Metal-Free Aryl Cross-Coupling Directed by Traceless Linkers

Article abstract of DOI:10.1002/chem.201903582

The metal-free, highly selective synthesis of biaryls poses a major challenge in organic synthesis. The scope and mechanism of a promising new approach to (hetero)biaryls by the photochemical fusion of aryl substituents tethered to a traceless sulfonamide linker (photosplicing) are reported. Interrogating photosplicing with varying reaction conditions and comparison of diverse synthetic probes (40 examples, including a suite of heterocycles) showed that the reaction has a surprisingly broad scope and involves neither metals nor radicals. Quantum chemical calculations revealed that the C?C bond is formed by an intramolecular photochemical process that involves an excited singlet state and traversal of a five-membered transition state, and thus consistent ipso–ipso coupling results. These results demonstrate that photosplicing is a unique aryl cross-coupling method in the excited state that can be applied to synthesize a broad range of biaryls.

Full text of DOI:10.1002/chem.201903582

A Journal of
Accepted Article
Title: Metal-Free Aryl Cross-Coupling Directed by Traceless Linkers
Authors: Veit G. Haensch, Toni Neuwirth, Johannes Steinmetzer,
Florian Kloss, Rainer Beckert, Stefanie Gräfe, Stephan
Kupfer, and Christian Hertweck
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To be cited as: Chem. Eur. J. 10.1002/chem.201903582
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FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Metal-Free Aryl Cross-Coupling Directed by Traceless Linkers
Veit G. Haensch,a Toni Neuwirth,a Johannes Steinmetzer,b Florian Kloss,c Rainer
Beckert,d Stefanie Gräfe,b Stephan Kupferb* and Christian Hertwecka,e*
a
Department of Biomolecular Chemistry, Leibniz Institute for Natural Product Research and Infection Biology, HKI,
Beutenbergstr. 11a, 07745 Jena, Germany
Institute for Physical Chemistry, Friedrich Schiller University Jena, Helmholtzweg 4, 07743 Jena, Germany
Transfer Group Antiinfectives, Leibniz Institute for Natural Product Research and Infection Biology (HKI), 07745 Jena,
b
c
Germany.
d
Institute for Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, 07743 Jena,
Germany
e
Chair of Natural Product Chemistry, Friedrich Schiller University Jena, 07743 Jena, Germany
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
Abstract. The metal-free, highly selective synthesis of Quantum chemical calculations revealed that the C–C bond
biaryls poses a major challenge in organic synthesis. We is formed by an intramolecular photochemical process that
report the scope and mechanism of a promising new involves an excited singlet state and the traverse of a five-
approach to (hetero)biaryls by the photochemical fusion of membered transition state, thus warranting consistent ipso
aryl substituents tethered to a traceless sulfonamide linker ipso coupling fidelity. These results demonstrate that
(photosplicing). Interrogating photosplicing with varying photosplicing is a unique aryl cross-coupling method in the
reaction conditions and comparison of diverse synthetic excited state that can be applied to synthesize a broad range
probes (40 examples, including a suite of heterocycles) of biaryls.
showed that the reaction has a surprisingly broad scope and
involves neither metals nor radicals.
Keywords: Biaryls; Cross-coupling; Photochemistry;
Synthetic methods; TDDFT
employed to generate radicals or aryl radical
cations,[10] which set the stage for C–C bond
formations. Regioselective biaryl syntheses have
been achieved via intramolecular aryl couplings and
rearrangements, yet parts of the linkers remain in the
product.[11] Recently, we found that bis-aryl-
substituted sulfonamides can be contracted
Introduction
The enormous economic role of biaryls in chemical
and pharmaceutical products[1] has propelled the
development of a large number of cross-coupling
methodologies.[2] In general, metal catalysis offers
great fidelity in terms of substrate tolerance and
scalability of biaryl synthesis.[3] Yet, in large-scale
manufacture and the production of bioactive
ingredients, removal of heavy metal impurities,
catalyst recycling, and the provision of suitable
starting materials can be challenging and costly.[4]
Thus, there is a high demand for alternative, metal-
free cross-coupling methods.[5] Most known metal-
free approaches, however, suffer from disadvantages
such as narrow scope and low selectivity because of
the harsh conditions required.[5a] Recent approaches
to transition metal-free biaryl syntheses involve, for
example, Grignard reagents,[6] oxidative couplings of
electron-rich arenes,[7] base-mediated couplings of
aryl halides,[8] and phosphorus ligand couplings.[9]
Furthermore, photolytic reactions have been
photochemically,
thereby
permitting
the
regioselective fusion of two aryl groups.[12] Using a
UV photoreactor, we demonstrated the preparative
applicability of the reaction in the synthesis of
building blocks for a series of clinically relevant
pharmaceuticals. Remarkably, all required anchor
groups to tether the aryls as sulfonamides are cleanly
extruded as volatile fragments (ammonia, sulfur
dioxide, formaldehyde), yielding the ipso-ipso-
coupled biaryl product (Figure 1A). Yet, the scope
and the molecular mechanism of this unusual
photosplicing reaction have remained elusive. Here
we address these important questions by
a
combination of chemical synthesis and quantum
mechanical considerations, and present photosplicing
1
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10.1002/chem.201903582
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as a highly versatile method for the selective, metal-
free preparation of biaryls.
