Chain-selective and regioselective ethylene and styrene dimerization reactions catalyzed by a well-defined cationic ruthenium hydride complex: New insights on the styrene dimerization mechanism

Article abstract of DOI:10.1021/om100468q

The cationic ruthenium hydride complex [(η⁶-C ₆H₆)(PCy₃)(CO)RuH]⁺BF ₄^- was found to be a highly regioselective catalyst for the ethylene dimerization reaction to give 2-butene products (TOF = 1910 h ^-1, >95% selectivity for 2-butenes). The dimerization of styrene exclusively produced the head-to-tail dimer (E)-PhCH(CH₃)CH=CHPh at an initial turnover rate of 2300 h^-1. A rapid and extensive H/D exchange between the vinyl hydrogens of styrene-d₈ and 4-methoxystyrene was observed within 10 min without forming the dimer products at room temperature. The inverse deuterium isotope effect of k _H/k_D = 0.77 ± 0.10 was measured from the first-order plots on the dimerization reaction of styrene and styrene-d ₈ in chlorobenzene at 70 °C. The pronounced carbon isotope effect on both vinyl carbons of styrene as measured by using Singletons method ( ¹³C(recovered)/¹³C(virgin) at C₁ = 1.096 and C₂ = 1.042) indicates that the C-C bond formation is the rate-limiting step for the dimerization reaction. The Eyring plot of the dimerization of styrene in the temperature range of 50-90 °C led to ΔH^? = 3.3(6) kcal/mol and ΔS^? = -35.5(7) eu. An electrophilic addition mechanism has been proposed for the dimerization of styrene.

Full text of DOI:10.1021/om100468q

Organometallics 2010, 29, 3413–3417 3413
DOI: 10.1021/om100468q
Chain-Selective and Regioselective Ethylene and Styrene Dimerization
Reactions Catalyzed by a Well-Defined Cationic Ruthenium Hydride
Complex: New Insights on the Styrene Dimerization Mechanism
Do W. Lee and Chae S. Yi*
Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201-1881
Received May 14, 2010
-
The cationic ruthenium hydride complex [(η6-C6H6)(PCy3)(CO)RuH]þBF4 was found to be a
highly regioselective catalyst for the ethylene dimerization reaction to give 2-butene products (TOF =
1910 h-1, >95% selectivity for 2-butenes). The dimerization of styrene exclusively produced the
head-to-tail dimer (E)-PhCH(CH3)CHdCHPh at an initial turnover rate of 2300 h-1. A rapid and
extensive H/D exchange between the vinyl hydrogens of styrene-d8 and 4-methoxystyrene was
observed within 10 min without forming the dimer products at room temperature. The inverse
deuterium isotope effect of kH/kD = 0.77 ( 0.10 was measured from the first-order plots on the
dimerization reaction of styrene and styrene-d8 in chlorobenzene at 70 °C. The pronounced carbon
isotope effect on both vinyl carbons of styrene as measured by using Singleton’s method
(13C(recovered)/13C(virgin) at C1 = 1.096 and C2 = 1.042) indicates that the C-C bond formation
is the rate-limiting step for the dimerization reaction. The Eyring plot of the dimerization of styrene in
the temperature range of 50-90 °C led to ΔHq = 3.3(6) kcal/mol and ΔSq = -35.5(7) eu. An
electrophilic addition mechanism has been proposed for the dimerization of styrene.
