Hydroxylation of aromatic amines with dioxygen in photooxidation sensitized by substituted phthalocyanines

Article abstract of DOI:10.1016/j.mencom.2020.01.021

Photooxidation of aniline and its methyl and chloro derivatives with dioxygen sensitized by substituted zinc (PcZn) and palladium (PcPd) phthalocyanines in solution and on the carrier surface upon visible light irradiation affords selectively the corresponding p-aminophenols. Active and the most stable PcPd derivative adsorbed on Amberlite XAD 7HP provides conversion of 2,6-dimethylaniline with selectivity over 90% without the loss of sensitizer activity at least in 8 repeated cycles, the overall turnover number of the sensitizer being greater than 25,000.

Full text of DOI:10.1016/j.mencom.2020.01.021

Mendeleev
Communications
Mendeleev Commun., 2020, 30, 64–66
Hydroxylation of aromatic amines with dioxygen in photooxidation
sensitized by substituted phthalocyanines
a
a
a
Tatyana M. Fedorova, Valentina M. Derkacheva, Ekaterina N. Shevchenko,
a
b
b
Evgeny A. Luk’yanets, Eduard B. Bordaev and Oleg L. Kaliya*
a
b
State Scientific Center ‘Research Institute of Organic Intermediates and Dyes’, 123995 Moscow,
Russian Federation
M. V. Lomonosov Institute of Fine Chemical Technologies, MIREA – Russian Technological University,
1
19571 Moscow, Russian Federation. E-mail: eduard9308@gmail.com, saborisenkova@mail.ru
DOI: 10.1016/j.mencom.2020.01.021
Photooxidation of aniline and its methyl and chloro
derivatives with dioxygen sensitized by substituted zinc
NH2
NH2
(PcZn) and palladium (PcPd) phthalocyanines in solution
R1
R2
R3
and on the carrier surface upon visible light irradiation
affords selectively the corresponding p-aminophenols. Active
and the most stable PcPd derivative adsorbed on Amberlite
XAD 7HP provides conversion of 2,6-dimethylaniline with
selectivity over 90% without the loss of sensitizer activity at
least in 8 repeated cycles, the overall turnover number of the
sensitizer being greater than 25,000.
R1
R2
hn, PcM
+
O2
+ H O
+ H O
2 2
2
M = Zn, Pd
R3
OH
5
examples
Keywords: photooxidation, singlet oxygen, arylamines, sensitizers, photocatalysis, hydroxylation, p-aminophenols, phthalocyanines.
p-Aminophenols are valuable compounds for fine organic
synthesis in the interests of pharmaceutics, agricultural chemistry
and dye technology. Modern methods for their preparation from
Herein, we report an unusual hydroxylation of benzene ring
of some anilines into the corresponding p-aminophenols under
visible light irradiation of substrate solutions containing
substituted PcM in both homogeneous and heterogenized forms
1
2
chlorobenzene, phenol or nitrobenzene comprise several steps
that require high temperatures, pressure and expensive Pt/Pd-
catalysts. In particular, 4-amino-3,5-dimethylphenol is prepared
from 3,5-dimethylphenol via nitration and reduction of thus
obtained nitro intermediate.3 The traditional stoichiometric
oxidation of aniline with oxidants (KHSO , K Cr O , KIO )
1
8
(Scheme 1). These results were previously patented and in part
1
9,20
1
were presented at the conferences on catalysis.
