One-pot reductive amination of aldehydes catalyzed by a hydrio-iridium(III) complex in aqueous medium

Article abstract of DOI:10.1016/j.tet.2007.11.079

A combination of sodium tetrakis[3,5-di(trifluoromethyl)phenyl]borate [NaBAr^F₄] and hydrio-iridium(III) complex efficiently catalyzed the one-pot reductive amination of aldehydes with various amines and ammonia in water under mild conditions in good to excellent yields.

Full text of DOI:10.1016/j.tet.2007.11.079

Available online at www.sciencedirect.com
Tetrahedron 64 (2008) 1213e1217
www.elsevier.com/locate/tet
One-pot reductive amination of aldehydes catalyzed
by a hydrio-iridium(III) complex in aqueous medium
*
Rung-Yi Lai, Chun-I Lee, Shiuh-Tzung Liu
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
Received 27 August 2007; received in revised form 21 November 2007; accepted 23 November 2007
Abstract
A combination of sodium tetrakis[3,5-di(trifluoromethyl)phenyl]borate [NaBArF4] and hydrio-iridium(III) complex efficiently catalyzed the
one-pot reductive amination of aldehydes with various amines and ammonia in water under mild conditions in good to excellent yields.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Reductive amination; Silane; Carbonyl; Iridium
1. Introduction
as NaBArF4) and a hydrio-iridium(III) complex 1 as a catalytic
system, we found that experiments on the reductive amination
of aldehydes can be carried out in aqueous medium with high
yields by using the silane as the reducing agent.
For the chemical transformations, one-pot reductive amina-
tion of aldehydes and ketones is an important methodology in
conversion of carbonyl compounds into the higher alkylated
amines.1 The success of this ‘one-pot’ fashion relies on the
acid-catalyzed formation of intermediate imines and the pref-
erential reduction of C]N. The two commonly used reduction
processes are based on either hydride reduction or catalytically
hydrogenation. Several reagents, which effect reductive ami-
nation have been recently developed.2 However, most of these
reactions require carrying out in organic or aqua mixed sol-
vents, which does not meet the context of green chemistry.
Few works concerning this context were reported.3 Therefore,
a more efficient method to carry out this transformation in
aqueous medium is still required.
In our previous works, we have found that sodium tetra-
kis[3,5-di(trifluoromethyl)phenyl]borate is able to show the
acidic property on the catalysis of Mannich reaction in aque-
ous medium.4 In addition, imine functionality can be success-
fully reduced by silanes in the presence of the iridium(III)
complex 1 as the catalyst.5 By using the combination of so-
dium tetrakis[3,5-di(trifluoromethyl)phenyl]borate (denoted
Ph
Ph
Cl
CO
P
Ir
H
N
1
2. Results and discussion
The reaction of benzaldehyde with aniline served as a model
reaction (Eq. 1) for studying the influence of catalysts, addi-
tives, and critical reaction parameters such as solvent and tem-
perature. Results are summarized in Table 1.
When a mixture of benzaldehyde (5 mmol), aniline (5 mmol),
triethylsilane (10 mmol), NaBArF4 (0.01 mmol), and
1
(0.005 mmol) in water (2.0 mL) under air atmosphere was heated
at 50 ꢀC for 1 h, the amine 2 was obtained quantitatively (entry
1). Running the reaction at room temperature took 2 h for the
completion (entry 2). The catalytic activities of other iridium
complexes in this reaction were also examined. [Ir(COD)Cl]2
proved a moderate catalytic activity affording the desired amine
* Corresponding author. Tel.: þ886 2 2366 0352; fax: þ886 2 2363 6359.
E-mail address: stliu@ntu.edu.tw (S.-T. Liu).
