Cyclic ruthenium-alkylidene catalysts for ring-expansion metathesis polymerization

Article abstract of DOI:10.1021/ja8037849

A series of cyclic Ru-alkylidene catalysts have been prepared and evaluated for their efficiency in ring-expansion metathesis polymerization (REMP). The catalyst structures feature chelating tethers extending from one N-atom of an N-heterocyclic carbene (NHC) ligand to the Ru metal center. The catalyst design is modular in nature, which provided access to Ru complexes having varying tether lengths, as well as electronically different NHC ligands. Structural impacts of the tether length were unveiled through ¹H NMR spectroscopy as well as single-crystal X-ray analyses. Catalyst activities were evaluated via polymerization of cyclooctene, and key data are provided regarding propagation rates, intramolecular chain transfer, and catalyst stabilities, three areas necessary for the efficient synthesis of cyclic poly(olefin)s via REMP. From these studies, it was determined that while increasing the tether length of the catalyst leads to enhanced rates of polymerization, shorter tethers were found to facilitate intramolecular chain transfer and release of catalyst from the polymer. Electronic modification of the NHC via backbone saturation was found to enhance polymerization rates to a greater extent than did homologation of the tether. Overall, cyclic Ru complexes bearing 5- or 6-carbon tethers and saturated NHC ligands were found to be readily synthesized, bench-stable, and highly active catalysts for REMP.

Full text of DOI:10.1021/ja8037849

Published on Web 08/27/2008
Cyclic Ruthenium-Alkylidene Catalysts for Ring-Expansion
Metathesis Polymerization
Andrew J. Boydston, Yan Xia, Julia A. Kornfield, Irina A. Gorodetskaya, and
Robert H. Grubbs*
Arnold and Mabel Beckman Laboratory of Chemical Synthesis, DiVision of Chemistry and
Chemical Engineering, California Institute of Technology, Pasadena, California 91125
Received May 27, 2008; E-mail: rhg@caltech.edu
Abstract: A series of cyclic Ru-alkylidene catalysts have been prepared and evaluated for their efficiency
in ring-expansion metathesis polymerization (REMP). The catalyst structures feature chelating tethers
extending from one N-atom of an N-heterocyclic carbene (NHC) ligand to the Ru metal center. The catalyst
design is modular in nature, which provided access to Ru complexes having varying tether lengths, as well
as electronically different NHC ligands. Structural impacts of the tether length were unveiled through 1H
NMR spectroscopy as well as single-crystal X-ray analyses. Catalyst activities were evaluated via
polymerization of cyclooctene, and key data are provided regarding propagation rates, intramolecular chain
transfer, and catalyst stabilities, three areas necessary for the efficient synthesis of cyclic poly(olefin)s via
REMP. From these studies, it was determined that while increasing the tether length of the catalyst leads
to enhanced rates of polymerization, shorter tethers were found to facilitate intramolecular chain transfer
and release of catalyst from the polymer. Electronic modification of the NHC via backbone saturation was
found to enhance polymerization rates to a greater extent than did homologation of the tether. Overall,
cyclic Ru complexes bearing 5- or 6-carbon tethers and saturated NHC ligands were found to be readily
synthesized, bench-stable, and highly active catalysts for REMP.
tathesis,7 and stereoselective cross-metathesis (CM) have each
benefited from breakthroughs in catalyst design, development,
Introduction
and application.8
The exploration of Ru-based metathesis catalysts has opened
doorways to multiple areas of synthetic and polymer chemistry.1,2
Advances in these areas have been made possible via develop-
ment of new catalyst scaffolds based upon bis(phosphine)
complex 1 (Figure 1) or those bearing N-heterocyclic carbene
(NHC) ligands such as 2 and 3. The introduction of catalysts
based on 1-3, but predisposed for specific tasks, has further
expanded the potential of olefin metathesis. For example, areas
such as solid-supported catalysis,3 asymmetric olefin metathesis,4
tandem catalysis,5 living polymerization,1a,6 acyclic diene me-
Recently, a Ru-based catalyst design was reported that
featured a chelating N-to-Ru tether (Figure 2).9 Whereas the
catalytic activities of 4cyc-6cyc have not been explored, 7cyc was
found to mediate the synthesis of cyclic polymers from cyclic
monomers (Scheme 1).10,11 This ring-expansion metathesis
polymerization (REMP) afforded the ability to produce cyclic
polymers on a large scale from diverse, readily available cyclic
monomers.12,13 While the high catalytic activity of 7cyc was
(6) (a) Choi, T.-L.; Grubbs, R. H. Angew. Chem., Int. Ed. 2003, 42, 1743.
(b) Love, J. A.; Morgan, J. P.; Trnka, T. M.; Grubbs, R. H. Angew.
Chem., Int. Ed. 2002, 41, 4035.
(1) For general reviews on Ru-catalyzed olefin metathesis, see:(a) Bielaw-
ski, C. W.; Grubbs, R. H. Prog. Polym. Sci. 2007, 32, 1–29. (b) Grubbs,
R. H. Tetrahedron 2004, 60, 7117. (c) Grubbs, R. H. Handbook of
Metathesis; Wiley-VHC: Weinheim, Germany, 2003. (d) Trkna, T. M.;
Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18. (e) Grubbs, R. H.; Chang,
S. Tetrahedron 1998, 54, 4413. (f) Ivin, K. J.; Mol, J. C. Olefin
Metathesis and Metathesis Polymerization; Academic Press: San
Diego, CA; 1997.
(7) (a) Matloka, P. P.; Wagener, K. B. J. Mol. Catal. A: Chem. 2006,
257, 89. (b) Baughman, T. W.; Wagener, K. B. AdV. Polym. Sci. 2005,
176, 1. (c) Schwendeman, J. E.; Church, A. C.; Wagener, K. B. AdV.
Synth. Catal. 2002, 344, 597.
(8) (a) Vougioukalakis, G. C.; Grubbs, R. H. J. Am. Chem. Soc. 2008,
130, 2234. (b) Plietker, B.; Neisius, N. M. J. Org. Chem. 2008, 73,
3218. (c) Vougioukalakis, G. C.; Grubbs, R. H. Organometallics 2007,
26, 2469. (d) Vehlow, K.; Maechling, S.; Blechert, S. Organometallics
2006, 25, 25. (e) Hansen, E. C.; Lee, D. Org. Lett. 2004, 6, 2035. (f)
Chatterjee, A. K.; Choi, T.-L.; Sanders, D. P.; Grubbs, R. H. J. Am.
Chem. Soc. 2003, 125, 11360.
(2) (a) Xia, Y.; Verduzco, R.; Grubbs, R. H.; Kornfield, J. A. J. Am. Chem.
Soc. 2008, 130, 1735. (b) Gorodetskaya, I. A.; Choi, T.-L.; Grubbs,
R. H. J. Am. Chem. Soc. 2007, 129, 12672. (c) Choi, T.-L.; Rutenberg,
I. M.; Grubbs, R. H. Angew. Chem., Int. Ed. 2002, 41, 3839.
(3) (a) Cope´ret, C.; Basset, J. M. AdV. Synth. Catal. 2007, 349, 78. (b)
Colacino, E.; Martinez, J.; Lamaty, F. Coord. Chem. ReV. 2007, 251,
726.
(9) For the synthesis and structural characterization of 4cyc and 5cyc, see:
Fu¨rstner, A.; Ackermann, L.; Gabor, B.; Goddard, R.; Lehmann, C. W.;
Mynott, R.; Stelzer, F.; Thiel, O. R. Chem.sEur. J. 2001, 7, 3236.
