Pt-catalyzed C-C activation induced by C-H activation

Article abstract of DOI:10.1021/ja406260j

The catalytic cleavage of two C-C single bonds is achieved by treatment of the hydrocarbon substrate spiro[bicyclo[2.2.1]hept-2-ene-7,1′- cyclopropane] with Pt(II) catalysts such as (Me₂bpy)PtPh(NTf ₂) (Me₂bpy = 4,4′-dimet

Full text of DOI:10.1021/ja406260j

Article
pubs.acs.org/JACS
Pt-Catalyzed C−C Activation Induced by C−H Activation
Miriam A. Bowring, Robert G. Bergman,* and T. Don Tilley*
Department of Chemistry, University of California, and Chemical Sciences Division, Lawrence Berkeley National Laboratory,
Berkeley, California 94720, United States
S
* Supporting Information
ABSTRACT: The catalytic cleavage of two C−C single bonds
is achieved by treatment of the hydrocarbon substrate
spiro[bicyclo[2.2.1]hept-2-ene-7,1′-cyclopropane] with Pt(II)
catalysts such as (Me2bpy)PtPh(NTf2) (Me2bpy = 4,4′-
−
dimethyl-2,2′-bipyridine, NTf2− = N(SO2CF3)2 ). The surpris-
ing rearrangement product 1,2,4,7,7a-pentahydroindene is
generated in good yield. The mechanism of C−C bond
activation is investigated using NMR spectroscopy, electro-
spray ionization mass spectrometry, and deuterium labeling,
along with density functional theory calculations. These studies
support an unusual catalytic mechanism in which an initial
masked C−H bond activation initiates successive C−C bond cleavage events.
for generalized application to catalytic hydrocarbon conver-
sions.
INTRODUCTION
■
C−H and C−C bond activations are potentially important
chemical transformations with impacts in fundamental
mechanistic chemistry and important synthetic applications.1−9
9 However, efficient homogeneous C−C activation reactions
are not well developed and are far less common than those
involving C−H bonds.5−9 Generally, C−H bond activation
reactions are used simply to install new functionality, but in
several recent examples, they have been shown to initiate
subsequent, more difficult bond activations.10−12 Transforma-
tions that use C−H bond activation to initiate C−C bond
activation in one overall conversion are especially rare.11,12 The
facilitation of catalytic C−C bond cleavage via C−H bond
activation would provide a new approach to catalytic hydro-
carbon transformations.
RESULTS AND DISCUSSION
■
Reaction Development. The bicyclic substrate spiro-
[bicyclo[2.2.1]hept-2-ene-7,1′-cyclopropane]13 (1) was origi-
nally envisioned as a mechanistic probe for hydrocarbon C−H
activation catalysis (Scheme 1a). Previous reports have revealed
the unexpected and crucial roles protons can play in reactions
ostensibly catalyzed by transition-metal complexes.14−16
Substrates designed to transform into divergent products
depending on the mechanism of reaction have been effective
in distinguishing between plausible reaction pathways.16,17 We
imagined that, in hydroarylation reactions, which can be either
metal18−20 or proton catalyzed,14,21 bicyclic substrates such as 1
could provide crucial mechanistic information.
We report herein a new hydrocarbon rearrangement of a
spirocyclopropane (1) to form a pentahydroindene (2),
catalyzed by a platinum(II) complex (3) (eq 1). We
We expected that protonation of 1 would initiate a well-
known Wagner−Meerwein carbocation rearrangement
(Scheme 1a)22 and that the spirocyclopropane moiety could
accelerate this rearrangement by donating electron density to
the developing neighboring carbocationic center in the
transition state and serve as a label to break the degeneracy
of the proposed carbocation rearrangement. The hydroarylation
product resulting from nucleophilic attack of an arene on the
rearranged carbocation would then be easily distinguishable
from the product predicted for metal-catalyzed hydroarylation
of 1 (Scheme 1a).
Substrate 1 was synthesized in three steps using a procedure
adapted from Adam and co-workers (Scheme 2).23 Cyclo-
pentadiene was treated with base and 1,2-dichloroethane to
give spiro[2.4]hepta-4,6-diene,24 which then underwent a
demonstrate that the likely mechanism includes a masked C−
H activation event, which facilitates two C−C activation steps
to generate the final rearrangement product. The use of C−H
activation to initiate C−C bond cleavage in a catalytic cycle
represents a remarkable new phenomenon, showing promise
Received: June 20, 2013
© XXXX American Chemical Society
A
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discovery of this rearrangement motivated a mechanistic study
designed to uncover the new means of hydrocarbon activation
that might be at play.
