Amphiphilic Polymeric Nanoparticles for Photoredox Catalysis in Water

Article abstract of DOI:10.1002/chem.202001767

Photoredox catalysis has recently emerged as a powerful synthesis tool in organic and polymer chemistry. In contrast to the great achievements realized in organic solvents, performing photocatalytic processes efficiently in aqueous media encounters several challenges. Here, it is presented how amphiphilic single-chain polymeric nanoparticles (SCPNs) can be utilized as small reactors to conduct light-driven chemical reactions in water. By incorporating a phenothiazine (PTH) catalyst into the polymeric scaffold, metal-free reduction and C?C cross-coupling reactions can be carried out upon exposure to UV light under ambient conditions. The versatility of this approach is underlined by a large substrate scope, tolerance towards oxygen, and excellent recyclability. This approach thereby contributes to a sustainable and green way of implementing photoredox catalysis.

Full text of DOI:10.1002/chem.202001767

Accepted Article
Accepted Article
Title: Amphiphilic Polymeric Nanoparticles for Photoredox Catalysis in
Water
Authors: Anja R.A. Palmans, Fabian Eisenreich, and E. W. Meijer
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202001767
Link to VoR: https://doi.org/10.1002/chem.202001767
01/2020

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Amphiphilic Polymeric Nanoparticles for Photoredox Catalysis in
Water
Fabian Eisenreich, E. W. Meijer, Anja R. Palmans*
Dr. F. Eisenreich, Prof. Dr. E. W. Meijer, Prof. Dr. A. R. A. Palmans
Laboratory of Macromolecular and Organic Chemistry, Institute for Complex Molecular Systems, Department of Chemical Engineering and Chemistry,
Eindhoven University of Technology
P.O. Box 513, 5600 MB Eindhoven, The Netherlands
E-mail: a.palmans@tue.nl
Supporting information for this article is given via a link at the end of the document.
Abstract: Photoredox catalysis has recently emerged as a powerful
synthesis tool in organic and polymer chemistry. In contrast to the
great achievements realized in organic solvents, performing
photocatalytic processes efficiently in aqueous media encounters
several challenges. Here, we present how amphiphilic single-chain
polymeric nanoparticles (SCPNs) can be utilized as small reactors to
conduct light-driven chemical reactions in water. By incorporating a
phenothiazine (PTH) catalyst into the polymeric scaffold, metal-free
reduction and C-C cross coupling reactions can be carried out under
exposure to UV light at ambient conditions. The versatility of this
approach is underlined by a large substrate scope, tolerance to
oxygen, and excellent recyclability. This approach thereby contributes
to a sustainable and green way of implementing photoredox catalysis.
Lipshutz and coworkers,[11] tailor-made micelles have been
applied as reaction compartments for efficient catalysis in water.
With the aid of an iridium-based photocatalyst covalently attached
to an amphiphilic micelle-forming reagent, alkenes were
functionalized with sulfonyl groups.[12] By capitalizing on the
recyclability of their catalyst system, the net usage of iridium was
reduced to the level of ppm over several reaction cycles. Recently,
the group of König has impressively shown that micellar
assemblies can be utilized to activate stable carbon-chloride
bonds with light for various chemical transformations, such as
reduction, cross coupling, and cyclization reactions.[13] Moreover,
the generation of hydrated electrons as versatile super-reductants
was thoroughly studied by the group of Goez based on
photoredox processes involving ruthenium complexes and the
support of micelle compartmentalization.[14–16]
In these seminal contributions in the field of photoredox
catalysis in aqueous media, the predominant utilization of metal
catalysts typically unfold their reactivity in the excited triplet state,
thus requiring oxygen-free conditions. Furthermore, the utility and
benefits of immobilizing a photoredox catalyst onto amphiphilic
polymer-based systems have not been explored to date in this
regard. By linking a catalytic unit to a polymeric support several
crucial advantages can be unlocked, namely high robustness,
facile separation and opportunities for recycling, and restricted
catalyst leaking.[17,18] The contamination of reaction products with
traces of catalyst can thus be prevented, which is particularly
important for biomedical[19] or microelectronic applications.[20]
In the late 90’s, Bergbreiter and coworkers reported on the
high activity and simple recovery of water-soluble polymer-bound
catalysts.[21,22] Beyond that, various synthetic macromolecules,
such as star polymers[23–25] and dendrimers,[26,27] have been
created with the aim to isolate incorporated catalytic units from
the aqueous surroundings.[28–30] Dynamic and crosslinked
micelles,[31–36] nanogels,[37,38] as well as various polymer-
supports[39,40] have also been investigated in detail to realize
efficient catalysis in water.
