Angewandte
Communications
Chemie
reaction yields (products 2b,u,J) and iodine was well toler-
(see Scheme S1 in the Supporting Information). Meanwhile,
ated (product 2p). Furthermore, when more strongly electron
withdrawing groups, such as aldehyde, cyano, trifluoromethyl,
and nitro groups, were present on the migrating aryl ring, the
the profile of the reaction yield was monitored with the light
turned on and off at intervals, and we found that the
transformation occurred smoothly under irradiation with
blue LEDs and that further conversion stopped in the absence
of a visible-light source (Figure 1a). Moreover, the formation
À
use of Acr+-Mes ClO4 led to significantly higher reaction
efficiency or good tolerance (products 2x–A).
Notably, when the reaction was carried out on a gram
scale, the excellent yield of 2a was maintained [Scheme 3,
À
Eq. (3)]. Owing to the importance of C O bond cleavage, we
Figure 1. [a] Profile of the formation of 2a with the light turned on or
off at regular intervals. Yields were determined by 1H NMR spectrosco-
py with Cl2CHCHCl2 as an internal standard. [b] Emission-quenching
experiments of 1a and 1a-K+.
of a carboxylic radical is suggested by EPR studies on benzoic
acid under the optimized conditions (see Figures S2 and S3).
Furthermore, a quantum yield value[9c,14] of f = 0.24 was
determined (see the Supporting Information). Thus, a con-
clusion could not be reached as to whether the reaction
proceeds through a photoredox catalytic pathway or a radical
chain pathway at this stage.
The quenching of the excited state PDI* by the acid or the
acid anion was conducted in MeOH because the potassium
salt of 1a has good solubility in MeOH. The results revealed
that both 1a and the anion of 1a could quench PDI*
(Figure 1b; see Figures S5–S7). However, the quenching rate
constant of the anion of 1a is much larger than the rate itself.
We deduced that the carboxylate anion, but not 1a, quenches
the PDI*.[9] The quenching of PDI* by 1a could be attributed
to the lower concentration of the 1a anion produced in
MeOH.
Scheme 3. Gram-scale aryl migration and subsequent transformatioÀn.
[a] With PDI as the photosensitizer for 32 h. [b] With Acr+-Mes ClO4
as the photosensitizer for 11 h.
À
conducted a one-pot, two-step reaction to cleave the C O
bond of the biaryl ether [Scheme 1, Eq. (2)]. To our delight, 3
and 4 were obtained in excellent yields (91 and 86%,
respectively) from a gram-scale reaction of 1a. Thus,
À
a novel approach to C O cleavage to give two phenolic
compounds was realized under mild conditions. This trans-
À
formation provides an alternative approach to C O bond
cleavage in biaryl ethers to the generation of a phenolic
product and an aryl product by hydrogenation.[7j,k,n] Further-
more, this approach might inspire visible-light photoredox-
À
catalyzed C O bond cleavage of general aryl ethers by aryl
ipso-substitution via aryl carboxylic radicals.[12] Moreover, to
demonstrate the potential application of this aryl migration,
we carried out a gram-scale reaction of commercially
available 1k and obtained guacetisal (5), a drug used to
treat inflammatory respiratory diseases,[13] in 92% yield in
a one-pot, two-step reaction [Scheme 3, Eq. (4)]. As com-
pared with conventional methods,[13b] toxic reagents, such as
SOCl2 and POCl3, and intermediate purification are avoided.
2-(Phenylthio)benzoic acid, 2-(phenylamino)benzoic acid,
and 2-(methyl(phenyl)amino)benzoic acid were also inves-
tigated as starting materials; however, no conversion was
observed.
The thermodynamic feasibility of the photoinduced
electron transfer was analyzed on the basis of the oxida-
tion–reduction potentials. The oxidation potential of
À
=PhOPhCO
EPhOPhCO
2C
ÀC
and the reduction potential of EPDI=PDI
2
in CH3CN were determined as + 1.90 V vs. SCE (see Fig-
ure S10) and À0.49 V vs. SCE (see Figure S11), respectively.
[15]
The excited-state energy E00 of PDI was read from the
cross-point of the absorption and luminescence spectra at
526 nm (see Figure S8) as 2.36 eV. Therefore, the reduction
ÀC
=PDI
potential of EPDI
was calculated as + 1.87 V vs. SCE
+ E00) in CH3CN, and thus displayed
*
ÀC
ÀC
=PDI
PDI=PDI
(EPDI
= E
*
ÀC
=PDI
[8i,16]
*
We conducted a series of experiments under the opti-
mized reaction conditions with PDI as the photosensitizer to
provide insight into the reaction mechanism. First, we
confirmed that only PDI can act as a photosensitizer from
the UV/Vis absorption spectrum of PDI and 1a (see Figure S1
in the Supporting Information). Second, when 2,2,6,6-tetra-
methyl-1-piperidinyloxy (TEMPO) or 2,6-di-tert-butyl-4-
methylphenol (BHT) was added as a radical scavenger to
the reaction mixture, 2a was obtained in less than 5% yield
a slight difference to EPDI
=+ 1.92 vs. SCE in DMF.
This reduction potential indicates the possibility of the
formation of PDIÀC and the carboxylic acid radical by single-
electron transfer (SET) between PDI* and the carboxylic acid
anion.[17]
On the basis of the results described above, we propose
the following reaction mechanism (Scheme 4): Initially,
irradiation of PDI with blue LEDs leads to the formation of
ÀC
=PDI
the excited state PDI* as a strong oxidant (EPDI
=
*
Angew. Chem. Int. Ed. 2017, 56, 13809 –13813
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim