Synthesis and Application of Stereoretentive Ruthenium Catalysts on the Basis of the M7 and the Ru-Benzylidene-Oxazinone Design

Article abstract of DOI:10.1021/acs.organomet.7b00836

A series of new stereoretentive ruthenium catalysts bearing the dithiocatecholate ligand was synthesiszed on the basis of the M7 and Ru-benzylidene-oxazinone design. The activity of the catalysts was tested in ring-opening cross-metathesis reactions, ring

Full text of DOI:10.1021/acs.organomet.7b00836

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis and Application of Stereoretentive Ruthenium Catalysts on
the Basis of the M7 and the Ru−Benzylidene−Oxazinone Design
Adrien Dumas,†,‡ Daniel S. Muller,† Idriss Curbet,† Loïc Toupet,§ Mathieu Rouen,‡ Olivier Basle,*,†
́
̈
and Marc Mauduit*,†
†Univ Rennes, Ecole Nationale Super
́
ieure de Chimie de Rennes, CNRS, ISCR-UMR 6226, F-35000 Rennes, France
‡DEMETA SAS, 6 rue Pierre-Joseph Colin, 35000 Rennes, France
§Institut de Physique de Rennes, Universite
́ ́ ́
Rennes 1, CNRS UMR 6251, 263 Av. General Leclerc, 35042 Rennes, France
S
* Supporting Information
ABSTRACT: A series of new stereoretentive ruthenium
catalysts bearing the dithiocatecholate ligand was synthesiszed
on the basis of the M7 and Ru−benzylidene−oxazinone
design. The activity of the catalysts was tested in ring-opening
cross-metathesis reactions, ring-closing metathesis reactions,
cross-metathesis reactions, and self-metathesis reactions using
Z- and E-configured starting materials. The desired metathesis
products were obtained with moderate to high yields and with
excellent stereoselectivities (>95%). Substrate-dependent
reactivity was found by carrying out kinetic studies on internal
and terminal olefins.
predisposition of the substrate in combination with a specific
metathesis catalyst to achieve high levels of stereoselectivity.
A structurally simple catalyst based on the selective synthesis
of (Z)-olefins was introduced in 2013 by Hoveyda (Scheme
1).9a Electronically modified catalyst Ru-1, obtained by simple
INTRODUCTION
■
Carbon−carbon double bonds reside within a large number of
molecules. Control of the stereochemistry of alkenes is of utter
importance, given that changing the configuration of a double
bond can significantly alter the biological and physical
properties of a molecule.
Scheme 1. Synthesis of Ru−Dithiocatecholate Ru-1
Historically, stereoselective synthesis of olefins has relied on
reactions producing stoichiometric amounts of waste material,
such as the renowned Wittig olefination.1 In the early 1950s
transition-metal-catalyzed olefin metathesis emerged as a
powerful tool for the generation of olefins, creating only
ethylene or other olefins as side products.2 Despite the many
advances in this field, the stereocontrol of the double bond has
been elusive until recently.3 Early studies of cross-metathesis
reactions revealed that certain substrates such as acrylates and
styrenes favor the thermodynamically more stable E double-
bond isomer whereas others, e.g. acrylonitrile, enynes, and
vinylsiloxanes, favor the Z-configured product.4 Moreover,
several examples of ring-closing metathesis reactions were
reported where the substrate can exert high levels of
stereocontrol, favoring one double bond isomer over the
other.5
The first well-defined stereoselective metathesis catalyst was
reported by Schrock and Hoveyda in 2009.6 Since then several
stereoselective metathesis catalysts based on Mo, W, and Ru
have been discovered and applied to ring-opening metathesis
polymerization (ROMP), ring-closing metathesis (RCM), and
cross-metathesis (CM) reactions.7,8 Importantly, many of the
reported methodologies require an electronic or stereochemical
substitution of the chloride ligands of Hoveyda−Grubbs
catalyst Ru-0 by a dithiocatecholate ligand, was subsequently
