Rhodium-catalyzed hydroformylation of 1,1-disubstituted allenes employing the self-assembling 6-DPPon system

Article abstract of DOI:10.1002/anie.201502086

Abstract A rhodium-catalyzed hydroformylation of 1,1-disubstituted allenes is reported. Using a Rh^I/6-DPPon catalyst system, one can obtain β,γ-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 conditions. 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 transformations of one of the obtained products. β,γ-Unsaturated aldehydes are obtained by a rhodium-catalyzed hydroformylation of 1,1-disubstituted allenes. For unsymmetrically 1,1-disubstituted allenes the Z-configured product is formed in up to about 95% selectivity. This is the first time that these building blocks are accessible by hydroformylation of allenes. The utility of this methodology is demonstrated by further transformations of one of the obtained products.

Full text of DOI:10.1002/anie.201502086

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201502086
German Edition: DOI: 10.1002/ange.201502086
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
under http://dx.doi.org/10.1002/anie.201502086.
Scheme 2. The self-assembling 6-DPPon catalyst system.
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Initial studies focused on the hydroformylation of the
symmetrically 1,1-disubstituted allene 4a. The substitution
pattern would allow for the optimization of the reaction
conditions omitting stereoselectivity issues. To identify a cata-
lyst system that is able to provide b,g-unsaturated aldehydes
in a chemo- and regioselective manner we screened different
ligands (Table 1). Besides PPh3, Xantphos and BIPHEPHOS,
diphenylallene 2c could only be isolated in moderate yields
due to degradation of the allene starting material under the
reaction conditions.[12]
To get further insights into the reaction, the kinetics were
monitored (Scheme 4). After a short period of catalyst
preformation the conversion linearly increases up to 60–
70% in less than 15 min. Within this range a turnover
frequency of 761 hÀ1 can be determined.
Table 1: Ligand screening.[a]
Entry
Ligand (mol%)
Selectivity [%][b]
Yield [%][c]
1
2
3
4
5
6
7
8
9
PPh3 (10)
Xantphos (5)
n.d.
65
89
77
85
>95
27
43
12
36
52
69
50
76
20
20
14
BIPHEPHOS (5)
2-DPPAT/3-DPICon (5 each)
6-BBmTFMPPon (10)
6-DPPon (10)
6-DPMePon (10)
2-MeODPP (10)
Ph2PPy (10)
37
[a] see SI for details; [b] selectivity toward 2a among all aldehydes,
determined by 1H NMR analysis of the crude product; [c] 1H NMR yield
of all aldehydes formed, trimethoxybenzene as internal standard.
n.d.=not determined.
Scheme 4. Reaction kinetics for the hydroformylation of 4b. The
conversion was determined by 1H NMR analysis (see SI for details).
With these results in hand, we wondered whether it was
possible to induce stereoselectivity in unsymmetrically 1,1-
disubstituted allenes. We envisioned that sterically demand-
ing substituents at the C1-position of an allene should reach
into one half room of the allene moiety thereby hindering the
hydrometalation step from this side (Scheme 5). This suggests
that the Z-product should be formed predominantly.
we also tested different self-assembling ligands such as the 2-
DPPAT/3-DPICon system[10] and the 6-BBmTFMPPon-
ligand, which was successfully applied in alkyne hydroformy-
lation.[3f] To our delight it was the parent 6-DPPon ligand,
which was superior to all other tested ligands in regard to both
reactivity and selectivity toward the desired b,g-unsaturated
aldehyde. O- and N-methylated analogues of 6-DPPon as well
as Ph2PPy were examined proofing the importance of the
hydrogen-bond interaction of the ligand (entries 7–9).
Scheme 5. Proposed reason for stereoinduction.
With this catalyst system in hand, the reaction conditions
were adjusted in terms of solvent, pressure, temperature, and
concentrations (see the Supporting Information (SI) for
details). The optimal conditions and a substrate scope for
the hydroformylation of symmetrically 1,1-disubstituted
allenes are shown in Scheme 3. The b,g-unsaturated alde-
hydes 2a and 2b could be obtained in excellent yields and
selectivity. As a limitation, the hydroformylation product of
Indeed, when we optimized the conditions for the hydro-
formylation of unsymmetrically 1,1-disubstituted allenes we
found that the expected stereoinduction is possible. Interest-
