Protection-Group-Free Synthesis of Sequence-Defined Macromolecules via Precision λ-Orthogonal Photochemistry

Article abstract of DOI:10.1002/anie.201901933

An advanced light-induced avenue to monodisperse sequence-defined linear macromolecules via a unique photochemical protocol is presented that does not require any protection-group chemistry. Starting from a symmetrical core unit, precision macromolecules

Full text of DOI:10.1002/anie.201901933

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Accepted Article
Title: Protection Group Free Synthesis of Sequence-Defined
Macromolecules via Precision λ-Orthogonal Photochemistry
Authors: Waldemar Konrad, Christian Fengler, Sarrah Putwa, and
Christopher Barner-Kowollik
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201901933
Angew. Chem. 10.1002/ange.201901933
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10.1002/anie.201901933
Angewandte Chemie International Edition
COMMUNICATION
Protection Group Free Synthesis of Sequence-Defined
Macromolecules via Precision λ-Orthogonal Photochemistry
Waldemar Konrad,[a,b] Christian Fengler,[a,b] Sarrah Putwa,[a] and Christopher Barner-Kowollik*[a,b]
4, 6]
Abstract: We report an advanced light-induced avenue to
monodisperse sequence-defined linear macromolecules via a unique
photochemical protocol, which does not require any protection group
exponential growth strategies,[2g,
for instance, have been
investigated. Lately, sequence-defined macromolecules draw
increased attention as potential data storage medium where
precise placement of functional groups is of crucial importance,[1h,
7] summarized in a recent review.[8] Herein, we are introducing a
versatile completely protection group free strategy for the
synthesis of monodisperse sequence-defined macromolecules by
combining two photosensitive groups activated in a λ-orthogonal
fashion (Scheme 1). A two monomer approach is employed
exploiting benzaldehydes and tetrazoles as light responsive
moieties resulting in monodisperse macromolecules as potential
chemistry. Starting from
a symmetrical core unit, precision
macromolecules with molecular weights up to 6257.10 g mol-1 are
obtained via a two-monomer system: (i) a monomer unit carrying a
pyrene functionalized visible light responsive tetrazole and a photo-
caged UV responsive diene, enabling an iterative approach for chain
growth; (ii) a monomer unit equipped with a carboxylic acid and a
fumarate. Both light-induced chain growth reactions are carried out in
a λ-orthogonal fashion – exciting the respective photosensitive group
selectively and thus avoiding protecting chemistry. In-depth
characterization of each sequence-defined chain, carried out via size-
exclusion chromatography (SEC), high-resolution electrospray
ionization mass spectrometry (ESI-MS) and NMR spectroscopy,
confirms the precision nature of the macromolecules.
materials for applications in data storage or as photoresists.[1h, 7a,
9]
Dimer
Non-Photoactive
Monomer
Core Unit
The unique class of perfect monodisperse sequence-defined
oligomers emerged inspired by nature’s precision and gains ever
since growing scientific interest. Such macromolecules consist of
monomers or building units placed at exact positions along the
polymer chain establishing monodispersity. Indeed, monomer
sequence regulation plays a key role in biology and is essential
for critical features of life, such as self-replication and complex
self-assembly. Guided by nature, the synthesis of sequence-
defined macromolecules requires non-statistical and highly
efficient synthetic processes and is envisioned as the key towards
highly functional materials, for instance, in molecular bar coding,
biological applications in synthetic enzyme design or precision
network synthesis.[1] Indeed, interesting concepts providing
simple, scalable and orthogonal strategies have been introduced
forming the foundation for the class of macromolecules that
feature sequence-definition including peptides, peptoids and
conjugated oligomers or dendrimers.[1j, 2] Single unit monomer
insertions (SUMIs),[2d, 3] liquid phase polymerizations[4] under bulk
or flow conditions as well as solid supports[5] exploiting the
iterative addition of single units or modular building blocks and
Photoactive
Monomer
Tetramer
Further Chain
Extension
Hexamer
Scheme 1. Sequential light-induced chain extension based on two monomers.
A symmetrical sequence-defined oligomer is synthesized starting from a core
unit avoiding protection chemistry by switching between NICAL and Diels–Alder
chemistry.
