Highly efficient heterobimetallic iron-magnesium-HMPA-catalyzed Michael-type Friedel-Crafts reactions of indoles and chalcones

Article abstract of DOI:10.1080/00397910801929721

A simple and novel bimetallic catalyst, an iron and magnesium complex (Fe-Mg-hexamethylphosphoramide (HMPA)), with dual activation was found to be an effective and promising catalyst for the Michael-type Friedel-Crafts reactions of indoles with chalcones.

Full text of DOI:10.1080/00397910801929721

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Highly Efficient Heterobimetallic
Iron‐Magnesium‐HMPA–Catalyzed Michael‐type
Friedel–Crafts Reactions of Indoles and
Chalcones
Wei Zhou a b , Li Li b , Li‐Wen Xu a b , Hua‐Yu Qiu b , Guo‐Qiao Lai b
Chun‐Gu Xia a & Anne‐Sophie Castanet c
,
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou
Institute of Chemical Physics, Chinese Academy of Sciences, and Graduate
School of the Chinese Academy of Sciences, Lanzhou, China
b Key Laboratory of Organosilicon Chemistry and Material Technology of
Ministry of Education, Hangzhou Normal University, Hangzhou, China
c Department of Organic Molecular Macromolecular Chemistry, Faculty of
Science, University of Main and CNRS, Le Mans, France
Published online: 24 Apr 2008.
To cite this article: Wei Zhou , Li Li , Li‐Wen Xu , Hua‐Yu Qiu , Guo‐Qiao Lai , Chun‐Gu Xia & Anne‐Sophie
Castanet (2008): Highly Efficient Heterobimetallic Iron‐Magnesium‐HMPA–Catalyzed Michael‐type
Friedel–Crafts Reactions of Indoles and Chalcones, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 38:10, 1638-1645
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Synthetic Communicationsw, 38: 1638–1645, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910801929721
Highly Efficient Heterobimetallic
Iron-Magnesium-HMPA–Catalyzed
Michael-type Friedel–Crafts Reactions
of Indoles and Chalcones
Wei Zhou,1,2 Li Li,2 Li-Wen Xu,1,2 Hua-Yu Qiu,2
Guo-Qiao Lai,2 Chun-Gu Xia,1 and Anne-Sophie Castanet3
1State Key Laboratory for Oxo Synthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
and Graduate School of the Chinese Academy of Sciences,
Lanzhou, China
2Key Laboratory of Organosilicon Chemistry and Material Technology of
Ministry of Education, Hangzhou Normal University, Hangzhou, China
3Department of Organic Molecular Macromolecular Chemistry, Faculty
of Science, University of Main and CNRS, Le Mans, France
Abstract: A simple and novel bimetallic catalyst, an iron and magnesium complex
(Fe-Mg-hexamethylphosphoramide (HMPA)), with dual activation was found to be
an effective and promising catalyst for the Michael-type Friedel–Crafts reactions of
indoles with chalcones. This novel catalytic system has the advantages of highly
efficient, mild reaction conditions and exhibits high reactivity and selectivity, which
make it a useful and attractive process for the synthesis of indole derivatives.
Keywords: Bimetallic catalysis, dual activation, Friedel–Crafts reaction, indoles,
Michael addition
Homogeneous bimetallic or multimetallic catalysis has now been recognized
as a promising tool in organic synthesis, because it is expected that
Received in Japan November 15, 2007
Address correspondence to Li-Wen Xu, Key Laboratory of Organosilicon
Chemistry and Material Technology of Ministry of Education, Hangzhou Normal
University, Hangzhou 310012, China. E-mail: licpxulw@yahoo.com
1638

