Diverse Visible-Light-Promoted Functionalizations of Benzotriazoles Inspired by Mechanism-Based Luminescence Screening

Article abstract of DOI:10.1002/anie.201609393

Three new visible-light-promoted functionalizations of benzotriazole substrates were discovered using a mechanism-based screening method. ortho-Thiolated, borylated, and alkylated N-arylbenzamide products were obtained under mild reaction conditions in a new denitrogenative synthetic approach to functionalized aniline derivatives. The functional group tolerance of the borylation reaction was further analyzed in the first application of an additive-based robustness screen in a photocatalytic transformation. All the functionalizations proceed via photocatalytically initiated chain mechanisms as indicated by determination of the reaction quantum yields and Stern–Volmer analyses.

Full text of DOI:10.1002/anie.201609393

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201609393
German Edition: DOI: 10.1002/ange.201609393
Photocatalysis
Diverse Visible-Light-Promoted Functionalizations of Benzotriazoles
Inspired by Mechanism-Based Luminescence Screening
Michael Teders, Adriꢀn Gꢁmez-Suꢀrez+, Lena Pitzer+, Matthew N. Hopkinson, and
Frank Glorius*
Abstract: Three new visible-light-promoted functionalizations
of benzotriazole substrates were discovered using a mecha-
nism-based screening method. ortho-Thiolated, borylated, and
alkylated N-arylbenzamide products were obtained under mild
we discovered benzotriazoles as a new quencher class. Aided
by data obtained, we designed and realized a novel photo-
catalytic hydro-denitrogenation reaction of various 1-(ben-
zoyl)benzotriazoles (1) leading to N-arylbenzamides (see
Scheme 1).[2] While benzotriazoles have demonstrated activ-
reaction conditions in
a new denitrogenative synthetic
approach to functionalized aniline derivatives. The functional
group tolerance of the borylation reaction was further analyzed
in the first application of an additive-based robustness screen in
a photocatalytic transformation. All the functionalizations
proceed via photocatalytically initiated chain mechanisms as
indicated by determination of the reaction quantum yields and
Stern–Volmer analyses.
R
ecently, the use of photocatalysts capable of effectively
transferring energy from visible light to non-absorbing
compounds has gained increasing attention. This approach
allows for the mild and selective generation of radicals in
a controlled fashion and has led to new transformations
involving unique bond constructions.[1] A fundamental step,
common to all photocatalytic processes, involves the direct
interaction of the excited catalyst with a quencher substrate
and many research programs are duly focused on identifying
and applying new classes of quenchers. We recently reported
a conceptually novel mechanism-based screening method that
accelerates this discovery process using luminescence spec-
troscopy.[2] Whereas most screening approaches are “reac-
tion-based” and look for the formation of an overall reaction
product,[3] a mechanism-based screening focuses directly on
a single key mechanistic step central to a general reaction
class.[4] While not providing individual working reactions, this
approach leads to the identification of new substrates,
potentially suitable for many transformations within that
reaction class, and can furthermore provide insights into the
key steps that are useful for reaction optimization and
mechanistic elucidation. During our mechanism-based
screen of one hundred potential substrates for photocatalysis,
Scheme 1. Quencher identification based on luminescence quenching
and diverse functionalization strategies. PC=photocatalyst, BT=ben-
zotriazole.
ity under irradiation with ultraviolet light such as in the
photolytic Graebe–Ullmann carbazole synthesis,[5] function-
alization of these compounds by visible-light photocatalysis
has, to the best of our knowledge, not been reported before.
Herein, building on the discovery of this new quencher class,
we greatly expand the scope of visible-light photocatalysis
with benzotriazoles by developing three new o-functionaliza-
tion reactions. Denitrogenative routes to o-borylated, -thio-
lated, and -alkylated N-arylbenzamide derivatives directly
from bench-stable substrates have been developed by a novel
disconnection strategy for the synthesis of these common
organic building blocks (Scheme 1).
Firstly, we turned to the results of the mechanism-based
screening study to obtain information on the operating
quenching mode and to identify the optimum photocatalyst
and substrate combination. By monitoring the luminescence
of eight widely employed polypyridyl metal photocatalysts in
the presence of 1-(benzoyl)benzotriazole (1a) using a stan-
dard luminescence spectrometer, we observed efficient
quenching with fac-[Ir(ppy)3] (ppy = 2-phenylpyridine) and
the NHC-containing iridium complex [Ir(ppy)2(NHC-F2)]
(A) (NHC-F2 = 1-(2,4-difluorophenyl)-3-methyl-2,3-dihydro-
1H-imidazolydene) as well as moderate quenching with fac-
[Ir(dF(ppy)3)] (dF(ppy) = 2-(2,4-difluorophenyl)pyridine).[6]
These results are highly indicative of an oxidative quenching
pathway since these catalysts possess high excited-state
[*] M. Teders, Dr. A. Gꢀmez-Suꢁrez,[+] L. Pitzer,[+] Prof. Dr. F. Glorius
Organisch-Chemisches Institut
Westfꢂlische Wilhelms-Universitꢂt Mꢃnster
Corrensstrasse 40, 48149 Mꢃnster (Germany)
E-mail: glorius@uni-muenster.de
Prof. Dr. M. N. Hopkinson
Institut fꢃr Chemie und Biochemie
Freie Universitꢂt Berlin
Takustrasse 3, 14195 Berlin (Germany)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201609393.
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reduction potentials. A mechanistic scenario involving photo-
induced single-electron reduction is consistent with literature
reports on benzotriazoles bearing electron-withdrawing sub-
stituents on the nitrogen, which are thought to stabilize the
ring-opened zwitterionic diazonium form of these compounds
in polar solvents.[7] Regarding benzotriazoles as masked
diazonium compounds led us to consider whether analogous
light-promoted functionalization reactions to those achieved
with commonly employed diazonium salt quenchers could be
developed.[8,9]
applied in cross-coupling chemistry.[11] In a preliminary
experiment, a solution of 1a (0.10m) in acetonitrile was
irradiated with visible light from blue LEDs in the presence of
bis(pinacolato)diboron (B2pin2 2; 3.0 equiv) and photocata-
lyst A (2.5 mol%), which was identified as an optimal
photocatalyst in the screening. Since only trace amounts of
the desired borylated product 3a were observed, we consid-
ered possible factors hindering a successful reaction. One
potential pitfall concerns the turnover of the photocatalytic
cycle, that is, the regeneration of the IrIII catalyst from the IrIV
species, obtained upon oxidative quenching. We hypothesized
that the addition of N,N-diisopropylethylamine (DIPEA, 4)
as a sacrificial electron donor should alleviate this problem
and, indeed, when 4 was added under otherwise the same
conditions, the desired product was obtained in 47% GC
yield. After optimization, the yield of isolated 3a could be
increased to 85%.[6]
Inspired by a report on a synthetic route to arylboronates
via visible-light-induced borylation of aryldiazonium salts,[10]
we first designed a denitrogenative borylation of benzotria-
zoles leading to o-borylated N-arylbenzamides
3 (see
Scheme 2). The obtained aromatic arylboronic esters are
important building blocks in organic synthesis that are widely
Having found suitable conditions, we next investigated the
À
scope of this C B bond-forming reaction (Scheme 2). The
presence of electron-donating or -withdrawing substituents on
the benzotriazole core was tolerated and products 3b and 3d
were isolated in 61% and 62% yield, respectively. para-MeO-
Substitution on the benzoyl fragment led to the borylated
product 3c in 64% yield, whereas the addition of an electron-
withdrawing para-CF3 substituent resulted in a lower yield
(41%).
For a deeper understanding of the scope and limitations of
this denitrogenative borylation, we conducted an additive-
based robustness screen,[12] which is the first application of
this method to a photocatalytic reaction. To our delight, most
of the additives representing various functional groups
neither inhibited product formation nor decomposed under
the reaction conditions. The addition of aniline (Scheme 2,
entry 3), however, led to the complete shutdown of product
formation and consumption of the additive. 1-Nonanol
(entry 7), 1-aminododecane (entry 10), 1-chlorododecane
(entry 11), and benzofuran (entry 15) were consumed under
the reaction conditions but did not affect the formation of the
product 3a. N-Methylimidazole (entry 13), indole (entry 19),
and 2-chloroquinoline (entry 21) slightly inhibited the photo-
catalytic transformation of 3a. These results indicate a high
functional-group tolerance of this transformation and suggest
that this approach may be useful for the synthesis of ortho-
borylated aniline derivatives.