Our previous approaches reported a photochemical protocol
relying on benzaldehydes and furan caged maleimides to afford
sequence-defined macromolecules.[2c,
Consequently, after
10]
every chain growth step, the obtained oligomers needed to be
thermally treated upon further chain extension. To reduce the
amount of reaction steps that are performed with the
macromolecules itself and to increase the overall yield, it is critical
to avoid protection group chemistry. One option to prevent
protecting groups is to introduce photolabile moieties which can
be excited independently from each other by irradiation with
different colors of light. A pyrene functionalized tetrazole is visible
light responsive and forms a nitrile imine upon irradiation at
410 nm, which can be exploited to perform a nitrile imine-
carboxylic acid ligation (NICAL).[11] α-Methyl benzaldehydes entail
[a]
[b]
W. Konrad, C. Fengler, S. Putwa, Prof. C. Barner-Kowollik
Macromolecular Architectures
Institut für Technische Chemie und Polymerchemie
Karlsruhe Institute of Technology (KIT)
Engesserstr. 18, 76131 Karlsruhe, Germany
E-mail: christopher.barner-kowollik@kit.edu
W. Konrad, C. Fengler, Prof. C. Barner-Kowollik
School of Chemistry, Physics and Mechanical Engineering
Queensland University of Technology (QUT)
2 George Street, QLD 4000, Brisbane, Australia
E-mail: christopher.barnerkowollik@qut.edu.au
photo-caged UV responsive dienes which undergo [4+2] Diels–
Alder (DA) cycloadditions[2c,
at 365 nm in presence of
10]
dienophiles such as fumarates. Combining both photoactive
groups in one molecule enables light-induced chain growth
reactions in a λ-orthogonal fashion by the selective excitation of
the respective photosensitive group. Our herein reported
synthesis of sequence-defined macromolecules is based on the
convergent light-induced chain extension of a symmetrical core
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201901933
Angewandte Chemie International Edition
COMMUNICATION
unit 1 employing two different monomer units in a flow system
using a custom made flow reactor. First, a monomer 2 equipped
with a pyrene functionalized visible light responsive tetrazole for
chain extension via NICAL and a photo-caged UV responsive
diene for DA cycloaddition and second, a monomer unit 4 carrying
a carboxylic acid and a fumarate were designed to enable iterative
chain growth. The synthetic strategy using two monomer building
blocks enables the synthesis of perfect macromolecules avoiding
protecting chemistry in-between the addition of the single
monomers. Using the symmetric bisfunctional carboxylic acid 1
as a core unit allows to grow sequence-defined macromolecules
in a bidirectional manner, thus, enabling a significant increase of
molecular weight per chain extension step. Therefore, high
molecular weights can be accessed in as few reaction steps as
possible by switching between the two monomer types.
time. Thus, we designed a flow reactor (refer to the SI) using
LEDs (λmax = 410 – 420 nm) as light source. As depicted in
Scheme 2, the symmetrical dimer 3a (1964.28 g mol-1, 89 %) was
obtained by NICAL of the core 1 and a slight excess of the
monomer 2 (2.3 eq.) in anhydrous DCM. While the tetrazole is
transformed into the reactive nitrile imine at 410 nm irreversibly,
the benzaldehyde moiety remains in its ground state and is
unreactive. The bidirectional growth leads to extension on both
chain ends resulting in sequence-defined macromolecules end
functionalized with α-methyl benzaldehyde moieties allowing
further growth by DA cycloaddition. In
a
subsequent
oligomerization step, an excess of monomer 4 (4.0 eq.) was
added to the dimer 3a employing a PL-L lamp (λmax = 365 nm) as
light source yielding the symmetrical carboxylic acid
functionalized tetramer 3b (2540.79 g mol-1). Excess of monomer
4 and the target compound 3b (85 %) can be selectively obtained
by extraction of the crude mixture. The irradiation was performed
under batch conditions employing photovials as reaction vessel
because the DA cycloaddition between α-methyl benzaldehyde
and fumarate is not feasible under our flow conditions, possibly
because an anhydrous inert atmosphere free of oxygen cannot
be established. Nevertheless, we wish to highlight that
photoreactions with α-methyl benzaldehyde and fumarate can be
Flow reactors present two significant advantages compared
to conventional reaction conduction under batch conditions. First,
according to the Beer-Lambert’s law, reactor tubes with a low
diameter can be employed maximizing light penetration and
therefore reaction efficiency. Second, reaction scaling can be
performed by increasing reaction times while retaining the very
same conditions in terms of volume, concentration and exposure
Scheme 2. Photochemical sequential strategy for the synthesis of sequence-defined macromolecules avoiding protection chemistry. The bisfunctional core unit 1
allows light-induced bidirectional chain extension. Switching between a photoactive monomer 2 and a fumarate based monomer 4 affords the macromolecules
equipped with terminal benzaldehydes (3a, 3c and 3e) or carboxylic acids (3b and 3d) in a λ-orthogonal fashion.