Introduction
an electrophilic Ir-vinyl catalyst, on the basis of both experi-
mental and computational analyses.4 Since the ethylene oli-
gomerization reaction typically produces a range of
olefin products (C4-C26) which require tedious and energy-
consuming separation steps, one of the enduring challenges is
to design effective catalytic processes that would result in
chain-selective and regioselective olefin products. The chain-
selective ethylene trimerization reaction by soluble Cr and Ti
catalysts has been a notable advance in this context.5
In recent years, considerable research has been focused on
utilizing electrophilic late-transition-metal catalysts to
achieve selective dimerization and oligomerization of
olefins.1d,6 Brookhart and co-workers investigated detailed
kinetic and mechanistic aspects of the ethylene dimerization
by using well-defined cationic Pd and Pt catalysts, where the
energy of activation for the turnover-limiting olefin insertion
Transition-metal-catalyzed olefin dimerization and oligo-
merization reactions constitute one of the most important
industrial processes for forming R-olefins.1-3 Ziegler-type Ti
and Ni catalysts have been successfully utilized for the com-
mercial processes of ethylene dimerization (IFP Alphabutol
process) and R-olefins (Dimersol process), respectively.2 Ex-
tensive research efforts have led to two distinctively different
dimerization reaction mechanisms: the Cosse-Arlman mecha-
nism of sequential alkene insertion for Ziegler catalysts1d,2b
and the oxidative coupling mechanism via the formation of
metallacycles for Ti and Ta catalysts.3 More recently, Periana
and co-workers proposed a novel ethylene dimerization
mechanism via vinyl C-H activation, which is mediated by
*To whom correspondence should be addressed. E-mail: chae.yi@
marquette.edu.
(5) Recent selected examples with Cr catalysts: (a) Dixon, J. T.;
Green, M. J.; Hess, F. M.; Morgan, D. H. J. Organomet. Chem. 2004,
689, 3641–3668. (b) McGuinness, D. S.; Wasserscheid, P.; Keim, W.;
Morgan, D.; Dixon, J. T.; Bollmann, A.; Maumela, H.; Hess, F.; Englert,
U. J. Am. Chem. Soc. 2003, 125, 5272–5273. (c) Blann, K.; Bollmann, A.;
Dixon, J. T.; Hess, F. M.; Killian, E.; Maumela, H.; Morgan, D. H.; Neveling,
A.; Otto, S.; Overett, M. J. Chem. Commun. 2005, 620–621. (d) Agapie, T.;
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(e) Arteaga-M€uller, R.; Tsurugi, H.; Saito, T.; Yanagawa, M.; Oda, S.;
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Ed. 2001, 40, 2516–2519. (g) Otten, E.; Batinas, A. A.; Meetsma, A.; Hessen,
B. J. Am. Chem. Soc. 2009, 131, 5298–5312. (h) Suzuki, Y.; Kinoshita, S.;
Shibahara, A.; Ishii, S.; Kawamura, K.; Inoue, Y.; Fujita, T. Organometallics
2010, 29, 2394–2396.
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(3) (a) Keim, W.; Behr, A.; Roper, M. In Comprehensive Organome-
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Press: New York, 1982; Vol. 12. (b) Datta, S.; Fischer, M. B.; Wreford,
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r
2010 American Chemical Society
Published on Web 07/08/2010
pubs.acs.org/Organometallics

3414 Organometallics, Vol. 29, No. 15, 2010
Lee and Yi
step has been estimated to be 19.5 kcal/mol for cationic
(P-N)Pd-alkyl complexes and 29.8 kcal/mol for (diimine)PtII
complexes.7 Jordan also found significant steric and electronic
effects of the bis(pyrazolyl) and diimine ligands in modulating
the activity of cationic (N-N)PdII-alkyl catalysts for the
ethylene dimerization and oligomerization reactions.8 While a
number of selective formations of 1-butene have been recently
reported by using well-defined Ziegler-type catalysts,9 most late-
transition-metal catalysts have been found to produce a mixture
of 1- and 2-butenes.2 In a notable case, Roddick reported the
regioselective formation of 2-butenes from the ethylene dimer-
ization reaction by using Pt and Pd catalysts with perfluorinated
diphosphine ligands in strongly acidic media (340 turnovers/h
with 100 psi of C2H4 at 25 °C).10 Regioselective formation of
2-butenes from the dimerization of ethylene is of considerable
synthetic importance in homogeneous catalysis, since 2-butenes
are a common precursor for both industrially significant SHOP
metathesis11 and Wacker-type olefin oxidation processes.12
Table 1. Catalyst Survey for the Ethylene Dimerization Reactiona
entry
catalyst
additive
2 (E:Z)b TOFc
1
1
82:18
1910
0
2d
3d
4
[RuH(CO)(PCy3)]4(O)(OH)2
[RuH(CO)(PCy3)]4(O)(OH)2
RuHCl(CO)(PCy3)2
RuH2(CO)(PPh3)3
HBF4 OEt2 70:30
HBF4 OEt2
HBF4 OEt2
790
0
3
3
3
3
5
20
0
6
RuCl2(PPh3)3
[(p-cymene)RuCl2]2
HBF4 OEt2
7
HBF4 OEt2
0
3
NH4PF6
8
Ru3(CO)12
[RuH(CO)(PCy3)2(CH3CN)2]þBF4
0
-
9
10 RuCl3 3H2O
0
0
3
11 HBF4 OEt2
0
3
a Reaction conditions: ethylene (0.74 g, 26 mmol), catalyst (5-7 mg,
0.033 mol %), HBF4 OEt2 (1-2 μL, 1.0 equiv), C6H5Cl (1 mL), 50 °C,
3
0.5 h. b Determined by 1H NMR. c TOF = (mol of ethylene consumed)-
(mol of catalyst)-1 h-1
complex.
