Application of coumarin dyes for organic photoredox catalysis

Article abstract of DOI:10.1039/C8CC04048F

Here we report the application of readily prepared and available coumarin dyes for photoredox catalysis, which are able to mimic powerful reductant [Ir(iii)] complexes. Coumarin derivatives 9 and 10 were employed as photoreductants in pinacol coupling and in other reactions, in the presence of Et₃N as a sacrificial reducing agent. As the electronic, photophysical, and steric properties of coumarins could be varied, a wide applicability to several classes of photoredox reactions is predicted.

Full text of DOI:10.1039/C8CC04048F

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Application of Coumarin Dyes for Organic Photoredox Catalysis.
Andrea Gualandi,a* Giacomo Rodeghiero,a,b Emanuele Della Rocca,a Francesco Bertoni,a Marianna
Marchini,a Rossana Perciaccante,b Thomas Paul Jansen,b Paola Ceroni,a and Pier Giorgio Cozzi*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Here we report the application of readily prepared and available cation formed in the photocatalytic process.10
coumarin dyes for photoredox catalysis able to mimic powerful
reductant [Ir(III)] complexes. The coumarin derivatives 9 and 10
R
were employed as photoreductants in the pinacol coupling and in
other reactions, in the presence of Et3N as sacrificial reducing
N
agent. As electronic, photophysical, and sterical properties of
N
N
coumarins could be varied, a wide applicability to several classes
S
N
of photoredox reactions is predicted.
BF4
2
1
Photoredox catalysis has become a hot topic in catalysis, due to the
facility to generate active radical species in controlled and mild
conditions, via electron or energy transfer from photocatalysts in
their excited states to organic molecules.1 In this context, organic
dyes are attracting a great interest as photoredox catalysts (PC).2,3,4
Although complexes of abundant and inexpensive metals have
recently found some applications in photoredox catalysis,5 organic
dyes have been used as valid alternatives to the expensive, but
widely used Ru(II)- and Ir(III)-complexes.6 Several classes of suitable
dyes have been explored in photocatalytic transformations.2 As a
prototypical example, the Fukuzumi catalyst (Figure 1, 1) has been
exploited in many interesting photoredox reactions, thanks to its
strong oxidant ability in its lowest singlet excited state (E1/2 = +2.06
V vs SCE, MeCN).7 However, organic molecules behaving as strong
reductants are much less common. Murphy described powerful
organic reductants able to promote radical coupling and other
reactions of substrates with reduction potentials < -1.8 V (vs SCE).8
Unfortunately, these molecules are rather reactive and difficult to
generate. Other organic photocatalysts used as reductants of
substrates with reduction potentials in the range −1.5 – −2.1 V (vs
SCE) are: 10-phenylphenothiazine 2,9a perylene 3,9b N-aryl
phenoxazines 49c and N,N-diaryl dihydrophenazines 5.9d These
molecules contain electron-rich motifs which stabilize the radical
R
N
5, R = OMe, H,
CF , CN
3
R
O
R
3
4, R = biphenyl
Figure. 1. Dyes employed in photocatalytic reactions.
Remarkably, although coumarins have been largely employed as
fluorescent bio labels,11a laser dyes,11b emitting materials in organic
light-emitting diodes (OLED)11c and dyes in solar cells,11d to the best
of our knowledge, their systematic employment in photoredox
reactions has not been yet explored. In addition, the low molecular
weight and their straightforward synthesis12 give the possibility to
vary their photophysical and redox properties,13 allowing to cover a
wide range of photoredox potentials. With all these potentialities,
the coumarin class attracted our interest for photoredox catalytic
applications. Herein, we report the use of coumarins 914 and 10 (see
ESI for synthesis, details, and for evaluation of other coumarins in
the model reaction) as powerful reductants in the photo-promoted
radical coupling of carbonyls and imines,15,16 and we illustrate the
further possibility to apply coumarin dyes to many other
photoredox reactions.
