Kinetic and thermodynamic preferences in aryl vs benzylic C-H bond activation with cationic Pt(II) complexes

Article abstract of DOI:10.1021/ja045078k

Anhydrous cationic Pt(II) complexes [(NN)Pt(CH₃)(CF₃CD₂OD)]⁺ (1, NN = ArN=C(Me)-C(Me)=NAr), which are obtained by reaction of (NN)Pt(CH₃)₂ with B(C₆F₅)₃ in

Full text of DOI:10.1021/ja045078k

Published on Web 10/28/2004
Kinetic and Thermodynamic Preferences in Aryl vs Benzylic C-H Bond
Activation with Cationic Pt(II) Complexes
Alan F. Heyduk,† Tom G. Driver, Jay A. Labinger,* and John E. Bercaw*
The Arnold and Mabel Beckman Laboratories of Chemical Synthesis,
California Institute of Technology, Pasadena, California 91125
Received August 16, 2004; E-mail: jal@its.caltech.edu
The past two decades have seen a number of detailed mechanistic
studies of C-H bond activation processes by transition metal
complexes,1 with the ultimate goal of new catalysts for selective
hydrocarbon functionalization. Our work has focused on platinum-
(II) complexes, which have yielded mechanistic insight into C-H
bond activation chemistry2 and pointed toward promising catalysts
for selective oxidation of alkanes3 and for regiospecific function-
alization of complex organic molecules.4 The cationic Pt(II)
complexes [(NN)Pt(CH3)(CF3CH2OH)]+ (1, NN ) ArNdC(Me)-
C(Me)dNAr), obtained by protonolysis of (NN)Pt(CH3)2 with
aqueous HBF4, provide a particularly fertile field for mechanistic
investigation.5 Thus prepared, however, 1 is obtained as an
equilibrium mixture with the aquo complex [(NN)Pt(CH3)(OH2)]+,
which is both thermodynamically favored (Keq ) 102-103) over,
and much less reactive than, 1. Furthermore, attempts to probe the
nature of the bond activation transition state were hampered by
the fact that effects of ligand variations are most strongly manifested
Figure 1. Plots showing the relative concentrations of platinum cations 1
in ground-state energies (Keq).5e To circumvent these limitations,
(s), 4a (- -), and 4b (- - -) during the reaction of 1 with (a) p-xylene-h10
(0.30 M) and (b) p-xylene-d10 (0.22 M) in CF3CD2OD at 25 °C.
we sought a water-free route to 1 and found not only the anticipated
rate accelerations but also unexpected isomerizations and prefer-
Table 1. Second-Order Rate Constants for C-H Bond Activation
Reactions of 1 (25 °C, CF3CD2OD)
ences in C-H activations of alkylbenzenes.
1
a
1
a
entry
substrate
kAr (M-1 s-
)
kH/kD
kBz (M-1 s-
)
kH/kD
1
2
3
benzene
mesitylene
p-xylene
1.6(2) × 10-2 1.8 ( 0.2
4.0(13) × 10-3 4.0 ( 1.8 2.0(9) × 10-3 0.8 ( 0.3
na
na
na
na
2.0(2) × 10-3 1.1 ( 0.2
a For perdeutero- vs d0-substrate.
first-order kinetic behavior, the observed rate constant was determin-
ed to be an order of magnitude lower than that for benzene and the
measured KIE was found to be essentially 1 (Table 1, entry 2).
p-Xylene reacted with 1 to give products of both aromatic (Pt-
(2,5-Me2C6H4), 4a) and benzylic (Pt-(CH2C6H4Me), 4b) activation,
similar to the behavior in the presence of water.5c,e,8 However, the
initial product distribution was not stable (Figure 1). After starting
complex 1 was completely consumed, 4b continued to grow at the
expense of 4a. This conversion displayed first-order dependence
on p-xylene concentration (k2 ) 0.0003(1) M-1 s-1; KIE ) 0.9-
(2)), indicating a reaction of 4a with p-xylene rather than an
intramolecular isomerization process (Scheme 1). The KIEs for
aromatic and benzylic C-H bond activation were quite different,
as can be readily seen by comparing Figures 1a and 1b; kinetic
parameters were extracted by fitting6 the data in Figure 1 (Table 1,
entry 3). The difference in KIE for sp2 and sp3 C-H bond activation
was unexpected and not obviously interpretable.8
Solutions of the strong Lewis acid B(C6F5)3 in CF3CD2OD
exhibited a 19F NMR spectrum consistent with reversible formation
of the adduct, D[(CF3CD2OB(C6F5)3].6 Addition of (NN)Pt(CH3)2
(Ar ) 3,5-dimethylphenyl) to such a solution resulted in the release
of methane and quantitative formation of 1, both produced as a
mixture of two isotopomers (CH4 and CH3D; [(NN)Pt(CH3)(CF3-
CD2OD)]+ and [(NN)Pt(CH2D)(CF3CD2OD)]+), indicating a pro-
tonolytic route to 1 (eq 1).7 Addition of benzene to this solution
resulted in rapid reaction of 1 to give the corresponding phenyl
cation, [(NN)Pt(C6H5)(CF3CD2OD)]+ (2) and a second equivalent
of methane. With excess benzene, the reaction followed clean
pseudo-first-order kinetics; the calculated second-order rate constant
(Table 1, entry 1) and kinetic isotope effect (KIE) were in accord
with previous results for the equilibrium mixture of solvento/aquo
complexes of closely related ligands.5e
Previous observations on reactions of methylbenzenes with Pt-
(II) complexes supported an apparent strong preference for aromatic
activation, which could be overridden by steric crowding.5c,e Thus,
1 reacted with mesitylene exclusively by benzylic activation, to
afford [(NN)Pt(CH2C6H3Me2)(CF3CD2OD)]+ (3). From the clean
Toluene behaved (qualitatively) analogously: an initial mixture
of benzylic and aromatic C-H activation products subsequently
converted to solely the former, although the complex mixture of
aryl isomers has not yet been completely analyzed. Similarly,
reaction of 1 with an equimolar mixture of benzene and mesitylene
afforded an initial mixture of 2 and 3 in ca. 6:1 ratio (consistent
† Current address: Department of Chemistry, University of California, Irvine,
Irvine, CA, 92697.
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J. AM. CHEM. SOC. 2004, 126, 15034-15035
10.1021/ja045078k CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
activation appears to result from stabilizing η3-bonding observable
in 5. Experimental and theoretical efforts are currently directed
toward understanding these novel patterns for benzylic versus
aromatic C-H bond activation and exploiting these selectivities in
C-H bond functionalization.
Figure 2. Diagnostic 1H NMR spectroscopic evidence in CF3CD2OD
showing that JPtH of Ha and Hb in 5 (Ar ) Mes) supports η3-bonding as the
thermodynamic preference for benzylic C-H activation.
Scheme 1
Acknowledgment. Funding for this work was provided by the
BP MC2 program and the NIH in the form of an NCI fellowship
to A.F.H. and an NRSA fellowship to T.G.D. We thank 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 experimental pro-
cedures, integrated rate laws for p-xylene reactions, proposed mecha-
nistic interpretations, and X-ray diffraction data for 4b′ (PDF, CIF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(1) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507-514 and references
therein.
with the independently measured rate constants), which slowly (kobs
∼ 10-5 s-1) converted entirely9 to 3.
(2) (a) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed.
1998, 37, 2180-2192 and references therein. (b) Fekl, U.; Goldberg, K.
I. AdV. Inorg. Chem. 2003, 54, 259-320 and references therein.
(3) (a) Gol’dschleger, N. F.; Es’kova, V. V.; Shilov, A. E.; Shteinman, A. A.
Zh. Fiz. Khim. 1972, 46, 1353-1354 (Engl. trans. 1972, 46, 785-786).
