Silica supported MoO3: A mild heterogeneous catalyst for the Beckmann rearrangement and its application to some sugar derived ketoximes

Article abstract of DOI:10.1016/j.tetlet.2004.04.068

Silica supported molybdenum(VI) oxide (20%) has been explored as a new solid catalyst for the Beckmann rearrangement and the results are compared in parallel with the known β-zeolite as a catalyst for the same transformation. Both catalysts were found to facilitate the rearrangement under mild conditions and the conditions employed were tolerable for protecting groups such as isopropylidene, cyclohexylidene and PMB are commonly employed in carbohydrate chemistry.

Full text of DOI:10.1016/j.tetlet.2004.04.068

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 4759–4762
Silica supported MoO3: a mild heterogeneous catalyst for the
Beckmann rearrangement and its application to some
sugar derived ketoximesq
Mohan K. Dongare,* Vivekanand V. Bhagwat, C. V. Ramana and Mukund K. Gurjar
National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411008, India
Received 5 January 2004; revised 7 April 2004; accepted 13 April 2004
Abstract—Silica supported molybdenum(VI) oxide (20%) has been explored as a new solid catalyst for the Beckmann rearrangement
and the results are compared in parallel with the known b-zeolite as a catalyst for the same transformation. Both catalysts were
found to facilitate the rearrangement under mild conditions and the conditions employed were tolerable for protecting groups such
as isopropylidene, cyclohexylidene and PMB are commonly employed in carbohydrate chemistry.
ꢀ 2004 Published by Elsevier Ltd.
The Beckmann rearrangement is a well-known trans-
formation of keto-oximes to N-substituted amides in the
presence of an acid.1 Generally, strong Brønstead/Lewis
acids ranging from stoichiometric to catalytic amounts
are employed in order to accomplish the Beckmann
rearrangement. To circumvent the harsh reaction con-
ditions generally used for the Beckmann rearrangement
coupled with a desire to use clean and environmentally
benign reusable catalysts, various heterogeneous cata-
lysts such as zeolites, mesoporous materials and metal
oxides,2;3 various solid acid catalysts4 including B2O3
supported on Al2O3,5 TiO2 doped on AlPO4,6 SiO2/
Al2O3,7;8 tungsten oxide,8;9 FSM-16 and MCM-4110
have been used either in solution or in the vapor phase.
the highest activity and selectivity for esterification.12
One added advantage of the MoO3/SiO2 catalyst is the
availability of standard protocols for the preparation of
catalysts with the molybdenum percentage varying from
5–30%. In this communication we would like to present
our results on the utilization of the MoO3/SiO2 catalyst
for the Beckmann rearrangement. In addition, we eval-
`
uated its efficiency vis-a-vis b-zeolite 2, the latter having
already been employed as a catalyst for the Beckmann
rearrangement.
The silica supported MoO3 catalyst was prepared by the
molybdate
sol–gel
method
using
ammonium
[(NH4)6Mo7O24Æ4H2O] and ethyl silicate-40 as molyb-
denum and silica sources, respectively, followed by dry-
ing and calcination at 500 ꢁC. The catalyst was
characterized for chemical composition as well as struc-
tural characterization by atomic absorption spectros-
copy and X-ray diffraction analysis. Chemical analysis of
MoO3/SiO2 showed the composition of the catalyst as
19.5 mol % MoO3 and 80.5 mol % SiO2. The XRD pat-
tern of MoO3/SiO2 showed the presence of a MoO3
crystalline phase supported on amorphous silica. The
BET surface area and pore size distribution measure-
ments showed the mesoporous nature (pore diameter of
Molybdenum oxide without support or supported on
SiO2 or Al2O3 is a well-known solid acid catalyst useful
for acid catalyzed reactions such as the oxidation of
11
ammonia to elemental N2 and the synthesis of diphen-
yl carbonate from dimethyl carbonate and phenol.12
Among the supported oxides, V2O5/SiO2, ZrO2/SiO2,
PbO/SiO2 and MoO3/SiO2 1, MoO3/SiO2 has revealed
Keywords: Beckmann rearrangement; Solid acid; b-Zeolite; Molybde-
num oxide; Glucose diacetonide.
q Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.tetlet.2004.04.068
* Corresponding author. Tel./fax: +91-20-25893761; e-mail:
dongare@cata.ncl.res.in
2
ꢀ
79 A) and the surface area of the material as 145 m /g
indicating the high dispersion of MoO3 on the amor-
phous silica support. The TPD analyses of MoO3/SiO2
and b-zeolite are shown in Figure 1. MoO3/SiO2 showed
a higher number of weaker acid sites than b-zeolite. The
0040-4039/$ - see front matter ꢀ 2004 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2004.04.068