Results and Discussion
Photosplicing permits the synthesis of a broad
range of (hetero)biaryls
To test the scope of the photochemical aryl coupling,
we prepared and compared a library of 30
sulfonamides with different substitution patterns and
chemically diverse substituents at both aryl residues
(Figure 1B). We found that irradiation (254 nm) of
p-alkyl carboxylate-substituted educts give best
results, in particular when this substituent is located
at the benzylic residue (blue). In lieu of a
carboxyester, various electron-withdrawing groups
(nitrile, trifluoromethyl, acyl, and formyl) can be
employed. Likewise, electron-releasing substituents
(methoxy, benzyloxy, dimethylamino, methyl) afford
the desired products, albeit in reduced yields.
As to the benzenesulfonyl residue (green),
photosplicing tolerates methyl, ethyl, methoxy,
benzyloxy, hydroxyl, fluoro, chloro, nitrile,
carboxymethyl, acetyl, dimethylamino, methyl
thioether, and trifluoromethyl moieties, except for
photolabile groups such as nitro moieties. It should be
highlighted that photosplicing is even possible in the
presence of two ortho-methoxy or benzyloxy groups
(2g, 2j). In contrast, two ortho-methyl groups (2ad)
and the lack of substituents (2p) hamper the reaction
and yield only traces of the corresponding biaryls. In
some cases, when the irradiation of sulfonamides
afforded low biaryl yields, diverse degradation
products resulting from S–N cleavage, oxidative
deamination and other oxidative processes could be
observed. Overall, photosplicing proved to be highly
versatile as it tolerates a broad range of photostable
substituents. Surprisingly, even substrates with
reversed polarity, such as 1i and 1aa, afford the
corresponding biaryls.
To evaluate the possibility of coupling
Figure 1. General scheme and scope of photosplicing. A)
General reaction scheme. B) Biaryl target compounds and
comparison of yields obtained by photosplicing. C)
General reaction scheme for heterobiaryls. D) Heterobiaryl
target compounds and comparison of yields obtained by
photosplicing. *Based on recovered starting material.
‡Known example, included for comparison.
heteroaromatic units we prepared
a suite of
sulfonamides with pyridine, imidazole, thiophene and
its benzannulated derivatives (Figure 1C). Irradiation
with UV light (254 nm) led to the expected
photosplicing products in moderate to excellent
yields (Figure 1D). Best results were obtained with
pyridine and imidazole moieties. These findings
illustrate the remarkably broad substrate scope for
photosplicing.
Experimental evidence for metal- and radical-free
reaction
For the photoactivated aryl coupling, various reaction
mechanisms are conceivable. UV light excitation
could potentially lead to the homolytic fission of the
linker followed by formation of carbon-centered
radicals and coupling of phenyl radicals (Figure 2A,
I),[13] or the observed ipso-ipso substitution could
involve intermolecular reactions favored by
dispersive interactions (Figure 2A, II).[14] Although
no metal additives were used, trace metal impurities
in solvents and reagents could be sufficient to
2
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10.1002/chem.201903582
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facilitate photocatalytic effects (Figure 2A, III).[15]
Alternatively, the biaryl could be formed in analogy
to a radical Smiles type rearrangement (Figure 2A,
IV).[11b, 16] Another plausible route could involve an
intramolecular ipso-ipso attack of the excited
sulfonamide leading to a five-membered transition
state or intermediate, which undergoes further
fragmentation (Figure 2A, V).
To unravel whether UV-irradiation of sulfonamide 1a
afforded reactive aryl radicals in a Norrish type I
reaction,[17] first we meticulously analyzed the
reaction mixture; yet, biaryls that would arise from
random radical recombination processes could not be
detected. Furthermore, neither radical starters such as
dibenzoyl peroxide (DBPO) or azobisisobutyronitrile
(AIBN), nor the mixture of tributyltinhydride and
AIBN, which is used for the radical Smiles
rearrangement,[11a] initiated the biaryl coupling
(Figure 2B). Moreover, we found that the course of
the photoreaction is not affected in the presence of
the radical quencher 2,6-di-tert-butyl-4-methylphenol
(BHT),[18] even when added in large excess (Figure
2C). To trap potential intermediary radicals, excess of
toluene to the photoreaction with sulfonamide 1s was
added. Even when using toluene as co-solvent in
acetonitrile (v/v = 50:50), no side products of the
coupling reaction were detectable (Figure 2D).
