.
Angewandte
Communications
DOI: 10.1002/anie.201407357
Carbon Dioxide Reduction
Carbon Dioxide Reduction to Methylamines under Metal-Free
Conditions**
Enguerrand Blondiaux, Jacky Pouessel, and Thibault Cantat*
Abstract: The first metal-free catalysts are reported for the
methylation of amines with carbon dioxide. Proazaphospha-
trane superbases prove to be highly active catalysts in the
reductive functionalization of CO2, in the presence of hydro-
agents such as methyliodide, dimethylsulfate, or dimethylcar-
bonate.[9] Using molecular zinc catalysts, our group has indeed
shown that CO2 could serve as an efficient carbon source for
the formation of N-CH3 groups, in the presence of PhSiH3,[6a]
and Beller et al. developed, in parallel, ruthenium catalysts to
promote the same transformation (Scheme 1).[6b] Klanker-
À
boranes. The new methodology enables the methylation of N
H bonds in a wide variety of amines, including secondary
amines, with increased chemoselectivity.
T
he use of CO2 as a C1-building block for the production of
fuels or chemicals has the advantage of creating added-value,
able to mitigate the capture costs of this greenhouse gas.[1] In
this context, increasing research efforts have been devoted to
promoting the reduction of CO2 to formic acid or methanol,
because these C1 molecules can either serve as H2 carriers or
directly as fuels.[2] In parallel, the scope of organic molecules
incorporating a CO2 molecule has rapidly increased with the
discovery of new methodologies to promote the conversion of
CO2 to a variety of heterocycles, carboxylic acids, amides, and
methylamines.[3–6] Importantly, the reduction of CO2 to fuels
or functional chemicals both require the use of stable catalysts
able to operate under mild conditions, with high turnover
numbers (TONs). While renewable reductants, such as H2 or
electrochemical cells, are required for the large-scale recy-
cling of CO2 to formic acid or methanol, the formation of fine
chemicals from CO2 can also be advantageously achieved
with mild hydrides such as hydrosilanes or hydroboranes,
Scheme 1. Methodologies for the methylation of amines with CO2.
mayer et al. and Beller et al. have shown later on that, using
Ru complexes, molecular hydrogen could replace the hydro-
silane reductant.[6c,d] Nonetheless, the metal catalysts perform
at elevated temperatures, ranging from 100 to 1508C, under
elevated pressure, with TONs lower than 100. Furthermore,
these new methodologies still suffer from a limited scope and,
which feature a polarized Si H or B H bond.[7,8] In fact, CO2
transformation to fine chemicals poses different constraints as
a large scope and a high chemoselectivity is anticipated for
the fixation of CO2 into functionalized organic molecules. In
addition, metal-free catalytic systems are desirable in this
strategy, to circumvent the problematic availability, costs, and/
or toxicity of metal ions.
À
À
À
while aromatic and benzylic N H bonds are the most active
substrates, electron-rich secondary aliphatic amines display
a low reactivity. To overstep these limitations, we describe
herein the first metal-free methylation of amines using CO2 as
À
First unveiled in 2013, the catalytic reduction of CO2 to
methylamines is a promising new method for recycling CO2 to
value-added chemicals, as it by-passes the classical method-
ologies involving formaldehyde or hazardous alkylating
a carbon source. Using phosphorus bases as catalysts, N H
bonds in aromatic and aliphatic amines are quantitatively
methylated, with CO2 and hydroborane reductants.
The methylation of amines with CO2 necessitates the use
À
of catalysts able to promote both the formation of N C bonds
and the 6-electron reduction of CO2. N-heterocyclic carbenes
(NHCs) are thus potential organocatalysts in this trans-
formation. Zhang et al. have indeed shown that NHCs are
efficient catalysts in the hydrosilylation of CO2 to methanol
and our group has demonstrated that they can also convert
amines and CO2 to formamides, in the presence of hydro-
silanes.[2b,4a] Nevertheless, reacting diphenylamine (1a) or N-
methylaniline (1b) with CO2, in the presence of PhSiH3 or
Ph2SiH2 and 5.0 mol% IPr, only afforded the corresponding
formamide products 2 and only trace amounts of the expected
methylamines 3 were detected at 1008C by GC/MS analyses
(< 1% yield) [Eq. (1)].
[*] E. Blondiaux, Dr. J. Pouessel, Dr. T. Cantat
CEA, IRAMIS, NIMBE, CNRS UMR 3299
91191 Gif-sur-Yvette (France)
E-mail: thibault.cantat@cea.fr
[**] For financial support of this work, we acknowledge CEA, CNRS,
ADEME, the CHARMMMAT Laboratory of Excellence and the
European Research Council (ERC Starting Grant Agreement
n.336467). T.C. thanks the Fondation Louis D.—Institut de France
for its support.
Supporting information for this article, including detailed descrip-
tions of experimental methods and results, is available on the WWW
12186
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 12186 –12190