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10.1002/anie.201901933
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COMMUNICATION
performed successfully in a commercially available Vapourtec
flow reactor. Applying similar reaction conditions according to the
dimer formation afforded the symmetric sequence-defined
hexamer 3c (4110.69 g mol-1, 79 %) by chain extension of the
tetramer 3b employing the UV-responsive monomer 2 in our flow
setup introducing benzaldehyde moieties at the chain ends for
further extension. The oligomer was further extended under batch
conditions using the hexamer 3c and monomer 4, for the
introduction of carboxylic acids as end groups, giving octamer 3d
(4687.20 g mol-1, 65 %) after UV-induced cycloaddition. In the
final chain growth step, the targeted symmetrical sequence-
defined macromolecule 3e (6257.10 g mol-1, 75 %) was obtained
by UV irradiation of octamer 3d with monomer 2 employing a LED
light source in a flow reactor. 3e also features benzaldehydes at
its chain ends which potentially enables further polymer growth
towards higher molecular weight. The low overall yield can be
explained by the necessary purification of the intermediates since
the photoreaction demands equimolar stoichiometry and a slight
excess of monomers was used. Due to their bidirectional growth,
the sequence-defined oligomers require quantitative conversion
of the chain ends, leading to a certain loss in overall efficiency in
addition. Compared to our previous report, purification is
simplified in-between the single chain extension steps because
the polarity of the macromolecule is drastically changed.[2c] Every
second extension reaction introduces carboxylic acids at the
macromolecules chain ends and enables the facile separation of
the single generations.
Successful formation of the targeted decamer 3e and
respective intermediates was verified by SEC, high resolution
Orbitrap ESI-MS and NMR. All SEC traces of the core unit 1, the
monomer units 2 and 4 and the oligomers 3a-d up to the final
monodisperse sequence-defined decamer 3e are depicted in
Figure 1a. The molecular weights were obtained relative to a
polystyrene calibration. Inspecting the SEC traces at each
synthetic step, a shift towards increased molecular weight can be
observed from the bisfunctional core unit 1 as well as the
monomers 2 and 4 to the iteratively generated sequence-defined
macromolecules 3a-e. The traces resulting from the sequential
approach confirm the purity of the oligomers and the increase in
molecular weight verifies the successfully conducted synthetic
strategy using different wavelengths of light to initiate chain
growth. The chromatographically determined dispersities of
Đ = 1.01 for all in Figure 1a depicted SEC traces representing the
building units and oligomers are evidencing the monodisperse
nature of the synthesized sequence-defined macromolecules 3a-
e. In addition, SEC analysis verifies the iterative chain extension
by switching between the two monomer types with an increase of
molecular weight after the addition of the respective monomers.
In Figure 1b, the high resolution ESI-MS spectrum of the targeted
symmetrical sequence-defined macromolecule 3e is depicted in
the m/z range from 1500 to 5000. The recorded spectrum only
reveals mass signals of respective charged sodium adducts at
a)
4
1
2
3a 3b 3c 3d 3e
1.0
0.5
0.0
m/z = 3151.3119
Δm/z = 0.0195),
([3e+2Na]2+,
m/z = 2108.5312
m/zcalc. = 3151.2924,
([3e+3Na]3+,
103
104
m/zcalc. = 2108.5247, Δm/z = 0.0065), and m/z = 1587.1443
([9c+4Na]4+, m/zcalc. = 1587.1408, Δm/z = 0.0035) all assigned to
the oligomer 3e. Recorded mass signals are in excellent
agreement with calculated molecular mass. In addition, Figure 1c
shows the experimental and simulated isotopic pattern of the
double charged species 3e containing two sodium counter ions in
excellent agreement. The characterization via SEC, the full mass
spectrum and the conformity of recorded and calculated isotopic
pattern clearly evidences monodispersity and the successful
synthesis of the sequence-defined decamer 3e (full MS spectra
and isotopic patterns of 3a-d can be found in the SI).
