Versatile and sustainable alcoholysis of amides by a reusable CeO 2 catalyst

Article abstract of DOI:10.1039/c4ra07419j

CeO₂ catalyzed the esterification between an equivalent molar ratio of primary amides and alcohols under neutral conditions, which provides the first versatile reusable catalytic system for direct alcoholysis of amides to esters with wider scope and 67 times higher turnover number (TON) than previous catalytic systems.

Full text of DOI:10.1039/c4ra07419j

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Versatile and sustainable alcoholysis of amides by a
reusable CeO2 catalyst†
Cite this: RSC Adv., 2014, 4, 35803
a
S. M. A. Hakim Siddiki, Abeda Sultana Touchy,b Masazumi Tamurac
*
and Ken-ichi Shimizuab
Received 22nd July 2014
Accepted 31st July 2014
CeO2 catalyzed the esterification between an equivalent molar ratio of primary amides and alcohols under
neutral conditions, which provides the first versatile reusable catalytic system for direct alcoholysis of
amides to esters with wider scope and 67 times higher turnover number (TON) than previous catalytic
systems.
DOI: 10.1039/c4ra07419j
www.rsc.org/advances
The amide bond is thermodynamically stable, and thus amide molar ratios of amide and alcohol under additive-free neutral
solvolysis usually requires heating under strongly acidic or conditions using CeO2 as reusable heterogeneous catalyst.
basic conditions because of the difficulty of cleaving an amide
First, we performed catalyst screening adopting a model
bond. Therefore, the synthetic application of amide solvolysis is reaction of equimolar amount of n-valeramide and benzylalco-
limited. Synthesis of ester by amide alcoholysis has attracted hol in mesitylene under reux conditions. Table 1 lists initial
the attention of organic chemists, but all of the reported rate per gram of catalyst under the conditions in which
methods are not effective in terms of generality and atom conversions were below 25%. Among various metal oxides and
economy.1–5 Although various non-catalytic methods of amide solid acids tested (metal oxides: CeO2, CaO, MgO, ZnO, Y2O3,
alcoholysis have been reported, they suffer from use of excess TiO2, ZrO2, Nb2O5, Al2O3, SiO2, solid acids: Amberlyst-15, HBEA,
amount of promoter (such as HCl and NaNO2), generation of niobic acid, mont. K10, Naon-SiO2), CeO2 showed 1–3 orders
inorganic or organic wastes, and limited substrate scope.1–10 of magnitude higher rate of ester formation than the other
Twisted amides undergo alcoholysis under mild and neutral metal oxides. Thermal reaction of n-valeramide and benzyl
conditions,11–13 but synthetic application of this reaction is quite alcohol in the absence of any catalysts was found completely
limited. Mashima and co-workers demonstrated the rst inactive for the ester formation.
example of catalytic amide alcoholysis using Zn(OTf)2,14 though
Using the most effective catalyst, CeO2, we studied general
the method is only applicable to the reaction of b-hydroxy- applicability of the present catalytic system. Table 2 shows the
ethylamides with n-BuOH, and stoichiometric amount of yields of the corresponding esters from the reaction of various
promoter was required. Very recently, Mashima et al.15 devel- primary amides with equimolar amount of benzylalcohol.
oped combined catalytic system of Sc(OTf)3 and boronic ester Various aliphatic linear amides with short to long carbon chain
for alcoholysis of primary amide to the corresponding ester. (entries 1–5) were tolerated, giving 100% conversion of amide
However, the method is only applicable to the ethanol and 1- and high isolated yield (86–93%) of corresponding esters.
propanol and has drawbacks such as inability of catalyst reuse, Benzylamide, benzamides with electron-donating and electron-
low TON, necessity of excess amount of alcohol, and difficulty in withdrawing groups (entries 6–11) and a heteroaromatic amide
catalyst/product separation. To the best of our knowledge, there (entry 12) were converted to the corresponding esters in good to
are no reports on reusable catalysts for the alcoholysis of amide excellent isolated yield (75–95%) at 100% conversions of alco-
to the corresponding ester. As a part of our continuing interest hols and amides. a, a-Disubstituted amide (entry 13) and
in CeO2-catalysed green organic syntheses,16–20 we report herein aliphatic diamide (entry 14) were also tolerated with good yield
a general catalytic system for esterication between equivalent (86% and 75%). Gram-scale experiments with 10 mg of CeO2
(0.58 mol% Ce) for the reaction of valeramide and benzylalcohol
(eqn (1)) and that of p-methoxybenzamide with 1-propanol (eqn
(2)) gave 95% and 85% yields, respectively. Based on the
number of Ce cations on the surface of CeO2 (1.067 mmol gꢀ1),21
turnover number (TON) for the latter reaction is 888, which is 67
times higher than the TON (13.2) of homogeneous catalytic
system with Sc(OTf)3 and boronic ester for the same reaction
under excess amount of 1-propanol (6.7 equivalent).
aElements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura,
Kyoto 615-8520, Japan. E-mail: hakim@cat.hokudai.ac.jp
bCatalysis Research Center, Hokkaido University, N-21, W-10, Sapporo 001-0021,
Japan
cDepartment of Applied Chemistry, School of Engineering, Tohoku University, 6-6-07,
Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra07419j
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Table 1 Ester formation from amide and alcohols by metal oxide
catalysts
centrifugation. The recovered catalyst was washed with acetone,
followed by drying in air at 90 ꢁC for 1 h. As shown in Fig. 1, the
recovered catalyst showed high yields (89–95%) in the succes-
sive 4 cycles. Although the yield was slightly decreased at 4th
run, the activity was recovered in the 5th and 6th cycles simply
by calcination (600 ꢁC, 0.5 h, in air) of the catalyst aer 4th run.
The present system is truly heterogeneous catalytic system as
evidenced by the following results. For the reaction of n-valer-
amide and benzylalcohol, we removed the catalyst from the
reaction mixture at the initial stage of the reaction (28% yield
aer 1 h). Further continuation of the reaction under reux
conditions did not increase the product yield. ICP analysis of
the ltrate conrmed that the content of Ce in the solution was
below the detection limit. These results indicate that CeO2 is a
leaching-resistant and reusable heterogeneous catalyst for this
reaction.
Catalyst
Va/mmol hꢀ1 gꢀ1
CeO2
41
CaO
2.7
MgO
ZnO
Y2O3
TiO2
ZrO2
Nb2O5
Al2O3
SiO2
Amberlyst-15
HBEA
0.22
0.07
0.33
0.32
0.37
0.29
0.10
0.03
2.17
1.06
1.39
0.16
0.72
0
Conclusion
Niobic acid
Mont-K10
Naon-SiO2
Blank
We found CeO2-catalyzed esterication between equivalent
molar ratios of amide and alcohol. This is the rst additive-free
and reusable heterogeneous catalytic system for direct alco-
holysis of amides to esters with wider scope and 67 times higher
TON than the previous homogeneous catalyst, and thus it
a
Initial rate per gram of catalyst under the conditions in which
conversions were below 25%.
provides
a sustainable, general, and economic synthetic
method of a wide range of esters.
Next we investigated the scope of alcohols. Table 3 shows the
reaction of various alcohols with equimolar amount of benza-
mide. Linear and branched aliphatic alcohols (entries 1–12) and
an allylalcohol were (entry 13) converted to the corresponding
ester with 100% conversions and high isolated yields (80–90%).
Benzyl alcohols with electron-donating or electron-withdrawing
group (entries 14–19) reacted to afford the corresponding ester
in good to high yields (75–89%). A heteroaromatic alcohol (entry
20) was also successfully converted to the ester with high yield
(83%). The GC-MS analysis for all the reactions in Tables 2 and 3
showed no formation of byproducts.
Experimental section
General
Commercially available organic and inorganic compounds
(from Tokyo Chemical Industry, Wako Pure Chemical Indus-
tries, Kishida Chemical, or Mitsuwa Chemicals) were used
without further purication. The GC (Shimadzu GC-14B) and
GCMS (Shimadzu GCMS-QP2010) analyses were carried out
with Ultra ALLOY capillary column UA+-5 (Frontier Laboratories
Ltd.) using nitrogen as the carrier gas.
Finally, we studied reusability of the catalytic system. Aer
the reaction of n-valeramide with benzylalcohol (entry 1 in Table
2), the catalyst was separated from the reaction mixture by
Catalyst
CeO2 (JRC-CEO3, surface area ¼ 81 m2 gꢀ1) pre-calcined at
600 ꢁC before being supplied from the Catalysis Society of Japan
(1)
(2)
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Table 2 CeO2-catalyzed ester formation from various amide and Table 3 CeO2-catalyzed ester formation from benzamide and various
benzylalcohol
alcohols
Isolated
yield (%)
Isolated
yield (%)
Entry
1
Amides
Products
Entry
1
Alcohols
Products
93 (95)a
95
88
90
2
3
4
5
2
90
91
86
3
88
85
80
82
75
90
93
85
78
88
86
83
4
5
6
7
8
86
89
75
6
7
8a
9a
10a
11
12
13
14
9
88
85
81
10
11
12
13
93
86
14b
75
a
GC yield. b 2 mmol alcohol.
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Table 3 (Contd.)
resin (Amberlyst-15) and Naon-SiO2 were purchased from
Aldrich.
Typical procedures of catalytic test
The mixture n-valeramide (1.0 mmol) and benzylalcohol (1.0
mmol) in mesitylene (1.5 g) was injected to the catalyst inside
the reactor (cylindrical glass tube), followed by lling N2. Then,
the resulting mixture was magnetically stirred for 22 h under
reux condition; the bath temperature was 180 ꢁC and reaction
temperature was ca. 165 ꢁC. Aer cooling the mixture, followed
by removal of the catalyst, the mixture was isolated by column
chromatography using silica gel 60 (spherical, 63-210 mm, Kanto
Chemical Co. Ltd.) and the eluting solvent of hexane–ethyl-
Isolated
yield (%)
Entry
15
Alcohols
Products
80
85
83
75
81
1
acetate (19 : 1) and analyzed by H NMR, 13C NMR and GCMS.
16
17
18
19
For the standard reaction of n-valeramide and benzyl alcohol for
catalyst screening, catalyst recycle, conversion and yields of
products were determined by GC using n-dodecane as an
internal standard adopting the GC sensitivity estimated using
the isolated product.
Acknowledgements
This work was supported by a MEXT program “Elements
Strategy Initiative to Form Core Research Center” (since 2012),
Japan.
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This journal is © The Royal Society of Chemistry 2014
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