Olefin metathesis catalysts containing acyclic diaminocarbenes

Article abstract of DOI:10.1021/om9008718

The first examples of ruthenium-based olefin metathesis catalysts containing acyclic diaminocarbene (ADC) ligands are reported. Complexes of the type (ADC)(SIMeS)Cl₂Ru=CHPh and (ADC)Cl₂Ru=CH(2- isopropoxy)Ph (ADC = N,N'-dimethylformamidin-2-ylidene or N,N'-bis(2,6-di- isopropylphenyl)-N,N'-dimethylformamidin-2-ylidene; SIMes = 1,3- dimesitylimidazolin-2-ylidene) were synthesized and studied in solution as well as in the solid state. Depending on their N-substituents and the metal center to which they were coordinated, the aforementioned ADC ligands were found to adopt different conformations. Preliminary investigations revealed that these Ru complexes exhibited high catalytic activities in a variety of olefin metathesis reactions at elevated temperatures and afforded cross-metathesis products with significantly lower E:Z ratios than catalysts containing analogous N-heterocyclic carbene ligands.

Full text of DOI:10.1021/om9008718

250 Organometallics 2010, 29, 250–256
DOI: 10.1021/om9008718
Olefin Metathesis Catalysts Containing Acyclic Diaminocarbenes
Evelyn L. Rosen, Daphne H. Sung, Zheng Chen, Vincent M. Lynch, and
Christopher W. Bielawski*
Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712
Received October 8, 2009
The first examples of ruthenium-based olefin metathesis catalysts containing acyclic diaminocar-
bene (ADC) ligands are reported. Complexes of the type (ADC)(SIMes)Cl2RudCHPh and
(ADC)Cl2RudCH(2-isopropoxy)Ph (ADC = N,N0-dimesityl-N,N0-dimethylformamidin-2-ylidene
or N,N0-bis(2,6-di-isopropylphenyl)-N,N0-dimethylformamidin-2-ylidene; SIMes = 1,3-dimesityli-
midazolin-2-ylidene) were synthesized and studied in solution as well as in the solid state. Depending
on their N-substituents and the metal center to which they were coordinated, the aforementioned
ADC ligands were found to adopt different conformations. Preliminary investigations revealed that
these Ru complexes exhibited high catalytic activities in a variety of olefin metathesis reactions at
elevated temperatures and afforded cross-metathesis products with significantly lower E:Z ratios
than catalysts containing analogous N-heterocyclic carbene ligands.
The olefin metathesis reaction1 has become an indispen-
sible tool for synthesizing small molecules2 as well as macro-
molecular materials.3 Although a variety of catalysts for
facilitating this useful transformation are known,4 those
based on late transition metals, particularly ruthenium, often
show the highest stabilities toward oxygen, moisture, and a
wide range of functional groups.5 As a bonus, many of these
same catalysts also display high activities and react with a
broad range of substrates.6 Representative examples of
commercially available Ru-based catalysts that have found
widespread utility in a variety of olefin metathesis reactions
are shown in Figure 1 (i.e., 1-3).
Realization of the next generation of catalysts that effectively
hurdle contemporary challenges, such as those that are able to
react with sterically hindered olefins and/or provide products
with high diastereo- or enantioselectivities, hinge on the devel-
opment of new ligands.7 Although a broad range of ligands
for metathesis-active Ru-based catalysts have been studied, N-
heterocyclic carbenes (NHCs)8 are among the most promising.9
Widely accepted to be stronger electron donors than typical
phosphines,10 NHCs often enhance the activities of Ru-based
catalysts upon coordination.11 In addition to exhibiting favor-
able electronic properties, the steric properties of NHCs can be
modified by varying the nature of their N-substituents using
straightforward methods.10,12 Indeed, efforts toward control-
ling the stereochemical outcomes of olefin metathesis reactions
catalyzed by complexes that contain unsymmetrical NHCs
have been reported.7,13 For example, catalysts 4 and 5 were
*To whom correspondence should be addressed. E-mail: bielawski@
cm.utexas.edu.
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pubs.acs.org/Organometallics Published on Web 11/06/2009
2009 American Chemical Society

Article
Organometallics, Vol. 29, No. 1, 2010 251
Figure 2. Structures of two acyclic diaminocarbenes (ADCs)
studied as ligands for Ru alkylidenes. The conformations shown
are consistent with their solution state structures.
catalytically active.17 In addition to their unique electronic
and steric properties, another remarkable aspect of ADCs is
their structure. Enabled by free rotation about their C-N
bonds, ADCs are capable of adopting multiple, unique con-
formations if their N-substituents are differentially substi-
tuted. For example, we reported that the conformations
adopted by various ADCs as well as their parent formamidi-
nium salts and derivative metal complexes may be controlled
by tuning the steric properties of their N-substituents.18
Motivated by the inherent features of ADCs, particularly
those that are differentially substituted, we envisioned these
ligands enhancing the activities and/or selectivities of Ru-
based olefin metathesis catalysts that contain them. With these
goals in mind, we describe herein the synthesis of the first
examples of olefin metathesis catalysts that contain ADCs and
give a preliminary account of their catalytic properties.19
Since NHCs with bulky N-aryl substituents often result in
complexes with superior stabilities due to protection pro-
vided by steric shielding about the metal center,20 efforts
were focused on coordinating ADCs 6 and 7 (Figure 2),
which feature N-DIPP and N-Mes substituents, respectively,
to various Ru alkylidenes (DIPP = 2,6-di-isopropylphenyl;
Mes = 2,4,6-trimethylphenyl). The respective conjugate
Figure 1. Representative examples of commercially available
Ru-based catalysts (1-3) and selected examples of Ru catalysts
containing unsymmetrical NHC ligands (4, 5). Cy=cyclohexyl,
Mes= 2,4,6-trimethylphenyl, DIPP = 2,6-di-isopropylphenyl.
investigated for their abilities to afford cross-metathesis pro-
ducts with different E:Z ratios than catalysts containing sym-
metric NHC ligands, many of which had varying degrees of
success.13 While a Ru-based olefin metathesis catalyst that is
universally applicable and provides only cis products has yet to
be developed,14 the use of unsymmetrical NHCs as ligands
remains a viable approach.
