Tuning the Steric Properties of a Metathesis Catalyst for Copolymerization of Norbornene and Cyclooctene toward Complete Alternation

Article abstract of DOI:10.1021/om100185g

We have recently published a mechanistic concept for the olefin metathesis reaction with ruthenium catalysts that explains the independent control of chemo- and stereoselectivity by substitution in two orthogonal planes. The basic structure from which we started the structural modifications for improved stereoselectivity had been optimized substantially for chemoselectivity, as compared to the prototype that had been published. We designed the catalyst according to a concept in which the factor that governs alternation is directly related to the size difference of the substituents on the bidentate phosphine/phenolate ligand 2a-e and in which the origin of chemoselectivity arises from diastereomeric site control. The most selective catalyst (11d) outperforms our prototype (3) in selectivity as well as reactivity. Synthetic problems, i.e., the undesired formation of 2:1-complexes (bidentate ligand-to-ruthenium ratio) (8) by reaction of the bidentate phosphine/phenolate ligands 2a and 2b with the first-generation Hoveyda-Grubbs catalyst 4, were solved by introduction of a larger carbene unit, which not only favors the formation of 1:1-complexes but also results in increased initiation rates. The work is supported by NMR data and X-ray crystallography, which give insights into the steric properties of the investigated system.

Full text of DOI:10.1021/om100185g

Organometallics 2010, 29, 2735–2751 2735
DOI: 10.1021/om100185g
Tuning the Steric Properties of a Metathesis Catalyst
for Copolymerization of Norbornene and Cyclooctene toward
Complete Alternation
€
Sebastian Torker, Andre Muller, Raphael Sigrist, and Peter Chen*
€
Laboratorium fu€r Organische Chemie, Eidgenossische Technische Hochschule (ETH),
Wolfgang-Pauli-Strasse 10, CH-8093 Zu€rich, Switzerland
Received March 9, 2010
We have recently published a mechanistic concept for the olefin metathesis reaction with ruthenium
catalysts that explains the independent control of chemo- and stereoselectivity by substitution in two
orthogonal planes. The basic structure from which we started the structural modifications for improved
stereoselectivity had been optimized substantially for chemoselectivity, as compared to the prototype
that had been published. We designed the catalyst according to a concept in which the factor that governs
alternation is directly related to the size difference of the substituents on the bidentate phosphine/
phenolate ligand 2a-eand in which the origin of chemoselectivity arises from diastereomeric site control.
The most selective catalyst (11d) outperforms our prototype (3) in selectivity as well as reactivity.
Synthetic problems, i.e., the undesired formation of 2:1-complexes (bidentate ligand-to-ruthenium ratio)
(8) by reaction of the bidentate phosphine/phenolate ligands 2a and 2b with the first-generation
Hoveyda-Grubbs catalyst 4, were solved by introduction of a larger carbene unit, which not only
favors the formation of 1:1-complexes but also results in increased initiation rates. The work is supported
by NMR data and X-ray crystallography, which give insights into the steric properties of the investigated
system.
Introduction
content of cis double bonds in the produced polymer. Here
we summarize now our efforts in improving the first type of
selectivity, i.e., the factor that governs alternation that went
into the design and optimization of the lead structure and
demonstrates that it is directly related to the size difference
between the two substituents on the phosphine ligand by a
series containing four catalysts (11a-d). Our simple ligand
exchange approach5 (in analogy to preparation of 3 from 1,
Scheme 1) with the first-generation Hoveyda-Grubbs cata-
lyst 4 was disappointing in the beginning, since the desired
1:1-complexes with the bidentate phosphine/phenolate li-
gand 2a could not be synthesized (Scheme 2). However,
through the solution of the synthetic problem we gained
insight into the steric requirements for catalyst formation,
which finally resulted in the introduction of a larger, faster
initiating carbene unit (10), which we employed as the lead
structure for our subsequent further developments.
In 2005 we reported a mechanistic concept for chemos-
electivity in the olefin metathesis reaction based on our
preceding computational and gas phase studies.1-3 We con-
firmed our mechanistic hypotheses by the design of a solu-
tion phase system (catalyst 3, Schemes 1 and 3), which, in
contrast to the first-generation Grubbs catalyst 1, is able to
produce a largely alternating copolymer from norbornene
and cyclooctene.4,5 A first improved catalyst was already
published in 2007 containing a Hoveyda-type carbene unit,
rather than PCy3 as dissociating ligand, without mentioning
the problems we encountered during synthesis.5 In a more
recent communication we have shown an essentially com-
pletely chemoselective catalyst (11d) on which we extended
our concept to stereoselectivity issues (see Chart 1).6 We were
able to demonstrate that replacement of the chloride anion
by sulfonates with increasing steric bulk can increase the
Experimental Section
*Corresponding author. Tel: (þ41) 44 632 2898. Fax: (þ41) 44 632
1280. E-mail: peter.chen@org.chem.ethz.ch.
(1) Adlhart, C.; Hinderling, C.; Baumann, H.; Chen, P. J. Am. Chem.
Soc. 2000, 122 (34), 8204–8214.
(2) Adlhart, C.; Chen, P. J. Am. Chem. Soc. 2004, 126 (11), 3496–
3510.
(3) Adlhart, C.; Chen, P. Angew. Chem., Int. Ed. 2002, 41 (23), 4484–
4487.
(4) Bornand, M.; Chen, P. Angew. Chem., Int. Ed. 2005, 44 (48), 7909–
7911.
General Remarks. Unless otherwise stated, all manipulations
were carried out under an argon atmosphere on a vacuum line
using standard Schlenk techniques. The solvents were dried by
distillation from the following drying agents prior to use and
were transferred under N2: diethyl ether (Na/K), n-hexane
(Na/K), THF (K), CH2Cl2 (CaH2), methanol (Mg). Flash
chromatography employed Fluka silica gel 60, type 60752
(230-400 mesh). NMR measurements were either done on
300 (1H: 300 MHz, 13C: 75 MHz, 31P: 121 MHz) or 600 MHz
instruments (1H: 600 MHz, 13C: 150 Hz). Chemical shifts
(5) Bornand, M.; Torker, S.; Chen, P. Organometallics 2007, 26 (14),
3585–3596.
(6) Torker, S.; Mueller, A.; Chen, P., manuscript submitted.
r
2010 American Chemical Society
Published on Web 06/03/2010
pubs.acs.org/Organometallics

2736 Organometallics, Vol. 29, No. 12, 2010
Torker et al.
Scheme 1. Synthesis of the Prototype of a Chemoselective
Catalyst (3)
(121 MHz, CDCl3): δ 0.54 (s). To the filtrate was added 13.8 mL
of BH3 in THF (1.0 M, 13.8 mmol, 1.5 equiv), and the solution
quenched by slow addition of MeOH until formation of H2 has
ceased. The crude product was subjected to a column (300 mL
silica) and eluted with hexane/ethyl acetate (9:1) to give 2.38 g
(71%) of 20. The compound can be recrystallized by vapor
diffusion of CH2Cl2 out of a 1:1 CH2Cl2/hexane solution into
hexane, mp=143-144 °C.
1H NMR (300 MHz, CDCl3): δ 8.00 (dd, 1H, JH,H = 7.8 and
12.6 Hz), 7.68 (m, 2H), 7.50 (m, 1H), 7.36 (m, 3H), 7.06 (m, 1H),
6.92 (dd, 1H, JH,H = 3.6 and 8.4 Hz), 3.60 (s, 3H), 2.03 (m, 9H),
1.71 (br s, 6H), 0.95 (v br m, 3H, BH3). 13C NMR (75 MHz,
CDCl3): δ 160.83 (s, 1C), 137.88 (d, 1C, JC,P = 12.8 Hz), 133.13
(s, 1C), 132.58 (d, 2C, JC,P = 7.9 Hz), 129.68 (s, 1C), 129.57 (d,
1C, JC,P = 53.9 Hz), 127.71 (d, 2C, JC,P = 9.2 Hz), 120.89 (d,
1C, JC,P = 11.6 Hz), 115.38 (d, 1C, JC,P = 48.5 Hz), 111.19 (d,
1C, JC,P = 4.3 Hz), 54.75 (s, 1C), 38.25 (s, 3C), 36.58 (s, 3C),
(δ-values) are reported in ppm and calibrated with respect to the
residual solvent signal for 1H and 13C NMR (CD2Cl2: 5.32 and
53.80 ppm; CDCl3: 7.26 and 77.00 ppm). An 85% aqueous
H3PO4 solution is used as an external standard for 31P NMR.
Coupling constants (J) are given in Hz. 13C NMR and 31P NMR
spectra were proton broad-band-decoupled. The multiplicities
of peaks are denoted by the following abbreviations: s: singlet, d:
doublet, dd: doublet of doublets, t: triplet, tm: triplet with an
additional unresolved m, m: multiplet, br: broad. Elemental
analysis was performed by the Microanalytical Laboratory of
34.99 (d, 1C, JC,P = 30.4 Hz), 28.58 (d, 3C, JC,P = 9.2 Hz). 31
P
NMR (121 MHz, CDCl3): δ 33.73 (br peak). Anal. Calcd (%)
for C23H30OBP (364.27 g/mol): C 75.84, H 8.30. Found:
C 75.65, H 8.37.
2-(Adamantylphenylphosphanyl)phenol, 21b. To 325 mg (0.89
mmol) of adamantyl-(o-methoxyphenyl)phenylphosphine BH3
3
(20) in 5 mL of dry CH2Cl2 was added at -78 °C 2.3 equiv of
BBr3 (0.51 g, 0.194 mL, 2.05 mmol). The reaction mixture was
allowed to warm to rt overnight, and the solvent was then
evaporated. To the residue was carefully added 8 mL of
degassed water, and the white suspension was heated to 90 °C
for 4 h. Afterward NaHCO3 was added to neutralize the acid
and the product extracted under argon three times with 20 mL of
ether. After drying the solution with MgSO4 and evaporation of
the solvent a white foam was obtained. The residue was taken up
in 2 mL of MeOH, upon which a white solid (21b) precipitated,
which was washed with 2 mL of MeOH (221 mg, 74%).
€
€
the Laboratorium fur Organische Chemie, ETH-Zurich.
Syntheses of ligands 2a and 2e as well as of complexes 10 and
11a have been reported previously.5
Polymerizations. A 150-200 mg amount of norbornene
(NBE) was polymerized in the presence of 20, 100, or 200 equiv
of cyclooctene (COE) under argon. Prior to polymerization, the
reaction volume was filled up to 20 mL with CH2Cl2 or hexane if
indicated. The catalyst (1:2000 with respect to NBE) was then
added and the reaction temperature kept at either room temp-
erature or 0 °C (investigating the temperature dependence with
catalysts 11a and 11b). The yields were usually around 90%, and
the reaction was stopped by precipitation with 100 mL of MeOH
when it was reasonably viscous. The coagulated polymer was
dried at HV for 2 h and analyzed by NMR in CDCl3. The NMR
measurements were conducted with 30 mg of polymer that was
left overnight in the NMR solvent.
Syntheses of Ligands 2b, 2c, and 2d. Adamantylphenylpho-
sphorus Chloride, 17b. A 5.37 g (25.0 mmol) sample of 1-ada-
mantylbromide (15b) and 9.03 g of Mg turnings (375 mmol, 15
equiv, the turnings were preactivated with I2 and a heat gun)
were gently refluxed in 100 mL of absolute ether without stirring
(!) for 17 h.7 The solution was filtered and titrated with 2-pro-
panol in toluene and phenanthroline as an indicator (45% yield,
11.25 mmol). Then 10.6 mmol of this Grignard solution was
added slowly to 1.90 g (10.6 mmol, 1.0 equiv) of phenylpho-
sphorus dichloride in 20 mL of dry ether at -78 °C, and the
solution was allowed to warm to rt overnight. The precipitate
was filtered and washed with ether, and the filtrate concentrated
in vacuo. The residual oil was taken up in CH2Cl2 (filtered and
concentrated) and once more in ether and filtered again to yield
4.04 g (100% estimated) of crude 17b (mixture of chloride
and bromide). 31P NMR (121 MHz, CDCl3): δ 104.22 (s) and
100.94 (s).
1H NMR (300 MHz, CDCl3): δ 7.61 (m, 3H), 7.34 (m, 5H
incl. OH), 6.98 (m, 2H), 2.00-1.65 (several m, 15H). 13C NMR
(75 MHz, CDCl3): δ 161.42 (d, 1C, JC,P = 21.9 Hz), 135.56
(s, 1C), 134.25 (d, 2C, JC,P = 17.6 Hz), 133.34 (d, 1C, JC,P =
9.2 Hz), 131.87 (s, 1C), 128.71 (s, 1C), 128.36 (d, 2C, JC,P = 7.3
Hz), 120.08 (s, 1C), 117.82 (d, 1C, JC,P = 5.3 Hz), 115.55 (s, 1C),
39.93 (d, 3C, JC,P = 10.4 Hz), 36.86 (s, 3C), 35.12 (d, 1C, JC,P
=
7.3 Hz), 28.76 (d, 3C, JC,P = 9.8 Hz). 31P NMR (121 MHz,
CDCl3): δ -21.95 (s). Anal. Calcd (%) for C22H25OP (336.41
g/mol): C 78.55, H 7.49. Found: C 78.07, H 7.76.
Sodium 2-(Adamantylphenylphosphanyl)phenolate, 2b. A 34.1 mg
amount of NaOH (0.85 mmol, 1.0 equiv) in 1.9 mL of MeOH was
added to 287 mg (0.85 mmol) of 21b in 4 mL of CH2Cl2. Thesolvent
was removed to yield a white foam in quantitative yield, which was
used without further purification.
1H NMR (300 MHz, CDCl3): δ 7.55 (m, 1H), 7.34 (m, 2H),
7.14 (m, 3H), 6.90 (t, 1H, JH,H = 7.4 Hz), 6.45 (t, 1H, JH,H =
7.2 Hz), 6.19 (m, 1H), 1.85-1.50 (several m, 15H). 31P NMR
(121 MHz, CDCl3): δ -6.71 (s).
Neopentylphenylphosphorus Chloride, 17c. A 5.00 g (33.1 mmol)
portion of neopentyl bromide (15c) in 20 mL of dry THF was
added dropwise to 1.22 g (50.0mmol, 1.5equiv) of Mg in 10 mL of
refluxing THF.9 The dry, solid Mg was preactivated with I2 and
application of heat (heat gun). After refluxing for 2 h the solution
was filtered and titrated with 2-propanol in toluene and phenan-
throline as indicator (70% yield, 23.1 mmol). A 21.8 mmol sample
of this Grignard solution was added to 3.90 g (2.96 mL, 21.8 mmol,
1.0 equiv) of phenylphosphorus dichloride in 10 mL of ether at
-78 °C. The mixture was allowed to warm to rt overnight;
afterward the solvent was evaporated, yielding 17c as a mixture
of chloride and bromide, which was used without workup. 31P
NMR (121 MHz, CDCl3): δ 91.16 (s) and 80.55 (s).
Adamantyl-(o-methoxyphenyl)phenylphosphine BH3, 20. This
3
compound was prepared by a different route than in a reported
literature procedure.8 A 5.74 mL portion of 1.6 M BuLi in
hexane (9.19 mmol, 1.0 equiv) was added dropwise to 2-bro-
moanisole in 25 mL of dry ether at 0 °C, and the solution was
stirred for 2.5 h at the same temperature. Then 3.50 g of 17b (9.19
mmol, as a mixture of chloride and bromide) in 20 mL of ether
was added at 0 °C, and the suspension was stirred overnight,
filtered, and washed with ether to yield crude 19b. 31P NMR
(7) Molle, G.; Bauer, P.; Dubois, J. E. J. Org. Chem. 1982, 47 (21),
4120–4128.
(8) Brown, J. M.; Laing, J. C. P. J. Organomet. Chem. 1997, 529
(1-2), 435–444.
(9) Chubb, J. E.; Richey, H. G. Organometallics 2002, 21 (17), 3661–
3666.

