Advances in Materials Physics and Chemistry, 2012, 2, 142-145
doi:10.4236/ampc.2012.24B037 Published Online December 2012 (http://www.SciRP.org/journal/ampc)
Facile and Green Synthesis of α,β-Unsaturated Ketone
Catalyzed by Air-Stable Organobismuth Complex
Renhua Qiu1, Yimiao Qiu1, Zhengong Meng1, Xingxing Song1, Zhenyong Jia1, Kun Yu1, Shuangfeng Yin1,
Chak-Tong Au2, Wai-Yeung Wong2
1Collenge of Chemistry and Chemical Engineering, Hunan University, Changsha, P.R. China
2Department of Chemistry, Hong Kong Baptist Universtiy, Hong Kong, China
Received 2012
ABSTRACT
Air-stable cationic organobismuth complexes (2-5) possessing both acidic and basic characters were synthesized. The catalyst system
that comprises an air-stable bifunctional Lewis acidic/basic organobismuth complex and [Bmim]BF4 showed high catalytic activity,
diastereoselec-tivity, stability, and reusability in the one-pot synthesis of (E)-α,β-unsaturat-ed ketones through highly selective
crossed-condensation of ketones and aldehydes. Through switching the reaction from homogeneous to heterogeneous, the system
shows facile separation ability and facile reusability.
Keywords: α,β-Unsaturated Ketone; Catalysis; Facile Seperation Catalytic System;Organobismuth; .Synthesis
1. Introduction
α,β-Unsaturated carbonyl compounds are widely used as sub-
strates for a number of reactions such as hydrogenation, epoxi-
dation, peroxidation, cycloaddition, and conjugate addition.
Aldol condensation reaction of carbonyl compounds is the most
common processfor the synthesis of α,β-unsaturated carbonyl
compounds. Claisen-Schmidt condensation, a crossed aldol
condensation of an aromatic aldehyde and an aliphatic ketone
or aldehyde under basic conditions, is traditionally used process.
In the reaction, a relatively strong base (such as metal hydroxide
or metal alkoxide) is employed, and selective mono-condensation
is often difficult due to side reactions such as bis-condensation
and aliphatic aldehyde dimerization. Application of the method
is further limited because substrates with base-sensitive func-
tional groups are not suitable. A better approach is by means of
the Mukaiyama-aldol reaction followed by subsequent dehy-
dration catalyzed by a Lewis acid. Recently, Yanagisawa et al.
reported the one-pot selective synthesis of α,β-unsaturated ke-
tones from alkenyl trichloroacetates and aldehydes; in this ap-
proach, ketones have to be converted to alkenyl trichloroace-
tates before the condensation reaction.
Catalytic direct crossed-condensation of ketones and alde-
hydes would be an ideal process for the synthesis of α,β-un-
saturated carbonyl compounds, because there is no need to
prepare reactive intermediates (e.g. silyl enol ether) and only
H2O is generated as a side-product (Scheme 1). Such a process
is significantly “energy-efficient” and “atom economic” since
multistep transformations and separation of product (from
by-products) is not necessary. Recently, use of organocatalysts
for direct crossed-condensation reaction was reported, while
high catalyst loading is necessary (20 mol%).
We are interested in the study of organobismuth complexes
because bismuth is a stable (green) heavy element. The utiliza-
tion of bismuth compounds in the field of catalysis and organic
synthesis has been studied intensively in recent years. Simple
bismuth Lewis acids such as bismuth halides and triflates are
catalysts highly efficient in a number of reactions. The use of
designed cationic organobismuth compounds in catalysis, how-
ever, is rarely reported partly due to the instability of the Bi-C
bond. In this paper, air-stable cationic organobismuth com-
plexes [S(CH2C6H4)2Bi(OH2)]+[X]– (2-5) possessing both
acidic and basic characters are synthesized [1-5] Furthermore,
we herein report a catalytic process that is based on the facile
separation approach. The catalyst system is composed of an air-
stable Lewis acidic/basic bifunctional complex [S(CH2C6H4)2Bi
(OH2)]+[BF4]– (1) and [Bmim]BF4 (1-buty-3-methylimida-
zolium tetrafluoroborate); it shows high catalytic efficiency for
the green synthesis of (E)-α,β-unsaturated ketones through
cross-condensation of aldehydes and ketones.
