Selective isomerization-hydroformylation sequence: A strategy to valuable α-methyl-branched aldehydes from terminal olefins

Article abstract of DOI:10.1021/cs400872a

For the first time, an original selective isomerization-hydroformylation sequence to convert terminal olefins bearing an anionic moiety to α-methyl-branched aldehydes with unprecedented selectivities is reported. This opens up new synthetic avenues to these valuable building blocks from inexpensive and bioavailable substrates. The catalytic system involves a suitable selective monoisomerization catalyst and a selective supramolecular catalyst that preorganizes a substrate molecule prior to the hydroformylation reaction via hydrogen bonding. In principle, the strategy can be extended to other classes of substrates, providing suitable catalysts for the hydroformylation of internal alkenes.

Full text of DOI:10.1021/cs400872a

Letter
pubs.acs.org/acscatalysis
Selective Isomerization−Hydroformylation Sequence: A Strategy to
Valuable α‑Methyl-Branched Aldehydes from Terminal Olefins
Paweł Dydio, Marten Ploeger, and Joost N. H. Reek*
van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
S
* Supporting Information
ABSTRACT: For the first time, an original selective isomerization-
hydroformylation sequence to convert terminal olefins bearing an
anionic moiety to α-methyl-branched aldehydes with unprecedented
selectivities is reported. This opens up new synthetic avenues to these
valuable building blocks from inexpensive and bioavailable substrates.
The catalytic system involves a suitable selective monoisomerization
catalyst and a selective supramolecular catalyst that preorganizes a
substrate molecule prior to the hydroformylation reaction via
hydrogen bonding. In principle, the strategy can be extended to
other classes of substrates, providing suitable catalysts for the
hydroformylation of internal alkenes.
KEYWORDS: isomerization−hydroformylation sequence, branched-regioselective hydroformylation, catalytic cascade reactions,
supramolecular chemistry, transition metals
lkene hydroformylation, the addition of the formyl group
(CHO) to a CC double bond to form an aldehyde
catalytic auxiliaries (with b/l up to 99)9 or for unfunctionalized
alkenes using a capsulary catalyst approach (b/l up to 1.7).10
We aimed at a new alternative approach that bypasses the
challenges imposed by the inherent properties of the terminal
double bond by combining a selective monoisomerization of
the terminal double bond and its subsequent C-2 regioselective
hydroformylation (Scheme 1). Herein, we report first examples
of such one-pot tandem reactions, using an isomerization
catalyst and a supramolecular hydroformylation catalyst
recently developed in our group,15a to convert terminal alkenes
with anionic groups to C-2 aldehyde products with
A
using syngas as the reagent, is key to various industrial
processes and results in a total production capacity of 107 ton/
year.1 Hence, this transformation has attracted considerable
research interest over the past decades.2 This brought a myriad
of examples of highly active and selective catalysts2,3 as well as
detailed knowledge of the reaction mechanism.2a,4 The
regioselectivity of the reaction, that is, the ratio between
regioisomeric products that can form, is a crucial parameter that
should be controlled. For aliphatic olefins, such as 1-octene and
1-hexene, rhodium catalysts generally produce the linear
aldehyde preferentially. The selectivity for the linear product
can be increased by using bulky ligands5 and is especially high
(l/b product ratio 50−100) when ligands with a wide bite
angle,6 such as BISBI and Xantphos, are applied.
Scheme 1. Selective Isomerization-Hydroformylation
Sequence for a Terminal Olefin to Access the α-Methyl-
Branched Aldehyde, And Possible Undesired Side Reaction
Pathways
In contrast, traditional catalysts that form dominantly
branched aldehyde products from aliphatic olefins are very
scarce,7 and rather moderate selectivities (b/l ratio up to 3 and
up to 10 for unfunctionalized and functionalized olefins,
respectively) have thus far been obtained.7b Access to branched
aldehydes by catalytic conversion of alkenes is highly desired,
considering the synthetic value of these building blocks for the
fine chemical and pharma industries.7a,8 As demonstrated, this
selectivity can be realized for the hydroformylation of specific
functionalized alkenes using reversible directing groups:
Received: October 1, 2013
Revised: November 1, 2013
Published: November 6, 2013
© 2013 American Chemical Society
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Letter
unprecedented selectivities: branched/linear ratios of up to 28
and with up to 85% yields of the α-methyl-branched products.
