CARBONYLATIVE REDUCTION OF NITROPHENOLS TO AMINOPHENOLS

Article abstract of DOI:10.1135/cccc19950514

Two- or three-component catalysts composed of (i) sulfur or sulfur compound (H2S, CS2, COS, Na2S), (ii) basic additive (triethylamine, CH3ONa, Na2S), and usually (iii) vanadium(V) compounds (e.g.NH4VO3) were found to catalyze efficiently the reaction of CO + H2O with isomeric nitrophenols to give the corresponding aminophenols.The reaction proceeds smoothly at 398 and 483 K and initial pressure of 7 MPa, and its rate increases from 2- to 4-nitrophenol.The selectivity to aminophenols exceeding 96 per cent was obtained at the water to nitrophenol molar ratio higher than 5.The solvents such as methanol and dioxane ensured better contact of the reactants, which was necessary for achievement of such a high selectivity.The effectiveness of the sulfur components (based on the S content) is expressed by the following sequence: S : CS2 : Na2S : H2S : COS = 1 : 1.2 : 2.5 : 10 : 11.The reaction takes place also under the reduced CO pressure to 0.1 - 0.35 MPa.Formation of side products and mechanism of the reaction are discussed.

Full text of DOI:10.1135/cccc19950514

514
Macho, Kucera, Kralik:
CARBONYLATIVE REDUCTION OF NITROPHENOLS
TO AMINOPHENOLS*
Vendelin MACHO, Milan KUCERA and Milan KRALIK
Department of Organic Technology,
Slovak Technical University, 812 37 Bratislava, The Slovak Republic
Received October 27, 1994
Accepted December 12, 1994
Two- or three-component catalysts composed of (i) sulfur or sulfur compound (H2S, CS2, COS,
Na2S), (ii) basic additive (triethylamine, CH3ONa, Na2S), and usually (iii) vanadium(V) compounds
(e.g. NH4VO3) were found to catalyze efficiently the reaction of CO + H2O with isomeric nitrophenols
to give the corresponding aminophenols. The reaction proceeds smoothly at 398 and 483 K and initial
pressure of 7 MPa, and its rate increases from 2- to 4-nitrophenol. The selectivity to aminophenols
exceeding 96 per cent was obtained at the water to nitrophenol molar ratio higher than 5. The sol-
vents such as methanol and dioxane ensured better contact of the reactants, which was necessary for
achievement of such a high selectivity. The effectiveness of the sulfur components (based on the S
content) is expressed by the following sequence: S : CS2 : Na2S : H2S : COS = 1 : 1.2 : 2.5 : 10 : 11.
The reaction takes place also under the reduced CO pressure to 0.1 – 0.35 MPa. Formation of side
products and mechanism of the reaction are discussed.
As aminophenols are important intermediates for syntheses of pharmaceuticals, antiox-
idants, dyes, developers, and polymers, the routes to their simple and selective produc-
tion are investigated. Thus, besides hydrogenation of nitrophenols on supported Group
VIII metal catalysts, especially on palladium and platinum1–5, and reactions with hy-
drogen “in statu nascendi”, especially important is a two-step synthesis of 4-amino-
phenol, in which nitrobenzene is first hydrogenated on a heterogeneous catalyst to the
intermediate phenylhydroxylamine, followed by the acid-catalyzed rearrangement of
the latter to the above product5,6. In the presence of CO and a catalyst formed from
Pd(II) salt, triarylphosphine, carbon monoxide, and sulfuric acid, water becomes the
hydrogen donor7–9, as shown in Eq. (A).
Pd cat.
C6H5NO2 + 2 CO + H2O
4-HOC6H4NO2 + 2 CO2
(A)
333 K
* Presented at the XXVIth Czech and Slovak Symposium on Catalysis, Prague, November 7 – 8, 1994.
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Carbonylative Reduction of Nitrophenols
515
However, only 4-aminophenol can be obtained by this way. The other procedures are
analyzed in more detail elsewhere5. It is worth mentioning that besides hydrogenation
of the nitro group on a palladium catalyst, still utilized for reduction of nitrophenols is
their reaction with polysulfides, particularly with hydrogen sulfides in alkaline aqueous
medium10,11. Although the reaction proceeds at 303 to 383 K, overstoichiometric
amounts of the sulfides are required to achieve the nitro group reduction.
In connection with our previous studies of the reductive carbonylation of nitroben-
zene12,13 and the carbonylative reduction of nitrobenzene to aniline and of dinitroben-
zenes to aminonitrobenzenes or diaminobenzenes14 using a novel catalyst, in the
present work we were interested in the selective carbonylative reduction of isomeric
nitrophenols to the corresponding aminophenols. Special attention has been paid to the
sulfur component of the catalytic system and to the effect of reaction conditions (water
to nitrophenol molar ratio and partial carbon monoxide pressure).
EXPERIMENTAL
Chemicals
2-, 3-, and 4-Nitrophenols were commercial chemical purity grade samples (of the higher than 99%
purity) as well as the corresponding aminophenol isomers (Lachema Brno, Apolda Buchs, and Al-
drich). The other chemicals were of analytical purity grade, and water was redistilled.
Procedures
Reductive carbonylation of individual nitrophenol isomers was carried out in a high pressure stainless
steel 250 ml autoclave, equipped with a stirrer (400 rpm) and an electrical heating controlled auto-
matically within ±2 K. After charging solid and liquid components, the autoclave was closed and
then hydrogen sulfide or carbonyl sulfide were introduced by means of the weighed rubber baloon,
followed by carbon monoxide pressuring to usually 7 MPa. With stirring, the reaction mixture was
heated to 398 or 423 K (ca 30 min). At this temperature the actual temperature and pressure were
recorded in 5 – 10 min intervals during 4 or 5 h. Then, stirring was stopped, the autoclave content
was cooled, the pressure and temperature were recorded, and the product was weighed. After that, the
product was analyzed by GLC (Chrom 5 instrument, flame ionization detector), using 2.4 m long
column (3 mm i.d.) packed with 5% silicone elastomer SE 30 on Chromaton N-AWD MCS as the
stationary phase. The solvent was N,N-dimethylformamide. At feeding point and detector temperature
of 423 K, that of the column 453 K, the nitrogen flow rate 40 cm3 min−1 and the hydrogen flow rate
30 cm3 min−1, the components had the following retention times (in min): methanol 0.52, triethyl-
amine 0.62, dimethylformamide 0.67, 2-nitrophenol 2.33, 2-aminophenol 1.50, 3-nitrophenol 3.40,
3-aminophenol 1.73, 4-nitrophenol 4.28, 4-aminophenol 4.00, and methyl N-(4-hydroxyphenyl)carba-
mate 4.03. Experiments were repeated and the data given in the tables are rounded off arithmetic
means.
Analysis of Reaction Mixture from Carbonylation Using Water to Nitrophenol Molar Ratio = 1
The reaction product consisted of a liquid and the solid, mostly crystalline portion. The solid (7.0 g)
contained 46% 4-aminophenol. Its sample (5 g) was mixed with the same amount of water and 10 g
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

516
Macho, Kucera, Kralik:
of methanol and the mixture was refluxed for 10 h. Then, the volatiles were evaporated at 323 K and
the solid residue was analyzed. In order to differentiate 4-aminophenol from methyl N-(4-hydroxy-
phenyl)carbamate, GLC conditions were modified (nitrogen flow rate reduced to 30 cm3 min−1, col-
umn temperature to 433 K, and after 4-aminophenol elution (3.67 min) increased again to 453 K at
which methyl N-(4-hydroxyphenyl)carbamate was eluted). The solid residue contained 41% 4-aminophe-
nol and 59% methyl N-(4-hydroxyphenyl)carbamate.
RESULTS AND DISCUSSION
As a continuation of our previous work on the reductive carbonylation of nitroben-
zene13 and the carbonylative reduction of nitro- and dinitrobenzenes14, in the present
work we have applied the catalytic system based on sulfur compounds in a basic me-
dium also on the carbonylative reduction of isomeric nitrophenols (Eq. (B)).
cat.
(2-, 3-, 4-)HOC6H4NO2 + 3 CO + H2O
(2-, 3-, 4-)HOC6H4NO2 + 3 CO2 (B)
Further data on reaction conditions and results obtained are presented in Table I.
The results show that also nitrophenol isomers can be reduced carbonylatively. In
this case the reaction rate under comparable conditions increases from the 2-nitro to
4-nitro derivative. The most effective sulfur compounds, in fact by more than by one
order of magnitude when compared to elemental sulfur or carbon disulfide, were
proved to be hydrogen sulfide and carbonyl sulfide. This agrees well with our previous
observation14 of a similar effect in the carbonylative reduction of nitrobenzene and
dinitrobenzenes. Furthermore, we have proved the applicability of sodium sulfide (and
likely also NaHS) which acts both as the donor and the base. The basic medium is a
prerequisite for the above catalytic reaction. With regard to the inhibition time of the
carbonylative reduction and the course of the pressure decrease, the efficiency of the
sulfur components (per mol of the compounds or the amount of the bound sulfur) is
following: S : CS2 : Na2S : H2S : COS = 1 : 1.2 : 5 : 10 : 11. We believe that the effect
of Na2S could have achieved that of hydrogen sulfide. The reason of its lower effective-
ness is likely the escape of a part of the hydrogen sulfide evolved by hydrolysis of Na2S
. 9 H2O added into the water present in the autoclave before its sealing and carbon
monoxide introduction.
