Catalytic alcoholysis of tetramethylsilane via Pt-mediated C-H bond activation

Article abstract of DOI:10.1021/ja029099v

Tetramethylsilane reacts with 2,2,2-trifluoroethanol (TFE) in the presence of a cationic platinum(II) catalyst [(NN)PtMe(TFE)]⁺ (NN = 1,2-bis(3,5-dimethylphenylimino)butane). Catalytic Si-C bond heterolysis results in the formation of the trimethylsilyl ether, Me₃SiOCH₂CF₃, accompanied by liberation of one equivalent of methane. Preliminary experiments suggest that a rate-determining C-H bond activation precedes rapid attack by solvent at silicon to yield the silyl ether product and regenerate the active platinum methyl cation. Copyright

Full text of DOI:10.1021/ja029099v

Published on Web 05/06/2003
Catalytic Alcoholysis of Tetramethylsilane via Pt-Mediated C-H Bond
Activation
Alan F. Heyduk,† Jay A. Labinger,* and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology,
Pasadena, California 91125
Received October 25, 2002; E-mail: jal@its.caltech.edu; bercaw@caltech.edu
Square-planar, d8 complexes of platinum have received consider-
Scheme 1
able attention as potential catalysts in selective alkane oxidation
schemes.1 Cationic platinum(II) complexes of the general formula
[(NN)PtMe(TFE)]+ (NN ) a chelating R-diimine ligand; TFE )
2,2,2-trifluoroethanol), generated by protonolysis of the corre-
sponding neutral dimethyl complex (NN)PtMe2 (1) in TFE, have
been particularly useful as mechanistic probes for individual steps
of the catalytic oxidation.2 Ongoing studies into the rate- and
selectivity-determining C-H activation step (Eq 1) have provided
a working mechanistic model, the details of which continue to
evolve. As part of a study into the kinetic and thermodynamic
preferences for aliphatic C-H bond activation, we examined the
reaction of tetramethylsilane (TMS) with [(NN)PtMe(TFE)][X] (2;
CF3. The 13C NMR spectrum of the reaction solution exhibited a
NN ) 2,3-bis(3,5-dimethylphenylimino)butane; X ) [(CF3CH2O)B-
new Si-CH3 resonance at δ -0.7 and a new CF3 resonance at δ
(C6F5)3]-, B[C6H3(CF3)2]4-). Although reaction at a C-H bond of
126.1 ppm (1JCF ) 277 Hz); an independently prepared sample of
TMS apparently occurs, the initial C-H activation product proved
5
Me3SiOCH2CF3 exhibited the same 1H and 13C NMR signals.
to be unstable, and catalytic conversion of TMS and TFE solvent
to methane and Me3SiOCH2CF3 was observed. We report here our
preliminary studies of this novel transformation.
Combined with the above-noted H/D exchange between Pt-CH3
and CF3CD2O-D, these observations suggest that a rate-determin-
ing C-H activation step,6 analogous to previously reported arene
activation reactions, initiates a rapid solvolysis reaction (Scheme
1).7,8
Protonolysis reactions of (NN)Pt(CH2SiMe3)2 (3) were used to
probe the mechanism of the fast, product-forming step of Scheme
1. The reaction of cis/trans-PtCl2(SMe2)2 with Li(CH2SiMe3) at low
temperature followed by addition of 1,2-bis(3,5-dimethylphenyl-
imino)butane afforded dark purple 3 as a semicrystalline solid (see
Supporting Information for details). 3 dissolved slowly into TFE-
d3 containing excess B(C6F5)3 to afford 2 equiv of Me3SiOCD2-
CF3 and one equivalent each of methane and 2, both partially
deuterated (eq 2). No free TMS was observed and no deuterium
Solutions of 2 in TFE-d3 were readily generated upon dissolution
incorporation was observed in the methyl groups of the silyl ether
product.
of 1 and either H(OEt2)2[B(C6H3(CF3)2)4] or B(C6F5)3.3 Following
addition of TMS (0.35 mmol, 0.52 M, 36 equiv relative to 2) to a
1
TFE-d3 solution of 2, the signature H NMR resonance for TMS
decayed with concomitant growth of a new signal at δ 0.18.
1
Integration ratios in the H NMR spectrum indicated 75% con-
sumption of TMS after 18 h at 26 °C. Throughout the reaction, 1H
NMR signals for 2 were unchanged except for diminution of the
resonance for the platinum-bound methyl group accompanied by
the appearance of upfield multiplets for the Pt-CH2D and Pt-
The same results were obtained for the protonation of 3 at -80
°C with D(OEt2)2[B(3,5-C6H3(CF3)2)4] in THF-d8 containing TFE-
CHD2 isotopomers of platinum cation 2. After 18 h, integration of
d3 ([TFE-d3] ) 0.2-1.1 M), except that 2 formed as the THF
ligand backbone resonances indicated that over 75% of 2 remained;
adduct. Significantly, addition of excess TMS to this reaction
afforded no additional silyl ether, indicating that the THF adduct
of 2 was not reactive toward TMS. Hence, both equivalents of Me3-
SiOCD2CF3 must be produced directly from protonolysis of 3 rather
than by liberation and subsequent activation of TMS. Finally,
solutions of 3 in THF containing TFE produced no silyl ether
without acid addition.
0.29 mmol of gas (30 equiv relative to 2, TOF ) 1.6 h-1) was
then collected from the reaction solution by Toepler pump and was
identified as a mixture of methane isotopomers by gas-phase IR
spectroscopy.4
Methane formation in tandem with TMS consumption suggests
that 2 catalyzes Si-C bond alcoholysis to produce Me3SiOCH2-
