Covalent Immobilization of (?)-Riboflavin on Polymer Functionalized Silica Particles: Application in the Photocatalytic E→Z Isomerization of Polarized Alkenes

Article abstract of DOI:10.1002/chem.201800231

The covalent immobilization of the biomimetic, photo-organocatalyst (?)-riboflavin on silica micro- and nanoparticles via atom transfer radical polymerization (ATRP) is disclosed. Given the effectiveness of (?)-riboflavin as a versatile, environmentally benign photocatalyst, an immobilization strategy based on acrylate-linker modification of the catalyst core and controlled polymerization on initiator pre-functionalized silica particles has been developed. Validation of this approach is demonstrated in the E→Z isomerization of a benchmark cinnamonitrile (Z/E up to 88:12) with 0.97 mol % catalyst loading. Characterization of the immobilized photocatalyst supports covalent embedding of the catalyst in the polymeric brushes on the silica particle surface.

Full text of DOI:10.1002/chem.201800231

A Journal of
Accepted Article
Title: Covalent Immobilization of (-)-Riboflavin on Polymer
Functionalized Silica Particles: Application in the Photocatalytic
E→Z Isomerization of Polarized Alkenes
Authors: Jan Metternich, Sven Sagebiel, Anne Lückener, Sebastian
Lamping, Bart Jan Ravoo, and Ryan Gilmour
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.201800231
Link to VoR: http://dx.doi.org/10.1002/chem.201800231
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Covalent Immobilization of (–)-Riboflavin on Polymer
Functionalized Silica Particles: Application in the Photocatalytic
E→Z Isomerization of Polarized Alkenes
Jan B. Metternich,§[a] Sven Sagebiel,§[a,b] Anne Lückener,[a] Sebastian Lamping,[a,b] Bart Jan Ravoo,*[a,b]
Ryan Gilmour*[a]
Abstract: The covalent immobilization of the biomimetic, photo-
organocatalyst (−)-riboflavin on silica micro- and nanoparticles via
atom transfer radical polymerization (ATRP) is disclosed. Given the
effectiveness of (–)-riboflavin as a versatile, environmentally benign
photocatalyst, an immobilization strategy based on acrylate-linker
modification of the catalyst core and controlled polymerization on
initiator pre-functionalized silica particles has been developed.
Validation of this approach is demonstrated in the E→Z
isomerization of a benchmark cinnamonitrile (Z:E up to 88:12) with
0.97 mol % catalyst loading. Characterization of the immobilized
photocatalyst supports covalent embedding of the catalyst in the
polymeric brushes on the silica particle surface.
excitation between the catalyst-substrate and catalyst-product
scenarios would differ and provide the structural basis for the
intrinsic directionality of the transformation (E→Z) (Figure 1,
center). Despite the relatively inexpensive nature of the vitamin
catalyst, we were cognizant of the pressing issue of
sustainability in contemporary catalysis.
Given the importance of stereodefined alkenes in contemporary
synthesis and the dependence of subsequent stereospecific
transformations on the configuration, efficient strategies to
isomerize pre-existing alkenes in a contra-thermodynamic sense
are conspicuous in their absence.[1] Recently, we communicated
a bio-inspired, contra-thermodynamic isomerization of activated
alkenes based on the mammalian visual cycle.[2] This study was
inspired by Walker and Radda’s observation that crystalline (–
)-riboflavin in the eyes of fish and mammals can isomerize E-
retinal to the corresponding Z-isomer by an energy transfer (ET)
manifold facilitated by visible light irradiation (Figure 1, upper).[3]
This biomimetic blueprint for the development of an efficient
isomerization was re-engineered by utilizing the cinnamoyl motif
as
a simplified retinal surrogate and exploiting structural
differences in the substrate and product to achieve selective
excitation. Addition of a substituent on the β-position of the
substituted alkene ensured that the planar, conjugated π-system
that is present in the substrate would be compromised by
developing allylic strain in the product.[4] This would generate
torsional stress about the C-Cipso-C-R torsional angle, thus
deconjugating the π-system. Necessarily, the efficiency of
Figure 1. Contra-thermodynamic isomerization of retinal in biology (upper),
the bio-inspired E → Z isomerization of polarized alkenes mediated by (–)-
riboflavin (center) and an immobilization strategy (lower).