0040-4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.11.079

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R.-Y. Lai et al. / Tetrahedron 64 (2008) 1213e1217
Table 1
Reductive amination of benzaldehyde with anilinea
Entry
Complex (mol %)
Additives (mol %)
Reducing agent
Solvent
t (h)
Temp (ꢀC)
Product distributionb (%)
2
3
4
1
1 (0.1)
1 (0.1)
NaBArF4 (0.2)
NaBArF4 (0.2)
NaBArF4 (0.15)
NaBArF4 (0.15)
NaBArF4 (0.1)
NaBArF4 (0.1)
d
NaBArF4 (0.2)
NaBArF4 (0.2)
NaBArF4 (0.2)
NaBPh4 (0.2)
NaBF4 (0.2)
NaPF6 (0.2)
NaBArF4 (0.2)
NaBArF4 (0.2)
Et3SiH
Et3SiH
Et3SiH
Et3SiH
H2
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
MeOH
THF
1
2
50
rt
100
100
50
d
27
d
60
20
5
d
d
d
d
d
d
6
2
3
[Ir(COD)Cl]2 (0.05)
[Ir(COD)Cl]2 (0.05)c
[Ir(COD)Cl]2 (0.05)
1 (0.1)
16
12
16
16
16
24
1
50
50
50
50
50
50
50
50
50
50
50
50
50
50
4
100
67
100
40
80
75
61
66
25
30
d
38
5
6
H2
d
d
7
1 (0.1)
d
Et3SiH
Et3SiH
NaBH4
Silaned
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
8
9
1 (0.1)
1 (0.1)
20
d
d
d
d
10
5
10
11
12
13
14
15
a
1
39
34
75
70
90
57
1 (0.1)
1 (0.1)
24
6
1 (0.1)
1 (0.1)
1 (0.1)
6
1
1
Reaction conditions: benzaldehyde (5 mmol), aniline (5 mmol), Et3SiH (10 mmol) in solvent (2 mL).
NMR yield.
b
c
d
Addition of 2-(diphenylphosphino)aniline ligand.
Et3SiH was replaced by polymethylhydrosiloxane.
CHO
+
NH2
CH2OH
HN
N
+
+
ð1Þ
2
3
4
Table 2
Reductive amination of carbonyl compounds in aqueous mediuma
product in 50% yield (entry 3). However, a mixture of 2-(diphe-
nylphosphino)aniline and [Ir(COD)Cl]2 did not show any
catalytic activity on the reduction of the imine 3. As for the re-
ducing agent, triethylsilane appears to be the best choice. The
use of hydrogen gas or sodium borohydride as the reducing agent
provided a less satisfactory result as indicated by the lower yields
and the formation of the direct reduction of carbonyl compounds
(entry 9). This observation is consistent with the known proce-
dure for the reduction of imines catalyzed by other iridium
complexes.5,6
It is of interest to note that the tetrakis[3,5-di(trifluorome-
thyl)phenyl]borate plays an important role in this catalysis. It
appears that the sodium salt of tetrakis[3,5-di(trifluoromethyl)-
phenyl]borate as the additive gave the best result. On the other
hand, other salts such as NaBF4 or NaPF6 only showed moder-
ate activities even for a longer reaction period. In the previous
work, we learned that this borate anion can act as a surfactant
and stabilize the metal complex.5,7 In a separate experiment,
we also found that the condensation of benzaldehyde and ani-
line in presence of NaBArF4 to yield the corresponding imine
was completed within 15 min. This might explain why the reac-
tion is able to carry out in a water solution. Therefore the influ-
ence of solvents on this catalysis is also investigated. Running
reaction in THF only offers 57% of the desired product or
90% in methanol (entries 14 and 15, Table 1).
R2
R2
R1-CHO +
R1
N
N
H
R3
R3
Entry
R1CHO
R2R3NH
Yieldb (%)
1
C6H5CHO
C6H5CHO
C6H5CHO
C6H5CHO
C6H5CHO
C6H5NH2
92
89
90
70
86
94
90
92
41d
0c
2
p-MeOC6H4NH2
p-ClC6H4NH2
o-MeC6H4NH2
3,5-Me2C6H3NH2
C6H5NH2
3
4
5
6
p-MeOC6H4CHO
p-ClC6H4CHO
o-MeC6H4CHO
7
C6H5NH2
C6H5NH2
8
9
2-Pyridinecarbaldehyde
1-Naphthylenecarbaldehyde
6-Methoxy-2-naphthylene-
carbaldehyde
C6H5NH2
C6H5NH2
C6H5NH2
10
11
90
12
13
14
15
16
17
18
19
20
a
C6H5CHO
C6H5CHO
C6H5CHO
C6H5CH2NH2
n-C6H13NH2
(C6H5)2NH
C6H5NH2
81
70
60d
70
C6H5CH]CHCHO
o-MeC6H5CHO
p-MeOC6H4CHO
p-ClC6H4CHO
C6H5CHO
C7H15CHO
(C6H5)2NH
(C6H5)2NH
(C6H5)2NH
Piperidine
55d
46d
49d
25d
56d
C6H5NH2
Reaction conditions: carbonyl compound (5 mmol), amine (5 mmol), 1
(0.005 mmol), NaBArF4 (0.01 mmol), and Et3SiH (10 mmol) in H2O (2 mL)
at 50 ꢀC.
