Flavin derivatives immobilized on mesoporous silica: a versatile tool in visible-light photooxidation reactions

Article abstract of DOI:10.1007/s00706-017-2127-1

Abstract: Anchoring riboflavin tetraacetate to mesoporous silica (MCM-41) provides a heterogenized flavin photocatalyst, which was proven to be an effective catalyst for chemoselective sulfoxidation, benzyl alcohol oxidation, and photoesterification reactions. Analogously heterogenized 1-butyl-7,8-dimethoxyalloxazine was shown to mediate sulfoxidation and benzylic oxidation reactions. This is the first application of an alloxazine photocatalyst in a redox reaction. The use of these heterogeneous catalysts allows simple operation and work-up of the reaction mixtures. Graphical abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s00706-017-2127-1

Monatsh Chem
https://doi.org/10.1007/s00706-017-2127-1
ORIGINAL PAPER
Flavin derivatives immobilized on mesoporous silica: a versatile
tool in visible-light photooxidation reactions
1
1
1
•
Martin Kurfirt Jessica Spackova Eva Svobodova Radek Cibulka1
ˇ
•
•
ˇ
ˇ
´
´
Received: 22 November 2017 / Accepted: 3 December 2017
Ó Springer-Verlag GmbH Austria, part of Springer Nature 2017
Abstract Anchoring riboflavin tetraacetate to mesoporous
silica (MCM-41) provides a heterogenized flavin photo-
catalyst, which was proven to be an effective catalyst for
chemoselective sulfoxidation, benzyl alcohol oxidation,
and photoesterification reactions. Analogously heteroge-
nized 1-butyl-7,8-dimethoxyalloxazine was shown to
mediate sulfoxidation and benzylic oxidation reactions.
This is the first application of an alloxazine photocatalyst in
a redox reaction. The use of these heterogeneous catalysts
allows simple operation and work-up of the reaction
mixtures.
Introduction
Photoredox catalysis utilizing an organic dye and visible
light (photoorganocatalysis) has already been established
as a useful tool in organic synthesis allowing mild and
effective procedures or even new chemical transformations
[1–5]. Within the last decade, a large number of photoac-
tive organic molecules have been involved in
photocatalysis. Nevertheless, most of them have been
applied as homogeneous catalysts [1]. The number of their
‘‘heterogenized’’ analogs has still been very limited despite
their advantages such as the ease of their separation from
the reaction mixture [6]. Rose bengal immobilized on
Wang resin [7] or iodo-Bodipy immobilized on siliceous
molecular sieves [8] can be given as examples. Recently,
we reported anchoring a flavin analog, 7,8-dimethoxy-3-
methylalloxazine (1a), to mesoporous silica MCM-41 to
provide an efficient visible-light photocatalyst 2 used for
[2?2] photocycloadditions [9]. Thus, we demonstrated that
mesoporous silica nanoparticles, which are transparent
over a broad range of wavelengths [10], could be a suit-
able solid matrix for photocatalyst immobilization.
Graphical abstract
irradiation
450 nm
tetraacetylribityl
irradiation
400 nm
R
N
H3
C
N
N
MeO
MeO
N
N
O
O
N
N
Me
H3
C
N
R
O
O
O
O
=
R
C
N
Si
O
H2
H
O
= MCM-41
Heterogeneous photocatalysts for: sulfoxidation,
benzylalcohol oxidation
esterification
Keywords Esterification Á Heterocycles Á
Heterogeneous catalysis Á Oxidation Á Photochemistry Á
Sulfoxidation
Alloxazines (such as compound 1) are considered as
flavin derivatives, however, the original flavins are based
on an isoalloxazine heterocyclic core (Fig. 1) [11, 12].
Among the isoalloxazines, riboflavin (3a; vitamin B2) and
its derivatives, namely riboflavin tetraacetates 3b and 3c
(Fig. 1) have been commonly used as photocatalysts
[13, 14]. When irradiated with blue light (450 nm), flavin 3
has been shown to mediate the aerobic oxidation of benzyl
alcohols and other benzylic compounds to benzaldehydes
[15–23], chemoselective oxidation of sulfides to sulfoxides
[24, 25], esterification under Mitsunobu reaction conditions
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s00706-017-2127-1) contains supple-
mentary material, which is available to authorized users.
