Catalytic C–O bond cleavage of ethers using group 5 or 6 metal
halide/acid chloride systems
Qiaoxia Guo, Taichi Miyaji, Guohua Gao, Ryuichiro Hara and Tamotsu Takahashi*
Catalysis Research Center and Graduate School of Pharmaceutical Science, Hokkaido University and
CREST, Science and Technology Corporation (JST), Kita-Ku, Sapporo 060-0811, Japan.
E-mail: tamotsu@cat.hokudai.ac.jp
Received (in Cambridge, UK) 24th January 2001, Accepted 19th April 2001
First published as an Advance Article on the web 15th May 2001
Ethers reacted with acid chlorides in the presence of a
catalytic amount of MCl5/6 (M = Mo, W, Nb or Ta) to give
esters in 75–98% yield; a stoichiometric reaction of dioctyl
ether with MoCl5 afforded 1-chlorooctane in 93% yield and
addition of benzoyl chloride to the resulting mixture gave
octyl benzoate in 49% yield.
The C–O bond of dibutyl ether was cleaved in the presence of
10 mol% of MoCl5 and n-caproyl chlorides at 80 °C for 24 h and
the caproic acid n-butyl ester was obtained in 96% yield (Run
2). The lower reactivity of aliphatic acid chlorides than that of
aromatic acid chlorides has been reported in the case of
graphite.13 In the reaction reported here, there is no significant
reactivity difference between the aromatic acid chloride and the
aliphatic acid chloride. Dioctyl ether also reacted with benzoyl
chloride and n-caproyl chloride to give octyl benzoate and n-
caproyl acid n-octyl ester, respectively, in high yield (Runs 3
and 4). It is interesting to note that some unsymmetric ethers
showed selective cleavage of the C–O bond. For example,
methyl tert-butyl ether reacted with benzoyl chloride to give
methyl benzoate in 99% yield (Run 5) i.e. the But–O bond in
methyl tert-butyl ether was selectively cleaved. Allyl propyl
ether was also selectively cleaved under the same conditions.
The allyl–O bond was cleaved to give caproyl acid propyl ester
(Run 6). In contrast, for allyl trimethylsilyl ether, selective
cleavage of the Si–O bond was observed (Run 7) and the allyl–
O bond was not cleaved. Other group 5 and 6 metal chlorides
were also quite active for acylative cleavage of ethers. Dibutyl
ether reacted with benzoyl chloride in the presence of 10 mol%
of WCl6 for 24 h to give butyl benzoate in 96% yield (Run 8).
When NbCl5 and TaCl5 were used as catalysts, catalytic C–O
bond cleavage was also successful and butyl benzoate was
obtained in 98 and 96% yield, respectively (Runs 9 and 10).
In order to investigate the reaction mechanism, a stoichio-
C–O bond cleavage is a versatile reaction in organic synthesis;
in particular, the catalytic cleavage of the C–O bond of ethers is
very attractive.1 There are several examples of catalytic C–O
bond cleavage of ethers in the presence of acid chlorides and
Lewis acids such as ZnCl2,2 FeCl3,3 SnBr2,4 CoCl2,5 AlCl3,6
YCl3,7 LnCl3,8 Al9 and Zn.10 As for group 6 metal compounds,
11
low-valent molybdenum compounds such as Mo(CO)6 and
ArMo(CO)312 have been used as catalysts for acylative cleavage
of ethers. It is generally believed that these catalytic reactions
proceed via a cationic mechanism in which the acid chloride
reacts with the catalyst first to generate a RCO+ ion. Addition of
+
an ether gives an oxonium salt (RCO2 RA2) followed by
cleavage of the C–O bond to give an ester. Here, we report a
novel catalytic system for acylative cleavage of ethers [eqn. (1)]
using group 5 or 6 metal halides and propose a novel catalytic
reaction mechanism.
(1)
A representative procedure for the catalytic C–O bond
cleavage is as follows: All reactions were carried out under
nitrogen and metal chlorides were handled under nitrogen.
Solvents were dried and distilled. To a mixture of molybden-
metric reaction of dioctyl ether with molybdenum( ) chloride
V
was carried out at 80 °C. After 3 h, 1-chlorooctane was obtained
in 93% yield [eqn. (2)]. To the resulting mixture, benzoyl
chloride was added and octyl benzoate was obtained in 49%
yield [eqn. (3)].
um( )chloride (0.1 mmol, 27 mg) and dichloroethane (DCE) (5
V
ml) was added dibutyl ether (1 mmol, 130 mg) and benzoyl
chloride (1 mmol, 141 mg). The reaction mixture was stirred at
80 °C for 3 h. GC analysis of the resulting mixture after
hydrolysis showed the formation of butyl benzoate in 95%
yield. Purification by column chromatography on silica gel
afforded butyl benzoate in 75% isolated yield. The results are
summarized in Table 1.
(2)
(3)
a
Table 1 Acylative cleavage of ethers with acid chlorides in the presence of MCl5/6
Run
Ether
RCOCl
MCl5/6
Time/h
Product
Yieldb (%)
1
2
3
4
5
Bu2O
Bu2O
C8H17OC8H17
C8H17OC8H17
CH3OC(CH3)3
PhCOCl
C5H11COCl
PhCOCl
C5H11COCl
PhCOCl
MoCl5
MoCl5
MoCl5
MoCl5
MoCl5
3
24
1
3
1c
PhCO2Bu
95(75)
96(78)
88(78)
94(82)
99(76)
C5H11CO2Bu
PhCO2C8H17
C5H11CO2C8H17
PhCO2CH3
6
C5H11COCl
MoCl5
1
C5H11CO2C3H7
98(81)
7
C5H11COCl
MoCl5
3
C5H11CO2CH2CHNCH2
75(58)
8
9
10
Bu2O
Bu2O
Bu2O
PhCOCl
PhCOCl
PhCOCl
WCl6
NbCl5
TaCl5
3
24
6
PhCO2Bu
PhCO2Bu
PhCO2Bu
96
98
96
a Conditions: MCl5/6, 0.1 mmol; ether, 1 mmol; acid chloride, 1 mmol, 80 °C. b GC yields: isolated yields in parentheses. c 50 °C.
1018
Chem. Commun., 2001, 1018–1019
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b100852h