The First Highly Active, Halide-Free Ruthenium Catalyst for Olefin Metathesis

Article abstract of DOI:10.1021/om030494j

New ruthenium alkylidyne and alkylidene complexes are prepared, in which aryloxide groups function as pseudohalide ligands. The selectivity for alkylidene or alkylidyne products is controlled by steric matching or mismatching between pseudohalide and anci

Full text of DOI:10.1021/om030494j

3634
Organometallics 2003, 22, 3634-3636
Communications
Th e F ir st High ly Active, Ha lid e-F r ee Ru th en iu m Ca ta lyst
for Olefin Meta th esis
J ay C. Conrad, Dino Amoroso,† Pawel Czechura, Glenn P. A. Yap, and
Deryn E. Fogg*
Center for Catalysis Research and Innovation, Department of Chemistry, University of Ottawa,
10 Marie Curie, Ottawa, Ontario, Canada K1N 6N5
Received J une 25, 2003
Ch a r t 1. Olefin Meta th esis Ca ta lysts
Summary: New ruthenium alkylidyne and alkylidene
complexes are prepared, in which aryloxide groups
function as pseudohalide ligands. The selectivity for
alkylidene or alkylidyne products is controlled by steric
matching or mismatching between pseudohalide and
ancillary donor ligands. Alkylidene 5 achieves up to
40 000 turnovers in ring-closing metathesis of diethyl
diallylmalonate.
Over the past decade, advances in catalyst design
have transformed olefin metathesis into a powerful
synthetic tool in organic1a-c and materials1d chemistry.
Two dominant lines of experimental progress have
emerged (Chart 1), centered around the Mo and Ru
catalysts pioneered by Schrock and Grubbs, respec-
tively. A key feature of the former systems is the
presence of aryloxide or alkoxide ligands that enable
steric and electronic tuning.1c Chiral biphenolate and
binaphtholate derivatives, in particular, have led to
impressive advances in asymmetric ring-closing me-
tathesis (ARCM).2 The simple chloride ligands ubiqui-
tous in the Ru chemistry offer no such opportunity, but
can enable3 bimolecular deactivation pathways4,5 which
limit the robustness that constitutes a key advantage
of the Ru systems.
Sch em e 1. Rea ction s of Ru Alk ylid en es w ith
P h en oxid es or Alk oxid es
Chart 1, IMes ) N,N′-bis(mesityl)imidazol-2-ylidene10),
modification of the anionic ligands is much less explored.
Metathesis catalysts of low to moderate activity are
obtained by replacing chloride with carboxylate,4,11 or
on use of heterobifunctional salicylaldimine12 or NHC-
naphtholate ligands.13 Installation of alkoxide ligands
affords the four-coordinate species 2a ,b (Scheme 1)14,15
whichsdespite their nominal coordinative unsaturations
exhibit near-zero metathesis activity14 (Table 1), prob-
While much effort has focused on modification of
neutral “L-donor” ligands in the ruthenium systems (an
important recent addition being highly reactive N-
heterocyclic carbene (NHC) species with labile donors;6-9
* To whom correspondence should be addressed. E-mail: dfogg@
science.uottawa.ca. Fax: (613) 562-5170.
† Present address: Promerus L.L.C., 9921 Brecksville Rd., Brecks-
ville, OH 44141-3829.
(8) Weskamp, T.; Kohl, F. J .; Hieringer, W.; Gleich, D.; Herrmann,
W. A. Angew. Chem., Int. Ed. 1999, 38, 2416.
(9) Conrad, J . C.; Yap, G. P. A.; Fogg, D. E. Organometallics 2003,
22, 1986.
(10) Arduengo, A. J .; Dias, H. V. R.; Harlow, R. L.; Kline, M. J . Am.
Chem. Soc. 1992, 114, 5530.
(11) (a) Buchowicz, W.; Ingold, F.; Mol, J . C.; Lutz, M.; Spek, A. L.
Chem. Eur. J . 2001 7, 2842. (b) Buchowicz, W.; Mol, J . C.; Lutz, M.;
Spek, A. L. J . Organomet. Chem. 1999, 588, 205.
(12) Chang, S.; J ones, L.; Wang, C.; Henling, L. M.; Grubbs, R. H.
Organometallics 1998, 17, 3460.
(1) Recent reviews: (a) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res.
2001, 34, 18. (b) Fu¨rstner, A. Angew. Chem., Int. Ed. 2000, 39, 3012.
(c) Schrock, R. R. Tetrahedron 1999, 55, 8141. (d) Buchmeiser, M. R.
Chem. Rev. 2000, 100, 1565.
(2) Hoveyda, A. H.; Schrock, R. R. Chem. Eur. J . 2001, 7, 945.
(3) (a) Amoroso, D.; Yap, G. P. A.; Fogg, D. E. Organometallics 2002,
21, 3335. (b) Amoroso, D.; Snelgrove, J . L.; Conrad, J . C.; Drouin, S.
D.; Yap, G. P. A.; Fogg, D. E. Adv. Synth. Catal. 2002, 344, 757.
(4) Wu, Z.; Nguyen, S. T.; Grubbs, R. H.; Ziller, J . W. J . Am. Chem.
Soc. 1995, 117, 5503.
(5) Ulman, M.; Grubbs, R. H. J . Org. Chem. 1999, 64, 7202.
(6) Sanford, M. S.; Love, J . A.; Grubbs, R. H. Organometallics 2001,
20, 5314.
(7) (a) Huang, J .; Stevens, E. D.; Nolan, S. P.; Peterson, J . L. J . Am.
Chem. Soc. 1999, 121, 2674. (b) Huang, J .; Schanz, H.-J .; Stevens, E.
D.; Nolan, S. P. Organometallics 1999, 18, 5375.
(13) Van Veldhuizen, J . J .; Garber, S. B.; Kingsbury, J . S.; Hoveyda,
A. H. J . Am. Chem. Soc. 2002, 124, 4954.
(14) Sanford, M. S.; Henling, L. M.; Day, M. W.; Grubbs, R. H.
Angew. Chem., Int. Ed. 2000, 39, 3451.
(15) Coalter, J . N.; Bollinger, J . C.; Eisenstein, O.; Caulton, K. G.
New J . Chem. 2000, 24, 925.
10.1021/om030494j CCC: $25.00 © 2003 American Chemical Society
Publication on Web 08/06/2003