Inspired by a report by Jacobi von Wangelin and Majek on
a visible-light-mediated synthesis of aryl sulfides from aryl
diazonium salts,[13] we next focused on the development of
a thiolation of benzotriazoles. The obtained ortho-thiolated
N-arylbenzamides 6a–n are useful intermediates for the
synthesis of bicyclic 4H-[1,2,4]thiadiazine heterocycles,[14]
which feature in several important drug molecules.[15] Initially,
1a was reacted with 10.0 equiv of dimethyl disulfide in DMSO
(0.05m) in the presence of photocatalyst A (2.5 mol%). After
15 h of visible-light irradiation, we observed the desired
product 6a in 29% GC yield. Optimization studies on
solvents, stoichiometries, and reaction conditions, however,
did not reveal an appropriate protocol with one of the three
highly reducing photocatalysts identified through the mech-
anism-based screening.[6]
Scheme 2. Substrate scope and robustness screen for the borylation.
[a] 1 (0.30 mmol), 2 (2.0 mmol), 4 (0.30 mmol), benzoic anhydride
(0.15 mmol), A (0.5 mol%), MeCN (0.20m). NMR yields in paran-
thesis. [b] GC yield. Piv=pivaloyl.
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At this stage, we considered whether an alternative
sec-butyl disulfide led to a lower yield of 29% of 6g, while the
reductive quenching manifold may lead to a more efficient
process. In this scenario, substrate 1 is not reduced by the
excited photocatalyst but, rather, in a subsequent step in the
cycle by a RuI or IrII species generated upon electron
reduction by a suitable reductive quencher. While our
mechanism-based screening method does not provide direct
information on this step, the insights obtained previously still
prove useful for selecting an appropriate catalyst and
quencher combination. For example, analysis of the excited-
state oxidation potentials of the three successful Ir complexes
for oxidative quenching suggest that a photocatalyst with
a ground-state reduction potential (E(MnÀ1ÀMn)) of around
À1.35 V vs. SCE in MeCN would be required. Additionally,
a fast screen of these complexes with various reductive
quenchers reveals which electron donors would be suitable
for each catalyst. Complex [Ir(dF(CF3)ppy)2(dtbbpy)]PF6 (B;
(dF(CF3)ppy = 2-(2,4-difluorophenyl)-3-trifluoromethylpyri-
dine, dtbbpy = 4,4’-di-tert-butyl-2,2’-bipyridine), which was
not quenched by 1a during screening, seemed to fit these
ester-containing disulfide was marginally better tolerated (6n,
46%). When a benzotriazole featuring a 1-(3-pyridoyl)
substituent was used, the bis-thiolated compound 6j was
identified as the major product (29%). When an intra-
molecular competition experiment was performed by using
the unsymmetrical methyl propyl disulfide, a 62:38 mixture of
products was obtained in an overall yield of 61% with the
methyl-substituted product 6a as the major compound.
After having applied benzotriazoles 1 in visible-light-
À
À
mediated C S and C B bond-forming reactions, we next
À
focused on the development of a novel C C bond-forming
reaction. As pioneered by the Kçnig group, aryl radicals
generated upon photoredox decomposition of aryldiazonium
salts can be engaged in various Meerwein-type addition
processes with alkenes.[8b] Inspired by this work, we designed
a reductive coupling of benzotriazoles with styrene deriva-
tives that we hoped would provide facile access to diverse
ortho-alkylated benzamides. As before, we started our
optimization using a photocatalyst identified during screen-
ing: photocatalyst A (2.5 mol%) in the presence of 4
(3.0 equiv) as an electron and hydrogen source and DMSO
(0.10m) as solvent. After 15 h irradiation, the alkylated
product 8a was observed in 9% GC yield. In analogy to the
thiolation reaction, switching to the reductive quenching
manifold using sacrificial reductive quencher 4 and photo-
catalyst B or [Ir(ppy)2(dtbbpy)]PF6 (C) led to an increase in
efficiency. After an optimization study, the yield of 8a could
be increased to 67% using the latter catalyst and generating
the 1-(benzoyl)-substituted substrates in situ from 1H-benzo-
triazoles and benzoic anhydride.[6] Exploring the scope
(Table 2) we found that substrates having methyl substituents
at the meta- and para-positions of the benzotriazole gave the
criteria, having
a
ground-state reduction potential
(E(IrIIÀIrIII)) of À1.37 V vs. SCE in MeCN and demonstrat-
ing efficient quenching with 4. Indeed, switching to this
photocatalyst in the presence of 4 resulted in formation of 6a
in 67% yield.[6]
With the optimized reaction conditions in hand, we
explored the scope and the limitations of this thiolation
reaction (Table 1). Changing the electronic properties of the
aroyl fragment by adding para-MeO or para-CF3 substituents
delivered the desired products 6b and 6c in good yields (62%
and 69%, respectively). Substrates with Cl or Me substituents
on the benzotriazole gave the desired isolated products in 59–
63% yield. In terms of the disulfide, alkyl and benzyl
derivatives gave the corresponding products in moderate to
good yields (6d–6 f; 48–75% yield). Aryl disulfides, however,
were not suitable coupling partners. The sterically demanding
Table 2: Substrate scope of the denitrogenative alkylation.[a]
Table 1: Substrate scope of the denitrogenative thiolation.[a]
[a] 1a–n (0.30 mmol), 5 (3.0 mmol), 4 (0.90 mmol), benzoic anhydride
(0.60 mmol), B (1.0 mol%), DMSO (0.05m).[b] NMR yield in paren-
thesis. [c] Unsymmetrical disulfide was used. [d] NMR yield. [e] With
benzotriazole featuring a 1-(3-pyridoyl) substituent.