Introduction
Photoredox catalysis has recently become a flourishing field in
modern organic chemistry as it paves the way for novel modes of
reactions under mild and biocompatible conditions (i.e., ambient
temperature and non-invasive light irradiation).[1–4] Light energy is
used as a fuel to accelerate chemical transformations by single-
electron redox processes, thereby giving rise to well-controlled
and improved synthetic procedures. Thus far, tremendous
progress has been achieved to synthesize value-added products
efficiently in organic solvents. Aside from these advances, there
is a strong interest in the field of nanomedicine or other biological
applications for the development of photoredox-active catalyst
systems that can be operated in pure water. The ability to perform
modern chemical reactions in aqueous media together with
effective recycling strategies for catalysts represent keystones for
sustainable and green chemistry.[5,6] To this end, nature’s
fascinating repertoire of tools to perform chemical transformations
with tremendous accuracy and reliability (e.g., enzymatic
reactions) can serve as a precious source of inspiration. In many
enzymes, hydrophobic domains that are shielded from the
aqueous environment based on well-defined tertiary architectures
provide an anchor point for efficient catalysis. This precise
compartmentalization of catalytic sites has the additional
advantage to result in a dramatic elevation of catalytic activity and
selectivity.
In our group, we have established that both, organocatalytic
as well as transition-metal based reactions, proceed well in water
when the catalytically active sites are shielded by amphiphilic
polymer chains that fold around the catalyst to form single-chain
polymeric nanoparticles (SCPNs; see Figure 1a).[41–43] As a result,
stable, structured, and hydrophobic interiors are created, in which
reactions can be performed that are typically not compatible with
water due to the lack of solubility of either the substrate or the
Hitherto, examples for photoredox catalysis in pure water are
scarce and mostly rely on water-soluble ruthenium complexes[7,8]
or micellar approaches.[9,10] For instance, in pioneering work of
1
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Figure 1. Photoredox-active single-chain polymeric nanoparticles: a) Illustration of the intramolecular hydrophobic collapse of an individual chain to a nanoparticle
in water, furnishing an apolar interior as platform for photoredox catalysis. b) Post-functionalization of poly(pentafluorophenyl) acrylate by the sequential addition of
amine-bearing compounds to yield the catalytically active polymers P1–P3.
catalyst. While most of the reported catalytic processes with
SCPNs are conducted thermally, it would be highly advantageous
to gain control over a catalytic reaction with light as an external
trigger. With the aim to avoid precious transition metal catalysts
(e.g., iridium and ruthenium complexes) we chose 10-
phenylphenothiazine (PTH) as an organic photocatalyst for our
study. Hawker and Read de Alaniz et al. have shown that PTH
efficiently facilitates dehalogenation reactions[44,45] and C-C cross
couplings.[45] PTH and derivatives thereof are also known to
catalyze controlled radical polymerizations of various
monomers.[46–50] Moreover, PTH bound to a hydrophobic polymer-
support was successfully applied in photocatalysis and recycling
studies in organic media.[51]
Here, we describe the use of SCPNs as a stable and
recyclable platform to perform metal-free photoredox catalysis in
water. The devised method is simple to operate as cheap and
commercially available light-emitting diodes (LEDs) are employed
as light sources. More importantly, the catalytic reactions take
place at ambient temperature and in the presence of air oxygen,
representing prerequisites for potential biological applications.
