Well-defined ruthenium olefin metathesis catalysts: Mechanism and activity

Article abstract of DOI:10.1021/ja963136z

Several ruthenium-based olefin metathesis catalysts of the formula (PR₃)₂X₂Ru=CHCHCPh₂ have been synthesized, and relative catalyst activities were determined by monitoring the ring-closing metathesis of the acyclic diene diethyl diallylmalonate. The following order of increasing activity was determined: X = I < Br < Cl and PR₃ = PPh₃ << P(i)Pr₂Ph < PCy₂Ph < P(i)Pr₃ < PCy₃. Additional studies were conducted with the catalyst (PCy₃)₂-Cl₂Ru=CH₂ to probe the mechanism of olefin metathesis by this class of catalysts. The data support a scheme in which there are two competing pathways: the dominant one in which a phosphine dissociates from the ruthenium center and a minor one in which both phosphines remain bound. Higher catalyst activities could be achieved by the addition of CuCl to the reaction.

Full text of DOI:10.1021/ja963136z

J. Am. Chem. Soc. 1997, 119, 3887-3897
3887
Well-Defined Ruthenium Olefin Metathesis Catalysts:
Mechanism and Activity
Eric L. Dias, SonBinh T. Nguyen, and Robert H. Grubbs*
Contribution from The Arnold and Mabel Beckman Laboratory of Chemical Synthesis,
DiVision of Chemistry And Chemical Engineering, California Institute of Technology,
Pasadena, California 91125
ReceiVed September 6, 1996X
Abstract: Several ruthenium-based olefin metathesis catalysts of the formula (PR3)2X2RudCHCHCPh2 have been
synthesized, and relative catalyst activities were determined by monitoring the ring-closing metathesis of the acyclic
diene diethyl diallylmalonate. The following order of increasing activity was determined: X ) I < Br < Cl and
PR3 ) PPh3 , PiPr2Ph < PCy2Ph < PiPr3 < PCy3. Additional studies were conducted with the catalyst (PCy3)2-
Cl2RudCH2 to probe the mechanism of olefin metathesis by this class of catalysts. The data support a scheme in
which there are two competing pathways: the dominant one in which a phosphine dissociates from the ruthenium
center and a minor one in which both phosphines remain bound. Higher catalyst activites could be achieved by the
addition of CuCl to the reaction.
Introduction
Scheme 1
The synthesis and isolation of the ruthenium vinylcarbene 1
opened the door to the development of well-defined, late
transition metal, low oxidation state complexes that catalyze
olefin metathesis.1 In addition to the activity of 1 in the
metathesis of strained cyclic1,2 and exocyclic3 olefins, the
remarkable functional group tolerance and stability toward
several conditions such as air, water, and acids1 has made this
class of catalyst particularly attractive for practical applications.
macrocycles and covalently stabilized â-turns,7 and likewise
ene-yne-ene systems to make fused or tethered bicyclic
molecules9 are examples of the particular reactivity and synthetic
utility of these catalysts.10
Until now, a systematic investigation of the factors governing
catalyst activity and the mechanism by which these catalysts
perform olefin metathesis has not been reported. The present
study was undertaken to address both of these topics in the
following ways. By varying the ligand sphere around the
ruthenium catalyst, we wished to determine how the electronic
and steric properties of the ligands affect catalyst activity.
Likewise, by examining the kinetics of olefin metathesis, we
hoped to gain some insight as to the mechanistic pathway(s)
by which these catalysts operate. Once the relationship between
the mechanism, ligands, and catalyst activity was understood,
we sought to apply this knowledge and tune catalyst activity in
a desired fashion.
By exchanging the triphenylphosphines in 1 for tricyclohex-
ylphosphines, it was found that the catalyst 2a could also be
easily synthesized and isolated.4 However, 2a proved to be a
much more active catalyst than 1, reacting with relatively low-
strain cyclic olefins4,5 as well as straight-chain alkenes,4 while
retaining the stability of 1 toward air and protic media.4 The
ring-closing metathesis of functionalized R,ω-dienes to produce
five-, six-, seven-,6,7 and eight-membered rings,7,8 as well as
X Abstract published in AdVance ACS Abstracts, April 1, 1997.
(1) Nguyen, S. T.; Johnson, L. K.; Grubbs, R. H. J. Am. Chem. Soc.
1992, 114, 3974-3975.
(2) Wu, Z.; Benedicto, A. D.; Grubbs, R. H. Macromolecules 1993, 26,
4975-4977.
(3) Wu, Z.; Nguyen, S. T.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem.
Soc. 1995, 117, 5503-5511.
Results and Discussion
(4) Nguyen, S. T.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1993,
115, 9858-9859.
To determine the relative activities of the ruthenium catalysts,
the ring-closing metathesis of commercially available diethyl
diallylmalonate was studied, as shown in Scheme 1. Diethyl
diallylmalonate was chosen as the substrate for two reasons:
(1) it has been observed that ring-closing metathesis to the
corresponding cyclopentene diester is quantitative and relatively
(5) Hillmyer, M. A.; Laredo, W. R.; Grubbs, R. H. Macromolecules 1995,
28, 6311-6316.
(6) (a) Holder, S.; Blechert, S. Synlett 1996, June, 505-506. (b)
Crimmins, M. T.; King, B. W. J. Org. Chem. 1996, 61, 4192-4193. (c)
Garro-Helion, F.; Guibe, F. Chem. Commun. 1996, 641-642. (d) Fu, G.
C.; Nguyen, S. T.; Grubbs, R. H. J. Am. Chem. Soc. 1993, 115, 9856-
9857.
(7) (a) Furstner, A.; Langemann, K. J. Org. Chem. 1996, 61, 3942-
3943. (b) Miller, S. J.; Grubbs, R. H. J. Am. Chem. Soc. 1995, 117, 5855-
5856. (c) Borer, B.; Deerenberg, S.; Bieraugel, H.; Pandit, U. K. Tetrahedron
Lett. 1994, 35, 3191-3194.
(8) Miller, S. J.; Kim, S.-H.; Chen, Z.-R.; Grubbs, R. H. J. Am. Chem.
Soc. 1995, 117, 2108-2109.
(9) Kim, S.-H.; Bowden, N.; Grubbs, R. H. J. Am. Chem. Soc. 1994,
116, 10801-10802.
(10) For a recent review of olefin metathesis catalysts in organic
synthesis, see Grubbs, R. H.; Miller, S. J.; Fu, G. C. Acc. Chem. Res. 1995,
28, 446-452.
S0002-7863(96)03136-8 CCC: $14.00 © 1997 American Chemical Society

3888 J. Am. Chem. Soc., Vol. 119, No. 17, 1997
Dias et al.
Table 1. Relative Activities of the Catalysts
(PR3)2X2RudCH-CHdCPh2 in the Ring-Closing Metathesis of
Diethyl Diallylmalonatea
of the phosphine ligands resulted in substantial changes in
catalyst activity. Catalysts 2a,b,c with PCy3 ligands were found
to be more active than the respective catalysts 4a,b,c with
PiPr3 ligands. Likewise, catalysts 3a,b,c with PCy2Ph ligands
are more active than the respective catalysts 5a,b,c with PiPr2-
Ph ligands. These results suggest that phosphines with larger
cone angles generate catalysts with greater activities.
A more dramatic electronic effect is observed. Catalysts
2a,b,c with PCy3 ligands are much more active than the
respective catalysts 3a,b,c with PCy2Ph ligands. Similarly,
catalysts 4a,b,c with PiPr3 ligands are much more active than
the respective catalysts 5a,b,c with PiPr2Ph ligands. Thus,
merely changing one cyclohexyl or ispropyl group to a phenyl
groupsand thereby making the phosphine less electron
donatingsresults in a marked decrease in catalyst activity. This
trend is even further illustrated by the fact that 1, which has
PPh3 ligands, is totally inactive for the metathesis of diethyl
diallylmalonate, and will only react with suitably strained
olefins.
catalyst
PR3
X
activity (turnovers/h)b
2a
2b
2c
3a
3b
3c
4a
4b
4c
5a
5b
5c
PCy3
Cl
Br
I
Cl
Br
I
Cl
Br
I
Cl
Br
I
19.0
15.4
1.4
8.0
4.5
c
17.5
13.9
1.1
5.5
2.3
c
PCy2Ph
PiPr3
PiPr2Ph
a Conditions: [diethyl diallylmalonate]0 ) 0.2 M; [catalyst] ) 0.010
M; temperature ) 20 °C. b Turnover numbers were obtained by fitting
data of [product] vs time to a double-exponential expression (see Figure
1) and using the product concentration from the 1-h time point of the
curve fit. c Catalyst showed no activity in the metathesis reaction over
several hours.
An interesting trend is observed when the halogens are varied.
Comparing catalyst 2a, 2b, and 2c, it is easily seen that going
down the series from Cl to Br to I corresponds to a decrease in
catalyst activity. It should be noted that in going from Cl to
Br, catalyst activity is depressed only slightly, while changing
to I has a precipitous effect. Similar effects are observed for
catalysts 3, 4, and 5, with catalysts 3c and 5c being sufficiently
slow that ring-closing is negligible at 20 °C. These observations
are puzzling, as they suggest that the more electron withdrawing
and smaller halogens generate more active catalystsstrends that
are exactly opposite to those observed when varying the
phosphines.14
While consistent trends are observed throughout this series
of catalysts, it appears that the steric and electronic effects of
the phosphines upon catalyst activity are opposite to those
observed for the halogens. Phosphines, which are larger and
more electron donating, and likewise halogens, which are
smaller and more electron withdrawing, lead to more actiVe
catalysts.
facile,11 and (2) the rates of ring-closing are slow enough to be
followed by H NMR but fast enough to be experimentally
1
feasible. We chose ring-closing olefin metathesis rather than
ring-opening polymerization or cis-2-pentene metathesis because
there is only one propagating species, as opposed to the other
systems in which more than one propagating species is
observed.4,12 Methylene chloride was found to be a suitable
solvent for these studies: ring-closing is approximately three
times faster than in benzene, and minimal catalyst decomposition
occurs in this solvent.
Catalysts 2a,b,c, 3a,b,c, 4a,b,c, and 5a,b,c were synthesized
to explore the changes in catalyst activity as the phosphine and
halogen ligands are systematically varied. PiPr3 has a slightly
Mechanism of Olefin Metathesis. Initially, two general
mechanisms for olefin metathesis by these catalysts were
proposed (Scheme 2). The top pathway, termed “associative”
(NOT in the classical ligand-exchange sense), assumes that the
olefin simply coordinates to the catalyst to form the intermediate
18-electron olefin complex, followed by the actual metathesis
steps to form the product. The bottom pathway, termed
“dissociative”, assumes that upon binding of the olefin, a
phosphine is displaced from the metal center to form a
16-electron olefin complex, which undergoes metathesis to form
the cyclized product, regenerating the catalyst upon recoordi-
nation of the phosphine. (It should be noted that the “associa-
tive” mechanism at first appeared more attractive, because all
of the intermediates have either 16 or 18 electrons. In the
“dissociative” pathway, all of the intermediates also have either
16 or 18 electrons, with the exception of the 14 electron
metallacyclobutane). In order to distinguish between these two
mechanisms, the kinetics of the reaction were examined by
monitoring product formation (or substrate disappearance) over
time.
smaller cone angle than PCy3, but similar electronic properties.
On the other hand, PiPr2Ph and PCy2Ph should have cone angles
similar to or perhaps slightly smaller than PiPr3 and PCy3,
respectively, with substantially different electronic properties
than the trialkylphosphines.13 In this way, the effects of
changing the steric or electronic properties of the phosphine
can be studied independently. The halogens, on the other hand,
pose a more complex problem. While Cl, Br, and I all have
different electronic properties, the size of the halogens also
varies substantially down the series. Because of this, the effects
of the halogens cannot readily be separated into predominantly
steric or predominantly electronic in nature. The catalyst
activities, measured under a standard set of conditions which
provided reasonable rates for study, are summarized in Table
1.
Ligand Effects upon Catalyst Activity. As previously
demonstrated by the remarkable difference in reactivities
between catalysts 1 and 2a, it was found that varying the nature
(11) Bowden, N.; Grubbs, R. H. Unpublished results.
(12) By propagating species, we refer to the resting state of the catalyst
as determined by NMR spectroscopy, which is the species (PR3)2X2RudCH2
in this ring-closing metathesis reaction.
(13) For background reading on the steric and electronic properties of
phosphines, see: (a) Brown, T. L.; Lee, K. J. Coord. Chem. ReV. 1993,
128, 89-116. (b) Wilson, M. R.; Woska, D. C.; Prock, A.; Giering, W. P.
Organometallics 1993, 12, 1742-1752. (c) Tolman, C. A. Chem. ReV. 1977,
77, 313-348.
(14) The same trend is observed for the polymerization of norbornene
by the compounds (PPh3)2X2RudCH-CHdCPh2 (X ) Cl, Br, I). To further
explore the relative electron withdrawing abilities of the X ligands, we
synthesized the two compounds (PPh3)2(Cl)(X)RudCH-CHdCPh2 (X )
CH3CO2, CF3CO2). We found that the trifluoroacetate-containing catalyst
polymerized norbornene at least ten times faster than the acetate-containing
catalyst, and therefore we conclude that more electron withdrawing X groups
produce more active catalysts.

Well-Defined Ruthenium Olefin Metathesis Catalysts
J. Am. Chem. Soc., Vol. 119, No. 17, 1997 3889
Scheme 2
Inspection of the plots of product versus time for the ring-
closing reaction with catalysts 2-5 indicated that the kinetics
did not exhibit first-order behavior with respect to diene. In
fact, the curves fit remarkably well to a double-exponential
expression, as shown in Figure 1. We considered the possiblity
that the unexpected kinetic behavior might be due to differences
between the initiating carbene (RudCH-CHdCPh2) and the
propagating carbene (RudCH2). The synthesis and isolation
of the ruthenium methylidene catalyst 6,15 which was used in
all of the following kinetic experiments, allowed us to resolve
this issue. We observed similar results with catalyst 6: the
important results were obtained from these experiments. First,
addition of 0.25-1.0 equiv (0.005-0.020 M) of phosphine (with
respect to 0.020 M catalyst) depresses the rate dramatically,
with the reaction proceeding up to 20 times slower upon the
addition of 1.0 equiv (0.020 M) of phosphine. Second, as shown
in Figure 2, the kinetics become pseudo first order with respect
to diene upon addition of phosphine. This fortuitous result
allows us to obtain a pseudo-first-order rate constant kobs, which
can be used to determine the relationship between kobs and
phosphine concentration. The plot of kobs vs. reciprocal
phosphine concentration in Figure 3 shows a linear correlation,
and the positive intercept indicates that there is an additional
phosphine-independent term in the rate expression.
To determine the complete rate expression for the ring-closing
reaction, experiments were conducted to determine the rate
dependence upon catalyst concentration. Because of the
complex kinetics in the absence of excess phosphine, we could
not measure an observed rate constant kobs for the ring-closing
reaction under these conditions. The fact that these curves fit
to a double-exponential expression was utilized to find the rate
at each point in time simply by taking the derivative of the
double-exponential curve fit. The plots of rate of diene
disappearance vs diene concentration for different catalyst
concentrations are shown in Figure 4. The data are obviously
not first ordersat least not initially. (It appears, however, that
the reaction eventually reaches a steady state, which occurs after
more than half of the substrate has been consumed.) By
comparing these calculated rates at identical diene concentra-
tions, as summarized in Table 2, we find an approximate square-
root dependence on catalyst concentration.
kinetics still did not exhibit first-order behavior with respect to
the diene, and instead fit very well to a double-exponential
expression. These observations led us to conclude that the
differences between the initiating and propagating carbenes were
not responsible for the unexpected kinetic behavior.
Because the dissociative pathway in Scheme 2 relies on a
phosphine dissociating upon olefin binding, we added excess
phosphine to the reaction, reasoning that addition of phosphine
would disfavor the equilibrium for olefin binding. Likewise,
if the associative pathway were active, adding excess phosphine
would have little or no effect upon the reaction kinetics. Two
(15) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996,
118, 100-110.

