Copper-catalyzed C-C bond-forming transformation of CO2 to alcohol oxidation level: Selective synthesis of homoallylic alcohols from allenes, CO2, and hydrosilanes

Article abstract of DOI:10.1039/c5cc03932k

A highly selective carbon-carbon bond-forming transformation of carbon dioxide (CO₂) to alcohol oxidation level has been disclosed. By employing a copper/bisphosphine catalyst system and hydrosilanes as mild and easy-to-handle reducing agents,

Full text of DOI:10.1039/c5cc03932k

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DOI: 10.1039/C5CC03932K
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Copper‐catalyzed C–C bond‐forming transformation of CO2 to the
alcohol oxidation level: selective synthesis of homoallylic alcohols
from allenes, CO2, and hydrosilanes
Received 00th January 20xx,
Accepted 00th January 20xx
Yosuke Tani, Kazunari Kuga, Tetsuaki Fujihara,* Jun Terao and Yasushi Tsuji*
DOI: 10.1039/x0xx00000x
www.rsc.org/
a. Hydrocarboxylation with CO2 to the Carboxylic Acid Oxidation Level
A highly selective carbon–carbon bond‐forming transformation of carbon
dioxide (CO2) to the alcohol oxidation level has been disclosed. By
employing a copper/bisphosphine catalyst system and hydrosilanes as the
mild and easy‐to‐handle reducing agents, various allenes react with CO2 to
give homoallylic alcohols regioselectively. Esters and other reducible
functionalities on the allenes remain intact during the reaction, while CO2
is reduced to the alcohol oxidation level.
O
OH
Pd, Ni or Cu cat.
C
C
ZnEt2, AlEt3
or HSiR3
C
C
CO2
+
+
H
(1 atm)
C
b. CO2 to the Aldehyde Oxidation Level
O
Ar
C
Pd cat.
CO2
(10 atm)
+
+
Si O
Ar
I
n
H
H
Carbon dioxide (CO2) is at the highest oxidation level of carbon.
Owing to its abundance and non‐toxic, stable, and inert nature,
the chemical transformation of CO2 to lower oxidation levels
(i.e., the carboxylic acid, aldehyde, and alcohol oxidation
levels)1 has been one of the central challenges in chemistry.2 In
particular, C–C bond‐forming reactions using CO2 have
afforded various value‐added organic compounds.3 However,
the products obtained from these reactions have been limited
to carboxylic acids or their derivatives, i.e., at the carboxylic
acid oxidation level.4,5 Even in the presence of stoichiometric
amounts of reactive reducing agents such as ZnEt2,5b–5d
AlEt3,5d,e and hydrosilanes,5a only carboxylic acids were
provided through hydrocarboxylation reactions of alkynes,5a,5b
styrenes,5c allenes,5d and 1,3‐dienes5e (Scheme 1a). Quite
recently, CO2 was converted to the aldehyde oxidation level;
benzaldehyde derivatives were afforded by the Pd‐catalyzed
reaction of aryl iodides, CO2 (10 atm), and a hydrosilane
(Scheme 1b).6
c. CO2 to the Alcohol Oxidation Level with C–C Bond Formation (This Work)
H
OH
R2
H
C
Cu cat.
H–Si(OMe)2Me
CO2
(1 atm)
+
+
C
R2
R1
•
R3
R3
R1
H
1
2
Scheme 1 C–C bond‐forming transformation of CO2 to different oxidation levels.
To the best of our knowledge, this is the first example of the
transformation of CO2 to the alcohol oxidation level with
concomitant C–C bond formation,12 Importantly, CO2 is
reduced to the alcohol oxidation level chemoselectively, with
esters or other reducible functionalities on the allenes being
intact. Previously, a formal addition of hydroxymethyl (CH2OH)
group onto allenes was reported by Krische, who used
methanol instead of CO2 with hydrosilanes in the presence of
an Ir catalyst.13 However, there has been no C–C bond‐forming
synthesis of alcohols directly from the CO2 oxidation level.
As for transformation of CO2 to the alcohol oxidation level,
its catalytic conversion to methanol has been intensively
studied in the presence of various reducing agents.7 However,
without a C–C bond‐forming event, the product is limited to
First, the reaction of 1‐methyl‐1‐(2‐phenethyl)allene (1a
,
0.25 mmol) with CO2 (1 atm) and a hydrosilane was examined
in the presence of a copper catalyst (Table 1). We used 1.5
mmol of hydrosilane, which corresponds to 2.0 equiv for the
required amount to provide the homoallylic alcohol 2a since
three H atoms were incorporated into the product. Initially,
various ligands were evaluated employing HSi(OMe)2Me as the
hydrosilane. Xy‐Xantphos14 was identified as the best ligand,
affording 2a in 99% GC yield and in 84% isolated yield after
column chromatography (entry 1). In stark contrast to
methanol. Herein, we report the selective synthesis of
8,9
homoallylic alcohols (
2
) from CO2 with allenes (
1
)
by
employing a copper catalyst and a hydrosilane10,11 (Scheme 1c).
Department of Energy and Hydrocarbon Chemistry, Graduate School of
Engineering, Kyoto University, Kyoto 615‐8510, Japan. E‐mail: ytsuji@scl.kyoto‐
u.ac.jp; tfuji@scl.kyoto‐u.ac.jp, FAX: +81‐75‐383‐2514
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Table 1 Effect of catalytic system on the synthesis of 2aa
Table 2 Synthesis of homoallylic alcohols (2) from 1a
Entry
Cu cat.
H–Si
Yield (%)b
1
2d
3
4
5
6
7
8
9
Xy-Xantphos / CuCl
Xy-Xantphos / CuCl
Xantphos / CuCl
rac-BINAP / CuCl
dppp / CuCl
HSi(OMe)2Me
HSi(OMe)2Me
HSi(OMe)2Me
HSi(OMe)2Me
HSi(OMe)2Me
HSi(OMe)2Me
HSi(OMe)2Me
HSi(OEt)3
99 (84)c
96
92
85
73
IPrCuCl
62
ClIPrCuCl
67
Xy-Xantphos / CuCl
Xy-Xantphos / CuCl
83
PMHS
35
a
Conditions: 1a (0.25 mmol), Cu cat. (3 mol %), KOtBu (5 mol %), a hydrosilane
(1.5 mmol), in THF (0.50 mL) under CO2 (1 atm, closed) at 70 °C for 18 h. b Yield
was determined by GC analysis using an internal standard. Isolated yield is
shown in parenthesis. d For 20 min.
c
reported hydrocarboxylation reactions,5 no product at the
carboxylic acid oxidation level was detected under the present
conditions. Present reaction was fast, reaching completion
within 20 min (entry 2). Other bidentate phosphine ligands
such as Xantphos,14 BINAP14 and dppp14 gave 2a in 92%, 85%
and 73% yields, respectively (entries 3–5). Employing an N‐
heterocyclic carbene (NHC) ligand IPr14 or ClIPr,14 the yield was
decreased (entries 6 and 7). In a survey of hydrosilanes using
Xy‐Xantphos as the ligand, HSi(OEt)3 afforded 2a in slightly
lower yield (entry 8), whereas PMHS14 was ineffective possibly
owing to its large steric bulk (entry 9).
With these optimized conditions in hand, various allenes
were employed in the reaction (Table 2). The corresponding
homoallylic alcohols (2b
and the resulting terminal olefin moieties of 2b
preserved without undergoing hydrosilylation by the
remaining hydrosilane. 1,1‐Disubstituted allenes 1b 1d
provided 2b 2d in good to high isolated yields (entries 1–3). It
is noteworthy that various ester functionalities on the allenes
1e 1h) remained intact without any indication of reduction to
–
2n) were obtained regioselectively,
–
2m were
(
–
)
–
(
–
the corresponding diols (judging from GCMS and 1H NMR
analyses), whereas CO2 was reductively incorporated into the
homoallylic alcohol moieties. Thus, homoallylic alcohols
bearing a primary alkyl acetate (2e, entry 4), primary alkyl
isobutyrate (2f, entry 5), methyl benzoate moiety (2g, entry 6),
a
Conditions: 1 (0.25 mmol), CuCl (3 mol %), Xy‐Xantphos (3 mol %), KOtBu (5
mol %), HSi(OMe)2Me (2.0 mmol), in THF (0.50 mL) under CO2 (1 atm, closed) at
70 °C for 18 h. b Isolated yield of 2. c HSi(OMe)2Me (1.5 mmol) was used. d By
e
modified workup procedure.15 DTBM‐Xantphos14 (3 mol %) was used as the
ligand. f dppBz14 (3 mol %) was used as the ligand.
2 | J. Name., 2012, 00, 1‐3
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further reduction (step 6), and protic workup VoiefwGArticaleffOonrldinse
DOI: 10.1039/C5CC03932K
carboxylic acid 3.
Scheme 2 Cu‐catalyzed hydrocarboxylation of allenes.
and an ethyl ester functionality in the proximity of the reaction
site (2h, entry 7) were obtained in good to high isolated yields.
