Unexpected, Latent Radical Reaction of Methane Propagated by Trifluoromethyl Radicals

Article abstract of DOI:10.1021/acs.joc.6b01903

Thorough mechanistic studies and DFT calculations revealed a background radical pathway latent in metal-catalyzed oxidation reactions of methane at low temperatures. Use of hydrogen peroxide with TFAA generated a trifluoromethyl radical (?CF₃), which in turn reacted with methane gas to selectively yield acetic acid. It was found that the methyl carbon of the product was derived from methane, while the carbonyl carbon was derived from TFAA. Computational studies also support these findings, revealing the reaction cycle to be energetically favorable.

Full text of DOI:10.1021/acs.joc.6b01903

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Article
An Unexpected, Latent Radical Reaction of
Methane Propagated by Trifluoromethyl Radicals
Nima Zargari, Pierre Winter, Yong Liang, Joo Ho Lee,
Andrew Cooksy, Kendall N. Houk, and Kyung Woon Jung
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b01903 • Publication Date (Web): 29 Sep 2016
Downloaded from http://pubs.acs.org on September 30, 2016
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An Unexpected, Latent Radical Reaction of Methane Propagated by
Trifluoromethyl Radicals
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Nima Zargari, Pierre Winter, Yong Liang, Joo Ho Lee, Andrew Cooksy,* K. N. Houk,* and Kyung
Woon Jung*1
1Loker Hydrocarbon Research Institute, Department of Chemistry, University of Southern California, 837 Bloom Walk, Los
Angeles, CA 90089
2Department of Chemistry and Biochemistry, San Diego State University, 5500 Campanile Drive, San Diego CA 92182
3Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095
CH
4
O
O
CF
3
2
H O
AcOH
+
TFA
F
3
C
O
CH
3
3
CF H
TFAA
H2O2
O
O
CH
3
O
O
F
3
C
O
CF
3
3
CH
F
3
C
O
CF
3
ABSTRACT: Thorough mechanistic studies and DFT calculations revealed a background radical pathway latent in metal catalyzed
oxidation reactions of methane at low temperatures. Use of hydrogen peroxide with TFAA generated a trifluoromethyl radical (·CF ),
which in turn reacted with methane gas to selectively yield acetic acid. It was found that the methyl carbon of the product was derived
from methane, while the carbonyl carbon was derived from TFAA. Computational studies also support these findings, revealing the
reaction cycle to be energetically favorable.
3
Using various palladium catalysts including our NHC-
INTRODUCTION
amidate complex,[8] we examined the oxidation of methane
Methane, in the form of natural gas, is the most abundant
under the conditions similar to the work done by Sen at 90
hydrocarbon but the least reactive due to its high bond-disso-
°C, and obtained methanol in similar yields as Sen reported.
ciation energy.[1] Because methane and its flared product, car-
However, at lower temperatures (60 °C) we observed acetic
bon dioxide, are major greenhouse gases,[2] it is of paramount
acid as the primary product. To our surprise, the same
importance to discover novel ways to functionalize methane.
amount of acetic acid was seen without the use of any palla-
Various oxidants with transition metals or even main-group
dium catalysts as well. As depicted in Scheme 1, acetic acid
catalysts have been investigated in order to oxidize methane
was selectively generated from methane at 60 °C while we also
into chemical feedstock products with varying degrees of suc-
observed the presence of another critical product, fluoroform,
cess.[3] Seminally, Sen, Mizuno, and Ingrosso used hydrogen
peroxide in trifluoroacetic acid (TFA) and trifluoroacetic an-
which stemmed from TFA/TFAA (vide infra). Hence, we em-
barked on the study of this background reaction to better un-
hydride (TFAA) to successfully convert methane gas to either
derstand the mechanistic pathway.
methyl trifluoroacetate or acetic acid as major liquid prod-
ucts.[4-6] Despite low yields based on methane, these method-
ologies were progressive and pioneering. These transfor-
mations required the use of transition metal catalysts; how-
ever, their mechanisms including the possible presence of non-
Scheme 1. Background radical reaction at a low temperature
metallic background processes were not comprehensively
studied. Herein, we report a background reaction which we
have identified as a radical process.[7] The purpose of this pa-
2
TFAA, TFA, H O2
CH4
CH CO H + CF H
3
2
3
60 °C, 16 h
3
.8%
0.6%
per is to provide clarity to the mysterious and underdeveloped
reaction of methane gas under radical conditions in TFA and
TFAA solution.
