Homogeneous catalysis with methane. A strategy for the hydromethylation of olefins based on the nondegenerate exchange of alkyl groups and σ-bond metathesis at scandium

Article abstract of DOI:10.1021/ja021341a

The scandium alkyl Cp*₂ScCH₂CMe₃ (2) was synthesized by the addition of a pentane solution of LiCH₂CMe₃ to Cp*₂ScCl at low temperature. Compound 2 reacts with the C-H bonds of hydrocarbons including methane, benzene, and cyclopropane to yield the corresponding hydrocarbyl complex and CMe₄. Kinetic studies revealed that the metalation of methane proceeds exclusively via a second-order pathway described by the rate law: rate = k[2][CH₄] (k = 4.1(3) × 10^-4 M^-1s^-1 at 26 °C). The primary inter- and intramolecular kinetic isotope effects (k_H/k_D = 10.2 (CH₄ vs CD₄) and k_H/k_D = 5.2(1) (CH₂D₂), respectively) are consistent with a linear transfer of hydrogen from methane to the neopentyl ligand in the transition state. Activation parameters indicate that the transformation involves a highly ordered transition state (ΔS? = -36(1) eu) and a modest enthalpic barrier (ΔH? = 11.4(1) kcal/mol). High selectivity toward methane activation suggested the participation of this chemistry in a catalytic hydromethylation, which was observed in the slow, Cp*₂ScMe-catalyzed addition of methane across the double bond of propene to form isobutane.

Full text of DOI:10.1021/ja021341a

Published on Web 06/04/2003
Homogeneous Catalysis with Methane. A Strategy for the
Hydromethylation of Olefins Based on the Nondegenerate
Exchange of Alkyl Groups and σ-Bond Metathesis at
Scandium
Aaron D. Sadow and T. Don Tilley*
Contribution from the Department of Chemistry and Center for New Directions in Organic
Synthesis (CNDOS), UniVersity of California, Berkeley, Berkeley, California 94720-1460
Received November 7, 2002
Abstract: The scandium alkyl Cp*2ScCH2CMe3 (2) was synthesized by the addition of a pentane solution
of LiCH2CMe3 to Cp*2ScCl at low temperature. Compound 2 reacts with the C-H bonds of hydrocarbons
including methane, benzene, and cyclopropane to yield the corresponding hydrocarbyl complex and CMe4.
Kinetic studies revealed that the metalation of methane proceeds exclusively via a second-order pathway
described by the rate law: rate ) k[2][CH4] (k ) 4.1(3) × 10-4 M-1s-1 at 26 °C). The primary inter- and
intramolecular kinetic isotope effects (kH/kD ) 10.2 (CH4 vs CD4) and kH/kD ) 5.2(1) (CH2D2), respectively)
are consistent with a linear transfer of hydrogen from methane to the neopentyl ligand in the transition
state. Activation parameters indicate that the transformation involves a highly ordered transition state (∆S‡
) -36(1) eu) and a modest enthalpic barrier (∆H‡ ) 11.4(1) kcal/mol). High selectivity toward methane
activation suggested the participation of this chemistry in a catalytic hydromethylation, which was observed
in the slow, Cp*2ScMe-catalyzed addition of methane across the double bond of propene to form isobutane.
Introduction
a number of interesting stoichiometric transformations, there
have been significantly fewer reports describing selective
The selective conversion of saturated hydrocarbons to func-
tionalized and more valuable products remains an important goal
in chemical research.1-7 Intense interest in this topic has led to
many important advances including the discovery of several
mechanisms by which transition metal species react with
unactivated C-H bonds.2 Although these studies have revealed
conversions of alkanes via homogeneous catalysis.3-5 Methane
is a particularly attractive substrate for such conversions since
it is cheap and readily available, and represents a potentially
useful reagent for the incorporation of methyl groups into
molecular structures. Research on homogeneous methane con-
version has focused on selective oxidations via activations with
electrophilic late metal complexes in acidic media, or with
reactive metal oxo species.6-8 Alternative strategies involving
non-oxidative mechanisms via electrocyclic transition states (i.e.,
σ-bond metathesis9,10 and 1,2-cycloaddition across metal-ligand
double bonds11) have until recently not been incorporated into
catalytic cycles.12
(1) (a) SelectiVe Hydrocarbon ActiVation; Davies, J. A., Watson, P. L., Liebman,
J. F., Greenberg, A., Eds.; VCH Publishers: New York, 1990. (b) ActiVation
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M. R. Science 2002, 295, 305.
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10.1021/ja021341a CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 7971-7977
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A R T I C L E S
Sadow and Tilley
Si-C bonds via σ-bond metathesis.12-15 This rich reaction
chemistry suggested that similar activation steps might be used
in catalytic hydrocarbon functionalizations, given appropriately
active and selective catalysts. We recently reported an initial
step in this direction with the description of a catalytic methane
dehydrosilylation, which appears to occur via σ-bond meta-
thesis.12 This discovery prompted further reactivity studies on
compounds of the type Cp*2ScR10a and a search for transforma-
tions that might be incorporated into a catalytic cycle.