ingly, the stereoselectivity showed a significant dependence
on the syngas pressure (Table 2). Gradually going from 20 to
50 bar led to an increasing preference for the Z-configured
product. This proposes that the stereoselectivity is deter-
mined kinetically. In contrast to low syngas pressures that are
applied in isomerizing hydroformylation of internal alkenes
to linear aldehydes[13] to facilitate reversible hydrometalation
by b-H-elimination in the 16-valence-electron complex HMd,
high pressures should accelerate the coordination of CO to
form an 18-valence-electron complex thereby suppressing
isomerization.
With these conditions in hand we screened more 1,1-
disubstituted allenes (Scheme 6). Gratifyingly, the reaction
Scheme 3. Hydroformylation of symmetrically 1,1-disubstituted allenes.
Yields are of isolated products (see SI for details).
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Table 2: Dependency of the Z/E-selectivity on the pressure of CO/H2.[a]
hydroformylated to yield the Z-configured product as a single
stereoisomer.[15] The BDMS-group is one of the most potent
substrates for Hiyama couplings[16] which can potentially be
used for derivatization at this position.
Since allene substrates are, with a few examples, not
commercially available, their accessibility determines the
synthetic utility of this methodology. Interestingly, allenes can
be readily prepared over a two-step sequence consisting of
alkynyl-Grignard addition and propargylic SN2’ reduction
potentially from any ketone (Scheme 7).
Entry
p [bar]
Z:E[b]
Yield [%][c]
10
11
12
13
20
30
40
50
5:1
20:1
22:1
30:1
85
97
92
89
[a] see SI for details; [b] determined by 1H NMR analysis of the crude
product; [c] 1H NMR-yield of 2d, trimethoxybenzene as internal stan-
dard.
Scheme 7. Two-step synthesis of allenic substrates from ketones (see
SI for details).
Furthermore, the reaction can be performed on a 10 mmol
scale with excellent yield, chemo-, regio-, and stereoselectiv-
ity (Scheme 8). To demonstrate the synthetic utility of the
obtained b,g-unsaturated aldehydes, a small scope of further
transformations was performed. Under Horner–Wadsworth–
Scheme 6. Hydroformylation of unsymmetrically disubstituted allenes.
Z/E-selectivities were determined by 1H NMR analysis of the crude
reaction mixtures and confirmed by NOE experiments. Yields are of
isolated products (see SI for details).
conditions proved to be very general and yielded the
corresponding aldehydes in very good to excellent yield.
Expectedly, the diastereoselectivity of alkene formation is
a function of the differentiation of the sterical demand of the
two substituents at the allene terminus: the larger the
difference the higher the stereoselectivity (see 2d,g,k,l,n,o).
Next, the hydroformylation of allenes was applied to more
complex substrates and high stereoselectivities could be
reached for all of them (2 f,m,p). A (À)-menthone derived
allene provided the corresponding b,g-unsaturated aldehyde
2 f in quantitative yield, a (+)-camphor and an estrone 3-
methyl ether derived product could be isolated in satisfying
yield.[14] Interestingly, the hydroformylation of 1-(benzyldi-
methylsilyl)-1-methylallene (4i) gave the E-configured prod-
uct predominantly. To limit the reason for this switch of
stereoselectivity to the properties of the silicon atom, the
analogous substrate with a carbon at this position (4j) was
Scheme 8. Synthesis and chemical transformations of 2d (see SI for
details). [a] Rh(CO)2acac (0.5 mol%), 6-DPPon (10 mol%), PhMe
(0.2m), CO/H2 (1:1, 30 bar), 808C, 1.5 h; [b] 2d (0.5 mmol),
BrMgCHCH2 (4.0 equiv), THF (0.2m); [c] 2d (1.0 mmol),
=
EtO2CCH2P( O)(OEt)2 (1.1 equiv), NaH (1.1 equiv), THF (0.33m);
[d] 2d (2.0 mmol), NEt3 (1.5 equiv), THF (1.0m), TBSTf (1.1 equiv);
[e] 2d (0.5 mmol), tBuOH (0.6m), 2-methyl-2-butene (6.6 equiv),
NaClO2 (1.3 equiv), KH2PO4 (1.3 equiv), H2O (0.9m); [f] 1H NMR yield,
trimethoxybenzene as internal standard; [g] 2d (0.5 mmol), LiAlH4
(1.0 equiv), THF (0.5m). acac=acetylacetonate, 6-DPPon=6-diphenyl-
phosphanyl-2-pyridone, TBSTf=tert-butyldimethylsilyltrifluoromethane-
sulfonate.
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Keywords: aldehydes · allenes · hydroformylation · rhodium ·
self-assembly
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Received: March 5, 2015
Published online: April 27, 2015
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