Efficient and recyclable monomeric and dendritic Ru-based metathesis catalysts

Article abstract of DOI:10.1021/ja001179g

Several highly active, recoverable and recyclable Ru-based metathesis catalysts are presented. The crystal structure of Ru complex 5, beating a 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene and styrenyl ether ligand is disclosed. The heterocyclic ligand significantly enhances the catalytic activity, and the styrenyl ether allows for the easy recovery of the Ru complex. Catalyst 5 promotes ring-closing metathesis (RCM) and the efficient formation of various trisubstituted olefins at ambient temperature in high yield within 2 h; the catalyst is obtained in >95% yield after silica gel chromatography and can be used directly in subsequent reactions. Tetrasubstituted olefins can also be synthesized by RCM reactions catalyzed by 5. In addition, the synthesis and catalytic activities of two dendritic and recyclable Ru-based complexes are disclosed (32 and 33). Examples involving catalytic ring-closing, ring-opening, and cross metatheses are presented where, unlike monomer 5, dendritic 33 can be readily recovered.

Full text of DOI:10.1021/ja001179g

8168
J. Am. Chem. Soc. 2000, 122, 8168-8179
Efficient and Recyclable Monomeric and Dendritic Ru-Based
Metathesis Catalysts
Steven B. Garber, Jason S. Kingsbury, Brian L. Gray, and Amir H. Hoveyda*
Contribution from the Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill,
Massachusetts 02467
ReceiVed April 4, 2000
Abstract: Several highly active, recoverable and recyclable Ru-based metathesis catalysts are presented. The
crystal structure of Ru complex 5, bearing a 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene and styrenyl ether
ligand is disclosed. The heterocyclic ligand significantly enhances the catalytic activity, and the styrenyl ether
allows for the easy recovery of the Ru complex. Catalyst 5 promotes ring-closing metathesis (RCM) and the
efficient formation of various trisubstituted olefins at ambient temperature in high yield within 2 h; the catalyst
is obtained in >95% yield after silica gel chromatography and can be used directly in subsequent reactions.
Tetrasubstituted olefins can also be synthesized by RCM reactions catalyzed by 5. In addition, the synthesis
and catalytic activities of two dendritic and recyclable Ru-based complexes are disclosed (32 and 33). Examples
involving catalytic ring-closing, ring-opening, and cross metatheses are presented where, unlike monomer 5,
dendritic 33 can be readily recovered.
Introduction
We recently reported the synthesis and catalytic activity of
Ru-based complex 1,1 which can efficiently catalyze the ring-
closing metathesis (RCM) of dienes that contain terminal
olefins.2 We demonstrated that 1 can be recovered from the
reaction mixture by silica gel chromatography in high yield and
reused in subsequent C-C bond forming reactions. Despite the
above unique attributes, several shortcomings remained to be
addressed: (1) Similar to benzylidene catalyst 2,3 complex 1 is
efficient mostly for substrates that contain terminal alkenes
(synthesis of disubstituted alkenes). (2) In certain cases, because
of adventitious coelution, isolation of 1 from the substrate is
problematic. We therefore decided to turn our attention to
alternative transition metal ligands that would enhance catalytic
activity so that, upon release from 2-isopropoxystyrene, the
propagating Ru-carbene is substantially more reactive. To
address the issue of ease of recovery, we set out to design,
synthesize, and examine Ru-based dendritic complexes.4,5
their solubility in common organic solvents, and the facility and
high level of certainty with which metal-containing sites can
(4) For the use of metal-containing dendrimers in homogeneous catalysis,
see: (a) Miedaner, A.; Curtis, C. J.; Barkley, R. M.; DuBois, D. L. Inorg.
Chem. 1994, 33, 5482-5490. (b) Knapen, J. W. J.; van der Made, A. W.;
de Wilde, J. C.; van Leeuwen, P. W. N. M.; Wijkens, P.; Grove, D. M.;
van Koten, G. Nature 1994, 372, 659-663. (c) Brunner, H.; Altmann, S.
Chem. Ber. 1994, 127, 2285. (d) Brunner, H.; Furst, J. Tetrahedron, 1994,
50, 4303. (e) Brunner, H.; Bublak, P. Synthesis 1995, 36. (f) Brunner, H.;
Net, G. Synthesis, 1995, 423. (g) Brunner, H. J. Organomet. Chem. 1995,
500, 39-46. (h) Bolm, C.; Derrien, N.; Seger, A. Synlett, 1996, 387-388.
(i) Seebach, D.; Marti, R. E.; Hintermann, T. HelV. Chim. Acta 1996, 79,
1710-1740. (j) Rheiner, P. B.; Sellner, H.; Seebach, D. HelV. Chim. Acta
1997, 80, 2027-2032. (k) Sellner, H.; Seebach, D. Angew. Chem., Int. Ed.
Engl. 1999, 38, 1918-1920. (l) Rheiner, P. B.; Seebach, D. Chem.-Eur.
J. 1999, 5, 3221-3236. (m) Reetz, M. T.; Lohmer, G.; Schwickardi, R.
Angew. Chem., Int. Ed. Engl. 1997, 36, 1526-1529. (n) Reetz, M. T. Top.
Catal. 1997, 4, 187-200. (o) Kollner, C.; Pugin, B.; Togni, A. J. Am. Chem.
Soc. 1998, 120, 10274-10275. (p) Schneider, R.; Kollner, C.; Weber, I.;
Togni, A. J. Chem. Soc., Chem. Commun. 1999, 2415-2416. (q) Yamago,
S.; Furukawa, M.; Azuma, A.; Yoshida, J.-I. Tetrahedron Lett. 1998, 39,
3783-3786. (r) Hu, Q.-S.; Pugh, V.; Sabat, M.; Pu, L. J. Org. Chem. 1999,
64, 7528-7536. (s) Bourque, S. C.; Maltais, F.; Xiao, W.-J.; Tardif, O.;
Alper, H.; Arya, P.; Manzer, L. E. J. Am. Chem. Soc. 1999, 121, 3035-
3038. (t) Bourque, S. C.; Alper, H.; Manzer, L. E.; Arya, P. J. Am. Chem.
Soc. 2000, 122, 956-957. (u) Oosterom, G. E.; van Haaren, R. J.; Reek, J.
N. H.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. J. Chem. Soc., Chem.
Commun. 1999, 1119-1120. (v) de Groot, D.; Eggeling, E. B.; de Wilde,
J. C.; Kooijman, H.; van Haaren, R. J.; van der Made, A. W.; Spek, A. L.;
Vogt, D.; Reek, J. N. H.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. J.
Chem. Soc., Chem. Commun. 1999, 1623-1624. (w) Hovestad, N. J.;
Eggeling, E. B.; Heidbuchel, H. J.; Jastrzebski, J. T. B. H.; Kragl, U.; Keim,
W.; Vogt, D.; van Koten, G. Angew. Chem., Int. Ed. Engl. 1999, 38, 1655-
1658. (x) Kleij, A. W.; Gossage, R. A.; Jastrzebski, J. T. B. H.; Boersma,
J.; van Koten, G. Angew. Chem., Int. Ed. Engl. 2000, 39, 176-178. (y)
Fan, Q.-H.; Chen, Y.-M.; Chen, X.-M.; Jiang, D.-Z.; Xi, F.; Chan, A. S. C.
J. Chem. Soc., Chem. Commun. 2000, 789-790. (z) Wijkens, P.; Jastrzebski,
J. T. B. H.; van der Schaaf, P. A.; Kolly, R.; Hafner, A.; van Koten, G.
Org. Lett. 2000, 2, 1621-1624.
Organometallic dendrimers offer an attractive avenue for
catalyst development because of their ease of characterization,
(1) (a) Kingsbury, J. S.; Harrity, J. P. A.; Bonitatebus, P. J.; Hoveyda,
A. H. J. Am. Chem. Soc. 1999, 121, 791-799. For the initial reports on the
synthesis of 1 and its related derivatives, see: (b) Harrity, J. P. A.; Visser,
M. S.; Gleason, J. D.; Hoveyda, A. H. J. Am. Chem. Soc. 1997, 119, 1488-
1489. (c) Harrity, J. P. A.; La, D. S.; Cefalo, D. R.; Visser, M. S.; Hoveyda,
A. H. J. Am. Chem. Soc. 1998, 120, 2343-2351.
(2) For reviews on catalytic metathesis, see: (a) Grubbs, R. H.; Miller,
S. J.; Fu, G. C. Acc. Chem. Res. 1995, 28, 446-452. (b) Schmalz, H.-G.
Angew. Chem., Int. Ed. Engl. 1995, 34, 1833-1836. (c) Schuster, M.;
Blechert, S. Angew. Chem., Int. Ed. Engl. 1997, 36, 2036-2056. (d)
Furstner, A. Top. Catal. 1997, 4, 285-299. (e) Armstrong, S. K. J. Chem.
Soc., Perkin Trans. 1 1998, 371-388. (f) Grubbs, R. H.; Chang, S.
Tetrahedron 1998, 54, 4413-4450. (g) Ivin, K. J. J. Mol. Catal. A: Chem.
1998, 133, 1-16. (h) Randall, M. L.; Snapper, M. L. J. Mol. Catal. A:
Chem. 1998, 133, 29-40. (i) Phillips, A. J.; Abell, A. D. Aldrichimica Acta
1999, 32, 75-89.
(3) (a) Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H. Angew.
Chem., Int. Ed. Engl. 1995, 34, 2039-2041. (b) Schwab, P.; Grubbs, R.
H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118, 100-110. (c) Belderrain, T.
R.; Grubbs, R. H. Organometallics 1997, 16, 4001-4003.
10.1021/ja001179g CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/12/2000