PTH by introducing an ethylamine linker at the 4-position of the
phenyl group. To increase the number of hydrophobic side groups
that furnish the inner compartments of the SCPNs, linear dodecyl
chains were selected for P1.[52] The use of chiral, non-racemic
BTA ((S)-BTA) units permits to investigate the structural integrity
of the SCPNs under operating conditions (P2).[42] These moieties
are capable of forming helical stacks due to threefold hydrogen
bonding[41,53] and thus render a structuring element within the
nanoparticle cores. The supramolecular helical assemblies can
be analyzed by circular dichroism (CD) spectroscopy and hence
provide the means to probe the integrity of the inner composition
upon exposure to the reaction conditions. Polymer P3 was solely
functionalized with PTH, but in relatively high amounts, and
Jeffamine. Overall, these three polymers were selected to
investigate whether composition and catalyst loading of the
polymer affect the catalytic performance of SCPNs for light-
mediated reactions.
To avoid differences in the degree of polymerization and
molar mass dispersity in the three polymers, we chose a post-
functionalization approach starting from poly(pentafluorophenyl)
acrylate to prepare the amphiphilic polymers.[52] This polymer with
activated ester pendants can be randomly functionalized with
various amine-bearing compounds as side groups (Figure 1b),[54]
making the stepwise synthesis of SCPNs easy and versatile. It is
important that the distribution of the hydrophobic moieties is
random since this ensures an intramolecular hydrophobic
collapse in water and hereby the formation of nanoparticles from
a single polymer chain.[52,55–57]
Results and Discussion
Molecular design, synthesis, and characterization of
photoredox-active SCPNs.
We designed three, differently substituted, amphiphilic polymers
(P1–P3, Figure 1b) that differ in the amount of hydrophobic alkyl
residues (P1 versus P3), the presence of structuring moieties
(benzene-1,3,5-tricarboxamide, BTA, P2) and the amount of
photoredox catalyst PTH (~10% in P1 and P2, and ~23% in P3).
In all cases, we selected long polyetheramine-based chains, so-
called Jeffamine®M-1000 (Jeff), to impart sufficient water-
solubility. In order to covalently attach the catalytic unit to the
polymer backbone, we slightly modified the chemical structure of
Poly(pentafluorophenyl) acrylate with an average degree of
polymerization of 100 and a molar mass dispersity Đ of 1.17
(determined by SEC, size exclusion chromatography, in THF
calibrated with polystyrene standards) was obtained after
reversible
addition−fragmentation
chain-transfer
(RAFT)
polymerization of pentafluorophenyl acrylate and subsequent
removal of the RAFT end group.[52] Each sequential post-
2
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modification step was monitored by 19F NMR spectroscopy and
the degree of amine-functionalization was determined by
comparing the signals originating from the released
pentafluorophenol and the polymer precursor (see Figures S1–S3
in the Supporting Information). Thereby, the set of three differently
substituted amphiphilic polymers (P1–P3) was synthesized and
fully characterized. The modified polymers have a theoretical
molecular weight of ca. 90 kDa and molar mass dispersities of
Đ = 1.11–1.13 (SEC in DMF with LiBr, calibrated with
poly(ethyleneoxide) standards).
Dynamic light scattering (DLS) measurements revealed on
the one hand that the hydrodynamic radii of the nanoparticles of
P1 and P3 are 5.2 and 5.6 nm in water, respectively, which is the
typical regime for nanoparticles consisting of individual polymer
strands[52] (see Figure S4 in the Supporting Information).
Interestingly, the presence of the aromatic PTH moieties appears
to suffice to induce an intramolecular hydrophobic collapse into a
SCPN. On the other hand, nanoparticles of P2 show a slightly
higher radius of 6.5 nm, a result of interactions between
nanoparticles and the formation of presumably dimeric
aggregates. This is consistent with our previous results where
BTA loadings above 10% were found to induce some clustering
of the polymer chains.[52]
respectively, after light exposure of 24 h (see entries 7 and 8 in
Table 1). Importantly, no side products were detected during the
light-driven dehalogenation of benzene derivatives 1–3. In control
experiments, we validated that product formation is indeed only
taking place when the photoredox-active nanoparticles and
triethylamine are simultaneously present in combination with the
required light activation (see entries 9–12 in Table 1). Instead of
using UV light (385 nm) it is also feasible to perform the
photocatalytic reaction with a blue light LED (405 nm) or even
sunlight, thereby contributing to sustainable and green chemistry
(see entries 13 and 14 in Table 1).
Table 1. Screening of different tertiary amines for the light-driven
dehalogenation of benzene halides in water.
Entry
Substrate
Deviation
from
procedure[a]
Irradiation
time
Conversion[c,d]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
2
3
1
as shown
NiPr2Et
NCyMe2
NPr3
1 h
1 h
1 h
1 h
1 h
1 h
24 h
24 h
1 h
99%
85%
80%
51%
27%
13%
99%
40%
0%
The optical properties of the polymers were studied via UV/vis
spectroscopy in water as well as organic solvent (i.e., acetonitrile;
see Figure S5 in the Supporting Information). The UV/vis spectra
show in both solvents the typical absorption bands of the
incorporated PTH catalyst[58] with maxima at around 258 and
320 nm, indicating the successful incorporation of the
photocatalyst within the polymer scaffold (in accordance with
NMR spectroscopy, see Figures S6–S8 in the Supporting
Information).
NBu3
NHex3
as shown
as shown
Optimization of reduction conditions for benzene halides.
With the characterized polymers in hand, we investigated their
catalytic activity towards the light-driven dehalogenation of
benzene halides. Previously reported mechanistic investigations
showed that the working principle is based on a single-electron-
transfer from the highly reducing excited catalyst to the substrate,
followed by a homolytic cleavage of the carbon-halide bond and
the formation of a benzene radical.[44] The presence of a tertiary
amine base is paramount to guarantee a successful outcome as
it regenerates the photoredox catalyst and also acts as the proton
source to yield the reduced benzene compound. The catalytic
reactivity of PTH primarily originates from its excited singlet state
and not from its oxygen-sensitive triplet state, thus reductions can
be performed without excluding oxygen.
First, we optimized the reaction conditions for the
dehalogenation of iodobenzene with the aim to reach full
conversion after 1 h of irradiation with a 385 nm LED (see Figure
S9 in the Supporting Information for the general reaction setup).
For the optimization we used 10 mg of P1 in 0.8 mL of deionized
water, which corresponds to a polymer concentration of 140 µM
and catalyst loading of 4 mol%, and examined several tertiary
amines (Table 1). We observed that a quantitative conversion to
benzene was obtained with triethylamine (see entry 1 in Table 1
and Figure S10 in the Supporting Information). The formation of
benzene decreased with an increasing hydrophobicity of the
amines (see entries 2–6 in Table 1). We also applied inherently
less reactive bromo- and chlorobenzene as substrates under
these conditions and measured conversions of 99% and 40%,
no NEt3,
no P1
10
11
12
13
1
1
1
1
no NEt3
no P1
1 h
1 h
1 h
1 h
traces
traces
0%
no light
blue light
LED
45%
(405 nm)
14
15
16
17
1
1
1
1
sun light
7 h
1 h
1 h
1 h
17%
0%
[b]
in CDCl3
[b]
in THF-d8
0%
[b]
in ACN-d3
41%
[a] Standard procedure: 30 μmol (1 eq.) of substrate, 5 eq. NEt3, and 10 mg of
P1 were taken up in 0.8 mL of deionized water and the mixture was irradiated
with a 385 nm LED. After the indicated time, the mixture was extracted with
CDCl3 and the conversion was determined by 1H NMR spectroscopy. [b] The
solution was analyzed directly by 1H NMR spectroscopy without extraction. [c]
A conversion of 99% indicates quantitative conversion by NMR. [d] When
conversion was below 99% only starting material remained, no side products
were observed.
