Arene-Immobilized Ru(II)/TsDPEN Complexes: Synthesis and Applications to the Asymmetric Transfer Hydrogenation of Ketones

Article abstract of DOI:10.1002/ejic.202000948

The Noyori-Ikariya (arene)Ru(II)/TsDPEN precatalyst has been anchored to amorphous silica and DAVISIL through the η⁶-coordinated arene ligand via a straightforward synthesis and the derived systems, (arene)Ru(II)/TsDPEN@silica and (arene)Ru(II)/TsDPEN@DAVISIL, form highly efficient catalysts for the asymmetric transfer hydrogenation of a range of electron-rich and electron-poor aromatic ketones, giving good conversion and excellent ee's under mild reaction conditions. Moreover, catalyst generated in situ immediately prior to addition of substrate and hydrogen donor, by reaction of silica-supported [(arene)RuCl₂]₂ with (S,S)-TsDPEN, was as efficient as that generated from its preformed counterpart [(arene)Ru{(S,S)-TsDPEN}Cl]@silica. Gratifyingly, the initial TOFs (up to 1085 h^?1) and ee's (96–97 %) obtained with these catalysts either rivalled or outperformed those previously reported for catalysts supported by either silica or polymer immobilized through one of the nitrogen atoms of TsDPEN. While the high ee's were also maintained during recycle studies, the conversion dropped steadily over the first three runs due to gradual leaching of the ruthenium.

Full text of DOI:10.1002/ejic.202000948

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doi.org/10.1002/ejic.202000948
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Arene-Immobilized Ru(II)/TsDPEN Complexes: Synthesis
and Applications to the Asymmetric Transfer
Hydrogenation of Ketones
Simon Doherty,*[a] Julian G. Knight,*[a] Hind Alshaikh,[b] James Wilson,[a] Paul G. Waddell,[a]
Corinne Wills,[a] and Casey M. Dixon[a]
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The Noyori-Ikariya (arene)Ru(II)/TsDPEN precatalyst has been
anchored to amorphous silica and DAVISIL through the η6-
coordinated arene ligand via a straightforward synthesis and
the derived systems, (arene)Ru(II)/TsDPEN@silica and (arene)
Ru(II)/TsDPEN@DAVISIL, form highly efficient catalysts for the
asymmetric transfer hydrogenation of a range of electron-rich
and electron-poor aromatic ketones, giving good conversion
and excellent ee’s under mild reaction conditions. Moreover,
catalyst generated in situ immediately prior to addition of
substrate and hydrogen donor, by reaction of silica-supported
[(arene)RuCl2]2 with (S,S)-TsDPEN, was as efficient as that
generated from its preformed counterpart [(arene)Ru{(S,S)-
TsDPEN}Cl]@silica. Gratifyingly, the initial TOFs (up to 1085 hÀ 1)
and ee’s (96–97%) obtained with these catalysts either rivalled
or outperformed those previously reported for catalysts
supported by either silica or polymer immobilized through one
of the nitrogen atoms of TsDPEN. While the high ee’s were also
maintained during recycle studies, the conversion dropped
steadily over the first three runs due to gradual leaching of the
ruthenium.
Introduction
ultimately reduce operating costs. One of the most popular
approaches to immobilize these systems has been covalent
attachment of a nitrogen-modified TsDPEN to an amorphous or
mesoporous silica, while retaining the essential key feature of
an ‘active NÀ H’.[6] In addition, while TsDPEN grafted by covalent
attachment of nitrogen to polystyrene[7] and PEG[8] have both
been used to immobilize these catalysts with varying levels of
success, Xiao developed an alternative approach to immobiliz-
ing TsDPEN by attachment to poly(ethylene glycol) via both of
its phenyl rings; the corresponding Noyori-Ikariya catalyst is
among the most efficient to be reported with fast reaction
rates, excellent ee’s and outstanding reusability.[9] Other
methods used to immobilize (arene)Ru(II)/TsDPEN and facilitate
its recovery and reuse include modification of TsDPEN with an
imidazolium[10] or phosphonium[11] group for use in ionic liquids
and water, respectively, and incorporation of the diamine into a
Fréchet-type core-functionalized dendrimer.[12] In more recent
developments, (arene)Ru(II)/TsDPEN has been anchored to a
support which also incorporates a palladium-based catalyst for
cross-coupling and the resulting switchable bifunctional system
The asymmetric hydrogenation of ketones to alcohols is a
pivotal transformation in organic synthesis which is widely used
in the production of intermediates and pharmaceuticals.[1] The
Noyori arene-Ru(II)/TsDPEN system is among the most versatile
and efficient catalysts for asymmetric transfer hydrogenation
(ATH),[2] using either an azeotropic mixture of formic acid and
triethylamine or propan-2-ol as the hydrogen source, as well as
asymmetric hydrogenation (AH)[3] and as such numerous
modifications have been reported.[4] Although the Noyori-Ikariya
catalyst is highly efficient and has been successfully applied in
synthetic methodology,[5] the catalyst can be quite costly due to
the high catalyst loadings that are often required (0.5–
1.0 mol%) coupled with the expense of the precious metal and
a chiral ligand. As such there has been considerable interest in
exploring strategies to immobilize this system onto a solid
support to facilitate catalyst separation, recovery and reuse as
well as improve product purification and enable integration
into a continuous flow process for scale up, all of which will
used as
a catalyst for enantioselective cascade reaction
sequences.[13]
[a] Dr. S. Doherty, Dr. J. G. Knight, J. Wilson, Dr. P. G. Waddell, Dr. C. Wills,
Dr. C. M. Dixon
In contrast to the myriad of examples of solid-supported
Noyori-Ikariya-type catalysts immobilized through the TsDPEN
ligand, there appears to be only a single report of immobiliza-
tion through the π-arene ring.[14] This system was prepared by
polymerization of methacrylate side chain-modified [(arene)
RuCl2]2 with ethyleneglycol dimethacrylate and the resulting
polymers combined with TsDPEN to form an efficient catalyst
for the ATH of ketones. However, to the best of our knowledge
there are no reports of silica-supported precatalysts tethered
via the η6-coordinated arene, which is somewhat surprising
considering there are numerous advantages associated with the
Newcastle University Centre for Catalysis (NUCAT)
School of Chemistry, Bedson Building, Newcastle University
Newcastle upon Tyne, NE1 7RU, UK
E-mail: simon.doherty@ncl.ac.uk
julian.knight@ncl.ac.uk
www.ncl.ac.uk/nes/staff/profile/simondoherty.html#background
[b] Dr. H. Alshaikh
Department of Chemistry, Science and Arts College, King Abdulaziz
University, Rabigh Campus, Jeddah 21911, Saudi Arabia
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202000948
Part of the“Supported Catalysts” Special Collection.
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use an ordered mesoporous silica as a support for anchoring
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chiral transition metal catalysts; these include control of surface
area and pore volume, tunable pore dimensions, potential for
functionalization and good thermal and mechanical integrity.[15]
Thus, our interest in developing such an arene-anchored
catalyst is four-fold, firstly, the straightforward and versatile
synthesis of a range of functionalized 1,4-cyclohexadienes via
cycloaddition would lend itself to catalyst modification and
diversification, secondly, a library of catalysts could be gen-
erated either before or after silanization by introduction of a
suitable chiral diamine or amino alcohol, thirdly, anchoring the
catalyst to the support via the arene ring avoids modification of
the basic nitrogen atom of Ts-DPEN which has been reported to
reduce catalyst activity and enantioselectivity and, finally, arene
ruthenium complexes have also been used in a host of other
transformations including; 1,4-additions to conjugated
enones,[16] hydrosilylations,[17] arene hydrogenation,[18] oxidation
of alcohols,[19] Diels-Alder cycloadditions,[20] cyclopropanation,[21]
the hydrocarboxylation of alkynes[22] and ring-opening and ring-
closing metathesis[23] and as such this strategy may well have
much broader applications. Herein we report the first examples
of an (arene)Ru(II)/TsDPEN-based Noyori-Ikariya precatalyst
anchored to silica through the coordinated η6-arene and their
application to the asymmetric transfer hydrogenation of
ketones. The conversions and ee’s obtained with these catalysts
either rivalled or outperformed those obtained with their
homogeneous counterparts as well as systems immobilized on
either silica, a polymer or PEG through one of the nitrogen
atoms of the TsDPEN.
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Scheme 1. Synthesis of silica-supported (arene)Ru(II) dimers 6 and 7.
ers that are indistinguishable by NMR spectroscopy. Crystallisa-
tion of 4 by slow evaporation of a concentrated ethanol
solution at room temperature gave crystals suitable for a single-
crystal X-ray study; a perspective view of the molecular
structure is shown in Figure 1. The molecular structure shows
that the crystal used to collect the data contains the (S,R) anti-
diastereoisomer although this does not conclusively prove that
4 exists as a single diastereoisomer in solution. The RuÀ C(arene)
bond lengths fall between 2.149(3) and 2.200(3) Å with a mean
value of ca. 2.18 Å, which is within the range reported for
related complexes such as [(p-cymene)RuCl2]2 (range: 2.13(1)–
2.18(2) Å; mean: ca. 2.16 Å),[25] [(C6H5OCH2CH2OH)RuCl2]2 (range:
2.14(1)–2.21(1) Å; mean: ca. 2.17 Å),[26] [1,4-C6H4(CH2CO2Et)2)
RuCl2]2 (range: 2.150(5)–2.182(5) Å; mean: ca. 2.17 Å)[27] and
[(C6H5CH2CO2H)RuCl2]2 (range: 2.15(1)–2.184(8) Å; mean: ca.
2.16 Å).[28] The RuÀ Cl bond lengths of 2.4152(8) Å (monoden-
tate) and ca. 2.44 Å (bridging) are also unexceptional and
similar to the mean value for those observed in this series of
(arene)Ru-dimers (ca. 2.40 Å (monodentate) and ca. 2.45 Å
(bridging).
Results and Discussion
Synthesis of Silica-Supported (Arene)Ru(II) Dimers (6and 7),
the Corresponding Precatalysts (8and 9) and Molecular
Precatalyst (10)
The key to immobilizing the (arene)Ru(II) fragment via the arene
ligand is straightforward access to an appropriately substituted
1,4-cyclohexadiene which can be further modified to introduce
a silanizable triethoxysilyl group after coordination to ruthe-
nium (Scheme 1). This was achieved via the cobalt catalyzed
cycloaddition between 2,3-dimethyl-1,3-butadiene 1 and 3-but-
The silanizable triethoxysilane-containing extension was
introduced by reaction of the alcohol 4 with triethoxy(3-
isocyanatopropyl)silane to afford the corresponding carbamate
5 in 80% yield after washing the crude product with dry hexane
1-ynol
2 to afford the 1,4-cyclohexadiene 3 as a clear
spectroscopically pure oil after purification by distillation
(Scheme 1).[24] The corresponding (arene)ruthenium(II) dimer 4
was prepared by reaction of 3 with ruthenium trichloride in 2-
methoxyethanol and isolated as a pale orange solid in near
quantitative yield by precipitation with diethyl ether. As each
ruthenium fragment has a stereogenic plane, 4 could exist as a
mixture of rac and meso diastereoisomers. Interestingly though,
the 1H NMR and 13C{1H} NMR spectra appear to contain only one
set of resonances with no evidence for a second diastereoisom-
er; this may be due to either (i) formation of a single
diastereoisomer, (ii) rapid interconversion of a mixture of
diastereoisomers via facile dissociation of the kinetically labile
chloride bridges or (iii) formation of a mixture of diastereoisom-
Figure 1. Crystal structure of [(2-(3,4-dimethylphenyl)ethan-1-ol)RuCl2]2 (4).
Hydrogen atoms have been omitted for clarity except those of the hydroxyl
groups. Ellipsoids are drawn at the 50% probability level.
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to remove excess isocyanate. The identity of 5 was established
by H NMR spectroscopy, IR spectroscopy and the electrospray
prepare Noyori-Ikariya-type precatalysts by reacting 6 and 7
with (S,S)-TsDPEN and evaluating their efficacy in the ATH of
ketones. While it would be most convenient and cost effective
to generate the catalyst in situ immediately prior to catalysis,
precatalysts 8 and 9 were also first prepared on a large scale to
accurately determine the ruthenium loading. Reactions were
conducted in dichloromethane by stirring either 6 or 7 with a
slight excess of (1S,2S)-TsDPEN and triethylamine for 4 h, in a
modification of a previously reported procedure (Scheme 2).[24a]
Precatalysts 8 and 9 were isolated as pale orange solids by
filtration and the ruthenium content determined to be 0.021
and 0.032 mmolgÀ 1, respectively, by ICP-OES giving loadings of
0.21 and 0.32 wt%. The solid state 13C cross-polarization magic
angle spinning NMR spectra of 8 and 9 confirmed incorporation
of the (arene)Ru(II)/TsDPEN precatalyst as signals between δ 4
and 51 ppm are characteristic of the SiCH2, the CH2 fragments
of the aliphatic linker and the CH3 substituents on the η6-arene
ring and the TsDPEN, while those at δ 64 and 75 ppm
correspond to the NCHPh carbon atoms of TsDPEN and the
OCH2 of the carbamate linker and those between δ 120 and
145 ppm belong to the carbon atoms of the aromatic ring of
the TsDPEN and the η6-arene. Moreover, the chemical shifts of
the structurally relevant carbon atoms map closely to those for
the homogeneous molecular benchmark 10, further confirma-
tion for incorporation of the (arene)Ru(II)/TsDPEN precatalyst.