Useful catalytic processes that might utilize σ-bond metathesis
steps involve the formation and cleavage of C-C bonds (e.g.,
hydrocarbon homologation and hydrocracking, respectively).
This possibility has seemed rather remote given some of the
apparent limitations associated with such steps. For example,
it seems that carbon is disfavored in the â-position of the four-
centered transition state for σ-bond metathesis, which should
prevent the direct formation (and cleavage) of C-C bonds.16,17
However, a potentially useful product-forming step could
involve the nondegenerate exchange of hydrocarbyl groups at
the metal center (eq 1)
Figure 1. ORTEP diagram of Cp*2ScCH2CMe3 (1).
that Cp*2ScMe (1) undergoes the thermoneutral exchange of
its methyl ligand with methane.10a Slow addition of a freshly
prepared pentane solution of LiCH2CMe3 (0.149 M, 1.01
equiv)20 to a pentane solution of Cp*2ScCl at -78 °C in the
dark, followed by extraction and repeated fractional crystal-
lization at -78 °C afforded yellow crystals of Cp*2ScCH2CMe3
(2) in 50% yield. All manipulations were performed in the dark,
because workup under ambient room lighting did not provide
2 and led to formation of deep red solutions and oily
decomposition products. The formation of 2 was quantitative
in benzene-d6 but preparative-scale reactions in benzene yielded
mixtures contaminated with Cp*2ScC6H5 (3)
Cp*2Sc-R + R′-H f Cp*2Sc-R′ + R-H
(1)
Very few reactions of this type have been reported, and the
majority of these form products that exhibit low reactivities
toward further bond activations (e.g., M-Ph, M-CtR, M-OR,
etc).9,10 Nevertheless, the possibility that highly active metal
centers may promote carbon-carbon interactions is suggested
by the work of Basset and co-workers on silica-supported
catalysts,18 and by the fact that alkene polymerization occurs
by an insertion process that passes through a 4-center transition
state with carbon in the â-position.19 Here, we describe a
nondegenerate alkyl exchange reaction involving scandium, and
the apparent participation of this reaction type in a catalytic
C-C bond formation, the hydromethylation of propene.
pentane
Cp*2ScCl + LiCH2CMe3 -78 °C, dark8
Cp*2ScCH2CMe
3 + LiCl (2)
2
Results and Discussion
Compound 2 thermally decomposes at room temperature and
its solutions are sensitive to ambient light; however, it can be
stored in the solid state at -30 °C in the dark for at least three
months. The CH coupling constant of the scandium-bound
methylene group (1JCH ) 108 Hz) and the infrared spectrum
(the absence of bands from 1600 to 2700 cm-1) suggest that
the neopentyl ligand is not R-agostic. For comparison, spec-
troscopic data suggest that Cp*2ScCH3 (1JCH ) 111 Hz) is not
R-agostic, whereas Cp*2ScCH2CH3 is â-agostic.10a However,
the structure of Cp*2Th(CH2CMe3)2 clearly possesses an R-ago-
stic CH group [Th-CR-Câ is 158.2(3)°].10b
The X-ray crystal structure of 2 was determined (Figure 1)
and key crystallographic data are listed in Tables 1 and 2. The
Sc1-C21-C22 angle of 128.3(3)° is consistent with a normal
σ-bond between scandium and the R-carbon. Furthermore,
calculated positions for the R-hydrogens refined to reasonable
locations that are beyond bonding distance to the metal. The
Sc1-C21 bond distance of 2.286(4) Å is slightly longer than
the corresponding distance in the scandium methyl 1 (2.243-
(11) Å).10a The wedge of the bent sandwich is slightly more
open in 2 than in 1 due to the steric demand of the larger -CH2-
CMe3 ligand. Thus, the Cpcent-Sc-Cpcent angle in 2 [138.78-
Synthesis of Cp*2ScCH2CMe3 (2). A search for new
catalytic methane activation chemistry began with attempts to
observe a nondegenerative alkyl-exchange reaction (eq 1). This
reaction seemed likely for the exchange of a sterically hindered
alkyl ligand for a methyl group, given the previous observation
(13) (a) Woo, H.-G.; Walzer, J. F.; Tilley, T. D. J. Am. Chem. Soc. 1992, 114,
7047. (b) Radu, N. S.; Tilley, T. D. J. Am. Chem. Soc. 1995, 117, 5863.
(14) (a) Castillo, I.; Tilley, T. D. J. Am. Chem. Soc. 2001, 123, 10526. (b) Sadow,
A. D.; Tilley, T. D. Organometallics 2001, 20, 4457.
(15) Sadow, A. D.; Tilley, T. D. J. Am. Chem. Soc. 2002, 124, 6814.
(16) Piers, W. E.; Shapiro, P. J.; Bunel, E. E.; Bercaw, J. E. Synlett. 1990, 74.
(17) (a) Steigerwald, M. L.; Goddard, W. A. J. Am. Chem. Soc. 1984, 106,
308. (b) Rappe´, A. K. Organometallics 1990, 9, 466. (c) Rappe´, A. K.;
Upton, T. H. J. Am. Chem. Soc. 1992, 114, 7507. (d) Ziegler, T.; Folga, E.
J. Organomet. Chem. 1994, 478, 57. (e) Maron, L.; Perrin, L.; Eisenstein,
O. J. Chem. Soc., Dalton Trans. 2002, 4, 534.
(18) (a) Vidal, V.; The´olier, A.; Thivolle-Cazat, J.; Basset, J.-M. J. Chem. Soc.,
Chem. Commun. 1995, 991. (b) Vidal, V.; The´olier, A.; Thivolle-Cazat,
J.; Basset, J.-M.; Corker, J. J. Am. Chem. Soc. 1996, 118, 4595. (c) Vidal,
V.; The´olier, A.; Thivolle-Cazat, J.; Basset, J.-M. Science 1997, 276, 99.
(d) Dufaud, V.; Basset, J.-M. Angew. Chem., Int. Ed. Engl. 1998, 37, 806.
(e) Coperet, C.; Maury, O.; Thivolle-Cazat, J.; Basset, J.-M. Angew. Chem.,
Int. Ed. Engl. 2001, 40, 2331.
(19) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organometallic Chemistry; University Science
Books: Mill Valley, CA, 1987; p 577. (b) Brintzinger, H. H.; Fischer, D.;
Mulhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl.
1995, 34, 1143. (c) Watson, P. L. J. Am. Chem. Soc. 1982, 104, 337. (d)
Soto, J.; Steigerwald, M. L.; Grubbs, R. H. J. Am. Chem. Soc. 1982, 104,
4479. (e) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Marks,
T. J. J. Am. Chem. Soc. 1985, 107, 8091. (f) Burger, B. J.; Thompson, M.
E.; Cotter, W. D.; Bercaw, J. E. J. Am. Chem. Soc. 1990, 112, 1566.
(20) Schrock, R. R.; Fellmann, J. D. J. Am. Chem. Soc. 1978, 100, 3359.
9
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Homogeneous Catalysis with Methane
A R T I C L E S
Table 1. Summary of Crystallographic Data for Cp*2ScCH2CMe3
(2)
formula
MW
ScC25H41
386.55
crystal color, habit
crystal dimensions
crystal system
yellow, blocks
0.30 × 0.25 × 0.24 mm
monoclinic
cell determination (2θ range)
2544 (3.5-50.9°)
a ) 10.4151(6) Å
b ) 15.7925(8) Å
c ) 14.2005(7) Å
â ) 95.133(2)°
V ) 2326.3(2) Å3
P21/n (#14)
lattice parameters
space group
Z value
Dcalc
µ(MoKR)
diffractometer
radiation
temperature
scan type
no. of reflections measured
4
1.104 g/cm3
3.21 cm-1
Siemens SMART
MoKR (λ ) 0.71069 Å)
-123.0 °C
ω (0.3° per frame)
total: 8151
unique: 3950 (Rint ) 0.054)
Lorentz-polarization
absorption (Tmax ) 0.90, Tmin ) 0.53)
direct methods (SAPI91)
full-matrix least-squares
2297
corrections
structure solution
refinement
no. observations (I > 3.00σ(I))
Figure 2. Representative second-order plots of ln{[CH4]/[Cp*2ScCH2-
CMe3]} vs time for the reaction of Cp*2ScCH2CMe3 (2) with CH4 in a
re-sealable J. Young NMR tube. The second-order rate constants, k, were
determined by dividing the slope of the linear least squares best fit line by
∆o (∆o ) [CH4]ini - [2]ini).
no. variables
241
R; Rw; Rall
0.050; 0.062; 0.090
max peak in final diff. map
min peak in final diff. map
0.26 e-/Å3
-0.36 e-/Å3
for the reaction of 2 with methane (and Me4C elimination).
Under the conditions employed (with g 5 equiv of CH4) the
elimination of Me4C is irreversible
Table 2. Selected Bond Distances (Å) and Angles (°) for
Cp*2ScCH2CMe3
bond distances
Sc-C21
2.286(4)
1.07(4)
2.2134(7)
C21-C22
C21-H32
Sc-Cpcent
1.549(6)
1.07(4)
2.2119(7)
C8D12 or C8D8
Cp*2ScCH2CMe
Cp* ScCH
3 + CH4
8
3 + Me4C
C21-H31
Sc-Cpcent
2
2
1
(3)
bond angles
Complex 2 is particularly more reactive toward methane (vs
other hydrocarbons), such that methane (10 equiv) is selectively
activated in benzene-d6 solvent. This reactivity trend is unusual
for C-H activation by transition metal complexes,1,2,21 and note
that the rate of H/D exchange catalyzed by Cp*2ScH (3) follows
the trend H-H . C6H6 > CH4 > cyclooctane.10a The complex
[η5-C5(CD3)5]2ScCH2CMe3 (2-d30) reacted with benzene-d6 to
form (Cp*-d15)2ScC6D5 and CMe4-d1, but significant rates
required elevated temperatures (70 °C, t1/2 ) 305 min).
Proteobenzene reacted more rapidly with 2-d30 (t1/2 ) 115 min;
ca. 3 times faster), and these rates are similar to those associated
with the reactions of Cp*2ScMe with benzene/benzene-d6.10a
The reaction of 2 with a large excess of cyclopropane (60 equiv,
cyclohexane-d12) proceeded slowly at room temperature with
formation of Cp*2SccPr and CMe4 (t1/2 ≈ 1 day). In contrast,
Cp*2LuMe is reported to react more rapidly with cyclopropane
than with methane (by a factor of ca. 4).9 Compound 2 reacted
with ethane (70 equiv, t1/2 . 1 week), but much more slowly
than with methane. The latter reaction produced neopentane,
but just as in the reaction of Cp*2MCH3 (M ) Sc, Lu) with
ethane,9,10a the product of â-H elimination (Cp*2ScH) was
formed, along with unidentified species (by 1H NMR spectros-
copy).9,10
Sc-C21-C22
Sc-C21-H32
C22-C21-H31
C21-Sc-Cpcent
Cpcent-Sc-Cpcent
128.3(3)
104(2)
108(2)
112.0(1)
138.78(3)
Sc-C21-H31
H31-C21-H32
C22-C21-H32
C21-Sc-Cpcent
105(2)
96(2)
108(2)
108.2(1)
(3)°] is smaller by 6.87° than the corresponding angle in
Cp*2ScCH3. Additionally, the Sc-Cpcent distances [2.2134(7)
and 2.2119(7) Å] in 2 are slightly longer than in 1 by (ca. 0.04
Å).
C-H Activation Reactions of Cp*2ScCH2CMe3 (2) with
Hydrocarbons. Addition of ca. 0.4 atm of CH4 to cyclohexane-
d12 or benzene-d6 solutions of 2 in an NMR tube with a Teflon-
valve at 77 K, followed by warming to room temperature,
quantitatively produced the scandium methyl 1 and C(CH3)4
(eq 3, t1/2 ) 45 min at room temperature in the dark). The
conversion of 2 to 1 could proceed either via intramolecular
â-methyl elimination or intermolecular C-H bond activation
of methane. The quantitative formation of neopentane rather
than isobutylene indicates that compound 2 does in fact react
with methane. This conclusion was confirmed by second-order
kinetics (first order in methane, vide infra) and a labeling study,
in which 2 reacted with CD4 to quantitatively form 1-d3. This
reaction is similar to the well-known 13CH4 exchange mediated
by Cp*2MCH3 (M ) Sc, Lu, Y).9,10a Although the latter
degenerate reactions are facile, significant reaction rates require
slightly elevated temperatures (ca. 70 °C).9,10a Thus, 2 is
considerably more reactive than 1 toward methane, and this
appears to be related to the greater thermodynamic driving force
Plots of ln{[CH4]/[2]} vs time (Figure 2) are linear for greater
than three half-lives, indicating a second-order rate law, rate )
(21) (a) Halpern, J. Inorg. Chim. Acta 1985, 100, 41. (b) Jones, W. D.; Feher,
F. J. Acc. Chem. Res. 1989, 22, 91. (c) Janowicz, A. H.; Bergman, R. G.
J. Am. Chem. Soc. 1982, 104, 352. (d) Jones, W. D.; Feher, F. J. J. Am.
Chem. Soc. 1986, 108, 4814.
9
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A R T I C L E S
Sadow and Tilley
Scheme 1. Two Observed Mechanisms for Methane Activationa
a Hydrogen catalysis proceeds through the cycle represented by A, whereas the direct reaction proceeds in the absence of added Cp*2ScH (B).
k3[2][CH4] (k3 ) slope/{[CH4]in - [2]in}; k3 ) 4.1(3) × 10-4
M-1s-1, 26 °C; k3 ) 2.0 × 10-3 M-1s-1, 50 °C). For
comparison, the second-order rate constants for methyl exchange
in the Cp*2MCH3/CH4 systems are 1 × 10-5 M-1s-1 (Sc, 70
°C), 4.6 × 10-4 M-1s-1 (Lu, 70 °C) and 2.6 × 10-3 M-1s-1
(Y, 70 °C).9,10a For the methyl exchange reactions, two
competitive processes were proposed: a second-order pathway
and a first order, two-step sequence involving a “tuck-in”
intermediate species, [η5-Cp*(η1:η5-C5Me4CH2)M] (eq 4; M )
Sc, Y, Lu).9,10a On the basis of kinetic and isotopic labeling
studies, the nondegenerate alkyl exchange of eq 3 does not occur
by such a metalation pathway below 50 °C. Thus, with 2-d30
as the reactant, no Me4C-d1 was observed in the product mixture
1
by H NMR spectroscopy. Furthermore, the second-order rate
constant for the reaction of 2-d30 with methane was identical to
that observed for reaction of the perproteo compound (k ) 4.1-
(3) × 10-4 M-1s-1).
Figure 3. Plot of Cp*2ScH vs the observed second-order rate constant
kobs, demonstrating the rate acceleration with added Cp*2ScH.
first order in both [CH4] and [2], but would also reflect first-
order dependence on [Cp*2ScH]: rate ) k[Cp*2ScH][CH4][2]
{k ) (k1k2)/(k-2[Cp*2ScMe]); k2 and k-2 are rate constants
associated with reversible methane metalation by Cp*2ScH and
k1 is the rate constant for hydrogenolysis of 2}. In fact, the
reaction is accelerated by added Cp*2ScH (Figure 3), and this
hydride-promoted reaction exhibits first-order dependence on
Cp*2ScH. Significantly, the observed rate enhancement is small
relative to the rate of reaction in the absence of added Cp*2-
ScH (e.g., with 0.5 equiv of Cp*2ScH, the observed rate constant
increases only 2-fold). Also, the experimentally determined rate
constant k3 (for the direct reaction of 2 with CH4) is identical
to the value of k3 obtained by extrapolation to zero [Cp*2ScH]
in the plot of Figure 3. This indicates that in the reaction of 2
with methane, the only pathway for alkyl exchange involves
direct interaction of the Sc-C bond of 2 with the C-H bond
of methane. With added Cp*2ScH, the rate expression becomes
rate ) kobs[2][CH4] where kobs ) k3 + k[Cp*2ScH].
A third possible mechanism is a chain reaction involving a
scandium hydride species, which would metalate CH4 with the
elimination of H2. Hydrogen would then react with 2 to produce
neopentane and reform the hydride (Scheme 1). This possibility
is suggested by the observation that the metalation of benzene
by Cp*2LuMe is accelerated by the addition of H2,9b and by a
study of the reaction of Cp*2SmCH(SiMe3)2 with H2Si(SiMe3)2,
which proceeds exclusively via a chain reaction involving Cp*2-
SmH.13b Similarly, trace quantities of Cp*2ScH or H2 could
promote the exchange of eq 3. The rate law for a mechanism
which proceeds solely through this hydride catalysis would be
9
7974 J. AM. CHEM. SOC. VOL. 125, NO. 26, 2003

Homogeneous Catalysis with Methane
A R T I C L E S
Scheme 2. Proposed Catalytic Cycle for Hydromethylation of
Interestingly, although methane metalation by Cp*2ScH could
not be observed directly,12,22 these kinetic studies of the hydride
catalysis indicate that the interaction of Cp*2ScH with CH4
occurs to rapidly produce a low, equilibrium concentration of
Cp*2ScMe. This direct reaction is also suggested by the Cp*2-
ScH-catalyzed deuteration of methane,10a and by the catalytic
dehydrosilation of methane by Cp*2ScH.12 Note that the direct
metalation of CH4 by Cp*2LuH was also not observed.23
Propene by Cp*2ScMe
As mentioned above, the reaction of 2 with CD4 yielded the
expected products Cp*2ScCD3 and Me4C-d1 (kD ) 4.0(1) ×
10-5 M-1s-1). Comparison of second-order rate constants for
the activations of CD4 and CH4 provided a large intermolecular
primary kinetic isotope effect of kH/kD ) 10.2. For comparison,
Wolczanski has measured similarly large primary isotope effects
(kH/kD ) 11.2) for the metalation of CH4 vs CD4 by the transient
zirconium imido complex [(SitBu3)NH]2ZrdN(SitBu3).24 These
large isotope effects were attributed to ground state energy
differences for proteo vs deutero compounds in highly sym-
metrical environments. Therefore, the intramolecular kinetic
isotope effect was measured by determining the ratio of Cp*2-
ScCHD2 vs Cp*2ScCH2D formed by the reaction of 2 with
CH2D2. The observed value of 5.2(1) is similar to the intramo-
lecular kinetic isotope effect reported for the metalation of
CH2D2 by [(SitBu3)NH]2ZrdN(SitBu3) [5.1(6)].24 These rela-
tively high values indicate that hydrogen is transferred in a linear
fashion in the â-position of the transition state. Interestingly,
the rate constant for the reaction of CH2D2 with 2 (k ) 2.3(1)
× 10-4 M-1s-1) is only 1.78 times slower than the reaction of
2 with CH4. The activation parameters (∆H‡ ) 11.4(1) kcal/
mol and ∆S‡ ) -36(1) eu, determined for the temperature range
of 10-50 °C) for the exchange of eq 3 are consistent with those
observed for the Cp*2LuCH3/CH4 system (∆H‡ ) 11.6 kcal/
mol; ∆S‡ ) -38.1 eu).9 Although such activation parameters
(a high entropy and a modest enthalpy of activation) are
frequently attributed to transition states in which bond cleavage
is a minor component of the activation barrier, the large, primary
isotope effect for the reaction of 2 with methane indicates that
the transition state involves significant C-H bond cleavage.
This suggests that the relatively small activation enthalpy results
from concurrent bond cleavage and bond formation processes.
[kH/kD ) 0.96(5)], as determined by measurements of the rate
constant for the reaction of Cp*2ScCD2CMe3 (2-d2) with CH4.
Thus, this σ-bond metathesis reaction occurs without R-agostic
assistance.
Catalytic Hydromethylation of Propene with Cp*2ScMe
(1). The facile and selective activation of methane by 2
suggested that a related nondegenerate alkyl exchange might
be incorporated into a catalytic cycle, if the resulting methyl
complex could be readily converted into a higher alkyl derivative
(e.g., via alkene insertion). Propene appeared to be a reasonable
substrate to test in this regard, since it is known to insert into
the Sc-Me bond of 1.10a Note, however, that the insertion
product, Cp*2ScCH2CHMe2, is reported to react with a second
equivalent of propene via σ-bond metathesis to form isobutane
and Cp*2ScCHdCHMe.10a
Propene and methane (9 and 10 equiv, respectively) were
added to a cyclohexane-d12 solution of 1 in a Teflon-sealed
NMR tube. Over the course of 3 days at room temperature,
Cp*2ScCH2CHMe2, Cp*2ScCHdCMe2 and isobutane (3 equiv
relative to 1), resulting from the catalytic hydromethylation of
propene, were observed in the reaction mixture. After heating
a similar mixture to 80 °C overnight, 4 equiv of isobutane
formed but the catalyst had completely decomposed to unidenti-
fied products (by NMR spectroscopy). Neither Cp*2ScH nor
propane is observed in the reaction mixture, suggesting that the
scandium hydride does not play a role in the observed catalysis.
Furthermore, the apparent lack of participation of Cp*2ScH in
the catalysis suggests that the C-H bond activation step involves
the alkyl complex Cp*2ScCH2CHMe2, in a step analogous to
the reaction of 2 with methane. Interestingly, â-hydride elimina-
tion from the scandium isobutyl complex does not occur, as
isobutylene was not observed. The proposed catalytic cycle
(Scheme 2) is based on the reactivity of 2 with methane, the
known insertion of propylene into 1, and the observed compo-
nents in the reaction mixture. The addition of only 4 equiv of
propene required 2 weeks to produce 3.5 equiv of isobutane,
thus the slow step in the catalytic cycle appears to be olefin
insertion into the Sc-Me bond.
Small, normal, secondary isotope effects in olefin polymer-
ization reactions have been attributed to R-agostic assistance
in the transition state of the insertion step.25 The similarities
between the mechanisms of olefin insertion and σ-bond meta-
thesis (four centered electrocyclic transition states, 2σ + 2π vs
2σ + 2σ) suggest the possibility of an R-agostic participation in
C-H bond activation.26 However, there is not a significant
secondary isotope effect associated with the R-hydrogens of 2
(22) Relative bond dissociation energies (B. D. E.'s) have been determined for
Sc-H and Sc-C bonds. (a) Bulls, A. R.; Bercaw, J. E.; Manriquez, J. M.;
Thompson, M. E. Polyhedron 1988, 7, 1409. (b) Labinger, J. A.; Bercaw,
J. E. Organometallics 1988, 7, 926.
(23) Castillo, I.; Tilley, T. D., unpublished results.
(24) Schaller, C. P.; Bonanno, J. B.; Wolczanski, P. T. J. Am. Chem. Soc. 1994,
116, 4133.
(25) (a) Grubbs, R. H.; Coates, G. W. Acc. Chem. Res. 1996, 29, 255. (b)
Clawson, L.; Soto, J.; Buchwald, S. L.; Stiegerwald, M. L.; Grubbs, R. H.
J. Am. Chem. Soc. 1985, 107, 3377. (c) Piers, W. E.; Bercaw, J. E. J. Am.
Chem. Soc. 1990, 112, 9406. (d) Krauledat, H.; Brintzinger, H. H. Angew.
Chem., Int. Ed. Engl. 1990, 29, 1412. (e) Cotter, D. W.; Bercaw, J. E. J.
Organomet. Chem. 1991, 417, C1. (f) Leclerc, M. K.; Brintzinger, H. H.
J. Am. Chem. Soc. 1995, 117, 1651.
(26) (a) Ziegler, T.; Folga, E.; Berces, A. J. Am. Chem. Soc. 1993, 115, 636.
(b) Ustynyuk, Y. A.; Ustynyuk, L. Y.; Laikov, D. N.; Lunin, V. V. J.
Organomet. Chem. 2000, 597, 182.
Attempts to extend this catalysis to the addition of other
hydrocarbons (e.g., arenes and cyclopropane) to propene have
been unsuccessful with 1. For example, propene did not insert
into the scandium-carbon bond of Cp*2SccPr. Both the alkene
insertion and C-H bond activation steps appear to be highly
sensitive to the nature of the reacting scandium alkyl species.
9
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A R T I C L E S
Sadow and Tilley
The attempted addition of methane to other alkenes or alkynes
(cis/trans-2-butene, 1-hexene, 1-butene, 2-methylpropene, nor-
bornylene, and 2-butyne) did not produce methylated products
over several days at room temperature or elevated temperatures
(70 °C). Potential difficulties may relate to the fact that internal
alkenes, such as cis/trans-2-butene and norbornylene, do not
insert into the scandium-carbon bond of 1. For example, no
reaction was observed between Cp*2ScMe and norbornylene
(10 equiv, cyclohexane-d12, room temperature, 1 week). Also,
larger R-olefins (1-butene, 2-methylpropene, 1-hexene) react
with Cp*2ScMe via σ-bond metathesis to form the corresponding
scandium vinyl complexes.10a Although 2-butyne (5 equiv)
reacted with Cp*2ScMe rapidly via a single insertion (<5 min),
the resulting scandium vinyl compound Cp*2ScC(Me)dCMe2
did not react with methane (12 equiv in solution), even upon
heating at 70 °C in cyclohexane-d12 for 4 days.
not occur in this system. Apparently, the higher rate for the
bimolecular reaction of 2 with methane favors direct C-H
activation over a two-step pathway involving a “tuck-in”
intermediate, as observed in the reaction of Cp*2ScMe with
methane.
Another aspect to the selectivity exhibited by 2 in bond
activations is seen in its reactivity toward silanes. Although
Si-H bonds typically react more rapidly than C-H bonds, when
cyclohexane-d12 solutions of 2 and Ph2SiH2 (0.5 equiv) were
exposed to methane (10 equivalents) at room temperature, Ph2-
MeSiH (0.5 equiv) and Me4C (1 equiv) were formed rather than
Ph2(CH2CMe3)SiH. Note that in contrast to 2, Cp*2ScMe reacts
rapidly with Ph2SiH2 to form Cp*2ScH and Ph2MeSiH.12 The
enhanced activity and selectivity of 2 in its reaction with
methane is unusual, in that complexes (such as Cp*2LuMe) that
are reactive toward metathesis with C-H bonds are also highly
reactive toward Si-H and Si-C bond activations.14
Concluding Remarks
On the basis of current mechanistic information, it seems that
carbon-carbon coupling reactions are disfavored by purely
σ-bond metathesis pathways because such reactions would
involve transition states with carbon in the â-position. Experi-
mental and theoretical studies indicate that such transformations
are prohibited by high energy transition states, limiting possible
strategies for hydrocarbon homologations.9,10,16,17,26 However,
the combination of methane activation and alkene insertion (as
in propene hydromethylation) provides an alternative approach
for catalytic carbon-carbon bond formations. Although catalytic
systems combining C-H bond activation with insertions of
unsaturated hydrocarbons have been reported,4,10c the scandium
system described here represents the only example involving a
methane conversion. Future efforts will target modifications of
the ancillary ligands and the substrates for the development of
new C-H bond activation chemistry.
The application of σ-bond metathesis chemistry in catalytic
hydrocarbon conversions requires metal complexes that are
active toward the cleavage of C-H bonds. Such complexes, of
the type Cp*2MR (M ) Sc, Lu, Y; R ) H, CH3), were reported
almost twenty years ago.9,10a However, productive catalysis also
depends critically on the selectivity exhibited by the catalyst
toward potential bond activations. For example, the hydro-
methylation of propene requires that the insertion product
(LnM-CHCHMe2) react with methane rather than another
equivalent of propene, or the solvent, by σ-bond metathesis. In
addition, an intramolecular ligand metalation via C-H activation
could lead to inactive or insoluble species.10a In this contribution,
we have described a highly selective activation of methane by
the scandium neopentyl complex 2, which suggested that a
related process might provide the basis for a new type of
catalytic methane conversion.
Experimental Section
The selective activation of methane by the scandium neo-
pentyl complex 2 is particularly interesting in light of compari-
sons to related systems. For example, whereas 2 reacts with
methane (0.55 M, 10 equiv in solution) in benzene-d6 to form
only Cp*2ScMe, the complexes Cp*2MCH3 (M ) Sc, Lu, Y)
react at approximately the same rate with benzene and
methane.9,10a Unlike the methyl complexes,9,10a 2 reacts with
methane more rapidly than with cyclopropane. The thoracyclo-
butane Cp*2Th(κ2-CH2CMe2CH2) exhibits the typical trend in
selectivities toward C-H bond activations: cyclopropane >
benzene > methane.10b
General. All manipulations were performed under an atmosphere
of argon using Schlenk techniques and/or a M. Braun glovebox. Dry,
oxygen-free solvents were employed throughout. Removal of thiophenes
from benzene and toluene was accomplished by washing each with
H2SO4 and saturated NaHCO3 followed by drying over MgSO4. Olefin
impurities were removed from pentane by treatment with concentrated
H2SO4, 0.5 N KMnO4 in 3 M H2SO4, saturated NaHCO3, and then the
drying agent MgSO4. All solvents were distilled from sodium benzo-
phenone ketyl, with the exception of benzene-d6 and cyclohexane-d12,
which were purified by vacuum distillation from Na/K alloy. The
compounds Cp*2ScCl, [η5-C5(CD3)5]2ScCl, Cp*2ScCH3 (1), Cp*2ScH
(2), Cp*2ScPh (7),10a LiCH2CMe3 and LiCD2CMe3 were prepared
20
Notably, the enhanced selectivity for methane by Cp*2ScCH2-
CMe3 is not associated with reduced activity; 2 reacts with
methane at a rate that is 2 orders of magnitude faster than that
of Cp*2ScMe, under similar conditions. The enhanced reactivity
of 2 (relative to Cp*2ScMe) in the C-H bond activation of
methane likely results from a Sc-C bond that is weakened by
steric pressure, possibly resulting from the presence of the bulky
Cp* and -CH2CMe3 ligands. Consistent with this, reactions of
the scandium neopentyl complex and larger hydrocarbon
substrates (e.g., benzene and cyclopropane) are comparatively
slow. Thus, it seems that the bulky ligands of 2 create a small,
reactive binding site that is selective for methane. Interestingly,
compound 2 also exhibits enhanced selectivity for intermolecular
vs intramolecular C-H bond activation, as mechanistic inves-
tigations reveal that an intramolecular “tuck-in” mechanism does
according to literature procedures. Commercial sources were used for
CH4 and propylene (Airgas), CD4 (Cambridge Isotope Labs), and
cyclopropane (Aldrich), and these materials were used as received.
Elemental analyses were performed by the microanalytical laboratory
at the University of California, Berkeley. Infrared spectra were recorded
using a Mattson FTIR spectrometer at a resolution of 4 cm-1. All NMR
spectra were recorded at room temperature in benzene-d6 unless
otherwise noted, using a Bruker AM-400 spectrometer at 400 MHz
(1H) or a Bruker DRX-500 at 500 MHz (1H) and 125 MHz (13C).
Cp*2ScCH2CMe3 (2). Cp*2ScCl (0.529 g, 1.51 mmol) was dissolved
in pentane (20 mL) and the resulting solution was cooled to -78 °C.
The reaction flask was wrapped in aluminum foil, and a solution of
LiCH2CMe3 in pentane (10.1 mL, 0.149 M) was added slowly with a
syringe. The flask was sealed and the solution was stirred at - 78 °C
in the dark for 2 h, and then the pentane was removed in vacuo. The
resulting solids were warmed to room temperature and extracted with
9
7976 J. AM. CHEM. SOC. VOL. 125, NO. 26, 2003

Homogeneous Catalysis with Methane
A R T I C L E S
50 mL of pentane. The resulting solution was concentrated to ca. 30
mL and cooled to -78 °C for 1 day. A white, pyrophoric precipitate
was isolated by filtration from the supernatant, which was further
concentrated to ca. 5 mL and cooled to -78 °C. Yellow crystals of
Cp*2ScCH2CMe3 were isolated by filtration (0.292 g, 0.75 mmol, 49%).
1H NMR (C6D6): δ 1.888 (s, 30 H, C5Me5), 1.342 (s, 9 H, CH2CMe3),
0.843 (s, 2 H, CH2CMe3). 13C{1H} NMR (C6D6): δ 121.370 (C5Me5),
53.2 (CH2CMe3), 36.842 (CH2CMe3), 12.431 (C5Me5). IR (Nujol, cm-1):
2908 s, 2859 s, 1440 m, 1379 m, 1249 w, 1200, 1087 w, 1022 w,
540 m. Anal. Calcd for C25H41Sc: C, 77.68; H, 10.69. Found: C, 77.82;
H, 10.77. Mp: 90-92 °C (dec).
was monitored with a thermocouple. Single scan spectra were acquired
automatically at preset time intervals. The peaks were integrated relative
to cyclooctane as an internal standard. Rate constants were obtained
by nonweighted linear least-squares fit of the integrated second-order
rate law, ln{[CH4]t/[2] ) ln{[CH4]0/[2]0} + k∆0t.27
Acknowledgment. This work was supported by the National
Science Foundation, and by the Director, Office of Energy
Research, Office of Basic Energy Sciences, Chemical Sciences
Division, of the U.S. Department of Energy under Contract DE-
AC03-76SF00098. We also thank John Bercaw, Richard Ander-
sen, and Mark Thompson for helpful discussions, and Allen G.
Oliver and Fred Hollander for assistance with the X-ray structure
determination. CNDOS is supported by Bristol-Myers as a
founding member.
[η5-C5(CD3)5]2ScCH2CMe3 (2-d30). The synthesis of 2-d30 was
performed in a manner analogous to that of 2, using [η5-C5(CD3)5]2-
ScCl.
Kinetic Measurements. Reactions were monitored by 1H NMR
spectroscopy, with a Bruker DRX500 spectrometer, using 5 mm
Wilmad NMR tubes with a Teflon-valve seal. Samples were prepared
by dissolution of 2 in cyclohexane-d12 containing a known concentration
of C8H16 standard. The samples were cooled to 77 K, the headspace of
the NMR tube was evacuated, and CH4 was admitted. The sample was
maintained at 77 K until immediately before being placed in the NMR
probe, which was preset to the required temperature. At the appropriate
time, the sample was carefully warmed to room temperature and shaken
to ensure maximum dissolution of CH4 into solution. The probe
temperature was calibrated using a neat ethylene glycol sample and
Supporting Information Available: Details for the kinetic
runs, representative kinetic data (PDF) and X-ray crystal-
lographic data for 2 (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA021341A
(27) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms, 2nd ed.;
McGraw-Hill: New York, 1995; p 155.
9
J. AM. CHEM. SOC. VOL. 125, NO. 26, 2003 7977