[a] 1a–j (0.30 mmol), 7 (3.0 mmol), 4 (0.90 mmol), benzoic anhydride
(0.60 mmol), C (2.5 mol%), DMSO (0.20m). [b] Using 4-methoxyben-
zoic anhydride. Cy=cyclohexyl.
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corresponding product 8b in 56% yield, while the addition of
a para-MeO substituent on the benzoyl fragment resulted in
the formation of product 8c in 56% yield. Various styrenes
with different electronic properties could be employed
affording alkylated products 8d–8i in moderate to good
yields (30–68%). The obtained products have, to the best of
our knowledge, not been described previously. However,
related structural motifs can be found in several novel drug
candidates for the regulation of estrogen receptors[16] and for
the inhibition of factor Xa.[17] In accordance with other
Meerwein-type addition processes of aryl radicals, aliphatic
alkenes, which are comparatively poor radical acceptors, were
not suitable coupling partners.
the major product-forming pathway in these transforma-
tions.[19]
In conclusion, we have applied benzotriazole substrates
identified by a mechanism-based screening approach, in three
À
new visible-light-mediated functionalization reactions. C S,
À
À
C B, and C C bond-forming denitrogenative transforma-
tions were achieved under mild conditions giving straightfor-
ward access to ortho-functionalized N-arylbenzamides via
a novel synthetic approach. In addition to providing the
inspiration for these processes through the discovery of the
benzotriazoles as new oxidative quenchers, the mechanism-
based screening also revealed insights into the quenching step
which were helpful in the optimization of each reaction,
especially when the alternative reductive quenching manifold
proved more efficient. Mechanism-based luminescence
screening is a useful tool for accelerating the development
of new photocatalytic reactions and further applications of
this method are under exploration in our laboratory.
A plausible mechanism for the photocatalytic alkylation
of benzotriazoles, which is consistent with the screening
results and a full Stern–Volmer analysis, is shown in
Scheme 3.[6] Upon irradiation, the excited photocatalyst
Acknowledgements
We thank Prof. B. J. Ravoo for use of the UV/Vis spectrom-
eter and S. Mukherjee and L. Schlatt (all WWU) for
experimental support. This work was supported by the
Deutsche Forschungsgemeinschaft (Leibniz Award) and the
Alexander von Humboldt Foundation (M.N.H).
Keywords: benzotriazoles · luminescence · photocatalysis ·
radicals · screening
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undergoes single-electron transfer with the reductive
quencher 4, resulting in the generation of an amine radical
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from this complex to the benzotriazole 1a regenerates the
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Indeed, quantum yield measurements of this alkylation
process (F = 24.8) and also of the borylation (F = 64.7) and
thiolation (F = 48.5) reactions indicate that radical chains are
4
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Manuscript received: September 24, 2016
Final Article published: && &&, &&&&
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Communications
Photocatalysis
M. Teders, A. Gꢁmez-Suꢂrez, L. Pitzer,
M. N. Hopkinson,
F. Glorius*
&&&& — &&&&
Diverse Visible-Light-Promoted
Functionalizations of Benzotriazoles
Inspired by Mechanism-Based
Luminescence Screening
Trinity! Three visible-light-promoted
functionalization reactions of benzotria-
zole substrates were discovered using
a mechanism-based screening method
and provided ortho-thiolated, -borylated,
and -alkylated N-arylbenzamide products.
The functional group tolerance of the
borylation reaction was further analyzed
by the first application of an additive-
based robustness screen in a photocata-
lytic transformation.
6
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Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!