Article
Organometallics, Vol. 29, No. 12, 2010 2737
Scheme 2. Syntheses of 1:1- and 2:1-Complexes
(o-Methoxyphenyl)neopentylphenylphosphine, 19c. A solution
and washed with ether, and the filtrate was concentrated in
vacuo. The residue was subjected to Kugelrohr distillation at 180 °C
under reduced pressure (0.1 mbar), giving the pure product 19c in
72% yield (4.50 g).
of 21.8 mmol of 2-methoxyphenyllithium (18) was prepared by
addition of 13.9 mL of 1.6 M BuLi in hexane (22.2 mmol, 1.02
equiv) to 4.08 g (21.8 mmol, 2.72 mL) of o-bromoanisole in
30 mL of dry ether at 0 °C (stirring for 2 h). This solution was
then added to the crude precursor 17c in 20 mL of ether, and the
mixture was stirred overnight. The precipitate was filtered off
1H NMR (300 MHz, CDCl3): δ 7.58 (t, 2H, JH,H = 7.8 Hz),
7.36 (m, 3H), 7.29 (t, 1H, JH,H = 7.8 Hz), 7.09 (t, 1H, JH,H =
6.5 Hz), 6.90 (t, 1H, JH,H = 7.5 Hz), 6.84 (dd, 1H, JH,H = 3.9

2738 Organometallics, Vol. 29, No. 12, 2010
Scheme 3. Syntheses of Bidentate Phosphine/Phenolate Ligands 2a-2e
Torker et al.
Chart 1. Exchange of Chloride by Various Sulfonates Shows an
Impact on the Stereoselectivity of Double Bonds
Chart 2. Several Isomers of trans (8a5, 8a6, 14) and cis (8a1, 8a2,
8a3, 8a4, 14a) 2:1-Complexesa
and 8.1 Hz), 3.81 (s, 3H), 2.17 (d, 2H, JH,P = 4.2 Hz), 1.06 (s, 9H).
13C NMR (75 MHz, CDCl3): δ 160.53 (d, 1C, JC,P = 13.4 Hz),
138.85 (d, 1C, JC,P = 11.6 Hz), 133.39 (d, 2C, JC,P = 20.7 Hz),
132.39 (d, 1C, JC,P = 5.5 Hz), 129.50 (s, 1C), 128.22 (s, 1C),
128.2-128.0 (hidden signal, 1C), 128.05 (d, 2C, JC,P = 7.3 Hz),
120.56 (d, 1C, JC,P = 2.3 Hz), 110.14 (s, 1C), 55.37 (s, 1C), 41.78
(d, 1C, JC,P = 15.2 Hz), 31.55 (d, 1C, JC,P = 15.8 Hz), 31.03
(d, 3C, JC,P = 9.1 Hz). 31P NMR (121 MHz, CDCl3): δ -33.01
(s). Anal. Calcd (%) for C18H23OP (286.35 g/mol): C 75.50, H
8.10. Found: C 75.29, H 8.22.
a In the trans complexes the carbene unit is not chelating.
J
H,H = 3.3 and 14.1 Hz), 1.04 (s, 9H). 13C NMR (75 MHz, CDCl3):
δ 159.11 (d, 1C, JC,P = 20.0 Hz), 137.97 (d, 1C, JC,P = 4.5 Hz),
133.25 (s, 1C), 131.79 (d, 2C, JC,P =17.6 Hz), 131.19 (s, 1C), 128.38
(d,2C, JC,P =6.6 Hz), 128.11 (s, 1C), 122.91 (d, 1C, JC,P =4.5 Hz),
120.87 (s, 1C), 115.14 (s, 1C), 43.17 (d, 1C, JC,P = 10.4 Hz), 31.50
(d, 1C, JC,P = 13.4 Hz), 31.04 (d, 3C, JC,P = 9.1 Hz). Anal. Calcd
(%) forC17H21OP (272.33 g/mol): C 74.98, H 7.77. Found: C 74.37,
H 8.03 (purity after distillation).
Sodium 2-(Neopentylphenylphosphanyl)phenolate, 2c. A 118 mg
(2.94 mmol) amount of NaOH in 7.5 mL of MeOH was added to
0.80 g (2.94 mmol) of phenol 21c. The solvent was evaporated,
yielding a white foam in quantitative yield, which was used
without further purification.
2-(Neopentylphenylphosphanyl)phenol, 21c. A 2.3 equiv sam-
ple of BBr3 (2.01 g, 0.76 mL, 8.03 mmol) was added to 1.00 g
(3.49 mmol) of 19c in 10 mL of dry CH2Cl2 at -78 °C. The react-
ion mixture was allowed to warm to rt overnight. The CH2Cl2
was then evaporated under reduced pressure followed by slow
addition of 10 mL of degassed H2O. The mixture was refluxed
for 45 min, followed by neutralization with NaHCO3. The
phenol was extracted with 20 and 15 mL of ether under argon
and the solvent evaporated. The residue was subjected to Kugel-
rohr distillation under reduced pressure (180 °C, 0.05 mbar),
which gave 0.82 g of product (86%).
2-Bromo-2,3,3-trimethylbutane, 15d. This was produced ac-
cording to a reported procedure.10 A 12 mL (8.4 g, 86 mmol)
amount of 2,3,3-trimethylbutene (16) in 20 mL of CH2Cl2 was
stirred vigorously with 25 mL of 48% HBr for 2 h. Phase
1H NMR (300 MHz, CDCl3): δ 7.44 (tm, 2H, JH,H = 7.8 Hz),
7.32 (m, 5H), 6.94 (t, 2H, JH,H = 7.4 Hz), 6.81 (br d, OH, JH,P =
8.7 Hz), 2.26 (dd, 1H, JH,H = 6.9 and 14.4 Hz), 2.19 (dd, 1H,
(10) Roberts, J. D.; Yancey, J. A. J. Am. Chem. Soc. 1955, 77 (21),
5558–5562.

Article
Organometallics, Vol. 29, No. 12, 2010 2739
Chart 3. Computed Structures and Relative Energies in kcal/mol (BP86/TZP) of Open (trans-Phosphines) (8a5, 8a6, 14) and Closed (cis,
Chelating) 2:1-Complexes (8a1, 8a2, 8a3, 8a4, 14a)
separation, extraction with CH2Cl2, and drying over MgSO4
yielded 10.1 g (66%) of 15d, which was used without further
purification.
1H NMR (300 MHz, CDCl3): δ 7.77 (m, 4H, incl. -OH), 7.32
(m, 3H), 7.26 (m, 1H), 6.91 (m, 2H), 1.21 (d, 3H, JH,P =10.5 Hz),
1.14 (d, 3H, JH,P = 10.2 Hz), 0.99 (s, 9H). 13C NMR (75 MHz,
CDCl3): δ 160.19 (d, 1C, JC,P=22.5 Hz), 135.79 (d, 2C, JC,P=21.3
Hz), 135.46 (d, 1C, JC,P = 7.3 Hz), 134.92 (d, 1C, JC,P =1.7 Hz),
1H NMR (300 MHz, CDCl3): δ 1.81 (s, 6H), 1.12 (s, 9H). 13
C
NMR (75 MHz, CDCl3): δ 79.78 (1C), 40.03 (1C), 30.72 (3C),
26.91 (2C).
133.32 (s, 1C), 129.07 (d, 1C, JC,P =1.2 Hz), 128.12 (d, 2C, JC,P
8.9 Hz), 121.35 (d, 1C, JC,P=5.0 Hz), 119.89 (s, 1C), 115.29 (d, 1C,
C,P =1.4 Hz), 42.46 (d, 1C, JC,P =14.0 Hz), 37.53 (d, 1C, JC,P
=
Phenyl-(1,1,2,2,tetramethylpropyl)phosphorus Chloride, 17d.
A 3.9 g (21.8 mmol) portion of 15d in 100 mL of dry ether was
added with a syringe pump to 10.6 g of Mg powder
(436 mmol) in 20 mL of ether at reflux over 45 min. Reflux
was continued for 15 min, and then the cooled Grignard
solution (0 °C) was slowly filtered via a cannula into a solution
of 1.56 g (8.72 mmol) of phenylphosphorus dichloride (PhPCl2)
in 10 mL of ether at -78 °C. The reaction was allowed to warm
to rt overnight and the ether evaporated to give crude 17d as a
mixture of chloride and bromide, which was used without purifica-
tion. 31P NMR (121 MHz, CDCl3): δ 106.05 (s) and 104.40 (s).
(o-Methoxyphenyl)phenyl-(1,1,2,2-tetramethylpropyl)phosphine,
19d. An aryllithium solution of 18 prepared from 3.26 g (17.4 mmol)
of o-bromoanisole and 10.9 mL of 1.6 M BuLi (hexane solution,
17.4 mmol) in THF at 0 °C (2 h stirring) was then added to 17d, and
the reaction was refluxed for 2 h. Quenching with 2 mL of MeOH
and filtration yielded crude 19d, which was purified by Kugelrohr
distillation (190-210 °C, 8 ꢀ 10-2 mbar), 2.05 g (75% based on
PhPCl2).
J
=
16.8 Hz), 27.04 (d, 3C, JC,P=10.4 Hz), 22.71 (d, 1C, JC,P=6.6 Hz),
22.05 (d, 1C, JC,P = 4.9 Hz). 31P NMR (121 MHz, CDCl3): δ
-35.44 (s). Anal. Calcd (%) for C19H25OP (300.38 g/mol): C 75.97,
H 8.39. Found: C 74.98, H 8.14 (purity after distillation).
Sodium 2-[Phenyl-(1,1,2,2-tetramethylpropyl)phosphanyl]phe-
nolate, 2d. The sodium salt was produced by reacting 21d with
1.0 equiv of NaOH in MeOH and used without further purifica-
tion after evaporation of the solvent.
Syntheses of Complexes. Crystallization of Grubbs First-
Generation Catalyst 1. Crystals suitable for X-ray analysis were
grown by slowly concentrating a hexane/ether solution of the
complex via solvent diffusion through a rubber septum.
Attempted Synthesis of 7a (Path B) and Isolation of 8a1. a. Slow
Addition of a Substoichiometric Amount of Ligand 2a. To 16.4 mg
(28 μmol) of complex 5 in 2 mL of CH2Cl2 was added dropwise
2.8 mg (10 μmol, 0.35 equiv) of ligand 2a in 2 mL of CH2Cl2. After
evaporation of the solvent the mixture was analyzed by NMR.
1H NMR (300 MHz, CD2Cl2, only carbene region): δ 16.69 (d,
1H NMR (300 MHz, CDCl3): δ 7.75 (m, 2H), 7.61 (m, 1H), 7.27
(m, 4H), 6.91(m, 1H), 6.85(m, 1H), 3.80(s, 3H), 1.26(d, 3H,JH,P=
7.5 Hz), 1.05 (s, 9H), 1.03 (d, 3H, JH,P = 9.9 Hz). 13C NMR
(75 MHz, CDCl3):δ=161.95 (d, 1C, JC,P=15.8 Hz), 139.22 (d, 1C,
JH,P=6.6 Hz, 5), 14.96 (t or dd, 1H, JH,P=6.2 Hz, 8a1), 14.53 (dd,
1H, JH,P = 6.0 and 7.8 Hz, 8a2); relative intensities of carbene
protons: 5:8a1:8a2 = 87:8:5. 31P NMR (121 MHz, CD2Cl2): δ
80.66 (d, JP,P =30 Hz, 8a2), 77.82 (d, JP,P =30 Hz, 8a1), 67.61 (d,
J
C,P=20.6 Hz), 136.72 (d, 1C, JC,P=7.4 Hz), 135.37 (d, 2C, JC,P
=
=
=
23.0 Hz), 129.99 (s, 1C), 127.88 (s, 1C), 127.53 (d, 2C, JC,P
7.9 Hz), 126.37 (d, 1C, JC,P = 21.2 Hz), 120.21 (d, 1C, JC,P
J
P,P =30 Hz, 8a1), 60.77 (s, 5), 57.42 (d, JP,P =30 Hz, 8a2), -5.34
(s, PPh3).
b. After Addition of More Than 2 equiv of Ligand 2a. To the
2.4 Hz), 110.55 (d, 1C, JC,P = 1.8 Hz), 55.41 (s, 1C), 42.63 (d, 1C,
C,P=24.9 Hz), 37.66 (d, 1C, JC,P =20.0 Hz), 27.31 (d, 3C, JC,P
J
=
same sample another 16.8 mg (2.1 equiv) of ligand 2a was added
and the conversion monitored by NMR. 1H NMR (300 MHz,
10.9 Hz), 22.61 (d, 1C, JC,P=3.6 Hz), 22.24 (d, 1C, JC,P=7.9 Hz).
31P NMR (121 MHz, CDCl3): δ -7.00 (s). Anal. Calcd (%) for
C20H27OP (314.41 g/mol): C 76.40, H 8.66. Found: C 75.34, H 8.56
(purity after distillation).
CD2Cl2, only carbene region): δ 14.97 (t or dd, 1H, JH,P
=
6.2 Hz, 8a1), 14.55 (dd, 1H, JH,P=6.0 and 7.8 Hz, 8a2); relative
intensity of carbene protons: 8a1:8a2 = 65:35. The sample was
subjected to column chromatography (10 g of silica gel) under
argon using hexane/Et2O as the eluent. Two yellow fractions
were collected with 50% and 70-100% ether, respectively. The
first one was pure and assigned to 8a1.
2-[Phenyl-(1,1,2,2-tetramethylpropyl)phosphanyl]phenol, 21d.
To a solution of 2.0 g (6.36 mmol) of 19d in 15 mL of CH2Cl2 was
added at -78 °C 3.66 g of BBr3 (14.6 mmol). The reaction was
allowed to warm to rt overnight, the solvent evaporated, and the
residue first quenched carefully with 10 mL of H2O and then
refluxed for 1 h. Solid NaHCO3 was added in small portions to
neutralize the reaction, and the product was extracted with
CH2Cl2 under argon via a cannula. After evaporation of the sol-
vent 1.5 mL of CH2Cl2 was added, and a precipitate was formed
upon addition of 20 mL of ether. The filtrate was concentrated
and purified by Kugelrohr distillation (175-190 °C, 10-2 mbar),
1.45 g (76%).
1H NMR (300 MHz, CD2Cl2): δ 14.95 (t or dd, 1H, JH,P
=
6.2 Hz), 7.98 (m, 2H), 7.56 (m, 3H), 7.40 (m, 2H), 7.30 (m, 2H),
7.03 (m, 5H), 6.84 (m, 2H), 6.52 (m, 3H), 6.36 (m, 2H), 5.85 (m,
1H), 5.07 (m, 1H), 1.47 (d, 3H, JH,H = 6.6 Hz), 1.16 (d, 9H,
J
H,P=14.7 Hz), 0.94 (d, 3H, JH,H=6.9 Hz), 0.69 (d, 9H, JH,P
=
=
14.7 Hz). 31P NMR (121 MHz, CD2Cl2): δ 77.80 (d, 1P, JP,P
30 Hz), 67.62 (d, 1P, JP,P =30 Hz).