The results support also our assumption that the catalytically active form of the sul-
fur components is carbonyl sulfide. It seems likely that it is its formation which starts
the carbonylative reduction. On the other hand, although ammonium metavanadate acts
as a promotor, it is not a prerequisite for the reaction, as it is observed in the case of the
reductive carbonylation of nitrobenzene to methyl N-phenylcarbamate13.
In this connection, the earlier finding by Kondo and coworkers15 concerning promo-
(−)
tion of the reaction Se + COR→O SeCO by an alcoholate anion is of interest. This finding
can be utilized also in the synthesis of ureas from aliphatic amines and carbon
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Carbonylative Reduction of Nitrophenols
517
monoxide as well as in the preparation of dimethyl carbonate15 from methanol and
carbon monoxide. An analogous situation can be also expected in the case of sulfur and
low molecular compounds thereof in a basic medium. Furthermore, Harper16 reacted a
mixture of carbonyl sulfide, nitrobenzene, carbon monoxide, and water to obtain N,N′-
diphenylurea in the nearly 50% yield.
As proved, the basic medium can be realized also by the use of an ash-free base. In
addition, triethylamine used for this purpose distills off easily from the reaction product
and can be thus recycled.
Of interest is also the finding that at high excess of water, methanol, and likely also
other water-soluble alcohols, can be used as the solvent of nitrophenols. This is made
TABLE I
Effect of catalyst components on carbonylative reduction of nitrophenols to aminophenols (12.5 g of
nitrophenol, 12.5 g water, solvent, 7 MPa CO at 298 K, reaction time 4 h, H2O : nitrophenol molar
ratio 7.7)
Catalytic system (mass%)a
Nitrophenol Solvent (mass%)a
Xb, %
Sb, %
S-compound
base
NH4VO3
2-NO2
3-NO2
dioxane (400)
dioxane (400)
dioxane (400)
dioxane (400)
methanol (100)
methanol (400)
dioxane (400)
methanol (400)
dioxane (400)
dioxane (400)
methanol (400)d
methanol (400)
methanol (400)
methanol (400)
methanol (100)e
methanol (100)e
methanol (100)e
S (14)
Et3N (12)
Et3N (12)
Et3N (12)
Et3N (12)
Et3N (12)
Et3N (12)
Et3N (12)
–
0.4
0.4
–
88
68
98
93
93
97
96
96
97
99
98
98
89
89
99
98
98
90
88
H2S (4)
H2S (4)
H2S (4)
H2S (2)
H2S (4)
CS2 (10)
Na2Sc (2)
S (14)
57
0.4
0.1
0.1
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
–
99
96
98
98
97
4-NO2
Et3N (12)
Et3N (1.2)
Et3N (1.2)
Et3N (1.2)
97
S (14)
99
H2S (0.8)
H2S (0.8)
100
100
100
99
COS (0.8) Et3N (12)
COS (0.7) Et3N (12)
Na2Sc (2)
Na2Sc (2)
Na2Sc (1)
–
–
–
96
84
0.4
75
a With respect to the nitrophenol; b X conversion of nitrophenol, S selectivity to aminophenol; c Na2S
as the nonahydrate (Na2S . 9 H2O); d water to 4-nitrophenol molar ratio 1.6; e at 398 K.
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

518
Macho, Kucera, Kralik:
possible by the fact that alkyl N-(hydroxyphenyl)carbamates are not formed, the only
products being aminophenols.
Table II presents the results of the carbonylative reduction of 4-nitrophenol at the
initial CO pressures 0.35 and 0.40 MPa. In place of usual 12.5 g, the weight amount of
the 4-nitro derivative was only 5 g, to ensure the possibility of the complete conversion
of the nitrophenol by the present carbon monoxide. The latter was achieved after 5 h,
with the selectivity to 4-aminophenol higher than 97 per cent. Also these results indi-
cate that the carbonylative reduction takes place even at the CO partial pressure 0.35 MPa.
Furthermore, the complete 4-nitrophenol conversion under these conditions allows to
conclude that the reduction proceeds even at the lower CO pressure of around 0.1 MPa.