Scheme 1. Design of Substrate 1 as a Mechanistic Probe for
Hydroarylation
To develop a maximally efficient catalytic system for
mechanistic investigations, the catalyst and reaction conditions
were optimized. Catalyst 4 was of limited use for mechanistic
study because of the counteranion. The BArF − counteranion
4
1
was not inert, giving new resonances in the 19F and H NMR
spectra during and after catalysis. In addition, the BArF −
4
complex 4 was difficult to crystallize, hindering purification
and crystallographic study of the catalyst and catalytic
intermediates. Finally, Gunnoe and co-workers showed that a
Lewis base such as THF or acetonitrile was necessary to
stabilize the catalyst when BArF − was the counteranion.18b,c
4
The equilibrium behavior of Lewis base coordination could
both slow the reaction and complicate mechanistic inves-
tigations.
27
−
−
−
Use of the counteranion NTf2 (NTf2 = N(SO2CF3)2 )
instead of BArF − resulted in an improved catalyst for the
4
rearrangement of 1 to 2. The improved catalyst Pt(tBu2bpy)-
Ph(NTf2) (3) was generated by protonolysis of one Pt−Ph
bond of Pt(tBu2bpy)Ph2 by HNTf2 (eq 2). The new complex
Scheme 2. Synthesis of Substrate 1
crystallized readily from toluene, and the X-ray crystal structure
shows that the counteranion coordinates to the Pt center in the
solid state (Figure 1). Furthermore, a resonance for the Pt-
Diels−Alder reaction with cis-1,2-dichloroethylene.25 The
chlorine−carbon bonds were reductively cleaved with sodium
in refluxing ammonia to give 1.23
When subjected to three different catalysts for hydro-
arylation, substrate 1 proved to be a useful probe for in situ
proton production by platinum catalysts as predicted, albeit
with unexpected products (Scheme 1b). Compound 1 was
stable in o-dichlorobenzene solution at 130 °C for four days in
the absence of catalyst. When treated either with HOTf or with
(tBu2bpy)Pt(OTf)2 (tBu2bpy = 4,4′-di-tert-butyl-2,2′-bipyri-
dine; OTf = SO3CF3), a platinum hydroarylation catalyst that
was shown to rely on proton production, substrate 1 yielded
similar product distributions.14 Electrospray ionization mass
Figure 1. Molecular structure of 3 determined by single-crystal X-ray
diffraction. The thermal ellipsoids are shown at the 50% probability
level. Hydrogen atoms and toluene molecules are omitted for clarity.
1
−
spectrometry (ESI-MS) and H NMR data for these products
bound NTf2 counteranion was observed by 19F NMR
were consistent with oligomers26 of 1 instead of the predicted
spectroscopy at −70 ppm in C6D6 solution. During and after
the rearrangement reaction or upon displacement by an added
Lewis base such as THF, the dissociated counteranion was
observed (−78 ppm). No anion decomposition was observed
by 19F NMR spectroscopy after the catalytic rearrangement
reaction to form 2 or after heating to 100 °C in a variety of
solvents. The weakly coordinating counteranion eliminates the
need for an additional Lewis base for stability.
hydroarylation product. In contrast, when treated with
[Pt(tBu2bpy)Ph(THF)][BArF ] (ArF = 3,5-(CF3)2C6H3)) (4),
4
a platinum hydroarylation catalyst that Gunnoe and co-workers
showed does not rely on proton production,18b,c substrate 1
generated the surprising rearrangement product 1,2,4,7,7a-
pentahydroindene (2) in 79% yield with no major side
products. Substrate 1 thus exceeded its intended use as a
mechanistic probe to differentiate hydroarylation mechanisms
by also undergoing an unusual rearrangement reaction. The
The new catalyst 3 afforded faster and more efficient
rearrangement of 1 than did the BArF − analogue 4 (Table 1,
4
B
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entries 4−6). At 80 °C with 5 mol % catalyst 5, 57% yield was
reached after 6 h in benzene, but 87% yield was reached after
15 min in o-difluorobenzene, and the highest observed yield,
94%, was reached after 5.5 min in o-dichlorobenzene. The
improved reactivity in polar aromatic solvents allowed the
reaction temperature to be lowered further, resulting in final
conditions optimized for ease of mechanistic study. The
rearrangement product 2 was produced in 87% yield after 1 h at
70 °C in o-dichlorobenzene using catalyst 5 (Table 1, entry 7).
The initial discovery and optimization of the catalytic
rearrangement reaction provide clues about its mechanism.
The inability of HOTf to catalyze the rearrangement implies
that the reaction is not acid catalyzed. (Simple Lewis acids
including B(C6F5)3 and ZnCl2 also failed to catalyze the
rearrangement.) The superiority of platinum catalysts lacking
coordinated THF and the observation of anion dissociation
during catalysis with 3 suggest that the open coordination site
on platinum is essential to the transformation. The ability of
complex 6, bearing a tridentate form of the ligand set, to
catalyze the rearrangement implies that only one open
coordination site is necessary for the transformation and that
the Ph group on catalysts 3 and 5 need not be eliminated
during the reaction. The relative sluggishness of catalyst 6, in
which the phenyl group cannot rotate out of the bipyridine
plane as it can in catalyst 3, suggests that an out-of-plane
geometry for the phenyl group is superior. Finally, the
significant rate acceleration in more polar solvents indicates
that a charge-separated state, such as anion dissociation to open
a coordination site at platinum, is active during the trans-
formation. Overall, these findings suggest that catalysis requires
a single open coordination site at platinum.
