Formation of carbanions from carboxylate ions bearing electron-withdrawing groups via photoinduced decarboxylation: Addition of generated carbanions to benzaldehyde

Article abstract of DOI:10.1071/CH15115

The photoinduced decarboxylation of carboxylate ions bearing electron-withdrawing groups using biphenyl and 1,4-dicyanonaphthalene leads to the efficient generation of carbanions under mild conditions. The efficiency of the carbanion generation is strongl

Full text of DOI:10.1071/CH15115

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem.
Communication
http://dx.doi.org/10.1071/CH15115
Formation of Carbanions from Carboxylate Ions Bearing
Electron-Withdrawing Groups via Photoinduced
Decarboxylation: Addition of Generated
Carbanions to Benzaldehyde
A
A
A
Yuta Kumagai, Takashi Naoe, Keisuke Nishikawa,
A
A
A B
,
Kazuyuki Osaka, Toshio Morita, and Yasuharu Yoshimi
A
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering,
University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan.
B
Corresponding author. Email: yyoshimi@u-fukui.ac.jp
The photoinduced decarboxylation of carboxylate ions bearing electron-withdrawing groups using biphenyl and
1,4-dicyanonaphthalene leads to the efficient generation of carbanions under mild conditions. The efficiency of the
carbanion generation is strongly dependent on the single-electron transfer from the photogenerated radical anion of
the electron-acceptor to the radical. In particular, the cyanomethyl anion formed using this photochemical method can be
added to benzaldehydes to give the corresponding adducts.
Manuscript received: 9 March 2015.
Manuscript accepted: 6 May 2015.
Published online: 25 May 2015.
Carbanions are one of the most common and useful reactive
intermediates in organic synthesis.[1] For example, carbanions
bearing electron-withdrawing groups, such as diethyl mal-
onate ion, are frequently used as valuable components for the
preparation of organic molecules.[2] However, the generation
of these carbanions typically requires the use of a strong base
and heat. Thus, the ability to generate carbanions under mild
conditions is desired.
to date. In this study, we developed a novel, simple process for
the generation of carbanions from carboxylate ions bearing
electron-withdrawing groups via photoinduced decarboxyl-
ation, and addition of the formed cyanomethyl anion to
benzaldehydes.
Initial exploratory studies were carried out regarding the
photoinduced decarboxylative addition of tetra-n-butylammo-
nium cyanoacetate (4a) with benzaldehydes 5 in dry CH3CN
using molecular sieves (MS4A) because water and the acidic
proton of the carboxy group quenched the photogenerated
carbanion (Table 1). Irradiation of a dry CH3CN solution
(30 mL) containing 4a (5 mM, 48.9 mg), benzaldehyde 5a
(5 mM, 15.9 mg), Phen (20 mM, 106 mg) as an arene, DCB
(20 mM, 76.8 mg) as an electron-acceptor, and MS4A (6 g) in
two Pyrex vessels (.280 nm, 15 ꢀ 180 mm) under an argon
atmosphere using a 100 W high-pressure mercury lamp for 10 h
at room temperature led to formation of adduct 6a in 46 % yield
(Entry 1). In addition to Phen and DCB, biphenyl (BP) and
1,4-dicyanonaphthalene (DCN) served as a viable arene and
electron-acceptor, respectively, in the photoreaction, and sig-
nificantly increased the yield of 6a (88 %) (Entry 2).[6] As we
previously reported,[3f,4h] the SET from the radical anion of
DCN to radical intermediate 1 occurs more smoothly than the
SET from the radical anion of DCB to 1 because of the relatively
long lifetime of the radical anion of DCN. A similar trend in the
SET from the radical anion of DCN to the generated cyano-
methyl radical was observed, and led to an increase in the yield
of 6a. In addition, lower concentrations of BP and DCN led to a
decrease in the yield of 6a, likely due to the lower concentration
of the radical anion of DCN (Entries 3 and 4). These results
indicated that the efficiency of carbanion generation was
Recently, we reported decarboxylative radical reactions[3]
of aliphatic carboxylic acids using phenanthrene (Phen) and
1,4-dicyanobenzene (DCB) via photoinduced electron transfer
(PET) (Scheme 1).[4] The process ispromotedby a single-electron
transfer (SET) from the carboxylate ion to the radical cation of
Phen via PET, and leads to the formation of carboxy radicals that
rapidly lose CO2 to produce alkyl radicals 1. The formed alkyl
radicals react with a variety of reagents, such as oxime ethers,
thiols, and the radical anion of DCB, to provide the respective
addition,[4e] reduction,[4a,f] and substitution[4b] products in high
yields. In particular, the addition of generated alkyl radical 1 to
an electron-deficient alkene, such as acrylonitrile, proceeds
efficiently to form electron-deficient radical 2.[4c,d,g-i] Subse-
quently, the SET from the radical anion of DCB to 2 generates
carbanion 3, which upon protonation, gives the corresponding
adduct in a high yield. The earlier observation regarding the SET
from the radical anion of DCB to electron-deficient radical 2
encouraged us to investigate the photoinduced decarboxylation
of carboxylate ions bearing electron-withdrawing groups to
form carbanions. Although a laser flash photolysis study of
decarboxylative carbanion formation from benzylic carboxylate
ions using dicyanoarenes was previously reported,[5] synthetic
methodologies based on these reactions have not been reported
Journal compilation Ó CSIRO 2015
www.publish.csiro.au/journals/ajc