The product 2h seemed labile during the workup procedure,
but could be isolated as the γ‐lactone 2h' in higher yield (entry
8).15 The carbamate functionality in 1i was also retained in the
reaction (entry 9). Unfortunately, a substrate bearing an amide
functionality afforded the corresponding product in low
yield.15 Aryl bromide (1j) and alkene (1k) moieties were
tolerated to give 2j and 2k in 75% and 72% yields, respectively
(entries 10 and 11). Phenyl‐substituted (1l) and mono‐
substituted (1m) allenes gave the corresponding alcohols (2l
and 2m) in low yields though substrates were fully consumed
(entries 12 and 13). A tri‐substituted allene (1n) also converted
to the product (2n) in moderate yield (entry 14).
Scheme 3 Possible reaction mechanisms.
In conclusion, we have developed a selective C–C bond‐
forming transformation of CO2 to the alcohol oxidation level
for the first time. Allenes react with CO2 and a mild and easy‐
to‐handle hydrosilane to afford homoallylic alcohols
regioselectively. A wide range of esters on the allenes remains
unchanged during the reaction although CO2 is reduced to the
alcohol oxidation level, representing the distinctive
chemoselectivity of the transformation. The oxidation level of
the products can be switched to give β,γ‐unsaturated
carboxylic acids as the products by simply using the
stoichiometric amounts of base with a tuned catalyst system.
Further experiments elucidating the reaction mechanisms and
developing enantioselective reactions are now on going.
We found that by increasing the amount of the base from 5
Cl
mol % to 1.0 equiv with IPr as the ligand, the product could
be completely switched from
2 to β,γ‐unsaturated carboxylic
acid
3
(Scheme 2).15 The hydrocarboxylation of allenes was
previously reported by Iwasawa and Takaya employing a Pd
catalyst and the highly reactive and pyrophoric AlEt3 as the
reducing agents.5d In the present work, we realized the
hydrocarboxylation with a Cu catalyst utilizing a mild and easy‐
to‐handle hydrosilane as the reducing agent. The product 3a
was isolated in 75% yield without the formation of 2a (Scheme
2). Similarly, other 1,1‐disubstituted allenes (1b
well as a mono‐substituted allene 1m, selectively provided the
corresponding acids (3b 3c 3o, and 3m) in moderate to good
yields. The use of a stoichiometric amount of base is a key for
the selective formation of . Thus, products at two different
oxidation levels ( and ) were selectively prepared by simply
, 1c and 1o), as
,
,
This work was supported by Grant‐in‐Aid for Scientific
Research (A) from MEXT, Japan. Y.Tani is grateful to a JSPS
Research Fellow. T.F. acknowledges a financial support from a
Grant‐in‐Aid for Young Scientists (A) (No. 25708017) from JSPS.
3
2
3
adjusting the equivalence of base in the Cu‐catalyzed reactions.
Possible catalytic cycles are shown in Scheme 3. Copper
hydride
a base and a hydrosilane (step 0).5a,16 Addition of
terminal double bond of allene with the Cu atom at the less
hindered site generates allylcopper intermediate (step 1),
which reacts with CO2 regioselectively at the position via six‐
membered transition state to give copper carboxylate (step
2).4a For the synthesis of homoallylic alcohols (
), carboxylate
could be further reduced15 by the hydrosilane to copper
alkoxide via the silyl copper ketal derivative (steps 3 and 4).
Finally,
silyl ether
A
is generated by the reaction of copper chloride with
A
across a
Notes and references
1
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B
γ-
C
2
2
C
E
D
σ‐bond metathesis of
E
A
with the hydrosilane provides
as the active catalyst species
,
F
and regenerates
3
(step 5). Protonation (workup) of
F affords homoallylic alcohol
2
as the alcohol product. In the hydrocarboxylation reaction
(Scheme 2), the base traps copper carboxylate
C to prevent
Sci., 2013,
4, 3395–3403; (c) K. Huang, C.‐L. Sun and Z.‐J. Shi,
This journal is © The Royal Society of Chemistry 20xx
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3
=
4,5‐bis(bis(3,5‐
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dimethylphenyl)phosphino)‐9,9‐dimethylxanthene; Xantphos
= 4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene; BINAP =
2,2'‐bis(diphenylphosphino)‐1,1'‐binaphthyl; dppp
=
1,3‐
bis(diphenylphosphino)propane;
IPr
=
1,3‐bis(2,6‐
diisopropylphenyl)imidazole‐2‐ylidene; ClIPr = 1,3‐bis(2,6‐
diisopropylphenyl)‐4,5‐dichloroimidazole‐2‐ylidene; PMHS =
poly(methylhydrosiloxane);
bis(bis(3,5‐di‐tert‐butyl‐4‐methoxyphenyl)phosphino)‐9,9‐
dimethylxanthene; dppBz 1,2‐
bis(diphenhylphosphino)benzene.
DTBM‐Xantphos
=
4,5‐
=
5
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