Magnesium bistrifluoromethanesulfonimide as an efficient catalyst for the hydroalkylation of aromatic olefins with 1,3-diketones under solvent-free conditions

Article abstract of DOI:10.1016/j.cclet.2011.01.036

An efficient magnesium bistrifluoromethanesulfonimide [Mg(NTf ₂)₂] catalyzed hydroalkylation of aromatic olefins with 1,3-diketones under solvent-free conditions has been developed. The reactions proceed smoothly to give the desired products in good yields in short reaction times.

Full text of DOI:10.1016/j.cclet.2011.01.036

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 911–914
www.elsevier.com/locate/cclet
Magnesium bistrifluoromethanesulfonimide as an efficient
catalyst for the hydroalkylation of aromatic olefins
with 1,3-diketones under solvent-free conditions
*
Hong She Wang , Wei Xing Zhao
Shaanxi Key Laboratory for Phytochemistry, Baoji University of Arts and Sciences, Baoji 721013, China
Received 8 November 2010
Available online 17 May 2011
Abstract
An efficient magnesium bistrifluoromethanesulfonimide [Mg(NTf2)2] catalyzed hydroalkylation of aromatic olefins with 1,3-
diketones under solvent-free conditions has been developed. The reactions proceed smoothly to give the desired products in good
yields in short reaction times.
# 2011 Hong She Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Hydroalkylation; Aromatic olefins; 1,3-Diketones; Magnesium bistrifluoromethanesulfonimide
The hydroalkylation of alkenes with 1,3-dicarbonyl compounds is one of the most common methods for carbon–
carbon bond formation in organic synthesis [1]. Generally, this transformation is carried out using alkyl halides in the
presence of a stoichiometric amount of base [2]. Although this method works well even on large scale, it is often
associated with many drawbacks including nonavailability of halogenated substrate, toxicity of halogenated
substrates, the use of strong base and production of large amounts of salts as by-products. Therefore, the development
of a practical and economical process for carbon–carbon bond formation between active methylene compounds and
unmodified substrates is an important task. Recently, there are some reports on the direct hydroalkylation of alkenes
with 1,3-dicarbonyl compounds in the presence of AuCl3/AgOTf [3], FeCl3 [4], I2 [5], Cu(OTf)2 [6], phosphotungstic
acid [7], SnBr4 [8] and InCl3 [9]. Although each of the above methods has its own merit, some of these methods are
limited by low yields and long reaction times. Therefore, the development of convenient and high yielding approaches
to the hydroalkylation of 1,3-dicarbonyl compounds with alkenes is still desirable.
In recent years, metal bistrifluoromethanesulfonimides [M(NTf2)n] have been successfully used for the acetylation
of phenols and alcohols [10], [2 + 2] cycloadditions of siloxy alkynes with carbonyl compounds [11], Friedel–Crafts
acylation reactions [12], cycloisomerization of 1,6-dienes [13] and aminolysis of lactones with amines [14].
Previously, we have reported the use of Eu(NTf2)3 [15] and Mg(NTf2)2 [16] as efficient catalysts for the organic
synthesis. Mg(NTf2)2 is commercially available, cheaper and not sensitive to air, and therefore, better suited for
catalytic use. Following our interest in the catalytic uses of M(NTf2)n, we herein report an efficient method for the
* Corresponding author.
E-mail address: hongshewang@126.com (H.S. Wang).
1001-8417/$ – see front matter # 2011 Hong She Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2011.01.036

9
12
H.S. Wang, W.X. Zhao / Chinese Chemical Letters 22 (2011) 911–914
O
O
O
Mg(NTf2)2 (3 mol%)
solvent-free, 80 oC
R1
R
R
R1
R1
+
O
R1
2
1
3
Scheme 1. Addition of 1,3-diketones to aromatic olefins catalyzed by Mg(NTf2)2.
hydroalkylation of aromatic olefins with 1,3-diketones in the presence 3 mol% of Mg(NTf2)2 under solvent-free
conditions (Scheme 1).
In order to establish the optimum conditions for the hydroalkylation of aromatic olefins with 1,3-diketones,
initially, using the reaction of acetylacetone with styrene as a model, we investigated the effect of the amount of
catalyst on this reaction. It was found that 3 mol% of Mg(NTf2)2 (with respect to acetylacetone) is sufficient to
promote reaction efficiently. In the absence of catalyst, the reaction did not proceed even after a long reaction time
(22 h). Excess of Mg(NTf2)2 beyond this amount did not show any further increase in conversion and yield. Use of less
than the required catalyst amount resulted in poor yield. Then, the effects of different solvents on the reaction were
studied. Unsatisfactory yields and long reaction times were observed in the solvents (CH2Cl2, CHCl3, CH3CN, THF,
DMF, DMSO, CH3NO2, H2O, 1,2-dichloroethane and toluene) screened. The reaction exhibited the best results in
terms of the yield and reaction time under solvent-free conditions. The reaction temperature has also a great influence
on the reaction. We found that the reaction did not take place within 20 h when it was carried out at room temperature.
Increasing the reaction temperature could remarkably enhance both reaction yield and rate. After the temperature was
elevated to 80 8C, the reaction could be completed within 3 h and the product 3-(1-phenylethyl)- pentane-2,4-dione
(3a) was isolated with high yield (91%). However, while increasing the temperature continuously, the yield has not
obviously changed.
To demonstrate the generality of this method, we investigated the hydroalkylation of various aromatic olefins with
1,3-diketones in the presence 3 mol% of Mg(NTf2)2 under solvent-free conditions at 80 8C [17] and the results are
summarized in Table 1. As shown in Table 1, in all cases, the reactions proceeded efficiently. Various aromatic olefins
having methyl, t-butyl, methoxy, chloro and bromo underwent smooth hydroalkylation with 1,3-diketones to afford
the corresponding products in high yields. In the case of 2-vinylnaphthalene, the corresponding products could also be
obtained in high yields (Table 1, entries 7 and 12). Furthermore, both aliphatic (Table 1, entries 1–7) and aromatic
(Table 1, entries 8–12) 1,3-diketones reacted smoothly with olefins under identical conditions.
Table 1
Mg(NTf2)2-catalyzed hydroalkylation reactions of aromatic olefins with 1,3-diketones.a
Entry
1
Aromatic olefin
1,3-Diketone
Product
Time (h)
3.0
Yieldb (%)
91
O
O
O
O
[TD$INLINE]
[DT$INLE]
[DT$INLE]
O
O
3a
2
3
2.5
2.0
93
96
O
O
O
O
[DT$INLE]
[TD$INLINE]
[DT$INLE]
3b
O
[DT$INLE]
[TD$INLINE]
[DT$INLE]
O
3c
4
4.5
82
O
O
O
[DT$INLE]
[TD$INLINE]
[DT$INLE]
MeO
MeO
O
3d

H.S. Wang, W.X. Zhao / Chinese Chemical Letters 22 (2011) 911–914
913
Table 1 (Continued )
Entry
5
Aromatic olefin
1,3-Diketone
Product
Time (h)
3.5
Yieldb (%)
88
O
O
O
O
O
O
O
[DT$INLE]
[TD$INLINE]
[DT$INLE]
Cl
Cl
Br
O
3e
6
7
4.0
3.0
84
91
O
Br
[DT$INLE]
[TD$INLINE]
[DT$INLE]
O
3f
O
[TD$INLINE]
[DT$INLE]
[DT$INLE]
O
3g
8
3.0
2.5
3.0
90
94
92
O
O
O
O
O
O
O
[TD$INLINE]
[DT$INLE]
Ph
Ph
Ph
Ph
Ph
[DT$INLE]
O
Ph
3h
9
O
[TD$INLINE]
[DT$INLE]
Ph
Ph
O
[DT$INLE]
O
Ph
3i
10
[DT$INLE]
[TD$INLINE]
Ph
Ph
[DT$INLE]
O
O
Ph
3j
3l
11
12
3.0
4.0
88
89
O
O
O
O
Cl
Cl
[DT$INLE]
[TD$INLINE]
Ph
Ph
Ph
Ph
[DT$INLE]
O
Ph
3k
O
[TD$INLINE]
[DT$INLE]
Ph
Ph
[DT$INLE]
Ph
O
a
All the reactions were carried out with 3 mol% of Mg(NTf2)2 under solvent-free conditions at 80 8C.
Isolated yields.
b
Table 2
Comparison of efficiency of various catalysts in the synthesis of 3 h.
Catalyst (mol%)
Conditions
Time (h)
Yielda (%)
Ref.
AuCl3 (5)/AgOTf (15)
FeCl3 (30)
Room temperature/CH2Cl2
80 8C/1,2-dichloroethane
110 8C/toluene
90 8C/dioxane
50 8C/ionic liquid
100 8C/MeNO2
Overnight
5.0
93
90
88
76
60
65
90
[3]
[4]
I2 (10)
5.5
[5]
Cu(OTf)2 (15)
SnBr4 (10)
20.0
6.0
[6]
[8]
InCl3 (10)
15.0
3.0
[9]
Mg(NTf2)2 (3)
80 8C/solvent-free
This work
a
Isolated yields.

914
H.S. Wang, W.X. Zhao / Chinese Chemical Letters 22 (2011) 911–914
In order to evaluate the efficiency of this work in comparison with previously reported procedures, synthesis of 1,3-
diphenyl-2-(1-phenylethyl)propane-1,3-dione (3 h) was considered as a representative example. As shown in Table 2,
the yield of our present method is better or comparable with others.
In conclusion, we have developed an efficient direct hydroalkylation reactions of aromatic olefins with 1,3-
diketones catalyzed by Mg(NTf2)2. The reactions proceeded in high yields and avoided the use of toxic organic
solvents. Simple workup, the use of a catalytic amount and commercially available catalyst, and short reaction time are
the advantages of the present protocol. The properties of Mg(NTf2)2 such as stability, commercial availability, and
high Lewis acidity, make Mg(NTf2)2 useful for catalytic applications [10].
Acknowledgments
This work was financially supported by the Special Scientific Research Project of Shaanxi Education Commission
(Nos. 09JK332, 2010JS069) and the Science Research Foundation of Baoji University of Arts and Sciences (No.
ZK1053).
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[17] General procedure: a mixture of aromatic olefin 1 (1.5 mmol), 1,3-diketone 2 (1.0 mmol) and Mg(NTf2)2 (3 mol%) was stirred at 80 8C. After
completion of the reaction (monitored by TLC), the reaction mixture was cooled to room temperature and CH2Cl2 (2 Â 10 mL) was added and
the combined organic phases were washed with water. Removal of the solvent under reduced pressure afforded crude product 3, which was
further purified by column chromatography.