Communications
oxidation state of rhodium species is possibly + 3.[16] The Rh(OH)x/
Al2O3 catalyst is ESR silent. All these results suggest that rhodium-
(III) hydroxide is highly dispersed on Al2O3. A detailed character-
ization of Rh(OH)x/Al2O3 is in progress.
suggests that both OH and H functionalities play an
important role in this transformation. It is probable that the
OH groups of aldoximes bind to the rhodium center to form
ꢀ ꢀ =
Rh O N CHR species (step 1 in Scheme 1), which then
A typical Rh(OH)x/Al2O3-catalyzed synthesis of amides is as
follows: n-Dodecanaldoxime (0.5 mmol), Rh(OH)x/Al2O3 (4 mol%
Rh), and water (2 mL) were placed in a Teflon vessel along with a
magnetic stir bar. The Teflon vessel was placed inside an autoclave
and heated at 1408C (bath temperature). After the reaction had
finished, the spent catalyst was separated by filtration, washed with
ethanol, and dried in vacuo prior to being reused. The products
(amides) were isolated by silica gel column chromatography with
ethanol as eluent. Amides were efficiently produced from aldoximes
and aldehydes in water, whereas nitriles were the major products in
common organic solvents such as toluene and o-xylene. The identity
of the products was confirmed by comparison of their GC retention
times, mass spectra, and 1H and 13C NMR spectra with those of
authentic samples.
eliminate H+ to give nitrile intermediates and the starting
Rh OH species (step 2 in Scheme 1).[12] The Rh(OH)x/Al2O3-
ꢀ
catalyzed hydration of nitriles likely proceeds by a mechanism
Received: March 23, 2007
Published online: May 30, 2007
Keywords: aldehydes · amides · heterogeneous catalysis ·
.
rhodium · supported catalysts
Scheme 1. A possible reaction mechanism for the conversion of
aldoximes into amides. The value of n is probably 3.
[1] a) C. E. Mabermann in Encyclopedia of Chemical Technology,
Vol. 1 (Eds.: J. I. Kroschwitz), Wiley, New York, 1991, pp. 251 –
266; b) D. Lipp in Encyclopedia of Chemical Technology, Vol. 1
(Eds.: J. I. Kroschwitz), Wiley, New York, 1991, pp. 266 – 287;
c) R. Opsahl in Encyclopedia of Chemical Technology, Vol. 2
(Eds.: J. I. Kroschwitz), Wiley, New York, 1991, pp. 346 – 356.
[2] For examples of classical Beckmann rearrangements, see: a) E.
Beckmann, Ber. Dtsch. Chem. Ges. 1886, 19, 988; b) L. Ruzicka,
M. Kobelt, O. Hafliger, V. Prelog, Helv. Chim. Acta 1949, 32, 544;
c) O. Meth-Cohn, B. Narine, Synthesis 1980, 133; d) R. E.
Gawley, Org. React. 1988, 35, 1. Efficient catalysts for the
Beckmann rearrangement under mild and neutral conditions
have been developed recently: e) L. DeLuca, G. Giacomelli, A.
Porcheddu, J. Org. Chem. 2002, 67, 6272; f) S. Chandrasekhar, K.
Gopalaiah, Tetrahedron Lett. 2003, 44, 755; g) Y. Furuya, K.
Ishihara, H. Yamamoto, J. Am. Chem. Soc. 2005, 127, 11240.
[3] For the rearrangement of aldoximes to amides with stoichio-
metric amounts of reagents, see: a) E. C. Horning, V. L. Strom-
berg, J. Am. Chem. Soc. 1952, 74, 5151; b) D. S. Hoffenberg,
C. R. Hauser, J. Org. Chem. 1955, 20, 1496; c) M. Oku, Y.
Fujikura, J. Etsuno (Jpn. Kokai Tokkyo Koho), JP 07138213,
1995; d) A. Loupy, S. Regnier, Tetrahedron Lett. 1999, 40, 6221.
