Light-Induced Tetrazole-Quinone 1,3-Dipolar Cycloadditions

Article abstract of DOI:10.1002/chem.201904138

Quinones were firstly used as dipolarophiles in a photoclick 1,3-cycloaddition with 2,5-diaryltetrazoles, as photoactivatable predipoles, providing a novel and efficient access to three types of pyrazole-fused quinones (indazoledione derivatives). Distinc

Full text of DOI:10.1002/chem.201904138

A Journal of
Accepted Article
Title: Light-Induced Tetrazole-Quinone 1,3-Dipolar Cycloadditions
Authors: Maria Ribagorda, Carmen Carreño, Laura Ortiz-Rojano,
Jaime Rojas-Martin, and Ciro Rodriguez-Diaz
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To be cited as: Chem. Eur. J. 10.1002/chem.201904138
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Light-Induced Tetrazole-Quinone 1,3-Dipolar Cycloadditions
Laura Ortiz-Rojano,[a],† Jaime Rojas-Martín,[a],† Ciro Rodríguez-Diaz,[a], M. Carmen Carreño,[a,b] Maria
Ribagorda*,[a,b]
Abstract: Quinones were firstly used as dipolarophiles in a photoclick
1,3-cycloaddition with 2,5-diaryltetrazoles, as photoactivatable
predipoles, providing a novel and efficient access to three types of
pyrazole-fused quinones (indazoledione derivatives). Distinctive
features of this protocol include the use of light as the unique reagent,
readily available, stable and easy to handle starting materials and
good to excellent yields. Photophysical and electrochemical
properties quinones and their potential application as photoredox
catalysts is also detailed.
Nitrile imine dipoles obtained from hydrazonoyl chloride
derivatives in the presence of base, have been reported to react
with naphthoquinones and also benzoquinones giving medicinal
relevant pyrazole fused-quinone type (I)20,21 or different mixtures
of (II) and (III)22 (Scheme 1, A). Given our long interest in
expanding the quinone´s map reactivity,23 we envisioned that the
application of photoclick reaction, using quinones as
dipolarophiles, could represent a new and efficient method for the
preparation of pyrazole fused-quinones types (I-III), using light as
the unique reagent (Scheme 1, B). Herein, we also report its
electrochemical and photophysical properties, and give
evidences of their potential use as photocatalysts in organic
transformations.
Quinones represent an important family of organic chromophores
that exhibit a broad spectrum of pharmacological activities.1,2,3
They have an active role as electron carriers or cofactors in
several biological routes,4 and are recognized organic oxidants.5
These redox-active organic molecules, and especially
benzannulated quinones, such as naphtho or anthraquinone, are
also powerful oxidants in the excited state and have been used in
visible light photo-oxidative catalytic reactions.6,7 Moreover, in an
oxygen-free atmosphere, suitable quinones can also participate
in photo-reductive transformations under visible light and with a
proper electron donor compound that enable the formation of the
semiquinone radical anion.8 Recently, König9 described the
functionalization of arylhalides by photoredox reductive
transformations using 1,8-dihydroxyanthraquinone as
a
photoredox catalyst and Et3N as the sacrificial electron donor
species. Due to their relevant biological and chemical applications,
novel preparation of quinones, including benzannulated nitrogen
containing derivatives,10,11 are always in demand.
UV-light
Scheme 1. Preparation of heterocyclic quinones (I-III) from hydrazonoyl
The light induced tetrazole-alkene 1,3-dipolar cycloaddition
has emerged as an efficient synthetic tool for the preparation of
fluorescent pyrazoline cycloadducts.12 The key step is the clean
photolysis of tetrazole into a nitrile imine dipole. This “photoclick”
chemistry have proven to be highly valuable with a variety of
alkenes13,14 and found application in the light-ligation and labelling
of biomolecules.15,16 Other dipolarophiles, including alkynes17 and
other counterparts,18 have evidenced the usefulness of this atom-
economical transformation. However, to our knowledge, the use
of quinones as dipolarophiles19 in this process remained
unexplored.
chloride predipoles and proposed work.