.
d See ref 14a for the synthesis and structure of the
Thus, the mixture of ethylene (26 mmol, 7 atm) and a Ru
catalyst (0.02-0.03 mol %) in C6H5Cl in a Fisher-Porter
pressure tube was stirred at 50 °C for 30 min (eq 1). The initial
turnover rate of the dimeric and oligomeric products was
determined by the pressure-volume method from a high-
vacuum line with a Hg manometer.15 Among the selected
ruthenium catalysts, complex 1 exhibited uniquely high
activity and selectivity for the formation of 2-butenes (2)
over 1-butene and other oligomers, as analyzed by both
NMR and GC (>95% selectivity, (E)-2/(Z)-2 = 4:1)
(Table 1). Among the initially screened solvents, C6H5Cl
was found to be most suitable for the catalyst 1; CH2Cl2
was also found to be an acceptable solvent (initial TOF
∼1000 h-1), but with a considerably lower selectivity for
2-butenes.
We have recently disclosed that the in situ formed cationic
ruthenium hydride complexes are highly effective catalysts
for the dehydrative coupling reaction of aryl ketones and
1-alkenes, in which the coupling reactions apparently in-
volved olefin isomerization and vinyl C-H activation
steps.13 In an effort to elucidate the coupling reaction
mechanism, we have undertaken a series of investigations
on the alkene dimerization/oligomerization reactions by
using well-defined electrophilic ruthenium hydride catalysts.
This report delineates a highly regioselective and chain-
selective ethylene and styrene dimerization reaction by using
a well-defined cationic ruthenium hydride catalyst, [(η6-C6H6)-
(PCy3)(CO)RuH]þBF4- (1).14
Ethylene Dimerization Reaction. The catalytic activity of 1
for the ethylene dimerization reaction was examined. The
rate of formation of the products was measured by GC in
10 min intervals from the treatment of ethylene (0.74 g,
26.4 mmol) with 1 (5 mg, 8.7 μmol) in C6H5Cl (2 mL) in a
100 mL Fisher-Porter pressure tube at 50 °C. The ini-
tial turnover rate, which was measured to be ca. 2440 h-1
after 10 min, was steadily decreased to 1340 h-1 after 1 h, at
which time ∼45% of ethylene was converted to 2-butenes.
Due to the product inhibition, the reaction rate slowed
further, giving only about 10 000 TON (50% conversion)
after 24 h. The rate of the dimerization reaction was found
to be linearly dependent on the pressure of ethylene gas
(Figure S1, Supporting Information), and the first-order plot
of ln[ethylene] vs time resulted in kobs = 0.76 h-1 (Figure 1).
In an effort to detect possible intermediate species, the
treatment of ethylene (4 mg, 4 equiv) with 1 (20 mg, 35 μmol)
in CD2Cl2 (0.5 mL) was monitored by NMR. The formation
of ethane (δ 0.23 ppm) along with a small amount of 1-butene
Results and Discussion
The catalytic activity of selected ruthenium complexes was
initially surveyed for the ethylene dimerization reaction.
(7) (a) DiRenzo, G. M.; White, P. S.; Brookhart, M. J. Am. Chem.
Soc. 1996, 118, 6225–6234. (b) Doherty, M. D.; Trudeau, S.; White, P. S.;
Morken, J. P.; Brookhart, M. Organometallics 2007, 26, 1261–1269. (c)
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(8) (a) Burns, C. T.; Jordan, R. F. Organometallics 2007, 26, 6726–
6736. (b) Conley, M. P.; Burns, C. T.; Jordan, R. F. Organometallics 2007,
26, 6750–6759.
€
(9) (a) Ajellal, N.; Kuhn, M. C. A.; Boff, A. D. G.; Horner, M.;
Thomas, C. M.; Carpentier, J.-F.; Casagrande, O. L., Jr. Organometal-
lics 2006, 25, 1213–1216. (b) Zhang, S.; Nomura, K. J. Am. Chem. Soc.
2010, 132, 4960–4965.
(10) (a) White, S.; Bennett, B. L.; Roddick, D. M. Organometallics
1999, 18, 2536–2542. (b) Basu, S.; Arulsamy, N.; Roddick, D. M. Organo-
metallics 2008, 27, 3659–3665.
(11) (a) Ivin, K. J.; Mol, J. C. Olefin Metathesis and Metathesis
Polymerization; Academic Press: London, 1997. (b) Taoufik, M.; Le Roux,
E.; Thivolle-Cazat, J.; Basset, J.-M. Angew. Chem., Int. Ed. 2007, 46, 7202–
7205.
1
was detected by H NMR after 10 min, but no detectable
(12) (a) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd ed.;
Wiley: New York, 1992. (b) Jira, R. In Applied Homogeneous Catalysis with
Organometallic Compounds, 2nd ed.; Cornils, B., Herrmann, W A., Eds.;
Wiley-VCH: Weinheim, Germany, 2002; Vol. 1.
(13) (a) Yi, C. S.; Lee, D. W. Organometallics 2009, 28, 4266–4268. (b)
Yi, C. S.; Lee, D. W. Organometallics 2010, 29, 1883–1885.
(14) (a) Yi, C. S.; Zeczycki, T. N.; Guzei, I. A. Organometallics 2006,
25, 1047–1051. (b) Yi, C. S.; Lee, D. W. Organometallics 2009, 28,
947–949.
amount of 2-butenes was formed at room temperature.
When the temperature was raised to 50 °C, 2-butenes were
formed rapidly within 5 min, without changing the amount
of 1-butene significantly over time (((E)- and (Z)-2):1-butene
= 95:5). Though the formation of free benzene molecules
(15) See the Supporting Information for experimental details.

Article
Organometallics, Vol. 29, No. 15, 2010 3415
tion. First, the formation of the styrene-coordinated com-
-
plex [(η6-C6H5CHdCH2)RuH(CO)(PCy3)]þBF4 (4) was
detected along with ethylbenzene and free benzene molecules
when the treatment of styrene (0.87 mmol) with 1 (17 μmol)
in CD2Cl2 (0.5 mL) was monitored by NMR at 40 °C (eq 3).15
Assuming [styrene] remains constant during the exchange
reaction, the equilibrium constant (Keq) for the reaction was
estimated to be 0.32 at 20 °C from the 31P NMR analysis.
Several attempts to isolate the complex 4 in pure form were
not successful.
Figure 1. Plot of ln [ethylene] vs time at 50 °C.
was observed (the ratio of free benzene to coordinated benzene
was 35:65 after 10 min), we were not able to detect/identify any
new ruthenium species under these conditions.
To examine the H/D exchange rate on the alkene sub-
strate, a 1:1 mixture of styrene-d8 and 4-methoxystyrene
(1.4 mmol) in the presence of 1 (3 mg) in CH2Cl2 (0.5 mL)
was monitored by NMR at room temperature. A rapid
and extensive H/D exchange between the vinyl hydrogens
of styrene-d8 and 4-methoxystyrene was observed within
10 min without formation of the dimer products (eq 4). In
a separate experiment, the partially deuterated complex 1-d
(40 mg, 69 μmol; 64% Ru-D) was treated with 5 equiv of
styrene in CH2Cl2 (0.5 mL) at room temperature (eq 5). A
complete H/D exchange at the vinyl positions of styrene
occurred rapidly within 30 min at room temperature along
with the formation of deuterated ethylbenzene (33% D) as
To gauge the rate of ethylene dimerization vs olefin
isomerization, 1-pentene (1.83 g, 26 mmol) was treated with
1 (0.02 mol %) in C6H5Cl at 50 °C. The initial turnover rate
for the isomerization of 1-pentene to 2-pentenes was found to
exceed 20 000 h-1, which is an order of magnitude higher
than the dimerization rate under comparable conditions.