Table 1. Catalysis Optimization: Effect of Catalyst Structure
and Solvent on Yield
a
entry catalyst [1], mM T, °C
solvent
C6D6
time
yield (%)
1
2
3
4
5
6
7
8
9
4
3
3
5
5
5
5
5
6
24
24
100
100
80
18 days
1.5 h
46
62
67
57
87
94
87
56
33
C6D6
200
200
200
200
200
200
200
C6D6
1.5 h
80
C6D6
6.0 h
80
o-C6H4F2
o-C6H4Cl2
o-C6D4Cl2
o-C6D4Cl2
o-C6D4Cl2
15 min
5.5 min
1.0 h
80
70
50
24 h
Mechanistic Study. To examine the mechanism further,
1H NMR spectroscopy was used to monitor the rearrangement
under a variety of reaction conditions. Observation of an
induction period, in which the rate of catalysis increases from a
slow initial rate (Figure 2), suggests in situ homogeneous
100
5 days
a
Yields were determined by 1H NMR spectroscopy integration relative
to 1,3,5-tris(trifluoromethyl)benzene internal standard.
entries 1 and 2). At low concentration (24 mM) of substrate 1
in C6D6 at 100 °C with 5 mol % catalyst 4, a 46% yield was
observed after 18 days, while catalyst 3 generated a higher yield
(62%) in 1.5 h.
Additional variants of the improved catalyst 3 were also
synthesized for comparison. The catalyst Pt(Me2bpy)Ph(NTf2)
(5) differs from 3 by a minor change in bipyridine substituents,
t
from Bu to Me (Me2bpy = 4,4′-dimethyl-2,2′-bipyridine).
Complexes 5 and 3 performed comparably as catalysts (Table
1, entries 3 and 4). At 80 °C in C6D6 with 5 mol % catalyst
loading and 200 mM substrate 1, 67% yield was reached after
1.5 h using catalyst 3 and 57% yield was reached after 6 h using
catalyst 5. A variant of 3 in which the phenyl group and
bipyridine ligand are covalently tethered together, complex 6,
was also synthesized. Complex 6 was a catalyst for the
rearrangement of 1 to 2, but it was inefficient, requiring five
days at 100 °C to reach 33% yield in o-dichlorobenzene-d4
(Table 1, entry 9). Both of the efficient catalysts 3 and 5 were
used for mechanistic investigations of the rearrangement
reaction.
1
Figure 2. Induction period observed by H NMR spectroscopy for
rearrangement of 1 to 2. Reaction conditions: 5 mol % catalyst 3, 80
°C, C6D6.
The activities of catalysts 3 and 5 were evaluated in several
solvents. Coordinating and reactive solvents such as THF or
CH2Cl2 were incompatible with the reaction, thwarting
substrate conversion or causing catalyst decomposition,
respectively. A comparison of benzene, o-difluorobenzene,
and o-dichlorobenzene revealed that the highest catalyst
activities were found in polar aromatic solvents (Table 1,
catalyst generation (vide infra). The induction behavior is more
pronounced for less polar solvents such as C6D6, supporting the
notion that the activation step involves charge separation.
Catalyst activation was further confirmed by observing the
rearrangement of 1 to 2 using recycled catalyst that had been
used for a previous run. In the case of this recycled catalyst, no
C
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induction period was observed. Taken together, the observed
induction period and activity of recycled catalyst further
support the necessity of anion dissociation for catalysis and
demonstrate that this dissociation takes the form of a kinetically
relevant catalyst activation step.
Eventual deactivation of the catalyst was indicated by a
slowing in the rate of reaction relative to simple first-order
behavior. The catalyst deactivation was competitive enough
with catalysis at lower reaction temperatures (50 °C) that the
reaction failed to reach complete conversion. In these cases,
activity was renewed upon addition of fresh catalyst. This
renewal of catalytic activity in the presence of product 2
suggests that catalyst deactivation, rather than product
inhibition, best explains the decreasing rate. While catalyst
activation and deactivation prevent the extraction of simple rate
laws from the kinetic data, they contribute to the development
of a suitable mechanistic model.
The active and inactive forms of catalyst 5 were compared by
ESI-MS characterization to identify catalytically active com-
pounds. During active catalysis, a diluted aliquot of the crude
catalytic mixture was collected and analyzed by ESI-MS (Figure
3a). After catalyst deactivation, a similarly prepared aliquot of
the crude reaction mixture was also analyzed by ESI-MS
(Figure 3b). Upon comparison, a single major peak present
only during active catalysis was identified. On the basis of
isotope distribution modeling, this peak (monoisotopic m/z =
575.20, Figure 3c) represents an adduct of the cationic portion
of the catalyst, (Me2bpy)PtPh+, with the hydrocarbon substrate
1 or an isomer thereof (Figure 3d). The most abundant
platinum complex observed after catalyst deactivation has a
monoisotopic m/z value of 499.16, which is consistent with loss
of a phenyl group relative to the active catalyst. This result
implies that the phenyl group is necessary for catalysis. Thus,
comparison of reaction mixtures containing active and inactive
catalysts shows that the active catalytic species takes the
straightforward form of a substrate-bound cationic Pt complex.
