The metal tin promoted cascade reaction of ketones in aqueous media for the construction of 2-bromo-4-aryl-1,3-pentadiene

Article abstract of DOI:10.1039/c4ob00584h

A novel type of transformation was discovered serendipitously during the Barbier-type allenylation reaction of aromatic ketones promoted by the metal, tin, in aqueous media. Additionally, a series of new, highly functionalized 2-bromo-4-aryl-1,3-pentadienes could be obtained with good yields in this reaction. This cascade reaction shows the unique properties of the metal, tin. Furthermore, it is actually a cascade reaction which involves two steps: one is the Barbier-type allenylation of the carbonyl compound, and the other is an S_N2′ type addition-elimination reaction. Notably, this reaction has the advantages of simple, mild conditions and is easy to operate. Furthermore, the corresponding product could be applied to various coupling reactions or other diversified transformations. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob00584h

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The metal tin promoted cascade reaction of
ketones in aqueous media for the construction of
2-bromo-4-aryl-1,3-pentadiene†
Cite this: Org. Biomol. Chem., 2014,
12, 5393
Lingyan Liu,* Yan Zhang, Hua Zhang, Kaimeng Huang, Bo-xin Gao, Min Zou,
Xin Zhou, Hongkai Wang and Jing Li*
A novel type of transformation was discovered serendipitously during the Barbier-type allenylation reac-
tion of aromatic ketones promoted by the metal, tin, in aqueous media. Additionally, a series of new,
highly functionalized 2-bromo-4-aryl-1,3-pentadienes could be obtained with good yields in this reac-
tion. This cascade reaction shows the unique properties of the metal, tin. Furthermore, it is actually a
cascade reaction which involves two steps: one is the Barbier-type allenylation of the carbonyl com-
pound, and the other is an SN2’ type addition–elimination reaction. Notably, this reaction has the advan-
tages of simple, mild conditions and is easy to operate. Furthermore, the corresponding product could be
applied to various coupling reactions or other diversified transformations.
Received 20th March 2014,
Accepted 28th April 2014
DOI: 10.1039/c4ob00584h
www.rsc.org/obc
The Barbier-type propargylation of carbonyl compounds is a
hot research field in organic reaction methodology. To date,
there have been many successful reports on the propargylation
of aldehydes promoted by metal (Sn, Zn, In, Ga, Mn, Ba etc.).1
In comparison, studies on the Barbier-type allenylation of car-
bonyl compounds are very scant.2 This is because in the
Barbier-type reaction, homopropargylic alcohol is mainly
obtained in most cases, whereas the allenylic alcohol is rarely
generated. To the best of our knowledge, the allenic alcohol
can only be produced in the case of a reaction between the
ketone and 1-bromobut-2-yne.3 Additionally, only active metals
such as indium, barium or metal alloys and so on, can
promote this Barbier-type propargylation or allenylation reac-
tion of aromatic ketones.4 Moreover, the regioselectivity is
poor relative to that of aromatic aldehydes, due to the low reac-
tivity and steric hindrance of the ketones.5
In continuation of our work on the propargylation of alde-
hydes mediated by the metal, tin, for the synthesis of homo-
propargylic alcohols, which has been further used to construct
complicated compounds,6 we embarked upon the propargyla-
tion reaction of less active ketone carbonyl compounds, to
prepare the tertiary homopropargylic alcohol. As a result, we
obtained an unexpected new compound, 2-bromo-4-phenyl-
1,3-pentadiene (Scheme 1).
Scheme 1 A novel transformation mediated by the metal, tin.
Initially, our developed, optimal protocol for the Barbier-
type propargylation of aldehydes was directly employed to the
acetophenone substrate.6c However, unfortunately, no reaction
occurred (Table 1, entry 1). When changed to use the analo-
gous additives, SnBr2 and SnI2, similarly poor results were
obtained (entries
2 and 3). To activate the propargylic
bromide, NaI or CuI was added into the reaction systems,
respectively. However, the reaction was still sluggish (entries
4 and 5), with a great amount of starting material recovered. In
view of the increased hydrophobicity of acetophenone com-
pared to that of benzaldehyde in aqueous media, we attempted
to add the surfactant C12H25SO3Na, or phase transfer catalyst
TBAI (tetrabutyl ammonium iodide) to the reaction mixture.