B
Y. Kumagai et al.
∗
Phen
DCB
hν
Phen;
CN
O
Phen
DCB; NC
ꢁ
ꢀ
DCB
CN
Phen
ꢀCO2
O
R
O
ꢀHꢁ
Hꢁ
CN
CN
Oꢀ
CN
R
O
R
R
R
R
OH
R
ꢀ
1
2
3
Scheme 1. Decarboxylative radical addition of aliphatic carboxylic acids to acrylonitrile by the photogenerated radical
cation of Phen.
Table 1. Photoinduced decarboxylative addition of 4a with 5A
hν
Arene
O
OH
Electron-Acceptor
CN
CO2ꢀTBAꢁ
NC
ꢁ
H
MS4A
Dry CH3CN
4a (5 mM)
R
R
5
(5 mM)
6
5a: R ꢂ H
5b: R ꢂ Cl
5c: R ꢂ Br
Nꢁ
TBAꢁ
ꢂ
5d: R ꢂ CH3
Entry
Arene
Electron-Acceptor
5
Irradiation. time [h]
Yield of 6 [%]B
1
2
3
4
5
6
7
Phen (20 mM)
BP (20 mM)
BP (10 mM)
BP (5 mM)
DCB (20 mM)
DCN (20 mM)
DCN (10 mM)
DCN (5 mM)
DCN (20 mM)
DCN (20 mM)
DCN (20 mM)
5a
5a
5a
5a
5b
5c
5d
10
10
10
10
6
46
88
63
36
67
39
16
BP (20 mM)
BP (20 mM)
BP (20 mM)
8
10
ADry acetonitrile solutions containing 4a (5 mM), 5 (5 mM), arene, electron-acceptor, and MS4A under an Ar atmosphere were irradiated at room temperature.
TBAþ ¼ tetra-n-butylammonium cation.
BIsolated yield.
strongly dependent on the SET from the radical anion of the
electron-acceptor to the generated radical. When 4-substituted
benzaldehydes 5b–d were subjected to the photoreaction, cor-
responding adducts 6b–d were obtained in moderate or low
yields because products 6b–d decomposed under the reaction
conditions employed (Entries 5–7). In fact, the photoreaction
of 6c (5 mM) for 3 h under the aforementioned conditions (BP:
20 mM; DCN: 20 mM) led to the recovery of 6c in only 58 %
yield. In contrast, similar photoreactions with 4-methoxy or
4-cyano-benzaldehydes did not give adduct 6, possibly because
methoxy and cyano-benzaldehydes disturb the PET process.
Subsequently, the effects of substituents on the carboxylate
ion on the photoreaction were explored (Table 2). The photoin-
duced decarboxylative addition of monoethyl malonate ion 4b,
alkylcyanoacetate ions 4c and 4d, and monoethyl aminomalo-
nate ion 4e to 5a did not take place, but gave 6a. In the case of 4e,
the decarboxylative reduction product 8 was also obtained
(76 %).[7] These results indicated that the carbanions generated
from 4b–e via photoinduced decarboxylation did not attack the
carbonyl group of aldehyde 5a, but abstracted a proton from
the solvent, acetonitrile, to form a cyanomethyl anion, which led
to the formation of 6a because the anions generated from 4b–e
are more basic than the cyanomethyl anion.[8] In order to
overcome this problem, other solvents were examined. Instead
of acetonitrile, propionitrile, isobutyronitrile, cyanobenzene,
and a 1 : 1 mixture of cyanobenzene and THF were evaluated;
however, they were not suitable for this photoreaction because
the low polarity of the solvents retarded the formation of the
radical cation of BP via PET to recover the starting materials.[4d]
Improvement of the reaction conditions to facilitate the use of
other carboxylate ions such as 4b–e in this photoreaction is in
progress in our laboratory.
Based on the obtained results, a plausible mechanism for the
photoreaction is shown in Scheme 2. First, absorption of light
(.280 nm) by electron-acceptor DCN produces the excited state
of DCN (DCN*). PET between BP and DCN* generates the
radical cation of BP and the radical anion of DCN. Subsequent-
ly, the SET from the carboxylate ion bearing an electron-
withdrawing group to the radical cation of BP leads to the
formation of an electron-deficient radical via decarboxylation.
The third electron transfer from the radical anion of DCN to
the electron-deficient radical generates the carbanion. With the
cyanomethyl anion (R ¼ CN), the formed anion reacts with
benzaldehydes 5 to give adduct 6. In contrast, other carbanions
derived from 4b–e abstract a proton from acetonitrile to form the
cyanomethyl anion, which yields 6a due to the higher basicity.
In conclusion, we have described the development of a new
and mild method for the generation of carbanion intermediates
via the PET-promoted decarboxylation of carboxylate ions
bearing electron-withdrawing groups. The efficiency of the