2. Results and Discussion
Shown in Scheme 2 is the synthetic route of the organobismuth
complexes 1-5. Treatment of S(CH2C6H4)2BiCl 1 with AgX in
THF afforded organobismuth complexes 2-5 quantitatively.The
results of 1H NMR spectroscopy and elemental analysis show
Scheme 1. A comparison between a common process and an ideal
process for the selective synthesis of α,β-unsaturated ketones.

R. H. QIU ET AL. 143
that samples 2-5 freshly obtained from recrystallization contain
one water molecule. They are air-stable and show good water
tolerance. They remained as dry colorless crystals or white
powder in ambient environment in a test period of one year.
The thermal behavior of complexes 2-5 was investigated by
TG-DSC in N2 (Figure 1). The materials show high thermal
stability, especially complexes 4 and 5 with perfluoroalkylsul-
fonate counter anions (stable up to 230oC). We also employed
the Hammett indicator method11a-b to determine acidity and
basicity, and found moderate acidity with acid strength of 3.3 <
Ho ≤ 4.8 for 3-5 and 4.8 < Ho ≤ 6.8 for 2. In terms of basicity,
the four complexes exhibit strength of 7.2 ≤ H- < 8.9. It is
worth pointing out that complex 1 shows no acidity but basicity
(7.2 ≤ H- < 8.9.). Despite Lewis acid/base pair exists in com-
plexes 2-5, there is no sign of self-quenching. With the steric
effect of the butterfly-shaped ligand structure and the presence
of bismuth-sulfur bifunctional centers, it is envisaged that com-
plexes 2-5 are efficient and stereoselective catalysts.
S
Bi
Cl
1.0eqAgX
THF, RT,3 h, Dark
S
Bi
X
-
+
OH2
,X=ClO
4
,X=BF
4
,X=OSO
2C4F9
,X=OSO
2C8F17
S
Br Br
Et2O, -60oC
S
Li Li
BiCl3
Et2O, -78oC
nBuLi
Scheme 2. Synthetic routes of butterfly-shaped sulfur-bridged or-
ganobismuth complexes 1-5.
Figure 1. TG-DSC analysis of organobismuth complexes 2-5.
We first investigated the catalytic performance of 3 towards
the direct three-component Mannich reaction of benzaldehyde,
cyclohexanone and propylamine in ionic liquid [Bmim]BF4.
However, a completely different product, (E)-α,β-unsaturated
ketone, was obtained. Since this process was efficient under
mild conditions, we further evaluated the catalytic performance
of 3 towards the synthesis of α,β-unsaturated ketones. Further-
more, during the course of reaction, the nature of catalysis
switches from homogeneous to heterogeneous (Figure 2). At the
beginning, benzaldehyde 6a, cyclohexanone 7a, propylamine,
complex 3 and [Bmim]BF4 merge together and the reaction
system is homogeneous (Figure 2(a)). By the end of the reac-
tion, the system becomes turbid, and after 5 min of settling, the
mixture separates into two phases. The upper consists of the
product and unconsumed reactants while the lower consists of
[Bmim]BF4, complex 3, and water (the only side product)
(Figure 2(b)). The findings were congruent to Leng groups
work, which it has a catalytic procedure that is monophasic at
the beginning and biphasic at the end. The overall reaction
occurs at room temperature and there is no need to change any
reaction condition. Previously, Leng’s group deduced the sys-
tem with such behavior and features as reaction- induced
self-separation catalyst system, which is a facile separation
catalyst system. The prominent feature of the system is its ex-
cellent solubility in water or polar solvents but immiscibility in
apolar α,β-unsaturated ketones. In other words, as a catalyst
complex 3 dissolves completely in [Bmim]BF4 as well as in the
reactants, but is insoluble in the product (α,β-unsatu- rated ke-
tones). Thus at the early stage, the mixture for the cross-con-
densation reaction is homogeneous. With the consumption of
reactants, the system becomes heterogeneous, and there is the
spontaneous separation of the catalyst system (complex 3 and
[Bmim]BF4) and product. Eventually, the catalyst system can
be easily recovered by simple decantation (Figure 2(c)). It is
apparent that the advantages of both homogeneous and hetero-
geneous catalysis are captured in this method.
