DOI: 10.1002/chem.201502982
Communication
&
Organic Chemistry
Efficient Domino Hydroformylation/Benzoin Condensation: Highly
Selective Synthesis of a-Hydroxy Ketones
[a]
[a, b]
[b]
[b]
[c, d]
[a]
[a]
[
8]
amines, diols, epoxides, or deoxygenated compounds. In
order to realize a domino hydroformylation/benzoin condensa-
tion sequence, the development of a suitable catalyst system
is crucial. While hydroformylations are known to be catalyzed
in the presence of various rhodium, cobalt, ruthenium, and
palladium complexes, benzoin condensations are known to be
catalyzed by bases or carbenes. Since the first isolation by Ar-
Abstract: An improved domino hydroformylation/benzoin
condensation to give a-hydroxy ketones has been devel-
oped. Easily available olefins are smoothly converted into
the corresponding a-hydroxy ketones in high yields with
excellent regioselectivities. Key to success is the use of
a specific catalytic system consisting of a rhodium/phos-
phine complex and the CO adduct of an N-heterocyclic
[9]
2
duengo and co-workers in 1991, N-heterocyclic carbenes
carbene.
(
NHCs) have not only become versatile ligands in organometal-
[
10]
lic chemistry, but also used as organocatalysts in various or-
[
11]
ganic transformations such as benzoin condensation,
the
[
12]
[13]
[14]
Hydroformylation reactions, which produce aliphatic aldehydes
from olefins and syngas, constitute the most important homo-
geneous transition-metal-catalyzed processes in the chemical
Stetter reaction, cyanosilylation, transesterification, and
[
15,11c]
[16]
other reactions.
Owing to their sensitivity,
NHCs are
generally in situ produced from the corresponding (dihydro)i-
midazolium salts by deprotonation with a strong base. Un-
fortunately, this method is not applicable for base-sensitive
substrates/products in most cases. Therefore, more stable NHC
[
1]
industry. Due to the versatile chemistry of the aldehyde
[
2]
group, in recent decades this transformation has been com-
[
3–7]
bined with other reactions in a one-pot manner.
Such
[
17]
[18]
[19]
[20]
tandem sequences follow a general trend in green chemistry
adducts with haloalkanes,
alcohols,
silver,
CO2,
and
[4]
[21]
considering the advantageous step economy. In this respect,
the development of productive novel or improved domino hy-
droformylation reactions has a great appeal for organic chem-
ists. In fact, hydroformylation/reduction (hydrohydroxymethyla-
H CO3 have been developed to liberate NHCs on demand
2
under base-free conditions. Among these masked NHCs, zwit-
terionic (dihydro)imidazolium-2-carboxylates ((H )IMes-CO )
2
2
gain much attention owing to their accessibility, good stability,
[
5]
[13b,14b,20c]
tion), hydroformylation/reductive amination (hydroaminome-
and easy generation of the respective free NHCs.
[
6]
[7]
thylation), and hydroformylation/aldol condensation have
become powerful synthetic methods in organic synthesis.
Combination of hydroformylation and benzoin condensation
would allow for a straightforward and atom-efficient access to
a-hydroxy ketones directly from easily available olefins and
A general problem for the use of NHCs in domino hydrofor-
mylation/benzoin condensations is the competitive coordina-
tion of such d-donor ligands to the transition-metal catalyst,
I
II
for example, Rh or Ru , needed for the hydrofomylation step.
Very recently, Vorholt and co-workers reported the first tandem
hydroformylation/benzoin reaction using the rhodium/BiPhe-
CO/H . The resulting products constitute important organic in-
2
termediates which can be further functionalized to valuable
Phos and vitamin B (in situ generated from its corresponding
1
[
22]
hydrochloride salt and Et N) as the catalytic system. Herein,
3
+
+
we present a more general combination of a specific rhodium/
phosphine complex and H IMes-CO , which allow for hydrofor-
[
a] Dr. K. Dong, R. Sang, Dr. R. Jackstell, Prof. Dr. M. Beller
Leibniz-Institut für Katalyse e.V. an der Universität Rostock
Albert-Einstein-Str. 29a, 18059 Rostock (Germany)
2
2
mylation/benzoin condensation of various aliphatic olefins,
providing linear a-hydroxy ketones in high yields with excel-
lent regioselectivities (Scheme 1).
E-mail: Matthias.Beller@catalysis.de
+
[
b] R. Sang, Prof. Dr. J.-F. Soule, Prof. Dr. C. Bruneau
Institut Sciences Chimiques de Rennes, UMR 6226
CNRS-UniversitØ de Rennes “OrganomØtalliques: MatØriaux et Catalyse”
Campus de Beaulieu, 35042 Rennes (France)
In cooperation with industry, we demonstrated earlier on
that H IMes-CO adduct C1 is an efficient organocatalyst for
2
2
[
c] Prof. Dr. R. Franke
Evonik Performance Materials GmbH
Paul-Baumann-Str. 1, 45772 Marl (Germany)
benzoin condensations of various aldehydes under base-free
[
23]
conditions. Consequently, in our initial studies we investigat-
ed the domino hydroformylation/benzoin condensation of 1-
[
d] Prof. Dr. R. Franke
octene (1a) with C1 and [Rh(acac)(CO) ] (acac=acetylacetone)
2
Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum
in the presence of different phosphine ligands (Table 1,
entries 2–8). Under standard conditions (808C, 20 h, THF), with-
out phosphine ligand no conversion of 1a was observed
(entry 1).
4
4780 Bochum (Germany)
+
[
] These authors contributed equally to this work.
Chem. Eur. J. 2015, 21, 18033 – 18037
18033
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim