Octahedral Ruthenium Complex with Exclusive Metal-Centered Chirality for Highly Effective Asymmetric Catalysis

Article abstract of DOI:10.1021/jacs.7b01098

A novel ruthenium catalyst is introduced which contains solely achiral ligands and acquires its chirality entirely from octahedral centrochirality. The configurationally stable catalyst is demonstrated to catalyze the alkynylation of trifluoromethyl ketones with very high enantioselectivity (up to >99% ee) at low catalyst loadings (down to 0.2 mol%).

Full text of DOI:10.1021/jacs.7b01098

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Communication
Octahedral Ruthenium Complex with Exclusive Metal-
Centered Chirality for Highly Effective Asymmetric Catalysis
Yu Zheng, Yuqi Tan, Klaus Harms, Lilu Zhang, and Eric Meggers
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b01098 • Publication Date (Web): 14 Mar 2017
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Octahedral Ruthenium Complex with Exclusive Metal-
Centered Chirality for Highly Effective Asymmetric Catalysis
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Yu Zheng, Yuqi Tan, Klaus Harms, Lilu Zhang, and Eric Meggers*
Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Strasse 4, 35043 Marburg, Germany
ands provides metal-centered Λ- (left-handed propeller)
ABSTRACT: A novel ruthenium catalyst is introduced
or ꢀ-configuration (right-handed propeller).
which contains solely achiral ligands and acquires its chi-
rality entirely from octahedral centrochirality. The con-
figurationally stable catalyst is demonstrated to catalyze
the alkynylation of trifluoromethyl ketones with very high
enantioselectivity (up to >99% ee) at low catalyst loadings
(down to 0.2 mol%).
Transition metal complexes represent one of the most
powerful and versatile classes of homogeneous catalysts.
Applied to asymmetric catalysis, metal ions are typically
combined with carefully tailored chiral ligands.1 In a more
simplistic design, only achiral ligands are employed but
their assembly around the central metal creates metal-
centered chirality2 which is then responsible for the
asymmetric induction during catalysis.3 We recently real-
ized this approach with the design of bis-cyclometalated
iridium4 and rhodium5 complexes as chiral Lewis acids
which provide excellent enantioselectivities and high
turnover numbers for a variety of reactions. However, at
the onset of this study it was unclear to what extend this
design principle is general and applicable to chiral octa-
hedral metal complexes of other elements. In pioneering
work, Fontecave reported that Λ- and ꢀ-[Ru(2,9-
dimethyl-1,10-phenanthroline)(MeCN)2]2+ catalyze the
oxidation of organic sulfides to their sulfoxides, albeit
with a maximum of just 18% ee.3a Much higher enantiose-
lectivities for the synthesis of sulfoxides were achieved by
Figure 1. Design of a ruthenium-based asymmetric catalyst
relying solely on octahedral centrochirality.
RuCl3 xH2O
Br
N
1.) 200 °C
2.) AgPF6, 60 °C
+
R
N
N
Mes
R = 3,5-Me2Ph
92%
1
R
(PF6)2
N
N
Ru
N
Me
C
N
N
Mes
Mes
N
N
C
N
Me
R
Ru1
rac-
Ye
using
chiral-at-metal
Λ-
and
ꢀ-[Ru(2,2’-
bipyridine)2(pyridine)2]2+ as recyclable chiral auxiliaries.6
Hartung and Grubbs reported a chiral-at-ruthenium cata-
lyst for diastereo- and enantioselective ring-
opening/cross-metathesis. The complex contains addi-
tional carbon-centered stereogenicity and catalysis is
supposed to occur via a trigonal bipyramidal intermedi-
ate.7 Here we demonstrate, that ruthenium complexes
featuring exclusive octahedral centrochirality can serve as
highly effective asymmetric catalysts for the enantioselec-
tive alkynylation of trifluoromethyl ketones.
O
N
O
N
Et3N
Et3N
32%
iPr
iPr
36%
OH
(S)-
OH
2
2
(R)-
R
R
N
N
N
N
Ru
N
N
N
O
O
N
N
N
Mes
Mes
Mes
Mes
Ru
N
N
N
N
O
O
iPr
iPr
Our design is shown in Figure 1 and based on a struc-
tural scaffold reported by Hahn and co-workers.8 Ruthe-
nium in the oxidation state +2 is coordinated by two N-(2-
pyridyl)-subsituted N-heterocyclic carbene (PyNHC) bi-
dentate ligands in addition to two acetonitrile ligands.9
The propeller-type arrangement of the two bidentate lig-
R
3
R
3
-(S)-
-(R)-
1.) TFA (10 eq)
2.) NH4PF6 (30 eq)
92%
95%
Ru1
Ru1
-
-
Figure 2. Synthesis of enantiopure complexes Λ- and ꢀ-Ru1.
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The chiral-at-metal ruthenium complex was synthe- phenyl moiety, 2-ethynylthiophene, the conjugated
1
2
3
4
5
6
7
8
sized by reacting RuCl3 hydrate with the N-(2-pyridyl)-
imidazolium salt 1 in ethylene glycol at 200 °C followed by
treatment with AgPF6 to afford the racemic complex rac-
Ru1 in 92% yield (see Supporting Information for a single
crystal X-ray structure of rac-Ru1) (Figure 2).8 This race-
mic mixture was reacted with the chiral salicyloxazoline
ligand (S)-2 to provide Λ-(S)-3 as a single diastereomer in
36% yield.10-12 In analogy, using instead the auxiliary (R)-2,
the complex ꢀ-(R)-3 was obtained. The individual dia-
stereomerically pure complexes were next treated with
TFA in MeCN to generate Ru1 as individual Λ- and ꢀ-
enantiomer. CD spectra of both enantiomers are shown in
Figure 3 and were used to assign the absolute configura-
tion by comparison with related enantiopure ruthenium
complexes,13 and confirmed by an X-ray crystal structure
of a derivative of ꢀ-Ru1. Revealingly, Λ- and ꢀ-Ru1 are
constitutionally and configurationally surprisingly stable.
At a temperature of 60 °C in THF, after 72 hours no signs
of isomerization or decomposition could be observed (see
Supporting Information for more details).
alkenyl acetylene 1-ethynylcyclohexene, aliphatic acety-
lenes, and trimethylsilylacetylene. Typically, catalyst load-
ings of just 0.5 mol% Λ-Ru1 are sufficient except for or-
tho-substituted phenylacetylenes which react more slug-
gish, presumably due to steric reasons.
Table 1. Initial catalysis experimentsa
PF6
(PF6)2
R
tBu
S
9
N
N
N
N
Ru
N
Me
Me
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C
C
C
N
N
N
N
Mes
Mes
M
N
N
N
C
Me
Me
tBu
S
R
-Ru1: R = 3,5-Me2Ph
-Ru2: R = H
-IrS: M = Ir
-RhS: M = Rh
Ph
O
cat.
HO
+
Ph
H
Ph
CF3
Ph
CF3
Et3N (0.2 eq)
THF, 60 °C
4a
5a
6a
-
80
60
entry catalyst loading (mol%) t (h) yield (%)b ee (%)c
Λ Ru1
ꢀ
-Ru1
1
3.0
1.0
16
16
16
30
16
16
20
20
97
93
95
98
95
93
15
99 (S)
99 (S)
99 (S)
99 (S)
99 (R)
97 (S)
15 (R)
93 (R)
Λ-Ru1
Λ-Ru1
Λ-Ru1
Λ-Ru1
ꢀ-Ru1
Λ-Ru2
ꢀ-IrS
40
2
3
4
5
6
7
20
0.5
0.2
0.5
0.5
3.0
3.0
0
-20
-40
-60
-80
8
28
ꢀ-RhS
200
250
300
λ
350
(nm)
400
450
500
a
Conditions: 4a and 5a (3.0 eq) with catalyst (0.2-3.0 mol%)
and Et3N (20 mol%) in THF at 60 °C. b Isolated yields. c Enan-
Figure 3. CD spectra (0.2 mM in CH3OH) of Λ- and ꢀ-Ru1.
tiomeric excess of 6a determined by chiral HPLC.
After some scouting experiments we found that Ru1 is
an excellent catalyst for the enantioselective alkynylation
of trifluoromethyl ketones.14,15 For example, the reaction
of trifluoroacetophenone (4a) with phenylacetylene (5a)
in the presence of Et3N (0.2 eq), catalyzed by 3.0 mol% Λ-
Ru1, provides the propargylalcohol (S)-6a with 97% yield
and 99% ee (Table 1, entry 1). The catalyst loading can be
reduced down to 0.2 mol% without any loss in yield or
enantioselectivity (entries 2-4). As to be expected, mirror-
imaged ꢀ-Ru1 provides the mirror-imaged product (R)-6a
with otherwise identical performance (entry 5). A refer-
ence catalyst devoid of the 3,5-Me2Ph substituents (Λ-Ru2)
leads to a reduced enantioselectivity of 97% ee (entry 6),
confirming the steric role of the substituents at the pyri-
dine ligands. Interestingly, previously reported chiral-at-
metal iridium4 and rhodium5 catalysts only display very
sluggish reactivity for the alkynylation of trifluoromethyl
ketones and a diminished enantioselectivity even at cata-
lyst loadings of 3.0 mol% (entries 7 and 8).
A substrate scope with respect to terminal alkynes is
shown in Figure 4, providing the propargylalcohols (S)-
6b-m in yields of 66-99% and with outstanding enanti-
oselectivities of 96 to >99% ee. The catalyst tolerates
equally well phenylacetylenes with substituents in the
a
Figure 4. Substrate scope with respect to terminal alkynes.
1.0 mol% catalyst loading instead.
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The scope of this reaction with respect to trifluorome-
1
2
3
4
5
6
7
8
thyl ketones is outlined in Figure 5. Trifluoroacetophe-
none with different substituents in the phenyl moiety
provided the corresponding propargylalcohols in high
yields and with almost perfect enantioselectivity except
for ortho-methyl trifluoroacetophenone which reacts
sluggish, reinforcing that the catalyst is sensitive to steric
effects. It is also noteworthy that an aliphatic trifluorome-
thyl ketone and ethyl trifluoropyruvate are not suitable
substrate for this catalysis. However, replacing one fluo-
rine of the trifluormethyl group with chlorine by using 2-
chloro-2,2-difluoroacetophenone as the substrate yields
the corresponding propargyl alcohol in 99% yield and
99% ee.
Figure 6. Catalytic asymmetric synthesis of (S)-8, a key in-
termediate in the synthesis of Efavirenz.
9
Mechanistically, we propose that the reaction proceeds
through an intermediate ruthenium acetylide which then
tranfers the acetylide to the presumable ruthenium-
coordinated trifluoroketone.16 The observed fluoromethyl
ketone substrate coordinates to the ruthenium ahead of
the acetylide transfer. During this transfer, the metal-
centered chirality provides a suprisingly high asymmetric
induction, thus reinforcing our catalyst design strategy.
The rigidity of the propeller-type coordination sphere
most likely contributes to the observed excellent enanti-
oselectivities but is also responsible for sensitivity to ste-
ric effects. It is worth noting that catalytic amounts of
base are necessary in this reaction,21 which apparently
serves as a proton shuttle.
In summary, we here reported the first example of an
octahedral chiral-only-at-metal ruthenium complex with
high catalytic activity and excellent enantioselectivity.