Also, the catalyst 1 was found to mediate a rapid isomeriza-
tion of 2-butenes to produce the same mixture as that formed
from the ethylene dimerization reaction (((E)- and (Z)-2):
1-butene = 94:6). These results showed that the rate of olefin
isomerization is much higher than that of the dimeriza-
tion reaction and further suggest that the initially formed
1-alkenes would have been rapidly isomerized under the
catalytic reaction conditions.
2
monitored by H NMR. In this case, the formation of the
dimer 3 was observed after ∼1 h. The formation of ethyl-
benzene suggested that the ruthenium hydride complex 1 is
acting as a hydrogenation agent, but we have not been able to
detect/identify any other ruthenium species under the reac-
tion conditions.
Styrene Dimerization Reaction. Among the initially
screened R-olefins, the catalyst 1 was found to be particularly
effective for regioselective and chain-selective dimerization
of styrene and vinylarenes. Thus, the treatment of styrene
(45 g, 9.4 mmol) with 1 (5 mg, styrene:1 = 50 000:1) in C6H5Cl
(1 mL) at 70 °C led to the head-to-tail dimer product (E)-3
exclusively at an initial TOF of 2300 h-1 (eq 2). No other oligo-
meric or polymeric products were detected by GC. Further-
more, the catalyst 1 was found to be active for a prolonged
reaction time, giving a TON of ∼40 000 in 48 h. Again, the
turnover rate was found to steadily decrease as the concentra-
tion of the dimer product 3 increased. Though regioselective
dimerization of styrene and related vinylarenes has been
achieved by using electrophilic palladium16 and ruthenium17
catalysts, the mechanism of the dimerization reaction has not
been clearly established. To the best of our knowledge, the
activity and selectivity of 1are uniquely high among non-Ziegler
types of late-transition-metal catalysts for the styrene dimeriza-
tion reaction.
The deuterium isotope effect of the dimerization reaction
was measured under relatively dilute conditions. Thus, the
rate of disappearance of styrene was analyzed by GC peri-
odically from the treatment of styrene (0.54 g, 5.2 mmol) with
1 (3 mg, 0.1 mol %) in C6H5Cl (2 mL) at 70 °C. The first-
order plots of the dimerization reaction rate vs time for both
Kinetics and Mechanistic Study for the Styrene Dimeriza-
tion Reaction. We performed the following experiments to
gain mechanistic insights into the styrene dimerization reac-
styrene and styrene-d8 led to kobs = 0.37 and 0.48 h-1
,
respectively, which translated to an inverse isotope effect of
kH/kD = 0.77 ( 0.10 (Figure 2). No significant temperature
effect on the isotope effect was observed in the range of
50-90 °C, giving the nearly equal kH/kD = 0.67 ( 0.10 at
50 °C and kH/kD = 0.68 ( 0.10 at 90 °C. In transition-metal-
mediated C-H activation reactions, inverse deuterium
isotope effects have been commonly observed for the step-
wise reactions involving rapid pre-equilibrium followed by a
(16) (a) Jiang, Z.; Sen, A. J. Am. Chem. Soc. 1990, 112, 9655–9657. (b)
Jiang, Z.; Sen, A. Organometallics 1993, 12, 1406–1415. (c) Kretschmer,
W. P.; Troyanov, S. I.; Meetsma, A.; Hessen, B.; Teuben, J. H. Organome-
tallics 1998, 17, 284–286. (d) Tsuchimoto, T.; Kamiyama, S.; Negoro, R.;
Shirakawa, E.; Kawakami, Y. Chem. Commun. 2003, 852–853. (e) Yamada,
S.; Obora, Y.; Sakaguchi, S.; Ishii, Y. Bull. Chem. Soc. Jpn. 2007, 80, 1194–1198.
