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SiO2 catalyzed amidation of alcohols with nitriles: A simple, cost-effective and recyclable catalytic system for Ritter reaction
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Catalysis Communications 9 (2008) 1297–1301 www.elsevier.com/locate/catcom
PMA/SiO2 catalyzed amidation of alcohols with nitriles: A simple, cost-effective and recyclable catalytic system for Ritter reaction J.S. Yadav *, B.V. Subba Reddy, T. Pandurangam, Y. Jayasudan Reddy, Manoj K. Gupta Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad 500007, India Received 27 June 2007; received in revised form 31 October 2007; accepted 19 November 2007 Available online 24 November 2007
Abstract Various benzylic, allylic and tertiary alcohols are converted into their corresponding amides in excellent yields, when reacted with various nitriles in the presence of 0.5 mol% of phosphomolybdic acid supported on silica gel at 80 °C under solvent-free conditions. The use of PMA–SiO2 makes it quite simple, more convenient and environmentally benign. This method offers several advantages such as high conversions, short reaction times, cleaner reaction profiles and the use of inexpensive and readily available PMA–SiO2. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Ritter reaction; Heterogeneous catalysis; Nitriles; Activated alcohols; Amides
1. Introduction The Ritter reaction is one of the most important carbonnitrogen bond forming reactions and is widely used to prepare N-tert-butyl amides, which are important amine precursors in pharmaceuticals [1–3]. The reaction of tertiary or benzylic alcohols with nitriles in the presence of conc. H2SO4 is known as the classical Ritter reaction but strongly acidic medium limits its use, especially for acid sensitive substrates. Subsequently, several modifications and improvements have been made to accomplish this reaction under mild conditions. As a result, a variety of acidic reagents such as (CF3SO2)2O [4], BF3 OEt2 [5], Fe(ClO4)3/ SiO2 [6], MeSO3H/Al2O3 [7], Nafion-H [8], CoCl2/Ac2O [9,10], Fe3+-K10 Montmorillonite [11], Zeolites [12], triflic anhydride [13,14] and P2O5–SiO2 [15] have been developed for this conversion. The modified Ritter reaction has been reported using Ph2 Cþ ClSbCl 6 [16] and orthoesters [17] or trimethylsilyl cyanide [18] instead of alcohols or nitriles, respectively. However, many of these methods involve the use of strongly acidic conditions, stoichiometric amounts *
Corresponding author. Tel.: +91 40 27193030 ; fax: +91 40 27160512. E-mail address: [email protected] (J.S. Yadav).
1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.11.021
of reagents, harsh conditions and extended reaction times. Therefore, the development of mild, efficient, convenient and benign reagents for the Ritter reaction is desirable. Recently, the use of phosphomolybdic acid has been increased as an environmentally friendly catalyst in organic synthesis. In addition to its abundance, economical and safety reasons, phosphomolybdic acid has naturally become as a substitute and an alternative solid acid. Phosphomolybdic acid (PMA) belongs to a class of heteropoly acids (HPA). Catalysis using HPAs and related polyoxometalate compounds is a field of growing importance [19,20]. HPAs are commercially cheap and environmentally friendly catalysts. They exhibit high activities and selectivities over conventional catalysts. HPAs are promising solid acids and act as bifunctional catalysts under homogeneous as well as in heterogeneous conditions. HPAs are very strong acids, approaching the super acid region, with a Bronsted acidity greatly exceeding that of ordinary mineral acids and solid acid catalysts. HPAs are several times more active than H2SO4, TsOH, BF3 OEt2 and ZnCl2 [21,22]. It has been shown that in organic media, the molar catalytic activity of HPAs is often 100–1000 times higher than that of H2SO4 [23,24]. This makes it possible to carry out a catalytic process at low concentrations and at lower
1298
J.S. Yadav et al. / Catalysis Communications 9 (2008) 1297–1301
temperatures. Supported HPAs are more active than bulk HPAs. Acidic or neutral substances such as silica gel [22], active carbon [25,26] or an acidic ion-exchange resin [27] are suitable supports, the more commonly used being silica gel [22]. Synthetically, a variety of methods has been developed and commercialized using HPAs as catalysts [28–30]. PMA–SiO2 is a stable, non-hygroscopic and the highly active catalytic system than unsupported PMA. However, there have been no reports on the use of PMA–SiO2 for the Ritter reaction.
