Enantioselective synthesis of cyclic enol ethers and all-carbon quaternary stereogenic centers through catalytic asymmetric ring-closing metathesis

Article abstract of DOI:10.1021/ja058428r

The first examples of catalytic asymmetric ring-closing metathesis (ARCM) reactions of enol ethers are reported. To identify the most effective catalysts, various chiral Mo- and Ru-based catalysts were screened. Although chiral Ru catalysts (those that do

Full text of DOI:10.1021/ja058428r

Published on Web 03/24/2006
Enantioselective Synthesis of Cyclic Enol Ethers and
All-Carbon Quaternary Stereogenic Centers Through Catalytic
Asymmetric Ring-Closing Metathesis
Ai-Lan Lee,† Steven J. Malcolmson,† Alessandra Puglisi,† Richard R. Schrock,‡ and
Amir H. Hoveyda*,†
Contribution from the Department of Chemistry, Merkert Chemistry Center, Boston College,
Chestnut Hill, Massachusetts 02467, and Department of Chemistry, Massachusetts Institute of
Technology, Cambridge, Massachusetts 02139
Received December 12, 2005; E-mail: amir.hoveyda@bc.edu
Abstract: The first examples of catalytic asymmetric ring-closing metathesis (ARCM) reactions of enol
ethers are reported. To identify the most effective catalysts, various chiral Mo- and Ru-based catalysts
were screened. Although chiral Ru catalysts (those that do not bear a phosphine ligand) promote ARCM
in some cases, such transformations proceed in <10% ee. In contrast, Mo-based alkylidenes give rise to
efficient ARCM and deliver the desired products in the optically enriched form. Thus, Mo-catalyzed
enantioselective transformations allow access to various five- and six-membered cyclic enol ethers in up
to 94% ee from readily available achiral starting materials. The first examples of catalytic ARCM that lead
to the formation of all-carbon quaternary stereogenic centers are also disclosed. Mechanistic models that
offer a plausible rationale for the identity of major enantiomers as well as the observed levels of
enantioselectivity are provided. Representative examples demonstrate that the enol ether moiety and the
unreacted alkene of the ARCM products can be discriminated with excellent site selectivity (>98%).
moallylic ethers.5 Metal-catalyzed ring-closing metathesis (RCM)
has been used to access small and medium ring cyclic enol
Introduction
Research in these laboratories has, in recent years, led to the
development of an assortment of Mo-1 and Ru-based2 precata-
lysts that can be used to promote olefin metathesis reactions;
several examples of such complexes are illustrated in Scheme
1. One noteworthy aspect of our programs relates to chiral
complexes that give rise to efficient enantioselective transforma-
tions. Within this context, the higher oxidation state (vs Ru-
based) systems have proven to be particularly effective for
asymmetric ring-closing metathesis (ARCM).1,3,4 Mo-based
complexes exhibit high reactivity (1-5 mol % loading) with
reactions that involve substrates bearing allylic or homoallylic
ethers or amines to deliver optically enriched unsaturated
heterocycles (>90% ee).1,3
ethers,6 and such protocols have been applied to the preparation
of natural products.7 In all the studies disclosed thus far,
however, optically enriched enol ethers are obtained by treatment
of a nonracemic substrate with an achiral Mo- or Ru-based
(3) For examples of Mo-catalyzed ARCM, see: (a) Alexander, J. B.; La, D.
S.; Cefalo, D. R.; Hoveyda, A. H.; Schrock, R. R. J. Am. Chem. Soc. 1998,
120, 4041-4042. (b) La, D. S.; Alexander, J. B.; Cefalo, D. R.; Graf, D.
D.; Hoveyda, A. H.; Schrock, R. R. J. Am. Chem. Soc. 1998, 120, 9720-
9721. (c) Zhu, S. S.; Cefalo, D. R.; La, D. S.; Jamieson, J. Y.; Davis, W.
M.; Hoveyda, A. H.; Schrock, R. R. J. Am. Chem. Soc. 1999, 121, 8251-
8259. (d) Weatherhead, G. S.; Houser, J. H.; Ford, J. G.; Jamieson, J. Y.;
Schrock, R. R.; Hoveyda, A. H. Tetrahedron Lett. 2000, 41, 9553-9559.
(e) Cefalo, D. R.; Keily, A. F.; Wuchrer, M.; Jamieson, J. Y.; Schrock, R.
R.; Hoveyda, A. H. J. Am. Chem. Soc. 2001, 123, 3139-3140. (f) Kiely,
A. F.; Jernelius, J. A.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc.
2002, 124, 2868-2869. (g) Dolman, S. J.; Schrock, R. R.; Hoveyda, A. H.
Org. Lett. 2003, 5, 4899-4902. (h) Jernelius, J. A.; Schrock, R. R.;
Hoveyda, A. H. Tetrahedron 2004, 60, 7345-7351. (i) Sattely, E. S.;
Cortez, G. A.; Moebius, D. C.; Schrock, R. R.; Hoveyda, A. H. J. Am.
Chem. Soc. 2005, 127, 8526-8533.
(4) For examples of Ru-catalyzed ARCM, see: (a) Seiders, T. J.; Ward, D.
W.; Grubbs, R. H. Org. Lett. 2001, 3, 3225-3228. (b) Van Veldhuizen, J.
J.; Kingsbury, J. S.; Garber, S. B.; Hoveyda, A. H. J. Am. Chem. Soc.
2002, 124, 4954-4955. (c) Van Veldhuizen, J. J.; Gillingham, D. G.;
Garber, S. B.; Kataoka, O.; Hoveyda, A. H. J. Am. Chem. Soc. 2003, 125,
12502-12508. (d) Funk, T. W.; Berlin, J. M.; Grubbs, R. H. J. Am. Chem.
Soc. 2006, 128, 1840-1846.
(5) Louie, J.; Grubbs, R. H. Organometallics 2002, 21, 2153-2164.
(6) For representative examples of cyclic enol ether synthesis through olefin
metathesis reactions, see: (a) Fujimura, O.; Fu, G. C.; Grubbs, R. H. J.
Org. Chem. 1994, 59, 4029-4031. (b) Sturino, C. F.; Wong, J. C. Y.
Tetrahedron Lett. 1998, 39, 9623-9626. (c) Clark, J. S.; Kettle, J. G.
Tetrahedron 1999, 55, 8231-8248. (d) Rainier, J. D.; Allwein, S. P.; Cox,
J. M. J. Org. Chem. 2001, 66, 1380-1386. (e) Liu, L.; Postema, M. H. D.
J. Am. Chem. Soc. 2001, 123, 8602-8603. (f) Sutton, A. E.; Seigal, B. A.;
Finnegan, D. F.; Snapper, M. L. J. Am. Chem. Soc. 2002, 124, 13390-
13391. (g) Hekking, K. F. W.; van Delft, F. L.; Rutjes, F. P. J. T.
Tetrahedron 2003, 59, 6751-6758.
Enol ethers, a versatile class of compounds in organic
chemistry, can participate in olefin metathesis reactions.1,2
Nonetheless, particularly for electronic reasons, the reactivity
of the derived Mo-alkylidenes and Ru-based carbenes can be
significantly different than those derived from allylic or ho-
† Boston College.
‡ Massachusetts Institute of Technology.
(1) For a review on achiral and chiral Mo-based complexes and their related
olefin metathesis reactions, see: Schrock; R. R.; Hoveyda, A. H. Angew.
Chem., Int. Ed. 2003, 42, 4592-4633.
(2) (a) Kingsbury, J. S.; Harrity, J. P. A.; Bonitatebus, P. J.; Hoveyda, A. H.
J. Am. Chem. Soc. 1999, 121, 791-799. (b) Garber, S. B.; Kingsbury, J.
S.; Gray, B. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2000, 122, 8168-
8179. For a comprehensive review on achiral and chiral Ru-based complexes
developed in these laboratories and their related olefin metathesis reactions,
see: (c) Hoveyda, A. H.; Gillingham, D. G.; Van Veldhuizen, J. J.; Kataoka,
O.; Garber, S. B.; Kingsbury, J. S.; Harrity, J. P. A. Org. Biomol. Chem.
2004, 2, 8-23.
9
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J. AM. CHEM. SOC. 2006, 128, 5153-5157
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A R T I C L E S
Lee et al.
Scheme 1. Representative Achiral and Chiral Metal Complexes
of the desired 10 in 89% yield after silica gel chromatography
(eluted with 1% Et3N).
b. Preliminary Screening of Achiral Mo- and Ru-Based
Olefin Metathesis Catalysts. We investigated the ability of Mo-
and Ru-based complexes (e.g., Scheme 1) to promote RCM.
These preliminary investigations (Table 1) indicated that Ru-
and Mo-based catalysts efficiently initiate the reaction; however,
studies involving some of the less facile RCM (e.g., 12)
suggested that Mo alkylidenes are somewhat more effective
(compare entries 1-3, Table 1) and that phosphine-free 22b
promotes RCM significantly more readily than the PCy3-bearing
second-generation Grubbs complex 1110 (compare entries 2-5
and 8-10, Table 1). Accordingly, we included the available
chiral Mo-based alkylidenes as well as phosphine-free Ru-based
carbenes (Scheme 1) in our investigations of catalytic ARCM
of enol ethers.
c. Mo-Catalyzed Enantioselective Synthesis of Tertiary
Stereogenic Centers Through ARCM of Enol Ethers. As the
data summarized in entries 1-2 of Table 2 indicate, secondary
acyclic enol ethers 12 and 10, in the presence of 5-10 mol %
binaphthyl-based chiral Mo complex 4a (see below for catalyst
complex. A strategically unique and valuable, but thus far
unexplored, alternative would be to access optically enriched
cyclic enol ethers through ARCM of achiral substrates.
Herein, we report the first examples of catalytic enantio-
selective olefin metathesis reactions of enol ethers, leading to
the formation of five- and six-membered ring heterocycles in
up to 94% ee. The present disclosure also contains the first
instances of enantioselective syntheses of all-carbon quaternary
stereogenic centers8 through olefin metathesis.
Table 1. Preliminary Examination of the Reactivity of Achiral Mo-
and Ru-Based Complexesa
Results and Discussion
a. Synthesis of Acyclic Enol Ether Substrates. To initiate
our investigations, we prepared various acyclic unsaturated enol
ethers (see Tables 1-3). A representative procedure is depicted
in eq 1.9 Treatment of bis(homoallylic) alcohol 9 with 0.5 mol
% of commercially available Pd(OCOCF3)2 and 4,7-diphenyl-
1,10-phenanthroline in the presence of 0.75 equiv Et3N and 20
equiv n-butyl vinyl ether (75 °C, 24 h) leads to the formation
entry
catalyst
temp (
°
C)
time (h)
conv (%)b
1
2
3
4
5
7
8
9
1
2
2
11
11
1
2
11
11
22
22
65
22
65
22
22
22
65
3
14
1
14
1
1
1
17
1
>98
<2
88
5
19c
86
92
47
>98
(7) For an example of enol ether RCM in natural product total synthesis, see:
Oliver, S. F.; Hogenauer, K.; Simic, O.; Antonello, A.; Smith, M. D.; Ley,
S. V. Angew. Chem., Int. Ed. 2003, 42, 5996-6000.
10
(8) For recent reviews of catalytic asymmetric methods for the formation of
all-carbon quaternary stereogenic centers, see: (a) Denissova, I.; Barriault,
L. Tetrahedron 2003, 59, 10105-10146. (b) Douglas, C. J.; Overman, L.
E. Proc. Natl. Acad. Sci. 2004, 101, 5363-5367.
a All reactions carried out under an atmosphere of N2. b Conversions
determined by 400 MHz 1H NMR analysis of the unpurified product mixture.
c 57% conv. after 4.5 h and 94% conv. after 14 h.
(9) Bosch, M.; Schlaf, M. J. Org. Chem. 2003, 68, 5225-5227.
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Enantioselective Synthesis Through Catalytic ARCM
A R T I C L E S
Table 2. Enantioselective Synthesis of Tertiary Carbon Stereogenic Centers by Mo-Catalyzed ARCM of Enol Ethersa
a All reactions performed under N2 atmosphere. b Conversions determined by analysis of the 400 MHz 1H NMR spectra of unpurified mixtures. c Isolated
yields after silica gel chromatography. d Enantioselectivities determined by chiral GLC analysis (see the Supporting Information for details). e Lower isolated
yields are due to product volatility. f Lower conversion (68%) but identical ee is observed at 22 °C.