Formation of Carbanions via Photoinduced Decarboxylation
C
Table 2. Substituent effect of carboxylate ions 4b–e on photoreactionA
hν, 10 h
BP (20 mM)
DCN (20 mM)
OH
R2
R2
CO2ꢀTBAꢁ
ꢁ
R1
5a (5 mM)
ꢁ 6a
MS4A
Dry CH3CN
R1
4b–e (5 mM)
7
ꢁ
BocHN
CO2CH2CH3
8
Entry
4
R1
R2
Yield of 7 [%]B
Yield of 6a [%]B
1
2
3
4
4b
4c
4d
4e
CO2CH2CH3
CN
H
0
39
24
10
42
CH2CH3
CH2CH2CH3
NHBocC
0
CN
0
CO2CH2CH3
0 (76)D
ADry acetonitrile solutions containing 4 (5 mM), 5a (5 mM), BP (20 mM), DCN (20 mM), and 4AMS under an Ar atmosphere were irradiated for 10 h at room
temperature.
BIsolated yield.
CBoc ¼ t-butoxycarbonyl.
DYield of 8.
∗
DCN
hν
BP
DCN
O
ꢁ
BP
ꢀ
H
DCN
ꢁ
R
5
ꢀCO2
CO2ꢀ
CH2
ꢀ
6
R
R
CO2
R
CH2
R
R ꢂ CN
ꢁ
5a
ꢁ CH3CN
R ꢂ CN
NC CH2ꢀ
6a
Scheme 2. Plausible mechanism for the photoinduced carbanion generation from carboxylate ion using BP
and DCN.
carbanion generation in this photoreaction is strongly influenced
by the SET from the radical anion of the electron-acceptor to the
electron-deficient radical. Although the addition to benzalde-
hydes is only achieved with the use of cyanoacetate ion 4a, we
found that a variety of carbanions from 4b–e can be generated
using this method. Further investigations regarding the applica-
bility of this methodology in reactions involving the photogen-
erated carbanions are underway in our laboratory.
[3] (a) D. Budac, P. Wan, J. Photochem. Photobiol., A 1992, 67, 135.
Recently, similar photoinduced decarboxylative radical reactions
of carboxylic acids using phthalimide,[3b-d] cyanoarene,[3e,f] TiO2,[3g,h]
Ir,[3i,j] and Fukuzumi[3k] catalyst have been widely explored.
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Supplementary Material
1H and 13C NMR spectral data of 4, 6, and 8 are available on the
Journal’s website.
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