Redox-economical radical generation from organoborates and carboxylic acids by organic photoredox catalysis

Article abstract of DOI:10.1039/c5ra01826a

A simple generation method of carbon radicals via 1e-oxidation of organotrifluoroborates and carboxylic acids by the action of an organophotoredox catalyst, 9-mesityl-10-methylacridinium perchlorate ([Acr⁺-Mes]ClO₄), has been developed. This organophotocatalytic protocol is amenable to radical C-C bond formation with electron-deficient olefins.

Full text of DOI:10.1039/c5ra01826a

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Redox-economical radical generation from
organoborates and carboxylic acids by organic
Cite this: RSC Adv., 2015, 5, 21297
photoredox catalysis†
Received 10th December 2014
Accepted 12th February 2015
Tatsuya Chinzei, Kazuki Miyazawa, Yusuke Yasu, Takashi Koike* and Munetaka Akita*
DOI: 10.1039/c5ra01826a
www.rsc.org/advances
+
A simple generation method of carbon radicals via 1e-oxidation of methylacridinium perchlorate ([Acr –Mes]ClO (1d)), an organo-
4
6,7
organotrifluoroborates and carboxylic acids by the action of an photoredox catalyst developed by Fukuzumi and Ohkubo. The
organophotoredox catalyst, 9-mesityl-10-methylacridinium perchlo- photoinduced electron-transfer state of 1d is known as an
+
rate ([Acr –Mes]ClO
4
2
), has been developed. This organo- oxidant (+1.65 V vs. Cp Fe) which is stronger than those of the Ru
photocatalytic protocol is amenable to radical C–C bond formation and Ir photoredox catalysts. Thus, 1d enables oxidative trans-
with electron-deficient olefins.
formation, which cannot be promoted by metal photoredox
catalysis. We will describe a new and simple protocol for the
generation of organic radicals from organoborates 2 and
carboxylic acids 3, which is followed by radical C–C bond
formation without noble metals and co-oxidants (Scheme 1b).
We initially examined the photocatalytic reaction of potas-
sium phenethyltriuoroborate (2a) with ethylidenemalonic acid
dimethyl ester (4a) in the presence of 1d (2 mol%) in a 1 : 1
Over the past few years, photoredox catalysis, which is
composed of visible-light-induced single-electron-transfer (SET)
processes, has become a powerful tool in the eld of synthetic
1
0
organic chemistry. In particular, a well-dened tris(2,2 -bipyr-
idine) ruthenium(II) complex (1a) and the relevant cyclo-
metalated iridium(III) derivatives (1b and 1c) have been proven
to be useful transition metal photocatalysts for invaluable
radical transformations. On the other hand, organic photoredox
catalysis has also been attracting great interest in terms of the
acetone-d
6 3
and CD OD mixture under visible light irradiation
(blue LEDs: lmax ¼ 425 nm) for 24 h (entry 1 in Table 1). As a
2
elements strategy initiative and green chemistry.
Organoboronic acid derivatives and organoborates are
regarded as convenient radical precursors in terms of high
stability, low toxicity and high compatibility with various func-
tional groups. One-electron oxidation of organoboron
compounds smoothly generates reactive carbon-centered radi-
cals through deboronation. Recently, some metal-catalyzed
radical reactions using organoboronic acid derivatives have
been reported, but they usually require an excess amount of co-
3
oxidants (the upper process in Scheme 1a). More recently, our
group and the groups of Chen and Molander have developed
radical reactions of organoborates designed by Ru or Ir photo-
4,5
redox catalysis (the lower process in Scheme 1a).
In this communication, we describe a simple generation
method of carbon-centered radicals from organotriuoroborates
2
and carboxylic acids 3 by the action of 9-mesityl-10-
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta,
Midori-ku, Yokohama 226-8503, Japan. E-mail: koike.t.ad@m.titech.ac.jp; makita@
res.titech.ac.jp; Fax: +81-45-924-5230; Tel: +81-45-924-5230
†
Electronic supplementary information (ESI) available: Experimental details and
Scheme 1 Generation of C-radicals through catalytic oxidation of
spectral data. See DOI: 10.1039/c5ra01826a
organoborons.
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a,b
Table 1 Optimization of the photocatalytic radical reaction of 2a with Table 2 The scope of the present photocatalytic reaction
a
4a
NMR yield of
Entry
Photocatalyst
Solvent
5aa/%
b
1
2
3
4
5
6
7
8
1d
1d
1d
1d
1d
1a
1b
1c
1d
None
Acetone-d
Acetone-d
6
/CD
3
OD (1/1)
64, 79
6
28
34
53
0
0
13
3
CD
CD
3
OD
CN
3
DMSO-d
6
Acetone-d
Acetone-d
Acetone-d
Acetone-d
Acetone-d
6
6
6
6
6
/CD
/CD
/CD
/CD
/CD
3
OD (1/1)
3
OD (1/1)
3
OD (1/1)
3
OD (1/1)
3
OD (1/1)
c
9
0
0
1
0
a
Reaction conditions: a reaction mixture of 2a (0.06 mmol), 4a (0.05
(an internal standard) and
mmol), photocatalyst (1.0 mmol), SiEt
4
solvent (0.50 mL) was irradiated by 3 W blue LEDs (l ¼ 425 ꢀ 15 nm)
b
c
at room temperature for 24 h. Reaction time ¼ 48 h. The reaction
was conducted in the dark.
result, the addition product 5aa was obtained in a 64% NMR
yield (24 h). An elongated reaction time (48 h) gave a better yield
(79%). The acetone–methanol mixed solvent system resulted in
the highest yield (entries 1–5). It should be noted that the typical
transition metal photoredox catalysts (1a–c) afforded the
product in considerably lower yields (entries 6–8). The reactivity
a
Reaction conditions:
a reaction mixture of 2 (0.36 mmol), 4
of the organocatalyst 1d was superior to that of metal catalysts (0.30 mmol), 1d (6.1 mmol), acetone (1.5 mL) and MeOH (1.5 mL) was