Combined, these observations indicate that the rearrangement
mechanism likely includes displacement of the triflimidate
anion by the substrate, followed by rearrangement at the single
open coordination site on platinum.
To probe the crucial rearrangement steps beyond substrate
coordination and to overcome limitations imposed by the
complex reaction kinetics, we designed a deuterium labeling
experiment. Substrate 1-d4 was synthesized from 1,2-dichloro-
ethane-d4 by a route analogous to that for substrate 1. When 1-
d4 was subjected to the optimized rearrangement reaction
conditions, product 2-d4 was produced in 89% yield with the
quantitative regioselectivity and diastereoselectivity illustrated
in eq 3.
The selectivity of the reaction and the positions of the
deuterium atoms in the product are instructive. The high
selectivity rules out mechanistic possibilities that would result in
scrambling, such as multiple reversible C−H activation,
elimination, or protonation events. The positions of the
deuterium atoms in the product are especially telling. The
four deuterium atoms in the starting material 1-d4 constitute
two adjacent methylene units (C8, C9). In the product, only
one deuterated methylene unit remains, bound to a deuterated
methine carbon. The fourth deuterium atom has undergone
stereoselective migration across the molecule. The transfer of a
deuterium to a third carbon atom in the formation of product
2-d4 indicates that C−D activation at one of the labeled
positions is an essential step of the rearrangement. This C−D
activation is a masked process, since the conversion of
unlabeled starting material (1) to product (2) does not in
itself necessitate C−H activation. The deuterium labeling study
allowed the identification of C−H activation as a key step.
The deuterium labels also allow us to track the rearrange-
ment of the carbon skeleton. Assuming that (a) the three
deuterium atoms that occupy neighboring carbon atoms in the
product do not participate in C−D activation and (b) the
tertiary center and unlabeled pair of vicinal methylene units are
conserved from reactant 1-d4 to product 2-d4, we can tentatively
map the position of each carbon atom from the reactant to
product (eq 3). This mapping indicates that three σ bonds
(C9−D, C2−C3, and C7−C9) and one π bond (C1−C2) are
broken. Two new σ bonds (C1−D and C2−C9) and two new
π bonds (C2−C9 and C3−C7) are formed. The stereo-
chemistry at C1 in the product 2-d4 indicates that the
deuterium is transferred to the face of C1 that was exo in
Figure 3. Isotopically resolved electrospray ionization mass spectra of
reaction mixtures during active catalysis (a) and after catalyst
deactivation (b). The highlighted peak was assigned as the cationic
catalyst−substrate complex by comparison of the measured isotopic
distribution (c) with the calculated natural-abundance isotopic
distribution (d).
D
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starting material 1-d4. Thus, in addition to revealing the C−H
activation step, the deuterium labeling study informs a putative
carbon skeleton rearrangement map.
Our empirical mechanistic findings and the atom map
developed from the deuterium labeling study facilitated the
construction of a proposed mechanism for the transformation
(Scheme 3). The catalyst initiation step, indicated by the
directly from empirical findings, the proposed mechanism is
consistent and plausible.
To evaluate any mechanistic proposal, it is essential to
consider alternative pathways and to understand the basis for
selectivity. Our best efforts yielded no productive alternative
mechanistic hypotheses that were consistent with the
experimental results. Most alternative pathways considered
might lead reversibly to new organometallic complexes, but
none feasibly form or release the observed organic product 2.
For example, initial C−C bond activation of the cyclopropane
moiety would yield a metallocyclobutane, but further reactivity
would not follow. A variety of C−C and C−H activations are
likely to occur reversibly, but only intermediate B is poised for
proton transfer and subsequent catalytic hydrocarbon rear-
rangement. Likewise, many nonproductive off-cycle intermedi-
ates may be formed reversibly throughout the transformation.
For example, in the proposed mechanism (Scheme 3),
intermediate E has the platinum atom and the C3 leaving
group in an antiperiplanar conformation that allows elimination
to occur to form F. If the stereochemistry at C9 is inverted at
any point such that the platinum atom and C3 are not
antiperiplanar in an intermediate such as E, no direct
elimination of product can occur. Instead, equilibrium with
the productive intermediate E will ultimately yield only the
observed product 2. Overall, the mechanism presented above is
the only mechanism considered that could catalytically produce
organic product 2.