[a]
Dr. J. B. Metternich, S. Sagebiel, A. Lückener, S. Lamping, Prof. Dr.
B. J. Ravoo,* Prof. Dr. R. Gilmour*
Organisch-Chemisch Institut
Westfälische Wilhelms-Universität Münster
Corrensstr. 40, 48149 Münster, Germany
E-mail: B.J.Ravoo@uni-muenster.de, ryan.gilmour@uni-
muenster.de
S. Sagebiel, S. Lamping, Prof. Dr. B. J. Ravoo*
Center For Soft Nanoscience (SoN)
To that end, we have devised a strategy that allows for the
covalent immobilization of (−)-riboflavin on silica micro- and
nanoparticles via atom transfer radical polymerization (ATRP)
(Figure 1, lower). Validation of this approach is demonstrated in
the (−)-riboflavin-catalyzed E→Z isomerization of a benchmark
cinnamonitrile[5] and report selectivities up to Z:E 88:12 using
0.97 mol % of the immobilized catalyst.
Whilst photocatalysis is being exploited with increasing
frequency to generate complex molecular scaffolds, scalability,
sustainability and catalyst recovery remain important
considerations. As an immobilization strategy to mitigate
sustainability concerns and facilitate catalyst recovery, we were
[b]
[§]
Westfälische Wilhelms-Universität Münster
Busso-Peus-Straße 10, 48149 Münster, Germany
J. B. Metternich and S. Sagebiel contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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attracted to the controlled generation of polymeric brushes on
particles which has proven to be a useful strategy to modulate
surface or particle properties.[6] Besides colloidal stabilization,
generation of self-assembling systems, and generation of Janus-
particles,[7] polymer grafting has been utilized in catalytic
applications to enhance the activity of transition metals.[6,8]
Arguably, one of the most prominent techniques utilized in
polymer-brush grafting is atom transfer radical polymerization
(ATRP) with radical initiator pre-modified particles.[9] Typical
initiators enabling controlled polymer growth are based on
α-bromoisobutyric acid linked to the surface via alkyl-siloxane
linkers, derived from chloro- or alkoxysilanes.[6] The properties of
the resulting polymers are determined by the chosen monomer
or copolymer composition. Employing various monomers to
generate a defined copolymer provides the opportunity to
covalently immobilizing a monomer-modified catalytically active
species on silica particles by simple surface initiated ATRP,
which would enable wide-spread immobilization of catalysts for
photochemical, organocatalytic and potentially even transition-
metal based transformations.[10]
As a benchmark reaction to evaluate the effect of immobilization
on the catalytic efficiency, the
E → Z isomerization of
cinnamonitriles catalyzed by (–)-riboflavin at 402 nm was
selected due to the operational simplicity of the process.[5]
Pursuant to work by König and co-workers, modification of the
flavin catalyst was performed at the free benzo[g]pteridine–N3
nitrogen site (see Figure 1).[11] To systematically explore the
effect of linker introduction, the efficiency of catalysis in the
model transformation was compared with (–)-riboflavin, (–)-
riboflavin tetraacetate and then with the core modified at the
pteridine core with various linker lengths (2-5, n = 1-4,
respectively) (Table 1).
Performing the isomerization of E-1 → Z-1with the reported
parent catalyst (−)-riboflavin, or (−)-riboflavin tetraacetate, in
MeCN under irradiation at 402 nm delivered selectivities Z/E
91:9 and 90:10, respectively (Table 1, entries 1 and 2). The
tetraacetate was chosen due to increased solubility which would
thus facilitate polymerization. Gratifyingly, appending the various
linkers required for surface-initiated ATRP to the core did not
significantly compromise catalytic efficiency. In all cases,
quantitative yields and similarly high selectivities were observed
(Table 1, entries 3-6, up to Z/E 90:10). Since the photostationary
state was mostly independent of the linker chain length, the least
expensive alkyl linkers (n = 1 and 4, entries 3 and 6) were used
in subsequent studies.
Table 1. Exploring the effects of linker introduction.[a]
Table 2. Isomerization of E-1 with catalyst 2 immobilized on silica nanoparticles
(Immob. 2).[a]
Entry
Co-monomer
Solvent
Conditions
Yield[a]
Z/E
Entry
Catalyst
n
Yield[a]
Z/E
1
2
3
4
5
HEA
MEA
MEA
MEA
MEA
MeOH
MeOH
MeCN
MeCN
MeCN
-
quant.
quant.
quant.
quant.
quant.
