Rhodium(I)-catalyzed direct carboxylation of arenes with CO2 via chelation-assisted C-H bond activation

Article abstract of DOI:10.1021/ja109097z

Rh-catalyzed direct carboxylation of unactivated aryl C-H bond under atmospheric pressure of carbon dioxide was realized via chelation-assisted C-H activation for the first time. Variously substituted and functionalized 2-arylpyridines and 1-arylpyrazoles underwent the carboxylation in the presence of the rhodium catalyst and a stoichiometric methylating reagent, AlMe ₂(OMe), to give carboxylated products in good yields. The catalysis is proposed to consist of methylrhodium(I) species as the key intermediate, which undergoes C-H activation to afford rhodium(III), followed by reductive elimination of methane to give nucleophilic arylrhodium(I). This approach demonstrates promising application of C-H bond activation strategy in the field of carbon dioxide fixation.

Full text of DOI:10.1021/ja109097z

COMMUNICATION
pubs.acs.org/JACS
Rhodium(I)-Catalyzed Direct Carboxylation of Arenes with
CO2 via Chelation-Assisted C-H Bond Activation
Hajime Mizuno, Jun Takaya, and Nobuharu Iwasawa*
Department of Chemistry, Tokyo Institute of Technology, O-Okayama, Meguro-ku, Tokyo 152-8551, Japan
S Supporting Information
b
Table 1. Screening of Reaction Conditions
ABSTRACT: Rh-catalyzed direct carboxylation of unacti-
vated aryl C-H bond under atmospheric pressure of carbon
dioxide was realized via chelation-assisted C-H activation
for the first time. Variously substituted and functionalized
2-arylpyridines and 1-arylpyrazoles underwent the carbox-
ylation in the presence of the rhodium catalyst and a
stoichiometric methylating reagent, AlMe2(OMe), to give
carboxylated products in good yields. The catalysis is
proposed to consist of methylrhodium(I) species as the
entry
MeMXn
PR3
2a/%
3a/%
key intermediate, which undergoes C-H activation to
afford rhodium(III), followed by reductive elimination of
methane to give nucleophilic arylrhodium(I). This approach
demonstrates promising application of C-H bond activa-
tion strategy in the field of carbon dioxide fixation.
b
1
2
3
4
5
6
7
ZnMe2
P(mes)3
P(mes)3
P(mes)3
P(mes)3
PPh3
0
41
67
48
5
14
16
13
22
22
17
21
AlMe3
AlMe2(OMe)c
d
AlMe(OMe)2
AlMe2(OMe)c
AlMe2(OMe)c
AlMe2(OMe)c
P(t-Bu)3
35
PCy3
73
a The product was obtained as its methyl ester after treatment of the
he catalytic nucleophilic carboxylation reaction using carbon
crude mixture with TMSCHN2. b P(mes)3 = Tris(2,4,6-trimethylphenyl)-
T
dioxide as a one-carbon source through C-H bond activa-
tion of aromatic molecules is still a formidable challenge in the
filed of synthetic chemistry.1,2 Very recently, Boogaerts and
Nolan reported the first example of such reaction, that is, the
gold(I)-NHC catalyzed carboxylation reaction of sp2 C-H
bonds of electron-deficient arenes; however, this reaction was
proposed to proceed via deprotonation by hydroxo-gold(I)
complex and thus was limited to substrates which have rather
acidic protons (pKa of about 30).3 In this paper, we report
another approach for the direct carboxylation reaction of aro-
matic C-H bonds, that is, Rh(I)-catalyzed carboxylation of
aromatic compounds via chelation-assisted ortho-metalation.
To realize such a reaction, we focused our attention on rhodium
catalysts, as rhodium(I) complexes are well-known as a catalyst
for C-H activation4 and arylrhodium(I) species are expected to
have sufficient nucleophilicity for carboxylation.5,6 As for the key
catalytic species, we have chosen methylrhodium(I), as there are
several precedents where methylrhodium(I) activates aryl C-H
bond to give Ar(Me)(H)Rh(III) species which undergoes reduc-
tive elimination of methane to give ArRh(I) species in stoichio-
metric reactions.7 It was also expected that, by carrying out the re-
action in the presence of an appropriate methylmetallic reagent,
the key methylrhodium(I) would be regenerated by transmetala-
tion with the rhodium(I) carboxylate.8
c
phosphine. Prepared by adding an equimolar amount of MeOH to a
toluene solution of AlMe3 before use. d Prepared by adding two molar
amounts of MeOH to a toluene solution of AlMe3 before use.
closed system in the presence of various methylmetallic reagents
(MeMXn). After extensive screening of the reagent, use of Me3Al
was found to give the desired o-carboxylated product 2 regiose-
lectively in 41% yield along with o-methylated product 3a in 16%
yield (Table 1, entry 2). Furthermore, methylaluminum alkox-
ides were found to promote the carboxylation more efficiently
(entries 3, 4). In particular, use of the aluminum reagent prepared
from equimolar amounts of Me3Al and MeOH before use gave
the best result (entry 3).9 Encouraged by these initial results, we
further examined the effect of phosphine ligands. Although use of
PPh3 gave a low yield of carboxylation product, bulkier arylpho-
sphines and alkylphosphines gave better results (entries 3, 5-7).
Use of 20 mol % amount of monodentate phosphines suppressed
the reaction and bidentate phosphine ligands such as dppm,
dppp, and dppf did not give the desired product either. Among
the ligands examined, PCy3 was found to be the best ligand for
this reaction (entry 7). Formation of dicarboxylated product was
not observed under these reaction conditions.
We next examined the generality of the reaction using several
substituted phenylpyridines (Table 2). A wide range of substrates
bearing an electron-donating or electron-withdrawing group at
In the first place, 2-phenylpyridine was chosen as substrate and
the reaction was examined using 5 mol % [Rh(coe)2Cl]2 (10 mol %
based on Rh), 12 mol % trimesitylphosphine (P(mes)3) in DMA
(N,N-dimethylacetamide) at 70 °C under CO2 atmosphere in a
Received: October 10, 2010
Published: December 30, 2010
r
2010 American Chemical Society
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|

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Table 2. Generality of the Reaction Using Phenylpyridinesa
Table 3. Generality of the Reaction Using Phenylpyrazolesa
a The reaction was carried out according to the conditions of Table 1,
entry 7. The products were obtained as their methyl esters after
treatment of the crude mixture with TMSCHN2.
a The reaction was carried out according to the conditions of Table 1,
entry 7. The products were obtained as their methyl esters after treat-
ment of the crude mixture with TMSCHN2. b P(mes)3 was employed.
c 10 mol % [RhCl(coe)2]2 and 24 mol % P(mes)3 were employed.
Scheme 1. Proposed Catalytic Cycle
the p-position smoothly underwent the carboxylation reaction in
moderate to good yield by carrying out the reaction under the
optimized conditions. It should be noted that styrene derivative
gave the desired carboxylation product 2d without affecting the
alkene moiety (entry 4). Substitution of fluorine at ortho-position
caused no problem (entry 6). The carboxylation of 2-(2-naphthyl)-
pyridine occurred regioselectively at 3-position probably due to
steric repulsion by peri-hydrogen (entry 7). Furthermore, hetero-
aromatic derivatives such as furan and benzofuran also gave the
corresponding carboxylation products 2h and 2i in good yield.
Introduction of an electron-donating substituent on the 4-position
of the pyridine ring did not affect the efficiency of the reaction,
while the reaction of sterically congested 3-methylpyridine derivative
required increased loading of the catalyst (entries 10 and 11).10
Thus, this protocol showed wide generality for preparation of
functionalized pyridylarene carboxylic acids.
Furthermore, it was found that the pyrazolyl group was also an
effective directing-group of this reaction (Table 3). In this case,
double carboxylation at both ortho-positions partially proceeded
using nonsubstituted and p-substituted benzene derivatives to
give 2-mono- and 2,6-dicarboxylated products 4 and 40 in the
combined yield of 80% (entries 1 and 2),11 while m-substituted
derivative gave the corresponding monocarboxylated product 4c
selectively in good yield (entry 3).
Although further studies were required to clarify the detailed
mechanism of the reaction, we currently believe the reaction
proceeds as follows (Scheme 1). First the methylrhodium(I) A
would be produced by the reaction of rhodium(I) chloride and
methylaluminum reagent, and then this would give the key aryl-
rhodium(I) Cvia arylrhodium(III) species Bgenerated by chelation-
assisted C-H bond activation (oxidative addition), followed by
reductive elimination of methane. Subsequently, nucleophilic
carboxylation of arylrhodium(I) C would afford rhodium carbo-
xylate D which would undergo transmetalation with methylalu-
minum reagent to give aluminum carboxylate E with regenera-
tion of methylrhoduim(I) A. o-Methylated product 3 would be
produced via C-C bond forming reductive elimination from the
arylrhodium(III) intermediate B.12
In summary, we have succeeded in developing a catalytic direct
carboxylation of aromatic compounds via chelation-assisted C-H
bond activation. By carrying out the reaction in the presence of
methylaluminum reagent, nucleophilic C-C bond formation was
achieved with C-H bond activation. This concept will find abundant
applications in C-H bond activation chemistry.
’ ASSOCIATED CONTENT
S
Supporting Information. Preparative methods and spec-
b
tral and analytical data of compounds 2-4. This material is
available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
azodicarboxylate; it is proposed that the reaction proceeds via the
ethoxycarbonyl radicals. (j) Yu, W.-Y.; Sit, W. N.; Lai, K.-M.; Zhou,
Z.; Chan, A. S. C. J. Am. Chem. Soc. 2008, 130, 3304. To formic acid
(k) Shibahara, F.; Kinoshita, S.; Nozaki, K. Org. Lett. 2004, 6, 2437. To
nitriles; (l) Zhou, C.; Larock, R. C. J. Org. Chem. 2006, 71, 3551.
(m) Zhou, C.; Larock, R. C. J. Am. Chem. Soc. 2004, 126, 2302.
(7) (a) Price, R. T.; Anderson, R. A.; Muetterties, E. L. J. Organomet.
Chem. 1989, 376, 407. (b) Boyd, S. E.; Field, L. D; Hambley, T. W.;
Partridge, M. G. Organometallics 1993, 12, 1720. (c) Fagnou, K.;
Lautens, M. Chem. Rev. 2003, 103, 169.
(8) For examples of catalyst regeneration from metal carboxylates
using organoaluminum or zinc reagents, see refs 2a, 2g-2j.
(9) No carboxylation product was obtained without rhodium cata-
lyst under the reaction conditions, and therefore, the possibility of
Al-promoted carboxylation of arenes was ruled out. For example of
Al-promoted carboxylation of arenes, see Olah, G. A.; T€or€ok, B.;
Joschek, J. P.; Bucsi, I.; Esteves, P. M.; Rasul, G.; Prakash, G. K. S.
J. Am. Chem. Soc. 2002, 124, 11379.
(10) In all cases except for the reaction of furan-derivative 2i
(Table 2, entry 9), formation of o-methylated products was observed
in about 4-15% yield.
(11) Small amounts of o-methylated products (4-7%) were ob-
served.
(12) C-H activation by rhodium carboxylate complex D followed
by transmetalation and reductive elimination should be considered as
another possible pathway to give arylrhodium(I) complex B. Prelimin-
ary mechanistic studies showed that the transmetalation of a rhodium
carboxylate complex with AlMe2(OMe) is very fast and proceeded even
at room temperature, whereas the C-H activation of phenylpyridine by
a rhodium carboxylate complex did not occur under the reaction
conditions. Therefore, we currently believe that the transmetalation
precedes the C-H activation as in Scheme 1. Details will be reported in
due course.
Corresponding Author
niwasawa@chem.titech.ac.jp
’ ACKNOWLEDGMENT
This research was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “Molecular Activation Directed
toward Straightforward Synthesis” (No. 22105006), a Grant-in-
Aid for Scientific Research (A) (No. 21245024) and a Grant-in-
Aid for Scientific Research on Innovative Areas (No. 20200049)
from the Ministry of Education, Culture, Sports, Science, and
Technology of Japan.
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