Under the assumption that singlet oxygen O can be an
2
effective oxidant in the photooxidation of aromatic amines,
substituted phthalocyanines PcM (M = Zn, Pd) known as effective
5
2
2
7
3
1
produces polyaniline. Catalytic oxidation of aniline and other
arylamines using hydrogen peroxide and hydroperoxides occurs
at the amino-group with the formation of nitroso, azoxy, nitro
sensitizers of O formation under the action of visible-range
2
radiation were tested. Halogen 300 W lamp with a filter that cuts
off light with l < 450 nm or laser MRL 250 (l = 671 nm) with
output power of 150 mW or four 0.2 W LEDs (l = 664 nm) were
4
and azobenzenes. The use of the Fenton reagent [hydrogen
1
peroxide and iron(ii) salts] and photo-Fenton process causes
used as a light sources. The participation of O2 in the
5
6
degradation of aniline and o-toluidine. Destruction of aniline
photooxidation of amines with these sensitizers was confirmed
by special experiments confirming acceleration of the process in
molecules up to CO occurs under sunlight irradiation of TiO
with formation of trace quantities of p-aminophenol as
2
2
7
1
D O compared to H O (since O lifetime is 20 times longer in
2
2
2
1
intermediate product.
D O) and retardation upon NaN ( O physical quencher)
2
3
2
The oxidation of organic substrates with singlet oxygen is
addition.
comprehensively reviewed.8
–12
However, its interaction with
aromatic amines has not been systematically studied. In works
Nꢁ2
Nꢁ2
13–15
where it is nevertheless mentioned,
of p-aminophenol is not even discussed. p-Aminophenol and
,5-dimethylphenol16 are formed in living organisms under the
the purposeful production
Rꢀ
R2
Rꢀ
R2
hn
ꢂ O2 ꢂ ꢁ O
ꢂ ꢁ O
2 2
2
3
Rꢃ
Rꢃ
action of enzymes (a series of cytochrome P450) as metabolites of
aniline and 2,6-dimethylaniline, respectively. In addition,
ꢀ
2
ꢃ
Oꢁ
1
aꢄe
a R ꢅ R ꢅ R ꢅ ꢁ
ꢀ
2
ꢃ
b R ꢅ Meꢆ R ꢅ R ꢅ ꢁ
c R ꢅ R ꢅ ꢁꢆ R ꢅ Me
d R ꢅ R ꢅ Meꢆ R ꢅ ꢁ
e R ꢅ Clꢆ R ꢅ R ꢅ ꢁ
2
aꢄe
3
,5-dimethylphenol is formed in liver as a metabolite of lidocaine,
ꢀ
2
ꢃ
17
and it can serve as a marker of the liver function. This allowed us
to hope that in the absence of radical-forming (one-electron)
oxidation systems, selective hydroxylation of aromatic amines
with oxygen can become possible. These expectations have been
accomplished in the present study.
ꢀ
2
ꢃ
ꢀ
2
ꢃ
Scheme 1 Reagents and conditions: PcM, MeCN–H O, 25 °C, visible light
2
irradiation.
©
2020 Mendeleev Communications. Published by ELSEVIER B.V.
–
64 –
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.

Mendeleev Commun., 2020, 30, 64–66
R3
R2
Table 1 Conversion of 1d and selectivity towards 2d in homogeneous
photooxidation in the presence of some zinc phthalocyanines.a
R4
N
R1
N
PcZn complex
Reaction characteristics
3
a
3c
78
70
3d
75
50
3e
25b
-
3f
20b
-
R1
R4
N
M
N
R2
R3
R3
Conversion (%)
Selectivity (%)
95
N
N
> 90
R2
a
–3
–5
–3
R4
N
N
R1
[1d] = 0.01 mol dm , [PcZn] = 0.25×10 mol dm , MeCN–H2O (9:1),
irradiation time 4 h, laser. b The reaction was complete after full solution
bleaching (~50 min irradiation).
R1
R4
Taking into account the advantages of the use of heterogeneous
sensitizers on homogeneous systems, the possibility of arylamine
photooxidation has been studied in the presence of heterogenized
PcM on a carrier. This possibility was not obvious because a
R2
R3
3
a–h
1
priori the loss of immobilized phthalocyanine activity in the O2
1
2
4
3
t
a M = Zn, R = PhSO , R = R = H, R = Bu
2
generation due to intermolecular interaction (aggregation) and
the inevitable quenching of PcM excited triplet states seemed
probable.