(10) Here we use the subscript “cyc” to denote the “cyclic” catalysts
and the subscript “acyc” to denote the precyclized, acyclic
complexes.
(11) (a) Bielawski, C. W.; Benitez, D.; Grubbs, R. H. J. Am. Chem. Soc.
2003, 125, 8424. (b) Bielawski, C. W.; Benitez, D.; Grubbs, R. H.
Science 2002, 297, 2041.
(4) (a) Funk, T. W.; Berlin, J. M.; Grubbs, R. H. J. Am. Chem. Soc. 2006,
128, 1840. (b) Berlin, J. M.; Goldberg, S. D.; Grubbs, R. H. Angew.
Chem., Int. Ed. 2006, 45, 7591.
(5) (a) Louie, J.; Bielawski, C. W.; Grubbs, R. H. J. Am. Chem. Soc.
2001, 123, 11312. (b) Bielawski, C. W.; Louie, J.; Grubbs, R. H. J. Am.
Chem. Soc. 2000, 122, 12872.
9
10.1021/ja8037849 CCC: $40.75
2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 12775–12782 12775

A R T I C L E S
Boydston et al.
We also hoped to further elucidate the mechanism by which
REMP proceeds through judicious catalyst design. Initially,
REMP was proposed to proceed via a ring-expansion initiation
event from a cyclic Ru-alkylidene catalyst (Scheme 1) and
propagate as cyclic monomers were incorporated into a growing
cyclic polymer. Upon consumption of monomer, a final catalyst
release step would provide the original catalyst and the
desired cyclic polymer. The polymerization mechanism
depicted in Scheme 1 has several intriguing features includ-
ing: (1) opening of a chelated Ru-alkylidene catalyst, (2)
propagation with the prospect of competing intramolecular
chain-transfer events, and (3) a final release of the original
catalyst via intramolecular CM.
Many scenarios are consistent with Scheme 1, depending on
the relative rates of initiation, propagation, intramolecular chain
transfer, and catalyst release. Initial studies using catalyst 7cyc
demonstrated the ability to control polymer molecular weight
(MW) using the monomer/catalyst loading.11 This corresponds
to a regime in which nearly complete initiation occurs, and
catalyst release does not take place prior to complete monomer
consumption. Another key observation was that, after complete
conversion of monomer, the MW of the cyclic polymers
progressively decreased in the presence of 7cyc, indicating
significant amounts of intramolecular chain transfer (Scheme
1). Alternatively, if the rate of propagation is much greater than
that of initiation, and the rates of intramolecular chain transfer
and catalyst release are negligible, then all monomer species
may be incorporated into a number of macrocycles equal to
the number of catalyst molecules that initiated. This latter
scenario would yield cyclic polymers in which Ru is incorpo-
rated into the backbone. Finally, a catalyst that shows relatively
rapid release from a propagating cyclic chain may be expected
to initiate, propagate (e.g., oligomerize), and then release,
ultimately providing multiple polymer rings from each catalyst
complex. Therefore, understanding how the catalyst design
influences the relative kinetics of these processes is central to
controlling the nature and distribution of products obtained via
REMP.
Figure 1. Representative Ru-based metathesis catalysts.
Figure 2. Cyclic Ru-alkylidene metathesis catalysts.
Scheme 1. Proposed REMP Catalytic Cycle
desirable, the synthesis and storage of this compound were
complicated by instability.
To realize the potential in the area of cyclic polymer
chemistry, catalysts should be readily synthesized in good yields,
be easily purified to eliminate any acyclic contaminants, and
have an appropriate balance of stability (e.g., during storage as
well as polymerizations) and activity. To address issues of
stability, we envisioned that catalysts with shorter tether lengths,
such as 4cyc, 5cyc, and 6cyc, which contain 4-, 5-, and 6-carbon
tethers, respectively, may be advantageous. A potential draw-
back, however, is that this may be accompanied by decreased
catalytic activities. Therefore, we designed catalysts to incor-
porate two key structural features, shortened tether lengths and
saturated NHC backbones, expected to synergistically provide
REMP catalysts of high stabilities and activities.14
To better understand each of the mechanistic aspects of
REMP and provide guidance for REMP catalyst design, we
sought to investigate a homologous series of cyclic catalysts of
varying N-to-Ru tether lengths (Figure 2). The tether length may
be central in controlling structural features of the catalyst such
as (1) inherent ring strain in the cyclic Ru complexes, (2) relative
orientations of the NHC and PCy3 ligands about the metal center,
and 3) rotation about the Ru-alkylidene (i.e., RudCsR) bond.
As will be discussed below, a combination of NMR spectros-
copy and single-crystal X-ray analyses of cyclic catalysts
ultimately revealed key connections between their structures and
activities.
(12) For additional cyclic polymer syntheses involving ring-expansion
approaches, see:(a) Culkin, D. A.; Jeong, W.; Csihony, S.; Gomez,
E. D.; Balsara, N. P.; Hedrick, J. L.; Waymouth, R. M. Angew. Chem.,
Int. Ed. 2007, 46, 2627. (b) Li, H.; Debuigne, A.; Je´rome, R.; Lecomte,
P. Angew. Chem., Int. Ed. 2006, 45, 2264. (c) He, T.; Zheng, G.-H.;
Pan, C.-Y. Macromolecules 2003, 36, 5960. (d) Kudo, H.; Makino,
S.; Kameyama, A.; Nishikubo, T. Macromolecules 2005, 38, 5964.
(e) Shea, K. J.; Lee, S. Y.; Busch, B. B. J. Org. Chem. 1998, 63,
5746.
Considering each step in the REMP cycle, it was expected
that the tether length ideal for polymerization activity might be
unfavorable for catalyst release. Specifically, intramolecular
metathesis to reform and release the initial catalyst from the
polymer is expected to be most efficient for shorter tether
lengths. In contrast, increased tether lengths may be beneficial
for polymerization rates, considering longer tethers may increase
ring strain of the catalyst or provide necessary flexibility within
the structure. Encouraged by the modular nature of the NHC
ligand, and the possibility of controlling REMP catalyst activities
via tether length, we prepared and analyzed a homologous series
of cyclic REMP catalysts (4cyc-7cyc, Figure 2), as well as
analogues possessing saturated imidazolinylidene ligands. Herein,
(13) For cyclic polymer syntheses involving ring-closing of telechelic
polymers, see:(a) Schappacher, M.; Deffieux, A. Science 2008, 319,
1512. (b) Laurent, B. A.; Grayson, S. M. J. Am. Chem. Soc. 2006,
128, 4238. (c) Oike, H.; Mouri, T.; Tezuka, Y. Macromolecules 2001,
34, 6229. (d) Alberty, K. A.; Tillman, E.; Carlotti, S.; Bradforth, S. E.;
Hogen-Esch, T. E.; Parker, D.; Feast, W. J. Macromolecules 2002,
35, 3856. (e) Roovers, J. J. Polym. Sci., Part B: Polym. Phys. 1988,
26, 1251. (f) Roovers, J.; Toporowski, P. M. Macromolecules 1983,
16, 843. (g) Geiser, D.; Hoeker, H. Macromolecules 1980, 13, 653.
(14) (a) Sanford, M. S.; Love, J. A.; Grubbs, R. H. J. Am. Chem. Soc.
2001, 123, 6543. (b) Bielawski, C. W.; Grubbs, R. H. Angew. Chem.,
Int. Ed. 2000, 39, 2903.
9
12776 J. AM. CHEM. SOC. VOL. 130, NO. 38, 2008

Cyclic Ruthenium-Alkylidene Catalysts
A R T I C L E S
Scheme 3. Synthesis of “Saturated” Catalysts 5cyc ·H2 and 6cyc ·H2
we report the study of their activities in various steps of the
REMP cycle, as well as key structure-activity relationships.
