Direct hydrogenation of biobased carboxylic acids mediated by a nitrogen-centered tridentate phosphine ligand

Article abstract of DOI:10.1002/cssc.201501461

A novel nitrogen-centered tridentate ligand was identified from a series of multidentate ligands and applied for the direct hydrogenation of 9 biogenic acids into alcohols, lactones and esters with high yields. Comparison of substrates and ruthenium precursors suggested that the Ru^II hydride cationic species was more active to transform acids than the corresponding lactone or esters.

Full text of DOI:10.1002/cssc.201501461

DOI: 10.1002/cssc.201501461
Communications
Direct Hydrogenation of Biobased Carboxylic Acids
Mediated by a Nitrogen-centered Tridentate Phosphine
Ligand
Li Deng,*[a] Bin Kang,[a] Ulli Englert,[b] Jürgen Klankermayer,[a] and Regina Palkovits*[a]
A novel nitrogen-centered tridentate ligand was identified
from a series of multidentate ligands and applied for the direct
hydrogenation of 9 biogenic acids into alcohols, lactones and
esters with high yields. Comparison of substrates and rutheni-
um precursors suggested that the RuII hydride cationic species
was more active to transform acids than the corresponding
lactone or esters.
have been reported. Moreover, the efficient hydrogenation of
several biogenic acids using complexes with trialkylphosphines
or triphos ligands required high reaction temperatures (ca.
2008C) or strong acid additive.[5–7,14] Using a L1 based complex,
only the efficient hydrogenation of levulinic acid to 1,4-pentan-
diol (PDO) and 2-methyltetrahydrofuran has been reported.[13b]
Considering the limited number of suitable catalysts and the
importance of a direct hydrogenation of carboxylic acids, the
investigation of more catalyst structures and ligands for this
challenging reaction becomes imperative. Therefore, we syn-
thesized a class of nitrogen-centered multidentate phosphine
ligands and applied them with ruthenium precursors to cata-
lyze the hydrogenation of biogenic acids (Scheme 1 and Sup-
porting Information, Scheme S1). Compared to the synthesis of
Carboxylic acids including lactic acid, levulinic acid, and itacon-
ic acid are an important class of building blocks produced in
biorefineries.[1–3] Catalytic hydrogenation is an ideal way to
convert oxygen-rich carboxylic acids into alcohols or lac-
tones.[4–7] However, the direct hydrogenation of carboxylic
acids remains a challenging task in homogeneous catalysis. Al-
though the green reductant H2 has been successfully applied
to hydrogenate esters and amides using various noble or non-
noble metal catalysts under mild conditions,[8–11] there are only
few excellent examples for carboxylic acid hydrogenation.[4–6]
A
very efficient homogeneous catalyst system was introduced by
Klankermayer and Leitner, using Ru(acac)3 and the tridentate
ligand triphos in combination with acidic additives.[5a] Based on
mechanistic investigations the molecularly defined complex
Ru(triphos)(TMM) (TMM= trimethylene methane) could be es-
tablished, demonstrating unprecedented performance in the
hydrogenation of carboxylic and carbonic acid derivatives.[5]
Very recently, the triphos-based catalytic system could be fur-
ther improved and established as a general method for carbox-
ylic acid and amide hydrogenation.[6]
Despite the success of the discussed catalytic systems and
the deep understanding of the role of metal complexes and
acid additives in reaction mechanisms, only a limited number
of ruthenium catalysts or ligands are effective for this reaction.
To the best of our knowledge, only ruthenium catalysts based
on three types of ligands, that is, trialkylphosphines,[7] tri-
phos,[12] and the nitrogen-centered triphos analogue L1,[13]
[a] Dr. L. Deng, B. Kang, Prof. Dr. J. Klankermayer, Prof. Dr. R. Palkovits
Institut für Technische und Makromolekulare Chemie
RWTH Aachen University
Scheme 1. Direct hydrogenation of carboxylic acids using homogeneous Ru-
catalysts containing triphos-type ligands.
Worringerweg 2, 52074 Aachen (Germany)
E-mail: deng.li@itmc.rwth-aachen.de
palkovits@itmc.rwth-aachen.de
[b] Prof. Dr. U. Englert
Institute of Inorganic Chemistry
RWTH Aachen University
Aachen (Germany)
the reported tripodal ligands L1 with C3 symmetry, using
amines instead of ammonia as starting material facilitates
access to more candidates for hydrogenation due to the diver-
sity of available amine building blocks. Herein, L2 was identi-
fied as a new ligand for the direct hydrogenation of various
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201501461.