Ru-Based Metathesis Catalysts
J. Am. Chem. Soc., Vol. 122, No. 34, 2000 8169
be incorporated within the macromolecular ensemble.6 We
suspected that Ru release and return (cf. Scheme 3) in the present
systems would be more efficient than those in related polymer-
bound analogues, largely because of the exposed binding sites
at the dendrimer periphery. In general, catalyst efficiency is
expected to be superior with a dendritic complex, since the more
densely coiled backbone of a linear polymer in a heterogeneous
solid support may hinder the availability and subsequent retrieval
of the transition metal.6
Herein, we report the results of our studies designed to address
the above issues. We disclose the synthesis of a new recyclable
Ru-based catalyst7 that is substantially more active than 1. The
first generation of dendritic and recyclable metathesis catalysts
are presented here as well. Our studies demonstrate that Ru-
based dendrimers are readily characterized and serve as
homogeneous metathesis catalysts that are highly active and
allow for significantly more facile catalyst recovery when
compared to their corresponding monomers.
the catalyst that allow it to be easily recyclable (by preserving
the styrenyl alkoxide bidentate ligation), we set out to synthesize,
characterize, and examine the catalytic activity of 5. This
strategy was based on the recently reported accelerating effect
of a variety of saturated imidazolin-2-ylidene8 and unsaturated9
imidazol-2-ylidene carbene ligands10 on the metathetic activity
of Ru-based complexes. Thus, we established that, as depicted
in eq 1, treatment of 38a with 1.0 equiv of CuCl and 0.97 equiv
of 4 in CH2Cl2 at 40 °C delivers 5 within 1 h; styrenyl complex
5 can be isolated in air as a bright green solid in 85% yield
after silica gel chromatography (mp ) 178-181 °C dec).11
Results and Discussion
Synthesis and Structure of Ru Complex 5. To address the
question of reactivity and maintain the structural features of
(5) For recent general reviews on dendrimers, see: (a) Peerlings, H. W.
I.; Meijer, E. W. Chem.sEur. J. 1997, 3, 1563-1570. (b) Smith, D. K.;
Diederich, F. Chem.sEur. J. 1998, 4, 1353-1361. (c) Majoral, J.-P.;
Caminade, A.-M. Chem. ReV. 1999, 99, 845-880. For a review of
organometallic and related metal-containing dendrimers, see: (d) Newkome,
G. R.; Moorefield, C. N.; Vogtle, F.; Dendritic Molecules: Concepts,
Syntheses, PerspectiVes; VCH: Weinheim, 1998; pp 210-222. (e) Hear-
shaw, M. A.; Moss, J. R. J. Chem. Soc., Chem. Commun. 1998, 1-8. (f)
Newkome, G. R.; He, E.; Moorefield, C. N. Chem. ReV. 1999, 99, 1689-
1746.
Single-crystal X-ray structure analysis of 5 (Figure 1; IMES
) 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) confirms
the structural assignment.12 Selected bond lengths and angles
are provided in Table 1. The overall geometry around the
transition metal center and most of the bond angles and bond
lengths in 5 are analogous to their related values in complex 6
(see Scheme 1).1a
(6) For general discussions on advantages and disadvantages of den-
drimers over solid-phase systems, see: (a) Tomalia, D. A.; Dvornic, P. R.
Nature 1994, 372, 617-618. (b) Haggin, J. Chem. Eng. News 1995, 73,
26-27.
(7) For other reports on recyclable Ru-based metathesis catalysts (in
addition to ref 1a), see: (a) Nguyen, S. T.; Grubbs, R. H. J. Organomet.
Chem. 1995, 497, 195-200. (b) Ahmed, M.; Barrett, A. G. M.; Braddock,
D. C.; Cramp, S. M.; Procopiou, P. A. Tetrahedron Lett. 1999, 40, 8657-
8662. (c) Barrett, A. G. M.; Cramp, S. M.; Roberts, R. S. Org. Lett. 1999,
1, 1083-1086.
(8) (a) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999,
1, 953-956. (b) Chatterjee, A. K.; Grubbs, R. G. Org. Lett. 1999, 1, 1751-
1753. (c) Chatterjee, A. K.; Morgan, J. P.; Scholl, M.; Grubbs, R. H. J.
Am. Chem. Soc. 2000, 122, 3783-3784.
(9) (a) Weskamp, T.; Schattenmann, W. C.; Spiegler, M.; Herrmann,
W. A. Angew. Chem., Int. Ed. Engl. 1998, 37, 2490-2493. (b) Weskamp,
T.; Kohl, F. J.; Hieringer, W.; Gleich, D.; Herrmann, W. A. Angew. Chem.,
Int. Ed. Engl. 1999, 38, 2416-2419. (c) Scholl, M.; Trnka, T. M.; Morgan,
J. P.; Grubbs, R. H. Tetrahedron Lett. 1999, 40, 2247-2250. (d) Huang,
J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J. Am. Chem. Soc. 1999,
121, 2674-2678. (e) Ackermann, L.; Furstner, A.; Weskamp, T.; Kohl, F.
J.; Herrmann, W. A. Tetrahedron Lett. 1999, 40, 4787-4790. (f) Furstner,
A.; Thiel, O. R.; Ackermann, L.; Schanz, H.-J.; Nolan, S. P. J. Org. Chem.
2000, 65, 2204-2207.
Figure 1. ORTEP diagram of Cl2Ru(dCH-o-OiPrC6H4)(4,5-dihy-
droIMES) (5). Thermal ellipsoids are drawn at the 30% probability
level. For selected bond distances and angles, see Table 1.
(10) For comprehensive reviews of nucleophilic carbenes, see: (a) Regitz,
M. Angew. Chem., Int. Ed. Engl. 1996, 35, 725-728. (b) Herrmann, W.
A.; Kocher, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 2162-2187. (c)
Arduengo, A. J., III.; Krafczyk, R. Chem.-Ztg. 1998, 32, 6-14. (d)
Arduengo, A. J., III. Acc. Chem. Res. 1999, 32, 913-921.
(11) The route shown below was also examined but proved to be less
efficient (32% isolated yield is unoptimized; MES ) 2,4,6-trimethylphenyl).
See the Experimental Section for details of the synthesis of the imidazo-
linium salt.
1
Comparison of the H NMR spectra of 1 and 5 sheds light
on some of the subtle structural attributes of these complexes.
As illustrated in Scheme 1, there are two distinct chemical shift
changes in the 400 MHz H NMR spectra of 1 and 5; one
variation is observed at the iso-Pr methine proton and another
at the carbene CH (HR). In both instances, the protons for the
imidazolin-2-ylidene system 5 are more shielded. These differ-
ences may be attributed to higher electron density at the
transition metal center of 5, caused by the stronger electron
1
(12) A single crystal of 5 was obtained by slow diffusion of hexanes
into a concentrated methylene chloride solution (-20 °C); crystal size 0.5
× 0.25 × 0.25 mm3; monoclinic; space group P21/n; a ) 13.7075(1) Å, b
) 10.4555(8) Å, c ) 22.846(2) Å; â ) 95.3170(1)°; V ) 3274.4(4) Å3,
Dcalcd ) 1.443 g/cm3 for Z ) 4; T ) 293(2) K. The crystal lattice contained
methylene chloride. Further details, including tables of crystallographic data,
are available in the Supporting Information.

8170 J. Am. Chem. Soc., Vol. 122, No. 34, 2000
Garber et al.
Table 1. Selected Bond Lengths and Angles for
Cl2Ru(dCH-o-OiPrC6H4)(4,5-dihydroIMES) (5)
Table 2. Ring-Closing Metathesis of Acyclic Dienes by Ru
Complex 5a
Bond Lengths (Å)
Ru(1)-C(1)
Ru(1)-C(2)
Ru(1)-O(1)
1.828(5)
1.981(5)
2.261(3)
Ru(1)-Cl(1)
Ru(1)-Cl(2)
C(2)-N(1)
C(2)-N(2)
2.328(12)
2.340(12)
1.351(6)
1.350(6)
Bond Angles (deg)
C(1)-Ru(1)-O(1)
C(1)-Ru(1)-C(2)
C(2)-Ru(1)-O(1)
79.3(17) O(1)-Ru(1)-Cl(1)
101.5(14) O(1)-Ru(1)-Cl(2)
176.2(14) C(2)-Ru(1)-Cl(1)
86.9(9)
85.3(9)
96.6(12)
90.9(12)
C(1)-Ru(1)-Cl(1) 100.2(15) C(2)-Ru(1)-Cl(2)
C(1)-Ru(1)-Cl(2) 100.1(15) Cl(1)-Ru(1)-Cl(2) 156.5(5)
N(1)-C(2)-N(2) 106.9(4)
Scheme 1
a Conditions: 5 mol % 5 for entries 1 and 3-7, 1 mol % 5 for entry
2, 22 °C, CH2Cl2 (entries 1-5); 24 h at 22 °C and 20 h at 40 °C,
CH2Cl2 for entry 6; toluene, 80 °C for entry 7. b Isolated yields after
silica gel chromatography.
donation by the heterocyclic ligand (relative to PCy3).8 Thus,
the weaker electron donation by the oxygen ligand to the Ru
center in 5 may be manifested by the more upfield appearance
of the isopropyl methine proton (4.90 versus 5.28 ppm). These
comparisons must be tempered however, because the difference
in the chemical shifts of HR may be partially due to an
anisotropic effect caused by the aryl units of the ligand in 5.
Catalytic Activity and Recovery of Complex 5. As il-
lustrated in Table 2, the Ru complex 5 serves as an effective
catalyst for the RCM of dienes; hetero- and carbocyclics bearing
trisubstituted alkenes are obtained from the corresponding
precursor dienes in the presence of 5 mol % catalyst at ambient
temperature within 10 min to 4 h. As shown in entries 1 and 2
of Table 2, both 1,1-disubstituted (7) and trisubstituted olefins
(9) may be utilized in the synthesis of trisubstituted cyclic
alkenes.13 The catalytic RCM in entries 3 and 4 indicates that
trisubstituted allylic alcohols (12)14 and acetates (14) can be
accessed in the presence of 5 mol % 5 within 2 h. As before,
the Ru catalyst is recovered with high efficiency (95% and
>98% yield, respectively). It must be noted that the recyclable
catalyst 1 would be significantly less efficient in promoting the
above transformations. As an example, treatment of 11 with 5
mol % 1 (22 °C, CH2Cl2) leads to only 15% conversion after 2
h (as judged by 400 MHz H NMR analysis).
Two important points in connection to the above data merit
mention: (1) In all instances, the catalyst is recovered along
(13) Ru complex 3 is equally efficient in effecting the RCM reactions
in Table 2. For example, catalytic RCM of 7, 11, and 13 proceeds in the
presence of 5 mol % 3 and in a similar length of time as that for 5 (22 °C)
to afford the desired products 8, 12, and 14 in 93%, 64%, and 72% yield,
respectively. It is worth noting that all previously reported reactions
promoted by 3 were carried out at 45 °C (see ref 8).
(14) For a recent report on the accelerating effect of an R-hydroxyl group
on Ru-catalyzed RCM reactions, see Hoye, T. R.; Zhao, H. Org. Lett. 1999,
1, 1123-1125.
with the desired cyclic product in high yield after simple silica
gel chromatography. Moreover, addition of 2 equiV of styrene
ether 4 (relatiVe to the catalyst) to a solution of a transformation
promoted by 3 at the end of the reaction time leads to the
isolation of the recyclable catalyst 5. As an example, treatment
of diene carbinol 11 with 5 mol % 3 (CH2Cl2, 22 °C, 1 h),
followed by the addition of 10 mol % 4 and additional stirring
for 1 h, leads to the formation of 12 and 5 in 98% and 82%
yields, respectively (after silica gel chromatography). (2)
Catalyst loadings lower than 5 mol % are sufficient. As
exemplified by the reaction in entry 2, catalytic RCM can readily
proceed to completion with only 1 mol % 5. As another
example, catalytic RCM of 7 occurs within 10 min at 22 °C in
the presence of 1 mol % 5 to afford 8 in 73% isolated yield
(>98% conversion); recovered 5 is obtained in 92% yield after
chromatography.
As the reaction in entry 5 of Table 2 indicates, catalytic RCM
involving 1,1-disubstituted alkenes of R,â-unsaturated carbonyls9f
are also efficiently promoted by 5. Tetrasubstituted olefins can
even be obtained through catalytic RCM promoted by 5, but
less efficiently (entry 6). Nonetheless, even in this instance, the
Ru catalyst is recovered in >80% yield. When toluene9f (80
°C) is used as the solvent in the catalytic RCM of 19 (entry 7),
tetrasubstituted alkene 20 is formed in 65% isolated yield within
30 min (70% conversion); the catalyst recovery, however, suffers
under these conditions (60%). Prolonged reaction times (1 h at
80 °C in toluene) lead to similar or lower yields of product and
<10% catalyst recovery. The lower levels of efficiency observed
in the synthesis of tetrasubstituted alkenes may be partly due
to the fact that the styrenyl ligand effectively competes with
the 1,1-disubstituted olefins, thus preventing the efficient
1

