Friedel-Crafts reactions catalyzed by samarium diiodide

Article abstract of DOI:10.1016/j.tetlet.2006.05.169

Samarium diiodide is an efficient precatalyst for the Friedel-Crafts reaction involving various aromatic substrates and chelating electrophiles. Alkyl 3,3,3-trifluoropyruvates are transformed into α-hydroxyesters in good yields with total regioselectivity

Full text of DOI:10.1016/j.tetlet.2006.05.169

Tetrahedron Letters 47 (2006) 5467–5470
Friedel–Crafts reactions catalyzed by samarium diiodide
Mohamad Soueidan, Jacqueline Collin* and Richard Gil*
ˆ
´
Laboratoire Catalyse Mole´culaire, UMR 8182, ICMMO, batiment 420, Universite Paris-Sud, 91405 Orsay Cedex, France
Received 7 April 2006; revised 23 May 2006; accepted 30 May 2006
Abstract—Samarium diiodide is an efficient precatalyst for the Friedel–Crafts reaction involving various aromatic substrates and
chelating electrophiles. Alkyl 3,3,3-trifluoropyruvates are transformed into a-hydroxyesters in good yields with total regioselectivity.
In reactions involving an ethyl glyoxylate or a glyoxylic imine, a-hydroxyesters or a-aminoesters are obtained with variable amounts
of products resulting from a double Friedel–Crafts reaction.
Ó 2006 Elsevier Ltd. All rights reserved.
Friedel–Crafts reaction1 is one of the oldest carbon–
O
OH
CO2R
Ar
x mol% SmI2(THF)2
CH2Cl2, r.t.
ArH
+
carbon bond forming processes, and is still an attractive
method to introduce substituents on aromatic rings. Ini-
tial works concerned Friedel–Crafts acylation from acyl
chlorides or alkylation from alkyl halides. To perform
acylations, Lewis acids are needed. More than stoichio-
metric amounts of AlCl3 or BF3 are required whereas
catalytic amounts of rare-earth triflates,2 more specially
scandium triflate,3 perfluorinated rare earth metals,4 gal-
lium triflate5 or bismuth triflate,6 allow the formation of
the expected products. However, it is sometimes neces-
sary to add LiClO4 and to carry out reactions in nitro-
methane to improve the activity of Sc(OTf)3 or
Yb(OTf)3.7 Lanthanide chlorides (30 mol %) catalyze
the alkylation of benzene with benzyl halides.8 Cata-
lyzed Friedel–Crafts reactions, involving electron-rich
aromatic compounds and electrophiles such as alde-
hydes, ketones, imines, or benzyl alcohols9 have been re-
cently described. Reactions with benzyl alcohol can be
improved using rare-earth triflates supported in meso-
porous silica.10 Electrophiles such as trifluoropyruvates
and activated alkenes were tested in asymmetric Fri-
edel–Crafts reactions using chiral copper11 or scandium
catalysts.12
F3C
CO2R
F3C
Scheme 1.
opening of epoxides by amines15 or aza-Michael reac-
tions.16 We now wish to report our first results concern-
ing the use of samarium diiodide as a precatalyst for
Friedel–Crafts reactions.
The catalytic activity of samarium diiodide was first
investigated in the reaction of ethyl and methyl pyru-
vates with indole 1, which is usually employed as a
strong nucleophile in alkylation reactions leading to
interesting molecules17 (Scheme 1). A solution of indole
in dichloromethane was added to a suspension of
10 mol % SmI2(THF)2 in CH2Cl2, followed by addition
of alkyl trifluoropyruvate. Then, the reaction mixture
changed from a blue suspension to a yellow solution,
indicating the trivalent state of samarium.18 The ex-
pected a-hydroxy-esters were obtained in good yields
after 18 h reaction time at room temperature (Table 1,
entries 1 and 3). Reactions of electron-rich aromatic
compounds such as N,N-dimethylaniline 2, N,N-di-
methyl-m-anisidine 3 or 1,3-dimethoxybenzene 4 with
ethyl and methyl trifluoropyruvates have been also
examined.
We have previously shown that SmI2(THF)2 in methyl-
ene chloride is an efficient Lewis acid precatalyst for
different reactions such as tandem Michael–aldol reac-
tions,13 tandem Michael–iminoaldol reactions,14 ring
In all reactions, a complete conversion was observed and
only one product was isolated. No trace of other isomers
was detected, indicating a total regioselectivity. Sub-
strates 1, 2 and 3 react faster than 1,3-dimethoxybenzene
4 and no reaction was observed with 1,4-dimethoxybenz-
ene in the same conditions. The strongly electron
Keywords: Friedel–Crafts reaction; Samarium diiodide; Catalysis;
Carbon–carbon bond formation.
*
Corresponding authors. Tel.: +33 1 6915 4740; fax: +33 1 6915 4680
(R.G.); e-mail: richardgil@icmo.u-psud.fr
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.05.169