3890 J. Am. Chem. Soc., Vol. 119, No. 17, 1997
Dias et al.
Figure 3. Plot of kobs vs reciprocal phosphine concentration for the
ring closing metathesis of diethyl diallylmalonate at varying phosphine
concentrations, with [diene]0 ) 0.2 M and [Ru]0 (6) ) 0.02 M. The
reactions were carried out in CD2Cl2 at 30 °C. The filled diamonds are
the data points, the solid line is the linear fit kobs ) K0 + K1(1/[PCy3],
where the constants Kn are generic constants calculated by the curve-
fitting procedure, and the dashed line is the extrapolation of the linear
fit to the intercept. Intercept ) (5.27 ( 0.13) × 10-4; slope ) (2.73 (
0.01) × 10-5; linear correlation coefficient ) 1.00.
Figure 1. (a) Representative plot of diene concentrations vs time for
catalyst 4a. The reaction was carried out with [diene]0 ) 0.2 M and
[catalyst (4a)] ) 0.01 M in CD2Cl2 at 20 °C. The filled diamonds are
the data points, and the solid line is the double-exponential fit: [diene]-
(t) ) K0 + K1 exp(-K2t) + K3 exp(-K4t). The dashed line is the best
first-order fit [diene](t) ) K0 + K1 exp(-K2t). The constants Kn are
generic constants that are calculated by the curve-fitting procedure. (b)
The residuals (crosses) from the double-exponential fit in part (a) were
found by taking the difference between the data and the curve fit at
each point.
Figure 4. Plot of reaction rate vs diene concentration for the ring-
closing metathesis of diethyl diallylmalonate at varying concentrations
of catalyst (6), with [diene]0 ) 0.2 M. The reactions were carried out
at 25 °C in CD2Cl2. The data points and concentrations of catalyst 6
are as follows: filled circles, 0.005 M; open diamonds, 0.010 M; filled
triangles, 0.015 M; open squares, 0.020 M. The data points were
obtained by first fitting the plots of [diene](t) vs time to the double-
exponential expression: [diene](t) ) K0 + K1 exp(-K2t) + K3, exp-
(-K4t), where the constants Kn are generic constants calculated by the
curve-fitting procedure. Using the derivative of this equation and the
calculated values for the constants Kn, the rate can be calculated as a
function of time. The diene concentration is already expressed as a
function of time in the double-exponential relationship above, so the
rate can be expressed more usefully as a function of diene concentration,
represented by the solid lines in the above figure. The vertical hash
marks are the diene concentrations at which the rates were compared,
for which the data are summarized in Table 2.
pseudo-first-order rate constants could be obtained. A plot of
kobs vs catalyst concentration is shown in Figure 5. A good
linear correlation is observed, and it can be concluded that the
reaction is first order with respect to catalyst concentration. The
final rate expression is shown in eq 1, where kobs is the
expression within parentheses, and [Ru]0 is the concentration
of catalyst (i.e., the total concentration of ruthenium in the
system).
Figure 2. Log plot of diene concentration vs time for the ring-closing
metathesis of diethyl diallylmalonate in the presence of 0.02 M PCy3,
where [Ru]0(6) ) 0.02 M and [diene]0 ) 0.2 M. The reactions were
carried out in CD2Cl2 at 30 °C. K0 and K1 are the constants from the
first-order fit [diene](t) ) K0 + K1 exp(-K2t), and K2 is the slope of
the line, where the constants Kn are generic constants calculated by
the curve-fitting procedure. The boxes are the data points and the line
is the linear fit. Intercept ) (6.45 ( 7.64) × 10-3; slope ) (-1.88 (
0.01) × 10-3; linear correlation coefficient ) 1.00.
d[diene]
A
-
)
+ B [Ru]0[diene]
(1)
(
)
Finally, to determine the actual catalyst order in the rate
expression, the reactions in which the catalyst concentration was
varied were repeated in the presence of a constant concentration
of excess phosphine. By adding the excess phosphine, the
dt
[PCy3]
In order to explain this result, we have proposed a mechanism
in Scheme 3 in which both the “associative” and “dissociative”

Well-Defined Ruthenium Olefin Metathesis Catalysts
J. Am. Chem. Soc., Vol. 119, No. 17, 1997 3891
Table 2. Ratios of Rates for Ring-Closing Metathesis with
Varying Catalyst Concentrations
complex with a sterically demanding environmentsthe metal-
lacyclobutane being the highest energy intermediate. The rate
of disappearence of diene based on Scheme 3 is shown in eq 2:
ratio of catalyst concns
(0.010 M)/
(0.005 M)
(0.015 M)/
(0.005 M)
(0.020 M)/
(0.005 M)
d[diene]
-
) k3[I2] + k4[I1]
(2)
averagea
1σb
1.33
1.61
1.81
dt
0.0741
0.148
0.222
0.111
0.222
0.333
0.185
0.370
0.555
2σb
By solving the equilibria for the concentrations of [I1] and [I2],
one easily obtains:
3σb
a The rates of olefin metathesis for different catalyst concentrations
were determined at the diene concentrations designated by vertical hash
marks in Figure 4. The rates of metathesis for the different catalyst
concentrations were then compared at several identical diene concentra-
tions (so that the rate dependence on diene concentration cancels out),
and the ratios calculated at these diene concentrations were averaged
accordingly. Thus, the ratios shown above show the effect of doubling,
tripling, and quadrupling (from left to right) the catalyst concentration
upon the rate of metathesis. By doubling the catalyst concentration,
the rate of metathesis increases by a factor of 1.33; by tripling the
catalyst concentration, the rate increases by a factor of 1.61; and by
quadrupling the catalyst concentration, the rate increases by a factor
of 1.81. This appears to demonstrate an approximate square-root
dependence upon catalyst concentration, where the rate would increase
by a factor of 1.41, 1.73, and 2.00, respectively. b σ ) standard deviation
calculated from the data.
[I1] ) K1[6][diene]
and
[I1]
2 [PCy3]
[6][diene]
[I2] ) K
) K1K2
(3)
[PCy3]
Substituting eq 3 into eq 2 yields the final rate expression:
K1K2
3[PCy3]
d[diene]
-
) k
+ k4K1 [6][diene]
(4)
(
)
dt
By comparing eq 4 with the empirical rate expression in eq 1,
where [6] ) [Ru]0, we find that the constants A and B are:
A ) k3K1K2
and
B ) k4K1
(5)
Referring back to eq 1, we can attribute the first part of kobs
to the “dissociative” pathwaysthe rate is inversely proportional
to the concentration of unbound phosphine, and directly
proportional to the concentrations of both catalyst and diene.
The second part of kobs can be attributed to the “associative”
pathwaysthe rate is directly proportional to the concentrations
of both catalyst and diene, and independent of the concentration
of unbound phosphine. From the data in Figure 3, however,
we concluded that B, the observed rate constant for the
“associative” pathway, is relatively small compared to the
quotient A/[PCy3], such that in the absence of excess phosphine,
the “dissociative” part of the expression clearly dominates
(>90-95%).
We can rationalize the kinetic behavior as follows. Because
the starting catalyst (with both phosphines) is the only species
observed by NMR during the reaction, and likewise no unbound
phosphine is observed, we can assume that the amount of
unbound phosphine does not exceed 5% of the catalyst
concentration. In considering the equilibrium for diene coor-
dination and phosphine dissociation, the concentration of
unbound phosphine at every point in time should be proportional
to the square root of the catalyst concentration, as shown in
Scheme 4. The approximate square-root dependence of the rate
on catalyst concentration, observed previously in the absence
of added phosphine (Table 2), is consistent with this analysis.
The addition of a mere 0.25-1.0 equiv (0.005-0.020 M) of
phosphine (with respect to 0.020 M catalyst) is sufficient to
swamp the concentration of unbound phosphine orginating from
the catalyst. Because the phosphine concentration is constant,
the kinetics become pseudo-first order with respect to diene, as
demonstrated by eq 1. Finally, when the catalyst concentration
is varied in the presence of added phosphine (and thus keeping
a constant phosphine concentration), the linear correlation
between kobs and catalyst concentration can be extracted.
