Rhodium-catalyzed Beckmann rearrangement

Article abstract of DOI:10.1021/ol006951z

equation presented Beckmann rearrangement of oxime is catalyzed by [RhCl(cod)]₂, trifluoromethanesulfonic acid, and tris(p-tolyl)phosphine in refluxing dichloroethane, giving the corresponding amide in good yield. Product/acid ratios of 10:20 can be attained in the reaction of benzophenone oximes.

Full text of DOI:10.1021/ol006951z

ORGANIC
LETTERS
2001
Vol. 3, No. 2
311-312
Rhodium-Catalyzed Beckmann
Rearrangement
Mieko Arisawa and Masahiko Yamaguchi*
Department of Organic Chemistry, Graduate School of Pharmaceutical Sciences
Tohoku UniVersity, Aoba, Sendai 980-8578, Japan
yama@mail.pharm.tohoku.ac.jp
Received December 4, 2000
ABSTRACT
Beckmann rearrangement of oxime is catalyzed by [RhCl(cod)]2, trifluoromethanesulfonic acid, and tris(p-tolyl)phosphine in refluxing
dichloroethane, giving the corresponding amide in good yield. Product/acid ratios of 10:20 can be attained in the reaction of benzophenone
oximes.
The rearrangement of a ketoxime to the corresponding amide
is a powerful method in organic synthesis1 and is known as
the Beckmann rearrangement. This reaction, however, gener-
ally requires a large amount of a strong Brønsted acid such
as sulfuric acid and forms ammonium sulfate as a byproduct.
Development of this important process promoted by a
catalytic amount of active species has been strongly desired
for a long time. In the vapor-phase process, a few examples
of the Beckmann rearrangement catalyzed by activators such
as boria-hydroxyapatite are reported.2 The Beckmann rear-
rangement in the supercritical water is also reported.3 As
for the liquid-phase process, the catalytic methods have been
developed by using O-alkyl-N,N-dimethylformamidium salt4
or tetrabutylammonium perrhenate.5 Antimony(V) salt was
reported to catalyze the reaction of silylated oximes.6 The
efficiency of the liquid-phase process, however, is not very
high, and turn over number (TON) based on the acid is
generally less than 5. Formation of the parent ketone often
is a serious problem. As an extension of our recent
investigations on the use of transition metal complexes and
sulfuric acid derivatives in organic synthesis,7 we examined
the Beckmann rearrangement. A small amount of rhodium
complex was found to promote the reaction of ketoxime and
trifluoromethanesulfonic acid.
When propiophenone oxime is treated with [RhCl(cod)]2
(cod ) 1,5-cyclooctadiene) (2.5 mol %), (p-tol)3P (15 mol
%), and trifluoromethanesulfonic acid (25 mol %) in reflux-
ing dichloroethane for 3 h, N-propionylaniline 2 is obtained
in 78% yield (Table 1, entry 1), which is accompanied by a
small amount of ethyl migrated N-ethylbenzamide 3 (2%).
Ketone is recovered in 7% yield. The rhodium complex and
the phosphine are essential for the rearrangement, and no
reaction occurs in the absence of either of the reagents
(entries 2 and 3). This reaction is effectively promoted by
trifluoromethanesulfonic acid, while the yield of product
decreases when methanesulfonic acid, p-toluenesulfonic acid,
or fluorosulfonic acid is used (entries 4-6). The reaction is
relatively insensitive to the substituent on the triarylphosphine
(entries 7-10). Alkyl phosphines (entries 11-13) and a
bidentate phosphine (entry 14) are not very effective except
for dppf (entry 15). The trifluoromethanesulfonic acid/
rhodium ratio of more than 5 is critical. Otherwise, the yield
of amide decreases, and a considerable amount of ketone is
formed. The catalytic activities of several metal complexes
are compared employing benzophenone oxime for the
substrate: while RhCl(PPh3)3 and [RhCl(cod)]2 + PPh3 are
(1) Gawly, R. E. Org. React. 1988, 35, 1 and references therein.
(2) Izumi, Y.; Sato, S.; Urabe, K. Chem. Lett. 1983, 1649.
(3) Ikushima, Y.; Hatakeda, K.; Sato, O.; Yokoyama,T.; Arai, M. J. Am.
Chem. Soc. 2000, 122, 1908. Sato, O.; Ikushima,Y.; Yokoyama, T. J. Org.
Chem. 1998, 63, 9100.
(4) Izumi, Y. Chem. Lett. 1990, 2171.
(5) Kusama, H.; Yamashita, Y.; Narasaka, K. Bull. Chem. Soc. Jpn. 1995,
68, 373. Narasaka, K.; Kusama, H.; Yamashita, Y.; Sato, H. Chem. Lett.
1993, 489.
(6) Mukaiyama, T.; Harada, T. Chem. Lett. 1991, 1653. Harada, T.; Ohno,
T.; Kobayashi, S.; Mukaiyama, T. Synthesis 1991, 1216.
(7) Arisawa, M.; Yamaguchi, M. J. Am. Chem. Soc. 2000, 122, 2387.
Arisawa, M.; Yamaguchi, M. AdV. Synth. Cat. 2001, in press.
10.1021/ol006951z CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/04/2001

Table 1. Effect of Sulfonic Acid and Phosphine on the
Table 2. Rh-Catalyzed Beckmann Rearrangement of Oximesa
Rh-Catalyzed Beckmann Rearrangement
yield (%)
entry
R
R′3P
(p-tol)3P
(p-tol)3P
none
(p-tol)3P
(p-tol)3P
(p-tol)3P
(p-MeOC6H4)3P
(C6H5)3P
(p-ClC6H4)3P
(C6F5)3P
2
3
1
2a
3
4
5
6
7
8
9
10
11
12
13
14
15
CF3
CF3
CF3
CH3
p-tol
F
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
78
-
-
2
-
-
-
-
-
5
13
16
23
68
58
59
40
39
-
3
3
-
-
-
-
-
2
(C6H5)Me2P
(n-C4H9)3P
(cycloC6H13)3P
dppeb
24
13
52
dppfc
a In the absence of Rh catalysis. b 1,2-Diphenylphosphinoethane. c 1,1′-
Bis(diphenylphosphino)ferrocene.
active, RhH(PPh3)4, RhH(CO)(PPh3)3, RuCl(CO)(PPh3)3,
RuH(OAc)(PPh3)3, Pd2(dba)3, and Pd(PPh3)4 are inactive. Use
of dichloroethane, chlorobenzene, toluene, or hexane as the
solvent gives better result than use of dichloromethane or
THF.
The catalytic Beckmann rearrangements of several oximes
are summarized in Table 2.8 When derivatives of benzophe-
none oxime are used, product/acid ratios of 10 to 20 can be
attained, and the ratio of product/Rh approaches 100. The
stereochemistry as shown in the case of p-fluoropropiophe-
none oxime is unimportant. Facile isomerization of oxime
in the presence of trifluoromethanesulfonic acid is known.5
As in the conventional Beckmann rearrangement, electron-
donating groups on the aromatic ring facilitate the reaction,
and electron-withdrawing groups retard it. Migration of an
aryl group predominates over that of an alkyl group. The
yield of the product decreases with unbranched aliphatic
ketone oximes; for example, 6-undecanone oxime gives the
product in 18%, which is accompanied by the regeneration
of the ketone (18%) and the recovery of the oxime (27%).
Although the mechanism of the present reaction is not
clear, the first step probably is the oxidative addition of
oxime to a low valent rhodium complex at the nitrogen-
oxygen bond.9 Then, the alkyl migration and the reductive
elimination take place, resulting in the amide.
(8) The rearrangement reaction of benzophenone oxime is representative.
In a one-necked flask were placed [RhCl(cod)]2 (0.5 mol %, 12.3 mg),
tris(p-tolyl)phosphine (3 mol %, 45.7 mg), benzophenone oxime (5.0 mmol,
985 mg), and trifluoromethanesulfonic acid (5 mol %, 0.022 mL) in
dichloroethane (4 mL) under an argon atmosphere, and the solution was
heated at reflux for 3 h. Then, water was added, and the organic materials
were extracted twice with ether. The combined organic layers were washed
with brine, dried over magnesium sulfate, and concentrated. The residue
was purified by flash column chromatography to give benzanilide (916 mg,
93%).
(9) Tsutsui, H.; Narasaka, K. Chem. Lett. 1999, 45. Ferreira, C. M. P.;
Guedes de Silva, M. F. C.; Kukushkin, V. Y.; Frau´sto da Silva, J. J. R.;
Pombeiro, A. J. L. J. Chem. Soc., Dalton Trans. 1998, 325. Deeming, A.
J.; Owen, D. W.; Powell, N. I. J. Organomet. Chem. 1990, 398, 299.
Hosokawa, T.; Shimo, N.; Maeda, K.; Sonoda, A.; Murahashi, S. Tetra-
hedron Lett. 1976, 383.
Acknowledgment. M. A. expresses her thanks to JSPS
for a Grant-in-Aid for Encouragement of Young Scientists
(11771374).
OL006951Z
312
Org. Lett., Vol. 3, No. 2, 2001