Electronic and bite angle effects in catalytic C-O bond cleavage of a lignin model compound using ruthenium Xantphos complexes

Article abstract of DOI:10.1039/c6cy00518g

Bite angle and electronic effects on the ruthenium-diphosphine catalysed ether bond cleavage of the lignin β-O-4 model compound 2-phenoxy-1-phenylethanol were tested. Enhanced conversion of the substrate was observed with increasing σ-donor capacity of the ligands. Kinetic and thermodynamic data suggest oxidative addition of the dehydrogenated model compound to the diphosphine Ru(0) complex to be rate-limiting.

Full text of DOI:10.1039/c6cy00518g

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Electronic and bite angle effects in catalytic C-O bond cleavage of
a lignin model compound using ruthenium xantphos complexes
Received 00th January 20xx,
Accepted 00th January 20xx
Luke Shaw,a,† D. M. Upulani K. Somisara,a,† Rebecca C. How,a Nicholas J. Westwood,a Pieter C. A.
Bruijnincx,b Bert M. Weckhuysen,b and Paul C. J. Kamera*
DOI: 10.1039/x0xx00000x
Bite angle and electronic effects on the ruthenium-diphosphine catalysed ether bond cleavage of the lignin β-O-4 model
compound 2-phenoxy-1-phenethanol were tested. Enhanced conversion of the substrate was observed with increasing σ-
donor capacity of the ligands. Kinetic and thermodynamic data suggest oxidative addition of the dehydrogenated model
www.rsc.org/
compound
to
the
diphosphine
Ru(0)
complex
to
be
rate-limiting.
lignocellulosic biomass depends on the plant species and
environmental factors, for example. In general, 15-30% of the
lignocellulosic biomass is lignin by weight.15a-b It is estimated that
the annual production of lignin as a non-commercial waste falls in
the range of 40-50 million tonnes,16 which results mainly from
paper and pulp industry.17
Introduction
The limited availability of fossil reserves and concerns about climate
change demand that alternative resources are investigated to meet
the future needs of energy and fine chemicals. Most importantly,
these alternatives should rank high in renewability and
sustainability and not be in direct competition with food
production.1-3 Hence, lignocellulosic biomass is considered as one of
the most promising resources for the sustainable production of
fuels, chemicals and materials.4-8
Where (hemi-)cellulose provides ample access to fuels and aliphatic
platform chemicals, lignin is anticipated to be a major source for the
sustainable production of aromatic compounds in the chemical
industry. Lignin is a three-dimensional, amorphous polymer that is
derived from methoxylated hydroxycinnamyl alcohol building
blocks, the prototypical monolignols. These units are
interconnected by various C-O and C-C linkages to give, e.g., the β-
O-4, β-β, and β-5 units.18 About 45-60% of the linkage structures in
both softwood and hardwood consists of β-O-4 linkages in the
native lignin structure. Cleavage of these linkages in combination
with further catalytic upgrading steps could potentially result in an
array of value-added, small aromatic molecules.15a-c Any method
used for lignin valorisation, including thermochemical19 or
mechanochemical degradation,20 oxidation21-23 homogeneous or
The estimated annual production of biomass amounts to an
impressive 56.8 billion tonnes of elemental carbon. Although only a
small part will be available as a feedstock, it will provide a
substantial sustainable resource for the chemical industries.9
Although much research and development is invested in the field of
biofuels as an energy source it should be noted that the total
energy contents of this produced biomass, which includes food
production, is not sufficient to cover the world energy demand.
Therefore, biofuels are expected to constitute only part of the
future of sustainable energy production, which will be strongly
dependent on other sources. However, the harvested biomass can
easily cover the volume of chemicals produced today and acquiring
chemical feedstocks from biomass conversion therefore seems a
promising way forward. A substantial research effort on valorisation
of the cellulose component has been carried out successfully over
the years producing bio-fuels and chemicals.10,11 In comparison to
this, catalytic lignin valorisation has only more recently been
receiving the same attention. Lignin is a highly cross-linked, poly-
aromatic macromolecular component that acts as a resin between
cellulose and hemicellulose fibres in plant cell walls, giving rigidity
to the plant structure12 and acting as an essential scaffold for the
plant nutrient and water transport system.13,14 The composition of
heterogeneous
catalysis,15a-c,24
acid/base-catalysed
depolymerisation,15b,25 solubilisation,26 liquefaction27 or enzymatic
modifications28 should preserve the aromatic nature of the end
product. In general, homogeneous catalysts can exhibit high
reactivity under mild reaction conditions, which provides excellent
opportunities to process such a notoriously recalcitrant material as
lignin. With respect to the three-dimensional structure of lignin,
homogeneous catalysis may be better equipped to reach linkages
buried in the macromolecule where heterogeneous catalysis would
suffer due to steric hindrance.8 The non-uniformity and complex
structure of lignin poses many challenges when a specific linkage
and its cleavage need to be studied in detail. The common practice
is to test the catalyst material on a simple chemical compound that
mimics the specific linkage of interest. These model compounds are
intensively studied to identify efficient catalyst systems and to
optimise the reaction conditions.15a-c,22,29-31
Bergman, Ellman and co-workers reported the catalytic, redox-
neutral cleavage of 2-aryloxy-1-arylethanols as lignin-related β-O-4
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A DFT study carried out by Beckham, Paton and co-workers,
provided detailed mechanistic insight into the ruthenium-
phosphine catalysed C-O bond cleavage of 2-phenoxy-1-
phenylethanol (1).40 They proposed that, as in earlier studies, the
reaction starts with a catalytic dehydrogenation pathway that
involves hydrogen migration from the substrate to the ruthenium
centre generating a ruthenium dihydride (8). The subsequent C-O
bond cleavage proceeds via oxidative addition across the ether
bond and dihydrogen coordination to the ruthenium centre forming
Ru-enolate (10), which will then eliminate the products, phenol and
acetophenone. Furthermore, they proposed an unusual 5-
membered metallacycle as the most likely transition state (TS9-10) of
the oxidative addition step. This kinetically favoured O-bound
enolate (10), which enables reaction completion, is preferred over a
thermodynamically more stable C-bound enolate. This study also
indicates a preference for phenol to be released via heterolytic
dihydrogen activation before acetophenone from the system
(Scheme 2).
Scheme 1: C-O bond cleavage in 2-phenoxy-1-phenlethanol (1) by
(Ru)(H)2(CO)(PPh3)3-Xantphos catalyst studied by Berman, Ellman and co-
workers.32
model compounds (Scheme 1).32 They have tested different catalyst
systems and varied reaction conditions to find that
Ru(H)2(CO)(PPh3)3-Xantphos (in xylenes at 135 °C) performed best
and gave 62-98% isolated yields of the cleavage products. The
PPh2
Ph2P
OC
H
Ru
H
PPh3
reaction was reported to proceed via
a tandem catalytic
OH
6
dehydrogenation and C-O ether bond cleavage. Subsequently, Wu
et al. applied this catalyst to more realistic model compounds and
isolated several intermediate complexes confirming the hydrogen
transfer mechanism.33 The reaction mechanisms proposed by both
OPh
Ph
1
PPh3, H2
of these studies suggested that the reaction starts with
a
O
ruthenium-catalysed dehydrogenation of the benzylic alcohol
leading to the corresponding ketone, followed by oxidative
addition. Further work by Leitner, Bolm and co-workers showed the
[Ru(TMM)triphos] system to be a highly efficient catalyst giving
higher yields of phenol and acetophenone than Bergman, Ellman
and co-workers reported for [Ru(methallyl)2(Xantphos)].34 Details of