2740 Organometallics, Vol. 29, No. 12, 2010
Torker et al.
Attempted Synthesis of 7a (Path C) and in Situ Observation
of 6, 9, and 7a. a. NMR Observation of Intermediate 6. A 12 mg
(20 μmol) amount of first-generation Hoveyda-Grubbs catalyst
4 and 45 mg (160 μmol, 8.0 equiv) of PCy3 weremixed inanNMR
tube in CD2Cl2. 1H NMR (300 MHz, CD2Cl2, only selected
peaks): δ 20.55 (s, carbene proton of 6), 17.36 (d, JH,P =4.5 Hz,
carbene proton of 4), 5.27 (m, (C)-H of isopropyl of 4), 4.67 (m,
(C)-H of isopropyl of 6); relative intensities of carbene protons:
6:4=70:30. 31P NMR (121 MHz, CD2Cl2): δ 59.64 (s, complex
4), 36.05 (s, intermediate 6), 11.16 (s, PCy3).
(d, 1C, CAr(PO),2(ORu), JC,P = 12.5 Hz), 157.07 (s, 1C, CAr(O)
(OR)), 146.45 (s, 1C, CAr(O)(CHdRu)), ∼134.7 (v br s, 2C,
CAr(P),1þ2(ortho or meta)), 133.11 (s, 1C, CAr(PO),1(H)), ∼132.8
-
(v br s, 1C, CAr(P),1þ2(ortho or meta)), 132.50 (s, 1C, CAr(PO),1
-
(H)), 132.42 (d, 1C, CAr(P),1(ipso), JC,P=40.7 Hz), 132.21 (s, 1C,
CAr(PO),2(H)), 131.72 (s, 1C, CAr(PO),2(H)), ∼131.0 (v br s, 1C,
CAr(P),1þ2(ortho or meta)), 129.95 (s, 1C, CAr(O)(H)), 129.24 (d,
1C, CAr(P),1(para), JC,P =1.9 Hz), 129.00 (d, 1C, CAr(P),2(para),
JC,P = 2.0 Hz), 128.26 (d, 1C, CAr(P),2(ipso), JC,P = 42.7 Hz),
∼128.0 (v br s, 4C, CAr(P),1þ2(ortho or meta)), 127.42 (s, 1C,
b. NMR Observation of Intermediate 9. To the same NMR
sample 4.4 mg of ligand 2a (16 μmol, 0.8 equiv) was added. 1H
NMR (300 MHz, CD2Cl2, only selected peaks): δ 20.54 (s,
carbene proton of 6), 19.94 (s, carben proton of intermediate
9), 17.35 (d, JH,P = 5.1 Hz, carbene proton of 4), very weak
signals for 8a1 and 8a2; relative intensities of carbene protons:
CAr(O)(H)), 123.30 (s, 1C, CAr(O)(H)), 121.48 (d, 1C, CAr(PO),1
-
(H), JC,P = 7.3 Hz), 121.29 (dd, 1C, CAr(PO),2(PRu), JC,P = 2.5
and 52.2 Hz), 118.10 (s, 1C, CAr(O)(H)), 115.95 (dd, 1C,
CAr(PO),2(H), JC,P = 2.6 and 9.4 Hz), 114.29 (d, 1C, CAr(PO),1
-
(PRu), JC,P = 46.5 Hz), 113.98 (d, 1C, CAr(PO),2(H), JC,P = 7.0
Hz), 112.31 (d, 1C, CAr(PO),1(H), JC,P = 5.9 Hz), 79.51 (s, 1C,
Cisopropoxy), 41.55 (d, 1C, Cadamantyl, JC,P = 23.1 Hz), 40.46 (d,
1C, Cadamantyl, JC,P=23.5 Hz), 37.66 (s, 3C, Cadamantyl), 36.75 (s,
3C, Cadamantyl), 36.71 (s, 3C, Cadamantyl), 36.44 (s, 3C, Cadamantyl),
28.86 (d, 3C, Cadamantyl, JC,P =9.4 Hz), 28.67 (d, 3C, Cadamantyl,
JC,P = 9.1 Hz), 22.72 (s, 1C, Cisopropoxy), 22.04 (s, 1C,
6:9:4=54:11:35. 31P NMR (121 MHz, CD2Cl2): δ 63.36 (d, JP,P
=
200 Hz, P-O ligand in 9), 59.64 (s, 4), 40.61 (d, JP,P = 200 Hz,
PCy3 in 9), 36.05 (s, 6), 11.15 (s, PCy3).
c. NMR Observation of 7a. The same NMR solution was
stirred with 39 mg (0.39 mmol, 20 equiv) of CuCl for 30 min and
then filtered. 1H NMR (300 MHz, CD2Cl2, only selected peaks):
Cisopropoxy). 31P NMR (121 MHz, CD2Cl2): δ 76.71 (d, JP,P
30 Hz), 55.08 (d, JP,P =30 Hz).
=
δ 17.36 (d, JH,P=4.2 Hz, carbene proton of 4), 15.16 (d, JH,P
=
8.7 Hz, carbene proton of 7a), 14.95 (m, carbene proton of 8a1),
14.52 (m, carbene proton of 8a2); relative intensities of carbene
protons: 4:7a:8a1:8a2 = 77:12:4:7. 31P NMR (121 MHz,
CD2Cl2): δ 83.31 (s, 7a), 59.62 (s, 4), 28.62 (br s, PCy3CuCl-
adduct), doublets of 8a1 and 8a2 very weak.
2:1-Complexes 8b1 and 8b2. A 100 mg amount of first-
generation Hoveyda-Grubbs catalyst 4 (166 μmol) and 131
mg (366 μmol, 2.2 equiv) of ligand 2b were stirred in 5 mL of
CH2Cl2 for 30 min and filtered afterward to remove NaCl. The
proton NMR shows two new carbene products in a ratio of
61:39 (8b1 and 8b2). 1H NMR (300 MHz, CD2Cl2, only carbene
region): δ 14.92 (t, JH,P=6.6 Hz, 8b1), 14.40 (dd, JH,P=5.8 and
8.5 Hz, 8b2). 31P NMR (CD2Cl2): δ 76.70 (d, JP,P =30 Hz, 8b2),
73.08 (d, JP,P=30 Hz, 8b1), 61.75 (d, JP,P=30 Hz, 8b1), 55.08 (d,
1:1-Complex 7d. A 250 mg sample of first-generation Hovey-
da-Grubbs catalyst 4 (0.42 mmol) and 148 mg of ligand 2d (0.46
mmol, 1.1 equiv) were stirred in 4.5 mL of CH2Cl2 for 1.5 h. An
excess of CuCl (91 mg, 0.92 mmol, 2.2 equiv) was added and the
reaction stirred for a further 20 min. A 22 mL amount of hexane
was added, and after 30 min the precipitate was filtered off.
After evaporation of the solvent the residue was adsorbed on
Celite and transferred onto a column (silica, hexane/ether,
85:15) under argon. The complex was eluted by increasing the
polarity to 40% ether. The yield was 125 mg (52%).
1H NMR (300 MHz, CD2Cl2): δ 15.65 (d, 1H, JH,P=7.5 Hz),
8.17 (br s, 2H), 7.92 (td, 1H, JH,H = 1.5 and 7.7 Hz), 7.54
(m, 4H), 7.18 (d, 1H, JH,H =8.7 Hz), 7.13 (m, 2H), 6.96 (t, 1H,
J
J
H,H = 7.2 Hz), 6.75 (tm, 1H, JH,H = 7.4 Hz), 6.63 (dd, 1H,
H,H=7.5 and 4.5 Hz), 5.34 (m, 1H), 1.87 (d, 3H, JH,H=5.7 Hz),
JP,P=30 Hz, 8b2). Leaving the two isomers in solution for 2 days
did not result in a change of the isomer ratio. Crystals suitable for
X-ray analysis were grown by diffusion of CH2Cl2 into ether out
of a 1:1 solution of CH2Cl2 and ether of the isomer mixture. Both
isomers crystallized, predominantly 8b1, of which the structure
could be resolved. Stirring the sample with excess of CuCl in
CH2Cl2 and filtration of the excess CuCl still shows both isomers.
However, after leaving the solution for 2 days only 8b2 was
detected, which was purified by column chromatography under
argon (substance adsorbed on Celite, column: silica, eluent: ether/
hexane, 1:4). Crystals suitable for X-ray analysis were grown by
diffusion of CH2Cl2 into ether out of a 1:1 solution of CH2Cl2 and
ether of the complex, giving the structure of 8b2.
1.79 (d, 3H, JH,H=6.6 Hz), 1.37 (d, 3H, JH,P=17.1 Hz), 1.28 (d,
3H, JH,P = 11.7 Hz), 1.12 (s, 9H). 13C NMR (150 MHz,
CD2Cl2): δ 284.58 (dd, 1C, C(dRu), JC,P = 37.5 and 12.0 Hz),
178.91 (d, 1C, CAr(PO)(ORu), JC,P = 14.4 Hz), 154.61 (d, 1C,
C
Ar(O)(OR), JC,P = 1.4 Hz), 143.93 (s, 1C, CAr(O)(CHdRu)),
∼139.0 and 133.0 (2 v br s almost in the baseline, 2C, CAr(P)
(ortho)), 133.18 (d, 1C, CAr(PO)(H), JC,P=1.6 Hz), 132.52 (d, 1C,
Ar(PO)(H), JC,P = 2.1 Hz), 130.66 (d, 1C, CAr(P)(para), JC,P
2.6 Hz), 129.50 (s, 1C, CAr(O)(H)), 128.15 (d, 2C, CAr(P)(meta),
C,P = 10.2 Hz), 127.98 (d, 1C, CAr(P)(ipso), JC,P = 47.7 Hz),
-
C
=
J
123.31 (s, 1C, CAr(O)(H)), 122.19 (s, 1C, CAr(O)(H)), 120.83 (d,
1C, CAr(PO)(PRu), JC,P = 41.6 Hz), 119.30 (d, 1C, CAr(PO)(H),
JC,P = 10.1 Hz), 117.62 (d, 1C, CAr(PO)(H), JC,P = 6.3 Hz),
113.81 (s, 1C, CAr(O)(H)), 75.29 (s, 1C, Cisopropoxy), 47.93 (d, 1C,
1H NMR (600 MHz, CD2Cl2): δ 14.42 (dd, 1H, CH(dRu),
JH,P = 5.9 and 8.4 Hz), 7.95 (v br s almost in the baseline, 2H,
Ar(P)1Hortho), 7.63 (t, 1H, Ar(PO)1H, JH,H = 7.1 Hz), 7.57 (t,
1H, Ar(O)H, JH,H =7.8 Hz), 7.41 (dd, 1H, Ar(O)H, JH,H =1.4
and 7.6 Hz), 7.37 (t, 1H, Ar(P)1Hpara, JH,H=6.0 Hz), 7.36 (t, 1H,
C
TMP, JC,P=24.1 Hz), 37.22 (d, 1C, CTMP, JC,P=5.3 Hz), 29.21
(d, 3C, CTMP, JC,P = 5.7 Hz), 22.33 (s, 1C, Cisopropoxy), 22.10
(s, 1C, Cisopropoxy), 21.57 (d, 1C, CTMP, JC,P=5.9 Hz), 20.50 (d,
1C, CTMP, JC,P = 4.5 Hz). 31P NMR (121 MHz, CD2Cl2): δ
81.62 (s).
Ar(P)2Hpara, JH,H = 6.0 Hz), 7.32 (tm, 1H, Ar(PO)1H, JH,H
=
7.2 Hz), 7.30 (d, 1H, Ar(O)H, JH,H = 7.8 Hz), 7.17 (ddd, 1H,
Ar(PO)1H, JH,H=1.1, 5.0, and 8.4 Hz), 7.09 (peak max., several
very broad peaks from 7.6 to 6.1 ppm, 6H, Ar(P)1þ2Horthoþmeta),
7.04 (t, 1H, Ar(O)H, JH,H = 7.4 Hz), 6.98 (tm, 1H, Ar(PO)2H,
JH,H = 7.8 Hz), 6.59 (m, 2H, Ar(PO)2H and Ar(PO)1H), 6.55
(dd, 1H, Ar(PO)2H, JH,H = 4.2 and 8.3 Hz), 6.29 (tm, 1H,
Ar(PO)2H, JH,H =7.2 Hz), 5.84 (m, 1H, CH(isopropoxy)), 1.91
(d, 3H, CH3(isopropoxy), JH,H = 6.7 Hz), 1.81 (br d, 3H,
H(adamantyl), JH,P =11.0 Hz), 1.74 (d, 3H, CH3(isopropoxy),
JH,H =6.7 Hz), 1.62 (br s, 3H, H(adamantyl)), 1.42 (br m, 12H,
H(adamantyl)), 1.30 (br t, 6H, H(adamantyl), JH,H =10.6 Hz),
1.06 (br d, 3H, H(adamantyl), JH,H =11.0 Hz), 0.65 (br m, 3H,
H(adamantyl)). 13C NMR (150 MHz, CD2Cl2): δ 282.93 (m, 1C,
C(dRu)), 177.89 (d, 1C, CAr(PO),1(ORu), JC,P=18.2 Hz), 177.17
1:1-Complex 7e. To 49 mg (81 μmol) of first-generation
Hoveyda-Grubbs catalyst 4 was added 24 mg (91 μmol, 1.1
equiv) of ligand 2e dropwise, and the solution was stirred for
15 min. An excess of CuCl (>2 equiv) was added and the
suspension stirred for additional 25 min. After filtration and
evaporation of the solvent the sample was transferred in hexane
onto a column (10 g of silica) prepared under argon and eluted
with gradually increasing polarity by adding up to 40% ether. A
brown-orange band was collected in 56% yield (24 mg).
1H NMR (300 MHz, CD2Cl2): δ 16.73 (d, 1H, JH,P=5.4 Hz),
7.65 (t, 1H, JH,H =8.7 Hz), 7.58 (d, 1H, JH,H =8.1 Hz), 7.41 (t,
1H, JH,H = 8.1 Hz), 7.18 (d, 1H, JH,H =8.4 Hz), 7.09 (m, 2H),
6.66 (m, 2H), 5.33 (m, 1H), 1.80 (d, 3H, JH,H = 6.0 Hz), 1.71

Article
Organometallics, Vol. 29, No. 12, 2010 2741
(d, 3H, JH,H=6.0 Hz), 1.70 (d, 9H, JH,P=14.1 Hz), 1.56 (d, 9H,
JH,P=14.1 Hz). 13C NMR (75 MHz, CD2Cl2): δ 271.94 (m, 1C,
C(dRu)), 178.13 (d, 1C, CAr(PO)(ORu), JC,P =12.2 Hz), 152.85
(s, 1C, CAr(O)(OR)), 144.23 (s, 1C, CAr(O)(CHdRu)), 132.43 (s,
1C, CAr(PO)(H)), 132.06 (s, 1C, CAr(PO)(H)), 128.83 (s, 1C,
CAr(O)(H)), 123.39 (s, 1C, CAr(O)(H)), 122.24 (s, 1C, CAr(O)(H)),
119.68 (d, 1C, CAr(PO)(PRu), JC,P = 38.7 Hz), 118.37 (d, 1C,
CAr(PO)(H), JC,P=9.5 Hz), 117.22 (d, 1C, CAr(PO)(H), JC,P=6.4
Hz), 113.55 (s, 1C, CAr(O)(H)), 75.73 (s, 1C, Cisopropoxy), 40.36 (d,
J
C,P =49.1 Hz), 114.07 (s, 1C, CAr(O)(H)), 93.47(s, 1C, C(O)CHCy2),
42.66 (d, 1C, CNp, JC,P = 28.9 Hz), 39.98 (s, 1C, CCy), 39.44 (s, 1C,
CCy), 32.60 (d, 3C, CNp, JC,P = 7.1 Hz), 31.81 (d, 1C, CNp, JC,P
1.4 Hz), 30.06 (s, 1C, CCy), 29.87 (s, 1C, CCy), 29.69 (s, 1C, CCy),
29.09 (s, 1C, CCy), 26.95 (s, 1C, CCy), 26.80 (s, 1C, CCy), 26.58 (s,
1C, CCy), 26.53 (s, 1C, CCy), 26.46 (s, 1C, CCy), 26.24 (s, 1C, CCy).
31P NMR (121 MHz, CD2Cl2): δ 62.42 (s).
1:1-Complex 11d. A 150 mg (0.20 mmol) sample of modi-
fied first-generation Hoveyda-Grubbs catalyst 10 and 72 mg
(0.22 mmol, 1.1 equiv) of ligand 2d were stirred in 5 mL of
CH2Cl2 for 30 min. After treatment with a few equivalents of
CuCl for 30 min, the solid was filtered off, and the residue was
transferred with hexane onto a column (10 g of silica) prepared
under argon. The complex was eluted with 5% ether/hexane in
35% yield (52 mg). Crystals suitable for X-ray analysis were
grown by diffusion of CH2Cl2 into hexane out of a 1:1 solution
of CH2Cl2 and hexane of the complex.
=
1C, Ctert-butyl, JC,P = 21.4 Hz), 34.63 (d, 1C, Ctert-butyl, JC,P
21.4 Hz), 31.53 (s, 3C, Ctert-butyl), 28.60 (d, 3C, Ctert-butyl, JC,P
=
=
P
2.8 Hz), 22.12 (s, 1C, Cisopropoxy), 21.57 (s, 1C, Cisopropoxy). 31
NMR (121 MHz, CD2Cl2): δ 92.62 (s).
1:1-Complex 11b. A 56 mg (156 μmol, 1.2 equiv) amount of
ligand 2b in 2 mL of CH2Cl2 was added dropwise to a solution of
105 mg (128 μmol) of modified first-generation Hoveyda-
Grubbs catalyst 10, and the mixture was stirred for 30 min.
After stirring with CuCl for another 25 min the solid was filtered
off. 31P NMR showed full conversion of 10 to 11b. After evapor-
ation of the solvent the substance was transferred with hexane onto a
column (12 g silica) under argon. An orange-brown spot was iso-
lated with 5-7% ether/hexane as the eluent, in 40% yield (39 mg).
1H NMR (300 MHz, CD2Cl2): δ 15.23 (d, 1H, JH,P = 7.8 Hz),
8.06 (t, 2H, JH,H = 9.0 Hz), 7.75 (t, 1H, JH,H = 7.1 Hz), 7.53 (m,
4H), 7.26 (d, 1H, JH,H = 8.7 Hz), 7.16 (t, 1H, JH,H = 7.7 Hz),
7.07 (d, 1H, JH,H = 7.2 Hz), 6.90 (t, 1H, JH,H = 7.4 Hz), 6.74 (m,
1H NMR (300 MHz, CD2Cl2): δ 15.51 (d, 1H, JH,P=7.2 Hz),
8.13 (br s, 2H), 7.93 (td, 1H, JH,H = 1.5 and 7.5 Hz), 7.51 (m,
4H), 7.24 (d, 1H, JH,H =9.0 Hz), 7.14 (tm, 1H, JH,H =7.8 Hz),
6.98 (dd, 1H, JH,H=1.8 and 7.5 Hz), 6.88 (td, 1H, JH,H=0.9 and
7.2 Hz), 6.74 (m, 2H), 4.84 (m, 1H), 2.40-1.20 (several m, 22H),
1.35 (d, 3H, JH,P =16.8 Hz), 1.26 (d, 3H, JH,P =12.3 Hz), 1.12
(s, 9H). 13C NMR (150 MHz, CD2Cl2): δ 284.62 (m, 1C,
C(dRu)), 179.39 (d, 1C, CAr(PO)(ORu), JC,P = 14.0 Hz),
158.51 (s, 1C, CAr(O)(OR)), 143.66 (s, 1C, CAr(O)(CHdRu)),
2H), 4.94 (m, 1H), 2.43 (m, 2H), 2.20-1.16 (several m, 35H). 13
C
∼137.5 and 132.5 (2 v br s almost in the baseline, 2C, CAr(P)
(ortho)), 133.21 (d, 1C, CAr(PO)(H), JC,P=1.3 Hz), 132.50 (d, 1C,
Ar(PO)(H), JC,P = 1.9 Hz), 130.57 (d, 1C, CAr(P)(para), JC,P
2.5 Hz), 129.46 (s, 1C, CAr(O)(H)), 128.65 (d, 1C, CAr(P)(ipso),
C,P = 48.2 Hz), 128.06 (d, 2C, CAr(P)(meta), JC,P = 10.1 Hz),
-
NMR (75 MHz, CD2Cl2): δ 276.98 (m, 1C, C(dRu)), 179.61 (d,
1C, CAr(PO)(ORu), JC,P = 14.1 Hz), 157.85 (s, 1C, CAr(O)(OR)),
143.34 (s, 1C, CAr(O)(CHdRu)), 134.62 (d, 2C, CAr(P)(ortho),
JC,P = 8.3 Hz), 133.72 (s, 1C, CAr(PO)(H)), 132.27 (s, 1C,
C
=
J
122.73 (s, 1C, CAr(O)(H)), 122.35 (s, 1C, CAr(O)(H)), 120.24 (d,
1C, CAr(PO)(PRu), JC,P = 42.0 Hz), 119.46 (d, 1C, CAr(PO)(H),
JC,P = 10.2 Hz), 117.36 (d, 1C, CAr(PO)(H), JC,P = 6.4 Hz),
114.53 (s, 1C, CAr(O)(H)), 94.06 (s, 1C, C(O)CHCy2), 48.31 (d, 1C,
C
Ar(PO)(H)), 130.64 (d, 1C, CAr(P)(para), JC,P = 2.4 Hz), 128.56
(s, 1C, CAr(O)(H)), 128.26 (d, 2C, CAr(P)(meta), JC,P = 10.2 Hz),
126.02 (d, 1C, CAr(P)(ipso), JC,P = 46.1 Hz), 122.72 (s, 1C,
CAr(O)(H)), 122.06 (s, 1C, CAr(O)(H)), 118.45 (d, 1C, CAr(PO)(H),
C
TMP, JC,P = 24.3 Hz), 40.21 (s, 1C, CCy), 39.38 (s, 1C, CCy),
JC,P = 10.2 Hz), 116.80 (d, 1C, CAr(PO)(H), JC,P = 6.3 Hz),
115.44 (d, 1C, CAr(PO)(PRu), JC,P = 45.6 Hz), 113.86 (s, 1C,
37.33 (d, 1C, CTMP, JC,P = 4.9 Hz), 30.49 (s, 1C, CCy), 30.01
(s, 1C, CCy), 29.83 (s, 1C, CCy), 29.30 (d, 3C, CTMP, JC,P = 5.6
Hz), 28.52 (s, 1C, CCy), 27.16 (s, 1C, CCy), 26.93 (s, 1C, CCy),
26.64 (s, 1C, CCy), 26.53 (s, 1C, CCy), 26.44 (s, 1C, CCy), 26.35
(s, 1C, CCy), 21.63 (d, 1C, CTMP, JC,P = 5.5 Hz), 20.95 (d, 1C,
C
(s). Anal. Calcd (%) for C39H52ClO2PRu (720.34 g/mol): C
65.03, H 7.28. Found: C 64.79, H 7.42.
CAr(O)(H)), 93.31 (s, 1C, C(O)CHCy2), 40.19 (s, 1C, CCy), 39.72 (s,
1C, CCy), 39.30 (d, 1C, CAd, JC,P = 26.9 Hz), 37.70 (s, 3C, CAd),
36.79 (s, 3C, CAd), 30.45 (s, 1C, CCy), 30.18 (s, 1C, CCy), 29.91 (2s
overlapping, 2C, CCy), 28.89 (d, 3C, CAd, JC,P = 9.7 Hz), 26.87
(3s overlapping, 3C, CCy), 26.77 (s, 1C, CCy), 26.36 (s, 1C, CCy),
26.25 (s, 1C, CCy). 31P NMR (121 MHz, CD2Cl2): δ 80.82 (s).
1:1-Complex 11c. A 56 mg (68 μmol) portion of modified first-
generation Hoveyda-Grubbs catalyst 10 (CH2Cl2 adduct) and
19 mg (64 μmol, 0.95 equiv) of ligand 2c were stirred for 20 min
in 4 mL of CH2Cl2. After treatment with a few equivalents of
CuCl for 25 min the solid was filtered off. 31P NMR showed full
conversion of 10 to 11c. The solvent was evaporated and the
residue taken up in hexane and transferred onto a column
(silica) prepared under argon. The desired complex was eluted
with 5-6% ether/hexane in 55% yield (26 mg). Crystals suitable for
X-ray analysis were grown by diffusion of CH2Cl2 into hexane out
of a 1:1 solution of CH2Cl2 and hexane of the complex.
TMP, JC,P = 4.3 Hz). 31P NMR (121 MHz, CD2Cl2): δ 81.62
1:1-Complex 11e. An 11.4 mg (43.8 μmol, 1.0 equiv) amount
of ligand 2e in 2 mL of CH2Cl2 was added dropwise to 31.9 mg
(43.3 μmol) of modified first-generation Hoveyda-Grubbs
catalyst 10. A few equivalents of CuCl were added, and the
suspension was stirred for 30 min. After filtration and evapora-
tion of the solvent, the residue was transferred in hexane onto a
column (silica) prepared under argon. An orange fraction was
collected in 70% yield (20 mg). Crystals were grown at rt via
diffusion of dichloromethane from a hexane/CH2Cl2 solution
(1:1) into hexane.
1H NMR (300 MHz, CD2Cl2): δ 16.78 (d, 1H, JH,P=5.1 Hz),
7.57 (t, 1H, JH,H =8.1 Hz), 7.52 (d, 1H, JH,H =7.5 Hz), 7.41 (t,
1H, JH,H = 7.7 Hz), 7.26 (d, 1H, JH,H = 8.7 Hz), 7.08 (t, 1H,
1H NMR (300 MHz, CD2Cl2): δ 14.98 (d, 1H, JH,P = 8.4 Hz),
7.80 (m, 2H), 7.52 (m, 4H), 7.41 (m, 1H), 7.27 (d, 1H, JH,H
=
8.4 Hz), 7.19 (m, 1H), 7.02 (dd, 1H, JH,H = 1.5 and 7.5 Hz), 6.90
(t, 1H, JH,H = 7.2 Hz), 6.78 (m, 2H), 4.94 (m, 1H), 2.88 (dd, 1H,
JH,H(P) = 10.2 and 14.4 Hz), 2.39 (m, 1H), 2.27 (m, 1H), 2.12
(dd, 1H, JH,H(P) = 14.1 and 17.1 Hz), 1.89-1.16 (several m, 20H),
1.11 (d, 9H, JH,P = 0.9 Hz). 13C NMR (150 MHz, CD2Cl2): δ
JH,H=6.9 Hz), 7.05 (t, 1H, JH,H=5.1 Hz), 6.70 (dd, 1H, JH,H
=
3.6 and 8.1 Hz), 6.63 (t, 1H, JH,H =7.4 Hz), 4.86 (m, 1H), 2.29
(m, 2H), 1.84-1.16 (several m, 20H), 1.69 (d, 9H, JH,P = 14.1
Hz), 1.55 (d, 9H, JH,P=14.1 Hz). 13C NMR (75 MHz, CD2Cl2):
δ 272.80 (1C, C(dRu), very weak signal), 178.42 (d, 1C,
CAr(PO)(ORu), JC,P = 11.9 Hz), 156.74 (s, 1C, CAr(O)(OR)),
143.91 (s, 1C, CAr(O)(CHdRu)), 132.41 (s, 1C, CAr(PO)(H)),
132.04 (s, 1C, CAr(PO)(H)), 128.83 (s, 1C, CAr(O)(H)), 122.85
(s, 1C, CAr(O)(H)), 122.60 (s, 1C, CAr(O)(H)), 118.99 (d, 1C,
CAr(PO)(PRu), JC,P = 39.6 Hz), 118.43 (d, 1C, CAr(PO)(H),
JC,P = 9.6 Hz), 116.87 (d, 1C, CAr(PO)(H), JC,P = 6.3 Hz),
114.03 (s, 1C, CAr(O)(H)), 93.42 (s, 1C, C(O)CHCy2), 39.83 (s, 1C,
CCy), 39.41 (s, 1C, CCy), 39.04 (d, 1C, Ctert-butyl, JC,P = 21.8 Hz),
279.49 (m, 1C, C(dRu)), 179.72 (d, 1C, CAr(PO)(ORu), JC,P
14.6 Hz), 158.25 (s, 1C, CAr(O)(OR)), 142.90 (s, 1C, CAr(O)
=
-
(CHdRu)), 133.12 (d, 1C, CAr(PO)(H), JC,P = 1.8 Hz), 132.22 (s,
1C, CAr(PO)(H)), 131.98 (d, 2C, CAr(P)(ortho), JC,P = 8.4 Hz),
131.67 (d, 1C, CAr(P)(ipso), JC,P = 55.5 Hz), 130.67 (d, 1C, CAr(P)
-
(para), JC,P = 2.6 Hz), 129.04 (d, 2C, CAr(P)(meta), JC,P = 10.6
Hz), 129.00 (s, 1C, CAr(O)(H)), 122.82 (s, 1C, CAr(O)(H)), 122.18 (s,
1C, CAr(O)(H)), 119.00 (d, 1C, CAr(PO)(H), JC,P = 10.8 Hz), 117.90
(d, 1C, CAr(PO)(H), JC,P = 7.5 Hz), 116.27 (d, 1C, CAr(PO)(PRu),

