Deactivation of ruthenium olefin metathesis catalysts through intramolecular carbene-arene bond formation

Article abstract of DOI:10.1002/anie.200702560

(Chemical Equation Presented) Oxygen knocks it out: Olefin metathesis catalysts without steric hindrance in the ortho positions of the N-aryl substituents can be transformed into catalytically inactive ruthenium complexes through C-H activation (see schem

Full text of DOI:10.1002/anie.200702560

Communications
DOI: 10.1002/anie.200702560
Ruthenium Alkylidene Complexes
Deactivation of Ruthenium Olefin Metathesis Catalysts through
Intramolecular Carbene–Arene Bond Formation
Kati Vehlow, Simon Gessler, and Siegfried Blechert*
Dedicated to Professor Ekkehard Winterfeldt on the occasion of his 75th birthday
Since the development of Grubbs catalysts of the first
During our study of diastereoselective ring rearrangement
metathesis reactions (dRRM), we found that the neutral
ligand that remains on the metal center has a pronounced
effect on conversion and diastereoselectivity.[8] While only
moderate diastereoselectivities could be seen for the first-
generation catalysts (1, 3), the sterically more demanding and
more active second-generation catalysts (2, 4) led to signifi-
cantly higher selectivities. Thus, we focused on the develop-
ment of a bulky ruthenium carbene complex to increase the
diastereoselective interaction between the olefine moiety and
the catalytically active ruthenium species. To do this, we
connected the N-aryl substituent with the N-heterocyclic
carbene (NHC) through a C2 unit. Unlike in 2 or 4, the
aromatic moiety in this new ruthenium complex (5, Scheme 1)
should exert a much stronger steric influence on the
ruthenium alkylidene moiety through torsion of approxi-
mately 458 and hindered rotation.
=
À
([RuCl2(PCy3)2( CH C6H5)] (1; Cy = cyclohexyl) and
second (2) generation, many modified ruthenium complexes
for olefin metathesis have been reported,[1] for which strik-
ingly different influences of the ligands on the efficiency of
À
C C bond formation have been observed. For example,
notable effects have been found in the class of alkoxy
benzylidene ligands first introduced by Hoveyda and co-
workers. Phosphine-free complexes such as the second-
generation Hoveyda–Grubbs catalyst (4; IMesH2 = 1,3-dime-
sityl-4,5-dihydroimidazol-2-ylidene)[2] are especially well-
suited for cross metathesis with electron-deficient olefins
such as a,b-unsaturated nitriles.[3] The stability and the
initiation rate of the precatalyst could be tuned by steric
and electronic effects of the substituents.[4]
The synthesis of the NHC ligand started from commer-
cially available 2,2’-biquinoline (6). The first step was hydra-
tion to octahydrobiquinoline; the result of the hydration was
strongly dependent on the catalyst system. PtO2 and H2 under
ambient pressure led to the formation of a 3:1 mixture of the
meso and racemic forms. Simple chromatography on silica gel
Additional intramolecular interactions of functional
groups or atoms with the metal center have been reported
for some ligands.[3c,5,6] Grubbs and co-workers recently found
an accelerating effect in olefin metathesis through fluorine–
ruthenium interaction. Besides knowledge of such ligand and
substituent effects, an understanding of catalyst decomposi-
tion and transformation to catalytically inactive ruthenium
species is of major importance for the rational design and
improvement of metathesis catalysts. Fundamental studies
have been presented by Grubbs and co-workers and by
Dinger and Mol.[7] Herein, we report a novel deactivation
reaction pathway transforming active ruthenium catalysts
with alkoxy benzylidene ligands into catalytically inactive
carbene complexes.
[*] Dr. K. Vehlow, Dr. S. Gessler, Prof. Dr. S. Blechert
Technische Universität Berlin
Institut für Chemie
Strasse des 17. Juni 135, 10623 Berlin (Germany)
Fax : (+49)30-314-23619
E-mail: blechert@chem.tu-berlin.de
Scheme 1. Synthesis of ruthenium complexes 5 and 11: a)PtO 2, H2,
CH2Cl2, RT, 4 h, 61%; b)HC(OMe) 3, HCOOH, toluene, 908C, 8 h,
98%; c)Ph 3CBF4, CH2Cl2, RT, 3 h, 85%; d)KOC(CH 3)2CH2CH3, hexane,
1, 508C, 16 h, 65%; e)2-Isopropoxystyrene, CH 2Cl2, RT, 1.5 h, 95%.