Stereochemistry of Intermediates. Before we can consider
an explanation for the ligand effects in Table 1, we must first
determine the stereochemistry of the postulated intermediates
I1 and I2 in our proposed mechanism (Scheme 3). Scheme 5
describes what we at first considered to be the two most
plausible stereochemical pathways for the “dissociative” path-
way in Scheme 3sthe “dissociative” pathway, being responsible
Figure 5. Plot of kobs vs catalyst concentration for the ring-closing
metathesis of diethyl diallylmalonate at varying catalyst concentrations
in the presence of 0.005M PCy3, with [diene]0 ) 0.2 M. The reactions
were carried out in CD2Cl2 at 30 °C. The filled circles are the data
points, the solid line is the linear fit kobs ) K0 + K1([Ru]0) where the
constants Kn are generic constants calculated by the curve-fitting
procedure, and the dashed line is the extrapolation of the linear fit to
the intercept. Intercept ) (2.42 ( 0.72) × 10-4; slope ) 0.323 ( 0.005;
linear correlation coefficient ) 1.00.
pathways from Scheme 2 are operating. Because no intermedi-
ates are observed during the course of reaction, the rate of
disappearance of diene is equal to the rate of product formation.
We assumed that in both cases, metallacycle formation is the
rate determining step16,17sin the “dissociative” pathway, the
metallacyclobutane is a 14-electron complex,18 and in the
“associative” pathway, the metallacyclobutane is a 16-electron
(16) It may be argued that breakdown of the metallacycle is the rate
determining step, in which case our differential equations are only slightly
altered, and still in agreement with our empirically derived rate expression.
However, we believe that metallacycle formation is rate determining, as it
corresponds to a formal two electron oxidation of the ruthenium center.
(17) We also believe that the second metathesis step, the intramolecular
reaction to form the cyclized product is faster than the first, intermolecular
metathesis, due to the decreased activation entropy. This is evidenced by
the fact that we never observe the intermediate that preceeds the cyclization
step even when excess phosphine is added, as opposed to the cyclization
of 1,7-octadiene to cyclohexene, during which this intermediate is observed
1
by H NMR in the presence of excess phosphine.
(18) The possiblity that the diene chelates the metal center to form a
16-electron complex cannot be ruled out, although entropically this seems
unlikely without any predisposition of the diene toward chelation as in the
case of butadiene, norbornadiene, or 1,5-cyclooctadiene.

3892 J. Am. Chem. Soc., Vol. 119, No. 17, 1997
Dias et al.
Scheme 3
Scheme 4
In a comparison of these two mechanisms, pathway 1
appeared to be more plausible. The olefin coordinates in what
appears to be the open coordination site, and phosphine
dissociates with olefin migration to a position from which
metallacycle formation can occur. However, after examining
the proposed intermediates in greater detail, we have concluded
that pathway 2 is more likely based on the following analysis.
Although we have not been able to directly observe olefin
complexes of the catalysts 2-6, olefin complexes of the
compound (PCy3)2Cl2Ru(CO) have been reported in the litera-
ture. Carbon monoxide is an excellent model for the carbene
moietyssubstantial π-bonding is expected for CO bound to an
electron rich Ru(II) metal center, such that a CO ligand will
occupy the same ruthenium orbitals as the carbene moiety (in
either orientation). Olefin complexes of strong π-acids such
as acrylonitrile and 1,2-dicyanoethylene bound to (PCy3)2Cl2-
Ru(CO) have been characterized by IR and 31P NMR spectros-
copy, and in all cases the olefin is coordinated as shown below
in 7a and 7b.20 In addition, a crystal structure of the ethylene
complex 821 depicts an identical ligand geometry, in which the
olefin is bound cis to the CO ligand. Based on these structures,
we believe that the olefin complex I1 is correctly depicted in
pathway 2 (Scheme 5).
for 95% of catalyst turnover, will also be responsible for the
differences in catalyst activity.
From the crystal structures of the vinylcarbene4 and ben-
zylidene15 ruthenium catalysts (PCy3)2Cl2RudCHR, we know
that they both have a raised square pyramidal geometry with
the carbene occupying the apical position. The carbene moiety
lies in the Cl-Ru-Cl plane, as opposed to the crystal structure
of catalyst 11 in which the carbene moiety lies in the P-Ru-P
plane. This is reflected by the coupling constant 3JHP between
HR on the carbene and the phosphorous nuclei in the 1H NMR
spectras3JHP is approximately zero in the spectrum of the
vinylcarbene and benzylidene catalysts (PCy3)2Cl2RudCHR,4,15
and 11-12 Hz in the spectrum of 1.1 From this we conclude
that 3JHP closely follows the Karplus relationship for the P-Ru-
CR-HR dihedral angle.4
Because 3JHP is approximately zero in the methylidene catalyst
6, we have concluded that the carbene moiety lies in the Cl-
Ru-Cl plane.19 Furthermore, because it is necessary that the
carbene moiety and olefin approach each other in a face-to-
face manner to effect metallacycle formation, the carbene
orientation must be considered in our analysis. According to
these arguments, an olefin coordinated in one of the positions
currently occupied by PCy3 will be in the required face-to-face
orientation with the ruthenium-carbene double bond.
In Scheme 5, pathway 1 depicts olefin coordination trans to
the carbene to make an 18-electron intermediate I1, followed
by phosphine dissociation with olefin migration to form a 16-
electron intermediate I2 in which the olefin is cis to the carbene.
Assuming that the carbene retains its orientation, this 16-electron
intermediate has the required geometry for metallacyclobutane
formation and subsequent metathesis.
Pathway 2 depicts I1 with the olefin coordinating cis to the
carbene and chloride migrating to the position trans to the
carbene. It should be noted that, with the olefin coordinated
as shown in I1 , the carbene must rotate 90° before metallacycle
formation can occur. Subsequent phosphine dissociation, along
with carbene rotation, forms the 16-electron intermediate I2,
which has the required geometry for metallacycle formation.
Further evidence that the olefin must coordinate cis to the
carbene moiety is provided by the second step in the ring-closing
metathesis reaction (eq 6):
For small to moderate sized rings, the pendant olefin can only
coordinate cis to the carbene to which it is tetheredsa trans
coordinated olefin of this type would almost certainly have a
prohibitive amount of strain energy. Excepting the unlikely
event in which the olefin coordinates in different places
depending on whether or not it is tethered to the carbene, these
geometric constraints indicate that pathway 2 (Scheme 5)
accurately depicts the olefin complex I1.
For symmetric catalysts such as 1-6, consideration of the
principle of microscopic reversibility22 has interesting conse-
quences for the possible mechanisms of olefin metathesis. For
(19) Chemical exchange decoupling of the phosphines has been ruled
out, since the ruthenium-bound phosphine peak is not averaged with the
unbound phosphine peak in the 31P NMR spectrum when excess PCy3 is
added. In fact, the 1H and 31P NMR resonances for 6 remain unchanged
from 20 to 80 °C in C6D6 (catalyst decomposition is significant at this
temperature) in both the presence and absence of excess PCy3, indicating
relatively high barriers to both phosphine exchange and carbene rotation.
(20) Moers, F. G.; Langhout, J. P. J. Inorg. Nucl. Chem. 1977, 39, 591-
593.
(21) Brown, L. D.; Barnard, C. F. J.; Daniels, J. A.; Mawby, R. J.; Ibers,
J. A. Inorg. Chem. 1978, 17, 2932-2935.

Well-Defined Ruthenium Olefin Metathesis Catalysts
J. Am. Chem. Soc., Vol. 119, No. 17, 1997 3893
Scheme 5
Scheme 6
the ruthenium catalysts, it is easiest to consider a degenerate
metathesis reaction, as shown in Scheme 6.
complex with two phosphines is a common intermedate in both
the associative and dissociative pathways. In pathway 1,
however, there is no plausible intermediate that can satisfy both
pathways. Either the ligands must rearrange to a species in
which the bulky tricyclohexylphosphines are cis to each other
or an altogether separate pathway must exist in which the olefin
coordinates as shown in pathway 2swhich in the end provides
another reason why pathway 2 is more likely.