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source of the observed acetic acid. The necessity of having an
RESULTS AND DISCUSSION
excess amount of TFAA has significant mechanistic implica-
tions, which include the trapping of acetic acid within a mixed
anhydride (vide infra).
In the absence of methane, no products were observed, in-
dicating that methane was the actual carbon source for the
intended reactions. When we employed 13C labeled methane,
we observed acetic acid as the only meaningful 13C incorpo-
rating product (Figure 1). However, the acetyl protons in the
1H NMR spectra exhibited a large one-bond 13C-H coupling
C-H = 130 Hz) but no two-bond coupling (i.e., 13C- C-H),
13
(J
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signaling only the methyl of the acetyl group came from me-
thane. In addition, fluoroform was constantly detected in var-
iable amounts, suggesting that the C-C bond of the trifluoro-
acetyl group was cleaved to offer the carbonyl moiety, which
would be incorporated into the produced acetic acid (vide in-
fra).
Figure 2. C NMR of 13C labeled methane reaction (DMSO
13
was added as an internal NMR standard after the reaction)
Table 1. Effects of TFAA and TFA
Entry
TFAA
TFA (mmol)
/
AcOH (μmol,
yield)
MeOH
(μmol)
1
2
3
4
5
0.0 / 11
1.4 / 8.1
2.8 / 5.4
4.3 / 2.7
5.7 / 0.0
-
-
33 (1.0%)
55 (1.7%)
121 (3.8%)
25 (0.77%)
2.0
0.7
-
Figure 1. Wet1D H NMR of 13C labeled methane reaction
1
(
DMSO was added as an internal NMR standard after the re-
action)
-
In the proton-coupled 13C NMR spectra (Figure 2), similar
Reaction conditions: Varying amounts of TFAA and TFA
were added to 130 μmol 30% H in a stainless steel reactor
with a high pressure valve and charged with 3200 μmol me-
thane. The mixture was stirred at 60 °C for 16 hours. D
patterns were demonstrated, where the methyl carbon of the
acetyl group showed a quartet with a large coupling constant
due to the aforementioned one-bond coupling (JC-H = 130
Hz). The carbonyl carbon peak was negligible due to low
abundance of 13C, suggesting this carbon would be naturally
abundant 12C. Both spectrum data confirmed that methane
was a key reactant to form the methyl portion of acetic acid,
and the carbonyl moiety would stem from another carbon
source.
2
O
2
2
O
was added to the reaction mixture and a wet1D NMR was
taken using DMSO as an internal standard. Yields were cal-
culated based on methane as the limiting reagent.
1
Since CF
3
H was observed in both H and 19F NMR, we
proposed that trifluoromethyl radical (·CF
chain carrier of this reaction. In conditions including hydro-
gen peroxide and TFAA, there are several ways in which ·CF
3
) would be the
As a potential candidate of the extra carbon sources, we first
decided to see if carbon dioxide would react since we usually
detected carbon dioxide in small amounts (Figure 2). How-
ever, the addition of either carbon dioxide or carbon monox-
ide did not affect the yield of the reaction. Thus, it was con-
cluded that TFAA/TFA would offer the carbonyl moiety to
the produced acetic acid since they were the only carbon-con-
taining compounds left in the reaction mixture.
3
can be produced. As depicted in Scheme 2, the radical pro-
cesses can be initiated by trifluoroperacetic acid 1 (TFPAA),
which is in accordance with previous studies performed with
TFAA and hydrogen peroxide at room temperature.[9] There-
fore, TFPAA (1) can react directly with methane to form in-
termediate 2.[10] Radical degradation of this complex would
We investigated the effect of TFA and TFAA in order to
determine which reagent participated in the formation of ace-
tic acid and fluoroform (Table 1). The reaction did not pro-
ceed when TFAA was excluded (entry 1). As the amount of
TFAA increased, the reaction afforded higher yields of acetic
acid and smaller quantities of methanol (entries 2 – 4). Use of
pure TFAA resulted in a lower yield, suggesting acidic anhy-
drous conditions are required (entry 5). These results sup-
ported the role of TFAA rather than TFA as the carbonyl
furnish methyl (·CH
Decarboxylation of TFA radical 3 leads to the facile produc-
tion of trifluoromethyl radical (·CF ), which plays a crucial
3
) and TFA radicals (3) along with water.
3
role in the subsequent radical propagation processes. Previous
studies also confirmed that trifluoroacetyl peroxides furnished
fluoroform along with carbon dioxide when they were subject
to thermal decomposition in hydrocarbon solvents.[11]
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Scheme 2. Formation of trifluoroperacetic acid and subse-
quent reaction with methane
H
O
CH4
3
0% H
2
O
2
O
TFAA
O
2
5 °C
CF3
1
H
O
H3C
O
O
H
+
CH
3
CF
3
O
F
3
C
O
-
H2O
- CO2
CF3
0
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0
3
2
Another source of ·CF
3
is bis(perfluoroacetyl) peroxide 4
(
Scheme 3). When hydrogen peroxide is added to a solution
of TFAA and trace amounts of base, 1 is no longer produced
and the primary product is bis(perfluoroacetyl) peroxide 4.[9]
Alternatively, when 30% hydrogen peroxide is introduced
dropwise into a biphasic system of an aqueous alkaline media
and Freon 113, 4 is also produced.[12] At elevated tempera-
tures, 4 readily undergoes homolysis, and the resulting TFA
radical 3 rapidly dissociates to give rise to the formation of
3
trifluoromethyl radical (·CF ).
Figure 3. 19F NMR of three methods of forming trifluorome-
thyl radical sources: A) addition of H to a TFA/TFAA so-
lution at room temperature; B) addition of H to a
TFA/TFAA solution with trace amounts of potassium hy-
droxide at room temperature; C) dropwise addition of H
into a biphasic system of an aqueous alkaline media and Freon
2
O
2
2
O
2
2
O
2
1
13 at -4 °C.
O
O
3
As illustrated in Scheme 4, trifluoromethyl radical (·CF ) re-
acts with methane to form methyl radical (·CH
3
) and fluoro-
30% H O , 0.1% KOH
2
2
TFAA
TFAA
F
3
C
O O
CF
3
form (CF H) because of the favorable difference in the bond
3
25 °C
bis(trifluoroacetyl) peroxide
dissociation energies between fluroform (446.4 kJ/mol) and
methane (439.7 kJ/mol).[13] After the produced methyl radical
4
O
O
(
·CH
radical dissociation to mixed anhydride 6, while trifluorome-
thyl radical (·CF ) is regenerated to propagate the radical cy-
3
) adds to TFAA, the resultant intermediate 5 undergoes
Na CO , NaCl, H O
2
3
2
F C
3
O O
CF3
30% H O , Freon 113, -3 °C
2
2
bis(trifluoroacetyl) peroxide
in Freon 113 solution
3
cle. Upon treatment with water during the work-up, mixed
anhydride 6 is ultimately liberated into acetic acid and TFA.
This mechanistic scheme is similar to the one proposed by Sen
while investigating the radical-initiated oxidative functionali-
zation of higher chain alkanes such as ethane and propane.[4b]
Interestingly, the trifluoromethyl radical might add to TFAA,
however we did not observe other potential dissociation prod-
ucts such as hexafluoroacetone. The adduct would dissociate
3
to ·CF more rapidly via the reversible process of the addition
than the dissociation of the anhydride C-O bond to hex-
afluoroacetone. Termination of this radical process may occur
O
O
3
CF3
F C
3
O
O
CF3
- CO2
4
Scheme 3. Formation of bis(perfluoroacetyl) peroxide and
subsequent homolysis
Our standard reaction conditions were applied using all
three different pathways of producing trifluoromethyl radical
initiators and all produced acetic acid as the selective product.
Most importantly, in the reaction which bis(perfluoroacetyl)
peroxide in Freon 113 solution was used as the trifluoromethyl
radical initiator, no additional hydrogen peroxide was intro-
duced into the solution. This supports our belief that the rad-
ical chain carrier of this methane functionalization reaction is
by the reaction of two trifluoromethyl radicals (·CF ), forming
3
hexafluoroethane (C F ) which was observed in 19F NMR.
2
6
Hexafluoroethane is commonly seen as a decomposition prod-
uct of reactions containing trifluoromethyl radicals (·CF3).[14]
Scheme 4. Methane functionalization by proposed radical
processes and termination of trifluoromethyl radical
3
trifluoromethyl radical (·CF ).
The 19F NMR of the solutions prior to reacting them with
methane clearly showed the selective formation of either 1 or
CH
4
O
O
4
(Figure 3). Since our standard reaction conditions did not
CF3
H2O
use any bases, trifluoroperacetic acid 1 (TFPAA) would be the
trifluoromethyl radical source of the reactions, not bis(per-
fluoroacetyl) peroxide 4.
F3C
O
O
CH3
CF3H
O
6
O
AcOH
+ TFA
CH3
O
F3C
O
CF3
CH3
F3C
O
CF3
5
CF3
+
CF3
C2F6
Termination
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O
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F C H CH
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F C
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CF3
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62.2
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CF3
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CF +
CH4
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CF H +
CH3
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0.0
-3.4
O
O
-
15.8
F C
O
CF3
3
CH3
5
-
66.1
O
O
+
CH3
CF3
F C
O
3
6
Figure 4. TFA Solvated relative free energy (kJ/mol) diagram for Scheme 4 reactions. Vibrational corrections were calculated at
333K and 27.0 atm.
This mechanistic approach takes into account several criti-
cal observations. Firstly, greater amounts of acetic acid were
observed as the amount of TFAA increased because TFAA
conditions (Table 1, entry 4), proton-coupled 13C NMR stud-
ies (Figure 2) showed the production of carbon dioxide, while
gas chromatography analysis quantified the amount to be 109
μmol. These results suggested that 130 μmol of hydrogen per-
oxide led to 109 μmol of CO
trifluoromethyl radical (·CF ). Acetic acid as the only liquid
product was obtained in slightly larger amounts (i.e., 121
μmol), implying the radical cycle proposed in Scheme 4 would
be in play.
3
provided both CF radical as the reagent and the carbonyl
group as the substrate. Secondly, acetic acid was not detected
until the aqueous work-up because the mixed anhydride 6 was
stable during the reaction conditions. As a consequence, at
low temperatures, the over-oxidation of acetic acid to carbon
dioxide was inhibited, while higher temperatures may have
degraded the mixed anhydride. The rare cases where trace
methanol was observed, may be caused by the direct oxidation
of methane via peroxide radicals. Lastly, since water could be
formed as a by-product of the radical degradation of complex
2
and the equimolar amount of
3
Rigorous computational studies were also performed to
support these experimental findings. The radical processes for
methane functionalization in Scheme 4, with the exception of
the hydrolysis of mixed anhydride 6 and the radical
termination reaction, were analyzed computationally using
Gaussian 09.[15] The bimolecular reaction between the
trifluoromethyl radical (·CF
as reaction 1. The bimolecular addition reaction between the
methyl radical (·CH ) and TFAA will be referred to as
reaction 2. Finally, the unimolecular decomposition reaction
of 5 into trifluoromethyl radical and 6 will be referred to as
reaction 3. Each of these reactions proceeds through a single
transition state which will be referred to as transition state 1,
2, and 3, respectively.
2
(Scheme 2), excess amounts of TFAA were used to maintain
anhydrous reaction conditions.
