Donor-Acceptor Fluorophores for Visible-Light-Promoted Organic Synthesis: Photoredox/Ni Dual Catalytic C(sp3)-C(sp2) Cross-Coupling

Article abstract of DOI:10.1021/acscatal.5b02204

We describe carbazolyl dicyanobenzene (CDCB)-based donor-acceptor (D-A) fluorophores as a class of cheap, easily accessible, and efficient metal-free photoredox catalysts for organic synthesis. By changing the number and position of carbazolyl and cyano groups on the center benzene ring, CDCBs with a wide range of photoredox potentials are obtained to effectively drive the energetically demanding C(sp³)-C(sp²) cross-coupling of carboxylic acids and alkyltrifluoroborates with aryl halides via a photoredox/Ni dual catalysis mechanism. This work validates the utility of D-A fluorophores in guiding the rational design of metal-free photoredox catalysts for visible-light-promoted organic synthesis.

Full text of DOI:10.1021/acscatal.5b02204

Letter
pubs.acs.org/acscatalysis
Donor−Acceptor Fluorophores for Visible-Light-Promoted Organic
Synthesis: Photoredox/Ni Dual Catalytic C(sp3)−C(sp2) Cross-
Coupling
Jian Luo and Jian Zhang*
Department of Chemistry, University of Nebraska−Lincoln, Lincoln, Nebraska 68588, United States
S
* Supporting Information
ABSTRACT: We describe carbazolyl dicyanobenzene (CDCB)-based
donor−acceptor (D−A) fluorophores as a class of cheap, easily accessible,
and efficient metal-free photoredox catalysts for organic synthesis. By
changing the number and position of carbazolyl and cyano groups on the
center benzene ring, CDCBs with a wide range of photoredox potentials are
obtained to effectively drive the energetically demanding C(sp3)−C(sp2)
cross-coupling of carboxylic acids and alkyltrifluoroborates with aryl halides
via a photoredox/Ni dual catalysis mechanism. This work validates the
utility of D−A fluorophores in guiding the rational design of metal-free
photoredox catalysts for visible-light-promoted organic synthesis.
KEYWORDS: donor−acceptor fluorophores, photoredox catalysis, nickel catalysis, visible light, cross-coupling
isible-light photoredox catalysis, the synthetic manifold
mostly centered on ruthenium (Ru)- and iridium (Ir)-
Herein, we describe carbazolyl dicyanobenzenes (CDCBs) as
a versatile design platform to facilely construct D−A
fluorophores at low cost. We choose CDCBs because
dicyanoarenes and carbazole derivatives are strong electron
acceptors8b,f and donors,17 respectively, with appropriately
aligned HOMO−LUMO energy levels. More importantly, by
changing the number and position of carbazolyl and cyano
groups on the center benzene ring, one can easily modify the
photoredox potentials of the resulting D−A fluorophores. We
further test the photocatalytic activities of CDCBs using two
photoredox/Ni dual catalytic C(sp3)−C(sp2) cross-coupling
reactions and demonstrate their wide potential utility in visible-
light-promoted organic synthesis.
Six CDCB-based D−A fluorophores, namely, 4CzIPN,
2CzIPN, 4CzPN, 2CzPN, 4CzTPN, and 2CzTPN (Figure 1)
were prepared using a previously reported one-step nucleo-
philic substitution reaction followed by the simple recrystalliza-
tion purification procedure.18 Note that 2CzIPN and 2CzTPN
are synthesized here for the first time. To demonstrate the
versatility of this synthetic protocol, we prepared 4DPAIPN as
an example in which a stronger electron donor diphenylamine
is incorporated. It is gratifying that the costs for synthesizing
these D−A fluorophores (e.g., ∼ $6/g for 4CzIPN and
4DPAIPN) were much lower than those of common Ru- and
Ir-complexes (Supporting Information, S-2).
V
based photoredox catalysts, has recently received resurgent
attention.1 Thanks to the large collection of Ru- and Ir-
complexes that cover a wide range of photoredox potentials for
driving the intermolecular single-electron transfer (SET)
processes,2 many complicated organic transformations have
now been realized,3 including dual catalytic reactions.4 Since
Ru- and Ir-complexes are expensive, photocatalysts based on
earth-abundant copper (Cu),5 chromium (Cr),6 and iron (Fe)7
have been investigated. Nevertheless, for large-scale synthesis,
organic dyes8 with low cost, ease of availability, and low toxicity
offer an intriguing alternative.9 However, the general synthetic
utility of organic dyes is still rather limited due to relatively few
catalyst options. Therefore, it is highly desirable to design new
organic photocatalysts with broad redox capabilities.
