Angewandte
Communications
Chemie
Coupling Reactions
Nitrous Oxide as a Hydrogen Acceptor for the Dehydrogenative
Coupling of Alcohols
Thomas L. Gianetti,* Samuel P. Annen, Gustavo Santiso-Quinones, Markus Reiher,*
Matthias Driess, and Hansjçrg Grützmacher*
Abstract: The oxidation of alcohols with N2O as the hydrogen
acceptor was achieved with low catalyst loadings of a rhodium
complex that features a cooperative bis(olefin)amido ligand
under mild conditions. Two different methods enable the
formation of either the corresponding carboxylic acid or the
ester. N2 and water are the only by-products. Mechanistic
studies supported by DFT calculations suggest that the oxygen
atom of N2O is transferred to the metal center by insertion into
plexes, has been reported. Under homogeneous conditions,
only few metal complexes catalyze reactions that employ N2O
as the oxygenation reagent.[11]
Rhodium(I) amido bis(olefin) complexes are efficient
transfer hydrogenation catalysts[12b] and catalyze dehydrogen-
ative coupling (DHC) reactions between primary alcohols
and water, methanol, ammonia, or amines.[12c] Mixtures of
aldehydes and primary amines or ammonia are also converted
into amides with excellent efficiency while the formation of
imines is avoided.[12d] Metal–ligand cooperativity between the
rhodium center and the amido moiety is responsible for the
high activity of these catalysts. A serious drawback of these
reactions is the use of hydrogen acceptors.[12b,d] Milstein and
others have developed remarkably efficient homogeneously
catalyzed DHC reactions without acceptors.[13] However,
these transformations require elevated temperatures, and
the search for catalytic systems that enable the use of
inexpensive hydrogen acceptors that allow for an easy
work-up of the reaction mixtures remains attractive. Unac-
tivated olefins can be used as hydrogen acceptors in the
presence of palladium nanoparticles supported on silica as
a co-catalyst.[14a] Oxygen from the air can also serve as
a hydrogen acceptor in the dehydrogenative coupling of
primary alcohols with water.[14b] However, in this case, an
oxygen acceptor is necessary as only one oxygen atom is
transferred to the substrate molecule. Herein, we report our
results on using N2O as a convenient hydrogen acceptor for
the efficient dehydrogenative coupling (DHC) of alcohols
under mild conditions.
À
the Rh H bond of a rhodium amino hydride species, generat-
ing a rhodium hydroxy complex as a key intermediate.
N
itrous oxide (N2O) is an industrial waste product[1] that
massively depletes ozone[2] and has a greenhouse gas effect
that is 300 times higher than that of CO2.[2b] Its transformation
into less harmful chemicals is therefore of great interest and
particularly attractive when useful/value-added products are
generated at the same time. N2O is thermodynamically
unstable (DHf =+ 82 kJmolÀ1) but kinetically inert.[3] Heter-
ogeneous catalysts can decompose N2O into nitrogen and
oxygen[1b,4] or enable the oxidation of hydrocarbons under
harsh conditions.[1c,5]
The use of oxygen as an oxygenating agent is often
difficult to control, and it forms explosive mixtures with
a wide range of organic reagents and solvents.[6] N2O would be
an attractive alternative but has been considered to be rather
unreactive and a poor ligand[7a] although the formation of
metal oxo species has been established.[7] In rare cases,
[8]
À
cleavage of the N N bond occurs. The insertion of the
oxygen atom of N2O into a metal–carbon[9] or a metal–
hydride bond,[10] yielding alkoxy, aryloxy, or hydroxy com-
Benzyl alcohol was fully converted into its coupling
product benzyl benzoate in the presence of 1 mol% of
[Rh(TMIY)(trop2NH)][OTf] (1;[14b] TMIY= 1,3,4,5-tetrame-
thylimidazole-2-ylidene, OTf = trifluoromethanesulfonate;
Scheme 1), N2O as the hydrogen acceptor, and activated
4 molecular sieves. The optimized conditions for the
formation of esters are referred to as anhydrous “condi-
tions 1”. During optimization (see the Supporting Informa-
tion), we observed that 1) other RhI complexes are not as
efficient as complex 1 (see below), 2) the yield of the reaction
was sensitive to the solvent mixture, and 3) the amount of
molecular sieves had a great influence on the yield. The
absence of molecular sieves and the use of non-activated
sieves or other Lewis acids, such as SiO2, Al2O3, TiO2, ZnCl2,
or anhydrous CeCl3, led to low conversions. When either the
catalyst loading was reduced to 0.5 mol% or the substrate
loading doubled under otherwise identical conditions 1, the
conversion dropped below 50%. These results suggest that
a precise amount of activated molecular sieves as a water
trapping agent is needed (1.2 g per mmol of substrate).
[*] Dr. T. L. Gianetti, Dr. S. P. Annen, Dr. G. Santiso-Quinones,
Prof. Dr. H. Grützmacher
Department of Chemistry and Applied Biosciences
ETH Zürich, Laboratory of Inorganic Chemistry
Wolfgang Pauli Strasse 10, 8093 Zürich (Switzerland)
E-mail: gianetti@inorg.chem.ehtz.ch
Prof. Dr. M. Reiher
Department of Chemistry and Applied Biosciences
ETH Zürich, Laboratory of Physical Chemistry
Wolfgang Pauli Strasse 10, 8093 Zürich (Switzerland)
E-mail: markus.reiher@phys.chem.ethz.ch
Prof. Dr. M. Driess
Institute of Chemistry: Metalorganics and Inorganic Materials
Technical University Berlin
Strasse des 17. Juni 135, 10623 Berlin (Germany)
Supporting information and ORCID(s) from the author(s) for this
1854
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1854 –1858