Visible Light [2+2] Photocycloaddition Mediated by Flavin Derivative Immobilized on Mesoporous Silica

Article abstract of DOI:10.1002/cctc.201601654

7,8-Dimethoxy-3-methylalloxazine was immobilized on mesoporous silica (MCM-41) to provide a heterogenized flavin photocatalyst. Thus, the prepared heterogeneous catalyst 2 was found to sensitize the visible light [2+2] cycloaddition of various types of di

Full text of DOI:10.1002/cctc.201601654

Accepted Article
Title: Visible Light [2+2] Photocycloaddition Mediated by Flavin
Derivative Immobilized on Mesoporous Silica
Authors: Jessica Špačková, Eva Svobodova, Tomáš Hartman, Ivan
Stibor, Jitka Kopecká, Jana Cibulková, Josef Chudoba, and
Radek Cibulka
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10.1002/cctc.201601654
ChemCatChem
COMMUNICATION
Visible Light [2+2] Photocycloaddition Mediated by Flavin
Derivative Immobilized on Mesoporous Silica
Jessica Špačková,[a] Eva Svobodová,[a] Tomáš Hartman,[a] Ivan Stibor,[b] Jitka Kopecká,[c] Jana
Cibulková,[d] Josef Chudoba,[d] and Radek Cibulka*[a]
Abstract: 7,8-Dimethoxy-3-methylalloxazine was immobilized on
mesoporous silica (MCM-41) to provide a heterogenized flavin
photocatalyst. Thus, the prepared heterogeneous catalyst 2 was
found to sensitize the visible light [2+2] cycloaddition of various
types of dienes to produce corresponding cyclobutanes in high
yields and diastereoselectivities. Use of 2 allows procedure which is
advantageous by its simple operation and workup, no additives
requirement and minimum waste generation.
aerial photosulfoxidation can be given[10] as one of the few
examples of an heterogeneous photoorganocatalyst utilizing
another material.
The UV light promoted [2+2] cycloaddition of alkenes
providing cyclobutane derivatives is a textbook example of the
usefulness of light in organic synthesis because this process is
thermally disfavoured.[11,12] Recently, [2+2] photocycloadditions
have been described to also occur under visible light, which is
allowed when using a suitable photocatalyst/sensitiser[2,11d,13]
that absorbs in the appropriate region. Initially, Yoon[14] reported
visible-light procedures utilising the transition metal complexes,
The rapid development of visible-light photocatalysis has
produced many procedures that are useful in organic synthesis;
most of them are based on transition metal chromophores.[1]
Nowadays, metal-free alternatives are of a growing interest
thanks to the lower price of organic dyes, which may even allow
other chemistry to occur.[2] Regardless of their nature, dyes have
mostly been used as homogeneous catalysts. The number of
“heterogenized” photocatalysts has still been limited despite
their advantages such as the ease of separation from the
reaction mixtures.[3]
2+
Ru(bpy)3 and Ir[(Fbpy)2(tBubpy)]+, which mediated the [2+2]
photocycloaddition via electron and energy transfer, respectively.
Later attempts to replace these metal ion chromophores with an
organic photocatalyst resulted in
a
few metal-free
procedures.[15,16] We described one using the flavin derivative, 1-
butyl-7,8-dimethoxy-3-methylalloxazine (1), selected as suitable
alloxazine as it has absorption maximum in visible region (400
nm) while keeping its triplet energy high enough to excite
activated double bonds by energy transfer. System with 1
excelled with its versatility since upon irradiation with visible light,
it promoted the [2+2] photocycloaddition of both styrene dienes,
which are considered as electron-rich substrates, and electron-
poor bis(arylenones).[16] Very recently, we showed catalyst 1
can be applied even for nitrogen, sulfur and anhydride function
containing dienes.[17] Despite the effectiveness of this
photoorganocatalytic method, it suffers from some drawbacks: i)
the homogeneous flavin catalyst 1 must be removed by
chromatography, which is, moreover, not fully tolerated by some
cyclobutane products and ii) less reactive substrates did not
achieve full conversion under the homogeneous conditions.
