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Origin of the High Activity of Second-Generation Grubbs Catalysts.

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Homogeneous Catalysis
DOI: 10.1002/anie.200501114
Origin of the High Activity of Second-Generation
Grubbs Catalysts**
Bernd F. Straub*
Dedicated to Professor Rolf Huisgen
on the occasion of his 85th birthday
Alkene metathesis has experienced a breathtaking success in
the last decade. Organic synthetic applications were mainly
facilitated by the development of highly active ruthenium
carbene catalysts.[1] The increase in activity from the catalysts
of the first generation 1[2] to those of the second generation 2
further broadened their significance (Scheme 1).[3] However,
Scheme 1. Alkene metathesis with first-generation and second-generation Grubbs catalysts.
“there is so far no unique explanation for the enhanced
reactivity of the (second-generation( Grubbs-type ruthenium
The influence of the ligands is reflected in Grubbs(
conclusion: “Phosphines, which are larger and more electron
donating, and likewise halogens, which are smaller and more
electron withdrawing, lead to more active catalysts.”[5] Since
N-heterocyclic carbenes (NHC) behave as if they were
extremely electron-donating phosphines, Grubbs( rule was
[*] Dr. B. F. Straub
Department of Chemie and Biochemie
Ludwig-Maximilians-Universit0t M1nchen
Butenandtstrasse 5–13 (Haus F)
81377 M1nchen (Germany)
Fax: (+ 49) 89-2180-77717
[**] This research was supported by a Liebig-Fellowship of the Fonds der
Chemischen Industrie. Generous support by the LMU Munich,
Prof. Dr. Thomas Carell, and Prof. Dr. Herbert Mayr is gratefully
acknowledged. Some of these results were presented in a lecture by
B.F.S. at the Chemiedozententagung in Munich on March 9, 2005.
The model structures 3 b,c to 11 b,c have been included in ref. [9] for
a comparison of the overall activation energies of alkene metathesis
and enyne metathesis.
Supporting information for this article is available on the WWW
under or from the author.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 5974 –5978
confirmed by the development of the second-generation
catalysts. A first assumption concerning the origin of their
high activity was based on the higher electronic transinfluence of NHC ligands compared to phosphines. An
expected, more favored phosphine dissociation would result
in higher concentrations of the active 14 valence-electron
complexes. These complexes undergo [2+2] cycloaddition
reactions with alkene substrates according to the Chauvin
mechanism.[6] However, NMR spectroscopy studies by Sanford and Grubbs demonstrated exactly the opposite behavior
for the ease of phosphine dissociation.[7] The least active firstgeneration iodide complex has the lowest barrier for phosphine dissociation, while the most active second-generation
complex has the highest barrier (Scheme 2).
conformer is active for an immediate rearrangement into a
Based on this almost trivial consideration, two findings by
Hansen and Hofmann are decisive for a deeper understanding
of the electronic origin of catalyst activity:[11] 1) “The
orientation of the carbene moiety allows for optimal backbonding from the metal fragments [of both Grubbs complexes
and Hofmann cis-diphosphine ruthenium complexes] to the
carbene acceptor orbital”. 2) The ruthenium–ligand unit (PRu-P, C-Ru-P, Cl-Ru-Cl, or Cl-Ru-P) “that interacts with the
p-acceptor orbital of the methylene carbon is more strongly
bent” than the unit which has an orthogonal orientation to the
carbene p orbital.
The formally ideal angles at the ruthenium center can be
derived from the classification of Grubbs carbene complexes
as 16 valence-electron d6-ML5 species. As the Cl-Ru-Cl angle
decreases from 1808, the nonbonding interaction between
occupied chloride orbitals and an occupied ruthenium
d orbital becomes increasingly antibonding (Figure 1, first
Scheme 2. Gibbs free activation energies for phosphine dissociation
and the efficiency of alkene metathesis according to Grubbs et al.[7]
A more efficient partitioning between the coordination of
the phosphine ligand and the alkene substrate by the
14 valence-electron ruthenium carbene complexes has been
proposed to account for the high activity of second-generation Grubbs catalysts.[7, 8] We encountered the challenging
question concerning the origin of their outstanding catalytic
activity in our recent quantum-chemical investigation on
enyne metathesis.[9] The key to the understanding of metathesis activities is the existence of active and inactive
conformations in the alkene–carbene intermediates
(Scheme 3). Three conformers are inactive, and only one
Scheme 3. Inactive and active alkene–carbene complexes.
Angew. Chem. Int. Ed. 2005, 44, 5974 –5978
Figure 1. Stabilization of inactive and active carbene orientations:
better backbonding by bending.
row). Intrafragment polarization (mixing of 5p and 4d
orbitals) of the ruthenium fragment minimizes this antibonding orbital interaction and leads to increased backbonding to
the empty p orbital of the carbene ligand. Overall, the
bending of the chloride ligands results in a stronger ruthenium–carbene double bond and thus to a stabilization of the
inactive carbene orientation. More electropositive iodide
ligands are expected to further stabilize the inactive carbene
conformation. An analogous line of argumentation can also
be applied to the decrease of the NHC-Ru-alkene angle
(Figure 1, second row). The alkene ligand is a worse s donor
than a chloride ligand, while the NHC ligand is a better
s donor and leads to a pronounced antibonding orbital
interaction. With increasing antibonding interaction between
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
the NHC sp2 orbital and a ruthenium d orbital, the backbonding of the ruthenium fragment to the carbene acceptor
orbital increases, and the active carbene conformation is
This qualitative hypothesis can be tested by the quantumchemical calculation of relative energies of catalytic intermediates, particularly of the four alkene–carbene conformers.[12] To our knowledge, the relative stabilities of the four
alkene carbene conformations have not yet been investigated
at the same level of theory. The catalytic cycle of alkene
metathesis has already been thoroughly studied theoretically.[4, 13] Thus, the predicted catalytic pathway in Scheme 4
comes as no surprise.
Even in the isomeric simplified Grubbs catalyst models
3 a–c and 4 a–c, the acceptor orbitals of the carbene fragments
prefer a parallel orientation with the stronger s-donor ligands
(2 I or NHC + PMe3 > 2 Cl > 2 PMe3). Intrinsic electronic
barriers for the rotation of the methylene unit between model
structures 3 and 4 are small.[14] Dissociation of PMe3 from the
models 4 a–c leads to the active carbene complexes 5 a–c.
Then, alkene association yields the alkene–carbene complexes 6 a–c,[15] or their conformers 7 a–c, 8 a–c, and 9 a–c.
[2+2] Cycloaddition via the transition states 10 a–c results in
the ruthenacycles 11 a–c.[16] Interestingly, the low relative
Gibbs free energy of the sterically simplified complex 5 c is in
contrast to the high, experimentally observed dissociation
barrier for PCy3 from complex 2. This result indicates that
electronic factors are not responsible for the experimental
anomaly. The overall Gibbs free activation energies in the
transition state models 10 a–c fit nicely to the analogous
experimental catalyst activities.
The computed Gibbs free energies of the alkene carbene
ruthenium complexes show little preference for a particular
alkene rotamer. The rotation of the alkene in the most stable
conformers 6 a–c to give the rotamers 7 a–c is only slightly
disfavored by 2.5 to 5.6 kJ mol 1.[17] The orientation of the
carbene fragment, however, strongly depends on the choice of
the spectator ligands. The rotation of the carbene ligand into
its active orientation in 8 a–c is highly disfavored for the firstgeneration iodide model complex (7 a versus 8 a, DG =
23.3 kJ mol 1), and it is also disfavored for the first-generation
chloride complex (7 b versus 8 b, DG = 21.0 kJ mol 1). In
contrast, carbene rotation is essentially degenerate for the
second-generation chloride complex (7 c versus 8 c, DG =
1.6 kJ mol 1). We were unable to localize minimum structures 9 a–c, because either alkene–carbene structures 6 a–c or
ruthenacycles 11 a–c were obtained after geometry optimization. The transition states 10 a–c, however, have the same
alkene and carbene conformation as the hypothetical structures 9 a–c. Indeed, the transition states 10 a–c with their
active carbene orientation have similar Gibbs free energies to
the active carbene conformers 8 a–c.
Thus, the high reactivity of second-generation Grubbs
catalysts mainly originates in the electronic stabilization of
the active carbene conformation by the N-heterocyclic
carbene ligands (Scheme 5). The computed total electronic
energies for phosphine dissociation of the models 12 a, 12 b,
and 14 to the model complexes 13 a, 13 b, and 15 correlate
nicely with experimental Gibbs free activation energies for
PCy3 loss (Scheme 6 versus Scheme 2).[18] The differences in
phosphine dissociation barriers for first- and second-generation complexes are caused almost exclusively by specific
steric congestion. In this case the repulsive interaction of aand b-hydrogen atoms of the cyclohexyl substituents with the
halogen ligands is of primary importance.
In addition to the electronic benefit of using the strong sdonor NHC ligands, their mesityl substituents also contribute
to a favored active carbene orientation (Scheme 7). The
association of ethene to complex 15 and the subsequent
carbene rotation proceed without enthalpic barrier to the
intermediate 16, which is formally a minimum structure
(NIMAG 0). The additional s-donor ligand ethene as well as
the steric repulsion between the hydrogen of an inactive
carbene ligand and a mesityl substituent enforce an active
carbene orientation. Finally, an essentially barrierless transformation leads to the cycloadduct 17.
Scheme 4. Computed Gibbs free energies of simplified catalyst intermediate models.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 5974 –5978
Scheme 5. Qualitative energy diagram for ruthenium carbene conformers. Sterically demanding phosphine ligands result in a destabilization of
the precatalyst.
In summary, second-generation alkene–carbene complexes are characterized by their high efficiency in partitioning between unproductive alkene dissociation and successful
[2+2] cycloaddition. The electronic and steric stabilization of
the active conformation of the carbene moiety in secondgeneration Grubbs catalysts is responsible for their exceptional alkene metathesis activity.
Received: March 29, 2005
Published online: August 11, 2005
Keywords: carbene ligands · cycloaddition ·
density functional calculations · metathesis · ruthenium
Scheme 6. Computed total energy differences for phosphine dissociation.
Scheme 7. Computed Gibbs free energies of non-simplified catalyst
Angew. Chem. Int. Ed. 2005, 44, 5974 –5978
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[6] J.-L. HGrisson, Y. Chauvin, Makromol. Chem. 1971, 141, 161.
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[8] For near-degenerate metathesis reactions, the different local
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proposed to be relevant. C. Adlhart, P. Chen, Angew. Chem.
2002, 114, 4668; Angew. Chem. Int. Ed. 2002, 41, 4484.
[9] J. J. Lippstreu, B. F. Straub, J. Am. Chem. Soc. 2005, 127, 7444.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
[10] On the 1H NMR spectroscopy time scale at room temperature,
carbene rotation is rapid in a ruthenium complex. See T.
Weskamp, F. J. Kohl, W. Hieringer, D. Gleich, W. A. Herrmann,
Angew. Chem. 1999, 111, 2416; Angew. Chem. Int. Ed. 1999, 38,
2573. This feature indicates that the rotation barrier is below
about 50 kJ mol 1. The assumption of a “free” carbene rotation
might have been a factor for considering the carbene rotation as
irrelevant for the overall catalytic barrier.
[11] a) S. M. Hansen, F. Rominger, M. Metz, P. Hofmann, Chem. Eur.
J. 1999, 5, 557; b) S. M. Hansen, diploma thesis, Ruprecht-KarlsUniversitJt Heidelberg, 1996; See also c) J. N. Coalter III, J. C.
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[12] The B3LYP/LACV3P** + //B3LYP/LACVP* level of theory as
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The transition state for rotation of the methylene unit from
model 3 b to model 4 b has a relative Gibbs free energy of Grel =
11.2 kJ mol 1. The transition state connecting structures 3 c and
4 c has a relative Gibbs free energy of Grel = 11.8 kJ mol 1; see
ref. [9].
A derivative of model 6 b has been characterized crystallographically; see J. A. Tallarico, P. J. Bonitatebus, Jr., M. L.
Snapper, J. Am. Chem. Soc. 1997, 119, 7157.
Recently, a derivative of model 11 c has been characterized by
NMR spectroscopy; see P. E. Romero, W. E. Piers, J. Am. Chem.
Soc. 2005, 127, 5032.
Owing to the low binding strength of the alkene ligand to
ruthenium, alkene reorientation may proceed either by alkene
dissociation and re-association or by intramolecular alkene
ligand rotation. Since the active alkene conformation is more
stable than the inactive conformation anyway, alkene rotation
should be irrelevant in the catalytic cycle.
Zero-point energy corrections to E0 values would result in a
decrease of the energy differences of about 5 kJ mol 1.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 5974 –5978
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