By accepting pathway 2 as the more probable mechanism,
we are left to rationalize the required 90° carbene rotation that
must precede metallacycle formation . This may not be as
energetically unfavorable as it may first appear.19 In catalyst
1, the conformation of the carbene is already rotated 90° (as
compared to 2-6). In compound 9, the carbene is oriented at
a 45° angle such that it bisects the Cl-Ru-Cl and P-Ru-P
planes as determined by X-ray crystallography,3 indicating that
linear combinations of the two available π-bonding orbitals on
the metal center exist such that the RudC π-bond is not broken
during the rotation process.
According to pathway 1, the square pyramidal intermediate
I2, in which the carbene occupies the apical position and the
olefin a basal position, directly precedes metallacyclobutane
formation. Productive cleavage of the metallacyclobutane places
the carbene in a basal position and the departing olefin in the
apical position, intermediate I2′. In order to regenerate the
starting catalyst 6, some type of ligand rearrangement must occur
along with phosphine recoordination and displacement of the
product olefin. Because this is a degenerate reaction, the
principle of microscopic reversibility requires that the reverse
reaction occur at the same rate as the forward reaction. In
regarding pathway 1, therefore, two pathways must actually be
occurring simultaneously to effect “productive” metathesissthe
direct result of having an asymmetric energy profile for a
degenerate reaction. The extension of this to nondegenerate
olefin metathesis results in there being two competing “dis-
sociative” pathways, which may or may not be kinetically
distinguishable from each other.
In pathway 2, on the other hand, formation of the metalla-
cyclobutane from I2 and subsequent cleavage produces the
intermediate I2′, which is actually the enantiomer of I2. Reco-
ordination of the phosphine and displacement of the product
olefin directly regenerates the starting catalyst 6 without the
need for any additional ligand rearrangements. Furthermore,
because the energy profile is symmetric, no additional pathways
are implicated by this mechanism.
1
Additionally, we have found by H NMR spectroscopy that
during ligand exchange reactions in which PCy3 (or another
exchangeable phosphine) is added to 1, the carbene in the mixed
phosphine intermediate is oriented as in 1. It is only upon
reaction of the second equivalent of PCy3 that the carbene
rotates. We therefore find it very feasible that, upon dissociation
of PCy3 from the intermediate I1 in pathway 2, the carbene
rotates 90° as the steric and/or electronic environment is
changed.
When the associative pathway that is implied by the reaction
kinetics is also taken into account, pathway 2 becomes even
more attractive. In Scheme 3, we have proposed that the olefin
(22) For a brief discussion of the principle of microscopic reversibility,
see for example: Laidler, K. J. Chemical Kinetics, 2nd ed.; McGraw-Hill:
New York, 1965; pp 110-112.

3894 J. Am. Chem. Soc., Vol. 119, No. 17, 1997
Dias et al.
Scheme 7
From the above analysis, the preponderance of evidence
supports pathway 2 as the more likely mechanism by which
the ruthenium catalysts 2-6 operate. We have depicted what
we propose to be the complete, detailed mechanism in Scheme
7. Based on this, the relationship between the ligand sphere
and catalyst activity can be rationalized.
Explanation of Ligand Effects Upon Catalyst Activity. To
explain the ligand effects upon catalyst activity, we will refer
to the “dissociative” (top) pathway shown in Scheme 7, and eq
7, which is the part of eq 4 that corresponds to the “dissociative”
pathway. Because the “dissociative” pathway accounts for
approximately 95% of the catalyst turnover, we will ignore the
“associative” pathway in our discussion of relative catalyst
activities.
halogens, we expect that larger halogens such as iodide would
disfavor olefin binding due to steric crowding in the halogen-
olefin-carbene plane, resulting in a decrease in K1. By the
same reasoning as above, we again predict that catalyst activity
will decrease down the series from Cl to I.
(b) Effect of Phosphines. The catalyst activities increase
as both the cone angle and the electron donating ability of the
phosphines increase. Although these effects are exactly contrary
to those observed for the halogens, the proposed mechanism
provides a reasonable explanation.
As the cone angle of the phosphine increases, it should be
obvious that phosphine dissociation from the sterically crowded
18-electron olefin complex I1 should be favored, corresponding
to an increase in K2. Although this steric crowding is expected
to destabilize I1, and therefore decrease K1, the relief of steric
crowding should stabilize the monophosphine olefin complex
I2 even furthersi.e. bulkier phosphines will favor the overall
equilibrium for olefin binding and phosphine dissociation,
represented by the product K1K2. The product K1K2, and hence
the rate, is therefore expected to increase as the phosphine cone
angle increases.
K1K2
d[diene]
-
) k3
[6][diene]
(7)
(
)
dt
[PCy3]
In deriving eqs 4 and 7, we have assumed that formation of
the 14-electron metallacyclobutane intermediate is the rate
determining step. All the steps following metallacycle formation
should be faster, including intramolecular reaction with the
second olefin of the diene to make the cyclic product and
regenerate the catalyst. (Independent investigations have con-
firmed that alkyl-substituted carbenes are more reactive than
the unsubstituted methylidene.)15 From (7), it is easily seen
that the rate, and therefore catalyst activity, is directly propor-
tional to three constants: K1, the equilibrium constant for olefin
binding; K2, the equilibrium constant for phosphine dissociation;
and k3, the rate constant for metallacylobutane formation from
the monophosphine olefin complex I2.
As the electron donating ability of the phosphines increases,
the relative trans influence also increases.24 Because of this,
we expect more electron donating phosphines to favor
dissociationsby stabilizing the vacant coordination site trans
to them in the 16-electron monophosphine olefin complex I2,
and especially in the 14-electron metallacyclobutane intermedi-
ate. This is analagous to the trans effect observed in dissociative
ligand substitutions at octahedral metal centerssthe rate of
substitution increases as the trans influence of the appropriate
ligand increases, due to weakening of the bond in the ground
state and/or stabilization of the five-coordinate intermediate.24,25
In addition, more electron donating phosphines may facilitate
the two-electron oxidation of the metal center to form the
metallacyclobutane. This will increase both K2 and k3, so we
expect the rate to increase substantially as the electron donating
ability of the phosphines is increased. The magnitude of these
two effects is manifested in the astonishing difference in activity
between catalysts 1 and 2a.
(a) Effect of Halogens. The catalyst activities decrease as
the halogens are changed from Cl to Br to I (Table 1). Because
the olefin binds trans to one of the halogens, their trans
influencing abilities will have a substantial effect upon the
relative ruthenium-olefin bond strengths. The trans influence
of the halogens increases down the series from Cl to Br to I,23
so the olefin should be bound tighest for Cl and weakest for I.
Therefore, K1 will decrease down the series from Cl to I, and
we expect a corresponding decrease in the rate. Since cis effects
are generally weak, phosphine dissociation should not be
affected by the change in halogens, and K2 should remain
relatively unchanged. We believe that k3 also remains relatively
unaffected, so an overall decrease in rate, and therefore catalyst
activity, will result when changing from Cl to Br to I.
Rate Enhancement by CuCl. The final and perhaps most
important aspect of understanding the mechanism of olefin
metathesis by these catalysts is the ability to rationally tune
catalyst activity in a desired fashion. For example, the rate of
metathesis by the ruthenium catalysts is slow compared to that
The size of the halogens should also affect the equilibrium
for olefin binding. Because the olefin binds cis to one of the
(24) Garlatti, R. D.; Tauzher, G. Inorg. Chim. Acta 1988, 142, 263-
267.
(23) For a brief discussion of the ligand trans influences and the kinetic
trans effect, see for example: Collman, J. P.; Hegedus, L. S.; Norton, J.
R.; Finke, R. G. Principles and Applications of Organotransition Metal
Chemistry; University Science: Mill Valley, 1987; pp 241-244 and
references therein.
(25) (a) Seibles, L.; Deutsch, E. Inorg. Chem. 1977, 16, 2273-2278.