Ru-Based Metathesis Catalysts
J. Am. Chem. Soc., Vol. 122, No. 34, 2000 8171
Scheme 2
Dendritic Ru Complexes. We judged that macromolecular
Ru complexes, because of their different polarities compared
to the monomeric species, could be more easily separated from
reaction products, and that one reasonable first step would be
to prepare and examine dendritic rather than polymer-bound
systems. This decision was also largely based on the relative
ease with which dendrimers can be spectroscopically analyzed,
allowing us to design and construct the molecular environment
within which the Ru complex resides. With Ru-containing
dendrimers, it would be possible to gauge rigorously the
efficiency with which the active Ru-carbene leaves the ligation
site and returns to the macromolecule (see below for details).
Such mechanistic information would be invaluable, pointing the
way to the design and synthesis of other more efficient dendritic
or polymer-supported Ru-based catalysts.
In addition to the above considerations, we elected to pursue
dendritic ensembles based on the proposed mechanism through
which this class of Ru complexes likely serve as metathesis
catalysts. As summarized in Scheme 3,1a a diene substrate
probably first reacts with the initial Ru complex to remove the
transition metal from the styrene ligand and release the styrene
ether 4. Upon consumption of the diene, the active Ru-carbene
reacts with the previously occupied styrenyl ether to cause
reformation of the initial complex (return). We argued that both
the release of the Ru center from the styrenyl ligand (initiation)
and the return of the active Ru to the initial site (recovery) would
be more efficient with the more accessible and exposed terminal
sites within a soluble dendrimer structure. Complications
regarding the rate of diffusion of diene substrates to the active
metal center have been previously noted in connection with
heterogeneous polymer-bound Ru-based metathesis catalysts
(processes promoted by Ru attached to solid support).7a
Consequently, the bound catalysts proved to be significantly
less efficient than related monomeric species. In a more recent
example,7b involving a polymer-bound Ru complex that, similar
to 1 and 5, operates through a release/return mechanism, catalyst
activity is reduced significantly after the first round of recycling.
This complication may be partly because styrenes are less
effective than bidentate alkoxy styrenyl systems in retrieval of
the active metal through olefin metathesis with the propagating
Ru-carbene. We speculate that the released and highly active
Ru-methylene undergoes competitive decomposition prior to its
return to the polymer-supported ligand. Entropy may thus play
a prominent role in the efficient recovery of Ru by bidentate
ligand 4.
formation of the requisite Ru-carbene derived from the triene
substrate. In addition, the catalyst (or the released Ru-methylene)
may not be stable under metathesis conditions at 80 °C for more
than a few minutes.15,16
The catalyst retrieved from the above transformations after
silica gel chromatography (recrystallization not needed) may
be used in subsequent metathesis reactions with equal efficiency.
For example, the catalyst recovered from the reaction in entry
1 of Table 2 was reused in the same reaction to afford the desired
product 8 in 71% isolated yield (10 min, 22 °C). Complex 5
was again recovered in 98% yield after chromatography.
As the representative transformations in Scheme 2 indicate,
5 is an efficient catalyst in ROM/RCM17 and ROM/CM18
processes as well. Both transformations are completed within
1 h (>98% conversion). However, due to coelution, repeated
and extensive attempts to accomplish a complete separation of
the desired products from the recovered catalyst proved unsuc-
cessful (>90% mass balance in both cases). At this point, we
turned our attention to the preparation of active Ru-based
catalysts that are more easily recoverable.
(15) To assess the thermal stability of complexes 3 and 5 in the absence
of substrate, 0.05 M toluene solutions of these catalysts (containing ferrocene
as an internal standard) were heated to 80 °C for 12 h in a temperature-
controlled oil bath. Resulting solutions were then analyzed by 400 MHz
1H NMR spectroscopy. Catalyst 5 gave a clean spectrum, indicating <2%
each of free 2-isopropoxybenzaldehyde and 2-isopropoxystyrene (or its
derived homodimer). In contrast, no carbene proton signal could be detected
for benzylidene 3. It should be stressed, however, that the results in Table
2 are a more appropriate measure of catalyst integrity, since propagating,
electron-deficient intermediates are involved. In addition, any direct
comparison of complexes 3 and 5 is complicated by the unique kinetic
profiles which govern catalysis in each system. This experiment is therefore
simply an indication of the relative facility with which initiation occurs in
3 (dissociative loss of the labile PCy3). For other studies on Ru catalyst
decomposition and longevity, see: (a) Ulman, M.; Grubbs, R. H. J. Org.
Chem. 1999, 64, 7202-7207. (b) Huang, J.; Schanz, H.-J.; Stevens, E. D.;
Nolan, S. P. Organometallics 1999, 18, 5375-5380.
(16) To ensure that initiation is facilitated, 10 mol % of diallyl ether
was added to react with 5 and release the transition metal from the styrene
alkoxide ligand. This strategy, however, did not improve reaction efficiency
probably because the resulting LnRudCH2 returns to its original styrene
ligand faster than reacting with a substrate-disubstituted olefin (i.e., initiation
is not the problem). For the application of diallyl ether or ethylene to metal-
catalyzed metathesis, see: (a) Reference 1b. (b) Reference 1c. (c) Johannes,
C. W.; Visser, M. S.; Weatherhead G. S.; Hoveyda, A. H. J. Am. Chem.
Soc. 1998, 120, 8340-8347. (d) Mori, M.; Sakakibara, N.; Kinoshita, A.
J. Org. Chem. 1998, 63, 6082-6083. (e) Lautens, M.; Hughes, G. Angew.
Chem., Int. Ed. Engl. 1999, 38, 129-131. (f) Weatherhead, G. S.; Ford, J.
G.; Alexanian, E. J.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc.
2000, 122, 1828-1829.
Synthesis of Ru-Containing Dendrimers. Initially, we set
out to prepare a dendrimer bearing siloxane-containing side
chains, based on the pioneering work of Van Koten.4b However,
this system proved susceptible to decomposition upon purifica-
tion, presumably because of hydrolysis of alkyl(Me2)Si-O
bonds. Accordingly, we opted to prepare the more robust
tetraalkylsilyl systems. The route for the synthesis of dendrimer
32 is illustrated in Scheme 4. The key features of the synthesis
include the attachment of the requisite vinyl group through a
Pd-catalyzed Stille coupling (f28)19 and preparation of the
dendrimer backbone by a Pt-catalyzed hydrosilation/alkylation20/
(18) For studies on Ru-catalyzed ROM/CM processes, see: (a) Randall,
M. L.; Tallarico, J. A.; Snapper, M. L. J. Am. Chem. Soc. 1995, 117, 9610-
9611. (b) Snapper, M. L.; Tallarico, J. A.; Randall, M. L. J. Am. Chem.
Soc. 1997, 119, 1478-1479. (c) Schneider, M. F.; Lucas, N.; Velder, J.;
Blechert, S. Angew. Chem., Int. Ed. Engl. 1997, 36, 257-259. (d) Cuny,
G. D.; Cao, J.; Hauske, J. R. Tetrahedron Lett. 1997, 38, 5237-5240. (e)
Cao, J.; Cuny, G. D.; Hauske, J. R. Mol. DiVersity 1998, 3, 173-179.
(19) (a) McKean, D. R.; Parrinello, G.; Renaldo, A. F.; Stille, J. K. J.
Org. Chem. 1987, 52, 422-424. For a related review, see: (b) Farina, V.;
Krishnamurthy, V.; Scott, W. J. Org. React. 1998, 50, 1-652.
(17) For studies on Ru-catalyzed ROM/RCM processes, see: (a) ref 16a-
b. (b) Zuercher, W. J.; Hashimoto, M.; Grubbs, R. H. J. Am. Chem. Soc.
1996, 118, 6634-6640. (c) Burke, S. D.; Quinn, K. J.; Chen, V. J. J. Org.
Chem. 1998, 63, 8626-8627.

8172 J. Am. Chem. Soc., Vol. 122, No. 34, 2000
Garber et al.
Scheme 3
Scheme 4
a Anhydrous HCl, i-PrOH, 98%. b2 equiv NaH, 2 equiv i-PrI, DMF, THF, 82%. c1.1 equiv Br2, HOAc, CH2Cl2, 98%. d1.1 equiv Bu3SnCHCH2,
3 mol % Pd(PPh3)4, <1 mol % BHT, tol, 110 °C, 67%. e1 M KOH (40 equiv), 100 °C, 91%. f4.1 equiv CH2CHCH2MgBr, Et2O, 35 °C, 88%. g4.6
equiv HMe2SiCl, 0.25 mol % H2PtCl6, THF, 65 °C. h4.2 equiv CH2CHCH2MgBr, Et2O, 90% overall for two steps. i4.7 equiv 9-BBN, THF; NaOH,
H2O2, EtOH, THF. j4.8 equiv EDC‚HCl, 4.4 equiv 28, 0.5 equiv DMAP, 5.0 equiv Et3N, CH2Cl2, 63% overall for two steps. k4.3 equiv 2, 4.8 equiv
CuCl, CH2Cl2, 87%.
hydroboration21 sequence (29 f 30 f 31). Coupling of 31 with
4 equiv of 28, followed by incorporation of the Ru center
through treatment with 2 in the presence of CuCl, affords the
desired 32 as an air-stable brown solid (mp ) 92-98 °C dec).22
The related Ru dendrimer 33 is prepared as an air-stable dark
green solid by the same sequence of reactions as shown in

Ru-Based Metathesis Catalysts
J. Am. Chem. Soc., Vol. 122, No. 34, 2000 8173
Scheme 4, except that the last step involves treatment of the
vacant dendritic structure with 4.8 equiv of 3 and 4.9 equiv of
CuCl in CH2Cl2 for 2 h (55% yield; mp ) 114-117 °C dec).23
Catalytic RCM, ROM, and CM Promoted by Dendritic
Ru Complexes 32 and 33. As illustrated in Table 3, treatment
of diene 34 with 1.25 mol % 32 (5 mol % Ru) leads to efficient
and catalytic RCM. The desired product (35) is first isolated in
99% yield by silica gel chromatography through elution with
CH2Cl2. Subsequent washing of the silica with Et2O leads to
the isolation of the dendritic catalyst (>98% mass balance).
Recovered 32 was analyzed by 400 MHz 1H NMR spectroscopy;
the resulting spectrum indicated that 13% of the styrenyl ligands
were vacant (13% Ru loss, 93% isolated yield of 32).24
Table 3. Utility of Dendritic 30 in Catalytic RCM
cycle
product yielda (%)
Ru contentb (%)
1
2
3
4
5
6
99
91
96
89
92
87
87
76
72
64
48
41
a Isolated yields after silica gel chromatography. b Determined by
1
analysis of the 400 MHz H NMR of the purified reaction mixture of
(20) For carbosilane dendimer structures based on a repetitive alkeny-
lation/hydrosilation sequence, see: (a) van der Made, A. W.; van Leeuwen,
P. W. N. M. J. Chem. Soc., Chem. Commun. 1992, 1400-1401. (b) van
der Made, A. W.; van Leeuwen, P. W. N. M.; de Wilde, J. C.; Brandes, R.
A. C. AdV. Mater. 1993, 5, 466-468. (c) Roovers, J.; Toporowski, P. M.;
Zhou, L.-L. Polym. Prepr. (Am. Chem. Soc., DiV. Polym. Chem.) 1992, 33,
182. (d) Zhou, L.-L.; Roovers, J. Macromolecules 1993, 26, 963. (e)
Roovers, J.; Zhou, L.-L.; Toporowski, P. M.; van der Zwan, M.; Iatrou,
H.; Hadjichristidis, N. Macromolecules 1993, 26, 4324. (f) Seyferth, D.;
Son, D. Y.; Rheingold, A. L.; Ostrander, R. L. Organometallics 1994, 13,
2682-2690.
(21) For carbosilane dendimers based on a repetitive alkenylation/
hydroboration/oxidation sequence, see: Kim, C.; Son, S.; Kim, B. J.
Organomet. Chem. 1999, 588, 1-8.
(22) An attractive attribute of this route is that it is amenable to
modification so that related and more highly branched dendrimers can be
prepared. For example, catalytic hydrosilation of 30 with HSiCl3 followed
by alkylation of the 12 Si-Cl bonds with allylmagnesium bromide should
lead to a 12-branch dendritic core structure. Synthesis and study of these
and related dendrimers are in progress.
dendrimer after silica gel purification (devoid of other Ru-containing
impurities).
As illustrated in Table 3, repeated use of 32 as a catalyst,
despite steady loss of Ru per reaction, results in complete
conversion of 34 to 35 and isolation of the desired product in
>86% isolated yield. These data thus illustrate that dendrimer
32 is effective in promoting the catalytic RCM of terminal dienes
in an efficient manner and can be easily recovered by simple
silica gel filtration and reused repeatedly in subsequent reactions.
Several additional points in connection with the data in Table
3 deserve comment: (1) After repeated use, the partially
depleted dendrimer complex can be easily remetalated upon
treatment with the appropriate equivalents of 2 and CuCl in
CH2Cl2. (2) The dendritic complex remains active even after
nearly 50% of the Ru content has been depleted (see cycle 6 in
Table 3). This level of reactivity may be attributed, at least
partially, to the fact that 32 (similar to 1 and 5) releases a highly
active monophosphine Ru complex (cf. 25, Scheme 3) into the
solution. In the absence of a second equivalent of PCy3 that
can recoordinate to Ru and retard its catalytic activity (which
(23) Dendritic complexes 32 and 33 and their corresponding precursors
1
were fully characterized by IR, H and 13C NMR spectroscopy, elemental
analysis, and mass spectrometry. See the Experimental Section for details.
(24) 13% loss of Ru constitutes a 7% loss of the total mass of the fully
metalated dendrimer. The reported quantitative (>98%) mass balance for
the purified macromolecule is based on a theoretical yield which has been
corrected for the minor loss of the metal and its associated ligands.