Supporting information for this article is available on the WWW
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yielded 61% of the pure meso compound. Subsequent
transformation into carbene precursor 8 turned out to be
quite problematic. The usual reaction conditions for cycliza-
tion with trimethyl orthoformate and formic acid led to the
formation of aminal 7 in near quantitative yield. The
reduction of the highly strained intermediate iminium salt
could not even be avoided by the use of acetic acid. The
synthesis of the desired imidazolium salt (8) finally succeeded
using tritylium tetrafluoroborate as a hydride abstractor.
From tetrafluoroborate salt 8, we synthesized complex 5
using the procedure of Nolan and co-workers.[9] Catalyst 5 was
obtained as a green microcrystalline solid in 65% yield. The
green color of this complex is unusual, as ruthenium
complexes of the second generation are usually purple or
brown. The novel catalyst 5 was used in different metathesis
reaction protocols (RCM, CM). Complex 5 was found to be of
limited stability in solution even in the absence of olefin
substrate. Thus, conversions were generally much lower than
for 2. Nevertheless, test reactions for diastereoselective ring
rearrangements showed very promising results. RRM of 9
with 5, for example, delivered the product with an E/Z
selectivity of 9:1 and a conversion of 58% (Scheme 2).[8]
Figure 1. Crystal structure of 12 (ORTEP drawing, thermal ellipsoids
are shown at 50% probability). Hydrogen atoms have been omitted
for clarity. Selected bond lengths [Š] and angles [8]: Ru–C1 1.842(4),
Ru–C8 1.907(4), Ru–O 2.255(3), C1–C10 1.461(6), C25···Ru 2.408(0);
C1-Ru-C8 93.3(2), C1-Ru-O 80.7(2), C8-Ru-O 173.8(2), Ru-C1-C10
125.7(3).
The crystal structure shows the formation of an intra-
molecular carbene–arene bond between the benzylidene
carbon atom (C1) and the ortho position of the N-aryl
À
ligand (C10). The length of the C1 C10 bond (1.461(6) Š) is
between that of a single and a double bond. The NHC ligand
with its boomerang shape is very close to the ruthenium
center, thus leading to a strong agostic interaction between
the hydrogen atom on C25and the ruthenium atom. The
Scheme 2. Diastereoselective RRM of 9 using catalysts 2 and 5.
TBDMS=tert-butyldimethylsilyl.
À
C25 Ru separation of 2.408(0) Š is clearly shorter than was
À
À
found for Ru F and Ru Cl interactions in comparable
ruthenium benzylidene complexes.[6]
À
We expected an increase in stability when introducing an
isopropoxy benzylidene ligand. Following the method of
Hoveyda and co-workers for the synthesis of the second-
generation Grubbs–Hoveyda catalyst 4,[2a] we obtained the
air-stable catalyst 11 in an unsatisfactory yield of only 12% by
stirring a mixture of 5, 2-isopropoxy styrene, and CuCl in
CH2Cl2. As byproducts, we observed the Hoveyda I complex
3 and the copper NHC complex in equal amounts (43%
each). The transformation of 5 to 3 is remarkable, as usually
the phosphine ligand is exchanged with the alkoxy benzyli-
dene ligand and the N-heterocyclic carbene ligand remains
untouched. Reaction without CuCl finally resulted in the
formation of the desired ruthenium catalyst 11 as an air-
stable, olive green solid in 95% yield (Scheme 1).
Investigations of the stability of 11 in solution showed
very remarkable results. Storage of samples in the presence of
air led to a decrease in the intensity of the benzylidene signal
at d = 17.33 ppm in the 1H NMR spectrum over the course of
two weeks. After this period, the signal disappeared com-
pletely, and an entirely new set of signals appeared along with
some minor signals from decomposition products. The new
complex 12 was fully characterized by mass spectrometry, IR
and NMR spectroscopy, and X-ray crystallographic analysis
(Figure 1).[10]
This unexpected C H insertion prompted us to synthesize
a phosphine-free second-generation ruthenium complex with
similar unsubstituted ortho positions on one of the N-aryl
ligands. We presumed that a bond might be formed between
the benzylidene carbon atom and the b-position of the N-aryl
substituent if oxygen is present and if the approach of the
reactive positions is not impeded by steric hindrance. Con-
sequently, we synthesized ruthenium complex 17. Carbene
precursor 15 was synthesized following the procedure we
reported for unsymmetrically substituted carbenes
(Scheme 3).[11]
The air-stable NHC–phosphine complex 16 can be
obtained in excellent yield from the addition of 1 to a
solution of the carbene generated in situ from 15 in hexane.
The reaction of 16 and 2-isopropoxy styrene afforded the
phosphine-free complex 17. Under inert conditions, the crude
product contained only 17, which is analogous to the
Hoveyda–Grubbs catalyst. But to our surprise, we could
isolate two different green compounds from the chromato-
graphic purification. The olive green complex 17 was isolated
(67% yield), but we also obtained the dark green, crystalline
À
C H insertion product 18 in 10% yield.
Both solids are air-stable. In CH2Cl2, 17 converts com-
pletely into 18 within a few hours.[12] The insertion could be
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sequence of a pericyclic reaction followed by oxidation and
re-aromatization steps (Scheme 4). In these complexes,
several mesomeric forms can be considered. Favored by the
geometry of the alkoxy benzylidene ligand, valence structure
Scheme 3. Synthesis of ruthenium complexes 16 and 17: a)[Pd 2-
(dba)3], (Æ)-binap, 2-bromomesitylene, NaOtBu, toluene, 808C, 72 h,
91%; b)NH 4BF4, CH(OEt)3, HCOOH, 1208C, 18 h; 85%; c) 1,
KO(CH3)2CH2CH3, hexane, 508C, 12 h, 97%; d)2-isopropoxystyrene,
CH2Cl2, 408C, 0.5 h, 67%. dba=trans,trans-dibenzylideneacetone,
binap=2,2’-bis(diphenylphosphino)-1,1’-binaphthyl.
Scheme 4. Proposed mechanism for the insertion reaction.
A can undergo a pericyclic cyclization reaction to form B. This
step is reversible, but the subsequent oxidation with oxygen
renders the reaction irreversible (C). Elimination and re-
aromatization finally yields the insertion product D.
These results are of importance for further development
of olefin metathesis catalysts. Complexes with N-aryl sub-
stituted NHC ligands, especially those of the Grubbs–
Hoveyda type lacking steric hindrance in the ortho position
monitored by NMR spectroscopy, which showed the decrease
of the benzylidene signal at d = 16.65ppm. The isolated
complexes 12 and 18 were found to be completely inactive in
various metathesis reaction protocols. The crystal structure
confirms the formation of the insertion product (Figure 2).[10]
Complex 18 crystallizes in the monoclinic space group C2/c.
The insertion reaction could only be observed in solution
in the presence of atmospheric oxygen. No insertion was
found for solutions stored under inert atmosphere. These
findings are in accordance with the proposed reaction
À
of the arene ligand, can give rise to intramolecular C H
insertion. This insertion leads to metathesis-inactive ruthe-
nium complexes. The transformation only occurs in the
presence of atmospheric oxygen. Olefin metathesis precata-
lysts of this kind should therefore be handled under inert
atmosphere. Their purification via column chromatography
and their recycling is thus more difficult.
Received: June 12, 2007
Revised: July 6, 2007
Published online: September 13, 2007
Keywords: carbenes · deactivation · insertion · metathesis ·
.
ruthenium
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Figure 2. Crystal structure of 18 (ORTEP drawing, thermal ellipsoids
are shown at 50% probability). Hydrogen atoms have been omitted
for clarity. Selected bond lengths [Š] and angles [8]: Ru–C1 1.841(2),
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