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Communications
biogenic acids into alcohols, lactones, and esters in the ab-
sence of acid additive, with overall yields from 82 to 99 mol%.
We began our investigation with synthesis of the ligands
listed in Scheme 1. Substitution with diphenylphosphino lithi-
um and Mannich-like reaction was employed to functionalize
ammonia, primary, and secondary amines to afford nitrogen-
centered multidentate phosphine ligands L1–L6. Among them,
L1 was reported as an analogue of triphos to form various
ruthenium complexes and applied to the hydrogenation of lev-
ulinic acid.[13] Herein, we chose L1 as benchmark ligand to ex-
amine the catalytic activities of the combination of Ru(acac)3
and these ligands in the hydrogenation of levulinic acid. As
shown in Table 1, only gamma-valerolactone (GVL) and trace
As shown in Table 1, entries 9~14, chloride-containing precur-
sors RuCl2(PPh3)3 afforded mainly GVL and low yields of PDO.
RuH2(CO)(PPh3)3 also catalyzed the conversion of levulinic acid
to GVL. The results indicate that the chloride and carbonyl co-
ordinating groups were detrimental for this catalytic hydroge-
nation, though hydrides were presented. The PPh3-free precur-
sor RuCOD(methylallyl)2 which was used to generate active
Ru(trimethylenemethane)triphos complex for catalytic hydro-
genation, was also tested. However, it did not exhibit activity
in the presence of L2. Using 0.2 mol% of the dihydride com-
plex RuH2(PPh3)4, the yield of PDO could be improved to
61 mol%. To our delight, close to full conversion could be ach-
ieved in the presence of 0.5 mol% RuH2(PPh3)4 and L2, which
is comparable to the performance of the ruthenium dihy-
dride complex generated from the benchmark L1.[13b]
Note that RuH2(PPh3)4 was not able to catalyze the reac-
tion efficiently without L2. In addition, the combination
of the other ligands and RuH2(PPh3)4 were also examined
and presented in Table S1.
Table 1. Catalytic hydrogenation of levulinic acid using ruthenium precursors
and various ligands.[a]
Entry Ligands Ruthenium
precursor
Amount of
Yield of
Yield of
precursor [mol%] GVL [mol%] PDO [mol%]
To further understand the above results at a molecular
level, the coordination of L2 with ruthenium complexes
RuCl2(PPh3)3, RuH2(PPh3)4, and Ru(methyallyl)2(COD) was
investigated by 31P{1H} NMR and single-crystal X-ray dif-
fraction (XRD) techniques. Owing to the presence of two
phosphorus species in L2 (Supporting Information, Fig-
ure S3), after treatment with RuCl2(PPh3)3 in toluene,
a doublet peak at 40 ppm and a triplet peak at 59 ppm
appeared with a peak area ratio of 2:1 (Supporting Infor-
mation, (Figure S11), indicating that all three diphenyl-
phosphine groups were coordinated to the ruthenium
center. The remote phosphine group took trans position
and the two identical phosphine groups attached to two
cis-positions based on the position of the possibly coordi-
nated solvent or vacancy (Scheme 2). However, after
column separation using a methanol/chloroform mixture
as mobile phase, the complex changed into a structure in
which the two near phosphine groups occupied a cis- and
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
–
Ru(acac)3
Ru(acac)3
Ru(acac)3
Ru(acac)3
Ru(acac)3
Ru(acac)3
Ru(acac)3
Ru(acac)3
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.5
0.2
0.2
0.2
99
75
70
88
95
88
91
33
95
78
97
97
32
2
<1
22
24
11
4
10
6
66
3
19
<1
2
61
98
3
L1
L2
L3
L4
L5
L6
L2
L2
L2
L2
L2
L2
L2
–
RuCl2(PPh3)3
RuHCl(PPh3)3
RuH2(CO)(PPh3)3
RuCOD(methylallyl)2 0.2
RuH2(PPh3)4
RuH2(PPh3)4
RuH2(PPh3)4
0.2
0.5
0.2
95
[a] Reaction conditions: 10 mmol substrate, 5 mL THF, 1.5 equiv ligand, 70 bar H2,
1608C, 18 h.
amount of PDO were generated, and no 2-mehyltetrahydrofur-
an was detected when solely using 0.2 mol% Ru(acac)3 as cata-
lyst. As reported previously, the benchmark ligand L1 dramati-
cally improved this reaction, giving 22 mol% of PDO and
75 mol% GVL. To our delight, the novel ligand L2, which has
one ethylene linker between the central nitrogen atom and
a terminal diphenyl phosphino moiety, exhibited comparable
performance (entry 3). Further modification of the linkers gave
another two novel structures, L3 containing two ethylene link-
ers and L4 containing a diphenyl phosphino propyl group.
