Efficient iron-catalyzed sakurai-michael addition of allyltrimethylsilane to chalcones

Article abstract of DOI:10.1080/00397910701860646

A general catalytic conjugate addition with allyltrimethylsilane and chalcones is described. Allyltrimethylsilane undergoes the Sakurai-Michael reaction smoothly with high chemoselectivity in the presence of a catalytic amount of FeCl3/TMSCl under very mi

Full text of DOI:10.1080/00397910701860646

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Efficient Iron‐Catalyzed Sakurai–Michael
Addition of Allyltrimethylsilane to Chalcones
Li‐Wen Xu a b , Ming‐Song Yang a , Hua‐Yu Qiu a , Guo‐Qiao Lai a & Jian‐Xiong
Jiang a
a Key Laboratory of Organosilicon Chemistry and Material Technology of
Ministry of Education, Hangzhou Normal University, Hangzhou, 310012,
China
b State Key Laboratory for Oxosynthesis and Selective Oxidation, Lanzhou
Institute of Chemical Physics, Chinese Academy of Sciences, and Graduate
School of the Chinese Academy of Sciences, Lanzhou, China
Published online: 12 Mar 2008.
To cite this article: LiWen Xu , MingSong Yang , HuaYu Qiu , GuoQiao Lai & JianXiong Jiang (2008) Efficient
Iron‐Catalyzed Sakurai–Michael Addition of Allyltrimethylsilane to Chalcones, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry, 38:7, 1011-1019, DOI:
10.1080/00397910701860646
To link to this article: http://dx.doi.org/10.1080/00397910701860646
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Synthetic Communicationsw, 38: 1011–1019, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701860646
Efficient Iron-Catalyzed Sakurai–Michael
Addition of Allyltrimethylsilane to
Chalcones
Li-Wen Xu,1,2 Ming-Song Yang,1 Hua-Yu Qiu,1
Guo-Qiao Lai,1 and Jian-Xiong Jiang1
1Key Laboratory of Organosilicon Chemistry and Material Technology of
Ministry of Education, Hangzhou Normal University, Hangzhou, China
2State Key Laboratory for Oxosynthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
and Graduate School of the Chinese Academy of Sciences, Lanzhou,
China
Abstract: A general catalytic conjugate addition with allyltrimethylsilane and
chalcones is described. Allyltrimethylsilane undergoes the Sakurai–Michael reaction
smoothly with high chemoselectivity in the presence of a catalytic amount of FeCl3/
TMSCl under very mild and convenient conditions to afford the corresponding
Michael adducts in high yields.
Keywords: Enones, iron, Lewis acid, Michael addition, Sakurai reaction
INTRODUCTION
The conjugate addition of allylsilanes to conjugated enones, referred to as the
Sakurai–type Michael reaction, has been recognized as one of the most
efficient methods of carbon–carbon bond formation and has been extensively
applied in organic synthesis, especially in the preparation of some heterocyclic
compounds and natural product synthesis.[1] The range of Lewis acids
Received December 12, 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
1011

1012
L.-W. Xu et al.
employed in Sakurai–type Michael addition is extensive, among which TiCl4,
AlCl3, and BF3-OEt2 have been employed as the most effective promoters for
these conjugate additions of allylsilanes.[2] However, many of these methods
involve the use of highly toxic compounds, expensive reagents, or procedures
that require a stoichiometric amount of catalyst to accelerate the reaction.
Although a few examples of catalytic Sakurai–type Michael reactions have
been reported recently,[3] the development of new and cheap catalyst
systems, which are more efficient and lead to convenient procedures with
improved yields, is well appreciated.
In our search for an economical, efficient Sakurai–type Michael addition
of allylsilane to enones, we became intrigued by the idea of using a catalytic
amount of cheap, readily available, and low-toxicity FeCl3 in this reaction. As
an extension of these studies of FeCl3-catalyzed organic transformations,[4]
herein we report a mild and convenient procedure for the Sakurai–type
Michael addition of allyltrimethylsilane to chalcones.
RESULTS AND DISCUSSION
The reaction of chalcone (1a) and allyltrimethylsilane was selected as a model
in the initial experiments. We screened some metal salts for this reaction. In
the first trials, we found many transition-metal salts, such as InCl3, ZnF2,
FeCl3, Fe(ClO4)3, PdCl2(CH3CN)2, and Mg(ClO4)2, were not effective in
this reaction, and only Trace adduct was obtained in the most cases (less
than 10% of yield). We also found the product was isomeric in the presence
of catalytic FeCl3 (Scheme 1, 2a and 2b); even when the amount of FeCl3
was increased to 1.0 equiv., the yield was still low. After the first screening,
a test was performed using the strategies of combination of two different
Lewis acids and the addition of different additives. As shown in Table 1,
FeCl3 was a highly effective catalyst in the presence of TMSCl and gave
excellent yields with good chemoselectivity for the desired product 2a
(entry 13). Other types of additives, such as ZnF2, Brønsted acid (1,10-Bi-2-
naphthol (BINOL)), and Lewis base (hexamethylphosphoramide (HMPA)
or amines), were not suitable promoters in the FeCl3-catalyzed Sakurai–
Michael reaction (entries 2, 5, 7) (Scheme 2). We reasoned that these
results may be due to the activation of Michael acceptors, which should be
feasible in the presence of TMSCl or the product of a stronger Lewis acid
Scheme 1. Sakurai–Michael reaction.

Allyltrimethylsilane to Chalcones
1013
Table 1. Catalytic activity of several Lewis acids in the Sakurai–type Michael
reaction
Isolated
Reaction yield
Entrya Allylsilane
Catalysts
Additive Solvent time (h)
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
CH255CHCH2SiMe3
InCl3
InCl3
ZnF2
FeCl3
FeCl3
Fe(ClO4)3
Fe(ClO4)3
Fe(ClO4)3
Mg(ClO4)2
PdCl2(CH3CN)2
L-Proline
—
—
CH2Cl2
4
3
0
CH255CHCH2SiMe3
CH255CHCH2SiMe3
CH255CHCH2SiMe3
CH255CHCH2SiMe3
CH255CHCH2SiMe3
CH255CHCH2SiMe3
CH255CHCH2SiMe3
CH255CHCH2SiMe3
CH255CHCH2SiMe3
CH255CHCH2SiMe3
CH255CHCH2SiMe3
CH255CHCH2SiMe3
CH255CHCH2SiMe3
CH255CHCH2SiMe3
CH255CHCH2SiMe3
BINOLb CH3NO2
Trace
0
Trace
Trace
Trace
NR
Trace
0
0
—
—
CH2Cl2 12
CH2Cl2
CH3NO2 12
3
ZnF2
—
HMPA CH2Cl2
CH3NO2
3
6
3
—
—
—
—
CH2Cl2
CH2Cl2 12
CH3CN
CH2Cl2 24
3
0
TMSCl CH2Cl2
TMSCl CH2Cl2
TMSCl Toluene
TMSCl CH3CN
TMSCl THF
TMSCl CH2Cl2 12
ZnF2 CH2Cl2 24
TMSCl CH2Cl2 12
3
3
4
4
12
Trace
88 (95)c
27c
FeCl3
FeCl3
FeCl3
FeCl3
52c
0
NR
NR
NR
CH255CHCH2Si(OEt)3 FeCl3
CH255CHCH2Si(OEt)3 FeCl3
CH255CHCH2Si(OEt)3 Fe(ClO4)3
aReaction conditions: 1.0 mmol of enone, 1.5 mmol of allylsilane, 10 mol% of
catalyst, 1.5 equiv. of additive, 3 mL of solvent, at room temperature.
b30 mol% of (S)-BINOL.
cGC yield.
catalyst of FeCl3–TMSCl complex. Further studies indicated that dichloro-
methane was the best solvent in our experiments (entries 13–16). It is
important to note that TMSCl (1.5 eq.) did not show any catalytic activity
in the absence of FeCl3 (entry 12). It is well known that the reactivity of allyl-
trialkloxysilanes was very low, and therefore the Sakurai–Michael reaction of
allyltriethoxysilane was also studied (entries 17–19); unfortunately, this
reaction did not occur in the presence of catalytic FeCl3 and TMSCl.
The success of the Sakurai–Michael addition between chalcone and ally-
trimethylsilane prompted us to extend the general scope of the reaction. Under
Scheme 2. Iron-catalyzed Sakurai–Michael reaction.