2742 Organometallics, Vol. 29, No. 12, 2010
Torker et al.
35.02 (d, 1C, Ctert-butyl, JC,P=21.5 Hz), 31.65 (s, 3C, Ctert-butyl), 29.85
(s, 1C, CCy), 29.71 (s, 1C, CCy), 29.62(s, 1C, CCy), 29.47 (s, 1C, CCy),
28.80 (s, 3C, Ctert-butyl), 26.82 (2s overlapping, 2C, CCy), 26.64 (s, 1C,
CCy), 26.34 (2s overlapping, 2C, CCy), 26.28 (s, 1C, CCy). 31P NMR
(121 MHz, CD2Cl2): δ 94.59 (s). Anal. Calcd (%) for C34H50ClO2-
PRu (658.26 g/mol): C 62.04, H 7.66. Found: C 61.98, H 7.72.
2:1-Complex 14. A 30 mg amount of PPh3-Hoveyda-Grubbs
catalyst5 (52μmol) and32.5 mg (125 μmol, 2.4equiv) ofligand2e
were stirred in CH2Cl2 for 30 min. A color change to deep purple
was observed. A few equivalents of CuCl were added, and the
sample was filtered after 5 min. The substance was adsorbed on
Celite and purified by a column under argon (silica, hexane/5%
ether as the eluent) to give the complex in 80% yield (30 mg).
Crystals suitable for X-ray analysis were grown out of an ether/
MeOH solution of the complex by diffusion of ether into MeOH.
1H NMR (600 MHz, CD2Cl2): δ 20.04 (d, 1H, JH,P=5.7 Hz),
8.27 (dd, 1H, JH,H =7.9 and 1.8 Hz), 7.52 (m, 2H), 7.41 (t, 1H,
with an intact chelate ring involving the carbene unit. We have
already shown in our previous paper that a 1:1-complex with
this ligand behaves similarly to the nonselective first-genera-
tion Grubbs catalyst 1.5
1. Attempted Syntheses of 7a. When the bidentate phos-
phine/phenolate ligand 2a is added dropwise to the first-
generation Hoveyda-Grubbs catalyst 4 (path A, Scheme 2),
one would expect a new singlet to appear in the 31P NMR
spectrum in case a 1:1-complex were to be formed. However,
we saw four doublets indicating the formation of two isomers
of a 2:1-complex (8a1: 77.82 and 67.61 ppm; 8a2: 80.66 and
57.42 ppm) with a cis coordination of the two phosphines,
suggested by the low coupling constants of JP,P =30 Hz. In
contrast, the bis-phosphine complex 3, where the two differ-
ent phosphines are trans, shows a coupling constant of JP,P
=
194 Hz.5 The corresponding peaks in the 1H NMR spectrum
are two doublet of doublets at 14.97 and 14.55 ppm that
show coupling of the carbene proton to both phosphorus
atoms. Disappointed by these results, we thought that the
PCy3 ligand in 4 could be too strongly bound, which would
favor attack of a second ligand molecule 2a on 7a over
replacement of the tricyclohexylphosphine in 4. There-
fore we subjected catalyst 5,11 which contains the more
loosely bound PPh3 ligand, to the exchange reaction with
ligand 2a (path B, Scheme 2). Also this approach was un-
successful, since we observed exactly the same two isomers of
the 2:1-complex (8a1 and 8a2 in Scheme 2), even with only
small amounts of ligand added (0.35 equiv; see Figures S4
and S5). Looking again at the 1H NMR spectrum suggested a
possible explanation: The chemical shifts of the carbene
protons of the two isomers are shifted upfield relative to
catalyst 4 (17.36 ppm) and are much lower than for the first-
generation Grubbs catalyst 1 (20.02 ppm),12 which we inter-
preted in such a way that the carbene unit must be chelating
and that electron density is donated from the oxygen to the
metal. Before we completely gave up this approach, we
added our bidentate phosphine/phenolate ligand 2a to a
mixture of catalyst 4 with an 8-fold excess of PCy3 (path C,
Scheme 2). The excess of phosphine is able to break the
oxygen/ruthenium chelate ring, to form an intermediate (6)
(1H NMR: singlet at 20.55 ppm; 31P NMR: singlet at 36.05
ppm; see Figure S6), which resembles the first-generation
Grubbs catalyst 1 (1H NMR: singlet at 20.02 ppm; 31P NMR:
singlet at 36.61 ppm).12 The ratio between 6 and 4 is 70:30.
Ligand 2a is now not able to fully displace PCy3, which
results in formation of an intermediate (9) in analogy with
J
H,H = 6.7 Hz), 7.09 (m, 2H), 6.84 (dd, 1H, JH,H = 8.2 and
3.6 Hz), 6.80 (dd, 1H, JH,H=8.4 and 3.4 Hz), 6.72 (d, 1H, JH,H
8.3 Hz), 6.55 (t, 1H, JH,H =7.6 Hz), 6.51 (t, 1H, JH,H=7.4 Hz),
6.46 (t, 1H, JH,H =7.3 Hz), 4.58 (m, 1H), 1.44 (dd, 9H, JH,P
=
=
11.3 and 2.0 Hz), 1.40 (dd, 9H, JH,P = 11.3 and 2.0 Hz), 1.29
(d, 3H, JH,H=6.0 Hz), 1.23 (d, 3H, JH,H=6.0 Hz), 1.03 (dd, 9H,
J
H,P =11.5 and 2.1 Hz), 0.99 (dd, 9H, JH,P =11.8 and 2.1 Hz).
13C NMR (150 MHz, CD2Cl2): δ 291.65 (m, 1C), 182.61 (dd, 1C,
C,P=5.3 and 10.3 Hz), 182.49 (dd, 1C, JC,P=5.3 and 10.3 Hz),
J
148.53 (s, 1C), 145.42 (s, 1C), 133.60 (s, 1C), 133.38 (s, 1C),
133.03 (s, 1C), 131.94 (s, 1C), 131.46 (s, 1C), 131.22 (s, 1C), 121.
97 (s, 1C), 121.57 (d, 1C, JC,P = 6.9 Hz), 121.51 (d, 1C, JC,P
6.9 Hz), 114.81 (dd, 1C, JC,P =29.7 and 3.8 Hz), 114.49 (d, 1C,
C,P = 4.8 Hz), 114.17 (s, 1C), 113.79 (dd, 1C, JC,P = 28.8 and
=
J
4.1 Hz), 112.89 (d, 1C, JC,P = 5.0 Hz), 70.50 (s, 1C), 38.41
(dd, 1C, JC,P = 14.3 and 3.8 Hz), 37.11 (dd, 1C, JC,P = 12.6
and 3.1 Hz), 36.97 (dd, 1C, JC,P = 13.2 and 3.9 Hz), 34.95 (dd,
1C, JC,P =12.6 and 3.5 Hz), 30.51 (d, 3C, JC,P =2.6 Hz), 30.08
(d, 3C, JC,P=3.0 Hz), 29.63 (d, 3C, JC,P=3.1 Hz), 29.11 (d, 3C,
J
C,P=2.5 Hz), 22.15 (s, 1C), 22.05 (s, 1C). 31P NMR (121 MHz,
CD2Cl2): δ 67.03 (d, 1P, JP,P=214 Hz), 65.07 (d, 1P, JP,P=213
Hz). The two signals show a very strong roof effect.
Computational Details
Density functional theory (DFT) calculations were performed
using ADF 2006 at the BP86/ZORA-TZP level of theory. All
structures were fully optimized without constraints and checked
with frequency calculations to ensure that they were minima.
Results
The results shown here reflect the different stages (1-4) of
catalyst development that guided us on our way toward
improved chemoselectivity. It should be mentioned that the
systematic optimization of the catalyst was severely hampe-
red in the beginning by 2:1-complexes (bidentate ligand-to-
ruthenium ratio), whose undesired, and initially unexpected,
formation needed to be suppressed. An overview of all experi-
ments is given in Scheme 2. Ligand syntheses are described in
Scheme 3. From unsuccessful direct ligand exchange with
catalyst 4 (1), via discovery of the similar, but unexpected,
behavior of the adamantyl substituent versus a tert-butyl
group (2)—the adamantyl group was assumed to disfavor 2:1-
complexes due to its size—we were led to the introduction of a
larger carbene unit, which favors 1:1-complexes (3). This stage
then allowed us to establish a series in which the size of
substituent R1 was systematically increased (4). Point 5 is not
directly related to catalyst improvement but demonstrates,
from a coordination chemistry point of view, that the larger
ligand 2e, upon ligand exchange, does not form a 2:1-complex
1
catalyst 3,5 which shows a singlet at 19.94 ppm in the H
NMR and two doublets at 63.36 and 40.61 ppm in the 31P
NMR (see Figure S7). The coupling constant JP,P =200 Hz
is indicative of a trans orientation. Intermediate 9 releases
tricyclohexylphosphine upon treatment with CuCl13 to form
complex 7a (see Figure S8). This route, however, did not
seem to be practical as a preparative procedure, since a large
excess of phosphine needs to be applied even though forma-
tion of 2:1-complexes 8a1 and 8a2 could be reduced (see
Experimental Section). However, CuCl13 also scavenges
ligand 2a and leaves large amounts of unreacted catalyst 4.
Another strategy was needed.
(11) Kingsbury, J. S.; Harrity, J. P. A.; Bonitatebus, P. J.; Hoveyda,
A. H. J. Am. Chem. Soc. 1999, 121 (4), 791–799.
(12) Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H. Angew.
Chem., Int. Ed. Engl. 1995, 34 (18), 2039–2041.
(13) Lipshutz, B. H.; Frieman, B.; Birkedal, H. Org. Lett. 2004, 6 (14),
2305–2308.

Article
Organometallics, Vol. 29, No. 12, 2010 2743
Figure 1. Crystal structure of 2:1-complex 8b1 (ORTEP plot,
20% probability ellipsoids). Hydrogen atoms are omitted for
˚
clarity. Selected distances (A) and angles (deg): Ru1—C49
Figure 2. Crystal structure of 2:1-complex 8b2 (ORTEP plot,
20% probability ellipsoids). Hydrogen atoms are omitted for
˚
clarity. Selected distances (A) and angles (deg): Ru1—C50
1.878(5), Ru1—O33 2.047(3), Ru1—O9 2.172(3), Ru1—O56
2.273(3), Ru1—P26 2.2742(11), Ru1—P2 2.3528(11), C49—
Ru1—O33 101.85(16), C49—Ru1—O9 164.29(17), O33—
Ru1—O9 83.53(13), C49—Ru1—O56 78.80(15), O33—Ru1—
O56 81.31(11), O9—Ru1—O56 87.56(12), C49—Ru1—P26
91.13(14), O33—Ru1—P26 82.16(9), O9—Ru1—P26 104.29(10),
O56—Ru1—P26 158.40(9), C49—Ru1—P2 95.82(13), O33—
Ru1—P2 160.92(10), O9—Ru1—P2 77.56(9), O56—Ru1—P2
95.27(9), P26—Ru1—P2 104.80(4).
1.877(4), Ru1—O2 2.052(2), Ru1—O23 2.139(3), Ru1—O57
2.270(2), Ru1—P9 2.2742(9), Ru1—P10 2.3742(10), C50—
Ru1—O2 104.36(13), C50—Ru1—O23 162.18(12), O2—
Ru1—O23 82.13(10), C50—Ru1—O57 79.47(12), O2—Ru1—
O57 77.54(9), O23—Ru1—O57 85.92(9), C50—Ru1—P9
91.73(11), O2—Ru1—P9 83.54(7), O23—Ru1—P9 105.61(7),
O57—Ru1—P9 156.33(7), C50—Ru1—P10 93.52(11), O2—
Ru1—P10 161.54(8), O23—Ru1—P10 79.42(7), O57—Ru1—
P10 101.71(7), P9—Ru1—P10 100.73(3).
2. Changing tert-Butyl (2a) for Adamantyl (2b). If forma-
tion of hexacoordinated 2:1-complexes is favored due to
steric effects, larger phosphine ligands could be expected to
work against 2:1-complex formation. In this sense we re-
placed the tert-butyl group in 2a with the larger adamantyl.
The synthesis of the new ligand 2b is similar to 2a, except that
formation of the adamantyl Grignard reagent required a
particular published experimental procedure.7 Subjecting
ligand 2b to the exchange reaction pushed us back to a stage
where we were already. We see again four doublets in the 31P
NMR (73.08 and 61.75 ppm for 8b1; 76.70 and 55.08 ppm for
8b2) with coupling constants of 30 Hz. The 1H NMR (14.92
ppm for 8b1 and 14.40 ppm for 8b2) also shows an almost
identical pattern to that obtained in experiments with ligand
2a, with the ratio between the two isomers (8b1 and 8b2)
being equal to 61:39. Crystallization of the mixture did not
provide a pure sample of the dominant isomer 8b1 (both
crystallize under the applied conditions; for the NMR spec-
trum see Figure S14), but at least gave single crystals of suf-
ficient quality to obtain an X-ray structure of 8b1 (Figure 1).
It indeed shows a hexacoordinated ruthenium with the two
bidentate phosphine/phenolate ligands 2b cis to each other.
Purification attempts after reaction with CuCl13 to remove
excess phosphine ligand gave another interesting result.
After filtration of the excess CuCl the isomer ratio remained
unchanged. However, after two days in solution isomer 8b1
has completely disappeared, leaving only isomer 8b2. We
think that ligand exchange, catalyzed by CuCl, may trans-
form a product formed under kinetic control (isomer 8b1) to
the thermodynamically more stable isomer 8b2. Purification
by column chromatography and crystallization allowed us now
to get an X-ray structure of isomer 8b2, for which the substi-
tuents on the equatorial phosphine are switched (Figure 2).
In any case, as a preparative procedure for 1:1 complexes, the
adamantyl approach failed and we needed again a new strategy.
3. Increasing the Size of the Carbene (4 . 10). Since in the
above-described 2:1-complexes 8a and 8b the carbene unit is
chelating, a larger substituent there could potentially prevent
this otherwise favorable interaction and destabilize the 2:1-
complexes in favor of the 1:1-complex. To obtain a cata-
lyst closely related to 3, we synthesized the modified first-
generation Hoveyda-Grubbs catalyst 10 by replacing the
isopropoxy substituent by dicyclohexylmethoxy (Scheme 2),
which had been reported previously.5 Indeed, we were suc-
cessful now, since the ligand exchange reaction with both 2a
and 2b gave the desired 1:1-complexes 11a and 11b as the
main products. Crystal structures for 10 and 11a are given in
Figures 5 and 6, but have been reported previously.5 We
tested 11a and 11b in copolymerization of NBE and COE
and obtained high yields of largely alternating copolymers.
The polymer microstructure was for both catalysts depen-
dent on (a) the temperature and (b) the norbornene-to-
cyclooctene ratio. Figures 3 and 4 show the olefinic region
of the 13C NMR spectra. Peak assignments have been done in
accordance with the literature.14-18 Lower temperatures and
(14) Vehlow, K.; Wang, D.; Buchmeiser, M. R.; Blechert, S. Angew.
Chem., Int. Ed. 2008, 47 (14), 2615–2618.
(15) Lichtenheldt, M.; Wang, D. R.; Vehlow, K.; Reinhardt, I.;
Kuhnel, C.; Decker, U.; Blechert, S.; Buchmeiser, M. R. Chem.—Eur.
J. 2009, 15 (37), 9451–9457.
(16) Al Samak, B.; Amir-Ebrahimi, V.; Corry, D. G.; Hamilton,
J. G.; Rigby, S.; Rooney, J. J.; Thompson, J. M. J. Mol. Catal. A: Chem.
2000, 160 (1), 13–21.
(17) Amir-Ebrahimi, V.; Rooney, J. J. J. Mol. Catal. A: Chem. 2004,
208 (1-2), 115–121.
(18) Al Samak, B.; Carvill, A. G.; Hamilton, J. G.; Rooney, J. J.;
Thompson, J. M. Chem. Commun. 1997, 21, 2057–2058.

2744 Organometallics, Vol. 29, No. 12, 2010
Torker et al.
Figure 3. Temperature and NBE/COE ratio dependence with
catalyst 11a.
Figure 5. Crystal structure of complex 10 (ORTEP plot, 20%
probability ellipsoids). Hydrogen atoms are omitted for clarity.
Selected distances (A) and angles (deg): Ru(1)-C(23) 1.825(4),
˚
Ru(1)-P(4) 2.2683(9), Ru(1)-Cl(2) 2.3385(9), Ru(1)-Cl(3)
2.3396(9), Ru(1)-O(30) 2.341(2), C(23)-Ru(1)-P(4) 98.86(12),
C(23)-Ru(1)-Cl(2) 98.66(12), P(4)-Ru(1)-Cl(2) 91.29(3),
C(23)-Ru(1)-Cl(3) 102.70(12), P(4)-Ru(1)-Cl(3) 90.61(3),
Cl(2)-Ru(1)-Cl(3) 158.00(4), C(23)-Ru(1)-O(30) 79.53(13),
P(4)-Ru(1)-O(30) 176.98(6), Cl(2)-Ru(1)-O(30) 86.45(6),
Cl(3)-Ru(1)-O(30) 92.23(6).
Figure 4. Temperature and NBE/COE ratio dependence with
catalyst 11b.
higher amounts of norbornene favor the norbornene homo-
polymer. Cooling from room temperature to 0 °C almost
entirely suppresses the cyclooctene homopolymerization.
Furthermore the substituents on the phosphine ligands have
an influence, although not very pronounced. Analysis of the
polymers (Table 1) shows that the degree of alternation goes
up to 76% with catalyst 11a. The overall performance of
catalyst 11b is slightly better, which gives up to 85% hetero-
junctions. The larger dicyclohexylmethoxy carbene has an-
other advantage besides providing enough steric bulk for a
successful formation of 1:1-complexes. We compared the
performance of 10 with commercially available catalysts by
performing NMR kinetics with cyclooctene, which demon-
strate the higher activity of 10 versus 4 (Figure 7).
Figure 6. Crystal structure of 1:1-complex 11a (ORTEP plot,
20% probability ellipsoids). Hydrogen atoms are omitted for
clarity. Selected distances (A) and angles (deg): Ru(1)-C(21)
1.825(4), Ru(1)-O(6) 1.997(2), Ru(1)-P(3) 2.2201(9), Ru(1)-
O(24) 2.310(2), Ru(1)-Cl(2) 2.3397(10), C(21)-Ru(1)-O(6)
104.49(14), C(21)-Ru(1)-P(3) 92.72(12), O(6)-Ru(1)-P(3)
84.51(7), C(21)-Ru(1)-O(24) 79.67(13), O(6)-Ru(1)-O(24)
92.10(9), P(3)-Ru(1)-O(24) 170.66(7), C(21)-Ru(1)-Cl(2)
102.07(12), O(6)-Ru(1)-Cl(2) 153.13(8), P(3)-Ru(1)-Cl(2)
98.31(3), O(24)-Ru(1)-Cl(2) 88.65(7).
˚
4. Improving the Selectivity toward Alternation to 100%
with Catalyst 11d. As a next step, we tried to improve the
selectivity of the catalyst even further and synthesized
ligands 2c and 2d (Scheme 3). Synthesis of ligand 2c was
straightforward, whereas 2d was more difficult to obtain. A
Grignard solution of 2-bromo-2,3,3-trimethylbutane was
finally prepared in about 40-50% yield by slow addition
of the alkyl bromide to a 15-fold excess of Mg powder in
refluxing ether. Both complexes 11c and 11d could be pre-
pared and also characterized by X-ray (Figures 8 and 9), and
the steric bulk of the TMP substituent can readily be seen in
the 1H and 13C NMR spectra (Figures S20 and S21). At room

Article
Organometallics, Vol. 29, No. 12, 2010 2745
Table 1. Polymerization Experiments Carried Out with 150-200
mg of Norbornene (NBE) and the Indicated Amount of COE
(Cyclooctene) with the Reaction Volume Filled up to 20 mL with
CH2Cl2 and a NBE/Catalyst Ratio of 2000:1
alternating cis content
[%]e
catalyst
T
NBE/COE t [h]b yield [%]c linkages [%]d
11a
11a
11a
11a
11b
11b
11b
11b
11c
rt
rt
0 °C
0 °C
rt
1:100
1:200
1:100
1:200
1:100
1:200
1:100
1:200
1:100
1:20
1.5
1.5
1.5
1.5
1
1
1
1
84
90
40
41
82
72
53
11
89
88
72
76
67
76
78
85
80
83
89
97
30
37
29
34
38
39
36
37
20
13
rt
0 °C
0 °C
rt
1.5
0.25a
11da rt
a Catalyst 11d produces a highly viscous polymer solution within
seconds after catalyst addition. b The polymerizations were quenched by
addition of 100 mL of MeOH after the times indicated. c Yields were
determined after drying the coagulated copolymer under high vacuum
for 2 h. d The percentage of alternating linkages has been determined by
integration of the olefinic region of the 13C NMR spectrum. e The cis
content was estimated by integration of the CH(CHdCHR) protons
on the norbornene terminus of the double bond (see Supporting
Information).
Figure 8. Crystal structure of 1:1-complex 11c (ORTEP plot,
20% probability ellipsoids). Hydrogen atoms are omitted for
clarity. The structure contains a heavily disordered hexane
molecule, which is refined with restrained geometry. Selected
˚
distances (A) and angles (deg): Ru1—C27 1.837(9), Ru1—O8
2.009(6), Ru1—P1 2.217(2), Ru1—O20 2.316(5), Ru1—Cl1
2.350(2), C27—Ru1—O8 105.6(3), C27—Ru1—P1 93.5(3),
O8—Ru1—P1 84.69(17), C27—Ru1—O20 79.9(3), O8—Ru1—
O20 91.8(2), P1—Ru1—O20 171.41(15), C27—Ru1—Cl1
97.6(3), O8—Ru1—Cl1 156.54(19), P1—Ru1—Cl1 97.58(8),
O20—Ru1—Cl1 88.78(15).
Figure 7. NMR kinetics showing the performance of our modi-
fied first-generation Hoveyda-Grubbs catalyst 10 versus
Grubbs 1 (1) and Hoveyda-Grubbs 1 (4) in polymerization of
cyclooctene (COE) in CDCl3 at room temperature. [C0]COE
=
0.13 M, [C0]cat./[C0]COE = 1/200.
temperature the phenyl ring cannot freely rotate in 11d, as
documented by strong broadening of the signals of the ortho
carbons, which almost disappear into the baseline. Similarly,
although less pronounced, the peaks of the ortho and meta
protons are broadened. Furthermore, the ortho protons do
not show correlation to any aromatic carbon in an HSQC
experiment. A similar effect is observed for complexes 7d
(Figures S9 and S10) and 22a-f (see the 2D spectrum for
complex 22a (Chart 1) in the Supporting Information,
Figure S25). Comparison of all four catalysts in copolymer-
ization shows a systematic trend (Figure 10). Catalyst 11c
shows higher selectivity than 11a and 11b, and almost no
polycyclooctene is obtained even at room temperature and a
norbornene-to-cyclooctene ratio of 1:100. Catalyst 11d out-
performs all and displays complete selectivity for the for-
mation of the alternating copolymer to within our detection
limits, even at a norbornene-to-cyclooctene ratio of 1:20.
Figure 9. Crystal structure of 1:1-complex 11d (ORTEP plot,
20% probability ellipsoids). Hydrogen atoms are omitted for
˚
clarity. Selected distances (A) and angles (deg): Ru1—C24
1.851(5), Ru1—O16 1.996(3), Ru1—P3 2.2320(13), Ru1—O31
2.355(3), Ru1—Cl2 2.3617(14), C24—Ru1—O16 104.52(19),
C24—Ru1—P3 93.53(17), O16—Ru1—P3 84.20(10), C24—
Ru1—O31 78.34(18), O16—Ru1—O31 89.80(13), P3—Ru1—
O31 168.40(9), C24—Ru1—Cl2 95.79(17), O16—Ru1—Cl2
158.68(10), P3—Ru1—Cl2 101.18(5), O31—Ru1—Cl2
87.98(9).
Furthermore it is much faster than the other three in this
series and is able to produce a copolymer in high yields within
seconds after catalyst addition, as seen by immediate gel
formation. Since the TMP group is sterically very demand-
ing, we also tried the synthesis of 1:1-complex 7d (Scheme 2),
which indeed could be obtained in good yield. Whereas 11d

2746 Organometallics, Vol. 29, No. 12, 2010
Torker et al.
Figure 11. Crystal structure of 2:1-complex 14 (ORTEP plot,
20% probability ellipsoids). Hydrogen atoms are omitted for
˚
clarity. Selected distances (A) and angles (deg): Ru1—C34
Figure 10. Dependence of the degree of alternation on the
substituents in copolymerization of NBE and COE with cata-
lysts 11a (tert-butyl), 11b (adamantyl), 11c (neopentyl), and 11d
(TMP). For polymer characterization see Table 1.
1.844(5), Ru1—O2 2.068(3), Ru1—O17 2.084(3), Ru1—P9
2.3774(10), Ru1—P10 2.3994(11), C34—Ru1—O2 89.5(2),
C34—Ru1—O17 101.3(2), O2—Ru1—O17 169.24(11), C34—
Ru1—P9 92.69(16), O2—Ru1—P9 82.79(8), O17—Ru1—P9
97.52(8), C34—Ru1—P10 98.78(16), O2—Ru1—P10 95.06(8),
O17—Ru1—P10 82.45(8), P9—Ru1—P10 168.32(4).
gives a highly viscous polymer solution within a few seconds,
7d, with the smaller and slower initiating carbene unit, needs
about a minute to reach that stage. Additionally, the cis
content of the polymers produced by 7d and 11d decreased to
13% (Table 1), which may be attributed to a more crowded
metallacyclobutane structure, which avoids additional steric
interaction by adopting a more favorable trans orientation of
the substituents.
5. Synthesis of the trans 2:1-Complex 14 with the Bulky
Ligand 2e. As a negative control for our mechanistic concept,
we synthesized complex 11e, and we could show that the
symmetric ligand 2e induces no chemoselectivity. The poly-
mer microstructure contains blocks of the two homopoly-
mers but no alternating units. We had already demonstrated
this previously with the analogous complex based on the
structure of 3.5 As a matter of interest we also synthesized
symmetric complex 7e, which could be obtained as a stable
1:1-complex. This demonstrates that the two tert-butyl groups
in 2e are too big for a stable 2:1-complex with a cis coordina-
tion of the phosphines and a chelating carbene unit (structure
14a in Chart 2). A crystal structure for 11e is given in Figure 12.
A 2:1-complex (14), however, could be prepared and also
characterized by X-ray crystallography (Figure 11). The co-
ordination is different from that of the other 2:1-complexes (8a
and 8b), with the phosphines being trans to each other and no
chelation of the carbene unit. The structure of 14 therefore
resembles that of the first-generation Grubbs catalyst 1.12 Also
the color of the complex is almost identical to that of 1, deep
purple in CH2Cl2, in contrast to all the other 1:1- and 2:1-
complexes in the present paper, which give orange-brown
solutions. Furthermore, the chemical shift of the carbene
proton in CD2Cl2 (20.02 ppm) is identical to that reported
for the Grubbs first-generation catalyst 1 (20.02 ppm),12
although it shows a doublet (JH,P = 5.7 Hz), indicating that
coupling to one of the phosphorus atoms is apparently stron-
ger than to the second one. The 31P NMR spectrum indicates
that the two phosphines are chemically different, although not
by much, since it shows two doublets with a very strong roof
effect at 67.03 and 65.07 ppm (Figure S24). The distinction
Figure 12. Crystal structure of 1:1-complex 11e (ORTEP plot,
20% probability ellipsoids). Hydrogen atoms are omitted for
clarity. Selected distances (A) and angles (deg): Ru(1)-C(19)
1.834(4), Ru(1)-O(6) 1.992(2), Ru(1)-P(3) 2.2468(10), Ru(1)-
O(26) 2.306(2), Ru(1)-Cl(2) 2.3481(10), C(19)-Ru(1)-O(6)
104.69(14), C(19)-Ru(1)-P(3) 99.55(12), O(6)-Ru(1)-P(3)
84.53(8), C(19)-Ru(1)-O(26) 79.06(13), O(6)-Ru(1)-O(26)
90.64(10), P(3)-Ru(1)-O(26) 174.47(7), C(19)-Ru(1)-Cl(2)
99.29(12), O(6)-Ru(1)-Cl(2) 154.63(8), P(3)-Ru(1)-Cl(2)
99.68(4), O(26)-Ru(1)-Cl(2) 85.83(7).
˚
arises from a hindered rotation of the carbene unit. The coupl-
ing constant (JP,P=213 Hz) is indicative of a trans orientation.
The Ru-P distances of 14 in the crystal are relatively large,
˚
2.3774(10) and 2.3994(11) A. They are, however, still shorter
than in the Grubbs first-generation catalyst 1 (2.4188(10) and
˚
2.4265(10) A), of which we also present a crystal structure
(Figure 13) without para substitution on the benzylidene car-
bene, as has been reported earlier.12 The carbene in 14 is not