Key components of this new class of asymmetric catalysts
are the two N-(2-pyridyl)-subsituted N-heterocyclic car-
bene (PyNHC) chelate ligands.8,9 First, the PyNHC ligands
are tightly coordinating ligands which provide a strong
ligand field important for the constitutional and configu-
rational stability of the bis-(PyNHC)Ru unit. Second, the
propeller shape and high rigidity of the bis-(PyNHC)Ru
provides an excellent asymmetric induction. And third,
the strong σ-donating NHC-ligands22 in trans to the coor-
dinated acetonitrile ligands are crucial for labilizing the
coordinated acetonitrile ligands (trans-effect23) thereby
ensuring a high catalytic activity.24 The application of this
novel class of chiral-at-metal ruthenium-based catalysts
to other catalytic, enantioselective reactions is underway
in our laboratory.
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Figure 5. Substrate scope with respect to trifluoromethyl
ketones. a 1.0 mol% catalyst loading instead.
The here introduced synthetic methodology should be
very valuable because propargylic alcohols constitute
highly versatile synthetic building blocks and furthermore
fluorinated compounds play an increasingly important
role in drug development.16,17 Searching for an application,
we turned our attention to Efavirenz,18 a potent HIV-1
reverse transcriptase inhibitor and a key drug for the
treatment of AIDS, which contains a quaternary stereo-
center bearing a CF3 and alkynyl group. Carreira reported
the first catalytic asymmetric conversion of the trifluoroa-
cetylanilide 7 into the key intermediate (S)-8.19,20 However,
the reaction requires a complicated cocktail out of Et2Zn,
chiral ligand, and the chiral product. Instead, reacting 7
with an excess of cyclopropylacetylene with 3 mol% ꢀ-
Ru1 under otherwise standard conditions provided (S)-8
in a yield of 58% and with 92% ee. Thus, our novel chiral-
at-ruthenium catalyst provides a very convenient access
to the chiral building block (S)-8.
ASSOCIATED CONTENT
Supporting Information. Experimental details and chiral
HPLC traces. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
meggers@chemie.uni-marburg.de
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We gratefully acknowledge funding from the Deutche For-
schungsgemeinschaft (ME 1805/13-1).
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H.; Jang, D.-J.; Chung, Y. K. Inorg. Chem. 2004, 43, 6896. (b)
Cheng, Y.; Sun, J.-F.; Yang, H.-L.; Xu, H.-J.; Li, Y.-Z.; Chen, X.-T.;
Xue, Z.-L. Organometallics 2009, 28, 819. (c) Dakkach, M.;
Fontrodona, X.; Parella, T.; Atlamsani, A.; Romero, I.; Rodríguez,
M. Adv. Synth. Catal. 2011, 353, 231. (d) Norris, M. R.; Concep-
cion, J. J.; Harrison, D. P.; Binstead, R. A.; Ashford, D. L.; Fang,
Z.; Templeton, J. L.; Meyer, T. J. J. Am. Chem. Soc. 2013, 135, 2080.
(e) Leigh, V.; Ghattas, W.; Lalrempuia, R.; Müller-Bunz, H.;
Pryce, M. T.; Albrecht, M. Inorg. Chem. 2013, 52, 5395. (f) Bar-
bante, G. J.; Francis, P. S.; Hogan, C. F.; Kheradmand, P. R.; Wil-
son, D. J. D.; Barnard, P. J. Inorg. Chem. 2013, 52, 7448. (g) Saha,
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33.
(10) For related auxiliary-mediated syntheses of enantiopure
transition metal complexes, see for example: (a) Gong, L.; Mul-
cahy, S. P.; Harms, K.; Meggers, E. J. Am. Chem. Soc. 2009, 131,
9602. (b) Chepelin, O.; Ujma, J.; Wu, X.; Slawin, A. M. Z.; Pitak,
M. B.; Coles, S. J.; Michel, J.; Jones, A. C.; Barran, P. E.; Lusby, P.
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