Angewandte
Chemie
Synthetic Methods
Rhodium-Catalyzed Hydroformylation of 1,1-Disubstituted Allenes
Employing the Self-Assembling 6-DPPon System**
Alexander Kçpfer and Bernhard Breit*
Abstract: A rhodium-catalyzed hydroformylation of 1,1-
disubstituted allenes is reported. Using a RhI/6-DPPon catalyst
system, one can obtain b,g-unsaturated aldehydes in high
regio- and chemoselectivity. The Z-configured product is
formed with up to > 95% selectivity when unsymmetrically
1,1-disubstituted allenes are submitted to the reaction con-
ditions. This is the first time that these interesting building
blocks are accessible by hydroformylation of allenes. The
utility of this methodology is demonstrated by further trans-
formations of one of the obtained products.
H
ydroformylation is one of the most important industrial
processes applying homogeneous catalysis.[1] In 2008 more
than 10 million tons of oxoproducts have been produced.[2]
Whereas hydroformylation of alkenes is highly developed,
only little is known about the corresponding reactions with
other p-unsaturated species such as alkynes,[3] 1,3-dienes,[4]
and allenes.[5]
Scheme 1. The hydroformylation of allenes.
Reports about the hydroformylation of allenes suggest that
it has the potential to deliver the missing equivalent at the C3-
position. In 1976 Fell and Beutler performed studies on the
hydroformylation of allene and found complex mixtures of
mono- and dialdehydes.[5a] DFT calculations by Jiao, Beller,
and co-workers suggested that coupling at the terminal sp2-
carbon of allene should be thermodynamically and kinetically
favored over the reaction at the sp-center.[5b] The only
chemoselective hydroformylation of allenes is in agreement
with this theory and converts a 1,2-allenyl-phosphine oxide
into the corresponding linear saturated aldehyde (aldehyde 3
in Scheme 1).[5c] However, maintaining one of the double
bonds during allene hydroformylation would yield syntheti-
cally more interesting products. In particular b,g-unsaturated
aldehydes 2 could be highly attractive building blocks for
organic synthesis.
Especially reports about the hydroformylation of allenes
are elusive, mainly due to the fact that one has to control
chemo-, regio-, and stereoselectivity in this cumulated
À
unsaturated system. The C C bond formation may occur at
three different positions resulting in several a,b- or b,g-
unsaturated aldehydes (Scheme 1). By p-bond isomerization,
alkene and/or aldehyde hydrogenation, or a second hydro-
formylation an even more complex array of products might be
formed. As an alternative to hydroformylation, reductive
couplings with formaldehyde have been reported[6] regiodi-
vergently for internal alkynes,[6b] 2-substituted 1,3-dienes,[6c]
and allenes.[6d,e] However, these methodologies require stoi-
chiometric amounts of reducing agents whereas the reducing
agent in hydroformylation (hydrogen) is incorporated in the
product making it an atom-economic reaction.[7] Further-
more, reductive couplings of allenes to formaldehyde have
In our group we developed the 6-DPPon ligand[9] which
can self-assemble[10] through complementary hydrogen bond-
ing (Scheme 2) and proved to form excellent catalysts in Rh-
catalyzed hydroformylation of terminal alkenes and alky-
nes.[3f]
À
only been reported with a C1-selectivity. C C bond formation
at the C2-sp-atom of an allene has been reported for Ni-
catalyzed reductive couplings with higher aldehydes.[8]
We were interested in applying this catalyst system to the
hydroformylation of allenes because of its unique properties
of flexibility combined with structural integrity that allow for
the adoption of different coordination geometries together
with maintaining the regiodiscriminating properties of a che-
lating ligand.[9d]
[*] A. Kçpfer, Prof. Dr. B. Breit
Institut für Organische Chemie
Albert-Ludwigs-Universität Freiburg
Albertstrasse 21, 79104 Freiburg im Breisgau (Germany)
E-mail: bernhard.breit@chemie.uni-freiburg.de
[**] This work was supported by the DFG, the International Research,
Training Group “Catalysts and Catalytic Reactions for Organic,
Synthesis” (IRTG 1038), the Fonds der Chemischen Industrie, and
the Krupp Foundation. We thank Umicore, BASF, and Wacker for
generous gifts of chemicals. Sonka Zeyner is acknowledged for
highly motivated and skillful technical assistance.
Supporting information for this article is available on the WWW
Scheme 2. The self-assembling 6-DPPon catalyst system.
Angew. Chem. Int. Ed. 2015, 54, 6913 –6917
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6913