Donor−acceptor (D−A) molecular dyads have been
extensively studied in chemical sensing,10 optoelectronic
materials,11 organic photovoltaics,12 and artificial photo-
synthetic systems.13 D−A fluorophores are particularly
interesting since their photoredox potentials are strongly
dependent on the energy level of acceptor’s LUMO
(determining E1/2(P+/*P) and E1/2(P/P−), P = photocatalyst,
* = excited state) and donor’s HOMO (determining E1/2(*P/
P−) and E1/2(P+/P)). In principle, by choosing donor−acceptor
pairs with different electronic properties, D−A fluorophores
with a wide range of photoredox potentials can be realized. In
fact, the highly oxidative photocatalyst, 9-mesityl-10-methyl-
acridinium (Acr+-Mes),14 is a D−A dyad.15 However, tuning
the photoredox potentials of Acr+-Mes via structural
modification is not straightforward.16
Received: September 30, 2015
Revised: December 24, 2015
© XXXX American Chemical Society
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Scheme 1. Proposed Mechanism of Photoredox/Ni Dual
Catalytic C(sp3)−C(sp2) Cross-Coupling20
Figure 1. Structure and photoredox potentials (vs SCE; all potentials
mentioned hereafter are against SCE) of the D−A fluorophores:
1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), 1,3-bis-
(carbazol-9-yl)-4,6-dicyanobenzene (2CzIPN), 1,2,3,4-tetrakis-
(carbazol-9-yl)-5,6-dicyanobenzene (4CzPN), 1,2-bis(carbazol-9-yl)-
4,5-dicyanobenzene (2CzPN), 1,2,4,5-tetrakis(carbazol-9-yl)-3,6-di-
cyanobenzene (4CzTPN), 1,4-bis(carbazol-9-yl)-2,5-dicyanobenzene
(2CzTPN), and 1,3-dicyano-2,4,5,6-tetrakis(N,N-diphenylamino)-ben-
zene (4DPAIPN). DCA (9,10-dicyanoanthracene) is depicted here as
a comparison. For simplicity, only E1/2(*P/P−) and E1/2(P/P−) are
shown here. See Supporting Information, Table S1 for comprehensive
values.
fluorophores are sufficiently oxidative (E1/2(*P/P−) = +1.10 ∼
+ 1.41 V) to obtain an electron from the amino acid N-tert-
butoxycarbonyl-proline (N-Boc-pro, E1/2 = +0.95 V23) to
red
generate the C(sp3) radical, which is supported by a Stern−
Volmer fluorescence quenching experiment between 4CzIPN
and N-Boc-pro (in the form of its cesium salt, Figure S10).
Encouraged by this result, we conducted the coupling reaction
of N-Boc-pro with methyl 4-iodobenzoate in a reaction mixture
consisting photocatalyst, bench stable NiCl2·DME (DME =
dimethoxyethane), bpy ligand (bpy = 2,2′-bipyridine), and
Cs2CO3 under irradiation from a 26W white CFL (compact
fluorescence lamp). This reaction is highly sensitive to the
photoredox catalyst (Table 1 and Supporting Information,
Table S2 and S4). Many common transition metal complexes
(e.g., [Ru(bpy)3]2+ and [Ru(bpz)3]2+ (bpz = 2,2′-bipyrazine))
and organic dyes (e.g., eosin Y, Rose Bengal, methylene blue,
and Acr+-Mes) showed no activity (Table S4), likely due to
Upon visible light excitation, the nanosecond scale excited
state of D−A fluorophores exhibits both strong oxidative
(E1/2(*P/P−) = +1.10 ∼ + 1.41 V) and reductive (E1/2(P+/*P)
= −0.99 ∼ − 1.41 V) capabilities that are comparable or
superior to many metal complexes and organic dyes
(Supporting Information, Table S1). Importantly, their
relatively wide distribution of photoredox potentials is achieved
simply by changing the number of donors (two or four
carbazolyl), the positions of acceptors (ortho-, meta-, or para-
dicyanobenzene), or the nature of donor (carbazolyl- or
diphenylamino-). For example, increasing the number of
carbazolyl groups from two to four anodically shifts E1/2(*P/
P−) and E1/2(P/P−) (∼0.29 V), while inducing a smaller anodic
shift on E1/2(P+/*P) and E1/2(P+/P) (<0.08 V) (Table S1).
