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Z. Tang et al. / Journal of Catalysis 380 (2019) 1–8
torted laser light and the radical anion CPÅ– ground-state bleach.
Interestingly, a broad negative A at ca. 726 nm was also observed
drastically (Table 1, entry 4), most likely due to quenching of CPÅ–
by molecular oxygen (Fig. S3). Light source was crucial for the reac-
tion. Irradiation of the reaction mixture with either blue LED or
green LED delivered only traces of the desired product (Table 1,
entries 5–6) as these individual LEDs could not cover all maximum
absorption spectra of CP and its excited-states (Figs. 2, S2). A series
of control experiments (i.e., omitting each individual component)
confirmed that all components, including CP as the photocatlyst,
the electron donor DIPEA and visible-light irradiation, are essential
(Table 1, entries 7–9) for the photocatalytic CAH arylation
activation.
Using the optimized conditions, different bromo- and chloro-
(hetero)aryl halides were used as (hetero)aryl radical precursors
(Scheme 1) to react with a broad range of (hetero)arenes. Aryl bro-
mides with electron–withdrawing (ACN, ACOCH3, ACO2Me, ACO2-
Et, 3aa–3ae and 3ag–3ai) as well as electron–donating (–OMe, 3af)
groups gave the corresponding coupling products in moderate
yields. Furthermore, partially fluorinated aryl bromides can also
be activated with the desired products 3aj–3al in good yields.
Although aryl chlorides are inexpensive and easily accessible, they
are not commonly considered to be activated by visible light
D
(Fig. 2d). Similarly, when excitated at 726 nm, the scattered laser
light distorted the spectra to varying degrees at the pump wave-
length in a manner as excitation at 430 m (Fig. 2e), meanwhile,
additional less negative
D
A at ca. 430 nm appeared owing to CPÅ–
ground-state bleach and stimulated emission (Fig. 2e). These data
suggest that the typical absorption bands of CPÅ– are centered at
ca. 430 nm and ca. 726 nm, but not 630 nm (Fig. 2b). Importantly,
the excited-state absorption of CPÅ– with strong positive
DA was
observed around 450–640 nm region in both 430 nm and 726 nm
excitation conditions, which can be assigned to the optically for-
bidden S1 state of CPÅ– and the so-called CPÅ–* state [33]. This con-
clusion can be further verified by the recovery of the CPÅ– ground
state as well as the decay of the transient spectra due to CPÅ–* with
a lifetime of 19 ps (Fig. 2f).
3.2. Cercosporin-catalyzed photoreductive activation of aryl halides
Since the excited radical anion CPÅ–* was truely observed here,
we next investigated whether it can activate bromo- and chloro-
(hetero)aryl halides with very low reactivities to obtain aryl radi-
cals for some important chemselective transformations. To test
our hypothesis, a mixture of 40-bromoacetophenone (1aa), N-
methyl pyrrole (2a) (acts as trapping reagent), CP as a photocata-
lyst, and DIPEA as sacrificial electron donors in CH3CN solution
was photoirradiated by a 23 W compact fluorescent light bulb
(CFL) at room temperature under nitrogen atmosphere (Tables 1
and S1). To our delight, when the reaction was conducted in the
presence of CP (5.0 mol%) and a slight excess of DIPEA (4 equiv)
in CH3CN solution at room temperature, the CAHA arylated pro-
duct 3aa was obtained within 48 h and isolated in 62% yield
(Table S1). When the amount of CP was reduced, the reaction time
was extended (Table S1, entries 1–4). Among the sacrificial elec-
tron donors, DIPEA was slightly more effective than Et3N (triethy-
lamine), and much more effective than TBA (tributylamine)
(Table S1, entries 5–6). Although all dipolar aprotic solvents
(DMSO, DMF, CH3CN, DMAC, pyridine, acetone) gave CAHA ary-
lated product, CH3CN gave the best yield of 3aa (Table S1, entries
7–11). For other solvents (EtOH, MeOH, DCE, DCM, THF, toluene),
trace product was produced (Table S1, entries 12–17). Other com-
mercially available perylenequinonoid pigments were also tested.