In a scale-up (x5) experiment, we found that catalyst loading
can be lowered to 0.1 mol% with the facile separation of cata-
lyst system almost unaffected. Furthermore, the resulting ILs
containing catalyst and water can be conveniently reused along
with the unconsumed reactants and newly added substrates.
Subject to desiccation treatment and owing to the air-stable,
water-tolerant features and special interaction effect of the or-
ganobismuth complex and ILs [Bmim]BF4, the catalyst system
can be recycled for at least ten times without significant decline
in product yield (96–100%) and stereoselectivity (E/Z = 100/0).
Furthermore, we examined the structure integrity of the recy-
cled catalyst (with [Bmim]BF4) by the NMR technique and
found that the structure of the recycled catalyst is consistent
with that of the freshly prepared one. In other words, the cata-
lyst is stable and suitable for reuse. It should be noted that the
total substrate molar ratio (6a : n-PrNH2 : 7a) for ten cycles is
1.0 : 0.19 : 1.2, and the TON is up to 9893.
Figure 2. Photographs of the cross-condensation reaction of ben-
zaldehyde 6a with cyclohexanone 7a over organobismuth complex 3
in the presence of propylamine in [Bmim]BF4. (a) Homogeneous
mixture during reaction; (b) the reaction system becomes hetero-
geneous at completion of reaction: the upper layer is composed of
the product (α,β-unsaturated ketones) and unconsumed reactants
while the lower layer [Bmim]BF4, complex 3, and water generated
in the reaction; (c) at the end of reaction, the layer of [Bmim]BF4,
catalyst 1, and water was removed by decantation.

R. H. QIU ET AL.
144
Usually, a catalyst system is only conveniently suitable for
certain substrates. However, the one depicted by us here can be
applied to enolizable aliphatic aldehydes as well as to aromatic
aldehydes with electron-donating and electron-withdrawing
groups (Table 1. In all cases, the phenomenon of facile separa-
tion was observed with high product yields. Although the reac-
tion of furfural occurs at 0oC (Table 1, entry 5), the E-selectiv-
ity for furfural is consistent with those of the other aldehydes. It
is worth pointing out that the reaction of enolizable aliphatic
aldehydes selectively produces (E)-α,β-unsaturated ketones in
almost quantitative yields without aldehyde facile condensation
product or any-other side-product formation (Table 1, entries
6–7). On the other hand, the active methylene compounds ap-
pear to be efficient substrates in the present scheme (Table 1,
entries 9–11). In all cases, no dibenzylidene byproduct is de-
tected in NMR analysis. We ascribe such phenomenon to the
special steric effect of monobenzylidene. In the catalyst system,
it is hard for large group such as monobenzylidene to approach
the active sites, and cross-condensation of monobenzylidene
with cyclohexanone to form dibenzylidene byproduct is unlikely.
Due to the fact that complex 3 plays a major role in this fac-
ile separation catalyst system, we studied the crystal structure
of 3 by X-ray analysis. An ORTEP representation of 3, and the
selected bond lengths and angles are shown in Figure 3. It is
clear that the organobismuth component in 3 is cationic. The
oxygen atom of the coordinating water occupies a vacant site of
the cationic bismuth centre, making the coordination geometry
distorted and equatorially vacant. One can see a trigonal
bipyramidal with the sulfur and the oxygen atoms in the apical
positions and the two carbon atoms in the equatorial positions.
Table 1. Synthesis of different α,β-unsaturated ketones catalyzed by
cationic organobismuth complex 3 in [Bmim]BF4.a
Entry R1CHO Ketone Product Yield (%)bE/Zc
1 6a 7a 8a 98 100/0
2 6b 7a 8b 93 100/0
3 6c 7a 8c 99 100/0
4 6d 7a 8d 100 100/0
5d 6e 7a 8e 98 100/0
6 6f 7a 8f 100 100/0
7 6g 7a 8g 98 100/0
8 6a 7b 8h 97 --
9 6a 7c 8i 99 --
10 6a 7d 8j 98 --
11 6a 7e 8k 95 --
a6, 20 mmol; n-PrNH2, 20 mmol; 7, 60 mmol; 3, 0.2 mmol; [Bmim]BF4, 1.0 mL;
RT. bIsolated yield. cDetermined by 1H NMR. d0 oC.
Figure 3. An ORTEP view (30% probability level) of 3. Selected
bond lengths (Å) and angles (deg): Bi(1)–C(1), 2.256(7); Bi(1)–C
(14), 2.262(7); Bi(1)–O(1), 2.499(6); Bi(1)–S(1), 2.6992(19); S(1)–C(7),
1.815(8); S(1)–C(8), 1.816(9); O(1)–H(1A), 0.8180; O(1)–H(1B),
0.7499; C(1)–Bi(1)–C(14), 97.2(2); C(1)–Bi(1)–O(1), 86.3(2); C(14)–
Bi(1)–O(1), 90.7(2); C(1)–Bi(1)–S(1), 78.01(19); C(14)–Bi(1)–S(1),
78.01(19); O(1)–Bi(1)–S(1), 159.20(14); C(7)–S(1)–C(8), 101.0(4);
C(7)–S(1)–Bi(1), 95.7(3); C(8)–S(1)–Bi(1), 94.7(3); Bi(1)–O(1)–
H(1A), 109.5; Bi(1)–O(1)–H(1B), 119.8.
The Bi–S(1) distance (2.699(19)) is shorter than that (2.845 Å)
of precursor 1, clearly suggesting stronger sulfur-to-bismuth
coordination in 3. The Bi–O(1) distance (2.499(6) for 3) is longer
than that of covalent Bi–O bonds (e.g., Bi–O bond distances of
monomeric diorganobismuth alkoxides within 2.15–2.20 Å),
indicating that the weakly coordinated water molecule can be
replaced by a substrate. The dihedral angle of the two phenyl
planes (ca. 107 degrees) is equal to the C(1)-Bi-C(14) angle
(97.2 degrees) and the C(7)-S(1)-C(8) angle (101 degrees). This
butterfly-shaped cationic organobismuth ion is similar to that of
1,1’-binaphthol template used as asymmetric catalyst in organic
synthesis.
Although further study is necessary to clarify the reaction
mechanism, the results mentioned so far suggest that the reac-
tion probably takes place through a Mannich-type mechanism
as shown in Scheme 3. In the reaction, the complex with the
above framework displays Lewis acidic/basic bifunctional
properties with the accessible bismuth centers acting as a Lewis
acid sites and the uncoordinated lone-pair electrons of sulfur as
Lewis base sites. However, it should be noted that when or-
ganobismuth complex 3 is used as catalyst in the presence of
propylamine in ILs, high catalytic activity was observed in
cross-condensation of benzaldehyde and cyclohexanone, dis-
playing high synthetic yield and diastereoselectivity (yield 98%,
E/Z = 100/0). Very different result is obtained when water is
used as solvent (yield 94%, E/Z = 90/10), suggesting that the
ILs play an important role in the control of diastereoselectivity.
We utilized NMR technique to investigate the interaction of
complex 1 with [Bmim]BF4 (Scheme 4).
Because of interaction such as hydrogen bonding and the
special phenyl planar geometry, we postulate that the 1H NMR
singlet of water molecules or ILs coordinated with the Bi cata-
lyst should shift to high field. The change of 1H NMR chemical
shift that is related to methyl and methylene group linked to the
nitrogen atom of [Bmim]BF4 is consistent with our hypothesis,

R. H. QIU ET AL.
145
catalyst in the ILs. The water generated is absorbed by the hy-
drophilic ILs, inducing stronger polarity of ILs that is beneficial
for the facile separation process. With the consumption of re-
actants and the generation of apolar α,β-unsaturated ketones,
facile separation of products from the polar solution occurs. In
other words, the product can be transferred to the apolar or-
ganic phase directly and efficiently, breaking the equilibrium of
cross-condensation reaction in a controlled manner.
3. Conclusion
We have developed a facile separation catalyst system (composed
of [Bmim]BF4 and air-stable organobismuth tetrafluoroborate 1)
that is highly efficient (showing high catalytic activity, stereo-
selectivity, stability, and reusability) for the synthesis of (E)-
α,β-unsaturated ketones from aldehydes and ketones through
direct crossed-condensation.
Scheme 3. A plausible catalytic cycle for the crossed-condensation
reaction of ketones and aldehydes catalyzed by 3 in the presence of
n-PrNH2. 4. Acknowledgements
This work was financially supported by the NSFC (Grant Nos.
20973056, 21003040, 20873038 and E50725825), and the 863
project (2009AA05Z319). Prof. C.-T. Au (adjunct professor of
Hunan University) and Prof. W.-Y. Wong thank the Hong
Kong Baptist University for a Faculty Research Grant (FRG/
08-09/II-09). Prof. Yin thanks Dr. S. Shimada of AIST in Japan
for helpful advice.
REFERENCES
[1] S. Yin, J. Maruyama, T. Yamashita, S. Shimada, “Efficient fixa-
tion of carbon dioxide by hypervalent organobismuth oxide, hy-
droxide, and alkoxide,” Angew. Chem. –Int. Ed., vol. 47, pp.
6590-6593, 2008
[2] S.-F. Yin, S. Shimada, “Synthesis and structure of bismuth
compounds bearing a sulfur-bridged bis(phenolato) ligand and
their catalytic application to the solvent-free synthesis of pro-
pylene carbonate from CO2 and propylene oxide,” Chem. Com-
mun., pp. 1136-1138, 2009
Scheme 4 Proposed interaction of complex 3 and imidazolium
cationic ion [Bmim]+ in ionic liquids.
[3] R. Qiu, Y. Qiu, S. Yin, X. Song, Z. Meng, X. Xu, X. Zhang, S.
Luo, C.-T. Au, W.-Y. Wong, “Facile separation catalyst system:
direct diastereoselective synthesis of (E)-α, β-unsaturated ke-
tones catalyzed by an air-stable Lewis acidic/basic bifunctional
organobismuth complex in ionic liquids,” Green Chem., vol. 12,
pp.1767-1771, 2010.
[4] R. Qiu, Y. Qiu, S. Yin, S. Luo, C.-T. Au, W.-Y. Wong, S. Shi-
mada, “Highly Efficient and Selective Synthesis of (E)-α,
β-Unsaturated Ketones by Crossed Condensation of Ketones and
Aldehydes Catalyzed by an Air-Stable Cationic Organobismuth
Perfluorooctanesulfonate,” Adv. Synth. Catal., vol. 352, pp.
153-162, 2010
Figure 3. Interaction of catalyst 3 with [Bmim]BF4 in the catalyst
system. (a) The upper layer contains reactants benzaldehyde, cyclo-
hexanone, and propylamine while the lower layer ILs and catalyst 3.
(b) Homogeneous mixture during reaction. (c) Heterogeneous mix-
ture at completion of reaction; the upper layer is α,β-unsaturated
ketones and unconsumed reactants while the lower layer ILs, cata-
lyst 1, and water generated during reaction.
[5] R. Qiu, S. Yin, X. Song, Z. Meng, Y. Qiu, N. Tan, X. Xu, X.
Zhang, S. Luo, F.-R. Dai, C.-T. Au, W.-Y. Wong, “Effect of
butterfly-shaped sulfur-bridged ligand and counter anions on the
catalytic activity and diastereoselectivity of organobismuth com-
plexes,” Dalton Trans., vol. 40, pp. 9482-9489, 2011
implying that hydrogen bonding is apparent. With hydrogen
bond formation, there should be enhancement of electron-
withdrawing ability, consequently enhancing the diastereose-
lectivity of the reaction (Figure 4). Furthermore, the adduct
formed from the Bi complex and ILs leads to miscibility of the
[6] R. Qiu, S. Yin, X. Zhang, J. Xia, X. Xu, S. Luo, “Synthesis and
structure of an air-stable cationic organobismuth complex and its
use as a highly efficient catalyst for the direct diastereoselective
Mannich reaction in water,” Chem. Commun., pp. 4759-4761,
2009.