Naphthochromenones: Organic Bimodal Photocatalysts Engaging in Both Oxidative and Reductive Quenching Processes

Article abstract of DOI:10.1002/anie.201912455

Twelve naphthochromenone photocatalysts (PCs) were synthesized on gram scale. They absorb across the UV/Vis range and feature an extremely wide redox window (up to 3.22 eV) that is accessible using simple visible light irradiation sources (CFL or LED). Their excited-state redox potentials, PC*/PC^.? (up to 1.65 V) and PC^.+/PC* (up to ?1.77 V vs. SCE), are such that these novel PCs can engage in both oxidative and reductive quenching mechanisms with strong thermodynamic requirements. The potential of these bimodal PCs was benchmarked in synthetically relevant photocatalytic processes with extreme thermodynamic requirements. Their ability to efficiently catalyze mechanistically opposite oxidative/reductive photoreactions is a unique feature of these organic photocatalysts, thus representing a decisive advance towards generality, sustainability, and cost efficiency in photocatalysis.

Full text of DOI:10.1002/anie.201912455

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Naphthochromenones: Organic Bimodal Photocatalysts
Engaging in Both Oxidative and Reductive Quenching Processes.
Authors: Javier Mateos, Francesco Rigodanza, Alberto Vega, Andrea
Sartorel, Mirco Natali, Tommaso Bortolato, Giorgio Pelosi,
Xavier Companyo, Marcella Bonchio, and Luca Dell'Amico
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201912455
Angew. Chem. 10.1002/ange.201912455
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Naphthochromenones: Organic Bimodal Photocatalysts
Engaging in Both Oxidative and Reductive Quenching Processes.
Javier Mateos,§[a] Francesco Rigodanza,§[a] Alberto Vega–Peñaloza, Andrea Sartorel, Mirco Natali,
[a]
[a]
[b]
[
a]
[c]
[a]
[a]
[a]
Tommaso Bortolato, Giorgio Pelosi, Xavier Companyó, Marcella Bonchio, and Luca Dell’Amico*
In memory of Dieter Enders (1946–2019) for his invaluable contribution to organic chemistry
Abstract: 12 different naphthochromenone photocatalysts (PCs)
have been synthesized at gram–scale, combining absorption features
across the UV–Vis spectrum, up to 440 nm with an extremely wide
redox window (up to 3.22 eV) that is accessible using simple visible
light irradiation sources (CFL or LED). Their excited state redox
of performing light–driven transformations under catalytic
2
routines. The utilization of these molecules has direct impact on
3
our every–day life, from the synthesis of polymers to the more
recent applications towards natural products synthesis or drug
4
development. Recently, the identification of novel photocatalytic
●–
●+
5
6,7
potentials, PC*/PC = up to 1.65 V and PC /PC* up to –1.77 V vs
SCE, are such that these novel PCs can engage in both oxidative and
reductive quenching mechanisms with strong thermodynamic
requirements. Converging absorption/emission spectroscopy and
cyclic voltammetry we delineate robust structure–properties
relationships, that are further supported by time–dependent density
functional theory (TD–DFT) calculations. The potential of these
bimodal PCs has been benchmarked in thermodynamically
challenging photocatalytic processes, were strong oxidative (> 1.46
V) and strong reductive power (< –1.96 V) is required. Further
advantages are given by their simple recovery and reuse – up to four
times, without any significant loss in their photocatalytic performances.
The ability of efficiently catalysing mechanistically opposite
oxidative/reductive photoreactions is a unique feature for organic
photocatalysts. This new class of molecules represents a decisive
advancement towards generality, sustainability and cost efficiency in
photoredox catalysis.
systems has become an extremely active filed of research.
Introduction
Solar light is an unlimited source of clean and renewable energy
on Earth. Its conversion into diverse energy forms is at the
forefront of intense research in diverse scientific areas from
biology to engineering. The chemical community, playing a
central role across disciplines, has devoted enormous efforts in
the identification of more efficient photocatalysts (PCs), capable
1
Figure 1. Organic photoredox catalysts (PCs) absorbing between UV– and
visible–light (bottom). Mes = mesityl. Ar = 2–naphtyl. Reported redox potentials
vs SCE.
New directions are pointing to the identification of more
sustainable purely-organic molecules,8
, 5e–g
able to conserve all
the advantages of the well-established metal-based PCs (Ru, Ir
or Cu complexes),5a–d including: i) the use of visible-light photons
[
a]
J. Mateos, F. Rigodanza, A. Vega–Peñaloza, A. Sartorel, T.
Bortolato, X. Companyó, M. Bonchio, L. Dell’Amico
Department of Chemical Sciences
(≥ 400 nm), abundant in the solar emission spectra and capable
2, 5a, b
of minimizing product decomposition,
ii) the use of readily
University of Padova Institution
available light sources such as CFL or LED, and finally iii) an
equal distribution of the excited state energy among the oxidation
and reduction potentials, thus delivering upon light irradiation both
a strong oxidant and a strong reductant. This bimodal way of
action is rare in organic photocatalysis, especially when
connected to high excites state energies (E0,0 > 2.5 eV).5 The
large majority of organic PCs with an E0,0 > 2.5 eV are able to
catalyse either oxidative or reductive photoreactions with extreme
thermodynamic requirements (see e.g. acridiniums of
phenoxazines in Figure 1),5e forcing the user to move from one
class of PCs to another one, when changing the operative
reaction mechanism. The combination within a single molecule of
Via Marzolo 1, 35131 Padova, Italy
E–mail: luca.dellamico@unipd.it
M. Natali
Department of Chemical and Pharmaceutical Sciences
University of Ferrara Institution
Via Luigi Borsari 46, 44121 Ferrara, Italy
G. Pelosi
Department of Chemistry, Life Sciences and Environmental
Sustainability, University of Parma
Parco Area delle Scienze 17, 43124 Parma, Italy
These authors contributed equally to this work.
[b]
[c]
[§]
e-g
Supporting information for this article is given via a link at the end of
the document.
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RESEARCH ARTICLE
a strong oxidative exited state power (up to 1.5 V), an equally
strong reductive power (up to -1.5 V), and an absorption under
visible-light (≥ 400 nm), will define an ideal class of bimodal
organic PCs, capable of performing both thermodynamically
challenging oxidative and reductive processed in a catalytic
routine. This class of molecules, having a wider operational
window, will impart new mechanistic avenues in photocatalysis.
We herein outline the identification of 6H–naphthochromen–6–
one (NT) as a novel family of bimodal organic PCs (Figure 1, right).
NTs present an absorption spectrum at the border of visible–light,
which guarantees the access of high energy excited state (up to
3.22 eV) with the use of simple visible light sources, maintaining
both strong oxidative and strong reductive properties,
E
●
–
●+
(
PC*/PC ) up to 1.65 V and E (PC /PC*) up to –1.77 V vs SCE.
Scheme 1. Synthesis of PCs 5a–f from carbonyl compounds 1a–c and
coumarins 2a–c.
The identification of a scalable synthetic plan together with easy
structural functionalisation lead to the generation of 12 diverse
PCs at gram scale. Structure–photoredox property relationships
are assessed in terms of
absorption/emission,
fluorescence quantum yield and excited state lifetime, allowing a
rational use to the intended purpose. Remarkably, the potential of
HOMO–LUMO
energy,
potentials,
We started the synthesis of differently substituted NTs 5, with the
goal of defining structure–property relationships while generating
a library of PCs with varied photoredox properties.
ground/excited
state
this new class of PCs is demonstrated for
a variety of
Table 1. Synthesis of PCs 5a–f from carbonyl compounds 1a–c and coumarins
2a–c.
mechanistically diverse transformations classically promoted by
either UV- or visible-light absorbing PCs. Furthermore, NTs can
be easily recovered and reused, up to four runs, in subsequent
catalytic cycles with comparable photoredox performances.
Results and Discussion
The identification of the NT scaffold 5 as potential organic PC was
stimulated by our previous works on the synthesis of the
9
tetracyclic scaffolds 3. (Scheme 1). We envisaged compound 3,
derived from the reaction between the photoenol, generated upon
light excitation of the corresponding benzophenones 1,10 and
coumarins 2, as precursor of NT
5
through an
elimination/aromatization sequence (Scheme 1). We must
consider that both starting materials benzophenone 1 and
coumarin 2 have distinctive absorptions under the UV region.11
This fact also suggested a possible absorption at longer
wavelengths of the more conjugated scaffold 5. Once identified a
general synthetic strategy (Scheme 1), suitable reaction
conditions were developed and applied to diverse readily
available starting materials 1a–c and 2a–c, allowing the easy
tuning of the NT scaffold at positions 3 and 7 (Table 1). The initial
light–driven [4+2]–cycloaddition reaction between benzophenone
1
a and coumarin 2a was performed under microfluidic
9
b
conditions. Tetracyclic product 3a formed rapidly within 35 min
residence time. Isolation of 3a and subsequent treatment with
triflic acid delivered directly NT scaffold 5a without the need of any
additional synthetic operation.12
A simple recrystallization
furnished compounds 5a–5c with variations at position 7 in good
overall yields 37%–82%.
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Scheme 2. Post–functionalization of PC 5e for the synthesis of PCs 5g,h and 5j–m. All the yields refer to isolated yields. TBAF = tetrabutylammonium fluoride. Cz
carbazole. For 5j–5l X = I, for 5k X = Br.