Scheme 3. Proposed Mechanism for the Catalytic
Rearrangement of 1 to 2
The possibility of heterogeneous catalysis was ruled out by a
combination of mass spectrometry, variable-temperature
studies, catalyst decomposition, and filtration. Positive-ion
ESI-MS revealed no clusters containing more than one
platinum atom during or after active catalysis. The reaction
proceeded, albeit slowly, at room temperature (24 °C) and
became inactive over time, both of which are atypical of
nanoparticle formation. The active catalyst remained active
when filtered through Celite, consistent with a homogeneous
mechanism.
Density Functional Theory. The proposed mechanism is
supported by density functional theory (DFT) studies (Figure
4). Gas-phase calculations demonstrated that each proposed
intermediate A through F can exist at experimentally accessible
energy levels. Transition-state calculations were performed, and
appropriate transition states were identified for every
elementary step, A−B, B−C, C−D, D−E, and E−F. Notably,
no transition state that directly connected C to E could be
identified, and this computational result supports inclusion of
intermediate D in the proposed mechanism. The overall energy
profile for the reaction (Figure 5) includes a relatively facile
initial C−H activation (29 kcal/mol) and a barrierless (1 kcal/
mol) final elimination from intermediate E to generate the
product complex F. Between these two transition states lie
three higher energy transition states, corresponding to proton
transfer (38 kcal/mol relative to A) and two consecutive C−C
bond cleavage events (36 kcal/mol relative to A in each case).
The fact that every intermediate and transition state was
located by calculation supports the proposed mechanism.
The calculated free energies exceed the value of 25 kcal/mol
that is approximately consistent with the measured rates.
Possible explanations for this discrepancy include general
systematic errors found in DFT calculations and the inability of
the calculations to account for direct solvent or counteranion
involvement that may assist in the cationic rearrangement.
Overall, the successful location by DFT of all proposed
−
observed induction period, is likely the displacement of NTf2
by neutral substrate 1 at the platinum center to form the
cationic platinum olefin complex A. The proposed first bond
activation is C−H activation at C9, since C−H activation is
known to be kinetically favored over C−C activation9,28,29 and
because initial C−C activation pathways would lead only to
unproductive intermediates. C−H activation at C9 yields the
cationic platinum(IV) alkyl hydride complex B. We propose
that the acidic proton then transfers from the platinum center
to the pendent olefin moiety, generating a carbocation, C.
Intermediate C could undergo cyclopropane ring-opening to
the carbocationic center, forming one of two rearranged
carbocation structures, D or E. If intermediate D, in which
the positive charge is stabilized α to the platinum center, is
formed initially, an additional rearrangement step could then
form the brexane-like30 structure E. From intermediate E, only
one bond, C2−C3, must break to form the product complex F.
This could occur easily by elimination from platinum to
introduce double bond character to the C9−C2 and C3−C7
bonds and formally restore the positive charge to the platinum
center. Olefin exchange of product 2 by substrate 1 would then
regenerate complex A, continuing the catalytic cycle. Derived
E
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that catalytic systems can be designed to incorporate this
feature, generating new means of activating and functionalizing
hydrocarbons in a controlled, catalytic manner.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations of air-sensitive
compounds were conducted under a nitrogen atmosphere using
standard Schlenk techniques or using a nitrogen atmosphere glovebox.
Solvents were stored in poly(tetrafluoroethylene) (PTFE)-valved
flasks after drying using Vacuum Atmospheres solvent purification
systems or by distillation under nitrogen from appropriate drying
agents. Deuterated solvents (Cambridge Isotopes) and 1,3,5-tris-
(trifluoromethyl)benzene were dried over appropriate drying agents
and vacuum-transferred prior to use. cis-5,6-Dichlorospiro[bicyclo-
[2.2.1]hept[2]ene-7,1′-cyclopropane],24,25 [Pt(tBu2bpy)Ph(THF)]-
[BArF ] (4),18c Pt(tBu2bpy)Ph2,31−33 PtPh2(SMe2)2,32 and Pt(κ3-6-
4
phenyl-4,4′-di-tert-butyl-2,2′-bipyridine)Cl34 were prepared according
to literature procedures. Ammonia (Scott Specialty Gases), 1,2-
dichloroethane-d4 (Cambridge Isotopes), and dicyclopentadiene, cis-
1,2-dichloroethylene, sodium, phenyllithium, and AgNTf2 (Aldrich)
were purchased and used without further purification. HNTf2 was
purchased from Aldrich and sublimed before use. NMR spectra were
recorded on Bruker spectrometers at room temperature unless
otherwise noted. Spectra were referenced internally by the residual
solvent proton signal for H NMR, solvent signal for 13C NMR, and
1
Figure 4. Calculated structures for all intermediates and transition
states. Ligands are truncated for visual clarity, but calculations were run
using the full ligand set present in catalyst 5.
1,3,5-tris(trifluoromethyl)benzene signal for 19F NMR experiments. X-
ray analyses were carried out at the University of California, Berkeley,
College of Chemistry X-ray Crystallography Facility. Measurements
were made on an APEX charge-coupled device (CCD) area detector
with Mo Kα radiation (λ = 0.71073 Å) monochromated with
QUAZAR multilayer mirrors. Structures were solved using the
SHELXTL (version 5.1) program library (G. Sheldrick, Bruker
Analytical Systems, Madison, WI). All software and sources of
scattering factors are contained in the SHELXTL (version 5.1)
program library. Elemental analyses were performed by the University
of California, Berkeley, College of Chemistry Microanalytical Facility.
ESI-MS data were acquired in the QB3/College of Chemistry Mass
Spectrometry Facility at the University of California, Berkeley, using a
Waters (Milford, MA) Q-Tof Premier quadrupole time-of-flight mass
spectrometer in the positive ion mode. Preparatory gas chromatog-
raphy was carried out using a GOW-MAC series 400P basic isothermal
gas chromatograph, DataApex Clarity Lite chromatography station
software, and a custom-made PTFE-stoppered glass receiving vessel.
Synthesis of 1. The synthesis was adapted from the literature
procedure23 as follows: cis-5,6-Dichlorospiro[bicyclo[2.2.1]-hept[2]-
ene-7,1′-cyclopropane] (2.00 g, 10.6 mmol) and sodium metal (3.20 g,
139 mmol) were put into a 250 mL round-bottom flask equipped with
a glass-coated stir bar and both a bath and a condenser at −77 °C.
Ammonia (150 mL) was condensed into the flask. The flask was
allowed to warm to reflux temperature under N2. After 2.5 h, the flask
was cooled to −77 °C. Pristane (5 mL) was added, and then water (5
mL) was added slowly. The cold bath was removed, and ammonia was
allowed to evaporate. The pristane layer was separated from the water
layer and dried over magnesium sulfate. Compound 1 was separated
from pristane by preparatory gas chromatography (sequential 0.4 mL
injections, 8 ft × 1/4 in. Carbowax 20 column, column temperature
125 °C, detector and injection port temperatures 140 °C, collection
vessel temperature −77 °C, UHP He carrier gas flow rate 100 mL/
min, retention time 10 min) to give 1 as a clear, colorless liquid (45%
Figure 5. Reaction energy profile diagram.
intermediates and transition states at approximately reasonable
energy levels supports the proposed catalytic mechanism in
which C−H activation of substrate 1 occurs first and enables
C−C bond activation.
CONCLUSION
■
In conclusion, we have discovered a new hydrocarbon
rearrangement using platinum triflimidate precatalysts (3, 5).
On the basis of experimental results from NMR, mass
spectrometry, and deuterium labeling studies, along with
DFT calculations, we propose an unusual new mechanism for
catalytic C−C bond activation. The proposed mechanism
includes initial C−H activation of the substrate, which leads to
subsequent carbocation generation and rearrangement to break
two C−C bonds and catalytically release a bicyclic product. The
reaction is noteworthy because it is a rare example of a catalytic
C−C bond activation reaction and because it introduces an
unprecedented mechanistic possibility: catalytic C−C bond
cleavage driven by a masked C−H bond activation. We expect
1
yield, >95% pure by H NMR).
Synthesis of 1-d4. Compound 1-d4 was synthesized identically to
1,23−25 except for substitution of 1,2-dichloroethane-d4 for 1,2-
1
dichloroethane. H NMR (o-dichlorobenzene-d4, 400 MHz): δ 6.09
2
(m, 2H), 2.10 (m, 2H), 1.78 (m, 2H), 1.06 (m, 2H). H NMR (o-
dichlorobenzene-d4, 400 MHz): δ 0.40 (s, 2D), 0.20 (s, 2D).
Representative Catalytic Procedure for Conversion of 1 to 2.
A stock solution was prepared containing substrate 1 (260 mM) and
1,3,5-tris(trifluoromethyl)benzene (22 mM) as an internal standard.
To a sample of catalyst 5 (2.2 mg, 3.0 μmol) was added the substrate
F
dx.doi.org/10.1021/ja406260j | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Article
solution (230 μL, 60 μmol, of substrate 1, 5 μmol of standard). The
resulting clear yellow solution was transferred to a medium-walled
PTFE-valved NMR tube. The tube was heated to 70 °C for 50 min.
The yield of 2 (87%) was determined by 1H NMR integration relative
to the internal standard.
3H). 13C NMR (o-C6D4Cl2, 600 MHz): δ 156.9, 153.5, 151.8, 151.6,
151.1, 148.1, 137.8, 136.3, 135.0, 125.6, 124.4, 123.3, 122.9, 120.8,
118.6, 21.6, 21.5. 19F NMR (C6D6, 400 MHz): δ −70.3 (s, 6F).
