Catalytic Aerobic Oxidation of Arylhydrazides with Iron Phthalocyanine

Article abstract of DOI:10.1002/adsc.201500459

A convenient method for the synthesis of 2-arylazocarboxylates from 2-arylhydrazinecarboxylates by aerobic oxidation with iron phthalocyanine is described. The reaction is applicable to oxidative activation of 1-acyl-2-phenylhydrazines. Some preliminary experiments suggest Michaelis-Menten kinetics and participation of radical species in the reaction mechanism.

Full text of DOI:10.1002/adsc.201500459

UPDATES
DOI: 10.1002/adsc.201500459
Catalytic Aerobic Oxidation of Arylhydrazides with Iron
Phthalocyanine
Takuma Hashimoto,+a Daisuke Hirose,+b and Tsuyoshi Taniguchia,*
a
School of Pharmaceutical Sciences, Institute of Medical, Pharmaceutical and Health Sciences, Kanazawa University,
Kakuma-machi, Kanazawa 920-1192, Japan
E-mail: tsuyoshi@p.kanazawa-u.ac.jp
Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
b
+
These authors contributed equally.
Received: May 9, 2015; Revised: July 23, 2015; Published online: October 14, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500459.
Abstract: A convenient method for the synthesis of
2-arylazocarboxylates from 2-arylhydrazinecarboxy-
lates by aerobic oxidation with iron phthalocyanine
is described. The reaction is applicable to oxidative
activation of 1-acyl-2-phenylhydrazines. Some pre-
liminary experiments suggest Michaelis–Menten ki-
netics and participation of radical species in the re-
action mechanism.
tert-butyl 2-phenylazocarboxylate derivatives via
a rhodium-catalyzed C H activation process.
[9]
À
1-Acyl-2-phenylhydrazines have similar reactivity
to that of aryl azocarboxylates.[10] In addition, they
are utilized for protecting groups in carboxylic acids,
linkers in solid-phase synthesis or potential acylation
reagents because they are converted to the corre-
sponding azo compounds by oxidation and because
a phenylazo group works as a good eliminating
group.[11]
2-Arylazocarboxylates and their derivatives are
usually prepared by oxidation of easily available 2-ar-
ylhydrazinecarboxylates and their derivatives. Metal
reagents such as manganese dioxide,[10b] potassium
permanganate,[12] mercury(II) oxide,[12] and lead(IV)
Keywords: azo compounds; iron; oxygen; phthalo-
cyanines; synthetic methods
Azo compounds possess versatile reactivity and an in- acetate[12,13] are often used as oxidants, though they
teresting physical nature.[1] Azobenzene derivatives are toxic. Tetra-n-butylammonium periodate,[6b] am-
are the oldest compounds in the class and have been monium hexanitratocerate(IV),[14] sodium nitrate/
historically used as dyes and pigments.[2] They are also acetic acid[14b,15] and N-bromosuccinimide[14b] serve as
promising compounds in material sciences.[1a,3] Azo more convenient oxidants. In the context of green oxi-
compounds having electron-withdrawing groups such dants, atmospheric molecular oxygen is an ideal one
as alkoxycarbonyl groups work as electrophiles or oxi- because it is safe and free.[16] There are several meth-
dants in synthetic chemistry. Diethyl azodicarboxylate ods for the synthesis of azobenzenes by aerobic oxida-
(DEAD), which is a reagent for the Mitsunobu reac- tion,[17] but there are few example of aerobic oxida-
tion,[4] is a representative example. It can react with tion of arylhydrazides. A few methods using metals or
anions or radicals as a nitrogen source.[5]
enzymes for aerobic oxidation of 1-acyl-2-phenylhy-
Aryl azocarboxylates can be regarded as hybrids of drazides are known.[11c,g] Pifferi and co-workers re-
the above compounds. They have properties that ported a convenient method for the synthesis of sev-
might be useful for synthetic chemistry. They can be eral 2-arylazocarboxylates from the corresponding 2-
electrophiles or dienophiles like DEAD.[6] The azo arylhydrazinecarboxylates with atmospheric oxygen
group activates the aromatic ring as an electron-with- and 10% palladium on charcoal, although the scope
drawing group to induce nucleophilic aromatic substi- of substrates has not been widely explored.[18]
tutions.[7] In addition, they can be precursors of aryl
Replacement of rare transition metals with inex-
radicals under appropriate conditions.[7] Recently, we pensive elements is important for the development of
reported that they worked as catalysts in catalytic practical synthesis. Cheap iron complexes are promis-
Mitsunobu reactions.[8] Glorius and co-workers report- ing candidates.[19] The combination of an iron catalyst
ed the synthesis of 1-aminoindoline derivatives from
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Catalytic Aerobic Oxidation of Arylhydrazides with Iron Phthalocyanine
Table 1. Aerobic oxidation of ethyl 2-phenylhydrazinecar-
boxylate (1).[a]
and atmospheric oxygen would be conceptually ideal
to realize practical oxidation methods.
We recently found that iron(II) phthalocyanine
[Fe(Pc)] is a good catalyst for aerobic oxidation of 2-
arylhydrazinecarboxylates in a study on catalytic Mit-
sunobu reactions.[8] Iron phthalocyanines have great
potential as oxidation catalysts.[20] They have the po-
tential ability to act as catalysts for activation of mo-
lecular oxygen. The catalytic property of iron phthalo-
cyanine complexes has not been sufficiently explored
compared with structurally related iron porphyrin
complexes.[21] In this paper, we describe in detail the
aerobic oxidation of 2-arylhydrazinecarboxylates and
their derivatives including the optimization, scope of
the reaction and the reaction mechanism (Scheme 1).
Entry
Catalyst
Solvent
Time [h]
Yield [%]
1
2
3
4
5
6
7
8
Fe(Pc)
Fe(Pc)
Fe(Pc)
Fe(Pc)
Fe(Pc)
Fe(Pc)
Fe(Pc)
Fe(Pc)
Fe(Pc)
Fe(Pc)
Fe(Pc)
Fe(Pc)
Fe(Pc)
Fe(Pc)Cl
Fe(Tpp)Cl
FeCl3
MeCN
THF
24
24
24
24
24
24
8
4.5
4.5
2.5
6
74
77
83
58
63
83
89
93
94
91
93
87
51
90
29
13
89
0
CPME
TBME
MeOH
EtOAc
toluene
(CH2Cl)2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
9
10[b]
11[c]
12[d]
13[e]
14[c]
15[c]
16[c]
17[c]
18[c]
19[c]
20[f]
21
12
48
5
8
24
35
24
24
2
Mn(Pc)
Co(Pc)
Cu(Pc)
CuBr
0
68
0
–
24
Scheme 1. A concept of a catalytic aerobic oxidation of 2-ar-
ylhydrazinecarboxylates with iron phthalocyanine.
[a]
Reaction conditions: 1 (0.2 mmol), catalyst (0.02 mmol),
solvent (1 mL) under an air atmosphere at room temper-
ature.
Under a pure oxygen atmosphere.
5 mol% (0.01 mmol) of catalyst.
[b]
[c]
[d]
[e]
[f]
When a mixture of ethyl 2-phenylhydrazinecarbox-
ylate (1) and a catalytic amount of iron phthalocya-
nine (10 mol%) was exposed to atmospheric oxygen,
ethyl 2-phenylazocarboxylate (2) was obtained in
good yield after purification by chromatography. The
reaction time and product yield seemed to depend on
solvents (Table 1, entries 1–9; CPME=cyclopentyl
2 mol% (0.004 mmol) of catalyst.
0.5 mol% (0.001 mmol) of catalyst.