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M. Soueidan et al. / Tetrahedron Letters 47 (2006) 5467–5470
Table 1. Alkylation of trifluoropyruvates catalyzed by samarium diiodide
Entry
ArH
R
x (mol %)
Time (h)
Product
Yielda,b (%)
1
2
3
Et
Et
Me
10
1
10
18
0.5
18
5a
5a
5b
81
79
91
OH
F3C
CO2R
N
H
1
N
H
4
5
Et
Me
10
5
5
1
6a
6b
90
99
OH
CF3
CO2R
Me2N
Me2N
2
3
Me2N
Me2N
HO
CF3
6
Et
10
18
7a
70
CO2R
OMe
OMe
HO
CF3
CO2R
MeO
63c
75
4
MeO
7
8
Et
Me
10
10
63
114
8a
8b
OMe
OMe
a See Ref. 14a for a typical procedure.
b Isolated yield. 100% conversion. All products were fully characterized by spectroscopic data and compared to literature.
c 75% conversion.
withdrawing trifluoromethyl group increases the electro-
philic property of the carbonyl. No reaction was indeed
observed between ethyl pyruvate and N,N-dimethylani-
line in the presence of 10 mol % samarium diiodide.
For the optimization of the experimental conditions,
the reaction between N,N-dimethylaniline 2 and methyl
3,3,3-trifluoropyruvate was performed using 5 mol %
SmI2(THF)2. Product 6b was obtained in high yield
and a complete conversion was observed in 1 h (entry
5). For this reaction, samarium diiodide appears as a
more active catalyst than copper, scandium or indium
triflates.19
by ytterbium and scandium triflates,3 and the hydrolysis
of the ester group was observed. Surprisingly, in the
reactions catalyzed by samarium diiodide, the presence
of a small amount of water does not inhibit the activity
of the catalyst since the conversion is complete. More-
over, the hydrolysis of the ester function was never
observed. We unsuccessfully tested a slow addition of
aromatic compounds to reduce the formation of the
double Friedel–Crafts product.
The reaction of aromatic compounds with glyoxylic
imine 12 has been studied in the same conditions as de-
scribed above, and afforded the Friedel–Crafts and the
double Friedel–Crafts products (Scheme 3 and Table 3).
Furthermore, the use of only 1 mol % SmI2(THF)2 for
the reaction of indole 1 with ethyl 3,3,3-trifluoropyru-
vate allowed a complete conversion into product 5a
within 30 min (entry 2), whereas 10 mol % Ga(OTf)3
had been needed.20 Under the same conditions, N,N-
dimethylaniline 2 led to 53% conversion, confirming
the stronger nucleophilicity of indole.
With indole 1, the reaction gave a mixture of the ex-
pected aminoester 13a and of 9b arising from a double
Friedel–Crafts reaction. These two products could not
be separated by chromatography and a 51:49 ratio was
1
evaluated by H NMR analysis of the crude product.
Elimination of amine had already been reported in reac-
tion of indole with N-benzylidene aniline catalyzed by
lanthanide triflates.21 By the use of 1 mol % samarium
diiodide, a complete conversion was observed after
30 min with a 68:32 ratio for compounds 13a and 9b,
respectively (entry 2).22 Similarly, for the reaction
involving N,N-dimethyl-m-anisidine 3 catalyzed by
10 mol % samarium diiodide, the ratio of products 15a
and 11b depends on the reaction time. Interestingly, Fri-
edel–Crafts reaction of imine 12 with N,N-dimethylani-
line 2 was highly selective since only aminoester 14a was
isolated in 75% yield (entry 3).
Friedel–Crafts alkylations with other chelating sub-
strates, such as ethyl glyoxylate and glyoxylic imines,
which can lead to interesting a-hydroxy- and a-amino-
esters, were also investigated. We first studied the reac-
tion of ethyl glyoxylate with aromatic compounds 1, 2
and 3 in the presence of 10 mol % samarium diiodide
(Scheme 2 and Table 2). Two products are formed, the
desired a-hydroxy-esters 9a–11a and products 9b–11b
identified as arising from a double Friedel–Crafts reac-
tion with dehydration. This double Friedel–Crafts reac-
tion had already been reported for reactions catalyzed
O
OH
CO2Et
9a-11a
Ar
CO2Et
9b-11b
Ar
H
Ar
H
10 mol% SmI2(THF)2
CH2Cl2, 18 h, r.t.
+
ArH
+
H
CO2Et
1-3
Scheme 2.

