Aromatic versus benzylic CH bond activation of alkylaromatics by a transient η2-cyclopropene complex

Article abstract of DOI:10.1021/om200199e

The methyl cyclopropyl hydrotris(3,5-dimethylpyrazolyl)borate complex Tp^Me2NbMe(c-C₃H₅)(MeCCMe) reacts smoothly with different alkylaromatics XH at 308 K to yield methane and Tp^Me2NbX(c- C₃H₅)(MeCCMe). NMR data show that for mesitylene and 1,4-dimethylbenzene, selective benzylic CH bond activation is observed, giving the benzyl cyclopropyl complexes Tp^Me2Nb(CH₂Ar′)(c- C₃H₅)(MeCCMe) (Ar′ = 3,5-Me₂C ₆H₃, 4-MeC₆H₄, respectively). Selective arene CH bond activation is observed with 1,2-dimethylbenzene, yielding Tp^Me2Nb(3,4-Me₂C₆H₃)(c- C₃H₅)(MeCCMe). With 1,3-dimethylbenzene, a 3:1 mixture of arene and benzylic CH activated products, Tp^Me2Nb(3,5-Me ₂C₆H₃)(c-C₃H₅)(MeCCMe) and Tp^Me2Nb(CH₂-3-MeC₆H₄)(c-C ₃H₅)(MeCCMe), is observed, translating to a 18:1 preference for aromatic versus benzylic CH bond activation on a per CH bond basis. Kinetic studies are consistent with rate-limiting intramolecular β-H abstraction of methane to yield a transient unsaturated η²- cyclopropene intermediate. This intermediate reacts rapidly with the aromatic or benzylic CH bond of the arene via 1,3-addition across a Nb(η²- cyclopropene) bond. DFT calculations suggest that the observed selectivities are a result of the steric influence of the methyl groups on the arene ring, which blocks activation of an ortho C-H bond.

Full text of DOI:10.1021/om200199e

ARTICLE
pubs.acs.org/Organometallics
Aromatic versus Benzylic CH Bond Activation of Alkylaromatics by a
Transient η2-Cyclopropene Complex
Cꢀedric Boulho,†,‡ Laure Vendier,†,‡ Michel Etienne,*,†,‡ Abel Locati,§ Feliu Maseras,§,^ and
John E. McGrady#
†LCC (Laboratoire de Chimie de Coordination), CNRS, 205 Route de Narbonne, F-31077 Toulouse, France
‡UPS, INPT, LCC, Universitꢀe de Toulouse, F-31077 Toulouse, France
§
€
Institute of Chemical Research of Catalonia (ICIQ), Avinguda Paisos Catalans 16, 43007, Tarragona, Spain
^
ꢀ
ꢀ
Unitat de Quimica Fisica, Edifici Cn, Universitat Autꢁonoma de Barcelona, 08193 Bellaterra, Catalonia, Spain
#Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QR,
United Kingdom
S Supporting Information
b
ABSTRACT: The methyl cyclopropyl hydrotris(3,5-dimethyl-
pyrazolyl)borate complex TpMe2NbMe(c-C3H5)(MeCCMe) re-
acts smoothly with different alkylaromatics XH at 308 K to yield
methane and TpMe2NbX(c-C3H5)(MeCCMe). NMR data show
that for mesitylene and 1,4-dimethylbenzene, selective benzylic
CH bond activation is observed, giving the benzyl cyclopropyl
complexes TpMe2Nb(CH2Ar0)(c-C3H5)(MeCCMe) (Ar0 =
3,5-Me2C6H3, 4-MeC6H4, respectively). Selective arene CH
bond activation is observed with 1,2-dimethylbenzene, yielding TpMe2Nb(3,4-Me2C6H3)(c-C3H5)(MeCCMe). With 1,3-dimethyl-
benzene, a 3:1 mixture of arene and benzylic CH activated products, TpMe2Nb(3,5-Me2C6H3)(c-C3H5)(MeCCMe) and TpMe2Nb-
(CH2-3-MeC6H4)(c-C3H5)(MeCCMe), is observed, translating to a 18:1 preference for aromatic versus benzylic CH bond activation
on a per CH bond basis. Kinetic studies are consistent with rate-limiting intramolecular β-H abstraction of methane to yield a transient
unsaturated η2-cyclopropene intermediate. This intermediate reacts rapidly with the aromatic or benzylic CH bond of the arene via 1,3-
addition across a Nb(η2-cyclopropene) bond. DFT calculations suggest that the observed selectivities are a result of the steric influence
of the methyl groups on the arene ring, which blocks activation of an ortho CꢀH bond.