3. Experimental section
2. Results and discussion In continuation of our efforts to explore the synthetic utility of phosphomolybdic acid supported on silica gel (PMA–SiO2) [31–33], we herein report a simple and efficient protocol for the Ritter reaction using a cheap and readily available PMA–SiO2 catalyst. Accordingly, treatment of 2-methylpropan-2-ol (1) with benzonitrile (2) in the presence of PMA/SiO2 under solvent free conditions gave the corresponding N-tert-butyl benzamide 3a in 95% yield (Scheme 1). Encouraged by the results obtained with 2-methylpropan-2-ol and benzonitrile, we turned our attention to various substituted alcohols and nitriles. Interestingly, several primary, secondary and tertiary alcohols reacted smoothly with a wide array of nitriles to give the corresponding amides relatively in good to excellent yields (entries a–p, Table 1). In all cases, the reactions proceeded efficiently in high yields at 80 °C. However, in the absence of catalyst, the reaction did not yield the desired product even an extended reaction time (12 h). PMA/SiO2 was found to be highly selective for benzylic-, or allylic secondary and tertiary alcohols, at general with regard to nitriles, which are the other ingredients in this reaction. Unlike Lewis acid catalyzed Ritter amidation, our method is applicable to electron deficient aryl carbinols and tertiary alcohols like tert-butanol and all these substrates gave amides in good to excellent yields. All the products were characterized by 1 H NMR, IR and mass spectrometry. Several functional groups could tolerate these reaction conditions and no dealkylation or dearylation were observed, which are otherwise commonly encountered under strongly acidic conditions. To know the efficacy of PMA supported silica, we have carried out the comparative experiments with some solid acids and the comparative results are summarized in Table 2. These results clearly show the advantages of our method over protic or Lewis acid catalyzed Ritter reactions. The catalyst was separated from the reaction
OH
CN
+
0.5 mol % PMA-SiO2 70 oC, 6 h
1
mixture by simple filtration. The recovered catalyst was washed with dichloromethane and dried in vacuo. Thus recovered catalyst was reused for further reactions without significant loss of activity. For instance, treatment of tbutanol with benzonitrile for 6 h gave N-tert-butyl benzamide 3a in 95, 92 and 90% yield, respectively over three cycles. This observation clearly shows the reusability of the catalyst. The scope and generality of this process is illustrated with respect to various alcohols and substituted nitriles and the results are summarized in Table 1.
O N H 3a
2
Scheme 1.
3.1. General remarks Melting points were recorded on Buchi R-535 apparatus and are uncorrected. IR spectra were recorded on a Perkin–Elmer FT-IR 240-c spectrophotometer using KBr optics. 1H NMR spectra were recorded on Varian-unity 300 spectrometer in CDCl3 using TMS as internal standard. Mass spectra were recorded on a Finnigan MAT 1020 mass spectrometer operating at 70 eV. 3.2. General procedure Cyclohexanol (0.27 g, 2.90 mmol) and benzonitrile (0.3 g, 2.6 mmol) were mixed with PMA–SiO2 (2.9 g, 0.5 mol%) under solvent free condition and mixture was heated at 70–80 °C (see Table 1). After completion of the reaction as indicated by TLC, the reaction mixture was diluted with EtOAc (15 mL) and H2O (15 mL) and aqueous layer was extracted with EtOAc (3 20 mL). The combined organic layers were washed with brine (20 mL), dried over anhydrous Na2SO4 and concentrated in vacuo. The resulting product was charged on a short silica gel column (Merck, 60–120 mesh) and eluted (EtOAc-n-hexane, 2:8) to afford the pure amide in crystalline form. Compound 3a [34], 3c [35], 3h [34] and 3o [16] are reported in the literature. Spectroscopic data for selected compounds. N-(1-phenylethyl) benzamide: (3f): Pale brown solid; m.p. = 114–116 °C; IR (KBr): mmax: 3355, 3029, 2974, 2932, 1634, 1517, 1488, 1270, 1071, 869, 695 cm1. 1H NMR (CDCI3, 300 MHz): d 7.78–7.16 (m, 10H), 6.19 (bs, 1H), 5.39–5.23 (m, 1H), 1.60 (d, 3H). 13C NMR (CDCI3, 300 MHz): d: 21.6, 49.1, 126.2, 127.3, 127.8, 128.4, 129.3, 131.5, 134.4, 143.5, 166.4. LC-MS: m/z: 248 (M+ + Na). HRMS calcd for C15H16NO: 226.1231. Found: 226.1233. N-(1-phenylethyl) acryl amide: (3h): pale reddish brown solid; m.p. = 63–65 °C; IR (KBr): mmax: 3265, 3062, 2976, 2927, 2870, 1656, 1625, 1549, 1404, 1245, 1130, 955, 699 cm1. 1H NMR (CDCI3, 300 MHz): d 7.32– 7.18 (m, 5H), 6.26 (dd, 1H), 6.05 (dd, 1H), 6.02 (bs, 1H), 5.59 (dd, 1H), 5.23–5.12 (m, 1H) 1.51 (d, J = 6.7 Hz, 3H). LC-MS: m/z: 198 (M+ + Na). HRMS calcd for C11H13NONa: 198.0894. Found: 198.0890. N-(2-adementyl)benzamide: (3n): White solid; m.p. = 143–1458 °C; IR (KBr): mmax: 3304, 3061, 2906, 2849, 1635, 1543, 1451,
J.S. Yadav et al. / Catalysis Communications 9 (2008) 1297–1301
1299
Table 1 PMA–SiO2 catalyzed amidation of alcohols with nitrilesa Entry a
Alcohol (1)
Amide (3)b
Nitrile (2) CN
OH
O
Time (h)
Yield (%)c
6.0
95
5.5
91
6.0
88
5.0
93
6.0
93
7.0
85
6.5
86
5.5
86
5.5
86
6.5
87
6.0
78
5.5
80
N H
b
H N
CN
OH
O
c
O
H3C CN
OH
N H
d
F
OH
F
OH
N H
O2N
O2N
e
O
CN
CN
O N H
f
OH
CN
O N H
g
OH
Cl
O
CN Cl
h
OH
N H
O
CN
N H
OH
i
H3C CN
O N H
H3C
j
OH
Cl
CN O Cl
k
O
H3C CN
OH
H3C OPh
N H
N H OPh
l
OH
NH2 O CN NH2
MeO
N H
OMe OMe
OMe
(continued on next page)
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J.S. Yadav et al. / Catalysis Communications 9 (2008) 1297–1301
Table 1 (continued) Entry
Alcohol (1)
m
Nitrile (2) Cl
Amide (3)b
CN
Time (h)
Yield (%)c
5.0
82
6.0
93
6.5
95
5.0
89
O Cl
OH
n
OH
N H
H N
CN O
o
OH
CN
O N H
p
OH
CN
H N O
a b c
The reactions were carried out at 80 °C under solvent-free conditions. All products were characterized by IR, 1H NMR spectra and mass spectrometry. Yield refers to pure products after column chromatography.
Table 2 A comparative study for the Ritter reaction using solid acidsa S. no.
Catalyst
1
PMA–SiO2 (2.9 g, 0.5 mol%)
Alcohol (2.9 mmol) OH
Nitrile (2.6 mmol) CN
Amide O
Time (h)
Yield (%)
6.0
95
6.5
80
5.0
85
6.5
68
N H
2
Amberlyst-15 (3 g) OH
CN
O N H
3
Montmorillonite KSF (3 g) OH
CN
O N H
4
Zeolite (3 g, HZSM-5) OH
CN
O N H
a
The reactions were carried out at 80 °C.
