Mechanism of C-H bond activation of alkyl-substituted benzenes by cationic platinum(II) complexes

Article abstract of DOI:10.1021/om050251m

While all methyl- and ethyl-substituted benzenes react with diimine Pt(II) methyl cations to give η³-benzyl products, they do not all get there by the same pathway. For toluene and p-xylene, isotopic labeling shows that initial activation occurs at aryl positions with subsequent intermolecular conversion to the benzyl product. For ethylbenzene and 1,4-diethylbenzene, initial activation takes place exclusively at aryl C-H bonds, and conversion to the η³-benzyl product takes place via intramolecular isomerization. Only in the most extreme case of steric crowding-the reaction of a bulky diimine platinum methyl cation (Ar = Mes) with triethylbenzene-does direct activation of the ethyl group become preferred to aryl activation.

Full text of DOI:10.1021/om050251m

3644
Organometallics 2005, 24, 3644-3654
Mechanism of C-H Bond Activation of Alkyl-Substituted
Benzenes by Cationic Platinum(II) Complexes
Tom G. Driver, Michael W. Day, Jay A. Labinger,* and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of
Technology, Pasadena, California 91125
Received April 4, 2005
While all methyl- and ethyl-substituted benzenes react with diimine Pt(II) methyl cations
to give η3-benzyl products, they do not all get there by the same pathway. For toluene and
p-xylene, isotopic labeling shows that initial activation occurs at aryl positions with
subsequent intermolecular conversion to the benzyl product. For ethylbenzene and 1,4-
diethylbenzene, initial activation takes place exclusively at aryl C-H bonds, and conversion
to the η3-benzyl product takes place via intramolecular isomerization. Only in the most
extreme case of steric crowdingsthe reaction of a bulky diimine platinum methyl cation (Ar
) Mes) with triethylbenzenesdoes direct activation of the ethyl group become preferred to
aryl activation.
Introduction
patterns of alkyl-substituted benzenes, to identify fac-
tors controlling kinetic and thermodynamic preferences
for activation of aryl vs alkyl C-H bonds.
Selective C-H bond activation and functionalization
is a long-standing and current goal in organic and
organometallic chemistry.1 In most cases to date, ob-
taining useful selectivities requires the assistance of
directing groups.2-5 In the absence of such direction,
previous research has suggested that initial C-H bond
activation is kinetically not very selectivesless selective
than hydrogen abstraction by radicals, for examples
although reversible activation often leads to selective
formation of products as a consequence of thermody-
namic stability.6-10 We have been examining reactivity
Much of our work on the detailed mechanism of C-H
bond activation has involved platinum methyl cations
[(NN)PtMe(solv)]+ (2),10-12 where (NN) represents a
bidentate diimine ligand and (solv) ) trifluoroethanol
(TFE). 2 is obtained by protonolysis of platinum dimeth-
11
yl 1 in TFE with either HBF4‚OH2 (which affords an
(6) For reports where the product ratio of transition metal-mediated
C-H bond activation changed as a function of time or temperature,
see: (a) Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1983, 105,
3929-3939. (b) Jones, W. D.; Feher, F. J. J. Am. Chem. Soc. 1984,
106, 1650-1663.
(7) For reports of nonselective H/D isotope exchange catalyzed by
transition metals via C-H bond activation, see: (a) Garnett, J. L.;
Hodges, R. J. J. Am. Chem. Soc. 1967, 89, 4546-4547. (b) Goldshleger,
N. F.; Tyabin, M. B.; Shilov, A. E.; Shteinman, A. A. Russ. J. Phys.
Chem. (Engl. Transl.) 1969, 43, 1222-1223. (c) Shilov, A. E.; Shtein-
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Green, M. L. H.; Izquierdo, A. J. Chem. Soc., Chem. Commun. 1981,
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Am. Chem. Soc. 2002, 124, 2092-2093. (f) Skaddan, M. B.; Yung, C.
M.; Bergman, R. G. Org. Lett. 2004, 6, 11-13. (g) Yung, C. M.;
Skaddan, M. B.; Bergman, R. G. J. Am. Chem. Soc. 2004, 126, 13033-
13043.
* To whom correspondence should be addressed. E-mail: bercaw@
caltech.edu.
(1) For recent reviews on C-H activation and functionalization,
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H. Acc. Chem. Res. 1995, 28, 154-162. (b) Shilov, A. E.; Shul’pin, G.
B. Chem. Rev. 1997, 97, 2879-2932. (c) Erker, G. Angew. Chem., Int.
Ed. 1999, 38, 1699-1712. (d) Davies, H. M. L.; Antoulinakis, E. G. J.
Organomet. Chem. 2001, 617-618, 47-55. (e) Crabtree, R. H. J. Chem.
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Inorg. Chem. 2003, 54, 259-320.
(2) Ryabov, A. D. Chem. Rev. 1990, 90, 403-424.
(3) Pfeffer, M. Pure Appl. Chem. 1992, 64, 335-342.
(8) For a broad list of C-H abstraction reactivities by radicals, see:
Ingold, K. U. Rate Constants for Free Radical Reactions in Solution.
In Free Radicals; Kochi, J. K., Ed.; Wiley: New York, 1973; pp 37-
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(a) Tan, K. L.; Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc. 2001,
123, 2685-2686. (b) Dangel, B. D.; Johnson, J. A.; Sames, D. J. Am.
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(9) For recent mechanistic reports of transition metal-mediated C-H
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Tilset, M. J. Am. Chem. Soc. 2001, 123, 6579-6590. (b) Northcutt, T.
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Chem. Soc. 2000, 122, 10846-10855.
(5) For recent reports of nondirected regioselective C-H bond
activations or functionalizations, see: (a) Bennett, J. L.; Wolczanski,
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10.1021/om050251m CCC: $30.25 © 2005 American Chemical Society
Publication on Web 06/22/2005

C-H Bond Activation of Alkyl-Substituted Benzenes
Organometallics, Vol. 24, No. 15, 2005 3645
Scheme 1
Scheme 2
of p-xylene by 2b to give 6; the latter occurs about twice
as fast. A subsequent, slower intermolecular reaction
converts 6 to 7 (Scheme 2).10b
equilibrium mixture with aquo complex 3)10a,12 or
B(C6F5)3 (which produces acid by coordinating to tri-
fluoroethanol),13,14 yielding anhydrous 2. In either case,
if deuterated solvent is used, 2 is formed as a mixture
of two isotopologs,15,16 as shown in Scheme 1. (In
subsequent schemes we will represent this mixture
simply as Pt-Me.)
