Solar-driven incorporation of carbon dioxide into α-amino ketones

Article abstract of DOI:10.1002/anie.201206166

Power of the sun: Carbon dioxide was incorporated into α-amino ketones through a consecutive process consisting of a solar-energy-harvesting photocyclization reaction and a nucleophilic CO₂ incorporation reaction. The single-flask operation produced amino-substituted cyclic carbonates, thereby presenting a simple model of the chemical utilization of solar energy for CO₂ incorporation. R=sulfonyl group. Copyright

Full text of DOI:10.1002/anie.201206166

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Angewandte
Communications
DOI: 10.1002/anie.201206166
Solar Energy Utilization
Solar-Driven Incorporation of Carbon Dioxide into a-Amino
Ketones**
Naoki Ishida, Yasuhiro Shimamoto, and Masahiro Murakami*
The recent manifestation of the potential risk inherent to
nuclear technologies has incited a strong demand for explo-
ration of innovative means to exploit energy from natural
sources, thus increasing the need for research on sustainable
energy in many scientific fields. Chemists can contribute by
developing chemical systems to utilize solar light, which is
undoubtedly the best source of sustainable energy available
on the planet.[1] Another imperative issue is the establishment
of carbon-neutral systems.[2] One synthetic approach to this
issue is the incorporation of CO2 into organic compounds as
a chemical feedstock.[3] High-energy compounds suit ener-
getically low CO2 as the reaction partner. Under these
circumstances, it presents a significant challenge to simulta-
neously tackle the two issues mentioned above. Thus, we tried
to develop a reaction that is promoted by solar light and
utilizes CO2 as a chemical feedstock (Figure 1).
structed through the insertion of a photo-excited carbonyl
group into the carbon–hydrogen bond of a pendant methyl
group on a nitrogen atom. In a formal sense, the intrinsically
inert carbon–hydrogen bond was cleaved[7] and added across
the carbon–oxygen double bond in a 4-exo-trig mode.[8]
Although the reaction was originally reported to require
irradiation with a mercury lamp, which used electricity,[5b] we
discovered that natural solar light successfully effected this
energetically uphill reaction; a-amino ketone 1a cyclized at
a reasonable rate upon exposure to solar light (Scheme 1).
Scheme 1. Solar-light-promoted cyclization of 1a. On the right, the
solution in the reaction vessel made of ordinary Pyrex glass is shown.
Ts =toluenesulfonyl.
Furthermore, ordinary Pyrex glass, not quartz was suitable for
the reaction vessel; light of wavelengths below 400 nm was
required for the photoreaction, and Pyrex glass transmitted
a sufficient amount of operative light even on a cloudy day.
Thus, simply setting a Pyrex tube containing a solution of 1a
in N,N-dimethylacetamide (DMA) on a balcony or rooftop
brought about the cyclization. The conversion was dependent
upon the amount of solar radiation. We defined it as “sunny”
when an hourly solar radiation over 0.4 kWmÀ2 was observed,
and as “cloudy” when it ranged from 0.1 to 0.4 kWmÀ2.
Typically, the total amount of solar radiation amounted to
5.5 kWhmÀ2 in eight hours on a sunny day to cause full
conversion of 1a on a 0.1 mmol scale (0.10 mmolLÀ1; Fig-
ure 2A). The reaction mixture was subsequently applied to
conventional column chromatography on silica gel and
analytically pure azetidinol 2a was isolated in 91% yield. In
contrast, an analogous experiment on a cloudy day produced
2a in 54% yield after eight hours (Figure 2B), when the total
amount of solar radiation reached 0.9 kWhmÀ2. Thus, it
turned out to be possible to transfer 1a into the product[9] by
using solar energy even on a cloudy day.
Figure 1. Solar-driven CO2 incorporation.
We report herein a solar-driven process that incorporates
CO2 into a-methylamino ketones. The resulting amino-
substituted cyclic carbonates are expected to be useful
building blocks of pharmaceuticals and fuel additives.[4] Our
attention was initially directed to a photochemical cyclization
reaction observed with a-methylamino ketones.[5] Of note was
that this photoreaction proceeded in an energetically uphill
direction. A high-energy four-membered ring[6] was con-
[*] Dr. N. Ishida, Y. Shimamoto, Prof. Dr. M. Murakami
Department of Synthetic Chemistry and Biological Chemistry
Kyoto University
Katsura, Kyoto 615-8510 (Japan)
E-mail: murakami@sbchem.kyoto-u.ac.jp
Homepage: http://www.sbchem.kyoto-u.ac.jp/murakami-lab/
We next attempted to incorporate CO2 into azetidinol 2a,
in which energy has been stored, by reacting it with CO2, and
a remarkably simple way was found; CO2 was successfully
incorporated upon exposure of a solution of 2a in DMA to
gaseous CO2 in the presence of a base. For example, stirring
a heterogeneous mixture of 2a and Cs2CO3 (4.0 equiv) in
DMA under an atmosphere of CO2 (1 atm) at 608C for ten
hours led to the quantitative production of the cyclic
[**] This work was supported in part by a Grant-in-Aid for Scientific
Research on Innovative Areas “Molecular Activation Directed
toward Straightforward Synthesis” and Grant-in-Aid for Scientific
Research (B) from MEXT and The Asahi Glass Foundation.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206166.
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Angewandte
Chemie
quently adds to CO2 to afford carbonate anion B. The
following nucleophilic attack of the anionic oxygen onto the
2-carbon atom prompts displacement of the tosylamide anion
through a 5-exo-tet process.[8] Thus, the four-membered ring is
opened, thereby releasing the energy originating from sun.
Finally, protonation of C affords 3a.
For comparison, pyrrolidinol 4 was subjected
to the identical reaction conditions. Unlike the
four-membered counterpart 2a, the five-mem-
bered compound 4 failed to react and was
recovered unchanged. The contrasting results
observed with 2a and 4 lend support to the
explanation that the major driving force for the
CO2-capturing reaction is the release of the energy
stored in the form of the strained four-membered ring.
The experimental procedures for both the photochemical
cyclization reaction and the CO2-capturing reaction are so
simple that it is possible to carry them out in a single flask
(Scheme 4). Initially, a DMA solution of 1a in an atmosphere
of CO2 (1 atm) was irradiated with solar light outside. After
completion of the photoreaction, Cs2CO3 was simply added to
the reaction mixture, which was then heated at 608C for ten
hours in a fume hood. Isolation by chromatography afforded
the analytically pure cyclic carbonate 3a in 83% yield based
on 1a.
Figure 2. Solar-light-promoted cyclization of 1a. The reactions were
*
conducted on a 0.1 mmol scale. A) Residual ratio of 1a ( ) and yield
~
~
of 2a ( ) during solar radiation. B) Yield of 2a over time on sunny (
)
&
and cloudy ( ) days.
carbonate 3a, which was isolated in 93% yield after
purification by chromatography (Scheme 2). Organic bases,
such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), also
afforded 3a, albeit in a lower yield (56%).
Scheme 4. Solar-driven CO2 incorporation into a-amino ketone 1a.
The broad generality of this consecutive process was
verified by application to various acetophenone derivatives
(Scheme 5). Both an electron-donating methoxy group and an
electron-withdrawing trifluoromethyl group were allowed at
the para-position of the benzoyl group, and the corresponding
products 3b and 3c were isolated in reasonable total yields.
The presence of possibly photoactive bromo and chloro
groups[10] had essentially no effect on the reaction (3d and
3e). Sulfonyl groups other than a p-toluenesulfonyl group
were also suitable as the substituent on the nitrogen atom
(3 f–h).
Scheme 2. CO2 incorporation.
The formation of 3a is explained by assuming the pathway
depicted in Scheme 3.[4] Azetidinol 2a is initially deprotonat-
ed by Cs2CO3 to produce alkoxide anion A, which subse-
In summary, we have developed the unique solar-driven
transformation of a-amino ketones. CO2 is incorporated to
afford amino-substituted cyclic carbonates, which are poten-
tially useful ingredients in industry.
Although photosynthesis is a complex assembly of
a number of elementary reactions, it can be divided into
two major reactions; the light reaction and the dark reaction.
In the former reaction, solar energy is captured to promote an
energetically uphill process with production of high-energy
molecules (adenosine triphosphate (ATP) and nicotinamide
adenine dinucleotide phosphate (NADPH)). The dark reac-
tion is an energetically downhill process that does not require
light. ATP and NADPH, which have chemically stored solar
Scheme 3. Plausible mechanism.
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Angewandte
Communications
(m, 4H), 7.34–7.43 (m, 3H), 7.69–7.73 ppm (m, 2H); 13C NMR: d =
21.5, 50.3, 72.4, 85.0, 124.2, 126.9, 129.2, 129.9, 136.5, 138.1, 143.9,
154.2 ppm; HRMS (ESI+): Calcd for C17H18NO5S, [M+H]+ 348.0900,
found m/z 348.0892. Elemental analysis calcd for C17H18NO5S: C,
58.78; H, 4.93; N, 4.03; O, 23.03; S, 9.23; found: C, 58.71; H, 4.96; N,
3.89; O, 22.84; S, 9.32; IR (ATR): 3261, 1773, 1435, 1319, 1045 cmÀ1
.
Received: August 1, 2012
Published online: November 6, 2012
À
Keywords: C H activation · carbon dioxide · photochemistry ·
photosynthesis · strained molecules
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Scheme 5. Scope of solar-driven CO2 incorporation into a-amino
ketone 1. Overall yields of isolated products are given. Reactions were
conducted on a 0.2 mmol scale with the following reagents and
conditions: 1 (0.20 mmol), DMA (1.0 mL), solar light, ambient temper-
ature, CO2 (1 atm); then Cs2CO3 (0.80 mmol), 608C. See the Support-
ing Information for full experimental details.
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present consecutive process does not involve CO2 reduction,
its mechanistic profile of energy resembles that of photosyn-
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Experimental Section
A typical procedure for the one-pot reaction of a-amino ketone 1a
with carbon dioxide: a-Amino ketone 1a (30.3 mg, 0.10 mmol) was
placed in a Pyrex tube. The vessel was subsequently filled with CO2
(1 atm) by vacuum–refill cycles (three times), and N,N-dimethylacet-
amide (1.0 mL) was added therein. The mixture was exposed to solar
light outside for eight hours. After completion of the photochemical
reaction, Cs2CO3 (130 mg, 0.40 mmol) was added to the reaction
mixture, which was then stirred at 608C in a fume hood. After ten
hours, the reaction mixture was treated with HCl aq. (2.0m) and the
aqueous layer was extracted with Et2O (three times). The combined
organic layer was washed with water (three times) and brine (once),
dried over MgSO4, and concentrated. The residue was purified by
preparative thin-layer chromatography on silica gel (hexane/ethyl
acetate = 3:1) to afford the analytically pure carbonate 3a (28.8 mg,
0.083 mmol, 83% yield). 1H NMR: d = 2.40 (s, 3H), 3.30 (dd, J = 14.4,
5.6 Hz, 1H), 3.40 (dd, J = 14.4, 8.8 Hz, 1H), 4.54 (d, J = 8.4 Hz, 1H),
5.08 (d, J = 8.4 Hz, 1H), 5.70 (dd, J = 8.4 Hz, 5.6 Hz, 1H), 7.26–7.32
À
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