The magic angle spinning solid state 29Si spectra of 8 and 9
both contained two groups of signals typical for silica; a Q-
series for inorganic silica (HO)nSi(OSi)4-n and a T-series for
organic silica RSi(OSi)3. The Q series appears as a set of three
poorly resolved signals at ca. δ À 91, À 102 and À 112 ppm for
Q2 ((HO)2Si(OSi)2), Q3 ((HO)Si(OSi)3) and Q4 (Si(OSi)4), respectively,
and is markedly more intense than the major T3 species which
is a broad ill-defined peak at δ À 66 ppm (Figure 2b); the
disparate intensities of these resonances confirms that these
catalysts are comprised mainly of inorganic silicate together
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mass spectrum which contained an ion with an m/z at 534.1011
corresponding to the [Ru(arene)Cl]+ fragment. (Arene)Ru(II)
dimer 5 is a highly versatile fragment that can be immobilised
on a range of silica supports and subsequently reacted with a
chiral ligand to generate a library of catalysts. Amorphous silica
and DAVISIL were identified as suitable supports for our
preliminary immobilisation studies; this was achieved by
heating 5 and the silica in a range of solvents at reflux. Reaction
times were varied and the extent of silanization determined as
a function of time by filtering the reaction mixture to remove
the silica and analysing the remaining solution by 1H NMR
spectroscopy, using 4-bromobenzonitrile as an internal
standard to quantify the amount of 5 that had not been
immobilised. The most efficient silanization was achieved in
toluene after 24 h. In addition, as the π-arene ligand is known
to undergo exchange at elevated temperatures, the stability of
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°
5 in toluene was monitored at 120 C for 24 h; under these
conditions there was no evidence for dissociation of the arene
in 5. Under these conditions, 5 was immobilized on amorphous
silica and DAVISIL to afford 6 and 7, respectively, in near
quantitative yields. DAVISIL was chosen as it is a high surface
area amorphous silica (pore size 6 nm, 450–560 m2 gÀ 1) and as
such the active sites are expected to be exposed and accessible
on the surface of the support, whereas for mesoporous
materials with a hierarchical structure such as MCM-41 and
SBA-15 the active sites are more likely to be encapsulated in
pores; moreover a wide range of pore diameters is commercially
available which will ultimately enable the influence of the pore
size on catalyst efficacy to be explored in a systematic manner.
The amorphous silica used for comparison has a slightly smaller
pore size of 4 nm and a correspondingly higher BET surface
area of >700 m2 gÀ 1. The ruthenium loadings of 6 and 7 were
determined to be 0.023 and 0.037 mmolgÀ 1, respectively, by
ICP-OES which corresponds to a loading of 0.23 and 0.37 wt-%,
respectively, and the solid state 13C and 29Si cross-polarization
magic angle spinning NMR spectra confirmed that the
ruthenium dimer was immobilized onto the silica walls of an
inorganosilicate network (Figure 2a).
with
a minor amount of organic silicate resulting from
immobilization of the silylated (arene)Ru(II)/TsDPEN on the walls
of the silicate network. The X-ray photoelectron spectra of 8
and 9 each contained a doublet with binding energies of
462.28 eV (Ru 3p1/2) and 484.58 eV (Ru 3p3/2), and 463.78 eV (Ru
3p1/2) and 486.68 eV (Ru 3p3/2), respectively; these values are
consistent with a Ru(II) species immobilized on the surface of
silica.
Our interest in anchoring (arene)Ru(II) to silica through the
η6-coordinated arene was to develop a practical system that
was amenable to diversification into a library of heterogeneous
catalysts by modification with a range of ligands. With the aim
of demonstrating the practicality of this approach, we chose to
While it is conventional to undertake comparative catalyst
testing against [(p-cymene)Ru{(1S,2S)-TsDPEN)}Cl] as a soluble
Scheme 2. Synthesis of silica-supported (arene)Ru(II)/TsDPEN precatalysts 8
and 9.
Figure 2. Solid state 29Si CP MAS NMR spectra of (a) 7 and (b) precatalyst 9.
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monomeric homogeneous pe-catalyst we chose to use [(2-(3,4-
evidence for the presence of multiple species in either the 1H or
13C{1H} NMR spectra; however, this is not conclusive as
resonances may be coincident or other diastereoisomers may
be present but only as a minor component. Figure 3 shows that
the ruthenium atom adopts a piano stool pseudo octahedral
geometry with the η6-arene, chloride and the (1S,2S)-DPEN
completing the coordination sphere. The RuÀ C(arene) bond
lengths fall in the range 2.162(5)-2.217(5) Å which is consistent
with those reported for related complexes such as [(p-cymene)
Ru{(1S,2S)-TsDPEN}(Cl)] (range: 2.141(7)–2.239(8) Å; mean: ca.
2.19 Å),[29] [{1,4-C6H4(Me)(C4H8OH)}Ru{(1S,2S)-TsDPEN(Cl)] (range:
1
2
3
4
5
6
7
8
9
dimethylphenyl)ethan-1-ol)Ru{(1S,2S)-TsDPEN}(Cl)] (10) as it
more closely represents a molecular analogue of 8 and 9 and,
as such, should provide a more realistic assessment of the
influence on catalyst performance of attachment to the support.
Precatalyst 10 was prepared by stirring a dichloromethane
solution of 4, (1S,2S)-TsDPEN and triethylamine for 1 h at RT.
Although the NMR spectra, the electrospray mass spectrum and
analytical data were all consistent with the formulation of 10,
its identity was conclusively established by a single-crystal X-ray
study; a perspective view of the molecular structure is shown in
Figure 3. The molecular structure shows that the crystal used to
collect the data contains a single diastereoisomer which is
consistent with the NMR spectroscopic data as there is no
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2.166(3)–2.236(3) Å;
mean:
ca.
2.20 Å),[30]
and
[{C6H5(OCH2CH2OH)}Ru{(1S,2S)-TsDPEN}(Cl)] (range: 2.18(2)–
2.23(2) Å; mean: ca. 2.2 Å).[31] The RuÀ N(1) and RuÀ N(2) bond
lengths of 2.104(5) Å and 2.162(4) Å, respectively, are also
unexceptional and similar to those in this representative
selection of precatalysts.