An emerging class of ligands that may also facilitate the
realization of new classes of Ru-based olefin metathesis
catalysts with enhanced activities or stereoselectivities is acyc-
lic diaminocarbenes (ADCs).15 Compared to NHCs, ADCs
typically possess wider N-C-N bond angles, are stronger σ-
donors, and can be generated in a straightforward manner via
deprotonation of readily accessible formamidinium salts.16
Furthermore, a broad range of metal complexes containing
ADCs are known, some of which have been reported to be
acids of these ADCs, formamidinium iodides 6 HI and
3
7 HI, were synthesized by independently treating aceto-
3
nitrile solutions of N,N0-bis(2,6-di-isopropylphenyl)form-
amidine or N,N0-dimesitylformamidine with excess iodo-
methane in 69% and 88% yields, respectively. We previously
determined that 6 HI adopts a pseudo trans conformation in
3
solution as well as in the solid state;18 7 HI appears
to follow suit. Two inequivalent signals attributed to the
3
N-methyl groups were observed in the 1H NMR spectrum of
(13) (a) Vehlow, K.; Maechling, S.; Blechert, S. Organometallics 2006,
25, 25. (b) Ledoux, N.; Linden, A.; Allaert, B.; Mierde, H. V.; Verpoort, F.
Adv. Synth. Catal. 2007, 349, 1692. (c) Ledoux, N.; Allaert, B.; Linden, A.;
Van Der Voort, P.; Verpoort, F. Organometallics 2007, 26, 1052.
(14) For recent examples of Z-selective catalysts based on Mo, see:
(a) Ibrahem, I.; Yu, M.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem.
Soc. 2009, 131, 3844. (b) Flook, M. M.; Jiang, A. J.; Schrock, R. R.; M€uller,
P.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131, 7962. For selected
examples of the observation of Z-selectivity in cross-metathesis reactions
involving Ru-based catalysts, see: (a) Randl, S.; Gessler, S.; Wakamatsu, H.;
Blechert, S. Synlett 2001, 430. (b) Hansen, E. C.; Lee, D. Org. Lett. 2004, 6,
2035. (c) Sashuk, V.; Samojlowicz, C.; Szadkowska, A.; Grela, K. Chem.
Commun. 2008, 2468.
7 HI at δ=3.50 and 2.64 ppm (DMSO-d6), chemical shifts
3
that were nearly identical to those exhibited by 6 HI (δ =
3
3.59 and 2.69 ppm in the same solvent). Additionally, two
distinct singlets were observed for the N-aryl protons of the
mesityl groups at δ = 7.14 and 7.07 ppm. The solid state
structure of 7 HI (see Figure S1) was consistent with the
3
solution state assessment, and the key bond lengths and
angles exhibited by this complex were similar to those
observed in the solid state structure of 6 HI.
3
(15) Alder, R. W.; Allen, P. R.; Murray, M.; Orpen, A. G. Angew.
Chem., Int. Ed. Engl. 1996, 35, 1121.
€
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E.; Oliva, J. M.; Orpen, A. G.; Quayle, M. J. Chem. Commun. 1999, 241.
(c) Alder, R. W.; Blake, M. E.; Bufali, S.; Butts, C. P.; Orpen, A. G.; Sch€utz, J.;
Williams, S. J. J. Chem. Soc., Perkin Trans. 1 2001, 1586. (d) Denk, K.;
Sirsch, P.; Herrmann, W. A. J. Organomet. Chem. 2002, 649, 219.
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Chem.;Eur. J. 2005, 11, 1833. (b) Moncada, A. I.; Manne, S.; Tanski, J. M.;
Slaughter, L. M. Organometallics 2006, 25, 491. (c) Dhudshia, B.; Thadani,
A. N. Chem. Commun. 2006, 668. (d) Wanniarachchi, Y. A.; Kogiso, Y.;
Slaughter, L. M. Organometallics 2008, 27, 21. (e) Hirsch-Weil, D.; Snead,
D. R.; Inagaki, S.; Seo, H.; Abboud, K. A.; Hong, S. Chem. Commun. 2009,
18, 2475.
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(e) Herrmann, W. A.; Ofele, K.; Preysing, D. V.; Herdtweck, J. J. Organomet.
Chem. 2003, 684, 235. (f) Otto, M.; Conejero, S.; Canac, Y.; Romanenko,
V. D.; Rudzevitch, V.; Bertrand, G. J. Am. Chem. Soc. 2004, 126, 1016.
(g) Alder, R. W.; Blake, M. E.; Chaker, L.; Harvey, J. N.; Paolini, F.; Sch€utz, J.
Angew. Chem., Int. Ed. 2004, 43, 5896. (h) Magill, A. M.; Cavell, K. J.;
Yates, B. F. J. Am. Chem. Soc. 2004, 126, 8717. (i) Lee, M.-T.; Hu, C.-H.
Organometallics 2004, 23, 976. (j) Frey, G. D.; Herrmann, W. A. J.
Organomet. Chem. 2005, 690, 5876. (k) Frey, G. D.; Herdtweck, E.;
Herrmann, W. A. J. Organomet. Chem. 2006, 691, 2465. (l) Wanniarachchi,
Y. A.; Slaughter, L. M. Chem. Commun. 2007, 3294. (m) Yoshitha, A.;
Wanniarachchi, T. K.; Slaughter, L. M. Organometallics 2008, 27, 1055.
(18) Rosen, E. L.; Sanderson, M. D.; Saravanakumar, S.; Bielawski,
C. W. Organometallics 2007, 26, 5774.
(19) Attempts to prepare a Ru-based catalyst containing an ADC via
treatment of (PCy3)2Cl2RudCHPh with N,N,N0,N0-tetra(isopropyl)-
formamidin-2-ylidene were reported to result in decomposition; see:
€
Herrmann, W. A.; Ofele, K.; Preysing, D. V.; Herdtweck, E. J. Orga-
nomet. Chem. 2003, 684, 235.
ꢀ
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Chem. 2005, 690, 5407.

252 Organometallics, Vol. 29, No. 1, 2010
Rosen et al.
experiments were performed in CDCl3. Irradiation of
the benzylidene proton at δ = 18.60 ppm (CDCl3) in 8a led
to enhancement of signals that were attributed to both
N-DIPP and N-Mes substituents present in this complex.
This result suggested to us that the benzylidene moiety
was positioned in between these two aromatic systems.