Different 2,5-diaryltetrazoles 1a-o were synthesized from
arylsulfonylhydrazides and diazonium salts following a reported
protocole.24 We started our study using naphthoquinone 2a and
tetrazole 1a (R1 = R2 = p-methoxyphenyl), as model substrates for
the photoclick reaction, using different solvents and light
sources.25 The best conditions were found running the reaction
with an equimolar mixture of 1a and 2a in an open air atmosphere
at rt, using EtOAc as solvent and using an ultraviolet high-
pressure lamp as the source of irradiation (Ultra-vitalux, Osram
lamp).26, 27 Gratifying, quinone 3a was quantitatively formed after
2 h and isolated in a 90% yield (Scheme 2). Compared with other
tetrazole-alkene 1,3-dipolar cycloadditions, in our case, no
excess of dipolarophile was required. It is also important to
mention that under the irradiation wavelength used for the
reaction, the primary absorber (higher ε) was the tetrazole.25
Formation of 3a included the photogeneration of the imino nitrile
[a]
L. Ortiz-Rojano, D. J. Rojas-Martín, C.o Rodríguez-Diaz, Prof. Dr. M.
C. Carreño, Prof. Dr. M. Ribagorda, Departamento de Química
Orgánica, Facultad de Ciencias, Universidad Autónoma de Madrid,
C/ Francisco Tomás y Valiente 7, 28049-Madrid, Spain. E-mail:
maria.ribagorda@uam.es
[b]
Prof. Dr. M. C. Carreño, Prof. Dr. M. Ribagorda, Institute for
Advanced Research in Chemical Sciences (IAdChem), Universidad
Autónoma de Madrid, 28049-Madrid, Spain.
†
These authors contributed equally
dipole
followed
by
the
sequential
1,3-dipolar
cycloaddition/enolization process and an oxidation. The starting
naphthoquinone 2a and the air might be acting as oxidants in the
Supporting information for this article is given via a link at the end of
the document.
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last step of the sequence. In fact, the reaction under argon
atmosphere and using 2 equiv of 2a was slower than in an open
air flask, obtaining after 7 hours a 60% conversion of 3a, together
with a 40% of 2a and 1,4-dihydroxynaphthalene, as a 1:1 mixture.
The hydronaphthoquinone reduced form of 3a was not observed
in any case.
cycloaddition on both quinone double bonds as a unique
regioisomer. Up to now, this is the best yield reported for this
regioisomer 6.22a In an attempt to provide an improved yield of
cycloadduct 5a, optimization was carried out changing the relative
amounts of 1a and 4 and the molarity.25 The best yield resulted
upon irradiation a 3/1 molar ratio of quinone/tetrazole (4/1a, 0.2 M
solution) using AcOEt as solvent, isolating 5a in 80% yield
(Scheme 3). Under these conditions, other tetrazoles were
examined. 5-(m-Methoxy)phenyl or 5-(p-cyano)phenyl (R2)
tetrazoles exclusively gave the desired compounds 5b and 5g in
moderate to good yields. The reaction with 5-phenyl substituted
tetrazole 1e gave cycloadduct 5e in a 72% yield, and a 25% of 6e
was also obtained in this case. The structure of 6e was confirmed
by single crystal X-ray diffraction.28 This structure was in
accordance with
a regiochemical course of the second
cycloaddition defined by the electron donating effect of N-1 in the
precursor 5e, making the C-5, of the quinone double bond, more
electron deficient than C-6.29 Cycloadducts 5k/6k were obtained
as an inseparable 63:37 mixture (55% yield), as well as 2-thienyl
derivatives 5n/6n (88:12 mixture, 72% yield). 2-Pyrrolyl adducts
5m and 6m, could be separated and obtained pure in 42% and
13% yields.
Scheme 2. 1,3-Dipolar cycloadditions between 2,5-diaryltetrazoles and 2a.