=
Compounds 5d–5f, bearing diverse groups at position 3 were
synthesized in overall yields spanning from 36% to 67%. In these
cases, a two–step elimination–aromatization sequence resulted
in improved yields and diminished reaction time (See SI).
Remarkably, the developed synthetic protocol was scaled up
without any yield drop, producing 5a, and 5e at gram scale and
up to 2.88 g within 40 h. The structure of 5a and 5e were
confirmed by X–ray analysis on single crystal. Prompted by the
simple synthetic route developed, we explored the impact of
diverse structural functionalization. We identified 5e, bearing an
OTf group at position 3, as a versatile precursor to diverse PCs
scaffolds. The hydroxyl functionality was easily deprotected with
tetrabutylammonium fluoride (TBAF) solution, delivering PC 5g in
quantitative yield (Scheme 2 top, left). Additionally, scaffold 5h
was synthetized (75% yield, 1.90 g) in order to evaluate the effect
of an increased conjugation. After a simple desilylation step, 5i
was obtained in high yield (95%) after simple filtration on a silica–
pad (Scheme 2, grey box). At this juncture, diverse aromatic
substituents bearing electron withdrawing or electron donating
groups were introduced by a simple synthetic operation (Scheme
between 300–330 nm (299, 313 and 327 nm). To get insight into
the electronic transitions responsible for the absorption, we used
time–dependent density functional theory (TD–DFT) at the
B3LYP/6–311+g(d,p) level of theory, including a polarizable
continuum model in polar solvent.13 As shown in Figure 2c (black
levels), the HOMO, LUMO and LUMO+1 orbitals are localized on
the NT core of 5a, with no contribution of the phenyl group at 7
position.14 These three orbitals are involved in the two low energy
transitions calculated with TD–DFT. In particular, the
HOMO→LUMO transition provides the highest contribution (91%,
Table S11 in SI) to the lowest energetic one, predicted at 362 nm
(Figure 2a, dotted line) and characterized by an oscillator strength
–1
–1
of 0.0693 ( = 5×103 M cm ). The second lowest energetic
transition is predicted at 323 nm (oscillator strength 0.1848), in
excellent agreement with the experimental data, and it is
characterised by a major HOMO→LUMO+1 contribution setting
to 71%. While substitutions at position 7 have relatively small
influence on the PCs absorption spectra (Figure S133 in SI), the
introduction of a bromine (5d) or a triflate (5e) at position 3, is
reflected in a slight shift of the peaks at longer wavelengths.
Conversely, compounds 5f and 5g, bearing electron donating
groups have highly enhanced absorption in the visible region, in
particular 5f (Figure 2b, orange line) being the most absorbing in
the series of PCs. Interestingly, this behaviour was consistent with
calculated absorptions by TD–DFT (Table S11 in SI). Focusing
on the two less energetic transitions, significant effects are
observed for 5f and 5g, where a 0.2–0.3 eV raising in the energy
of the HOMO orbitals is observed, consistent with the electron–
donating character of the methoxy and hydroxy group at position
3.
2, right and bottom). PCs 5j–5m were synthesized in excellent
yields at gram scale (up to 1.80 g) spanning from 81% to 91% by
using 5i and selected aryl halides through a Sonogashira cross–
coupling. With a wide library of novel organic PCs in hand, we
next examined their photoredox properties.
Analysis of the Photochemical and Redox Properties. We first
looked at the properties of PCs 5 in both the excited and ground
states, crucial in effecting the photocatalytic performances. 5a
showed a distinctive absorption spectrum with a peak centred at
–
1
–1
3
80 nm ( = 5100 M cm ) and a shoulder at 360 nm, tailing up
to 440 nm (Figure 2a, black line), and three distinct peaks
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RESEARCH ARTICLE
80000
60000
40000
20000
0
5a
1
5000
0000
5f
experimental
calculated
80000
60000
40000
20000
0
1
5j
5m
5
000
0
375
400
425
250
275
300
325

350
/ nm
375
400
425
450
300
325
350
375
 / nm
400
425
450
Figure 2. a) Measured and calculated absorption spectra of PC 5a. b) Extinction coefficients of PC 5a, 5f, 5j and 5m measured in ACN. c) Selected frontier molecular
orbitals of PC 5a, 5f, 5j and 5m and relative energies. Cz = carbazole.
The calculated absorption spectra of these PCs show a red shift
to 387 and 385 nm for 5f and 5g, respectively, in agreement with
the experimental values. A different scenario is observed by
introducing conjugation at position 3. For PCs 5h–m the extinction
coefficient increases significantly (See 5j and 5m in Figure 2b).