3 mol% (0.006 mmol) of catalyst in the presence of pyri-
dine (0.018 mmol).
methyl ether, TBME=tert-butyl methyl ether). The tion (entries 18 and 19). A combination of copper(I)
use of chlorinated solvents gave good results (en- bromide and pyridine, which is known to show strong
tries 8 and 9). Although the reaction under a pure catalytic activity in aerobic oxidation reactions,[17c]
oxygen atmosphere shortened the reaction time gave the product, but the yield was lower than that of
(entry 10), the use of atmospheric oxygen is better be- the reaction with iron phthalocyanine (entry 20). The
cause it is more economical and convenient. When reaction was not induced in the absence of the cata-
a half amount (5 mol%) of the catalyst was loaded, lyst (entry 21). Thus, iron phthalocyanine was clearly
the good result was maintained (entry 11). However, the best catalyst for aerobic oxidation of 2-arylhydra-
further decrease in the amount of the catalyst caused zinecarboxylates.
prolonged reaction time and a decrease in product
A procedure without chromatography was success-
yield (entries 12 and 13). Next, replacement of iron ful in the reaction of 1. After completion of the reac-
phthalocyanine with other catalysts was tested (en- tion, Fe(Pc) could be removed by filtration through
tries 14–20). Iron(III) phthalocyanine chloride worked Celiteꢁ. Removal of the solvent provided the product,
similarly to iron(II) phthalocyanine (entry 14). The and 1H NMR analysis of the product showed high
use of an iron(III) tetraphenylporphyrin (Tpp) com- purity (see the Supporting Information).
plex and iron(III) chloride resulted in poor yield (en-
When the reaction on a gram scale (5.55 mmol of
tries 15 and 16). Manganese phthalocyanine showed 1) was performed, a prolonged reaction time (48 h)
moderate catalytic activity (entry 17), whereas cobalt was required for completion of the reaction (99%
and copper phthalocyanines did not induce the reac- yield). Fortunately, bubbling air quantitatively afford-
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Takuma Hashimoto et al.
Scheme 2. The reaction on a gram scale.
ed the product in a shorter reaction time (7 h)
(Scheme 2).
With optimum reaction conditions having been de-
termined, the scope of the reaction was investigated
(Figure 1). Phenylazo compounds having various elec-
tron-withdrawing groups could be synthesized by the
present method. N’-Phenylbenzohydrazide was con-
verted to the corresponding azo compounds in good
1
yield. We confirmed by H NMR analysis of crude
products that some 2-phenyl-1-acylhydrazines having
aliphatic acyl groups also produced the corresponding
azo compounds in good yield, but isolation of these
products was often difficult because they were sensi-
tive to moisture. N’-Phenylisobutyrohydrazide and N’-
phenylpivaloylhydrazide were relatively stable. The
yield of a 2-phenylazo compound having a p-toluene-
sulfonyl group was modest because degradation of
the product was observed in the reaction mixture, al-
though the isolated product was stable. Various p-
and m-substituted 2-phenylhydrazinecarboxylates
showed that the impact on the electron density of aro-
matic rings was not great. A long reaction time in the
reaction of ethyl p-nitrophenylhydrazinecarboxylate
was due to poor solubility of the substrate in CH2Cl2.
In fact, dodecyl p-nitrophenylhydrazinecarboxylate
having good solubility in CH2Cl2 afforded the corre-
sponding azo product in excellent yield (96%) and in
a shorter reaction time (6 h) (not shown in Figure 1,
see the Supporting Information). A p-amino group
was tolerated under the oxidation conditions. Reac-
tions of o-substituted 2-phenylhydrazine derivatives
needed a long reaction time, probably due to their
steric factors. Reactions of multisubstituted ethyl 2-
phenylhydrazinecarboxylates also provided the corre-
sponding azo compounds in good yields. The method
was applicable to the preparation of azobenzene from
1,2-diphenylhydrazine. Thus, the reaction was shown
Figure 1. Synthesis of various azo compounds. Reaction con-
ditions shown in Table 1, entry 11 were used unless other-
wise noted.