Fe-Mg-HMPA Catalyst
1639
cooperative or successive interaction of two (or more) different metal centers
with the substrate molecules gives rise to enhanced catalytic activities. Thus in
some cases, bimetallic catalysts are superior to monometallic catalysts and
promote new reactions that cannot be achieved by monometallic catalyst
systems.[1] Recently, bimetallic catalysts have been attracting significant
attention for organic transformations such as oxidation,[2] hydroxylation,[3]
dihydroxylation,[4] carbonylation,[5] cross-coupling reactions,[6] reduction,[7]
ring-opening,[8] multi-component coupling reactions,[9] and other
reactions.[10] However, to the best of our knowledge, there are no reports on
Michael-type Friedel–Crafts addition of indoles with electron-deficient
olefins catalyzed by bimetallic complexes.
The Michael-type Friedel–Crafts addition of indoles to electron-deficient
olefins and a,b-unsaturated enones is a very important process because it is
involved in the total synthesis of bioactive indole alkaloids such as the hapa-
lindoles and other 3-substituted indoles, which are important substructures and
building blocks for the synthesis of natural products and therapeutic agents.[11]
During past years, a variety of transition-metal salts, such as Zr(OTf)4,[12]
[14]
Bi(OTf)3,[13] CeCl3 7H2O-NaI,
InBr3,[15] and other transition-metal-
.
based Lewis acid catalysts[16] or organic molecular catalysts[17] have been
used as efficient catalysts in the Michael-type Friedel–Crafts addition of
indoles to electron-deficient olefins or a,b-unsaturated enones. However,
some drawbacks still exist in previous synthetic methods, for example, the
use of expensive stoichiometric amounts of Lewis acidic catalysts and toxic
heavy metals. Hence, there is still a strong need for cheaper, simpler, and
more efficient catalysts and procedures. In our continuing interest in develop-
ing new synthetic protocols for Michael-type Friedel–Crafts addition of
indoles to enones, we report here an efficient method using a homogeneous
heterobimetallic catalyst.
RESULTS AND DISCUSSION
The reaction of chalcone and indole was selected as a model (Scheme 1). We
screened a series of transition-metal salts as catalyst (10 mol%) such as
PdCl2(CH3CN)2, Mg(ClO4)2, MgCl2, FeCl3, ZnCl2, and Cu(OAc)2. Unfortu-
nately, no addition product was detected with these transition-metal salts at
room temperature, so we then combined these transition-metal salts as
Scheme 1.

1640
W. Zhou et al.
Table 1. Michael-type Friedel–Crafts reaction of indole with chalcone promoted by
bimetallic complex (Fe-Mg-HMPA) catalyst
Reaction Yield
Solvent time (h) (%)b
Entrya
Metal salt 1
Metal salt 2
Additive
1
2
Mg(ClO4)2
FeCl3
ZnCl2
—
Mg(ClO4)2
Noc or HMPA CH2Cl2
48
24
24
24
24
48
48
48
48
24
24
12
12
12
12
12
12
12
NRd
Trace
NR
Noc
CH2Cl2
3
Mg(ClO4)2 Noc or HMPA CH2Cl2
Cu(OTf)2 Noc or HMPA CH2Cl2
4
FeCl3
Trace
NR
NR
5
PdCl2(CH3CN)2 Cu(OTf)2
Cu(OAc)2
Mg(ClO4)2 Noc or HMPA CH2Cl2
PdCl2(CH3CN)2 Cu(OAc)2 Noc or HMPA CH2Cl2
Noc or Et3N CH3CN
6
7
NR
Trace
NR
8
InCl3
AgNO3
PdCl2(CH3CN)2
Cu(OAc)2
FeCl3
Mg(ClO4)2 Noc or HMPA CH2Cl2
Mg(ClO4)2 Noc or HMPA CH2Cl2
9
10
11
12
13
14
15
16
17
18
FeCl3
FeCl3
Noc or HMPA CH2Cl2
Noc or HMPA CH2Cl2
Trace
Trace
63
Mg(ClO4)2
Mg(ClO4)2
Mg(ClO4)2
Mg(ClO4)2
Mg(ClO4)2
Mg(ClO4)2
Mg(ClO4)2
HMPA
HMPA
HMPA
HMPA
HMPA
HMPA
Et3N or
pyridine
Noc
CH2Cl2
CH3CN
Toluene
Et2O
FeCl3
FeCl3
55
53
FeCl3
FeCl3
92
15
THF
CH3OH
CH3OH
FeCl3
FeCl3
96
Trace
19
20
21
22
23
24
FeCl3
FeCl3
FeCl3
Mg(ClO4)2
Mg(ClO4)2
Mg(ClO4)2
—
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
12
12
12
12
12
12
30
72e
58f
HMPA
HMPA
HMPA
HMPA
HMPA
Fe(ClO4)3
FeCl3
Mg(ClO4)2
48
45
—
—
Trace
aReactions conditions: 10 mol% of transition-metal salt 1, 10 mol% of transition-
metal salt 2, and 30 mol% HMPA: 0.55 mmol/0.5 mmol, of indole/chalcone; and
2 mL of solvent at room temperature for 12 h.
bIsolated yields.
cNo addition of HMPA.
dNo reaction.
e5 mol% of FeCl3 and 5 mol% of Mg(ClO4)2.
f1 mol% of FeCl3 and 1 mol% of Mg(ClO4)2.
bimetallic systems (some representative results are listed in Table 1, entries
1–11). Incidentally, we found that 63% of desired product 3a was obtained
when 10 mol% of Mg(ClO4)2 and 10 mol% of FeCl3 were used as cocatalysts
in the presence of 30 mol% HMPA in CH2Cl2 at room temperature for 12 h.
With this encouraging result, we carried out the model reaction in different
solvents. (Table 1, entries 12–18). Among the different solvents tested,
CH3OH and Et2O were the most effective ones, and tetrahydrofuran (THF)

Fe-Mg-HMPA Catalyst
1641
gave the worst result with yields of 96%, 92%, and 15%, respectively.
Moderate yields (50–60%) were obtained in lesser or nonpolar solvents
such as CH2Cl2 and toluene. Interestingly, we found that HMPA was a
crucial additive for this bimetallic catalytic system, other organic bases
(Et3N and pyridine) were inefficient (entry 18), and 30 mol% of HMPA
exhibited better effect in this reaction after many trials (entry 17). In
addition, even when the amounts of bimetallic catalysts [both Mg(ClO4)2
and FeCl3] were reduced to 1 mol%, there was moderate yield (entry 21),
which showed this bimetallic catalytic system has a dramatic effect in
promoting the Michael-type Friedel–Crafts addition of indoles to chalcone.
To assess whether the counteranions of Mg and Fe affect the course of this
Michael reaction, the next studies were carried out employing Fe(ClO4)3,
FeCl3, or Mg(ClO4)2 alone under the same conditions (in CH3OH). As
expected, lower yields were obtained (entries 22–24). These results showed
the remarkable synergistic effect between FeCl3 and Mg(ClO4)2.
Under the optimized mild reaction conditions [10 mol% of Mg(ClO4)2,
10 mol% of FeCl3, and 30 mol% HMPA, 1 equiv. chalcone, and 1.1 equiv.
indole in CH3OH at room temperature], we then investigated various
indoles and enones for Michael-type Friedel–Crafts reactions (Scheme 2).
As shown in Table 2, the yields of reaction are good except in the case of
5-Br substituted indole 2d with enones (entry 11). For reaction of indole 2a
with enones bearing H-group, Cl-group, or methoxyl, moderate to good
isolated yields could be obtained (entries 1 to 5). It is worth emphasizing
that reacting N-substituted indole 2b with different enones afforded the
addition products with good to excellent yields (up to almost quantitative,
entries 6 to 9). Nearly all reactions are clean, and the target compounds are
obtained in good yields with no formation of side products such as dimers
or trimers, which are normally observed when using strong acids.
Although the correct mechanism of how the bimetallic iron and
magnesium complex (Fe-Mg-HMPA) works is not yet clear, it seems that
the catalyst acts as an efficient Lewis acid or the dual activation occurred
with bimetallic iron and magnesium complex with both enone and indole.
Further study of the mechanism of heterobimetallic iron–magnesium–
catalyzed Michael-type Friedel–Crafts reactions is in progress.
In summary, we have demonstrated that simple bimetallic catalyst with
dual activation was an effective and promising catalyst for the Michael-type
Friedel–Crafts reactions of indoles with chalcones. This novel protocol,
Scheme 2.

1642
W. Zhou et al.
Table 2. Mg(ClO4)2-FeCl3-based bimetal complex (Fe-Mg-HMPA)–catalyzed
Michael-type Friedel–Crafts reactions of indoles with chalcones
Entrya
Enone
Indole
Product Yield (%)b
3a
3b
3c
3d
96
64
83
70
1
2
3
4
3e
3f
82
99
5
6c
7d
8c
9c
3g
3h
90
94
3i
3j
98
76
32
10
11
3k
aUnless noted, reactions conditions are 10 mol% of Mg(ClO4)2, 10 mol% of FeCl3,
and 30 mol% HMPA; 0.55 mmol/0.5 mmol of indole/chalcone; and 2 mL of CH3OH
at room temperature for 12 h.
bIsolated yields.
cReaction time ¼ 6 h.
dReaction time ¼ 8 h.

Fe-Mg-HMPA Catalyst
1643
based on cheap and available Lewis acidic transition-metal catalysts, has the
advantages of high efficiency, mild reaction conditions, high reactivity, and
high selectivity, which make it a useful and attractive process for the
synthesis of indole derivatives. Extension of this protocol to asymmetric
catalysis is also in progress in our laboratories.
EXPERIMENTAL
All reaction flasks and solvents were used directly. Flash-column chromato-
graphy was performed over silica (100–200 mesh). NMR spectra were
recorded on a 400-MHz spectrometer. 13C NMR spectra were obtained with
broadband proton decoupling. For spectra recorded in CDCl3, unless noted,
chemical shifts were recorded relative to the internal TMS (tetramethylsilane)
reference signal. Infrared spectrometer (IR) spectra were recorded using a
fourier transform infrared spectrometer (FT-IR) apparatus. Thin layer chrom-
atography (TLC) was performed using silica gel.
Typical Michael-type Friedel–Crafts Reaction Procedure
FeCl3 (0.1 mmol), Mg(ClO4)2 (0.1 mmol), and HMPA (0.3 mmol) were added
into a solution of enone (1.0 mmol) and indole (1.1 mmol) in freshly distilled
CH3OH (4 mL). After stirring at room temperature for 12 h, the mixture was
diluted with H2O (10 mL) and extracted with EtOAc (3 Â 15 mL). The
combined organic layers were dried (Na2SO4), concentrated in vacuo, and
purified by column chromatography on silica gel (EtOAc–petroleum ether,
1:5) to gain the pure product. All the products are known,[17b] and structures
were confirmed by mass spectrum (MS), nuclear magnetic resonance (NMR),
and IR.
ACKNOWLEDGMENTS
This study was supported by the Natural Science Foundation of China (NSFC
No. 20572114) for financial support of the work (NFSF No. 20572114) and
Hangzhou Normal University.
REFERENCES
1. (a) Shibasaki, M.; Yamamoto, Y. Eds. Multimetallic Catalysts in Organic
Synthesis; Wiley-VCH: Weinheim, 2004; (b) Tsuji, J. Ed. Transition Metal
Reagents and Catalysts; Wiley: New York, 2000.

1644
W. Zhou et al.
2. (a) Ru/Pd: Kuiper, J. L.; Shapely, P. A. Aerobic alcohol oxidation catalyzed by a
new, oxygen-bridged heterometallic compound [PPh4][Ru(N)Me2(m2-O)2Pd((-)-
sparteine)]. J. Organomet. Chem. 2007, 692, 1653; (b) RuCl3/CeCl3: Plietker, B.;
Niggemann, M. RuCl3/CeCl3/NaIO4: A new bimetallic oxidation system for the
mild and efficient dihydroxylation of unreactive olefins. J. Org. Chem. 2005, 70,
2402; (c) Pd/Co: Mares, F.; Diamong, S. E.; Regina, F. J.; Solar, J. P.
Formation of glycol monoacetates in the oxidation of olefins catalyzed by metal
nitro complexes: mono- vs. bimetallic system. J. Am. Chem. Soc. 1985, 107, 3545.
3. Pd/Cu: Shen, C.; Garcia-Zayas, E. A.; Sen, A. A bimetallic system for the catalytic
hydroxylation of remote primary C-H bonds in functionalized organics using
dioxygen. J. Am. Chem. Soc. 2000, 122, 4029.
´
¨
4. Os-V and Os-Re: Johansson, M.; Linden, A. A.; Backvall, J. E. Osmium-catalyzed
dihydroxylation of alkenes by H2O2 in room temperature ionic liquid co-catalyzed
by VO(acac)2 or MeReO3. J. Organomet. Chem. 2005, 690, 3614.
5. (a) Pd/Co: Misumi, Y.; Ishii, Y.; Hidai, M. Homogeneous multimetallic catalysts.
11. Carbonylation of aryl iodides with HSiEt3 catalyzed by Pd-Co bimetallic
systems. Organometallics 1995, 14, 1770; (b) Pd/Ag: Santi, R.;
Romano, A. M.; Panella, F.; Santini, C. Palladium and silver reductive
catalyzed carbonylation of nitrobenzene to methyl N-phenylcarbamate. J. Mol.
Catal. A: Chem. 1997, 127, 95.
6. (a) Pd2(bda)3/NiBr2 in Heck reaction: Bozell, J. J.; Vogt, C. E. Bimetallic
catalysis. A new method for the activation of chloroarenes toward palladium-
catalyzed coupling with olefins. J. Am. Chem. Soc. 1988, 110, 2655; (b) Ni/Mg
in Suzuki reaction: Yoshikai, N.; Mashima, H.; Nakamura, E. Nickel-catalyzed
cross-coupling reaction of aryl fluorides and chlorides with Grignard reagents
under nickel/magnesium bimetallic cooperation. J. Am. Chem. Soc. 2005, 127,
17978; (c) Chouzier, S.; Gruber, M.; Diakovitch, L. New hetero-bimetallic Pd-
Cu catalysts for the one-pot indole synthesis via the Sonogashira reaction.
J. Mol. Catal. A: Chem. 2004, 212, 43.
7. Rh/Mo: Behr, A.; Brehme, V. A. Bimetallic-catalyzed reduction of carboxylic
acids and lactones to alcohols and diols. Adv. Synth. Catal. 2002, 344, 525.
8. (a) Ga/Co: Li, W.; Thakur, S. S.; Chen, S. W.; Shin, C. K.; Kawthekar, R. B.;
Kim, G. J. Synthesis of optically active 2-hydroxy monoesters via-kinetic resol-
ution and asymmetric cyclization catalyzed by heterometallic chiral (salen) Co
complex. Tetrahedron Lett. 2006, 47, 3453; (b) Co/Ti: Yang, M.; Zhu, C.;
Yuan, F.; Huang, Y.; Pan, Y. Enantioselective ring-opening reaction of meso-
epoxides with ArSeH catalyzed by heterometallic Ti-Ga-Salen system. Org.
Lett. 2005, 7, 1927.
9. Pd/Cu: Kamijo, S.; Jin, T.; Huo, Z.; Yamamoto, Y. Synthesis of triazoles from
nonactivated terminal alkynes via the three-component coupling reaction using a
Pd(0)-Cu(I) bimetallic catalyst. J. Am. Chem. Soc. 2003, 125, 7786.
10. (a) Sammis, G. M.; Danjo, H.; Jacobsen, E. N. Cooperative dual catalysis: appli-
cation to the highly enantioselective conjugate cyanation of unsaturated imides.
J. Am. Chem. Soc. 2004, 126, 9928; (b) Peng, J.; Li, J.; Qiu, H.; Jiang, J.;
Jiang, K.; Mao, J.; Lai, G. Dimerization of styrene to 1,3-diphenyl-1-butene
catalyzed by palladium–Lewis acid in ionic liquid. J. Mol. Catal. A: Chem.
´
2006, 255, 16; (c) Alcaide, B.; Almendros, P.; Rodrıguez-Acebes, R. Pd-Cu bime-
tallic catalyzed domino cyclization of -allenols followed by a coupling reaction:
new sequence leading to functionalized spirolactams. Chem. Eur. J. 2005, 11,
5708; (d) Robles-Machin, R.; Adrio, J.; Carretero, J. C. Gold-catalyzed
synthesis of alkylidene 2-oxazolidinones and 1,3-oxazin-2-ones. J. Org. Chem.

Fe-Mg-HMPA Catalyst
1645
¨
2006, 71, 5023; (e) Lofberg, C.; Grigg, R.; Derrick, A.; Sridharan, V.; Kilner, C.
Sequential one-pot bimetallic Ir(III)/Pd(0) catalysed mono-/bis-alkylation and
spirocyclisation processes of 1,3-dimethylbarbituric acid and allenes. Chem.
Commun. 2006, 5000, and references cited therein.
11. Toyota, M.; Ihara, N. Recent progress in the chemistry of non-monoterpenoid
indole alkaloids. Nat. Prod. Rep. 1998, 15, 327, and references therein.
12. Shi, M.; Gui, S. C.; Li, Q. J. Zirconium triflate-catalyzed reactions of indole, 1-
methylindole, and pyrrole with a, b-unsaturated ketone. Tetrahedron 2004, 60,
6679.
13. Reedy, A. V.; Ravinder, K.; Gaud, T. V.; Krishnaiah, P.; Raju, T. V.;
Venkateswarlu, Y. Bismuth triflate catalyzed conjugate addition of indoles to a,
b-enones. Tetrahedron Lett. 2003, 44, 6257.
14. Bartoli, G.; Bartolacci, M.; Bosco, M.; Foglia, G.; Giuliani, A.; Marcantoni, E.;
Sambri, L.; Torregiani, E. The Michael addition of Indoles to a,b-unsaturated
ketones catalyzed by CeCl3 7H2O NaI combination supported on silica gel.
J. Org. Chem. 2003, 68, 4594.
15. Agnusdel, M.; Bandini, M.; Melloni, A.; Umani-Ronchi, A. New versatile route to
the synthesis of tetrahydro-b-carbolines and tetrahydro-pyrano[3,4-b]indoles via
an intramolecular michael addition catalyzed by InBr3. J. Org. Chem. 2003, 68,
7126.
16. (a) Manabe, K.; Aoyama, N.; Kobayashi, S. Friedel-Crafts-type conjugate addition
of indoles using a Lewis Acid-surfactant-combined catalyst in water. Adv. Synth.
Catal. 2004, 343, 174; (b) Evans, D. A.; Scheidt, K. A.; Fandrick, K. R.;
Lam, H. W.; Wu, J. Enantioselective indole Friedel-Crafts alkylations catalyzed
by bis(oxazolinyl)pyridine-scandium(III) triflate complexes. J. Am. Chem. Soc.
2003, 125, 10780; (c) Bandini, M.; Fagioli, M.; Garavelli, M.; Melloni, A.;
Trigari, V.; Umani-Ronchi, A. Can simple enones be useful partners for the
catalytic stereoselective alkylation of indoles? J. Org. Chem. 2004, 69, 7511;
(d) Firouzabadi, H.; Iranpoor, N.; Nowrouzi, F. The facile and efficient Michael
addition of indoles and pyrrole to a,b-unsaturated electron-deficient compounds
catalyzed by aluminium dodecyl sulfate trihydrate [Al(DS)3] W 3H2O in water.
Chem. Commun. 2005, 789.
17. (a) Paras, N. A.; MacMillan, D. W. C. The enantioselective organocatalytic 1,4-
addition of electron-Rich benzenes to b-unsaturated aldehydes. J. Am. Chem.
Soc. 2002, 124, 7894; (b) Zhou, W.; Xu, L. W.; Yang, L.; Zhao, P. Q.;
Xia, C. G. Novel Brønsted acid-catalyzed Michael-type Friedel-Crafts reactions
of indoles and acetalization of aldehydes. J. Mol. Catal. A: Chem. 2006, 249,
129; (c) Zhou, W.; Xu, L. W.; Li, L.; Yang, L.; Xia, C. G. Enantioselective
Michael-type Friedel-Crafts reactions of indoles to enones catalyzed by a chiral
camphor-based Brønsted acid. Eur. J. Org. Chem. 2006, 23, 5225.