Mesoporous silica nanoparticles[4] seem to be suitable solid
supports for photocatalysts because of their thermal stability,
versatile surface functionalisation, large surface area and,
importantly, good transparency over
a
broad range of
wavelengths.[5] Yet their use in the immobilization of molecular
photocatalysts is still uncommon. He[6] used Ru(bpy)32+ anchored
on mesoporous silica for the photooxidation of thioanisole. Mori
attached [Ir(Mebib)(ppy)Cl][7] and [Pt(tpy)Cl]Cl[8] with several
types of mesoporous silica, including MCM-41, MCM-48 and
SBA-15 with differing in pore dimensions and structures, and
used them for the photooxidation of trans-stilbene, styrene and
1-naphtol. Regarding organic photocatalysts, iodo-Bodipy on the
siliceous mesoporous molecular sieve, KIT-1, was used for an
aerobic oxidation – [3+2] cycloaddition cascade and is a rare
example.[9] Rose-bengal immobilized on Wang resin used for
Herein, we report anchoring flavin
1 (Figure 1) on
mesoporous silica nanoparticles, which we envisage will solve
the aforementioned specific problems with homogeneous
photoorganocatalytic [2+2] photocycloaddition. From a more
general point of view, we expect it could be a proof of concept
for the immobilization of flavin photocatalysts. To the best of our
knowledge, the use of a flavin-thiourea derivative anchored on
silica gel for benzyl alcohol oxidation[18] has still been the only
application of an immobilized flavin photocatalyst. Notably, dark
oxidations with flavin immobilized in an ionic liquid[19] or gold
nanoparticles[20] have been described.
[a]
[b]
J. Špačková, Dr. E. Svobodová, M.Sc. T. Hartman, Dr. R. Cibulka
Department of Organic Chemistry
University of Chemistry and Technology Prague
Technická 5, 166 28 Prague 6, Czech Republic
E-mail: cibulkar@vscht.cz
Prof. I. Stibor
Institute for Nanomaterials, Advanced Technologies and Innovations
Technical University of Liberec
Studentská 2, 461 17 Liberec, Czech Republic
Dr. J. Kopecká
Department of Physics and Measurements
Dr. J. Chudoba, M.Sc. J. Cibulková
[c]
[d]
Central laboratories
University of Chemistry and Technology Prague
Technická 5, 166 28 Prague 6, Czech Republic
Supporting information for this article is given via a link at the end of
the document.
Figure 1. Homogeneous (1) and heterogeneous (2) alloxazine photocatalyst
designed for [2+2] photocycloadditions.
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Mesoporous silica MCM-41 was selected as a readily
available siliceous matrix to support flavin; it was prepared in two
steps via the graft method.[21] Initially, a silica polymer was
formed after adding tetraethoxysilane (TEOS) to a water–
ammonia solution of hexadecyltrimethylammonium bromide
(CTABr). Then, the surfactant was removed using ethanolic
hydrogen chloride. Next, the surface of the nanoparticles was
With the prepared heterogeneous catalyst 2, we optimized
the reaction conditions for the visible light [2+2]
photocycloadditions using the same apparatus (including light
source) and model substrate (styryl ether 5) as in our previous
study with homogeneous catalyst 1.[16] As previously described,
the initial experiments were performed in a Schlenk tube in 1 mL
of deuterated solvent under an argon atmosphere (Table 1). The
solvents were selected to be easily removable by evaporation
since this single operation was thought to be needed for the
product isolation when using heterogeneous catalyst. The
amount of catalyst 2 (2 mg) was chosen as it contains
approximately the same amount of anchored alloxazine as that
used in the homogeneous catalysis with 1 (2.5-2.7 mol% relative
to the substrate).[23]
modified[6,22]
with
an
aminopropyl
group
using
aminopropyltriethoxysilane (APTES), followed by flavin unit
attachment using acyl chloride 3 to form an amide bond
(Scheme 1; for details, see Supporting Information). The catalyst
(Figure 2a) was analyzed by SEM showing spherical shaped
particles with diameter in the range approximately 450-650 nm
(Figure 2b). A hexagonal pore structure was confirmed by
powder X-ray diffraction (XRD). Using this method, each step
was monitored to prove a degree of structural ordering and
invariability of the (100) interplanar spacing correlating with
mesopore diameter (Figure 2d).[4d] Absorption and emission
maxima of alloxazine dye were found not to be affected by its
anchoring on solid phase (see Figure 2c).