1112, 695 cm1; 1H NMR (CDCI3, 300 MHz): d 7.42-7.21 (m, 5H), 5.55 (bs, 1H), 3.98 (bs, 1H), 3.56 (s, 2H), 1.85–1.32 (m, 14H). LC-MS: m/z: 292.1 (M+ + Na). HRMS calcd for C18H24NONa: 270.1857. Found: 270.1860.
and high yielding protocol for the amidation of alcohols. This method offers significant advantages such as inexpensive reagents, solvent free conditions and ease of separation of the catalyst for achieving quick and clean conversions, which makes it very useful and eco-friendly.
4. Conclusion
Acknowledgments
In conclusion, silica gel supported PMA catalyzed Ritter reaction presented in this paper is an efficient, convenient
TP, YJR and MKG thank CSIR, New Delhi, for the award of fellowships.
J.S. Yadav et al. / Catalysis Communications 9 (2008) 1297–1301
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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[19] F. Tamaddon, M.A. Amrollahi, L. Sharafat, Tetrahedron Lett. 46 (2005) 7841. [20] I.V. Kozhevnikow, Chem. Rev. 98 (1998) 171. [21] N. Mizuno, M. Misono, Chem. Rev. 98 (1998) 199. [22] Y. Izumi, R. Hasebe, K. Urabe, J. Catal. 84 (1983) 402. [23] I.V. Kozhevnikov, Russ. Chem. Rev. 56 (1987) 811. [24] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113. [25] Y. Izumi, K. Urabe, Chem. Lett. (1981) 663. [26] M.A. Schwegler, H. van Bekkum, N.A. de Munck, Appl. Catal. 74 (1991) 191. [27] T. Baba, Y. Ono, Appl. Catal. 22 (1986) 321. [28] Y. Izumi, K. Urabe, M. Onaka, Zeolite, Clay and Heteropoly Acid in Organic Reactions, Kodansha/VCH, Tokyo, 1992, p. 99. [29] E.F. Kozhevnikova, E.G. Derouane, I.V. Kozhevnikov, Chem. Commun. (2002) 1178. [30] J. Kaur, K. Griffin, B. Harrison, I.V. Kozhevnikov, J. Catal. 208 (2002) 448. [31] J.S. Yadav, M. Satyanarayana, E. Balanarsaiah, S. Raghavendra, Tetrahedron Lett. 47 (2006) 6095. [32] K. Nagaiah, D. Sreenu, R. Srinivasa Rao, G. Vashishta, J.S. Yadav, Tetrahedron Lett. 47 (2006) 4409. [33] J.S. Yadav, S. Raghavendra, M. Satyanarayana, E. Balanarsaiah, Synlett (2005) 2461. [34] H.M.S. Kumar, B.V.S. Reddy, S. Anjaneyulu, E.J. Reddy, J.S. Yadav, New J. Chem. 23 (1999) 955. [35] E. Callens, A.J. Burton, A.G.M. Barrett, Tetrahedron Lett. 47 (2006) 8699.
