Rapidly initiating ruthenium olefin-metathesis catalysts

Article abstract of DOI:10.1002/anie.200461374

Vacancies: Protonation of the ruthenium carbide compounds [Cl ₂(L)(PR₃)Ru≡C:] gives the 14-electron four-coordinate ruthenium phosphonium alkylidenes [Cl₂(L)Ru= CH(PR₃)]⁺[B(X)₄]^- (see scheme). These compounds which already have a vacant coordination site provide direct access to the active species in olefin metathesis catalysis and thus very fast initiation.

Full text of DOI:10.1002/anie.200461374

Angewandte
Chemie
Olefin Metathesis
Rapidly Initiating Ruthenium Olefin-Metathesis
Catalysts**
Patricio E. Romero, Warren E. Piers,* and
Robert McDonald
Olefin metathesis is arguably the most powerful carbon–
carbon bond breaking and making reaction in chemical
synthesis.[1] Depending on the nature of the reacting partners,
olefin metathesis can be used for ring-opening polymerization
(ROMP),[2] to create advanced polymeric materials,[3] trans-
formation of acyclic diene substrates into complex cyclic
organic molecules (in ring-closing metathesis (RCM))[4] or
polymers (in acyclic diene metathesis (ADMET))[5] or in
cross metathesis (CM) to generate unsymmetrical olefins.[6,7]
Although olefin metathesis is fully reversible, RCM,
ADMET, and CM rely on the elimination of ethylene, the
simplest olefin, as a thermodynamic driving force. Used by
itself or in tandem with other synthetic transformations,[8,9]
olefin metathesis is a versatile method for the modern
synthetic chemist.
Figure 1. Commercially available olefin-metathesis catalysts. Cy=cyclo-
hexyl, Mes=2,4,6-trimethylphenyl.
=
general formula [Cl2(L)(L’)Ru C(H)R] (compounds 1)
which are significantly more functional-group tolerant, but
do not exhibit the same levels of activity or longevity as the
Schrock catalysts.
It is generally acknowledged that a metal carbene species,
=
{LnM CRR’}, is required and that interaction with an olefin
The root of the lower activities of the Grubbs systems lies
in their mode of initiation and the accessibility of the reactive
species, which has been shown experimentally[13,14] and
computationally[15,16] to be the 14-electron alkylidene
substrate leads to four-membered metallacyclobutane inter-
mediates or transition states, {LnM(CRR’)3}, by a 2+2 cyclo-
addition; cleavage of this intermediate in the opposite sense
by which it was formed leads to olefin metathesis, creating a
new carbon–carbon double bond and regenerating an active
metal carbene. Metal carbenes are generally classified as
being nucleophilic (electron rich) or electrophilic (electron
poor) in character at the carbene carbon atom, but an
effective olefin-metathesis catalyst exhibits behavior between
these two extremes. Two carefully tuned classes of mediator
have evolved into the catalysts of choice for olefin metathesis.
Schrock catalysts[10,11] are molybdenum- or tungsten-based
alkylidenes with a fairly specific ligand set designed to
modulate the properties of the carbene (Figure 1). These
catalysts display high activities and stabilities, but are
sensitive to ambient air and moisture and are relatively
intolerant of polar functionalities. The Grubbs-catalyst port-
folio[12] consists of a variety of ruthenium-based systems of
=
[Cl2(L)Ru C(H)R] formed upon reversible dissociation of
L’. The reaction temperatures required to overcome this
initiation step can lead to decreased catalyst lifetimes.
Successful improvements to the “Grubbs first-generation”
=
catalyst [Cl2(PCy3)2Ru C(H)Ph] (1b) are modifications that
either encourage loss of L’[17] or reduce the tendency of
[Cl2(L)Ru = C(H)R] to re-capture the liberated L’,[18] which
competes with the olefin substrate for the unsaturated metal
=
center in [Cl2(L)Ru C(H)R]. Alternatively, Hoveyda and co-
workers have developed catalysts 1d, in which L’ is incorpo-
rated into a loosely chelating group associated with the
carbene ligand that is removed upon the first metathesis
event.[19,20] Fast-initiating improvements on this motif have
been devised by Blechert and Wakamatsu.[21]
Herein we report a surprising twist on the Grubbs-catalyst
motif that improves the kinetics of initiation dramatically by
circumventing the initiation step in commercially available
Grubbs catalysts completely, thus providing direct access to
the reactive 14-electron catalyst species with no free L’
present to interfere with the operation of the catalyst. This
new generation catalyst precursor is easily prepared from
existing Grubbs catalysts and brings their activity into the
realm of the Schrock family, while retaining the favorable
functional-group tolerance associated with the ruthenium
systems.
[*] P. E. Romero, Prof. Dr. W. E. Piers
Department of Chemistry
University of Calgary
2500 University Drive N. W., Calgary, Alberta T2N1N4 (Canada)
Fax: (+1)403-289-9488
E-mail: wpiers@ucalgary.ca
Dr. R. McDonald
X-Ray Structure Laboratory, Department of Chemistry
University of Alberta, Edmonton, Alberta T6G2G2 (Canada)
[**] Financial support for this work was provided by the Natural
Sciences and Engineering Research Council of Canada through a
discovery grant to W.E.P. Materia Inc. is acknowledged for generous
donations of catalysts shown in Figure 1.
Recently, Heppert and co-workers reported a stoichio-
metric metathesis reaction between catalyst precursors 1b
and 1c and a methylene cyclopropane olefin known as Feistꢀs
ester.[22] Metathesis and elimination of diethyl fumarate
provides the unusual ruthenium carbides 2a–c (Scheme 1).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200461374
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6161

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Figure 2) is fast on the NMR timescale, with a free energy
barrier of 11.5(5) kcalmolÀ1 at 236 K calculated from the
coalescence behavior of the NMR signals arising from the
inequivalent mesityl groups at low temperature. This con-
trasts with the behavior for the parent compound 1c, which
exhibits slow rotation with a barrier of 21.8 kcalmolÀ1.[26]
Remarkably, when solutions of 3c are exposed to the
atmosphere, the NMR spectra remain unchanged for several
hours, which indicates that the compound is stable to oxygen
and ambient moisture. Indeed, introduction of water into
these samples results only in minor perturbations to the
spectra, owing to reversible coordination of H2O to the
ruthenium center. The compounds are thermally stable
indefinitely in the solid state at room temperature and can
be heated under refluxing CD2Cl2 (458C, sealed tube) for
several hours with no evidence of decomposition. Heating at
758C in C6D5Br results in clean decomposition to an
unidentified species (31P{1H} NMR d = 37 ppm) with a half-
life of approximately 5 h; the nature of this process is under
investigation.[27] Preliminary studies show that the carbides
can be converted into more economically viable BF4 salts of
cations 3a–c by protonation with [H(Et2O)x]+[BF4]À with
little change in the behavior of the compounds.
Scheme 1. Synthesis of rapidly initiating metathesis catalysts 3;
H2IMes=N-heterocyclic carbene ligand.
The precise structure of 3c was determined by X-ray
crystallography (Figure 2).[28] Compounds 3 represent the
closest structural models for the proposed four-coordinate,
Heppert and co-workers reported the preparation of carbides
2b and 2c in low yields; we have found this to be fairly a
general reaction and a variety of carbides containing other
combinations of L and L’ can be prepared in improved yields
(90–96%) from 2b and 2c. We became interested in the
reactivity of these carbides with electrophiles and discovered
that protonation with [H(OEt2)2]+[B(C6F5)4]À, Jutziꢀs acid,[23]
leads to the cationic four-coordinate, 14-electron phosphoni-
um alkylidenes 3a–c by transfer of a trialkylphosphine ligand
(that is, L’, Scheme 1) to the protonated carbide carbon atom
in high yields (87–95%).[24]
Evidently, protonation of the carbide carbon atom
switches it from being electron rich to electron poor; the
electrophilicity of the protonated carbide carbon atom is
quenched by transfer of the phosphine (L’) from the metal
center, generating the observed phosphonium alkylidenes.