Quite recently, Rueping reported a photoredox mediated coupling
of aldehydes, ketones, and imines mediated by Ir(III) photocatalyst,
in the presence of Et3N as sacrificial reductant.16a,b Photoexcitation
of the Ir(III) complex in the presence of Et3N yielded the reduced Ir
complex (E1/2 = −1.69 V vs Fc), which is responsible for the
reduction of carbonyl and imine groups. The so-formed radical
+
cation Et3N• , a Lewis acid, is supposed to coordinate to the
carbonyl oxygen and to activate its reduction. This transformation is
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quite challenging for organic dye photocatalysts, due to the high only it is possible to use a cleavable benzyl as protecting group, but
negative potential necessary for the aldehyde reduction (e.g., chiral benzylimines are suitable substrates and give access to chiral
benzaldehyde E1/2 = −2.11
V
vs Fc).17 We commenced our protected diamines.20
investigation from the pinacol coupling of 4-chlorobenzaldehyde
(6a, Scheme 1), as a model reaction, and investigating a series of
coumarins readily prepared or commercially available.
Scheme. 1. Reaction model for pinacol coupling (yields after chromatographic
purification).
Scheme 2. Pinacol coupling of selected aldehydes (yields after chromatographic
purification).
Table 1. Photophysical and electrochemical properties (E1/2 in V vs SCE) of coumarins 8,
9 and 10 in DMF solution at 298 K.
Absorption
Emission
Electrochemistry
λ
ε
λ
Φem
τ
E00
E(A+/A)
E(A+/A*)
(nm)
400
427
413
(M-1 cm-1
2.97
)
(nm)
476
497
482
(ns)
3.0
3.3
2.9
(eV)
2.79
2.66
2.72
(V)
(V)
8
9
0.82
0.50
0.57
+0.92
+0.79
+0.83
−1.87
−1.87
−1.89
3.30
10
3.03
Among all the coumarins investigated (see ESI for more details) in
the model reaction, only few coumarins were active, with
coumarins 8,18 9, and 10 being the best performing photocatalysts.
Coumarin 9 was readily obtained in 46% isolated yield from the
reaction of diethylamino)-salicylaldehyde and thiophene acetic acid,
performing the reaction with acetic anhydride and triethyl amine.
The preparation of 10 is a straightforward sulfonylation of 9 with
sulfur trioxide N,N-dimethylformamide complex in DMF, that gave
the desired product 10 in 33% yields after purification by reverse
phase chromatography. The reaction was further optimized in the
presence of coumarin 10 by varying solvents, reductive reagents,
and conditions (see SI for more details), as 10 was proved to be the
most efficient catalyst.19 The optimal conditions for the reaction
were reached by using DMF as reaction solvent ([substrate] = 0.2
M) in the presence of 4 equiv. of Et3N as sacrificial reductant. The
generality of these conditions was investigated with different
aldehydes: Scheme 2 reports the salient results. Other aldehydes
were tested as well, but only low conversions or no reaction were
Scheme 3. Pinacol coupling of selected ketones and imines (yields after
chromatographic purification).
To get insights into the mechanism of the pinacol reaction, a
photochemical study was performed. The absorption spectrum of
the photocatalyst is not significantly changed under the reaction
observed (see ESI for further details). Only in the case of naphthyl conditions reported in Scheme 2 (ca. 1% degradation of coumarin
10 at the end of irradiation), demonstrating its significant
photostability.
aldehyde we have obtained excellent yields, and this could be due
to the efficient formation of the ketyl radical coupled with an
effective reaction. Our conditions could also be applied to ketones
without further optimization (Scheme 3 A). We were pleased to find
that benzophenone 11a (E1/2 = −1.87 V vs. SCE)16 and its derivatives
were suitable substrates for the reaction. Imines were also proper
substrates and under the standard reaction conditions, different
benzyl and aryl imines react in satisfactory yields (Scheme 3B). Not
Fluorescence of coumarin 10 is not quenched by Et3N (Figure S6,
and comments), while it is quenched by aldehydes (Figure S7, and
comments).21 This behavior suggests a photoinduced oxidative
quenching of the coumarin with direct formation of ketyl radicals.