Results and Discussion
Catalyst Syntheses. The syntheses of complexes 4cyc and 5cyc
were previously described by Fu¨rstner.9 To our knowledge,
however, their catalytic activity has not been reported. Catalysts
6cyc and 7cyc were prepared analogously, as described in Scheme
2. The corresponding imidazolium salts (8) were first obtained
by alkylation of 1-mesitylimidazole.15 Ligand exchange was then
achieved via deprotonation of the imidazolium salt, followed
by addition of bis(phosphine) complex 1 (8/1 molar ratio )
2:1) to give “open” complexes 4acyc-7acyc.
10,16 In general, ligand
signals characteristic of 1, 4acyc-7acyc, or 4cyc-7cyc. These signals
may be attributed to CM products such as those arising from
CM involving styrene (formed as a product in the cyclization
step), or the terminal olefin on an NHC ligand with another Ru
complex. As expected, these intermolecular metathesis events
were significantly diminished at lower concentration. Prolonged
reaction times did not result in increased yields of the desired
cyclic species; rather decomposition was observed. It is worth
noting that, in the case of 7acyc, no product was observed when
cyclization was conducted at 0.01 M. Therefore, catalyst release
during REMP may be slow in comparison with other chain-
transfer events when 7cyc is employed.19
exchange proceeded smoothly and the desired nonchelated
complexes were isolated in good yields after chromatography
on silica gel.17,18 Intramolecular metathesis/cyclization was
conducted in a PhH/pentane mixture (1:15 v/v) at 70 °C and
0.001 M to give the final “closed” complexes 4cyc-7cyc. Each
of the catalysts could be purified by chromatography on silica
gel; however, purification of 4cyc-6cyc was more efficiently
accomplished via recrystallization from Et2O/pentane.15
Scheme 2. Synthesis of Cyclic REMP Catalysts 4cyc-7cyc
Considering the enhanced activity observed from saturation
of the NHC backbone (cf. 2 and 3),14 we were motivated to
investigate cyclic catalysts incorporating this design feature. As
depicted in Scheme 3, a PhCH3 solution of N-mesitylethylene-
diamine (9)20 was treated with HC(OEt)3 in the presence of
catalytic PTSA and stoichiometric bromo-olefin at 110 °C. This
one-pot procedure effected cyclization and alkylation to provide
the imidazolinium salts 10 in excellent yields.21 Unfortunately,
attempts at direct deprotonation of 10 using KHMDS in the
presence of bis(phosphine) complex 1 were complicated by
NHC dimerization and provided low yields of the desired
products.22 Alternatively, treatment of 10 with NaH in CHCl3
cleanly provided neutral adducts 11.23 Heating THF solutions
of 11 (0.001 M) in the presence of 1 (11/1 molar ratio ) 2:1)
accomplished ligand exchange as well as cyclization to provide
the desired cyclic catalysts 5cyc ·H2 and 6cyc ·H2 in 46 and 57%
overall yields, respectively.24 Although 5cyc ·H2 and 6cyc ·H2
were each isolable via chromatography on silica gel, both were
found to be crystalline solids and were routinely recrystallized
by slow addition of pentane into saturated PhH solutions of the
complexes. Similar to 5cyc and 6cyc, the saturated catalysts
5cyc ·H2 and 6cyc ·H2 displayed good stability both in the solid-
state and in solution.25
a
Table 1. Cyclization to Give Cyclic Catalysts 4cyc-7cyc
yield (%)b
cyclic
catalyst
tether
length
0.01 M
0.001 M
4cyc
5cyc
6cyc
7cyc
4
5
6
7
62
76
81
0
81
97
97
63
a Reactions conducted in dry C6D6 under N2 atmosphere at 80 °C for
1 h. b Determined by 1H NMR spectroscopy.
We noted that the efficiency of cyclization of open complexes
4acyc-7acyc to give cyclic catalysts 4cyc-7cyc is highly dependent
on the tether length (Table 1). Ostensibly, the ability of an open
complex to undergo intra- versus intermolecular metathesis
events gives some indication of the tendency for the proposed
catalyst release step in Scheme 1. Table 1 summarizes the results
of cyclization reactions for each catalyst at 0.01 and 0.001 M.
In each case, yields were markedly improved at lower concen-
(19) Additional considerations should be noted: (1) Intramolecular cycliza-
tion may proceed to directly give an acyclic methylidene complex
that is identical to the product that would be obtained between CM of
styrene and the cyclic catalyst, and (2) in rare cases, olefin isomer-
ization and cyclization to give small amounts of cyclic catalysts bearing
a one-carbon shorter tether were observed (e.g., 7acyc f 6cyc).
(20) Perillo, I.; Caterina, M. C.; Lo´pez, J.; Salerno, A. Synthesis 2004, 851.
(21) For an alternative, two-step procedure for the synthesis of imidazo-
linium salts bearing one N-aryl and one N′-alkyl group, see ref 8d.
(22) Attempts at ligand exchange via deprotonation of imidazolinium salts
10 in hexanes were also unsuccessful.
1
tration (0.001 M) as determined by H NMR analysis of the
crude reaction mixtures. At 0.01 M, additional alkylidene peaks
were observed via H NMR spectroscopy that were upfield of
1
(15) Supporting Information.
(16) In solution, complexes 4acyc-7acyc appeared to exist as a mixture of
two rotamers as shown by two benzylidene signals (ratio ca. 10:1).
The major isomer appeared as a singlet, analogous to complex 2,
whereas a second signal was observed further downfield as a doublet
(23) (a) Courchey, F. C.; Sworen, J. C.; Ghiviriga, I.; Abboud, K. A.;
Wagener, K. B. Organometallics 2006, 25, 6074. (b) Trnka, T. M.;
Morgan, J. P.; Sanford, M. S.; Wilhem, T. E.; Scholl, M.; Choi, T.-
L.; Ding, S.; Day, M. D.; Grubbs, R. H. J. Am. Chem. Soc. 2003,
125, 2546.
3
15
(e.g., JH,P ) 12.9 Hz for 6cyc
) .
(17) Purchased from TSI Scientific, 230-400 mesh, neutral pH.
(18) Notably, “open” complexes 4acyc-7acyc were routinely used in
subsequent cyclization steps following only filtration through a short
silica gel plug and concentration under vacuum. Residual tricyclo-
hexylphosphine was efficiently removed from the cyclized catalysts
(4cyc-7cyc) during purification.
(24) This reaction was found to be solvent-dependent, and PhCH3, PhH,
pentane, and PhH/pentane mixture gave inferior results.
(25) Unfortunately, attempts to isolate pure samples of 7cyc · H2 were
unsuccessful.
9
J. AM. CHEM. SOC. VOL. 130, NO. 38, 2008 12777

A R T I C L E S
Boydston et al.
Table 2. Selected 1H NMR and Single-Crystal X-ray Data for
4cyc-6cyc, 5cyc ·H2, and 6cyc ·H2
saturated analogues 5cyc ·H2 and 6cyc ·H2.26 The crystal structures
of these complexes confirmed a variable degree of rotation about
the Ru1-C2 bond, as determined from the Cl2-Ru-C2-C3
dihedral angles (Table 2). Overall, for 4cyc-6cyc, decreased
3JH2,P1 values corresponded to decreased dihedral angles sug-
gesting that the solution and solid-state structures of the
catalysts are similar. It should be noted that while the 3JH2,P1
values observed from 5cyc · H2 and 6cyc · H2 were consistent
with each complex’s respective unsaturated analogue, solid-
sate analysis revealed that the Cl2-Ru-C2-C3 dihedral
angles were not consistent with the trend observed from the
unsaturated series.
b
4cyc
b
5cyc
catalyst
6cyc
5cyc ·H2
6cyc ·H2
NMR data
20.50
10.5
δ H2 (ppm)a
3JH2,P1 (Hz)a
19.70
14.1
19.71
5.1
20.39
9.3
19.61
5.0
dihedral angles (deg)
Cl2-Ru1-C2-C3
N1-C1-Ru1-C2
51.3
156.5
26.6
162.2
16.2
153.5
18.3
160.0
21.1
151.2
bond angles (deg)
Stepwise increase in the tether lengths was found to cause
increasing nonlinearity in the C1-Ru1-P1 bond angles. Specif-
ically, catalysts 4cyc, 5cyc, and 6cyc have C1-Ru1-P1 bond
angles of 171.0°, 166.0°, and 163.3°, respectively. One rationale
for this trend may be that increasing the tether length caused
the NHC ligand to tilt to accommodate the increased steric
demand of the tether. A consequence of this tilt is that the Mes
group is forced closer to the PCy3 group which may facilitate
dissociation of the phosphine during initiation (see below for a
comparison of catalyst activities).14a In addition, the tilted
conformation may be difficult to restore which would hinder
catalyst release and thus manifest in more rapid polymerization.
Steric demands also resulted in discernible increases in the
Ru1-P1 and Ru1-C1 bond lengths as the tether length was
increased. For example, upon extension of the tether, the
Ru1-C1 bond length increased from 2.076 Å for 4cyc to 2.113
Å for 6cyc. The saturated catalysts, 5cyc ·H2 and 6cyc ·H2, showed
changes in their Ru1-P1 and Ru1-C1 bond lengths that were
consistent with those observed in the unsaturated series.
Ring-Expansion Metathesis Polymerization. To our knowl-
edge, the catalytic activities of complexes 4cyc-6cyc have not
been explored in either small-molecule or polymer syntheses.
In addition, electronic variants such as saturation of the NHC
backbone have not been investigated. To compare the activities
of 4cyc-7cyc in REMP, we examined their relative efficiencies
in the polymerization of cyclooctene (COE) to poly(cyclooctene)
(PCOE). As can be seen from the data presented in Figure 4,
the relative efficiencies of the catalysts showed a strong
dependence on the length of the chelating tether.28 In general,
increased tether length was accompanied by an increase in
catalytic activity. For example, comparison of the unsaturated
catalysts revealed the time required to reach >95% conversion
was nearly 24 h for 4cyc (green line), approximately 8 h for 5cyc
(purple line) and 6cyc (blue line), and less than 1 h for 7cyc (red
line).
C1-Ru1-P1
N1-C1-Ru1
N2-C1-Ru1
N1-C1-N2
171.0
128.8
127.8
103.4
166.0
127.0
128.9
104.1
163.3
126.6
129.4
103.6
165.3
124.3
128.1
107.5
168.8
124.0
129.5
107.2
bond lengths (Å)
Ru1-C1
Ru1-C2
Ru1-P1
2.076
1.812
2.402
2.091
1.806
2.421
2.113
1.823
2.421
2.072
1.821
2.417
2.084
1.800
2.423
a Data taken in C6D6 at ambient temperature. b See ref 9.
Structural Analyses. The structural impacts of changing the
tether lengths of catalysts 4cyc-7cyc resulted in significant
differences in catalyst activities (see polymerization studies
below for more discussion). In addition to understanding the
structure-activity relationships pertaining to REMP catalysts,
a more general understanding of catalyst architecture may lead
to breakthroughs in catalyst design as well as fundamental
mechanistic insights of olefin metathesis. Cyclic catalysts
4cyc-6cyc were found to show tether length-dependent trends
in three key structural parameters summarized in Table 2: (1)
rotation about the Ru1-C2 bond, (2) the C1-Ru1-P1 bond
angle, and (3) the Ru-C1 bond length (Figure 3).26
Although many structural features of 4cyc-7cyc are best
observed via solid-state analysis, rotation about the Ru1-C2
bond is manifested in the coupling constants between the P1
and H2 (i.e., the alkylidene proton, bonded to C2) atoms in the
1H NMR spectra (Table 2 and Figure 3). Complexes 4cyc and
5
cyc, which were previously characterized in solution and solid
state,9 displayed coupling constants of 3JH2,P1 ) 14.1 and 10.5
Hz (solvent ) C6D6), respectively. The smaller coupling
constant observed from complex 5cyc, in comparison with 4cyc
,
indicated that the corresponding atoms in the former are closer
to a perpendicular arrangement. Consistent with this trend, a
smaller coupling constant was observed in the 1H NMR
spectrum of 6cyc (i.e., JH2,P1 ) 5.1 Hz), indicating that the
alkylidene proton (H2) was projected nearly perpendicular to
Saturation of the NHC backbone was found to dramatically
increase catalyst activity. As expected, 5cyc ·H2 and 6cyc ·H2 each
displayed faster polymerization rates than their unsaturated
analogues 5cyc and 6cyc, respectively (Figure 4). Surprisingly,
the rate acceleration resulting from NHC saturation appeared
to be greater than that for homologation of the tether length.
Specifically, 6cyc ·H2 was found to achieve >95% conversion
in shorter reaction times than did 7cyc. Similarly, 5cyc ·H2 was
3
1
the Ru1-P1 bond. The H NMR spectrum of 7cyc revealed a
coupling constant of 3JH2,P1 ) 10.2 Hz, which may be ascribed
to the increased ring size (cf. 6cyc) inducing twist about the
Ru1-C2 bond.27
To further investigate the structures of the cyclic catalysts,
we compared single-crystal X-ray data of 4cyc-6cyc, as well as
found to be a more active polymerization catalyst than 6cyc
.
Overall, the data revealed that judicious combinations of shorter
tether lengths (i.e., 5- or 6-carbon tethers) and NHC backbone
saturation (e.g., 5cyc ·H2 and 6cyc ·H2) provided a desirable
balance of catalyst stabilities, synthetic accessibility, and
activities.
(26) Attempts to obtain X-ray quality crystals of 7cyc were met with limited
success.
(27) Since single-crystal X-ray data were not obtained for 7cyc, it was not
possible to determine the direction of the rotation about the Ru1-C2
bond with respect to complexes 4cyc-6cyc. We speculate that the long
tether of 7cyc may allow for conformations that collectively frustrate
crystallization.
(28) Additional representations of the data in Figure 4 are provided.15
9
12778 J. AM. CHEM. SOC. VOL. 130, NO. 38, 2008

Cyclic Ruthenium-Alkylidene Catalysts
A R T I C L E S
Figure 3. (Top) X-ray crystal structures of 5cyc ·H2, 6cyc, and 6cyc ·H2. Solvent molecules and hydrogens have been removed for clarity. Ellipsoids are drawn
at the 50% probability level. (Bottom) 1H NMR spectra (C6D6) of alkylidene proton of 5cyc ·H2, 6cyc, and 6cyc ·H2.
Figure 5. Log plots for REMP of COE using catalysts 4cyc (green), 5cyc
Figure 4. REMP of COE using catalysts 4cyc (green), 5cyc (purple), 5cyc ·H2
(purple), 5cyc ·H2 (orange), 6cyc (blue), 6cyc ·H2 (black), and 7cyc (red). Linear
(orange), 6cyc (blue), 6cyc ·H2 (black), and 7cyc (red). Conditions: CD2Cl2,
least-squares fitting gave R2 values of the following: 4cyc, 0.997; 5cyc, 0.969;
40 °C, [M/C]0 ) 1000:1, [M]0 ) 0.5 M. Conversion determined by 1H
5cyc ·H2, 0.991; 6cyc, 0.998; 6cyc ·H2, 0.990; and 7cyc, 0.991. Conditions:
NMR spectroscopy.