Finally, we installed a cyclopropyl, which serves as a
radical trap,[19] at the linker and noted that this unit
does not hamper the photoreaction (Figure 2G). Thus,
the involvement of (long-living) radicals in the course
of the aryl coupling can be excluded.
To investigate the potential involvement of metal
catalysts in the aryl coupling, we added different late
d-block metals (Fe, Co, Ni, Pd, Pt, Ir, Ru, Cu, Rh,
Os) that are used for cross couplings[20] to
sulfonamide 1a. In the absence of light, biphenyls
were not formed, and after irradiation no increased
ratio of sulfonamide to biphenyl could be observed
(Figure 2E). These findings provide strong support
for a metal-free reaction mechanism.
Figure 2. Evaluation of potential reaction
mechanisms. A) General reaction scheme and
different mechanistic scenarios for the photosplicing
process I) radical, II) intermolecular, III) metal-
catalyzed, IV) Smiles-like rearrangement, V)
regiochemically controlled intramolecular reaction;
B) impact of radical initiators (~1 eq. of radical
initiator with 1a in benzene at 70 °C for 25 h), C)
radical quenchers (1 eq. or 10 eq. BHT with 1a in
MeOH, 254 nm, 15 min, 20 °C), D) reactive solvent
(1a in MeCN/toluene (v/v = 50:50), 254 nm, 15 min,
20 °C), and E) addition of metals (5 mol% or
saturated solution of metal salts with 1a in MeOH,
254 nm, 15 min, 20 °C), F) cross-reactivity, G)
modification of linkers to hamper tautomer formation
and to install a radical trap.
As an alternative to the templating effect of metals,
we reasoned that the aromatic rings of the
sulfonamide educts could be positioned by dispersive
interactions. To address the possibility of potential
intermolecular reactions, we prepared a pair of
3
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10.1002/chem.201903582
Chemistry - A European Journal
sulfonamides with complementary substitution and
monitored the course of the photoreaction using a
mixture of educts. The photoreaction of sulfonamide
1a and 1ak in one batch with the same molar ratio
showed no photoproducts with mixed substituents
(Figure 2F). This cross-reactivity experiment would
be in line with an intramolecular recombination.
Consequently, the linker plays a key role in the aryl
coupling.
To estimate the impact of C- and N-substitutions in
the linker and thus the facilitation of the ipso-carbon
attack by tautomer formation, we exchanged all
protons by methyl groups (Figure 2G). Therefore, we
prepared an N-methyl sulfonamide (1al), as well as
sulfonamides with two methyl groups at the carbon of
the linker (1am). As both substrates afforded the
corresponding biphenyl (2a), we concluded that the
influence of potential tautomers on the reaction is
negligible.
relaxation towards the product state, and thus
formation of the photoproduct (2a), the S–N and C1–
C2 distances are increased, indicating the extrusion of
SO2 and CH2NH, which can hydrolyze into ammonia
and formaldehyde (Figure 3B). According to our
calculations, the formation of the TS requires an
activation energy of 4.08 eV in the ground state.
–double-spaced figure at end of article–
Theoretical model of the reaction mechanism
To elucidate the mechanism underlying the
photosplicing at a molecular level, we performed
quantum chemical simulations, density functional
theory (DFT) and its time-dependent extension (TD-
DFT). As models we investigated the light-driven
formation of biphenyl 2a from sulfonamides 1a and
1aa, respectively, featuring inverse electronic
structures. Initially, we calculated the potential
energies of the educt states (1a and 1aa) by GFN-
xTB,[21] which permits a detailed analysis of the
possible conformers in the ground state. The energies
of selected conformers were verified by coupled
cluster calculations,[22] and were subsequently
analyzed by means of a principal component analysis
(Figure S9-S12).
Relaxed scans along the dihedral angle φ (given by
C2-N3-S4-C5) revealed that the flexible sulfonamide
linker adapts preferably linear and U-shaped
horseshoe conformers (Figure 3A). The relaxed
potential energy curves (PECs) obtained by DFT
(Figure 3A) indicated that in 1a and 1aa the
horseshoe conformer (φ = 80°, φ = 70° for 1aa) is
energetically more favored (by 10 kJ/mol) than the
linear conformer (φ = –70°). The 1a horseshoe
conformer exhibits a short C1–C5 distance (326 pm),
which is pivotal for the C–C coupling of the aromatic
rings.