Obviously, even though at the slower but still measurable rate, the carbonylative reduc-
tion could proceed in the presence of the above catalyst already at the CO partial press-
ure equaling to or being higher than 0.1 MPa.
With regard to the competition reactions (C) and (D) and likely due to the higher
reactivity of the intermediate nitrene12 with water than with methanol, even at the water
to 4-nitrobenzene molar ratio equaling to 1, the selectivity to 4-aminophenol equals to
TABLE II
Effect of initial CO pressure (in MPa) and water to nitrophenol molar ratio (Y) on carbonylative re-
duction of 4-nitrophenol (temperature 398 K, 12.5 g 4-nitrophenol, 25 g solvent, x g water, 0.0125 g
NH4VO3, reaction time 5 h)
Catalytic system (mass%)
CO pressure
Solvent
Xa, %
Sa, %
Y
S-compound
base
0.35b
0.40b
0.35b
0.35b
12.7
12.7
7.7
7.7
7.7
7.7
5
methanol
dioxane
CS2 (10)
S (10)
CH3ONa (10)
Et3N (10)
Et3N (10)
Et3N (10)
Et3N (10)
Et3N (5)
100
100
100
100
100
99
98
97
98
97
99
99
97
91
88
74
methanol
dioxane
H2S (1)
S (10)
7
7
7
7
7
7
7
dioxane
H2S (1)
H2S (1)
CS2 (10)
CS2 (10)
CS2 (10)
CS2 (10)
CS2 (10)
dioxane
methanol
methanol
methanol
methanol
methanol
Et3N (10)
Et3N (10)
Et3N (10)
Et3N (10)
Et3N (10)
99
3
100
99
2
1
100
100
–
0.2d
a For X and S see footnotea in Table I; b batch contained only 5 g of 4-nitrophenol; c at 423 K;
d the selectivity to methyl N-(4-hydroxyphenyl)carbamate was 81 per cent.
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Carbonylative Reduction of Nitrophenols
519
73.4 per cent. To achieve the more than 96% selectivity, the above ratio has to be equal
or higher than 5. The side products formed are above all
CH3OH
HOC6H4NHCOOCH3 + 2 CO2 (C)
cat.
(2-, 3-, 4-)HOC6H4NO2 + 3 CO
H2O
(2-, 3-, 4-)HOC6H4NH2 + 3 CO2 (D)
methyl N-(4-hydroxyphenyl)carbamate (Eq. (C)), and to the less extent also the compe-
titive reductive carbonylation of 4-nitrophenol to N,N′ -bis(4-hydroxyphenyl)urea (Eq. (E)).
cat.
4-HOC6H4NO2 + 4-HOC6H4NH2 + 3 CO
HOC6H4NHCONHC6H4OH + 2 CO2
(E)
However, also other reductive carbonylations (Eqs (F) and (G)) cannot be excluded,
although by us their products have not been detected.
cat.
2 4-HOC6H4NO2 + 3 CO
HOC6H4NHCOOC6H4NO2 + 2 CO2
cat.
(F)
2 4-HOC6H4NO2 + 6 CO + H2O
HOC6H4NHCOOC6H4NH2 + 5 CO2
(G)
In the absence of water, the main reaction is the reductive carbonylation of 4-nitrophe-
nol to methyl N-(4-hydroxyphenyl)carbamate (81% selectivity).The other products in
this case are likely those formed by Eqs (E) and (F).
The reaction product from the experiment in which water and 4-nitrophenol were
used in the molar ratio 1 : 1 was a mixture of liquid and crystalline portion. The liquid
phase (27.8 g) contained 69% of a mixture of methanol and triethylamine and 31% of
4-aminophenol. The solid portion (7.0 g) contained 46% of 4-aminophenol. Its further
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Macho, Kucera, Kralik:
treatment (for details see Experimental) and GLC analysis showed that the final solid
consists of 41% 4-aminophenol and 59% of methyl N-(4-hydroxyphenyl)carbamate.
This also documents that we deal here with the compounds formed according to Eqs
(C) to (E).
Summarizing, the catalytic system described in the present work thus catalyzes not
only the reductive carbonylation of nitrobenzene to alkyl N-(hydroxyphenyl)carba-
mates13 but also the oxidative carbonylation of aniline to N,N′ -diphenylurea and methyl
N-phenylcarbamate17, carbonylative reduction of nitrobenzene to aniline, and dinitroben-
zenes to diaminobenzenes14 as well as reduction of nitrophenols to aminophenols.
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