8174 J. Am. Chem. Soc., Vol. 122, No. 34, 2000
Garber et al.
Scheme 5
observations indicate that the Ru metal, after reacting with the
diene substrate and leaving the dendrimer, can be trapped again
by a styrenyl ether. Thus, in the absence of 4, the catalytically
active Ru-monophosphine would likely return to a styrene unit
within the dendritic structure. Moreover, these data suggest that
indeed a significant portion of the active Ru-carbene is released
into the reaction mixture.
Dendrimer 33 exhibits catalytic activity higher than that
observed for 32. Unlike 1 or 32, but similar to 5, dendritic 33
efficiently promotes the formation of trisubstituted allylic alcohol
12 (eq 2); in addition to the desired product (78%), the
dendrimer is recovered in 90% yield after silica gel chroma-
tography, with 8% loss in Ru loading (judged by analysis of
1
400 MHz H NMR spectrum). Moreover, as shown in eq 3,
similar to 5, dendrimer 33 effectively catalyzes tandem ROM/
RCM of 21 and the formation of 22 (94%). However, in contrast
to the corresponding monomer 5, dendrimer 33 can be easily
separated from 22 and recovered in 90% yield (8% Ru loss).28
The transformation in eq 4 indicates that 33 effectively promotes
catalytic ROM/CM reactions as well and, as before, it can be
recovered readily and in good yield (>98% trans-olefins in 24
is the case when 2 or 3 is used as a catalyst),25 and since styrene
ethers probably do not kinetically reassociate with Ru as
efficiently as PCy3 during propagation,26 even a small amount
of Ru release can lead to substantial amounts of metathesis
activity.
The data in Table 3 and the above mechanistic considerations
raise the possibility that 32 and 33 may simply be releasing
small but sufficient amounts of highly active monophosphine
Ru complexes (e.g., 25, Scheme 3) into solution that effect
complete conversion, and that little or none of the released metal
complexes return to the dendritic complex.
1
and 36, as judged by 400 MHz H NMR analysis).29 In each
case (eqs 2-4), owing to its high polarity, isolation of the
dendrimer is a straightforward operation. Reaction mixtures are
loaded onto a small plug of silica gel, and products are eluted
with CH2Cl2. The column is then washed with Et2O to retrieve
33. Thus, dendritic catalyst 33 retains the high actiVity of
monomeric 5 and proVides the Valuable practical adVantage
of being readily separable from metathesis products (compare
eqs 3 and 4 with reactions in Scheme 2).
To address this possibility, the metal crossover experiments
depicted in Scheme 5 were carried out. These studies were
facilitated by a minor chemical shift difference for the carbene
proton signals of dendritic (32) and monomeric (1) Ru-carbenes.
Thus, the amount of Ru bound to the dendritic versus mono-
meric ligands is readily determined by integration of the
Similar to monomeric 5, lower loadings of 33 are sufficient
for efficient catalytic metathesis. As an example, when triene 7
(cf. Table 2, entry 1) is treated with 0.25 mol % 33 (CH2Cl2,
22 °C) for 10 min, RCM adduct 8 is formed with >98%
conversion. In addition to dihydrofuran 8, isolated in 84% yield,
1
appropriate downfield signals in the H NMR spectrum of the
mixture. Treatment of 4 with dendritic Ru complex 32 (1.0
equiv) results in no metal crossover (<2% 1 is formed by 400
(28) Analysis of the unpurified reaction mixture indicates that ap-
proximately 3% of the Ru loss (3% loss of the total Ru sites) may be due
to the isolation of the cross-metathesis product ii, which is presumably
formed upon release of the transition metal from the dendrimer (33). The
formation of compounds such as ii only occurs with substrates that do not
contain terminal alkenes. Otherwise, as shown in Scheme 2, the terminal
styrenyl alkoxide is formed upon Ru departure. Presumably, most of ii is
reverted back to dendritic 33 through reaction with the Ru-methylene
complex. Ru return may occur via ii through formation of the 1,1-
disubstituted Ru-carbene followed by intramolecular RCM. Alternatively,
cross-metathesis at the conjugated alkene can either afford 33 and the diene
precursor to 22 or give the terminal styrene olefin and the Ru-carbene
precursor to 22. Regardless, all of the above possibilities eventually afford
22 and recover 33. The recovery of 3% ii may be due to competitive Ru-
methylene decomposition.
1
MHz H NMR analysis).27 When diene substrate 34 is treated
with 1.25 mol % of fully loaded 32 and 4 mol % of 4, RCM
product 35 is obtained within 15 min. The amount of recovered
32, however, bears 42% less Ru compared to 13% metal
reduction when the reaction is carried out in the absence of 4
(see Table 3, cycle 1). In addition, ∼30% of uncomplexed 4 is
isolated after the reaction; the remainder of the monomeric
styrenyl isopropyl ether is recovered as Ru complex 1. These
(25) (a) Dias, E. L.; Nguyen, S. T.; Grubbs, R. H. J. Am. Chem. Soc.
1997, 119, 3887-3897. (b) Reference 1a. For a report on a related
monophosphine and catalytically active Ru complex, see: (c) Tallarico, J.
A.; Bonitatebus, P. J.; Snapper, M. L. J. Am. Chem. Soc. 1997, 119, 7157-
7158.
(26) Recoordination to Ru by styrene ether probably requires olefin
metathesis, and the styrenyl CdC and the neighboring isopropyl ether are
inferior Lewis bases in comparison to PCy3. Our previous studies (ref 1a)
indicate that 1 initiates approximately 30 times slower than and propagates
four times faster than the bis(phosphine)Ru complex 2. The slower rate of
initiation of 1 is likely due to the less facile dissociation of the bidentate
ligand from the metal center; the higher rate of propagation of 1 is caused
by the absence of the competing PCy3 ligand. The unusual stability of 1 is
a result of steric congestion at the transition metal center (the corresponding
methyl ether complex is significantly less stable) and thermodynamic
stability provided by a styrene ether ligand.
(27) The relative inability of 4 to remove Ru from the dendritic structure,
in contrast to 34, is noteworthy and may be due to steric factors; electronic
issues may also play a role in this reactivity difference, where electronically
mismatched systems undergo cross metathesis more effectively. See:
Blackwell, H. E.; O’Leary, D. J.; Chatterjee, A. K.; Washenfelder, R. A.;
Bussmann, D. A.; Grubbs, R. H. J. Am. Chem. Soc. 2000, 122, 58-71 and
references therein.
(29) It is not clear whether diene 36 is formed in the reaction of
monomeric catalyst 5 (see Scheme 2); analysis of the outcome of the reaction
promoted by monomeric 5 was complicated by the presence of the
inseparable Ru complex (5).

Ru-Based Metathesis Catalysts
J. Am. Chem. Soc., Vol. 122, No. 34, 2000 8175
positive attributes have already provided us with important
information. For example, comparison of our previously pub-
lished data1a and those in Tables 2 and 3 and Scheme 5 suggests
that dendritic systems 32 and 33 might be less efficient in Ru
retrieval than the corresponding monomers; whereas 5-13%
Ru loss is observed with dendritic structures (cf. Table 3 and
eqs 2-4), 5% or less metal loss is detected with monomeric
units (cf. Table 2 and Scheme 5). Coiling of the relatively
flexible arms of 32 and 33, leading to slower capture of the
free Ru-carbene by the styrenyl ether ligand, may be responsible
for this difference. The above studies thus imply that more
efficient dendritic or other macromolecular complexes may be
designed that bear rigid side chains; as such, coiling may be
minimized and less Ru loss may occur per transformation.
Accordingly, the design, synthesis, and reactivity of various
other dendritic and subsequent polymer-supported recyclable
Ru-based metathesis catalysts are underway in these laboratories.
It is hoped that the studies described above will lead to the
development of even more active and more readily recoverable
and recyclable catalysts. In addition, synthesis of various chiral
versions of the above monomeric, dendritic, and related
polymer-bound complexes and their application to asymmetric
catalytic metathesis31 are being actively pursued.
(2)
(3)
(4)
Experimental Section
General. Infrared (IR) spectra were recorded on a Perkin-Elmer 781
spectrophotometer, with νmax in inverse centimeters. Bands are char-
acterized as broad (br), strong (s), medium (m), and weak (w). 1H NMR
spectra were recorded on a Varian Unity 300 (300 MHz), a Gemini
2000 (400 MHz), or an INOVA 500 (500 MHz) spectrometer. Chemical
shifts are reported in parts per million from tetramethylsilane with the
solvent resonance as the internal standard (CDCl3: δ 7.26 ppm. CD3-
CN: δ 1.94 ppm). Data are reported as follows: chemical shift,
multiplicity (s ) singlet, d ) doublet, t ) triplet, br ) broad, m )
multiplet), coupling constants (Hz), integration, and assignment. 13C
NMR spectra were recorded on a Varian Unity 300 (75 MHz), a Gemini
2000 (100 MHz), or an INOVA 500 (125 MHz) spectrometer with
complete proton decoupling. Chemical shifts are reported in parts per
million from tetramethylsilane with the solvent as the internal reference
(CDCl3: δ 77.00 ppm. CD3CN: δ 1.19 ppm). 31P NMR spectra were
recorded on a Varian Gemini 2000 (162 MHz) spectrometer with
complete proton decoupling. The chemical shifts of the phosphorus
resonances were determined relative to phosphoric acid as an external
standard (H3PO4: δ 0.0 ppm). Mass spectra were recorded at the
University of Illinois (Urbana-Champaign, IL) and Harvard University
(Cambridge, MA). Combustion analyses were performed by Robertson
Microlit Laboratories (Madison, NJ).
All reactions were carried out under an atmosphere of dry Ar in
oven- (135 °C) and flame-dried glassware with standard Schlenk or
vacuum-line techniques. In most instances, solid organometallic
compounds were purified and recovered in air and later stored in a
drybox under an atmosphere of argon. (PCy3)2Cl2RudCHPh (2)3 and
tetraallylsilane (29)32 were prepared according to literature procedures.
(4,5-DihydroIMES)(PCy3)Cl2RudCHPh (3) and its requisite starting
materials were prepared by a modification of the published method8a
(see below for further details). 2-Isopropoxystyrene (4) was prepared
from salicylaldehyde (Aldrich) by alkylation and Wittig olefination.
All other materials were obtained from commercial sources and typically
purified before use. Tetrahydrofuran, diethyl ether, benzene, and toluene
were distilled from sodium metal/benzophenone ketyl. Dichlo-
romethane, pentane, hexanes, 2-propanol, triethylamine, and ethanol
were distilled from calcium hydride under Ar. Methanol was distilled
over Mg under Ar. 2,4,6-Trimethylaniline (Aldrich) was vacuum
distilled. Triethylorthoformate (Aldrich) was distilled from MgSO4
under reduced pressure. 3-(4-Hydroxyphenyl)propionic acid (Aldrich)
was recrystallized from water. 2-Iodopropane (Aldrich) was distilled
from MgSO4 under argon. Dimethylformamide (Fisher) was stored over
4 Å molecular sieves prior to use. Tributyl(vinyl)tin (Aldrich) was
vacuum distilled from MgSO4. Allylmagnesium bromide was freshly
recovered 33 is obtained in 88% yield after silica gel chroma-
tography (22% Ru loss).30
Conclusions
We present a new class of highly active and recyclable Ru-
based metathesis catalysts. These complexes, both monomeric
(5) and dendritic (33), promote RCM, ROM, and CM reactions
in a highly efficient manner. Unlike the first generation
recoverable Ru-based complexes (1 and 32), 5 and 33 effect
the efficient formation of trisubstituted alkenes through catalytic
RCM. Tetrasubstituted olefins can be catalytically accessed too,
but less efficiently than trisubstituted olefins. The related
dendritic systems are recyclable and as active as their corre-
sponding monomers but offer the added advantage of being
more readily isolable (cf. eqs 3 and 4 and Scheme 2).
The appreciable levels of efficiency observed with Ru
dendrimers is due to the fact that the active Ru species are
readily released and recaptured. The fact that dendritic structures
can be readily characterized by various spectroscopic methods
and serve as useful mechanistic probes renders these macro-
molecular systems excellent starting points for the ultimate
design and synthesis of more efficient recyclable catalysts. Such
(30) Although the recovered dendrimer is relatively less loaded with lower
catalyst loading (78% return for 0.25 mol % 33 versus 90-95% for 1.25
mol % 33), the amount of recovered Ru per mmole of product formed is
nearly identical for both conditions.
(31) For Mo-catalyzed asymmetric RCM reactions, see: (a) Alexander,
J. B.; La, D. S.; Cefalo, D. R.; Hoveyda, A. H.; Schrock, R. R. J. Am.
Chem. Soc. 1998, 120, 4041-4042. (b) La, D. S.; Alexander, J. B.; Cefalo,
D. R.; Graf, D. D.; Hoveyda, A. H.; Schrock, R. R. J. Am. Chem. Soc.
1998, 120, 9720-9721. (c) Zhu, S. S.; Cefalo, D. R.; La, D. S.; Jamieson,
J. Y.; Davis, W. M.; Hoveyda, A. H.; Schrock, R. R. J. Am. Chem. Soc.
1999, 121, 8251-8259. (d) Fujimura, O.; Grubbs, R. H. J. Am. Chem. Soc.
1996, 118, 2499-2500. (e) Fujimura, O.; Grubbs, R. H. J. Org. Chem.
1998, 63, 824-832. For application of catalytic asymmetric RCM in target-
oriented synthesis see: (f) Burke, S. D.; Muller, N.; Beudry, C. M. Org.
Lett. 1999, 1, 1827-1829. For Mo-catalyzed asymmetric ROM reactions
see: (g) La, D. S.; Ford, J. G.; Sattely, E. S.; Bonitatebus, P. J.; Schrock,
R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 1999, 121, 11603-11604. (h)
Reference 16f.
(32) Abel, E. W.; Rowley, R. J. J. Organomet. Chem. 1975, 84, 199-
229.

8176 J. Am. Chem. Soc., Vol. 122, No. 34, 2000
Garber et al.
prepared from distilled allyl bromide (Aldrich) and Mg turnings (Strem)
and titrated before use. Silicon tetrachloride (Strem) and chlorodim-
ethylsilane (Aldrich) were distilled under argon. 9-Borabicyclo[3.3.1]-
nonane (9-BBN) was freshly prepared from distilled 1,5-cyclooctadiene
(Aldrich), borane-dimethyl sulfide complex (Aldrich), and anhydrous
dimethoxyethane (Aldrich, distilled from sodium metal/benzophenone
ketyl).33 4-(Dimethylamino)pyridine (DMAP) (Aldrich) was recrystal-
lized from anhydrous toluene. The following materials were purchased
from commercial sources and used as received: glyoxal (40 wt %
solution in water) (Aldrich), sodium cyanoborohydride (Aldrich),
bromocresol green (Fisher), ammonium tetrafluoroborate (Aldrich),
potassium tert-butoxide (Strem), copper(I) chloride (Strem), anhydrous
HCl (Aldrich), sodium hydride (Aldrich), bromine (Aldrich), acetic acid
(Fisher), sodium thiosulfate (Aldrich), 2,6-di-tert-butyl-4-methylphenol
(Aldrich), tetrakis(triphenylphosphine)palladium(0) (Strem), activated
carbon (Aldrich), chloroplatinic acid hexahydrate (Speier’s catalyst)
(Strem), platinum-divinyltetramethylsiloxane complex in xylene
(Karstedt’s catalyst) (Gelest), hydrogen peroxide (30 wt % solution in
water) (Aldrich), citric acid (Aldrich), and 1-(3-dimethylaminopropyl)-
3-ethylcarbodiimide (EDC) (Advanced Chemtech).
MeOH in a 500 mL round-bottom flask. Several crystals of bromocresol
green were added as a pH indicator, and the mixture was cooled to 0
°C. NaCNBH3 (10.0 g, 159 mmol, 6.4 equiv) was added to the reaction
mixture in one portion as a solid. Vigorous bubbling was observed,
and the reaction mixture turned a deep blue-green color (alkaline pH).
After 10 min, concentrated HCl was added dropwise to the mixture,
restoring its original yellow color. Additional reduction slowly occurred,
causing the mixture to again become basic. The acidification process
was repeated (typically two more times) until the yellow color persisted.
The reaction mixture was warmed to 22 °C and stirred for 1 h. A
solution of 2 M KOH was added dropwise until the mixture was weakly
alkaline (pH ) 8-9). The mixture was then diluted with water (300
mL), transferred to a separatory funnel, and washed three times with
Et2O (500 mL). The combined organic layers were washed with 800
mL of a saturated solution of sodium chloride, dried over MgSO4,
filtered, and concentrated to a yellow oil. Silica gel chromatography
(TLC Rf ) 0.32 in 4:1 pentane/Et2O) afforded the product as a colorless
oil (7.13 g, 24.1 mmol, 96%). IR (NaCl): 3367 (br), 2996 (s), 2916
(s), 2854 (s), 2729 (w), 1612 (w), 1485 (s), 1446 (s), 1373 (m), 1344
(w), 1228 (s), 1207 (m), 1154 (m), 1110 (m), 1062 (w), 1030 (m),
1012 (m), 853 (s), 822 (w), 801 (w), 738 (m), 563 (m). 1H NMR (400
MHz, CDCl3): δ 6.85 (s, 4H, aromatic CH), 3.30 (br, 2H, NH), 3.17
(s, 4H, NCH2CH2N), 2.30 (s, 12H, mesityl o-CH3), 2.25 (s, 6H, mesityl
p-CH3). 13C NMR (100 MHz, CDCl3): δ 143.24, 131.35, 129.65,
129.38, 49.19, 20.60, 18.50. HRMS Calcd for C20H28N2: 296.2252.
Found: 296.2258. Anal. Calcd for C20H28N2: C, 81.03; H, 9.52.
Found: C, 81.28; H, 9.41.
All silica gel column chromatography was driven with compressed
air and performed with silica gel 60 (230-400 mesh; pH (10%
suspension) 6.5-7; surface area 500 m2/g; pore volume 0.75 mL/g)
obtained from TSI Chemical Co. (Cambridge, MA). The use of other
commercially available silica gels repeatedly gave unsatisfactory results
in the purification of the Ru complexes reported herein.
Similar to the original monomer (1), dendritic catalyst 32 forms a
dark brown solution in organic solvents. In contrast, the more active
catalysts bearing the 4,5-dihydroIMES ligand form bright green-colored
organic solutions. The purification of the above complexes can be easily
monitored visually, since they appear as dark brown or green bands
on the column. Denritic complexes 32 and 33 are significantly more
polar than the corresponding monomers. Following a metathesis reaction
mediated by 32 or 33, isolation of both product and catalyst typically
involved simply a filtration of the crude mixture through a silica gel
plug in 100% CH2Cl2 followed by a column “flush” in 100% Et2O
(TLC Rf of 32 and 33 < 1.0 in CH2Cl2). See below for further details.
((2,4,6-Trimethylphenyl)NCH)2.34 Glyoxal (3.73 mL of a 40%
weight solution in water, 32.5 mmol) was dissolved in 325 mL of
reagent-grade methanol in a 500 mL flask. 2,4,6-Trimethylaniline (8.25
mL, 58.8 mmol, 1.81 equiv) was added dropwise to this solution by
syringe. The mixture was stirred for 12 h at 22 °C as a bright yellow
precipitate slowly formed. The mixture was diluted with CH2Cl2,
dissolving the solid. The resulting yellow solution was dried over
MgSO4, filtered, and concentrated to a yellow-orange solid residue.
The unpurified product was recrystallized from anhydrous methanol
(for every 10 g, 850-900 mL of MeOH was required for complete
dissolution at reflux). After slow cooling to 22 °C followed by
subsequent storage of the sample at -20 °C for 12 h, long canary yellow
crystals formed. The product was recovered by vacuum filtration,
washed with pentane, and dried under high vacuum (7.40 g, 25.3 mmol,
86%). IR (NaCl): 3005 (m), 2946 (s), 2916 (s), 2854 (m), 2725 (w),
1617 (s), 1595 (w), 1476 (m), 1438 (w), 1374 (m), 1265 (m), 1202 (s),
1,3-Dimesitylimidazolinium tetrafluoroborate.35 A 25 mL round-
bottom flask was charged with ((2,4,6-trimethylphenyl)NHCH2)2 (7.81
g, 26.4 mmol) and ammonium tetrafluoroborate (2.77 g, 26.4 mmol,
1.0 equiv). Triethylorthoformate (4.39 mL, 26.4 mmol, 1.0 equiv) was
added by syringe. The flask was equipped with a reflux condenser and
submerged into a preheated oil bath at 120 °C. The mixture was refluxed
for 3 h and cooled to 22 °C. A tan-colored solid precipitated, leaving
a cloudy suspension. This mixture was recrystallized from hot
anhydrous ethanol. The resulting bright white crystals of product were
recovered by vacuum filtration, washed with pentane, and dried under
high vacuum (5.62 g, 14.3 mmol, 54%). Additional product could be
obtained by further recrystallization of the mother liquor. IR (NaCl):
3091 (w), 2979 (br), 2941 (br), 1633 (s), 1487 (w), 1459 (w), 1393
(w), 1313 (w), 1269 (m), 1214 (w), 1092 (m), 1054 (s), 1036 (s), 965
1
(w), 880 (w), 852 (m). H NMR (400 MHz, CD3CN): δ 8.14 (s, 1H,
NCHN), 7.08 (s, 4H, aromatic CH), 4.43 (s, 4H, NCH2CH2N), 2.37 (s,
12H, mesityl o-CH3), 2.32 (s, 6H, mesityl p-CH3). 13C NMR (100 MHz,
CD3CN): δ 160.41 (d, JNC ) 10.3 Hz), 141.54, 136.50, 131.36, 130.61,
52.21, 21.16, 17.92. HRMS Calcd for C21H27N2: 307.2174 (cation only).
Found: 307.2175. Anal. Calcd for C21H27BF4N2: C, 63.97; H, 6.90.
Found: C, 63.79; H, 6.85.
(4,5-DihydroIMES)(PCy3)Cl2RudCHPh (3). The ligand salt 1,3-
dimesitylimidazolinium tetrafluoroborate (2.94 g, 7.46 mmol, 1.2 equiv)
was suspended in 50 mL of THF in a 250 mL round-bottom flask.
This mixture was then treated with a solution of potassium tert-butoxide
(840 mg, 7.49 mmol, 1.2 equiv) in 50 mL of THF via cannula at 22
°C. This mixture was immediately transferred by cannula (20 mL of
THF used as rinse) to a second vessel containing a solution of (PCy3)2-
Cl2RudCHPh (2) (5.01 g, 6.09 mmol, 1.0 equiv) in 100 mL of benzene
(additional stirring of the ligand salt mixture at 22 °C prior to exposure
to the Ru-carbene often resulted in incomplete conversion to the desired
product). The resulting mixture was refluxed at 80 °C for 30 min and
then cooled to 22 °C. All manipulations from this point forward were
carried out in air with reagent-grade solvents. The solvents were
removed at reduced pressure, leaving a red-brown solid residue. The
unpurified residue was dissolved in 8:1 hexanes/Et2O and loaded onto
a wide plug of silica gel. Elution with the same solvent system slowly
removed a pink-red band of the desired product from the column.
Concentration of the product fractions in vacuo removed the more polar
and volatile Et2O and resulted in spontaneous precipitation of the
catalyst from hexanes as a cranberry red, microcrystalline solid (3.78
g, 4.45 mmol, 73%). These crystals were dried under high vacuum. IR
1
1141 (m), 1031 (w), 850 (s), 780 (m), 739 (s), 705 (w), 609 (w). H
NMR (400 MHz, CDCl3): δ 8.12 (s, 2H, NCH), 6.93 (s, 4H, aromatic
CH), 2.31 (s, 6H, mesityl p-CH3), 2.18 (s, 12H, mesityl o-CH3). 13C
NMR (100 MHz, CDCl3): δ 163.31, 147.29, 134.13, 128.86, 126.44,
20.83, 18.28. HRMS Calcd for C20H23N2: 292.1861 (M-H)+. Found:
291.1862. Anal. Calcd for C20H24N2: C, 82.15; H, 8.27. Found: C,
81.99; H, 8.12.
((2,4,6-Trimethylphenyl)NHCH2)2. The bis(imine) ((2,4,6-trimeth-
ylphenyl)NCH)2 (7.30 g, 25.0 mmol) was suspended in 250 mL of
(33) Soderquist, J. A.; Alvin, N. Org. Synth. 1991, 70, 169-176.
(34) Grubbs and co-workers recently reported (see ref 8a) the syntheses
of the first four compounds described in this Experimental Section. However,
experimental details and complete characterization for all of these materials
were not provided. A recent disclosure (Arduengo, A. J., III; Goerlich, J.
R.; Marshall, W. J. J. Am. Chem. Soc. 1995, 117, 11027-11028) also
describes 1,3-dimesitylimidazolinium chloride, but characterization is only
mentioned for the corresponding 1,3-dimesitylimidazolin-2-ylidene. We have
found the procedures described above satisfactory for the efficient synthesis
of these materials in multigram quantities.
(35) Saba, S.; Brescia, A.-M.; Kaloustain, M. K. Tetrahedron Lett. 1991,
32, 5031-5034.