These results appear to be most consistent with the mechanism
presented in Scheme 2 for the catalytic functionalization of TMS.
† Current address: Department of Chemistry, University of California, Irvine,
CA 92697.
9
6366
J. AM. CHEM. SOC. 2003, 125, 6366-6367
10.1021/ja029099v CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
Supporting Information Available: Detailed experimental pro-
cedures including the synthesis and spectroscopic characterization of
3 (PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
References
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(3) B(C6F5)3 dissolves in TFE to afford an acidic solution capable of
protonating 1 to release methane and form 2. Isotopic scrambling patterns
are inconsistent with methide abstraction from 1. Heyduk, A. F.; Labinger,
J. A.; Bercaw, J. E., manuscript in preparation.
An initial, rate-determining C-H activation of TMS by 2 (step a)
reflects the established reactivity pattern of these platinum cations
with hydrocarbon substrates.2 Protonolysis reactions of 3 support
a subsequent solvolytic cleavage of the Si-C bond via nucleophilic
attack by TFE (step b). Two factors suggest that this nucleophilic
attack at silicon must be fast relative to both reductive coupling9
and alkane elimination:10,11 (1) protonation of 3 in the presence of
TFE gave no TMS, but rather 2 equiv of the silyl ether product,
and (2) incorporation of deuterium was not observed in the SiMe3
group under any circumstances. Elimination of the silyl ether in
step b would produce a transient, neutral methylidene hydride
complex that rearranges to 1 by an R-hydrogen migration (step c).
In an acidic environment, 1 readily loses methane (with isotopic
scrambling; steps d and e) to regenerate the catalyst resting state,
2.
Nucleophilic attack at the silicon of cationic [PtIV-CH2SiMe3]
is reminiscent of the established reactivity of silyl groups â to an
electropositive carbon center.12 A related mechanism, in which the
alcohol adds across the Si-C bond via a four-centered transition
state, is also consistent with our observations. Two other mecha-
nisms for the Si-C bond cleavage have precedent. â-methyl
migration from the activated silane to platinum13 followed by
addition of TFE to the coordinated silylene14 may be ruled out,
since this would incorporate deuterium into the methyl groups of
the silyl ether product. A rapid R-silyl migration15,16 to give a PtIV-
(silyl)methylidene species does not account for the observed
reactivity: if it follows alkane elimination, the protonolysis of 3
would give TMS (not observed), whereas if it precedes alkane
elimination, the putative intermediate would require PtVI; moreover,
3 does not react with TFE to extrude the silyl ether product in the
absence of added acid.
(4) Isotopomers were readily observed in the C-H bending region of the IR
spectrum: 1302 (CH4), 1157 (CH3D), 1090 (CH2D2), and 1034 (CHD3)
cm-1
.
(5) Prepared by the reaction of Me3SiCl with TFE in the presence of base.
(6) Alternatively, a C-Si bond activation could initiate the solvolysis of TMS;
however, based on the established reactivity of cationic platinum
complexes with hydrocarbons and the reactivity of the putative [Pt-CH2-
SiMe3] species (vide infra), we currently favor an initial, rate-determining
C-H bond activation.
(7) The persistence of 2 as well as its formation in stoichiometric reactions
strongly suggests that it, rather than an impurity or decomposition product,
participates in the catalytic cycle. Control experiments comprising solutions
of TMS and B(C6F5)3 in TFE-d3, or of TMS in the presence of Zeise’s
dimer, H2PtCl6 or platinum metal, did not lead to formation of Me3SiOCH2-
CF3.
(8) At longer reaction times (after ca. 90% consumption of the TMS)
secondary activation is observed, forming Me2Si(OCH2CF3)2 and another
equivalent of methane.
(9) Here, reductive coupling is used to describe the conversion of a platinum-
(IV) alkyl hydride species to a platinum(II) alkane σ species; a mechanism
that could lead to isotope scrambling into the SiMe3 group.
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proposed to be faster than alkane elimination, see refs 2c, 2e and 11.
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While stoichiometric cleavage of unactivated alkyl-Si bonds by
transition metal centers, including platinum, have been reported,13,15-17
to our knowledge this is the first example of a catalytic Si-C bond
cleavage and functionalization catalyzed by a transition metal
complex.18 The coupling of C-H activation to nucleophilic attack
at the activated alkylsilane affords a novel route for selective
functionalization at the silicon center. We are currently exploring
the scope and synthetic utility of this transformation.
Acknowledgment. We thank Mike Sailor (UCSD) and T. Don
Tilley (UCB) for helpful recommendations, and we are grateful to
NIH (CA94589 to A.F.H.) and BP for financial support.
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