In view of the above results, the catalytic system generated
from 1 and NaBArF4 in water rendered the best yield in reduc-
tive amination of benzaldehyde and was followed in the
subsequent studies. Various aldehydes and amines were tested
under this reaction condition and the results are summarized in
b
Isolated yield.
The corresponding imine was isolated.
c
d
All the aldehydes were converted into the corresponding imine as indicated
by the 1H NMR studies.

R.-Y. Lai et al. / Tetrahedron 64 (2008) 1213e1217
1215
Table 2. Substituted anilines when reacted with benzaldehyde
afforded the corresponding N-benzyl derivatives in excellent
yields except o-toluidine (entries 1e5). Similarly, reactions
of a range of substituted benzaldehydes with aniline proceeded
smoothly, indicating that the substituents have less effect on
this catalysis. Under the reductive conditions, reaction of
1-naphthylenecarbaldehyde with aniline only provided the for-
mation of the corresponding imine, showing that the reduction
step did not occur (entry 10). On the other hand, the substituted
2-naphthylenecarbaldehyde gave the desired amine in 90%
yield. These results clearly demonstrate that the catalyst is quite
sensitive toward the steric environment of the imine substrates.
To screen the overall compatibility of this protocol, the di-
rect reductive amination of benzaldehydes with secondary
amines was also studied. However, the desired amine products
were obtained in moderate yields (entries 16e19). Further-
more, the ketone substrates were investigated. Under the
same conditions, a mixture of cyclohexylbenzylamine (48%
yield) and cyclohexanol (32% yield) were obtained by using
cyclohexanone and aniline as the substrates. In the case of ace-
tophenone, the formation of 1-phenylethanol by the reduction
of the ketone is found to be the major pathway of the catalysis.
All of these observations suggest that the catalyst is quite sen-
sitive to the steric bulkiness of the substrates. Thus, a mixture
of benzaldehyde (5 mmol) and cyclohexanone (5 mmol) re-
acted with aniline (5 mmol) in the presence of Et3SiH
(7.5 mmol) to give the N-benzylaniline as the only product,
and the cyclohexanone remained intact. This outcome clearly
shows the selectivity of this method.
The mechanistic pathway of the reductive amination cata-
lyzed by 1/NaBArF4 is shown in Scheme 1. Basically, this path-
way is quite similar to that reported by Messerle and
co-workers in the reduction of imine.8 Accordingly, the cata-
lytic pathway leading to the amine products via the iridiu-
m(III) complex involves the generation of a vacant site for
the coordination of an imine substrate followed by the reduc-
tion of the imino group to the coordinating amineesilane
product, which was subsequently hydrolyzed to yield the final
product. In addition, the borate salt also assisted the imine
formation and stabilization in aqueous medium.
[Ir] H
RCHO + R'NH2
1
H
N
Cl
NaBArF
4
R
R'
H2O
SiEt3
H
R
RCH=NR'
[Ir]
H
N
[Ir] H
R'
H
RCH=NR'
Et3Si [Ir] H
Et3SiH
N
R
R'
H
Scheme 1. Pathway for catalysis.
In summary, a new and chemoselective method for the
reductive amination of aldehydes in water under mild reaction
conditions has been developed in this work. The advantages of
this method are the general applicability with aldehydes in
high yields, the observed selectivity, and a very mild reaction
conditions such as 0.1 mol % of catalyst in water at 50 ꢀC.
Since the reaction can be carried out in water, we envi-
sioned that the use of aqueous ammonia for the reductive ami-
nation of the carbonyl compounds, might offer a method to
prepare primary amines. Results are summarized in Table 3.
In all cases, the major product from each reaction is the direct
reduction of the carbonyl function. For example, reaction of
benzaldehyde with ammonia followed by the reduction gave
60% yield of the benzyl alcohol, with only 40% of the desired
amine product.
3. Experimental
3.1. General
Nuclear magnetic resonance spectra were recorded in
CDCl3 or acetone-d6 on a Bruker AVANCE 400 spectrometer.
