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Table 3 The effect of different metals on the yield of succinimidea
showed the highest yield more than 99% (Table 1, entry 4 and
5). The results demonstrate the effect of different persulfates on
yields and the difference may be due to their different solubility.
Under the conditions described in the experimental section
(water 3 mL, persulfate 2.2 mmol), the concentration of per-
sulfate theoretically was about 0.7 mol Lꢀ1. However, at room
temperature, the concentration of persulfate was less than 0.2
mol Lꢀ1. As a result, K2S2O8 can not be completely dissolved by
calculation so that the ratio theoretically required of pyroglu-
tamic acid and persulfate can not be achieved. Whereas, the
solubility of Na2S2O8 or (NH4)2S2O8 can be far from meeting the
ratio to obtain a high yield.18
With the optimized oxidant (NH4)2S2O8, we then explored
the effect of different silver salts on the yield and the results
were summarized in Table 2. To investigate the catalytic effi-
ciency of different silver salts, we used AgOAc (1 mol%), Ag2SO4
(0.5 mol%), Ag2CO3 (0.5 mol%), Ag3PO4 (0.3 mol%) and AgCl (1
mol%) instead of AgNO3 (1 mol%) with all other conditions
unchanged. When the Ag(I) ion was added with an amount of 1
mol%, most of the silver catalyst efficiency remained at a high
level (Table 2, entries 1–4) except AgCl. The yield of AgCl was
markedly reduced to 34% (Table 2, entry 5). Based on these
results, taking into account the major differenence of these
catalysts on solubility, we predicted that the solubility of the
catalyst would largely affect the catalytic efficiency when the
amount of dissolved catalyst was insufficient to achieve the
required amount of catalytic theory. Aer calculation, based on
the amount of catalyst added and their respective Ksp,19 the
actual concentration of Ag(I) from AgCl dissolved in the aqueous
solution was only 1.34 ꢁ 10ꢀ5 mmol mLꢀ1. For Ag2CO3 (0.5
mol%) and Ag3PO4 (0.3 mol%), whose yields remained at more
than 99%, the actual amount of water-soluble salt was only
Entry Cat. (mol%)
Oxidant (eq.)
Time/h Conv.b/% Yieldb/%
1
2
3
4
5
6
Cu(NO3)2 (1%)c (NH4)2S2O8 (2.2)
2
2
2
2
2
2
39
40
36
38
36
38
6
7
6
5
3
3
CuSO4 (1%)d
(NH4)2S2O8 (2.2)
Cu(OAc)2 (1%)e (NH4)2S2O8 (2.2)
Ni(NO3)2 (1%)f (NH4)2S2O8 (2.2)
Ni(OAc)2 (1%)g (NH4)2S2O8 (2.2)
Al(NO3)3 (1%)h (NH4)2S2O8 (2.2)
a
Reaction conditions: apyroglutamic acid (130 mg, 1 mmol), Ag(I) ion
(0.01 mmol), water (3 mL), (NH4)2S2O8 (0.50 g, 2.2 mmol), room
b
c
temperature. Determined by HPLC. Cu(NO3)2 (1.9 mg, 0.01 mmol).
d
f
e
CuSO4$5H2O (2.5 mg, 0.01 mmol). Cu(OAc)2 (1.8 mg, 0.01 mmol).
g
Ni(NO3)2$6H2O (2.9 mg, 0.01 mmol). Ni(OAc)2$4H2O (2.5 mg, 0.01
mmol). h Al(NO3)3$9H2O (3.7 mg, 0.01 mmol).
entries 1–5), even though they have a good solubility in water
without considering the factor that the dissolved amount is too
small to exert its effect. In addition, we selected Al(NO3)3 as a
typical example of short-period mental elements as a catalyst, but
an unsatisfactory result was also presented with the yield of 3%
(Table 3, entry 6). These results all support the involvement redox
2ꢀ
cycling reaction of silver mechanism in the oxidation by S2O8
.
We know that the reduction potential of S2O82ꢀ is about 2.01 V. At
thispoint, silverwasselectedasabridgeconnectingpyroglutamic
acid and oxidant S2O82ꢀ thanks to the reduction potential of 1.98
V to promote the oxidative decarboxylation based on the litera-
tures where oxidation of carboxylate anions by Ag(II) complexes
was mentioned.20 Until now, we have chosen silver as the best
catalyst for the oxidative pyroglutamic of pyroglutamic acid.
about 2.96 ꢁ 10ꢀ4 mmol mLꢀ1 and 1.43 ꢁ 10ꢀ4 mmol mLꢀ1
.
Conclusions
Seen in this light, the amount of catalyst required in the process
of oxidative decarboxylation using silver as catalyst and per-
sulfate as oxidant is minimal (high TON up to 104), which is
crucial for practical applications.
In summary, we reported a sustainable succinimide preparation
method without the use of additional nitrogen. Using silver as
the catalyst, we achieved the conversion from a biobased
chemical (glumatic acid) to target product (succinimide) in
excellent yields under mild conditions. We observed the yield
up to 96% in aqueous solution at room temperature aer only 2
h with the minimal amount of catalyst. At the same time, this
process was simple and green with only two steps and water as
the solvent instead of some organic solvents which were toxic or
expensive. In this way, it opened a green way for succinimide
production from biomass and pushed the skylight to see the
broad application prospects of biomass resources.
The above silver-catalyzed oxidative decarboxylation was then
extended to other transition metal such as copper and nickel.
Nevertheless, these compounds, such as Cu(NO3)2 (1 mol%),
CuSO4 (1 mol%), Cu(OAc)2 (1 mol%), Ni(NO3)2 (1 mol%) and
Ni(OAc)2 (1 mol%), did not exhibit good catalytic effect (Table 3,
Table 2 The effect of different silver salts on the yield of succinimidea
Entry Cat. (mol%)
Oxidant (eq.)
Time/h Conv.b/% Yieldb/%
Experimental section
Preparation of pyroglutamic acid from glutamic acid
1
2
3
4
5
AgOAc (1%)c
(NH4)2S2O8 (2.2)
2
2
2
2
2
100
100
100
100
48
>99 (96)
96 (92)
99 (93)
>99 (94)
34
Ag2SO4 (0.5%)d (NH4)2S2O8 (2.2)
Ag2CO3 (0.5%)e (NH4)2S2O8 (2.2)
Ag3PO4 (0.3%)f (NH4)2S2O8 (2.2)
In a typical experiment, a beaker was lled with glutamic acid
(100 g) and placed in an oil bath (145–150 ꢂC). The dehydration
reaction incubated for 45 minutes and the solution gradually
turned brown. Aer the dehydration reaction, the solution was
poured into boiling water (350 mL) and dissolved in the water.
When the temperature droped to 40–50 ꢂC, the solution was
bleached by activated carbon twice and became colorless. The
AgCl (1%)g
(NH4)2S2O8 (2.2)
a
Reaction conditions: apyroglutamic acid (130 mg, 1 mmol), Ag(I) ion
(0.01 mmol), water (3 mL), (NH4)2S2O8 (0.50 g, 2.2 mmol), room
temperature. Determined by HPLC, yield of isolated product is in
parenthesis. AgOAc (1.7 mg, 0.01 mmol). Ag2SO4 (1.7 mg, 0.005
mmol). Ag2CO3 (1.4 mg, 0.005 mmol). Ag3PO4 (1.4 mg, 0.003
mmol). g AgCl (1.5 mg, 0.01 mmol).
b
c
d
e
f
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 27541–27544 | 27543