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M. K. Dongare et al. / Tetrahedron Letters 45 (2004) 4759–4762
obtained when beta zeolite was used instead of MoO3/
SiO2 as the catalyst.
Representative procedure for the rearrangement of benzo-
phenone oxime: A suspension of the catalyst (12.3 mg)
and benzophenone oxime (5.0 mmol) in ethanol (10 mL)
under argon was heated under reflux for 18 h. The
reaction mixture was diluted with water and repeatedly
extracted with ether. The combined organic layer was
washed with brine, dried (magnesium sulfate) and con-
centrated. The residue was purified by flash column
chromatography on silica gel to give benzanilide in
97% yield (with catalyst 1) and 94% yield (with catalyst
2).
Encouraged by the results obtained with aromatic as
well as aliphatic oximes, we next focused our attention
with the oximes derived from carbohydrates. To the best
of our knowledge, there is no report concerning the
Beckmann rearrangement of sugar oximes. However,
oximes derived from erythronolides14 and cyclohexitol15
derivatives have been shown to undergo the Beckmann
rearrangement yielding valuable drug candidates such as
azythromycin, and polyhydroxycyclic lactams with
varying ring sizes. Nonetheless, the use of catalytic
methods is scarce. We chose the oximes 3–6, which can
be easily derived from glucose (Table 2; 3 and 4) and
xylose (Table 2; 5 and 6).
Figure 1. NH3-TPD of (a) Mo/SiO2 and (b) b-zeolite.
total acidity of b-zeolite was found to be 0.781 mmol/g
while that of MoO3/SiO2 was 1.104 mmol/g.
After characterization of catalyst 1 in comparison with
2, we proceeded further to investigate its applicability
for Beckmann rearrangements. Attempted Beckmann
rearrangement of various oximes derived from simple
ketones (Table 1) with catalyst 1 gave excellent yields,
and the regioselectivities in the cases of unsymmetrical
ketoximes were high as expected.13 Similar results were
The rearrangement of sugar-oxime 3 was carried out in
the presence of catalyst 1 or 2 in refluxing ethanol. With
both catalysts, the reactions are clean and resulted in
inseparable regioisomeric mixtures. The amides 7 and 8
were obtained in very good yields and with moderate
regioselectivity (3:2). The structures of the major 7 and
the minor 8 isomers were determined with the help of 1H
Table 1. Beckmann rearrangement using solid acid catalysts 1 and 2
Entry Starting
material
Product
Yield (%)
b-Zeolite MoO3/
SiO2
1
NMR, COSY and NOESY spectra.16 In the H NMR
spectra, H(3) of the major isomer 7 resonated downfield,
ca. 0.25 ppm, with respect to the minor isomer 8.
Whereas, H(6) of the minor isomer 8 resonated
0.48 ppm downfield with respect to the major isomer 7.
This clearly established that in isomer 7, C(5) is a car-
bonyl carbon while C(4) is a carbonyl carbon in the
minor isomer 8. Additionally, the long-range coupling
HO
N
H
N
1
9295
O
HO
O
N
N
2
3
94
81
97
91
5
H
observed J3;6 ¼ 1:3 Hz (confirmed by selective irradia-
tion experiments) in both isomers clearly indicated a
trans-relationship between these protons thus excluding
epimerization/inversion at C(6) during the rearrange-
ment. This was further substantiated by the NOESY
spectrum, where there was no coupling between H(6)
and H(2)/H(3) for both isomers.
O
N
N
OH
H
O
O
OH
4
83
87
NH
N
Having established the configuration of the two isomers
resulting from oxime 3, we next focused on oximes 4–6.
As indicated in Table 2, with oximes 4 and 5, the rear-
rangement was smooth and resulted in regiomeric mix-
tures. In the case of the TBDPS oxime 6 (Table 2, entry
4), we encountered a complex reaction mixture presum-
ably because of the cleavage of TBDPS under these
conditions. The regioselectivity for oximes 3–5 was
found to be independent of the catalyst used. This indi-
cated that both catalysts can facilitate the rearrangement
but the regioselectivity is governed by stereoelectronic
O
HO
O
N
NH
5
6
79
83
89
96
Ph
N
Ph
N
H
Ph
Ph
H
N
O
OH
H
N

M. K. Dongare et al. / Tetrahedron Letters 45 (2004) 4759–4762
Table 2. Beckmann rearrangement of sugar oximes
4761
Entry
Starting material
Products
Yield (%)a
MoO3/SiO2
b-Zeolite
O
O
O
O
O
O
1
O
O
O
O
O
O
O
6
HN
1
2
85 (3:2)
82 (3:2)
87 (3:2)
HO
O
N
O
O
N
H
O
8
3
7
O
O
O
O
O
O
O
O
O
O
O
O
O
84 (3:2)
O
HN
O
N
H
9
O
HO
N
O
10
4
O
PMBO
HO
O
O
O
O
PMBO
PMBO
O
O
3
4
71 (3:1)
70 (3:1)
HN
O
N
H
O
N
O
12
O
11
5
TBDPSO
HO
O
Complex reaction mixture
O
N
O
6
a Ratio of isomers in parentheses.
2. O’Sullivan, P.; Forni, L.; Hodnett, B. K. Ind. Eng. Chem.
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To conclude, a simple and convenient solid acid (b-zeo-
lite and MoO3/SiO2) catalyzed Beckmann rearrangement
has been reported. For the first time the Beckmann
rearrangement on carbohydrate templates using solid
catalysts has been evaluated. Commonly employed pro-
tecting groups in carbohydrate chemistry such as iso-
propylidene and cyclohexylidene systems and benzyl
ethers were found to be stable under the conditions
employed. Work in the direction of exploring the
potential of the resulting [1,3]-oxazinone and morpholi-
none derivatives as chiral intermediates is in progress.
Supplementary material: 1H, 1H–1H COSY, NOESY
and ESI-MS of compounds 7/8.
Acknowledgement
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16. Major isomer 7: IR (CHCl3) 1740 cmÀ1 1H NMR
;
(300 MHz, CDCl3): d 1.35 (s, 3H), 1.41 (s, 3H), 1.43 (s,
3H), 1.51 (s, 3H), 3.96–4.10 (m, H-8, H-80), 4.30 (dt, H-7,
J ¼ 6:3, 4.9 Hz, irrdn. at 4.77 fi t, J ¼ 6:3 Hz), 4.76 (dd,
H-6, J ¼ 4:7, 1.2Hz, irrdn. at 5.28 fi d, J ¼ 4:7 Hz), 5.28
(dd, H-3, J ¼ 4:4, 1.2Hz, irrdn. at 4.76 fi d, J ¼ 4:3 Hz,
irrdn. at 5.94 fi d, J ¼ 1:2Hz), 5. 94 (d, H-2, J ¼ 4:4 Hz,
irrdn. at 5.28 fi br s), 8.87 (br s, H–N); 13C NMR
(75 MHz, CDCl3) 25.3, 26.3, 27.4, 65.4, 74.3, 76.9, 77.4,
104.8, 110.2, 113.8, 157.9 ppm. Minor isomer 8: IR
1
(CHCl3) 1740 cmÀ1; H NMR (300 MHz, CDCl3): d 1.33
(s, 3H), 1.39 (s, 3H), 1.44 (s, 6 H), 3.96–4.10 (m, H-8, H-80,
irrdn. at 5.24 fi change), 4.49 (dt, H-7, J ¼ 6:9, 2.3 Hz,
irrdn. at 5.24 fi t, J ¼ 6:7 Hz), 5.03 (dd, H-3, J ¼ 4:4,
1.3 Hz, irrdn. at 5.24 fi d, J ¼ 4:1 Hz, irrdn. at 6.01 fi d,
J ¼ 1:3 Hz), 5.24 (dd, H-6, J ¼ 2:4, 1.3 Hz, irrdn. at
5.03 fi d, J ¼ 2:0 Hz, irrdn. at 4.49 fi d, J ¼ 1:3 Hz, irrdn.
at 6.01 fi no change), 6.01 (d, H-2, J ¼ 4:4 Hz, irrdn. at
5.03 fi s), 8.80 (br s, H–N); 13C NMR (75 MHz, CDCl3)
26.1, 27.4, 27.6, 64.9, 77.4, 77.7, 79.1, 104.6, 109.8, 114.1,
158.5 ppm. MS-ESI: 274.2 (49%, [M+1]þ).
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Anal. Calcd for C12H19NO6:C, 52.74; H, 7.01; N, 5.13.
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