In order to substantiate the importance of the formation of
SCPNs acting as nanoreactors, we executed the reduction of
3
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iodobenzene in organic solvents, in which the hydrophobic
collapse and thus nanoparticle generation is precluded. Under
otherwise identical conditions, either no product was detected (in
deuterated chloroform and tetrahydrofuran) or moderate yields
were obtained (in deuterated acetonitrile; see entries 15–17 in
Table 1). This emphasizes the value of providing a defined
nanocompartment for catalysis – similar to enzymes – in which
high local concentrations of reagents lead to an acceleration of
product formation.
not required. Moreover, the differences in catalyst loading, which
ranged between 3-8 mol% depending on the applied polymer, did
not result in a significantly altered catalytic efficiency. This
strongly indicates that only a fraction of the catalyst was
simultaneously excited with light to populate the reactive excited
state.
Investigation of substrate scope and tolerance to functional
groups.
Based on these promising initial results, we further checked for
the substrate scope of this chemical transformation as well as
potential differences between the set of polymers. To this end, we
merely focused on iodobenzene derivatives due to their relatively
high reactivity (Scheme 1). It turned out that a variety of functional
groups are tolerated, ranging from electron-poor derivatives
(substrates 4–6) that expectedly gave higher conversions than
electron-rich derivatives (substrates 7–10) to heterocyclic
compounds (substrates 11–13). By comparing the individual
results of P1–P3 for each substrate, slight deviations can be
identified, while they all follow the same trend in compliance with
the reactivity of the iodobenzene derivatives. Contrary to our
expectations, the use of polymer P3 with the highest number of
catalysts attached (catalyst loading of 8 mol% under the applied
conditions) resulted in the highest product formation for
compounds 8 and 10, but not for the others. Interestingly, polymer
P2, which is modified with the fewest PTH functionalities (catalyst
loading of 3 mol%), showed the best performance for substrates
Scheme 1. Illustration of the reaction scope for the dehalogenation of differently
substituted (hetero)aryl iodides. The indicated conversions are average results
of at least two experiments following the general protocol. The reduction of
2-iodobenzoic acid 5 was carried out in deuterated water.
6
and 11. By employing an acidic compound, such as
4-iodophenol 9, polymer P1 gave the best result, which in turn
generally yielded the lowest conversions for the other starting
materials. Although the reduction of 2-iodobenzoic acid 5 was
carried out in deuterated water, no signals for the reduced
deuterated product were detected by NMR spectroscopy (see
Figure S11 in the Supporting Information). This is consistent with
the proposed mechanism stating that the tertiary amine is the
primary proton source and not the solvent.[44] Remaining starting
material can be completely converted by extending the
illumination period if needed, hence product purification steps by
column chromatography are not required.
Overall, polymers P1–P3 display a high activity for the light-
mediated reduction of (hetero)aryl iodides in water based on the
(almost) quantitative conversion of many substrates after 1 h. As
examples in organic solvents under comparable conditions
require longer reaction times,[44,59,60] it becomes apparent that the
compartmentalization and thus high local concentration of
reagents in SCPNs accelerate the product formation.
Temporal control, recyclability, and concentration study.
Benzoic acid derivative 5 shows near complete conversion to
benzoic acid in 1 h for all polymers P1–P3. In order to assess
differences in the activity of the different polymer catalysts for this
substrate, we performed a typical step experiment. Herein, the
conversion of 5 to benzoic acid is followed as a function of time
while the light source is repetitively turned ON and OFF (see
Figure 2a and Figure S12 in the Supporting Information). On the
one hand, the results for this experiment underline that the
dehalogenation reaction is only taking place when the mixture is
irradiated with light and paused when the LED is switched OFF,
thereby giving rise to a temporally well-controlled catalytic
process. On the other hand, we observe that the use of P1 yielded
the highest conversion (55% after in total 3 min of irradiation)
compared to P2 (41%) and P3 (38%).