The presence of the triple bond significantly lowers the energy of
the LUMO+1 orbital: moving from –1.77eV for 5a, to –2.05 and –
characterization by its decay pathway and its lifetime, is
instrumental to predict its reactivity.15 High excited state energy
means higher applications in a number of diverse PET reactions.
Long lifetime will match with greater chances of encountering the
desired reactant and the higher the fluorescence quantum yield,
the greater the likelihood of PET happening.6c We started
1
analysing 5a looking at the modifications of S properties caused
2
.07 eV for 5j and 5m (Figure 2c). Thus, causing a red shift of the
by the different substitutions. PC 5a has an emission maximum at
421 nm, consistent with an excited state energy of 3.29 eV, an
emission quantum yield (QY) of 4% and an excited state lifetime
of 1.44 ns (Table 2). Removing the phenyl ring in position 7 (5c)
determines an emission shift of 10 nm to shorter wavelengths with
an excited state energy of 3.35 eV, the highest in the series. The
more compact core led to longer lifetime (3.86 ns) and higher QY
(8%). While substitutions with electron withdrawing groups did not
alter significantly the properties of 5, PCs 5f and 5g, bearing
electron–donating groups (Table 2), showed an increased QY to
12% for 5g and to 20% for 5f and a lifetime extension to 8.50 ns
and 7.78 ns, respectively.
second less energetic transition at 323 for 5a, to 371 and 387 nm
for 5j and 5m, respectively. The further conjugation with a phenyl
group increases the red shift of the absorption with a less
energetic peak at 385–390 nm, this effect being ascribed to the
increase in the HOMO energy. 5m has the most red–shifted
absorption (Figure 2b, azure line) ascribed to a distinctive HOMO
higher in energy and located predominantly on the carbazole
moiety, suggesting an intramolecular charge transfer contribution.
Excited state analysis. For organic PCs, the singlet excited state
(
S
1
) is the most likely to react in bimolecular reactions to perform
photoinduced electron transfer (PET).15 Therefore, the
S
1
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Table 2. Excited– and Ground–State Photoredox Properties Summary.
a
E b
(PC/PC●–)
E b
(PC●+/PC)
E b
(PC*/PC●–)
λ
maxem.
E
0,0
QY
%

PC
E b (PC●+/PC*)
(nm)
(eV)
3.02
3.15
3.22
3.16
3.20
2.97
2.89
3.13
3.12
3.14
3.08
3.12
(ns)
1
5
5
5
5
5
5
5
5
5
5
5
5
a, (R = 7–Ph)
421
421
411
421
413
462
461
426
434
427
449
459
4
9
1.44
2.07
3.86
1.49
0.81
7.78
8.50
2.53
3.34
3.05
4.21
2.49
–1.74
–1.68
–1.67
–1.61
–1.55
–1.76
–1.89
–1.71
–1.70
–1.58
–1.65
–1.59
1.75
1.81
1.80
1.78
1.86
1.47
1.44
1.73
1.64
1.69
1.44
1.35
1.28
-1.27
-1.34
-1.42
-1.38
-1.34
-1.50
-1.45
-1.40
-1.48
-1.45
-1.64
-1.77
1
b, (R = 7–o–F)
1.47
1.55
1.55
1.65
1.21
1.00
1.42
1.42
1.56
1.43
1.53
1
c, (R = 7–H)
8
2
d, (R = 3–Br)
5
2
e, (R = 3–OTf)
4
2
f, (R = 3–OMe)
20
12
15
17
23
16
6
2
g, (R = 3–OH)
3
h, (R = TMS)
3
j, (R = Ph)
3
k, (R = p–CN–Ph )
3
l, (R = p–OMe–Ph )
3
m, (R = p–Cz–Ph)
[
a] Estimated by emission onset at 77 K. [b] All potentials in V vs SCE. Measurements were performed in ACN (See SI). TMS = trimethylsilyl; Cz = carbazole.
Hence, these PCs are very good candidates towards demanding
reductive PET processes. Finally, the increasing of the
conjugation by a triple bond in compounds 5h–5m also generated
promising properties in between 5a and 5f–5g. The emission
maximum was observed in a range from 427 nm (5k) to 459 nm
V for 5k up to a remarkable –1.77 V for 5m (Table 2). Moving to
●
–
the (PC*/PC ) reductive process we immediately discern the
wider variability of the PCs covering the window from 1.00 V for
5e to 1.65 V for 5e. As observed for the potential regarding
●
+
(PC /PC*), the electron–donating ability of the substituents have
a clear effect on the redox potentials, increasing the energy of the
HOMO. Interestingly, addition of EWGs such as the case of 5b,
5d, 5e and 5k stabilizes the HOMO of the PCs bringing the
potential above 1.65 V.