The scope of this dipolar reaction was next examined with a
wide array of 2,5-diaryltetrazoles bearing electron-donating and
withdrawing substituents (Scheme 2). Tetrazoles bearing 3-
methoxy, 2-methoxy or 3,4,5-trimethoxy phenyl substituents at C-
5 (R2) reacted with 2a giving 1H-benzo[f]indazole-4,9-diones 3b-
d in very good yields and short reaction times. Similarly, 5-phenyl
and electron deficient 5-p-bromo or 5-p-cyano phenyl substituted
(R2) tetrazoles gave quinones 3e-g in very good yields, showing
that the electronic nature of aryl groups in the tetrazoles had no
influence on the overall process. 2,5-Bis(2-naphthyl) and 5-p-
fluorophenyl substituted tetrazoles evolved into 3h and 3i in good
to moderate yields. p-Tolyl (R1) substituted quinones 3j or 3k were
obtained in 60% and 62% isolated yield respectively. 5-Phenyl-
tetrazoles 1e,i,k with substituents with different electronic
properties at N-2 (R1) showed that electron donating groups
favored the reaction.15a Tetrazoles bearing hetereoaromatic
groups (R2) at C-5 worked also very well regardless of the
electronic properties of the heterocyclic group, giving novel
heteroaromatic quinones 3l-o in good yields (74-85%).
Scheme 3. Reactions between 2,5-diaryltetrazoles and benzoquinone 4.
Next, we measured the electronic spectra (absorption and
emission) of prepared pyrazole fused-quinones 3, 5 and 6 using
CH2Cl2 as solvent (see Fig. 1a for selected examples). Intense
absorption bands centered at 245-279 nm due to π–π* transitions
of the quinone moiety, appeared in all cases. Additional less
intense broad bands, assigned to n-π* transitions, were observed
in the region 400-480 nm. Similar band shifts were observed
regardless the heterocyclic quinone skeleton 3, 5 or 6, whereas
the electronic effects of the substituents influenced the position of
these bands within each structural framework. For instance, the
n-π* transition band suffered a bathochromic effect from max 418
nm of 3o, with the electro deficient R2 = 3-pyridyl group, to a max
= 480 nm of compound 3m, due to the electron donating pyrrole
moiety (R2). Compound 3m had an additional absorption band at
Then, attention was turned to the reaction between
benzoquinone 4 and photoactivatable tetrazoles. When the
reaction was carried out with an equimolar mixture of 4 and 1a,
cycloadducts 5a and 6a, were isolated in a 20% and 49% yield
respectively. Compound 6a resulted from a double 1,3-dipolar

max = 321 nm, that could be ascribed to an intramolecular charge
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transfer transition from the heteroaryl group to the quinone
acceptor, through the π-extended conjugated heteroaromatic
moiety. Similar trend where observed between derivatives 5a or
5e (max 433 and 400 nm) and 6a or 6e (max 450 and 400 nm).
Emission spectra are also depicted in Figure 1 (dotted lines). In
general, all of them showed a luminescence band in the yellow-
red light region (λem 577-680 nm) when excited by irradiation at
their lower energy absorptions (λmax~400-450 nm, see Table S1).
Different emission bands were observed from yellow-orange to
red (3o λem 577, 3a λem 590 nm), having the most red shifted
emission of the series the pyrrole substituted 3m (λem 680 nm),
evidencing again the efficient donation effect of pyrrole. In
contrast, compounds 5a presented weak or no fluorescence,
whereas 5e, 6a and 6e showed red emission bands (λem 600-620
nm). The redox potentials of selected derivatives were measured
in CH2Cl2 (0.1 M, referenced with ferrocene, see Table S2 for SCE
and NHE potentials)30 and compared with 9,10-anthraquinone
(AQ) and naphthoquinone (NQ). All compounds exhibited two
couples of well-defined anodic and cathodic peaks that
corresponded to the two monoelectronic reversible processes.31
Quinones 3 and 6 had a reduction potential in between AQ and
NQ, whereas 5 had more positive reduction potential than NQ.