Synthesis of 6. To AgNTf2 (39 mg, 100 μmol) was added a
solution of Pt(κ3-6-phenyl-4,4′-di-tert-butyl-2,2′-bipyridine)Cl (57 mg,
100 μmol) in o-dichlorobenzene (5 mL). The clear red solution was
stirred in the dark for three days. A gray precipitate formed, which was
removed by filtration through Celite. Pentane (30 mL) was added to
the filtrate. The resulting yellow precipitate was collected on a frit,
washed with pentane (8 × 2 mL), and then dried under vacuum (66
mg, 81% yield). Crystals suitable for X-ray diffraction were obtained
from benzene at 24 °C after three days. Anal. Calcd for
C26H27F6N3O4PtS4: C, 38.14; H, 3.32; N, 5.13; S, 7.83. Found: C,
NMR Characterization of Product 2..35,36 Product 2 was
isolated by filtration of the catalytic mixture through a silica plug
followed by preparatory gas chromatography (0.3 mL injection, 8 ft ×
1/4 in. DC710 column, column temperature 135 °C, detector and
injection port temperatures 150 °C, collection vessel temperature 0
°C, UHP He carrier gas flow rate 100 mL/min, retention time 30 min)
to give 2 as a clear, colorless liquid. The following assignments were
based on HSQC, COSY, and NOESY analyses. The abbreviation “ax”
designates axial, or trans to C6−H, while “eq” represents equatorial, or
cis to C6−H. The numbering of the carbon atoms is defined in eq 3.
1H NMR (CDCl3, 400 MHz): δ 5.74−5.67 (m, 1H, C2−H), 5.67−
5.61 (m, 1H C9−H), 5.38 (m, 1H, C3−H), 2.85 (m, 2H, H−C8−H),
2.71−2.59 (m, 1H, C6−H), 2.44−2.23 (m, 3H, H−C4−H and C1−
Heq), 2.22−2.11 (m, 1H, C5−Heq), 1.85−1.74 (m, 1H, C1−Hax),
1.50−1.40 (m, 1H, C5−Hax). 13C NMR (CDCl3, 400 MHz): δ 142.5
(C7), 127.1 (C2), 126.2 (C9), 121.3 (C3), 41.9 (C6), 34.7 (C1), 31.1
(C4), 30.4 (C5), 28.3 (C8).
1
37.88; H, 3.17; N, 4.94; S, 8.09. H NMR (o-C6D4Cl2, 500 MHz): δ
8.70 (d, J = 5.5, 1H), 7.70 (s, 1H), 7.64 (d, J = 7.61, 1H), 7.40 (s, 1H),
7.36 (s, 1H), 7.34 (m, 1H), 7.30 (d, J = 7.8, 1H), 7.24 (m, 1H), 7.06
(m, 1H), 1.38 (s, 9H), 1.26 (s, 9H). 13C NMR (o-C6D4Cl2, 600
MHz): δ 167.3, 165.7, 165.2, 156.2, 154.7, 150.1, 146.8, 136.0, 131.1,
125.0 (br), 124.6, 124.4, 121.1, 119.5, 118.9, 115.8, 114.7, 36.0, 35.6,
30.4, 30.1. 19F NMR (C6D6, 400 MHz): δ −73.1 (s, 6F).
Computational Details. All calculations were performed using the
Gaussian ’09 suite of programs37 in the Molecular Graphics and
Computing Facility of the College of Chemistry, University of
California, Berkeley. The crystallographically determined atomic
coordinates of 4 were used as starting points for gas-phase geometry
optimization calculations for catalyst 5 and its derivatives, after
NMR Characterization of Product 2-d4. 1H NMR (o-
dichlorobenzene-d4, 400 MHz): δ 5.65 (s, 1H, C2−H), 5.32−5.28
(m, 1H, C3−H), 2.64−2.56 (m, 1H, C6−H), 2.35−2.22 (m, 2H, H−
C4−H), 2.16−2.07 (m, 1H, C5−Heq), 1.76−1.69 (m, 1.85−1.74, C1−
t
2
transformation of Bu groups into Me groups. The B3LYP hybrid
Hax), 1.44−1.35 (m, 1H, C5−Hax). H NMR (o-dichlorobenzene-d4,
functional was used with the 6-31G(d,p) basis set for all main-group
elements and the SDD basis set for platinum. Stationary points were
confirmed by frequency calculations, showing all positive vibrations for
minima and one imaginary vibration for transition states. Frequency
calculations were carried out at 298.15 K and 1 atm. Molecular images
were rendered and exported from Gaussview. Transition-state
structures were located using a combination of relaxed coordinate
scans and geometry optimizations, and they were all confirmed by
intrinsic reaction coordinate calculations or by geometry minimization
after perturbation of the transition-state structure along the imaginary
vibrational mode. Calculations for key steps were repeated with either
solvent corrections (o-dichlorobenzene, benzene) or an alternate
functional (ωB97XD), and the results were similar to those from the
original calculations.
400 MHz): δ 5.61 (m, 1D, C9−D), 3.79 (s, 2D, D−C8−D), 2.37−
2.19 (m, 1D, C1−Deq).