Catalysis Communications 9 (2008) 1297–1301 www.elsevier.com/locate/catcom
PMA/SiO2 catalyzed amidation of alcohols with nitriles: A simple, cost-effective and recyclable catalytic system for Ritter reaction J.S. Yadav *, B.V. Subba Reddy, T. Pandurangam, Y. Jayasudan Reddy, Manoj K. Gupta Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad 500007, India Received 27 June 2007; received in revised form 31 October 2007; accepted 19 November 2007 Available online 24 November 2007
Abstract Various benzylic, allylic and tertiary alcohols are converted into their corresponding amides in excellent yields, when reacted with various nitriles in the presence of 0.5 mol% of phosphomolybdic acid supported on silica gel at 80 °C under solvent-free conditions. The use of PMA–SiO2 makes it quite simple, more convenient and environmentally benign. This method offers several advantages such as high conversions, short reaction times, cleaner reaction profiles and the use of inexpensive and readily available PMA–SiO2. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Ritter reaction; Heterogeneous catalysis; Nitriles; Activated alcohols; Amides
1. Introduction The Ritter reaction is one of the most important carbonnitrogen bond forming reactions and is widely used to prepare N-tert-butyl amides, which are important amine precursors in pharmaceuticals [1–3]. The reaction of tertiary or benzylic alcohols with nitriles in the presence of conc. H2SO4 is known as the classical Ritter reaction but strongly acidic medium limits its use, especially for acid sensitive substrates. Subsequently, several modifications and improvements have been made to accomplish this reaction under mild conditions. As a result, a variety of acidic reagents such as (CF3SO2)2O [4], BF3 OEt2 [5], Fe(ClO4)3/ SiO2 [6], MeSO3H/Al2O3 [7], Nafion-H [8], CoCl2/Ac2O [9,10], Fe3+-K10 Montmorillonite [11], Zeolites [12], triflic anhydride [13,14] and P2O5–SiO2 [15] have been developed for this conversion. The modified Ritter reaction has been reported using Ph2 Cþ ClSbCl 6 [16] and orthoesters [17] or trimethylsilyl cyanide [18] instead of alcohols or nitriles, respectively. However, many of these methods involve the use of strongly acidic conditions, stoichiometric amounts *
Corresponding author. Tel.: +91 40 27193030 ; fax: +91 40 27160512. E-mail address: [email protected] (J.S. Yadav).
1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.11.021
of reagents, harsh conditions and extended reaction times. Therefore, the development of mild, efficient, convenient and benign reagents for the Ritter reaction is desirable. Recently, the use of phosphomolybdic acid has been increased as an environmentally friendly catalyst in organic synthesis. In addition to its abundance, economical and safety reasons, phosphomolybdic acid has naturally become as a substitute and an alternative solid acid. Phosphomolybdic acid (PMA) belongs to a class of heteropoly acids (HPA). Catalysis using HPAs and related polyoxometalate compounds is a field of growing importance [19,20]. HPAs are commercially cheap and environmentally friendly catalysts. They exhibit high activities and selectivities over conventional catalysts. HPAs are promising solid acids and act as bifunctional catalysts under homogeneous as well as in heterogeneous conditions. HPAs are very strong acids, approaching the super acid region, with a Bronsted acidity greatly exceeding that of ordinary mineral acids and solid acid catalysts. HPAs are several times more active than H2SO4, TsOH, BF3 OEt2 and ZnCl2 [21,22]. It has been shown that in organic media, the molar catalytic activity of HPAs is often 100–1000 times higher than that of H2SO4 [23,24]. This makes it possible to carry out a catalytic process at low concentrations and at lower
1298
J.S. Yadav et al. / Catalysis Communications 9 (2008) 1297–1301
temperatures. Supported HPAs are more active than bulk HPAs. Acidic or neutral substances such as silica gel [22], active carbon [25,26] or an acidic ion-exchange resin [27] are suitable supports, the more commonly used being silica gel [22]. Synthetically, a variety of methods has been developed and commercialized using HPAs as catalysts [28–30]. PMA–SiO2 is a stable, non-hygroscopic and the highly active catalytic system than unsupported PMA. However, there have been no reports on the use of PMA–SiO2 for the Ritter reaction.