Seeking evidence to corroborate the proposed mechanism,
we carefully analyzed products by using NMR and GC meth-
ods mainly to identify important product species such as fluo-
roform, hexafluoroethane, carbon dioxide, and mixed anhy-
dride 6. We confirmed the presence of fluoroform by both
wet1D H (Figure 1) and 19F NMR. Hexafluoroethane was ob-
served as another fluorine containing product besides fluoro-
3
) and methane will be referred to
3
1
form by 19F NMR analysis. By employing wet1D 1H and 13C
NMR techniques, we detected the mixed anhydride at the end
of the reaction as well as its rapid hydrolysis to acetic acid and
TFA upon the addition of water. Under standard reaction
The COSMO model was used to estimate the solvent effects
of TFA on the single-point energies of all reactants, transition
states, and products. The COSMO results predict that the free
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energies of reaction for reactions 1 and 3 are both lowered by
7.7 kJ/mol when using TFA as a solvent, but increase by 1.8
kJ/mol for reaction 2. For reactions 2, the energy of activation
did not change significantly when using the COSMO model.
The free energies of activation for reaction 1 and 3, however,
were each reduced by 3.0 kJ/mol when the COSMO model
was employed. From these data we conclude that solvation
plays an important role in the reaction thermodynamics of
Scheme 4, if not the kinetics (Figure 4).
0
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Frequency calculations were performed on the optimized
geometries to obtain zero-point and thermal corrections at
3
33K and 27.0 atm. These calculations also confirm that our
transition state geometries are first-order saddle points and
that our reactant and product geometries are minima on the
potential energy surface. The single imaginary frequency
calculated for each transition state was verified to correspond
to the reaction coordinate for that step.
Figure 6. Transition state structure of reaction between the
methyl radical (·CH ) and TFAA; carbon: gray, hydrogen:
white, oxygen: red, and fluorine: blue (reaction 2).
3
The transfer of a hydrogen atom to proceed from ·CF
CH to CF H and ·CH is found to occur through transition
state 1 (Figure 5). The C-H bond length in this structure is
.37 Å between ·CF and the shared hydrogen atom and 1.33
Å between ·CH and the shared hydrogen atom. The FCH
3
and
4
3
3
1
3
3
bond angle is 109.8° and the HCH bond angle is 104.3°. The
solvated free energy of activation for this transition state was
found to be 62.2 kJ/mol.
Figure 7. Transition state structure of decomposition
reaction of 5 into trifluoromethyl radical and 6; carbon: gray,
hydrogen: white, oxygen: red, and fluorine: blue (reaction 3).
Figure 5. Transition state structure of reaction between
trifluoromethyl radical (·CF ) and methane; carbon: gray,
3
hydrogen: white, and fluorine: blue (reaction 1).
In an effort to validate the radical process, we evaluated the
feasibility of the methane functionalization by using various
radical inhibitors and promoters (Table 2). When various rad-
ical inhibitors and initiators including butylated hydroxytolu-
ene (BHT), TEMPO, and azobisisobutyronitrile (AIBN) were
added (10 mole percent to hydrogen peroxide), yields of acetic
acid were reduced by 75% (entries 2 – 4 vs entry 1). As ex-
pected, a stoichiometric amount of BHT shut down the reac-
tion completely (entry 5). When radical initiators such as
AIBN were solely utilized in the absence of hydrogen perox-
ide, no products were observed. These data support a radical
mechanism through the necessity of TFPAA (1).
Of the three reactions modeled, the transition state for re-
action 2 has the highest solvated activation free energy of 67.2
kJ/mol, indicating that this is the slowest reaction in the mech-
anism. The C-C bond distance in this structure is 2.22Å be-
3
tween the carbonyl of TFAA and the ·CH carbon atom. The
CCO bond angle between the previously mentioned carbon
atoms and the respective carbonyl oxygen atom is 100.5° (Fig-
ure 6).
The structure of transition state 3 shows the removal of the
·CF
product (Figure 7). The C-C bond length in this structure is
2.07 Å between ·CF and the carbonyl carbon of the mixed
anhydride. The CCO bond angle between the ·CF moiety
3
group from the reactant to form the mixed anhydride
3
3
and the carbonyl group is 99.0°. The solvated free energy of
activation for this transition state was found to be 35.4 kJ/mol.
