Homogeneous catalysed hydrogenation of HMF

Article abstract of DOI:10.1039/c8gc01025k

In this report, hydroxymethylfurfural (HMF) is used as a bio-based feedstock for homogeneous metal-catalysed hydrogenation. Several ligand classes and metals are employed to reduce the aldehyde and aromatic ring of HMF. The previously unknown homogeneous catalysed hydrogenation of HMF to tetrahydrofuran-dimethanol (THFDM) was investigated using different catalyst systems. NHCs and phosphites give higher trans/cis ratios (between 1:1.25 and 1:3.95) of the product THFDM, but low conversions of only up to 17% accompanied by up to 92% yield of bis(hydroxymethyl)furan at 10 bar H₂ and 120 °C. Conversely, di-phosphine ligated ruthenium catalysts in up to 87% yield lead to the highest overall conversion but only moderate trans/cis ratios of only 1:3.1-1:5.

Full text of DOI:10.1039/c8gc01025k

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DOI: 10.1039/C8GC01025K
Homogeneous Catalysed Hydrogenation of HMF †
Alban Cadu,a Kohei Sekine b, Jaroslaw Mormul,c Dominik M. Ohlmann,c Thomas Schaub,a,c A. Stephen K.
Hashmia,b,d
aCatalysis Research Laboratory (CaRLa), Im Neuenheimer Feld 584, 69120 Heidelberg, Germany
bInstitut für Organische Chemie, Universität Heidelberg, Im Neuenheimer Feld 270,
69120 Heidelberg, Germany.
cBASF SE, Synthesis and Homogeneous Catalysis, CarlꢀBoschꢀStrasse 38, 67056 Ludwigshafen,
Germany
dHeidelberg Center for the Environment (HCE), Universität Heidelberg, Im Neuenheimer Feld 229,
69120 Heidelberg, Germany. Eꢀmail: hashmi@hashmi.de; Fax: +49ꢀ(0)6221ꢀ544205
†Electronic supplementary information (ESI) available: Full catalysts screening
tables, detailed procedures and characterisation of the compounds.
Abstract
In this report, hydroxymethylfurfural (HMF) is used as a bioꢀbased feedstock for homogeneous metalꢀ
catalysed hydrogenation. Several ligand classes and metals are employed to reduce the aldehyde and
aromatic ring of HMF. The previously unknown homogeneous catalysed hydrogenation of HMF to
tetrahydrofuranꢀdimethanol (THFDM) was investigated using different catalysts systems. NHCs and
phosphites give higher trans/cis ratios (between 1:1.25 and 1:3.95) of the product THFDM, but low
conversions of only up to 17% accompanied by up to 92% of bis(hydroxymethyl)furan at 10 bar H2 and
120°C. Conversely diꢀphosphine ligated ruthenium catalysts in up to 87% yield lead to the highest overall
conversion but only moderate trans/cis ratios of only 1:3.1ꢀ1:5.
Introduction

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Scheme 1: HMF and related compounds
Hydroxymethylfurfural (HMF) is readily available from hexoses by catalytic dehydration of fructose, as
such HMF stands to become a platform chemical, which has for over 10 years been hailed as one of the
“top value-added chemicals from biomass”.1 The utilisation of HMF as a renewable feedstock has
generated much academic interest,2 with the hope of replacing petroleumꢀderived buildingꢀblocks with
renewable plantꢀbased ones. Moreover, it gives a straightforward access to molecules, such as FDCA,
which are not readily available from petrochemical sources. Based on HMF, the diol tetrahydrofuranꢀ
dimethanol (THFDM) can be accessed, which is discussed as a bioꢀbased monomer for polyesters or
polyurethanes.3 The reduction of HMF to bis(hydroxymethyl)furan (BHMF) has been reported using both
homogeneous4 and heterogeneous methodologies.5 Further reduction to THFDM was observed, either as
an overꢀreduced side product or as the target molecule. Of particular interest, in one report where THFDM
was formed after Pd/Al2O3ꢀcatalysed hydrogenation of HMF under 70 bar of H2. The ratio of cis to transꢀ
THFDM was analysed,6 this ratio was 9:1 in favour of the cis isomer, which can be explained by a
preferential syn-facial hydrogenation, likely following the HoriutiꢀPolanyi mechanism.7 Additionally,
heterogeneous hydrogenation of bioꢀderived furans,8 can in some cases lead to overꢀreduction and ring
opening.9 The mixture of cis/transꢀTHFDM has been used in patented bioꢀbased polymers,10 however the
use of purely transꢀTHFDM is likely to show interesting properties as a bioꢀbased monomer due to the
possibility of a linear alignment of the monomeric transꢀblocks compared to that of a corkscrewꢀlike
arrangement of the cusped cisꢀmonomers. A recent report of the reduction of levoglucosenone yielded,
amongst other products, THFDM in at best 2.5:1 cis:trans ratio,3 which was, as the authors point out, the
highest ratio of trans:cis obtained to date. However, this approach requires an elevated temperature
(150 ºC) and yields a mixture of products (58 % combined yield of cisꢀ and transꢀTHFDM). In order to
achieve a more transꢀselective reaction, we investigated homogeneous catalysis for the hydrogenation of
HMF to THFDM, as this reduction had so far been limited to heterogeneous catalysis. Furans remain a
difficult class of substrates with comparatively few examples of hydrogenation: monoꢀsubstituted furans
were hydrogenated using bioꢀpolymer heterogenised noble metals,11 as well as homogeneous systems.12
The Ruꢀcatalysed homogeneous reduction of HMF and related furans was studied in one dissertation,13
though it led only to the reduction and/or hydrogenolysis of the aldehyde group, but not to the formation
of THFDM. The reduction of a few furans using homogeneous catalysts is known and the cisꢀ
diastereomer is normally favoured.14 However, the combination of the aldehyde and alcohol groups of
HMF (which can act as guiding groups) and through a choice of ligands (to modify the catalysts and its
selectivity/activity) a preferred trans-hydrogenation could be envisaged.

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Results and Discussions
A key element in the study of reactions involving HMF is the variable quality of the starting material.
Initial screenings were performed with a large supply of HMF synthesised by BASF (kept at 3 ºC, under
air) which gradually showed a decrease in activity when reproducing experiments. Subsequently, different
commercial sources of 99 % pure HMF were used, leading to irreproducible results due to variance from
one batch of HMF to the next as well as exhibiting a gradual decrease in activity over time. The most
consistent results were obtained by subliming the HMF (irrespective of the source) at 3 ·10ꢀ1 mbar at ca.
130 °C (see Supporting Information for more details on the sublimation).
As will be seen in the tables below, standard screenings and reaction optimisation are analysed by
comparing the relative integration of the NMR peaks of the starting material, intermediate and product.
Since all three compounds are known and characterised, it is not necessary to isolate each compound after
every screening, this enables rapid identification of promising catalytic systems, which were in turn more
thoroughly investigated. For ease of comparison, in each case the amount of BHMF was normalised and
the amount of remaining HMF and formed THFDM expressed as a ratio to the amount of BHMF. In the
tables, top entries were repeated with an internal standard, in order to determine an NMR yield, as in many
cases dark precipitates were formed in the autoclave.15 These insoluble precipitates were most likely
humins which also occur in the synthesis of HMF.16
1. NHC as ligands
The use of NHCꢀligated Ru complexes has been reported by Wysocki et al. to hydrogenate diꢀsubstituted
furans.14b While the reaction led to cisꢀdiastereomers being formed preferentially, this method was
nonetheless investigated for its suitability to reduce HMF. As can be seen from Table 1, the unsaturated
backbone proved to be more active compared to its saturated homologs (entries 1ꢀ2). Aromatic
substitution on the carbene provided much more active catalysts than the alkyl substituted variants (entries
1ꢀ2, 6ꢀ7 vs 3ꢀ5), likely due to electronic effects. Interestingly, the recently reported mixed DiPP/cycloalkyl
carbene17 only gave trace amounts of THFDM (entry 8). Within the carbene family, IPr offered the highest
trans:cis selectivity. However the highest conversions were observed for the bulky naphthyl containing
substituents (entries 6ꢀ7).
Table 1: Ruthenium catalysed hydrogenation of HMF using NHC ligands
Entry
Ligand
HMFa
0.03
(1%)
BHMFa
THFDMa
Cis:transb
Unsaturated, R=diisopropylphenyl
1
0.37
(17%)
1
1.26:1
(IPr), X=Cl
(43%)
Saturated, R=diisopropylphenyl (SIPr),
ꢀ
1 (92%)
0.08 (8%)
2.3:1
2
X=Cl