2642
J. Am. Chem. Soc. 1997, 119, 2642-2646
High-Yield, Radical-Initiated Oxidative Functionalization of
Ethane by Perfluorocarboxylic Acid Anhydrides. Role of Metal
Ions in Catalytic Alkane Oxidations in the Presence of
Perfluorocarboxylic Acid Anhydrides
Terrence Hogan and Ayusman Sen*
Contribution from the Department of Chemistry, The PennsylVania State UniVersity,
UniVersity Park, PennsylVania 16802
ReceiVed NoVember 25, 1996X
Abstract: Hydrogen peroxide and a trace of either ethene or propene initiated the conversion of ethane to propionic
acid and its mixed anhydride (CH3CH2CO2H + CH3CH2COOCOCF3) and trifluoromethyl ethyl ketone, CH3CH2-
COCF3, by trifluoroacetic anhydride at 80 °C. For a fixed amount of H2O2, the amount of products formed increased
with increasing amount of trifluoroacetic anhydride employed and was always higher than the amount of H2O2
added. These products were also obtained when H2O2 was replaced by other radical initiators: m-chloroperbenzoic
acid, azobisisobutyronitrile, and PbEt4. With PbEt4, ethene or propene was not required for product formation and
close to 500 equiv of products was formed for every equivalent of PbEt4 employed! Longer chain perfluorocarboxylic
acid anhydrides reacted analogously; however, as the Rf group increased in length, a corresponding increase in
mixed anhydride to ketone selectivity was observed. Methane gave very little product under the reaction conditions
whereas propane underwent simple stoichiometric oxidation to 2-propanol and acetone by H2O2. The addition of
(CF3CO2)2Pd to the ethane reaction resulted in simple oxidation to ethanol and acetaldehyde in amounts lower than
that corresponding to the H2O2 present. In complete contrast to the ethane reaction, the yield of products from
methane increased significantly (although less than the H2O2 added) upon the addition of (CF3CO2)2Pd, with methanol
being the principal product.
The selective oxidative functionalization of primary C-H
perfluorocarboxylic acid anhydrides. Hitherto, the generation
of perfluoroalkyl radicals generally necessitated the use of either
ultraviolet light or high temperatures (>250 °C).3 The current
system operates at 80 °C and requires only a source of alkyl
radicals and a perfluorocarboxylic acid anhydride.
bonds of alkanes in solution under mild conditions continues
to be a challenging problem.1 Herein we report the very
surprising radical-initiated conversion of ethane to, principally,
propionic acid by perfluorocarboxylic acid anhydrides. An
interesting feature of the reaction is that because of the nature
of the steps involved, much higher conversion was observed
for ethane than propane despite the presence of weaker
secondary C-H bonds in the latter. It is also worth noting that
while several groups have reported the metal-mediated func-
tionalization of alkanes in trifluoroacetic acid and/or trifluoro-
acetic anhydride,2 the ability of the solvent system to act as an
oxidant for alkanes has never been suspected.
Results and Discussion
In a typical reaction (reaction 1), 0.2 mL of 30% (w/w)
aqueous H2O2 (1.8 mmol of H2O2) was dissolved in 3.0 mL
(21.2 mmol) of (CF3CO)2O (note that excess (CF3CO)2O
remained after mixing). The mixture was then exposed to
500 psi of C2H6 containing 1600 ppm of ethene and 500 psi
of N2, and heated at 75-80 °C for 1 d. At the end of this
A second aspect of this system which merits comment is the
ease with which the perfluoroalkyl radical was formed from
1
period, the products identified by H- and 13C-NMR spectros-
copy (and confirmed by GC/MS and/or by comparison with
authentic samples) were propionic acid and the mixed anhydride,
CH3CH2CO2H + CH3CH2COOCOCF3, 1 (1.85 mmol); the
mixed ketone trifluoromethyl ethyl ketone, CH3CH2COCF3, 2
(0.68 mmol); ethanol and its derivative, C2H5OH + C2H5O2-
CCF3 (0.06 mmol); acetaldehyde and its derivative, CH3CHO
+ CH3CH(O2CCF3)2 (0.09 mmol); and acetic acid and its
derivative, CH3CO2H + CH3COOCOCF3 (0.31 mmol). GC/
MS of the solution revealed the additional presence of CF3-
CO2CF3 and CF3H. CF3H and a small amount of CO2 were
found in the gas phase. Most noteworthy is that the total amount
of ethane functionalized significantly exceeded the H2O2 and
alkene originally present. Additionally, with the same amount
of added H2O2, the amounts of 1 and 2 (but not the simple
X Abstract published in AdVance ACS Abstracts, March 1, 1997.
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