Table 1. [2+2] Photocycloaddition of diene 5 mediated by heterogeneous
catalyst 2 and visible light (400 nm) on an analytical scale.[a] A result[16] for
homogeneous conditions with 1 is given for comparison.
Entry
Solvent
Time
[min]
Conversion[b]
[%]
Impurities[c]
1
CD3CN
Toluene-D8
CDCl3
60
quant.
quant.
55
no
no
yes
yes
no
no
yes
-
2
60
3
60
4
CDCl3
120
60
70
Scheme 1. Synthesis of solid phase catalyst 2.
5
CD3OD
CD3OD
CD3OD
CD3CN
CD3CN
CD3CN
CD3CN
CD3CN
55
6
120
60
60
c)
1000
7[d]
8[e]
9[f]
10[g]
11[h]
12h[i]
quant.
0
2.0
1
Irel
A
1.0
0.0
500
120
120
60
0
-
1000
500
0
1.0
0.5
0.0
2
quant.
87
no
no
no
10
200
300
400
500
600
10
55
(nm)
d)
[a] Reaction was performed in a Schlenk tube in 1 mL of deuterated solvent
under an argon atmosphere; m(2) = 2 mg, i.e. n(2) = 5.4 x 10-4 mmol, c(5)
[b]
[c]
[d]
= 2 x 10-2 mol.L-1. Conversion determined by 1H NMR. GC-HRMS.
MCM-41
m(2) = 10 mg, i.e. n(2) = 2.7 x 10-3 mmol. [e] In the presence of mesoporous
silica MCM-41 instead of catalyst. [f] In the presence of 4 instead of catalyst
2. [g] Reaction performed under air. [h] In the presence of catalyst 1 from ref.
16. [i] m(2) = 1.8 mg, i.e. n(2) = 5 x 10-4 mmol.
4
2
2
4
6
8
10 12 14
Analogously to homogeneous conditions, the best results
were achieved in acetonitrile and also in toluene where the pure
cycloaddition product 6 was quantitatively formed within 60 min
of irradiation (Entries 1 and 2); quantitative conversion in
acetonitrile was observed even under aerial conditions (entry 10).
Two theta / degrees
Figure 2. Photograph (a) and SEM image (b) of the catalyst 2. Comparison of
UV-VIS absorption and emission spectra of 1 and 2 (solid state) (c). CuK X-
ray diffraction patterns of the catalyst 2 and intermediates MCM-41 and 4 (d).
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On the other hand, in chloroform and methanol (Entries 3 and 5), cis:trans-benzoylcyclobutanes 15 was 4:1 (Entry 3), which was
the reaction proceeded more slowly and did not give complete
conversion even after a prolonged reaction time (Entries 4 and
6). In methanol, attempts to improve the conversion by using a
higher amount of catalyst resulted in the disappearance of all the
starting diene but at the expense of purity (Entry 7). Importantly,
the blank experiments showed the reaction does not occur either
with mesoporous silica MCM-41 or with its APTES-modified
version 4 used instead of 2 (Entries 8 and 9). Short experiments
(10 min) demonstrated that the efficiency 2 is somewhat lower
when compared to 1 under homogeneous conditions (cf. Entries
11 and 12) nevertheless one should take in mind that the result
with the heterogeneous catalyst can be underestimated since
some alloxazine units may not participate in the catalysis.
