On the Functional Group Tolerance of Ester Hydrogenation and Polyester Depolymerisation Catalysed by Ruthenium Complexes of Tridentate Aminophosphine Ligands

Article abstract of DOI:10.1002/chem.201500907

The synthesis of a range of phosphine-diamine, phosphine-amino-alcohol, and phosphine-amino-amide ligands and their ruthenium(II) complexes are reported. Five of these were characterised by X-ray crystallography. The activities of this collection of catalysts were initially compared for the hydrogenation of two model ester hydrogenations. Catalyst turnover frequencies up to 2400 h^-1 were observed at 85 °C. However, turnover is slow at near ambient temperatures. By using a phosphine-diamine Ru^II complex, identified as the most active catalyst, a range of aromatic esters were reduced in high yield. The hydrogenation of alkene-, diene-, and alkyne-functionalised esters was also studied. Substrates with a remote, but reactive terminal alkene substituent could be reduced chemoselectively in the presence of 4-dimethylaminopyridine (DMAP) co-catalyst. The chemoselective reduction of the ester function in conjugated dienoate ethyl sorbate could deliver (2E,4E)-hexa-2,4-dien-1-ol, a precursor to leaf alcohol. The monounsaturated alcohol (E)-hex-4-en-1-ol was produced with reasonable selectivity, but complete chemoselectivity of C=O over the diene is elusive. High chemoselectivity for the reduction of an ester over an alkyne group was observed in the hydrogenation of an alkynoate for the first time. The catalysts were also active in the depolymerisation reduction of samples of waste poly(ethylene terephthalate) (PET) to produce benzene dimethanol. These depolymerisations were found to be poisoned by the ethylene glycol side product, although good yields could still be achieved. A simple catalyst for difficult reductions: Ruthenium complexes of P,N,N and P,N,O ligands catalyse the reduction of esters with high activities. The Ru complex of a phosphine-diamine ligand (see scheme) has been found to be a good catalyst for reducing alkene-, diene-, and alkyne-functionalised esters, displaying good activity and chemoselectivity. This catalyst was also active in the hydrogenation of waste poly(ethylene terephthalate) (PET).

Full text of DOI:10.1002/chem.201500907

DOI: 10.1002/chem.201500907
Full Paper
&
Homogeneous Catalysis
On the Functional Group Tolerance of Ester Hydrogenation and
Polyester Depolymerisation Catalysed by Ruthenium Complexes
of Tridentate Aminophosphine Ligands
JosØ A. Fuentes, Samuel M. Smith, M. Theresa Scharbert, Ian Carpenter, David B. Cordes,
Alexandra M. Z. Slawin, and Matthew L. Clarke*[a]
Abstract: The synthesis of a range of phosphine-diamine,
phosphine-amino-alcohol, and phosphine-amino-amide li-
gands and their ruthenium(II) complexes are reported. Five
of these were characterised by X-ray crystallography. The ac-
tivities of this collection of catalysts were initially compared
for the hydrogenation of two model ester hydrogenations.
Catalyst turnover frequencies up to 2400 hÀ1 were observed
at 858C. However, turnover is slow at near ambient tempera-
tures. By using a phosphine-diamine RuII complex, identified
as the most active catalyst, a range of aromatic esters were
reduced in high yield. The hydrogenation of alkene-, diene-,
and alkyne-functionalised esters was also studied. Substrates
with a remote, but reactive terminal alkene substituent
could be reduced chemoselectively in the presence of 4-di-
methylaminopyridine (DMAP) co-catalyst. The chemoselec-
tive reduction of the ester function in conjugated dienoate
ethyl sorbate could deliver (2E,4E)-hexa-2,4-dien-1-ol, a pre-
cursor to leaf alcohol. The monounsaturated alcohol (E)-hex-
4-en-1-ol was produced with reasonable selectivity, but com-
plete chemoselectivity of C=O over the diene is elusive.
High chemoselectivity for the reduction of an ester over an
alkyne group was observed in the hydrogenation of an alky-
noate for the first time. The catalysts were also active in the
depolymerisation reduction of samples of waste poly(ethy-
lene terephthalate) (PET) to produce benzene dimethanol.
These depolymerisations were found to be poisoned by the
ethylene glycol side product, although good yields could
still be achieved.
Introduction
However, we observed fairly moderate results when using
1 for ester hydrogenation with procedures that we had opti-
mised for enantioselective ketone reduction.[6a] More recently,
after noticing the tendency for more modern ester hydrogena-
tion catalysts to need rather large base/catalysts ratios, we
communicated the use of achiral catalyst 4 (Figure 1) that can
reduce aromatic esters at near ambient temperatures provided
that base/catalyst ratios of 30 to 50 are employed.[7] Here, we
give a full account of this work, including comparisons with
other related ligand systems. We also now address the unre-
solved question of the relative rates of reduction of esters,
alkene, diene, and alkyne functional groups, and study the de-
polymerisation of a polyester.
The catalytic reduction of esters to alcohols by using molecular
hydrogen can be considered an attractive method relative to
the use of stoichiometric reducing agents. Improved atom
economy, reduced cost, and safer, easier product isolation are
key advantages. Heterogeneous metal surface catalysts can ac-
complish this task rather well, albeit generally at high pres-
sures.[1] Moreover, many useful functional groups such as
alkene, halide, ketone, benzyl, and sometimes even phenyl
groups are hydrogenated under typical ester hydrogenation
conditions. Homogeneous catalytic ester hydrogenation has
been put forward as a potentially valuable reaction that could
be chemoselective and operate under mild conditions.[2,4] For
a long time, and until quite recently, this was just a possibility,[4]
Results and Discussion
but in recent years ester hydrogenation has come of age.[5]
A
variety of catalysts have now given encouraging results. Our
interest in this reaction started around ten years ago with
a study of the chiral phosphine-diamine catalyst 1 (Figure 1).
The chiral ketone hydrogenation catalysts 1, 2 and achiral 3
were prepared as described before;[6] an optimised synthesis of
2 and 4 at multigram scale is archived in the Experimental Sec-
tion (the Supporting Information). The synthesis of new Ru
complexes 5–7 is also described. The crystal structure of 2 has
now been solved and is shown in Figure 2. Although the for-
mula has always been clear from the NMR spectrum, MS, and
microanalytical data, this structure confirms several aspects
that were previously unknown. The structure shows the meri-
dional coordination of the P,N,O ligand. The DMSO ligand is co-
[a] Dr. J. A. Fuentes, S. M. Smith, M. T. Scharbert, Dr. I. Carpenter,
Dr. D. B. Cordes, Prof. A. M. Z. Slawin, Dr. M. L. Clarke
School of Chemistry, University of St. Andrews
EaStCHEM, St. Andrews, Fife, KY16 9ST (UK)
E-mail: mc28@st-andrews.ac.uk
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500907.
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Figure 1. Ruthenium complexes of phosphine-diamine, phosphine-amino-
alcohol and phosphine-amino-amide ligands used in this study.
ordinating through sulfur, rather than oxygen, and the chlor-
ides adopt mutually trans orientations. The phosphine amino-
alcohol is in a neutral coordination mode with the nitrogen
atom becoming chiral at N on coordination to Ru. In many
ways this structure is similar to the structure of 1 that we pre-
viously reported.[6a]
Figure 3. X-ray structure of complex 4. One molecule of acetonitrile and hy-
drogen atoms (except NH and NH2) are omitted for clarity. Selected bond
lengths [Š] and bond angles [8]: RuÀP(1) 2.2806(7), RuÀN(1) 2.130(2), RuÀ
N(2) 2.164(2); N(2)-Ru-P(1) 170.84(6), N(1)-Ru-P(1) 91.10(5), N(1)-Ru-N(2)
79.89(7).
This ligand was explored because it has a very low steric
demand around the amide oxygen, but this terminus also
cannot be readily deprotonated or form hydrogen bonds
under the reaction conditions. The RuII complex was also struc-
turally characterised (Figure 4). As expected, this is a PNO
donor ligand that prefers a meridional coordination. The rela-
tive orientation and coordination mode of the chloride and
DMSO ligands is as discussed above. The nitrogen of the
amide is, as expected, planar as clearly indicated by the short
Csp2ÀNsp2 bond length of 1.326(7) Š and the N-C-O angle
(120.6(4)8). With this series of complexes and the previously
published structure of 1,[6a] it can be seen that the longer RuÀ
P bond lengths across the series correspond to PNN complexes
1 and 4, 2.2912 (13) and 2.2806(7) Š, respectively. The PN-alco-
hol complexes 2 and 6 and PN-amido complex 7 all have
shorter RuÀP bonds because the amide and alcohol are
weaker trans labilising ligands. In the PN-amido complex 7,
binding through a sp2 oxygen, the RuÀO bond is shorter,
2.156(3) Š, in comparison with those of complexes 2 and 6,
2.205(3) and 2.188(3) Š, respectively. Geometry around the
metal centre exerts an important effect in catalysis.
Figure 2. X-ray structure of complex 2. Two acetonitrile molecules and hy-
drogen atoms (except NH and OH) are omitted for clarity. Selected bond
lengths [Š] and bond angles [8]: RuÀP(1) 2.256(1), RuÀN(20) 2.151(5), RuÀ
O(21) 2.205(3); O(21)-Ru-P(1) 171.31(9), N(20)-Ru-P(1) 92.3(1), N(20)-Ru-O(21)
79.2(1).
The PNN and PNO ligands used in complexes 1–5 and 7 are
chelating the Ru centre through a P atom and the N or the O
atoms, thus forming one six-membered and one five-mem-
bered chelate ring. Complex 6 (Figure 5), with two six-mem-
bered chelate rings, was prepared to study the effect of in-
creasing the size of the chelate ring of the pre-catalyst in the
ester hydrogenation reaction. Ru complex 6 and its ligand pre-
cursor are, to the best of our knowledge, new species (full de-
tails in the Supporting Information). The N-Ru-O angle in com-
plex 6 was, as expected, the largest (87.2(1)8), followed by
PNO complex 2 (79.2(1)8) and amido complex 7 (77.2(1)8). The
N-Ru-N angle in PNN complex 4 (79.89(7)8) was very similar to
the N-Ru-O angle in 2 (79.2(1)8).
The synthesis of the achiral ethylene-diamine-derived cata-
lyst 4 is described in the Supporting Information and works
well providing the optimised procedure is followed. Catalyst 4
was characterised by using X-ray crystallography (Figure 3).
This complex has a rather similar structure to the phosphine
amino-alcohol complex 2; a similar meridional neutral coordi-
nation mode, sulfur-bound DMSO ligand, and trans chloride li-
gands are all observed. RuII complex 7 and its ligand precursor,
are, to the best of our knowledge, new species and also can
be synthesised conveniently at multigram scale (the Support-
ing Information).
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Table 1. Hydrogenation of para-fluorobenzoic acid methyl ester using
[RuCl2(P^N^X)(dmso)] complexes as pre-catalysts.
Entry[a]
Cat.
Load
[mol%]
tBuOK/
cat.
t
[h]
T
[8C]
Product[b]
[%]
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
4
4
4
4
4
4
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.2
0.5
0.5
0.5
40:1
40:1
50:1
40:1
40:1
40:1
40:1
50:1
30:1
50:1
50:1
50:1
50:1
2
65
65
100
65
65
65
65
100
50
50
50
37
97
43
94
84
25
23
97
93
2
16
2
2
2
2
2
9
16
16
16
64
100
10
11[c]
12
13
>99
0
69
30
30
Figure 4. X-ray structure of complex 7. Hydrogen atoms (except NH) are
omitted for clarity. Selected bond lengths [Š] and bond angles [8]: RuÀP(1)
2.254(1), RuÀN(20) 2.152(4), RuÀO(2) 2.156(3); O(2)-Ru-P(1) 170.66(9), N(20)-
Ru-P(1) 93.4(1), N(20)-Ru-O(2) 77.2(1).
>99
[a] Unless otherwise stated, the reactions were carried out by using
0.5 mol% [RuCl2(P^N^X)(dmso)], tBuOK as base (50:1, base/catalyst) at
1008C at an initial pressure of 50 bar with 0.4 mmol of para-fluorobenzoic
acid methyl ester in Me-THF (3 mL). [b] Conversion determined by
19F NMR spectroscopy. [c] Use of KOH or MeONa as base gives no conver-
sion.
ditions in terms of turnover or minimising pressures; gas
mixing relative to a large vessel with direct stirring will not be
as effective, hence the use of 50 bar pressure to minimise
mass transport problems. Catalysts 1–7 were initially examined
at 658C for just 2 h using base/catalyst ratios of 50:1 (Table 1,
entries 1–7).
Catalyst 3 was tested at 1008C initially and not tested fur-
ther due to its low activity (Table 1, entry 3). The new catalysts
5–7 were then benchmarked in the ester hydrogenation reac-
tion (Table 1, entries 5–7). PNO Complex 7 proved to be a poor
catalyst under the reaction conditions employed (Table 1,
entry 7), but the achiral PNO complex 5, structurally similar to
complex 4, achieved good levels of activity (entry 5). Increasing
the size of the chelate ring as in complex 6 proved to be detri-
mental to the activity of the complex in the ester hydrogena-
tion reaction (Table 1, entry 6 vs. 5). This type of reactivity has
been observed in transfer hydrogenation.[8] Complexes 2 and 4
showed the highest level of activity, achieving near full conver-
sion within 2 h. Reaction conditions were next studied using
relatively cheap achiral catalyst 4 (Table 1, entries 8–13). Cata-
lyst loadings down to 0.2 mol% (Table 1, entry 10) and base/
catalyst ratios down to 30:1 (Table 1, entry 9) could be used al-
lowing the reduction of aryl esters at temperatures from
1008C to as low as 308C (Table 1, entry 14). Catalyst 4 was
used in the hydrogenation of other aromatic esters with differ-
ent electronic properties (Table 2). Notably, 3-pyridyl ester 16
could be successfully hydrogenated, but 2-pyridyl and 2-pyrrol-
yl esters were not suitable (Table 2, entries 8 and 9). A possible
explanation for this fact is the ability of both ester and alcohol
of the corresponding compounds to function as a bidentate
N,O ligand for Ru and inhibiting catalysis. Two reactions were
Figure 5. X-ray structure of complex 6. Hydrogen atoms (except NH and OH)
are omitted for clarity. Selected bond lengths [Š] and bond angles [8]: RuÀ
P(1) 2.254(1), RuÀN(20) 2.200(4), RuÀO(22) 2.188(3); O(22)-Ru-P(1) 175.5(1),
N(20)-Ru-P(1) 92.1(1), N(20)-Ru-O(22) 87.2(1).
With a range of pre-catalysts in hand, it was felt that some
useful information on catalyst design could be obtained from
testing this family in the catalytic hydrogenation of esters. Our
initial screening methodology used a simple aromatic ester,
para-fluorobenzoic acid methyl ester 8 as a substrate (Table 1).
To compare the performance of catalysts under similar condi-
tions, several small reactor vials were placed inside larger pres-
sure vessels. It can be seen that each of these stirs to the same
extent and will be under the same pressure and heat. These
conditions are suited to comparing a single variable, keeping
other conditions constant, and have been used in many other
studies in our group. These are not necessarily optimised con-
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set up in which the hydrogenation of ester 8 was carried out
(508C, 50 bar H2, 0.5 mol% cat., 25% base) in the presence of
20 mol% of either ester 17 or 2-pyridyl-CH2OH. The reactions
only gave 23 and 9% conversions, respectively, compared to
full conversion without these additives. This is therefore consis-
tent with the substrate and especially the product acting as an
inhibitor.
Table 3. Optimisation studies using Ru pre-catalysts for hydrogenation of
ethyl acetate.
Entry
Cat.
tBuOK/
cat.
t
[h]
T
[8C]
Conv.[d]
[%]
TOF
[hÀ1
]
1[a]
4
5
2
7
1
4
4
4
4
4
5
40:1
40:1
40:1
40:1
40:1
40:1
118:1
118:1
240:1
118:1
118:1
3
3
3
3
65
65
65
65
65
75
75
75
85
75
75
46
24
21
2
465
2[a]
242
212
20
3[a]
4[a]
5[a]
3
4
40
6[b]
7[b]
8[c]
1.5
1.5
1.5
1.5
1.5
1.5
40
56
71
63
28
37
808
1131
1434
2470
566
748
Table 2. Hydrogenation of various methyl esters using catalyst 6.
9[c][e]
10[f]
11[c]
[a] The reactions were carried out by using 0.033 mol% Ru catalyst and
1.3 mol% of tBuOK, at an initial pressure of 50 bar using 7.7 mmol of
ethyl acetate in Me-THF (3 mL) (general method A, the Supporting Infor-
mation). [b] The reactions were carried out using 0.033 mol% Ru pre-cat-
alyst, at an initial pressure of 50 bar using 30.7 mmol of ethyl acetate
(3 mL) in Me-THF (3 mL) (general method B, the Supporting Information).
[c] Reactions were carried out in neat ethyl acetate, otherwise using con-
ditions as in footnote b. [d] Ethanol produced determined by 1H NMR
spectroscopy using 1-methylnaphthalene as internal standard.
[e] 0.017 mol% of catalyst and 3.9 mol% of tBuOK were used. [f] Reaction
carried out at constant pressure of 15 bar H2, otherwise as in footnote b.
Entry[a]
Ester
T
[8C]
Product
[%][c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
50
50
50
50
50
>99
>99
>99
>99
>99
0
100
100
100
100
>99
0
(Table 3, entry 9) giving a TOF of 2470 hÀ1 by using an ester to
catalyst loading of 0.017 with 3.9 mol% of base. Lower pres-
sures (15 bar) could be used in the hydrogenation of ethyl ace-
tate although the TOF is reduced somewhat.
0
[a] Unless otherwise stated, the reactions were carried out by using
0.5 mol% of cat., tBuOK as base 25 mol% (50:1, base/catalyst) at an initial
pressure of 50 bar with 0.4 mmol of ester in Me-THF (3 mL). [b] Conver-
sion determined by 1H NMR spectroscopy.
3-(Z)-Hexen-1-ol (leaf alcohol) occurs in the green parts of
many plants and gives them the characteristic “green smell”
odour. This molecule and other structural analogues are used
in the perfumery industry to give a green note to fragrances.
This naturally occurring homoallylic alcohol is also frequently
used in the flavours industry in minty, fruity and green herbal
tea aromas.[9] The reduction of the conjugated dienoate ethyl
sorbate could be part of a very direct route to the fragrance
molecule leaf alcohol involving a stereospecific 1,4-cis-hydro-
genation of sorbic alcohol using [(Cp*)Ru(cod)][BF4]-type com-
plexes (Cp*=1,2,3,4,5-pentamethylcyclopentadienyl; cod=1,5-
cyclooctadiene) to produce the monounsaturated alcohol with
high selectivity.[9a,c] However, the chemoselective ester hydro-
genation of sorbate esters to unsaturated alcohols, to our
knowledge, has never been achieved, and conjugated esters
remain a challenge even for the most chemoselective cata-
lysts.[9a,10]
Next we examined the catalytic activity of the PNN and PNO
Ru complexes for the hydrogenation of ethyl acetate (Table 3).
Initially, to compare performance of catalysts under the same
conditions, several small reactor vials were placed inside larger
pressure vessels (Table 3, entries 1–5). Under 50 bar of hydro-
gen at 658C for 3 h with a 0.033 mol% of catalyst loading and
in Me-THF as solvent, less than 5% conversion was achieved
with phosphine diamine complex 1 and phosphine amino-
amide complex 7 (Table 3, entries 4 and 5). On the other hand,
both phosphine amino-alcohol-based complexes, 5 and 2, dis-
played similar performance achieving the conversion of ethyl
acetate to ethanol with average turnover frequencies (TOF) of
242 and 212, respectively (Table 3, entries 2 and 3).
Under the same mild conditions, the best performance was
achieved using the ruthenium PNN complex
4 (Table 3,
entry 1) to give a conversion of 46% and an average TOF
number of 465 hÀ1. We further investigated the activity of com-
plex 4 in a 50 mL stainless steel, glass-lined autoclave. By using
this vessel and increasing the temperature to 758C doubles
the conversion (Table 3, entry 6). Increasing the base loading
from 1.3 to 3.9 mol% (base/catalyst ratio of 118) enhanced the
performance of complex 4 to produce ethanol with an average
TOF number of 1131 hÀ1 (Table 3, entry 7). The ester hydroge-
nation reaction can also be run in neat ethyl acetate without
solvent, increasing further the reaction rate (Table 3, entry 8).
Complex 4 is also active at a lower catalyst loadings at 858C
We attempted the hydrogenation of different sorbate esters
to achieve a chemoselective reduction of the ester to produce
sorbic alcohol (Table 4), but unfortunately sorbic alcohol was
not observed under the reaction conditions used in this study.
On the other hand, the monounsaturated alcohol 23, possibly
interesting because other isomers of leaf alcohol also possess
a similar type of odour,[9b] could be obtained with moderate
chemoselectivity. We first examined the influence of the ester
moiety in the sorbate ester reduction using Ru catalyst 4
(Table 4, entries 1–5). The reaction using commercially available
ethyl sorbate 20 afforded the highest amount of (E)-hex-4-en-
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1-ol 23 with a 38% conversion (Table 4, entry 5). Next, we ex-
amined the performance of some of the most active catalysts
of the phosphine-containing counterparts. The substitution of
DMSO proved to be more energetically demanding than initial-
ly expected.[11] The treatment of complex 2 with PPh3, even
when using forcing conditions (1508C; see the Supporting In-
formation), was very sluggish, affording the substitution com-
plex only with 40% conversion. Next, we examined the substi-
tution of DMSO with PPhMe2; the smaller cone angle of this
phosphine should facilitate the reaction. This was indeed the
case and the complex was prepared under the conditions
shown in Scheme 1.
(Table 4, entries 6–8). Phosphine-amino-amide catalyst
7
showed the highest selectivity towards 23 with a 51:49 selec-
tivity (Table 4, entry 6). The use of additives (Table 4, entries 9–
13) produced different results. Catalyst 7, in the presence of
1.5 mol% of DMAP as co-catalyst (Table 4, entry 7) gave virtual-
ly no change in the selectivity of the process, but the use of
PPh3 as additive afforded compound 23 with high (80%) selec-
tivity (Table 4, entry 13). We also examined the effect of reduc-
ing pressure and temperature on the selectivity of the reaction.
By reducing the pressure to 30 bar (Table 4, entries 14–16), re-
ducing T to 508C (entries 17–19) or by reducing both (Table 4,
entries 20–22) had a different effect depending on the catalyst
used in the reaction. In the case of complex 7, the selectivity
in the reaction was similar to that achieved under the initial re-
action conditions (50 bar and 658C).
The positive effect of adding a coordinating additive to the
hydrogenation has been shown to improve chemoselectivity in
some cases (Table 4, entries 6 and 13). It was thought that
a variation of these catalysts, in which the labile DMSO is re-
placed by a phosphine or DMAP, could lead to more chemose-
lective catalysts. We were unable to isolate any DMAP-contain-
ing complex, but we succeeded in the synthesis and isolation
Scheme 1. DMSO substitution in Ru complex 2. The isolated pure complex
24 was obtained with 95% conversion and in 64% yield.
The 31P{1H} NMR spectrum of 24 shows two characteristic
groups of doublets at d=62.4 and 21.0 ppm with JPP =38.1 Hz
corresponding to the two different phosphorus atoms. We
then performed an X-ray diffraction study to determine unam-
biguously the structure of complex 24 (Figure 6). The structure
confirms that PPhMe2 has in fact replaced the DMSO ligand.
The coordination mode around the ruthenium atoms remains
broadly the same as that in complex 2, an octahedral rutheni-
um complex with the tridentate neutral PNO ligand and the
two chlorine atoms in apical positions. When compared with
its precursor 2, complex 24 has slightly longer RuÀN and RuÀO
bond lengths (Figures 2 and 6); on the other hand, the RuÀ
P(1) bond is shorter (2.239(1) versus 2.256(1) Š) in complex 2.
Both complexes also show similar N-Ru-O angles: 78.8(1) in
complex 24 versus 79.2(1)8 in complex 2.
Table 4. Optimisation studies using Ru pre-catalyst and sorbate esters.
Entry[a]
Ester
Cat.
P
t
T
Conv.[b]
Conv.[b]
[bar]
[h]
[8C]
22 [%]
23 [%]
1
2
3
4
5
6
7
8
9[c]
10[c]
11[c]
12[d]
13[e]
14
15
16
17
18
19
20
21
22
19
20
21
19
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
4
4
4
4
4
7
2
1
4
7
2
7
7
4
7
2
4
7
2
4
7
2
50
50
50
50
50
50
50
50
50
50
50
50
50
30
30
30
50
50
50
30
30
30
21
21
21
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
65
65
65
65
65
65
65
65
65
65
65
65
65
65
65
65
50
50
50
50
50
50
67
70
58
65
62
49
65
79
67
48
16
39
20
64
54
39
25
31
33
25
31
33
33
30
15
35
38
51
35
21
33
52
53
61
80
36
46
61
37
55
67
37
55
67
2
2
2
2
[a] Unless otherwise stated, the reactions were carried out using
0.5 mol% of cat., tBuOK as base 20 mol% at an initial pressure of 50 bar
using 1.0 mmol of ester in Me-THF (3 mL). [b] Conversion determined by
1H NMR spectroscopy using 1-methylnaphthalene as internal standard.
[c] DMAP (1.5 mol%) used as co-catalyst. [d] DBU (1.5 mol%) used as co-
catalyst. [e] PPh3 (1.5 mol%) used as co-catalyst.
Figure 6. X-ray structure of complex 24. THF and hydrogen atoms (except
NH and OH) are omitted for clarity. Selected bond lengths [Š] and bond
angles [8]: RuÀP(1) 2.239(1), RuÀP(2) 2.278(1), RuÀN(20) 2.187(4), RuÀO(21)
2.236(4); O(21)-Ru-P(1) 170.3(1), N(20)-Ru-P(1) 92.0(1), O(21)-Ru-P(2) 90.5(1),
P(2)-Ru-P(1) 98.89(4), N(20)-Ru-O(21) 78.8(1).
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A direct synthesis involving the use of ligands (Scheme 2)
and [RuCl2(PPh3)3] was also performed. Complexes 25 and 26
were prepared in 94 and 92% yields, respectively. The 31P{1H}
NMR spectrum of 25 and 26 shows the two characteristic
groups of doublets corresponding to the two different phos-
phorus atoms in a similar way to those of complex 24 (see the
Supporting Information). These complexes were compared in
a selection of ester hydrogenation reactions.
phosphine ligand, in the reduction of 27. The reaction proved
to be very unselective towards the reduction of the ester
moiety. These catalysts produced noticeable amounts of other
products that were not observed before, like methyl (Z)-4-styr-
ylbenzoate, the alkene obtained from the selective cis-reduc-
tion of 27, and methyl 4-phenethylbenzoate, the product ob-
tained from the full reduction of the alkyne moiety. Attempts
to use complexes 24–26 in a cis-selective semi-hydrogenation
of the alkyne led to significant amounts of the fully reduced
alkane methyl 4-phenethylbenzoate, so these complexes were
not pursued further.
Table 5. Chemoselective reduction of esters in the presence of alkynes.
Scheme 2. Direct synthesis of complexes 25 and 26.
Entry[a]
Cat.
t
27[b]
28[b]
29[b]
30[b]
[h]
[%]
[%]
[%]
[%]
The chemoselective reduction of esters in the presence of al-
kynes is a challenging process with several possible products
being formed in the reaction and, to the best of our knowl-
edge, no literature precedent. The main products obtained in
these reductions, using methyl 4-(phenylethynyl)benzoate 27
as model substrate, are shown in Table 5. Other possible prod-
ucts such as methyl (Z)-4-styrylbenzoate and methyl 4-phene-
thylbenzoate would be detectable but were not observed in
most cases. Compounds 28–30 are well-known literature com-
1[c]
2[c]
3[d]
4[e]
5
6[c]
7[f]
8[g]
9[c]
10[h]
4
4
4
4
4
2
24
25
7
17
5
5
5
5
17
5
5
17
5
99
97
99
99
98
93
63
66
87
66
61
71
72
78
70 (53)
42
9
17
16
9
6
4
2
9
6
14
3
4
10
3
30
4
5
2
2
5
0
1
3
0
26
1
[a] Reactions were carried out using 0.5 mol% Ru pre-catalyst, at an initial
pressure of 50 bar at 508C using 0.5 mmol of alkyne in Me-THF (1.5 mL)
(general method A, the Supporting Information). [b] Determined by
1H NMR spectroscopy by using 1-methylnaphthalene as internal standard;
the yield of the pure isolated 28 is given in parentheses. [c] PPh3 was
used as additive (1 mol%). [d] PPh3 was used as additive (2 mol%).
[e] DMAP was used as additive (1 mol%). [f] Methyl (Z)-4-styrylbenzoate
(21%), methyl 4-phenethylbenzoate (4%). [g] Methyl (Z)-4-styrylbenzoate
(21%), methyl 4-phenethylbenzoate (8%). [h] Methyl (Z)-4-styrylbenzoate
(22%), methyl 4-phenethylbenzoate (13%).
pounds, and their formation can be detected by H NMR spec-
troscopy. We decided to study the performance of the most
active and selective catalysts in the presence of coordinating
additives and the new phosphine-containing Ru complexes
24–26 in the chemoselective reduction of compound 27. We
initiated our study with complex 4 in the presence of 1 mol%
of PPh3, hoping that the presence of this phosphine could
compete with the alkyne in the coordination with the metal
and thus avoid the reduction of the latter (Table 5, entry 1).
The reaction afforded a positive result; the alkyne formed from
the selective reduction of the ester functionality (28) being the
main product in the reaction (61% conversion after 17 h). The
reaction also afforded a relative large amount of the fully re-
duced product (30). PNO Complexes 2 and 7 were also tested
(Table 5, entries 6 and 9), but their selectivity towards 28 was
inferior. Small amounts of alkene 29 (6–14%) were detected
with all three pre-catalysts. Reducing the reaction time to 5 h
increased the selectivity towards alkyne 28 (71%) with a small
amount of the fully reduced product being formed (4%;
Table 5, entry 2). Increasing the amount of PPh3 did not afford
any noticeable effect in the reaction (Table 5, entry 3). The use
of DMAP as an additive afforded similar results to those ob-
tained by using PPh3 (Table 5, entry 4). Remarkably, using com-
plex 4 with no PPh3 present in the reaction media offered very
similar chemoselectivity in the reaction (Table 5, entry 5). We
also tested pre-catalysts 24–26, which incorporate an extra
We also studied the reduction of an ester with a non-conju-
gated monosubstituted alkene moiety, methyl hex-5-enoate 31
(Table 6), using our best performing catalyst complex 4. Termi-
nal alkene-functionalised esters are more challenging sub-
strates when compared with esters containing internal or more
substituted alkenes, but less of a challenge than dienoates or
a,b-unsaturated esters.[3e] The reduction proceeded with full
conversion in all cases under the reaction conditions shown in
Table 6, affording a mixture of both the saturated and unsatu-
rated alcohols. Catalyst 4 gave the unsaturated alcohol as
major product in all cases. The introduction of DMAP as co-cat-
alyst had a small beneficial effect on the chemoselectivity of
the ester reduction with 4, affording unsaturated alcohol 25
with good chemoselectivity (82%, Table 6, entry 2).
Complexes 25 and 26 were also tested in the reduction of
methyl hex-5-enoate 31. These complexes did not promote
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Table 6. Ru-catalysed reduction of methyl hex-5-enoate.
Table 7. Ru-catalysed reduction of diesters derived from ethylene glycol.
Entry[a]
Solvent
Conv.
31[b]
22[b]
[%]
32[b]
[%]
1
THF
THF
THF
MeTHF
MeTHF
>99
>99
>99
>99
>99
11
10
20
12
13
78
82
72
77
76
2[c]
3[d]
4
Entry Substrate Cat. Load
T
tBuOK/ Conv.[b] 10[b,f] or
[mol%] [8C] cat.
[%]
34b[b] [%]
1[a]
33a
33a
33a
33a
33a
33a
33b
33b
33b
4
5
2
7
4
4
4
5
2
7
4
4
1.0
1.0
1.0
1.0
0.5
1.0
2.0
2.0
2.0
2.0
1.0
2.0
70 20:1
70 20:1
70 20:1
70 20:1
70 40:1
110 20:1
100 10:1
100 10:1
100 10:1
100 10:1
100 40:1
110 10:1
>99
>99
>99
>99
>99
>99
82
37
41
32
55
76[f]
65[f]
5[c]
2[a]
[a] Unless otherwise stated, reactions were carried out using 0.5 mol% of
cat., tBuOK as base 20 mol%, at an initial pressure of 50 bar using
0.5 mmol of ester in MeTHF (1.5 mL). [b] Conversion [%] determined
by1H NMR spectroscopy using 1-methylnaphthalene as internal standard.
The remaining balance is traces of isomerised internal alkene ester/alco-
hol. [c] DMAP was used as additive (1.5 mol%). [d] PPh3 was used as addi-
tive (1.5 mol%).
3[a]
60[f]
4[a,e]
5[a]
60[f]
>99
>99 (96)
73[f]
6[d]
7[a,c]
8[a,c]
9[a,c]
8[f]
26[f]
10[a,c] 33b
11[a,e] 33b
12[d]
15[f]
19[f]
33b
88
74[f] (71)
a selective reduction of the ester and in all cases hex-5-en-1-ol
32 was not observed. The hydrogenations afforded mixtures of
the product obtained from the reduction of the terminal
alkene, methyl hexanoate and the fully reduced hexanol 22,
suggesting that the alkene reduction was actually promoted
relative to the ester functionality under these reaction condi-
tions.
[a] Reactions were carried out at an initial pressure of 50 bar using
0.50 mmol of dimer in 3.0 mL of THF for 18 h. [b] Determined by 1H NMR
spectroscopy using 1-methylnaphthalene as internal standard; the yields
of the isolated products are in parentheses. [c] Reaction carried out for
21 h. [d] Reaction carried out for 23 h. [e] 40:1 base to catalyst. [f] Remain-
ing mass balance corresponds to partially reduced esters shown in the
equation above.
We also considered some other related applications for our
catalysts. Recently, the fundamental reactivity of ester hydroge-
nation catalysts has been applied towards some interesting
ideas for harnessing renewable feedstocks.[12] We were attract-
ed by the possibility of the depolymerisation of oligomers and
polymers. An exciting proof-of-concept paper[12d] recently ap-
peared in which the polyester polymer PET was depolymerised
by hydrogenation to give 1,4-benzene dimethanol and ethyl-
ene glycol. This raises the possibility of a different mode of re-
cycling: instead of melting PET and making off-white recycled
polyesters, it could be recycled into chemicals that are other-
wise derived from petrochemical resources. There is also a lot
of PET that is currently not recycled, so this could be a good
renewable feedstock. Only two examples using Milstein’s cata-
lysts are discussed in this initial work; the reactions were car-
ried out at 1608C and used 2 mol% of catalyst relative to the
repeat unit (or 1 mol% relative to each ester). The publication
of these results, which show that a step-change in reactivity
would be needed prior to implementation, focused our studies
on PET depolymerisation.
Scheme 3. Hydrogenation of ester 8 in the presence of ethylene glycol 35.
ethylene glycol has a negative effect on the rate of this hydro-
genation, at least at moderate temperatures.
With 4 identified as the best catalyst, we studied the depoly-
merisation–hydrogenation of real samples of waste PET. In the
hydrogenation of PET flakes (Table 8), we were pleased to find
that at 110 8C, a very significant conversion to benzene dime-
thanol was obtained by using 1 mol% catalyst relative to each
ester unit (entry 1). The reaction can be performed in toluene
(Table 8, entry 6), but the presence of anisole as co-solvent (as
was used in ref. [12d]) is needed to increase polymer solubility
(entry 5). In common with almost every report on the use of
catalysis on renewable substrates, the amount of catalyst used
is likely to be significantly greater than the amounts that
would be needed for research leading to implementation. Be-
cause such processes are generally producing relatively low
value products, the catalyst (metal and ligand) cost is critical.
Further studies on new catalysts to increase the productivity
beyond this new benchmark are needed.
To identify the best catalyst and most suitable conditions
and to study the effect of ethylene glycol production on the
catalyst, we chose to study two model diesters first, 33a and
33b. The results are shown in Table 7; catalyst 4 emerges as
the best catalyst for this transformation once again.
More forcing conditions are required even for these soluble
and simple substrates relative to the conditions used in the hy-
drogenation of other esters in this study. To probe this further,
we carried out the hydrogenation of ester 8 with varying
amounts of ethylene glycol. The results (Scheme 3) show that
Chem. Eur. J. 2015, 21, 10851 – 10860
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base/catalyst ratio of around 2:1 is actually optimum for highly
enantioselective ketone hydrogenation using catalyst 1.[6] In
both the hydrogenation of para-fluorobenzoic acid methyl
ester 8 and ethyl acetate, it can be seen that activity increases
significantly when the base/catalyst ratio is increased. More
formally, this should perhaps be the case when the base con-
centration is increased, although we note experimentally that
quite high concentrations were required to observe any con-
version under mild conditions for the benzoate 8 (Table 1, en-
tries 9 and 11), whereas in ethyl acetate hydrogenation, the ab-
solute concentration of base (0.017 mol% Ru cat., 3.9 mol%
base) is quite low even when average TOF are above 2000 hÀ1.
Clearly, there will be differences based on the solvating
medium, the trace impurities in the substrate and the identity
of the substrate. Recently, a revised mechanism has been put
forward for the enantioselective hydrogenation of ketones
using Noyori catalysts.[13] We have previously found our cata-
lysts to have some mechanistic similarities to Noyori cata-
lysts,[6c] so it seems reasonable that some of these insights
might apply here. In particular, the revised mechanism postu-
lates that the secondary alkoxide, produced after hydride
transfer to a ketone, hydrogen bonds to an NH function while
deprotonating a dihydrogen ligand coordinated to Ru. In ester
hydrogenation, the initial product is a deprotonated hemi-
acetal. Hemi-acetal anions are far less basic than simple alkox-
ides; it may be the case that a higher concentration of base is
needed either to facilitate the product deprotonating dihydro-
gen or to directly deprotonate dihydrogen itself. An alternative
or addition role that also requires more base than in ketone
hydrogenation could be required to assist the removal of
bound hemi-acetal anion from ruthenium, or to assist catalyst
stability.[3a] Catalyst 4 can be operated at relatively low
amounts of base when catalyst loadings are low but significant
amounts do seem to be needed for high productivity. This also
seems to be the case for other catalysts. The effect of DMAP is
somewhat mysterious, since it does not seem to be in suffi-
cient concentration to be simply acting as more base. It was
designed to act as a co-ligand for Ru that would prevent C=C
bond coordination and subsequent reduction. However, we
have not found it possible to make a DMAP complex in earlier
studies and here we show that exchange of DMSO with a phos-
phine group is a difficult process, at least from the dichloride
pre-catalysts. Moreover, the phosphine-ligated Ru complexes
24–26 are actually not just poorly active catalysts but tend to
reduce the C=C bonds to a greater extent. Ru/BINAP/DPEN cat-
alysts have also been reported to fully reduce the C=C bonds
of cinnamate esters. Whether there is some outer-sphere
mechanism possible for C=C reduction, or that unexpected
and less saturated species form during hydrogenation is not
clear. In any case, the use of catalyst 4 enables the ester hydro-
genation in the presence of isolated double and triple bonds
with good chemoselectivity. Catalyst 4 is therefore a useful
ester hydrogenation catalyst, and we have identified some
structural features and experimental protocols that appear to
be beneficial for chemoselective and productive ester hydroge-
nation catalysis.
Table 8. Depolymerisation of poly(ethylene terephthalate) (PET).
Entry[a]
Cat.
Load
tBuOK/
t
T
34b[b]
[%]
[mol%]
cat.
[h]
[8C]
1
2
3
4
5[c]
6[d]
4
4
4
4
4
4
2.0
2.0
1.0
1.0
2.0
2.0
20:1
20:1
20:1
40:1
20:1
20:1
48
24
24
24
24
24
110
110
120
110
110
110
73 (53)
41
4
26
49
17
[a] Reactions were carried out at an initial pressure of 50 bar in a 50/50%
mixture of THF/anisole. [b] Conversion determined by 1H NMR spectrosco-
py using 1-methylnaphthalene as internal standard; yield of the isolated
product is in parentheses. [c] Reaction run in a 50/50% mixture of tolu-
ene/anisole. [d] Reaction run in toluene.
Conclusion
Ten different Ru complexes of phosphine-diamine, phosphine-
amino-alcohol and phosphine-amino-amide ligands have been
prepared and examined in the hydrogenation of a range of
esters. The simple ethylene-diamine-derived complex 4 emerg-
es as significantly more active than the other catalysts. Com-
plexes 2 and 5, which are especially easy to prepare at signifi-
cant scale, also give encouraging catalytic activity. Our main
focus has been to establish how catalyst structure and sub-
strate structure impact on the chemoselectivity of these ester
hydrogenations; on this basis, complex 4 again outperforms
other catalysts. In addition, the hydrogenation of ethyl acetate,
carried out at larger scale, allows us to draw several conclu-
sions. The average turnover frequencies observed in neat ethyl
acetate are rather high relative to some of the more complex
substrates. This is promising in terms of future developments
with this catalyst system towards commercially viable catalyst
loadings for other hydrogenations. The extra-high reactivity
obtained when using the simple substrate ethyl acetate is
partly because it is an easy, sterically unencumbered and very
pure substrate, but also might relate to the high substrate con-
centrations used, since ketone hydrogenation using catalysts 4
was first order in substrate, and that is likely to be the case
here. There is also a strong pressure dependence: it is possible
that lower pressures could be used with a very efficiently
stirred pressure vessel, but we have never been successful in
carrying out ester hydrogenations at or near atmospheric pres-
sure using these catalysts. Entries 7 and 10 in Table 3 show
a significant increase in productivity when hydrogen pressure
is increased from 15 to 50 bar pressure.
The effect of the base co-catalyst certainly merits discussion.
Previous studies showed us that this type of catalyst was
nearly inactive at these quite low temperatures if base/catalyst
ratios of 2:1 are used. Some activity was observed at 1408C. A
Chem. Eur. J. 2015, 21, 10851 – 10860
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1
alysed by H NMR spectroscopy to calculate the conversion. Purifi-
cation by column chromatography on silica gel using EtOAc as
eluent afforded 1,4-benzene dimethanol 34b as a white solid
(73 mg, 0.53 mmol, 53%).
Experimental Section
Synthesis of ligands and catalysts along with their characterisation
data can be found in the Supporting Information.
Hydrogenation of ethyl acetate using Ru catalysts
Single-crystal X-ray structural determination
Reactions were carried out by using a Parr 50 mL stainless steel au-
toclave equipped with a pressure gage, gas inlet, safety valve and
injection port equipped with rubber septum. A glass lined auto-
clave containing a magnetic stirring bar (crosshead) was charged
with the appropriate amount of catalyst (0.033 mol%), purged
with three vacuum/argon cycles and left under argon atmosphere.
Ethyl acetate (30.7 mmol, 3.0 mL), MeTHF (3 mL) and 1-methyl-
naphthalene (0.2 mL) as an internal standard were added to the
autoclave through the injection port using a syringe. Finally, potas-
sium tert-butoxide (3.9 mol%; 1m solution in 2-methyl-2-propanol;
1.2 mmol, 1.2 mL) was added. The autoclave was then purged
three times with H2, pressurised to 50 bar and immersed into an oil
bath preheated to the desired temperature. After the desired reac-
tion time, the autoclave was cooled down to room temperature,
the pressure slowly released and opened. A small sample was
CCDC 1049167 (2), 1049168 (4), 1049169 (6), 1049170 (7), and
1049171 (24) contain the supplementary crystallographic data for
this paper. These data are provided free of charge by The Cam-
bridge Crystallographic Data Centre.
Acknowledgements
The authors would like to thank the EPSRC National Mass
Spectrometry Service, the EPSRC Impact Acceleration Scheme
for partial funding (with Johnson Matthey). Miss Katy Scott
(University of St. Andrews) is thanked for assistance with
alkyne synthesis. Dr. Mick O’Neill from Polymer Extrusion Tech-
nologies (UK) Ltd. is thanked for kindly donating PET samples,
and all the technical staff in the School of Chemistry are ac-
knowledged for their assistance.
1
taken, diluted with CDCl3 and analysed by H NMR spectroscopy to
calculate the conversion.
Hydrogenation of methyl 4-(phenylethynyl)benzoate (27)
using Ru catalysts
Keywords: bifunctional catalysis
hydrogenation · ligands · ruthenium
·
depolymerisation
·
A Biotage 5 mL microwave vial containing a stirrer bar was charged
with catalyst 4 (0.5 mol%). The vial was sealed with a crimp cap,
purged with three vacuum/argon cycles and left under argon at-
mosphere. The corresponding ester (1.0 mmol, 236 mg) was added
using a syringe from a stock solution in Me-THF containing 1-
methylnaphthalene as internal standard (3.0 mL). Finally, potassium
tert-butoxide (20 mol%; 1m solution in 2-methyl-2-propanol;
0.2 mmol, 0.2 mL) was added, then two needles were pierced into
the vial and this was introduced into an autoclave, which had
been previously purged with three vacuum/argon cycles. The auto-
clave was then purged three times with H2, pressurised to 50 bar
and immersed into an oil bath preheated to the desired tempera-
ture. After the desired reaction time, the autoclave was cooled
down to room temperature, the pressure slowly released and
opened. A small sample was taken, diluted with CDCl3 and ana-
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1
lysed by H NMR spectroscopy to calculate the conversion. Purifica-
tion by column chromatography on silica gel using petroleum
ether/EtOAc 3:1 as eluent afforded (4-(phenylethynyl)phenyl)me-
thanol 28 as a white solid (110 mg, 0.53 mmol, 53%).
Depolymerisation of PET (36) using Ru catalyst 4
A glass insert was charged with the appropriate amount of catalyst
(1–2 mol%) and substrate (1 mmol) and was introduced into an
autoclave, which had been previously purged with three vacuum/
argon cycles. A round bottom flask was charged with 1-methyl
naphthalene (ꢀ0.08 g) and quickly purged with three vacuum/
argon cycles and left under argon atmosphere. THF (3 mL), anisole
(3 mL) and potassium tert-butoxide (20 mol%; 1m solution in 2-
methyl-2-propanol; 0.4 mmol, 0.4 mL) were added to the round
bottom flask. The solution was then added to the autoclave and
the autoclave was then purged three times with H2, pressurised to
50 bar and immersed into an oil bath preheated to the desired
temperature. After the desired reaction time, the autoclave was
cooled down to room temperature, the pressure slowly released
and opened. A small sample was taken, diluted with CDCl3 and an-
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim