Nickel-catalyzed carboxylation of organozinc reagents with CO2

Article abstract of DOI:10.1021/ol800764u

(Chemical Equation Presented) An efficient nickel catalyst system for the carboxylation of organozinc reagents with CO₂ under very mild conditions has been developed. The catalyst system complements the conventional methods and enables the dire

Full text of DOI:10.1021/ol800764u

ORGANIC
LETTERS
2008
Vol. 10, No. 13
2681-2683
Nickel-Catalyzed Carboxylation of
Organozinc Reagents with CO2
Hidenori Ochiai, Minsul Jang, Koji Hirano, Hideki Yorimitsu,* and
Koichiro Oshima*
Department of Material Chemistry, Graduate School of Engineering, Kyoto UniVersity,
Kyoto-daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
yori@orgrxn.mbox.media.kyoto-u.ac.jp; oshima@orgrxn.mbox.media.kyoto-u.ac.jp
Received April 3, 2008
ABSTRACT
An efficient nickel catalyst system for the carboxylation of organozinc reagents with CO2 under very mild conditions has been developed. The
catalyst system complements the conventional methods and enables the direct synthesis of various saturated carboxylic acid derivatives
from the corresponding alkylzinc reagents and CO2.
Fixation of carbon dioxide has received significant attention
from the viewpoint of organic synthesis as well as C1
chemistry that utilizes compounds containing only one carbon
since CO2 is a cheap and abundant source of carbon.1
Reaction of carbon nucleophiles with CO2 is one of the most
fundamental processes for CO2 fixation and is an attractive
way to synthesize carboxylic acids. Highly reactive orga-
nometallic reagents such as organolithiums and Grignard
reagents can react with the relatively unreactive CO2.
Organocopper and -manganese reagents are known to react
with CO2.2 The reactions mentioned above are restricted in
scope and generality, and the development of new methods
for carboxylation of much milder organometallic reagents
with CO2 is strongly desired. In 2006, Iwasawa succeeded
in rhodium-catalyzed carboxylation of aryl- and alkenylbo-
ronic esters with CO2.3 While the catalytic reaction provided
an efficient route to a variety of benzoic and cinnamic acid
derivatives, the corresponding alkylboronic esters could not
participate in the reaction. Catalytic synthesis of saturated
aliphatic carboxylic acids under mild conditions has still
remained a challenge. Herein, we report nickel-catalyzed
carboxylation of organozinc reagents with CO2. The catalyst
system allows alkylzinc reagents to serve as carbon nucleo-
philes toward CO2, providing an efficient synthesis of various
saturated carboxylic acid derivatives directly from CO2 and
the corresponding alkylmetals of mild reactivity.4
Treatment of a solution of hexylzinc iodide-lithium
chloride complex (1a) in THF5,6 with CO2 (1 atm, balloon)
in the presence of 5 mol % of Ni(acac)2 and 10 mol % of
P(c-C6H11)3 in THF at room temperature for 3 h afforded
heptanoic acid (2a) in 70% yield (Table 1, entry 1).7 The
procedure was so simple that the catalytic reaction was
practical. Interestingly, the addition of lithium chloride was
(1) Reviews: (a) Behr, A. Angew. Chem., Int. Ed. Engl. 1988, 27, 661–
678. (b) Braunstein, P.; Matt, D.; Nobel, D. Chem. ReV. 1988, 88, 747–
764. (c) Aresta, M.; Armor, J. N.; Barteau, M. A.; Beckman, E. J.; Bell,
A. T.; Bercaw, J. E.; Creutz, C.; Dinjus, E.; Dixon, D. A.; Domen, K.;
Dubois, D. L.; Eckert, J.; Fujita, E.; Gibson, D. H.; Goddard, W. A.;
Goodman, D. W.; Keller, J.; Kubas, G. J.; Kung, H. H.; Lyons, J. E.; Manzer,
L. E.; Marks, T. J.; Morokuma, K.; Nicholas, K. M.; Periana, R.; Que, L.;
Rostrup-Nielson, J.; Sachtler, W. M. H.; Schmidt, L. D.; Sen, A.; Somorjai,
G. A.; Stair, P. C.; Stults, B. R.; Tumas, W. Chem. ReV. 2001, 101, 953–
996.
(3) Ukai, K.; Aoki, M.; Takaya, J.; Iwasawa, N. J. Am. Chem. Soc. 2006,
128, 8706–8707.
(4) CO2 fixation through oxidative cyclization of CO2 and carbon-carbon
unsaturated molecules on Ni: (a) Mori, M.; Takimoto, M. In Modern
Organonickel Chemistry; Tamaru, Y., Ed.; Wiley-VCH: Weinheim, 2005;
Chapter 7, pp 205-223. (b) Mori, M. Eur. J. Org. Chem. 2007, 4981–
4993. (c) Louie, J.; Gibby, J. E.; Farnworth, M. V.; Tekavec, T. N. J. Am.
Chem. Soc. 2002, 124, 15188–15189. (d) Murakami, M.; Ishida, N.; Miura,
T. Chem. Commun. 2006, 643–645. (e) Murakami, M.; Ishida, N.; Miura,
T. Chem. Lett. 2007, 476–477. (f) Aoki, M.; Izumi, S.; Kaneko, M.; Ukai,
K.; Iwasawa, N. Org. Lett. 2007, 9, 1251–1253.
(2) Reactions of organocopper with CO2: (a) Normant, J. F.; Cahiez,
G.; Chuit, C.; Villieras, J. J. Organomet. Chem. 1973, 54, C53–C56.
Reactions of organomanganese with CO2: (b) Friour, G.; Cahiez, G.;
Alexakis, A.; Normant, J. F. Bull. Soc. Chim. Fr. II 1979, 515–517.
10.1021/ol800764u CCC: $40.75
Published on Web 06/04/2008
2008 American Chemical Society

