Communications
DOI: 10.1002/anie.200801857
Homogeneous Catalysis
Carboxylation of Organoboronic Esters Catalyzed by N-Heterocyclic
Carbene Copper(I) Complexes **
Takeshi Ohishi, Masayoshi Nishiura, and Zhaomin Hou*
Carbon dioxide (CO2) is an attractive, cheap, and nontoxic C1
source.[1] However, because of its high thermodynamic
as conjugate reduction of a,b-unsaturated carbonyl com-
pounds,[6b] hydrosilylation of ketones,[6c] and also for the
reduction of CO2.[7] In addition, manycopper compounds
have also been reported to promote nucleophilic addition of
organoboron compounds to electrophiles, such as a,b-unsa-
turated carbonyls and allylic carbonates.[8] These results
encouraged us to examine the carboxylation of organobor-
stabilityand low reactivit,y the use of CO
as a C1 source
2
À
for C C bond formation usuallyrequires highlynucleophilic
organometallic reagents, such as alkyllithium compounds and
Grignard reagents. Less nucleophilic organoboron com-
pounds, though easilyavailable, usuallydo not react with
CO2. Recently, transition-metal-catalyzed addition of carbon onic esters with CO2 byuse of N-heterocyclic carbene copper
nucleophiles to CO2 has attracted much attention.[2,3] In this
context, Iwasawa and co-workers have reported the catalytic
carboxylation of aryl- and alkenylboronic esters with CO2 in
the presence of a rhodium(I) compound and additives.[3b] This
reaction is potentiallyuseful for the synthesis of functional-
ized carboxylic acid derivatives because of the easy avail-
complexes as catalysts. At first we examined the reaction of 4-
methoxyphenylboronic acid 2,2-dimethyl-1,3-propanediol
ester (1a) with CO2 using N-heterocyclic carbene copper
species generated in situ from CuCl, IPr·HCl[9]and tBuOK. In
[4]
abilityof various functionalized organoboronic esters.
Unfortunately, however, the Rh catalyst systems showed
onlylimited tolerance toward functional groups. Although
carbonyl and cyano groups survived the reaction conditions,
more reactive functional moieties, such as bromo, iodo, and
vinyl groups, seemed intolerant. Moreover, little information
about the active catalyst species and the reaction mechanism
was available because of the complexityof the catalsyt
systems. These difficulties have limited the application scope
of the Rh catalyst systems. The search for new catalysts for
more efficient, selective CO2 transformation as well as the
clarification of the catalytic process is therefore of interest
and importance. We report herein an excellent N-heterocyclic
carbene copper(I) catalyst system for the carboxylation of
aryl- and alkenylboronic esters with CO2. This Cu catalyst
system not only showed higher functional-group tolerance,
but could also afford structurallycharacterizable active
catalyst species, thus offering unprecedented insight into the
mechanistic aspects of the catalytic process.[5]
the presence of 5 mol% of CuCl with 5 mol% of IPr·HCl and
2 mmol of tBuOK in THF 708C, the reaction of 1a (1 mmol)
with CO2 (1 atm) took place smoothlyto afford the carbox-
ylation product 4-methoxybenzoic acid 2a almost quantita-
tively after acidic hydrolysis (Table 1, entry 1). The reaction
did not occur in the absence of CuCl or an NHC ligand or
tBuOK under the same conditions. The use of a less bulky
NHC ligand, such as IMes·HCl, instead of IPr·HCl led to a
lower yield of 2a (Table 1, entry4). CuBr, CuI, and the Cu II
compounds CuCl2 and Cu(OAc)2 were less effective than
CuCl (Table 1, entries 5–9). Further screening revealed that
the isolated carbene copper(I) chloride complex [(IPr)-
CuCl][6b,c] could also show high activityfor this reaction, and
the catalyst loading could be reduced to 1 mol% with
1.05 mmol of tBuOK (Table 1, entries 9 and 10).
We then examined the reactions of various functionalized
organoboronic esters with CO2 byusing [(IPr)CuCl] as a
catalyst precursor. Representative results are summarized in
Table 2. Awide range of aryl- and alkenylboronic esters could
be used in this reaction. The reaction was not affected by
either electron-donating (e.g., OMe) or electron-withdrawing
(e.g., NO2 and CF3) groups (Table 2, entries 1, 15, 16). A
variety of reactive functional groups, such as vinyl, propargyl
Copper complexes bearing N-heterocyclic carbene
(NHC) ligands have been reported to act as efficient catalysts
for the transformation of various carbonyl compounds,[6] such
[*] Dr. T. Ohishi, Dr. M. Nishiura, Prof. Dr. Z. Hou
Organometallic Chemistry Laboratory RIKEN
(The Institute of Physical and Chemical Research)
Hirosawa 2-1, Wako, Saitama 351-0198 (Japan)
Fax: (+81)48-462-4665
E-mail: houz@riken.jp
index.html)
[**] This work was partly supported by a Grant-in-aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science
and Technology of Japan. We thank Mrs. Hu (RIKEN) for elemental
analysis.
Supporting information for this article is available on the WWW
5792
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 5792 –5795