Ru-Based Metathesis Catalysts
J. Am. Chem. Soc., Vol. 122, No. 34, 2000 8177
(NaCl): 3057 (m), 3039 (m), 3015 (m), 2927 (s), 2850 (s), 1608 (w),
1479 (s), 1446 (s), 1421 (s), 1380 (m), 1328 (w), 1266 (s), 1243 (m),
1205 (w), 1174 (m), 1129 (w), 1036 (w), 1005 (m), 909 (m), 849 (m),
hydride (71.0 mg, 2.96 mmol. 0.75 equiv) in THF (5 mL) and
2-iodopropane (0.30 mL, 3.0 mmol, 0.75 equiv) were added. This
procedure was repeated if necessary until no starting material could be
detected by TLC analysis (we suspect that competing elimination of
the electrophile is reponsible for incomplete product conversions,
requiring us to resubject the reaction mixture). The mixture was then
diluted with Et2O (150 mL) and water (200 mL) and transferred to a
separatory funnel. The organic layer was removed, and the aqueous
layer was washed twice with Et2O (100 mL). The combined organic
layers were washed with three volumes of water to remove residual
DMF. The organic solution was then dried over MgSO4, filtered, and
concentrated in vacuo to a pale yellow oil. The product was passed
through a short column of silica gel in 7:1 hexanes/Et2O, affording
811 mg (3.24 mmol, 82%) of a colorless oil (TLC Rf ) 0.30 in 7:1
hexanes/Et2O). IR (NaCl): 2978 (m), 2934 (w), 1731 (s), 1612 (w),
1510 (s), 1452 (w), 1383 (m), 1295 (w), 1242 (s), 1182 (m), 1109 (s),
1
737 (s), 703 (m), 687 (m), 624 (w), 578 (w). H and 31P NMR data
were in agreement with those reported by Scholl et al.8a Anal. Calcd
for C46H65Cl2N2PRu: C, 65.08; H, 7.72. Found: C, 65.18; H, 7.71.
(4,5-DihydroIMES)Cl2RudCH-o-OiPrC6H4 (5). (4,5-DihydroIMES)-
(PCy3)Cl2RudCHPh (3) (895 mg, 1.05 mmol, 1.03 equiv) and CuCl
(104 mg, 1.05 mmol, 1.00 equiv) were weighed into a 100 mL round-
bottom flask in a glovebox and dissolved in 20 mL of CH2Cl2.
2-Isopropoxystyrene (4) (166 mg, 1.02 mmol, 0.97 equiv) was
cannulated into the resulting deep red solution in 20 mL of CH2Cl2 at
22 °C. The flask was equipped with a condenser, and the solution was
stirred at 40 °C for 1 h. From this point forth, all manipulations were
carried out in air with reagent-grade solvents. The reaction mixture
was concentrated in vacuo to a dark brown solid residue. The unpurified
material was dissolved in a minimal volume of 1:1 pentane/CH2Cl2
and loaded onto a plug of silica gel. Insoluble copper-phosphine
precipitates can complicate chromatography and must be removed
during loading of the sample. This was readily accomplished by passing
the solution through a second Pasteur pipette containing a plug of cotton
as the column was loaded. Elution with 1:1 pentane/CH2Cl2 removed
a bright green band from the column. Removal of solvent and drying
under high vacuum afforded 543 mg (0.87 mmol, 85%) of a bright
green crystalline solid. The addition of hexanes just prior to complete
removal of the chromatography solvent will result in spontaneous
precipitation of the product. Alternatively, crystallization can be effected
by layering concentrated CH2Cl2 solutions with pentane or hexanes.
IR (NaCl): 2922 (br), 2853 (m), 1730 (w), 1606 (w), 1589 (m), 1575
(w), 1478 (s), 1452 (s), 1420 (s), 1397 (m), 1384 (m), 1295 (m), 1263
(s), 1217 (m), 1160 (w), 1113 (s), 1098 (w), 1035 (w), 938 (m), 852
(w), 801 (w), 746 (m), 737 (m), 580 (m). 1H NMR (400 MHz,
CDCl3): δ 16.56 (s, 1H, RudCHAr), 7.48 (m, 1H, aromatic CH), 7.07
(s, 4H, mesityl aromatic CH), 6.93 (dd, J ) 7.4, 1.6 Hz, 1H, aromatic
CH), 6.85 (dd, J ) 7.4, 7.0 Hz, 1H, aromatic CH), 6.79 (d, J ) 8.6
Hz, 1H, aromatic CH), 4.90 (septet, J ) 6.3 Hz, 1H, (CH3)2CHOAr),
4.18 (s, 4H, N(CH2)2N), 2.48 (s, 12H, mesityl o-CH3), 2.40 (s, 6H,
mesityl p-CH3), 1.27 (d, J ) 5.9 Hz, 6H, (CH3)2CHOAr). 13C NMR
(100 MHz, CDCl3): δ 296.83 (q, J ) 61.5 Hz), 211.13, 152.04, 145.13
(d, JOC ) 3.9 Hz), 145.09, 138.61, 129.39 (d, JNC ) 3.9 Hz), 129.35,
129.17, 122.56, 122.11, 112.75, 74.86 (d, JOC ) 10.7 Hz), 51.42, 30.86,
25.93, 21.08. HRMS Calcd for C31H38Cl2N2O99Ru: 623.1421. Found:
623.1411. Anal. Calcd for C31H38Cl2N2ORu: C, 59.42; H, 6.11; Cl,
11.32; N, 4.47. Found: C, 59.28; H, 6.35; Cl, 11.36; N, 4.12.
1
957 (w), 829 (w). H NMR (400 MHz, CDCl3): δ 7.09 (d, J ) 8.6
Hz, 2H, aromatic CH), 6.80 (d, J ) 8.6 Hz, 2H, aromatic CH), 5.00
(septet, J ) 6.3 Hz, 1H, (CH3)2CHO2C), 4.50 (septet, J ) 6.3 Hz, 1H,
(CH3)2CHOAr), 2.87 (t, J ) 7.8 Hz, 2H, CH2CO2iPr), 2.55 (t, J ) 7.8
Hz, 2H, ArCH2), 1.32 (d, J ) 6.3 Hz, 6H, (CH3)2CHOAr), 1.20 (d, J
) 6.3 Hz, 6H, (CH3)2CHO2C). 13C NMR (100 MHz, CDCl3): δ 172.39,
156.12, 132.43, 129.15, 115.81, 69.86 (d, JOC ) 3.4 Hz), 67.59 (d, JOC
) 9.8 Hz), 36.59, 30.26, 22.14, 21.88. HRMS Calcd for C15H22O3:
250.1569. Found: 250.1566. Anal. Calcd for C15H22O3: C, 71.97; H,
8.86. Found: C, 72.26; H, 9.04.
Isopropyl-1-(m-bromo-p-isopropoxyphenyl)propionate. A 50 mL
round-bottom flask was charged with isopropyl-1-(p-isopropoxyphenyl)-
propionate (27) (1.09 g, 4.34 mmol) and CH2Cl2 (20 mL, 0.20 M).
Acetic acid (10 µL, 0.18 mmol) was added to the solution. Bromine
(0.235 mL, 4.56 mmol, 1.05 equiv) was then slowly added dropwise
via syringe, forming a red solution. Over the course of 0.5 h the solution
gradually turned pale yellow as the bromine was consumed. After 1 h,
the reaction was quenched with 5 mL of saturated sodium thiosulfate.
The mixture was diluted with water (200 mL) and Et2O (200 mL) and
partitioned in a separatory funnel. The aqueous layer was washed with
2 × 150 mL of Et2O. The combined organic layers were dried over
MgSO4, filtered, and concentrated to a yellow oil. This material could
be purified by vacuum distillation or silica gel chromatography (TLC
Rf ) 0.23 in 10:1 hexanes/Et2O) to deliver the product as a colorless
oil (1.40 g, 4.25 mmol, 98%). Crucial to the success of this reaction is
the use of exactly 1.0-1.1 equiv of bromine; an excess of the reagent
leads to dibrominated adducts. If these impurities are generated, a CH2-
Cl2/pentane solvent system must be used as eluant to effect purification
of the desired product on silica gel (TLC Rf ) 0.30 in 3:2 CH2Cl2/
pentane). The halogenated solvent mix also promotes a facile separation
of the product and the starting material (27) in the event that the reaction
does not proceed to completion (<1.0 equiv of Br2). IR (NaCl): 2979
(m), 2936 (w), 1729 (s), 1604 (w), 1493 (s), 1384 (m), 1373 (m), 1281
(m), 1253 (s), 1240 (m), 1180 (m), 1140 (m), 1109 (s), 1046 (w), 954
Isopropyl-1-(p-hydroxyphenyl)propionate. Through a stirring solu-
tion of 3-(4-hydroxyphenyl)propionic acid (26) (5.00 g, 30.1 mmol) in
2-propanol (167 mL, 72.0 equiv) was bubbled anhydrous HCl for 50
min. The flask was sealed under Ar, and the solution was stirred for
12 h at 22 °C. The solvent was removed under reduced pressure with
gentle heating, leaving a thick, colorless oil. Removal of residual
2-propanol under high vacuum at 22 °C resulted in spontaneous
precipitation of the desired product as a bright white crystalline solid
(6.12 g, 29.4 mmol, 98%). IR (NaCl): 3412 (br), 3024 (w), 2981 (m),
2930 (w), 2873 (w), 1712 (m), 1613 (s), 1595 (m), 1519 (s), 1449 (m),
1377 (s), 1298 (m), 1266 (s), 1225 (s), 1149 (m), 1108 (s), 904 (m),
1
(m), 812 (w). H NMR (400 MHz, CDCl3): δ 7.38 (d, J ) 2.2 Hz,
1H, aromatic CH), 7.06 (dd, J ) 8.4, 2.2 Hz, 1H, aromatic CH), 6.83
(d, J ) 8.4 Hz, 1H, aromatic CH), 4.96 (septet, J ) 6.2 Hz, 1H,
(CH3)2CHO2C), 4.50 (septet, J ) 6.2 Hz, 1H, (CH3)2CHOAr), 2.85
(dd, J ) 7.7, 7.3 Hz, 2H, CH2CO2iPr), 2.55 (dd, J ) 7.7, 7.3 Hz, 2H,
ArCH2), 1.36 (d, J ) 5.9 Hz, 6H, (CH3)2CHOAr), 1.20 (d, J ) 6.6 Hz,
6H, (CH3)2CHO2C). 13C NMR (100 MHz, CDCl3): δ 172.06, 152.84,
134.36, 133.09 (d, JOC ) 7.3 Hz), 128.03, 115.98, 113.63, 72.34 (d,
JOC ) 3.9 Hz), 67.80 (d, JOC ) 12.2 Hz), 36.29, 29.90, 22.15 (d, JOC
) 2.4 Hz), 21.89 (d, JOC ) 3.4 Hz). HRMS Calcd for C15H21BrO3:
328.0674. Found: 328.0671. Anal. Calcd for C15H21BrO3: C, 54.72;
H, 6.43. Found: C, 54.84; H, 6.43.
1
837 (m), 820 (m), 609 (m). H NMR (400 MHz, CDCl3): δ 7.04 (d,
J ) 8.4 Hz, 2H, aromatic CH), 6.74 (d, J ) 8.4 Hz, 2H, aromatic
CH), 5.80 (s, 1H, ArOH), 5.00 (septet, J ) 6.3 Hz, 1H, (CH3)2CHO),
2.87 (t, J ) 7.6 Hz, 2H, CH2CO2iPr), 2.57 (t, J ) 7.6 Hz, 2H, ArCH2),
1.20 (d, J ) 6.3 Hz, 6H, (CH3)2CHO). 13C NMR (100 MHz, CDCl3):
δ 172.97, 154.04, 132.19, 129.28, 115.19, 68.07 (d, JOC ) 9.8 Hz),
36.64, 30.23, 21.85. HRMS Calcd for C12H16O3: 208.1099. Found:
208.1099. Anal. Calcd for C12H16O3: C, 69.21; H, 7.74. Found: C,
69.43; H, 7.88.
Isopropyl-1-(p-isopropoxyphenyl)propionate (27). A solution of
isopropyl-1-(p-hydroxyphenyl)propionate (0.822 g, 3.95 mmol) in THF
(10 mL) was treated via cannula with a suspension of sodium hydride
(104 mg, 5.92 mmol, 1.1 equiv) in THF (10 mL) at 0 °C. After gas
evolution subsided, DMF (20 mL) and 2-iodopropane (0.40 mL, 4.0
mmol, 1.0 equiv) were syringed into the reaction mixture. The resulting
suspension was stirred at 22 °C for 6 h, at which time additional sodium
Isopropyl-1-(p-isopropoxy-m-vinylphenyl)propionate. Pd(PPh3)4
(166 mg, 0.144 mmol, 3 mol %) and 2,6-di-tert-butyl-4-methylphenol
(1 mg, 0.005 mmol) were weighed into a 50 mL pear-shaped flask in
a glovebox and dissolved in 25 mL of dry toluene. This solution was
transferred through a cannula into a neat sample of isopropyl-1-(m-
bromo-p-isopropoxyphenyl)propionate (1.58 g, 4.79 mmol) in a 50 mL
round-bottom flask. The resulting pale yellow solution was stirred for
15 min at 22 °C. Tributyl(vinyl)tin (1.54 mL, 5.27 mmol, 1.1 equiv)
was then added dropwise to the reaction mixture through a syringe.