(b) Trogler, W. C.; Stewart, R. C.; Marzilli, L. G. J. Am. Chem. Soc. 1974,
96, 3697-3699. (c) Tauzher, G.; Dreos, R.; Costa, G.; Green, M. J. Chem.
Soc., Chem. Commun. 1973, 413-414. (d) Crumbliss, A. L.; Wilmarth,
W. K. J. Am. Chem. Soc. 1970, 92, 2593-2594.

Well-Defined Ruthenium Olefin Metathesis Catalysts
J. Am. Chem. Soc., Vol. 119, No. 17, 1997 3895
chloride ligands. Such bimetallic catalysts with bridging
chlorides have been isolated, and are under current investigation.
Conclusions
In exploring the reactivity of many analogous ruthenium
catalysts 2-5, some surprising features were uncovered. It was
found that the ligand effects upon catalyst activity were exactly
opposite for the phosphines and the halogens. Larger and more
electron donating phosphines produced more active catalysts,
while smaller and more electron withdrawing halogens likewise
produced more active catalysts. These apparently contradictory
effects could not be easily explained on the basis of pure steric
and/or electronic arguments, prompting further investigation into
the mechanism of olefin metathesis by these catalysts.
Mechanistic studies allowed us to formulate an empirical rate
equation for the ring-closing of diethyl diallylmalonate by
catalyst 6. The major pathway was found to involve phosphine
dissociation from the metal center, such that a minor associative
pathway in which both phosphines remain bound can be
considered to operate only at higher phosphine concentrationssi.e.
when excess phosphine is added to the reaction mixture. We
were surprised by the relative importance of the “dissociative”
pathway, as it suggests that there is a 14-electron metallacy-
clobutane intermediatesan electron defficient intermediate for
a late transition metal such as ruthenium.
We have concluded that the mechanism in Scheme 7 is in
agreement with all of the kinetic evidence, as well as additional
evidence found in the literature. The olefin binding site is
presumed to be cis to the carbene, based upon analagous
compounds characterized in the literature employing carbon
monoxide in place of the carbene moiety, in addition to the
geometric constraints required by the ring-closing metathesis
reaction. Furthermore, metallacycle formation and breakdown
is thought to occur in a symmetric fashion. If it were to occur
in an asymmetric fashion, the mechanism would involve two
competing “dissociative” pathways, as well as complex ligand
rearrangements about the metal center. One very interesting
implication of this mechanism is that in order for metathesis to
occur, the carbene must rotate 90° sometime during or after
the olefin coordination or phosphine dissociation steps. By
using this mechanism as a guide, a self-consistent picture
emerges in which the ligand effects can be explained in terms
of well-established principles, and a full understanding of the
precise nature of metathesis in these systems is gained.
Regarding the “dissociative” pathway, the ligand effects could
be rationalized in terms of well-studied systems. Bulkier
phosphines favor phosphine dissociation by relief of steric
crowding around the ruthenium center. Likewise, the greater
trans influence of more electron donating phosphines favors
phosphine dissociation by stabilizing the 16-electron mono-
phosphine olefin complex, and more importantly the electron
defficient 14-electron metallacyclobutane. Halogens, on the
other hand, find their primary effects in their relative trans
influencing abilities. Because the olefin binds trans to one of
the halogen ligands, more electron withdrawing halogens with
a smaller trans influence will stablilize the ruthenium-olefin
complex. Because the olefin binds cis to the other halogen
ligand, larger halogens should destabilize the olefin complex
due to unfavorable steric crowding.
Figure 6. Plot of diene concentrations vs time for catalyst 2c without
(filled circles) and with (open squares) 10 equivalents of CuCl added
to the reaction. The reactions were carried out with [diene]0 ) 0.2 M
and [catalyst (2c)] ) 0.01 M in CD2Cl2 at 20 °C.
of the well-known molybdenum and tungsten alkylidenes,26 and
it would be desirable to tailor the system in such a way as to
increase catalyst activity.
With these goals in mind, we wish to report a substantial
increase in the rate of olefin metathesis by ruthenium catalysts
upon the addition of CuCl. We reasoned that if a “dissociative”
pathway accounts for 95% of metathesis in our systems, the
equilibrium for phosphine dissociation could be driven by the
addition of CuCl, which is known to react with phosphines to
make a marginally soluble, ill-defined complex.27 We found
that the addition of 10 equiv of CuCl to a ring-closing reaction
effects a rate-enhancement in the case of catalysts 2-5. In some
cases, however, the catalyst dies before the reaction goes to
>95% completion. The most dramatic effect was observed
when CuCl was added to a reaction employing the catalyst
(PCy3)2I2RudCH-CHdCPh2 (2c), shown in Figure 6. In the
absence of copper chloride, this catalyst was relatively slow
(Table 1). Upon addition of CuCl, however, the activity of this
catalyst rivals that of the most active catalyst 2a, (PCy3)2Cl2-
RudCH-CHdCPh2san increase by a factor of 20. Further-
more, the addition of CuCl to previously inactive catalysts such
as 5c initiates product formation.28
A control experiment employing CuCl2 as the phosphine
scavenger was performed, since it is possible that the increase
in catalyst activity is due to a redox process involving trace
amounts of Cu(II). Because CuCl2 reacts with phosphines in a
manner similar to CuCl, we expected either a further increase
in catalyst activity if a redox process were operating or a similar
increase in catalyst activity if the copper were scavenging the
phosphine. We obtained the same results with CuCl2, lending
support to the belief that it is the phosphine being taken up by
copper, and not a redox process involving Cu(II), that is
responsible for the observed rate-enhancement.
The nature of the species generated by adding CuCl to a
ruthenium catalyst is unknown. It is possible that a highly
reactive, 14-electron monophosphine compound is simply
generated, but some recent results suggest that the CuCl‚PR3
adduct may actually chelate the ruthenium center via bridging
(26) For a discussion of molybdenum and tungsten catalyst activities,
see: Progress in Inorganic Chemistry; Lippard, S. J., Ed.; Wiley: New
York, 1991; Vol. 39, and references therein.
(27) ComprehensiVe Coordination Chemistry; Wilkinson, G., Ed.; Per-
gamon: New York, 1987; Vol. 5.
(28) It is likely that there is some degree of halogen exchange when
catalysts containing Br or I are used. However, complexation of free
phosphine is undoubtedly the predominant effect, since Cl-containing
compounds such as 2c are activated by addition of CuCl to the extent that
the ring-closing reaction is too fast to study under the standard reaction
conditions.
Finally, reactions were carried out in the presence of CuCl,
which is known to complex phosphines. By adding CuCl to
the reaction mixture, we hoped to increase the amount of
monophosphine species present in the system, and thereby
increase the rate of metathesis. Dramatic increases in catalyst
activity resulted. The rates of metathesis by slower catalysts

3896 J. Am. Chem. Soc., Vol. 119, No. 17, 1997
Dias et al.
1
(d of t, RudC, JCH ) 149 Hz). FAB-HRMS: m/z calcd for C51H66-
Cl2P2Ru (M+) 912.3060, found 912.3023.
in the presence of CuCl rival that of the fastest catalysts studied,
and previously inactive catalysts could now perform the ring-
closing reaction. It may be the case that a bimetallic copper-
ruthenium species is being formed in these reactions, analagous
to other bimetallic catalysts that are currently under investiga-
tion.
Synthesis of (PiPr3)2Cl2Ru(CHCHCPh2) (4a). The procedure for
the synthesis of catalyst 2a was followed with use of triisopropylphos-
phine, and the product was obtained as a red solid. 1H NMR: δ 19.19
(d, 1 H, RudCH, JHH ) 11 Hz), 8.79 (d, 1 H, CHdCPh2, JHH ) 11
Hz). 31P NMR: δ 46.71 (s). 13C NMR: δ 290.7 (d of t, RudC, 1JCH
) 152 Hz). FAB-HRMS: m/z calcd for C33H54Cl2P2Ru (M+) 684.2121,
found 684.2126.