to be a reliable method for the synthesis of 2-arylazo- ly reproduced and was comparable to that of the reac-
carboxylates and their derivatives. tion with Fe(Pc) (entry 1). The reaction of ethyl 2-(4-
Incidentally, aerobic oxidation of ethyl 2-butylhy- chlorophenylhydrazine)carboxylate also gave the cor-
drazinecarboxylate occurred under the same condi- responding azo product in excellent yield and in
1
tions, and H NMR analysis of the crude product indi- a short reaction time (entry 2). On the other hand, re-
cated production of the corresponding hydrazone actions of ethyl 2-(3,4-dichlorophenyl)hydrazinecar-
compound. This would be because the corresponding boxylate and ethyl 2-(4-nitrophenyl)hydrazinecarbox-
azo product was rapidly tautomerized.
ylate were sluggish. Reactions did not go to comple-
We compared Pifferiꢂs method using 10% Pd-C/O2 tion in 48 h and gave the corresponding azo com-
with our method (Table 2).[18] The result of the reac- pounds in modest yield (entries 3 and 4). Those sub-
tion of ethyl 2-phenylhydrazinecarboxylate was readi- strates were converted to azo products in better yields
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Catalytic Aerobic Oxidation of Arylhydrazides with Iron Phthalocyanine
Table 2. Reproducing the result of the reaction with 10%
Pd-C and air and the effect of substituted groups.[a]
Entry
X
Y
Time [h]
Yield [%]
1
2
H
Cl
Cl
NO2
H
H
Cl
H
2
1
48
48
98
93
53
61
3
4[b]
[a]
Reaction conditions: 1 (0.2 mmol), catalyst (6.0 mg), sol-
vent (1 mL) under an air atmosphere at room tempera-
ture.
[b]
1,4-Dioxane was used instead of toluene.
Figure 2. A zero-order kinetic plot of the reaction of aero-
bic oxidation of ethyl 2-phenylhydrazinecarboxylate
(200 mM) with Fe(Pc) (10 mM) in CDCl3 at 228C.
and in shorter reaction times when the Fe(Pc)/O2
method was employed (Figure 1). Clearly, the reac-
tion using Pd-C/O2 is more sensitive to electron densi-
ty of the aromatic ring than is the reaction using under air atmosphere was monitored by 1H NMR
Fe(Pc)/O2. This might imply a difference in the mech- analysis.
anism.
The plot of product yield against reaction time gave
We expected that the present reaction would serve a straight line, indicating a zero-order kinetic profile
as a mild method for acylation by oxidative activation (Figure 2). Zero-order dependence in catalytic reac-
of N-phenylacylhydrazides.[11] Thereby, we tested tions suggests saturation kinetics such as enzyme ki-
some model experiments. When N-phenylhydrazides netics. In this case, the reaction process involves irre-
3 and 4 were oxidized under aerobic conditions in the versible and fast complexation between a substrate
presence of 2-phenylethanol and molecular sieve 5Š, and a catalyst, which causes saturation of the cata-
ester compounds 5 and 6 were produced in good lyst.[22] The results of some preliminary experiments
yields (Scheme 3). The method was applied to synthe- supported this consideration. For instance, the plot of
sis of amides. Amines inhibited the catalytic activity an amount of the product in a short time (3 min)
of Fe(Pc) by strong coordination to a metal center. against concentration of the substrate showed a satura-
Therefore, p-bromoaniline was added to the reaction tion curve that was a typical Michaelis–Menten kinet-
mixture after completion of oxidation of N-phenylhy- ic model (see Figure S1 in the Supporting Informa-
drazides 3 and 4, and the corresponding amides 7 and tion). A linear correlation was observed between
8 were isolated in good yields.
product yield and concentration of the catalyst (Fig-
With intent to obtain an insight into the mecha- ure S2 in the Supporting Information). These results
nism, we conducted kinetic studies of the reaction. are consistent with saturation kinetics.