(
5m) with associated excited state energies between 3.18–3.23
eV. Quantum yield was observed in the range 15% mb – 23%,
increased with respect to 5a. The lifespan of the excited state was
generally longer than 2 ns with a maximum for 5l reaching 4.21
ns.
Analysis of the Photocatalytic Performances, Stability and
Recyclability of the NT Photocatalysts. After having
established general structure–property relationships across the
12 PCs, we first evaluated their photocatalytic performances with
respect to the Povarov–type addition of N,N–dimethylaniline 6a to
phenylmaleimide 7 (Scheme 3). The choice of this photoreaction
was made on the bases of three main considerations: (i) to relay
on a well–known reaction mechanism where the PC is involved in
two catalytic steps. The reported mechanism has been studied for
Redox properties analysis. One of the advantages of the NTs 5
is the associated wide redox window which allows both highly
demanding reductive and oxidative quenching cycles. PC 5a
●
+
exhibited a reversible reduction at 1.75 V vs SCE (PC /PC) and
●
–
an oxidation (PC/PC ) at -1.74 V vs SCE. Moreover, the
●
–
potentials of the excited states calculated 1.28 V (PC*/PC ) and
●
+
-
1.27 (PC /PC*) are well suited for a variety of different
photoreactions requiring strong oxidative or reductive power.
Modulating the substituents within the NT core, it is possible to
both metal16 and organic PCs,17 facilitating comparisons; (ii) for
●
+
●+
reach more negative values for E (PC /PC*) with electron
donating groups in position 3, as in the case of 5f with –1.50 V
ang 5g –1.45 V vs SCE. Even more highly negative values were
registered for PCs with extended conjugation ranging from –1.45
0 2
the redox potential of 6a (E 6a /6a = +0.80 V vs SCE) and O
●–
= –0.64 V vs SCE),19 which are within the operational
(E
0
O /O
2 2
windows of the 12 developed PCs, thus ensuring their comparison.
and (iii) to assess the physicochemical stability of the NTs. The
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RESEARCH ARTICLE
presence of oxygen and hydrogen peroxide under basic
5
g
10
conditions has been documented to be detrimental for the stability
of different PCs.18 As reported, the first step of the reaction
5f
(
Scheme 3) involves a reductive quenching of the PC yielding its
reduced form and the radical cation 9.20 Indeed, N,N–
dimethylaniline 6 can efficiently quench 5*.
5l
5
5j 5c
5b 5h 5a
5k 5d 5m
5e
0.00
0.01
0.02
0.03
0.04
0.05
[
Q] / M
Figure 3. Stern–Volmer quenching between PCs 5 and N,N–dimethylaniline 6a
in ACN.
100
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
Scheme 3. Povarov–type addition of N,N–dimethylaniline 6a to
phenylmaleimide 7 – reported reaction mechanism.
The rate of this process was evaluated by Stern–Volmer plot
(
Figure 3). Considering a pure dynamic quenching, the slope
PC) of the Stern–Volmer plot is defined as: KPC = k PC, where
is the bimolecular rate constant and τPC the excited state
lifetime of the photocatalyst. Interestingly, the k of the different
PCs 5 (Table S3 in SI) fall in a narrow range from 1.2 to 3.5 x 1010
(
K
q
τ
5a 5b 5c 5d 5e 5f 5g 5h 5j 5k 5l 5m
k
q
q
Figure 4. Observed performances of the PCs 5 for the reaction between N,N–
dimethylaniline 6a and phenylmaleimide 7. Reported yields represent the media
of two independent runs.
–
1 –1
M s , ascribable to diffusion-controlled processes. PCs 5f and
g, having a  of 7.78 and 8.5 ns, respectively, exhibited the
steepest slopes. PCs with an extended conjugation such as 5h–
l with a  ranging from 2.53 to 4.21 ns also showed an efficient
5
5
PCs 5a, 5d, 5j and 5k furnished the product in good yields
spanning from 76% to 80%. In these cases, the fastest quenching
is observed for 5j, supported by Stern–Volmer experiments and
reflected in a  of 3.34 ns and a high extinction coefficient. The
least performing PCs were 5e and 5c, with 51% and 46% yield,
respectively. While 5e performances were predicted by the lowest
quenching constant of the series, 5c performances were affected
by the lowest absorption among the 12 PCs. Other PCs 5b, 5d,
quenching. The trend was confirmed also considering the PCs
with the shortest , such as 5d and 5e, which revealed the least
efficient. A second indication was found with respect to the QY.