20:80
18:82
18:82
17:83
2:98
1
2
3
4
5
6
(−)-Riboflavin
-
quant.
quant
quant
quant
quant
quant
91:9
-
Riboflavin tetraacetate
-
90:10
86:14
84:16
90:10
86:14
-
2
3
4
5
1
2
3
4
Ar
450 nm
1st quant.
2nd quant.
15:85
5:95
6
MEA
MeOH
2 runs
[a] Reactions were performed with 0.1 mmol of substrate in the specified solvent
using 0.5 ml of immobilized catalyst suspension (~10 mg particles) under UV-light
irradiation (402 nm, unless otherwise noted under cond.) for 12 h at ambient
temperature. The Z/E ratios were determined by 1H NMR spectroscopy.
[a] Reactions were performed with 0.1 mmol of substrate in MeCN using
5 mol% catalyst loading under UV-light irradiation (402 nm) for 12 h at ambient
temperature. The Z/E ratios were determined by 1H NMR spectroscopy.
To validate this strategy, the covalent immobilization of a (–)-
riboflavin derivative modified with an acrylate-based alkyl linker
by ATRP with the 2-hydroxyethyl acrylate comonomer via
surface initiated polymerization was envisioned (Figure 1, lower).
Polymerization with initiator modified silica nanoparticles
(commercially available particles with diameter d = 500 nm)[12]
was performed according to a procedure reported by Ravoo and
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Du Prez[7,13] with 2-hydroethyl acrylate (HEA) as co-monomer,
3 mol % catalyst loading 2 (97% HEA), and a triazolinedione
(TAD) based initiator that was immobilized silica particles
modified with n-octenyltrichlorosilane (for details see SI). As
expected, the particles exhibited the bright yellow color of
riboflavin even upon removal of residual monomeric catalyst by
extensive centrifugation and redispersion. Subsequently, the
immobilized catalyst was tested in the flavin catalyzed
isomerization of E-1 (Table 2).
amount of catalyst on the nanoparticles to 6 mol % also led to
full conversion with a selectivity of 88:12 (entry 7).
Table 3. Optimization of the catalyst and particle composition.[a]
The isomerization of cinnamonitrile E-1 with immobilized
catalyst 2 (Immob. 2) and HEA as the co-monomer gave
quantitative mass recovery and a Z/E ratio of 20:80 after
irradiation (402 nm) for 12 h in methanol (see Table 2, entry 1).
Interestingly, significant loss of the particle color was noted
following irradiation, suggesting decomposition of the catalyst.
Given the propensity of flavins to undergo oxidative
decomposition in alcoholic solvents, a switch in reaction medium
from MeCN to a mixture of MeCN/MeOH (1:1) was attempted.
However, this resulted in precipitation of the particles and which
compromised reactivity. Consequently, the co-monomer was
changed to 2-methoxyethyl acrylate (MEA) to increase the
solubility of the polymer brush modified particles in acetonitrile.
Performing the isomerization of E-1 with immobilized 2 (Immob.
2) and MEA co-monomer did not improve selectivity with Z/E
ratios of 18:82 being observed in both MeOH and MeCN (see
Table 2, entries 2 and 3). To reduce the rate of oxidative
decomposition, the reaction was repeated under Ar(g) and at
450 nm, giving a Z/E ratio of 17:83 and 2:98, respectively (entry
4). Finally, two reaction cycles were performed with the same
particle batch to confirm the loss of catalytic activity (see Table 2,
Entry 6, 1st run Z/E 15:85 and 2nd run Z/E 5:95).
Entry
Catalyst
T [h]
12
12
24
12
24
12
12
12
12
Cond.
Yield[a]
quant.
quant
quant
quant
quant
quant
quant
quant
quant
Z/E
1
2
3
4
5
6
7
8
9
Immob. 2
Immob. 6
Immob. 6
Immob. 7
Immob. 7
Immob. 7
Immob. 7
Immob. 7
Immob. 7
-
-
-
-
-
20:80
50:50
57:43
75:25
79:21
85:15
88:12
85:15
86:14
1.0 mL particle
suspension
6 mol% Immob. 7
Mass spectroscopic analysis of the particle suspension
revealed decomposition of the catalyst presumably by cleavage
of the ribityl side-chain and subsequent decomposition of the
flavin core. To address the pressing issue of catalyst stability,
modification of the protecting group strategy and/or synthesis of
the linker containing parent (−)-riboflavin derivative 7 were
considered. According to a literature report, (−)-riboflavin was
protected as the dimethyl acetal with dimethoxypropane and
acetone.[14] Following introduction of the desired linker for
polymerization (catalyst 6), the protecting groups were removed
under acidic conditions to yield the final catalyst scaffold Immob.