3. Experimental section
2. Results and discussion In continuation of our efforts to explore the synthetic utility of phosphomolybdic acid supported on silica gel (PMA–SiO2) [31–33], we herein report a simple and efficient protocol for the Ritter reaction using a cheap and readily available PMA–SiO2 catalyst. Accordingly, treatment of 2-methylpropan-2-ol (1) with benzonitrile (2) in the presence of PMA/SiO2 under solvent free conditions gave the corresponding N-tert-butyl benzamide 3a in 95% yield (Scheme 1). Encouraged by the results obtained with 2-methylpropan-2-ol and benzonitrile, we turned our attention to various substituted alcohols and nitriles. Interestingly, several primary, secondary and tertiary alcohols reacted smoothly with a wide array of nitriles to give the corresponding amides relatively in good to excellent yields (entries a–p, Table 1). In all cases, the reactions proceeded efficiently in high yields at 80 °C. However, in the absence of catalyst, the reaction did not yield the desired product even an extended reaction time (12 h). PMA/SiO2 was found to be highly selective for benzylic-, or allylic secondary and tertiary alcohols, at general with regard to nitriles, which are the other ingredients in this reaction. Unlike Lewis acid catalyzed Ritter amidation, our method is applicable to electron deficient aryl carbinols and tertiary alcohols like tert-butanol and all these substrates gave amides in good to excellent yields. All the products were characterized by 1 H NMR, IR and mass spectrometry. Several functional groups could tolerate these reaction conditions and no dealkylation or dearylation were observed, which are otherwise commonly encountered under strongly acidic conditions. To know the efficacy of PMA supported silica, we have carried out the comparative experiments with some solid acids and the comparative results are summarized in Table 2. These results clearly show the advantages of our method over protic or Lewis acid catalyzed Ritter reactions. The catalyst was separated from the reaction
OH
CN
+
0.5 mol % PMA-SiO2 70 oC, 6 h
1
mixture by simple filtration. The recovered catalyst was washed with dichloromethane and dried in vacuo. Thus recovered catalyst was reused for further reactions without significant loss of activity. For instance, treatment of tbutanol with benzonitrile for 6 h gave N-tert-butyl benzamide 3a in 95, 92 and 90% yield, respectively over three cycles. This observation clearly shows the reusability of the catalyst. The scope and generality of this process is illustrated with respect to various alcohols and substituted nitriles and the results are summarized in Table 1.
O N H 3a
2
Scheme 1.
3.1. General remarks Melting points were recorded on Buchi R-535 apparatus and are uncorrected. IR spectra were recorded on a Perkin–Elmer FT-IR 240-c spectrophotometer using KBr optics. 1H NMR spectra were recorded on Varian-unity 300 spectrometer in CDCl3 using TMS as internal standard. Mass spectra were recorded on a Finnigan MAT 1020 mass spectrometer operating at 70 eV. 3.2. General procedure Cyclohexanol (0.27 g, 2.90 mmol) and benzonitrile (0.3 g, 2.6 mmol) were mixed with PMA–SiO2 (2.9 g, 0.5 mol%) under solvent free condition and mixture was heated at 70–80 °C (see Table 1). After completion of the reaction as indicated by TLC, the reaction mixture was diluted with EtOAc (15 mL) and H2O (15 mL) and aqueous layer was extracted with EtOAc (3 20 mL). The combined organic layers were washed with brine (20 mL), dried over anhydrous Na2SO4 and concentrated in vacuo. The resulting product was charged on a short silica gel column (Merck, 60–120 mesh) and eluted (EtOAc-n-hexane, 2:8) to afford the pure amide in crystalline form. Compound 3a [34], 3c [35], 3h [34] and 3o [16] are reported in the literature. Spectroscopic data for selected compounds. N-(1-phenylethyl) benzamide: (3f): Pale brown solid; m.p. = 114–116 °C; IR (KBr): mmax: 3355, 3029, 2974, 2932, 1634, 1517, 1488, 1270, 1071, 869, 695 cm1. 