Our strategy for α-methyl-branched selective hydroformyla-
tion of terminal olefins is based on a two-catalyst system
involving a single isomerization step followed by a
regioselective hydroformylation (Scheme 1). The isomerization
of the terminal double bond to the internal alkene product is
driven by the higher thermodynamic stability of the latter;11
however, the primary product can, in principle, enter further
isomerization cycles, leading to a mixture of alkenes,11 which
would be detrimental for the overall selectivity of the two-step
process. Recent studies reported that a PdII catalyst12 and a RuII
“zipper” catalyst13 are rather selective for monoisomerization of
a few terminal alkenes. In contrast, common IrI and RuII
catalysts14 led to the formation of mixtures of products with
the double bond scrambled along the alkyl chain.12 The
terminal alkene substrates of the current study also contain a
carboxylic group at the other terminal end of the alkyl chain
because these are the typical functional groups that are required
to control the regioselectivity in the subsequent hydro-
formylation step (Scheme 2). In principle, the anionic group
with a carboxylic group represent a class of important building
blocks in synthesis of some pharmaceuticals and natural
products.17
We first optimized the isomerization reaction before
attempting the cascade reaction. Initial experiments revealed
that both the PdII and RuII “zipper”catalysts are active in the
isomerization of the carboxyl-containing substrates, showing
that the carboxylic group does not interfere with the alkene
isomerization. Interestingly, the activity of the Pd catalysts can
be readily switched off at the optimal alkene product
distribution by the addition of base (e.g., triethylamine), and
the Ru catalyst remains active under these conditions. For one-
pot cascade reactions this turning off the isomerization activity
is important because it allows one to limit the formation of later
isomerization products that are detrimental for the overall
selectivity, and as such, the Pd catalytic system was used in
further studies. Further experiments reveal that a mixture of
[(allyl)PdCl]2, PPh3, and AgOTf provides a catalytically active
system already at room temperature in CH2Cl2 without the
necessity of using any activating additives (e.g., ethylene or
diallyl ether).12 4-Pentenoic acid (1) is smoothly isomerized to
3-pentenoic acid (1a), reaching a 3:97 substrate-to-product
ratio (Scheme 3). The reverse reaction, that is, starting from
Scheme 2
Scheme 3. Isomerization of 4-Pentenoic Acid (1) by the Pd-
PPh3 Catalyst
pure 3-pentenoic acid, leads to the same product distribution,
confirming that this is the equilibrium. The catalyst loading can
be significantly lowered (see Supporting Information Figure
S1), although the equilibrium is reached after a longer reaction
time, that is, 20 h with 0.5 mol % Pd catalyst. Importantly, in
none of these experiments is overisomerization to 2-pentenoic
acid (1b) observed, even upon prolonged reaction time and
high catalyst loadings.
Next, we studied the isomerization of longer substrates for
which the primary products can also be expected to tend to
undergo subsequent isomerization steps as a result of the
unsubstituted allylic and homoallylic positions in the substrate
carbon chain.11−14 Indeed, the isomerization of 5-hexenoic acid
(2) leads to a mixture of isomeric products, 4-hexenoic (2a)
and 3-hexenoic (2b) acids (Figure 1). However, double
isomerization is observed only after most of the 5-hexenoic
acid (2) was converted. From the plot of product distribution
versus time, a clear window is visible in which 2a is the
dominant product in the mixture, with the best ratio of 1:22:2
for components 2 to 2a to 2b. Similarly, isomerization of 6-
heptenoic acid (3) leads to a mixture of components, yet it can
be stopped at 92% of the primary product, 5-heptenoic acid
(3a), with only 6% of the secondary product, 4-heptenoic acid
(3b), and 2% of the substrate left (Supporting Information
Figure S2). This clearly shows that the primary isomerization of
the terminal double bond is much faster than the subsequent
one (around 60 times faster in the case of substrate 2). Such a
high relative reaction rate, in principle, allows for the kinetic
control of the monoisomerized product formation.
of the substrate is involved in hydrogen bonding with the
supramolecular catalyst used, allowing for precise substrate
preorganization at the catalytic center. The restricted freedom
of the reactive double bond allows for its highly regioselective
hydroformylation (Scheme 2).15 In addition, this class of
substrates is easily accessible, also via isomerizing olefin cross-
metathesis of bioavailable fatty acids.16 The carboxylic group is
also one of the most common functional groups in organic
molecules, and as such, it can be useful in further product
synthesis. Indeed, the targeted α-methyl-branched aldehydes
With optimized conditions for alkene monoisomerization in
hand, we approached the hydroformylation of a series of
terminal alkenes with a carboxylic group, 1−5. In a typical
reaction, we stir a 1 M solution of the substrate with 0.5 mol %
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Letter
formed from the terminal alkene is identified as a minor
product, (3−12%).
In conclusion, we report here a new strategy for the synthesis
of α-methyl-branched aldehydes via a one-pot stepwise selective
isomerization-hydroformylation protocol. Whereas the more
classic one-pot simultaneous isomerization-hydroformylation
gives access to the linear aldehydes from internal alkenes,19 the
current strategy leads to branched aldehydes from terminal
alkenes. The two transformations, the terminal alkene isomer-
ization and the regioselectively hydroformylation, need to be
done sequentially because of the much higher reactivity of the
terminal alkenes in hydroformylation. For the hydroformylation
step, we have applied our previously developed supramolecular
DIMPhos ligand that preorganizes the substrate on the metal
complex such that only one of the two possible regioisomers
forms. The overall selectivity for the α-methyl-branched
aldehyde from the terminal alkene is the highest reported in
the literature to date,7 which makes this in combination with
the accessibility of the starting materials from renewable
resources an attractive route to these valuable intermediates for
synthetic targets.7a,8 In principle, this strategy can also give
access to the C-3 branched aldehydes utilizing appropriate
hydroformylation catalysts.3d,20 It can also be combined with
reactions of alkenes other than hydroformylation. Current
efforts will include the extension of the cascade protocol to
other selective olefin transformations.
Figure 1. Isomerization of 5-hexenoic acid (2) by the Pd-PPh3 catalyst
(10 mol %).
of palladium catalyst for 24 h, after which we add triethylamine
(90 mol %) as the base to stop the isomerization reaction. Next,
we add a solution of Rh(acac)CO2 (1 mol %) and the ligand 1
(1.1 mol %) and pressurize the autoclave with 10 bar of syngas
(H2/CO 1:1) to initiate the hydroformylation reaction
performed at 50 °C. Under these conditions, in the case of
all the substrates studied, the α-methyl-branched aldehyde was
the major product after the cascade reaction, confirming the
feasibility of the developed strategy (Table 1).18 Substrates 1−
ASSOCIATED CONTENT
* Supporting Information
■
S
Details concerning materials and methods, catalytic and kinetic
experiments. This information is available free of charge via the
Internet at http://pubs.acs.org/.
Table 1. Isomerization−Hydroformylation Sequence for
a
Substrates 1-5 with the Pd-PPh3 and Rh-L1 Catalysts
AUTHOR INFORMATION
Corresponding Author
*E-mail: j.n.h.reek@uva.nl.
■
Notes
regioselectivity
The authors declare no competing financial interest.
subs
n
% linear % α-Me-branched % isomers % conv. to aldehydes
ACKNOWLEDGMENTS
b
■
1
2
3
4
5
1
2
3
4
5
7
3
88
85
85
69
5
12
10
26
40
92
100
81
b
We kindly acknowledge the NRSC-C and the NWO for
financial support and Prof. B. de Bruin and Dr. J. I. van der
Vlugt for helpful discussions and valuable suggestions.
5
6
56
12
48
29
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a
■
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