’ INTRODUCTION
abstraction reactions in dialkyl complexes of groups 4ꢀ6, offers
an alternative route. These complexes are capable of activating
hydrocarbon CH bonds by a 1,2-addition across the MdC
bond.5 This reaction type was recently extended to transient
titanium alkylidyne intermediates (a 1,2-addition of the CH
bond across a TitC bond).6 β-H abstraction pathways such as
that illustrated in Scheme 1 are, in contrast, extremely rare.7 The
best characterized examples occur in alkyl phenyl or diphenyl
complexes LnMR(Ph), where β-H abstraction of a hydrocarbon
RH from an ortho hydrogen yields benzyne complexes. These
unsaturated benzyne intermediates are also capable of the reverse
reaction, a 1,3-addition of a hydrocarbon CH bond across a
Mꢀbenzyne bond.8 η3-Allyl alkyl and vinyl alkyl complexes of
molybdenum and tungsten also generate unsaturated η2-allene,
η2-diene, and η2-alkyne complexes, respectively, by β-H abstrac-
tion, and these intermediates are also able to cleave CH bonds of
various hydrocarbons.9 However, simple unsaturated η2-alkene
intermediates such as A have clearly been underestimated as
Even though CH bond activation reactions have become a
practical tool for synthetic chemists in recent years,1 the search
for new reactions and a deeper understanding of the underlying
mechanisms remains an active area of research.2 We recently
reported (Scheme 1) that the R-CC agostic methyl cyclopropyl
complex TpMe2NbMe(c-C3H5)(MeCCMe) (1) undergoes in-
tramolecular elimination of methane at room temperature to
yield an unsaturated η2-cyclopropene intermediate, [TpMe2Nb-
(η2-c-C3H4)(MeCCMe)] (A). This then reacts with benzene to
give the phenyl cyclopropyl complex TpMe2NbPh(c-C3H5)-
(MeCCMe) (2) via a 1,3-addition of the CH bond across a
NbꢀC bond of the Nb(η2-c-C3H4) moiety.3
There are a number of well-established pathways that lead to
intermolecular activation of unactivated hydrocarbon CH bonds
by complexes of early transition metal ions. For example, σ-bond
metathesis at d0 complexes, where no formal coordination of the
hydrocarbon occurs prior to formation of a kite-shaped four-
center transition state, has been established by seminal work on
scandium derivatives.4 The formation of intermediates with
unsaturated MdC bonds, such as those generated by R-H
Received: March 4, 2011
Published: July 06, 2011
r
2011 American Chemical Society
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Scheme 1. CH Bond Activation of Benzene by a Transient η2-Cyclopropene Species
Scheme 2. Reactions of TpMe2NbMe(c-C3H5)(MeCCMe) (1) with Alkylaromatics
competent CH bond activating species. Given the novelty and
the very mild conditions of the reaction summarized in Scheme 1,
we decided to study its scope in more detail. In this paper we
report a joint experimental and computational study of the
selectivity of intermediate A for Csp3ꢀH versus Csp2ꢀH bond
activation in alkyl aromatics. Our results offer an interesting
comparison to related studies for 1,2-CH bond activations at
MdC, MdNR0, and TitC bonds and to oxidative addition
reactions.
’ RESULTS AND DISCUSSION
Reactions of Alkyl Aromatics. The reactivity of the methyl
cyclopropyl complexTpMe2NbMe(c-C3H5)(MeCCMe) (1) with
various alkylaromatics is summarized in Scheme 2. 1 reacts
smoothly at 308 K with mesitylene (either pure or in cyclohexane
solution) over the course of 12 h to give the substituted benzyl
cyclopropyl derivative TpMe2Nb(CH2-3,5-Me2C6H3)(c-C3H5)-
(MeCCMe) (3). Monitoring the reaction in cyclohexane-d12 in
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We now turn to the reaction of 1 with dimethylbenzenes, as
depicted in Scheme 1. Reaction of 1 with 1,4-dimethylbenzene
under similar conditions to those described for mesitylene again
selectively yields a benzylic-type product, TpMe2Nb(CH2-4-
C6H4Me)(c-C3H5)(MeCCMe) (4), with similar key spectro-
scopic data to those for 3 (see Experimental Section). Both
benzylic complexes 3 and 4 readily react with benzene to yield
the phenyl cyclopropyl complex 2.3 At room temperature, and
again despite some decomposition, a single product, 5, results
from the reaction of 1 with one equivalent of 1,2-dimethyl-
benzene.12 Note that under similar conditions 1 decomposes
even more extensively in the absence of arene. Characteristic 1H
and 13C NMR signals of benzylic products are conspicuously
absent, and 5 can be identified as the aryl cyclopropyl complex
TpMe2Nb(c-C3H5)(3,4-Me2C6H3)(MeCCMe). In the 1H NMR
spectrum of 5, two aromatic signals at δ 7.04 and 6.81 integrating
for 2H and 1H, respectively, are observed at room temperature,
indicating a fluxional process. A characteristic 13C NMR signal at
δ 190.9 points to the presence of an aromatic ring bound to
niobium. Decoalescence of the 1H NMR signals is observed upon
cooling at 193 K in dichloromethane-d2. The characteristic
pattern of one singlet (δ 8.16) and two doublets (δ 6.44 and
5.71, 3J = 14 Hz) integrating to one H each gives evidence that
methyl groups are in positions 3 and 4 of the aryl ring in 5, as
shown in Scheme 1.
In marked contrast to either 1,2- or 1,4-dimethylbenzene, CH
bond activation in 1,3-dimethylbenzene is not selective (Scheme 1).
The reaction between 1 and 1,3-dimethylbenzene yields an insep-
arable 3:1 mixture of aryl TpMe2Nb(3,5-Me2C6H3)(c-C3H5)-
(MeCCMe) (6-Ar) and benzylic TpMe2Nb(CH2-3-MeC6H4)-
(c-C3H5)(MeCCMe) (6-Bz) complexes resulting from aromatic
or benzylic CH bond activation, respectively, along with uni-
dentified decomposition products as above. Despite these draw-
backs, 1H and 13C NMR data can be analyzed. In the 13C NMR
spectrum, a single aryl carbon resonance at δ 193 and a benzylic
resonance at δ 77.1 are assigned to 6-Ar and 6-Bz, respectively.
The niobium-bound carbon of the c-C3H5 rings is observed at
δ 70.5 and 66.3, respectively. This 3:1 ratio translates to an 18:1
preference for aromatic activation of the 5-CH bond over
benzylic CH activation on a per CH bond basis.
Overall these experiments indicate that aromatic CH bond
activation is preferred over benzylic CH bond activation in
alkylbenzenes, the latter only becoming competitive when steric
congestion is present. Selective benzylic CH activation is ob-
served for 1,4-dimethylbenzene and mesitylene but not for 1,2-
dimethylbenzene, establishing that activation of an aromatic CH
bond or of a benzylic CH bond ortho to at least one methyl group
is strongly disfavored. The case of 1,3-dimethylbenzene is
particularly significant in this regard since it is the only case
where both aromatic and methyl CꢀH groups that do not lie
ortho to an Me substituent are available. Consequently in this
case, competitive benzylic and aromatic activation are observed.
Kinetic Studies. The kinetic behavior of the reaction between
1 and mesitylene has been studied in detail to verify that the
mechanism previously established for the reaction between 1 and
benzene was still operational. The reaction converting 1 to 3
(1H NMR, cyclohexane-d12, T = 308 K) is first-order in 1 and
zeroth-order in mesitylene (established using 1, 2, 8, and 15 equiv
of mesitylene versus 1) with kobs = (5.67 ( 0.07) ꢁ 10ꢀ5 sꢀ1, a
rate constant equal to that of the reaction of 1 with benzene
under the same conditions.3 The same value of kobs was observed
when 1 was reacted with 1,4-dimethylbenzene, and no kinetic
Figure 1. Plot of the X-ray molecular structure of TpMe2Nb(CH2-3,
5-Me2C6H3)(c-C3H5)(MeCCMe) (3).
Teflon-valve sealed NMR tubes reveals that, along with some
unidentified decomposition products, complex 3 is formed selec-
tively. No signals attributable to any aryl complex (i.e., a broad
yet prominent niobium-bound aryl carbon in the δ 190 region;
see below) were observed. This establishes that selective benzylic
1
CH bond activation occurs in mesitylene. Key H NMR data
include an AB-type pattern for the diastereotopic benzylic
protons of NbCH2 at δ 2.93 and 2.85 (both d, 2JHH = 11.5 Hz)
and two singlets at δ 6.52 and 6.45 in a 2:1 ratio, assigned to ortho
and para protons on the aryl ring. The appearance of a single
aromatic ring methyl resonance at δ 2.20 integrating for six
protons confirms free rotation about the CH2ꢀarene bond. In
the 13C NMR spectrum, the benzylic carbon appears as a triplet
at δ 77.2 with a 1JCH of 117 Hz establishing η1-benzyl coordina-
tion. Protons of the cyclopropyl group give 1H NMR multiplets
in the δ 1.94ꢀ0.93 region, and the niobium-bound carbon
appears as a prominent doublet at δ 65.3 (1JCH = 138 Hz).
Both the nature of complex 3 and the coordination mode of the
benzylic ligand were confirmed by an X-ray diffraction analysis
(Figure 1). An obtuse NbꢀCRꢀCipso angle of 134.3(2)ꢀ and
large Nb Cipso (3.4 Å) and Nb Cortho (4.1, 4.4 Å)
3 3 3
3 3 3
distances testify to an η1-bound 3,5-dimethylbenzyl ligand. The
obtuse NbꢀCRꢀCipso angle and the conformation of the ben-
zylic ligand, which directs the 3,5-dimethylaryl ring away from
the TpMe2 ligand (Nb(1)ꢀC(8)ꢀC(9)ꢀC(10) 106ꢀ), are most
likely due to steric reasons. The NbꢀCR bond length of
2.220(3) Å for the benzyl ligand is notably greater than the value
of 2.188(3) Å for the NbꢀCR of the cyclopropyl ligand. As
observed in related cyclopropyl complexes,3,10 the cyclopropyl
ligand adopts a characteristic conformation with the CRꢀCβ
bond oriented roughly in the CRꢀNbꢀCR0 plane (C(2)ꢀ
C(1)ꢀNbꢀC(8) ꢀ37ꢀ). The C(1)ꢀC(2) distance (1.539(5) Å)
is slightly longer than C(1)ꢀC(3) (1.518(5) Å), although the
difference is barely significant here. With the distortion revealed
by different NbꢀCRꢀCβ angles [NbꢀC(1)ꢀC(2), 118.6(2)ꢀ;
NbꢀC(1)ꢀC(3), 130.2(3)ꢀ], these data are indicative of a
possible R-CC agostic structure.11
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Scheme 3. Aromatic versus Benzylic CH Bond Activation of 1,3-Dimethylbenzene by Intermediate A
isotope effect or incorporation of deuterium in evolved methane
(δ 0.21 in cyclohexane-d12) was seen when 1,4-dimethylben-
zene-d10 was activated under identical conditions (kH/kD = 1.0 (
0.1 at 308 K). These experiments indicate that whatever the
nature of the arene (benzene, mesitylene, or 1,4-dimethyl-
benzene), the rate-determining step consists of intramolecular
elimination of methane from 1 by β-H abstraction to yield an
unsaturated η2-cyclopropene intermediate A. A then rapidly
activates an aromatic or benzylic CH bond of the arene to give
the products.3 The available pathways in the case of the
nonselective activation of 1,3-dimethylbenzene are depicted in
Scheme 3.
similar to the X-ray structure, while the second, which connects
directly to the important CꢀH bond activation transition state
(vide infra) lies 7 kJ molꢀ1 higher. The third rotamer, where the
3
mesityl ligand lies in the wedge defined by two pyrazolyl rings,
lies 5 kJ molꢀ1 above the lowest energy structure. Apart from the
3
conformation of the mesityl ring, all three structures have very
similar bond lengths and angles.
A representative potential energy surface for the reaction of
intermediate A with 1,4-dimethylbenzene is shown in Figure 3.
The surfaces for the other substrates are qualitatively similar and
are shown in the Supporting Information. The energies of the key
stationary points are collected in Table 1. Our focus here is on
selectivity issues for aromatic versus benzylic CH bond activa-
tions by a common intermediate A, so we do not discuss the
details of the elementary step originating from 1 except to note
We have also monitored the reaction between 1 and 15 equiv
of 1,3-dimethylbenzene by 1H NMR in cyclohexane-d12 at 308 K.
The ratio between 6-Ar and 6-Bz remains constant throughout
the reaction, suggesting that the final product distribution results
from kinetic rather than thermodynamic selectivity.3 This view
is further substantiated by computational studies (see below)
that intermediate A (plus methane) lies 48 kJ molꢀ1 above 1 and
3
the transition state for the β-H abstraction of methane leading to
A is 116 kJ molꢀ1 above 1.
3
that indicate aryl complexes are preferred by ca. 20 kJ molꢀ1
.
Table 1 reveals that the benzylic activation pathway is very
similar for all substrates: the four key transition states have
3
It could not be established whether interconversion was occur-
ring at longer reaction times or higher temperatures due to
extensive decomposition, but again computational studies sug-
gest it would not.
relative energies between 69 and 74 kJ molꢀ1. Thus the selec-
3
tivity arises primarily from variations in the aromatic pathway,
which are determined by the number of methyl groups ortho to
the metalated carbon. Where there are no ortho methyls (1,
2-dimethylbenzene and 5-H activation in 1,3-dimethylbenzene)
Computational Studies. In this section we report a series of
calculationsperformedwithdensity functionaltheory(PBE1PBE/
LanL2DZ(+f), 6-31G(d)), the aim being to confirm the struc-
ture of the key intermediates and transition states iden-
tified above and to identify the origin of the energetic differences
between them. The optimizations on the mesitylene system
allow us to benchmark the calculations since the X-ray mole-
cular structure of 3 is known. We have identified three distinct
rotamers of the mesitylene complexes, all of which are presented
in Figure 2. The first and most stable one has a conformation very
the transition states have relative energies of 60 and 61 kJ molꢀ1
.
3
The presence of one ortho methyl (1,4-dimethylbenzene and 4-H
activation in 1,3-dimethylbenzene) increases this to 84 kJ molꢀ1
a value that exceeds the narrow range of benzylic activation.
,
3
Where two ortho methyls (mesitylene) are present the barrier
rises to 137 kJ molꢀ1. On this basis, the presence of a single ortho
3
methyl group should be sufficient to block aromatic CꢀH acti-
vation and cause a switch to the benzylic pathway. For the
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Figure 2. DFT-optimized structures of TpMe2Nb(CH2-3,5-Me2C6H3)(c-C3H5)(MeCCMe) (3). Left, rotamer related to the X-ray structure; center,
rotamer linked to the TS for benzylic CH bond activation (+7 kJ molꢀ1); right, rotamer with the mesityl group in the wedge (+5 kJ molꢀ1). Hydrogens
3
3
and TpMe2 backbone omitted for clarity. Bond distances in Å, bond angles (italicized) in deg.
Figure 3. Potential energy (with ZPE corrections) diagram for the benzylic and aromatic CH bonds of 1,4-dimethylbenzene by [TpMe2Nb(η2-
C3H4)(MeCCMe)].
Table 1. Computed Potential Energies (kJ molꢀ1) for the Activation of Benzylic and Aromatic CH Bonds of Alkylbenzenes by
3
Intermediate Aa
transition state for
benzylic activation
transition state for
aromatic activation
arene
mesitylene
benzyl complex
aryl complex
ꢀ32
ꢀ31
ꢀ32
69
73
73
45
137
84
1,4-dimethylbenzene
1,3-dimethylbenzene
ꢀ16
4-H activation
ꢀ17
5-H activation
4-H activation
84
5-H activation
61
ꢀ52
1,2-dimethylbenzene
ꢀ25
74
ꢀ54
60
a Energies include zero-point energy corrections and are relative to A + arene.
reaction of 1 with mesitylene, for example, the transition states
still in favor of benzylic activation (73 vs 84 kJ molꢀ1). The
3
for benzylic or aromatic activation are 69 or 137 kJ molꢀ1 above
difference of 11 kJ molꢀ1 is small but still compatible with the
3
3
A, respectively, with a ΔΔEq of 68 kJ molꢀ1, and only benzylic
experimental observation of only benzylic activation.
3
activation is observed. The barriers for the two competing pro-
cesses are closer for 1,4-dimethylbenzene (Figure 3), although
In the case of 1,3-dimethylbenzene, both benzylic and aro-
matic activation are experimentally observed. The aromatic
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Figure 4. DFT-computed transition states for benzylic and aromatic CH bond activations of 1,2-dimethylbenzene by TpMe2Nb(η2-C3H4)(MeCCMe)
(alkyne and TpMe2 backbone omitted for clarity). Bond distances in Å, bond angles (italicized) in deg.
Figure 5. DFT-computed transition states for aromatic CH bond activation of mesitylene (A), 4-CH (B), and 5-CH (C) bonds of 1,3-dimethylbenzene
by TpMe2Nb(η2-C3H4)(MeCCMe) (alkyne and TpMe2 backbone omitted for clarity). Bond distances in Å, bond angles (italicized) in deg.
activation is exclusively in the 5-position (no ortho CH3, barrier
The transition states for CH activation/formation of the
benzylic and aromatic CH bond of 1,2-dimethylbenzene are
depicted in Figure 4 and have broadly similar structures. There is
of 61 kJ molꢀ1) rather than in the 4-position (and one ortho
3
CH3, barrier of 84 kJ molꢀ1). The benzylic activation pathway
3
has a barrier of 73 kJ molꢀ1, and the difference in activation
an almost linear C
H
C geometry for the H transfer
3
3
3 3 3 3 3 3
energies, ΔEa, of 12 kJ molꢀ1 should be sufficient to lead to full
between the coordinated and entering hydrocarbons, most
notably in the case of the aromatic interaction where the TS is
slightly unsymmetrical with a shorter CH bond on the arene
side (activation of an aromatic sp2 CH bond) than on the
η2-cyclopropene side. At the same time the arene approaches
selectivity. However, we note that benzylic activation is favored
statistically over aromatic activation by a ratio of 6:1, and this
difference apparently reduces the free energy of activation suffi-
ciently to allow both products to form. We note here that in the
cases of mesitylene and 1,4-dimethylbenzene the statistical factor
only serves to reinforce the enthalpic preference for benzylic
activation.
the metal more closely (Nb C 2.47 versus 2.54 Å) in the
3 3 3
aromatic activation pathway, leading to a more compressed
transition state. The transferred hydrogen carries a similar small
positive Mulliken charge in both transition states (0.111 and
0.100; it is the least charged H in both structures), in both cases
despite a more negative benzylic (ꢀ0.674) versus aromatic
(ꢀ0.208) carbon. This suggests that the key step can be viewed
as a hydrogen atom transfer between the two carbons.
The computed potential energy profile for 1,2-dimethylben-
zene is very similar to that for the 1,3-isomer. However in this
case only aromatic activation (at the 4-position) is observed
experimentally, a difference that seems inconsistent with the very
similar computed barriers to benzylic and aromatic activation
(ΔEa = 14 kJ molꢀ1 for 1,2-dimethylbenzene versus 12 kJ molꢀ1
Variations of transition-state geometries for the aromatic CH
bond activation with the number of ortho methyl groups are
pictured in Figure 5. In all cases the alkyne lies roughly parallel
to the η2-cyclopropene. The NbꢀC(Ar) distance is markedly
longer (2.63 Å) in the case of mesitylene (two ortho methyl
groups) than in the cases of 1,3-dimethylbenzene (2.50 Å)
(one ortho methyl group), 1,3-dimethylbenzene (2.47 Å), and
1,2-dimethylbenzene (2.47 Å) (no ortho methyl group), which
suggests a steric role for the ortho methyl groups. The contraction
3
3
for 1,3-dimethylbenzene). While the statistical factor is 6:1 in
favor of benzylic activation for 1,3-dimethylbenzene, the ratio
is reduced to 3:1 in the case of 1,2-dimethylbenzene, where
two aromatic CꢀH bonds are available. Thus it appears that
statistical factors play a key role in fine-tuning the general
reactivity patterns that are determined primarily by the influence
of ortho methyl groups on the barrier to aromatic CꢀH acti-
vation.
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of the NbꢀC(Ar) distance in the transition state correlates with
For the isomers of dimethylbenzene, product distributions are
rather more variable. For 1,2-dimethylbenzene, the intermediate
Cp*W(NO)(dCHC6H5)5f generates a 94:6 mixture of aryl
Cp*W(NO)(CH2C6H5)(3,4-Me2C6H3) and benzyl Cp*W-
(NO)(CH2C6H5)(CH2-2-MeC6H4) complexes. Our niobium
systems show 100% selectivity for aryl activation, suggesting that
the benzylic position is slightly more accessible in the tungsten
chemistry. In contrast, the 1,4-isomer reacts with the unsaturated
alkylidene Cp*Mo(NO)(dCHCMe3) (generated by R-H ab-
straction of neopentane from Cp*Mo(NO)(CH2CMe3)2) to
give a 73:27 kinetic mixture of the aryl and benzyl complexes
Cp*Mo(NO)(CH2CMe3)(2,5-Me2C6H3) and Cp*Mo(NO)-
(CH2CMe3)(η2-CH2-4-MeC6H4).5i The dominance of aryl
product here stands in stark contrast to the 100% benzylic selec-
tivity displayed by 1. With 1,3-dimethylbenzene, the tungsten
species Cp*W(NO)(dCHC6H5) generates aryl and benzyl
activation products in a 90:10 ratio, not dissimilar to the 3:1
ratio observed with 1.
Thus in cases where the arene hydrogens are sterically blocked
by ortho methyl groups on both sides (i.e., mesitylene), all
reagents with unsaturated carbon- and nitrogen-based ligands
give the same result: activation at the benzylic position. Similarly,
in the least sterically crowded cases, where an arene hydrogen has
no ortho methyl groups, all unsaturated intermediates give aryl
CꢀH activation, albeit with some competitive benzylic reactivity.
Only for the intermediate case of 1,4-dimethylbenzene, where all
arene hydrogens lie ortho to one methyl group, does A show
significantly different reactivity from the alkylidene intermediates
reported by Legdzins and co-workers. The switch to benzylic
activation with intermediate A suggests that the balance between
steric and electronic factors is subtly different from that in
Cp*Mo(NO)(dCHCMe3).
the energy barrier (A, 137 > B, 84 > C, 61 kJ molꢀ1). The
3
transition states appear rather asynchronous, as judged by an
inverse trend for the variation of the distances between the two
carbons and the hydrogen. For 1,2-dimethylbenzene (Figure 4),
the value of 1.39 Å for the C(Ar)ꢀH distance can be compared
with values of 1.40 and 1.49 Å for 1,4-dimethylbenzene and
mesitylene, respectively. This increase in C(Ar)ꢀH distance is
accompanied by a decrease of the C(cyclopropyl)ꢀH distances
from 1.49 Å (1,2- and 1,3-dimethylbenzene, activation at C5) to
1.45 Å (1,3-dimethylbenzene, activation at C4) and 1.43 Å for
mesitylene.
Having established the key role for ortho methyl groups in
determining the barrier to aromatic CꢀH activation, it is impor-
tant to establish whether this effect has steric or electronic ori-
gins. We were able to resolve this question by performing a
parallel set of ONIOM(PBE1PBE:UFF) calculations on the
aromatic activation in 1,4-dimethylbenzene where one of the
methyl substituents is described by a molecular mechanics force
field. Within this protocol, the steric demand of the methyl group
is retained, but its electronic effect is that of hydrogen. The results
of this test are conclusively in favor of a purely steric effect: the
computed barrier to aromatic activation is 88 kJ molꢀ1, very
3
close to the 84 kJ molꢀ1 obtained from a full DFT calculation of
3
this system and far from the ca. 60 kJ molꢀ1 characteristic of
3
cases with no ortho methyl groups.
The seemingly diverse CꢀH activation chemistry of these
substituted benzenes can therefore be rationalized by a simple set
of rules. Overall, there is a small kinetic preference to activate
aromatic over aliphatic CH bonds, but only when steric effects
are negligible. The presence of ortho methyl groups raises the
barrier above that for the alternative benzylic activation, causing a
switch in mechanism.
Summary. Intramolecular β-H abstraction of methane from
TpMe2NbMe(c-C3H5)(MeCCMe) generates an unsaturated in-
termediate [TpMe2Nb(η2-c-C3H4)(MeCCMe)] (A) that acti-
vates benzylic or aryl CH bonds of various dimethylbenzenes and
mesitylene. The aromatic activation pathway is preferred, both
kinetically and thermodynamically, in sterically unhindered
systems. However, the presence of an ortho methyl group raises
the barrier above that for activation of a benzylic CꢀH bond. As a
result, benzylic activation dominates in cases where all aromatic
groups lie ortho to a methyl group.
Comparisons with Related Systems. In this section we
briefly compare our selectivity results with those seen in related
systems. Selectivity in CꢀH bond activation of alkyl aromatics
varies across a very broad spectrum. At one extreme, σ-bond
metathesis of toluene by Cp*2ScMe is extremely unselective,
yielding products resulting from CH activation at all benzylic and
aromatic sites.4 Reactions involving unsaturated intermediates
related to A have, however, proven to be rather more selective.
The groups of Legzdins5,9 and Mindiola6 have studied compe-
titive CH activation reactions of hydrocarbons by unsaturated
molybdenum and tungsten alkylidene complexes and unsatu-
rated titanium alkylidyne complexes, respectively, i.e., 1,2-CH
bond addition across MdC (M = Mo, W) and TitC bonds.
Bergman, Horton, and Wolczanksi have explored similar reaction
chemistry with transient unsaturated d0 imido complexes of
groups 4 and 5, [LnMdNR],13 while Jones has reported CꢀH
activation chemistry for Cp*- and TpMe2-based complexes of
rhodium, in this case via coordinatively unsaturated intermedi-
ates such as TpMe2Rh(CNCMe3).14,15
With mesitylene as substrate, benzylic CꢀH activation is the
exclusive product with all MdC, MtC, and MdN intermedi-
ates, precisely the outcome found in our own experiments. Only
with the coordinatively unsaturated rhodium complexes is there
any evidence for competitive aryl CꢀH activation (a 1:3 kinetic
distribution of TpMe2RhH(CNCMe3)(CH2C6H2-3,5-Me2) and
TpMe2RhH(CNCMe3)(C6H2-2,4,6-Me3)).14 Steric congestion
around Nb resulting from the presence of two η2-coordinated
ligands, 2-butyne and cyclopropene, in A appears to block aryl
CH bond activation, both kinetically and thermodynamically.
’ EXPERIMENTAL SECTION
All experiments were carried out under a dry argon atmosphere using
either Schlenck tube or glovebox techniques. Benzene, pentane, mesi-
tylene, and all dimethylbenzenes were dried by refluxing over CaH2
under argon. Deuterated NMR solvents were dried over molecular
sieves, degassed by freezeꢀpumpꢀthaw cycles, and stored under argon.
1H and 13C NMR spectra were obtained on Bruker ARX 250, DPX 300,
or Avance 300 and 500 spectrometers. Only pertinent 1JCH are quoted in
the 13C spectra. TpMe2NbMe(c-C3H5)(MeCtCMe) (1) was prepared
according to a published procedure.3 Elemental analyses were per-
formed in the Analytical Service of our laboratory.
Synthesis of TpMe2Nb(CH2-3,5-Me2C6H3)(c-C3H5)(MeCtCMe)
(3). TpMe2NbMe(c-C3H5)(MeCtCMe) (1) (0.143 g, 0.286 mmol)
was gently heated in mesitylene (5 mL) at 308 K for 16 h. The color of
the solution changed from yellow to dark orange. The solvent was
evaporated to dryness. The residue was then extracted with pentane, and
the slurry was filtered through a pad of Celite to give a dark orange
solution, which was dried under vacuum. The residue was washed with
2 ꢁ 5 mL of cold pentane (ꢀ80 ꢀC). The powder was isolated and dried
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dx.doi.org/10.1021/om200199e |Organometallics 2011, 30, 3999–4007

Organometallics
ARTICLE
under vacuum (0.089 g, 0.147 mmol, 51%). Crystals were obtained
by cooling a concentrated pentane solution of 3 at 253 K. H NMR
The residue was extracted with 15 mL of pentane and filtered through
a pad of Celite. An orange powder was obtained, which contained
(1H NMR) 1 (32%), 6-Ar (34%), and 6-Bz (12%) together with 22% of
ill-defined complexes. Separated kinetic experiments have been carried
out, and the ratio 6-Ar:6-Bz ≈ 3:1 remained constant throughout the
course of the reaction. Out of the complex mixture, 6-Ar and 6-Bz were
unambiguously and repeatedly identified, although several NMR signals
are missing/obscured and could not be assigned with confidence. For 6-
1
(300 MHz, benzene-d6, 288 K): δ 6.52 (s, 1H, 4-C6H3), 6.45 (s, 2H,
2-C6H3), 5.77, 5.65, 5.61 (all s, 1H each, TpMe2CH), 2.77, 2.28 (both s,
2
3H each, CH3Ct), 2.93, 2.84 (both d, JHH = 11.3 Hz, 1H each,
NbCH2), 2.66, 2.29, 2.19, 2.09, 2.08, 1.71 (all s, 3H each, TpMe2CH3),
2.20 (s, 6H, C6H3CH3), 1.94, 1.74, 1.36, 0.93 (all m, 1:1:2:1 H, CH2β,
CHR, CH2β, CH2β0, CH2β0). 13C NMR (75.47 MHz, benzene-d6, 288
K): δ 240.9, 240.0 (MeCt), 152.6 (Cipso), 151.7, 151.1, 150.2, 143.7,
143.6, 143.4 (TpMe2CMe), 135.6 (m-C6H3), 125.3 (o-C6H3), 123.0 (p-
1
Ar. H NMR (300 MHz, cyclohexane-d12, 288 K): δ 6.53 (s, NbC6-
H3Me2, 3-H), 5.71, 5.66, 5.58 (all s, 1H each, TpMe2CH), 3.02, 2.22
(all s, 3H each, CH3Ct), 2.47, 2.40, 2.38, 2.10, 1.64, 1.19 (all s, 3H each,
TpMe2CH3), 1.95, 1.81, 1.46, 1.24, 0.75 (all m, 1H each, CH2β, NbCHR,
CH2β, CH2β, CH2β). 13C{1H} NMR (75.47 MHz, cyclohexane-d12,
288 K): δ 241.4, 239.0 (MeCt), 193.0 (NbC6H3Me2), 141ꢀ154
1
C6H3), 107.9, 107.1, 106.9 (TpMe2CH), 77.2 (br t, JCH = 117 Hz,
NbCH2), 65.6 (d, 1JCH = 138 Hz, NbCRH), 22.3, 20.0 (CH3Ct), 21.5
(t, 1JCH = 156 Hz, NbCHCH2), 11.7 (t, 1JCH = 159 Hz, NbCHC0H2),
16.3, 15.3, 14.7, 12.8, 12.7, 12.7 (TpMe2CH3). Anal. Calcd for C31H44-
BN6Nb: C, 61.60; H, 7.34; N, 13.90. Found: C, 62.04; H, 7.28; N, 13.62.
Synthesis of TpMe2Nb(CH2-4-MeC6H4)(c-C3H5)(MeCtCMe)
(4). TpMe2NbMe(c-C3H5)(MeCtCMe) (1) (0.163 g, 0.326 mmol)
was heated in 1,4-dimehylbenzene (3 mL) at 315 K for 7 h. The color of
the solution changed from yellow to green. The solvent was evaporated
to dryness. The residue was extracted with 5 mL of pentane and filtered
through a pad of Celite. The solution was concentrated and cooled at
253 K. The resulting precipitate was washed three times with 3 mL of
cold pentane (ꢀ80 ꢀC) to give an orange powder, which was dried under
vacuum (0.088 g, 0.150 mmol, 46%). 1H NMR (300 MHz, benzene-d6,
288 K): δ 7.01, 6.93 (d, 2JHH = 2.4 Hz, 2 H each, NbCH2-4-MeC6H4,
2- and 3-Hs), 5.80, 5.63, 5.62 (all s, 1H each, TpMe2-CH), 2.65, 2.26 (all
s, 3H each, CH3Ct), 2.92, 2.78 (d, 2JHH = 11.4 Hz, 1H each, NbCH2),
2.71, 2.28, 2.16, 2.09, 2.04, 1.77 (all s, 3H each, TpMe2CH3), 2.28 (s, 3H,
C6H4CH3),1.88, 1.84, 1.36, 0.91 (all m, 1:1:2:1 H, CH2β, NbCHR,
CH2β, CH2β0, CH2β0). 13C{1H} NMR (75.47 MHz, benzene-d6,
288 K): δ 241.9, 241.2 (MeCt), 150.6 (Cipso), 151.8, 151.1, 150.4,
144.0, 143.8, 143.5 (TpMe2CMe), 130.0 (4-C6H4), 128.1, 127.0 (m,
(TpMe2CCH3),127ꢀ135 (C6H3Me2), 107.3, 107.1, 106.7 (TpMe2
-
CH), 70.5 (br, NbCR), 21.5, 19.1 (CH3Ct), 15.4, 15.1, 14.7, 13.1,
13.0, 13.0 (TpMe2CH3). For 6-Bz. 1H NMR (300 MHz, cyclohexane-
d12, 288 K): δ 6.80 (pt, 1H, 3JHH = 7.5 Hz, C6H4Me, 4-H), 6.48, 6.46 (d,
1H each, 3JHH = 7.5 Hz, C6H4Me, 3- and 5-H), 6.21 (s, 1H, C6H4Me,
2-H), 5.79, 5.65, 5.62 (all s, 1H each, TpMe2CH), 3.14, 2.35 (all s, 3H
2
each, CH3Ct), 2.64, 2.45 (d, JHH = 8.9 Hz, 1H each, NbCH2C6H4-
Me), 2.61, 2.45, 2.38, 2.33, 1.99, 1.65 (all s, 3H each, TpMe2CH3), 2,09
(s, 6H, C6H4CH3). 13C{1H} NMR (75.47 MHz, cyclohexane-d12,
288 K): δ 127.9 (C6H4Me2, 2-C, 127.0 (C6H4Me2, 4-C, 124.5, 122.0
(C6H4Me2, 5- and 6-C), 77.1 (NbCH2C6H4Me2), 66.3 (NbCR).
Kinetics of the Reaction between 1 and Mesitylene, 1,4-
Dimethylbenzene, or 1,4-Dimethylbenzene-d10. The same
procedure was used for the three cases; that with mesitylene is described
herein. Under argon, a weighted amount of 1 (ca. 10ꢀ4 mmol) was
introduced in a screw-cap NMR tube. Cyclohexane-d12 was added so
that the total volume (cyclohexane-d12 + arene) was 0.5 mL. The appro-
priate amount of mesitylene (see text) was added dropwise with a
microsyringe. The tube was then placed in the preheated (T = 308 K)
NMR probe of the AV300 spectrometer. Acquisitions of 1H NMR
spectra started 10 min later to allow temperature equilibration. Each
acquisition was composed of eight scans with a 13 s delay between scans
(total acquisition time 2 min).
1
o-C6H4), 108.2, 107.3, 107.0 (TpMe2CH), 76.5 (br t, JCH = 117 Hz,
NbCH2Ar), 67.2 (d, 1JCH = 138 Hz, NbCRH), 22.4 (C6H4CH3), 21.0,
20.2 (CH3Ct), 21.7, 12.5 (t, 1JCH = 158, 160 Hz, Cβ0, Cβ), 16.5, 15.4,
15.2, 13.0, 12.9, 12.8 (TpMe2CH3). Anal. Calcd for C30H42BN6Nb: C,
61.03; H, 7.17; N, 14.23. Found: C, 61.55; H, 7.52; N, 13.92.
Synthesis of TpMe2Nb(3,4-Me2C6H3)(c-C3H5)(MeCtCMe)
(5). TpMe2NbMe(c-C3H5)(MeCtCMe) (1) (0.138 g, 0.28 mmol)
was stirred in 1,2-dimethylbenzene (3 mL) at ambient temperature for
20 h. The color of the solution changed from yellow to orange. The
solvent was evaporated to dryness. The residue was extracted with 5 mL
of pentane and filtered through a pad of Celite. An orange powder
containing an inseparable mixture of 5 and the diaryl derivative (9:1
ratio) was obtained after drying under vacuum (0.168 g, 0.28 mmol,
X-ray Crystallography. Data for 3 were collected at 180 K on a
Bruker Kappa Apex II diffractometer using graphite-monochromated
Mo KR radiation (λ = 0.71073 Å) and equipped with an Oxford
Instrument cooler device. The final unit cell parameters have been
obtained by means of a least-squares refinement on a set of 7452 well-
measured reflections. Orange crystal, C31H44BN6Nb; fw 604.4 g molꢀ1
,
3
triclinic, P1, a = 10.3669(5) Å, b = 11.0238(5) Å, c = 14.7350(7) Å, R =
93.515(2)ꢀ, β = 95.803(2)ꢀ, γ = 114.082(2)ꢀ,V = 11519.72(12) Å3, T =
180(2) K, Z = 2, final R indices [I >2σ(I)]: R1 = 0.0427, wR2 = 0.1114,
goodness-of-fit on F2: 1.301.
1
100%). H NMR (300 MHz, benzene-d6, 288 K): δ 6.81 (br s, 3H,
NbC6H3Me2), 5.75, 5.74, 5.62 (all s, 1H each, TpMe2CH), 3.06, 2.28
(all s, 3H each, CH3Ct), 2.28, 2.20, 2.18, 2.16, 1.90, 1.40 (all s, 3H each,
TpMe2CH3), 2.06, 2.05 (s, 3H each, C6H3CH3), 2.27, 2.06, 1.65, 1.42,
0.93 (all m, 1H each, CH2β, NbCHR, CH2β0, CH2β0, CH2β). 1H NMR
The structure has been solved by direct methods using SHELXS-8616
and refined by means of least-squares procedures on F2 with the aid of
the program SHELXL9716 included in the software package WinGX
version 1.63.17 The atomic scattering factors were taken from the Inter-
national Tables for X-ray Crystallography.18 All hydrogen atoms were
geometrically placed and refined by using a riding model. All non-
hydrogen atoms were anisotropically refined, and in the last cycles of
refinement a weighting scheme was used, where weights were calculated
(500 MHz, dichloromethane-d2, 193 K) only dynamic aromatic protons
2
are quoted: δ 8.16 (s, 1H, NbC6H3Me2 2-H), 6.43, 5.71 (d, JHH
=
13.8 Hz, 1H each, NbC6H3Me2, 5- and 6-H). 13C{1H} NMR (75.47 MHz,
benzene-d6, 288 K): δ 241.6, 239.5 (MeCt), 190.9 (NbC6H3Me2), 153.2,
151.1, 151.0, 143.8, 143.5, 143.3 (TpMe2CCH3), 138.9, 135.4, 134.0, 133.2,
132.6 (C6H3Me2), 107.6, 107.1, 107.1 (TpMe2CH), 70.7 (br, NbCRH),
20.0, 19.8 (C6H3(CH3)2), 22.5, 19.4 (CH3Ct), 21.9, 12.9 (Cβ0, Cβ),
15.5, 15.5, 14.9, 13.0, 12.8, 12.8 (TpMe2CH3).
2
from the following formula: w = 1/[σ2(Fo ) + (aP)2 + bP] where P =
(Fo2 + 2Fc2)/3. Plot of the molecular structure was performed with the
program ORTEP3219 with 30% probability displacement ellipsoids for
non-hydrogen atoms.
Reaction of 1 with 1,3-Dimethylbenzene to Give a Mixture
of TpMe2Nb(c-C3H5)(3,5-Me2C6H3)(MeCtCMe) (6-Ar) and
TpMe2Nb(3-MeC6H4)(c-C3H5)(MeCtCMe) (6-Bz). TpMe2NbMe-
(c-C3H5)(MeCtCMe) (1) (0.200 g, 0.40 mmol) was stirred in
1,3-dimethylbenzene (4 mL) at 35 ꢀC for 6 h. The color of the solution
changed from yellow to orange. The solvent was evaporated to dryness.
Computational Details. Calculations were performed by DFT
methods with the PBE1PBE functional,20 as implemented in Gaussian03.21
Nb was described using the LANL2DZ effective core potential for the
inner electrons and its associated basis set for the outer ones.22 An f
polarization shell was added (exponent 0.952).23 The standard 6-31G(d)24
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dx.doi.org/10.1021/om200199e |Organometallics 2011, 30, 3999–4007

Organometallics
ARTICLE
basis set was used for all other atoms. Stationary points and transition
states were fully optimized without any symmetry restriction. Transition
states were identified by having one imaginary frequency. All reported
values correspond to potential energies with zero-point energy corrections
unless otherwise stated. In the case of aromatic activation of 1,4-dimethyl-
benzene, we performed an additional set of calculations with the
ONIOM(PBE1PBE:UFF) method,25,26 where the ortho methyl substituent
was placed in the MM region.
Tsyba, I.; Bau, R. J. Am. Chem. Soc. 2003, 125, 7035. (j) Andino, J. G.;
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’ ASSOCIATED CONTENT
S
Supporting Information. Full ref 21, computed reac-
b
tion profiles with potential and Gibbs free energy values for all
methylbenzene compounds, absolute energies and Cartesian
coordinates for all computed structures, key NMR spectra for
the reactions giving 5 and 6-Bz and 6-Ar, and a CIF file for the
X-ray structure of 3. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: michel.etienne@lcc-toulouse.fr.
’ ACKNOWLEDGMENT
M.E. thanks the ANR program (BLAN07-2_182861) and the
CNRS (LEA 368, LTPMM) for funding and the Universitꢀe Paul
Sabatier for a sabbatical leave. M.E. and J.E.M. thank the Royal
Society for a Short Visit incoming Grant and the Royal Society of
Chemistry for a Grant to International Authors. A.L. and F.M.
thank the Spanish MICINN (projects CTQ2008-06866-CO2-
02/BQU and Consolider Ingenio 2010 CSD2006-0003) and the
ICIQ Foundation for financial support.
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