Irradiation of the N-methyl signal at δ = 3.62 ppm in 8a
led to enhancement of two singlets at 6.87 and 6.82 ppm,
which were assigned to the N-mesityl’s aryl protons. Collec-
tively, these spectroscopic results were consistent with the
ADC ligand in 8a adopting a pseudo trans conformation,
which was similar to the structure of its respective free ADC
and 6 HI.18
3
Different spectroscopic results were obtained with 8b.
When the benzylidene signal (found at δ = 19.16 ppm;
CDCl3) was irradiated, a positive enhancement to a signal
found at δ=2.22 ppm was observed, suggesting the presence
of a juxtaposed N-methyl group. This interaction was con-
firmed after irradiating that same group (δ = 2.22 ppm),
which, in addition to enhancing the signal attributed to the
benzylidene moiety, also enhanced the signals found at δ =
2.00 and 1.62 ppm. These latter chemical shifts were assigned
to the 2,6-dimethyl groups on the N-mesityl substituent
connected to same nitrogen atom as the N-methyl
group under interrogation. Irradiation of the signal at δ =
3.10 ppm, which was assigned to the other N-methyl sub-
stituent, led to enhancement of the aromatic N-mesityl
protons found at δ = 6.90 ppm as well as two singlets at
2.08 and 1.86 ppm. This interaction was attributed to a NOE
contact with a mesityl group on the SIMes fragment. Collec-
tively, these results were consistent with the ADC ligand in
8b adopting a pseudo cis conformation where both N-methyl
substituents were oriented toward the Ru center. This geo-
metry differs from its respective free ADC22 and formami-
dinium salt, and indicated that 7 underwent a C-N bond
rotation either prior to or after metallation.23
Additional support for the unique conformation adopted
by the ADC ligand in 8b was obtained using X-ray crystal-
lography. X-ray quality crystals were obtained by vapor
diffusion of hexanes into a benzene solution saturated with
this complex.24 The ORTEP diagram shown in Figure 4
revealed that the ADC ligand in 8b adopted a pseudo cis
conformation in the solid state, a result that was consistent
with the aforementioned assessment of its solution state
structure. Close inspection of the crystal data revealed
that the distance between the adjoining25 arenes of the
Figure 3. Structures of Ru alkylidenes containing ADC ligands.
The conformations shown are consistent with their structures
observed in solution and, for 8b, 9a, and 9b, in the solid state.
With the aforementioned formaminidinum salts in hand,
efforts shifted toward synthesizing Ru alkylidenes that con-
tained the respective ADCs as ligands. Initial attempts to
synthesize (ADC)(PCy3)Cl2RudCHPh-type complexes by
treating independent benzene solutions of 6 HI or 7 HI with
3
3
NaN(SiMe3)2 (to generate their respective ADCs in situ)
followed by the addition of (PCy3)2Cl2RudCHPh were
promising. In addition to the observation of free PCy3,
new diagnostic signals were observed in the NMR spectra
of the crude product mixtures and tentatively assigned to
(6)(PCy3)Cl2RudCHPh (1H: 19.20 and 31P: 31.90 ppm,
CDCl3) and (7)(PCy3)Cl2RudCHPh (1H: 18.91 and 31P:
32.26 ppm, CDCl3), respectively. Unfortunately, pure mate-
rials could not be isolated from analogous preparation-scale
reactions in reasonable quantities, despite numerous at-
tempts using various techniques and conditions, including
chromatographic methods (using silica or alumina with
hexanes/ethyl acetate, hexanes/ether, hexanes/CH2Cl2, etc.),
precipitations, and triturations.
We reasoned that the apparent instabilities of the afore-
mentioned (ADC)(PCy3)Cl2RudCHPh-type complexes
could be related to the lability of the phosphine ligands
trans to the ADCs. As such, efforts shifted toward preparing
mixed NHC-ADC Ru complexes since analogous bis-
(NHC)-type Ru complexes are known to be relatively
stable.13,27 Gratifyingly, addition of (SIMes)(pyridine)2Cl2-
RudCHPh21 (SIMes = 1,3-dimesitylimidazolin-2-ylidene)
to a solution of the ADC (generated in situ via deprotonation
of 6 HI or 7 HI with NaN(SiMe3)2) afforded the desired
(22) Upon in situ deprotonation of 7 HI using NaHMDS, ADC 7
3
appears to adopt a pseudo trans conformation in solution, as determined
by NMR spectroscopy. 1H NMR (75 MHz, C6D6): δ 6.78 (s, 2H), 6.64 (s,
2H), 3.48 (s, 3H), 2.28 (s, 6H), 2.24 (s, 6H), 2.18 (s, 3H), 2.15 (s, 3H), 2.08
(s, 3H). 13C NMR (75 MHz, C6D6): δ 248.7, 149.1, 144.9, 136.6, 134.8,
134.1, 132.5, 129.4,þ128.8, 48.8, 35.5, 20.9, 18.5, 18.4. HRMS: calcd for
C21H29N2 [(M þ H )] 309.2331; found 309.2332.
(23) As noted in the text, the ADC ligand found coordinated to the
Ru center in 8b adopted a different ground state conformation than its
respective formidinium precursor, both in solution and in the solid state.
To determine if this complex was capable of displaying multiple con-
formations, a solution of 8b in toluene-d8 was examined by variable-
temperature 1H NMR spectroscopy. Unfortunately, no changes were
observed, even at 100 °C. Furthermore, the lack of Overhauser effects
between the benzylidene protons in 8 and 9 and the N-methyl groups
pointing toward the Ru centers in these complexes suggest that rotation
about the Ru-ADC bond may be relatively slow on the NMR timescale.
(24) Unfortunately, all efforts to obtain X-ray quality crystals of 8a
have been unsuccessful.
3
3
complexes, 8a and 8b, in 35% and 66% isolated yields,
respectively, after purification by column chromatography.
The characteristic benzylidene signals in these complexes
were observed at δ = 18.59 and 19.16 ppm (CDCl3) in their
respective 1H NMR spectra. To elucidate the solution struc-
tures adopted by complexes 8a and 8b, a series of NOESY
(21) Sanford, M. S.; Love, J. A.; Grubbs, R. H. Organometallics 2001,
20, 5314.
(25) The angle between the planes of these arenes was calculated to be
19.4°.

Article
Organometallics, Vol. 29, No. 1, 2010 253
Figure 5. (Left) ORTEP diagram of 9a showing ellipsoids at
50% probability. H atoms have been removed for clarity. Key
˚
Figure 4. ORTEP diagram of (7)(SIMes)Cl2RudCHPh (8b)
showing 50% probability ellipsoids. H atoms and solvent mole-
˚
cules have been removed for clarity. Selected bond lengths (A)
and angles (deg): Ru-C1, 2.112(3); Ru-C2, 2.132(3); Ru-C5,
1.840(3); N1-C1, 1.356(3); N2-C1, 1.356(4); N3-C2, 1.360(4);
N4-C2, 1.343(3); N1-C1-Ru, 110.6(2); N2-C1-Ru,
130.6(2); N3-C2-Ru, 125.4(2); N4-C2-Ru, 128.3(2); C1-
Ru-C5, 104.0(1); C2-Ru-C5, 95.7(1); C1-Ru-C2, 160.1(1);
Cl1-Ru-Cl2, 170.76; N1-C1-N2, 118.7(3); N3-C2-N4,
106.3(2).
bond lengths (A) and angles (deg): Ru-C1, 2.015(2); Ru-C2,
1.834(2); Ru-O, 2.333(2); N1-C1, 1.359(3); N1-C2, 1.350(3);
N1-C1-Ru, 110.4; N2-C1-Ru, 130.2(2); C1-Ru-C2,
106.56(9); C1-Ru-O, 175.75(7); Cl1-Ru-Cl2, 155.70(2);
N1-C1-N2, 119.4(2). (Right) ORTEP diagram of 9b showing
ellipsoids at 50% probability. H atoms and solvent molecules
˚
have been removed for clarity. Key bond lengths (A) and angles
(deg): Ru-C1, 2.012(2); Ru-C2, 1.838(1); Ru-O, 2.307(1);
N1-C1, 1.355(2); N1-C2, 1.481(3); N1-C1-Ru, 110.2(1);
N2-C1-Ru, 130.1(1); C1-Ru-C2, 107.361(7); C1-Ru-O,
174.47(6); Cl1-Ru-Cl2, 158.97(2); N1-C1-N2, 119.6(2).
˚
N-mesityl substituents in the ADC (3.58 A) was in accord
with a favorable π-π* interaction,26 which may facilitate
formation of the unusual conformation observed. Regard-
less, the ADC ligand possessed a relatively wide N-C-N
angle (118.7(3)°), which may impose additional steric con-
straint on the Ru center than may be otherwise expected and
explain why the observed CNHC-Ru-CADC bond angle
(C1-Ru-C2: 160.1(1)°) in 8b was significantly more acute
than the analogous angles observed in other reported bis-
(NHC) Ru complexes (162.0(2)-171.2(1)°).27,28 The other
structural metrics of 8b were relatively similar to those found
in reported bis(NHC) Ru complexes. For example, the
30.54 ppm and δ = 19.62 and 30.54 ppm for the reactions
involving 8a and 8b, respectively. These signals were nearly
identical to those observed in C6D6 solutions of 2 (δ=19.63
and 30.53 ppm). While the exchange reaction between 8a and
PCy3 was complete within 2 h, the analogous reaction with 8b
took nearly 8 h. The rate difference may be related to the
increased sterics associated with 6 as compared to 7, facilitat-
ing dissociation of the former. Regardless, these results sug-
gested to us that the relatively bulky ADC ligands in 8a and 8b
were dissociating from their respective metal centers in pre-
ference to SIMes. As such, the catalytically active species
generated from these catalysts would be effectively the same as
those generated from commercially available catalyst 2 or 3.
Although the catalytic activities of 8a and 8b were still
investigated (see below), efforts were also directed toward
the synthesis of ADC-containing derivatives with more labile
ligands. In particular, efforts focused on synthesizing
Hoveyda-Grubbs-type catalysts, which contain a weakly
bound aryl ether trans to either an NHC or phosphine.29
(ADC)Cl2RudCH(2-isopropoxy)Ph-type complexes, 9a
and 9b, were synthesized via treatment of (PCy3)-
Cl2RudCH(2-isopropoxy)Ph with 6 or 7 (generated in situ
from their respective formamidinium salts), respectively. As
observed with 8a and 8b, complexes 9a and 9b were stable
toward column chromatography, which facilitated their
isolation. On the basis of a series of NOESY measurements,
the ADCs in these complexes were determined to adopt
pseudo trans conformations in which one N-aryl ring was
juxtaposed with the benzylidene moieties. This assessment
was confirmed by analyzing single crystals of the aforemen-
tioned complexes using X-ray diffraction (see Figure 5).
Compared to NHC analogues 4,13a,b the N-C-N bond
˚
Ru-Cbenzylidene bond distance (Ru-C5, 1.840(3) A) was
comparable to the analogous bond distances reported for
13,27
˚
related bis(NHC) Ru complexes (1.818(4)-1.835(2) A).
˚
The Ru-CNHC bond distance (Ru-C2, 2.132(3) A) ob-
served in 8b, however, was slightly longer than analogous
distances observed in a range of other bis(NHC) Ru com-
13,27
˚
plexes (2.115(3)-2.122(3) A).
With catalytic applications in mind, efforts were directed
toward determining whether the ADC or the NHC ligand in
8a and 8b was more likely to dissociate. It has been pre-
viously reported that the addition of excess PCy3 to
(NHC)2Cl2RudCHPh-type complexes can be used to deter-
mine ligand lability.13c Heating a mixture of either 8a or 8b
(20 mg in 0.8 mL of C6D6) in the presence of a 10-fold molar
excess of PCy3 at 100 °C (sealed tube) resulted in a dramatic
color change from olive green to tan-red and resulted in the
formation of (SIMes)(PCy3)Cl2RudCHPh (2). 1H and
31P NMR spectroscopic analysis of the crude reaction mix-
tures showed the formation of new signals at δ= 19.60 and
(26) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112,
5525.
angles observed in the solid state structures of
9
(27) (a) Weskamp, T.; Schattenmann, W. C.; Spiegler, M.;
Herrmann, W. A. Angew. Chem., Int. Ed. 1998, 37, 2490. (b) Conrad,
J. C.; Yap, G. P. A.; Fogg, D. E. Organometallics 2003, 22, 1986.
(28) This angle is similar to that observed for (1-methyl-3-(2,6-di-
isopropylphenyl)imidazolinylidene)2Cl2RudCHPh (162.0(2)°).13c
were significantly more obtuse (119.4(2)-119.6(2)° versus
(29) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H.
J. Am. Chem. Soc. 2000, 122, 8168.

254 Organometallics, Vol. 29, No. 1, 2010
Rosen et al.
Figure 6. E:Z ratio of 12 versus conversion of 11 to 12 using 3 (O), 4a (0), 9a (9), and 9b (b) as catalysts. Conditions are shown in eq 2.
The reaction using 9a was performed at 60 °C; all other reactions were run at 23 °C. Ratios and conversions were determined by gas
chromatography.31
107.2(5)-107.6(3)°). These relatively large angles may im-
pose additional steric constraints on the ligated metal cen-
ters30 and explain why the 1H NMR signals attributed to the
benzylidene moieties of these complexes (δ=15.94 and 15.92
ppm, respectively; CDCl3) were upfield compared to those
observed in 4 (δ = 16.13-16.22 ppm).13a,b In addition, the
Ru-O bond lengths in 9 were longer than analogous dis-
be 89%, 62%, 100%, and 100%, for 8a, 8b, 9a, and 9b,
respectively, after 12 h (conditions: [DDM]0 = 0.1 M, 1.0
mol % catalyst, CD2Cl2). More impressively, diethyl bis(2-
methylallyl)malonate, a sterically encumbered olefin, was
quantitatively converted to the expected product in less
than 1 h when the reactions were performed at 100 °C
using catalysts 9a and 9b (conditions: [substrate]0 = 0.1 M,
5.0 mol % catalyst, toluene-d8). Under otherwise identical
conditions, the analogous RCM reaction involving 8a or 8b
required up to 12 h to reach similar conversions.
Having established that 8 and 9 showed reasonable acti-
vities at elevated temperatures, subsequent efforts focused
on determining if the unsymmetrical conformations adopted
by their ADC ligands would affect the inherent E:Z selecti-
vities displayed by these complexes in various metathesis
reactions. Emphasis was placed on catalysts 9a and 9b since
it was concluded from the aforementioned phosphine ex-
change experiments that the ADC ligands in 8 likely
dissociate in preference to the NHC ligands present.
Catalysts 9a and 9b were studied in two representative
cross-metathesis (CM) reactions to allow comparison of
their inherent E:Z selectivities to those exhibited by the
analogous NHC catalysts 4a and 4b. Under the conditions
summarized in eq 1, 9a and 9b afforded 10 with E:Z ratios=
1.2:1 and 0.6:1, respectively, at relatively early stages of the
reaction (30% and 32% conversion of allylbenzene (11) to
the desired CM product, respectively). For comparison,
catalysts 4a and 4b, which contain NHCs analogous to the
ADCs in 9a and 9b, were reported to afford the same product
with higher E:Z ratios (E:Z=2.8:1 and 1.8:1, respectively) at
similar conversions (<33%) and under similar conditions.13b
Analogous results were obtained when the CM of 11 with
1,4-diacetoxy-2-butene to afford 12 was explored (see eq 2).
As shown in Figure 6, 9a afforded 12 in a nearly 1:1 ratio
of its E and Z isomers to conversions that exceeded 75%.
˚
tances found in the crystal structures of 4 (2.307(1)-2.333(2) A
versus 2.269(3)-2.281(4) A),13 which may reflect the relatively
˚
strong electron donicities of the ADCs as compared to NHCs.
After the synthesis and characterization of 8 and 9, a
preliminary investigation of their catalytic activities was con-
ducted.31 Under standardized conditions,32 these complexes
showed relatively low catalytic activities in various ring-closing
metathesis (RCM) reactions, compared to 1 and 2. Although
the highest activity in the RCM of diethyl diallyl malonate
(DDM) was observed when 9b was used (23% conversion after
1 h), this catalyst was less active than 2 (∼100% conversion
within 30 min)32 (conditions: [DDM]0 = 0.1 M, 1.0 mol %
catalyst, CD2Cl2, 30 °C). In accord with results observed with
Ru complexes containing exceptionally bulky NHC ligands
(e.g., 1-(1-adamantyl)-3-mesityl-4,5-dihydroimidazol-2-ylidene),
the relatively slow kinetics displayed by 8 and 9 may be due
to the increased steric bulk of the ADCs interfering with olefin
coordination or the catalytic mechanism.33
However, significantly enhanced catalytic activities were
observed at elevated temperatures. For example, at 40 °C,
conversions of DDM to its cyclic product were determined to
(30) The distances between the benzylidene proton and the centroid
of the juxtaposed arenes are significantlyshorter for 9a and 9b (2.321 and
˚
˚
˚
˚
2.365 A, respectively) than for 3 (3: 2.489 A, 4a: 2.567 A, 4b: 2.595 A).
(31) See the Supporting Information for additional details.
(32) Ritter, T.; Hejl, A.; Wenzel, A. G.; Funk, T. W.; Grubbs, R. H.
Organometallics 2006, 25, 5740.
(33) Dinger, M. B.; Nieczypor, P.; Mol, J. C. Organometallics 2003,
22, 5291.

Article
Organometallics, Vol. 29, No. 1, 2010 255
For comparison, 4a also afforded 12, but with an E:Z
ratio = 2.5:1 at similar conversions. Likewise, catalyst 9b
yielded 12 with a lower E:Z ratio (ca. 4:1) than that re-
ported13 for 4b (6:1) under otherwise identical conditions
and conversions (80%).34
N,N0-bis(DIPP)-N,N0-dimethylformamidinium iodide (6 HI),18
3
N,N0-dimesitylformamidine,35 (SIMes)Cl2RudCH(2-isopro-
poxy)Ph (3),29 and (1-DIPP-3-methylimidazolin-2-ylidene)-
Cl2RudCH(2-isopropoxy)Ph
(4a)13c
were
synthesized
according to literature procedures. All other materials
and solvents were of reagent quality and were used as
received. Unless otherwise noted, all manipulations were
performed under an atmosphere of nitrogen using drybox
or Schlenk techniques.
While the origin of the selectivities observed in the reac-
tions described above is presumed to be due to the increased
sterics of the ADC-containing catalysts as compared to their
NHC analogues, it may also be related to a reduced ability of
the former to facilitate olefin isomerization and other secon-
dary metatheses. To investigate, a CM reaction involving 11
and 1,4-diacetoxy-2-butene was performed as described in eq
2 (60 °C), using 9a as the catalyst. When the conversion of 11
to 12 reached 95% (24 h; E:Z of product = 1.5:1), the
reaction mixture was split into two fractions. One fraction
was left untreated and a second bolus of catalyst was added
to the other ([Ru]final = 5.0 mol %). After heating both
fractions to 60 °C for an additional 24 h, they were analyzed.
The E:Z of 12 found in these fractions increased to 3.0:1
and 7.0:1, respectively. Hence, while 9a facilitates secon-
dary metathesis reactions, these processes appear to be
relatively slow.
In summary, the first examples of Ru-alkylidene olefin
metathesis catalysts containing acyclic diaminocarbene
ligands have been prepared. Compared to related NHC
analogues, the ADCs imposed unique steric and elec-
tronic constraints on the metal centers to which they were
ligated, as evidenced by relatively wide N-C-N bond
angles, long Ru-O bond lengths, and other spectroscopic
data. Furthermore, ADCs exhibited unsymmetrical con-
formations when coordinated to Ru, except for complex
8b, for which, surprisingly, its ADC ligand 7 underwent
a C-N rotation prior to or after complexation. This re-
sulted in a pseudo cis conformation of the ADC ligand
with the N-methyl substituents oriented toward the coordi-
nated metal. The conformations of the ADC moieties
were assigned in solution and found to be consistent with
their solid state structures, where applicable. Finally, Ru-
based catalysts containing ADCs afforded CM products
with lower E:Z ratios than analogous NHC-containing
catalysts.
1
Instrumentation. H and 13C{1H} NMR spectra were recor-
ded using a Varian 300, 400, 500, or 600 MHz spectrometer.
Chemical shifts δ (in ppm) were referenced to tetramethylsilane
using the residual solvent as an internal standard. For 1H NMR:
CDCl3, 7.24 ppm; C6D6, 7.15 ppm; toluene-d8, 2.09 ppm;
CD2Cl2, 5.32 ppm; DMSO-d6, 2.49 ppm. For 13C NMR: CDCl3,
77.0 ppm; DMSO-d6, 39.5 ppm. Coupling constants (J) are
expressed in hertz (Hz). 31P NMR spectra were recorded using a
Varian 300 MHz spectrometer, with chemical shifts δ (in ppm)
referenced to H3PO4. To determine the proton-decoupled che-
mical shift corresponding to the benzylidene 13C nuclei in
complexes 8 and 9, the decoupling frequencies were set to the
1H chemical shifts of their respective benzylidene signals. High-
resolution mass spectra (HRMS) were obtained with a VG
analytical ZAB2-E or a Karatos MS9 instrument (ESI or CI)
and are reported as m/z (relative intensity). Gas chromatogra-
phy (GC) was performed on an Agilent 6850 gas chromato-
graph. Elemental analyses were performed by Midwest
Microlabs, LLC (Indianapolis, IN).
N,N0-Dimesityl-N,N0-dimethylformamidinium Iodide (7 HI).
3
Under an atmosphere of air, a 30 mL pressure vessel equipped
with a stir bar was charged with N,N0-dimesitylformamidine
(2.79 g, 9.96 mmol), NaHCO3 (4.20 g, 49.8 mmol), and CH3CN
(20 mL). Methyl iodide (4.24 g, 29.9 mmol) was added to the result-
ing suspension, and the vessel was sealed with a Teflon-lined
cap. The reaction mixture was stirred for 12 h at 85 °C. After the
mixture was allowed to cool to ambient temperature, it was
filtered. The filtrate was concentrated to dryness and then
triturated with diethyl ether. The resulting solid was recovered
via vacuum filtration and dried under high vacuum to afford the
desired product as a pale yellow powder (3.82 g, 88% yield).
Crystals of 7 HI were grown as colorless prisms by slow
3
evaporation from ethyl acetate. Mp: 220-222 °C. 1H NMR
(300 MHz, DMSO-d6): δ 8.46 (s, 1H), 7.12 (s, 2H), 7.04 (s, 2H),
3.52 (s, 3H), 2.61 (s, 3H), 2.36 (s, 6H), 2.26 (s, 9H), 2.24 (s, 3H).
13C NMR (100 MHz, DMSO-d6): δ 156.0, 140.7, 139.7, 138.8,
134.3, 134.0, 133.8, 129.4, 129.3, 45.8, 37.4, 20.5, 20.4, 17.4, 17.0.
HRMS: calcd for C21H29N2 ([Mþ]) 309.2329; found 309.2325.
Anal. Calcd (%) for C21H29N2I: C, 57.80; H, 6.70; N, 6.42.
Found: C, 57.77; H, 6.73; N, 6.47.
Experimental Section
Materials and Methods. Benzene was distilled from sodium
and benzophenone under an atmosphere of nitrogen. Dichloro-
methane (CH2Cl2) and toluene were distilled from CaH2 under
an atmosphere of nitrogen. All solvents were degassed by
three, consecutive freeze-pump-thaw cycles. Allylbenzene
(11), acrylonitrile, cis-1,4-diacetoxy-2-butene, and n-octane
were purchased from Aldrich and degassed using three consecutive
freeze-pump-thaw cycles before use. (PCy3)2Cl2RudCHPh
(1) and (SIMes)(PCy3)Cl2RudCHPh (2) were generously
donated by Materia, Inc. The Hoveyda-Grubbs first-genera-
tion catalyst, (PCy3)Cl2RudCH(2-isopropoxy)Ph, was pur-
chased from Aldrich. (SIMes)(pyridine)2Cl2RudCHPh,21
(N,N0-Bis(2,6-di-isopropylphenyl)-N,N0-dimethylformamidin-
2-ylidene)(SIMes)Cl2RudCHPh (8a). A 6 mL glass vial equip-
ped with a stir bar was charged with 6 HI (213 mg, 0.409 mmol)
3
and NaN(SiMe3)2 (85 mg, 0.464 mmol). Toluene (5 mL) was
added to the mixture, and the vial was sealed with a Teflon-lined
cap. The solution was stirred for 30 min at ambient temperature
and then filtered through a 0.2 μm PTFE filter into a second
clean vial equipped with a stir bar. (SIMes)(pyridine)2-
Cl2RudCHPh (85 mg, 0.117 mmol) was then added, and the
vial was sealed with a Teflon-lined cap. The reaction was
allowed to stir for 1.5 h at 60 °C. The solvent was then removed
under reduced pressure. Hexanes (2 mL) was added to the
resulting green solid, which resulted in the formation of a
yellow-brown precipitate that was later removed by filtration.
The green filtrate was then loaded onto a short column of silica
gel. The silica gel was washed with hexanes (20 mL) followed by
ethyl acetate (10 mL), and the green band which eluted was
collected. The green solution was concentrated under reduced
pressure and dried under high vacuum to afford the desired
(34) We also monitored the ring-opening metathesis polymerization of
1,5-cyclooctadiene (COD) over time using 4a and 9a as catalysts by
quantitative 1H and 13C NMR spectroscopy (conditions: C6D6 as solvent,
75 °C (sealed tube), [COD]0 = 0.50 M, [catalyst]0 = 0.016 M).31 At low
monomer conversions (<35%), both catalysts afforded predominantly cis
polybutadiene (E:Z = 1.0:4.8 for 9a at 32% monomer conversion and
1.0:3.2 for 4a at 34% were observed). At conversions approaching 50%, 9a
afforded polymer with a higher cis content(E:Z=1.0:4.0 at 49% monomer
conversion) than 4a (E:Z=1.0:2.8 at 48%).
(35) Taylor, E. C.; Ehrhart, W. A. J. Org. Chem. 1963, 28, 1108.

256 Organometallics, Vol. 29, No. 1, 2010
Rosen et al.
1
product as a green solid (73.6 mg, 66% yield). H NMR (300
MHz, CDCl3): δ 18.59 (s, 1H), 8.97 (d, 1H, J=7.8), 7.27-7.22
(m overlapping with solvent, 1H, J=15.3), 7.09-7.03 (m, 3H),
6.94 (s, 1H), 6.87-6.74 (m, 3H), 6.66-6.61 (m, 2H), 6.36 (t, 1H,
J = 7.5), 5.99 (t, 2H, J = 6.0), 5.52 (s, 1H), 4.02-3.92 (m, 2H),
3.79-3.74 (m, 2H), 3.61 (s, 3H), 3.57-3.50 (m, 3H), 2.88-2.83
(m, 1H), 2.62 (s, 3H), 2.56 (s, 3H), 2.45 (s, 3H), 2.35 (s, 3H), 2.19
(s, 3H), 1.88 (s, 3H), 1.84 (s, 3H), 1.40 (d, 3H, J=6.6), 1.25 (d,
6H, J=6.6), 1.18 (d, 3H, J=6.9), 1.13 (d, 3H, J=6.3), 1.05 (d,
3H, J = 6.9), 0.81 (d, 3H, J = 6.6), 0.62 (d, 3H, J = 6.6). 13C
NMR (100 MHz, CDCl3): δ 296.1, 222.9, 216.6, 149.8, 148.8,
146.5, 146.0, 145.8, 143.8, 143.2, 139.5, 138.3, 137.9, 137.71,
137.66, 137.3, 136.8, 136.1, 132.6, 130.9, 129.4, 128.94, 128.90,
128.5,128.3, 128.2, 126.8, 126.3, 125.6, 123.5, 123.42, 123.39,
40.7, 27.8, 27.7, 27.6, 27.4, 27.2, 26.5, 26.0, 25.6, 25.2, 24.1, 22.5,
22.4, 22.3, 20.95, 20.94, 19.8, 19.1, 18.6, 18.4. HRMS: calcd for
C55H72Cl2N4Ru ([Mþ]) 960.4178; found 960.4163. Anal. Calcd
(%) for C55H72Cl2N4Ru: C, 68.73; H, 7.55; N, 5.83. Found: C,
68.63; H, 7.41; N, 5.94.
form. The green precipitate was collected and dried under
vacuum to afford the desired product as a dark green micro-
crystalline solid (34 mg, 28% yield). Crystals of 9a were grown as
green prisms by vapor diffusion of pentane into a C6H6 solution
of the complex. 1H NMR (300 MHz, CDCl3): δ 15.94 (s, 1H),
7.58-7.48 (m, 2H), 7.37-7.33 (m, 3H), 7.20 (d, 2H, J = 7.5),
6.95 (d, 1H, J = 7.2), 6.85 (t, 2H, J = 7.5), 5.05-5.01 (m, 1H),
4.97 (s, 3H), 3.98-3.90 (m, 2H), 3.30-3.21 (m, 2H), 2.88 (s, 3H),
1.69 (d, 6H, J=6.0), 1.36-1.34 (2 overlapping d, 12H), 1.06 (d,
6H, J=6.9), 0.94 (d, 6H, J=6.6). 13C NMR (125 MHz, CDCl3):
δ 299.3, 202.1, 152.0, 148.4, 146.6, 146.1, 145.3, 144.9, 129.5,
129.0, 128.7, 124.9, 124.2, 122.9, 122.4, 133.0, 74.5, 46.9, 46.8,
28.0, 27.7, 26.2, 26.1, 23.6, 23.0, 22.4. HRMS: calcd for
C37H52Cl2N2ORu ([Mþ]) 712.2500, found 712.2500. Anal.
Calcd (%) for C37H52Cl2N2ORu: C, 62.35; H, 7.35; N, 3.93.
Found: C, 62.41; H, 7.29; N, 4.03.