shown to promote various types of metathesis reactions of
stereochemically pure olefins with very high levels of Z or E
selectivity (Scheme 2).9,8f,j−m
Catalyst Ru-1 was initially described as a stereoselective
catalyst.9 However, subsequent work by Grubbs and co-workers
Special Issue: Organometallic Chemistry in Europe
Received: November 30, 2017
© XXXX American Chemical Society
A
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Scheme 2. Stereoretentive Catalysis with Ru-1
clearly showed that Ru-1 is a stereorententive catalyst capable
of transforming (E)- or (Z)-alkenes into the corresponding E
or Z products without significant loss of the stereochemical
information on the starting olefins (Scheme 2).7d,8k,l
In contrast to the parent catalyst Ru-0, the dithiocatecholate-
substituted catalyst Ru-1 shows much lower activity and high
catalyst loadings (5−10 mol %) are typically required for cross-
metathesis reactions. It is well-known that replacing the
chloride ligands on ruthenium by more electron donating
ligands leads in general to less active catalysts.10 Furthermore,
Hoveyda attributed the low turnover number (TON) to
premature degradation of catalyst Ru-1 via a 1,2-sulfur shift.9e
In addition to the limited catalyst lifetime, another reason for
the low efficiency of catalyst Ru-1 was identified by Grubbs and
Ahmed.8l 1H NMR studies demonstrated that the initiation rate
of catalyst Ru-1 is very slow, requiring more than 24 h for full
initiation. Therefore, only small amounts of precatalyst enter
the catalytic cycle, thus requiring a high catalyst loading.
To overcome the lack of reactivity of dithiocatecholate
catalysts, scientists at Materia Inc. and the Grubbs group
studied a series of new ruthenium catalysts relying on the 2-
isopropoxy-3-phenylbenzylidene motif, a structural modifica-
tion introduced by Blechert in 2000.8k,l,11 These studies showed
the following. (a) (Z)-Alkenes react considerably more quickly
in comparison to their (E)-alkene counterparts in the presence
of dithiocatecholate catalysts such as Ru-1. (b) Strategies to
increase the initiation rate of catalyst Ru-0 seem also to apply
to the derived dithiocatecholate catalysts. In other words, the
Blechert modification which was previously shown to increase
the initation rate of Ru-0 also improved the reactivity of Ru-1.
(c) Replacement of the SIMes NHC ligand by the SIPr motif
resulted in strong rate enhancement for (Z)-alkenes.8k,l In
contrast, no significant rate increase was observed with (E)-
alkenes when catalyst Ru-4 was used (Figure 1). The lower
reactivity of bulky Ru-4 catalyst with (E)-alkenes is in
agreement with the proposed models for Z and E selectivity
reported by Grubbs and Pederson.8k Grubbs actually
demonstrated that sterically less demanding NHC ligands
increased the reaction rates for stereoretentive cross-metathesis
with (E)-alkenes. Hoveyda and co-workers also prepared a
library of dithiocatecholate ligands and assessed their reactivity
for the ROMP of norbornene in a qualitative study.8j They
found that the dithiocatecholate-substituted Grela catalyst12
Ru-2 and Zannan catalyst Ru-313 were among the most active
catalysts (Figure 1), in apparent agreement with the previous
finding by Grubbs that modifications for the rate enhancement
of Ru-0 also apply to Ru-1.
Figure 1. Recently reported bisthiolate Ru catalysts and new catalysts
presented herein.
a kilogram scale.15 The M7 series benefits from fast initiation
due to an electron-withdrawing group in the para position of
the benzylidene moiety similarly to the Grela or Zannan
catalyst (Ru-2 and Ru-3).12 Furthermore, we modified Ru−
a‑c
benzylidene−oxazinone catalysts, which we reported recently.14
In contrast to the M7 series the benzylidene−oxazinone type
catalysts rely on a combination of electronic and steric
activation.