1014
L.-W. Xu et al.
the optimized conditions, we were pleased to find that a series of chalcones
with allyltrimethylsilane gave the desired Michael adduct in good to
excellent isolated yields (Table 2). In this reaction, chalcones with electron-
donating groups as CH3 and Cl (entries 4 and 5) gave better yields, but
those with electron-withdrawing groups such as NO2 afforded lower yields
(entry 2). Interestingly, p-OCH3-substituted chalcones worked to give low
to moderate isolated yields (entries 3 and 6–8).
With the basic reaction conditions established, questions regarding the
mechanism warranted further attention. Although it is known that TMSCl
could activate the carbonyl group by the initial interaction of TMSCl with
enones to activate the next step of the reaction,[5] the complete reaction
mechanism of transition-metal-salt/TMSCl–catalyzed Sakurai–Michael
Table 2. Sakurai–type Michael reactions of allyltrimethylsilane with chalcones
catalyzed by FeCl3 in the presence of TMSCl
Enone
number
Product
number Yieldb
Entrya
1
Enone
Product
1a
2a
88
2
3
1b
1c
2d
2c
39
55
4
5
1d
1e
2d
2e
87
67
6
1f
2f
21
7
8
1g
1h
2g
2h
46
54
aReaction conditions: 1.0 mmol of enone, 1.5 mmol of allylsilane, and 3 mL of
solvent at room temperature for 3 h.
bIsolated yield (%) by silica column chromatography.

Allyltrimethylsilane to Chalcones
1015
reaction of allyltrimethylsilane with enones was not clear in the past. A series
of control experiments with chalcone and allyltrimethylsilane gave some
mechanistic insight to better understanding the roles of the catalyst and
additive. The results in Tables 1 and 2 show that transition-metal-salt-based
Lewis acid and TMSCl should work synergistically in the hetero-Lewis
acidic catalyst system. First, the importance of FeCl3 and TMSCl was
assessed by carrying out reactions in their absence. As shown in Table 1
(entries 4 and 12), both FeCl3 and TMSCl are required for good reactivity.
The results of these experiments were consistent with our working hypothesis
based on the enhancement of the activity of FeCl3 in the preþsence of TMSCl.
The activation of Lewis acid by the formation of Fe2Cl4 Si Me3 from FeCl3
.
and TMSCl,[6,7] resulted in the improvement of catalytic activity of iron in
Sakurai–Michael reaction.
In conclusion, we have developed a new catalytic Sakurai–Michael
reaction of chalcones and allyltrimethylsilane promoted by the combination
of iron(III) salts and TMSCl. This work offers good examples of the combi-
nation of Lewis acid and TMSCl catalyst for novel Sakurai–Mchael
reactions of allylsilane with chalcones. Further studies are currently under
way to expand the application of the reaction to asymmetric catalysis and
establish the high reactivity and stereoselectivity with chalcones and allylsilane.
EXPERIMENTAL PART
General
All reaction flasks and solvent 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. IR spectra were recorded using an FTIR apparatus. Thin-
layer chromatography (TLC) was performed using silica.
Typical Sakural–Michael Allylation Reaction Procedure
FeCl3 (0.1 mmol), chalcone (1.0 mmol), allyltrimethylsilane (1.1 mmol), and
TMSCl (1.5 mmol) in anhydrous CH2Cl2 (3 mL) were added to a Schlenk tube
under argon. The mixture was stirred for 3 h at room temperature. After com-
pletion of the reaction, the mixture was quenched with water, and the aqueous
layer was extracted with CH2Cl2 (3 ꢀ 30 mL). The combined organic layers
were washed with 2 N HCl, dried over MgSO4, filtered, and evaporated.
The crude product was purified by column chromatography (petroleum
ether/EtOAc 10:1) to give the pure products.

1016
L.-W. Xu et al.
Selected Spectra Data of Products
1,3-Diphenylhex-5-en-1-one (2a): 1H NMR (400 MHz, CDCl3, ppm),
d ¼ 7.88 (m, 2H), 7.52 (m, 1H), 7.42 (m, 2H), 7.17–7.30 (m, 5H), 5.69
(m, 1H), 4.99 (m, 2H), 3.48 (m, 1H), 3.29 (d, J ¼ 8 Hz, 2H), 2.46 (m, 2H).
13C NMR (100 MHz, CDCl3, ppm): d ¼ 198.78, 144.20, 137.06, 136.12,
132.79, 128.37, 128.27, 127.87, 127.41, 126.22, 116.64, 44.40, 40.59,
40.54. GC-MS: m/z 250 (M, 4), 130 (58), 105 (100), 77 (39). IR (KBr):
3060, 3022, 2978, 2917, 1678, 1642, 1589, 1449, 1343, 1257, 1217, 984,
906 cm21
.
3-(3-Nitrophenyl)-1-phenylhex-5-en-1-one (2b): 1H NMR (400 MHz, CDCl3,
ppm), d ¼ 8.12 (s, 1H), 8.04 (d, J ¼ 12 Hz, 1H), 7.90 (d, J ¼ 8 Hz, 2H), 7.58
(m, 2H), 7.45 (m, 3H), 5.68 (m, 1H), 5.02 (m, 2H), 3.63 (m, 1H), 3.37 (m, 2H),
2.50 (m, 2H). 13C NMR (100 MHz, CDCl3, ppm): d ¼ 197.79, 148.22,
146.38, 136.62, 135.02, 134.30, 133.14, 129.11, 128.51, 127.80, 122.06,
121.42, 117.63, 43.80, 40.38, 40.13. GC-MS: m/z 295 (M, 3), 120 (37),
105 (100), 77 (41). IR (KBr): 3068, 2978, 2920, 1686, 1597, 1528, 1448,
1349, 1266, 1204, 1098, 1001, 920 cm21
.
1-(4-Methoxyphenyl)-3-phenylhex-5-en-1-one (2c): 1H NMR (400 MHz,
CDCl3, ppm), d ¼ 7.89 (d, J ¼ 8 Hz, 2H), 7.53 (t, J ¼ 8 Hz, 1H), 7.42
(t, J ¼ 8 Hz, 2H), 7.15 (d, J ¼ 8 Hz, 2H), 6.82 (d, J ¼ 8 Hz, 2H), 5.69
(m, 1H), 4.98 (m, 2H), 3.76 (s, 3H), 3.42 (m, 1H), 3.26 (m, 2H), 2.42
(m, 2H). 13C NMR (100 MHz, CDCl3, ppm): d ¼ 198.94, 157.84, 137.08,
136.25, 136.22, 132.76, 128.36, 128.29, 127.87, 116.55, 113.63, 55.02,
44.65, 40.70, 39.85. GC-MS: m/z 280 (M), 160 (13), 105 (100), 77 (24). IR
(KBr): 3064, 2998, 2929, 2830, 1684, 1610, 1597, 1513, 1448, 1300, 1248,
1178, 1036, 1001 cm21
.
1
1-Phenyl-3-p-tolylhex-5-en-1-one (2d): H NMR (400 MHz, CDCl3, ppm),
d ¼ 7.89 (d, J ¼ 12 Hz, 2H), 7.52 (t, J ¼ 4 Hz, 1H), 7.42 (t, J ¼ 8 Hz, 2H),
7.11 (m, 4H), 5.69 (m, 1H), 4.97 (m, 2H), 3.43 (m, 1H), 3.26 (m, 2H), 2.44
(m, 2H), 2.29 (s, 3H). 13C NMR (100 MHz, CDCl3, ppm): d ¼ 198.87,
141.15, 137.08, 136.26, 135.65, 132.75, 128.97, 128.36, 127.88, 127.24,
116.54, 44.53, 40.58, 40.18, 20.86. GC-MS: m/z 264 (M), 144 (31), 129
(10), 105 (100), 77 (27). IR (KBr): 3060, 3022, 2978, 2921, 1686, 1597,
1515, 1448, 1360, 1267, 1200, 1114, 1001, 915 cm21
.
3-(2-Chlorophenyl)-1-phenylhex-5-en-1-one (2e): 1H NMR (400 MHz,
CDCl3, ppm), d ¼ 7.93 (d, J ¼ 8 Hz, 2H), 7.53 (t, J ¼ 4 Hz, 1H), 7.44
(t, J ¼ 8 Hz, 2H), 7.35 (d, J ¼ 8 Hz, 1H), 7.24 (m, 1H), 7.13 (t, J ¼ 4 Hz,
1H), 5.70 (m, 1H), 4.98 (m, 3H), 4.04 (m, 1H), 3.33 (d, J ¼ 8 Hz, 2H), 2.49
(t, J ¼ 8 Hz, 2H); 13C NMR (100 MHz, CDCl3, ppm): d ¼ 198.28, 141.20,
136.85, 135.60, 133.87, 132.88, 129.69, 128.41, 127.90, 127.28, 126.67,