irradiated by 3 W blue LEDs (l ¼ 425 ꢀ 15 nm) at room temperature
1
a–c and can be ascribed to the oxidation potentials of the
b
c
d
for 24 h. Isolated yields. Reaction time ¼ 12 h. Reaction time ¼
photoactivated species (1a: +0.43 V, 1b: +0.91 V, 1c: +0.80 V,
1
potential: +1.41 V vs. Cp Fe) is readily oxidized by the photo-
e
f
g
3
6 h. Reaction time ¼ 18 h. Reaction time ¼ 60 h. Reaction
1d,f
h
d: +1.65 V vs. Cp Fe).
The organoborate 2a (oxidation time ¼ 15 h. Reaction time ¼ 48 h.
2
2
activated 1d but not by the excited 1a–c. Both the photocatalyst
and visible light were essential for the present reaction (entries
alkenes bearing a substituent at the reaction site (4e, 4f and 4g),
provided the corresponding C–C coupled products (5fc–5fg: 38–
9
and 10), strongly supporting the hypothesis that the photo-
88%). These results reveal that the present photocatalytic system
activated species of the photocatalyst is involved in the reaction.
Next, we examined the scope of the present photocatalytic
reaction (Table 2). The reaction of tertiary-alkyltriuoroborates
enables the selective generation of various alkyl radicals with/
without functionalities from the corresponding organoborates.
Remarkably, sunlight, which includes visible light as the
main component, can be utilized as the light source. To our
surprise, the sunlight-driven system was more efficient than the
blue LED system. The reaction of 2f with 4b was completed
within 18 h under daylight (eqn (1)).
(2b and 2c) smoothly proceeded with the construction of a
quaternary carbon center to give the corresponding products (5bb
and 5ca) in 61% and 94% yields, respectively. Secondary-
alkyltriuoroborates (2d–l), regardless of cyclic or acyclic struc-
tures, afforded the products (5db–lb) in good yields (58–85%).
Primary-alkyltriuoroborates (2m–o) were also applied to this
photocatalytic system (38–93% yields). It is noteworthy that the
addition of more substituted alkyl groups is rather efficient in the
present C–C bond formation. Tolerance to a variety of function-
alities such as Br (2k), ester (2l), Boc-protected amines (2h and 2n)
and ethers (2i and 2o) is particularly signicant. Furthermore, the
reaction of potassium cyclohexyltriuoroborate (2f) with a range
of electron-decient alkenes, acrylate derivatives (4c and 4d) and
(1)
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Control experiments in CD
3
OH revealed that the a-H atom in
the product 5 does not result from hydrogen abstraction from
the CD group but from protonation with the OH group (see the
3
ESI†), supporting the involvement of a carboanionic interme-
diate 7.
In conclusion, we have developed a novel redox-economical
photocatalytic radical reaction via oxidative generation of
carbon radicals from organotriuoroborates and carboxylic
+
acids by the action of an organophotoredox catalyst, [Acr –Mes]
Scheme 2 Photocatalytic radical addition of carboxylic acids 3.
ClO . The present organocatalytic strategy turns out to be free of
4
a noble metal catalyst, co-oxidant and toxic/explosive chemicals.
In particular, owing to the high oxidation potential of photo-
+
induced electron-transfer state, [Acr –Mes]ClO
4
can generate
Encouraged by these results, we next investigated the
generation of organic radicals from carboxylic acids by the
present organophotoredox system. Carboxylic acids are readily
available and known to serve as C-radical sources through 1e-
oxidation followed by decarboxylation. We optimized the
reaction of pivalic acid (3b) with 4b (see the ESI†). To our
organic radicals from a variety of organoborates, which cannot
be smoothly mediated by the Ru and Ir species. Further devel-
opment of the synthetically valuable organic photoredox catal-
ysis and redox-economical reaction is a continuing effort in our
laboratory.
8
delight, the reaction of 3b with 4b in the presence of Na
2 3
CO
(
0.1 equiv.) and organophotocatalyst 1d (2 mol%) proceeded
Acknowledgements
smoothly to give the corresponding product 5bb in an 81%
isolated yield (Scheme 2). However, the efficiency of the reac-
tions
hexanecarboxylic acid (3f) and 2-methylbutyric acid (3j)
declined (60 h). In addition, the reactions of 3-phenylpropionic
The nancial support from the Japanese government (Grant-in-
Aid for Scientic Research: no. 26288045) is gratefully
acknowledged.
of
1-adamantanecarboxylic
acid
(3c),
cyclo-
9
acid (3a) and n-pentanoic acid (3m) did not proceed at all. These
results show that this organophotoredox catalysis can be
applied to the oxidation of carboxylic acids, but the scope of the
reaction is rather limited compared to the reaction of
organoborates.
Notes and references
1
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4,6,7
On the basis of our study and previous reports,
a plausible
reaction scheme through a redox-neutral process is illustrated
in Scheme 3. First, visible light irradiation (blue LEDs or
sunlight) causes excitation of the organocatalyst 1d into a
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+
photoinduced electron-transfer state, Acrc–Mesc , which serves
+
as a strong oxidant, through the excited state, *[Acr –Mes]. A
carbon-centered radical is generated via the 1e-oxidation of
+
organoborate 2 or carboxylic acid 3 by Acrc–Mesc accompa-
2
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7
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21300 | RSC Adv., 2015, 5, 21297–21300
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