For 3a and 3b (L = PR3, R = iPr or Cy), the signal for the
alkylidene proton appears as a distinctive doublet of doublets
in the proton NMR spectrum at d = 17.35 ppm for 3a and
17.46 ppm for 3b (2JH,P = 36 Hz; 3JH,P = 1.5 Æ 3 Hz). In 3c,
where L = H2IMes, the three-bond hydrogen–phosphorus
coupling is lost and the alkylidene hydrogen resonance
signal appears as a doublet at d = 17.70 (2JH,P = 36 Hz). In
the 31P{1H} NMR spectra, the signals for the phosphonium
phosphorus atoms appear in the region around d = 54–
58 ppm, while those of the phosphorus nuclei bonded to
ruthenium appear at d = 97.6 (3a) and 88.7 ppm (3b). These
latter resonance signals, while shifted about 60 ppm downfield
relative to those found for 1b and 1c, are similar to that
Figure 2. ORTEP diagram of 3c (thermal ellipsoids set at 50% proba-
bility; counteranion and hydrogen atoms omitted for clarity). Selected
bond lengths [Š] and angles [8]: Ru-C1 1.817(2), Ru-C2 1.988(2), Ru-Cl1
2.2951(5), Ru-Cl2 2.2809(5), P-C1, 1.805(2); C1-Ru-C2 100.07(7), Cl1-
Ru-Cl2 150.51(2), Cl1-Ru-C1 103.15(6), Cl2-Ru-C1 102.79(6), Cl1-Ru-C2
96.14(5), Cl2-Ru-C2 92.90(5). Selected torsion angle [8]: C2-Ru-C1-P,
À175.06(11).
14-electron active species in the Grubbs-catalyst family; the
only other structurally characterized example, the bis-tert-
butoxide derivative [(tBuO)2(PCy3)Ru C(H)Ph],[25] relies on
=
the replacement of the chloride ligands with sterically bulky
groups for kinetic stabilization, but is essentially inactive as an
olefin metathesis catalyst. As in the bis-tert-butoxide com-
plex, the geometry about the ruthenium center in 3c is a
distorted trigonal pyramid with the H2IMes ligand occupying
the apex; however, the Cl1-Ru-Cl2 angle is at 150.51(2)8
substantially larger than the O1-Ru-O2 angle of 133.19(6)8 in
observed for the four coordinate complex [(tBuO)2(P-
[25]
=
Cy3)Ru C(H)Ph] prepared by Grubbs and co-workers. In
3c, rotation of the IH2Mes ligand about the Ru-C2 bond (see
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Chemie
=
[(tBuO)2(PCy3)Ru C(H)Ph]. The Ru-C1 and Ru-C2 bonds
of 1.817(2) and 1.988(2) Š, are slightly shorter than those in
1c,[17] a difference accounted for by the reduced coordination
number in 3c. The phosphonium alkylidene unit is oriented
such that the PCy3 group is pointing away from the IH2Mes
ligand. Angles and separations are normal within this part of
the molecule and although C32 is positioned near a vacant
ruthenium coordination site, any stabilization of the ruthe-
À
nium center from the C H sigma bonds of C32 is weak
À
(Ru···C32 3.001 Š). No evidence for a C H agostic inter-
action is observed in the proton NMR spectra, nor are low-
À
frequency C H vibrations apparent in the IR spectra of
À
compounds 3, which would be expected if any of the C H
bonds were donating electron density to the unsaturated
ruthenium center.[29] Four-coordinate ruthenium complexes
[30]
À
not stabilized by C H agostic interactions are rare; thus,
Figure 3. Relative rates of conversion for RCM of diethyldiallylmalonate
&
^
at 273 K by ( ) 1c, ( ) Schrock’s molybdenum alkylidene, ( ) the
J
apart from being models for the 14-electron active species in
olefin metathesis, compounds 3 are unusual coordination
compounds.
=
Grubbs fast-initiating catalyst [(IH2Mes)Cl2(3-Br-py)2Ru CHPh],
~
*
( ) 3b, and ( ) 3c.
Metal carbenes are classified according to the donor
qualities of the atoms directly bonded to the carbene carbon
atom. Fischer carbenes incorporate p-donating groups (OR,
NR2), while the nucleophilic Schrock carbenes have p-neutral
groups, such as H or alkyl substituents.[1] The phosphonium
substituted carbenes of compounds 3 are a less common third
class since the [PR3]+ group is a p acceptor.[31] Formally, this
CHPR3 ligand can be viewed as a deprotonated phosphorus
ylid species (ylene resonance structure A, inset Scheme 1),
but according to experimental[32] and computational[33] studies
and the parameters for 3c discussed above, a more realistic
depiction is the dicarbanionic ylid resonance structure B. The
ruthenium centers in compounds 3 may thus be viewed as
formally RuIV with the carbene ligand functioning as a four-
electron donor.
The electron-withdrawing nature of the phosphonium
substituent in the carbene ligands of compounds 3 does not
impede their ability to conduct olefin metathesis; they are
exceptionally active RCM catalysts in comparison to catalyst
precursors 1. Using the RCM of diallyldiethylmalonate as a
benchmark reaction, catalysts 1c (Grubbs 2nd Generation),
Schrockꢀs molybdenum-based catalyst (Figure 1), the fast-
We have also explored additional substrates for the RCM
process with catalyst 3c and initial results are summarized in
Table 1. The reactions are completed within 2–60 min at room
temperature (1% mol catalyst loading, except entry 2 and 6).
Diethyldiallylmalonate (entry 1) is ring-closed in under
2 min, this is determined as an upper limit since the reaction
time is too fast to be accurately measured at room temper-
ature. Using 0.1% mol catalyst loading, a 100% conversion is
reached within 30 min (entry 2). Similar results are found in
the formation of a six-membered ring (entry 4 and 5), while
higher catalyst loadings are required to push the closing of a
seven-membered ring to acceptable conversions (entry 6).
Notably, the closing of trisubstituted olefins proceeds to high
conversion, under very mild conditions (entries 3 and 5). In
particular, formation of the trisubstituted cyclopentene is
complete in less than 10 min (entry 3); this particular
substrate allows a more direct comparison with Blechertꢀs
catalyst, which accomplished this conversion in 40 min, under
otherwise identical conditions.[21]
=
initiating Grubbs catalyst [(IH2Mes)Cl2(3-Br-py)2Ru
The reason 3c is so active is that the need to dissociate a
ligand for the ruthenium complex to enter the catalytic
cycle[13,18] has been completely obviated and initiation is now
the much more energetically favorable binding of the
CHPh],[34] and compounds 3b and 3c were compared at
08C by monitoring reaction progress to the cyclized product
(Figure 3). At 08C 1c is a poor initiator[18] and only reaches
approximately 25% conversion after 4 h. Compound 3b fares
somewhat better, providing about 90% conversion after 4 h,
while Schrockꢀs catalyst mediates the reaction to a similar
point in this time. The sigmoidal shape of the curve for 3b is
reflective of the different activities of initiating versus
propagating species at 08C for this catalyst.[13] The trans-
formation is very rapid for compound 3c, however, which
brings the reaction to > 90% conversion after only 2 h at
08C, twice as fast as the Schrock catalyst under these
conditions and, significantly, it out-performs the fast-initiating
Grubbs catalyst incorporating more labile 3-bromopyridine
ligands. Furthermore, the rate of RCM for 3c is qualitatively
similar to the best Blechert catalyst,[21b] a less conveniently
available metathesis catalyst.
=
substrate C C bond to the ruthenium center. Rate constants
for this process have been measured by monitoring the
reaction of 3c with varying excesses of ortho-isopropoxy
styrene (10–40 equiv) at À30 to 08C by proton NMR
spectroscopy. This substrate was chosen since it produces
the Grubbs–Hoveyda catalyst 1d (Figure 1), which is inactive
towards further metathesis reactions at the temperatures
=
used; the other product is [(CH2 CH)PCy3][B(C6F5)4], which
was identified by separate synthesis and comparison of 1H and
31P NMR spectroscopy data. Use of this less-active substrate
slows the reaction down, but also avoids complications in the
kinetics arising from the presence of multiple active species in
the reaction.[35] These studies indicate that the reaction is
second order, first order in each of [3c] and [styrene]
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Table 1: Ring-closing metathesis using 3c[a] at room temperature.
provide an opportunity to probe the
mechanism of the propagating steps
in the olefin metathesis reaction
without the problem of excess phos-
phine in the reaction medium, as
well as catalyst decomposition proc-
esses that do not involve the dis-
sociated phosphine ligand of cata-
lysts 1.[40]
[b]
Entry
Substrate
Product
RCL
t [min]
Conversion [%][c]
1
2
3
H
H
Me
1.0
0.1
1.0
<2
30
<10
100
100
100
4
5
H
Me
1.0
1.0
<10
60
100
98
Experimental Section
Complete experimental details, includ-
ing synthesis and characterization of all
the new compounds, can be found in the
Supporting Information.
6
–
5.0
<10
85
[a] Conditions: room temperature, toluene, [diene]=0.23m. [b] Catalyst loading. [c] Conversions
determined by H NMRspectroscopy.
Received: July 21, 2004
Revised: August 27, 2004
1
Keywords: carbene ligands ·
.
homogeneous catalysis · metathesis ·
ruthenium
substrate, with a second order rate constant of 5.9 Æ 0.3 ”
10À4 mÀ1 sÀ1 at À108C.[36] By evaluating this rate constant as
a function of temperature (30 equiv of styrene), the thermo-
dynamic activation parameters of the reaction were measured
as DH° = 8.6(4) kcalmolÀ1 and DS° = À55(6) eu. The large,
negative activation entropy is consistent with a bimolecular
initiation step, which supports the notion that the measured
barrier corresponds to rate-limiting olefin binding. Never-
theless, even for this very unreactive substrate,[20] the magni-
tude of the second-order rate constant at À108C is compa-
rable to the initiation rate of 4.6 Æ 0.4 ” 10À4 sÀ1 measured at
358C for the widely used Grubbs second-generation catalyst
1c.[18]
These results are an advance in the evolution of ruthe-
nium-based olefin-metathesis catalysis. Slow initiation has
been a limiting factor for the existing Grubbs-catalyst
portfolio and the phosphonium alkylidene compounds dis-
closed herein circumvent this problem by effectively elimi-
nating the phosphine-dissociation initiation step altogether.
In essence, all of the ruthenium added is actually operating as
a catalyst in these reactions as compared to the traditional
Grubbs catalysts, where the majority of ruthenium is tied up
as the non-active 1c. In compounds 3, initiation consists of the
much lower barrier olefin-binding event; subsequent meta-
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thesis to liberate [(CH2 CH)PR3][B(C6F5)4] provides rapid
and direct access to the propagating 14-electron ruthenium
alkylidene complexes without any free phosphine present to
drain the catalyst pool of active ruthenium.
The benefits of fast-initiating catalysts for a variety of
metathesis applications are many. Lower catalyst loadings can
be achieved which improves the economics and the environ-
mental impact associated with use of these catalysts. Reac-
tions can be performed at much lower temperatures, poten-
tially providing greater enantioselectivity in asymmetric
RCM reactions[37] and better chemoselectivity in commer-
cially important CM processes.[38] In ROMP applications,
rapid initiation leads to polymer products with narrower
molecular-weight distributions.[39] Finally, the new compounds
6164
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[28] Crystal data for 3c: C64H60BCl2F20N2PRu, Mr = 1450.89, triclinic,
¯
P1, a = 12.0671(6) Š, b = 16.4448(9) Š, c = 17.6951(9) Š, a =
102.1701(8)8, b = 108.1299(8)8, g = 102.6938(9)8, V=
3104.4(3) Š3, Z = 2, 1calcd = 1.552 gcmÀ3, MoKa radiation, l =
0.71073 Š, T= À808C, 23973 measured reflections, 12683
unique (Rint = 0.0168), 11455 reflections with Inet > 2.0(Inet), m =
0.467 mmÀ1, min/max transmission = 0.7732 and 0.8803, R1 (I >
2s) = 0.0310, wR2 0.0822, GoF = 1.037, No. of parameters = 820,
final difference map within + 0.686 and À0.392 eŠ3. CCDC-
243766 (3c) contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
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[35] Attempts to determine rate constants by monitoring the ring
closing of diallyldiethylmalonate gave nonlinear plots because,
even at À608C, the alkylidene product of initiation was active for
turning the reaction over, as evidenced by the slow build-up of
ethylene in the sample over time.
[36] The pseudo first-order plots from which the rate constants were
obtained are linear for the duration of the reaction after a region
of curvature at the beginning of the reaction (see Supporting
Information). The origins of this kinetic behavior are not
currently understood; further investigations are underway.
[37] T. J. Seiders, D. W. Ward, R. H. Grubbs, Org. Lett. 2001, 3, 3225.
[38] R. L. Pederson, I. M. Fellows, T. A. Ung, H. Ishihara, S. P.
Hajela, Adv. Synth. Catal. 2002, 344, 728.
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Angew. Chem. Int. Ed. 2004, 43, 6161 –6165
www.angewandte.org
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