Figure 3. In silico analysis of photosplicing. A)
Relaxed scan along the dihedral angle of the linker
from sulfonamide 1a and 1aa with energy differences
and C1–C5 distance, B) distance of C1–C5 related to
C5–S4 and distance of S4–N3 related to C1–C2
during the photosplicing reaction, C) reaction
coordinate in the ground state and low-lying excited
(singlet) states of the photosplicing reaction with the
calculated absorption spectra and the emission of the
light source, D) deduced model for the reaction
mechanism of photosplicing.
To gain insight into the excited states involved in the
photoreaction, we first studied the low-lying excited
singlet states of 1a and 1aa within the Franck-
Condon region. The simulated absorption spectrum of
1a (Figure 3C) reveals several bright states, i.e. S2 at
5.30 eV (234 nm) and S3 at 5.40 eV (230 nm), in the
vicinity of the irradiating light source centered at 4.88
To evaluate the potential course of the aryl coupling,
we subsequently assessed the formation of
photoproducts 2a and 2aa along a reaction pathway
given by an intrinsic reaction coordinate (IRC). The
IRC connects the educt and product states via a cyclic, eV (254 nm). Light-driven coupling of the aryl
C1-C2-N3-S4-C5-containing transition state (TS)
optimized by DFT. Within the five-membered TS of
the reaction from 1a to 2a the C1–C5 distance
decreases to 164 pm, which is associated to C–C-
bond formation. Simultaneously, partial cleavage of
the C–S bond is indicated by the increasing bond
length from 176 to 239 pm (Figure 3B). In stark
contrast, changes within the linker are less
pronounced until the TS is reached. Only upon
moieties is feasible by the leading transition
underlying S2 of 1a, which features a bonding
character between the carbon atoms C1 and C5
(Figure 3C). In addition, the S2 state coincides with
the experimentally determined maximum turnover at
5.28 eV (235 nm).[12] Evolution of the adiabatic PECs
of the low-lying excited states reduces the energetic
barrier to enable photosplicing of 1a within the
excited states to only 0.69 eV along the IRC. It is
4
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10.1002/chem.201903582
Chemistry - A European Journal
very remarkable that reversal of the substrate polarity
(1aa in lieu of 1a) leads to a congruent model and
activation barrier (0.77 eV for S1).
is an important addition to the current toolbox for
metal-free biaryl syntheses.
To corroborate the quantum chemical simulations, we
chose 1p as reference, since the bis-phenyl-
substituted sulfonamide yields merely traces of the
photoproduct (2p). Compared to 1a and 1aa, we
found that in 1p the determined excited states are
shifted hypsochromically and with lowered oscillator
strengths (Table S3). Therefore, the disadvantageous
excited state properties of 1p prevent an efficient
photoreaction. The computational results are in full
accordance with the experimental observations and
thus allowed to elucidate the mechanism of
photosplicing.
Experimental Section
Experimental Details are given in Supplemental
Information
Acknowledgements
We thank A. Perner for MS analyses and H. Heinecke for NMR
measurements. We thank the Deutsche Forschungsgemeinschaft
(Leibniz Prize, for C.H.), the Federal Ministry of Education and
Research (BMBF) of Germany within the program InfectControl
2020 (FKZ 03ZZ0803 for F.K.) and the State of Thuringia (2018
FGI 0039) for financial support.
Taken together, our quantum chemical simulations
show that electron-withdrawing and electron-
donating groups are
a
prerequisite of the
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The course of the photoreaction can be illustrated by
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the activation energy substantially and enables the
formation of 2a. The small energy gap (1.0 eV)
between the ground and the excited state (S1) in the
vicinity of the TS facilitates the relaxation into the
product state.
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photosplicing is a highly versatile method for the
selective synthesis of a broad range of biaryls. With
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Chemistry - A European Journal
Figure 3. In silico analysis of photosplicing. A) Relaxed scan along the dihedral angle of the linker from
sulfonamide 1a and 1aa with energy differences and C1 C5 distance, B) distance of C1–C5 related to C5–S4
and distance of S4–N3 related to C1–C2 during the photosplicing reaction, C) reaction coordinate in the ground
state and low-lying excited (singlet) states of the photosplicing reaction with the calculated absorption spectra
and the emission of the light source, D) deduced model for the reaction mechanism of photosplicing.
7
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FULL PAPER
Metal-Free Aryl Cross-Coupling Directed by
Traceless Linkers
Chem. Eur. J.. Year, Volume, Page – Page
V. G. Haensch, T. Neuwirth, J. Steinmetzer, F.
Kloss, R. Beckert, S. Gräfe, S. Kupfer*, C.
Hertweck*
8
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