Further, when the carbazolyl moiety is changed to diphenyl-
amine (as in 4DPAIPN), a cathodic shift of 0.18−0.31 V was
observed for photoredox potentials (Table S1).
Table 1. Screening of Photocatalysts for Photoredox/Ni-
Catalyzed Decarboxylative Cross-Coupling
With their strong photoredox potentials in mind, we envision
that the D−A fluorophores are suitable to promote organic
transformations with high energetic demand such as the
photoredox/Ni dual catalytic C(sp3)−C(sp2) cross-coupling19
recently developed by the Molander group and the MacMillan/
Doyle group independently.20 The proposed reaction mecha-
nism (Scheme 1) suggests that a highly oxidative photocatalyst
(E1/2(*P/P−) > + 1.10 V) is necessary to generate an
organoradical (R·) from the C(sp3) precursor (R−X, where
% residual
a
b
entry
photocatalyst
% yield
photocatalyst
1
2
3
4
5
6
7
8
4CzIPN
4DPAIPN
85
87
56
15
20
12
<5
0
58
78
11
0
2CzIPN
4CzPN
−
X is CO2 , BF3K, or H), which is subsequently captured by an
2CzPN
0
aryl-NiII complex, forming a NiIII species that readily undergoes
reductive elimination.21 Meanwhile, the reduced photocatalyst
with a large E1/2 (P/P−) (<−1.10 V) facilitates a second SET to
release a Ni0 species, which further undergoes oxidative
addition to Ar−X′, synchronizing and completing the two
catalytic cycles.22
4CzTPN
0
2CzTPN
0
DCA
0
c
d
9
[Ir[dF(CF3)ppy]2(dtbbpy)]+
83
65
a
b
c
Isolated yield. Measured by HPLC. 24 h to complete the reaction.
[dF(CF3)ppy] = 2-(2,4-difluorophenyl)-5-trifluoromethyl-pyridine,
We first investigated the C(sp3)−C(sp2) cross-coupling of α-
amino acids with aryl halides. We expect that the D−A
d
dtbbpy =4,4′-ditert-butyl-2,2′-bipyridine.
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Letter
their inferior redox potentials (Table S1). The D−A
fluorophores, however, promoted this reaction with a variety
of efficiencies. In particular, 4CzIPN and 4DPAIPN exhibited
the best catalytic activity with isolated product yields of 85%
and 87%, respectively, and 2CzIPN gave a moderate yield of
56%. The other four CDCB D−A fluorophores were weakly
effective (5−20% yields). To our surprise, despite its strong
redox potentials (E1/2(*P/P−) = +2.06 V and E1/2 (P/P−) =
−1.00 V),8b DCA appeared to be ineffective. In fact, to the best
of our knowledge, there have been no reports using organic
photocatalysts for photoredox/Ni dual catalytic C(sp3)−C(sp2)
cross-coupling reactions before our study. When the reaction
was carried out using a less powerful blue LED (465 nm) in the
presence of 4CzIPN, 77% yield can be achieved (Table S2,
entry 6).
NiI reduction induced by electron-donating tert-butyl sub-
stituent. Moreover, bpy ligands with strong electron-donating
(methoxy-) or strong electron-withdrawing (trifluoromethyl
and methoxylcarbonyl) as well as the electron-deficient bpz
ligand gave no coupling product.
With optimized photocatalyst (4CZIPN) and Ni catalyst
(NiCl2·DME + bpy), we tested the scope of the decarboxylative
arylation reaction (Table 2). A wide range of electron-rich
Table 2. Photoredox/Ni-Catalyzed Decarboxylative Cross-
a
Coupling of α-Amino Acids and Aryl Halides
From a thermodynamic perspective, 4CzPN, 2CzPN,
4CzTPN, 2CzTPN, and DCA should favorably promote this
coupling reaction. Thus, we tentatively attribute their low
activities to the inferior stability under the reaction conditions.
Indeed, the fluorescence spectra of the reaction mixture
containing the four CDCB fluorophores exhibited a large
blue shift (∼100 nm) of the emission maximum and the
reaction solution containing DCA became nearly nonfluor-
escent (Figure S11). Moreover, HPLC analysis indicated that
none of the four CDCB fluorophores and DCA can be
recovered (entries 4−8, Table 1 and Supporting Information,
S-8). On the other hand, 58%, 78%, and 11% of 4CzIPN,
4DPAIPN, and 2CzIPN, respectively, remained intact, which
corresponds well with their product yields (entries 3−5, Table
1). Despite of its high activity, only 65% of [Ir[dF(CF3)-
ppy]2(dtbbpy)]+ can be recovered (entry 9, Table 1).