3
3
Experimental Section
Synthesis of (PiPr2Ph)2Cl2Ru(CHCHCPh2) (5a). The procedure
for the synthesis of 3a was followed with use of diisopropylphen-
ylphosphine, and the product was obtained as a reddish-brown solid.
All manipulations were performed using standard Schlenk tech-
niques. Argon was purified by passage through columns of BASF R3-
11 catalyst (Chemalog) and 4 Å molecular sieves (Linde). Solid
organometallic compounds were transferred and stored in a nitrogen-
filled Vacuum Atmospheres drybox. All 1H, 13C, and 31P NMR spectra
were recorded in CD2Cl2 on a JEOL JNM-GX400 (399.80 MHz H).
All NMR tubes and septa used were dried under vacuum and stored in
a drybox.
3
1H NMR: δ 19.12 (d, 1 H, RudCH, JHH ) 11 Hz), 8.95 (d, 1 H,
CHdCPh2, 3JHH ) 11 Hz). 31P NMR: δ 55.96 (s). 13C NMR: δ 290.5
1
1
(d of t, RudC, JCH ) 153 Hz). FAB-HRMS: m/z calcd for C39H50-
Cl2P2Ru (M+) 752.1808, found 752.1840.
Synthesis of (PCy3
) Br Ru(CHCHCPh ) (2b). Inside the drybox,
2
2
2
200 mg of LiBr were weighed into a small Schlenk flask equipped
with a stirbar and dissolved in 1-2 mL of THF. (PCy3)2Cl2Ru-
(CHCHCPh2) (2a) (100 mg) was then added, followed by 3-4 mL of
CH2Cl2 . The flask was capped with a rubber septum, removed from
the drybox, and stirred for 3-4 h on the Schlenk line under argon at
room temperature. The solvents were removed in Vacuo, and the
product was extracted with 3 × 3 mL portions of benzene. The
supernatant was collected by cannula filtration into a small Schlenk
flask, and the benzene was removed by a freeze-drying procedure in
which the flask was placed in a bath of liquid nitrogen to freeze the
solution, evacuated, and placed in an ice-water bath. The frozen
benzene was sublimed at 0 °C, usually overnight, and the reddish-
brown solid was collected and stored inside the drybox. Freeze drying
the product in this manner reduces static such that the solid is easily
collected. Yields are typically between 90 and 100%. 1H NMR: δ
18.88 (d, 1 H, RudCH, 3JHH ) 11 Hz), 8.79 (d, 1 H, CHdCPh2, 3JHH
) 11 Hz). 31P NMR: δ 37.82 (s). 13C NMR: δ 291.7 (d of t, RudC,
1JCH ) 152 Hz). Anal. Calcd for C51H78Br2P2Ru: C, 60.41; H, 7.75.
Found: C, 60.66; H, 7.70.
All solvents were vacuum transferred from sodium benzophenone
ketyl, except for chlorinated solvents (including CD2Cl2) which were
vacuum transferred from CaH2. All solvents were degassed by several
freeze-pump-thaw cycles.
Diethyl diallylmalonate obtained from Aldrich was purified by
repeated passage through activated alumina, until all discoloration was
gone. The liquid was placed in a Kontes flask with a Teflon stopcock
and degassed by several freeze-pump-thaw cycles. The pure,
degassed reagent was stored inside the drybox.
Lithium bromide and sodium iodide were dehydrated by placing the
solid inside of a large Schlenk flask and heating at 150-160 °C under
vacuum overnight. 1H NMR spectra of the salts were obtained in d8-
THF to verify that all excess water had been removed. However, it
should be mentioned that water does not harm the reactions or cause
catalyst decomposition under reaction conditions.
(PPh3)2Cl2Ru(CHCHCPh2) was synthesized from Ru(PPh3)4Cl2 ac-
cording to published procedures.1 The bromine and iodine containing
catalysts (2b-5b, 2c-5c) were synthesized from their chlorine
containing analogs (2a-5a) by Finklestein type chemistry, described
below. All catalysts synthesized below can be used without further
purification. If necessary, they can be recrystallized from CH2Cl2/
pentane at low temperature.
Mass spectral analysis was performed at the Southern California
Mass Spectrometry Facility at the University of California at Riverside.
Elemental analyses were performed by Quantitative Technologies Inc.
Synthesis of (PCy3)2Cl2Ru(CHCHCPh2) (2a). Inside the drybox,
2.35 g (2.64 mmol) of (PPh3)2Cl2Ru(CHCHCPh2) (1) were weighed
into a 150-mL Schlenk flask equipped with stirbar and dissolved in 70
mL of CH2Cl2. Tricyclohexylphosphine (2.50 g, 5.35 mmol) were
added to the green solution. The flask was capped with a rubber
septum, removed from the drybox, placed under argon on the Schlenk
line, and stirred overnight at room temperature, during which time the
solution changed from green to deep red. The solvent was removed
in Vacuo, and the product was washed liberally with pentane to remove
excess phosphines. A small amount of benzene may also be added to
help break up the solid. The solid is isolated by cannula filtration,
and the washing procedure is repeated. After three or four washes,
the remaining red solid is dried in vacuo.
Synthesis of (PCy2Ph)2Br2Ru(CHCHCPh2) (3b). The procedure
for the synthesis of catalyst 2b was followed with use of 100 mg of
(PCy2Ph)2Cl2Ru(CHCHCPh2) (3a), and the product was obtained as a
reddish-brown solid. 1H NMR: δ 18.93 (d, 1 H, RudCH, 3JHH ) 11
Hz), 8.91 (d, 1 H, CHdCPh2, 3JHH ) 11 Hz). 31P NMR: δ 44.81 (s).
1
13C NMR: δ 293.2 (d of t, RudC, JCH ) 149 Hz). FAB-HRMS:
m/z calcd for C51H66Br2P2Ru (M+) 1002.2030, found 1002.2088.
Synthesis of (PiPr3)2Br2Ru(CHCHCPh2) (4b). The procedure for
the synthesis of catalyst 2b was followed with use of 100 mg of (P-
iPr3)2Cl2Ru(CHCHCPh2) (4a), and the product was obtained as a
reddish-brown solid. 1H NMR: δ 19.03 (d, 1 H, RudCH, 3JHH ) 11
Hz), 8.88 (d, 1 H, CHdCPh2, 3JHH ) 11 Hz). 31P NMR: δ 46.21 (s).
1
13C NMR: δ 293.3 (d of t, RudC, JCH ) 152 Hz). FAB-HRMS:
m/z calcd for C33H54Br2P2Ru (M+) 774.1091, found 774.1078.
Synthesis of (PiPr2Ph)2Br2Ru(CHCHCPh2) (5b). The procedure
for the synthesis of catalyst 2b was followed using 100 mg of (PiPr2-
Ph)2Cl2Ru(CHCHCPh2 (5a), and the product was obtained as a reddish-
3
brown solid. 1H NMR: δ 18.94 (d, 1 H, RudCH, JHH ) 11 Hz),
9.01 (d, 1 H, CHdCPh2, 3JHH ) 11 Hz). 31P NMR: δ 54.59 (s). 13C
1
NMR: δ 293.1 (d of t, RudC, JCH ) 147 Hz).
It can easily be determined if there is any remaining starting material
1
1 by H or 31P NMR spectroscopy. If there is any starting material
Synthesis of (PCy3)2I2Ru(CHCHCPh2) (2c). Inside the drybox,
200 mg of NaI were weighed into a small Schlenk flask equipped with
a stirbar and suspended in 1-2 mL of THF. (PCy3)2Cl2Ru(CHCHCPh2)
(2a) (100 mg) was then added, followed by 3-4 mL of CH2Cl2 . The
flask was capped with a rubber septum, removed from the drybox, and
stirred for 4-5 h on the Schlenk line under argon at room temperature.
The solvents were removed in Vacuo, and the product was extracted
with 3 × 3 mL portions of benzene. The supernatant was collected
by cannula filtration into a small Schlenk flask, and the benzene was
removed by the freeze-drying procedure described above for 2b. The
greenish-brown solid was collected and stored inside the drybox. Yields
are typically between 90 and 100%. (Note: It has been found that if
this reaction is stirred for too long, some catalyst decomposition can
occur as evidenced by the appearance of the carbene coupling product
Ph2CdCH-CHdCH-CHdCPh2 in the 1H NMR spectrum. This can
be removed by washing the product with pentane, although care should
remaining, the above procedure can be repeated (using as much PCy3
as deemed necessary) until it is all converted to product. The desired
product (PCy3)2Cl2Ru(CHCHCPh2) (2.08 g, 85% yield) was collected
and stored inside the drybox. 1H NMR: δ 19.07 (d, 1 H, RudCH,
3
3JHH ) 11 Hz), 8.68 (d, 1 H, CHdCPh2, JHH ) 11 Hz). 31P NMR:
1
δ 37.59 (s). 13C NMR: δ 289.3 (d of t, RudC, JCH ) 150 Hz).
Synthesis of (PCy2Ph)2Cl2Ru(CHCHCPh2) (3a). The procedure
for the synthesis of catalyst 2a outlined above was followed, with the
exception that a larger excess of dicyclohexylphenylphosphine was used
(at least 2.5 equiv) and the procedure had to be repeated three times to
get complete conversion, due to the poorer equilibrium for phosphine
exchange with PPh3. The product was obtained as a reddish-brown
solid. The yields in these cases were typically lower (ca. 60-75%).
3
1H NMR: δ 19.16 (d, 1 H, RudCH, JHH ) 11 Hz), 8.84 (d, 1 H,
CHdCPh2, 3JHH ) 11 Hz). 31P NMR: δ 45.48 (s). 13C NMR: δ 290.9

Well-Defined Ruthenium Olefin Metathesis Catalysts
J. Am. Chem. Soc., Vol. 119, No. 17, 1997 3897
1
be taken as the catalyst is partially soluble in pentane.) H NMR: δ
A phosphine stock solution was made immediately prior to use by
dissolving 14.0 mg of tricyclohexylphosphine (0.05 mmol) in 0.5 mL
of CD2Cl2. The four reactions were set up simultaneously by adding
30.4 µL of diene/mesitylene solution to either 25, 50, 75, or 100 µL of
phosphine solution, and CD2Cl2 was added to bring the total volume
to 400 µL. Catalyst stock solution (100 µL) was finally added. The
NMR tubes were capped, removed from the drybox, wrapped well with
Parafilm, and placed in an oil bath preheated to 30 °C. The final
concentrations areas follows: diene, 0.20 M (10 equiv); catalyst, 0.020
M (1 equiv); and phosphine, 0.005, 0.010, 0.015, or 0.020 M.
18.54 (d, 1 H, RudCH, 3JHH ) 11 Hz), 8.81 (d, 1 H, CHdCPh2, 3JHH
) 11 Hz). 31P NMR: δ 38.51 (s). 13C NMR: δ 297.9 (d of t, RudC,
1JCH ) 150 Hz).
Synthesis of (PCy2Ph)2I2Ru(CHCHCPh2) (3c). The procedure for
the synthesis of catalyst 2c was followed with use of 100 mg of (PCy2-
Ph)2Cl2Ru(CHCHCPh2) (3a), and the product was obtained as a
greenish-brown solid. 1H NMR: δ 18.52 (d, 1 H, RudCH, 3JHH ) 11
Hz), 8.87 (d, 1 H, CHdCPh2, 3JHH ) 11 Hz). 31P NMR: δ 42.78 (s).
13C NMR: δ 299.6 (d of t, RudC, JCH ) 149 Hz). FAB-HRMS:
1
m/z calcd for C51H66I2P2Ru (M+) 1096.1773, found 1096.1817.
Synthesis of (PiPr3)2I2Ru(CHCHCPh2) (4c). The procedure for
the synthesis of catalyst 2c was followed with use of 100 mg of (P-
iPr3)2Cl2Ru(CHCHCPh2) (4a), and the product was obtained as a
greenish-brown solid. 1H NMR: δ 18.62 (d, 1 H, RudCH, 3JHH ) 11
Hz), 8.84 (d, 1 H, CHdCPh2, 3JHH ) 11 Hz). 31P NMR: δ 45.74 (s).
(3) Catalyst-Dependence Experiments (No Phosphine). The
diene/mesitylene and catalyst 6 stock solutions were made exactly as
above in (2) immediately prior to use. The catalyst stock solution was
stored in the drybox freezer in a vial with a Teflon-lined screw cap,
and the diene/mesitylene solution was stored in a vial with a Teflon-
backed septum screw cap and removed from the drybox. The reactions
were performed sequentially as follows. Inside the drybox, 25, 50,
75, or 100 µL of catalyst stock solution were added to the NMR tube,
and CD2Cl2 was added to bring the volume to 400 µL. The NMR
tube was sealed with a Teflon-backed septum screw cap, removed from
the drybox, and wrapped well with Parafilm. The diene/mesitylene
solution was added via gas-tight syringe immediately before the sample
was dropped in the NMR probe, with the temperature preset to 25 °C.
The final concentrations are as follows: diene, 0.20 M; and catalyst,
0.005, 0.010, 0.015, or 0.020 M.
1
13C NMR: δ 299.8 (d of t, RudC, JCH ) 152 Hz).
Synthesis of (PiPr2Ph)2I2Ru(CHCHCPh2) (5c). The procedure for
the synthesis of catalyst 2c was followed with use of 100 mg of (P-
iPr2Ph)2Cl2Ru(CHCHCPh2 (5a), and the product was obtained as a
greenish-brown solid. 1H NMR: δ 18.52 (d, 1 H, RudCH, 3JHH ) 11
Hz), 8.92 (d, 1 H, CHdCPh2, 3JHH ) 11 Hz). 31P NMR: δ 50.62 (s).
1
13C NMR: δ 300.1 (d of t, RudC, JCH ) 153 Hz).
Ring-Closing Metathesis of Diethyl Diallylmalonate. Reactions
for kinetic studies were performed inside the drybox in screw-cap NMR
tubes available from Wilmad, sealed with Teflon lined screw caps.
Product formation and diene disappearance were monitored by integrat-
ing the allylic methylene peaks, using mesitylene as an internal standard.
(1) Relative Catalyst Activity Experiments. A 5X stock solution
of the diene was made by diluting 1.21 mL of diethyl diallylmalonate
(5 mmol) with 3.79 mL of CD2Cl2, with 3.86 µl of mesitylene (5.55
µmol) added as an internal standard, and stored at -40 °C inside the
drybox freezer. All reactions were performed inside the drybox by
weighing 0.005 mmol of catalyst into the screw-cap NMR tube and
dissolving the solid in 400 µL of CD2Cl2 added by gas-tight syringe.
A 5X diene stock solution (100 µL) was added by gas-tight syringe
and the tube was capped, shaken, removed from the drybox, and
wrapped with Parafilm. The resulting concentration of catalyst is 0.010
M (1 equiv) and diene is 0.20 M (20 equiv).
(4) Catalyst-Dependence Experiments (Excess Phosphine). The
diene/mesitylene, catalyst 6, and tricyclohexylphosphine stock solutions
were made exactly as above in (2) immediately prior to use. The
reactions were set up simultaneously by adding 25 µL of phosphine
solution to 25, 50, 75, and 100 µL of catalyst solution. CD2Cl2 was
added to bring the volume to 470 µL, and 30.4 µL of diene/mesitylene
solution were finally added. The NMR tubes were capped, removed
from the drybox, wrapped well with Parafilm, and placed in an oil
bath preheated to 30 °C. The final concentrations are as follows: diene,
0.20 M; phosphine, 0.005 M; and catalyst, 0.005, 0.010, 0.015, or 0.020
M.
Acknowledgment. Support has been provided by the
National Science Foundation, Rohm and Haas, and the National
Institute of Health.
(2) Phosphine-Dependence Experiments. Diethyl diallylmalonate
was placed in a vial with Teflon lined cap, and 37.2 µL of mesitylene
was added. A 5X catalyst stock solution was made immediately prior
to use by dissolving 37.3 mg (0.05 mmol) of 6 in 0.5 mL of CD2Cl2.
JA963136Z