1
CD2Cl2, 40 °C, [M/C]0 ) 1000:1, [M]0 ) 0.5 M. Data recorded using H
NMR spectroscopy.
Catalyst stability takes on particular importance in REMP as
decomposition of the catalyst before, during, or after polym-
consumption. This observation can be rationalized by a relatively
erization could potentially lead to linear polymers, instead of
slow initiation period which would manifest in a gradual increase
the envisioned macrocycles. In addition, relative stabilities are
in the number of propagating polymer chains and concomitant
important factors in the general development of new metathesis
increase in the rate of conversion.
catalysts, especially considering that catalyst stability and
activity are often inversely related.29 To explore the stabilities
Catalyst Release. A unique aspect of REMP, in comparison
with ring-opening metathesis polymerization (ROMP), is the
of REMP catalysts 4cyc-7cyc during polymerization reactions,
requirement for an intramolecular chain-transfer event with the
we plotted ln([COE]) versus time for REMP of COE (Fig-
ure 5). The logarithmic plots were found to be linear (R2 values
olefin nearest to the NHC to release the initial cyclic catalyst
and provide a cyclic polymer free of Ru (Scheme 1). While
ranged from 0.969 to 0.997) between 20 and 80% conversion
removal of Ru from linear polymers obtained via ROMP can
of COE to PCOE, indicating that catalyst decomposition was
be done efficiently using a terminating agent, such as ethyl vinyl
ether, this method is incompatible with REMP as it may result
in linear polymer formation.30 Given the importance of catalyst
release from the cyclic polymers, we investigated each catalyst’s
propensity to undergo intramolecular cyclization during polym-
erization that would be indicative of the catalyst’s ability to be
released from a polymer.
negligible in all cases during the time of the polymerization
reactions. Closer examination of the plots revealed that the only
discernible deviations from linearity (i.e., pseudo-first-order rate
kinetics) involved apparent increases in the rate of monomer
(29) Ritter, T.; Hejl, A.; Wenzel, A. G.; Funk, T. W.; Grubbs, R. H.
Organometallics 2006, 25, 5740.
9
J. AM. CHEM. SOC. VOL. 130, NO. 38, 2008 12779

A R T I C L E S
Boydston et al.
Scheme 4. Proposed Species Observable upon ROMP of COE
Using Open Catalysts 5acyc-7acyc
Figure 6. Left axis: Conversion of COE to PCOE using 5acyc (black). Right
axis: Mole fraction of 5cyc (red), 5acyc (blue), and A (green). Conditions:
We envisioned that conducting polymerizations using “open”
catalysts 5acyc-7acyc would provide insight into each catalyst’s
ability to perform intramolecular CM to release “closed”
CD2Cl2, 40 °C, [COE/5acyc
determined by H NMR spectroscopy.
] ) 250:1, [COE]0 ) 0.5 M. Conversion
0
1
catalysts 5acyc-7acyc.
31 Propagation via growing Ru-alkylidene
Scheme 5. Equilibration of Cyclic Catalyst and Linear PCOE
species A (Scheme 4) would inherently compete with catalyst
cyclization (e.g., A f 5cyc + B) and provide an indication of
each catalyst’s propensity to be released from the polymer chain.
To investigate, we conducted polymerizations of COE using
open catalysts 5acyc-7acyc in CD2Cl2 at 40 °C ([M/C]0 ) 250:
1, [M]0 ) 0.5 M) and monitored the alkylidene region of the
1H NMR spectrum as the reactions progressed. Each Ru
complex shown in Scheme 4 was identified by characteristic
chemical shifts of the corresponding alkylidene protons. In
CD2Cl2, complexes 5acyc-7acyc gave sharp benzylidene reso-
nances as singlets at δ ) 19.30 ppm, whereas propagating
species (A) displayed broad multiplets at δ ) 18.69 ppm. Cyclic
catalysts 5cyc, 6cyc, and 7cyc displayed signals at δ ) 20.23, 19.35,
and 19.67 ppm, respectively, with multiplicities matching those
in Table 2.
analogues 6acyc and 7acyc, polymerization reactions using 5acyc
revealed much faster cyclization (to give 5cyc) relative to polym-
erization. Figure 6 shows the mole fraction of each catalytic species
(5acyc, A, and 5cyc) as well as the conversion of COE to PCOE
over time. As can be seen, almost complete formation of cyclic
catalyst 5cyc was observed after ca. 45 min, at which time the
polymerization had reached only 48% conversion. Moreover, the
amount of catalytic species within the polymer chains (A) quickly
diminished to nearly undetectable amounts. The data presented in
Figure 6 support that cyclization to form 5cyc is favored over
propagation and that the background rate of cyclization (i.e., 5acyc
f 5cyc) is also significant for this catalyst. In addition, the persistent
amount of 5cyc that is observed relative to propagating species (A)
suggested that incorporation of 5cyc into existing polymer chains
is unlikely. The continued conversion of COE to PCOE, with
persistent observation of only 5cyc, suggested that this cyclic catalyst
reached an equilibrium between propagating (e.g., A) and resting
(i.e., 5cyc) states that strongly favored the latter.
Collectively, the experiments investigating the behavior of
open catalysts 5acyc-7acyc revealed that controlling the tether
lengths of cyclic catalysts may dictate polymerization kinetics
with regard to polymer MWs and polydispersities. For example,
shorter tether lengths may facilitate catalyst release during
polymerization (Scheme 1), ultimately leading to multiple
macrocycles produced from each catalyst molecule. Alterna-
tively, REMP catalysts displaying little tendency to be released
from a cyclic polymer may provide access to cyclic block
copolymers or other advanced macrocycles.
We first examined open catalysts 6acyc and 7acyc as these were
representative of the most active cyclic catalysts (6cyc and 7cyc
,
respectively) for this series. Catalysts 6acyc and 7acyc gave similar
results, and polymerization of COE was found to reach completion
faster than did cyclization of 6acyc and 7acyc to give 6cyc and 7cyc
,
respectively. Specifically, complete conversion of COE was
achieved in less than 5 min for each catalyst.32 The mole fraction
of cyclic catalyst (6cyc/7cyc) observed at this point, however, was
only ca. 10%, relative to 6acyc/7acyc (ca. 30%) and A (ca. 60%).
Continued heating resulted in diminished amounts of 6acyc/7acyc
and 6cyc/7cyc in each case, with concomitant increases in the relative
amounts of A. As will be discussed in the next section, the
continued progression to form A may have been due to incorpora-
tion of free cyclic catalyst into the polymer chains. After ca. 1 h,
only trace amounts of cyclic species 6cyc/7cyc could be observed.
Overall, these results suggested that cyclization is not favored over
polymerization for catalysts bearing 6- or 7-membered tethers and
that cyclization to release catalyst 6cyc or 7cyc after polymerization
is not likely.