7 (Table 3, for full details see SI).
Nanoparticles
(Stöber method)
Commercial initiator
[a] Reactions were performed with 0.1 mmol of substrate in MeOH using
0.5 ml of immobilized catalyst suspension (~10 mg particles) under UV-light
irradiation (402 nm, unless otherwise noted) for the specified time at ambient
temperature. The Z/E ratios are the average of two experiments and were
determined by 1H NMR spectroscopy.
Having identified a competent catalyst species and optimized
loading, improvement of the immobilization sustainability was
addressed by preparing silica nanoparticles according to the
Stöber method:[15] The resulting particles exhibited an average
diameter of 60 nm (for details see SI), and subsequent
functionalization and immobilization of 7 proceeded smoothly.
Gratifyingly, only a slight decrease in isomerization efficiency
was noted using these nanoparticles modified with catalyst 7
(Table 3, entry 8, Z/E 85:15 vs 88:12). Finally, the need for a
complex and expensive TAD-anchor for particle pre-
functionalization was circumvented by exchange with the simple,
Immobilization of 6 and 7 was performed according to the
standard conditions with TAD-initiator and HEA co-monomer
(3 mol% catalyst, 97 mol% HEA) on commercially available
nanoparticles (diameter 500 nm).[12] The isomerization with
immobilized, dimethylacetal protected catalyst Immob. 6 gave a
significantly increased selectivity to Z/E ratios of 50:50 and
57:43 after 12 h and 24 h, respectively (entries 2 and 3). In
comparison, the modified parent catalyst Immob. 7 gave a
ration Z/E 75:25 after 12 h of irradiation (entry 4). Increasing the
reaction time to 24 h led to a modest improvement in selectivity
Z/E 79:21 (entry 5). Based on these results, catalyst Immob. 7
was selected for further particle optimization. It was noted that
doubling the amount of particles used in the reaction mixture led
to a more favorable Z/E ratio of 85:15 (entry 6). Increasing the
commercially
available
3-dimethylethoxy
silylpropyl-2-
bromoisobutyrate initiator.[16] Immobilization was achieved on the
60 nm nanoparticles and the isomerization of E-1 performed with
the corresponding (poly)riboflavin-(poly)HEA silica nanoparticles
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(PRF-PEA SiNP). This resulted in a respectable isomerization
selectivity of Z/E 86:14 (entry 9). Unfortunately, catalyst
recycling proved challenging and a significant erosion of
selectivity was observed after the second cycle (ca. Z/E 20:80).
It is conceivable that prolonged exposure to acidic silica induces
photodecomposition:[17]This limitation will be addressed as part
of future investigations. It is pertinent to mention that all attempts
to improve selectivity by exchanging the acrylate linker with
simple, terminal alkenes failed due to ineffective radical
polymerization (for full details see compounds 8-10 in the
Supporting Information).
In
a
final series of experiments, the retention of
photophysical properties of the catalyst species and
determination of catalyst concentration on the particles PRF-
PHEA SiNP was performed by UV/Vis-spectroscopic analyses.
The absorption spectra of the parent catalyst (−)-riboflavin, linker
modified catalyst 7 and the immobilized catalyst Immob. 7
(PRF-PHEA SiNP) are shown in Figure 3, providing support for
the retention of photochemical activity. Importantly, no significant
shift of the absorption maxima in the UV/vis region was
observed. The catalyst concentration on the particles was
determined using UV/Vis spectroscopy with external standard
With this highly efficient and inexpensive immobilization
strategy in hand, i.e. using the commercially available initiator,
HEA and 7 on 60 nm nanoparticles, a detailed characterization
of the particles was conducted to confirm that the photocatalyst
was covalently immobilized on the surface of the silica
nanoparticles. Fluorescence and optical microscopy analysis
revealed the formation of a chromophore-containing polymer
shell around the particle core (Figure 2A and B). This was
further confirmed by dynamic light scattering experiments of
unfunctionalized particles (bare SiNP), initiator functionalized
particles (initiator SiNP), silica particles with polymeric brushes
consisting of HEA (PHEA SiNP) and the catalyst containing
particles (PRF-PHEA SiNP). In accordance with the growth of
polymer brushes, a size increase of the particles from 60 nm
and 70 nm for bare SiNP and initiator SiNP, respectively, to
approximately 130 – 140 nm for polymer PHEA SiNP and
catalyst functionalized PRF-PHEA SiNP particles was observed
(for full details, see SI).
calibration of catalyst 7 and PRF-PHEA SiNP diffraction
correction with PHEA modified particles PHEA SiNP (for full
details see SI). Based on these experiments, the mass fraction
of
catalyst
on the particles
was found to be
w(mcat./mparticles) = 4.8%, thus giving a catalyst loading of 0.97%
under standard reaction conditions (0.1 mmol substrate, 0.5 mL
particle suspension with 22 mg/mL particles).
Figure 3. UV/Vis-absorption of (−)-riboflavin, the linker containing flavin
derivative 7 and flavin derivative immobilized on silica nanoparticles Immob. 7
(PRF-PHEA SiNP) in MeOH [concentrations: (−)-riboflavin: 0.5 mg/mL; 7;
0.2 mg/mL; PRF-PHEA SiNP: 0.22 mg/mL].
In conclusion,
a versatile and inexpensive immobilization
Figure 2. Fluorescence (A) and optical (B) microscopy images of PRF-PHEA
platform for organic photocatalysis has been developed. The
immobilization process is flexible and can easily be adapted to
accommodate a variety of different systems by simple pre-
functionalization of the catalyst with the appropriate acrylate
linker. Immobilization proceeds by polymerization with the co-
monomer HEA and initiator functionalized silica nanoparticles.
The retention of photocatalytic activity of the functionalized and
immobilized (–)-riboflavin analogs was demonstrated by the
isomerization of cinnamonitrile E-1 → Z-1, in combination with
comparative UV/vis absorption and fluorescence analyses of the
tested catalysts. Furthermore, DLS, XPS and microscopy
revealed the embedment of the catalyst in the backbone of the
generated polymeric brushes on the silica particle. We envisage
that the modular nature of this approach will enable the
immobilization of more complex, organic or transition metal
catalysts for photochemical and catalysis applications.
silica microparticles (4.6 µm) in MeCN (scale bar = 10 µm).
Furthermore, comparative XPS measurements indicated the
formation of a polymeric layer around the silica particles with an
oxygen containing hydrocarbon scaffold consistent with a HEA
polymer (for full details, see SI). Only the particles PRF-PHEA
SiNP with immobilized (–)-riboflavin catalyst Immob. 7 exhibited
a clear signal in the binding energy region indicative for nitrogen
containing compounds (binding energy = 399.9 eV), thus
confirming the introduction of the catalyst in the polymeric
backbone. This was further exemplified by fluorescence
anisotropy experiments giving an anisotropy A = 0.102±3.93*10-3
for the immobilized catalyst PRF-PHEA SiNP and
A = 0.014±2.33*10-3 for the free catalyst 7 in solution.[18] Finally,
NMR analysis of a corresponding copolymer generated in
solution showed copolymerization of HEA and 7 in the expected
ratio of 1:11.1 (for full details see SI).
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Acknowledgements
We acknowledge generous financial support from the WWU
Münster, Deutsche Forschungsgemeinschaft (SFB 858 and
Excellence Cluster EXC 1003 “Cells in Motion”), the Volkswagen
Foundation and Fonds der Chemischen Industrie (Fellowship to
JBM).
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Keywords: Photocatalysis • Immobilization • Isomerization •
ATRP • Nanoparticles • Riboflavin
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Entry for the Table of Contents
COMMUNICATION
Jan B. Metternich, Sven Sagebiel, Anne
Lückener, Sebastian Lamping, Bart Jan
Ravoo,* Ryan Gilmour*
Page No. – Page No.
Covalent Immobilization of (–)-
Riboflavin on Polymer Functionalized
Silica Particles: Application in the
Photocatalytic E→Z Isomerization of
Polarized Alkenes
Riboflavin on a bead: Immobilization of riboflavin in polymer brushes on silica
beads enables the efficient photoisomerization of a model alkene!
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