To gain more insight into the drastic differences in catalytic
efficiencies of the D−A fluorophores, we examined the effect of
solvent on their photostability. The least active catalyst,
2CzTPN, was chosen for this study. UV−vis spectra of a
deaerated solution of 2CzTPN (0.5 mM) before and after light
irradiation (26 W CFL, 4h) showed that 2CzTPN is fairly
stable in most common solvents except DMF (Figure S13).
Further examination via UV−vis spectroscopy revealed that the
photostability of the D−A fluorophores in DMF generally
follows in the order of meta > ortho > para and
tetrakiscarbazolyl > dicarbazolyl dicyanoarenes (Figure S12).
DCA, with a para-dicyano structure and no carbazolyl-
substitution, is extremely unstable: after 30 min irradiation, a
significant decrease of absorption intensity accompanied by a
blue shift (∼25 nm) was observed (Figure S12). Such
photoinduced decomposition of cyanoarenes in DMF likely
involves a nucleophilic substitution that can be inhibited in
meta-dicyanoarenes and/or by bulky substitution groups.24
Indeed, D−A fluorophores with these characteristics such as
4CzIPN and 4DPAIPN exhibit a significantly higher photo-
stability in DMF (Figure S12). It should be emphasized here,
however, that the inferior photostability of the D−A
fluorophores based on ortho- or para-dicyanobenzene in
DMF should not prevent their photocatalytic applications in
other solvents.
a
b
Isolated yield. NaOH was used as the base.
(methoxy, 1c) and electron-deficient (methoxycarbonyl 1b,
cyano 1d) aryl halides was suitable to this dual catalysis
manifold (72−88%). Electron-deficient pyrazinyl (1e, 66%)
and pyridinyl (1f, 74%) chlorides were also effective substrates.
On the other hand, both N-Boc protected cyclic and N-benzyl
carbamoyl (N-Cbz) protected acyclic α-amino acids (1g−1j)
proved to be competent coupling partners. In particular, sulfur-
containing N-Boc-4-thioproline was tolerant (1j, 92%), and no
deactivation of the Ni catalyst was observed. Furthermore,
decarboxylative arylation of tetrahydro-2-furoic acid and 2-
phenylacetic acid proceeded smoothly forming the α-arylated
ether (1k, 76%) and alkane (1l, 84%), respectively. Finally,
4CzIPN also exhibited an excellent activity for amine-based
C(sp3) precursors such as dimethylaniline (1m, 90%).
Using 4CzIPN as the photoredox catalyst, we further
optimized the Ni catalyst (Supporting Information, Table
S3). No coupling product was detected in the absence of the
ligand and the ligand’s electronic effect is also significant.19c,25
For example, 1,10-phenanthroline gave a comparably good
yield (82%) as that of bpy; however, dtbbpy led to a
significantly lower yield (30%), possibly due to the sluggish
We next tested the photocatalytic activities of D−A
fluorophores in the cross-coupling of alkyltrifluoroborates
with aryl halides.20a,d Using the coupling of potassium
benzyltrifluoroborate and methyl 4-bromobenzoate, we opti-
mized the synthetic condition and found that a reaction
consisting 4CzIPN as the photocatalyst, 2,2′-bpy and NiCl2·
DME as the metal catalyst in combination with 2,6-lutidine as
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an additive proceeded smoothly with 87% yield of the coupling
product (2a (also 1l), Table 3). Compared to the previously
the energetically demanding photoredox/Ni dual catalytic
C(sp3)−C(sp2) cross-coupling of α-amino acids/alkyltrifluor-
oborates with aryl halides. The concept of D−A fluorophores
can certainly be used to design other new metal-free
photoredox catalysts, and this work validates its wide utility
in visible-light-promoted organic synthesis.
Table 3. Photoredox/Ni Dual-Catalyzed Cross-Coupling of
Trifluoroborates and Aryl Bromides
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b02204.
■
S
Materials, general procedures, synthesis, the cost
calculations for D−A fluorophores, physical measure-
ments, spectroscopic characterizations, CV diagrams,
NMR spectra, and HPLC measurements (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: jzhang3@unl.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank the support from the University of
Nebraska-Lincoln.
■
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DOI: 10.1021/acscatal.5b02204
ACS Catal. 2016, 6, 873−877