Article
Organometallics, Vol. 29, No. 12, 2010 2747
Scheme 4. Catalytic Cycle for the Alternating Copolymerization
of Norbornene and Cyclooctene with Catalyst 3a
Figure 13. Crystal structure of the first-generation Grubbs
catalyst 1 (ORTEP plot, 30% probability ellipsoids). Hydrogen
˚
atoms are omitted for clarity. Selected distances (A) and angles
(deg): Ru1—C37 1.834(4), Ru1—Cl1 2.3953(10), Ru1—Cl2
2.4012(10), Ru1—P1 2.4188(10), Ru1—P2 2.4265(10), C37—
Ru1—Cl1 105.04(13), C37—Ru1—Cl2 86.94(13), Cl1—Ru1—
Cl2 167.96(4), C37—Ru1—P1 100.23(12), Cl1—Ru1—P1
89.90(4), Cl2—Ru1—P1 89.03(4), C37—Ru1—P2 97.66(12),
Cl1—Ru1—P2 89.07(4), Cl2—Ru1—P2 88.19(4), P1—Ru1—
P2 161.72(4).
a The catalyst switches between two different carbene states (A
and D).
part of a chelate ring, and we found only a few other crystal
structures in the literature, where the alkoxy substituent on the
benzylidene does not coordinate to the metal.19-21
step is actually not formation of the metallacyclobutane
itself, but rather a rotation of the tricyclohexylphosphine
in 1 at a metallacyclobutane structure, which then led to an
interpretation of the isotope effects for which the gas phase
and computational results were in agreement.2,3 A conse-
quence of the new interpretation was the realization that
during one productive turnover, i.e., carbene to trans-me-
tallacyclobutane to carbene, the carbene switches from one
“side” of the catalyst to the other. This was not obvious at the
time in the case of catalyst 1 due to ligand rotation of the
3-fold symmetric PCy3 ligand, which permits the generation
of a carbene configuration of the same energy. In 2005 the
group of Piers detected a metallacyclobutane intermedi-
ate with a modified second-generation Grubbs catalyst at
low temperature, which was trans to the N-heterocyclic car-
bene (NHC) ligand,28 which confirmed our previous com-
putational results.2
As a further consequence, we reported later in the same
year the design of a catalyst (3)4 that is able to produce a
largely alternating copolymer from norbornene and cyclooc-
tene, based on a simple, general mechanistic picture, taking
advantage of an asymmetric, bidentate phosphine ligand,
which cannot rotate (Scheme 4). The asymmetry is given by
the two substituents on the phosphorus which differ in size:
Two cyclic olefins are distinguished by their ring strain,
which was explained by two diastereomeric carbene states
(A and D) of the catalyst that ideally interchange after each
productive metathesis step.4 This means that the step from
one diastereomeric carbene (A . D) proceeds only if one can
pay the energetic price by release of ring strain, whereas the
next transformation (D . A) proceeds for either strained or
Discussion
In 1971 Chauvin was the first to postulate a metathesis
mechanism involving (i) [2þ2] cycloaddition of an olefin to a
metal carbene to form (ii) a metallacyclobutane, which then
undergoes (iii) cycloreversion to form a new olefin and a new
metal carbene species.22 The breakthrough of this reaction
into synthetic organic chemistry, however, started with the
report of a well-defined, stable, functional-group-tolerant
ruthenium catalyst (1) by Grubbs in 1995,12,23-25 which we
took as a starting point to perform, among other things,
kinetic isotope effect studies in the gas phase to elucidate the
nature of the metallacyclobutane in ruthenium metathesis.
We concluded at that time that it must be a transition state.1
This was in contradiction with our own computational
results, which predicted a minimum for it.2,26,27 After a more
careful investigation we realized that the rate-determining
(19) Occhipinti, G.; Bjorsvik, H. R.; Tornroos, K. W.; Furstner, A.;
Jensen, V. R. Organometallics 2007, 26, 4383–4385.
(20) Furstner, A.; Davies, P. W.; Lehmann, C. W. Organometallics
2005, 24 (16), 4065–4071.
(21) Gatard, S.; Nlate, S.; Cloutet, E.; Bravic, G.; Blais, J. C.; Astruc,
D. Angew. Chem., Int. Ed. 2003, 42 (4), 452–456.
(22) Herisson, J. L.; Chauvin, Y. Makromol. Chem. 1971, 141, 161–
176.
(23) Dias, E. L.; Nguyen, S. T.; Grubbs, R. H. J. Am. Chem. Soc.
1997, 119 (17), 3887–3897.
(24) Sanford, M. S.; Ulman, M.; Grubbs, R. H. J. Am. Chem. Soc.
2001, 123 (4), 749–750.
(25) Sanford, M. S.; Love, J. A.; Grubbs, R. H. J. Am. Chem. Soc.
2001, 123 (27), 6543–6554.
(26) Cavallo, L. J. Am. Chem. Soc. 2002, 124 (30), 8965–8973.
(27) Vyboishchikov, S. E.; Buhl, M.; Thiel, W. Chem.—Eur. J. 2002,
8 (17), 3962–3975.
(28) Romero, P. E.; Piers, W. E. J. Am. Chem. Soc. 2005, 127 (14),
5032–5033.

2748 Organometallics, Vol. 29, No. 12, 2010
Torker et al.
Table 2. Comparison of Various Bond Lengths of Complexesa
and Grubbs reported a catalyst structure with a fluorine-
ruthenium interaction.29 Further hexacoordinated struc-
tures are reported by Grela for a dormant ruthenium catalyst
bearing a chelating carboxylate ligand30,31 and a complex
containing a chelating trifluoroacetate ligand replacing the
chloride anion by Buchmeiser.32 The longer Ru-Ocarbene
bond distances in 8b1 and 8b2 compared to our sulfonate
complexes 22a-f6 are not surprising due to a more crowded
and more electron-rich hexacoordinated ruthenium center.
It is also seen by the rather long Ru-Ccarbene bond lengths of
Ru-
Ocarbene [A]
Ru-
Ccarbene [A]
˚
˚
˚
complex
Ru-P [A]
1
no chelating oxygen 1.834 (4) 2.4265 (10) and 2.4188 (10)
8b1
8b2
10
11a
11c
11d
11e
14
22a
22d
22e
22f
2.273(3)
2.270(2)
2.341(2)
2.310(2)
2.316(5)
2.355(3)
2.306(2)
no chelate ring
2.242(6)
2.250(2)
2.232(4)
2.258(3)
1.878(5) 2.2742(11) and 2.3528(11)
1.877(4) 2.2742(9) and 2.3742(10)
1.825(4) 2.2683(9)
1.825(4) 2.2201(9)
1.837(9) 2.217(2)
1.851(5) 2.2320(13)
1.834(4) 2.2468(10)
1.844(5) 2.3774(10) and 2.3994(11)
1.831(7) 2.231(2)
1.833(3) 2.2311(7)
˚
1.878(5) and 1.877(4) A, respectively. This is in agreement
1
with the H NMR signals that appear at rather high field
(see Results). The Ru-P bond lengths are quite a bit longer
than for monophosphine complexes 10,5 11a,5 11b-e, and
22a-f,6 however, on average distinctly shorter than for the
bisphosphine complexes 1 and 14. The initial observation
that 8a2 is the thermodynamic product (this has been
demonstrated for 8b2; see Results) is reproduced by calcula-
tions with BP86/TZP for the truncated tert-butyl model.
They suggest isomer 8a2 to be in fact more stable than isomer
8a1 (by 0.4 kcal/mol). Charts 2 and 3 show structures of
various chelating, hexacoordinated (8a1-4) and nonchelat-
ing, pentacoordinated (8a5,6) complexes. All are predicted
to be higher in energy than 8a2. If one has a closer look at the
crystal structure, there might be a favored stabilization in 8b2
by a π-π stacking interaction, since the two phenyl rings are
aligned in a parallel fashion, although they are not on top of
each other. 1H and 13C NMR spectra further show that both
phenyl rings are not freely rotating at room temperature,
since we observe broad peaks for the ortho and meta posi-
tions for the protons as well as for the corresponding carbons
(Figures S15 and S16). In contrast, an NMR spectrum of
complex 8a1 does not show this effect, where π-π stacking is
not possible (Figures S12 and S13). The calculations in
Charts 2 and 3 also predict, in the case of the more bulky
ligand 2e, that nonchelating complex 14 should be more
stable than structure 14a, in which the phosphines are cis to
each other and the carbene unit forms a chelate ring. This is
again in perfect agreement with our experimental findings.
Larger Carbene Unit . Faster Initiation. The oxygen/
ruthenium chelation for complexes containing the larger
dicyclohexylmethoxy-substituted benzylidene is weakened
due to steric repulsion. If one takes a look at the Ru-Ocarbene
1.840(7) 2.2493(17)
1.837(4) 2.2373(11)
a Complexes 10 and 11a have been published elsewhere,5 as well as
crystal structures of 22a, 22d, 22e, and 22f.6
unstrained cycloolefins. Our prototype of such a chemose-
lective catalyst 3 was synthesized by simple ligand exchange
of the asymmetric bidentate phosphine/phenolate ligand 2a
with the first-generation Grubbs catalyst 1 (Schemes 1 and
3). In contrast to the latter one, which polymerizes first the
more strained and more reactive norbornene, followed by
blocks of polycyclooctene, the new complex exhibited the
ability of inducing alternation. The new catalyst, however,
showed low stability in solution and tended to form small
amounts of first-generation Grubbs catalyst upon decom-
position, which is also reflected in the produced copolymer
that contains considerable amounts of polycyclooctene blocks
(ca. 20%).5 To test whether the lower selectivity comes from a
decomposition product of the catalyst or an insufficient ener-
getic difference between the two carbene states (A and D), we
synthesized a PCy3-free catalyst with a Hoveyda-type, chelat-
ing carbene unit (11a). We have already shown previously that
this complex is able to suppress homopolycyclooctene at 0 °C,
which is better than with 3, which even at this low temperature
gives a significant amount of polycyclooctene. We interpret the
result as confirmation that 3 forms indeed nonselective catalyst
1 upon decomposition.5 However, first attempts to synthesize
1:1-complex 7a by direct ligand substitution from complex 4
failed because of formation of stable 2:1 complexes. We were
successful, however, with the introduction of a larger carbene
unit in 10, which not only allowed for a clean formation of 1:1-
complexes 11a,b but also exhibits faster initiation kinetics.
The origin of these two effects can readily be seen by structural
analysis of 2:1-complexes 8b1 and 8b2 and 1:1-complexes 10,
11a, 11c, and 11d. Various bond lengths are compared in
Table 2.
˚
bond distance of 2.341(2) A in 10, it is quite a bit longer than
in sulfonate complexes 22a-f,6 and even longer than in the
very crowded 2:1-complexes 8a and 8b (Table 2). Also for all
other complexes with this larger carbene unit (11a-e) this
˚
distance is larger than 2.30 A. Furthermore the Ru-Ocarbene
˚
bond distance of 2.355(3) A for 11d is the longest of all the
Stability of 2:1-Complexes. The unsuccessful attempts of
the direct substitution reaction between ligands 2a,b and the
first-generation Hoveyda-Grubbs catalyst 4 are attributed
to the formation of stable 2:1-complexes. The Ru-Ocarbene
complexes in the present paper, which also reflects the great
steric bulk of the TMP substituent. We are aware of only two
longer Ru-Ocarbene distances in the literature.19,33 We
checked the CCDC database and found actually only seven
ortho alkoxy-substitued benzylidene complexes with Ru-
˚
bond distances are 2.273(3) and 2.270(2) A for 8b1 and 8b2,
a bit longer than in pentacoordinated sulfonate complexes
22a-f6 (between 2.232(4) and 2.258(3) A), indicating a
˚
˚
Ocarbene distances g 2.30 A: catalyst 5 by Hoveyda (2.309(2)
11
A), a complex with a four-membered NHC ligand by
˚
reasonably strong chelate ring (Table 2). A similar hexacoor-
dinated 2:1-complex with two bidentate NHC/phenolate
ligands has been reported by Jensen. There, the Ru-Ocarbene
(30) Gawin, R.; Makal, A.; Wozniak, K.; Mauduit, M.; Grela, K.
Angew. Chem., Int. Ed. 2007, 46, 7206–7209.
(31) Bieniek, M.; Bujok, R.; Cabaj, M.; Lugan, N.; Lavigne, G.; Arlt,
D.; Grela, K. J. Am. Chem. Soc. 2006, 128 (42), 13652–13653.
(32) Kumar, P. S.; Wurst, K.; Buchmeiser, M. R. J. Am. Chem. Soc.
2009, 131 (1), 387–395.
˚
distance is 2.38 A, probably due to the more electron-rich
NHC ligands.19 A hexacoordinated Ru dimer with a
Ru-Ocarbene chelate ring has been published by Hoveyda,11
(29) Ritter, T.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2006,
128 (36), 11768–11769.
(33) Despagnet-Ayoub, E.; Grubbs, R. H. Organometallics 2005,
24 (3), 338–340.