Surprisingly, neither the expected homopropargylic alcohol, nor
allenic alcohol was obtained, but the reaction resulted in a new
unknown compound. Through employing a series of determi-
1
nation methods, such as H NMR, 13C NMR, DEPT (Distortion-
less Enhance by Polar Transfer) (θ = 90° and 135°), HMBC
(Heteronuclear Multiple-Bond Correlation), HMQC (Hetero-
nuclear Multiple-Quantum Coherence) and EI-MS (Fig. S1–S7†),
this new compound was finally defined as a conjugated diene
containing a bromide atom. Moreover, the elemental analysis
(EA) results further confirmed the exact structure of this new
compound (Anal. calcd for C11H11Br, C 59.22, H 4.97; found:
The State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai
University, Collaborative Innovation Center of Chemical Science and Engineering
(Tianjin), Wiejin Road 94#, Nankai District, Tianjin, 300071, China.
E-mail: lijing@nankai.edu.cn, liulingyan@nankai.edu.cn; Tel: +86-22-23501410
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4ob00584h
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Inspired by the above primary results, we tried to further
optimize the reaction conditions. The amounts of tin powder
and propargylic bromide were then investigated (Table 2). For
example, by using 1.0 or 1.5 equiv. of tin powder to acetophe-
none, the reaction yield was slightly decreased compared to
that obtained when using 0.75 equiv. Sn (Table 2, entries 1–3).
Additionally, when using 1.5 equiv. of tin powder, there was a
small amount of iodide 3a obtained as well as the conjugated
diene bromide 2a. When further increasing the amount of
metal tin to 3.0 equiv., no expected product was obtained
(entry 4). This might be because the excess tin powder led to a
nearly dry reaction system in the same volume of water, which
failed to stir and react with each other completely. In addition,
the product yield was reduced in the presence of 0.5 equiv. of
metal tin (entry 5). When the metal was changed from tin to
indium, almost equivalent amounts of homopropargylic
alcohol and allenic alcohol (1 : 1) were obtained, in a total iso-
lated yield of 26% (Table 2, entry 6). Then, on increasing the
amount of propargylic bromide from 3.0 to 5.0 equiv., the reac-
tion gave a decreased yield of the product (Table 2, entry 7).
When the amount of propargylic bromide was reduced to
1.5 equiv., not only was there an obvious decrease in the reac-
tion yield, but the competitive conjugated diene iodide 3a was
Table 1 The effect of the additive on the new cascade reaction of
acetophenone mediated by the metal, tina
Entry
Additive (equiv.)
Diene bromideb
1
2
SnCl2·2H2O (0.2)
SnBr2 (0.2)
—
—
3
4
5
6
7
8
SnI2 (0.2)
NaI (0.5)
CuI (0.5)
Trace
Trace
Trace
80
85
62
26
57
20
11
SnCl2·2H2O (0.2) + C12H25SO3Na (0.5)
SnCl2·2H2O (0.2) + TBAI (0.5)
SnCl2·2H2O (0.2) + TBAI (0.2)
SnBr2 (0.2) + TBAI (0.5)
ZnCl2 (0.2) + TBAI (0.5)
SnCl2·2H2O (0.2) + TBABr (0.5)
SnBr2 (0.2) + TBABr (0.5)
9c
10
11
12
a Procedure: In a tube, 0.5 mL H2O, tin powder (45 mg, 0.375 mmol,
0.75 equiv.), the additive, propargylic bromide (176 mg, 1.5 mmol, 3.0
equiv.) and acetophenone (60 mg, 0.5 mmol) were added sequentially.
The mixture was stirred at 30 °C for a certain time and traced by TLC.
b Isolated yield. c The product was a mixture of bromide (26% yield)
and iodide (9% yield).
C 58.90, H 5.37). Namely, a new compound, 2-bromo-4-phenyl-1,3- also generated (entry 8). When using 1.0 equiv. of propargylic
pentadiene, was produced in this novel Barbier-type allenylation bromide, only allenic alcohol 4a was obtained (entry 9). It was
of acetophenone, promoted by the metal, tin, in aqueous media. surmised that the small bromine source failed to perform the
Thus, it was calculated that this new compound was obtained in next addition–elimination reaction.
high yields of 80% or 85%, respectively (Table 1, entries 6 and 7).