Platinum methyl cation 2 reacts with benzene to form
phenyl complex 5 via benzene π-complex 4.12 Kinetics
studies indicate that the identity of the rate-determining
step depends on the degree of steric crowding: for
relatively congested complexes with 2,6-disubstituted
aryl substituents (Ar) on the diimine ligand, as for 2a
(Ar ) mesitylene), coordination of benzene is rate
limiting, whereas in the absence of ortho substitution
(as for 2b and 2c), C-H bond activation becomes the
slowest step (Scheme 1).10a
The present work seeks to extend these patterns to
other methyl- and ethyl-substituted benzenes. Questions
of interest include the following: Is an η3-complex
always the thermodynamic product? Do aryl and ben-
zylic activation always compete? What are the relative
reactivities of methyl and ethyl C-H bonds? For eth-
ylbenzenes, is there direct secondary C-H bond activa-
tion of the benzylic methylene group? We show through
the use of isotopic labeling and other experiments that
while the η3-complex is apparently always thermody-
namically favored, there is not one single common
pathway for getting there; rather the detailed mecha-
nism depends on the steric nature of both the platinum
diimine ligand and the substrate in an unexpectedly
complex manner.
Results and Discussion
Anhydrous 2 reacts with alkyl-substituted benzenes
(p-xylene, mesitylene, p-diethylbenzene) to form η3-
benzyl complexes as the sole final products, the appar-
ent result of selective benzylic C-H bond activation.10b,17
Kinetics experiments revealed that the reaction of 2b
with p-xylene involves competing direct benzylic activa-
tion to give η3-complex 7 and aryl C-H bond activation
1,4-Diethylbenzene: Reactivity and Products.
Extensive studies of reactivity and product properties
were performed using 1,4-diethylbenzene as substrate,
both because symmetry provides (relatively) straight-
forward interpretation and for comparison to the base-
line p-xylene case. Irrespective of the steric nature of
platinum methyl cation 2, η3-complex 9 was the sole
final product obtained from the reaction of 2 with 1,4-
diethylbenzene (eq 1, Figure 1). For less bulky diimine
ligands (2b, 2c), an intermediate aryl complex 8 could
(13) Hill, G. S.; Manojlovic-Muir, L.; Muir, K. W.; Puddephatt, R.
J. Organometallics 1997, 16, 525-530.
(14) Heyduk, A. F.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
2003, 125, 6366-6367.
1
be observed by H NMR spectroscopy before complete
(15) For a discussion on the proper usage of the terms isotopolog
and isotopomer, see: Seeman, J. I.; Paine, J. B. Chem. Eng. News 1992,
70, 2.
conversion to the η3-complex, analogous to the behavior
of p-xylene. In contrast when a sterically bulky aryl
group was employed (2a), only the disappearance of
starting cation and formation of product could be
detected. Qualitatively, the rate of the reaction de-
pended on the identity of the diimine aryl group: the
half-life for the reaction (with 2.5 equiv of substrate)
was less than 1 h for 2b or 2c but around 8 h for 2a.
(16) Johansson, L.; Tilset, M. J. Am. Chem. Soc. 2001, 123, 739-
740.
(17) For recent, leading reports where η3-benzylic complexes have
been isolated, observed, or proposed as intermediates, see: (a) Pd: Rix,
F. C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996, 118, 2436-
2448. (b) Pd: LaPointe, A. M.; Rix, F. C.; Brookhart, M. J. Am. Chem.
Soc. 1997, 119, 906-917. (c) Ni: Djukic, J.-P.; Do¨tz, K. H.; Pfeffer,
M.; De Cian, A.; Fischer, J. Organometallics 1997, 16, 5171-5182. (d)
Re: Wang, T.-F.; Hwu, C.-C.; Tsai, C.-W.; Wen, Y.-S. J. Chem. Soc.,
Dalton Trans. 1998, 12, 2091-2096. (e) Pt: White, S.; Kalberer, E.
W.; Bennett, B. L.; Roddick, D. M. Organometallics 2001, 20, 5731-
5737. (f) Pd: Nettekoven, U.; Hartwig, J. F. J. Am. Chem. Soc. 2002,
124, 1166-1167. (g) Ni: Fassina, V.; Ramminger, C.; Seferin, M.;
Mauler, R. S.; de Souza, R. F.; Monteiro, A. L. Macromol. Rapid
Commun. 2003, 24, 667-670. (h) Pd: Kuwano, R.; Kondo, Y.; Mat-
suyama, Y. J. Am. Chem. Soc. 2003, 125, 12104-12105. (i) Re: Casey,
C. P.; Boller, T. M.; Kraft, S.; Guzei, I. A. J. Am. Chem. Soc. 2002,
124, 13215-13221. (j) Ni: Albers, I.; AÄ lvarez, E.; Ca´mpora, J.; Maya,
C. M.; Palma, P.; Sa´nchez, L. J.; Passaglia, E. J. Organomet. Chem.
2004, 689, 833-839. (k) Pd: Vo, L. K.; Singleton, D. A. Org. Lett. 2004,
6, 2469-2472. (l) Mo: Mork, B. V.; Tilley, T. D.; Schultz, A. J.; Cowan,
J. A. J. Am. Chem. Soc. 2004, 126, 10428-10440.
Although aryl activation products were not observed
in the reaction of 1,4-diethylbenzene with the crowded

3646 Organometallics, Vol. 24, No. 15, 2005
Driver et al.
1
Figure 1. Diagnostic H NMR peaks for 9a, showing 195Pt satellites10b
Scheme 3
platinum methyl cation 2a, exposure of platinum phenyl
cation 5a to an excess of diethylbenzene resulted in the
clean formation of the η3-complex 9a (eq 2). Aryl C-H
bond activation thus appears to be reversible under
these reaction conditions, but formation of the product
η3-benzyl complex is irreversible: 9a-d9 does not react
with an excess of 1,4-diethylbenzene-d0 (eq 3).18
doublet. This isotope exchange at Ha was reversible:
removal of the deuterated solvent and replacement with
methanol-d0 resulted in reappearance of the doublet,
along with some concomitant decomposition (see below).
Two possible mechanisms for isotope exchange can
be envisioned: â-hydride elimination or direct depro-
tonation (Scheme 3). â-Hydride elimination (path A)
from 9 would generate an ethylstyrene-hydride-Pt(II)
complex 10-d0, which could exchange with solvent to
afford 10-d1. 2,1-Insertion of 4-ethylstyrene into 10-d1
would regenerate the benzyl product. The insertion
would have to be completely regioselective19 since the
signal intensity of Hb remains unchanged. In the Bro¨n-
sted acid-base mechanism (path B), deprotonation
would generate ethylstyrene-Pt(0) complex 11, which
would reprotonate with D+, giving exchange solely at
While the η3-complexes are resistant to further C-H
bond activation, they do undergo slow isotope exchange
with solvent at the methyl (Ha) position (eq 4). For
example, after reaction of 2a with 1,4-diethylbenzene,
the methyl-Ha resonance for 9a appeared as a doublet
at 0.30 ppm in the 1H NMR. In CF3CD2OD solution this
resonance slowly became a multiplet as it lost intensity
and eventually disappeared. No other signal’s intensity
was affected. Addition of methanol-d4 increased the rate
of isotope exchange. For smaller 3,5-disubstituted di-
imine ligands, exchange was competitive with C-H
activation: after complete reaction, η3-complex 9c ex-
hibited a multiplet at 0.30 ppm instead of the expected
(19) The insertion of styrene into related palladium hydride com-
plexes typically occurs somewhat regioselectively, but not with com-
plete selectivity. For leading references, see: (a) Iggo, J. A.; Kawash-
ima, Y.; Liu, J.; Hiyama, T.; Nozaki, K. Organometallics 2003, 22,
5418-5422. (b) Nozaki, K.; Komaki, H.; Kawashima, Y.; Hiyama, T.;
Matsubara, T. J. Am. Chem. Soc. 2001, 123, 534-544. (c) Milani, B.;
Anzilutti, A.; Vicentini, L.; Sessanta o Santi, A.; Zangrando, E.;
Geremia, S.; Mestroni, G. Organometallics 1997, 16, 5064-5075. (d)
Brookhart, M.; Rix, F. C.; DeSimone, J. M.; Barborak, J. C. J. Am.
Chem. Soc. 1992, 114, 5894-5895. (e) Barsacchi, M.; Consiglio, G.;
Medici, L.; Petrucci, G.; Suter, U. W. Angew. Chem., Int. Ed. Engl.
1991, 30, 989-991.
(18) This experimental result was not unique to complex 9a: the
η3-complexes formed from reaction with p-xylene and ethylbenzene did
not react further with excess different isotopologs of the starting
substrate either.

C-H Bond Activation of Alkyl-Substituted Benzenes
Organometallics, Vol. 24, No. 15, 2005 3647
examined how the Pt center discriminates between
substituents in mixed (methyl-ethyl) benzenes. In ad-
dition to the issue of regioselectivity, there is also the
possibility of constitutional isomerism, depending on the
nature of the substrate, because the η3-benzyl structures
appear to be nonfluxional on the NMR time scale.10b
This is illustrated for the case of 3-ethyltoluene in eq 7,
where products derived from both ethyl (14) and methyl
(15) C-H bond activation were observed, each existing
in two isomeric forms (denoted as x and y; we have no
way of knowing which is which).
Figure 2. X-ray structure of 13a.
the methyl position. The latter appears more likely,20
but the former cannot be ruled out at this time.
Both the regioselectivity of benzylic C-H bond activa-
tion and the isomeric selectivity of the resulting η3-
complex appear to depend on the steric natures of both
the diimine ligand and the substrate, although the
dependence is not completely straightforward (Table 1).
In general we found a range from a slight preference to
a substantial preference for ethyl activation, except for
the case of the bulkiest diimine ligand (Ar ) Mes) and
substrate (3,5-dimethylethylbenzene), where activation
of the methyl C-H bond was preferred by 13:87 (entry
1). With the less sterically encumbered substrate 4-eth-
yltoluene, the selectivity of C-H bond activation was
reversed to give a strong preference for activation of the
ethyl C-H bond (entry 2), a preference eroded only
slightly for the ortho- or meta-analogues (entries 3 and
4). In most cases where constitutional isomerism is
possible there is a strong preference for one isomer, with
the exception of the products of ethyl activation of
3-ethyltoluene.
There appears to be a general preference for ethyl
activation, with some reversals, most notably for the
reaction of the most crowded complex with the trisub-
stituted arene (entry 1). However, a steric model clearly
cannot explain all the behavior; for example, note the
lower proportion of ethyl activation in entries 7, 8, 11,
and 12 (smaller ligands) compared to entries 3 and 4.
Since ethyl- and methyl-substituted benzenes react by
different mechanisms (see below), the absence of any
straightforward trend may not be surprising. In any
case, the fact that ligand control of selectivity can be
obtained is encouraging and suggests the possibility of
useful transformations, such as asymmetric C-H acti-
vation, with the appropriate ligand choice.
The η3-bonding in 9 is not affected by the presence of
oxygen Lewis bases such as Et2O, MeOH, or H2O. Better
ligands, such as the nitrogen Lewis bases acetonitrile,
benzonitrile, and aniline, are able to displace the
benzene ring π-bond to form η1-complexes 12 (eq 5), as
1
revealed by significant differences in the H NMR of
12: the diagnostic Pt-H coupling for the Hc of 9 was
no longer visible and its peak was shifted downfield.21
9 decomposes gradually, especially upon concentra-
tion or exposure of η3-complex to water or air, to form
13, resulting from aryl transfer from the counterion (eq
6). No 13 was observed spectroscopically during reaction
of 2a (Ar ) Mes) with 1,4-diethylbenzene to form η3-
complex 9a, but it was found as a competitive byproduct,
in up to 20-30% yield, at low substrate concentrations,
with a sterically less congested complex (2c) or substrate
(ethylbenzene). The structure of 13a (R ) H) was
determined crystallographically (Figure 2).22,23
Experiments with Regiospecifically Isotopically
Labeled Substrates: General Comments. Although
reaction of platinum methyl cations 2 with alkyl-
substituted benzenes always leads to η3-complexes as
Regioselectivity of Benzyl C-H Bond Activa-
tion: Methyl vs Ethyl Substituents. η3-Complexes
were obtained as sole final products from all the methyl-
and/or ethyl-substituted benzenes studied. We further
(21) The crystal structure of an analogous (η1-benzyl)(CH3CN)
complex from p-xylene has been determined; see ref 10b.
(22) Refer to the Supporting Information for crystallographic data.
(23) Crystallographic data have been deposited at the CCDC, 12
Union Road, Cambridge CB2 1EZ, UK, and copies can be obtained on
request, free of charge, by quoting the publication citation and the
deposition number 248006.
(20) When deuterated alcohols more Lewis basic than TFE-d3 were
employed, isotope exchange was observed in the backbone methyl
groups of the diimine ligand; see ref 10a.

3648 Organometallics, Vol. 24, No. 15, 2005
Driver et al.
Table 1. Comparison of Regioselectivity of C-H
is at most a minor contributor unless a more basic
medium, such as methanol, is employed.10a
Bond Activation
Isotopic exchange in unreacted substrate can arise
from two routes. The process outlined in Scheme 5
scrambles isotopes between substrate and the Pt-bound
methyl group (and perhaps with solvent as well); hence,
if substrate coordination and activation are reversible,
exchanged substrate would be released, accompanied by
exchange in the methyl group of unreacted 2. The other
route is a secondary reaction such as that seen for
p-xylene (Scheme 2); for the case of deuterated methyl
groups and undeuterated benzene ring (or the reverse)
the conversion of 6 to 7 would change the isotope at one
aromatic position. An additional possibility would be via
the exchange with solvent at Ca of 9 (eq 4), but since
other evidence indicates formation of η3-benzyl product
is completely irreversible (see above), this would not be
expected to lead to any exchange in free substrate.
The isotopic composition of liberated methane and
unconverted substrate was determined by 1H NMR
spectroscopy and mass spectrometry, respectively. For
convenience, the methane formed during the initial
protonolysis of 1 (a roughly equimolar mixture of CH4
and CH3D) was not separated, so that the measurement
represents the sum of that sample and the methane
formed during substrate activation (Scheme 4). The
ratio of peak intensities for [M + 1]+/[M]+ (for D
replacing H) or [M - 1]+/[M]+ (for H replacing D)
provides an estimate of the extent of exchange in
recovered free substrate; a value of 1,25 for example,
would indicate that a replacement has occurred in about
half the molecules. Mass spectrometry can determine
not only the extent but also the location of exchange,
since most substrates exhibit two identifiable mass
fragments: the parent ion ([M]+) and the fragment ([M
- Me]+). If a ratio for the parent peak is the same as
the corresponding ratio in [M - Me]+, then the exchange
must be exclusively on the arene ring; if they differ,
some or all of the exchange is on the methyl group.
Because of the different pathways available for multiple
H-D substitution, which complicates quantitative analy-
sis of these measurements, the discussion will be mostly
qualitative.
Scheme 4
final product, the mechanism by which they are reached
is far from clear. In some cases competitive aryl C-H
activation may be observed, but failure to observe such
species does not necessarily imply they are not inter-
mediates. Evidence bearing on this question may be
obtained from reactions of regiospecifically isotopically
labeled substrates, followed by analysis of isotope
distribution in the liberated methane, recovered sub-
strate, and η3-benzyl product.
The composition of methane isotopologs indicates the
site of initial activation, as illustrated in Scheme 4 for
the reaction of benzene with platinum methyl cation 2.
Activation of a C-H bond results in only two isotopologs
of methane, CH4 and CDH3 (recall that a mixture of 2-d0
and 2-d1 is generated in the protonolysis of the starting
platinum dimethyl complex).16 In contrast, when a
substrate C-D bond is activated, all four isotopologs of
methane are generated.
In the latter case, multiple deuterium incorporation
into methane occurs because dissociation of methane
from the σ-complex 16 is slower than the reversible
C-H bond cleavage/formation reactions via platinum
hydride 17 (Scheme 5), in most cases leading to complete
or near-complete statistical scrambling among the posi-
tions in 5 and methane.10a,12,24 Additional incorporation
of deuterium can be effected by isotope exchange
between platinum hydride 17 and solvent. This latter
process is influenced by basicity of reaction media and
Evidence of isotope migration within final η3-benzyl
products can also provide information on steps following
initial activation. As we shall see, such observations
apply only to ethyl-substituted benzenes.
Examination of Methyl-Substituted Benzene Iso-
topologs. The results for reactions between variously
labeled methyl-substituted benzenes and platinum meth-
yl cations 2 (eq 8) are shown in Table 2. Let us first
look in detail at the findings for the various toluene
(24) For leading reports on the role of σ-complexes in C-H activa-
tion, see: (a) Mobley, T. A.; Schade, C.; Bergman, R. G. J. Am. Chem.
Soc. 1995, 117, 7822-7823. (b) Stahl, S. S.; Labinger, J. A.; Bercaw,
J. E. J. Am. Chem. Soc. 1996, 118, 5961-5976. (c) Bromberg, S. E.;
Yang, H.; Asplund, M. C.; Lian, T.; McNamara, B. K.; Kotz, K. T.;
Yeston, J. S.; Wilkens, M.; Frei, H.; Bergman, R. G.; Harris, C. B.
Science 1997, 278, 260-263. (d) Geftakis, S.; Ball, G. E. J. Am. Chem.
Soc. 1998, 120, 9953-9954. (e) Janak, K. E.; Parkin, G. J. Am. Chem.
Soc. 2003, 125, 6889-6891. (f) Castro-Rodriguez, I.; Nakai, H.; Gantzel,
P.; Zakharov, L. N.; Rheingold, A. L.; Meyer, K. J. Am. Chem. Soc.
2003, 125, 15734-15735. (g) Lawes, D. J.; Geftakis, S.; Ball, G. E. J.
Am. Chem. Soc. 2005, 127, 4134-4135.
(25) There will be some [M - 1]+ or [M - 1]+ intensity resulting
from fragmentation, natural abundance 13C, etc.; all values (Tables 2,
3, 5) are corrected for the values observed in unexchanged substrates.

C-H Bond Activation of Alkyl-Substituted Benzenes
Organometallics, Vol. 24, No. 15, 2005 3649
Scheme 5. Mechanisms for Multiple Methane Isotopolog Formation
1
isotopologs reacting with 2a (entries 1-4).26 The meth-
ane isotopolog distribution depends only on the aryl
labeling: if the aryl positions are protonated, CH4 and
CH3D are obtained in about 3:1 ratio, while if the aryl
positions are deuterated, all four methane isotopologs
are observed. This indicates that the initial site of C-H
activation is exclusively the arene sites. We would
expect scrambling between the five aryl positions and
three Pt-Me positions to be rapid, analogous to the
benzene case shown in Schemes 4 and 5. The Pt-Me
consists of about 1:1 CH3 and CH2D, so the methane
liberated by reaction with aryl-protonated toluenes
should be predominantly (∼15/16) CH4 (neglecting any
isotope effects); when combined with the 1:1 mixture of
CH4 and CH3D obtained in protonolysis, the overall
ratio should be close to 3:1 CH3:CH2D. In contrast, aryl-
deuterated toluenes should give a preponderance of
deuterated methanes, as observed.
monitoring the reaction progress using H NMR spec-
troscopy: formation of additionally deuterated platinum
methyl cation 2a before its consumption was observed,
but only when alkyl-substituted benzenes deuterated in
aryl positions, such as toluene-d5, were employed as
substrates (eq 9). The platinum methyl isotopologs 2a-
d1 and 2a-d2 were increased by about 25% at 60%
conversion.27
What happens if the aryl and methyl positions of
toluene have opposite labels? In the sequential mech-
anism shown (for p-xylene) in Scheme 2, initially formed
Pt-aryl 6 reacts with the benzylic position of a second
molecule of substrate to give the η3-complex 7. In the
case of toluene, the isotopolog distribution for liberated
methane indicates that all of the reaction follows this
route. Hence, for this case, we would expect that every
product-forming event will be accompanied by liberation
of an aryl-exchanged molecule of toluene, and the extent
of exchanged free substrate should be much higher. The
exact level of exchange is difficult to calculate, as it
involves not only this mechanism along with the scram-
bling described in the preceding paragraph, but also the
extent to which exchanged molecules liberated at low
conversion levels can re-react with another molecule of
2. Qualitatively, though, it is clear from Table 2
(compare entries 3 and 4 to 1 and 2) that the observa-
tions agree well with this model.
The extent of isotope exchange in free substrate
depends on whether the aryl and benzylic positions have
the same or opposite isotopes. In the first case, exchange
will result from the above scrambling process followed
by loss of toluene, either by reversible activation (i.e.,
loss of benzene from 4) or by secondary reaction (as in
Scheme 2). Consider entry 1: since 2 contains on
average half a D per Pt, and the reaction was carried
out with 2.5 equiv of substrate per Pt, full statistical
scrambling would place a D in about 1/4 of the substrate
present at the completion of reaction, in rough agree-
ment with the observed values, which correspond to
exchange in 1/5 of the molecules. Evidence for some
reversible aryl C-H bond activation was obtained from
(26) With the sterically smaller platinum methyl complex 2c (Ar )
3,5-di-tert-butylbenzene), similar trends were observed. For presenta-
tion of these data, refer to the Supporting Information.
(27) The distribution of platinum methyl cation isotopologs was
estimated from comparison of the Pt-Me resonance with an aryl proton
resonance. See the Supporting Information for more details.

3650 Organometallics, Vol. 24, No. 15, 2005
Table 2. Examination of Methyl-Substituted Benzene Isotopologs
MS analysis of recovered substrateb
Driver et al.
[M + 1]+/
[M - 1]+/
[(M + 1) - Me]+/
[(M - 1) - Me]+/
entry
LnPtMe+
substrate
CH4:CDH3:CD2H2:CD3Ha
[M]+
[M]+
[M - Me]+
[M - Me]+
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2a
2a
2b
2b
CH3C6H5
76:24:0:0
0.26
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
0.13
CD3C6D5
19:36:32:13
18:42:32:8
75:25:0:0
0.25
1.09
CH3C6D5
CD3C6H5
1,4-(CH3)2C6D4
1,4-(CD3)2C6H4
1,4-(CH3)2C6D4
1,4-(CD3)2C6H4
0.85
0.37
0.34
27:39:26:8
62:34:4:<1
15:32:35:18
64:31:4:1
0.13
0.33
0.39
0.34
0.38
a The reported ratio includes the methane isotopologs produced from the protonation of the starting platinum dimethyl complex as
determined using 1H NMR spectroscopy. b Ion ratios corrected by corresponding values for unexchanged starting substrate.
The findings for xylene isotopologs are less clear, as
expected from the fact that initial activation involves
competition between the aryl and benzyl positions.10b
Consistent with that earlier finding, both p-xylene-
R,R,R,R′,R′,R′-d6 and p-xylene-d4 led to all four isoto-
pologs of methane (entries 5-8), confirming that initial
benzylic C-H bond activation is competitive with aryl
C-H bond activation. Quantitatively, the degree of
incorporation of label from benzylic positions appears
initially to be less than expected: for 2b, aryl C-H bond
activation had been reported to proceed only twice as
fast as benzylic CH3 bond activation. Again, though, any
calculation is complicated by the factors discussed
above, the possibility of differential kinetic isotope
effects (previously a KIE of 4.0 was found for aryl C-H
bond activation only),10b and the further likelihood that
reversible C-H activation applies only to aryl, not
benzyl, positions; once a benzyl position has reacted, it
is probable that the adjacent benzene π-bond coordi-
nates immediately and prevents further scrambling (if
benzyl C-H bond activation were reversible, the result-
ant η3-benzyl complex could be exchanged with a di-
fferent isotopolog, which was not observed; see above).18
This factor would increase (possibly very substantially)
the relative incorporation of H or D into liberated
methane from aryl rather than benzyl positions. Isotope
exchange in recovered substrate is (qualitatively) in
agreement: comparison of the parent and (parent minus
methyl) peaks shows clearly that all such exchange
takes place in aryl, not benzylic, positions.
To summarize, these data suggest that the mecha-
nism of C-H bond activation of methyl-substituted
benzenes is dependent on the steric nature of the
substrate (Scheme 6). Direct benzylic C-H bond activa-
tion to form 26 is competitive with aryl C-H bond
activation only when the aryl sites are sterically con-
gested, as in p-xylene. For the more accessible aryl C-H
bonds in toluene, aryl C-H activation occurs rapidly.
The activation of the benzylic methyl C-H bond pre-
sumably occurs via the corresponding σ-complex. For-
mation of this σ-complex could occur directly between
reaction of platinum aryl complex 25 and the benzylic
methyl C-H bond or more likely from initial π-coordi-
nation followed by migration to the σ-complex. Upon
formation of the benzylic platinum(II) species 26, the
complex relaxes rapidly to η3-product 18, a thermody-
namic well.
Scheme 6
results of analysis of the methane isotopologs and
recovered substrate are shown in Table 3 and are mostly
analogous to those found for toluene substrates. The
methane isotopolog ratio depends only on the isotope
in the aryl positions, and recovered ethylbenzene shows
low levels of exchange, in the aryl positions only, arising
from scrambling with D in the starting Pt methyl group.
No participation of label in the ethyl group could be
observed (entries 4-6). Together, these results show
that initial C-H bond activation occurs exclusively at
the sterically accessible aryl C-H bonds. By following
the reaction progress using 1H NMR spectroscopy, a
mixture (87:13) of intermediate platinum aryl complexes
(28) can be observed, presumably corresponding to
activation of para- and meta-aryl C-H bonds (eq 10).
Examination of Ethyl-Substituted Benzene Iso-
topologs. Formation of η3-complexes from the reaction
of 2a26 with ethyl-substituted benzene isotopologs was
investigated in the same manner. For ethylbenzene,
In striking contrast to the toluene case, though, there
is no additional exchange in recovered substrate when

C-H Bond Activation of Alkyl-Substituted Benzenes
Organometallics, Vol. 24, No. 15, 2005 3651
Table 3. Examination of Ethylbenzene Isotopologs
MS analysis of recovered substrateb
[(M + 1) - Me]+/
[(M - 1) - Me]+/
[M - Me]+
entry
substrate
CH4:CDH3:CD2H2:CD3Ha
[M + 1]+/[M]+
[M - 1]+/[M]+
[M - Me]+
1
2
3
4
5c
6
CH3CH2C6H5
CD3CD2C6D5
CH3CH2C6D5
CH3CD2C6H5
CH3CD2C6H5
CD3CH2C6H5
76:24:0:0
12:35:41:12
14:37:38:11
73:27:0:0
100:0:0:0
73:27:0:0
0.21
0.20
0.23
0.22
0.22
0.23
0.22
0
0.19
0.21
0
0.19
a The reported ratio includes the methane isotopologs produced from the protonation of the starting platinum dimethyl complex as
determined using 1H NMR spectroscopy. b Ion ratios corrected by corresponding values for unexchanged starting substrate. c CF3CH2OH
employed as solvent.
Table 4. Examination of Ethylbenzene Isotopologs
nation to styrene complex 31. Instead of reverting to
30 (1,2-insertion), 31 can alternatively undergo irrevers-
ible 2,1-insertion to form a benzylic structure, R-phen-
ethyl platinum(II) complex 32, which would rapidly
relax to the η3-product 27.29 This sequence would
scramble label between positions Hc and Ha and would
also move label from Hb to Ha, but would never move
label into Hb. The fact that less label is transferred from
Hb to Hc than to Ha suggests that 31 goes to 32 at least
as fast, or perhaps somewhat faster, than it reverts to
30.
Results for 1,4-diethylbenzene isotopologs (eq 11,
Table 5) are generally consistent with this mechanism,
but exhibit some interesting differences. As above,
introduction of D from substrate into methane was
observed only when 1,4-diethylbenzene-d4 was employed
as substrate (entry 4), implying that a platinum aryl
complex is formed before subsequent rearrangement to
the η3-complex, although in this case no intermediates
can be detected by NMR spectroscopy. In contrast to the
ethylbenzene case, however, the recovered substrate
showed only very small amounts of exchange, suggest-
ing that aryl activation is not reversible, but that
rearrangement to η3-benzyl is relatively much faster.
Since an aryl product from 1,4-diethylbenzene must
have Pt ortho to an ethyl group, Scheme 7 would indeed
predict just such behavior: not only is the aryl product
always in the right configuration to undergo oxidative
addition of a methyl C-H bond, but also one would
expect slower aryl C-H activation as a consequence of
crowding (see below).30 1H NMR spectroscopy of the
product η3-complex again shows exchange between Ha
and Hc (entries 3 and 4).
% deuterium in 27
entry
substrate
Ha
18
>95
64
47
46
Hb
Hc
1
2
3
4
5
CH3CH2C6H5
CD3CD2C6D5
CH3CH2C6D5
CH3CD2C6H5
CD3CH2C6H5
0
>95
0
>95
0
0
>95
44
trace
66
the aryl and ethyl groups (at the methyl and/or meth-
ylene position) are oppositely labeled (entries 3-6). This
result implies that conversion of 28 to 27 does not
involve reaction with a second molecule of ethylbenzene,
but rather some alternate pathway. It further implies
that the exchange that was found for recovered sub-
strate is due to reversible aryl C-H activation alone,
in agreement with the observation of additional deu-
teration of unreacted 2, at partial reaction, when the
aryl ring was deuterated.
How then does the conversion of 28 to 27 take place?
The pattern of isotope exchange in the product η3-
complexes (Table 4) is highly informative on this ques-
tion. A small amount of isotope exchange at Ha was
always seen, irrespective of the isotopolog of ethylben-
zene, as a consequence of exchange with deuterated
solvent (see above). Substantial isotope exchange be-
tween the aryl Hc and methyl Ha positions was evident,
whenever these two positions contained different labels
in the starting ethylbenzene isotopolog. Exchange into
Hb was never observed, nor from Hb into Hc (entry 4),
but exchange from Hb into Ha was seen, as revealed by
comparing entries 1 and 4. The process effecting this
exchange must precede exchange of Ha with solvent;
otherwise, significant amounts of D would show up at
the aryl position Hc in entries 1 and 4.
A mechanism that can account for these data is shown
in Scheme 7. As with toluene, initial activation occurs
exclusively, and reversibly, at an aryl C-H bond in
ethylbenzene, resulting in NMR-detectable amounts of
both p-28 and m-28. It is not clear whether intercon-
version between p-28, m-28, and o-28 occurs bimolecu-
larly or through an intramolecular migration. Unlike
toluene, however, conversion to η3-complex 27 must be
intramolecular, not intermolecular (to explain the ab-
sence of additional exchange in recovered substrate for
entries 3-6 in Table 3). We propose this occurs through
the ortho-ethylphenyl complex o-28, which undergoes
a series of rapid and reversible steps: intramolecular
C-H activation at the methyl position to give platinum-
(IV) metallocycle 29;28 reductive elimination to â-phen-
ethyl platinum(II) complex 30; and â-hydrogen elimi-
With 1,3,5-triethylbenzene isotopologs (Table 6) we
see an effect of the steric environment of platinum
methyl cation 2 on the selectivity of initial C-H bond
(28) For a recent report of a similar rearrangement from an ortho-
arylmetal complex to an η3-complex via a palladocycle, see: Jones, G.
D.; Anderson, T. J.; Chang, N.; Brandon, R. J.; Ong, G. L.; Vicic, D. A.
Organometallics 2004, 23, 3071-3074.
(29) A similar â-hydride elimination followed by a 2,1-methylstyrene
insertion to afford an analogous Pd-η3-benzyl complex was proposed
by Brookhart and co-workers; see ref 17b.
(30) When a sterically smaller ligand was employed (Ar ) 3,5-
dialkylbenzene), substantially more isotope exchange was observed in
the recovered 1,4-diethylbenzene isotopologs.

3652 Organometallics, Vol. 24, No. 15, 2005
Scheme 7. Proposed Mechanism for Formation of η3-Complex 27
Driver et al.
Table 5. Examination of 1,4-Diethylbenzene Isotopologs
MS analysis of recovered substrateb
% deuterium in 9
CH4:CDH3:
CD2H2:CD3Ha [M + 1]+/[M]+ [M - 1]+/[M]+
[(M + 1) - Me]+/ [(M - 1) - Me]+/
entry
substrate
[M - Me]+
[M - Me]+
Ha
16
n.a.
87
73
Hb
Hc
12
n.a.
84
27
1
(CH3CH2)2C6H4
(CH3CH2)2C6H4
(CD3CD2)2C6H4
(CH3CH2)2C6D4
69:31:0:0
100:0:0:0
70:30:0:0
21:45:28:6
0.08
,0.01
0.02
0.08
,0.01
0.02
0
2c
3
n.a.
0
4
0.19
0.20
0
a The reported ratio includes the methane isotopologs produced from the protonation of the starting platinum dimethyl complex as
determined by 1H NMR spectroscopy. b Ion ratios corrected by corresponding values for unexchanged starting substrate. c CF3CH2OH
was employed as the solvent.
