Easy to Synthesize, Robust Organo-osmium Asymmetric Transfer Hydrogenation Catalysts

Article abstract of DOI:10.1002/chem.201500534

Asymmetric transfer hydrogenation (ATH) is an important process in organic synthesis for which the Noyori-type Ru^II catalysts [(arene)Ru(Tsdiamine)] are now well established and widely used. We now demonstrate for the first time the catalytic a

Full text of DOI:10.1002/chem.201500534

DOI: 10.1002/chem.201500534
Communication
&
Asymmetric Catalysis
Easy To Synthesize, Robust, Organo-Osmium Asymmetric Transfer
Hydrogenation Catalysts
James P. C. Coverdale, Carlos Sanchez-Cano, Guy J. Clarkson, Rina Soni, Martin Wills,* and
Peter J. Sadler*[a]
Abstract: Asymmetric transfer hydrogenation (ATH) is an
important process in organic synthesis for which the
Noyori-type RuII catalysts [(arene)Ru(Tsdiamine)] are now
well established and widely used. We now demonstrate
for the first time the catalytic activity of the osmium ana-
logues. X-ray crystal structures of the 16-electron OsII cata-
lysts are almost identical to those of RuII. Intriguingly the
precursor complex was isolated as a dichlorido complex
with a monodentate amine ligand. The OsII catalysts are
readily synthesised (within 1 h) and exhibit excellent enan-
tioselectivity in ATH reactions of ketones.
Maintaining control over chirality is essential during many
Figure 1. Ruthenium pre-catalyst (R,R)-1, active amido catalyst (R,R)-2 and
chemical syntheses and so it has become necessary to obtain
enantiopure products cost-effectively. Selective reduction of
prochiral starting materials provides a simple route to such
compounds.[1] The Noyori catalyst [Ru(p-cymene)(TsDPEN)Cl],
(H)TsDPEN = N-p-tosyl-1,2-diphenylethylenediamine, 1 celebra-
tes two decades of use in the asymmetric transfer hydrogena-
tion (ATH) of ketones and imines, often achieving high enan-
tioselectivities (ee) and full conversions in under 24 h.[1–2]
hydrido species (R,R)-3 shown together with the transition state for reduc-
tion of ketones by (R,R)-3. The catalyst derived from the diamine of R,R con-
figuration gives the acetophenone product of R-configuration.
X-ray crystallographic structures of the pre-catalyst, catalyst
and hydrido species have been reported.[3a]
Although research on transfer hydrogenation catalysts has
largely been centred on RuII complexes,[2–4] and to some extent
those of Rh and Ir,[5] little attention has been paid to the po-
tential of closely related osmium(II) complexes. The first exam-
ples of osmium complexes for asymmetric transfer hydrogena-
tion (and the reverse reaction of alcohol oxidation) were re-
ported by Faller et al., using cis-aminoindanol as the chiral
ligand.[6] Several chiral OsII complexes have been reported for
asymmetric reductions including those containing l-a-amino
carboxylates[7] and pybox ligands.[8] Monophosphinite com-
plexes,[9] iminopyridine complexes,[10] and pincer complexes[11]
have also been described. Many of these reduce ketones with
high enantioselectivities at very low loadings. In addition, OsII
catalysts containing nonchiral ligands have been reported.[12]
Other low-spin d6 organometallic analogues [M(Cp*)(Tsdiami-
ne)Cl] with M=RhIII or IrIII have shown promise in their robust-
ness and performance whilst maintaining the high enantiose-
lectivities and conversions observed with the RuII catalysts,[5]
yet the chemistry of an Os-centred analogue of the Noyori cat-
alyst has never been explored. Here we report the first prepa-
ration of osmium analogues (4/5) of Noyori-type catalysts (1/
2). Importantly we find that the osmium catalysts are easy to
synthesise, are stable, and yet still retain high enantioselectivity
in asymmetric reductions of prochiral ketones.