In acetonitrile, we investigated the substrate scope of our
method with heterogeneous catalyst 2 both on an analytical and
semipreparative scale using substrates studied previously[16,17]
under homogeneous conditions with 1 (Table 2): electron-rich
non-symmetrical and symmetrical styrenes 5 and 7, electron-
poor bis(arylenone) 8, sulfur (9) and nitrogen (10, 11) containing
dienes and anhydrides 12 and 13. Using immobilized catalyst 2,
quantitative conversions were achieved in semipreparative (0.2
mmol) experiments within 120 min of irradiation with the
exception of bis(arylenone) 8, sulfone 9 and amide 11 (Entries 3,
4 and 6) which required longer reaction times or a higher
catalyst loading (in the case of 8). On the other hand, due to its
high reactivity, methoxyphenyl derivative 13 cyclised almost
quantitatively after 30 min of irradiation (Entry 8). We found this
time as optimal for 13 to avoid decomposition of the product 20
observable after longer irradiation.
similar to that obtained using 1 (5:1).[16]
Table 2. Substrate scope investigation for [2+2] photocycloadditions mediated
by 2 and visible light (400 nm) on an analytical[a] and semipreparative scale[b]
Data for homogeneous reactions with 1 (ref.16,17) are given for comparison.
.
Heterogeneous
reaction with 2 (2.7 %)
Homogeneous
reaction with 1[f]
(2.5 %)
Entry Substrate
Cyclobutane
Conv.
60min
[%][a,c]
Yield
[%][b,d]
(time
[min])
d.r.[e] Yield
[%]
d.r.[e]
(time
[min])
1
quant.
96
>10:1
82[m]
(10)
>10:1
(60)
2
3
quant. [g] 98
(120)
-
80[m]
(25)
-
quant. [g] 93[h]
(120)
4:1
70[m]
(45)
5:1
4
5
quant. [g] 91
(300)
>10:1
10:1
51[I,n]
(40)
>10:1
10:1
quant.
96
70[m]
(40)
Compared to reactions under homogeneous conditions with
1 (see Table 2, last two columns) cycloaddition with 2 required
longer reaction times with some exceptions: (i) cyclization of
anhydrides 12 and 13 which was faster with heterogenized
catalyst 2, (ii) cyclization of 9 which did not reach quantitative
conversion under homogeneous conditions even after longer
reaction time while complete formation of 16 was observed with
solid-supported catalyst 2.
(120)
6
7
70[g]
96
(300)
-
53[m]
(40)
-
Importantly, contrary to the crude product obtained from the
quant.
93
>10:1
96[k,l,m] >10:1
(180)
homogeneous photocycloaddition using
1 which required
(90)
chromatographic purification to remove the catalyst,
cyclobutanes with acceptable purity were obtained by simply
filtering off the catalyst 2 followed by solvent evaporation after
8
85
80
>10:1
46[l,n]
(120)
>10:1
the cycloaddition under heterogeneous conditions (for
a
(30)
comparison of 13C NMR data; see the Supporting Information).
Neither alloxazine dye nor possible products of its
decomposition were observed as impurities in the product. This
is advantageous mainly from the point of view of preparative
yields of cyclobutanes which were always higher using new
procedure with heterogenized catalyst 2 compared to method
with 1 (cf. data in Table 2). The differences in the yields are
significant mainly when less stable cyclobutanes 16, 18 and 20
were prepared (Entries 4, 6 and 8).
[a]
For conditions, see Table 1. [b] Reaction was performed in a Schlenk tube in
10 mL of anhydrous acetonitrile under an argon atmosphere; m(2) = 20 mg, i.e.
n(2) = 5.4 x 10-3 mmol, n(substrate) = 0.2 mmol.
[c]
Relative conversion
[d]
[e]
determined using 1H NMR.
Preparative yield after filtration of catalyst.
Diastereoisomeric ratio determined from 1H NMR spectra of the reaction
[f]
mixtures.