Article
Organometallics, Vol. 29, No. 12, 2010 2749
Chart 4. All Rotamers for the the Two Diastereomeric
Carbene Statesa
Table 3. Comparison of the Energies of the Two Diastereomeric
Carbene States in Chart 4 (BP86/TZP)
energy of carbene state 1
[kcal/mol]b
energy of carbene state 2
[kcal/mol]b
R
tert-butyl
S1
0.0
T1
0.0
S2
3.4
T2
3.4
adamantyl
neopentyl U1-1
0.0
U1-2
1.4
V1-2
1.7
U1-3
4.5
V1-3
5.5
U2-1
6.4
V2-1
7.7
U2-2
3.8
V2-2
5.3
U2-3
1.7
V2-3
4.0
TMPa
V1-1
0.0
a TMP = 1,1,2,2-tetramethylpropyl. b In the case of R = neopentyl or
TMP, there are three rotamers for each carbene state.
Chart 5. Calculated Structures for Methylidene Carbenes U
(R = Neopentyl as in Chart 4) and V (R = TMP)
a Truncated model using a methylidene carbene. Letter S for ligand
2a, T for 2b, U for 2c, and V for 2d.
33
Grubbs (2.360 (3) A), the above-mentioned hexacoordi-
˚
19
nated 2:1 complex by Jensen (2.38 A), a complex with a
˚
29
˚
chloride-substituted NHC ligand by Grubbs (2.300(4) A),
one with a RudCRR ligand by Lehmann (2.301(3) A),20 a
0
˚
complex with a nitro substituent on the carbene by Buch-
34
meiser (2.3103(16) A), and a catalyst with a backbone-
˚
35
substituted NHC ligand by Grubbs (2.3068(14) A). The
˚
shortest distances of such chelate rings are reported by Grela
for a dormant ruthenium catalyst bearing a chelating car-
30,31
˚
boxylate ligand (2.207(2) A)
and for one pentacoordi-
nated structure, where the oxygen of the benzylidene carbene
structures show bond distances between those extre-
mes.11,20,29,32-56 Additionally the chelate ring in 10 is easily
opened by tricyclohexylphosphine, since during formation
of this catalyst by cross-metathesis with styrene 12 in the
absence of CuCl intermediate 13 is almost exclusively ob-
served (Scheme 2; see NMR spectrum in Figure S17), in
contrast to catalyst 4, which can be opened to a greater extent
only by an excess of PCy3 (Figure S6). As our NMR kinetics
with cyclooctene confirm, initiation with 10 is faster than
with 4 (see Figure 7). Initiation via phosphine loss in 1 is,
however, still more favorable. Other, also faster initiating
chelating carbene units with further substituents on the
aromatic carbene ring in 4 have been reported by Hoveyda,38
Blechert,57,58 and Grela.59,60 Further direct evidence for this
effect is seen by comparison of catalyst 7d versus 11d in
copolymerization experiments. The latter one is about 30
times faster in yielding the copolymer quantitatively.
coordinates cis to the NHC ligand (2.205(6) A).36 All other
˚
(34) Yang, L. R.; Mayr, M.; Wurst, K.; Buchmeiser, M. R. Chem.—
Eur. J. 2004, 10 (22), 5761–5770.
(35) Chung, C. K.; Grubbs, R. H. Org. Lett. 2008, 10 (13), 2693–2696.
(36) Barbasiewicz, M.; Bieniek, M.; Michrowska, A.; Szadkowska,
A.; Makal, A.;Wozniak, K.;Grela, K.Adv. Synth. Catal. 2007, 349 (1-2),
193–203.
(37) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H.
J. Am. Chem. Soc. 2000, 122 (34), 8168–8179.
(38) Van Veldhuizen, J. J.; Gillingham, D. G.; Garber, S. B.; Kataoka,
O.; Hoveyda, A. H. J. Am. Chem. Soc. 2003, 125 (41), 12502–12508.
(39) Vougioukalakis, G. C.; Grubbs, R. H. Organometallics 2007,
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2750 Organometallics, Vol. 29, No. 12, 2010
Torker et al.
the similar behavior of the two ligands 2a and 2b in catalyst
formation. Both give stable 2:1-complexes with catalyst 4.
The larger adamantyl group, which is pointing “up” (away
from the carbene), has an additional effect only if the poly-
mer chain gets larger, where it can impose steric repulsion
better than the tert-butyl group. Calculations using BP86/
TZP therefore show similar energetics for the two ligands,
but they were run with a small carbene unit (Chart 4, Table 3).
The neopentyl substituent was at first thought to be less
selective since it can be oriented in such a way to avoid steric
interaction with the carbene, but obviously other factors play a
similar important role and increase selectivity. Calculations of
all three possible rotamers for both carbene states predict an
energy difference of only 1.7 kcal/mol between the most stable
conformers (Chart 4, Chart 5, Table 3). Two rotamers for one
state seem to be too high in energy (U2-1 and U2-2), whereas
for the other state this is the case only for U1-3. Hence, entropic
contributions may play a role but would only add about 0.4
kcal/mol to the overall ΔG (ΔG=ΔH - TΔS; ΔS=R ln(W);
W=2/1; T=298 K). For the 1,1,2,2-tetramethylpropyl sub-
stituent (TMP) calculations predict an energy difference of
4.0 kcal/mol between the most stable conformers of the two
diastereomeric 14-electron carbene states (V1-1 and V2-3).
Accordingly the TMP substituent should give higher selec-
tivity than the tert-butyl and adamantyl substituent, for
which only a difference of 3.4 kcal/mol between the two
states is predicted. Additionally entropy may contribute to
the overall steric energy. The temperature and concentration
dependence with catalysts 11a-d shows that the steric price
for the energetically disfavored step (A . D) can still be paid
to some extent by cyclooctene, at least when polymerizations
are carried out at room temperature (Figures 3 and 4).
Wiberg has given a value for the ring strain in cyclooctene
being equal to 4.2 kcal/mol,61 which is in reasonable relation-
ship to our computed number for the energy difference
between the two carbene states (3.4 kcal/mol). For the very
bulky TMP substituent it seems that the less strained
cyclooctene cleanly reacts efficiently fast with only one
diastereomeric form of the carbene, whereas norbornene
shows the same high activity toward both. The computed
number of 4.0 kcal/mol between the two carbene states seems
to present a lower bound, and all possible rotamers and the
actual transition states need to be considered. A polymer
produced from a 1:5 mixture of norbornene and cyclooctene
contains about 10% polynorbornene units, as expected for
this high selectivity that seems to be controlled only by
strain61 release of monomers (purely diastereomeric site
control; see below). The TMP substituent obviously provides
enough steric energy that cyclooctene can compete equally
with norbornene in the energetically favored step of the
catalytic cycle (D . A in Scheme 4).
mechanism for alternating copolymerization in our ROMP
systems. The experimental embodiment of enantiomorphic
site control can be seen in chiral ethylene-bridged ansa-
metallocene catalysts at room temperature and also in an
unbridged metallocene catalyst with unsubstituted Cp rings
(Cp2TiPh2/MAO) below -40 °C, both of which form highly
isotactic polypropylene. The catalyst itself controls the ster-
eochemistry. The alternative mechanism, growing chain-end
control, relies on the preference for one or the other prochiral
face of an inserting propylene unit being governed by the
stereochemical result of the immediately preceding insertion.
Blechert’s proposed mechanism for alternating copolymeri-
zation would resemble this kind of control (see below,
example b). Our catalysts operate unambiguously by the
mechanism analogous to enantiomorphic site control, which
we should properly designate as diastereomeric site control,
because the chelating bidentate ligand prevents any rotation
of the phosphine bearing the substituents that control selec-
tivity. Logically, the two stereogenic centers, at P and at Ru,
necessitate four stereoisomers, which appear as two enantio-
meric pairs of diastereomers. The diastereomers have neither
the same energy nor the same reactivity, which is the under-
lying basis for the chemoselectivity in the present work. For
the growing chain-end control, several proposals can be
found in the literature:
(a) Hoveyda et al. published stereogenic-at-metal ruthe-
nium complexes bearing enantiomerically pure bidentate
NHC/binaphthol ligands that effectively catalyze ROCM
of norbornene derivatives with two equivalents of
styrene.38,63 He also claimed that chiral Mo catalysts would
readily give polymers under these conditions.43 One might
therefore hypothesize other effects than pure ring strain
that could account for this high chemoselectivity. Since this
catalyst is stereogenic at Ru, one could expect, on the basis
of our mechanistic picture, two different diastereomeric
carbenes taking part in the catalytic cycle. This could lead to
the following conclusion: If the highly strained norbornene
would react faster than styrene with both sides, a ROMP
polymer would be the expected result. The presence of only
two equivalents of styrene, however, could be a strong
indication that the release of ring strain is needed only to
effect a change of the carbene’s position to the energetically
disfavored side. Once there, the norbornene unit on the
carbene would provide too much steric crowding so that
only the smaller styrene can react (diastereomeric site
control with a strong contribution of “chain-end” or sub-
strate control).
(b) Blechert and Buchmeiser use unsymmetrical, monoden-
tate, chiral, and achiral NHC ligands for the alternating
copolymerization of norbornene and cyclooctene.14,15 The
high tendency for alternation was explained by an enhanced
cyclooctene insertion rate into a norbornene-initiator-derived
terminus and, vice versa, an enhanced norbornene insertion
rate into a cyclooctene-initiator-derived terminus. In other
words they speculated that the norbornene-initiator-derived
terminus makes the catalyst more crowded, providing steric
energy for a faster cyclooctene insertion, but too bulky, with
the effect that it slows down consecutive norbornene incor-
poration. The chain-end control would arise from a relatively
Diastereomeric Site Control versus Chain-End Control. If
one compares our system to literature precedents, one finds
an analogous situation in stereoselective Ziegler-Natta
polymerization of propylene.62 Two possible mechanisms
for stereoselective polypropylene formation were identified:
enantiomorphic site control and growing chain-end control.
The former mechanism relies on the alternate switching
of the catalyst between two enantiomorphic states with
each productive turnover, very much in analogy with the
(61) Wiberg, K. B. Angew. Chem., Int. Ed. Engl. 1986, 25 (4), 312–322.
(63) Hoveyda, A. H.; Gillingham, D. G.; Van Veldhuizen, J. J.;
Kataoka, O.; Garber, S. B.; Kingsbury, J. S.; Harrity, J. P. A. Org.
Biomol. Chem. 2004, 2 (1), 8–23.
€
(62) Brintzinger, H. H.; Fischer, D.; Mulhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143–1170.

Article
Organometallics, Vol. 29, No. 12, 2010 2751
fast rotation of the NHC ligand, which would not allow for
distinction of the two otherwise diastereomeric sites.
(c) A similar effect may be seen in the alternating co-
polymerization of cyclopentene and norbornene using RuCl3 in
the presence of phenol as a cocatalyst, for which it has been
proposed that the catalyst is made more crowded by formation
of hydrogen-bonded solvent cages.16,17 Due to the great steric
bulk around the complex, the more reactive but bulkier
norbornene cannot perform two consecutive metathesis steps.
The alternation would arise purely from the appropriate
combination of the two monomers (chain-end or substrate
control).
Conclusion
We have shown that careful, systematic tuning of the
sterics of ligands of a ruthenium metathesis catalyst bearing
a bidentate phosphine/phenolate ligand resulted, step by
step, in more selective variants. We were able to demonstrate
by a four-point series that chemoselectivity is directly related
to the size difference between the substituents on the phos-
phine ligand. A strong indication is provided by the fact that
the ring strain in cyclooctene lies in the same range as the
computed energy differences between the two diastereomeric
carbene states. Calculations predict some activity toward
cyclooctene homopolymerization for catalysts 11a,b and
complete shut-down of this channel for catalyst 11d, which
is in perfect agreement with our experiments. Further investi-
gations are underway that focus on the different control
mechanism between our system and the one reported by
Blechert and Buchmeiser, who use second-generation cata-
lysts with a rotating NHC ligand.
To return to the precedent for a mechanistic duality, a
clean, experimentally accessible distinction between these
two cases can be made by assignment of pentad signals in the
13C NMR spectrum. A single defect in the polymer chain
would give the following microstructures: LLLLLRRRRR
for chain-end control and LLLLLRLLLL for enantio-
morphic site control (L and R stand for propylene units
where the methyl group is left or right in a Fischer pro-
jection). In the latter one the catalyst center of fixed chirality
overrules the error (R) and forces subsequent olefin inser-
tions to return to the previous preference (L). Taken over to
the ruthenium ROMP systems a single defect would result in
the following structure: NCNCNNCNCN for chain-end
control and NCNCNNNCNCN for diastereomeric site
control (N and C stand for norbornene and cyclooctene).
We have examined the 13C spectra carefully to evaluate the
possibility that the consequences of sequence errors could
provide a tool to distinguish between the two mechanisms,
but the spectra are unfortunately less informative than in
the corresponding case of stereoregular polypropylene (see
Figures S26-S29). Nevertheless, we are designing further
mechanistic tests that should help determine which mechan-
ism is operative in chemoselective, alternating ROMP for
catalysts where rotation is, in principle, possible. These
experiments are ongoing.
Acknowledgment. We thank Dr. Bernd Schweizer and
Dr. Michael Solar from the X-ray crystallographic service
of the ETH and Dr. Philip Zumbrunnen and Dr. Rainer
Frankensteiner from the NMR service of the Labora-
€
torium fur Organische Chemie (ETH) for performing the
corresponding experiments.
Supporting Information Available: 1H NMR spectra of poly-
mers (Figures S1-S3), NMR spectra showing the ligand
exchange reaction of 2a with 5 (Figures S4, S5), NMR spectra
for observation of intermediates 6 and 9 and complex 7a
(Figures S6-S8), NMR spectra of several 1:1- and 2:1-com-
plexes (Figures S9-S16 and S18-S24), NMR observation of
intermediate 13 (Figure S17), HSQC experiment for complex
22a (Figure S25), peak assignment in 13C NMR spectra of
homo- and copolymers (Figures S26-S29), and coordinates of
calculated structures. This material is available free of charge via
the Internet at http://pubs.acs.org.