Silver-catalyzed enantioselective carbon dioxide incorporation into bispropargylic alcohols

Article abstract of DOI:10.1021/ja1007118

(Figure Presented) The combined catalyst system of silver acetate with a chiral Schiff base ligand achieved asymmetric carbon dioxide Incorporation into bispropargylic alcohols with desymmetrization to afford the corresponding cyclic carbonates with good-to-excellent enantiomeric excesses. Copyright

Full text of DOI:10.1021/ja1007118

Published on Web 03/03/2010
Silver-Catalyzed Enantioselective Carbon Dioxide Incorporation into
Bispropargylic Alcohols
Shunsuke Yoshida, Kosuke Fukui, Satoshi Kikuchi, and Tohru Yamada*
Department of Chemistry, Keio UniVersity, Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
Received January 26, 2010; E-mail: yamada@chem.keio.ac.jp
Table 1. Examination of Various Reaction Conditions
Since carbon dioxide is expected to be an ubiquitous and
abundant C1 feedstock from industrial wastes, much effort has been
made to develop effective processes for various chemicals from
carbon dioxide.1 Because of its thermodynamic stability, however,
high pressure and/or high temperature are required to react carbon
dioxide and transform it into various chemicals. Especially, for the
enantioselective synthesis using carbon dioxide, mild reaction
conditions and efficient catalytic systems should be applied, though
few examples of enantioselective CO2 fixation reactions have been
proposed. It has been reported that the enantioselective CO2
incorporation into epoxides proceeded with kinetic resolution to
afford the corresponding optically active cyclic carbonates or
polycarbonates along with unreacted epoxides using optically active
salen-cobalt complexes2 or ketoiminatocobalt complexes3 as cata-
lysts. Another example is the asymmetric alternating copolymer-
ization of the meso-epoxide, such as cyclohexeneoxide, and carbon
dioxide using optically active zinc-based complex catalysts.4 It was
also reported that a nickel-catalyzed enantioselective carboxylative
cyclization of bis-1,3-dienes proceeded to afford the corresponding
cyclic products with high selectivity using CO2 as a reactant.5
Although these catalyst systems exhibited a high activity and
enantioselectivity, the enantioselective CO2 incorporation still
remains a challenging area.
AgOAc
(mol %)
L*
(mol %)
temp
(°C)
time
(h)
yieldb
(%)
eec
(%)
entrya
L*
1d
2
3
4
5
6
7
8
3a
10
10
5
5
2
3
5
3
12
12
1
1
1
1
2
1
r.t.
r.t.
20
5
5
5
2
5
96
92
90
91
92
97
90
98
40
70
75
83
88
90
89
92
20
96
22
22
12
48
3b
5
0
a Reaction conditions: The reaction was carried out in 1.5 mL of
CHCl3 with 0.25 mmol of substrate under 1.0 MPa of CO2 pressure .
b Isolated yield. c Enantiomeric excess was determined by HPLC
analysis using Daicel Chiralcel OD-H. d Carried out in CH2Cl2.
responding cyclic carbonates with good-to-excellent enantiomeric
excesses (Scheme 1).
Scheme 1. Enantioselective Chemical Incorporation of CO2 into
Bispropargylic Alcohols
Various optically active ligands were first examined, such as
ferrocene derivatives, BINAP, BOX, and Py-BOX. In the presence
of 10 mol % of silver acetate with the ligand, a model compound
1a was treated with a stoichiometric amount of DBU in CH2Cl2
under 1.0 MPa of CO2 pressure at room temperature. Although all
reactions proceeded to afford the corresponding cyclic carbonates,
the corresponding products were obtained without any enantiose-
lectivity. It was assumed that the DBU could strongly coordinate
the silver catalyst to block the coordination by the chiral ligand,
while (-)-sparteine was employed as a chiral ligand without DBU
to afford the corresponding cyclic product with 26% ee. Without
DBU, various chiral ligands were then screened and the product
was obtained with 40% ee by using the chiral Schiff base ligand
3a7 derived from 1,2-diaminocyclohexane and 2-pyridinecarboxy-
aldehyde (Table 1, entry 1). Various reaction conditions involving
the solvent, catalyst loading, and temperature were examined in
the presence of the ligand 3a (entries 2-4). When CHCl3 was
employed as the solvent, the selectivity was dramatically improved
to afford the cyclic product in 92% yield with 70% ee (entry 2). In
the presence of excess amounts of silver acetate with respect to
the chiral ligand, the enantiomeric excess was slightly improved
(entry 3) and the product was obtained with 83% ee at 5 °C (entry
4). It was eventually found that the chiral ketimine ligand 3b was
better than the ligand 3a for both reactivity and selectivity (entries
5-8). By using the ligand 3b, the bispropargylic alcohol 1a was
In a previous communication, we reported that the combined
use of a catalytic amount of silver salt and DBU efficiently catalyzed
the incorporation of CO2 under mild reaction conditions into a wide
range of propargylic alcohols to afford the corresponding cyclic
carbonates in high-to-excellent yields.6 In this reaction, it was
assumed that the silver catalysts effectively activated the C-C triple
bond as a π-Lewis acid on the side opposite the carbonate anion,
and then the intramolecular cyclization proceeded to selectively
afford the corresponding cyclic product with the (Z)-exo-alkene.
Based on these observations, the corresponding optically active
cyclic carbonates were obtained by using an optically active silver
catalyst, which is in rapid equilibrium between the two alkynes in
bispropargylic alcohols, and a nucleophilic attack selectively
proceeds to one of the C-C triple bonds, with asymmetric
desymmetrization. In this communication, we report that the
combined catalyst system of silver acetate with a chiral Schiff base
ligand achieved the asymmetric carbon dioxide incorporation into
bispropargylic alcohols with desymmetrization to afford the cor-
9
4072 J. AM. CHEM. SOC. 2010, 132, 4072–4073
10.1021/ja1007118  2010 American Chemical Society

C O M M U N I C A T I O N S
completely consumed to afford the corresponding product in
excellent yield with a high enantiomeric excess (entries 5-7). For
the reaction with over 2 equiv of AgOAc versus the chiral ligand
less reactive and the corresponding product 2f was obtained in 58%
yield with 82% ee (entry 6). Bispropargylic alcohols with a
substituted phenyl 1g-1j were also good substrates that afforded
the corresponding cyclic carbonates 2g-2j in excellent yields with
good-to-high enantiomeric excesses regardless of the electron-
donating and -withdrawing substituents (entries 7-10). The enan-
tiomer-enriched conjugated cyclic carbonate 2k was also obtained
in high yield with 80% ee (entry 11). The alkyl-substituted alkynes
with a protective group in the molecule 1l was converted into the
corresponding cyclic product 2l in excellent yield with a moderate
enantioselectivity (entry 12). As for the exo olefin structure, all
products were obtained as the sole isomer based on an NMR
spectroscopic analysis, and they were suggested to be the Z isomer
by NOE experiments. The optically active cyclic carbonate was
readily hydrolyzed in the presence of aqueous NaOH to afford the
corresponding R-hydroxyketone in high yield without any loss of
optical purity.8
1
3b, almost the same enantiomeric excess was observed. The H
NMR spectroscopic analysis suggested that the silver acetate and
chiral ketimine ligand 3b would form the corresponding 2:1
complex. After examination of the loading ratio of the ligand/silver,8
a combination of 3 mol % of silver acetate and 1 mol % of 3b was
found to be optimum to realize a high selectivity (entry 7). When
the reaction was carried out at 0 °C, the CO2-incorporated carbonate
was obtained at up to 92% ee. The absolute configuration of
carbonate 2a was determined by VCD measurement in combination
with the corresponding spectra predicted by DFT calculations.9 As
a result, it was found that (S)-2a was obtained in the reaction
catalyzed by the complex with (R,R)-3b.8
Table 2. Enantioselective Carbon Dioxide Incorporation into
Various Bispropargylic Alcohols
It is noted that enantioselective chemical CO2 incorporation into
various bispropargylic alcohols was achieved by the combined use
of a catalytic amount of silver acetate and chiral Schiff base ligand.
Further investigations involving the mechanical study and trans-
formation of the chiral cyclic carbonates are now under way.
Acknowledgment. This was supported by a Grant-in-Aid for
Science Research on Priority Areas “Advanced Molecular Trans-
formations of Carbon Resources” from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
entrya
R1
R2
time (h)
yield (%)b
ee (%)c
1d
2d
3e
Ph
Me
Et
2a
2b
2c
48
44
40
140
44
168
46
46
78
168
120
46
98
98
93
91
94
58
99
97
96
90
91
97
92
93
91
90
90
82
90
87
79
79
80
47
Supporting Information Available: Experimental procedure and
spectra data of the new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
CH2F
CH2CH2Ph 2d
i-Pr
t-Bu
CH2F
Me
4f
5e
2e
2f
2g
2h
2i
2j
2k
2l
6g
7d
8d
9e
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a Reaction conditions: The reaction was carried out in 1.5 mL of
CHCl3 with 0.25 mmol of substrate under 1.0 MPa of CO2 pressure.
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JA1007118
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J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010 4073