PHOTOCHEMICAL HYDROXYLATION OF SALICYLIC ACID DERIVATIVES WITH HYDROGEN PEROXIDE, CATALYZED WITH Fe(III) AND SENSITIZED WITH METHYLENE BLUE

Article abstract of DOI:10.1135/cccc19940905

Substitution of hydrogen in the carboxy or hydroxy group of a salicylic acid molecule with a methyl group, which hinders the coordination of Fe(III), results in a pronounced reduction of photocatalytic effects.The complex of Fe(III) with salicylic acid is the precursor of the thermal catalyst arising on irradiation.

Full text of DOI:10.1135/cccc19940905

Photochemical Hydroxylation
905
PHOTOCHEMICAL HYDROXYLATION
OF SALICYLIC ACID DERIVATIVES
WITH HYDROGEN PEROXIDE, CATALYZED WITH Fe(III)
AND SENSITIZED WITH METHYLENE BLUE
a
b
a
Stanislav LUNAK , Jacques MUZART and Jirina BRODILOVA
a Institute of Inorganic Chemistry,
Academy of Sciences of the Czech Republic, 160 00 Prague 6, The Czech Republic
b Universite de Reims Champagne-Ardenne, 51062 Reims, France
Received June 25, 1993
Accepted September 14, 1993
Substitution of hydrogen in the carboxy or hydroxy group of a salicylic acid molecule with a methyl
group, which hinders the coordination of Fe(III), results in a pronounced reduction of photocatalytic
effects. The complex of Fe(III) with salicylic acid is the precursor of the thermal catalyst arising on
irradiation.
In our previous papers we described the marked photocatalytic effects of Fe(III) com-
pounds in photo-initiated hydroxylation of salicylic1,2 and benzoic3,4 acids. We demon-
strated1 that a thermal hydroxylation catalyst, viz. Fe(II), is photochemically generated
during the reaction.
Two alternative explanations of iron-catalyzed hydroxylations have been suggested
in the literature:
1. Conventional explanation in terms of the Fenton reaction with the participation of
free hydroxyl radicals, which are active hydroxylation agents5 (Eq. (A)).
Fe2+ + H2O2
Fe3+ + OH− + OH•
(A)
The same hydroxylation agent, however, must be taken into account in the photo-
initiated dissociation of hydrogen peroxide6.
hν
H2O2
2 OH•
(B)
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906
Lunak, Muzart, Brodilova:
For the hydroxylation of benzoic acid, Ogata and coworkers6 suggest Eqs (C) and (D).
(C)
(D)
+
H2O
It is, however, questionable whether a reaction of two radical intermediates (reaction
(D)) can play a major role.
2. The alternative mechanism suggested involves the formation of ferryl ions, FeO2+
(Eq. (E)), which interact with the hydroxyl substrate, often giving a complex.
Fe2+ + H2O2
FeO2+ + H2O
(E)
For instance, Groves and coworkers7,8 used reaction (F) to describe the hydroxylation
of cyclohexanol.
(F)
This mechanism is strongly supported by the occurrence of stereoselective hydroxyla-
tion products but it fails to account for the occurrence of free hydroxyl radicals.
The aim of the present work was to contribute to the insight into the mechanism by
examining the role of coordination of the catalyzing iron ion by the substrate hydoxy-
lated. Salicylic acid was chosen as the substrate, and the effect of decrease in the coor-
dination affinity as a result of substitution of the hydrogen atom in the carboxy and/or
hydroxy group by a methyl group was investigated.
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Photochemical Hydroxylation
EXPERIMENTAL
Chemicals
907
Hydrogen peroxide p.a. (Chemical Works, Sokolov), FeCl3 p.a. (Cambrian Chemicals), Fe(III) acetyl-
acetonate (Merck, Darmstadt), methylene blue (Merck, Darmstadt), salicylic acid (Reanal, Budapest),
and 2,3- and 2,5-dihydroxybenzoic acids (Merck, Suchhardt) were used as received. Redistilled water
was additionally purified by a process involving photooxidative degradation of organic impurities as
described previously1.
Apparatus
The photochemical reaction was conducted in a 1 cm cell accommodated in an aluminium block
thermostatted to 25 °C. An SHC 250 W sodium high-pressure discharge lamp (Tesla Holesovice)
with the luminous flux maximum at 589 nm was used for irradiation. In the solutions studied, radia-
tion of 589 nm wavelength is predominantly absorbed by methylene blue; Fe(III) compounds absorb
a minor fraction, whereas the hydroxylated derivatives do not absorb radiation in this spectral region.
Absorption spectra of methylene blue, hydrogen peroxide and salicylic acid and its derivatives are
shown in Fig. 1.
Irradiation was discontinued periodically and the reaction mixture was analyzed on a liquid chro-
matograph equipped with a column 150 × 3.2 mm packed with Separon SCX C18, 7 µm (Tessek,
Prague). A UV detector (Unicam Philips) was used, measuring absorbance at 300 nm. The mobile
phase was constituted by a methanol–water–1 M phosphoric acid 53 : 46 : 1 mixture. Its flow rate
was 0.5 ml min−1 at a pressure of 2.5 – 3 MPa. A volume of 5 µl was invariably injected.
RESULTS AND DISCUSSION
The photocatalytic effect of Fe(III) compounds is most marked in the hydroxylation of
salicylic acid with hydrogen peroxide initiated by visible radiation and sensitized with
methylene blue. The photocatalytic effect of iron(III) acetylacetonate (Fe(acac)3) is
FIG. 1
Absorption spectra (substance, concentration
in mmol l−1): 1 methylene blue, 0.04; 2 hy-
drogen peroxide, 10; 3 methyl salicylate, 1;
4 salicylic acid, 1; 5 methyl o-methoxyben-
zoate, 1; 6 o-methoxybenzoic acid, 1
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