Significant yield drop was observed with hypocrellin A as a cata-
lyst, but hypocrellin B gave slightly less yields of 3aa (Table 1,
entries 2–3). Under O2 atmosphere, the yield of product decreased
driven-photocatalysis owing to their low reactivity [34–36]. Based
red
on the reduction potential of E CP/CPÅ– [–0.46 V versus saturated
1=2
calomel electrode (SCE)] [13,14] and the E0,0 transition energy of
CPÅ– [1.39 eV] (DFT calculation data is provided in Supporting Infor-
mation), the reducing power of the excited radical anion CPÅ–* was
estimated according to the Rehm-Weller equation [37] as Eox (CP/
CPÅꢀ*) = ꢀ1.85 V vs SCE, showing that it reaches or exceeds the
reduction potentials of substituted aryl chlorides. Indeed, we
showed that aryl chlorides with electron–withdrawing functional
groups such as ACN, ACO2Me or ACO2Et (3ba–3bb, 3bd–3be,
3bg–3bi), or chlorinated N-heterocycles including pyridine (3bc),
quinoline (3bf) and pyrimidines (3bj–3bl) were able to be
sufficiently activated. They gave the corresponding products in
moderate yields when pyrrole, N-methyl pyrrole, N-benzyl pyrrole,
and 1,3,5-trimethoxy-benzene were used as reaction partners.
Therefore, here we developed a green and atom economic method
to activate aryl halides with a broad range of functional groups by
visible light-driven photocatalysis with cercosporin as the
photocatalyst.
3.3. Mechanistic studies
Last, mechanistic studies were investigated. First, since the
obvious solvent effect was relative to the photocatalytic activity
of cercosporin (Table 1), it prompted us to study the formation
ability and stability of CPÅ– in different solvent conditions. Previous
studies show that electrochemical reduction of quinone undergo a
two-step consecutive single-electron process rather than a one-
step two-electron process, and the decision of radical anion or
dianion highly depends on the nature of solvent [38–40]. Thus, it
indicates that CP can selectively form monovalent radical anion
CPÅ– or divalent anion CP2– on the control of solvents. In fact, we
found that the obvious color change of sample and the typical
absorption bands of CPÅ– were only observed in dipolar aprotic sol-
vents, such as DMSO, DMF, CH3CN, DMAC, pyridine and acetone
(Fig. S4), but not in other solvents (Fig. S5), which was correlated
to the photocatalytical activity of cercosporin as show in
Table S1. It suggested that both of effect of dipole moment and
effect of proton are crucial for the generation and the stability of
the semiquinone radical anion CPÅ–. The strong dipole solvent can
reduce the barrier of the intramolecular hydrogen transfer (IPT)
process of CPÅ– and accelerate the IPT rate, which is beneficial to
the stabilization of CPÅ– [41]. However, according to previous stud-
ies [38,39], the OAH bond in protic solvents, such as MeOH and
Table 1
Optimization of the reaction conditions.
Entry
Deviation from abovea
Yield of 3aab
1
None
62
43
59
6
Trace
Trace
Trace
Trace
Trace
2c
3d
4
HA instead of CP
HB instead of CP
O2 instead of N2
Blue LED instead of 23 W CFL
Green LED instead of 23 W CFL
Without light
5e
6f
7
8
9
Without CP
Without DIPEA
a
Reaction conditions: 1a (0.2 mmol), N-methyl pyrrole (20 equiv), CP (5 mol%),
DIPEA (4 equiv), CH3CN (2 mL), N2, under 23 W CFL irradiation at room tempera-
ture. See Supporting Information for catalyst loading and DIPEA equivalent.
b
Yield of isolated product.
5 mol% of hypocrellin A as a photocatalyst.
5 mol% of hypocrellin B as a photocatalyst.
5 W blue LED was used.
c
d
e
f
5 W green LED was used.