We next investigated the behavior of 5acyc under the same
conditions as described above. In contrast to the longer tethered
Interaction Between Free Catalyst and Polymer. As men-
tioned previously, it may be possible for a cyclic catalyst to
equilibrate with poly(olefin)s and become incorporated (or rein-
corporated) into a polymer chain. This equilibrium, depicted in
Scheme 5, may be tether length-dependent given that ring-opening
of the catalyst may be a driving force toward incorporation into
the polymer. With regard to REMP, the reversibility of intramo-
lecular chain transfer and catalyst release (Scheme 1) would result
in an equilibrium amount of Ru species contained within the final
cyclic polymers. Therefore, understanding each catalyst’s affinity
toward polymer incorporation is important for understanding the
(30) Other terminating agents have also been employed; see:(a) Matson,
J. B.; Grubbs, R. H. Macromolecules 2008, 41, 5626. (b) Hilf, S.;
Berger-Nicoletti, E.; Grubbs, R. H.; Kilbinger, A. F. M. Angew. Chem.,
Int. Ed. 2006, 45, 8045. (c) Owen, R. M.; Gestwicki, J. E.; Young,
T.; Kiessling, L. L. Org. Lett. 2002, 4, 2293.
(31) In light of the relatively low activity of 4cyc, 4acyc was not evaluated
in these experiments.
(32) Under identical conditions, 5acyc-7acyc gave faster conversions of COE
to PCOE than the corresponding “closed” systems (5cyc-7cyc). The
higher polymerization activities of 5acyc-7acyc versus 5cyc-7cyc may
reflect restricted conformations of the latter. Further investigations are
underway.
9
12780 J. AM. CHEM. SOC. VOL. 130, NO. 38, 2008

Cyclic Ruthenium-Alkylidene Catalysts
A R T I C L E S
with neutral silica gel (TSI Scientific, 230-400 mesh, pH 6.5-7.0).
Crystallographic data have been deposited at the CCDC, 12 Union
Road, Cambridge CB2 1EZ, U.K., and copies can be obtained on
request, free of charge, by quoting the publication citation and the
deposition numbers 687290 (5cyc ·H2), 683585 (6cyc), and 687247
(6cyc ·H2).
potential purity of the cyclic polymers. To investigate, we prepared
linear PCOE via ROMP using acyclic catalyst 3 in the presence
of 3-hexene as a chain-transfer agent. This provided a hydrocarbon
polymer (Mn ) 150 kDa, PDI ) 2.1) which closely resembled the
PCOE obtained via REMP in composition.33 The linear PCOE was
then treated with each of the cyclic catalysts 5cyc-7cyc (olefin/
catalyst molar ratio ) 100:1) in CD2Cl2 at 40 °C. The equilibration
of catalyst and polymer was monitored via 1H NMR spectroscopy
using anthracene as an internal standard; key NMR signals of the
cyclic catalysts and incorporated species were similar to those
observed in the previous section (Scheme 4).
Cyclic Complex 5cyc ·H2. In a Schlenk tube, chloroform adduct
11a (200 mg, 0.51 mmol) was dissolved in dry THF (515 mL)
under an atmosphere of dry N2. To the solution was added Ru-
complex 1 (210 mg, 0.26 mmol). The flask was sealed, and the
reaction mixture was stirred in an oil bath at 70 °C for 2 h.
Afterward, the cooled reaction mixture was concentrated under
vacuum, redissolved in a minimal amount of PhH, and treated
dropwise with pentane until crystallization ensued (X-ray analysis
was performed on crystals obtained in this manner). The solids were
collected by vacuum filtration, rinsed with 5% Et2O/pentane, and
dried under vacuum to provide 147 mg (81% yield) of the desired
As expected, incorporation of cyclic catalyst into the polymer
chain was dependent on the tether length of the catalyst.
Specifically, after 1 h ca. 11% of catalyst 7cyc had become
incorporated into the polymer, whereas catalyst 6cyc showed only
3% incorporation over the same time period. Catalyst 5cyc
,
1
complex as a tan solid. H NMR (500 MHz, C6D6): δ 20.39 (dt,
however, revealed no incorporation even after extended periods
(up to 6 h). To compare, catalyst 5cyc ·H2 was also studied and
gave similar results as 5cyc. Overall, although the amount of
incorporated catalyst was small in each case, there appeared to
be some equilibration of free catalyst into the poly(olefin)
depending upon the length of the catalyst tether. Notably,
equilibration appeared to be slow in comparison with polym-
erization data presented above. Therefore, avoiding prolonged
polymerization reaction times may be sufficient for minimizing
the amount of Ru in the resulting polymers.
3JH,P ) 9.3 Hz, JH,H ) 4.5 Hz, 1H), 6.93 (s, 2H), 3.17 (t, J ) 10.0
Hz, 2H), 3.10-3.09 (m, 2H), 2.85 (t, J ) 10.0 Hz, 2H), 2.75 (br,
2H), 3.71 (s, 6H), 2.60-2.53 (m, 3H), 2.32 (br 2H), 2.23 (s, 3H),
1.95-1.92 (m, 6H), 1.78 (br, 6H), 1.68 (br, 2H), 1.47-1.45 (m,
6H), 1.34-1.29 (m, 10H), 1.19-1.17 (m, 2H). 13C NMR (125
MHz, C6D6): δ 216.0 (JCP ) 85.9 Hz), 138.4, 137.6, 136.6, 129.9,
57.9 (JCP ) 4.9 Hz), 51.2 (JCP ) 3.5 Hz), 48.5 (JCP ) 2.7 Hz),
47.9, 32.1 (JCP ) 15.1 Hz), 29.8, 28.2 (JCP ) 10.4 Hz), 27.3, 26.8,
26.7, 21.1, 20.0. 31P NMR (121 MHz, C6D6): δ 27.0. HRMS m/z
calcd for C35H57Cl2N2PRu [M+] 708.2680, found 708.2659.
[1-(6-Heptenyl)-3-mesitylimidazolylidene]RuCl2(dCHPh)(PC-
y3) (6acyc). Imidazolium bromide 8n)5 (400 mg, 1.10 mmol) was
suspended in dry PhCH3 (7 mL) under dry N2. To the solution was
added NaOtBu (106 mg, 1.10 mmol), and the resulting mixture
was stirred at RT for 12 h. Ru-complex 1 (453 mg, 0.55 mmol)
was then added in a single portion, and the resulting mixture was
stirred for 1 h during which time a color change from purple to
brown was observed. Upon completion, the mixture was filtered
through a thin pad of TSI silica gel using Et2O/pentane (1:4 v/v)
as eluent. The filtrate was concentrated under vacuum without
heating. Purification by column chromatography on TSI silica gel
under N2 pressure (10% Et2O/pentane) provided 408 mg (90% yield)
of the desired compound as a red-purple powder. 1H NMR (major
isomer) (300 MHz, C6D6): δ 19.85 (s, 1H), 7.05-6.94 (m, 2H)
6.56-6.55 (m, 1H), 6.31-6.15 (m, 3H), 5.90-5.76 (m, 1H),
5.15-5.03 (m, 2H), 4.70 (t, J ) 7.7 Hz, 2H), 2.65-2.53 (m, 4H),
2.13 (s, 3H), 1.97-1.08 (m, 37H), 1.80 (s, 6H). 31P NMR (major
isomer) (121 MHz, C6D6): δ 34.4. HRMS m/z calcd for
C44H65Cl2N2PRu [M+] 824.3306, found 824.3298.
Conclusions
In summary, we describe the synthesis and characterization
of a series of cyclic Ru-alkylidene catalysts with particular focus
on their ability to mediate ring-expansion metathesis polymer-
ization. Both catalyst tether length and NHC electronics were
found to significantly impact different aspects of the polymer-
ization mechanism. Whereas shorter tether lengths were more
efficient for catalyst release from the polymer, the caveat for
these systems was found to be slower polymerization rates.
Saturation of the NHC backbone, however, increased polym-
erization efficiency and effectively balanced activity loss due
to shortening of the tether. Overall, catalyst stabilities were found
to be good over the course of the polymerization experiments.