8178 J. Am. Chem. Soc., Vol. 122, No. 34, 2000
Garber et al.
The flask was equipped with a condenser and heated at 110 °C for 12
h. As the reaction progressed, a shiny mirrorlike film of Bu3SnBr salts
was gradually deposited onto the walls of the flask. After cooling to
22 °C, the reaction mixture was passed through a small plug of Celite
and activated carbon using Et2O as the eluant and concentrated in vacuo
to give a yellow oil. Purification by silica gel chromatography (TLC
Rf ) 0.27 in 8:1 hexanes/Et2O) afforded 888 mg of a colorless oil (3.22
mmol, 67%). IR (NaCl): 2978 (s), 2936 (m), 2873 (w), 1731 (s), 1627
(w), 1491 (s), 1373 (m), 1246 (s), 1180 (m), 1109 (s), 996 (w), 957
was sufficiently pure for the subsequent alkylation step. Thus, the
product was dissolved in 20 mL of Et2O and transferred by cannula
into a solution of freshly prepared allylmagnesium bromide (0.936 M,
17.8 mL, 16.7 mmol, 4.2 equiv). The reaction was stirred for 12 h at
22 °C and quenched with 10 mL of a saturated solution of ammonium
chloride. The mixture was diluted with water (200 mL) and Et2O (150
mL) and partitioned in a separatory funnel. The aqueous layer was
washed with 2 × 100 mL of Et2O. The combined organic layers were
washed with a volume of saturated sodium chloride, dried over MgSO4,
and vacuum filtered through a coarse frit funnel containing Celite.
Removal of volatiles gave a light orange oil which was purified by
silica gel chromatography (TLC Rf ) 0.63 in hexanes). The product
was recovered as a colorless oil (2.11 g, 3.56 mmol, 90%). IR (NaCl):
3077 (w), 2954 (m), 2913 (s), 2876 (m), 1630 (m), 1418 (w), 1250 (s),
1153 (m), 1034 (w), 990 (w), 932 (w), 893 (s), 844 (s), 698 (w), 629
(w). 1H NMR (400 MHz, CDCl3): δ 5.78 (ddt, J ) 16.8, 10.2, 8.2 Hz,
4H, CHdCH2), 4.87-4.79 (m, 8H, CHdCH2), 1.51 (d, J ) 8.2 Hz,
8H, SiCH2CHdCH2), 1.32 (m, 8H, SiCH2CH2CH2Si), 0.62-0.53 (m,
16H, SiCH2CH2CH2Si), -0.02 (s, 24H, Si(CH3)2). 13C NMR (100 MHz,
CDCl3): δ 135.21, 112.47, 23.49, 19.93, 18.55, 17.54, -3.52. HRMS
Calcd for C32H67Si5: 591.4089 (M-H)+. Found: 591.4072. Anal. Calcd
for C32H68Si5: C, 64.78; H, 11.55. Found: C, 64.98; H, 11.55.
1
(m), 904 (w), 814 (w). H NMR (400 MHz, CDCl3): δ 7.31 (d, J )
2.3 Hz, 1H, aromatic CH), 7.07-6.99 (m, 2H, aromatic CH and
ArCHCH2), 6.80 (d, J ) 8.2 Hz, 1H, aromatic CH), 5.71 (dd, J )
17.8, 1.6 Hz, 1H, ArCHCH2), 5.22 (dd, J ) 11.3, 1.6 Hz, 1H,
ArCHCH2), 5.00 (septet, J ) 6.3 Hz, 1H, (CH3)2CHO2C), 4.49 (septet,
J ) 6.3 Hz, 1H, (CH3)2CHOAr), 2.88 (dd, J ) 7.8, 7.4 Hz, 2H, CH2-
CO2iPr), 2.57 (dd, J ) 8.2, 7.4 Hz, 2H, ArCH2), 1.33 (d, J ) 6.3 Hz,
6H, (CH3)2CHOAr), 1.21 (d, J ) 6.3 Hz, 6H, (CH3)2CHO2C). 13C NMR
(100 MHz, CDCl3): δ 172.37, 153.50, 132.52, 131.81, 128.38, 127.67,
126.21 (d, JOC ) 5.4 Hz), 114.42, 113.76 (d, JOC ) 8.3 Hz), 70.01 (d,
JOC ) 3.4 Hz), 67.64 (d, JOC) 11.2 Hz), 36.58, 30.40, 22.28, 21.93.
HRMS Calcd for C17H24O3: 276.1725. Found: 276.1716. Anal. Calcd
for C17H24O3: C, 73.88; H, 8.75. Found: C, 73.71; H, 8.73.
1-(p-Isopropoxy-m-vinylphenyl)propionic Acid (28). A 100 mL
round-bottom flask was charged with isopropyl-1-(p-isopropoxy-m-
vinylphenyl)propionate (462 mg, 1.67 mmol) and 66.8 mL of 1 M KOH
(66.8 mmol, 40 equiv). The reaction vessel was equipped with a
condenser and heated at 100 °C for 12 h. The mixture was then cooled
to 0 °C and neutralized by the dropwise addition of concentrated HCl.
At a pH of ∼7-8, a white cloudy precipitate began to form in the
reaction mixture. At this point, the mixture was transferred to a
separatory funnel and washed with 150 mL of Et2O (an emulsion may
form, requiring extended time for phase separation). The aqueous layer
was acidified further with HCl, inducing additional precipitation. The
aqueous layer was washed with additional Et2O. This process was
repeated until no further precipitation was observed upon addition of
the acid (pH 3-4). The combined organic layers were dried over
MgSO4 and concentrated in vacuo to afford 355 mg (1.52 mmol, 91%)
of a light yellow solid which proved to be >98% pure as judged by 1H
NMR spectroscopy (400 MHz). If necessary, the acid could be further
purified by silica gel chromatography (TLC Rf ) 0.31 in 3:2 hexanes/
Et2O). It is recommended that the above procedure be followed with
care, since the product is quite prone to acid-catalyzed polymerization
of the styrene moiety. IR (NaCl): 2979 (m), 2935 (w), 2860 (w), 2760
(w), 1686 (s), 1600 (s), 1243 (s). 1H NMR (500 MHz, CDCl3): δ 11.26
(br, 1H, CO2H), 7.32 (d, J ) 2.3 Hz, 1H, aromatic CH), 7.06-7.00
(m, 2H, aromatic CH and ArCHCH2), 6.81 (d, J ) 8.5 Hz, 1H, aromatic
CH), 5.72 (dd, J ) 17.8, 1.5 Hz, 1H, ArCHCH2), 5.23 (dd, J ) 11.0,
1.5 Hz, 1H, ArCHCH2), 4.50 (septet, J ) 6.1 Hz, 1H, (CH3)2CHOAr),
2.90 (dd, J ) 8.0, 7.6 Hz, 2H, CH2CO2iPr), 2.67 (dd, J ) 8.0, 7.6 Hz,
2H, ArCH2), 1.34 (d, J ) 6.2 Hz, 6H, (CH3)2CHOAr). 13C NMR (125
MHz, CDCl3): δ 178.78, 153.77, 132.13, 131.88, 128.40, 127.87,
126.29, 114.48, 113.99, 70.99, 35.80, 29.88, 22.19. HRMS Calcd for
C14H18O3: 234.1256. Found: 234.1257. Anal. Calcd for C14H18O3: C,
71.77; H, 7.74. Found: C, 71.71; H, 7.68.
Si[(CH2)3Si(Me)2CHdCH2]4 (30). A 0.1 M solution of H2PtCl6‚
6H2O (Speier’s catalyst)36 was freshly prepared in anhydrous 2-pro-
panol. The hydrosilation could also be effected with Karstedt’s
catalyst.37 A 25 mL round-bottom flask was charged with the tetraene
(29) (762 mg, 3.96 mmol), HMe2SiCl (2.00 mL, 18.0 mmol, 4.6 equiv),
and THF (0.5 M, 8.0 mL). The platinum catalyst (10.0 µL, 0.010 mmol,
0.0025 equiv) was added dropwise by syringe, and the colorless solution
was heated at reflux (65 °C) for 12 h. After 20 min, the mixture had
turned dark green. Reaction progress was monitored readily by TLC;the
starting material (TLC Rf ) 0.9 in hexanes) stains bright yellow with
KMnO4. Following the removal of solvent and excess silane in vacuo,
1H NMR analysis (400 MHz) of the unpurified mixture indicated that
<5% of the R-substituted product was present and that the material
[(p-Isopropoxy-m-vinylphenyl)(CH2)2CO2(CH2)3Si(Me)2-
(CH2)3Si]4. Si[(CH2)3Si(Me)2CHdCH2]4 (30) (587 mg, 0.989 mmol)
was weighed into a 50 mL round-bottom flask and dissolved in 10 mL
of THF. This solution was treated by cannula with freshly prepared
9-BBN (527 mg, 4.69 mmol, 4.74 equiv) in 10 mL of THF. After 12
h at 22 °C, 10 mL each of H2O2 (30 wt % solution in water), 2 M
NaOH, and ethanol were added. The mixture was then allowed to stir
an additional 12 h at 22 °C. Water (100 mL) and Et2O (100 mL) were
added, and the organic layer was removed. The aqueous layer was
washed with 2 × 100 mL of Et2O. The combined organic layers were
dried over MgSO4 and filtered. Removal of volatiles gave a crude oil
that was purified by silica gel chromatography (TLC Rf ) 0.36 in
EtOAc). 1H NMR analysis (400 MHz) indicated that the product
contained minor impurities (including cyclooctadiol) which made
characterization of the material difficult. Thus, the crude product was
carried directly into the next step. The tetraol was transferred to a 25
mL round-bottom flask, dissolved in 15 mL of CH2Cl2, and cooled to
0 °C. 1-(p-Isopropoxy-m-vinylphenyl)propionic acid (28) (1.02 g, 4.35
mmol, 4.4 equiv), 1-(3-dimethylaminopropyl)-3-ethyl carbodiimide
(EDC) (912 mg, 4.76 mmol, 4.8 equiv), and DMAP (61 mg, 0.50 mmol,
0.50 equiv) were then directly added in succession to the mixture as
solids. Et3N (0.69 mL, 4.95 mmol, 5.0 equiv) was added through a
syringe. The resulting mixture was stirred for 4 h and quenched with
2 mL of a 10% citric acid solution. Additional water was added (200
mL), and the aqueous layer was washed with 3 × 100 mL of Et2O.
The combined organic layers were washed with 1 volume each of a
saturated solution of sodium chloride and water. Drying over MgSO4,
filtration, and concentration gave a yellow oil which was purified by
silica gel chromatography (TLC Rf ) 0.36 in 4:1 hexanes/EtOAc). The
desired tetra(ester) was recovered as a colorless oil (954 mg, 0.623
mmol, 63%). IR (NaCl): 2974 (m), 2951 (m), 2919 (s), 2873 (m),
2855 (m), 1735 (s), 1627 (w), 1491 (m), 1451 (w), 1384 (w), 1372
(w), 1293 (w), 1247 (s), 1139 (m), 1119 (m), 958 (w), 906 (w), 837
(m). 1H NMR (400 MHz, CDCl3): δ 7.30 (d, J ) 2.4 Hz, 4H, aromatic
CH), 7.02 (d, J ) 6.4 Hz, 4H, aromatic CH), 7.02 (dd, J ) 19.8, 9.4
Hz, 4H, ArCHCH2), 6.79 (d, J ) 8.8 Hz, 4H, aromatic CH), 5.71 (dd,
J ) 17.8, 1.6 Hz, 4H, ArCHCH2), 5.21 (dd, J ) 11.4, 1.6 Hz, 4H,
ArCHCH2), 4.48 (septet, J ) 6.2 Hz, 4H, (CH3)2CHOAr), 4.01 (t, J )
7.0 Hz, 8H, CO2CH2), 2.88 (t, J ) 7.8 Hz, 8H, ArCH2CH2CO2), 2.59
(t, J ) 7.8 Hz, 8H, ArCH2CH2CO2), 1.58 (m, 8H, CO2CH2CH2CH2-
Si(Me)2), 1.36-1.25 (m, 8H, SiCH2CH2CH2Si(Me)2), 1.33 (d, J ) 5.6
Hz, 24H, (CH3)2CHOAr), 0.58-0.52 (m, 16H, CH2Si(Me)2CH2), 0.47-
0.42 (m, 8H, Si(CH2)4), -0.04 (s, 24H, Si(Me)2). 13C NMR (100 MHz,
CDCl3): δ 172.77, 153.44, 132.40, 131.76, 128.30, 127.61, 126.14,
114.30, 113.78, 70.96, 67.16, 36.30, 30.38, 23.33, 22.33, 20.20, 18.65,
17.63, 11.30, -3.26. LRMS Calcd for C88H140O12Si5K (M+K)+:
1569.9. Found: 1569.5. Anal. Calcd for C88H140O12Si5: C, 69.06; H,
9.22. Found: C, 69.31; H, 9.36.
(36) (a) Speier, J. L.; Webster, J. A.; Barnes, G. H.; J. Am. Chem. Soc.
1957, 79, 974-979. (b) Musolf, M. C.; Speier, J. L. J. Org. Chem. 1964,
29, 2519.
(37) Karstedt, B. D. U.S. Patent 3,775,452, 1973.