We were furthermore interested in the possibility of recycling
the photoredox-active SCPNs and thus performed a series of
consecutive reactions using polymer P1 and 2-iodobenzonitrile 4
as the substrate (see Figure 2b and Figure S13 in the Supporting
Information). Using SCPNs has the advantage that organic
compounds can conveniently be extracted from the aqueous
phase with diethyl ether, while the polymer remains in the water
layer.[42] After each run, we extracted the water mixture
accordingly, determined the conversion by 1H NMR spectroscopy,
charged the polymer-containing aqueous solution with 4 and fresh
triethylamine, and illuminated it again with light of 385 nm for 1 h.
In a previous report on SCPNs, it was shown that the
presence of BTA groups within the nanoparticles was paramount
to render a proline catalyzed aldol reaction successful.[42] The
study presented here, however, displays that the catalytic
performance of the PTH photoredox catalyst is only marginally
influenced by the composition of the SCPNs. As a positive
consequence, the functionalization of the polymer backbone can
be simplified, as demonstrated with P3, by only attaching the
catalytic moiety and Jeffamine for water-solubility. Elaborate
synthesis of additional pendant groups, such as (S)-BTA, is thus
4
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Figure 2. Experiments demonstrating the versatility of the photoredox-active SCPNs: a) Substrate conversion over time during the dehalogenation reaction of 2-
iodobenzoic acid 5 in the presence of triethylamine and polymer P1 (orange line), P2 (blue line), and P3 (green line) in deuterated water. The 385 nm LED was
alternatingly turned ON (purple background) for 0.5 or 1 min and switched OFF for 1 min. Each experiment was performed at least in duplicate. b) Recycling
experiment based on the consecutive dehalogenation reaction of 2-iodobenzonitrile 4 in the presence of triethylamine and polymer P1 in water. c) Dehalogenation
reaction of 2-iodobenzoic acid 5 in the presence of triethylamine and polymer P3 with different polymer concentrations and reaction times in deuterated water.
The dehalogenation reactions were quantitative after each run
and we did not observe a decrease of reactivity over the course
of five recycling steps. These results highlight the robustness and
reusability of these PTH-comprising SCPNs.
Another benefit of SCPNs, besides their recyclability, is that
the nanoparticles also form at highly diluted concentrations. This
could be beneficial in those cases where highly dilute conditions
are desired. We capitalized on this by performing the reduction of
2-iodobenzoic acid 5 with varying amounts of polymer P3 and
observed catalytic activity even at a polymer concentration of
1.4 µM, which corresponds to a catalyst loading of 0.08 mol%
(see Figure 2c). Even at this very low catalyst loading, a near
quantitative conversion of 99% was observed after 22 h.
pathway. However, we envisioned that the quantity of coupling
partner and thereby the amount of waste can be reduced
significantly by benefiting from the high local concentrations of
reagents within the hydrophobic compartment created by the
SCPNs. Indeed, we achieved comparable conversions by only
adding 5 equivalents of 14 to the aqueous mixture (see entries 1–
3 in Table 2). In addition, the reaction time of 1 h is notably shorter
compared to the reaction time reported in literature.[59] In order to
reduce the formation of the dehalogenated side product 16, we
applied less triethylamine, on the one hand, as it is the major
proton source for the reduction pathway[44] (see entry 4 in Table
2). On the other hand, we doubled the amount of 14 added to the
reaction (see entry 5 in Table 2). Both strategies led to a slight
increase of the desired coupling product 15.
Investigation of nanoparticle stability by DLS and CD
measurements.
At this point, we wanted to confirm that the nanoparticles are in
fact stable under the reaction conditions applied. DLS
measurements showed marginally increased hydrodynamic radii
of the formed SCPNs in the presence of high excess of
triethylamine and subsequently added 4-iodophenol 9, which can
be attributed to a swelling effect of the nanoparticles (see Figure
S4 in the Supporting Information). We additionally analyzed an
aqueous solution of polymer P2 via circular dichroism
spectroscopy and detected a negative CD effect originating from
the formation of helical stacks of the BTA pendants within the
nanoparticles (Figure 3). This supramolecular assembly can be
used as a molecular probe to investigate whether the inner
composition remains intact or is disturbed by the accommodation
of substrate. To this end, we added again high excess of
triethylamine to the aqueous spectroscopy solution and as a result
neither intensity nor shape of the CD signal were impaired,
indicating that the nanoparticles’ inner structure remains
untouched.