Table 3. Representative Data from Catalyst Screening Studiesa
extensive screening studies involving a range of Mo- and Ru-
based chiral complexes. Several points regarding these studies
deserve mention. (1) Ru-based complexes 74b-c and 8,12 shown
in Scheme 1, do not promote RCM of substrates shown in Table
2 (<2% conv at 65 °C in 18 h). (2) In the case of substrates
entry
catalyst
conv (%)b
ee (%)c
that bear 1,1-disubstituted olefins, significant levels of catalyst
activity are observed in the presence of only one chiral Mo
complex (4a). The transformations shown in entries 1-3 of
Table 1 proceed to <5% conversion in the presence of
nonbinaphthyl-based Mo complexes 3a-c, 5, 6 as well as Ru
carbenes 7a,b4b-c and 8a,b;12 such findings underline the
significance of the availability of various structurally distinct
catalyst candidates.13 (3) As representative data in Table 3
indicate, when the substrate bears a 1,2-disubstituted olefin,
appreciable levels of reactivity are detected in the presence of
several chiral Mo complexes; however, product optical purity
can vary significantly.
e. Enantioslective Synthesis of All-Carbon Quaternary
Stereogenic Centers Through Mo-Catalyzed ARCM of Enol
Ethers. With the above findings in hand, we set out to
investigate whether Mo-catalyzed ARCM of enol ethers can be
applied to the enantioselective synthesis of all-carbon quaternary
stereogenic centers. The results of this aspect of our investigation
are summarized in Table 4. As shown in entry 1 of Table 4, in
the presence of 15 mol % of chiral alkylidene 4a, catalytic
ARCM of cyclohexyl-substituted triene 21 proceeds to 85%
conversion (84% isolated yield) after 15 h at 22 °C to afford
22 in only 23% ee. In contrast, as illustrated in entries 2-4 of
Table 4, the more synthetically versatile aryl-substituted trienes
23a-c readily undergo ARCM in the presence of 15 mol % 4a
(22 °C, 18-20 h) to afford unsaturated pyrans 24a-c in >90%
isolated yield and in 87, 85, and 83% ee, respectively. Although
Ru-based chiral complexes 7 and 8 promote RCM of substrates
1
2
3
4
5
6
3a
3b
3c
4a
5
62
72
<5
31
29
28
56
<10
nd
22
57
6
58
a All reactions carried out under N2 atmosphere. b Conversions deter-
1
mined by 400 MHz H NMR analysis of the unpurified product mixture.
c Enantioselectivities determined by chiral GLC analysis (chiraldex-GTA).
screening studies), undergo ARCM to afford dihydrofuran 13
and dihydropyran 14 in 90 and 83% ee and 70 and >98%
isolated yield, respectively.11 Mo-catalyzed ARCM of primary
vinyl ether 15 (entry 3) proceeds to 80% conversion within 24
h at 22 °C to afford 16 in 90% ee.
In contrast to transformations involving 1,1-disubstituted
alkenes (entries 1-3), trienes 17 and 19 bearing trans- and cis-
disubstituted olefins undergo catalytic ARCM to afford dihy-
drofurans 18 and 20 with lower asymmetric induction (41 and
62% ee, respectively). Catalytic ARCM of 1,2-disubstituted
alkenes 17 and 19 proceed with diminished enantioselectivity
(26 and 22% ee, respectively) when chiral alkylidene 4a, the
optimal catalyst for catalytic ARCM of 1,1-disubstituted olefins,
is used.
d. Screening of Chiral Mo- and Ru-Based Catalysts. The
identity of 4a as the optimal catalyst was established by
(10) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953-
956.
(11) The identity of major enantiomers was established through conversion and
comparison with an authentic optically enriched compound. See the
Supporting Information for details.
(12) Van Veldhuizen, J. J.; Campbell, J. E.; Giudici, R. E.; Hoveyda, A. H. J.
Am. Chem. Soc. 2005, 127, 6877-6882.
(13) Hoveyda, A. H.; Schrock, R. R. Chem.-Eur. J. 2001, 7, 945-950.
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A R T I C L E S
Lee et al.
Table 4. Enantioselective Synthesis of Quaternary Carbon Stereogenic Centers by Mo-Catalyzed ARCM of Enol Ethersa
a-c See Table 2 (all reactions performed in C6H6). d Enantioselectivities determined by chiral GLC and HPLC analysis (see the Supporting Information
for details).
The Mo-catalyzed ARCM illustrated in entry 6 of Table 4,
involving carbamate 27, proceeds to afford unsaturated pyran
28 in 97% isolated yield but in 54% ee.15 The latter finding, as
well as the difference in selectivity in reaction of 21 vs 23a-c
(entries 1-4), highlights the notable variations in selectivity
that can arise, unpredictably, as a result of seemingly minor
structural modifications.
f. Mechanistic Rationale for Mo-Catalyzed ARCM of Enol
Ethers. The above trends in enantioselectivity can be explained
through the transition state model depicted in Figure 2. The
reaction is proposed to proceed via a more Lewis acidic anti-
Mo alkylidene16 formed by reaction of the chiral complex with
the less substituted enol ether olefin; intermediacy of the cyclic
complex I illustrated in Figure 2, where one of the enantiotopic
unsaturated side chains is disposed equatorially (vs II), leads
to the formation of the major enantiomer.17 This mechanistic
scenario is consistent with lower enantioselectivities observed
in reactions of trans and cis-disubstituted olefins 17 and 19
(entries 4-5, Table 3). Thus, in the absence of the substituent
(Me) of the 1,1-disubstituted olefin that is coordinated to the
Mo center, reaction involving an axially situated unsaturated
side chain becomes competitive with the mode of addition I
Figure 1. p-Bromobenzoate derivative 29 and its X-ray structure. Note:
(S)-29 was derivatized from (S)-26 obtained through catalytic ARCM of
(R)-4a; reactions in Table 4 correspond to products of reactions promoted
by (S)-4a.
illustrated in Table 4 at elevated temperatures (65 °C, 4 h), these
reactions afford products in <10% ee.
As depicted in entry 5 of Table 3, enol ether 25, bearing a
functionalizable methyl ester substituent, readily undergoes
ARCM at 22 °C (21 h) to afford 26 in 94% ee and 94% isolated
yield. The stereochemical identity of the major enantiomer of
26 was determined through X-ray structure analysis14 of
p-bromobenzoate ester 29 (Figure 1), prepared through reduction
of the ester group with DIBAL-H (89% yield), followed by
esterification of the resulting primary alcohol (90% yield). The
assignment of the major enantiomers formed in reactions shown
in Table 4 is thus by inference to the stereochemical outcome
of Mo-catalyzed ARCM of 25. To the best of our knowledge,
transformations illustrated in entries 2-5 of Table 4 represent
the first instances of efficient enantioselective synthesis of all-
carbon quaternary stereogenic centers8 by catalytic asymmetric
olefin metathesis.
(15) Typically, in catalytic asymmetric desymmetrization reactions involving
trienes or tetraenes, high enantioselectivities are attained if initiation
(formation of the MdC bond) occurs at the central unsaturation site
followed by enantioselective ring closure (e.g., reactions in Tables 2 and
4). As a result, reactions of all-terminal olefins typically proceed with low
enantioselectivity. An example is illustrated below:
(16) For evidence pointing to the higher reactivity (and likely intermediacy) of
antialkylidenes in Mo-catalyzed olefin metathesis reactions, see ref 1 and
references therein.
(17) The corresponding pseudoboat transition structure would lead to unfavorable
steric interactions between the substrate moiety and the binaphtholate unit
of the chiral complex.
(14) See the Supporting Information for details.
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Enantioselective Synthesis Through Catalytic ARCM
A R T I C L E S
Scheme 2. Representative Functionalization of an Optically
Enriched Enol Ether Obtained through Mo-Catalyzed ARCM
developed in these laboratories for the first time allow access
to five- and six-membered cyclic enol ethers in up to 94% ee
in a single catalytic step from achiral starting materials.
Importantly, some of the products bear an all-carbon quaternary
stereogenic center; to the best of our knowledge, these are the
first instances of synthesis of all-carbon quaternary stereogenic
centers through enantioselective olefin metathesis reactions. Our
investigations illustrate that, in contrast to some Mo-based chiral
complexes, the available Ru-based carbenes either do not
provide significant levels of reactivity (substrates in Table 2)
or promote reactions readily without inducing high asymmetric
induction (substrates in Table 4). The optically enriched products
obtained bear readily differentiable olefin sites that can be
functionalized to access a wide range of nonracemic cyclic
structures.