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1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 2. Effects of radical inhibitors and initiators
the carbonyl carbon originated from TFAA. Experimental re-
sults from the use of radical initiators and inhibitors along with
careful analysis of products and intermediates supported our
hypothesis on a radical mechanism. Though similar reaction
conditions for methane oxidation have frequently been em-
ployed, a thorough study of this intriguing background reac-
tion has not been performed. Thus, we hope that this report
will bring some clarity to radical transformations and metal
catalyzed oxidations of methane, the most abundant carbon
source.
En-
try
H
2
O
2
Additive
AcOH
(μmol)
121
34
MeOH
(μmol)
(
μmol)
1
2
3
4
5
130
130
130
130
130
-
-
BHTa
1.4
2.1
4.3
-
TEM-
POa
22
46
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
AIBNa
BHTb
-
a13 μmoles. 130 μmoles. Reaction conditions: 2.7 mmol
b
EXPERIMENTAL SECTION
Methods. All glassware and reactor components were
2 2
TFA, 4.25 mmol TFAA, 130 μmol 30% H O , and varying
amounts of an radical inhibitor and initiator were added to a
stainless steel reactor with a high pressure valve and charged
with 3200 μmol methane. The mixture was stirred at 60 °C
for 16 hours. D O was added to the reaction mixture and a
2
wet1D NMR was taken using DMSO as an internal standard.
oven-dried prior to use. All chemicals were purchased as rea-
1
gent grade and used without further purification.. H (400 or
00 MHz), 13C (126 MHz), and 19F (470 MHz) NMR spectra
5
were referenced to DMSO or TFA.
A TCD detector for gas chromatography was set at 160 °C
and the inlet temperature was set at 200 °C. Initial oven tem-
perature was 40 °C for one minute and was then ramped to
50 °C at 10 °C/minute. The flow rate was set to 3 mL/minute
by an argon high purity carrier. After the reaction was com-
plete, the reactor was cooled at -20 °C for half an hour. The
vessel was opened and the expelled gas was passed through
potassium hydroxide and Drierite and collected in a screw cap
vial with septum. Resulting gas was then injected into the GC
column.
As represented in Table 3, we varied the amounts of hydro-
gen peroxide and compared the aforementioned results
(
shown again in Table 3, entry 2). Reducing the amount of
hydrogen peroxide by half produced acetic acid in a lower
yield as expected (entry 1). However, doubling the amount of
hydrogen peroxide failed to furnish additional acetic acid (en-
try 3). Both of these modified amounts of hydrogen peroxide
produced methanol in trace amounts. Therefore, there
seemed to be an appropriate stoichiometry between hydrogen
peroxide and TFAA for optimal radical processes (entry 2).
General Method for Methane Functionalization.
To a 0.5 dram vial equipped with a stir bar, was added a so-
lution of 0.2 mL (2.69 mmol) TFA, 0.6 mL (4.25 mmol)
TFAA, and 13 μL (130 μmol) 30% H O . The solution was
Table 3. Effect of hydrogen peroxide used
2
2
stirred at room temperature for one minute. The reaction was
then placed in a stainless steel reactor with a high pressure
valve and was subjected to eight purge cycles and charged
with 27 psi (3200 μmol) methane gas. The reactor was stirred
at 60 °C for 16 hours. The reaction vessels were then held at
Entry
H
μmol)
2
O
2
AcOH
MeOH
(μmol)
(
(μmol, yield)
75 (2.4%)
121 (3.8%)
109 (3.4%)
1
2
3
65
1.9
-
130
260
-
2
20 °C for half an hour. 0.5 mL D O was added to the reaction
1.8
mixture and a wet1D NMR was taken using DMSO as an in-
ternal standard.
Reaction conditions: 2.7 mmol TFA, 4.25 mmol TFAA
were added to varying amounts of 30% H and were added
2
O
2
Synthesis of Bis(perfluoroacetyl) peroxide (4). 0.21
g (2.0 mmol) sodium carbonate and 0.11 g (2.0 mmol) sodium
chloride were added to 1.9 mL of deionized water and the re-
action was stirred at 0 °C for five minutes. 0.2 mL of 30%
to a stainless steel reactor with a high pressure valve and
charged with 3200 μmol methane. The mixture was stirred at
6
2
0 °C for 16 hours. D O was added to the reaction mixture
and a wet1D NMR was taken using DMSO as an internal
standard.