Synthesis of 3. Pt(tBu2bpy)Ph2 (40 mg, 64 μmol) was dissolved in
a minimum of benzene (2 mL). To this dark orange solution was
added a solution of HNTf2 (17 mg, 60 μmol) in benzene (2 mL). The
color changed to yellow. After 1.5 h, pentane was added (16 mL). The
resulting yellow precipitate was collected on a frit, dried under vacuum,
then dissolved in a minimum of toluene (20 mL), and cooled to −30
°C. After 44 h, product 4 was collected as yellow crystals and dried
under vacuum (38 mg, 77% yield). Crystals suitable for X-ray
diffraction were grown by cooling a solution of 4 (10 mg) in toluene
(7 mL) to −30 °C for 18 h. Anal. Calcd for C26H29F6N3O4PtS2·C7H8:
1
C, 43.42; H, 4.09; N, 4.60. Found: C, 43.06; H, 4.27; N, 4.24. H
NMR (C6D6, 400 MHz): δ 9.16 (d, J = 6 Hz, 1H), 8.16 (d, J = 8 Hz,
2H), 7.87 (d, J = 6 Hz, JPt−C = 32 Hz, 1H), 7.35 (m, 3H), 7.07 (s, 1H),
6.94 (s, 1H), 6.68 (d, J = 6 Hz, 1H), 5.82 (d, J = 6 Hz, 1H), 0.88 (s,
9H), 0.79 (s, 9H). 19F NMR (C6D6, 400 MHz): δ −70.3 (s, 6F). 13C
NMR (o-C6D4Cl2, 600 MHz): δ 164.4, 164.2, 157.4, 153.8, 151.6,
148.7, 137.8, 124.5, 124.3, 123.9, 123.0, 120.8, 119.2, 119.0, 118.6,
35.6, 35.5, 30.1, 29.9.
Synthesis of Pt(Me2bpy)Ph2. To PtPh2(SMe2)2 (1.1 g, 2.4
mmol) were added Me2bpy (0.50 g, 2.7 mmol) and diethyl ether (150
mL). The suspension was stirred for 21 h, and Pt(Me2bpy)Ph2 was
collected by filtration as a fine yellow powder, then washed with
diethyl ether, and dried under vacuum (1.0 g, 77% yield). Anal. Calcd
for C24H22N2Pt: C, 54.03; H, 4.16; N, 5.25. Found: C, 53.64; H, 3.66;
N, 5.03. 1H NMR (CDCl3, 500 MHz): δ 8.49 (d, J = 6 Hz, 2H), 7.86
(s, 2H), 7.56 (d, J = 7 Hz, JPtH = 34 Hz, 4H), 7.18 (d, J = 6 Hz, 2H),
7.09 (t, J = 7, 4H), 6.96 (t, J = 7 Hz, 2H), 2.44 (s, 6H). 13C NMR
(CDCl3, 500 MHz): δ 156.0, 150.0, 149.3, 145.5, 138.5, 127.7, 127.2,
122.8, 121.9, 21.9.
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional experimental information, NMR characterization
details for 1-d4 and 2-d4, and X-ray crystallographic details and
CIF files for 3 and 6. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
rbergman@berkeley.edu; tdtilley@berkeley.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Synthesis of 5. To a suspension of Pt(Me2bpy)Ph2 (34 mg, 64
μmol) in benzene (12 mL) was added a solution of HNTf2 (17 mg, 60
μmol) in benzene (3 mL). The color changed from dark to light
yellow. After 1 h of stirring, the solvent volume was reduced to 5 mL
under vacuum, and pentane was added (10 mL). The resulting yellow
precipitate was collected on a frit, then rinsed with a 1:2 benzene/
pentane mixture (5 × 3 mL), and dried under vacuum (34 mg, 77%
yield). Anal. Calcd for C20H17F6N3O4PtS2: C, 32.61; H, 2.33; N, 5.70;
We gratefully acknowledge financial support from the Director
of the Office of Energy Research, Office of Basic Energy
Sciences, Chemical Sciences Division, of the U.S. Department
of Energy under Contract DE-AC02-05CH11231. For the
synthesis of starting materials, we thank Kathryn Liu. For
technical assistance and helpful discussion, we thank Dr.
Kathleen A. Durkin and Dr. Xinzheng Yang of the College of
Chemistry Molecular Graphics and Computation Facility, Dr.
Anthony T. Iavarone of the QB3/Chemistry Mass Spectrom-
etry Facility, and Dr. Antonio DiPasquale of the College of
1
S, 8.71. Found: C, 33.32; H, 2.40; N, 5.54; S, 9.10. H NMR (C6D6,
400 MHz): δ 8.96 (d, J = 6 Hz, 1H), 8.08 (d, J = 7 Hz, 2H), 7.98 (d, J
= 6 Hz, JPt−C = 31.4, 1H), 7.30 (m, 3H), 6.59 (s, 1H), 6.46 (s, 1H),
6.16 (d, J = 6 Hz, 1H), 5.53 (d, J = 6 Hz, 1H), 1.62 (s, 3H), 1.45 (s,
G
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Journal of the American Chemical Society
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