1H NMR (CDCI3, 300 MHz): d 7.78–7.16 (m, 10H), 6.19 (bs, 1H), 5.39–5.23 (m, 1H), 1.60 (d, 3H). 13C NMR (CDCI3, 300 MHz): d: 21.6, 49.1, 126.2, 127.3, 127.8, 128.4, 129.3, 131.5, 134.4, 143.5, 166.4. LC-MS: m/z: 248 (M+ + Na). HRMS calcd for C15H16NO: 226.1231. Found: 226.1233. N-(1-phenylethyl) acryl amide: (3h): pale reddish brown solid; m.p. = 63–65 °C; IR (KBr): mmax: 3265, 3062, 2976, 2927, 2870, 1656, 1625, 1549, 1404, 1245, 1130, 955, 699 cm1. 1H NMR (CDCI3, 300 MHz): d 7.32– 7.18 (m, 5H), 6.26 (dd, 1H), 6.05 (dd, 1H), 6.02 (bs, 1H), 5.59 (dd, 1H), 5.23–5.12 (m, 1H) 1.51 (d, J = 6.7 Hz, 3H). LC-MS: m/z: 198 (M+ + Na). HRMS calcd for C11H13NONa: 198.0894. Found: 198.0890. N-(2-adementyl)benzamide: (3n): White solid; m.p. = 143–1458 °C; IR (KBr): mmax: 3304, 3061, 2906, 2849, 1635, 1543, 1451,
J.S. Yadav et al. / Catalysis Communications 9 (2008) 1297–1301
1299
Table 1 PMA–SiO2 catalyzed amidation of alcohols with nitrilesa Entry a
Alcohol (1)
Amide (3)b
Nitrile (2) CN
OH
O
Time (h)
Yield (%)c
6.0
95
5.5
91
6.0
88
5.0
93
6.0
93
7.0
85
6.5
86
5.5
86
5.5
86
6.5
87
6.0
78
5.5
80
N H
b
H N
CN
OH
O
c
O
H3C CN
OH
N H
d
F
OH
F
OH
N H
O2N
O2N
e
O
CN
CN
O N H
f
OH
CN
O N H
g
OH
Cl
O
CN Cl
h
OH
N H
O
CN
N H
OH
i
H3C CN
O N H
H3C
j
OH
Cl
CN O Cl
k
O
H3C CN
OH
H3C OPh
N H
N H OPh
l
OH
NH2 O CN NH2
MeO
N H
OMe OMe
OMe
(continued on next page)
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J.S. Yadav et al. / Catalysis Communications 9 (2008) 1297–1301
Table 1 (continued) Entry
Alcohol (1)
m
Nitrile (2) Cl
Amide (3)b
CN
Time (h)
Yield (%)c
5.0
82
6.0
93
6.5
95
5.0
89
O Cl
OH
n
OH
N H
H N
CN O
o
OH
CN
O N H
p
OH
CN
H N O
a b c
The reactions were carried out at 80 °C under solvent-free conditions. All products were characterized by IR, 1H NMR spectra and mass spectrometry. Yield refers to pure products after column chromatography.
Table 2 A comparative study for the Ritter reaction using solid acidsa S. no.
Catalyst
1
PMA–SiO2 (2.9 g, 0.5 mol%)
Alcohol (2.9 mmol) OH
Nitrile (2.6 mmol) CN
Amide O
Time (h)
Yield (%)
6.0
95
6.5
80
5.0
85
6.5
68
N H
2
Amberlyst-15 (3 g) OH
CN
O N H
3
Montmorillonite KSF (3 g) OH
CN
O N H
4
Zeolite (3 g, HZSM-5) OH
CN
O N H
a
The reactions were carried out at 80 °C.
1112, 695 cm1; 1H NMR (CDCI3, 300 MHz): d 7.42-7.21 (m, 5H), 5.55 (bs, 1H), 3.98 (bs, 1H), 3.56 (s, 2H), 1.85–1.32 (m, 14H). LC-MS: m/z: 292.1 (M+ + Na). HRMS calcd for C18H24NONa: 270.1857. Found: 270.1860.
and high yielding protocol for the amidation of alcohols. This method offers significant advantages such as inexpensive reagents, solvent free conditions and ease of separation of the catalyst for achieving quick and clean conversions, which makes it very useful and eco-friendly.
4. Conclusion
Acknowledgments
In conclusion, silica gel supported PMA catalyzed Ritter reaction presented in this paper is an efficient, convenient
TP, YJR and MKG thank CSIR, New Delhi, for the award of fellowships.
J.S. Yadav et al. / Catalysis Communications 9 (2008) 1297–1301
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