Next, the solvent was also examined. As shown in Table 3,
Reducing the amount of TBAI from 0.5 equiv. to 0.2 equiv. or alter- on either increasing the water amount to 1 mL or 2 mL, or
ing the Lewis acid to SnBr2 or ZnCl2 dramatically decreased the decreasing it to 0.2 mL, the product was obtained in reduced
reaction yields (entries 8–10). This demonstrated that the reaction yields (entries 2–4). Moreover, the reaction yield decreased
was sensitive to the acidity of the micro-environment in the gradually with increased water. To improve the heterogeneous
system. When using TBABr, reaction yields were lower, whether in nature of the reaction system, we attempted to use a THF–H2O
the presence of SnCl2·2H2O or SnBr2 (Table 1, entries 11 and 12).
mixture as the solvent. Consequently, it was found that the
Table 2 The amounts of metal tin and propargylic bromide screened in this reaction
Entrya
The amount of tin (equiv.)
Product
Yieldc (%)
1
0.75
1.0
1.5
3.0
0.5
2a
2a
85
62
80
—
72
—
2
3e
4
2a + 3a (3 : 1)
—
2a
—
5
6d
In (0.75)
The amount of propargylic
bromide (equiv.)
Entryb
Product
Yieldc (%)
7
5.0
1.5
1.0
2a
76
66
30
8e
9
2a + 3a (2 : 1)
4a
a Entries 1–7: propargylic bromide 3.0 equiv. b Entries 8–10: tin 0.75 equiv. 4a was an allenyl alcohol product. c Isolated yield. d Almost equivalent
amounts of homopropargylic alcohol and allenic alcohol (1 : 1) were obtained in a total isolated yield of 26%. e The ratio of 2a/3a was determined
by the 1H NMR.
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Table 3 The effect of various solvents, or the amount of water, on the
reaction
Table 4 The scope of ketone substrates used in the reaction with the
optimal protocol a
Entry
1
R
Product
Yieldb (%)
85
Entry
Solvent (mL)
Product
Yielda (%)
—
1
2
3
4
5
6
7b
8
H2O (0.5)
H2O (1)
H2O (2)
H2O (0.2)
THF–H2O (1 : 1) (0.5)
THF (0.5)
THF (0.5)
2a
2a
2a
2a
2a
4a
2a
2a
85
77
64
79
27
43
—
58
2
3
4-F
67
58
Brine (0.5)
a Isolated yield (%). b Left standing overnight.
4-Br
reaction gave the product in a sharply decreased yield (Table 3,
entry 5, only 27%). Additionally, when pure THF solvent was
used, the allenic alcohol was mainly obtained (entry 6). Inter-
estingly, when we let the reaction mixture system stand in THF
overnight, part of the allenic alcohol compound was converted
to the 2-bromo-4-phenyl-1,3-pentadiene product (Fig. S8, entry
7†). Additionally, the brine was also tested. However, no
improved result was obtained (Table 3, entry 8). Therefore, the
optimal reaction conditions were as follows: 0.75 equiv. metal
tin, 3.0 equiv. propargylic bromide, 20 mol% SnCl2·2H2O and
50 mol% Bu4NI, 0.5 mL water, 30 °C.
4c
5
4-NO2
4-CH3
42
65
6
7
4-CH3O
71
54
The various carbonyl compounds were then employed to
demonstrate the efficiency and scope of the present method.
The results are summarized in Table 4. In most cases, for aro-
matic ketone derivatives, good or moderate yields were
achieved, while the chemical yields dramatically changed
depending on the substituent on the phenyl ring. Regardless
of the substituted position on the acetophenones and substi-
tution pattern, all reactions could proceed smoothly, except for
the 4-hydroxy and 2,4-dimethyl acetophenones (Table 4). Sur-
prisingly, no conjugated diene bromide was obtained, except
with the allenic alcohol, in 42% yield, when 4-nitroacetophe-
none was used in this reaction (Table 4, entry 4).14 The reason
for this remains unclear at present. In addition, a small
amount of allenic alcohol was also obtained as well as the con-
jugated diene product, in the example of 3-bromoacetophe-
none (Table 4, entry 9). These two cases indicated that the
aromatic ketone firstly conducted a Barbier-type allenylation
in our methodology. Moreover, the reaction between pro-
pargylic bromide and ketones which were unsubstituted, or
substituted with an electron-donating group, could afford the
conjugated diene bromide products in better yields (entries 1,
5–7, 12 and 13) than those obtained with acetophenones sub-
stituted with electron-withdrawing groups (entries 2, 3 and
9–11). Nevertheless, for 4-hydroxy or 2,4-dimethyl acetophe-
none, the reaction could hardly run, and a large amount of
starting material was recovered (Table 4, entries 8 and 14).