M. Soueidan et al. / Tetrahedron Letters 47 (2006) 5467–5470
Table 2. Alkylation of ethyl glyoxylate catalyzed by samarium diiodide
5469
Entry
ArH
Product a
Product b
Yield aa,b (%)
Yield ba,b (%)
a/bc
H
N
HO
CO2Et
1
1
38
22
75/25
CO2Et
CO2Et
9a
N
H
9b
N
H
OH
CO2Et
10b
10a
Me2N
Me2N
2
3
2
3
14
32
50
16
22/78
81/19
Me2N
Me2N
NMe2
CO2Et
OH
CO2Et
OMe
OMe
11b
11a
OMe
NMe2
a Isolated yield. Compounds a and b have been separated by chromatography.
b All products were fully characterized by spectroscopic data and compared to literature except 11b which is a new compound.
c Ratio determined by 1H NMR spectroscopy on the crude product.
MeO
MeO
N
Ar
CO2Et
Ar
H
10 mol% SmI2(THF)2
CH2Cl2, r.t.
ArH
+
NH
+
H
CO2Et
Ar
CO2Et
1-3
12
13a-15a
9b-11b
Scheme 3.
Table 3. Alkylation of ethyl glyoxylic imine 12 catalyzed by samarium diiodide
Entry
ArH
Product a
Product b
Time (h)
a/ba
Yield (%)
H
N
H
N
EtO2C
OMe
1
2
1
1
18
51/49
68/32
46b
44b
CO2Et
0.5d
N
H
13a
N
H
9b
HN
CO2Et
OMe
Me2N
Me2N
3
2
—
18
100/0
75c
14a
CO2Et
OMe
OMe
HN
CO2Et
OMe
4
5
3
3
0.5
18
31/69
0/100
69b
35c
OMe
Me2N
NMe2
15a
11b
a Ratio determined by 1H NMR spectroscopy on the crude product.
b Yield given for the mixture of products.
c Isolated yield.
d 1 mol % SmI2(THF)2 was used.
In conclusion, samarium diiodide is an efficient precata-
lyst for a variety of Friedel–Crafts reactions and
compares well to other catalysts in terms of activity. Sev-
eral selective preparations of aromatic a-hydroxyesters

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M. Soueidan et al. / Tetrahedron Letters 47 (2006) 5467–5470
10. Mantri, K.; Komura, K.; Kubota, Y.; Sugi, Y. J. Mol.
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Acknowledgements
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14. (a) Jaber, N.; Assie, M.; Fiaud, J.-C.; Collin, J. Tetra-
´
We thank CNRS for financial support and Universite
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3090.
Paris-Sud for a master grant for M.S.
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