& Radek Cibulka
cibulkar@vscht.cz
1
Department of Organic Chemistry, University of Chemistry
and Technology, Prague, Czech Republic
123

ˇ
M. Kurfirt et al.
Scheme 1
0.8
0.6
0.4
0.2
0.0
200
150
100
50
Fig. 1 Homogeneous and heterogeneous flavin photocatalysts used
in this study: alloxazines 1 and 2, isoalloxazines 3 and 4
[26], or photoisomerization of electron-poor styrene
derivatives [27–29]. Thus, question appeared whether also
riboflavin derivatives could be heterogenized by our
approach with MCM-41 similarly as alloxazines. Herein,
we report the preparation of the heterogeneous catalyst 4
with riboflavin tetraacetate acting as the photoactive unit
and its participation in visible-light aerobic sulfoxidation,
benzyl alcohol oxidation and esterification reactions.
Moreover, since alloxazine catalysts have only been tested
in [2?2] photocycloaddition and photoisomerization reac-
tions [9, 30–32], we have proven that both homogeneous
catalyst 1b and its heterogeneous analog 2 participate in the
aforementioned photooxidation reactions.
0
1.3
1.2
1.1
1.0
0.9
0.8
200
150
100
50
0
200
300
400
500
600
700
λ / nm
Fig. 2 Comparison of UV–Vis absorption and emission spectra of 3b
and 4 (solid state)
The photooxidation of sulfides to sulfoxides
Two effective procedures have been described using ribo-
flavin tetraacetate 3b for the chemoselective aerobic
oxidation of sulfides to sulfoxides. At first, singlet oxygen
sulfoxidation (photooxidation—type II [34, 35]) was
described to occur in alcohols (Scheme 2a) [25]. This
procedure utilizes the ability of 3b to sensitize the pro-
duction of singlet oxygen (UD = 0.6 in ethanol). The
highest efficiency for this sulfoxidation procedure was
Results and discussion
Heterogeneous catalyst 2 with an alloxazine core was
prepared in our preliminary communication [9], starting
from acyl chloride 5 and mesoporous silica 7 with its
surface modified with aminopropyl groups attached by a
silane function (the silane function was introduced using
aminopropyltriethoxysilane [33]). Heterogenized isoallox-
azine 4 was synthesized analogously, i.e., by flavin unit
attachment in the form of acyl chloride 6 (Scheme 1). The
catalysts were analyzed with SEM, powder X-ray diffrac-
tion, and elemental analysis (see Supporting Information
for details). As observed for alloxazine catalyst 2 [9], the
positions of the absorption and emission bands of isoal-
loxazine dye 4 were not significantly affected by its
anchoring to the solid phase (Fig. 2), although a change in
its absorption band shape caused a small bathochromic
shift of the maxima position.
Scheme 2
123

Flavin derivatives immobilized on mesoporous silica: a versatile tool in visible-light…
Table 1 Aerial oxidation of
sulfides on analytical scale
Entry
Sulfide
Catalyst
Time/min
Solventa
Conversion/%
Note
1
Thioanisole
3b
3b
4
30
15
EtOH/H2O
CH3CN/H2O
BTF
84
Ref. [25]
Ref. [24]
2
Quant.
14
3
30
4
4
30
EtOH/H2O
CH3OH/H2O
CH3OH/H2O
CH3OH/H2O
CH3CN
16
5
4
30
26
6
4
420
10
94
b
b
b
7
1b
1b
1b
2
88
8
10
19
9
10
CH3CN/H2O
CH3CN
100
66
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
30
2
30
CH3CN/H2O
CH3CN/H2O
CH3OH/H2O
CH3CN
Quant.