Indeed, energy transfer from the lowest excited state of coumarin
to populate the lowest triplet excited state of aldehydes is ruled
out, being endoergonic.22 Based on the reduction potentials of
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aldehydes (Table S13) and of the S1 excited state of coumarins
(Table 1), oxidative quenching is exoergonic, apart from 4-
chlorobenzaldehyde 6a. However, as evident from data reported in
Table S13, the presence of Lewis acids can substantially decrease
the reduction potentials of aldehydes.23 Under the conditions
reported in Scheme 2, the radical cation Et3N•+ can act as Lewis acid
or Brønsted acid, as previously discussed by Rueping, making the
investigated photoinduced electron transfer process exoergonic
(see SI for more details). In the conditions reported by Rueping, the
presence of 20 mol% K2CO3 or K3PO4 completely suppressed the
pinacol coupling, due to the essential role of the Brønsted acidic α-
ammonium radical in the C=O activation event. In our case, by
performing the model reaction in the presence of 20 mol% K2CO3
and K3PO4, we observed
a lower conversion, 16% and 12%
respectivily, compare to the model reaction performed under
standard conditions. In our case, the improved reduction potential
of coumarin dyes respect to Ir complex (-1.89 V vs -1.69 V) is
probably still favoring the reaction that was, however, less efficient,
confirming the importance of the Brønsted acidic α-ammonium
radical. We have also investigated cross-pinacol couplings, carring
out the reaction of 6a in the presence of different aldehydes (6b,
6c, 6e, and p-MeOC6H4CHO, 6l; see SI for full details). The cross
products, obtained as inseparable mixtures with the homo pinacol
products 7, were observed for the reactions with 6b and 6c but not
with 6e and 6l, probably due to the major stabilization of their ketyl
radicals. Based on the photochemical investigation we suggest the
mechanism depicted in Scheme 4: aldehydes are directly reduced
by the photocatalyst in the excited state (I) to form the ketyl radical
(III), at variance with the reductive quenching of the Ir(III)
photocatalyst reported by Rueping.16 The ketyl radical is then
coupled to another ketyl radical to give the pinacol product. In the
case described by Rueping, and in other examples reported in
literature, the photocatalyst in its photoexcited state is quenched
by the sacrificial reductant (normally a tertiary amine) and then is
able to perform the electron trasfer to the organic substrate in this
reduced state. Remarkably, coumarins are able to directly transfer
electrons to carbonyls, forming ketil radicals.
Scheme 5. ATRA reaction promoted by coumarin 9 (yields after chromatographic
purification).
To further highlight the capability of coumarins 9 and 10 to
promote a variety of different photoredox reactions, we have
briefly investigated their use in: (i) the trifluoromethylation of
alkenes by Umemoto reagent (Scheme 6, A),26 (ii) the MacMillan
stereoselective α-alkylation of aldehydes (Scheme 6, B),27 promoted
by the synergistic cooperation of photo- and enamine catalysis.28
The enantiomeric excesses obtained for the reactions were in line
with the reported values and the organic photocatalyst does not
influence or reduce the enantiomeric excess of the reactions. It was
also possible to use the coumarin catalyst in the reductive
protonation of bromoketones, in the presence of the Hantzsch
ester (Scheme 6, c).29 These selected examples demonstrate that
the potentiality of coumarins in photoredox catalysis could be
explored in many different reactions, allowing the replacement of
ruthenium(II) or iridium(III) complexes.
Scheme 4. Proposed reaction mechanism for the pinacol coupling reaction.
Recently, we have introduced BODIPY dyes for the atom transfer
radical addition (ATRA) reaction.24a Investigating the coumarins 9
and 10 as possible catalyst for the ATRA reaction,24b we found quite
promising results (Scheme 5), without the need of sacrificial Et3N,
or other reducing agent. It is noteworthy the possibility to use
ethylbromoacetate (16d) for the ATRA reaction promoted by
coumarin, a substrate quite challenging for other photocatalysts.25
Scheme 6. Application of coumarins dyes 8 and 9 in different chemical reactions.
In summary, we have introduced coumarins dyes as powerful
photoreductants in the photoredox catalysis arena. These new
photo-reductive catalysts shown broad applicability and further
studies about their application in other photoredox reactions are in
progress. The possibility of tailoring redox and photophysical
properties of coumarin dyes by introducing different functional
groups, their simple synthesis and their affordable cost, can be
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Acknowledgements
P. C. and M. M. acknowledge the European Commission ERC Starting Grant
(PhotoSi, 278912). A. G, L. M, and P. G. C. are grateful to Fondazione Del Monte,
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