Upon treatment with base in the course of a catalytic appli-
cation, Ru pre-catalyst 1 eliminates HCl, forming the active 16-
electron amido complex 2. During the reduction, the catalyst
cycles between 2 which contains a pseudo-planar chelated
ligand and an 18-electron ‘hydride’ species 3 (bearing hydrido
and amine ligands).[2,3] The catalysis is considered to be an
outer-sphere ligand-assisted process[3] since the substrate has
no direct interaction with the metal centre (Figure 1). Instead,
chiral components on the diamine ligand bring about a six-
membered transition state using both steric and electronic ef-
fects,[2,3] allowing hydrogenation from both the RuÀH and NÀH
centres to occur directly with the substrate. Because a favoured
diastereoisomer of hydride is regenerated during each cycle,
the catalyst is configurationally defined at the metal centre,
and this chirality is relayed to the product in the reduction.[2c]
[a] J. P. C. Coverdale, Dr. C. Sanchez-Cano, Dr. G. J. Clarkson, Dr. R. Soni,
Prof. Dr. M. Wills, Prof. Dr. P. J. Sadler
Department of Chemistry, University of Warwick
Coventry, CV4 7AL (UK)
E-mail: m.wills@warwick.ac.uk
p.j.sadler@warwick.ac.uk
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500534.
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Ruthenium Noyori-type catalysts are readily pre-
pared using the dimer precursor [RuCl2(h6-arene)]2,
(arene most commonly p-cymene) forming a chlorido
pre-catalyst when combined with the chiral ligand
(1S,2S)-(H)TsDPEN in 2-propanol with a base.[2e] This
method was unsuccessful for the Os complexes since
the dimer was poorly soluble in 2-propanol and de-
composition products were observed by NMR spec-
troscopy. However, OsII pre-catalysts were readily syn-
thesised by microwave methods, combining the
dimer [OsCl2(h6-p-cymene)]2 with a chiral diamine:
either (1R,2R)-(H)TsDPEN or (1S,2S)-(H)TsDPEN, to pro-
duce both enantiomers: (R,R)-4a and (S,S)-4b
(Figure 2).
Owing to the absence of base, it was anticipated
that these complexes would differ structurally from
the chelated ruthenium species.[3a] Indeed, NMR spec-
troscopy, mass spectrometry, infrared spectroscopy
Figure 3. Facile route for direct formation of 16-electron amido OsII catalysts, with X-ray
crystal structures of both enantiomers: 5a (left) and 5b (right). All hydrogen atoms
except those on the diamine and amide have been omitted for clarity.
Though adapted ruthenium
protocols to isolate the hydride
using 2-propanol[3a] and direct
hydrogenation were not success-
ful, the osmium hydride inter-
mediate was observed in solu-
tion by NMR spectroscopy by
treatment of 4 or 5 with triethyl-
1
amine and formic acid. H NMR
resonances of OsÀH species
were observed as two singlets at
d=À5.89 and À6.04 ppm each
with 187Os satellites (1J(187Os,1H)=
44 Hz). This suggests that hy-
dride transfer to 5 is not fully
stereoselective.
Subsequent
Figure 2. Noyori Ru pre-catalyst formation (top)[2c,3a] The route to the osmium pre-catalyst (bottom) differs in the
absence of base to form 4b. Upon subsequent treatment with a base they both form structurally similar 16-elec-
tron active catalysts (2b/5b).[2c,3a] Osmium catalysts 4a/5a (the opposite configuration) were synthesised by using
(1R,2R)-(H)TsDPEN.
enantioselectivity in substrate re-
duction may result from the
greater reactivity of one of the
two hydride diastereomers, as
previously speculated in the
and elemental analysis suggested that 4 contained the dia-
mine coordinated only by the terminal amine, while two chlori-
do ligands remained complexed to the metal centre (Figure 2
and Supporting Information).
case of RuII complexes.[4a,13] Alternatively, selectivity may be
controlled by an alternate interaction of the substrate with the
chiral ligand rather than a dependence on the configuration of
the metal hydride.
Treatment of either enantiomer of 4 with base yielded a red
solution of active amido catalysts 5a and 5b, highly stable an-
alogues of reported ruthenium complexes.[3a] The ability of 4
to still undergo activation by a Noyori-type HCl elimination
mechanism demonstrated that it was a suitable Os pre-catalyst
for ATH reactions. Alternatively, direct reaction of the Os-dimer
with either enantiomer of (H)TsDPEN with a base at room tem-
perature provided a facile route to 5a and 5b (Figure 3), syn-
thesised in less than an hour. Hence, although the osmium
chloride complexes analogous to 1 remained elusive, their 16e
amido derivatives 5 could be readily prepared. Moreover there
is no need for an inert atmosphere during synthesis or storage.
The X-ray crystal structures of catalysts 5a and 5b were de-
termined (Figure 3). The distorted octahedral OsII structures are
remarkably similar to the Ru analogue 2.[3a] Particularly, the ter-
minal amido NÀH bond was found to be similar in length
(0.96(3) ꢁ compared to 0.88(6) ꢁ in the Noyori catalyst). This
bond is known to be important in the RuII catalysis mecha-
nism[14] and so its comparable length may help to explain the
activities observed with OsII.
The catalytic activity of 4a and 4b and their 16-electron
active counterparts 5a and 5b towards acetophenone reduc-
tion were studied in a formic acid/triethylamine azeotrope to
give irreversible kinetic enantioselectivity.[1–2] Catalyst loading
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was defined by the substrate/catalyst molar ratio, S/C. A
Noyori-type ATH cycle and hydrogen-transfer mechanism was
assumed based on the structural similarities between amido
complex 5b and its RuII counterpart 2. Complexes 4a/4b
would be first converted into 5a/5b, respectively, before enter-
ing the catalytic cycle, hence the use of either 4 or 5 would be
expected to give identical results.
lectivities seem to be the result of both steric and electronic
factors, consistent with observations for Ru,[1–2] and a slight re-
duction of enantioselectivity for substrates bearing electron-
withdrawing substituents was observed.
Since reactions using the amido catalysts remained bright
yellow, the reduction of acetophenone using 4b was contin-
ued for a longer time. As anticipated, the conversion contin-
ued to increase with time (to 88% after 48 h and 94% after
72 h, Table 2), as the catalyst had not become deactivated. A
high enantiomeric excess (95%) was maintained, providing evi-
dence for ATH irreversibility when using formic acid/triethyla-
mine. The enantioselectivity compares well to ruthenium ana-
logues but the reaction proceeded at a slower rate, for exam-
ple, the conversion of ketone 6 at S/C=200 for Os complex
5b to 94% took 72 h, whilst Ru analogue 2 reached 99% after
20 h at 288C (Table 2). In general the kinetics of substitution re-
actions at OsII arene centres are approximately 40–100ꢂ slower
than at RuII.[15]
Complexes 4 and 5 were highly enantioselective for ATH,
with >94% ee controlled by the fixed chirality of the diamine.
(S,S)-Configured 4b/5b established reduction on the Si-face,
similarly to 2,[1–2] while R,R-configured 4a/5a selected for the
Re-face (reduction through an analogous transition state to
that in Figure 1).
Further reductions were undertaken using 4a/4b and select-
ed acetophenone derivatives 6–10 (Tables 1 and 2). Enantiose-
Table 1. Reduction of prochiral ketones 6–10 by R,R pre-catalysts 4a/5a.
Though the osmium catalysts may not compete with the
extent of conversion observed for ruthenium, they can be pre-
pared easily in under an hour, and do not require an inert at-
mosphere during synthesis or storage. The 16-electron amido
complexes 5a and 5b are particularly stable in air, and can be
prepared well in advance of requirement. This may be particu-
larly useful in a laboratory setting, in which a chiral alcohol can
be produced overnight (with high enantioselectivity) using
a catalyst that may be synthesised and stored well in advance.