(17) (a) Kondo, T.; Takagi, D.; Tsujita, H.; Ura, Y.; Wada, K.;
Mitsudo, T. Angew. Chem., Int. Ed. 2007, 46, 5958–5961. (b) Higashimura,
M.; Imamura, K.; Yokogawa, Y.; Sakakibara, T. Chem. Lett. 2004, 33,
728–729.

3416 Organometallics, Vol. 29, No. 15, 2010
Lee and Yi
Scheme 1. Proposed Mechanism of the Styrene Dimerization
Reaction
Figure 2. First-order plots of the dimerization of styrene (9)
and styrene-d8 (b) at 70 °C.
Scheme 2
plot of -ln{[styrene]t/[styrene]0} vs time in the treatment of
styrene (0.54 g, 5.2 mmol) with 1 (3 mg, 5.2 μmol) in C6H5Cl
(2 mL) for the temperature range of 50-90 °C. The Eyring
plot of -ln(kobs/T) vs 1/T led to ΔHq = 3.3(6) kcal/mol and
ΔSq = -35.5(7) eu. A relatively large negative ΔSq value is
consistent with an organized transition state formed from
combining two styrene molecules.
Proposed Mechanism of the Styrene Dimerization Reac-
tion. These results provide support for an electrophilic addi-
tion mechanism for the styrene dimerization reaction
(Scheme 1). We propose that the benzylic carbocation ruthe-
nium arene species 5 is generated from the initial arene
exchange reaction and the hydride migration to the coordi-
nated styrene. Both rapid H/D arene exchange results and
the observation of styrene complex 4 suggest that the initial
hydride migration and elimination steps are facile and
reversible. The electrophilic addition of the second styrene
molecule would form the chain-extended carbocation species
6. The inverse deuterium isotope effect and the carbon
isotope effect studies imply that the C-C bond formation
is the rate-limiting step for the dimerization reaction. The
subsequent deprotonation and arene exchange steps should
result in the dimer product 3 and the regeneration of the
Ru-H species 4.
Figure 3. Eyring plot for the styrene dimerization.
rate-limiting step.18 In our case, the observation of an inverse
isotope effect on the dimerization reaction is consistent with
rapid and reversible styrene coordination and hydride inser-
tion steps followed by a rate-limiting C-C bond formation
step.19
To further discern the rate-limiting step of the dimeriza-
tion reaction, we next examined the carbon isotope effect for
the styrene dimerization by employing Singleton’s isotope
measurement technique at natural abundance.20 The treat-
ment of styrene (4.5 g, 44 mmol) with 1 (10 mg, 17 μmol) in
C6H5Cl (1 mL) at 70 °C was stopped after 5-15 h at 71-88%
conversion (eq 6). A pronounced carbon isotope effect was
observed on both vinyl carbons when the 13C ratio of
recovered styrene was compared to that of the virgin sample
(13C(recovered)/13C(virgin) at C1 = 1.096 and C2 = 1.042,
average of three runs) (Table S1, Supporting Information).
The observation of a significant carbon isotope effect on both
vinyl carbons clearly indicates that the C-C bond formation
is the rate-limiting step for the dimerization reaction.
The proposed mechanism can readily explain the exclusive
formation of (E)-3 product resulting from the deprotonation
step of the benzylic carbocation species 6 (Scheme 2). From a
purely steric point of view, the formation of the “anti” benzyl
carbocation species anti-6 would be favored over the “syn”
isomer syn-6, and the deprotonation of a methylene hydro-
gen from anti-6 should lead to the trans product (E)-3
preferentially. Similar electrophilic addition mechanisms
have been commonly proposed for the dimerization/oligo-
merization reactions mediated by electrophilic metal cata-
lysts, but the reactions typically have been found to give a
E/Z mixture of the dimeric products.16
An Eyring plot was constructed to determine the thermo-
dynamic parameters for the styrene dimerization reaction
(Figure 3). The kobs value was determined from a first-order
(18) (a) Jones, W. D. Acc. Chem. Res. 2003, 36, 140–146. (b) Parkin, G.