this work, extended to dilignol model compounds resulting in both
C-C and C-O bond cleavage in β-O-4 linkage, was subsequently
published in 2015.35 Concurrently, Bolm and co-workers reported
on the base-catalysed cleavage of the β-O-4 linkage in the same
dilignol model compound using readily available bases in dimethyl
carbonate as solvent.36 In 2014, Jameel et al. suggested that
Xantphos can be omitted in Ru-Xantphos catalyst systems when a
relatively high concentration of KOH (0.4 M) is applied.37
PPh2
Ph2P
OC
OPh
Ph
H migration from
substrate to Ru
Ru
O
2
PPh2
H
Ph2P
OC
Ph
Ru
O
H
PPh2
7
Ph2P
H
H
Ru
O
OC
OH
Ph
11
OPh
OPh
HOPh
3
Ph
Ph
8
1
PPh2
Ph2P
OC
O
H
Ph
H2
Ru
O
PPh2
H
Ph2P
OC
OPh
Ru
O
Ph
10
In 2013, Weickmann and Plietker reported RuCl2(PPh3)3 activated by
potassium t-amylate as an efficient catalyst to cleave the β-O-4
linkage in 2-phenoxy-1-phenylethanol (1).38 Furthermore, they
showed that when more electron-donating groups were introduced
into the monodentate phosphine ligand in the RuCl2(PAr3)3 complex
an improved reactivity was observed. Interestingly, the product
ketone was prone to efficient α-alkylation using primary alcohols.
Recently, a density functional theory (DFT) study on the cleavage of
the β-O-4 linkage of lignin using group 8 pincer complexes, carried
out by Liu and Wilson, revealed a similar trend. Their study involved
the C-O bond cleavage of β-O-4 model compound 2-phenoxy-1-
phenylethanol (1) by pincer complexes including all possible
combinations of three group 8 metals Fe, Ru and Os with five
different pincer ligands. They have concluded that C-O bond
activation by transition metal catalysis depends on the ability of the
metal centre to donate electron density to the C-O bond and,
therefore, cleavage of the C-O bond is promoted by electron
donating ligands.39
H2
Ph
9
PPh2
Ru
Ph2P
OC
OPh
O
Ph
TS9-10
Scheme 2: Proposed mechanism for the C-O bond cleavage in 2-phenoxy-1-
phenylethanol.32,33,40
From the previous studies on 2-phenoxy-1-phenylethanol (1) as a
lignin model compound, we reasoned that the most likely rate
determining step for this reaction is the oxidative addition of the C-
O bond, which results in the formation of Ru(II) complex (10) from
Ru(0) complex (9) via (TS9-10). This is in line with the electronic
effect observed in work carried out by both Weickmann and
Plietker,38 and also by Liu and Wilson.39 Rates of oxidative addition
reactions are dependent on the electronic properties of the ligand
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in the catalyst system of interest, which are mainly governed by the
σ-donor or π-acceptor ability of the phosphorous moiety. In the
case of bidentate ligands, such as Xantphos, steric effects in the
form of the ligand bite angle can also have a dramatic effect on
catalyst performance.41-44 In most reactions the individual
contribution of these effects is difficult to determine as the overall
bite angle effect can influence several steps in a catalytic cycle.44
Subtle changes in the bite angle of Xantphos-type ligands can be
applied by changing the size of the substituent X of the xanthene
backbone (Table 1 and associated structures, 5, 14a-e), while
electronic properties can be changed by adding electron-donating
or electron-withdrawing groups to the phenyl rings of the
coordinating phosphine groups (Scheme 4).45
n-BuLi
TMEDA
Et2O, -78 °C
O
O
Li
Li
15
16
ClP(NEt2)2
hexanes, -78 °C
HCl
Et2O, -78 °C
O
O
PCl2
PCl2
P(NEt2)2
P(NEt2)2
18
Increasing the σ-donor ability and reducing the bite angle in a ligand
can increase the reduction potential and stabilise higher oxidation
states of the metal centres, promoting oxidative addition in
organometallic complexes,46 although the effect of reducing the
bite angle can be obscured by steric factors. Generally, oxidative
addition is facilitated by less steric hindrance; hence a ligand with a
moderately small bite angle with higher σ-donor ability would be
the obvious candidate for an oxidative addition reaction. In order to
optimise the catalytic activity and better understand the unique
reactivity of Xantphos in the Ru-catalysed ether cleavage we have
investigated the influence of small variations in the ligand bite angle
on catalyst performance. In addition, we have applied subtle
changes in the electron-donating properties to further improve
catalyst performance.