Figure 3. CD spectra of a solution of P2 in water (c = 0.1 mg/mL, V = 3 mL)
before (red curve) and after the addition of 1 μL of triethylamine (ca. 2300 eq.,
blue curve) at 20 °C. The negative CD signal, which originates from
supramolecular helical stacks of the BTA units[41,53] within the nanoparticles’
core, is not affected by the presence of triethylamine.
Cross coupling reaction with N-methylpyrrole.
Lastly, we explored the C-C cross coupling between
2-iodobenzonitrile 4 and electron-rich N-methylpyrrole 14 to
expand the versatility of our photoredox-active SCPNs (see
Table 2 and Figure S14 in the Supporting Information). Previously
reported protocols in organic media typically use high excess of
pyrrole (25–50 equivalents)[45,59,61] in order to trap the generated
benzene radical efficiently and thus compete with the reduction
5
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Table 2. C-C cross coupling reaction with N-methylpyrrole.
temperature. Afterwards, the aqueous mixture was extracted with CDCl3
and the conversion was determined by 1H NMR spectroscopy.
Acknowledgements
F.E. acknowledges the Alexander von Humboldt Foundation for
providing a Feodor Lynen research fellowship.
Entry
Polymer
NEt3 (eq.)
14 (eq.)
Conversion
Ratio
15:16
Keywords: Photoredox catalysis • Nanoparticles • Enzyme
mimics • Supramolecular chemistry • Green chemistry
1
2
3
4
5
P1
P2
P3
P1
P1
5
5
5
1
5
5
99%
99%
99%
99%
99%
64:36
65:35
66:34
78:22
70:30
5
5
[1]
[2]
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[3]
[4]
M. H. Shaw, J. Twilton, D. W. C. MacMillan, J. Org. Chem. 2016, 81,
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[5]
[6]
B. H. Lipshutz, S. Ghorai, Green Chem. 2014, 16, 3660–3679.
D. K. Romney, F. H. Arnold, B. H. Lipshutz, C.-J. Li, J. Org. Chem.
2018, 83, 7319–7322.
In conclusion, we utilized robust and easily recyclable single-
chain polymeric nanoparticles for efficient and well-controlled
metal-free photoredox catalysis in water under ambient conditions
(including the presence of air oxygen). The modular and facile
synthetic approach to prepare SCPNs allows to study a set of
differently substituted amphiphilic polymers, which were
functionalized with a phenothiazine group as the photocatalytic
unit. The SCPNs were successfully employed as tailor-made
nanoreactors for the reduction of (hetero)aryl halides upon
irradiation with light of 385 nm, thereby showing a high tolerance
towards a wide array of functional groups. In addition, the
nanoparticles show a pronounced stability even upon exposure to
high excess of reagents and still act as catalyst at low micromolar
concentrations. On this basis, we applied this reaction setup for
an arylation reaction with an electron-rich pyrrole. By providing
hydrophobic compartments and thus high local concentrations of
reagents within the nanoparticles an acceleration of product
formation could be observed. As a result, most of the light-driven
reactions were (almost) completed within 1 h, which is generally
faster compared to examples in organic media under similar
conditions.[44,59,60] By assessing the influence of the polymer
composition, it turns out that the incorporation of the aromatic
PTH catalyst as the only hydrophobic side group is sufficient to
induce the formation SCPNs without compromising the catalytic
activity. Overall, these results are an important step towards a
green and sustainable approach to conduct photoredox catalysis
in aqueous media, which has tremendous potential for biological
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Entry for the Table of Contents
Photoredox catalysis takes a bath: Single-chain polymeric nanoparticles are utilized as an efficient and recyclable platform to perform
photoredox catalysis in water. The reduction of aryl halides and C-C cross coupling reactions are studied upon UV light illumination at
ambient conditions.
8
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