These novel osmium–arene TsDPEN catalysts have been fully
characterised and catalyse ATH reductions of acetophenone-
derived substrates with high enantioselectivity. The Os cata-
lysts appear to follow a Noyori-type ATH mechanism, with simi-
lar X-ray crystal structures, hydrido intermediates observed by
NMR and mass spectrometry, and behaviour in catalytic reduc-
tions. The ease and speed of preparation combined with high
stability on storage of the osmium catalysts makes them at-
tractive alternatives to existing ruthenium catalysts.
Ketone
R1
R2
Cat.
S/C
Conv.[a]
ee [%][a]
6
6
6
6
7
8
9
10
H
H
H
H
Me
Me
Me
Me
Me
Me
CH2Cl
Et
4a
4a
5a
5a
4a
4a
4a
4a
100
200
100
200
100
100
100
100
98
62
83
67
98
67
97
56
98
98
98
97
96
p-Cl
p-MeO
H
[b]
–
94
96
H
[a] Determined by GC analysis. [b] ee undetermined for the reduction of 8
due to poor separation of enantiomers.
Table 2. Reduction of ketones 6–10 by S,S Os (pre)catalysts 4b/5b com-
pared to Ru analogue 2.[2a,b]
Experimental Section
General
The synthesis and characterisation of complexes 4a, 4b, 5a and
5b, catalytic reductions, H NMR spectroscopy of osmium hydride
species, and determination of the X-ray structures of 5a and 5b
are described in the Supporting Information.
Ketone
R1
R2
Cat.
S/C
100
Conv.[a]
ee [%][a]
1
6
6
6
6
6
7
7
8
8
9
9
10
10
6
H
H
H
H
Me
Me
Me
Me
Me
Me
Me
Me
Me
CH2Cl
CH2Cl
Et
4b
4b
5b
5b
2[2a,
4b
2[2a,
4b
2[2a,
4b
2[2b]
4b
2[2a,
5b
5b
93
54
79
59
99
97
99
70
99
92
36[b]
54
98
95
94
95
98
94
95
99
97
84
91[b]
95
97
95
95
200
100
200
200
100
200
100
200
100
1000
100
200
200
200
X-ray crystallographic data
2b]
2b]
2b]
H
p-Cl
p-Cl
p-MeO
CCDC-1035611 (5a), -1035612 (5b) and -686925 (2) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
p-MeO
H
H
H
H
H
H
2b]
Acknowledgements
Et
Me
Me
96
88[c]
94[d]
6
We thank the ERC (grant no. 247450 to P.J.S.), EPSRC (grant no
EP/F034210/1), WCPRS and Bruker Daltonics (Studentship for
J.P.C.C.) and the Leverhulme Trust (Fellowship for R.S. grant no.
RGP 374). We also thank Dr L. Song and P. Aston (Warwick) for
[a] Determined by GC analysis. Ru reductions for 20 h at 288C. [b] Reduc-
tion of 9 with ruthenium catalyst 2 was conducted in 1.0m EtOAc with S/
C=1000. [c] After 48 h. [d] After 72 h.
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Keywords: asymmetric catalysis · hydrogen transfer · ketone
reduction · organometallic complexes · osmium
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Received: February 9, 2015
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Communication
COMMUNICATION
Transferable methods: Asymmetric
transfer hydrogenation (ATH) is an im-
portant process in organic synthesis for
which the Noyori-type RuII catalysts
[(arene)Ru(Tsdiamine)] are now well es-
tablished and widely used. We now
demonstrate for the first time the cata-
lytic activity of the osmium analogues
(see scheme).
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Asymmetric Catalysis
J. P. C. Coverdale, C. Sanchez-Cano,
G. J. Clarkson, R. Soni, M. Wills,*
P. J. Sadler*
&& – &&
Easy To Synthesize, Robust, Organo-
Osmium Asymmetric Transfer
Hydrogenation Catalysts
Chem. Eur. J. 2015, 21, 1 – 5
www.chemeurj.org
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