Chemical shifts are given in parts per million relative to Me4Si
1
for H. Infrared spectra were measured on a Nicolet Magna-
Table 3
Reductive amination of carbonyl compounds in aqueous mediuma
IR 550 spectrometer (Series-II) as KBr pellets, unless
otherwise noted. Chemicals and solvents were of analytical
grade and used as received unless otherwise stated. The irid-
ium complex 1 was prepared according to the procedure
reported.5
CHO
OH
NH2
+
R
R
R
Entry
R
Product yieldc (%)
Benzylamine
3.2. Catalysis: general procedure
Benzyl alcohol
1
2b
3
4
5
6
a
H
40
38
20
18
16
0
60
62
A mixture of 0.1 mol % iridium complex and 0.21 mol %
NaBArF4 was placed in flask under air atmosphere. Then, a mix-
ture of aldehyde (5 mol) and amine (5 mol) in water (2 mL)
was added to the above iridium complex. The mixture was
stirred at room temperature for 10 min and Et3SiH (10 mol)
was added into the solution. The resulting mixture was heated
and kept at 50 ꢀC by an oil bath. The reaction was then mon-
H
4-Cl
4-F
80
82
2-OMe
2-Me
84
100
Reaction conditions: carbonyl compound (5 mmol), 30% ammonia solu-
tion (5 mL), 1 (0.005 mol), NaBArF4 (0.01 mmol), and Et3SiH (10 mmol) in
H2O (2 mL) at rt.
1
b
At 50 ꢀC.
itored by H NMR. After completion of the reaction, brine
(3 mL) and CH2Cl2 (5 mL) were added. The organic layer
c
NMR yield with the use of anisole as the internal standard.

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R.-Y. Lai et al. / Tetrahedron 64 (2008) 1213e1217
was separated and the aqueous layer was extracted with
CH2Cl2 (5 mLꢁ2). The combined organic extracts were dried
over magnesium sulfate and concentrated. The residue was
chromatographed on the silica gel with the elution of a mixture
of hexane and ethyl acetate. Products obtained in this work
were characterized by spectral methods particularly with H
and 13C NMR, and the data were consistent with those
reported.
3.2.8. N-(o-Methylphenyl)benzylamine15
1H NMR (400 MHz, CDCl3): d 7.40e7.28 (m, 5H, phenyl),
7.08 (m, 2H, phenyl), 6.68 (m, 1H, phenyl), 6.63 (m, 1H, phenyl),
4.37 (s, 2H, eCH2NHe), 3.95 (br, 1H, eNHe), 2.17 (s,
3H, eMe). 13C NMR (100 MHz, CDCl3): d 145.9, 139.3,
130.1, 128.6, 127.6, 127.2, 127.1, 122.0, 117.3, 110.1, 48.3, 17.5.
1
3.2.9. N-(3,5-Dimethylphenyl)benzylamine16
1H NMR (400 MHz, CDCl3): d 7.37e7.31 (m, 4H, phenyl),
7.29e7.26 (m, 1H, phenyl), 6.39 (s, 1H, phenyl), 6.29 (s, 2H,
phenyl), 4.30 (s, 2H, eCH2NHe), 4.10 (br, 1H, eNHe), 2.25
(s, 6H, eMe). 13C NMR (100 MHz, CDCl3): d 148.1, 139.5,
138.9, 128.6, 127.6, 127.2, 119.7, 110.9, 48.5, 21.5.
3.2.1. N-Phenylbenzylamine
1H NMR (400 MHz, CDCl3): d 7.39e7.26 (m, 5H, phenyl),
7.19 (t, J¼8 Hz, 2H, phenyl), 6.78 (t, J¼7 Hz, 1H, phenyl),
6.67 (d, J¼8 Hz, 2H, phenyl), 4.35 (s, 2H, eCH2NHe), 4.06
(br, 1H, eNHe). These data are similar to those reported.8
3.2.10. (6-Methoxynaphthalen-2-ylmethyl)phenylamine
1H NMR (400 MHz, CDCl3): d 7.70 (m, 3H, phenyl), 7.44
(dd, J¼4, 8 Hz, 1H, phenyl), 7.19e7.13 (m, 3H, phenyl), 7.12
(s, 1H, phenyl), 6.72 (t, J¼8 Hz, 1H, phenyl), 6.67 (d, J¼8 Hz,
2H, phenyl), 4.44 (s, 2H, eCH2NHe), 4.20 (br, 1H, eNHe),
3.91 (s, 3H, eOMe). 13C NMR (100 MHz, CDCl3): d 157.6,
148.0, 134.4, 133.8, 129.3, 129.2, 128.9, 127.2, 126.4,
126.0, 118.9, 117.7, 113.0, 105.6, 55.3, 48.5. Anal. Calcd
for C18H17NO: C, 82.10; H, 6.51; N, 5.32. Found: C, 81.90;
H, 6.54; N, 5.05.