100
93
4-Methoxythioanisole
3b
4
15
Ref. [24]
240
10
b
b
1b
1b
2
93
10
CH3CN/H2O
CH3CN
Quant.
Quant.
Quant.
Quant.
49
30
2
30
CH3CN/H2O
CH3CN/H2O
CH3OH/H2O
CH3OH/H2O
CH3CN
4-Nitrothioanisole
3b
3b
4
90
Ref. [24]
1440
600
120
120
120
120
1
Ref. [25]
b,c
64
b
b
1b
1b
2
45
CH3CN/H2O
CH3CN
93
57
2
CH3CN/H2O
CH3CN/H2O
CH3OH/H2O
CH3CN
43
Dibutylsulfide
3b
4
100
100
90
Ref. [24]
60
b
b
1b
1b
2
10
10
CH3CN/H2O
CH3CN
78
15
99
2
15
CH3CN/H2O
100
Conditions: reaction was performed in vial in 1 cm3 of solvent under oxygen (balloon); csubstr. = 1 9 10-2
mol dm-3, ccat. = 2 9 10-4 mol dm-3
aMixture of solvents: CH3CN/H2O 85:15 v/v, CH3OH/H2O 95:5 v/v, b approx. 5% of sulfone was detected.
c
10% of the catalyst
observed in an ethanol/water mixture (95:5, v/v) producing
sulfoxides within several hours (Table 1, entry 1). Later,
both an electron transfer (photooxidation—type I, Sche-
me 2b) and singlet oxygen mechanism were shown to
contribute to the aerobic photooxidation of sulfides pro-
vided by 3b in acetonitrile/water mixtures (entry 2). This
combination of both pathways allows an unusually broad
substrate scope when compared to other photocatalytic
oxidation procedures [24]. The reaction was even faster
and more efficient when compared to that observed in
alcohols achieving a quantum yield of up to 1.3 in ace-
tonitrile/water (85:15, v/v) for thioanisole (8b) oxidation.1
When using the heterogeneous isoalloxazine catalyst 4,
analogous reaction conditions suitable for both the electron
transfer and singlet oxygen processes were used. Unfortu-
nately, we observed an unexpected release of the
chromophore from the matrix during irradiation of the
catalyst in acetonitrile-based solvents. On the other hand,
catalyst 4 was found to be stable in alcohols and thus,
methanol or ethanol/water mixtures (per analogy to the
reactions performed with homogeneous catalyst 3b) were
used. In addition, trifluoromethylbenzene (BTF) was tested
as a solvent known to be beneficial from the point of view
of the stability of the photocatalyst [36]. A mixture of
methanol/water (95:5, v/v) was shown to be the superior
medium for the oxidation of thioanisole (model substrate)
1
The maximum quantum yield of photosulfoxidation is 2.
123

ˇ
M. Kurfirt et al.
affording the sulfoxide in an almost quantitative yield after
a prolonged reaction time (c.f. entries 3–6). Complete
conversion to the sulfoxide was also shown for other sub-
strates such as 4-methoxythioanisole or aliphatic dibutyl
sulfide (entries 13 and 26). Similar to homogeneous cata-
lyst 3b, the oxidation of electron poor 4-nitrothioanisole in
a methanol/water mixture occurred with 4 with a moderate
conversion (c.f. entries 19 and 20). Importantly, almost
sulfoxidation reactions (with the exception of 4-nitroth-
ioanisole oxidation) provided by 4 were chemoselective
and did not give the corresponding sulfone products.
Alloxazines are weaker oxidizing agents when com-
pared to the isoalloxazines,2 however, they are known to be
excellent singlet oxygen sensitizers achieving quantum
yields for singlet oxygen production higher than 0.7 [38].
Despite this, they have never been tested either in sulfox-
idation or other oxidation reactions. At first, we performed
the sulfoxidation reaction using homogeneous catalyst 1b,
observing an almost quantitative conversion of thioanisole
to its corresponding sulfoxide after 10 min of irradiation in
both methanol/water and acetonitrile/water systems.