Acc. Chem. Res. 2009, 42, 315–325.
(19) We previously observed a similar inverse deuterium isotope
effect in ruthenium-catalyzed hydrogenation of olefins: Yi, C. S.; Lee,
D. W. Organometallics 1999, 18, 5152–5156.
(20) (a) Singleton, D. A.; Thomas, A. A. J. Am. Chem. Soc. 1995, 117,
9357–9358. (b) Frantz, D. E.; Singleton, D. A.; Snyder, J. P. J. Am. Chem.
Soc. 1997, 119, 3383–3384.
The Cosse-Arlman type of insertion mechanism is not
likely for the styrene dimerization reaction, because the
mechanism would require a regioselective insertion of the

Article
Organometallics, Vol. 29, No. 15, 2010 3417
second styrene molecule to form a sterically hindered sec-
ondary alkyl species and the subsequent β-H elimination
would result in an E/Z mixture of the dimeric products.1,2
Previously, Periana and Goddard proposed a novel dimer-
ization mechanism invoking a vinyl C-H activation on the
basis of synthetic and computational studies of the ethylene
dimerization reaction mediated by electrophilic Ir cata-
lysts.4 However, in their case, the reaction was performed
at a relatively high temperature (150 °C), and the forma-
tion of Ru-vinyl species under our reaction conditions
(50-70 °C) is less likely, though it cannot be rigorously
excluded at this time. We also observed that the rate of olefin
isomerization is much higher than the rate of ethylene
dimerization, and this result provides support for the Cosse-
Arlman type of insertion and isomerization mechanism for
the ethylene dimerization reaction.2 It is certainly possible
that the cationic ruthenium hydride catalyst 1 could assist
both vinyl C-H activation and Cosse-Arlman insertion/
olefin isomerization mechanistic pathways under certain
conditions, and we are currently looking into this mechan-
istic possibility for the ethylene dimerization reaction.
immediately cooled in a dry ice/acetone bath. After it was
filtered through a small silica gel column (2:1 hexanes/EtOAc),
the solution was analyzed by GC to measure the disappearance
of the styrene. The kobs value was determined from a first-order
plot of -ln{[styrene]t/[styrene]0} vs time (Figures S2 and S3,
Supporting Information).
Carbon Isotope Effect Study. In a glovebox, complex 1 (10 mg,
17.4 μmol) and styrene (4.52 g, 43.5 mmol) were dissolved in
chlorobenzene (1 mL) in three separate 25 mL Schlenk tubes
equipped with a Teflon screw cap stopcock and a magnetic
stirring bar. The tubes were brought out of the box, and the
contents were stirred for 5, 10, and 15 h, respectively, in an oil
bath which was preset at 70 °C. After it was filtered through a
small silica gel column (2:1 hexanes/EtOAc), the solution was
analyzed by GC (71, 80, and 88% conversion). Unreacted
styrene was collected separately via vacuum transfer for the
13C{1H} NMR analysis.
The 13C{1H} NMR analysis of the recovered and virgin
samples of styrene was performed by following Singleton’s 13
C
NMR method.20 The NMR sample of virgin and recovered
styrene was prepared identically by dissolving styrene (100 mg)
in CDCl3 (0.5 mL) in a 5 mm high-precision NMR tube. The
13C{1H} NMR spectra were recorded with H-decoupling and
45° pulses. A 60 s delay between pulses was imposed to minimize
T1 variations (d1 = 60 s at 5.0 s, np = 245 098, nt = 704). The
data are summarized in Table S1 in the Supporting Information.
Conclusions
Formation of [(η6-C6H5CHdCH2)RuH(CO)(PCy3)]þBF4
-
A highly efficient and regioselective dimerization of ethy-
lene and styrene has been achieved by using the well-defined
cationic ruthenium hydride catalyst 1. Kinetic and mecha-
nistic studies for the styrene dimerization reaction provide
support for an electrophilic addition mechanism involving
the formation of benzylic carbocation species and the rate-
limiting C-C bond formation step. Efforts to establish a
detailed mechanism for the ethylene dimerization reaction as
well as to extend the synthetic utility of the dimerization
reaction are currently being pursued.
(4). In a glovebox, complex 1 (20 mg, 34.8 μmol) and styrene
(18 mg, 5 equiv) in a J. Young NMR tube equipped with a screw
cap were dissolved in CD2Cl2 (0.5 mL). The NMR tube was
1
brought out of the box and was analyzed by both H and 31P
NMR in the temperature range of 0-50 °C. Two sets of isomers
(3:1) for 4 as well as a ∼1:1 ratio of free benzene and ethylben-
zene were detected by 1H and 31P NMR.
Selected spectroscopic data are as follows. Major isomer of 4:
1H NMR (400 MHz, CD2Cl2) δ 6.75 (m, CHd), 6.6-6.4 (m,
Ar), 6.04 (d, J = 17.5 Hz, dCH2), 5.65 (d, J = 10.9 Hz, dCH2),
-10.75 (d, JPH = 26.5 Hz, Ru-H); 31P{1H} NMR (162 MHz,
1
CD2Cl2) δ 73.7 (s, PCy3). Minor isomer of 4: H NMR (400
Experimental Section
MHz, CD2Cl2) δ -10.66 (d, JPH = 26.4 Hz, Ru-H); 31P{1H}
NMR (162 MHz, CD2Cl2) δ 73.1 (s, PCy3).
Representative Procedure of the Ethylene Dimerization Reac-
tion. In a glovebox, complex 1 (5 mg, 8.7 μmol) was dissolved in
chlorobenzene (2 mL) in a 100 mL Fisher-Porter pressure tube
equipped with a magnetic stirring bar. The tube was brought out
of the box and was degassed three times by freeze-pump-thaw
cycles. Ethylene gas (0.74 g, 26.4 mmol) was condensed into the
tube via a vacuum line. The tube was slowly warmed to room
temperature, and the contents were stirred for 30 min in an oil bath
which was preset at 50 °C. After the tube was cooled in a dry ice/
ethylene glycol bath (-25 °C) for 10 min, the reaction tube
stopcock was slowly opened to a vacuum line connected to a Hg
manometer. The turnover number was determined by measuring
the difference of the vapor pressure exerted by the ethylene gas
at -25 °C. After evaporation of unreacted ethylene at -25 °C
under high vacuum, a ∼4:1 mixture of 2-butene products was
obtained (>95% pure as analyzed by NMR and GC).
General Procedure for Rate Measurement and Eyring Plot.
In a glovebox, complex 1 (3 mg, 5.2 μmol) and styrene (0.54 g,
5.2 mmol) were dissolved in chlorobenzene (2 mL). The solution
was divided into equal amounts (∼0.45 g) and placed in six
different 25 mL Schlenk tubes, each equipped with a Teflon
stopcock and a magnetic stirring bar. The tubes were brought
out of the box and were stirred in an oil bath preset at 50-90 °C.
Each reaction tube was taken out of the oil bath in 10 min
intervals and was immediately cooled in a dry ice/acetone bath.
After the mixture was passed through a small silica gel column
(2:1 hexanes/EtOAc), the disappearance of the styrene was
analyzed by GC. The kobs value was determined from a first-
order plot of -ln{[styrene]t/[styrene]0} vs time (Figure S4,
Supporting Information).
Deuterium Isotope Effect Study. In a glovebox, complex 1
(3 mg, 5.2 μmol) and styrene (0.54 g, 5.2 mmol) or styrene-d8
(0.58 g, 5.2 mmol) were dissolved in chlorobenzene (2 mL). The
solution was divided into equal amounts (∼0.45 g), and each
solution was placed into six different 25 mL Schlenk tubes
equipped with a Teflon stopcock and a magnetic stirring bar.
The tubes were brought out of the box, and the contents were
stirred in an oil bath (preset at 50, 70, and 90 °C). Each reaction
tube was taken out of the oil bath in 10 min intervals and was
Acknowledgment. Financial support from the National
Institutes of Health, General Medical Sciences (Grant No.
R15 GM55987), is gratefully acknowledged.
Supporting Information Available: Text, figures, and tables
giving experimental procedures and spectroscopic data of organic
products. This material is available free of charge via the Internet at
http://pubs.acs.org.