However, they were inferior to L2. The replacement of the
remote diphenyl phosphino group with amines or pyridine af-
forded ligands L5 and L6, which were also less effective in this
reaction. Based on the L2/Ru(acac)3 catalyst, we examined the
reaction conditions for the hydrogenation of levulinic acid
(Supporting Information, Table S1). Under optimized condi-
tions, 66 mol% PDO and 33 mol% GVL could be obtained in
the presence of 0.5 mol% of the in situ-generated catalyst at
1608C.
a
trans-site of ruthenium, respectively. Correspondingly,
a
31P NMR spectrum exhibited three triplet peaks with equal
peak areas from 22 to 30 ppm. Interestingly, the two isomers
have a reversible transformation according to the 31P NMR
Further optimization of the L2-based catalytic system was
carried out on the basis of the ruthenium precursors screening.
Scheme 2. Isomers of the complex obtained from L2 with RuCl2(PPh3)3,
RuH2(PPh3)4 and RuCOD(methylallyl)2.
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spectra. In the reaction solvent THF, signals of both isomers
could be observed in 31P NMR (Figure S11). According to ESI–
MS analysis, both cations of dimers and monomers were de-
tected (m/z calculated for [Ru2(m-Cl)3(L2)2]+ 1559.1581, found
1559.1499; m/z calculated for [RuCl(L2)]+ 762.0944, found
762.0906). Single-crystal XRD analysis revealed that the RuII
complex contains a dinuclear [Ru2(m-Cl)3(L2)2] cation (Support-
ing Information, Figure S8) and a chloride counteranion. The
compound crystallizes as a chloroform solvate; further details
are given in the Supporting Information. In agreement with
the performance of L2/RuCl2(PPh3)3, 0.2 mol% [Ru2(m-
Cl)3(L2)2]Cl was only able to convert 11 mol% levulinic acid to
PDO.
13C NMR spectra, the trimethylenemethane acts as the counter
anion ligand (Supporting Information, Figure S14), indicating
the formation of Ru(trimethylenemethane)L2. The molecular
structure of the complex was also identified by single-crystal
XRD analysis (Supporting Information, Figure S15); it crystalli-
zes as a THF solvate. In the presence of 0.2 mol% RuH2(PPh3)L2
complex, 15 mol% PDO was generated. Adding three equiva-
lents of PPh3 improved the yield to 27 mol%, respectively. Sim-
ilarly, Ru(trimethylenemethane)L2 delivered 14 mol% PDO. The
promoting effect of PPh3 was also observed in the presence of
one or four equivalent of PPh3 (Table S2). These results suggest
that PPh3 released from the in situ-generated catalyst
RuH2(PPh3)4/L2 is beneficial for the hydrogenation reaction.
The transformation of levulinic acid to PDO in dry THF did
not proceed via GVL as intermediate in the presence of L2 and
ruthenium precursors, because GVL as substrate decreased the
PDO yield (Supporting Information, Table S3). Water formation
presents the major difference between the reduction of levu-
linic acid and GVL or alkyl levulinates, respectively. Therefore,
we added one equivalent of water with GVL into the reaction
system, improving the PDO yield significantly to above
90 mol%. In the presence of water, GVL is likely to be hydro-
lyzed to 4-hydroxyl pentanoic acid, which may act as real inter-
mediate of PDO. More evidence could be found by comparing
lactic acid, succinic acid, and butyric acid with their corre-
sponding esters. These results suggested that the L2/Ru cata-
lysts are more reactive towards the hydrogenation of carboxyl-
ic acids rather than esters or lactones as substrates. A plausible
reason for the discrimination between acids and esters is that
the RuII-hydride cation generated in the presence of a proton
source is responsible for the catalytic hydrogenation of carbox-
ylic acid in anion form.
Owing to the flexible coordination structures, L2 and
RuH2(PPh3)4 in THF also generated two isomers by readily re-
placing three PPh3 moieties according to the 31P NMR spectra
in which two groups of peaks were found (Supporting Infor-
mation, Figures S12 and S13). The major group contains four
peaks with equal areas while the other group has three peaks
with an area ratio of 2:1:1. This suggest that the RuII center of
the major isomer was coordinated by the remote phosphine
group at cis position. The other two phosphine groups occu-
pied a cis-site and a trans-site related to the position of the
PPh3 moiety while the minor isomer contained two identical
phosphine groups at cis-sites and a remote phosphine at
1
a trans-site. Moreover, H NMR also shows two unequal hydride
peaks at 8.1 and 8.7 ppm. Unlike complexes from L2/
RuCl2(PPh3)3, no dimeric product was found by ESI–MS. A peak
at m/z of 990.2236 indicates the formation of a monomeric
cation [RuH(PPh3)L2]+ (calculated m/z: 990.2245), which is
a possible active species regarding the mechanism of the hy-
drogenation of acids with Ru(acac)3/triphos.[6a] Therefore, the
complex RuH2(PPh3)L2 should be identified as two monomeric
dihydride structures (Scheme 2). Unlike the reported complex
RuH2(PPh3)L1,[13b] RuH2(PPh3)L2 did not give single crystals due
to the coexistence of the two isomers. To prepare a PPh3-free
complex, Ru(methylallyl)2(COD) with one equivalent of L2 was
heated in toluene for 6 days. The resulting complex shows
a triple peak at 33 ppm and a double peak at 37 ppm in
To expand the substrate scope, several biogenic acids were
hydrogenated using L2 and ruthenium precursors (Table 2).
Like the hydrogenation of levulinic acid, lactic acid could be
fully converted to 1,2-propanediol under the same conditions.
For dicarboxylic acid substrates (entries 2~5), 1 mol% of ruthe-
nium precursors, higher temperature, and longer reaction time
were required to achieve high yields of lactones. Although no
diols were generated from these dicarboxylic acids, butyrolac-
1
31P NMR spectra with an area ratio of 1:2. According to H and
Table 2. Hydrogenation of biogenic acids using L2/Ru precursors.[a]
Entry]
Ruthenium
precursor
Amount of
precursor [mol%]
Substrate
T [8C]
t [h]
Products
Yield
[mol%]
1
RuH2(PPh3)4
RuH2(PPh3)4
Ru(acac)3
Ru(acac)3
Ru(acac)3
0.5
1
1
1
1
lactic acid
160
160
170
170
170
170
18
48
48
48
48
48
1,2-propanediol
methyl-g-butyrolactone
butyrolactone
butyrolactone
butyrolactone
octyloctanoate
octanol
99
95
93
95
88
78
5
2[b]
3[c]
4[c]
5[c]
6
itaconic acid
succinic acid
fumaric acid
maleic acid
RuH2(PPh3)4
2
octanoic acid
7
8
RuH2(PPh3)4
RuH2(PPh3)4
2
2
butyric acid
acetic acid
170
170
48
48
butylbutanoate
butanol
ethylacetate
ethanol
47
35
45
39
[a] Reaction conditions: 5 mmol substrates, 5 mL THF, 1.5 equivalent of L2 based on Ru precursors, 70 bar H2. [b] The products are a-methyl-g-butyrolac-
tone and b-methyl-g-butyrolactone with a ratio of 9:5. [c] 5 mL 2-methyltetrahydrofuran was used as solvent.
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In summary, a class of nitrogen-centered multidentate phos-
phine ligands is tested for the hydrogenation of carboxylic
acids. Among them, catalysts based on the novel compound
L2 exhibit good activity for this challenging reaction. Compari-
son of various ruthenium precursors and different substrates
suggests the L2-based RuII-hydride cation as active species.
The coordination structure of L2 with RuCl2(PPh3)3 and
RuH2(PPh3)4 reveals that L2 chelates with the RuII centre in two
manners: two equal phosphines at cis sites, or at both cis and
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99 mol% in the absence of acidic additives. Considering the ef-
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ually achieve the efficient and benign hydrogenation of car-
boxylic acids.
Acknowledgements
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Keywords: carboxylic acids
hydrogenation · ruthenium · tridentate ligands
·
homogeneous catalysis
·
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Received: October 28, 2015
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Published online on January 8, 2016
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