate alcohol or alkene generates a carbenium ion
which reacts with nitrile. While the successful course
of the reaction certainly depends on the reactivity of
nitrile, a major factor is the stability and reactivity of
the carbocationic intermediates.1 Ritter reaction is a
well-known reaction in the synthetic organic processes
which provides versatile synthetic method for prepara-
tion of a variety of amides.2 It is also used in industrial
processes as it can be effectively scaled up from labo-
ratory experiments to large-scale applications while
maintaining high yield. Real world applications include
Merck’s industrial-scale synthesis of anti-HIV drug
crixivan (indinavir),3 the production of the falcipain-2
inhibitor PK 11195, the synthesis of the alkaloid aristo-
telone,4 and synthesis of Amantadine, an antiviral
and antiparkinsonian drug.5 Other applications of the
Ritter reaction include synthesis of dopamine receptor
ligands4 and production of amphetamine from allylben-
zene.6 A problem with the Ritter reaction is the need
for an extremely strong acid catalyst in order to pro-
duce the carbocation. This poses an environmental risk,
as the acids are extremely corrosive and also cannot
∗For correspondence
1025

1026
Maryam-Sadat Shakeri et al.
ꢁ
ꢁ
number of small nucleophiles such as OH and F) and with SBNPSA (0.1 g) under solvent free condition and
the surface area could be extended up to 600 m2/g or the mixture was heated to 80◦C (see table 1, entry
higher. Also the size of surface pores could be changed 3i). After completion of the reaction as indicated by
between 5–500 Å. The surface of silica gel has two TLC, the reaction mixture was diluted with EtOAc
functional groups that include siloxane (Si–O–Si) and (10.0 mL). The catalyst was separated from the reaction
silanol (Si–OH). Hence, silica gel is modified and mixture by simple filtration. Then, the resulting product
its structure is changed via two different nucleophile was diluted (EtOAc-n-hexane, 2:8) to prepare the pure
substitutions on Si, or direct reaction with hydroxyl amide in crystalline form. Compounds 3a, 3b,38 3c, 3d,
group in silanol. The second method (hydroxyl group 3m, 3n,44 3g, 3p,47 3q, 3r, 3s, 3t39 are reported in the
reaction) for modified silica gel is often selected as the literature. Spectroscopic data for selected compounds
main approach. In view of this, several types of solid are presented below:
sulphonic-acid-functionalized silica (both amorphous
and ordered) have been synthesized and applied as
2.2a N- (t-phenyl methano)-2- methoxy ethyl carbox-
amide: (3f): White solid; m.p. = 160–161◦C; IR
(KBr): νmax: 3500, 3250, 3000, 1640, 1520, 1480, 1430,
1120 cm−1. 1H NMR (CDCl3, 500 MHz): δ (ppm) 2.55
(t, 2H, J = 5.5 Hz), 3.41 (s, 3H), 3.71 (t, 2H, J = 5.5 Hz),
7.26 (m, 7H), 7.29 (m, 2H), 7.33 (m, 6H), 7.74 (s, 1H).
13C NMR (CDCl3, 125 MHz): δ (ppm) 38.6, 59.24,
69.30, 70.72, 127.33, 128.31, 129.1, 145.3, 170.8.
an alternative to traditional sulphonic acid resins and
homogeneous acids in catalyzing chemical transforma-
tions.11–14,17,18 As a result, a variety of acidic reagent
such as: H2SO4,31 TfOH,32 Zeolits,33,34 MnO2.SiO2,35
BF3.OEt2,36 TiCl4,37 Bi(OTF)3,38 Ca(HSO4)2,39
DNBSA,40 Fe3+-K10 Montmorillonite,41 P2O5.SiO2,42
43
44
H2PW12O40 and PMA.SiO2 have been reported for
the synthesis of amides by using Ritter reaction. There-
fore, the development of mild, efficient, convenient and
benign reagents for the Ritter reaction is desirable. In
this research, we have developed a clean and environ-
mentally friendly protocol for the synthesis of amides
by using different benzylic or tertiary alcohols and
different nitriles in the presence of silica-bonded N-
propyl sulphamic acid (SBNPSA) under solvent-free
conditions with good to excellent yields. It is inter-
esting to note that SBNPSA was first prepared by the
authors of this study in 2009.45
2.2b N- (d-phenyl methano)-2- methoxy ethyl car-
boxamide: (3h): white solid; m.p. = 96–97◦C; IR
(KBr): νmax: 3500, 1640, 1540, 1120. 1H NMR (CDCl3,
500 MHz): δ (ppm) 2.57 (t, 2H, J = 5.5 Hz), 3.40 (s,
3H), 3.71 (t, 2H, J = 5.5 Hz), 6.29 (d, 1H, J = 10 Hz),
6.93 (d, 1H, J = 10 Hz), 7.26 (d, 4H, J = 7.8 Hz), 7.29
(m, 2H), 7.35 (m, 4H). 13C NMR (CDCl3, 125 MHz):
δ (ppm) 37.59, 57.16, 59.20, 69.20, 127.75, 127.77,
129.1, 142.21, 170.98.