1
2
3
4
b M = Zn, R = PhSO , R = R = R = H
2
1
3
4
2
c M = Zn, R = R = R = H, R = PhSO
d M = Zn, R = PhS, R = R = R = H
e M = Zn, R = PhS, R = R = H, R = Bu
f M = Zn, R = R = R = H, R = Bu
g M = Zn, R = R = H, R = R = C(O)NBu
h M = Pd, R = R = H, R = R = C(O)NBu
2
1
2
3
4
1
2
4
3
t
Heterogenization of phthalocyanines 3a,b,d,f–h was carried
out in two ways. First, equilibrium adsorption onto amino-
modified silica gel (NH -SiO ) or Amberlite XAD7HP (Amb)
1
3
4
2
t
1
4
2
3
2
2
2
1
4
2
3
2
provided adsorbed heterogeneous sensitizers (AHSs). Second,
covalently grafting of compounds 3b,d,e to the NH groups of
the carrier afforded covalently grafted heterogenized sensitizers
2
2
1
Synthesis of zinc phthalocyanines 3a–f and palladium ones
g,h was reported previously.
2
2
3
(GHSs). Heterogeneous sensitizers are characterized by the
–1
Photooxidation of anilines 1a–e in water-organic mixtures
H O–MeCN, H O–MeOH) was carried out in a glass reactor
35 mm in diameter) at continuous stirring and saturation with
number of PcM moles per 1 g of the carrier (a, mol g ), while
the substrate/PcM molar ratio (a) is used for characterization of
the photooxidation process.
(
(
2 2
dioxygen. The reaction did not occur in anhydrous MeOH or
MeCN. The reaction progress was monitored by seeding of
sample aliquots at the certain time intervals. Conversion of
In fact, the heterogeneous process occurred with specific (per
molePcM)rateandselectivityveryclosetothoseinhomogeneous
systems. The regularities of the influence of both the nature of R
†
substrate and yield of the products were measured by HPLC.
in R -PcZn and the central metal atom in the PcM (M = Zn, Pd)
4
Photooxidation products 2a–e were identified by coincidence
of their retention volumes and retention times with those of
authentic samples purchased from Sigma-Aldrich. Compounds 2
were mostly the only products of the photooxidation. The
on the efficiency of arylamine photooxidation also retained.
However, PcM aggregation in heterogeneous sensitizer
1
surface layer occurs, and the part of active in O generation
2
monomeric and inactive aggregated PcM forms can be clearly
monitored by the ratio between absorption band intensities of
‡
structure of product 2d was additionally confirmed by GC/MS.
The mass spectrum of isolated compound 2d contained a
molecular ion peaks with m/z 136 [M–H] and 107 [M–2Me] .
monomeric (Q-band at lmax = 668–720 nm, A ) and dimeric
1
+
+
§
(609–654 nm, A ) PcZn forms (Table 2) in UV-VIS spectra of
2
The influence of substituent (R) nature in R -PcZn on the
AHS (on NH -SiO ) and GHS suspensions in glycerol.
4
2
2
sensitizer effectiveness was investigated on the example of
photooxidation of substrate 1d (Table 1). The selectivity of 2d
formation in the presence of 3a exceeded 90%.
The introduction of electron-withdrawing substituents, such
as PhSO , into the PcZn molecule leads to decrease in the
2
aggregation of the complexes on the NH -SiO surface and,
2
2
The substrate conversion was determined based on the
sensitizerstabilityandwashigheratgreaterelectron-withdrawing
properties of the substituents. In full compliance with these data,
the highest conversions were achieved in the case of 3a,b and the
lowest ones with sensitizers containing donor substituents.
The effect of the nature of the central metal atom in PcM was
defined by comparing the efficiencies of 3g and 3h bearing the same
consequently, to their higher photoactivity in the oxidation (see
Table 2). The higher values of 1a conversion with GHS compared
to that withAHS with the use of the same PcZn are also consistent
with UV-VIS spectral data (see Table 2). Selectivity towards 2a
being 90% in the initial reaction period (up to 45 min) further
reduces to 60–70% due to insufficient stability of the product
under the reaction conditions.
