Angewandte
Chemie
Synthetic Methods
Effective Formylation of Amines with Carbon Dioxide and
Diphenylsilane Catalyzed by Chelating bis(tzNHC) Rhodium
Complexes**
Thanh V. Q. Nguyen, Woo-Jin Yoo, and Shu¯ Kobayashi*
Abstract: The reductive formylation of amines using CO2 and
hydrosilanes is an attractive method for incorporating CO2 into
valuable organic compounds. However, previous systems
required either high catalyst loadings or high temperatures to
achieve high efficiency, and the substrate scope was mostly
limited to simple amines. To address these problems, a series of
alkyl bridged chelating bis(NHC) rhodium complexes
(NHC = N-heterocyclic carbene) have been synthesized and
applied to the reductive formylation of amines using CO2 and
Ph2SiH2. A rhodium-based bis(tzNHC) complex (tz = 1,2,3-
triazol-5-ylidene) was identified to be highly effective at a low
catalyst loading and ambient temperature, and a wide substrate
scope, including amines with reducible functional groups, were
compatible.
iron[6e]) as active catalysts. However, in all cases, a high
catalyst loading and/or high temperature were necessary for
this reductive coupling process. Furthermore, the substrate
scope for this transformation is mostly limited to simple
amines. Therefore, the development of new types of catalysts,
which can facilitate the reductive formylation reaction both at
a low catalyst loading and ambient temperature, is of great
interest.
To achieve this goal, we turned our attention to the
application of N-heterocyclic carbene (NHC)/metal com-
plexes as potential catalysts. Based on the fact that the
common intermediate in metal-catalyzed hydrosilylation
reactions is the metal hydride complex,[7] we rationalized
that the strong electron-donating ability of NHC ligands may
increase the nucleophilicity of the metal hydride species and
facilitate the reduction of the weakly electrophilic CO2.
Moreover, the ability of NHCs to form strong bonds with
various transition metals would lead to more robust catalysts
and would permit lower catalyst loadings without causing
catalyst decomposition.[8] Although the use of electron-rich
NHC/metal complexes to facilitate nucleophilic additions to
CO2, to afford carboxylic acids or esters, is well known,[1]
there are few examples of its use for catalytic hydrosilylation
reactions of CO2.[6i,9] To the best of our knowledge, a selective
NHC/metal-catalyzed formylation of amines with CO2 has
not been reported.
Recently, bis(NHC) complexes of rhodium were found to
be effective catalysts for the hydrosilylation of ketones at
ambient temperature.[10] These bis(carbene) ligands not only
improved the stability of the rhodium complexes against
decomposition, but also enabled better control of the steric
and electronic properties around the metal center.[10a] We
hypothesized that these types of metal complexes could be
promising catalysts for hydrosilylation reactions of CO2.
Furthermore, since the high electron density of NHC/metal
complexes may be crucial for the favorable interaction
between CO2 and the key metal hydride intermediate, we
envisioned that modifying the ligands from normal to
abnormal NHCs should generate better catalysts because of
the superior electron-donating ability of the latter.[11] Such an
effect was previously observed for the copper-catalyzed
carboxylation reaction of benzoxazoles and benzothiazoles
with CO2,[12] and the iridium-catalyzed transfer hydrogenation
of CO2 with 2-propanol.[13] Among various types of abnormal
NHCs, tzNHCs (tz = 1,2,3-triazol-5-ylidene) stand out as
excellent candidates because they are easily accessed and
diversified by the [3+2] cycloaddition of azides and
alkynes.[14] Herein, we report the modular synthesis of alkyl-
bridged bis(tzNHC) rhodium complexes and their application
T
he incorporation of CO2 into organic compounds is highly
desirable since CO2 is a low-cost, abundant, and nontoxic raw
material. However, CO2 represents the highest oxidation
state of carbon and this limits its synthetic utility to the
formation of low-energy synthetic targets by CO2 insertion
reactions or to multielectron chemical reductive processes to
generate energy-rich small molecules such as formic acid,
carbon monoxide, methanol, and methane.[1,2] With respect to
catalytic reduction of CO2, hydrogen gas is the cleanest and
most atom-economical reductant, but harsh reaction condi-
tions, such as a high pressure of a mixture of gases and high
reaction temperature, prevents broad application of this
reducing agent. In contrast, the use of boranes[3] and hydro-
silanes[4] as reductants facilitates the catalytic reduction of
CO2 under much milder reaction conditions. In a related
process, the reduction of CO2 in the presence of amines
ultimately affords formamides or methylamines, which are
value-added bulk and fine chemicals.[5,6] Since the initial
report by Cantat and co-workers on the catalytic reductive
formylation of amines with CO2 and silanes,[6a] rapid develop-
ment in this field has led to the disclosure of various
organocatalyts[6a,b] and organometallic complexes (copper,[6c,d]
[*] T. V. Q. Nguyen, Dr. W.-J. Yoo, Prof. Dr. S. Kobayashi
Department of Chemistry, School of Science
The University of Tokyo
7-3-1 Hongo, Bunkyo, Tokyo (Japan)
E-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp
[**] This work was partially supported by a Grant-in-Aid for Scientific
Research from the Japan Society for the Promotion of Science
(JSPS), the Japan Science and Technology Agency (JST), and the
Ministry of Education, Culture, Sports, Science and Technology
(MEXT). tzNHC=1,2,3-triazol-5-ylidene N-heterocyclic carbene.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2015, 54, 9209 –9212
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9209