2. Experimental
2.2c N- (d-phenyl methano)-cyclo propan carboxami-
de: (3i): white solid; m.p. = 173–175◦C; IR (KBr):
2.1 General remarks
1
νmax: 3500, 3300, 3050, 1620, 1540, 1480. H NMR
(CDCl3, 500 MHz): δ (ppm) 0.77 (m, 2H), 1.03 (m, 2H),
1.45 (m, 1H), 6.30 (d, 1H, J = 5.6 Hz), 6.31 (d, 1H,
J = 5 Hz) 7.3 (m, 6H), 7.36 (m, 4H). 13C NMR (CDCl3,
125 MHz): δ (ppm) 7.78, 15.24, 57.51, 127.83, 127.86,
129.1, 142.16, 173.1.
Chemicals were purchased from Merk and Fluka Com-
panies. IR spectra were run on a Shmadzu IR-435 FTIR
spectrophotometer. 1H NMR was run on Bruker Avence
(DRX 500 MHz) and Bruker UltraShield (400 MHz).
Melting points were recorded on a Melting Point SMP1
apparatus in open capillary tubes. The progress of reac-
tion was followed with TLC, using silica gel SILG/UV
254 plates. All the products are known and were cha-
racterized by comparison of their spectra (IR, 1H
NMR), TLC and physical data with those reported in
the literature.46
2.2d N- (d-phenyl methano)-buthyl carboxamide (3j):
white solid; m.p. = 122–124◦C; IR (KBr): νmax: 3800,
1
3400, 2800, 1670, 1550, 1480, 700. H NMR (CDCl3,
500 MHz): δ (ppm) o.95 (t, 3H, J = 7.5 Hz), 1.38(m,
2H), 1.68 (m, 2H), 2.29(t, 2H, J = 7.5), 6.11 (d, 1H,
J = 5 Hz), 6.3 (d, 1H, J = 5 Hz), 7.26 (d, 4H, J =
7.3 Hz), 7.3 (d of d, 2H, J1 = 8.5 Hz, J2 = 5 Hz),
7.36 (t, 4H, J = 7.1 Hz). 13C NMR (CDCl3, 125 MHz):
δ (ppm) 14.22, 22.86, 28.24, 36.98, 57.22, 127.84,
129.07, 142.1, 172.56.
2.2 General procedure
Diphenyl methanol (1.0 mmol, 0.18 g) and cyclo-
propanecarbonitrile (1.0 mmol, 0.07 mL) were mixed

Preparation of different amides via Ritter reaction
1027
Table 1. SBNPSA catalysed amidation of alcohols with nitriles (the reactions were carried out at 80◦C under solvent-free
conditions).
Time
(min)
Yieldb
(%)
Entry
Alcohol (1)
Nitrile (2)
CN
Amide (3)a
M.P(◦C)
133–135
Me
O
3a38
45
80
98
96
Me
OH
(Me)3-C-NH-C
Me
Me
O
3b38
3c44
3d44
140–141
77–78
Me
OH
CN
(Me)3-C-NH-C
HN
Me
OH
Cl
Cl-CH2-CN
70
40
94
92
O
Me
Me
OH
HN
O
CN
114–116
O
3e
3f
CH3CN
40
94
91
199–200
160–161
C-NH-C
CH3
C-OH
C-OH
O
CH3O-CH2-CH2-CN
170
C-NH-C
OCH3
2.2e N- (d-phenyl methano)-3- choloro benzo car- 1.81 (m,1H), 1.84 (m, 2H), 1.94 (m, 2H), 2.20 (m, 1H),
boxamide: (3k): white solid; m.p. = 153–155◦C; IR 6.08 (d, 1H, J = 5 Hz), 6.28 (d, 1H, J = 5 Hz), 7.24
1
(KBr): νmax: 3500, 1640, 1520, 1480, 700. H NMR (d, 4H, J = 7.2 Hz), 7.29 (t, 2H, J = 5 Hz), 7.35 (t, 4H,
(CDCl3, 500 MHz): δ (ppm) 6.47 (d, 1H, J = 5 Hz), J = 5.6 Hz).13C NMR (CDCl3, 125 MHz): δ (ppm)
6.66 (d, 1H, J = 5 Hz), 7.32 (m, 6H), 7.40 (m, 5H), 26.14, 30.13, 45.95, 56.94, 127.81, 129.1, 142.18,
7.51 (d, 1H, J = 7.8 Hz), 7.72 (d, 1H, J = 7.5 Hz), 175.41.
7.83 (s, 1H). 13C NMR (CDCl3, 125 MHz): δ (ppm)
58.04, 125.59, 127.79, 127.90, 128.13, 129.24, 130.41,
132.16, 135.27, 136.45, 141.56.
3. Results and discussion
In this paper, an efficient, complete and high yield-
2.2f N- (d-phenyl methano)-cyclo hexane carboxamide ing Ritter reaction of alcohols with nitriles in the pre-
(3o): white solid; m.p. = 184–186◦C; IR (KBr): νmax
:
sence of Silica-bonded N-propyl sulphamic acid has
1
3500, 3200, 2900, 1640, 1540, 1480, 1440. H NMR been examined as a catalyst under solvent-free con-
(CDCl3, 500 MHz): δ (ppm) 1.28 (m, 3H), 1.52 (m, 2H), ditions (figure 1). Silica-bonded N-propyl sulphamic

1028
Maryam-Sadat Shakeri et al.
Table 1. (continued)
Time
(min)
Yield
(%)
Entry
Alcohol
Nitrile
Amide
M.P
O
CH2-CN
3g47
180
60
93
91
173–174
C-OH
C-NH-C
O
C-NH-C
OH
OH
3h
CH3O-CH2-CH2-CN
96–97
OCH3
O
3i
3j
25
45
94
96
173–175
122–124
CN
C-NH-C
O
OH
OH
CH3-(CH2)3-CN
C-NH-C-(CH2)3-CH3
O
Cl
CN
3k
25
20
15
95
94
98
153–155
146–148
143–144
C-NH-C
Cl
CN
O
OH
OH
3l
C-NH-C
Br
Br
O
3m44
CH3CN
C-NH-C-CH3

Preparation of different amides via Ritter reaction
1029
Table 1. (continued)
Time
(min)
Yield
(%)
Entry
Alcohol
Nitrile
Amide
M.P
O
OH
3n44
Cl-CH2-CN
20
20
96
94
123–125
184–186
C-NH-C-CH2-Cl
O
CN
OH
3o
C-NH-C
O
CN
OH
OH
OH
3p47
3q39
3r39
3s39
3t39
50
150
120
60
96
92
93
93
133–135
93–95
NH
Cl
Cl
Cl
Br
Br
Cl
O
CH3CN
Cl-CH2-CN
CH3CN
NH
CH3
CH2Cl
CH3
Cl
O
110–111
101–102
NH
Cl
O
OH
OH
NH
Br
O
Cl-CH2-CN
NH
CH2Cl
90
91
115–116
Br
aAll products were characterized by IR, NMR spectra.
bYield refers to pure products after re-crystallization with EtOH/n-hexane(2:8).
acid (SBNPSA) preparation is described as illustrated various solvents (table 3, entry 5) and different temper-
in figure 2.45
atures (table 4, entry 2).
In order to optimize the reaction conditions, we have
The optimized condition with respect to the
studied the reaction of diphenyl methanol with acetoni- ratio of nitrile/alcohol/cat./temperature was 1.0 mmol/
trile (table 1, entry 3 m) in the presence of SBNPSA 1.0 mmol/0.1 g/80◦C. Accordingly, treatment of diphe-
at different amounts of catalyst (table 2, entry 2) and nyl methanol with acetonitrile in the presence of opti-

1030
Maryam-Sadat Shakeri et al.
O
SiO2
NH-SO H
(0.1g)
3
R-NH-C-R1
(3)
R-OH
(1)
+
R1CN
(2)
Solvent -free, 80°C
(X=Br,Cl)
R= (Me)3C, Ph -CH-CH3, (Ph)3C, (Ph)2CH, X-Ph-CH2
R1= CH3, Br-Ph-, Cl-Ph-, CH3O-CH2-CH2-, ClCH2, CH3-(CH2)3-, cyclohexan, cyclopropan
Figure 1. General scheme of Ritter reaction.
O
O
O
NH2
Toluene, reflux, 18h
Table 4. Optimization of temperature.
OH
OH
OH
(MeO)3Si
Si
NH2
Entry
Temperature (◦C)
Time (min)
Yield (%)
1
2
3
4
50
80
100
r.t
120
15
10
80
98
93
20
O
1) ClSO3 / CHCl3 / 4h
2) Washing with EtOH
24 h
O
O
Si
NHSO3H
NHSO3H
SiO2
-H2O
Figure 2. Preparation of silica bonded N-propyl sulphamic
acid (SBNPSA).
SiO2
NHSO3
(1)
SiO2
N
R
OH
H2O-R
O3S
(2)
R
H
R1C
R
N
O
R1C
N R
H2O
NHR
R1C
N
R1
OH2
SO3
SO3
NH
Table 2. Optimization of catalyst’s amount.