Asymmetric Transfer Hydrogenation of Ketones
Preliminary catalyst screening and optimization focused on
acetophenone as the substrate of choice as this is often
employed as the benchmark for evaluating new catalysts;
optical purity and yields were determined by GC analysis and
full details are listed in Table 1. Reactions were initially
conducted using catalyst generated in situ by reaction of either
6 or 7 with a slight excess of (S,S)-TsDPEN and triethylamine in
°
dichloromethane for 1 h at 55 C, prior to addition of the
Figure 3. Crystal structure of [(2-(3,4-dimethylphenyl)ethan-1-ol)Ru{(1S,2S)-
TsDPEN}(Cl)] (10). Hydrogen atoms have been omitted for clarity with the
exception of those bound to heteroatoms. Ellipsoids are drawn at the 50%
probability level.
reducing agent and substrate; comparative testing was also
undertaken with pre-prepared catalysts 8 and 9. Table 1 shows
Table 1. Asymmetric transfer hydrogenation of acetophenone using precatalysts generated from silica-supported ruthenium dimers 6 and 7 their
precatalysts 8 and 9 or molecular precatalyst 10.[a]
ee [%][b,c]
°
Entry
Catalyst system
H-donor
[Mol%] cat
Temp [ C]
Solvent
Conv. [%]
(TOF [hÀ 1])[b]
1
2
3
4
5
6
7
8
(S,S)-TsDPEN/6
(S,S)-TsDPEN/7
8
9
HCO2H/NEt3
HCO2H/NEt3
HCO2H/NEt3
HCO2H/NEt3
HCO2H/NEt3
HCO2H/NEt3
HCO2H/NEt3
HCO2H/NEt3
HCO2H/NEt3
Me2NHBH3
NaBH4
KO2CH
NH4O2CH
HCO2H
HCO2H/NEt3
HCO2H/NEt3
HCO2H/NEt3
HCO2H/NEt3
HCO2H/NEt3
HCO2H/NEt3
HCO2H/NEt3
0.22
0.34
0.22
0.34
1.0
55
55
55
55
55
55
55
55
55
55
55
55
55
55
55
50
45
25
55
55
55
–
–
–
–
–
–
–
–
90 (272)
99 (194)
88 (266)
98 (192)
99 (66)
2 (3)
96
97
97
97
96
nd
nd
90
97
7
10
(R,R)-Ph-pyBOX/7
(1R,2S)-1-amino-2-indanol/7
(S,S)-TsCYDN/7
0.34
0.34
0.34
0.17
0.17
0.17
0.17
0.17
0.17
0.085
0.17
0.17
0.17
0.17
0.17
0.17
3 (6)
49 (96)
77 (392)
99 (388)
98 (384)
5 (20)
0 (0)
0 (0)
26 (203)
46 (180)
18 (71)
50 (12)
0 (0)
14 (55)
12 (47)
9
9
9
9
9
9
9
9
9
9
9
9
9
9
–
–
10
11
12
13
14
15
16
17
18[d]
19
20
21
water
water
water
–
–
–
3
91
nd
nd
99
98
98
98
nd
49
54
–
–
water
EtOH
MeOH
[a] Reactions were carried out with 0.5 mmol of acetophenone using precatalyst generated from silica-supported ruthenium dimers 6 and 7 or pre-prepared
silica-supported (arene)Ru/TsDPEN precatalysts 8, 9 or 10 in neat HCOOH-NEt3 azeotrope for 90 min (unless otherwise stated) under the specified conditions
of temperature, S/C ratio, hydrogen donor and solvent. [b] Determined by gas chromatography equipped with a CP-Chirasil-DEX CB column using decane as
internal standard. [c] Configuration was determined to be S from the sign of the optical rotation. [d] Reaction time of 24 h.
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that catalysts generated from either 6 or 7 and (S,S)-TsDPEN
gave TOFs and ee’s that either matched or outperformed those
obtained with their soluble molecular counterpart 10, after
gave a much lower conversion of 26% but with an enantiose-
lectivity of 99%. As there have been several reports of efficient
ATH of ketones and imines in water using catalysts generated
from silica-supported TsDPEN[6c,d,h,32a,b] a series of reactions were
conducted in water, ethanol and methanol; however, in each
case conversions were either low or negligible and, as such, all
further reactions were performed in neat HCOOH-NEt3 azeo-
trope. A study of the conversion and ee as a function of time
1
2
3
4
5
6
7
8
9
°
1.5 h at 55 C using formic acid-triethylamine azeotrope as the
hydrogen donor (Table 1, entries 1–2 and 5). Moreover, the ee’s
and yields obtained with both silica-supported systems match
those reported for catalyst generated from [(p-cymene)Ru{(S,S)-
TsDPEN}(Cl)] and triethylamine at a reaction temperature of
[2b]
°
°
60 C, and either compete with or outperform existing silica-
using 0.17 mol% 9 in neat HCOOH/NEt3 azeotrope at 50 C
supported (arene)Ru/TsDPEN-based catalysts immobilized
through the nitrogen atom of the TsDPEN ligand, including
magnetically retrievable mesoporous silica microcapsules[6c,h]
and (arene)Ru/TsDPEN confined in amphiphilic-modified nano-
cages of SBA-16,[32a,b] or supported on silica gel, mesoporous
MCM-41, SBA-15,[6e,d,f,g] or siliceous mesocellular foam (SMF).[6h,i]
Reassuringly, the ee’s and TOF’s obtained with preformed
catalysts 8 and 9 also match those obtained with catalyst
generated in situ which suggests that in situ formation of the
catalyst under these conditions is efficient and essentially
quantitative (Table 1, entries 3 and 4). The slightly higher yield
obtained with 9 compared to 8 is most likely due to the
increased efficiency in ruthenium loading, however, at this early
stage, speculation as to the origin of minor differences in
activity between catalyst supported on amorphous silica and
Davisil is not warranted. However, DAVISIL-supported 9 was
identified as the system of choice to undertake further studies
and substrate screening (vide infra) as the more tightly
controlled chemical and structural properties of DAVISIL silicas
will enable a systematic investigation of the influence of the
support on catalyst efficacy to be conducted. A survey of
catalysts generated in situ by addition of various chiral diamines
and amino alcohols including (S,S)-TsDPEN, (R,R)-Ph-pyBOX,
(1R,2S)-1-amino-2-indanol and (S,S)-TsCYDN revealed (S,S)-
TsDPEN generated the most efficient catalyst. Following this,
precatalyst 9 was used to further explore the influence of
varying reaction conditions and reagents on catalyst perform-
ance including solvent, temperature, time, catalyst loading and
hydrogen donor. Variation of the hydrogen donor revealed that
formic acid-triethylamine azeotrope gave the best combination
of TOF and ee whereas NaBH4 and Me2NHBH3 both gave near
quantitative yields but very poor ee’s and formate salts gave
negligibly low conversions. As expected, lowering the reaction
temperature resulted in a reduction in activity such that a
revealed that complete conversion was achieved after 2 h,
which corresponds to an initial TOF of 1085 hÀ 1 (Figure 4,
conversion & and ee ~). For comparison, the corresponding
conversion-time profile using 0.17 mol% 10 gave a markedly
lower initial TOF of 260 hÀ 1 (Figure 4, conversion * and ee ♦).
The improved performance of 9 compared with 10 could be
due to either ‘confinement’,[33] site isolation[34] or preorganiza-
tion of the CÀ H/π interaction arising from attachment of the η6-
arene ring to the silica[4m,35] and further catalyst modifications
are currently underway to explore the origin of this
enhancement. Gratifyingly, the TOF obtained with 9 also
appears to be significantly higher than Noyori-Ikariya catalysts
immobilized on mesoporous silica,[6f,h–i,7g,9a,32b] polystyrene,[7b,d,f]
or polyethylene glycol.[8a–c,9b] Based on the above screening
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
°
study, a temperature of 50 C and reaction time of 5 h was
considered to be the best compromise to explore the substrate
scope and efficacy of DAVISIL silica-supported 9.