All the PCs 5 with a quantum yield higher than 8% showed the
fastest quenching. We next examined the photoredox
performances of the 12 PCs in the Povarov–type reaction
monitoring the yields of product 8a after 24 h reaction time (Figure
4
).17 The best performing PCs were 5f and 5g which furnished the
5
h and 5m gave average performances from 57% to 66% yield. It
is worth mentioning that PC 5f (89% yield) outperformed the
previously reported yield values of eosin Y17 and Ru(bpy) ,16
product 8a in 91% and 86% yield (corresponding to 89% and 82%
isolated yield, respectively) – in agreement with the quenching
experiments. As predicted, also PCs 5l showed excellent
performances with 82% isolated yield (85% yield by GC-FID).
3
which furnished 8a in 82% and 83% yield. At this point we
evaluate the possibility of reuse our PCs over iterative
photoreactions. An important issue related to the use of
homogeneous PCs is the stability and recyclability of these
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RESEARCH ARTICLE
molecules. Gratefully, all the 12 PCs 5 were quantitatively
recovered at the end of the reaction. Most importantly,
recyclability studies performed on PC 5a revealed the unique
possibility of reuse the PC up to four times with negligible
variations in its catalytic performances (Section F in SI).
PCs exhibited the best performances with 5k furnishing 8d in 70%
yield. It is interesting to note how PCs 5f and 5g (PC*/PC● – = 1.21
and 1.00 V, respectively) showed inferior performances while
moving from 6a–b to the more oxidant substrate 6d. This
indicating an increased impact of the excited state oxidative
●
–
power (5*/5 ) while moving towards the thermodynamic oxidation
limits of the PC. On the other hand, more oxidant PCs 5b and 5k
(PC*/PC● – = 1.47 and 1.56 V) resulted in improved performances.
Eosin Y, the previous PC of choice for this type of reactions,
turned out completely inefficient. These experiments confirmed
how the identification of novel PCs with wide redox windows can
open the way to previously inaccessible substrates.
Table 3. Observed performances of PCs
5 beyond the state–of–the–art
synthetic applications.
Synthetic Applications and Comparison of the Photocatalytic
Performances. At this juncture, we decided to evaluate PCs 5 in
diverse photochemical transformations. We started by testing
their performances under reductive quenching methods, in the
decarboxylative Giese–type addition between proline 9a and
dimethyl maleate 10 (Scheme 4). This reaction has been used as
an evaluation probe for the synthetic performances of diverse
photocatalytic systems, including: Ir–based PCs,27 acridiniums8c
and cyanoarenes.6c What makes this transformation difficult to be
efficiently catalysed is the need of an optimal balance between
the oxidative excited state properties of the PC and the reductive
power of its radical anion. Thus ensuring: (i) an efficient
decarboxylation step – with a PC* reductive quenching (9b–Cs
E
ox = 0.95 V vs SCE)27 –generating the reactive –amino radical
1
2 and (ii) a fast reduction of the formed –acyl radical 13 (Ered
=
0.60 V vs SCE)27 – with a PC oxidation step – which closes
●–
–
the photocatalytic cycle. It is worth mentioning that only specific
Ir–based and organic PCs were able to catalyse this
transformation.8c,27 In fact, diverse PCs despite being
characterised by competent redox potentials turned out highly
inefficient. Interestingly, by using 5 mol% of 5a and readily
available CFL bulbs, the expected product 11 was obtained in
6
3% yield (Table 4, entry 1). Quite unexpectedly, PCs 5b and 5d
Next, we selected substrates 6b–d, because of their high
oxidation potentials (Eox = 1.05–1.33 V vs SCE, see SI). Due to
their thermodynamically demanding oxidations these compounds
are beyond the state–of–the–art synthetic applications reported
so far, catalysed by Ru–,16 Cu–,22 Co,23 Ir,24 Pt–complexes,25
furnished the product in reduced yields, 48% and 53%,
respectively (Table 4, entries 2 and 3). Inferior results were
obtained for 5e (Table 4, entry 4), in agreement with his very short
excited state lifetime of 0.81 ns (Table 1). PC 5f, having inferior
●
–
excited state oxidative power (5f*/5f
superior  (7.78 ns) showed superior performances with
2% yield (Table 4, entry 5). Finally, PCs with an extended
= 1.21 V) and
eosin Y17 and different supported metal oxides including TiO .26 In
2
all these cases only halogens were partially tolerated as EWGs.
When a different EWG was placed on the aromatic ring it
completely shut down the reactivity of the system. We started
using 6b, bearing an ester group at the p–position (Eox = 1.05 vs
SCE). Two of the best performances were registered for 5f and
6
conjugation such as 5k and 5m efficiently catalysed this
reaction with 75% and 70% yield, respectively. In particular
5
the series (Table 3, entry 6). Selected metal–based PCs
were also compared under the same reaction conditions,
with poor results (Table 3, entries 8–10)
repertoire of reaction catalysed by NTs with respect to
reductive quenching, we explored a synthetically valuable
light–driven acylation reaction (Table 5). The mechanism
●
–
k with a 5k*/5k = 1.65 and  = 3.05 resulted the best of
5
g, with 50% and 67% yield respectively. Remarkably, the highly
oxidant PC 5k (PC*/PC● – = 1.56 V) furnished the product in 63%
yield. Subsequently, 6c was selected not only for the higher
oxidation potential (Eox = 1.12 V vs SCE), but also for the
presence of an aldehyde moiety particularly sensitive to oxidative
conditions. Gratefully, PCs 5b and 5k, characterized by high
excited state reduction potentials delivered 8c in 52% and 61%
yield, respectively. Additional experiments were carried out with
2
7
.
To expand the
2
8
involves the oxidation of acylsilane 15 (Eox = 1.46 V) by
*, generating after the silyl group cleavage the reactive
acyl radical 19. Trapping of 19 by crotonate 16 and
5
3
6d (Eox = 1.33 vs SCE), bearing two CF groups. The most oxidant
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
following reduction of radical 20 by 5● –, delivers the -
ketoester 17.
nBu
yield).28
4
N)
4
[W10
O32]) PC under deep UV irradiation, 310 nm (60%
Table 4. Observed performances of PCs 5 for the Giese–type addition between
Table 5. Light–driven acylation of methyl crotonate 16.
9
and dimethyl maleate 10.
entry
PC
5a
yield %
entrya
yield %b
1
2
3
4
5
6
7
8
9
69 (67)
PC
5a
5b
70
66
1
2
3
5
6
7
8
9
66 (63)
48
5f
5b
5g
28
5d
53
5k
88 (85)
43
5f
62
5m
5k
77 (75)
72 (70)
<5
Ru(bpy)
3
-
5m
Ir(ppy)
3
-
Ru(bpy)
3
+
Acr -Mes
<5%
60
Ir(ppy)
Ir(dFppy)
Ir[dF(CF )ppy] (dtbbpy)+
3
<5
1
0c
TBADT @ 310 nm
10
3
10
[
a] Reactions in MeOH at rt for 15h illuminated by LED using PCs
1
1c
3
2
93
1
5
(5 mol%), see SI. [b] Yields determined by H NMR analysis of
[
a] Reactions in DMF for 20h illuminated by CFL using PCs 5 (5
the crude mixture using pyrazine as internal standard. In
parenthesis isolated yields. [c] Result as in reference 28 (reaction
time 8 h).
1
mol%), see SI. [b] Yields determined by H NMR analysis of the
crude mixture using trimethoxybenzene as internal standard.
Isolated yields in parenthesis. [c] As reported in reference 27
(
reaction time 36 h).
To further evaluate the generality of the PCs with respect to
mechanistically reversed photochemical methods, we assessed
them under oxidative quenching mechanisms. Hence, we
The reaction has significant thermodynamic restrictions. It is
needed a PC oxidation potential > 1.46 V and a reduction potential
selected diverse benzyl halides 22 (Table 6). Following previous
reports on dehalogenation reactions,29 we assumed the reaction
to proceed through the single-electron reduction of 22 by 5*.30
<
- 0.6 V.27,28 Remarkably, different PCs 5 performed smoothly
with 5k (PC*/PC●– = 1.56 V, PC /PC = -1.45 V) deliver the
product in 85% yield. Metal-based and acridinium PCs revealed
completely unproductive (entries 7-10). The only manner to
access 17, before the present report, was by using
●+
When PC 5a, was irradiated under 400 nm light the benzyl iodide
22a and benzyl bromide 22b were efficiently reduced to the
tutetrabutylammonium
decatungstate
(TBADT
=
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RESEARCH ARTICLE
corresponding toluene derivative 23 in 72% and 80% yield
their use in photocatalysis is often restricted by their difficult
single–electron reduction. Notably, when the reaction was
performed in the presence of 5a product 23 formed in 15% yield.
This prompted us to screen other PCs with respect to this
unprecedented process. The most reducing PCs 5f, 5l and 5m
showed promising results, with 47%, 51% and 45% yield,
respectively (Table 6, entries 8–10).
respectively (Table 6, entry 1 and 2).
Table 6. Observed performances of PCs
5 under direct light–driven
dehalogenation reaction of benzyl halides 22a–c.