Table 6. Examination of 1,3,5-Triethylbenzene
Isotopologs
appearance of some multiply deuterated methane in
entry 6 indicates that some competitive aryl activation
may take place as well.) In the absence of substrates
with partially labeled ethyl groups, it is not possible to
determine whether the initial activation takes place at
the methyl position, followed by isomerization as in
Scheme 7, or at the methylene position.
% deuterium
in 33
CH4:CDH3:
entry
substrate
LnPtMe+ CD2H2:CD3Ha Ha Hb Hc
1
2
3
4
5
6
(CH3CH2)3C6H3
(CD3CD2)3C6H3
2c
2c
2c
2a
2a
2a
69:31:0:0
64:36:0:0
24:46:24:6
66:34:0:0
30:49:18:3
46:40:13:1
16
85
41
6
>95
0
0
0
0
0
0
0
0
75
55
7
c
(CH3CH2)3C6D3
(CH3CH2)3C6H3
(CD3CD2)3C6H3
0
c
(CH3CH2)3C6D3
90c
a The reported ratio includes the methane isotopologs produced
from the protonation of the starting platinum dimethyl complex.
c
b ∆([M + 1]+/[M]+) measured. Et3C6D3 is 94% deuterated.
activation. For both 2a and 2c the product was consis-
tently the η3-complex 33 (eq 12). Analysis of methane
isotopologs reveals that for the sterically smaller com-
plex 2c aryl C-H bond activation is the exclusive initial
step (entries 1 and 2), although again no intermediate
aryl complex can be detected. The expected exchange
between positions Ha and Hc was observed (entries 2
and 3). In contrast, direct activation of the ethyl group
appears to be the dominant path for bulkier 2a, as
revealed by the extensive deuteration of methanes in
entry 5 as well as the virtual absence of isotope
exchange between positions in entries 5 and 6. (The
Comparative Reactivities
Rate constants for reactions of di- and triethylbenzene
with 2a are compared to those for several related
reactions in Table 7. Comparison of entries 1 and 4
shows that the bulkier ligand with Ar ) mesityl slows
the benzene reaction by nearly 2 orders of magnitude.
The different KIE values were previously interpreted
in terms of a change in RDS, benzene coordination being
rate-limiting for 2a, but C-H activation for 2c.10a Ethyl
substitution on benzene retards the reaction rate only

C-H Bond Activation of Alkyl-Substituted Benzenes
Organometallics, Vol. 24, No. 15, 2005 3653
Table 7. Comparison of Rate Constants for
Various Alkyl-Substituted Benzenes and Steric
Nature of Diimine Ligands
which ethyl position is reactive cannot be determined
from available data.
While the accessibility of multiple pathways for the
same overall transformation will unquestionably make
it difficult to predict selectivity patterns, the fact that
preferences for activation of different C-H bonds can
be so strongly affected by the steric properties of
substrate and activating complexes is encouraging for
the larger program of designing selective C-H activa-
tion processes for useful organic synthetic goals.
k (M-1‚s-1),
entry LnPtMe+
substrate
benzene
20 °C
kH/kD
1
2a
2a
2a
2b
2b
2b
4.1(4) × 10-4 1.1a
2
1,4-diethylbenzene
2.4(2) × 10-4 0.9,b 0.9c
3
1,3,5-triethylbenzene 1.3(1) × 10-4 3.0,d 1.1e
4f
5f
6f
benzene
p-xylene (Ar CH)
1.6(2) × 10-2 1.8
4.0(13) × 10-3 4.0
p-xylene (benzyl CH3) 2.0(9) × 10-3 0.8
a From ref 10a. b 1,4-Diethylbenzene-d10 (25 °C). c 1,4-Diethyl-
benzene-d4 (25 °C). d 1,3,5-Triethylbenzene-d15 (40 °C). e 1,3,5-
Triethylbenzene-d3 (40 °C). f From ref 10b.
Experimental Section
General Procedures. 1H NMR and 13C NMR spectra were
recorded at ambient temperature using Varian 600 or 300
spectrometers. The data are reported as follows: chemical shift
in ppm from internal tetramethylsilane on the δ scale,
multiplicity (br ) broad, s ) singlet, d ) doublet, t ) triplet,
q ) quartet, m ) multiplet), coupling constants (Hz), and
integration. Mass spectra were acquired on a Finnigan LCQ
ion trap or Agilent 5973 Network mass selective detector and
were obtained by peak matching. All reactions were carried
out under an atmosphere of nitrogen in glassware that had
been oven-dried. Unless otherwise noted, all reagents were
commercially obtained and, where appropriate, purified prior
to use. Tris(pentafluorophenyl)borane [B(C6F5)3] was purified
by sublimation (90 °C, 0.5 mmHg). Trifluoroethanol-d3 was
dried over 3 Å molecular sieves for at least 5 days and then
vacuum distilled onto B(C6F5)3. After 6 h, the trifluoroethanol-
d3 was vacuum distilled and stored in a valved reaction vessel.
The platinum dimethyl complexes were synthesized following
earlier reported procedures.10b,14 Trifluoroethanol-d3, B(C6F5)3,
and platinum dimethyl complexes were stored in a Vacuum
Atmospheres nitrogen atmosphere drybox. A representative
procedure and diagnostic characterization of an η3-complex
(9a) are included below. For procedures and characterization
of other new complexes, refer to the Supporting Information.
Representative Preparation and Characterization of
an η3-Complex. To a light orange solution of platinum methyl
cation 2a (prepared by dissolving 0.010 g of platinum dimethyl
complex 1a (0.018 mmol) and 0.017 g of B(C6F5)3 (0.033 mmol)
in 0.700 mL of trifluoroethanol-d3) was added 0.056 mL of 1,4-
diethylbenzene (0.36 mmol). The progress of the reaction was
slightly: diethylbenzene reacted nearly half as fast as
benzene (entry 2), while triethylbenzene slowed the rate
further but only by another factor of 2 (entry 3). The
shift from initial aryl activation for diethylbenzene to
primarily side-chain activation for triethylbenzene (see
above) is accompanied by a significant KIE for deutera-
tion in the ethyl groups, but not in the aryl positions.
This result may also suggest that the RDS has been
changed from coordination to oxidative addition of the
ethyl C-H bond. It should be noted, though, that for
the reactions of p-xylene with 2a (entries 5 and 6) the
opposite result was found: deuteration of aryl positions
gave a significant KIE, while deuteration of the methyl
groups did not.10b We do not at this time have a fully
satisfying interpretation of that observation.
Conclusions
While all methyl- and ethyl-substituted benzenes
react with Pt(II) cation 2 to give η3-benzyl products, they
do not all get there by the same pathway. For toluene
and p-xylene, initial activation at aryl positions is
evidenced by the buildup of NMR-detectable amounts
of aryl-platinum complexes during the course of the
reaction, with subsequent conversion to the benzyl
product.10b Isotopic labeling studies in the present work
reveal that the latter conversion is intermolecular,
involving reaction with a second molecule of substrate,
and that there is competitive initial activation at the
benzyl position for p-xylene but not for toluene, presum-
ably a consequence of greater crowding in the former.
Previous findings from kinetics experiments agree with
this conclusion.10b
For ethylbenzene and 1,4-diethylbenzene, isotopic
labeling shows that initial activation takes place exclu-
sively at aryl C-H bonds, although the intermediate
aryl complexes are detectable only for the former, and
that conversion to the η3-benzyl product takes place via
intramolecular isomerization rather than secondary
reaction with additional substrate. Comparison of 1,4-
diethylbenzene to p-xylene suggests that direct C-H
activation of a substituted benzylic methylene C-H
bond is disfavored relative to that of a benzylic methyl,
presumably a result of the increased steric interactions
present for activation of a secondary C-H bond, and
that activation at the nonbenzylic methyl position is not
competitive either, despite the steric retardation of
reaction at the aryl positions. Only in the most extreme
case of steric crowdingsthe reaction of bulky 2a with
triethylbenzenesdoes direct activation of the ethyl
group become preferred to aryl activation, although
1
monitored periodically using H NMR spectroscopy. After 14
h, the reaction mixture was heated to 45 °C. After 13 h, the
reaction mixture was cooled and 1H NMR spectroscopy analy-
sis revealed complete consumption of 2a and formation of η3-
product 9a (in situ characterization data): 1H NMR (600 MHz,
CF3CD2OD) δ 7.13 (s, 1H), 7.12 (s, 1H), 7.06 (s, 1H), 6.96 (AB
splitting, 1H), 6.91 (s, 1H), 6.05 (s, 1H), 6.05 (d, J ) 5.8 Hz,
1H), 5.62 (d, J ) 6.0 Hz, JPtH ) 46 Hz, 1H), 2.76 (q, J ) 6.6
Hz, JPtH ) 66 Hz, 1H), 2.37 (s, 3H), 2.34 (s, 6H), 2.18 (s, 3H),
2.16 (m, 1H), 2.07 (m, 1H), 1.96 (s, 3H), 1.89 (s, 3H), 1.85 (s,
3H), 1.46 (s, 3H), 1.03 (t, J ) 7.2 Hz, 3H), 0.32 (d, J ) 6.6 Hz,
3H).
The η3-complex could also be prepared with BF4-, a more
robust counterion. To a suspension of platinum dimethyl 1a
(0.092 g, 0.15 mmol) in 8 mL of CF3CH2OH was added 0.032
mL of a 54 wt % solution of HBF4 in Et2O (0.19 mmol). Upon
homogeneity, 0.065 mL of 1,4-diethylbenzene (0.42 mmol) was
added to the reaction solution. After 6 days at 25 °C, the
reaction mixture was warmed to 45 °C. After 18 h, the reaction
mixture was cooled to 25 °C and was concentrated in vacuo.
The resulting red residue was redissolved in 5 mL of methanol
and concentrated in vacuo to afford 0.180 g of a dark red
residue: 1H NMR (600 MHz, CD2Cl2) δ 7.11 (s, 1H), 7.10 (s,
1H), 7.04 (s, 1H), 6.92 (s, 1H), 6.91 (s, 1H), 6.90 (s, 1H), 5.99
(d, J ) 6.0 Hz, 1H), 5.63 (dd, J ) 7.0, 1.8 Hz, JPtH ) 38 Hz,
1H), 2.75 (q, J ) 6.6 Hz, JPtH ) 48 Hz, 1H), 2.39 (s, 3H), 2.37
(s, 3H), 2.36 (s, 3H), 2.21 (s, 3H), 2.14 (m, 1H),a 2.45 (m, 1H),a

3654 Organometallics, Vol. 24, No. 15, 2005
Driver et al.
2.01 (s, 3H), 1.98 (s, 3H), 1.97 (s, 3H), 1.46 (s, 3H), 1.03 (t, J
) 7.8 Hz, 3H), 0.30 (d, J ) 6.6 Hz, 3H); 13C NMR (150.8 MHz,
CD2Cl2) δ 180, 177, 143.4, 143.2, 141, 140.7, 140, 138, 135.0,
134.9, 130.22, 130.15, 129.9, 129.8, 129.7, 129, 128, 127, 116
(b), 112, 42, 29, 19.5, 19.3, 18.8, 18, 17.73, 17.68, 17.3, 16.7,
by comparison of the average signal intensity of the methyl
groups present on the diimine ligand with the resonances
associated with Ha, Hb, and Hc, respectively. The methane
isotopolog distribution was also determined using 1H NMR
spectroscopy analysis (0.10-0.15 ppm). After spectroscopic
analyses, the reaction mixture was filtered through SiO2, and
the resulting clear filtrate, containing recovered substrate, was
analyzed by GC-MS.
14, 12; IR (KBr pellet) 2968, 1610, 1386, 1059, 851 cm-1
;
HRMS (FAB) m/z calcd for C32H41N2Pt+ (M - BF4)+ 648.2918,
found 648.2932. aIdentified using 2D NMR COSY experiment.
General Comments on Isotopolog Experiments. A light
orange solution of platinum methyl cation 2a (prepared by
dissolving 0.010 g of platinum dimethyl complex 1a (0.018
mmol) and 0.017 g of B(C6F5)3 (0.033 mmol) in 0.700 mL of
trifluoroethanol-d3) was analyzed using 1H NMR spectroscopy.
At this time the amount of deuterium incorporation into the
Pt-Me resonance was determined by comparison of its peak
area to the average peak area of both the aryl protons and
methyl groups present on the diimine ligand. After spectro-
scopic analysis, 2.5 equiv of substrate was added. The progress
of the reaction was monitored periodically using 1H NMR
spectroscopy. After 10 h, the reaction mixture was heated to
45 °C. After 13 h, the reaction mixture was cooled and 1H NMR
spectroscopy analysis revealed complete consumption of the
aryl complexes and formation of η3-product. The amount of
deuterium incorporation into the η3-product was determined
Acknowledgment. Funding for this work was pro-
vided by the bp MC2 program and the NIH in the form
of a NRSA fellowship to T.G.D. (GM070272-02). We
thank Jonathan S. Owen for insightful discussions, Dr.
Scott Ross for assistance with NMR spectrometry, Dr.
Mona Shahgholi for mass spectrometry data, and Mr.
Larry M. Henling for crystallographic analysis.
Supporting Information Available: Detailed experi-
mental procedures, characterization data, integrated rate laws,
tables of GC-MS isotope exchange, and X-ray diffraction data
for 12 are available free of charge via the Internet at
http://pubs.acs.org.
OM050251M