Zhang et al.
TABLE 1. Conditions and Results of the Reaction
Recently, lanthanide amides Ln[N(SiMe3)2]3 (Ln ) Y,
lanthanide), which can either be prepared from a simple one-
step synthesis in very high yields or are commercially available,
have received much attention for their applications as active
catalysts for Tishchenko reactions,8 intramolecular alkene and
alkyne hydroaminations,9 hydrosilylations,10 and ring-opening
polymerizations of ꢀ-caprolactone and δ-valerolactone.11 We
have recently reported that lanthanide amides [(Me3Si)2N]3Ln-
(µ-Cl)Li(THF)3 can work as efficient catalysts for the aldol-
condensation reaction12 and the stereoregularity of polymeric
methyl methacrylate.13 The lanthanide amides [(Me3Si)2N]3Ln-
(µ-Cl)Li(THF)3 have been proven to be valuable starting
materials in lanthanide chemistry because of the facile cleavage
of the silylamine group.14 As part of our continuous interests
in developing lanthanide compounds as catalysts or initiators
in organic synthesis, we have tried to study the catalytic activity
of lanthanide amides on the Baylis-Hillman reaction of
aromatic aldehydes with methyl acrylate. To our surprise, the
corresponding aromatic amides and alcohols, instead of the
expected Baylis-Hillman reaction products, were isolated.
In this paper, we wish to report a Cannizzaro-type dispro-
portionation reaction of aromatic aldehydes to the corresponding
amides and alcohols by using stoichiometric or catalytic amounts
of the lanthanide compounds. The reaction provides a very
simple and efficient method for the preparation of aromatic
amides.
T
(°C)
cat.
(%)
t
yield of 2gb
ratioa
solvent
(d)
(%)
1:1
2:1
3:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
0
toluene
toluene
toluene
C6H6
Et2O
THF
CH2Cl2
hexane
toluene
toluene
toluene
toluene
toluene
toluene
toluene
5
5
5
5
5
5
5
5
0
2
10
5
5
5
2
2
2
2
2
2
2
2
2
2
2
2
2
1
3
40
85
85
85
65
40
0
32
47
78
80
48
40
75
88
70
rt
rt
5
a Ratio of aldehyde/lithium amide. b Isolated yield based on the lithium
amide.
completed when the reaction time was less than 2 days. Only a
little increase in yield was observed when the reaction was
prolonged. From Table 1, it could also be found that the amount
of aldehyde had a remarkable influence on the formation of the
amide, and this provided a useful clue to understanding the
reaction mechanism. The results showed that only a 40% yield
of amide could be obtained when the aldehyde to lithium amide
ratio was 1:1. A satisfactory yield of amide was obtained when
the ratio of aldehyde to lithium amide reached 2:1. The yield
of amide had almost not been changed when the aldehyde to
lithium amide ratio was 3:1. Thus, the suitable condition was
selected by the treatment of 2 equiv of the aldehyde with lithium
amide LiN(SiMe3)2 catalyzed by 5% YCl3 in toluene at room
temperature for 2 days.
A variety of aromatic aldehydes could be successfully
converted to the corresponding amides in good to excellent
yields (Table 2), and almost the same amount of the corre-
sponding alcohols could be isolated as well. As shown in Table
2, whether the R groups on the phenyl ring were electron-
donating groups (such as CH3O-, CH3-, (CH3)2N-) or the R
groups on the phenyl ring were electron-withdrawing groups
(such as O2N-, F3C-, X-), the isolated yields were satisfac-
tory. But the yields of the corresponding amides having electron-
withdrawing groups were higher than those having electron-
donating groups even though the reaction time was prolonged.
For example, when the aldehyde was the 4-nitrobenzaldehyde,
the yield of the product could reach as high as 85%, but when
the substitutents were 4-CH3O- or 4-(CH3)2N-, the yields of
the products were only 58 or 52%, respectively, indicating
electronic effects from the substituted groups on the reactions.
The influence of the rare earth metal on the yields of the
products was also investigated, and the results are given in Table
3. It was found that lanthanide chlorides generally had good
catalytic activities on the reaction, but the catalytic activity of
PrCl3 was relatively poor compared to that of other LnCl3
catalysts. The results are in accordance with the Lewis acidity
of the Ln3+ cations.15 The results (Tables 2 and 3) are also in
Results and Discussion
To select a favorable reaction condition, the reaction of 2
equiv of 4-nitrobenzaldehyde with lithium amide LiN(SiMe3)2
catalyzed by YCl3 under different conditions was examined
(Table 1). At first, different solvents such as dichloromethane,
benzene, toluene, diethyl ether, and THF were used to initiate
the reaction. It was found that toluene and benzene were the
most suitable solvents for the reaction. The isolated yield of
the amide was only 40% when THF was used as a solvent. No
amide could be obtained when the reaction was performed in
CH2Cl2, which may be a result of the lithium amide LiN(SiMe3)2
reacting with the CH2Cl2. A lower conversion was observed
upon heating the reaction mixture to 70 °C or from performing
the reaction at 0 °C when the reactions were carried out in
toluene. The mole ratios of the catalyst to 4-nitrobenzaldehyde
have some influences on the isolated yields of the reaction. For
example, the yields changed from 78% to 85% as the catalyst
to lithium amide mole ratios were changed from 2% to 10%
when the reaction was performed in toluene at room tempera-
ture. The experimental data indicated that the reaction was not
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