(b) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii,
H. Science 1998, 280, 560-564. (c) Lin, M.; Shen, C.; Garcia-Zayas, E.
A.; Sen, A. J. Am. Chem. Soc. 2001, 123, 1000-1001.
(4) (a) Johnson, J. A.; Li, N.; Sames, D. J. Am. Chem. Soc. 2002, 124, 6900-
6903. (b) Dangel, B. D.; Johnson, J. A.; Sames, D. J. Am. Chem. Soc.
2001, 123, 8149-8150.
(5) (a) Johansson, L.; Ryan, O. B.; Tilset, M. J. Am. Chem. Soc. 1999, 121,
1974-1975. (b) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw, J. E.
J. Am. Chem. Soc. 2000, 122, 10846-10855. (c) Johansson, L.; Tilset,
M. J. Am. Chem. Soc. 2001, 123, 739-740. (d) Johansson, L.; Ryan, O.
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C-H activating systems generally exhibit thermodynamic10 as
well as kinetic preferences for aromatic over benzylic activation.
In stark contrast, while aromatic activation here was favored
kinetically, the observations indicated a decided thermodynamic
preference for the products of benzylic activation. One possible
explanation is that 4b is [(NN)Pt(η3-CH2C6H4Me)]+ rather than
[(NN)Pt(η1-CH2C6H4Me)(CF3CD2OD)]+ where the additional con-
tribution of η3-bonding might stabilize it relative to 4a. We have
been unable to crystallize 4b, but addition of acetonitrile gave a
crystallizable complex (4b′) whose structure is clearly [(NN)Pt-
(η1-CH2C6H4Me)(NCCH3)][BF4].6
(6) Experimental details, including detailed procedures, spectroscopic data,
kinetics analyses and X-ray crystallographic data are provided in Sup-
porting Material.
(7) The alternative, methide abstraction by B(C6F5)3, would lead to
[MeB(C6F5)3]- (which is stable to reaction conditions) and no deuterium
incorporation into the platinum methyl cation.
Evidence for the formation of a η3-complex was obtained when
platinum methyl cation 1 was exposed to 1,4-diethylbenzene in
CF3CD2OD (eq 2). In contrast to 4, the Pt-H coupling could be
observed for both Ha and Hb (JPtH ) 20 and 30 Hz, respectively)
using 1H NMR spectroscopy. Behavior similar to that for methyl-sub-
stituted benzenes was observed with diethylbenzene: reaction with
1 also afforded aryl activation products, which were converted to
η3-complex 5 over time.11 Diagnostic Pt-H coupling of an analog-
ous η3-complex was also observed when a more substituted diimine
ligand (Ar ) Mes) was employed (Figure 2). The instability of 5
(or 5‚BF4) as a solid precluded structural analysis using X-ray
diffraction.
(8) Kinetic ratio of aromatic/benzylic activation here, around 2:1, is signifi-
cantly different from that found for a close analogue of 1 in the presence
of water, g10:1.5e The reason for this difference is not clear; (hopefully)
plausible explanations for this observation as well as the unusual isotope
effect behavior are offered in Supporting Information.
1
(9) Under the limitations inherent to the H NMR experiment, a lower limit
of Keq g 20 can be assigned to the equilibrium constant relating [2] and [3].
(10) For example, with the Tp′RhL fragment (Tp′ ) hydrotris(3,5-dimethyl-
pyrazolyl)borate, L ) neopentyl isocyanide) aromatic activation of benzene
is favored over benzylic activation of mesitylene by 6.6 kcal mol-1 (Jones,
W. D.; Hessell, E. T. J. Am. Chem. Soc. 1993, 115, 554-562).
(11) While methyl C-H activation was not directly observed, exposure of 5
to deuterated alcohols caused deuterium exchange into the methyl group
of the complex.
In summary, we have observed selectivity trends in the C-H
bond activation of substituted benzenes using anhydrous cationic
Pt(II) complexes. The thermodynamic preference for benzylic C-H
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