RESULTS AND DISCUSSION
■
Herein, we report the synthesis and application of new
stereoretentive ruthenium catalysts on the basis of the M7
and the benzylidene−oxazinone design series. We set out by
synthesizing several bis-thiolate ligands following the straight-
forward protocol reported by Hoveyda and co-workers shown
in Scheme 1 (Figure 2). The yields obtained by this procedure
are very high, typically ranging between 93 and 99% for the five
isolated complexes Ru-6−Ru-10.16 Complexes Ru-8−Ru-10
contain two stereocenters (see Figure 2). A racemic mixture of
the benzylidene moiety was used for complex preparation.
Hence, complexes Ru-8−Ru-10 were obtained as mixtures of
diastereoisomers (see the Supporting Information for further
details).
Furthermore, we were able to obtain X-ray crystal structures
for Ru-7 and Ru-8 (Figure 3). Both complexes show relatively
short Ru−S2 bond distances in comparison to Ru-1 (2.276 and
2.260 Å vs 2.283 Å in Ru-1). The shortening of the Ru−S2
bond distance is probably due to the electron-withdrawing
subsitutents on the opposite benzylidene, which minimize the
trans influence. Hence, these complexes should rank within the
most active complexes in comparison to the ranking recently
reported by Hoveyda.8j
Complementarily to the work carried out by Grubbs and
Hoveyda, our group was interested in the introduction of the
dithiocatecholate ligand to aminocarbonyl-group-containing
Hoveyda−Grubbs-type complexes, typically referred to as the
M7 series (Figure 1).14 These in-house-developed catalysts
were recently commercialized by Umicore and are available on
First, we tested our catalysts to the stereoretentive ring-
opening cross-metathesis (ROCM) of norbornene (Table 1).9a
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Table 1. Stereoretentive ROCM of Norbornene with Styrene
a
2 in the Presence of Bisthiolate-Substituted Catalysts
a
b
c
[Ru]
NMR yield (%)
yield (%)
Z:E ratio
Ru-1
Ru-4
Ru-6
Ru-7
Ru-8
Ru-9
Ru-10
75
90
93
80
82
92
60
98:2
98:2
97:3
99:1
99:1
98:2
99:1
90
>98
90
85
>98
60
a
b
NMR yield detetermined with anthracene as standard. Yields refer
c
to isolated yields after column chromatography. Z:E ratios were
determined by H NMR analysis of purified compounds.
1
Table 2. Stereoretentive Cross-Metathesis of cis-Butenediol
with 1-Dodecene in the Presence of Bisthiolate-Substituted
Catalysts
Figure 2. Synthesized dithiocatecholate-substituted Ru catalysts based
on the M7 and Ru−benzylidene−oxazinone motifs.
a
a
b
[Ru]
conversn (%)
yield (%)
Z:E ratio
Ru-1
Ru-4
Ru-6
Ru-7
Ru-8
Ru-9
Ru-10
81
53
80
62
60
71
60
72
42
76
51
55
67
50
96:4
95:5
98:2
98:2
98:2
98:2
97:3
a
Conversions and NMR yield were determined from the crude
mixture by 1H NMR analysis with 1,1,2,2-tetrachloroethane as
b
standard. Z:E ratios were determined by 1H NMR analysis of
purified compounds (see the Supporting Information).
In order to test the functional group tolerance and the
generality of the newly developed catalysts, we chose the highly
active ruthenium catalyst Ru-6 to explore the scope of the CM
reaction with cis-2-butene-1,4-diol (4) and cis-1,4-diacetoxy-2-
butene (7) (Figure 4). We were delighted to observe excellent
Z selectivity for almost all cases of cross-metathesis with cis-2-
butene-1,4-diol (>97:3), except for cross-metathesis with allyl
butyl ether, which afforded poor Z:E ratios (12).9e It appears
that unhindered alkenes bearing a remote electron-withdrawing
group such as bromo-substituted compound 11 and carboxylic
acid 13 react particularly well. Lower yields were observed with
α-branched alkene starting materials (15, 16, and 18).17 Cross-
metathesis with cis-1,4-diacetoxy-2-butene (7) furnished 18 and
19 in good yields and excellent selectivity.
We also tested the activity and selectivity of the M7 catalysts
for Z-stereoretentive macrocylization (Scheme 3). Ru-7
provided yuzu lactone 21, a camphor-like and minty-odored
substance,16 in almost identical yields and selectivity in
comparison to Ru-4, previously reported by Grubbs (with
Ru-4: 68% yield, 95:5 Z:E).8m
Figure 3. X-ray structures of dithiocatecholate Ru complexes Ru-5 and
Ru-7. H atoms and the solvent molecules are omitted for clarity.
We found that all catalysts are highly stereoretentive, furnishing
the products in good to excellent yields. In particular, SiMes-
substituted catalysts (Ru-6 and Ru-9) showed greater activity in
comparison to their SiPr-substituted derivatives (Ru-7 and Ru-
10).8k,l Next, we tested a selection of stereoretentive catalysts
for the Z-stereoretentive cross-metathesis of cis-butendiol and
1-dodecene as reported by Hoveyda and co-workers (Table
2).9e Again the SiMes-substituted catalysts showed better
performance in comparison to their SiPr-substituted counter-
parts with only minor erosion of stereoselectivity (Table 2). It
is noteworthy that the high Z selectivity varies very little among
the ruthenium catalysts tested.
C
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Figure 5. Self-metathesis of methyl (E)-9-octadecenoate (MO). There
was an equilibrium among MO, hydrocarbon HC and diester DE.
Product distribution and selectivity were analyzed by GC using
tetradecane as internal standard.
Figure 4. Further examples of CM of various alkenes with cis-2-
butene-1,4-diol (4) or cis-1,4-diacetoxy-2-butene (7). Conversions
were determined by 1H NMR analysis of the crude mixture with
1,1,2,2-tetrachloroethane as standard. The Z:E ratios were determined
by 1H NMR analysis of purified compounds. Legend: (a) 12 h reaction
time.
Table 3. Self-Metathesis of Methyl (E)-9-Octadecenoate
(MO): E:Z Ratios after 24 h
Scheme 3. Z-Stereoretentive RCM Using Ru-7
entry
[Ru]
MO (%) (E:Z)
DE (%) (E:Z)
1
2
3
4
5
6
7
Ru-1
Ru-4
Ru-6
Ru-7
Ru-8
Ru-9
Ru-10
50 (>99:1)
52 (>99:1)
66 (>99:1)
46 (89:11)
60 (>99:1)
50 (>99:1)
74 (>99:1)
25 (>99:1)
24 (90:10)
17 (>99:1)
23 (88:12)
19 (>99:1)
25 (>99:1)
13 (94:6)
Furthermore, we tested several ruthenium catalysts for the
self-metathesis of methyl (E)-9-octadecenoate (E-MO, >99%)
to produce dimethyl 9-octadecenedioate (DE) and 9-
octadecene (HC) (Figure 5). The parent ruthenium complex
Ru-1 showed the fastest kinetics, achieving equilibrium in
almost 9 h. Ru-9 and Ru-4 also performed well, leaving all
other catalysts tested behind. Interestingly, the activity of
ruthenium catalyst Ru-6 dropped significantly in a change from
a Z- to an E-configured substrate (Table 2 vs Figure 5).18 It is
noteworthy that Ru−bisthiolate catalysts are stable in the
presence of internal alkenes for more than 24 h (Figure 5). In
contrast, Hoveyda reported that terminal alkenes lead to full
catalyst degradation in less than 4 h.9e On the other hand, while
the SiMes-substituted catalysts showed highly stereoretentive
reactions (Table 3, entries 1, 3, and 6), most of the SiPr-
substituted catalysts provided the diester DE with lower
stereochemical purity (Table 3, entries 2, 4, and 7), with the
exception of catalyst Ru-8.
a
Methyl (E)-9-octadecenoate of >99% GC purity was used as starting
material. Product distribution and selectivity analyzed by GC using
tetradecane as internal standard. Note: We were unable to separate the
stereoisomers of the hydrocarbon component.
1
catalysts with ethyl vinyl ether with H NMR spectroscopy
(Figure 6). SiPr-substituted catalysts showed high initiation
rates even at −20 °C (Figure 6a). In contrast, reactions with
SiMes-substituted catalysts were much slower, necessitating
higher temperatures (10 °C; Figure 6b). Values of kinit are
highest for catalysts Ru-8 and Ru-10 in the SiPr series, whereas
parent catalyst Ru-1 initiates significantly more quickly in
comparison to Ru-9 and Ru-6 in the SiMes series. We also
determined the initiation rates in tetrahydrofuran at 45 °C for
Ru-6 and Ru-7 with trans-2-hexenyl acetate (Figure S30),8l
which contains an internal double bond. The initiation rates are
significantly lower in comparison to those observed for ethyl
To get further insight concerning the initiation rate of our
bisthiolate catalysts, we followed the reaction of a selection of
D
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with methyl (E)-9-octadecenoate showed that SiMes-substi-
tuted catalysts not only increase the reaction rate in comparison
to their SiPr-substituted congeners but are also crucial for
highly stereoretentive metathesis reactions with (E)-olefins.
Further studies concerning highly practical stereoretentive
catalysts are currently being pursued in our laboratories and
will be reported in due course.
EXPERIMENTAL SECTION
[add experimental section here]
■
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00836.
Experimental procedures and characterization data for all
products and catalysts, including 1H and 13C, NMR
spectra and X-ray structures (PDF)
Cartesian coordinates for calculated structures (XYZ)
Cartesian coordinates for calculated structures (XYZ)
Accession Codes
CCDC 1562960−1562961 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for M.M.: marc.mauduit@ensc-rennes.fr.
*E-mail for O.B.: olivier.basle@ensc-rennes.fr.
■
ORCID
Olivier Basle: 0000-0002-4551-473X
́
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are gprateful to the CNRS, the Ecole Nationale Super
Figure 6. Plot of ln([Ru]/[Ru]0) versus time for initiation
■
experiments conducted with a selection of bisthiolate catalysts. The
́
ieure
1
disappearance of the benzylidene signal was monitored by H NMR
de Chimie de Rennes. This work was supported by the ANRT
and Demeta SAS (CIFRE no 2014/0702, grant to A.D.), the
FASO (grant to D.S.M and I.C.), and the region Bretagne
(SAD 2016 no. 9639 “RZ-SELECT” grant to D.S.M). M.M.
thanks Umicore AG & Co. for a generous gift of M1 and M2
ruthenium complexes.
spectroscopy.
vinyl ether, indicating that the initiation kinetics are highly
substrate dependent. Interestingly, in this case the calculated
values for kinit were found to be very similar for the SiMes-
substituted catalyst Ru-6 (1.42 × 10−5 s−1) and the SiPr-
substituted complex Ru-7 (1.10 × 10−5 s−1).19,20
REFERENCES
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■
Though we did not achieve the original set goal to develop a
significantly more active stereoretentive catalyst, our study has
nevertheless added to the knowledge concerning bisthiolate-
modified catalysts. First, we were able to show that the
bisthiolate modification to obtain stereoretentive catalysts is
quite general and the high selectivity is conserved even upon
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E
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(17) We were unable to fully attribute the difference of yield and
conversion to other products. Approximately 5% of self-metathesis of
the alkene was typically observed. We suspect that some of the more
volatile starting materials evaporated during the reaction and during
workup.
(18) We attribute the lower performance of our catalyst in
comparison to Ru-1 to the more facile attack of the trans to NHC
thio ligand at the electron-poor benzylidene moiety as previously
described by Hoveyda.9e
(8) A list of recent reports on Z- and E-selective or stereoretentive
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DOI: 10.1021/acs.organomet.7b00836
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