Allyltrimethylsilane to Chalcones
1017
116.92, 43.00, 38.92, 36.55. GC-MS: m/z 284 (M), 249 (12), 164 (18), 129
(32), 105 (100), 77(40). IR (KBr): 3072, 2957, 2892, 2827, 1681, 1595,
1476, 1446, 1436, 1350, 1263, 1215, 1034, 985, 924 cm21
.
1
1,3-Bis(4-methoxyphenyl)hex-5-en-1-one (2f): H NMR (400 MHz, CDCl3,
ppm), d ¼ 7.88 (d, J ¼ 8 Hz, 2H), 7.14 (d, J ¼ 8 Hz, 2H), 6.90
(d, J ¼ 12 Hz, 2H), 6.82 (d, J ¼ 6 Hz, 2H), 5.68 (m, 1H), 4.98 (m, 2H),
3.85 (s, 3H), 3.77 (s, 3H), 3.41 (m, 1H), 3.20 (m, 2H), 2.42 (m, 2H); 13C
NMR (100 MHz, CDCl3, ppm): d ¼ 197.48, 163.17, 157.81, 136.36,
136.31, 130.21, 130.15, 128.27, 116.43, 113.60, 113.47, 55.27, 55.01,
44.33, 40.65, 40.05. IR (KBr): 3073, 3000, 2920, 2837, 1675, 1639, 1600,
1575, 1511, 1463, 1419, 1363, 1305, 1248, 1169, 1032, 997, 915 cm21
.
3-(4-Methoxyphenyl)-1-(4-nitrophenyl)hex-5-en-1-one (2g): 1H NMR
(400 MHz, CDCl3, ppm), d ¼ 8.25 (d, J ¼ 8 Hz, 2H), 8.00 (d, J ¼ 12 Hz,
2H), 7.13 (d, J ¼ 8 Hz, 2H), 6.81 (d, J ¼ 8 Hz, 2H), 5.70 (m, 1H), 5.02
(m, 2H), 3.78 (s, 3H), 3.39 (m, 1H), 3.30 (m, 2H), 2.45 (m, 2H); 13C NMR
(100 MHz, CDCl3, ppm): d ¼ 197.56, 158.04, 150.01, 141.51, 135.94,
135.57, 128.83, 128.21, 123.60, 116.89, 113.75, 55.02, 45.12, 40.71, 39.93.
IR (KBr): 3064, 2999, 2931, 2835, 1685, 1639, 1611, 1597, 1513, 1448,
1362, 1301, 1248, 1201, 1179, 1036, 1001, 916 cm21
.
3-(4-Methoxyphenyl)-1-phenylhex-5-en-1-one (2h): 1H NMR (400 MHz,
CDCl3, ppm), d ¼ 7.88 (d, J ¼ 12 Hz, 2H), 7.51 (t, J ¼ 4 Hz, 1H), 7.42
(t, J ¼ 8 Hz, 2H), 7.15 (d, J ¼ 8 Hz, 2H), 6.81 (d, J ¼ 12 Hz, 2H), 5.69
(m, 1H), 4.98 (m, 2H), 3.76 (s, 3H), 3.42 (m, 1H), 3.25 (m, 2H), 2.42
(m, 2H); 13C NMR (100 MHz, CDCl3, ppm): d ¼ 198.93, 157.86, 137.11,
136.25, 136.23, 132.74, 128.36, 128.29, 127.87, 116.53, 113.64, 55.02,
44.66, 40.69, 39.88. IR (KBr): 3073, 3000, 2920, 2837, 1675, 1639, 1600,
1575, 1511, 1463, 1419, 1363, 1304, 1248, 1169, 1112, 1032, 997, 915 cm21
.
ACKNOWLEDGMENTS
This study was partially supported by the National Natural Science Foun-
dation of China (Project 20572114) and Hangzhou Normal University.
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