+
N
H
NHSO3H
SiO2
SiO2
Entry
Catalysis (g)
Time (min)
Yield (%)
(3)
SiO2
(4)
1
2
3
4
0.05
0.1
0.2
90
15
15
15
98
98
93
93
Figure 3. Proposed mechanism for the conversion of alco-
hols and nitriles into the corresponding amides by using
SBNPSA.
0.3
Table 5. Recyclability of SBNPSA (0.1 g).
Entry
Time (min)
Yield (%)
Table 3. Selection of the appropriate solvent.
1
2
3
4
5
15
15
30
60
360
98
98
95
90
70
Entry
Solvent
Time (h)
Yield (%)
1
2
3
4
5
6
n-hexane (reflux)
CH2Cl2 (reflux)
Water (80◦C)
EtOAc (reflux)
Solvent-free (80◦C)
CHCl3(reflux)
24
24
24
24
15 min
<10
20
10>
10>
98
Based on the presented results in table 1, it can be
concluded that:
24
20
(i) The entire reactions in the presence of SBNPSA
are completed.
(ii) Generally, amidation of alcohols with nitriles in
the presence of an SBNPSA catalyst could be
done in a short time.
(iii) Diphenyl alcohols perform amidation reaction
directly and in a short time compared to tertiary,
triphenyl and benzyl alcohols.
mized condition gave the corresponding N-(diphenyl
methano)-2-methyl carboxamide with 98% yield. Then,
we applied this ratio for the reaction of other nitriles
with alcohols. All the reactions were completed and
proceeded well in high yields (91–98%) as summarized
in table 1.

Preparation of different amides via Ritter reaction
1031
Table 6. A comparative study for the Ritter reaction using solid acids.
Time /yield (%)
Entry
1
Alcohol
Nitrile
I
II
III
IV
V
CN
CH3
45 min/98 2 h/91 17 h/85 10 h/99 6.5 h/95
H3C
C
OH
CH3
OH
2
3
CH3CN
15 min/98
–
–
–
–
–
6.5 h/87
6.5 h/87
0H
ClCH2C
N
70 min/94 2 h/85
CH3
I= SiO2-CH2-CH2-CH2-NH-SO3H; II= Ca(HSO4)2; III= Bi(OTf)3 (20 mol%); IV=
H3PW12O40; V= PMA/SiO2.
(iv) This catalyst selectively performs the amidation 10 h and PMA/SiO2 for 6.5 h were resulted in 91%,
reaction of tertiary alcohols, benzyl, diphenyl and 85%, 99% and 95% yield, respectively. So, the achieved
triphenyl alcohols.
yield with the application of SBNPSA was superior to
(v) The SBNPSA is a clean catalyst and can be sepa- other catalysts. On the other hand, the time of reaction
rated simply from the reaction mixture.
for the implemented catalyst of this study was shorter
than the other ones and in the other cases, the reported
times for finishing the reaction were longer. Hence, high
speed reaction with a higher yield was the strength and
advantage for the applied catalyst (SBNPSA).
The Ritter reaction proceeds by the electrophilic addi-
tion of either the carbenium ion or covalent species48,49
to the nitrile. The resulting nitrilium ion is hydrolyzed
by water to the desired amide.
Based on the obtained results, the proposed mecha-
nism could be explained as follows (figure 3). Initially,
alcohol reacts with the catalyst (SBNPSA) (1). Then,
alcohol taking a proton and releasing water constructs
carbocation intermediate (2). Nitrile reacts with car-
bocation intermediate and results in N-alkyl nitronium
intermediate (3). Finally, by hydrolysis of N-alkyl nitro-
nium intermediate and tautomery (proton transfer), the
product will be produced (4).
The possibility of recycling the catalyst was exa-
mined using the reaction of diphenyl methanol and
acetonitrile under the optimized conditions. On com-
pletion, the reaction mixture was washed with EtOAc
(10 mL).The catalyst was separated by filtration. The
recovered catalyst was washed with ethanol, dried and
reused for subsequent runs. The recycled catalyst could
be reused four times without any additional treatments
(table 5).
4. Conclusions
Based on the findings of this study, we introduced
SBNPSA as a solid acid catalyst to increase the speed
of synthesis of amides. The proposed catalysis showed
a good ability to speed up these reactions. The applied
method with SBNPSA had several advantages such as
high yields, taking less reaction time, simple separation
and purification.
Acknowledgements
The first author would like to especially thank Dr. E Zia
Hosseinipour, Mr. Roozbeh Daneshvar and also Mrs.
Leila Hasheminik for their help and support.
The results obtained from SBNPSA catalyst was
compared with the results of other catalysts reported
by other researchers (table 6). For instance, the reac-
tion of t-butanol with benzonitrile (entry 1) in the
presence of SBNPSA for 45 min was performed with
98% yield. The same reactions in the presence of Ca
(HSO4)2 for 2 h, Bi (OTf)3 for 17 h, H3PW12O40 for
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