Yields (after column chromatography) and selectivities of
cycloadditions with 1 under homogeneous conditions[16,17] obtained on the same
apparatus. [g] m(2) = 5 mg, i.e. n(2) = 1.4 x 10-3 mmol. [h] m(2) = 40 mg and new
Notably, the stereoselectivity of the cycloadditions using 2
was the same as that found under homogeneous conditions
using 1 affording the exclusive formation of exo-cyclobutanes
with a relative cis-configuration of the aryl groups (if appropriate)
with diastereoselectivity d.r. >10:1 (Table 2). The ratio of the
[i]
[k]
portion (m(2) = 20 mg) of 2 after 1 hour. 10 % of 1.
Without column
[l]
[m]
chromatography which causes decomposition of the product. 5 % of 1.
Quantitative conversion. [n] Conversion 90%.
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[4]
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We also attempted to reuse catalyst 2 in subsequent
experiments but we observed significant drop of its activity
becoming less active (12%) and inactive for the second and third
round,
respectively.
Deactivation
is
caused
by
photodecomposition of flavin dye as evident from UV-VIS and
fluorescence spectra (see Supporting Information) which also
show that any aromatic, probably flavin decomposition product,
remains anchored on MCM-41 after photocycloaddition. Thus
the stability remains an issue to be solved in photocatalysis with
flavins regardless they are used as homogeneous[16,17] or
heterogenized catalysts.
[5]
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heterogenisation of the flavin photocatalyst 1 by its covalent
bonding on mesoporous silica MCM-41, which is still rarely used
to anchor molecular organocatalysts. We demonstrated the
usefulness of the prepared heterogeneous alloxazine catalyst 2
in the visible light [2+2] photocycloaddition of various dienes with
a special focus on the transformations characterized by lower
conversion and/or problematic product isolation under
homogeneous conditions. We believe this approach to prepare
the solid supported catalyst may be useful for other flavin
derivatives whose photocatalytic properties have been of high
research interest.[17a,24] It should bring new environmentaly
benign photoorganocatalytic procedures.
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Experimental Section
[2+2] Photocycloadditions on preparative scale
In a representative experiment, the reactions were performed in a
Schlenk tube. Solid phase catalyst 2 (20 mg, containing 5.4 x 10-3 mmol
of active units) and a substrate (0.2 mmol) were suspended/dissolved in
acetonitrile (10 mL), tube was closed and air was removed by three
freeze-pump-thaw cycles using argon as an inert gas. Schlenk tube was
irradiated for an appropriate time (usually 30 - 300 minutes) on apparatus
with six high power LED emitors (LED Engin LZ4-00UA00) directly
shining on the tube from three directions at distance of 5 mm from the
tube wall (see Supporting Information). The cycloaduct was obtained by
removing the catalyst by filtration and by evaporation of solvent.
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Acknowledgements
[20] Y. Imada, M. Osaki, M. Noguchi, T. Maeda, M. Fujiki, S. Kawamorita, N.
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This project was supported by the Czech Science Foundation
(Grant No. 14-09190S) and by the Czech Ministry of Education,
Youth and Sports in the framework of the targeted support of the
National Programme for Sustainability I (project LO1201).
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Keywords: Cycloadditions • Heterogeneous catalysis • MCM-41
• Photocatalysis • Sustainable chemistry
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[23] The amount of anchored alloxazine (0.27 mmol per
1 g of
heterogeneous catalyst 2) was estimated on the basis of subsequent
elemental analysis (C, N, H and Si) after each step of MCM-41
modification (see Supporting Information for data).
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
We proved immobilization of flavin on
mesoporous silica MCM-41 as
effective way to prepare a
Špačková J., Svobodová E., Hartman T.,
Stibor I., Kopecká J., Cibulková J.,
Chudoba J., and Cibulka R.*
heterogeneous flavin photocatalyst. Its
possible application was
Page No. – Page No.
demonstrated in visible light [2+2]
photocycloaddition which is
advantageous compared to analogous
homogeneous procedure.
Visible Light [2+2]
Photocycloaddition Mediated by
Flavin Derivative Immobilized on
Mesoporous Silica
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