Table 1. Optimization Studies of Nickel-Catalyzed
Table 2. Nickel-Catalyzed Carboxylation of Organozinc
Reagents 1 with CO2
a
Carboxylation of Hexylzinc Iodide-Lithium Chloride Complex
a
(1a) with CO2
entry
ligand
1H NMR yield of 2a (%)
1
P(c-C6H11
P(c-C6H11
P(t-Bu)3
P(n-Bu)3
DBU
)
)
70
5
3
2b
3
3
38
31
27
44
58
4
5
6c
7d
dtbpy
P(c-C6H11
)
3
a A mixture of Ni(acac)2 (0.025 mmol), ligand (0.050 mmol), and 1a
(0.70 M in THF solution, 0.71 mL, 0.50 mmol) was stirred in THF (5.0
mL) for 3 h under CO2 atmosphere (1 atm, balloon). b With LiCl-free
hexylzinc iodide instead of hexylzinc iodide-lithium chloride complex (1a).
c In the presence of 5 mol % of dtbpy. d Ni(cod)2 was used istead of
Ni(acac)2.
essential for the carboxylation. Without lithium chloride, the
desired carboxylic acid was obtained in only 5% yield (entry
2).8 The effects of other ligands were also examined. The
use of P(t-Bu)3 and P(n-Bu)3 instead of P(c-C6H11)3 failed
to furnish 2a in satisfactory yield (entries 3 and 4). Amine
ligands such as 1,8-diazabicyclo[5.4.0]-7-undecene (DBU)
and 4,4′-di(tert-butyl)-2,2′-bipyridine (dtbpy), which were
frequently employed as supported ligands in nickel-mediated
CO2 fixation,4 also showed lower activities (entries 5 and
6). In the absence of the nickel catalyst, no reaction took
place. We could use Ni(cod)2 as the precursor of the catalyst,
albeit with a lower yield (entry 7).
a A mixture of Ni(acac)2 (0.025 mmol), P(c-C6H11)3 (0.050 mmol), and
1 (ca. 0.7 M in THF solution, 0.50 mmol) was stirred in DME (5.0 mL) for
3 h under CO2 atmosphere (1 atm, balloon). b Yields of 2 determined by
1H NMR analysis are in parentheses. c In THF. d Reaction time was 8 h.
e Reaction time was 6 h. f At 50 °C. g The product was too unstable to
isolate. h The acetal protection was removed upon work up with 1.0 M aq
HCl.
With the optimized conditions in hand, we performed the
carboxylation of an array of alkylzinc reagents (Table 2).9
Isolation of highly polar carboxylic acids 2 often resulted in
significantly lower yields than those determined by 1H NMR
analysis of the crude products, and most of the products were
isolated as the corresponding benzyl esters 2′. The larger
phenylethylzinc reagent 1b reacted with CO2 smoothly to
furnish 2b in good yields (entry 2). The olefin moiety of 1c
did not interfere with the reaction (entry 3). To our delight,
the reactions of not only primary alkylzincs but also
secondaryoneswerecarriedout.Cyclohexylzinciodide-lithium
chloride complex (1d) was converted to cyclohexanecar-
boxylic acid (2d) in 69% yield (entry 4). By taking advantage
of high functional group compatibility of organozinc re-
agents, alkylzincs bearing various functional groups were
prepared and subjected to the carboxylation reaction. The
silyl and benzyl ether moieties were tolerated under the
reaction conditions (entries 5 and 6). The carbon-chloride
bond of 1g did not suppress the formation of 2g despite its
conceivable oxidative addition to the zerovalent nickel (entry
7). It is noteworthy that the reaction of 1h with CO2 gave
2h in good yield, leaving the ester linkage untouched (entry
8). Although preparation of alkylzinc reagents having alde-
hyde and ketone groups was quite difficult, the corresponding
acetal-protected reagents were readily accessible and reacted
with CO2 without any difficulties (entries 9 and 10).
We are tempted to assume the reaction mechanism as
follows (Scheme 1). A zero-valent nickel species 3 is initially
formed by reduction of Ni(acac)2 with organozinc reagent
1. The active Ni(0) 3 reacts with CO2 to produce η2-
coordinated complex 4.10 The subsequent transmetalation
with 111 followed by rapid reductive elimination yields 6
along with the starting nickel complex 3 to complete the
catalytic cycle. Protonolysis of 6 upon workup furnishes
(5) (a) Krasovskiy, A.; Malakhov, V.; Gavryushin, A.; Knochel, P.
Angew. Chem., Int. Ed. 2006, 45, 6040–6044. (b) Ren, H.; Dunet, G.; Mayer,
P.; Knochel, P. J. Am. Chem. Soc. 2007, 129, 5376–5377. (c) Boudet, N.;
Sase, S.; Sinha, P.; Liu, C.-Y.; Krasovskiy, A.; Knochel, P. J. Am. Chem.
Soc. 2007, 129, 12358–12359.
(6) LiBr worked as well (70% yield), but LiI was inferior (41% yield).
(7) An addition of TMEDA completely inhibited the reaction.
(8) Preparation of LiCl-free alkylzinc iodide: (a) Knochel, P.; Yen,
M. C. P.; Berk, S. C.; Talbert, J. J. Org. Chem. 1988, 53, 2390–2392.
(9) Except for the reaction of 1a, the use of DME as a solvent gave
better results.
(10) Aresta, M.; Nobile, C. F.; Albano, V. G.; Forni, E.; Manassero,
M. J. Chem. Soc., Chem. Commun. 1975, 636–637.
2682
Org. Lett., Vol. 10, No. 13, 2008