17
ArMgBr
THF, 0 °C
5a: Ar = p-OMe-C6H4
5b: Ar = p-Me-C6H4
5c: Ar = p-F-C6H4
O
5d: Ar = p-CF3-C6H4
PAr2
PAr2
Scheme 4: Synthesis of a series of Xantphos-type ligands with electron-
donating or electron-withdrawing p-substituents.
Ligand bite angle effect on C-O bond cleavage of 2-phenoxy-1-
phenylethanol (1)
Results and Discussion
The rate of oxidative addition is affected by the ligand bite angle
but the effects are not always straightforward and sometimes
difficult to predict.45,48 Therefore, the bite angle was changed from
102° (Homoxantphos, 14a) to 121° (Benzoxantphos, 14e) (Table 1).
Synthesis of Xantphos-type ligands
4,6-Bis(diphenylphosphanyl)phenoxathiine (Scheme
3
(14c)
Thixantphos) was prepared by selective dilithiation of phenoxathiin
(12) followed by reaction with chlorodiphenyl-phosphine.42
Scheme 3: Synthesis of Thixantphos.
The electronically modified Xantphos ligands (5a-d) were
synthesised
by
reacting
(9,9-dimethyl-9H-xanthene-4,5-
diyl)bis(dichlorophosphane) (18) with the freshly prepared Grignard
reagent of the corresponding p-substituted aryl bromide (Scheme
4).47
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Table 1: Bite angle effect on C-O bond cleavage of 2-phenoxy-1-
Table 2: Electronic effect on C-O bond cleavage of 2-phenoxy-1-
phenylethanol.a
phenylethanol.a
Conversion
Yield (%)d
χi,c
Conversion
(%)c
Yield
R
σp
5a OCH3 -0.27
(%)d
1
b
βn
(%)c
3d
(cm-1)
X
(°)b
2
3
4
1
2
4
e
e
-
3.4
3.5
4.3
5.0
6.6
47.6
46.5
44.6
26.2
5.1
35.7
32.0
29.4
13.2
2.2
31.6
27.6
25.3
9.8
11.1
13.9
13.5
11.5
2.6
14a
14b
14c
5
14d
14e
C2H4
102°
Si(CH3)2 109°
20.3
28.5
27.7
44.6
26.6
8.4
-
5.0
5b
5
5c
5d
CH3
H
F
-0.17
0.00
0.06
0.54
15.8
14.8
29.4
24.8
4.1
10.6
10.9
25.3
19.0
1.4
11.8
12.2
13.5
1.8
S
C(CH3)2
NH
110°
111°
114°
121°
CF3
1.0
-
4.2
a) Conditions: 0.25 mmol of 2-phenoxy-1-phenylethanol (1) with 2 mol%
catalyst loading (0.005 mmol of Ru(H)2(CO)(PPh3)3, 0.005 mmol of ligand (5,
5a-d)) and 0.125 mmol of 1,2,4,5-tetramethylbenzene as the internal
standard in anhydrous xylenes in a closed microwave vial, 135 °C, 45 min. b)
Hammett σp values.49 c) Tolman χi values.50 d) Determined by gas
chromatography. Results from 3 sets of experiments.