The ability to control catalyst activity by a combination of tether
length and ligand electronics may lead to new opportunities in
olefin metathesis and catalyst design.
Cyclic Complex 6cyc. Ru-complex 6acyc (400 mg, 0.48 mmol)
was dissolved in dry PhH (30 mL) and pentane (450 mL) in a
Schlenk tube under dry N2. The mixture was then placed in a oil
bath at 70 °C and stirred for 1 h. Upon completion, the solution
was cooled to RT, transferred to a round-bottom flask, and
concentrated under vacuum without heat. The crude material was
triturated with 20% Et2O/pentane (50 mL) for ca. 20 min. The solids
were then collected via vacuum filtration, rinsed with pentane, and
dried under vacuum to provide 335 mg (96% yield) of the desired
compound as a red-brown powder. X-ray quality crystals were
obtained by slow addition of pentane to a PhH solution of the
complex. 1H NMR (300 MHz, C6D6): δ 19.71 (dt, 3JH,P ) 5.1 Hz,
JH,H ) 5.5 Hz, 1H), 6.90 (s, 2H), 6.35 (d, J ) 1.8 Hz, 1H), 6.14 (s,
1H), 2.74-2.51 (m, 3H), 2.51 (s, 6H), 2.23 (s, 3H), 1.99-1.31
(m, 40H). 13C NMR (125 MHz, C6D6): δ 186.0 (JCP ) 84.7 Hz),
138.2, 137.1, 129.4, 128.3, 123.6 (JCP ) 2.3 Hz), 120.4, 62.9, 47.0,
32.4 (JCP ) 16.9 Hz), 31.5, 29.8, 28.1 (JCP ) 10.0 Hz), 26.8, 22.9,
21.1, 19.7. 31P NMR (121 MHz, C6D6): δ 33.3. HRMS m/z calcd
for C36H57Cl2N2PRu [M+] 720.2680, found 720.2671.
Experimental Section
Materials and Methods. 1H and 13C NMR spectra were
recorded using a Varian Mercury 300 or Varian Inova 500
spectrometer and were routinely run using broadband decoupling.
Chemical shifts (δ) are expressed in ppm downfield from tetram-
ethylsilane using the residual protiated solvent as an internal
standard (CDCl3 1H: 7.26 ppm and 13C: 77.0 ppm; C6D6 1H: 7.20
ppm and 13C: 128.0 ppm). 31P NMR spectra were externally
referenced to 85% H3PO4 (0.00 ppm). Coupling constants are
expressed in hertz (Hz). THF, CH2Cl2, Et2O, pentane, PhH, PhCH3,
and C6D6 were obtained from solvent purification columns.34
CD2Cl2 used for NMR-scale experiments was distilled over CaH2
under N2 prior to use. CHCl3 was distilled over P2O5 under N2
prior to use. Ru-complex 1 was obtained from Materia, Inc. All
other solvents and reagents were of reagent quality and used as
obtained from commercial sources. Chromatography was performed
(33) Molecular-weight data were obtained from triple-angle laser light-
scattering and refractive index measurements.
(34) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–1520.
Cyclic Complex 6cyc ·H2. This compound was prepared analo-
gously to 5cyc ·H2 from chloroform adduct 11b (280 mg, 0.69 mmol)
and Ru-complex 1 (285 mg, 0.35 mmol) in THF (650 mL) to
9
J. AM. CHEM. SOC. VOL. 130, NO. 38, 2008 12781

A R T I C L E S
Boydston et al.
provide 369 mg (74% yield) of the desired complex as a red-brown
solid. X-ray quality crystals were obtained by slow diffusion of
129.7, 115.2, 50.9, 48.7, 48.1, 32.8, 26.4, 25.2, 20.8, 17.9. HRMS
m/z calcd for C18H27N2 [M+] 271.2174, found 271.2161.
1
pentane into a Et2O/PhH (20:1 v/v) solution of the complex. H
1-(6-Heptenyl)-3-mesitylimidazolinium Bromide (10b). This
compound was prepared analogously to 10a from HC(OEt)3 (7.0
mL), PhCH3 (7.0 mL), PTSA ·H2O (27 mg, 0.14 mmol), N-
mesitylethylenediamine (9) (500 mg, 2.80 mmol), and 7-bromo-
1-heptene (0.51 mL, 3.36 mmol) to provide 951 mg (93% yield)
of the desired compound. 1H NMR (500 MHz, CDCl3): δ 9.31 (s,
1H), 6.77 (s, 2H), 5.70-5.61 (m, 1H), 4.90-4.81 (m, 2H),
4.20-4.16 (m, 2H), 4.12-4.08 (m, 2H), 3.78 (t, J ) 7.3 Hz, 2H),
2.16 (s, 3H), 2.15 (s, 6H), 1.96-1.91 (m, 2H), 1.64-1.58 (m, 2H),
1.35-1.30 (m, 2H), 1.27-1.21 (m, 2H). 13C NMR (125 MHz,
CDCl3): δ 158.5, 139.7, 138.1, 134.9, 130.3, 129.5, 114.4, 50.8,
48.6, 48.0, 33.1, 27.9, 26.7, 25.3, 20.7, 17.8. HRMS m/z calcd for
C19H29N2 [M+] 285.2325, found 285.2310.
1-(5-Hexenyl)-3-mesityl-2-(trichloromethyl)imidazolidine (11a).
Under an atmosphere of dry N2, imidazolinium bromide 10a (443
mg, 1.26 mmol) was dissolved in dry CHCl3 (6 mL). NaH (95 wt%,
38 mg, 1.51 mmol) was then added portionwise under a stream of
N2. The resulting mixture was placed in an oil bath at 55 °C and
stirred for 10 h. Afterward, the cooled reaction mixture was diluted
with Et2O (100 mL), filtered through a thin pad of silica gel, and
concentrated to provide 305 mg (62% yield) of the desired product
as a pale yellow oil. 1H NMR (300 MHz, CDCl3): δ 6.86 (s, 1H),
6.85 (s, 1H), 5.91-5.78 (m, 1H), 5.07-4.95 (m, 2H), 4.73 (s, 1H),
3.86-3.78 (m, 1H), 3.62-3.55 (m, 1H), 3.41-3.32 (m, 1H),
3.23-3.16 (m, 1H), 3.10-3.02 (m, 1H), 2.98-2.90 (m, 1H), 2.35
(s, 3H), 2.70 (s, 3H), 2.25 (s, 3H), 2.16-2.09 (m, 2H), 1.68-1.43
(m, 4H). 13C NMR (75 MHz, CDCl3): δ 147.8, 147.7, 142.8, 138.8,
138.7, 134.9, 132.6, 129.9, 129.5, 114.5, 108.2, 94.2, 58.2, 52.9,
52.6, 33.7, 29.7, 26.1, 20.7, 19.8, 19.3. HRMS m/z calcd for
C19H27Cl3N2 [M+] 388.1240, found 388.1225.
1-(6-Heptenyl)-3-mesityl-2-(trichloromethyl)imidazolidine (11b).