–3
substituents under the same conditions ([1d] = 0.01 mol dm ,
Table 2 Characteristics of the AHS and GHS absorption spectra and
–5
–3
[
PcM] = 0.25×10 mol dm , MeCN–H O, 9 : 1, LEDs). The
2
conversion of 1a in the presence of heterogeneous sensitizers.a
conversions of substrate 1d upon 90 min irradiation with 3g and
h were 56.8 and 96.3%, respectively. The higher activity of 3h
AHS
Conversion
GHS
3
PcZn
Conversion
(%)
compared to its Zn analogue 3g is determined not only by higher
stability of 3h (absence of degradation) but also, probably, by the
l1/ l2
A /A
l1/ l2
A /A
1 2
1
2
(%)
1
higher quantum yield of O for carbamoyl derivatives of PcPd
3a
3b
671/609 5.5
668/630 2.4
711/654 1.4
677/635 2.1
–3
90
45
15
14
-
-
-
2
23
24
than those of PcZn, 0.78±0.08 and 0.56, respectively.
669/620 3.6
720/654 2.7
684/640 3.0
76
38
59
3d
†
3f
An HP 1100 Agilent series chromatograph equipped with Zorbax
a
EclipseXDB-C8 column (4.6 x 150 mm, 5 µm) and UV-detector
[1a] = 0.4 mmol dm , 10% aq. MeOH (35 ml), irradiation time 2 h,
(
at 250 nm) was used [the mobile phase was H O–MeCN (40 : 60), a flow
a = 40, halogen lamp.
2
–1
rate was 1 ml min , internal standard was naphthalene].
‡
§
GC/MS spectra were measured on a GC-MS Varian Saturn 2100
UV-visible absorption spectra were recorded on an HP-8453 UV-Vis
instrument.
spectrophotometer.
–
65 –

Mendeleev Commun., 2020, 30, 64–66
The overall turnover number exceeded 25,000 at the selectivity
for 2d being higher than 90%.
The effect of substituents in the substrate molecule was
investigated for the series of ortho- and meta-methyl-substituted
anilines, the observed rate constants of pseudo first order reaction
are given in Table 3.
1
00
2
8
0
0
0
0
6
1
4
2
The effect of electronic properties of substituents in aniline
molecules on kobs values is consistent with the electrophilic
1
nature of the oxidant, O .
2
0
0
20
40
60
t/min
80
100 120
In conclusion,
substituted PcZn and PcPd in both
homogeneous and heterogenized versions are effective
sensitizers for photooxidation of anilines, the best sensitizers
providing selective formation of p-aminophenols.
Figure 1 Conversion of 1d in the presence of AHSs (1) 3g/Amb and (2)
h/Amb. [1d] = 0.01 mol dm , LEDs, MeCN–H O (1:1), a = 3950,
–
3
3
2
selectivity towards 2d > 90%.
This study was supported by Russian Foundation for Basic
Research (grant nos. 16-29-10674 and 16-03-00298).
Despite the more ordered structure of GHS-3b surface layer
compared with AHS-3b, these Bu -containing materials are two
t
times more active than their heterogenized analogues based on
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3
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2
2
2
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1
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2
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1
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2
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and quasi continuous experiments, and it was used in eight
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1
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2
015, 8, 143.
2
–1
Substrate
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Hammett s-constants
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2
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sp = –0.14
sm = –0.06
d
e
sp = 0.24
a
–3
–6
–1
[
1a–e] = 1 mmol dm , AHS-3a/Amb, a = 0.8×10 mol g , a = 1795,
MeCN–H O (1:1), LEDs, product selectivity > 80%.
Received: 12th July 2019; Com. 19/5985
66 –
2
–