Having identified optimum conditions and obtained encour-
aging conversions and ee’s for the benchmark transfer hydro-
genation of acetophenone, catalyst testing was extended to a
range of aryl and heteroaryl ketones to explore and assess the
scope and limitations of DAVISIL-supported precatalyst 9 and
its molecular counterpart 10 (Table 2). Good to excellent
conversions and high ee’s to the corresponding secondary
alcohol were obtained across a range of electron deficient 2-, 3-
and 4-substituted ketones (Table 2, entries 1–10) and in most
cases, silica-supported 9 either rivalled or outperformed its
molecular counterpart 10. Moreover, the TOFs and ee obtained
with 9 are comparable to or better than those previously
reported for (arene)Ru/TsDPEN precatalysts supported on
mesoporous silica[6c,d,e,g] and siliceous mesocellular foam[6h,i] and
encapsulated within nanocages of amphiphilic SBA-16[32] as well
as PEG-based polymers[8a,b,c,9b] and styrene-based systems such
°
conversion of 46% was reached at 50 C with a slight improve-
ment in the ee to 98%; the conversion could be improved by
extending the reaction time with no loss in ee. Lowering the
°
reaction temperature further to 45 C resulted in a marked
reduction in conversion to 18% after 1.5 h which increased to
°
81% after 5 h, in both cases with an ee of 98%. Finally, at 25 C
a reaction time of 24 h was required to reach 50% conversion
with an ee of 98%; although more sluggish than reactions
conducted at higher temperatures, the enantioselectivity and
TOF were comparable to those obtained by Noyori using
0.5 mol% [(η6-mesitylene)Ru{(S,S)-TsDPEN)(Cl)] as precatalyst.[2b]
A reduction in the catalyst loading to 0.17 mol% resulted in a
slightly lower conversion of 77% with an enantioselectivity of
97% while a further reduction in the loading to 0.085 mol%
Figure 4. Reaction profile as a function of time for the asymmetric transfer
hydrogenation of acetophenone in neat HCOOH/NEt3 azeotrope using (a)
0.17 mol% 9 (%conversion & and %ee ~) and (b) 0.17 mol% 10 (%
conversion * and % ee ♦).
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disparate and varied between 87–98%. For example, reduction
Table 2. Asymmetric Transfer Hydrogenation of Ketones in Formic Acid-
Triethylamine Azeotrope using Silica-Supported Precatalyst 9 and Molec-
ular Precatalyst 10.[a]
1
2
of 2-bromoacetophenone gave 1-(2-bromophenyl)ethan-1-ol
with an ee of 98% whereas its 2-chloro-substituted counterpart
gave the corresponding secondary alcohol in 87% ee. High
conversions and excellent ee’s were also obtained for arylke-
tones substituted with electron donating groups at the 2-, 3-
and 4-positions (Table 2, entries 11–13) as well as 2-acetonaph-
thone (Table 2, entry 14) all of which gave the corresponding
alcohol in 99–100% ee. Even though the p-meth-
oxyacetophenone only reached 60% conversion after 5 h under
these conditions (Table 2, entry 11), the ee of 99% is an
improvement on that reported for [(η6-mesitylene)Ru{(S,S)-
TsDPEN}Cl][2b] as well as the majority of silica,[6] polymer[7,14] and
PEG-supported[8] catalysts; moreover, complete conversion was
obtained by extending the reaction time to 12 h with no loss in
ee. The same protocol was extended to the asymmetric transfer
hydrogenation of 2-acetylfuran and 2-acetylthiophene which
gave 91% and 78% conversion to (S)-1-(2-furyl)ethanol and (S)-
1-(2-thienyl)ethanol, respectively, both with 100% ee (Table 2,
entries 15 and 16). While 9 tolerated the steric hindrance of a
variety of ortho-substituted acetophenones, negligible conver-
sions were obtained with 2,2-dimethyl-1-phenylpropan-1-one,
1-tetralone, 1-acetonaphthone, and cyclopropyl(phenyl)
methanone, which are sterically much more demanding
substrates. Unfortunately, 9 was also unable to reduce 3- and 4-
acetylpyridine as quantitative amounts of starting material were
consistently recovered even after an extended reaction time of
10 h. Reasoning that the presence of a large excess of nitrogen
donor could result in ligand substitution of either the TsDPEN
or chloride and afford a less active or inactive pyridine-
saturated species, pre-treatment of a mixture of 0.17 mol% 9
and HCO2H/NEt3 azeotrope with 0.5 mmol of 3-acetylpyridine at
3
4
5
6
7
8
9
Precatalyst 9
Conv [%]/(TOF
[hÀ 1])[b]
Precatalyst 10
Entry Substrate
ee
Conv (%)/(TOF ee
[%][b,c] [hÀ 1])[b]
[%][b,c]
1
100 (118)
93
95 (112)
92
2
3
4
5
6
99 (116)
91 (107)
100 (118)
100 (118)
80 (92)
98
94
94
92
87
100 (118)
100 (118)
100 (118)
100 (118)
95 (112)
75
94
95
90
83
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
7
8
9
100 (118)
100 (118)
99 (116)
96
80
97
100 (118)
98 (115)
96 (113)
93
79
97
10
11
99 (116)
60 (71)
93
100 (118)
90 (106)
93
>99
>99
°
50 C for 15 min resulted in a significant reduction in activity as
12
13
100 (118)
98 (115)
>99
100 (118)
99 (116)
>99
a conversion of only 14% with an ee of 98% was obtained for
the reduction of acetophenone, compared with complete
conversion and an ee of 98% under the same conditions but in
the absence of 3-acetylpyridine. In a modification of this
investigation, the conversion of acetophenone as a function of
3-acetylpyridine addition time was investigated by running a
series of hydrogenations in parallel and adding 3-acetylpyridine
after 0.5 h, 1 h and 2 h and working each reaction up after 5 h;
the conversion profile of 54% (0.5 h), 81% (1 h) and 98% (2 h)
shows that addition of 3-acetylpyridine results in near instanta-
neous deactivation of the catalyst as the conversions obtained
at each time interval are similar to those obtained for the same
reaction in the absence of 3-acetylpyridine. In a scale-up
experiment, the asymmetric transfer hydrogenation of 2-
bromoacetophenone on a 10 mmol scale in 5.0 mL of the
HCO2H/NEt3 azeotrope gave complete conversion to 1-(2-
99
99
14
15
100 (118)
91 (107)
97
100 (118)
95 (112)
98
99
100
16
78 (92)
100
88 (103)
97
[a] Reactions conditions: 0.5 mmol of acetophenone, 0.17 mol% silica
supported (arene)Ru/Ts-DPEN precatalyst 9, in neat 5:2 formic acid:
triethylamine (0.25 mL, 3.0 mmol of HCO2H), 50 C, 5 h. [b] Determined by
gas chromatography equipped with a CP-Chirasil-DEX CB column using
decane as internal standard. [c] Configuration determined to be S from the
sign of the optical rotation.