a) Conditions: 0.25 mmol of 2-phenoxy-1-phenylethanol with
2
mol%
catalyst loading (0.005 mmol of Ru(H)2(CO)(PPh3)3, 0.005 mmol of ligand (5,
14a-e)) and 0.125 mmol of 1,2,4,5-tetramethylbenzene as the internal
standard in anhydrous xylenes in a closed CEM microwave vial, 135 °C, 45
42
min. b) Calculated natural bite angle of the ligand. c) Determined by gas
The electronic effect on the conversion of the substrate can be a
result of either improved catalyst activity or catalyst stability, as
chromatography. Results from 3 sets of experiments. d) Lower phenol yield
due to degradation in GC inlet. e) Not observed in GC.
both will lead to higher conversions. To acquire
a better
understanding of the activity and stability of the catalyst complexes
containing the ligands 5 and 5a, reaction profiles were obtained by
monitoring separate reactions as a function of time for a period of 4
hours. The complete kinetic profiles of the two catalysts obtained
using ligands 5 and 5a is consistent with a change in activity and not
stability of the catalyst (Figure 1). As demonstrated in Figure 1, an
increase in the initial rate of reaction for ligand 5a relative to 5 is
observed, which could indicate a rate limiting oxidative addition
step within the catalytic cycle.
The bite angle effect on the efficiency of catalytic ether bond
cleavage showed a clear optimum in the conversion of 2-phenoxy-
1-phenylethanol (1) to phenol and acetophenone. The highest
activity was obtained using Xantphos (5) as the ligand, and both
smaller and larger bite angles resulted in lower activities.
Additionally, in terms of selectivity to the desired ether cleavage
products, Nixantphos (14d) showed promising results. Obtaining a
clear optimum for Xantphos with varying bite angles does not
provide decisive information about whether the oxidative addition
step or the hydrogen transfer is the rate-determining step.
However, when subtle changes in electronic properties of the
ligands were made, by increasing the σ-donor ability of the ligand, a
clear trend between electron-donating capacity of the ligand and
catalyst activity was observed. The most electron-donating 4-
methoxyphenyl derivative 5a shows higher activity than the parent
Xantphos ligand (Table 2, entries 5a and 5). These results are in
accordance with the expected trend that stronger σ-donor
character in the ligands stabilises higher oxidation states of metal
centres, promoting oxidative addition, as also observed by
Weickmann and Plietker using monodentate ligands38 and Liu,
Wilson and co-workers using group 8 pincer ligands.39 These results
are in line with a rate-determining oxidative-addition step.
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which is in line with expectations for a monometallic catalytic
system.
100
90
80
70
60
50
40
30
20
10
0
120
100
y = 24.889x
R² = 0.9924
80
60
40
20
0
0
50
100
150
Time (min)
200
250
300
0
1
2
3
4
5
Figure 1: Conversion of substrate 1 with xantphos-type ligands. Blue –
reaction with 5a as ligand. Red – reaction with 5 as ligand. Conditions: 1
mmol of 2-phenoxy-1-phenylethanol (1) with 2 mol% catalyst loading (0.02
mmol of Ru(H)2(CO)(PPh3)3, 0.02 mmol of ligand (5 or 5a)) and 0.125 mmol
of 1,2,4,5,-tetramethylbenzene as the internal standard in anhydrous
xylenes, 135 °C, samples were analysed by gas chromatography. Results
from duplicates of experiments.
Catalyst loading (mol%)
Figure 3: Initial Turnover Frequency (TOF) vs. catalyst loading for C-O bond
cleavage of 2-phenoxy-1-phenylethanol (1) by Ru(H)2(CO)(PPh3)3 and
Xantphos (ligand 5). Conditions: 1 mmol of 2-phenoxy-1-phenylethanol (1)
with a given catalyst loading (With a ruthenium:ligand ratio of 1:1) 0.125
mmol of 1,2,4,5-tetramethylbenzene as the internal standard in anhydrous
xylenes at 135 °C. Samples withdrawn at regular intervals and were analysed
by gas chromatography. Results from duplicates of experiments.
Kinetic studies on the reaction of 2-phenoxy-1-phenylethanol (
1)
with Ru(H)2(CO)(PPh3)3 and Xantphos ( ) were carried out with
5
varying substrate concentrations and catalyst loadings (Figures 2
and 3). An induction period was observed during the course of the
reactions, which is due to the required build-up of the intermediate
ketone prior to ether bond cleavage (ESI, Figures S2 – S11). This was
confirmed by the addition of 2 mol% intermediate ketone, which
saw a reduction in the observed induction period (ESI, Figure S12).
In order to provide additional evidence to support our proposal of a
rate limiting oxidative addition step, we investigated the
temperature dependence of the reaction. An Eyring plot for the
ether cleavage of substrate
1
was produced (ESI, Figure S14), and a
G‡. Attempts were
value of 156 KJ mol-1 was obtained for the
ꢀ
made to produce an Eyring plot for the initial dehydrogenation
step; however, due to a high initial rate this was unfortunately not
possible. Lowering the temperature below 25 °C in order to
moderate the high rate for the initial dehydrogenation led to
substrate precipitation. The concentration was also lowered in
order to moderate the initial rate, which led to issues of large
scatter in the analysis due to GC detection limits. Therefore we
A reaction using substrate
1 in the presence of hydrogen saw an
increase in the induction period (ESI, Figure S13) as the alcohol-
ketone equilibrium would be pushed towards the alcohol,
confirming that the required build-up of ketone is the cause of the
induction period. A plot of initial TOF vs substrate concentration
(Figure 2) shows a first-order rate dependency on substrate (
expected from the proposed catalytic cycle in which the only
1), as
calculated the initial TOF for the formation of ketone
4 at the
starting material is the substrate (1).
lowest possible temperature, 25 °C. An estimated value of 62.6 h-1
was obtained. However, we believe this to be an underestimate of
the true initial rate as our first data points were already outside the
pseudo first order initial rates regime (ESI Figure S15) and the
substrate already contained a small amount of residual ketone, i.e.
0.2 mol %. Even at concentrations approaching equilibrium a value
of 4.1 h-1 was obtained (ESI Figure S15). This is already higher than
the initial TOF obtained for the lowest temperature at which ether
cleavage is observed (105 °C), a value of 0.8 h-1 (ESI Figure S16). It is
clear that the initial dehydrogenation has a much higher rate than
the corresponding ether cleavage. This again is indicative of a rate
limiting oxidative addition step in the catalytic cycle.
100
90
80
70
60
50
40
30
20
10
0
y = 46.794x
R² = 0.9916
0
0.5
1
1.5
2
2.5
[1] (mmol)
Conclusions
Profound effects of the ligand bite angles and electronic properties
on the catalytic ether bond cleavage of the lignin model compound
Figure 2: Initial Turnover Frequency (TOF) vs Substrate concentration for C-
O bond cleavage of 2-phenoxy-1-phenylethanol (1) by Ru(H)2(CO)(PPh3)3 and
Xantphos (ligand 5). Conditions: 2-phenoxy-1-phenylethanol (1) with a 0.02
mmol catalyst loading (with a ruthenium:ligand ratio of 1:1) and 0.125 mmol
of 1,2,4,5-tetramethylbenzene as the internal standard in anhydrous xylenes
at 135 °C, samples withdrawn at regular intervals and were analysed by gas
chromatography. Results from duplicates of experiments.
2-phenoxy-1-phenylethanol
(1) have been observed. Newly
synthesised ligands (9,9-dimethyl-9H-xanthene-4,5-diyl)bis(bis(4-
methoxyphenyl)phosphane) (5a) and (9,9-dimethyl-9H-xanthene-
4,5-diyl)bis(di-p-tolylphosphane)
capacity, gave higher conversions compared to Xantphos (
(
5b
)
with stronger σ-donor
). The
5
kinetic profile for 5a showed that the improved conversion was a
result of an intrinsically higher initial rate and not caused by
The effect of catalyst loading on initial TOF was also investigated
(Figure 3) and a clear first-order rate dependency was observed,
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increasing catalyst stability. Furthermore, kinetic studies showed 124.6, 119.9. IR (KBr, cm-1) 3052 (m), 2851 (w), 1565 (m), 1477 (m),
first order kinetics with respect to the substrate and catalyst 1433 (s), 1394 (S), 1227 (s), 1204 (m), 1089 (m), 1025 (m). MS: APCI
loading. A higher initial TOF was observed for the dehydrogenation [M+H]+ Calcd.: 569.1252 Found: 569.1242 (%). Anal. Calcd. For
at low temperature compared to the initial TOF for the ether C36H26OP2S: C, 76.04; H, 4.61. Found: C, 75.97; H, 4.52.
cleavage at a higher temperature. This, along with increased
Representative procedure for the synthesis of electronic
xantphos-type ligands
conversion for electron-donating ligands indicate a likely rate
limiting oxidative addition step. Further in-depth mechanistic
studies are currently underway.
At room temperature, a solution of the corresponding aryl bromide
(6 mmol) in THF (4 mL) was added dropwise to a stirred mixture of
magnesium turnings (350 mg, 12 mmol) activated with 1,2-
dibromoethane (0.05 mL, 0.06 mmol) in THF (3 mL). The reaction
mixture was stirred for 3 h, filtered and added dropwise to a stirred
Experimental
General experimental
solution
of
(9,9-dimethyl-9H-xanthene-4,5-diyl)bis(dichloro-
All reactions were carried out using standard Schlenk techniques
under an argon atmosphere or in an inert atmosphere glove-box at
ambient temperature. Toluene and TMEDA were distilled from
sodium, THF and diethyl ether were distilled from
sodium/benzophenone, hexanes from sodium/benzophenone/
triglyme and dichloromethane from CaH2. 2-Phenoxy-1-
phosphane) (18) (0.5 g, 1.2 mmol) in THF (10 mL) at 0 °C and
allowed to warm to room temperature and stirred for another 3 h.
The resulting mixture was hydrolysed with water (5 mL) and the
solvents were removed in vacuo. The obtained residue was
dissolved in dichloromethane and washed with dilute hydrochloric
acid. The organic layer was separated and the aqueous layer was
extracted with dichloromethane (3 × 10 mL). The combined organic
fractions were dried over MgSO4, filtered and the dichloromethane
was removed in vacuo to give the corresponding solid.
phenylethanol
benzo[k,l]xanthene
(
1
),32
6,7-bis(diphenyl-phosphino)
and (10,10-di-methyl-10H-
46
(14e)
42
dibenzo[b,e][1,4]oxasiline-4,6-diyl)bis(diphenyl-phosphane) (14b
were synthesised according to literature procedures. The
)
intermediate
diyl)bis(dichlorophosphane)
(9,9-dimethyl-9H-xanthene-4,5-
18) was synthesised by modifying
Catalytic reactions
(
literature proceedures.51-55 All reagents were purchased from
commercial suppliers and used as received, unless otherwise noted.
Representative procedure for testing the ligand effect
NMR spectra were obtained on
a Bruker AVANCE III 500
Reaction mixture preparations were carried out in a glove box. 2-
phenoxy-1-phenylethanol (53.6 mg, 0.25 mmol), Ru(H)2(CO)(PPh3)3
spectrometer. For 1H and 13C NMR the chemical shifts were
referenced to the residual solvent signal. All NMR shifts are
reported as δ in parts per million (ppm). Mass spectrometry was
carried out at National Mass Spectrometry Facility (NMSF-
Swansea). Infrared spectra were measured on a Perkin Elmer 2000
FT-IR system and elemental analysis was carried out in the facility at
London Metropolitan University. Gas chromatography was
performed on a Thermo Scientific Trace GC Ultra equipment
(split/splitless injector, Restek Rtx®-1, 100% dimethyl polysiloxane
column with 30m×0.25mm×0.25µm dimensions, carrier gas – He,
F.I.D. detector). Data was analysed using Chromeleon data system.
