DOI: 10.1002/cctc.201500907
Communications
Enantioselective Hydrogenation of Olefins Enhanced by
Metal–Organic Framework Additives
Beµta Vilhanovµ,[a, c] Marco Ranocchiari,*[b] and Jeroen A. van Bokhoven*[a, b]
The use of nonprotic solvents (e.g., dichloromethane, toluene)
increases the enantioselectivity of the asymmetric hydrogena-
tion of olefins with chiral [Rh(Me-BPE)(cod)]OTf [Me-BPE=1,2-
bis(2,5-dimethylphospholano)ethane; cod=1,5-cyclooctadiene;
OTf=triflate]. Readily available achiral metal–organic frame-
works (MOFs) as additives yielded substantially enhanced reac-
tivity. In toluene (but not dichloromethane), the MOFs ad-
sorbed the homogeneous catalyst, which directly reduced rho-
dium contamination in the products of the reaction. The in situ
formed heterogeneous catalyst was reused without loss in se-
lectivity.
Scheme 1. Substrates 1–3 and homogeneous catalyst 4 used for asymmetric
hydrogenation.
larly, with a catalyst loading of 0.1 mol%, olefins 2 and 3 react-
ed to give the products with 91.4 and 85%ee, respectively.[3c]
To the best of our knowledge, examples of 4 as a catalyst for
the hydrogenation of 1, 2, or 3 in aprotic solvents are not
known. However, positive solvent effects have been reported
for the Rh(PP*)-catalyzed asymmetric hydrogenation of 1 with
an increase in enantioselectivity from 60%ee in methanol up
to 98%ee in dichloromethane.[4] In this publication, we demon-
strate the influence of metal-organic frameworks (MOFs) as ad-
ditives to the reaction mixture.
Asymmetric hydrogenation, one of the most common routes
towards enantiomerically enriched products, is pivotal in
modern synthetic chemistry and in the fine-chemicals indus-
try.[1] RhI catalysts that have the general formula [Rh(PP*)L2]X
{PP*=chiral diphosphane; L=olefin, methanol, or other sol-
À
vent; X=OTfÀ, BF4
, [3,5-bis(trifluoromethyl)phenyl]borate
(BArF), and others} belong to the most popular catalyst
classes.[2] For commonly studied substrates (Scheme 1) such as
dimethyl itaconate (1), methyl 2-acetamidoacrylate (2), and
methyl (Z)-a-acetamidocinnamate (3), a variety of ligands [e.g.,
DIPAMP,[3a] Josiphos,[3b] R-DuPHOS[3c–e] (R=Me, Et, iPr)] yield
high enantioselectivity at very low catalyst loadings. Other li-
gands, such as 1,2-bis(2,5-dimethylphospholano)ethane (Me-
BPE), give enantioselectivities that are high but still not compa-
rable with those obtained with the ligands above. The hydro-
genation of olefins 1–3 by using [Rh(Me-BPE)(cod)]OTf (4,
cod=1,5-cyclooctadiene; OTf=triflate) as the catalyst was
originally reported with methanol as the solvent. Substrate
1 was hydrogenated with full conversion and up to 91% enan-
tiomeric excess (ee) at a catalyst loading of 0.02 mol%.[3f] Simi-
MOFs are now well established in the area of gas storage
and separation[5] and are becoming increasingly popular in cat-
alysis owing to properties such as structural flexibility, high sur-
face area, tunable pore size, and stability.[6] Asymmetric cataly-
sis with the use of MOFs has been described with homochiral
frameworks,[7] the synthesis of which requires expensive enan-
tiomerically pure organic precursors prepared in multistep pro-
cedures. There are several asymmetric catalysts based on ho-
mochiral MOFs,[8] but their expensive synthesis combined with
their catalytic performance, which is often lower than that ob-
tained with analogous homogeneous catalysts, have hampered
their practical use. Alternatively, an achiral MOF can be func-
tionalized with a chiral molecule,[9] which is a modular and
cost-efficient solution also employed in this work. Given that
MOFs feature a high surface area and big pores, they can be
used as additives in catalysis to adsorb big molecules.[10] For in-
stance, this has been shown in a homogeneous esterification
reaction catalyzed by the heteropolyacid H3PW12O40 (HPW)
with the Cu3(btc)2 (HKUST-1; btc=benzene-1,3,5-tricarboxylate)
MOF as an adsorbent. The heterogeneous catalyst
HPW@HKUST-1 obtained by precipitation was reused at ap-
proximately 50% conversion over four runs.[10a] We explored
this principle for asymmetric hydrogenation with RhI catalysts.
In the process of designing our strategy, it was necessary to
choose MOFs with appropriate properties. We envisaged that
the use of such materials in asymmetric hydrogenation with
catalyst 4 required them to have the following features: one,
[a] B. Vilhanovµ, Prof. J. A. van Bokhoven
Department of Chemistry and Applied Biosciences,
Institute for Chemical and Bioengineering, ETH Zürich
Vladimir-Prelog-Weg 1
CH-8093 Zürich (Switzerland)
[b] Dr. M. Ranocchiari, Prof. J. A. van Bokhoven
Laboratory for Catalysis and Sustainable Chemistry
Paul Scherrer Institute (PSI)
CH-5232 Villigen (Switzerland)
[c] B. Vilhanovµ
Department of Organic Technology
University of Chemistry and Technology Prague
Technickµ 5
CZ-166 28 Prague 6 (Czech Republic)
Supporting Information for this article is available on the WWW under
ChemCatChem 2016, 8, 308 – 312
308
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