°
°
bromophenyl)ethan-1-ol with an ee of 98% after 9 h at 50 C.
The reusability of 9 was investigated for the benchmark
transfer hydrogenation of acetophenone under the conditions
described above to assess the robustness and longevity of the
catalyst and the potential for integration into a continuous flow
reactor set-up. Recycle experiments were conducted on a larger
scale to try and overcome the practical problems associated
with recovering the catalyst from a small-scale reaction.
as a poly(styrene-1-phosphonate styrene) inorganic zirconium
phosphate–phosphonate hybrid,[7a] phosphonate-containing
polystyrene copolymer,[7b] cross-linked polystyrene,[7d] and am-
phiphilic polystyrene.[7f] While high ee’s were obtained with
each of the 4-substituted acetophenones examined, the ee’s
obtained with their 2-substituted counterparts were more
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Reactions were run for 4 h to avoid complete conversion which
TsDPEN immediately prior to addition of substate and hydrogen
donor, either competed with or outperformed its preformed
counterpart [(arene)Ru{(S,S)-TsDPEN)Cl]@silica, which presents
numerous practical advantages for catalyst optimization, sub-
strate screening and reaction diversification. Gratifyingly, the
TOFs and ee’s obtained with these catalysts rivalled those
previously reported for catalysts immobilized on either silica or
polymer through a nitrogen atom of the Ts-DPEN ligand. High
ee’s were maintained during recycle studies, however, the
conversion dropped steadily over the first three runs due to
gradual leaching of the ruthenium. These are encouraging
results and provide a platform for further studies that will apply
this immobilization strategy to prepare (arene)Ru(II)/(Ts-DPEN)
precatalysts supported on a range of DAVISIL silicas as well as
functionalized ordered mesoporous silicas that can be inte-
grated into a continuous flow process for scale-up or that can
be used to develop multifunctional catalysts for cascade
reaction sequences and the conversion of biomass derived
substrates into value-added products.
1
2
3
4
5
6
7
8
9
would enable any change in activity to be observed. The
catalyst was recycled by quenching the reaction with a large
excess of ethyl acetate, recovering the catalyst by centrifuga-
tion, and removing the organic phase with a syringe, then
recharging the flask with additional portions of HCO2H/NEt3
azeotrope and acetophenone.[7d] Following this protocol, 9 gave
consistently high ee’s (>99%) across five cycles although
conversions dropped gradually from 99% in the first run to
77% and 63% in the second and third runs, respectively, after
which they remained constant (Figure 5). ICP analysis of the
organic phase collected during the first three runs revealed
leaching of the ruthenium to be the primary reason for the
decrease in conversion as the ruthenium content dropped by
18% in run 1 and 8% and 5% in runs 2 and 3, respectively;
which closely correlates with the reduction in conversion. To
this end, Hintermair has recently provided convincing evidence
for two deactivation/inhibition pathways for (arene)(TsDPEN)
RuÀ H, one of which involves gradual dissociation of the arene
ligand while the other involves competitive inhibition of the
unsaturated intermediate by excess base.[36] Even though the
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
activity dropped during the first two runs, the stable activity Experimental Section
profile during successive runs suggests that the remaining
Synthesis of 2-(4,5-Dimethylcyclohexa-1,4-dien-1-yl)ethan-1-ol
(3). According to the literature method,[24b] Zn powder (0.28 g,
4.22 mmol), ZnI2 (1.40 g, 4.38 mmol), CoBr2 (0.48 g, 2.19 mmol), and
DPPE (0.86 g, 2.15 mmol) were added to a three-neck flask (250 mL)
and stirred in dry THF (30 mL) at room temperature. 1,3-Dimethyl-
butadiene (13.5 mL, 118 mmol) was added to the reaction mixture
after 5 minutes, followed by but-3-yn-1-ol (7.9 mL, 105 mmol). The
supported ruthenium sites are robust with respect to leaching
and/or deactivation and inhibition. Future studies will aim to
establish why the remaining active sites are more robust after
initial leaching and further improve the stability profile to
integrate the system into a continuous flow process.
°
resulting mixture was stirred for 5 mins and then heated to 50 C
for one hour, after which the solvent was removed under reduced
Conclusion
pressure. The crude mixture was purified by vacuum distillation
1
°
(1 mmHg, 115–121 C) to afford diene 3 in 63% yield (10.1 g). H
NMR (300 MHz, CDCl3, δ): 5.49–5.41 (m, 1H, =CH), 3.70–3.53 (m, 2H.
CH2OH), 2.63–2.39 (m, 4H, CH2), 2.21–2.14 (m, 2H, CH2CH2OH), 1.60
(s, 6H, CH3). 13C{1H} NMR (75 MHz, DMSO, δ): 104.1, 102.6, 95.8, 87.5,
87.2, 83.4, 60.4, 36.2, 16.9, 16.3.
In conclusion, this paper describes the first example of a
Noyori-Ikariya precatalyst anchored to amorphous silica and
DAVISIL by immobilization through the η6-coordinated arene
ligand, which was prepared via a straightforward and versatile
cobalt-catalyzed [4+2] cycloaddition between a homopropar-
gylic alcohol and a diene. The derived catalysts (arene)Ru(II)/
TsDPEN@silica and (arene)Ru(II)/TsDPEN@DAVISIL exhibit excel-
lent activity for the asymmetric transfer hydrogenation of a
range of electron-rich and electron-poor aromatic ketones,
giving good conversions and high ee’s under mild reaction
conditions. Catalyst generated in situ, by reaction of the
corresponding silica-supported (arene)Ru(II) dimer with (S,S)-
Synthesis of [RuCl2{2-(3,4-dimethylphenyl)ethan-1-ol}]2 (4). Ac-
cording to the literature,[37] a suspension of RuCl3.H2O (1.1 g,
5.3 mmol) and NaHCO3 (0.45 g, 5.3 mmol) in a mixture of 2-
methoxyethanol:H2O (11 mL, 10:1) was added 2-(4,5-dimeth-
ylcyclohexa-1,4-dien-1-yl)ethan-1-ol 3 (3.2 g, 21.2 mmol). The result-
°
ing mixture was heated at 120 C for 2.5 h after which time half of
the solvent was removed under reduced pressure and diethyl ether
added (10 mL) to precipitate an orange solid. The solid was filtered,
washed with Et2O and dried to obtain the dimer 4 as an orange
1
powder in 49% yield (0.9 g). H NMR (300 MHz, DMSO, δ): 5.81 (d,
J=5.6 Hz, 1H, ArH), 5.70 (s, 1H, ArH), 5.58 (d, J=5.6 Hz, 1H, ArH),
4.76 (t, J=5.1 Hz, 1H, OH), 3.68 (qd, J=6.5, 3.3 Hz, 2H, CH2OH) 2.65-
2.53 (m, 1H, CHaCHbCH2OH), 2.47-2.34 (m, 1H, CHaCHbCH2OH), 2.05
(s, 3H, CH3), 1.96 (s, 3H, CH3). 13C{1H} NMR (75 MHz, DMSO, δ):104.1,
102.7, 95.7, 87.5, 87.2, 83.3, 60.4, 36.2, 17.0, 16.3. IR: νmax cmÀ 1 735,
863, 899, 1023, 1044, 1081, 117, 1211, 1297, 1377, 1438, 2858, 2914,
3039, 3429 (br). HRMS (ESI) calculated for C12H17O3Ru [Ru(arene)
OAc]+: 311.0220, Found: 311.0217.
Synthesis of [RuCl2 O-(3,4-dimethylphenethyl N-(3-(triethoxysilyl)
propyl) carbamate)]2 (5). In a modification of a previously reported
literature procedure,[24a] 3-(triethoxysilyl)propyl isocyanate (0.26 mL,
1.1 mmol) was added to [RuCl2(2-(3,4-dimethylphenyl)ethan-1-ol)]2
(0.456 g, 0.70 mmol) and NEt3 (0.49 mL, 3.54 mmol) in dry dichloro-
Figure 5. Recycle study for the transfer hydrogenation of acetophenone
using precatalyst 9 and a reaction time of 4 h.
°
methane (10 mL). The reaction mixture was heated to 38 C for
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48 h. The solvent was removed under reduced pressure to obtain a
for the allocated time. The orange mixture was filtered through a
short silica plug, washed through with diethyl ether (2×10 mL) and
the solvent removed under reduced pressure. The residue was
dissolved in CDCl3 (0.7 mL), decane (97 μL, 0.5 mmol) added as
internal standard, and the solution analysed by 1H NMR spectro-
scopy and gas chromatography to determine the conversion and
enantioselectivity.
1
2
3
4
5
6
7
8
9
brown oil which was triturated with hexane (2×10 mL). The
resulting crude oil (0.645 g, 77%) was re-dissolved in dry dichloro-
methane (10 mL) to give a dark red solution which was used
1
without further purification. H NMR (300 MHz, CDCl3, δ): 5.28–4.82
(m, 3H, ArÀ H), 4.38–4.11 (m, 2H, CH2CH2O), 3.84–3.64 (m, 6H,
(CH3CH2O)3Si), 3.15–2.97 (m, 2H, CH2NH) 2.90–2.69 (m, 2H,
C6H3À CH2CH2O), 2.19–2.06 (m, 3H, CH3), 2.07–1.99 (m, 3H, CH3) 1.62–
1.43 (m, 2H, NHCH2CH2CH2), 1.21–1.07 (m, 9H, (CH3CH2O)3Si), 0.60–
0.43 (m, 6H, CH2Si). IR: νmax cmÀ 1 3264, 2973, 2927, 2882, 1682, 1254,
1069, 951, 768. HRMS (ESI) calculated for C20H35O5NClRuSi, [Ru-
(arene)Cl]+: 534.1011, found: 534.1011.
ATH of Acetophenone Using In-Situ Generated Silica-Supported
Precatalysts. A flame-dried Schlenk flask was charged with silica-
supported ruthenium dimer 6 (0.22 mol%, 0.048 g, 0.0011 mmol) or
7 (0.34 mol%, 0.046 g, 0.0017 mmol), ligand (1.3 equivalents based
on ruthenium) and triethylamine (2.0 equivalents based on
ruthenium) and the resulting suspension stirred for 1 h under
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
Synthesis of Silica-Supported Ruthenium Dimers 6and 7. A two-
necked flame-dried round bottom flask was charged with 5 (4.88%
w/w solution in dichloromethane; 6 mL, containing 0.34 mmol of
Ru), the solvent was removed under reduced pressure and toluene
(6 mL) added. Amorphous silica or DAVISIL (7.10 g) was then added
°
nitrogen at 55 C. After this time, HCO2H/NEt3 (0.25 mL, 3.0 mmol of
HCO2H) and the ketone (0.5 mmol) were added and the mixture
°
stirred at 55 C for the allocated time, after which it was filtered
through silica and washed through with diethyl ether (2×10 mL)
and the solvent removed under reduced pressure. The residue was
dissolved in CDCl3 (0.7 mL), decane (97 μL, 0.5 mmol) was added as
internal standard and the solution analysed by 1H NMR spectro-
scopy and gas chromatography to determine the conversion and
enantioselectivity.
°
to the solution and the mixture was heated to 110 C with rapid
stirring for 24 h. After this time the mixture was left to cool and the
solid was filtered, washed with ethyl acetate (3×10 mL) and dried
°
at 45 C overnight to afford 6 and 7 as orange solids in 93% (7.45 g)
and 99% (7.49 g) yield, respectively. ICP-OES data for 6: 0.23 wt%
ruthenium corresponding to
a
ruthenium loading of
ATH of Acetophenone and its Derivatives Using Precatalyst 10. A
flame-dried Schlenk flask containing 5:2 formic acid:triethylamine
(0.25 mL, 3.0 mmol of HCO2H) and the ketone (0.5 mmol) was
charged with precatalyst 10 (0.0036 g, 5.5 μmol, 1 mol%) and the
0.023 mmolgÀ 1. ICP-OES data for 7: 0.37 wt% ruthenium corre-
sponding to a ruthenium loading of 0.037 mmolgÀ 1
.
Synthesis of Silica-Supported [{2-(3,4-Dimethylphenylethyl propyl
carbamate)}Ru{(S,S)-T-DPEN)Cl] Precatalysts 8and 9. In a typical
procedure, triethylamine (0.094 mL, 0.68 mmol), silica-supported
ruthenium dimer 6 or 7 (mass corresponding to 0.34 mmol of Ru
calculated from the ruthenium loading) and (S,S)-TsDPEN (0.155 g,
0.42 mmol) were stirred in dichloromethane (20 mL) for 4 h at room
temperature after which time the solid was filtered, washed with
°
resulting mixture stirred at 50 C for 5 h. After this time, the
resulting orange mixture was filtered through silica and flushed
with diethyl ether (2×10 mL) and the solvent removed under
reduced pressure. The residue was dissolved in CDCl3 (0.7 mL) and
decane (97 μL, 0.5 mmol) added as internal standard and the
solution analysed by 1H NMR spectroscopy and gas chromatog-
raphy to determine the conversion and enantioselectivity.
°
dichloromethane (3×5 mL) and dried in an oven at 40 C for 5 h to
obtain silica-supported precatalysts 8 and 9 as orange solids in
99% (3.7 g) and 98% (3.7 g) yield, respectively. ICP-OES data for 8:
0.21 wt% ruthenium corresponding to a ruthenium loading of
0.021 mmolgÀ 1. ICP-OES data for 9: 0.32 wt% ruthenium corre-
ATH of Acetophenone Using Precatalyst 10 Generated In-Situ
from 4. A Schlenk flask was charged with 5:2 formic acid:triethyl-
amine (0.25 mL, 3.0 mmol of HCO2H), 4 (0.0035 g, 5.0 μmol) and
°
(S,S)-TsDPEN (2.8 mg, 7.6 μmol) and the mixture stirred at 55 C for
sponding to a ruthenium loading of 0.032 mmolgÀ 1
.
15 min. After this time, acetophenone (0.058 mL, 0.50 mmol) was
added and stirring continued for a further 90 min. The resulting
orange mixture was filtered through silica and flushed through with
diethyl ether (2×10 mL) and the solvent removed under reduced
pressure. The residue was dissolved in CDCl3 (0.7 mL), decane
(97 μL, 0.5 mmol) was added as internal and the solution analysed
by 1H NMR spectroscopy and gas chromatography to determine the
conversion and enantioselectivity.
Synthesis of RuCl(S,S)-TsDPEN[(2-(3,4-dimethylphenyl)ethan-1-ol]
(10). In modification of previously reported literature
procedure,[24a] (S,S)-TsDPEN (0.100 g, 0.28 mmol),
(0.089 g,
a
a
4
0.14 mmol) and triethylamine (0.056 g, 0.55 mmol) were dissolved
in dichloromethane (3 mL) and the reaction mixture was stirred at
room temperature for 1 h. The resulting orange solid was filtered,
washed with dichloromethane (1 mL) and dried to obtain precata-
1
lyst 10 as an orange solid (0.103 g, 56%). H NMR (300 MHz, DMSO,
General Procedure for Catalyst Recycling. A centrifuge tube was
charged with precatalyst 9 (0.17 mol%, 0.106 g, 0.0034 mmol), 5:2
formic acid:triethylamine (1.0 mL, 12.0 mmol of HCO2H) and
acetophenone (0.232 mL, 0.2 mmol) and the reaction mixture
δ): 7.24–7.32 (m, 1H, NH), 7.14–7.06 (m, 5H, ArH+NH), 6.83-6.54 (m,
10H, ArH), 5.74 (s, 1H, ArH), 5.62–5.59 (m, 1H, ArH), 5.44-5.43 (m, 1H,
ArH), 4.91–4.89 (m, 1H, OH), 3.80–3.75 (m, 2H, CH2OH), 3.62-3.60 (m,
1H, CHNTs), 3.16-3.10 (m, 1H, CHN), 2.92–2.86 (m, 1H, CHaHbCH2OH),
2.69-2.64 (m, 1H, CHaHbCH2OH), 2.22 (s, 3H, Me), 2.20 (s, 3H, Me),
2.08 (m, 3H, Me). 13C{1H} NMR (75 MHz, DMSO, δ): 143.7, 140.5,
140.2, 138.6, 129.3, 128.5, 127.9, 127.9, 127.5, 127.5, 127.1, 126.4,
94.6, 94.4, 94.3, 88.7, 81.2, 79.4, 71.9, 69.3, 61.3, 36.7, 21.2, 17.1, 16.9.
IR: νmax cmÀ 1 535, 575, 695, 699, 813, 918, 1064, 1129, 1265, 1422,
1453, 1575, 2875, 2925, 3028, 3056, 3243, 3301, 3433 (br). HRMS
(ESI) calculated for C31H35N2O3RuS [MÀ Cl]+: 611.1439; found:
611.1440. M.P.: 221–223 °C.
°
heated at 55 C for 5 h under a nitrogen atmosphere. After this time
water was added (1.0 ml) and the tube was placed in a centrifuge
at 5000 rpm for 5 min and the formic acid:triethylamine carefully
removed by pipette. Following this the solid was re-suspended in
formic acid:triethylamine azeotrope and water, centrifugation
repeated and the formic acid:triethylamine removed. After a third
washing the solid was dried in vacuum before adding further
portions of formic acid:triethylamine azeotrope and acetophenone.
The combined aqueous washings were extracted with diethyl ether
1
ATH of Acetophenone and its Derivatives Using Preformed Silica-
Supported Precatalysts 8and 9. Ketone (0.5 mmol) was added to a
suspension of either 8 (0.22 mol% Ru, 0.052 g, 0.0011 mmol) or 9
(0.34 mol% Ru, 0.053 g, 0.0017 mmol) and 5:2 formic acid:triethyl-
amine azeotrope (0.25 mL, 3.0 mmol of HCO2H) in a flame-dried
Schlenk tube and the mixture stirred at the specified temperature
to obtain a sample for analysis by H NMR spectroscopy and gas
chromatography.
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doi.org/10.1002/ejic.202000948
Yang, F. Chen, Y. He, N. Yang, Q.-H. Fa, Angew. Chem. Int. Ed. 2016, 55,
X-ray Crystallography
1
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4
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8
9
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Crystal structure data were collected at 150 K on a Rigaku Oxford
Diffraction Xcalibur, Altas, Gemini Ultra diffractometer using
equipped with a sealed tube X-ray source (λCuKα =1.54184 Å) and an
Oxford CryostreamPlus open-flow N2 cooling device. Intensities
were corrected for absorption using a multifaceted crystal model
created by indexing the faces of the crystal for which data were
collected.[38] Cell refinement, data collection and data reduction
were undertaken via the software CrysAlisPro.[39] All structures were
solved using XT[40] and refined by XL[41] using the Olex2 interface.[42]
All non-hydrogen atoms were refined as anisotropic and hydrogen
atoms were positioned with idealised geometry, with the exception
of those bound to heteroatoms, the positions of which were
located using peaks in the Fourier difference map. The displace-
ment parameters of the hydrogen atoms were constrained using a
riding model with UH set to be an appropriate multiple of the Ueq
value of the parent atom.
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Deposition Numbers 2045014 (for 4) and 2045015 (for 10) contain
the supplementary crystallographic data for this paper. These data
are provided free of charge by the joint Cambridge Crystallographic
Data Centre and Fachinformationszentrum Karlsruhe Access Struc-
tures service www.ccdc.cam.ac.uk/structures.
Acknowledgements
We gratefully acknowledge the Deanship of Scientific Research
(DSR) at King Abdulaziz University, Jeddah, Saudi Arabia for
funding this project (grant no: KEP-37-130-41). High-resolution
mass spectra were obtained at the EPSRC National Mass
Spectrometry Service in Swansea. We also thank Dr Kathryn White
for the SEM images (Faculty of Medical Sciences, Newcastle
University).
Conflict of Interest
The authors declare no conflict of interest.
Keywords: arene-immobilised · silica · (arene)Ru(II)/TsDPEN ·
asymmetric hydrogenation · ketones
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Manuscript received: October 12, 2020
Revised manuscript received: November 20, 2020
Accepted manuscript online: December 2, 2020
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