Communications
Organometallics, Vol. 22, No. 18, 2003 3635
Ta ble 1. Rin g-Closin g Meta th esis via
Ru -Alk ylid en e Ca ta lystsa
time conversn TOF
entry cat.
S
[S]/[C]b solvent (min)
(%)
(h-1
)
1c
2c
3
4
5
6
7
8
9
2a
2b
5
5
5
5
5
5
5
6a
6a
6a
6a
6a
6b
7
5
5
C6D6
C6D6
5760
720
15
120
20
20
90
180
1440
<5
40
,1
,1
F igu r e 1. ORTEP representations of 4c and 5, with
hydrogen atoms and solvates omitted. Thermal ellipsoids
are given at 30% probability level. Aryloxide and alkylidyne
ligands in 4c are related by a center of inversion.
10 C6D6
20 C6F6
20 CDCl3
20 CDCl3
20 CDCl3
2000 CDCl3
56d
22d
10
>99
>99
>99
>99
>99
20
60
60e
13
667
333
despite the acidity of the perfluorophenol coproduct,16
implies a powerful driving force for reaction. Modeling
studies point toward steric crowding within the five-
coordinate intermediate (cf. 3), sufficient to promote
interaction between the alkylidene proton and the
phenoxide oxygen. This is ultimately relieved by elimi-
nation of perfluorophenol. Failure to observe the corre-
sponding reaction for the tert-butoxide system,14,15
despite the thermodynamically more favorable libera-
tion of tert-butyl alcohol, is consistent with prohibition
of the alkylidene-alkoxide interaction by the bulk of
the tert-butyl substituent. Relief of steric pressure can
then only be accommodated by phosphine loss.
6a
6a 200000 CDCl3
a
Conditions: [S] ) 0.1-0.5 M; [C] ) (2.5 × 10-6) - 0.01 M;
reactions under Ar, at 60 °C (C6D6) or at reflux; conversions
determined by 1H NMR. TOF ) turnover frequency. [S]/[C] )
b
substrate-to-catalyst ratio. c Reference 14. Catalyst incompletely
d
dissolved. e 92% selectivity for the expected 2,5-dihydropyrrole
product.21
Sch em e 2. Ster ica lly Deter m in ed Selectivity in
Rea ction s of Ru Alk ylid en e P r ecu r sor s w ith
Ar yloxid e
This analysis suggested that alkylidene complexes of
simple aryloxide ligands could potentially be accessed
by attenuating the bulk of the neutral ligands as well
as the pseudohalide. We therefore turned our attention
to NHC complexes of type 1d ,9 containing an ap-
proximately two-dimensional IMes ligand. Reaction of
TlOC6F5 with 1d is selective for transmetalation, yield-
ing solely alkylidene 5. Complex 5 was isolated in 92%
yield and characterized by spectroscopic, microanalyti-
cal, and crystallographic analysis. Its geometry is square
pyramidal, with alkylidene occupying the apical site
(Figure 1), consistent with the high trans influence of
this ligand.
Retention of the potentially labile pyridine ligand
within 5 attests to the absence of steric constraints in
this five-coordinate complex. Importantly, it also signi-
fies the presence of an incipient coordination site for
incoming substrate in 5, in contrast with the sterically
encumbered alkoxide complexes of type 2.14,15 Indeed,
5 exhibits dramatically increased activity for ring-
closing metathesis (Table 1). Metathesis activity is
retained in fluorocarbon solvents (entry 4) and at
exceptionally low catalyst loadings (entries 8 and 9). Up
to 40 000 turnovers are observed for RCM of the
benchmark substrate diethyl diallylmalonate, using only
5 × 10-4 mol % of 5. A lower limit of 0.05 mol % has
been established for chlororuthenium catalyst 1c under
comparable conditions.18 Catalyst loadings of 5-20 mol
% are typical in Ru-catalyzed RCM, owing to the short
lifetimes of the active Ru-methylidene intermedi-
ate.5,19,20 Even at 20 mol %, however, complexes 2a ,b
achieve turnover numbers of only 0.25 and 2, respec-
tively.14
ably owing to steric crowding, compounded by thermal
instability.
While use of planar aryloxide ligands, in place of
alkoxide, could potentially relieve these steric con-
straints, reaction of 1a or RuCl2(PiPr3)2(CHPh) with
phenoxide anion unexpectedly yields alkylidynes 4a ,b,
via deprotonation of the benzylidene ligand and libera-
tion of phenol.15 Given the higher basicity of tert-
butoxide, vs phenoxide,16 we speculated that the product
selectivity in Scheme 1 might be driven by steric
interactions between incoming pseudohalide and ancil-
lary donor ligands. We now report that steric matching/
mismatching strategies enable selective synthesis of
four-coordinate carbyne or five-coordinate alkylidene
complexes. The latter catalyzes RCM of diethyl diallyl-
malonate at very low catalyst loadings.
Consistent with our view that the nucleophilicity of
the aryloxide is extraneous to alkylidyne formation, we
find that treatment of 1a with TlOC6F5 effects quantita-
tive conversion to 4c within 3 h at 22 °C (Scheme 2).17
The complex is isolated as a green, air-stable, ether-
soluble powder; high solubility limits yields to ca. 60%.
The approximately square-planar molecular structure
(Figure 1; for details see Supporting Information) closely
resembles that reported for 4b.15 Formation of 4c,
(16) pKa values: perfluorophenol, 5.52 (Birchall, J . M.; Haszeldine,
R. N. J . Chem. Soc. 1959, 3653); phenol, 10; tert-butyl alcohol, 18
(Streitwieser, A. J .; Heathcock, C. H. Introduction to Organic Chem-
istry, 2nd ed.; McMillan: New York, 1981).
(17) The utility of thallium salts in such reactions is described in
ref 12.
(18) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999,
1, 953.

3636 Organometallics, Vol. 22, No. 18, 2003
Communications
In summary, we have established strategies for selec-
tive synthesis of alkylidene or alkylidyne derivatives of
ruthenium, via steric matching/mismatching between
two- or three-dimensional “L-donor” ligands and an
incoming pseudohalide. Alkylidene complex 5 exhibits
excellent activity for ring-closing metathesis, even at
very low catalyst loading. This chemistry affords con-
venient access to a new, active, and potentially broad
class of ruthenium catalysts for olefin metathesis.
Investigation of the selectivity and activity conferred by
other pseudohalide ligands, as well as immobilization
strategies, is in progress.
Ack n ow led gm en t. This work was supported by the
NSERC of Canada, the Canada Foundation for Innova-
tion, and the Ontario Innovation Trust.
Su p p or tin g In for m a tion Ava ila ble: Text and tables
giving synthetic, spectroscopic, and crystallographic data for
4c and 5. This material is available free of charge via the
Internet at http://pubs.acs.org.
(19) Sanford, M. S.; Love, J . A.; Grubbs, R. H. J . Am. Chem. Soc.
2001, 123, 6543.
(20) Turnover numbers are considerably increased for cross-
metathesis or RCM in neat substrate, with TON values for 6a in some
cases exceeding 200 000. Dinger, M. B.; Mol, J . C. Adv. Synth. Catal.
2002, 344, 671.
(21) Fu¨rstner, A.; Ackermann, L. Chem. Commun. 1999, 95.
OM030494J