COMMUNICATION
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Iron-catalysed green synthesis of carboxylic esters by the
intermolecular addition of carboxylic acids to alkenes{{
Jun-Chul Choi,* Kazufumi Kohno, Daisuke Masuda, Hiroyuki Yasuda and Toshiyasu Sakakura*
Received (in Cambridge, UK) 11th September 2007, Accepted 22nd November 2007
First published as an Advance Article on the web 18th December 2007
DOI: 10.1039/b713951a
Iron triflate, in situ-formed from FeCl
3
and triflic acid, or FeCl
3
Table 1 The effect of catalyst on ester formation via the addition of
benzoic acid to norbornene
a
and silver triflate efficiently catalyse the intermolecular addition
of carboxylic acids to various alkenes to yield carboxylic esters;
the reaction is applicable to the synthesis of unstable esters,
such as acrylates.
b
Entry
Catalyst
FeCl3
Yield (%)
Transition metal catalysts are powerful tools in organic synthesis.
2
For example, most C–C bond formations via cross-coupling,
1
2
3
4
5
6
7
8
9
a
5
3
,4
FeCl
Fe(OTf)
FeCl + 3 TfOH
3
+ 3 AgOTf
99
99
99
2
asymmetric hydrogenation, asymmetric epoxidation and hydro-
5
6
3
xylation, stereo-controlled polyolefin synthesis, and CLC bond
metathesis are all conducted in the presence of transition metal
3
7
AgOTf
TfOH
c
catalysts. However, disadvantages of transition metal catalysts
include high toxicity, high cost and uneven distribution of the
transition metal. In order to circumvent these problems, the
3
CoCl
NiCl
2
+ 2 AgOTf
+ 2 AgOTf
24
12
20
2
d
Sc(OTf)
3
8
development of iron-based catalysts has been strongly desired.
Reaction conditions: benzoic acid (10 mmol), norbornene
(10 mmol), catalyst (0.2 mmol), Bu O (20 ml), 80 uC, 18 h. GC
b
On the other hand, ester formation is a key reaction in organic
9,10
synthesis.
2
c
yield. TfOH (0.6 mmol). Many unknown products.
d
The addition of carboxylic acids to alkenes is an
especially attractive green procedure because the reaction is
coproduct-free and energy saving (eqn. (1)), unlike common
esterifications, which generally need the coproducts, typically water
or HCl, to be removed (eqn. (2)). The conventional procedure of
eqn. (1) uses a relatively large amount of sulfuric acid as a catalyst
FeCl
3
has been reported as an active catalyst for the intramolecular
19
cyclization of amino-alkenes. However, FeCl
3
poorly catalysed
the intermolecular addition of carboxylic acids to alkenes (Table 1,
entry 1). Interestingly, we found that the addition of a catalytic
amount of silver triflate dramatically accelerated the reaction
11
and requires its neutralization before purification of the ester.
12
Meanwhile, high catalytic activities of coinage metals (copper,
13,14
(Table 1, entry 2). The active species is presumably in situ-formed
1
2
silver and gold
attracted great attention. Complexes of precious metals like
) for eqn. (1), especially gold, have recently
Fe(OTf) . A similar enhancement has been reported for the
3
15
ruthenium-catalysed reaction using AgOTf. Separately, Fe(OTf)
was synthesized from FeCl and TfOH according to a literature
procedure. The isolated Fe(OTf)3 exhibited nearly the same
catalytic activity as the Fe(OTf) in situ-formed from FeCl and
3
15,16
ruthenium are also very effective as catalysts for eqn. (1).
An
3
intramolecular version of eqn. (1) has also been catalysed by
20
17
18
precious metal complexes such as platinum and silver. Herein,
we report the efficient catalysis of eqn. (1) by using iron-based
catalysts.
3
3
AgOTf (Table 1, entries 2 and 3). The catalyst formed in situ from
FeCl3 and TfOH also gave nearly the same high ester yields
(
Table 1, entry 4). On the other hand, using solely AgOTf or TfOH
ð1Þ
12
resulted in a poor catalyst (Table 1, entries 5 and 6). Other metal
triflates were much less active than iron triflate (Table 1, entries 7
12
to 9), as reported previously.
ð2Þ
The present reaction is applicable to a wide range of carboxylic
acids and alkenes, as summarized in Table 2 and Table 3. The
compatibility of the Fe(OTf)
functional groups are tolerant to the presence of catalytic
Fe(OTf) . The Fe(OTf) -catalysed reaction of reactive norbornene
3
system is shown in Table 2. Various
First, the effect of catalyst structure was investigated on the
reaction of benzoic acid and norbornene (Table 1). Recently,
3
3
with acetic acid proceeded smoothly in slightly polar solvents like
National Institute of Advanced Industrial Science and Technology
(AIST), Tsukuba, Ibaraki 305-8565, Japan.
Bu O and chloroform (Table 3, entry 1). This is remarkable
2
E-mail: junchul.choi@aist.go.jp; t-sakakura@aist.go.jp;
Fax: +81 29-861-4719; Tel: +81 29-861-4719
because the ruthenium catalyst failed to catalyse the reaction of
15
norbornene with aliphatic carboxylic acids. For less-strained
alkenes, higher yields are generally obtained under solventless
conditions. For example, cyclohexyl acetate was obtained from
neat acetic acid and neat cyclohexene in moderate to good yields
{
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A relevant patent has already been applied for; see ref. 1.
Electronic supplementary information (ESI) available: Representative
experimental procedure and characterization of the reaction products. See
DOI: 10.1039/b713951a
This journal is ß The Royal Society of Chemistry 2008
Chem. Commun., 2008, 777–779 | 777
Choi, Jun-Chul