These results defined an oxidant capacity trend directly related to
the heterocyclic quinone skeleton, as follows: AQ < 3 < 6 < NQ <
5 (Figure 1b). This trend also rationalized that the starting
quinones can be consumed for the oxidation of the annulated
hydroquinone cycloadducts, and can be reoxidized again to the
starting quinone by air oxygen.32
(1 mol%) as a catalyst under blue light irradiation and aerobic
conditions. In this case, Ph3PO was formed in 80% yield, whereas
no Ph3PO was detected under the same reaction conditions but
without 3a. The above results indicated that 3a could act as a
photosensitizer, and under blue light irradiation, the photoexcited
3a could enable the formation of 1O2 by an energy transfer
process with oxygen (3O2).25 We next examined the photo-
reductive transformation described by König, for the direct
arylation of toluene with fluorinated aryl bromides using eosine Y
as photocatalyst.34 The reaction between 7 and toluene, using
quinone 3a (5 mol%) as catalyst and Et3N, in an inert atmosphere
gave biaryl derivative 8 in good yield upon blue LEDs irradiation.
Control experiment without 3a indicated that only traces of the
final product were formed under the same conditions and
irradiation time. Formation of 8 can be explained by the reductive
fragmentation of the bromoarene enabled by the oxidation
process of semiquinone intermediate (SQ) to the intial quinone 3a.
SQ could be accessible from 3a upon blue-light irradiation to give
excited state [3a]* that is reduced to SQ in the presence of Et3N,
as the sacrificial electron donor specie.25 These experiments
enable us to discern potential usefulness of this type of quinones
as organic photocatalysts.
a)
Scheme 4 Photoxidation and a photoreductive transformations catalyzed by 3a.
In summary, we have demonstrated that a variety of pyrazole-
fused quinones can be efficiently prepared by a 1,3-dipolar
reaction from tetrazoles and quinones. Compared to other 1,3-
dipolar methodologies, the main advantages of this protocol lies
in the clean reaction conditions that only required irradiation with
light. Moreover, all the differently substituted diaryl and heteroaryl
tetrazoles were easily prepared and bench stable starting
materials. This methodology constitutes a new application of the
photoclick chemistry using diaryltetrazoles as photomasked nitrile
imine dipoles, expanding their utility to the direct preparation of
heterocyclic quinones in good yields. The emission properties and
reduction potentials revealed that 3 and 6 are fluorescent
quinones with reduction potentials in between AQ and NQ, while
the indazole-4,9-diones 5 have low or no emission properties and
are better oxidants than NQ itself. In addition, compound 3a can
participate as organic photocatalyst in photoredox processes,
making the described method suitable for the synthesis of
interesting photoactivatable quinones.
b)
Figure 1. a) UV-Vis (solid lines) and emission (dot lines) spectra and b)
reduction potentials of selected quinones (ferrocene internal standard).
Finally, with the aim of evaluating the potential use of these
quinones as photoactivatable organic catalysts, we tested the
catalytic activity of 3a in two different processes: a photoxidation
and a photo-reductive transformation. Based on a literature
precedent,33 photoxidation of phosphines can be assisted by
singlet oxygen (1O2). We examined the oxidation of Ph3P using 3a
Acknowledgements
We thank MINECO (Grants CTQ2014-53894-R and CTQ2017-
85454-C2-2-P) for financial support. J. R. and L. O. R. thanks
Ministerio de Educación y Ciencia (MEC) for FPI fellowships.
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Keywords: Quinones • 2,5-Diaryltetrazoles • 1,3-dipolar
cycloaddition • Photoclick • Photocatalyst
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Entry for the Table of Contents
COMMUNICATION
Quinones were firstly used as
dipolarophiles in a photoclick 1,3-
cycloaddition with 2,5-
Laura Ortiz-Rojano, Jaime Rojas-
Martín, Ciro Rodríguez-Diaz, M.
Carmen Carreño, Maria Ribagorda*
diaryltetrazoles, as photoactivatable
predipoles, providing a novel and
efficient access to three types of
pyrazole-fused quinones. This
protocol include the use of light as
the unique reagent, readily available,
stable and easy to handle starting
materials and good to excellent
yields. Photophysical and
Page No. – Page No.
30 examples
Yields up to 95%
Title
electrochemical properties quinones
and their potential application as
photoredox catalysts is also detailed.
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