Rate acceleration in olefin metathesis through a fluorine-ruthenium interaction

Article abstract of DOI:10.1021/ja064091x

The synthesis, structure, and performance of new ruthenium-based olefin metathesis catalysts, featuring fluorinated NHC ligands are presented. The introduction of halogen atoms into the N-heterocyclic carbene ligand profoundly alters the catalytic activit

Full text of DOI:10.1021/ja064091x

Published on Web 08/22/2006
Rate Acceleration in Olefin Metathesis through a Fluorine-Ruthenium
Interaction
Tobias Ritter, Michael W. Day, and Robert H. Grubbs*
The Arnold and Mabel Beckman Laboratory of Chemical Synthesis, DiVision of Chemistry and
Chemical Engineering, California Institute of Technology, Pasadena, California 91125
Received June 21, 2006; E-mail: rhg@caltech.edu
Ruthenium complexes 1,1 2,2 and 33 (Chart 1) are common
Chart 1. Benchmark Olefin Metathesis Catalysts
catalysts for a variety of different olefin metathesis transformations.4
In this communication, we describe the synthesis, structure, and
performance of new ruthenium-based olefin metathesis catalysts
featuring fluorinated N-heterocyclic carbene ligands. The introduc-
tion of ortho halogen atoms profoundly alters the catalytic meta-
thesis performance. Structural investigation suggested that an
Scheme 1
uncommon fluorine-ruthenium interaction is responsible for the
significant rate enhancement. This is the first example of such an
interaction resulting in increased catalytic activity.
The synthesis of the fluorinated complexes 5 and 6 commenced
with formation of the dihydroimidazolium carbene precursor 4,
accomplished in three steps from commercially available 2,6-
difluoroaniline (Scheme 1, eq 1). Deprotonation of 4 in the presence
of ruthenium source 1 did not afford the desired complex 5,
presumably because of a short lifetime of the intermediate carbene.5
The use of a carbene transfer agent, however, generated through
the reaction of silver(I) oxide with 4, furnished 5 in 60% yield
after transmetalation (Scheme 1, eq 2, yield based on 1).6 The
phosphine-free analogue 6 was prepared from 5 by olefin metathesis
with o-isopropoxy-â-methylstyrene and obtained in 75% yield after
crystallization (Scheme 1, eq 3). Compounds 5 and 6 are air stable
complexes in the solid state and can be purified by chromatography
on silica gel.
Figure 1 illustrates how catalysts 2, 3, 5, and 6 perform in the
ring-closing olefin metathesis of diethyldiallyl malonate under
standard reaction conditions.7 The phosphine-containing catalyst 5
heterocycle. This arrangement is nicely illustrated by the top view
shows a significantly increased reaction rate compared to the two
standard second-generation catalysts 2 and 3, which behave virtually
identically. Interestingly, however, the phosphine-free analogue 6
catalyzes the reaction at a slower rate than 2 and 3. The dissimilar
behavior of 5 and 6 is intriguing, given the fact that the relative
difference (exchange of a phosphine for a chelating ether ligand)
between the two pairs of catalystss2 and 3 versus 5 and 6s is the
same. One would therefore expect a change of activity in identical
directions for 5 and 6, not in opposing directions. Recent theoretical
investigations suggested the σ-withdrawing capacity of the elec-
tronegative fluorine atoms would lead to a decrease in catalyst
activity,8 but this is in contrast to what we observe with 5. Moreover,
if inductive effects were the single source of the reactivity
enhancement in 5, it should also be observed for 6, but this is not
the case.
To gain further insight into the unexpected difference in
efficiency, crystals of 5 and 6 were grown (Figure 2). Crystal-
lographic analysis revealed two significant differences in the
structures. Complex 5 displays a distorted square pyramidal
geometry similar to those observed with most other related
complexes such as 2 and 3. In particular, the plane of the NHC
heterocycle bisects the Cl-Ru-Cl angle and the aryl substituents
on the nitrogen atoms are perpendicular to the plane of the
of the catalysts given below the crystal structures in Figure 2. The
phosphine-free complex 6, however, shows a rotation of one of
the two fluorinated aryls by 26° around the N-C(aryl) bond and a
rotation of the entire NHC ligand around the Ru-C bond by 40°
compared to 5. These two structural differences in 6 position one
of the fluorine atoms in close proximity to the ruthenium atom,
and the complex might therefore be more correctly represented with
a hexacoordinate ruthenium center in a distorted octahedral environ-
ment with an additional fluorine-ruthenium interaction. Although
relatively weak in complex 6 (Ru-F distance ) 3.2 Å) we have
observed much stronger ruthenium-fluorine interactions (Ru-F
distance ) 2.5 Å) in related ruthenium benzylidene complexes, in
which both phosphine ligands of 2 have been replaced by the
fluorinated NHC ligands (for X-ray structures see Supporting
Information (SI)). The Ru-F distances reported here are short
compared to similar interactions in other complexes.9 The absence
of such an interaction in the solid state in 5 could be explained by
steric congestion of the fluoroaryl substituent with the large
tricyclohexylphosphine ligand upon fluorine coordination. The
isopropoxy group in 6 is significantly smaller, leaving space for
the fluorine atom to coordinate.
While ruthenium-fluorine interactions have been observed, they
are rare and have, to the best of our knowledge, never been used
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10.1021/ja064091x CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
originate from a steric interaction of the fluoroaryl with the
phosphine ligand upon fluorine coordination.
Chlorine atoms generally coordinate better to ruthenium than
fluorine atoms.10a Therefore, analogues of 5 and 6, in which fluorine
has been replaced with chlorine, were prepared as well because
the favorable halogen-ruthenium interaction in the chlorine case
should lower the free energy of activation even further. As expected,
the crystal structure of the chloro analogue of 6 shows a strong
ruthenium-chlorine interaction (see SI). The chloro-substituted
catalysts, however, were less stable than the corresponding fluoro-
counterparts and are hence not as suitable for catalysis. A low
quality crystal structure of a ruthenium(III) decomposition product
in which both of the aryl substituents of the NHC are C-bound to
ruthenium to form a tridentate ligand is in accord with Caryl-Cl
activation as a decomposition pathway.
In conclusion we have reported a ruthenium complex bearing a
fluorinated NHC ligand. Its increased efficiency is attributed to an
unusual fluorine-ruthenium interaction, which reduces the activa-
tion energy of rate-limiting phosphine dissociation and catalyst
initiation. This is the first example of the use of such an interaction
to enhance catalytic activity. We plan to use the beneficial effect
of this interaction in related catalyst systems for olefin metathesis.
Figure 1. Relative activities of 2, 3, 5, and 6 in RCM.
Acknowledgment. Larry M. Henling is acknowledged for X-ray
crystallographic analysis. We thank Donde Anderson and Prof.
Daniel J. O’Leary for helpful discussions and NMR studies. T.R.
thanks the German Academic Exchange Service (DAAD) for a
postdoctoral fellowship.
Supporting Information Available: Experimental procedures and
characterization for 4, 5, and 6. Characterization for the chloro-
substituted NHC-derived catalysts as well as two additional crystal
structures illustrating strong ruthenium-fluorine interactions. This
material is available free of charge via the Internet at http://pubs.acs.org.
References
(1) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118,
100-110.
Figure 2. Structures of 5 and 6 with top-view of catalysts.
(2) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953-
Scheme 2
956.
(3) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am. Chem.
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(4) Grubbs, R. H., Ed. Handbook of Metathesis; Wiley-VCH: Weinheim,
Germany, 2003.
(5) The in situ deprotonation of dihydroimidazolium salts in the presence of
ruthenium benzylidenes is a common procedure for the preparation of
NHC-containing ruthenium benzylidenes, see Trnka, T. M.; Morgan, J.
P.; Sanford, M. S.; Wilhelm, T. E.; Scholl, M.; Choi, T.-L.; Ding, S.;
Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2003, 125, 2546-2558.
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Organometallics, 2006, submitted for publication.
(8) Occhipinti, G.; Bjørsvik, H.-R.; Jensen, V. R. J. Am. Chem. Soc. 2006,
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to enhance catalytic activity as we present here.10 We believe that
a fluorine-ruthenium interaction accelerates the rate-limiting step
in catalyst initiation and can explain the increased efficiency of 5.
To probe this hypothesis, we determined the activation parameters
for catalyst initiation, known to be the rate-limiting phosphine
dissociation for 2.11 This was accomplished by reaction of 5 with
butyl vinyl ether, which irreversibly affords a Fischer carbene
(Scheme 2, eq 4). Indeed, the free energy of activation of 5 (∆Gq298
) 20.4 kcal‚mol-1) is ca. 2.6 kcal‚mol-1 lower than for 2 (∆Gq298
(9) A histogram obtained from the Cambridge database (CCDC) showing
ruthenium-fluorine distances is given in the SI.
(10) (a) Kulawiec, R. J.; Crabtree, R. H. Coord. Chem. ReV. 1990, 99, 89-
115. (b) Perera, S. D.; Shaw, B. L. Inorg. Chim. Acta 1995, 228, 127-
131.
(11) (a) Sanford, M. S.; Love, J. A.; Grubbs, R. H. J. Am. Chem. Soc. 2001,
123, 6543-6554.
(12) Love, J. A.; Sanford, M. S.; Day, M. W.; Grubbs, R. H. J. Am. Chem.
Soc. 2003, 125, 10103-10109.
(13) The corresponding activation energies for 3 and 6 are ∆Gq298 ) 20.7
kcal‚mol-1 and ∆Gq298 ) 21.4 kcal‚mol-1, respectively. In this case,
however, conclusions about the rate cannot be made because the rate-
determining step in catalyst initiation has not yet been investigated, but
is likely not oxygen dissociation.
) 23.0 kcal‚mol-1 12
orders of magnitude (Scheme 2, eq 5).13 This enhancement can
) corresponding to a rate increase of roughly 2
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