Unfortunately, the oxidation reaction was not chemose-
lective producing a significant amount of the corresponding
sulfone (entries 7–9). Interestingly, using heterogeneous
catalyst 2, the reaction provided the sulfoxide as the sole
product of the reaction (entries 10, 11). Similar favorable
results were observed for electron-rich 4-methox-
ythioanisole and aliphatic dibutyl sulfide (entries 29 and
30); lower yields were observed for 4-nitrothioanisole
(entries 23 and 24).
Scheme 3
Interestingly, both with homogeneous catalyst 3b and
heterogeneous catalyst 4, a significant positive effect using
acetic acid and molecular sieves was observed (c.f. entries
1–4 and 5–8, Table 2). The reaction catalyzed by 3b
afforded up to 90% conversion to benzaldehyde without
over-oxidation to the carboxylic acid. Heterogeneous cat-
alyst 4 showed a lower but still reasonable reactivity. The
presence of acetic acid does not influence the spectrum of
the chromophore (see Supporting Information) and its role
can be attributed to supporting the proton transfer process
[19]. The positive effect of the molecular sieves can be
explained by the decomposition of hydrogen peroxide
[26, 39], which is formed as a side product of the pho-
tooxidation reaction. In addition, with the ‘‘less
demanding’’ substrates, benzyl alcohol and 4-methoxy-
benzyl alcohol, both isoalloxazine catalysts 3b and 4
showed high efficiency achieving from good to high con-
versions (entries 17, 18, 21, and 22).
We found also the alloxazine catalysts can accomplish
the photooxidation of benzyl alcohols. With both homo-
geneous and heterogeneous alloxazine, electron-rich
4-methoxybenzyl alcohol and non-substituted benzyl
alcohol were oxidized with remarkable conversions (en-
tries 19, 20, 23, and 24). Using homogeneous alloxazine
1b, 4-chlorobenzyl alcohol was also oxidized to the alde-
hyde with high efficiency (entry 10).
The photooxidation of benzyl alcohols
Aerobic oxidation of benzyl alcohols mediated by ribo-
flavin tetraacetate 3b and visible light represents an
evergreen example among photooxidation reactions
(Scheme 3) [19, 21–23]. 4-Methoxybenzyl alcohol can be
easily oxidized by 3b in acetonitrile, acetonitrile/water as
well as in water. Since acetonitrile was not tolerated by
catalyst 4, we proved methanol and trifluoromethylbenzene
as the reaction media for the oxidation reactions catalyzed
using this heterogeneous catalyst. To our delight, our pre-
liminary experiments showed that catalyst 4 in both
solvents oxidizes even electron-deficient 4-chlorobenzyl
alcohol, which is described to be oxidized with 3b in the
presence of Sc(III) only [17, 23]. Therefore, we used this
challenging substrate to screen the reaction conditions.
Application of the heterogeneous catalysts
To demonstrate the potential practical applications of the
heterogeneous flavin photocatalysts 2 and 4 in photooxi-
dation reactions, we performed semipreparative
experiments with selected substrates including sulfides and
benzyl alcohols. Both 2 and 4 were shown to provide good
to excellent yields of sulfoxides derived from aromatic
(thioanisole) and aliphatic (dibutyl sulfide) sulfides
(Scheme 4). In the benzyl alcohol oxidation reactions, 4
was found to be the superior catalyst achieving high
preparative yields of 4-methoxybenzaldehyde (Scheme 5).
The yields of 4-chlorobenzaldehyde were lower, never-
theless, one should take in mind the high oxidation
potential of the substrate. Interestingly, in some cases,
when a quantitative conversion was achieved, the product
was isolated only by filtration to remove the catalyst and
evaporation of the solvent, i.e., in the oxidation of
2
A comparison of the oxidizing abilities of the alloxazines and
isoalloxazines is given by the redox potentials of - 1.3 and - 0.7 V
vs. SCE in acetonitrile for 7,8-dimethylalloxazine and 7,8,10-
trimethylisoalloxazine, respectively [37].