2 2
H O (20 mmol) and 1.9 mL of Freon 113 were then added to
the aqueous solution and stirred at -3 °C for five minutes. 0.35
mL (2.5 mmol) TFAA was added dropwise over 30 minutes.
The organic layer was separated and 19F NMR was taken, us-
ing trifluorotoluene as an internal standard. 19F NMR; -72.76
ppm.
CONCLUSION
In summary, we found a background radical reaction latent
in metal catalyzed oxidative functionalizations of methane.
Hydrogen peroxide together with TFAA produced the trifluo-
Gaussian Parameters. In all cases the reactant,
product, and transition state geometries were optimized using
the B3LYP hybrid functional,[16] and Dunning’s aug-cc-
pVDZ basis set.[17] Diffuse functions were included in the basis
set in order to account for long-range interactions, particularly
between the fluorine and carbon atoms.
romethyl radical (·CF
was also shown that ·CF
ysis of bis(perfluoroacetyl) peroxide 4. The ·CF
3
) by trifluoroperacetic acid 1, though it
can also be produced by the homol-
in turn re-
3
3
acted with methane in a favorable reaction forming fluroform
and methyl radical. The resulting methyl radical reacted with
TFAA to generate a mixed anhydride, which was subse-
quently hydrolyzed into acetic acid. NMR data indicated that
the methyl group of acetic acid originated from methane while
COSMO Paramenrts. Energies were calculated for the
gas-phase optimized geometries both with and without the
conductor-like polarizable continuum solvent model
(
COSMO).[18] Values for the dielectric constant and solute
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radius were manually configured in Gaussian to most
accurately represent TFA.
(7) Radical processes were reported using persulfate and
peroxodisulfate: (a) Basickes, N.; Hogan, T. E.; Sen, A. J.
Am. Chem. Soc. 1996, 118, 13111-13112. (b) Lin, M.; Sen,
A. J. Chem. Soc., Chem. Commun. 1992, 892-893.
(
8) Sakaguchi, S.; Yoo, K. S.; O’Neil, J.; Lee, J. H.; Stewart,
T.; Jung, K. W. Angew. Chem. Int. Ed. 2008, 47, 9326-9329.
9) Krasutsky, P. A.; Kolomitsyn, I. V.; Carlson, R. M. Org.
Lett. 2001, 3, 2997-2999.
ASSOCIATED CONTENT
Supporting Information
(
Experimental procedures, computational data, spectroscopic
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data, copies of wet1D H, 13C, and 19Fspectra, and gas chroma-
1
(
10) Camaioni, D. M.; Bays, J. T.; Shaw, W. J.; Linehan J. C.; Birn-
tography data
baum, J. C.; J. Org. Chem. 2001, 66, 789-795.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(11) Serov, S. I.; Zhurarlev, M. V.; Sass, V. P.; Sokolov, S.
V. J. Org. Chem. USSR (Engl. Transl.) 1980, 16, 1360-1362.
AUTHOR INFORMATION
Corresponding Author
(12) Sawada, H.; Nakayama, M. J. Fluorine Chem. 1990, 46,
423-431.
*
kwjung@usc.edu
(13) McGivern, W. S.; Derecskei-Kovacs A.; North, S. W.
J. Phys. Chem. A 2000, 104, 436-442.
*houk@chem.ucla.edu
*acooksy@mail.sdsu.edu
(
2
14) Kopitzky, R.; Willner, H. Inorg. Chem. 2001, 40, 2693-
698.
15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria,
ACKNOWLEDGMENT
(
We acknowledge Dr. Richard Giles for helpful discussion as well
as the Hydrocarbon Research Foundation, the NIH (S10
RR025432), and the NSF (Grant CHE-1361104) for generous fi-
nancial support.
G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Bar-
one, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.;
Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.;
Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara,
M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.;
Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven,
T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.;
Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.;
Staroverov, V. N.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S.
S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene,
M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jara-
millo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Aus-
tin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin,
R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Sal-
vador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.;
Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.;
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