(N,N0-Dimesityl-N,N0-dimethylformamidin-2-ylidene)Cl2Rud
CH(2-isopropoxy)Ph (9b). A 6 mL glass vial equipped with a stir
bar was charged with 7 HI (191 mg, 0.438 mmol), NaN(SiMe3)2
3
(N,N0-Dimesityl-N,N0-dimethylformamidin-2-ylidene)(SIMes)-
Cl2RudCHPh (8b). A 6 mL glass vial equipped with a stir bar
was charged with 7 HI (158 mg, 0.363 mmol), NaN(SiMe3)2
(80.0 mg, 0.438 mmol), and benzene (4 mL) and then sealed
with a Teflon-lined cap. The solution was stirred for 30 min
at ambient temperature, after which it was filtered through a
0.2 μm PTFE filter into a second clean glass vial containing
a stir bar. (PCy3)Cl2RudCH(2-isopropoxy)Ph (105 mg, 0.175
mmol) was added, and the vial was sealed with a Teflon-lined
cap. The reaction mixture was stirred at 50 °C for 3 h. The
solution was then concentrated under reduced pressure to
afford a dark green solid. The resulting solid was purified by
column chromatography on silica gel using hexanes/ethyl
acetate (5:1 v/v) as the eluent. A dark green band eluted first,
which was determined to be an intermediate where PCy3 had
displaced the coordinating isopropoxy moiety.36 This solu-
tion was concentrated, dissolved in 5 mL of CHCl3, and then
stirred at ambient temperature for 10 h to induce phosphine
dissociation. A second band was eluted from the aforemen-
tioned column, which contained the desired product. After the
first band had finished stirring, it was combined with the second
band and the resulting solution was concentrated under reduced
pressure. Diethyl ether (5 mL) was then added, which caused a
light green solid to precipitate upon standing. After decanting
the mother liquor, the residual solids were collected by vacuum
filtration and washed with hexanes (5 mL). The solids were then
dried under high vacuum to afford the desired product as a light
3
(66.0 mg, 0.363 mmol), and toluene (4 mL) and then sealed with
a Teflon-lined cap. The reaction mixture was stirred for 30 min
at ambient temperature. A cloudy mixture formed, which was
subsequently filtered through a 0.2 μm PTFE filter into a clean
glass vial equipped with a stir bar (in a drybox). (SIMes)-
(pyridine)2Cl2RudCHPh (251 mg, 0.346 mmol) was added,
and the vial was sealed with a Teflon-lined cap. The solution
was stirred at ambient temperature for 12 h and then concen-
trated under reduced pressure to afford a green solid. The solid
was triturated with pentane and filtered. The recovered solid
was then dissolved in ethyl acetate and filtered through a short
column of silica gel. Removal of residual solvent under high
vacuum afforded the desired product as a light green solid
(104.9 mg, 35% yield). Crystals of 8b C5H12 were grown as
3
green plates by slow evaporation from pentane. 1H NMR
(300 MHz, CDCl3): δ 19.16 (s, 1H), 9.50 (br s, 2H), 7.31
(t, 1H, J = 7.3), 7.07 (br s, 2H), 6.90 (s, 2H), 6.85 (s, 1H), 6.43
(s, 1H), 6.30 (s, 1H), 6.18 (s, 1H), 6.07 (s, 1H), 5.87 (s, 1H),
4.06-3.89 (m, 3H), 3.78-3.73 (m, 1H), 3.11 (s, 3H), 2.77 (s, 3H),
2.62 (s, 3H), 2.53 (s, 3H), 2.22 (s, 3H), 2.19 (s, 3H), 2.07 (s, 3H),
2.02 (s, 3H), 1.99 (s, 3H), 1.97 (s, 3H), 1.94 (s, 3H), 1.86 (s, 3H),
1.62 (s, 3H). 13C NMR (125 MHz, CDCl3): δ 295.8, 227.4, 222.7,
152.0, 144.0, 142.1, 139.2, 139.0, 138.3, 138.0, 137.6, 137.5,
136.8, 136.6, 135.6, 135.1, 135.0, 134.4, 133.9, 133.8, 129.4,
129.3, 129.1, 128.9, 128.2, 128.0, 127.9, 127.8, 127.4, 51.8,
51.6, 47.4, 40.6, 21.1, 21.0, 20.6, 20.4, 19.8, 19.66, 19.63, 19.5,
18.8, 18.65, 18.57, 18.3. HRMS: calcd for C49H60Cl2N4Ru
([Mþ]) 876.3239; found 876.3237. Anal. Calcd (%) for
C49H60Cl2N4Ru: C, 67.11; H, 6.90; N, 6.39. Found: C, 66.82;
H, 6.83; N, 6.40.
green powder (59 mg, 54% yield). Crystals of 9b 1.5(C6H6) were
3
grown as green needles by vapor diffusion of pentane into a
CH2Cl2 solution of the complex. 1H NMR (300 MHz, CDCl3):
δ 15.92 (s, 1H), 7.52 (t, 1H, J=8.7), 7.04 (s, 2H), 7.00-6.97 (m,
1H), 6.93 (s, 2H), 6.89-6.86 (m, 2H), 5.06 (h, 1H, J=6.0), 4.80
(s, 3H), 2.82 (s, 3H), 2.54 (s, 6H), 2.46 (s, 3H), 2.30 (s, 3H), 2.26
(s, 6H), 1.67 (d, 6H, J = 6.00). 13C NMR (125 MHz, CDCl3):
δ 302.8, 201.2, 151.7, 148.4, 145.4, 144.9, 138.0, 137.5,
136.7, 135.6, 129.9, 129.8, 129.0, 123.3, 122.6, 113.0, 74.4,
44.6, 42.7, 22.2, 21.1, 21.0, 18.7, 18.3. HRMS: calcd for
C31H40Cl2N2ORu ([Mþ]) 628.1561, found 628.1559. Anal.
Calcd for C31H40Cl2N2ORu: C, 59.23; H, 6.41; N, 4.46. Found:
C, 58.91; H, 6.29; N, 4.60.
(N,N0-Bis(2,6-di-isopropylphenyl)-N,N0-dimethylformamidin-
2-ylidene)Cl2RudCH(2-isopropoxy)Ph (9a). A 6 mL glass vial
equipped with a stir bar was charged with 6 HI (185 mg, 0.355
3
mmol), NaN(SiMe3)2 (65.0 mg, 0.355 mmol), and benzene
(4 mL) and then sealed with a Teflon-lined cap. The mixture
was stirred for 30 min at ambient temperature, after which it was
filtered through a 0.2 μm PTFE filter into a second clean glass
vial containing a stir bar. (PCy3)Cl2RudCH(2-isopropoxy)Ph
(100 mg, 0.166 mmol) was added, and the vial was sealed with a
Teflon-lined cap. The resulting purple-red solution was then
stirred for 5 h at ambient temperature, after which the color had
changed to green-brown. The solution was then concentrated
under reduced pressure to afford a dark green solid. The solid
was dissolved in a minimal amount of dichloromethane (ca.
2 mL) and then filtered through a short column of silica gel with
the aid of additional dichloromethane (ca. 4 mL). The filtrate
was then evaporated to a volume of approximately 1 mL, and
pentane (5 mL) was added, which caused a green precipitate to
Acknowledgment. We are grateful to the National
Science Foundation (CHE-0645563), the Welch Founda-
tion (F-1621), the Sloan Foundation, and Materia, Inc.,
for their generous support.
Supporting Information Available: 1H and 13C NMR spectra,
crystallographic data, and kinetics details are available free of
charge via the Internet at http://pubs.acs.org.
(36) An analogous intermediate has been reported in the syn-
thesis of related NHC-containing Hoveyda-Grubbs-type catalysts;
see ref 13b.