123

Flavin derivatives immobilized on mesoporous silica: a versatile tool in visible-light…
Table 2 Aerial oxidation of
Entry
Alcohol
Catalyst
Time/min
Solvent
Conversion/%
Note
benzyl alcohols on analytical
scale
1
4-Chlorobenzyl alcohol
3b
3b
3b
3b
4
720
720
720
720
720
720
720
720
400
400
600
600
720
720
720
720
720
720
720
720
720
720
720
720
BTF
37
64
84
90
25
20
21
48
34
90
16
45
19
19
26
22
80
32
68
70
79
95
16
71
2
CH3OH
BTF
a
a
b
3
4
CH3OH
BTF
5
6
4
CH3OH
BTF
a
a
7
4
8
4
CH3OH
CH3CN
CH3CN
BTF
9
1b
1b
1b
1b
2
a
a
a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
BTF
CH3CN
CH3CN
BTF
2
2
a
a
a
a
a
a
a
a
a
2
BTF
Benzyl alcohol
3b
4
CH3OH
CH3OH
CH3CN
CH3CN
CH3OH
CH3OH
CH3CN
CH3CN
1b
2
4-Methoxybenzyl alcohol
3b
4
1b
2
Conditions: reaction was performed in vial in 1 cm3 of solvent under oxygen (balloon); csubstr. = 1 9 10-1
mol dm-3, ccat. = 5 9 10-3 mol dm-3
a
b
˚
Additives: molecular sieves 4 A and acetic acid (1.2 equiv.), over-oxidation (\ 3% of benzoic acid)
Scheme 4
Scheme 5
A
B
the presence of triphenylphosphine. The role of the flavin
catalyst was to oxidize triphenylphosphine via single-
electron transfer to the reactive Ph3P•? species, which
subsequently activates the carboxylic group towards the
esterification reaction [26]. We performed the esterification
reaction using 3-(trifluoromethyl)benzoic acid and octanol
in the presence of catalyst 4 achieving 40% conversion
after 24 h of irradiation (Scheme 6). Using homogeneous
catalyst 3b, the conversion was not significantly higher
4-methoxythioanisole and dibutyl sulfide catalyzed by 2
and in 4-methoxybenzyl alcohol oxidation mediated by 4.
Riboflavin derivative 3c has been shown to be capable,
upon irradiation, of catalyzing esterification reactions in
123

ˇ
M. Kurfirt et al.
Scheme 6
(63%) [26]. The photoesterification procedure using 4 has
limitations when using non-oxidizable alcohols because the
benzylic substrates undergoes a side oxidation reaction to
give the aldehyde (c.f. previous chapter). Alloxazine 2 was
not found to be active in this reaction because of its lower
oxidation force (c.f. Ref. [37]).
reagents (temperature stabilized at 25 °C) was irradiated
with light-emitting diode (LED; 400 nm for catalysts 1b
and 2, 450 nm for catalyst 3b and 4) and stirred under
oxygen atmosphere (balloon). The reaction course was
monitored by HPLC (see Supporting Information for
details).
Visible-light-mediated oxidation of benzylalcohols
on analytical scale
Conclusion
We have proven that the concept of immobilizing an
organic dye on mesoporous silica MCM-41 is a general
method to prepare heterogeneous photocatalysts. Our pre-
liminary findings anchoring alloxazine derivative 1 on
MCM-41 [9] was extended to flavins 3 (isoalloxazine
derivatives), which are well established in pho-
toorganocatalysis. Thus, the as-prepared heterogenized
derivative 4 was shown to mediate sulfoxidation, benzyl
alcohol oxidation and photoesterification reactions, which
is similar to its homogeneous analog, riboflavin tetraacetate
3b. Nevertheless, preparing the heterogeneous catalyst by a
dye attachment to MCM-41 may be limited by the insta-
bility of the anchoring interaction under certain conditions.