(7)5a with CO2 (eq 1). On exposure of 7 to the standard
reaction conditions, benzoic acid (8) was obtained in 36%
yield. The use of P(n-Bu)3 instead of P(c-C6H11)3 was found
to improve the yield to 46%. However, any changes of
reaction parameters such as solvent, temperature, and additive
gave no positive effect on yield. We then turned our attention
to diphenylzinc (9) (eq 2). Gratifyingly, the reaction of 9
with CO2 proceeded smoothly to form the desired carboxylic
acid in 90% yield based on zinc.13 The result shows the high
potential of the catalyst system, although further modifica-
tions are essential.
Scheme 1. Plausible Reaction Mechanism
carboxylic acid 2. Strongly electron-donating tricyclohexy-
lphosphine increased the electron density of the oxygens of
4, which would allow for the following transmetalation. The
bulky tricyclohexylphosphine would also promote the reduc-
tive elimination without suffering from ꢀ-hydride elimination.
Although further investigation is required for the clarification
of the exact role of lithium chloride, it could change the
association state of organozinc iodide via its coordination
to the zinc center, leading to the monomeric species, which
could more readily undergo the transmetalation.12
In conclusion, we have found an efficient nickel catalyst
system for the carboxylation of organozinc reagents, in
particular alkylzincs, with CO2. The catalyst system comple-
ments the rhodium-catalyzed process3 and provides a new
access to highly functionalized saturated carboxylic acid
derivatives from CO2 under mild conditions.
Acknowledgment. This work was supported by Grants-
in-Aid for Scientific Research from MEXT and JSPS. K.H.
acknowledges JSPS for financial support.
Finally, we applied the nickel catalyst system to the
carboxylation of phenylzinc iodide-lithium chloride complex
Supporting Information Available: Experimental details
and characterization data of the products. This material is
available free of charge via the Internet at http://pubs.acs.org.
(11) Similar transmetalations between η2-coordinated nickel complexes
with aldehyde and organometallic reagents were suggested. (a) Biswas, K.;
Prieto, O.; Goldsmith, P. J.; Woodward, S. Angew. Chem., Int. Ed. 2005,
44, 2232–2234. (b) Hirano, K.; Yorimitsu, H.; Oshima, K. Org. Lett. 2005,
7, 4689–4691. (c) Hirano, K.; Yorimitsu, H.; Oshima, K. AdV. Synth. Catal.
2006, 348, 1543–1546. (d) Arao, T.; Kondo, K.; Aoyama, T. Tetrahedron
Lett. 2007, 48, 4115–4117.
OL800764U
(12) Knochel proposed a similar effect of lithium chloride in a halogen/
magnesium exchange reaction. (a) Krasovskiy, A.; Knochel, P. Angew.
Chem. Int. Ed. 2004, 43, 3333–3336.
(13) In the case of the reaction of phenylzinc iodide-lithium chloride
complex (7), diphenylzinc (9) would be generated in situ through Schlenk
equilibrium and participate in the reaction.
Org. Lett., Vol. 10, No. 13, 2008
2683