This compound was prepared analogously to 11a from imidazo-
linium bromide 10b (730 mg, 2.0 mmol), CHCl3 (8 mL), and NaH
(95 wt %, 101 mg, 4.00 mmol) to provide 670 mg (83% yield) of
the desired product as a pale yellow oil. 1H NMR (300 MHz,
CDCl3): δ 6.88 (s, 1H), 6.86 (s, 1H), 5.92-5.79 (m, 1H), 5.08-4.96
(m, 2H), 4.75 (2, 1H), 3.87-3.80 (m, 1H), 3.65-3.58 (m, 1H),
3.42-3.33 (m, 1H), 3.25-3.17 (m, 1H), 3.10-3.02 (m, 1H),
3.00-2.92 (m, 1H), 2.37 (s, 3H), 2.29 (s, 3H), 2.27 (s, 3H),
2.14-2.05 (m, 2H), 1.68-1.54 (m, 2H), 1.50-1.38 (m, 4H). 13C
NMR (75 MHz, CDCl3): δ 142.8, 139.0, 138.6, 134.8, 132.6, 129.9,
129.4, 114.3, 108.2, 94.2, 58.4, 52.9, 52.6, 33.8, 30.1, 28.8, 26.4,
20.7, 19.8, 19.3. HRMS m/z calcd for C20H29Cl3N2 [M+] 402.1396,
found 402.1382.
3
NMR (500 MHz, C6D6): δ 19.61 (dt, JH,P ) 5.0 Hz, JH,H ) 5.7
Hz, 1H), 6.93 (s, 2H), 3.33-2.83 (m, 4H), 2.70 (s, 6H), 2.63-2.56
(m, 3H), 2.22 (s, 3H), 1.93-1.30 (m, 40H). 13C NMR (125 MHz,
C6D6): δ 215.3 (JCP ) 80.1 Hz) 137.5, 137.0, 129.9, 129.5, 62.8,
51.7 (JCP ) 3.3 Hz), 47.7, 46.9, 32.2 (JCP ) 16.5 Hz), 29.7, 28.1
(JCP ) 10.1 Hz), 27.9, 26.8, 25.6, 23.6, 21.1. 31P NMR (121 MHz,
C6D6): δ 30.4. HRMS m/z calcd for C36H59Cl2N2PRu [M+]
722.2837, found 722.2808.
Cyclic Complex 7cyc. This compound was prepared analogously
to 6cyc from open complex 7acyc (150 mg, 0.18 mmol) in PhH (10
mL) and pentane (170 mL). Upon completion, the cooled reaction
mixture was concentrated under vacuum without heat and then
triturated with 20% Et2O/pentane (10 mL) for ca. 20 min. The solids
were collected via vacuum filtration, and further purification via
column chromatography on TSI silica gel under N2 pressure (30%
Et2O/pentane) provided 56 mg (42% yield) of the desired compound
as a light brown powder. 1H NMR (500 MHz, C6D6): δ 19.37 (dt,
3JH,P ) 10.2 Hz, JH,H ) 5.5 Hz, 1H), 6.92 (s, 2H), 6.38 (d, J ) 1.5
Hz, 1H), 6.19 (s, 1H), 3.66 (br, 2H), 2.62-2.57 (m, 3H), 2.54 (s,
6H), 2.24 (s, 3H), 2.08-1.61 (m, 24H), 1.50-1.42 (m, 2H),
1.34-1.28 (m, 12H), 1.22-1.17 (m, 2H). 13C NMR (125 MHz,
C6D6): δ 184.7 (JCP ) 97.5 Hz), 138.1, 137.6, 129.2, 128.4, 128.3,
128.1, 127.9, 123.3 (JCP ) 3.3 Hz), 119.9, 60.4, 47.1, 32.7 (JCP
)
16.1 Hz), 29.9, 28.2 (JCP ) 9.5 Hz), 26.8, 26.5, 24.7, 21.1, 20.7,
19.7. 31P NMR (121 MHz, C6D6): δ 26.3. HRMS m/z calcd for
C37H59Cl2N2PRu [M+] 734.2837, found 734.2814.
1-(6-Heptenyl)-3-mesitylimidazolium Bromide (8n)5). This com-
pound was prepared analogously to 8n)3,4,6 from N-mesitylimidazole
(1.00 g, 5.37 mmol) and 7-bromo-1-heptene (1.0 mL, 6.55 mmol)
in PhCH3 (20 mL).9 Upon completion, the reaction mixture was
concentrated under vacuum, and the crude material was suspended
in Et2O (100 mL) and vigorously stirred for 12 h to produce a fine
white suspension. The solids were collected via vacuum filtration
under a stream of N2 to provide 1.81 g (93% yield) of the desired
compound as an off-white powder.35 1H NMR (300 MHz, CDCl3):
δ 10.31 (dd appearing as t, J ) 1.5 Hz, 1H), 7.94 (dd apearing as
t, J ) 1.7 Hz, 1H), 7.21 (dd apearing as t, J ) 1.7 Hz, 1H), 6.94
(s, 2H), 5.76-5.63 (m, 1H), 4.96-4.84 (m, 2H), 4.67 (t, J ) 7.4
Hz, 2H), 2.29 (s, 3H), 2.01 (s, 6H), 2.01-1.91 (m, 4H), 1.43-1.30
(m, 4H). 13C NMR (125 MHz, CDCl3): δ 141.1, 138.1, 137.8,
134.0, 130.6, 129.7, 123.2, 123.1, 114.7, 50.1, 33.2, 30.2, 28.0,
25.3, 21.0, 17.5. HRMS m/z calcd for C19H27N2 [M+] 283.2174,
found 283.2186.
1-(5-Hexenyl)-3-mesitylimidazolinium Bromide (10a). To a solu-
tion of HC(OEt)3 (10 mL) and PhCH3 (10 mL) in a 50 mL round-
bottom flask was added PTSA ·H2O (39 mg, 0.20 mmol), N-mes-
itylethylenediamine (9) (729 mg, 4.09 mmol), and 6-bromo-1-
hexene (0.66 mL, 4.91 mmol). The flask was fitted with a H2O-
jacketed condenser, and the reaction mixture was stirred under N2
in an oil bath at 110 °C for 10 h. Afterward, the cooled reaction
mixture was concentrated under vacuum. The crude product was
treated with Et2O (40 mL) and vigorously stirred for 2 h to produce
an off-white slurry. The solids were collected via vacuum filtration,
rinsed with Et2O, and dried under vacuum to provide 1.31 g (91%
Acknowledgment. We gratefully acknowledge Materia, Inc. for
the generous gift of catalyst 1. We thank the DOE (DE-FG02-
05ER46218), NIH (5RO1GM-31332), and the California Institute
of Technology for generous financial support. A.J.B. thanks the
NIH/NCI for a postdoctoral fellowship. We thank Professor Gregory
B. McKenna for helpful discussions, as well as Matthew T. Whited,
Larry M. Henling, and Dr. Michael W. Day for help in obtaining
X-ray data.
Supporting Information Available: Additional representations
of Figure 4, NMR spectra for all new compounds, CIF files
containing X-ray structural data, atomic coordinates, thermal
parameters, bond distances, and bond angles for complexes
5cyc · H2, 6cyc, and 6cyc · H2. This material is available free of
charge via the Internet at http://pubs.acs.org.
1
yield) of the desired compound. H NMR (300 MHz, CDCl3): δ
9.50 (s, 1H), 6.87 (s, 2H), 5.80-5.67 (m, 1H), 5.03-4.92 (m, 2H),
4.30-4.11 (m, 4H), 3.94 (t, J ) 7.2 Hz, 2H), 2.26 (s, 6H), 2.24 (s,
3H), 2.12-2.05 (m, 2H), 1.76-1.66 (m, 2H), 1.49-1.42 (m, 2H).
13C NMR (125 MHz, CDCl3): δ 158.7, 139.9, 137.6, 135.0, 130.4,
(35) The compound appeared to be hygroscopic, producing a thick, viscous
material when collected under air.
JA8037849
9
12782 J. AM. CHEM. SOC. VOL. 130, NO. 38, 2008