908
Lunak, Muzart, Brodilova:
demonstrated in Fig. 2. As in the case of FeCl3 described previously, a high photocata-
lytic activity and an autocatalytic reaction course were also observed in the present
study. The autocatalytic nature is due to a gradual accumulation of Fe(II) which is the
photochemically generated thermal reaction catalyst. A photocatalytic effect was also
found, as expected, in the photochemical interactions of the other salicylic acid deriva-
tives examined. The photocatalytic effect of Fe(acac)3 on the hydroxylation of methyl
salicylate is documented by Fig. 3.
When considering the reaction mechanism, the high stability constants of Fe(II)
(ref.9) and, in particular, Fe(III) (refs10,11) with salicylic acid must be taken into ac-
count. In the reaction conditions where virtually all Fe(III) (as well as the generated
Fe(II)) is bonded in complexes with salicylic acid, the complexes apparently play a
decisive role in the hydroxylations studied. This assumption is borne out by the results
shown in Fig. 4, comparing the courses of hydroxylations, under identical conditions,
of salicylic acid and of its derivatives in which the hydrogen atoms in the carboxy
and/or hydroxy groups are replaced by methyl groups. The results indicate unambigu-
ously that the hydroxylation rate increases with increasing strength of Fe(III)/Fe(II)
coordination.
t, min
FIG. 2
FIG. 3
Effect of photocatalyst (Fe(acac)3) concentra-
tion on salicylic acid (S) loss. Starting concen-
trations (mmol
methylene blue 0.01, hydrogen peroxide 10.
Fe(acac)3 concentration (µmol l−1): 1 1, 2 2.5,
3 5, 4 7.5, 5 10
Effect of photocatalyst (Fe(acac)3) concentra-
tion on methyl salicylate (S) loss. Starting con-
centrations (mmol l−1): methyl salicylate 0.7,
methylene blue 0.01, hydrogen peroxide 10.
Fe(acac)3 concentration (µmol l−1): 1 0, 2 3, 3
10, 4 50
l
−1): salicylic acid 0.7,
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Photochemical Hydroxylation
909
Substitution of the hydrogen atom in the carboxy group by a methyl group not only
affects reactivity (photocatalyst activity) but also has a significant effect on the reaction
products. Hydroxylation of salicylic acid under the conditions applied (visible radia-
tion, sensitization with methylene blue, Fe(III) photocatalyst) provides 2,3-dihydrox-
ybenzoic acid as the main products, with a significant fraction of 2,5-dihydroxybenzoic
acid (Table I). On the other hand, hydroxylation of methyl salicylate in position 3 does
not occur, the −OH group entering into positions 4 and 5. The 2,6-dihydroxy derivative
is not formed either, this compound, however, was also nearly absent in the case of
salicylic acid. The effect of the Fe(acac)3 photocatalyst concentration on the formation
of the 2,4- and 2,5-dihydroxy derivatives is illustrated by Figs 5 and 6, respectively.
While the formation of the 2,5-dihydroxy derivative can be observed in the absence of
Fe(III), the 2,4-dihydroxy derivative is not detected. This is consistent with the fact that
the 2,5-derivative is a typical product of direct interaction of excited hydrogen peroxide
with salicylic acid, whereas the 2,4-derivative is not formed in appreciable quantities
during that interaction. Hence, 2,4-dihydroxybenzoic acid can be regarded as a product
of the reaction catalyzed by the photochemically generated catalyst.
t, min
FIG. 4
FIG. 5
Effect of substitution of the hydrogen atom in
the hydroxy/carboxy group by a methyl group
on the loss of substrate. Starting concentrations
(mmol l−1): substrate 0.7, methylene blue 0.01,
hydrogen peroxide 10, Fe(acac)3 0.01. Sub-
strate: 1 methyl o-methoxybenzoate, 2 methyl
salicylate, 3 o-methoxybenzoic acid, 4 salicylic
acid
Effect of photocatalyst (Fe(acac)3) concentra-
tion of the formation of methyl 2,4-dihydroxy-
benzoate (P). Starting concentrations (mmol l−1):
methyl salicylate 0.7, methylene blue 0.01, hy-
drogen peroxide 10, Fe(acac)3: 1 0, 2 0.003, 3
0.01, 4 0.05
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