The reaction of ethyl phenylhydrazinecarboxylate
The substituent effect on the reaction rate was in-
(200 mM) with Fe(Pc) (10 mM) in CDCl3 at 228C vestigated by employing several ethyl p-substituted
phenylhydrazine carboxylates under the same condi-
tions. As shown in Table 3, rate constants in zero-
order reactions of substrates having p-methyl, p-me-
thoxy, p-phenyl and p-halogen were larger than that
of the unsubstituted phenylhydrazine 1. This tendency
is apparently unusual if the mechanism of oxidation is
cationic. Hammettꢂs substituent constant (s+) of p-
chloro- and p-bromophenyl compounds has positive
values,[23] and the rate constants of these substrates
should be smaller than that of the unsubstituted sub-
strate (X=H) in cationic reactions. Promotion of the
reaction by p-halogen groups suggests formation of
intermediary radical species rather than cation spe-
cies. This consideration is consistent with the relative-
Scheme 3. Examples of acylation using N-phenylhydrazides. ly large rate constant of the substrate having a p-
Adv. Synth. Catal. 2015, 357, 3346 – 3352
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Takuma Hashimoto et al.
Table 3. The reaction rate of p-substituted phenylhydrazine-
carboxylates.
Entry
X
s+
kobs [10À3 MminÀ1]
kX/kH
1
2
3
4
5
6
7
OMe
Me
Ph
F
H
Cl
À0.78
À0.31
À0.18
À0.07
0
1.94
1.10
1.20
1.16
0.85
1.32
1.38
2.28
1.29
1.41
1.36
1.00
1.55
1.62
0.11
0.15
Br
Scheme 4. An outline of the proposed reaction mechanism.
phenyl group, which strongly stabilizes radicals. Al- short, the reaction might show saturation kinetics for
though the “radical Hammett rule” is known, it is still oxygen.
preliminary because its applicable range is vague.[24]
Based on these experimental results, we propose
A clear correlation was not obtained when it was ap- a mechanism of the reaction (Scheme 4). An Fe(III)
plied to our case. Further studies will be needed to species would basically participate in a catalytic cycle.
fully certify the radical mechanism.
Probably, Fe(II)(Pc) also has a catalytic activity for
We also tested a substrate having p-cyano (s+ = oxidation, but it would give Fe(III) species in the pro-
0.66), which worked as a strong electron-withdrawing cess of aerobic oxidation. This simple assumption is
group. However, a plot of this reaction did not denote supported by the fact that Fe(Pc)Cl could work as
a linear correlation (see Figure S9 in the Supporting a catalyst. Zero-order kinetics indicated that the cata-
Information). In this case, formation of the catalyst- lytic cycle started from rapid and irreversible com-
substrate complex would be reversible because the plexation between the iron catalyst and substrates 9
Lewis basicity of the nitrogen atom was decreased by to provide iron complex 10 maintaining a steady con-
the effect of strong electron-withdrawing groups. In centration until consuming most of the substrate. The
short, the kinetic model of the reaction would shift to small impact of oxygen pressure might be rationalized
“pre-equilibrium kinetics” from “saturation kinetics” similarly because it is known that metal phthalocya-
when substrates having electron-withdrawing groups nines form complexes with molecular oxygen reversi-
are employed. Therefore, the substituent effect of bly.[25] Adsorption of oxygen to the iron complex
strong electron-withdrawing groups could not be com- would be very fast, and 1 atm of oxygen would be suf-
pared with that of the p-substituted substrates show- ficient to fully form the iron-oxygen complex. Iron
ing zero-order dependence. On the other hand, strong complex 10 would be converted into the starting
electron-withdrawing groups do not seem to inacti- Fe(III) species with generation of hydrazyl radical 11
vate the oxidation step. In fact, the substrate having and hydroperoxy radical (HOOC), which are convert-
p-cyano was almost fully converted to the correspond- ed into azo product 12 and hydroperoxide. Stability of
ing azo compound within 6 h.
hydrazyl radical 11 would be affected by p-substitu-
The impact of oxygen pressure was tentatively in- ents of the aromatic ring.[26] A pair of Fe(III) radical
À
vestigated. The reaction was performed under 1, 3, or cation and superoxide anion (O2 C) is thought to be
5 atm of oxygen for 20 min, and yield of the product formed in the course of the conversion.[25c] Since hy-
1
was estimated by H NMR analysis. A significant dif- droperoxide would be decomposed to oxygen and
ference in product yield in the experiments (1 atm: water by the catalase activity of Fe(Pc), the hazardous
49%; 3 atm: 45%; 5 atm: 37%, average of three inde- problem of peroxide waste is probably avoided in the
pendent experiments, see Table S4 in the Supporting reaction on a large scale.[22]
Information) was not observed. This suggested that
Since reactions of o-substituted phenylhydrazine
the reaction rate did not depend on pressure of compounds were relatively slow, we assumed that the
oxygen when the pressure was larger than 1 atm. On steric effect would be a cause of the slow reaction.
the other hand, the conversion rate of the substrate Surprisingly, a kinetic profile of the reaction of ethyl
was lower under air atmosphere (ca. 14%, Figure 2). o-methylhydrazinecarboxylate still showed zero-order
Therefore, the reaction rate might depend on the con- dependence, indicating a saturation kinetic model.
centration of oxygen under low pressure of oxygen. In Therefore, formation of the catalyst-substrate com-
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Catalytic Aerobic Oxidation of Arylhydrazides with Iron Phthalocyanine
plex does not seem to be significantly inhibited by an Acknowledgements
o-methyl group. On the other hand, the observed rate
constant (kobs =0.24”10À3 MminÀ1) was smaller than
that of unsubstituted substrate 1 (see Figure S10 in
the Supporting Information). This observation implies
that the o-methyl group might affect the step of for-
mation of the proposed hydrazyl radical. This could
be rationalized by considering steric repulsion be-
tween a hydrazyl group and the o-methyl group. The
steric repulsion might partly disturb delocalization of
electrons on a nitrogen atom onto the aromatic ring
by causing slight distortion. This effect would unfavor-
ably work for formation of the hydrazyl radical be-
cause stabilization of the radical by delocalization is
inhibited. On the other hand, the effect might favora-
bly work for formation of the iron-hydrazine complex
because it would increase Lewis basicity of the nitro-
gen atom.
In conclusion, we have developed a practical
method for the preparation of synthetically useful 2-
arylazocarboxylates and their derivatives from the
corresponding 2-arylhydrazinecarboxylates and their
derivatives by aerobic oxidation. The reaction is cata-
lyzed by iron phthalocyanine, which is inexpensive
and non-toxic. Kinetic studies revealed that pre-for-
mation of a robust complex between a substrate and
a catalyst occurs, and radical species seem to mediate
the reaction. A single-electron transfer process be-
tween the iron complex and oxygen is probably in-
volved. The reaction is a good model of practical syn-
thesis because the reaction system is economically
and environmentally friendly. The results of the pres-
ent study will serve as a motif to design a new catalyt-
ic system with iron phthalocyanine in the future.
Authors are thankful to Profs. Shigeyoshi Kanoh, Katsuhiro
Maeda and Tomoyuki Ikai (Kanazawa University) for their
kind support. This work was supported by MEXT/JSPS/KA-
KENHI Grant-in-Aid for Scientific Research(C) (Grant No.
25460011) and Grant-in-Aid for JSPS Fellows (Grant No.
14J02441).
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Experimental Section
General Procedure for Aerobic Oxidation of
Hydrazines
To a stirred solution of a hydrazine compound (0.20 mmol)
in CH2Cl2 (1.0 mL) was added iron phthalocyanine (5.7 mg,
0.010 mmol), and the mixture was stirred at room tempera-
ture under an air atmosphere until completion of the reac-
tion. After removal of the solvent from the reaction mixture,
the crude material was purified by silica gel chromatography
(n-hexane/EtOAc) to give the corresponding azo compound.
Alternatively, iron phthalocyanine could be removed by fil-
tration of the reaction mixture through Celiteꢁ after comple-
tion of the reaction. After removal of the solvent under
[10] a) I. Sapountzis, P. Knochel, Angew. Chem. 2004, 116,
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1
vacuum, H NMR analysis of the resultant product showed
sufficient purity without further purification in many cases.
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