Ru-Based Metathesis Catalysts
J. Am. Chem. Soc., Vol. 122, No. 34, 2000 8179
[(PCy3)Cl2RudCH-o-OiPrC6H3(CH2)2COO(CH2)3Si(Me)2-
(CH2)3Si]4 (32). (PCy3)2Cl2RudCHPh (2) (792 mg, 0.962 mmol, 4.3
equiv) and CuCl (106 mg, 1.07 mmol, 4.8 equiv) were added to a 25
mL round-bottom flask and suspended in 12 mL of CH2Cl2. [(p-
Isopropoxy-m-vinylphenyl)(CH2)2CO2(CH2)3Si(Me)2(CH2)3Si]4 (341 mg,
0.223 mmol, 1.0 equiv) was added to this mixture through a cannula
in 10 mL of CH2Cl2. The mixture was stirred for a period of 3 h at 22
°C, during which time the original purple solution turned dark brown.
The following workup procedures were conducted in air with reagent-
grade solvents. The mixture was concentrated at reduced pressure and
passed through a short plug of silica gel in 3:2 hexanes/Et2O (brown
band rapidly elutes). Product fractions were pooled and concentrated.
This material was passed through a second column of silica gel, this
time with a gradient elution (1:1 hexanes/CH2Cl2 to 2:3 hexanes/CH2-
Cl2 to 1:3 hexanes/CH2Cl2 to 100% CH2Cl2). Finally, the column was
flushed with Et2O, at which point the product elutes (brown band).
Solvent removal afforded a dark brown crystalline solid (637 mg, 0.194
mmol, 87%). IR (NaCl): 2927 (s), 2852 (s), 1955 (w), 1733 (s), 1684
(w), 1610 (w), 1582 (w), 1488 (m), 1447 (m), 1417 (w), 1385 (m),
1296 (w), 1247 (m), 1222 (m), 1204 (m), 1134 (m), 1104 (m), 913
(w), 891 (w), 849 (m), 774 (w), 735 (m), 702 (w). 1H NMR (400 MHz,
CDCl3): δ 17.38 (d, J ) 4.0 Hz, 4H, RudCHAr), 7.52 (s, 4H, aromatic
CH), 7.46 (d, J ) 8.8 Hz, 4H, aromatic CH), 6.98 (d, J ) 8.8 Hz, 4H,
aromatic CH), 5.23 (septet, J ) 6.2 Hz, 4H, (CH3)2CHOAr), 4.03 (t, J
) 7.1 Hz, 8H, CO2CH2), 3.03 (t, J ) 7.7 Hz, 8H, ArCH2CH2CO2),
2.64 (t, J ) 7.7 Hz, 8H, ArCH2CH2CO2), 2.32 (m, 12H, PCH), 2.20-
1.20 (m, 136H, CO2CH2CH2CH2Si(Me)2, SiCH2CH2CH2Si(Me)2, and
P(CH(CH2)5)3), 1.79 (d, J ) 6.2 Hz, 24H, (CH3)2CHOAr), 0.60-0.52
(m, 16H, CH2Si(Me)2CH2), 0.50-0.45 (m, 8H, Si(CH2)4), -0.03 (s,
24H, Si(Me)2). 13C NMR (100 MHz, CDCl3): δ 279.24, 172.60, 151.29,
143.79, 134.69, 129.42, 122.51 (d, JOC ) 5.9 Hz), 113.19, 75.50 (d,
JOC ) 7.8 Hz), 67.27, 36.36, 35.67 (d, JPH ) 24.4 Hz), 30.14, 29.73,
27.80 (d, JPH ) 10.7 Hz), 26.33, 23.26, 22.14, 20.14, 18.58, 17.56,
11.26, -3.33. 31P NMR (162 MHz, CDCl3): δ 59.17 (s, PCy3). LRMS
Calcd for C156H264Cl8O12P4Ru4Si5Na2 (M + 2Na)+: 3331.2. Found:
3331.8. Anal. Calcd for C156H264Cl8O12P4Ru4Si5: C, 57.05; H, 8.10.
Found: C, 56.80; H, 8.00.
18.48, 17.47, 11.12, -4.13. LRMS Calcd for C168H240Cl8N8O12Ru4Si5
(M + 4H)+: 3393.1. Found: 3393.1. Anal. Calcd for C168H236Cl8N8O12-
Ru4Si5: C, 59.56; H, 7.02. Found: C, 59.55; H, 6.96.
Representative Experimental Procedure for RCM Catalyzed by
Monomeric (4,5-DihydroIMES)Cl2RudCH-o-OiPrC6H4 (5). Triene
(7) (50.1 mg, 0.329 mmol, 1.0 equiv) was weighed out in a 25 mL
round-bottom flask and dissolved in 3 mL of CH2Cl2 (0.1 M). (4,5-
DihydroIMES)Cl2RudCH-o-OiPrC6H4 (5) (9.80 mg, 0.0156 mmol,
0.0474 equiv) was added as a solid, and the resulting deep green solution
was stirred at 22 °C. TLC analysis after 10 min indicated completion
of the reaction. As usual, workup procedures were conducted in air
using reagent-grade solvents. The mixture was concentrated at reduced
pressure and passed through a short column of silica gel in 2:1 hexanes/
CH2Cl2, affording diene (8) (33.4 mg, 0.269 mmol, 82%) as a colorless
oil (TLC Rf ) 0.46 in 9:1 hexanes/Et2O). The catalyst was then retrieved
as a green solid by flushing the silica column with 100% CH2Cl2 (9.60
mg, 0.0153 mmol, 98%).
Representative Experimental Procedure for RCM Catalyzed by
Dendritic [(PCy3)Cl2RudCH-o-OiPrC6H3(CH2)2COO(CH2)3Si(Me)2-
(CH2)3Si]4 (32). Tosyl amide (34) (250 mg, 0.995 mmol, 1.0 equiv)
and dendritic catalyst 32 (43.9 mg, 0.0140 mmol, 0.014 equiv) were
weighed into a 50 mL round-bottom flask. The flask was equipped
with a reflux condenser, evacuated, and filled with an atmosphere of
argon. The vessel was charged with CH2Cl2 (20 mL, 0.05 M) and
submerged into an oil bath preheated to 40 °C. The reaction was stirred
for 15 min, at which point TLC analysis indicated completion of the
reaction. Removal of the solvent in vacuo afforded a dark brown oil
that was purified by silica gel chromatography (100% CH2Cl2),
affording 35 as a white solid (219 mg, 0.983 mmol, 99%). The column
was then flushed with 100% Et2O to recover the dendritic catalyst as
a brown solid residue (42.7 mg, 0.0130 mmol, 93%). The recovered
catalyst was transferred directly into a new flask for a subsequent
reaction. As discussed above, Ru recovery on the dendrimer can be
analyzed upon inspection of the 1H NMR (400 MHz) spectrum.
Integration of the benzylic methylene protons at 3.03 ppm (metal-
occupied sites) and 2.88 ppm (metal-vacant sites) provided a ratio of
87:13, respectively.
[(4,5-DihydroIMES)Cl2RudCH-o-OiPrC6H3(CH2)2COO(CH2)3Si-
(Me)2(CH2)3Si]4 (33). The unmetalated dendrimer (227 mg, 0.148
mmol, 1.0 equiv) was weighed into a 25 mL round-bottom flask and
dissolved in 15 mL of CH2Cl2. (4,5-DihydroIMES)(PCy3)Cl2RudCHPh
(3) (606 mg, 0.714 mmol, 4.8 equiv) and CuCl (72.0 mg, 0.731 mmol,
4.9 equiv) were added directly to this solution as solids. The mixture
was stirred for 2 h at 22 °C, during which time the original purple
solution turned a dark green/brown color. The following workup
procedures were conducted in air with reagent-grade solvents. The
mixture was concentrated at reduced pressure and passed through a
short column of silica gel (gradient elution: 100% CH2Cl2 to 4:1
hexanes/Et2O to 1:1 hexanes/Et2O to 100% Et2O). The green band was
collected and concentrated, affording a green microcrystalline solid (277
mg, 0.0816 mmol, 55%). IR (NaCl): 3432 (b), 2915 (m), 1732 (s),
1632 (w), 1607 (w), 1595 (w), 1487 (s), 1418 (m), 1259 (s), 1221 (m),
Representative Experimental Procedure for RCM Catalyzed by
Dendritic [(4,5-DihydroIMES)Cl2RudCH-o-OiPrC6H3(CH2)2COO-
(CH2)3Si(Me)2(CH2)3Si]4 (31). Diene (11) (32.7 mg, 0.233 mmol, 1.0
equiv) was weighed into a 25 mL round-bottom flask and dissolved in
5 mL of CH2Cl2 (0.05 M). Dendritic catalyst 33 (12.4 mg, 0.00366
mmol, 0.016 equiv) was added as a solid, and the solution was allowed
to stir at 22 °C. TLC analysis after 2 h indicated completion of the
reaction. Workup procedures proceeded in air with reagent-grade
solvents. The mixture was concentrated at reduced pressure and passed
through a short plug of silica gel in 100% CH2Cl2, affording 12 (20.4
mg, 0.1819 mmol, 78%) as a colorless oil (TLC Rf ) 0.25 in 4:1
hexanes/Et2O). The catalyst was then flushed off of the column with
100% Et2O, affording 11.2 mg (0.00329 mmol, 90%) of a green solid.
Ru recovery on the dendrimer was assessed using 1H NMR spectroscopy
(400 MHz). Integration of the isopropoxy methine proton for both
metal-occupied (4.90 ppm) and metal-vacant (5.71 ppm) sites gave a
ratio of 92:8 respectively, indicative of 8% metal loss.
1
1132 (m), 1104 (m), 1034 (w), 912 (w), 849 (w), 579 (w). H NMR
(300 MHz, CDCl3): δ 16.51 (s, 4H, RudCHAr), 7.33 (d, J ) 7.0 Hz,
4H, aromatic CH), 7.07 (s, 16H, mesityl aromatic CH), 6.74-6.66 (m,
8H, aromatic CH), 4.85 (septet, J ) 6.3 Hz, 4H, (CH3)2CHOAr), 4.17
(s, 16H, N(CH2)2N), 4.02 (t, J ) 7.0 Hz, 8H, CO2CH2), 2.91 (t, J )
7.8 Hz, 8H, ArCH2CH2CO2), 2.53 (t, J ) 7.8 Hz, 8H, ArCH2CH2-
CO2), 2.47 (s, 48H, mesityl o-CH3), 2.41 (s, 24H, mesityl p-CH3), 1.61
(m, 8H, CO2CH2CH2CH2Si(Me)2), 1.40-1.25 (m, 8H, SiCH2CH2CH2-
Si(Me)2), 1.24 (d, J ) 6.3 Hz, 24H, (CH3)2CHOAr), 0.61-0.45 (m,
24H, CH2Si(Me)2CH2 and Si(CH2)4), -0.02 (s, 24H, Si(Me)2). 13C NMR
(75 MHz, CDCl3): δ 297.07 (d, J ) 166.0 Hz), 211.32, 172.81, 150.80,
145.16, 138.76, 134.28, 130.42, 130.29, 129.80, 129.39, 128.74, 128.22,
74.41, 67.19, 51.44, 36.03, 29.54, 23.14, 21.53, 20.96, 20.60, 20.03,
Acknowledgment. This research was supported by the NSF
(Grant CHE-9905806 and a predoctoral fellowship to J.S.K.).
We thank the NIH (Grant 1S10RR09008) for the Siemens
SMART X-ray crystallography instrumentation grant.
Supporting Information Available: Crystallographic details
for 5 (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
JA001179G