3.2.2. N-(p-Methylphenyl)benzylamine9
1H NMR (400 MHz, CDCl3): d 7.37e7.30 (m, 4H, phenyl),
7.27 (m, 1H, phenyl), 6.97 (d, J¼8 Hz, 2H, phenyl), 6.56 (d,
J¼8 Hz, 2H, phenyl), 4.30 (s, 2H, eCH2NHe), 4.11 (br,
1H, eNHe), 2.23 (s, 3H, eMe).
3.2.3. N-Phenyl-(p-chlorobenzyl)amine10
1H NMR (400 MHz, CDCl3): d 7.29 (s, 4H, phenyl), 7.16
(t, J¼8 Hz, 2H, phenyl), 6.72 (d, J¼8 Hz, 1H, phenyl), 6.60
(d, J¼8 Hz, 2H, phenyl), 4.30 (s, 2H, eCH2NHe), 4.15 (br,
1H, eNHe). 13C NMR (100 MHz, CDCl3): d 147.7, 137.9,
132.8, 129.3, 128.7, 128.6, 117.8, 112.9, 47.6.
3.2.11. Dibenzylamine14
1H NMR (400 MHz, CDCl3): d 7.36e7.30 (m, 8H, phenyl),
7.26e7.23 (m, 2H, phenyl), 3.80 (s, 4H, eCH2NHe). 13C
NMR (100 MHz, CDCl3): d 140.1, 128.4, 128.2, 127.0, 53.4.
3.2.4. N-(p-Chlorophenyl)benzylamine11
1H NMR (400 MHz, CDCl3): d 7.34e7.25 (m, 5H, phenyl),
7.10 (d, J¼8 Hz, 2H, phenyl), 6.54 (d, J¼8 Hz, 1H, phenyl),
4.29 (s, 2H, eCH2NHe), 4.19 (br, 1H, eNHe).
3.2.12. Hexylbenzylamine15
1H NMR (400 MHz, CDCl3): d 7.34e7.20 (m, 5H, phenyl),
3.77 (s, 2H, eCH2NHe), 2.60 (t, J¼8 Hz, 2H), 1.50e1.45 (m,
2H), 1.32e1.24 (m, 6H), 0.86 (t, J¼7.2 Hz, 3H). 13C NMR
(100 MHz, CDCl3): d 140.6, 128.4, 128.1, 126.8, 54.1, 49.5,
31.8, 30.1, 27.0, 22.6, 14.1.
3.2.5. N-Phenyl-(p-methoxybenzyl)amine12
1H NMR (400 MHz, CDCl3): d 7.28 (d, J¼8 Hz, 2H, phe-
nyl), 7.17 (t, J¼8 Hz, 2H, phenyl), 6.87 (d, J¼8 Hz, 2H, phe-
nyl), 6.63 (d, J¼8 Hz, 2H, phenyl), 6.17 (t, J¼8 Hz, 1H,
phenyl), 4.24 (s, 2H, eCH2NHe), 4.19 (br, 1H, eNHe),
3.79 (s, 3H, eOMe). 13C NMR (100 MHz, CDCl3): d 158.8,
148.0, 131.2, 129.2, 128.8, 117.6, 114.0, 112.9, 55.3, 47.9.
3.2.13. N,N-Diphenylbenzylamine17
1H NMR (400 MHz, CDCl3): d 7.38e7.22 (m, 9H, phenyl),
7.10e7.07 (m, 4H, phenyl), 6.97e6.93 (m, 2H, phenyl), 5.02
(s, 2H, eCH2Ne). 13C NMR (100 MHz, CDCl3): d 148.1,
139.2, 129.2, 128.5, 126.7, 126.5, 121.3, 120.6, 56.3.