Molar Mass / g mol-1
[M+3Na]3+
2108.5312
b)
c)
[M+4Na]4+
1587.1443
[M+2Na]2+
3151.3119
1500
2000
2500
3000
3500
4000
4500
5000
m/z
[M+2Na]2+
Further characterization was carried out via NMR
spectroscopy. The proton spectrum and structure of the decamer
3e are depicted with colored frames for signal highlighting in
Figure 2. Intramolecular rearrangement after NICAL formation
from a hydrazonic anhydride to a hydrazide (see Supporting
Information) results in a complex proton spectrum. Typical signal
broadening, generally observed for macromolecules, and the
formation of different isomers after DA cycloaddition further
complicates the analysis via NMR. Nevertheless, characteristic
resonances can be assigned to the targeted oligomer 3e. Most
importantly, a proton resonance detected at 10.73 ppm (a)
[M+2Na]2+
calculated
3149
3150
3151
3152
3153
3154
3155
3156
m/z
Figure 1. SEC traces (a) of the core unit 1, the monomers 2 and 4, and the
sequence‐defined oligomers 3ꢀa–e recorded in THF. Full MS spectrum of 3e in
the mass range of m/z 1500 to 5000 (b) recorded in positive ion mode.
Experimental isotopic pattern (c) of 3e (top) and calculated isotopic pattern
(bottom).
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10.1002/anie.201901933
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resulting from the benzaldehyde moiety implies the successful
chain extension employing monomer 2 and the option for further
chain growth. Broad singlets at 11.06 and 10.84 ppm (a)
additionally confirm the formation of hydrazonic anhydride and
hydrazide after conducting the NICAL reaction. The introduction
of pyrenes into the backbone of the sequence-defined oligomers
can be verified by proton resonances ranging from 8.80 to
7.50 ppm (b). Resonances detected between 5.60-4.90 (d) and
3.50-2.50 ppm (e) indicate successful formation of the
tetrahydronaphthalene structure formed by DA cycloaddition
(detailed resonance assignments can be found in the SI, refer to
Figure S52 and S55).
evolution of molar mass obtained via SEC, proton NMR, recorded
mass spectra and the conformity of recorded and calculated
isotopic pattern underpin the precision nature of the
macromolecule, with monodisperse character and sequence
definition. Thus, the efficiency of our λ-orthogonal photochemical
protection group free avenue has been established.
Acknowledgements
C.B.-K. is grateful to the German Research Council (DFG) for
funding the current study in the context of the SFB 1176 (project
A3). C.B.-K. additionally acknowledges the Australian Research
Council (ARC) for a Laureate Fellowship underpinning his
photochemical research program as well as key support from the
Queensland University of Technology (QUT).
b
b
c
c
e
d
a
d
d
a
Conflict of Interest: The authors declare no conflict of interest.
b
CHCl3
Keywords: photochemistry • cycloaddition • sequence-defined
macromolecules • sequence-definition • sequence control
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Figure 2. 1H-NMR spectrum of the obtained decamer 3e recorded in CDCl3 at
400 MHz. Characteristic signals are highlighted in colored frames and refer to
the depicted structure.
In summary, we pioneer a photochemical protocol carried out
in a λ-orthogonal fashion and thus avoiding protection chemistry
for the synthesis of monodisperse precision sequence-defined
macromolecules with a molecular weight up to 6257.10 g mol-1. A
symmetrical bisfunctional carboxylic acid was chosen as a core
unit enabling bidirectional chain growth and thus providing a
significant increase of molecular weight per chain extension step.
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using a monomer unit carrying two photosensitive groups as key
molecule, which can be selectively activated for chain extension
by exposure to UV light at disparate wavelengths without
protecting groups. Consecutive chain growth employing a
complementary monomer led to the desired symmetric sequence-
defined macromolecules in an iterative approach. Successful
formation of the macromolecules was evidenced by in depth
characterization employing SEC, ESI-MS and NMR. The
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
No protection needed: A strategy
Waldemar Konrad, Christian Fengler,
Sarrah Putwa, Christopher Barner-
Kowollik
employing
orthogonal photochemical protocol
by selective excitation of
photosensitive groups paves the way
a
highly efficient λ-
Page No. – Page No.
to
precision
monodisperse
linear
Protection Group Free Synthesis of
Sequence-Defined Macromolecules
via Precision λ-Orthogonal
Photochemistry
sequence-defined
1.0
0.5
0.0
macromolecules without any need for
protection group chemistry. The
chain growth is based on a two-
monomer system enabling protection
103
104
group
free
convergent
chain
Molar Mass / g mol-1
extension on-demand.
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