Table 7. Direct light–driven dehalogenation of methyl 4–iodobenzoate 25.
entry
PC
5a
yield %
86 (83)
16
1
2
E
red
5d
entry
15, X
PC
yield
b
(
V vs SCE)
–1.25
–1.64
–1.64
–1.64
–1.64
–1.64
–1.81
–1.81
–1.81
–1.81
–1.81
–1.81
–1.81
3
5e
13
1
2
3
4
5
6
7
8
9
22a, I
22b, Br
22b, Br
22b, Br
22b, Br
22b, Br
22c, Cl
22c, Cl
22c, Cl
22c, Cl
22c, Cl
22c, Cl
22c, Cl
5a
5a
82 (72)
82 (80)
9
4
5f
81
5
5m
96 (93)
<5
5e
6
Ru(bpy)
3
5f
68 (67)
62
7c
Ir(ppy)
92
3
5l
[
a] Reactions in ACN at rt for 20h under illumination by CFL using
1
5m
5a
46
PCs 5 (3 mol%), see SI. [b] Yields determined by H NMR analysis
of the crude mixture using trimethoxybenzene as internal
standard. Isolated yields in parenthesis. [c] Result as in reference
29 (reaction time 6 h).
15
5f
48 (47)
54 (51)
48 (45)
–
5l
The fact that only the most reductant PCs 5 were able to catalyse
this reaction corroborates the reported oxidative quenching
mechanism.29 The difficulty of catalysing this reaction was
confirmed with respect to diverse metal–based PCs (Table 6,
10
11
12
13
5m
Ru(bpy)
3
3
entries 11–13). Only the highly reductant Ir(dFppy) furnished the
Ir(ppy)
Ir(dFppy)
3
<5
product to some extent (19% yield). It is worth mentioning that the
only way to directly photo-generate this type of radicals starting
from benzyl chlorides31 is under deep–UV photochemistry.32
Prompted by the results obtained for the reduction of benzylic
chlorides, we next examined an additional oxidative-quenching-
based reaction such as the deiodination of arenes. With methyl
3
19
[
a] Reactions in ACN at rt for 16 h under 400 nm LED strips using
PCs 5 (3 mol%), see SI. [b] Yields determined by GC–FID
analysis of the crude mixture using mesitylene as internal
standard. In parenthesis isolated yields.
4–iodobenzoate 25 (Ered = –1.96 V vs SCE, see SI) we further
challenged the reductive excited state potential of our PCs.
As expected, the less reductant PC 5e showed much inferior
results, while more electron–rich/more–reductant PCs 5f and 5m
performed well, delivering 23 in 67% and 62% yield, respectively
Interestingly, 5a performed smoothly delivering 26 in 83% yield
(
Table 7, entry 1). Less reducing PCs, such as 5d and 5e gave
6 in poor yields (Table 7, entries 2 and 3). Contrarily, highly
2
(
(
Table 6, entries 4 and 5). We next examined benzyl chloride 22b
red = –1.81 V vs SCE, See SI). Importantly, benzyl chlorides are
reducing PC 5f and 5m produced 26 in 81% and 93% yield,
E
respectively (Table 7, entries 4 and 5), reaching the best results
generally unexpansive and readily available starting materials but
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RESEARCH ARTICLE
Suzuki, J. Xie, O. Tomit, D. J. Martin, M. Higashi, D. Kong, R. Abe, J.
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obtained so far by more expanses rare Earth-metal complexes
(
entry 7). It should be noted that this type of reactions can be
catalysed only by precious Ir–complexes29 or trough consecutive
visible light–induced electron transfer processes.33 Thanks to
their ambivalent oxidative/reductive nature the NT photocatalysts
[
[
2]
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window (up to 3.22 eV), is well distributed across highly energetic
exited states potentials ranging from –1.77 V to 1.65 V. This
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thus covering a vast applications spectrum. Interestingly, NTs
outperformed diverse well–established PCs including eosin Y,
acridinium, Ru– and Ir–complexes, while efficiently catalysing
unprecedented Povarov-type reactions with electron-deficient
dimethylanilines 6b-d and photo-dehalogenation of benzyl
chlorides 22c (up to 70% and 51% yield, respectively). The
merging of a redox window up to 3.22 eV, with an ambivalent
oxidative/reductive nature, guarantees to NT photocatalysts the
ability to unlocking previously inaccessible light–driven reactivity.
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#10BIRD2018–UNIPD. Stefano Mercanzin, Mauro
Meneghetti and Alberto Doimo are acknowledged for
technical assistance.
[
[
Keywords: photoredox catalysis • organic catalyst • synthetic
methods • light–driven reactions • radical chemistry
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RESEARCH ARTICLE
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1
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1
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1
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2
1
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
RESEARCH ARTICLE
Javier Mateos, Francesco Rigodanza,
Alberto
Sartorel,
Bortolato,
Vega–Peñaloza,
Andrea
Tommaso
Xavier
Mirco
Natali,
Pelosi,
Giorgio
Companyó, Marcella Bonchio, and Luca
Dell’Amico*
Page No. – Page No.
Naphthochromenones:
Bimodal Photocatalysts Engaging in
Both Oxidative and Reductive
Quenching Processes.
Organic
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