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A. Nomoto et al. / Tetrahedron Letters 51 (2010) 6580–6583
SmI2/Sm
SmI2/Sm
SmI2/Sm (1.5 mmol/0.5 mmol)
CO2 atmosphere
Path A
COOH
RX
R
R
SmI2
Br
CO2
hν, THF, rt, 1.5 h
O
O
92% (Isolated)
0.5 mmol
O
O
H+, H2O
SmI2
SmI2, CO2
R
OSmI2
R
OH
5-exo
Path B
O
O
A
O
C
SmI2
h
ν
Scheme 6.
CO2
SmI2(CO2)n
OSmI2
Scheme 3.
proceeded via the generation of the aryl radical, 5-exo cyclization,
and carboxylation with CO2.
anion species of CO2; (3) radical coupling of the alkyl radical spe-
cies with the radical anion species of CO2 and the following proton-
ation to produce carboxylic acids.
In summary, atmospheric CO2 is found to react with alkyl
halides efficiently using SmI2/Sm mixed system. Photoirradiation
is indispensable for this reaction and resides the potential to devel-
op various synthetic reactions by following natural photosynthetic
systems.
To get insight into the reaction pathway, we examined the reac-
tion of n-dodecyl iodide with CO2 (1 atm) in the presence of SmI2/
Sm in the dark, which provided n-dodecane (44%) without forma-
tion of the desired tridecanoic acid (Scheme 4). The formation of
n-dodecane can be explained by the protonation of n-dodecylsa-
marium diiodide (nC12H25SmI2) during aqueous workups. If Path
A is present, n-dodecylsamarium diiodide (nC12H25SmI2) may react
with CO2 to give tridecanoic acid. However, no formation of tri-
decanoic acid suggests that Path A is not a real pathway for this
reductive carbonylation.13 Although the reaction of Scheme 4 also
involves the generation of n-dodecyl radical as an intermediate, the
dark condition inhibits the generation of radical anion of CO2,
resulting in no formation of tridecanoic acid.
Acknowledgments
This research was supported by JST Research Seeds Quest Pro-
gram (Lower Carbon Society), from the Ministry of Education, Cul-
ture, Sports, Science and Technology, Japan.
This work is supported by Grant-in-Aid for Scientific Research
on Priority Areas (Area 444, No. 18037064) and Scientific Research
(B, 19350095), from the Ministry of Education, Culture, Sports, Sci-
ence and Technology, Japan.
To get some information about the rate for the carboxylation
process, we next examined the photoinduced reductive carboxyla-
tion of 5-hexenyl bromide with CO2 in the presence of SmI2/Sm,
which afforded 6-heptenoic acid in good yield without the forma-
tion of cyclic carboxylic acid (Scheme 5). Since the rate for 5-exo
cyclization of 5-hexenyl radical is reported to be 2.3 Â 105 sÀ1
(25 °C),14 the rate for the carboxylation of carbon radical is esti-
mated to be more than 107 sÀ1 (25 °C).
Since 5-exo cyclization of aryl radicals such as A is known to be
very fast process (k = 6.3 Â 109 sÀ1 (30 °C)),15 it is of great interest
to examine the reductive carbonylation of o-allyloxy-bromoben-
zene (Scheme 6). Upon photoirradiation, o-allyloxy-bromobenzene
reacted with CO2 in the presence of SmI2/Sm to afford the corre-
sponding cyclic carboxylic acid in excellent yield. The reaction
References and notes
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5. In a 20 mL of three-necked glass flask (Pyrex) equipped with a cooler, an N2
inlet and CO2 bubbling tube were placed under the atmosphere of nitrogen, Sm
powder (6 mmol, 0.902 g), 1,2-diiodoethane (2 mmol, 0.846 g), and THF
(10 mL). The resulting mixture was stirred magnetically for 1.5 h to prepare a
SmI2/Sm reagent, and then CO2 was introduced into the vessel via tube. Upon
irradiation with a Xe lamp (500 W), haloalkane (0.5 mmol) was added to the
mixture, and then the reaction was continued for another 6 h. The reaction
SmI2/Sm (3mmol/3 mmol)
CO2 bubbling
nC12H25I
1.5 mmol
nC12H26
44%
THF (20 mL), rt, 20 h, dark
nC12H25COOH
not formed:
Scheme 4.
SmI2/Sm (1.5mmol/0.5 mmol)
CO2 atmosphere
Br
COOH
hν, THF, rt, 3 h
0.5 mmol
75%
COOH
not detected:
Scheme 5.