This can be the main problem for compounds with strong
oxidizing properties upon irradiation. To date, alloxazines
have only been used in [2?2] photocycloaddition reactions
[30, 31]. We have shown for the first time that they have
great potential in chemoselective photooxidation reactions,
especially in those involving singlet oxygen. Therefore,
their application in photooxidations both in their homoge-
neous and heterogenized forms is expected to be a subject
of future research.
The experiments were performed in vials. Aliquots of stock
solution of reactants in appropriate solvents were added
directly into the vial to achieve concentration of flavin and
substrate 5 9 10-3 and 1 9 10-1 mol dm-3, respectively
(total volume was 1 cm3). Alternatively, additional com-
ponents were added: acetic acid (to achieve concentration
1.2 9 10-1 mol dm-3) and crushed molecular sieves 4 A
˚
(50 mg). The vial with reagents (temperature stabilized at
25 °C) was irradiated with LED (400 nm for catalysts 1b
and 2, 450 nm for catalyst 3b and 4) and stirred under
oxygen atmosphere (balloon). The reaction course was
monitored by HPLC (oxidation 4-chlorbenzyl alcohol) or
1H NMR (see Supporting Information for details).
Visible-light-mediated Mitsunobu-type esterification
on analytical scale
The experiments were performed in vials. Alcohol
(0.05 mmol), acid (0.06 mmol, 1.2 eq.), PPh3 (0.1 mmol),
˚
crushed molecular sieves 4 A (25 mg) and catalyst
(5 mol%) were added into the vial and 0.5 cm3 solvent was
added. Reaction mixture (temperature stabilized at 40 °C)
was irradiated with LED (400 nm for catalysts 1b and 2,
450 nm for catalyst 3b and 4) and stirred under oxygen
atmosphere (balloon). The reaction course was monitored
Experimental
1
by H NMR (see Supporting Information for details).
Visible-light-mediated sulfoxidations on analytical
scale
Visible-light-mediated sulfoxidation
on semipreparative scale
The experiments were performed in vials. Aliquots of stock
solution of reactants in appropriate solvents were added
directly into the vial to achieve concentration of flavin and
Sulfide (50 mg, 0.3–0.7 mmol), solvent (CH3OH/H2O
95:5, CH3CN or CH3CN/H2O 85:15, and catalyst 2 or 4 (2
or 10 mol%) were added into the double-neck flask.
Reaction mixture was bubbled with oxygen and irradiated
substrate 2 9 10-4 or 5 9 10-4 and 1 9 10-2 mol dm-3
,
respectively (total volume was 1 cm3). The vial with
123

Flavin derivatives immobilized on mesoporous silica: a versatile tool in visible-light…
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with four LEDs (450 nm). Reaction course was monitored
by TLC or by 1H NMR. Catalyst was filtered out through a
syringe filter (PTFE 25 mm, 0.45 lm) or celite, solvent
was evaporated. In some cases (see Supporting Informa-
tion), product was purified by column chromatography.
¨
¨
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Visible-light-mediated oxidation of benzyl alcohols
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Into each of four vials, aliquot of stock solution prepared
from benzylic alcohol (50 or 100 mg, 0.35–0.72 mmol),
methanol, glacial acetic acid (1.2 equiv.) and catalyst 4 (5
or 10 mol%) were added. After addition of crushed
˚
molecular sieves 4 A (50 mg), the reaction mixture was
¨
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bubbled with oxygen and irradiated with four LEDs
1
(450 nm). Reaction course was monitored by TLC or H
¨
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(2001) J Phys Chem A 105:10501
NMR. Catalyst and molecular sieves were filtered out
through syringe filter (PTFE 25 mm, 0.45 lm) or celite
and solvent was evaporated. In some cases (see Supporting
Information), product was purified by column
chromatography.
´
´
24. Nevesely T, Svobodova E, Chudoba J, Sikorski M, Cibulka R
(2016) Adv Synth Catal 358:1654
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Acknowledgements This project was supported by the Czech Sci-
ence Foundation (Grant No. 16-09436S).
27. Metternich JB, Gilmour R (2015) J Am Chem Soc 137:11254
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