[4] a) T. Setsuda (Jpn. Kokai Tokkyo Koho), JP 52128302, 1977;
b) S. Park, Y. Choi, H. Han, S. H. Yang, S. Chang, Chem.
Commun. 2003, 1936; c) S. Komiya, H. Shimazu, T. Tamashima
(Jpn. Kokai Tokkyo Koho), JP 2003342245, 2003; d) N. A.
Owston, A. J. Parker, J. M. J. Williams, Org. Lett. 2007, 9, 73.
[5] R. A. Sheldon, H. van Bekkum, FineChmeicals through
Heterogeneous Catalysis, Wiley, Weinheim, 2001.
[6] Organic reactions in water have recently attracted great interest
because water is much safer than standard organic solvents,
which are often inflammable, explosive, or carcinogenic, and can
reduce pollution problems. See: a) C. Li, T.-H. Chan, Organic
Reactions in Aqueous Media, Wiley, New York, 1997; b) P. T.
Anastas, J. C. Warner, Green Chemistry: Theory and Practice,
Oxford University Press, London, 1998; c) P. A. Grieco, Organic
Synthesis in Water, Blackie Academic and Professional, London,
1998; d) R. A. Sheldon, Green Chem. 2000, 2, G1; e) P. T.
Anastas, L. B. Bartlett, M. M. Kirchhoff, T. C. Williamson,Catal.
Today 2000, 55, 11; f) Thematic issue on “Organic Reactions in
Water”: Adv. Synth. Catal. 2002, 3–4, 219 – 451.
similar to that of a previously reported Ru(OH)x/Al2O3-
catalyzed hydration (steps 3 and 4 in Scheme 1).[13] The
reaction rate is independent of the concentration of D2O,
ꢀ
which indicates that O H bond dissociation is not the rate-
=
limiting step. The reaction of 6a (PhCH NOH) and a-
=
deuteriobenzaldoxime (PhCD NOH) gave a kinetic isotope
effect (kH/kD) of 3.1 ꢁ 0.4 under the conditions described in
ꢀ
Table 1, which suggests that C H bond cleavage (step 2 in
scheme 1) is the rate-limiting step.[14]
In summary, Rh(OH)x/Al2O3 is an efficient heterogeneous
catalyst for the one-pot synthesis of primary amides from
activated and unactivated aldoximes or aldehydes in water in
high yields.
Experimental Section
The Rh(OH)x/Al2O3 catalyst was prepared as follows. Powdered
Al2O3 (KHS-24, supplied by Sumitomo Chemical Co., Ltd.; BET
surface area: 160 m2 gꢀ1; 2.0 g) was calcined at 5508C for 3 h and was
then vigorously stirred with 60 mL of an aqueous solution of RhCl3
(6.5 mm) at room temperature. After 15 min, the pH value of the
solution was quickly adjusted to 13 with an aqueous solution of
NaOH (1.0m) and the resulting slurry was stirred for 24 h at room
temperature. The solid was then collected by filtration, washed with a
large volume of deionized water, and dried in vacuo to afford 2.0 g of
Rh(OH)x/Al2O3 as a yellow powder with a rhodium content of
2.1 wt%. The rhodium content can be varied by changing the
concentration of the starting rhodium solution. The IR spectrum of
Rh(OH)x/Al2O3 shows a broad n(OH) band in the range 3000–
3770 cmꢀ1. The XRD pattern of Rh(OH)x/Al2O3 was the same as that
of the parent Al2O3 support and no peaks for rhodium metal (clusters)
or rhodium oxides were observed.[15] The X-ray photoelectron
spectrum of Rh(OH)x/Al2O3 showed Rh3d5/2 and Rh3d3/2 binding
energies of 310.0 (full width at half maximum (FWHM): 1.4 eV) and
314.8 eV (FWHM: 1.4 eV), respectively, which indicate that the
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 5202 –5205