910
Lunak, Muzart, Brodilova:
TABLE I
Photohydroxylation of salicylic acid (c0 = 0.7 mmol l−1) with hydrogen peroxide (c0 = 10 mmol l−1
)
sensitized with methylene blue (c0 = 0.01 mmol l−1) and photocatalyzed by Fe(acac)3. Products:
2,5-dihydroxybenzoic acid (upper values) and 2,3-dihydroxybenzoic acid (lower values)
Product concentration, µmol l−1, in a time of
cFe(III)
µmol l−1
60 min
120 min
180 min
240 min
300 min
0
0
0
0
0
traces
traces
traces
traces
4
6
1.0
2.5
5.0
7.5
10.0
0
0
0
0
traces
traces
2
3
7
8
traces
traces
7
7
24
29
37
51
44
67
2
4
30
35
43
60
46
69
47
75
12
13
44
61
48
73
48
76
49
78
28
34
–
–
36
62
38
70
38
69
FIG. 6
Effect of photocatalyst (Fe(acac)3) concentration
of the formation of methyl 2,5-dihydroxyben-
zoate (P). Starting concentrations (mmol l−1):
methyl salicylate 0.7, methylene blue 0.01, hy-
drogen peroxide 10, Fe(acac)3: 1 0, 2 0.003, 3
0.01, 4 0.05
t, min
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Photochemical Hydroxylation
911
Based on published as well as our data we suggest that the hydroxylation proceeds
by two different mechanisms in the reaction systems under study. In the absence of
catalyst the excited substrate interacts directly with hydrogen peroxide. The noncata-
lyzed photohydroxylation is described by Eqs (G) – (I), where S is substrate (salicylic
acid or its methyl derivative), Sox is the substrate hydroxylation product, and MB is
methylene blue.
hν
MB
(MB)*
(G)
(H)
(I)
(MB)* + S
S* + H2O2
MB + S*
Sox + H2O
It should be emphasized that the hydroxylation species which is most frequently
considered, i.e. the hydroxyl radical, does not form by interaction according to Eqs (J)
and (K). This conclusion is warranted by our results of investigation of hydrogen per-
oxide photolysis initiated by visible radiation and sensitized by methylene blue12,13
which does not take place if no photocatalyst (such as Fe(III)) is present.
,
(MB)* + H2O2
(H2O2)*
MB + (H2O2)*
2 OH•
(J)
(K)
In the presence of Fe(acac)3 the interaction of substrate with hydrogen peroxide in-
volves, in addition to the above direct mechanism, also the photocatalytic mechanism,
which starts to predominate even at low photocatalyst concentrations (see effect of
Fe(acac)3). The photocatalytic mechanism consists of photochemical generation of the
thermal reaction catalyst (i.e. Fe(II)) which subsequently catalyzes the thermal interac-
tion of substrate with hydrogen peroxide. This mechanism can be schematically de-
scribed by Eqs (L) – (P) in which MBox is the oxidized leuco form of methylene blue.
1. Photochemical generation of catalyst, i.e. of Fe(II)
(MB)* + Fe(III)Sn
MBox + Fe(II)Sn
(L)
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912
2. Catalyzed thermal reaction
Fe(II)Sn + H2O2
Lunak, Muzart, Brodilova:
Fe(II)S(n − 2)(Sox)2
(M)
(N)
Fe(II)S(n − 2)(Sox)2 + 2 S
Fe(II)Sn + 2 Sox
3. Catalyst deactivation
Fe(II)Sn + H2O2
Fe(II)Sn + OH•
Fe(III)Sn + OH− + OH•
Fe(III)Sn + OH−
(O)
(P)
The hydroxyl radical formed by reaction (O) can, naturally, also attack the benzene ring
and cause its hydroxylation (Eq. (C)).
REFERENCES
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4. Lunak S., Sedlak P., Brodilova J., Lederer P.: Tetrahedron Lett. 30, 2277 (1989).
5. Walling C.: Acc. Chem. Res. 8, 125 (1975).
6. Ogata Y., Tomizawa K., Yamashita Y.: J. Chem. Soc., Perkin Trans. 2 1980, 616.
7. Groves J. T., van der Puy M.: J. Am. Chem. Soc. 96, 5274 (1974).
8. Groves J. T., Mc Clusky C. A.: J. Am. Chem. Soc. 98, 859 (1976).
9. Perrin D. D.: Nature 182, 741 (1958).
10. Colleter J. C.: Ann. Chim. (Paris) 5, 415 (1960).
11. Ogawa K., Tobe N.: Bull. Chem. Soc. Jpn. 39, 227 (1966).
12. Lunak S., Sedlak P.: J. Photochem. Photobiol., A 68, 1 (1992).
13. Sedlak P., Kubat P., Engst P., Lunak S.: Proc. 9th ISPPCC, Fribourg 1991, p. 69. University of
Fribourg, Perolles, Fribourg 1991.
Translated by P. Adamek.
Collect. Czech. Chem. Commun. (Vol. 59) (1994)