3.2.6. N-Phenyl-(o-methylbenzyl)amine13
1H NMR (400 MHz, CDCl3): d 7.33 (m, 1H, phenyl), 7.22e
7.17 (m, 5H, phenyl), 6.73 (t, J¼8 Hz, 2H, phenyl), 6.64 (d,
J¼8 Hz, 1H, phenyl), 4.27 (s, 2H, eCH2NHe), 3.97 (br, 1H,
eNHe), 2.38 (s, 3H, eMe). 13C NMR (100 MHz, CDCl3):
d 148.1, 136.9, 136.3, 130.4, 129.6, 128.2, 127.4, 126.1,
117.5, 112.7, 46.4, 18.9.
3.2.14. (2-Methylbenzyl)diphenylamine17
1H NMR (400 MHz, CDCl3): d 7.38 (m, 1H, phenyl), 7.24
(t, J¼7.6 Hz, 4H, phenyl), 7.10e7.19 (m, 3H, phenyl), 7.06
(m, 4H, phenyl), 6.94 (t, J¼7.6 Hz, 2H, phenyl), 4.90 (s, 2H,
eCH2Ne), 2.31 (s, 3H, eMe). 13C NMR (100 MHz, CDCl3):
d 147.8, 136.2, 134.7, 130.0, 129.1, 126.4, 126.1, 126.0,
121.2, 120.4, 54.4, 19.1. Anal. Calcd for C20H19N: C, 87.87;
H, 7.01; N, 5.12. Found: C, 87.77; H, 7.13; N, 4.94.
3.2.7. N-Phenyl-(20-pyridinemethyl)amine14
1H NMR (400 MHz, CDCl3): d 8.57 (d, J¼4 Hz, 1H,
phenyl), 7.63 (t, J¼8 Hz, 1H, phenyl), 7.33 (d, J¼8 Hz, 1H,
phenyl), 7.18e7.14 (m, 3H, phenyl), 6.70 (t, J¼8 Hz, 1H, phe-
nyl), 6.65 (d, 2H, phenyl), 4.46 (s, 2H, eCH2NHe), 4.10 (br,
1H, eNHe). 13C NMR (100 MHz, CDCl3): d 158.4, 149.0,
147.8, 136.8, 129.2, 122.1, 121.6, 117.6, 113.0, 49.2.
3.2.15. (4-Chlorobenzyl)diphenylamine
1H NMR (400 MHz, CDCl3): d 7.21e7.27 (m, 8H, phenyl),
7.03 (m, 4H, phenyl), 6.94 (m, 2H, phenyl), 4.95 (s, 2H,
eCH2Ne). 13C NMR (100 MHz, CDCl3): d 147.6, 137.5,
132.3, 129.2, 128.6, 127.8, 121.5, 120.5, 55.8. Anal. Calcd

R.-Y. Lai et al. / Tetrahedron 64 (2008) 1213e1217
1217
3. Few recent examples: (a) Huber, C.; Wachtershauser, G. Tetrahedron Lett.
¨
2003, 44, 1695; (b) Robichaud, A.; Ajjou, A. N. Tetrahedron Lett. 2006,
¨
for C19H16ClN: C, 77.68; H, 5.49; N, 4.77. Found: C, 77.48;
H, 5.81; N, 4.30.
47, 3633; (c) Miyazawa, A.; Saitou, K.; Tanaka, K.; Gadda, T. M.;
¨
3.2.16. (4-Methoxybenzyl)diphenylamine18
Tashiro, M.; Prakash, G. K. S.; Olah, G. A. Tetrahedron Lett. 2006, 47,
1437; (d) Sato, S.; Sakamoto, T.; Miyazawa, E.; Kikugawa, Y. Tetra-
hedron 2004, 60, 7899 and references therein.
1H NMR (400 MHz, CDCl3): d 7.22 (m, 6H, phenyl), 7.04
(d, J¼7.6 Hz, 4H, phenyl), 6.91 (t, J¼7.6 Hz, 2H, phenyl),
6.82 (d, J¼8.4 Hz, 2H, phenyl), 4.92 (s, 2H, eCH2Ne), 3.76
(s, 3H, eOMe). 13C NMR (100 MHz, CDCl3): d 158.2,
147.9, 130.9, 129.1, 127.5, 121.2, 120.6, 113.8, 55.7, 55.3.
4. Chang, C.-T.; Liao, B.-S.; Liu, S.-T. Tetrahedron Lett. 2006, 47, 9257.
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Acknowledgements
This work was partially supported by the National Science
Council, Taiwan, ROC for the financial support (NSC95-2119-
M002-038).
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