(4.59 mg, 0.005 mmol), desired ligand (0.005 mmol) and 1,2,4,5-
tetramethylbenzene (16.8 mg, 0.125 mmol) were diluted in
anhydrous xylenes (2 mL), in a 10 mL CEM microwave vial. The
reaction mixture was sealed and heated to 135 °C for 45 min, after
which the reaction mixture was cooled to room temperature,
filtered through silica and analysed by gas chromatography (stock
solutions for each of the reagents were prepared prior to the
reactions).
Representative procedure for obtaining kinetic profiles
Synthesis of Xantphos-type ligands
2-phenoxy-1-phenethanol (214.25 mg, 1 mmol), Ru(H)2(CO)(PPh3)3
(18.39 mg, 0.02 mmol), Xantphos (11.57 mg, 0.02 mmol) or 5a
(13.97 mg, 0.02 mmol) and 1,2,4,5-tetramethylbenzene (0.125
mmol from stock solution) were diluted in anhydrous xylenes (20
mL), in a 100 mL three necked round bottom flask fitted with a
reflux condenser and connected to Schlenk. The reaction mixtures
were heated to 135 °C. 0.1 mL Aliquots were taken from the
reaction mixture every 10 min for 2 h and every 15 min thereafter
for 2 h more. Each sample taken was diluted with acetone (0.5 mL),
filtered through silica and analysed by gas chromatography. For the
Eyring plots the same above procedure was used, but at varying
temperatures (see ESI for further details).
4,6-bis(diphenylphosphanyl)phenoxathiine (Thixantphos) – (14c)
At −78 °C, sec-butyllithium (1.4 M in cyclohexane, 10.7 mL, 14.97
mmol) was added dropwise to a stirred solution of phenoxathiin
(1.00 g, 4.99 mmol) and TMEDA (2.25 mL, 14.97 mmol) in dry
diethyl ether (50 mL). The reaction mixture was allowed to reach
room temperature and stirred for 16 h. Then a solution of
chlorodiphenylphosphine (2.92 mL, 14.97 mmol) in hexanes (15 mL)
was added dropwise to the reaction mixture, which was cooled to -
78 °C and stirred for 16 h. Solvents were removed in vacuo and the
resulting solid was dissolved in dichloromethane. This solution was
washed with water (3 × 10 mL) and the organic fraction was dried
with MgSO4, filtered and the volatiles were removed in vacuo. The
resulting solid was crystallised from dichloromethane to give a
white crystalline solid. Yield = 2.2 g (77.5%). Mp 244-246 °C. 1H
Acknowledgements
The authors would like to thank the EPSRC (Global Engagement
NMR (500 MHz, CDCl3): δ 7.24-7.34 (m, 12H), 7.23-7.17 (m, 8H), grant EP/K00445X/1 and critical mass grant EP/J018139/1) and the
European Union (Marie Curie ITN ‘SuBiCat’ PITN-GA-2013-607044)
7.10 (dd, J = 7.6, 1.5 Hz, 2H), 6.90 (t, J = 7.6 Hz, 2H), 6.50 (dq, J = 7.6,
1.6 Hz, 2H). 31P{1H} NMR (202 MHz, CDCl3): δ – 17.86 (s). 13C NMR for financial support. NMSF-Swansea and Mr. Stephen Boyer are
(126 MHz, CDCl3) δ 154.2 (t, J = 10.1), 136.9 (t, J = 6.3), 133.9 (t, J = kindly acknowledged for mass spectrometry and elemental analysis,
respectively.
10.6 Hz), 132.5, 128.3, 128.2 (t, J = 3.4 Hz), 128.0 (t, J = 12.6), 127.2,
6 | J. Name., 2012, 00, 1-3
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