This may be due to the hydrogen bond, or slight pH effect
from the hydroxy group, and the relative steric hindrance from
4-PhCH2O
8
9
4-OH
3-Br
—
—
60
10
11
12
3-CF3
2-Cl
48
55
71
2-CH3O
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homopropargylic alcohol is mainly obtained for the highly
active aromatic aldehydes in the Barbier-type propargylation or
allenylation reaction. Additionally, aliphatic aldehydes or
ketones were hitherto untouched in this type of reaction. In
addition, it should be noted that the (E)-configuration diene
products were mainly obtained, with small amounts of the
(Z)-isomers, in all of the above reactions.
Table 4 (Contd.)
Entry
13
R
Product
Yieldb (%)
69
2,4-(CH3O)2
With respect to this novel Barbier-type allenylation reaction,
we paid more attention to investigating the reaction mechan-
ism. A 1H NMR trace method was employed. Propargylic
bromide showed two peaks a and b in Fig. S8(1)† in D2O
(2.92 ppm, CH; 3.99 ppm, CH2). After tin powder and
SnCl2·2H2O were added sequentially to the D2O solution, the
peaks of propargylic bromide showed no obvious changes,
with only very small new allenic peaks appearing (c and d)
(Fig. S8(2) and (3)†). With continued addition of TBAI to the
above mixture, it was observed that the peaks of propargylic
bromide almost disappeared, and new characteristic peaks for
the allenic tin intermediate appeared at 4.61 ppm and
5.24 ppm (Fig. S8(4)† c″, d″),7 which are consistent with the
spectrum of allenyltin trichloride, prepared in our group by
the exchange reaction of tributyl allenyltin and tin tetrachlor-
ide (Fig. S9,† CH2, 5.14–5.16 ppm; CH, 5.65–5.68 ppm). This
phenomenon demonstrated that the TBAI accelerated the for-
mation of the allenyltin intermediate by combining with the
Lewis acid SnCl2·2H2O to form the adduct Bu4N+SnCl2I−,
which displays stronger nucleophilicity than SnCl2·2H2O.2a
Ma et al. and other research groups have ever reported an
unexpected product (1,3-diene-3-bromide) in the reaction of
allenic alcohol and hydroquinone. The reaction is believed to
14
15
2,4-(CH3)2
—
—
3,4,5-(CH3O)3
50
16d
4-CH3O–C6H4COCH2CH3
62
17e
18
PhCHO
35
CH3COCH3
—
—
a Procedure: In a tube, 0.5 mL H2O, tin powder (45 mg, 0.375 mmol,
0.75 equiv.), SnCl2·2H2O (23 mg, 0.1 mmol, 0.2 equiv.), nBu4NI (92 mg,
0.25 mmol, 0.5 equiv.), propargylic bromide (176 mg, 1.5 mmol, 3.0
equiv.) and the aromatic ketone (0.5 mmol) were added sequentially.
The mixture was then stirred at 30 °C for 6 h. After completion, proceed through an SN2′ type addition–elimination process.8
saturated aqueous sodium bicarbonate was added to quench the
Therefore, it was inferred that the reaction herein may firstly
reaction, followed by extraction with ethyl acetate (15 mL × 3) three
times. This was combined with the organic phase, and washed with
result in an allenic alcohol, or similar structural intermediate,
sequential brine (5 mL) and water (5 mL), then dried by anhydrous and then afford the 1,3-diene-3-bromide via a similar reaction
MgSO4, filtered and concentrated to obtain the residue, which was
process.
purified by silica gel chromatography, finally affording the pure
product. b Isolated yield. c Allenic alcohol. d The substrate is propiophenone.
In order to verify our reaction process, we performed an
e The substrate is benzaldehyde, the major product is homopropargylic additional four reactions using the pure allenic alcohol or pre-
alcohol in 60% yield, the side product is 2-bromo-4-phenyl-1,3-
synthesized homopropargylic alcohol, in our reaction system,
pentadiene in 35% yield.
and in the presence of LiBr/HOAc, respectively (Scheme 2). For
the allenic tert-alcohol, the reaction proceeded smoothly under
the 2-methyl group, respectively. However, interestingly, this
reaction could afford the corresponding 2-bromo-1,3-penta-
diene in moderate yields in the cases of 2-methoxy and 2,4-
dimethoxy acetophenone, even for the multi-substituted aceto-
phenone, 3,4,5-(CH3O)3C6H2COCH3 (Table 4, entries 12, 13
and 15). The reason for this remains unclear at present. For
4-methoxy propiophenone, a good yield of diene product
could be obtained (entry 16). Additionally, this reaction
resulted in the major homopropargylic product (yield 60%)
with a small amount of the 2-bromo-4-phenyl-1,3-pentadiene
in 35% yield, when benzaldehyde was used as the substrate
(entry 17). For aliphatic ketones, such as acetone, the reaction
hardly ran (entry 18). From these results we could see that aro-
matic ketones were the most applicable substrates in this
cascade reaction, whereas aromatic aldehydes or aliphatic
ketones had poor performance. This may be because the
two different conditions, and afforded the corresponding
2-bromo-4-phenyl-1,3-pentadiene product in good and high
yields (Fig. S11,† Scheme 2 (1)). In contrast, no reaction
Scheme 2 The SN2’ type addition–elimination reaction.
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occurred in the case of homopropargylic alcohol, no matter verted to an unstable, but more active and kinetically favored
what conditions were used (Scheme 2 (2)). propargyltin reagent 6, followed by subsequent allenylation
In the Barbier-type propargylation reaction of carbonyl com- and the addition–elimination cascade reaction to give the final
pounds mediated by metal, the metal firstly reacts with the conjugated diene 2. In addition, the α-carbon of the propargyl-
propargylic halide to form the allenyl organometallic or pro- tin intermediate can freely rotate, but the α-carbon of allenyl-
pargylic metal reagent in situ. The allenyl metal reagent is a tin cannot. Furthermore, the methyl group of the aromatic
thermo-dynamically controlled intermediate, whereas the pro- ketone may cause a repulsion when attacking the γ-carbon of
pargylic metal reagent belongs to be the kinetic controlled the allenyltin with two hydrogen atoms. Therefore, the steric
intermediate.9 These two organometallic reagents could inter- hindrance has an important influence on this reaction.
convert, depending on the steric hindrance, substitution of the
Based on the reported literature, in the presence of the
organometallic substrate, solvation and nature of the metal. In metal indium, in general the homopropargylic alcohol is
addition, the chemo-selectivity of Barbier-type propargylation mainly obtained in most Barbier-type propargylation reactions
or allenylation of carbonyl compounds is generally higher for of carbonyl compounds, whether aromatic aldehydes or aro-
aldehydes with a sole homopropargylic alcohol. However, for matic ketones. This is because the organometallic indium
the inert ketone, both the homopropargylic alcohol and reagent formed in situ is so highly active that it is able to react
allenic alcohol could be obtained at the same time in most with both aldehydes and ketones.1–5 In comparison, for the
cases.1,4,5 Based upon that, we proposed a possible mechan- metal tin, the case is different for the aldehyde and ketone
ism for this cascade transformation (Scheme 3).
(Scheme 4). In our reaction system, in the absence of TBAI,
In our protocol, the allenyltin intermediate was observed by only the aldehyde could smoothly undergo the Barbier-type
1H NMR. In this way, the homopropargylic alcohol should be propargylation reaction with a high yield of homopropargylic
mainly produced. Nevertheless, only the allenic alcohol could alcohol and a trace of the conjugated diene. For the aromatic
result in the consecutive addition–elimination under the reac- ketone, no reaction occurred. However, when TBAI was added
tion conditions. This apparent contradiction could be into the reaction mixture, the reactions of the aldehyde and
explained by the equilibrium between the allenyltin (5) and the ketone exhibited both similarities and differences. In more
propargyltin reagents (6) under the reaction conditions.10 In detail, the similarity is that the same structural conjugated
the case of benzaldehyde, due to its high reactivity, the alde- dienes can be obtained in two cases, for benzaldehyde and
hyde can quickly attack the allenyltin intermediate 5 via acetophenone. This may be due to the relatively strong acidic
γ-addition to produce a large amount of homopropargylic conditions of the Sn-promoted Barbier-type propargylation
alcohol 8, as well as the minor conjugated diene 9 via the inter- (pH = 1–2), which thus leads to the subsequent SN2′-type
mediate B. However, for the inert acetophenone, which has a addition–elimination from the allenic alcohol generated. The
certain amount of steric hinderance, the major reaction difference is that in the case of the aldehyde, the major homo-
pathway may be dependent on the stability of the transition propargylic alcohol is obtained, but the diene is the single
state under kinetic control, or on the orientation and steric product for the acetophenone. In addition, when the metal tin
factors (Scheme 3). Namely, the stable allenyltin 5 was con- was replaced with indium in our protocol, the reaction pro-
duced the homopropargylic alcohol and allenic alcohol in a
1 : 1 ratio with a total isolated yield of 26%. All of these results
demonstrated that the IV main group metal tin displays
unique properties which are different from the metal indium.
Scheme 4 Possible reactions of carbonyl compounds promoted by
Scheme 3 The possible mechanism for this reaction.
different metals.
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but also as an auxiliary to accelerate the formation of the alle-
nyltin intermediate. Meanwhile, the corresponding product
2-bromo-4-phenyl-1,3-pentadiene could be further applied to
conduct various possible reactions for the synthesis of func-
tional compounds. Further studies on this new process cata-
lyzed by the metal, tin, and its application for the synthesis of
natural products are under way.
Scheme 5 Possible coupling reaction of 2-bromo-4-phenyl-1,3-penta-
diene. The reaction conditions were as follows: (i) Pd(OAc)2 (2.5 mol%),
In (1.0 equiv.), LiCl (3.0 equiv.), DMF, 100 °C, 2 h; (ii) Pd(PPh3)4 (5 mol%),
Cs2CO3 (3.0 equiv.), toluene–H2O (3 : 1), 80 °C, 6 h; (iii) TBAF·3H2O
(5.0 equiv.), THF, 60 °C, 26 h.
Acknowledgements
We are grateful for financial support from the National Natural
Science Foundation of China (21202084 and 21372120) and
the Tianjin Natural Science Foundation of China
(12JCQNJC03500) for this work.
Next, the synthetic utility of the corresponding product 1,3-
pentadiene was examined (Scheme 5). In view of its structural
features, for example, containing the double bond, bromine
and phenyl, 2-bromo-4-phenyl-1,3-pentadiene was firstly
used to attempt to perform the intramolecular Heck reaction,
catalyzed by the transition-metal Pd. Unfortunately, no cycliza-
tion product was obtained. On the contrary, a new, highly
cross-conjugated polyene derivative ([4]dendralene) was
generated via an intermolecular homo-coupling reaction in
77% yield.11 Simultaneously, a small amount of oligomers,
which were bright fluorescent yellow, was observed on the thin
layer chromatography (TLC) plate. Apart from dimerization,
this diene could also undergo Suzuki cross-coupling with
p-MeO-C6H4B(OH)2.12 Interestingly, it was easy to transform
this conjugated diene bromide to 1,3-enyne with high yield
(88%) in the presence of Bu4NF (TBAF).13 Research to obtain
further insight on the cascade cyclization or other possible
reactions of this conjugated diene are currently in progress in
our group.
Notes and references
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Conclusions
In summary, a novel Barbier-type allenylation reaction of aro-
matic ketones was developed, and a series of new multi-substi-
tuted 1,3-diene derivatives were afforded in good to moderate
yields. Employing the metal tin, and the environmentally
benign water as media, the ketone carbonyl compounds could
react with propargyl bromide to yield 2-bromo-4-aryl-1,3-penta-
diene in the presence of TBAI in one pot. This reaction avoids
separation of the intermediate, and the application of expens-
ive and complicated catalysts, such as transition metal com-
plexes, in the synthesis of conjugated dienes. This reaction has
a wide scope and applicability of the substrates. In addition,
the mechanism was also explored. It was concluded that the
metal tin possesses a distinct nature which is different from
the other main group metals (Zn or In). In addition, the differ-
ence in activity between aromatic aldehydes and ketones was
also well illustrated in our protocol. Moreover, TBAI plays an
important role in this reaction. It acts not only as a phase
transfer catalyst to improve the heterogeneous reaction system,
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