Enantioselectivities of the catalytic olefin metathesis reactions
of enol ethers are at times <90% ee, and often 10-20 mol %
catalyst loading is required for high (>80%) conversion. The
results of the investigation disclosed herein thus clearly indicate
that new and more effective chiral catalysts must be developed,
and will be required, if the full potential of catalytic ARCM
reactions of enol ethers is to be realized.
Figure 2. Proposed transition state structures for Mo-catalyzed ARCM of
enol ethers.
(Figure 2). The above considerations suggest that catalytic
ARCM of 25 proceeds through transition state model III (Figure
2) to afford 26. Unfavorable syn-pentane interactions are likely
minimized as the substituent that bears the less sterically
demanding sp2-carbon is situated pseudoaxially (e.g., carboxylic
ester or an aryl group). Such considerations account for the lower
enantioselectivity observed in the reaction of Cy-substituted 21
vs aryl-substituted 23a-c and carboxylic ester 25 (see Table
4), because the cycloalkane moiety of 21 likely leads to more
severe steric interactions with the Me group of the Mo-bound
olefin; the size difference between the unsaturated side chain
and the cyclohexane unit is therefore not sufficient to lead to
high asymmetric induction. The low asymmetric induction in
the Mo-catalyzed ARCM of carbamate 27 (entry 6, Table 4)
may be rationalized through competitive intermediacy of
transition structure IV, whereas the Lewis basic carbamate is
chelated with the transition metal center through interaction with
the low energy Mo-N σ* orbital.18
g. Representative Functionalization of Optically Enriched
Cyclic Enol Ethers. Enantioselective desymmetrizations of
trienes bearing allylic or homoallylic ethers through metal-
catalyzed ARCM leads to products that bear two olefin residues
that may not be easily differentiable. In contrast, in reactions
involving enol ethers, the resulting optically enriched products
carry olefins that are electronically distinct and can be easily
functionalized site-selectively. Two representative examples are
illustrated in Scheme 2. Treatment of optically enriched 14 with
aqueous HCl leads to the formation of the derived cyclic
hemiacetal, which can be easily oxidized to the corresponding
lactone 30 in 67% overall yield (1:1 dr). Regio- and diastereo-
selective dihydroxylation of the cyclic enol ether with AD-mix-
â,19 followed by a Dess-Martin oxidation, delivers lactone 31
in 69% overall isolated yield and >95:5 dr.
Studies directed toward the design and development of more
efficient catalysts for catalytic asymmetric olefin metathesis,
as well as further development of catalytic ARCM of enol ethers
and demonstration of their utility in organic synthesis are in
progress.
Acknowledgment. Financial support was provided by the
NIH (GM-59426 to R.R.S. and A.H.H.). Postdoctoral fellow-
ships to A.-L.L. (Lindemann Trust) and A.P. (Universita’ degli
Studi di Milano) are acknowledged. The X-ray facility at Boston
College is generously supported by Schering-Plough.
Supporting Information Available: Experimental procedures
and spectral data for substrates and products. This material is
available free of charge via the Internet at http://pubs.acs.org.
Conclusions
We disclose the first examples of catalytic ARCM reactions
involving enol ether substrates. The chiral Mo complexes
JA058428R
(19) Dihydroxylation with OsO4 leads to low levels of regioselectivity. For use
of chiral dihydrtoxylation catalysts in enantioselective synthesis, see: (a)
Kolb, H. C.; Van Nieuwenhze, M. S.; Sharpless, K. B. Chem. ReV. 1994,
94, 2483-2547. For catalytic dihydroxylation of enol ethers with chiral
catalysts, see: (b) Curran, D. P.; Ko, S.-B. J. Org. Chem. 1994, 59, 6139-
6141.
(18) For related Mo- and W-based chelate complexes (including X-ray structure
data), see: (a) Fox, H. H.; Lee, J.-K.; Park, L. Y.; Schrock, R. R.
Organometallics 1993, 12, 759-768. (b) Tsang, W. C. P.; Hultzsch, K.
C.; Alexander, J. B.; Bonitatebus, P. J., Jr.; Schrock, R. R.; Hoveyda, A.
H. J. Am. Chem. Soc. 2003, 125, 2652-2666.
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