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in a stoichiometric manner.[8] Other less effective examples of
the functionalization of methane have been described by
using trifluoroacetic acid as the reactant (route E).[9,10,11] In addi-
tion to methyl bisulfate, acetic acid derivatives were observed
under Catalytica-like conditions with a palladium-based cata-
lyst (route F).[12]
been proposed to occur without any metal–alkane contact
through a direct interaction of the CÀH bond with the carbenic
carbon atom (Scheme 2c) Therefore the trend in reactivity usu-
ally observed, tertiary>secondary>primary>methane CÀH
bonds, follows that of the bond-dissociation energies (BDEs).[14]
This trend has a tremendous impact when focussing on the
functionalization of methane: the reaction must be carried out
in the absence of any other CÀH bond that would react more
easily than that of methane. In this contribution, we report[21]
the use of a series of silver complexes bearing perfluorinated
hydrotris(indazolyl)borate ligands that have been found to cat-
alyse the reaction of methane and ethyl diazoacetate (N2 =
CHCO2Et, EDA) to give ethyl propionate in supercritical carbon
dioxide (scCO2) as the reaction medium.
The need of harsh conditions for the functionalization of
methane (and for alkanes in general)[13] can be attributed to
two thermodynamic and kinetic factors: the high CÀH bond
dissociation energy (BDE ca. 105 kcalmolÀ1 for methane)[14] and
the very low polarity of the CÀH bond derived from the similar
electronegativity of C and H (cC =2.55, cH =2.20, Pauling scale).
With a few exceptions, the activation of the weakly nucleophil-
ic CÀH bond of an alkane requires a powerful electrophile.[15]
Not surprisingly, the examples shown in Scheme 1 are termed
electrophilic activation.
There are alternative ways of functionalizing methane with Results and Discussion
tuneable transition-metal catalysts. An attractive catalyst would
Catalytic functionalization of methane with silver-based cat-
alysts
be a metallocarbene complex (MC), generated in situ upon re-
acting the appropriate transition-metal complex and a diazo-
compound, a strategy that has ample precedents in organic
synthesis.[16] Complexes of metals from Groups 8 to 11 have
been described for the transfer of CR1R2 units from N2 =CR1R2
(Scheme 2a) to a wide array of nucleophiles from olefins and
As mentioned above, the election of methane as the target
substrate brings the need of the absence of other, more reac-
tive, CÀH bonds in the reaction mixture. Therefore, we decided
to develop a catalytic system for the transfer of carbene units
from ethyl diazoacetate (EDA) to methane by using
supercritical carbon dioxide (scCO2) as the reaction
medium, leaving methane as the only compound
bearing CÀH bonds available to react. In addition to
the requirement of an electrophilic metal centre for
the catalyst, we must add the capability of the latter
to dissolve in scCO2. Both features could be fulfilled
with the series of complexes [Ag(FNÀTp4Bo,3C F )(L)]
(N=6n+15; n=1, L=acetone, 1; n=2, L=THF, 2;
n=3, L=acetone, 3; n=4, L=acetone, 4; n=6, L=
acetone, 5),[22] bearing a perfluoroalkyl chain CnF2n+
1 in the 3-position of the indazolyl ring as well as four
additional fluorine atoms per indazolyl group
(Scheme 3). It is well known that a high degree of flu-
orination of the catalyst facilitates dissolution in
scCO2.[23] In addition, the donor capabilities of the N
atoms in those ligands are considerably lowered due
n
2n+1
Scheme 2. a) General metal-catalyzed carbene transfer from diazocompounds. b) CÀH
bond functionalization by carbene insertion. c)Transition state for carbene insertion into
C–H bonds.
alkynes to XÀH bonds (X=N, O, Si, among others), in either
inter- or intramolecular reactions. The reactive metallocarbenes
MCs have been detected or isolated in a few cases.[17,18] The
functionalization of carbon–hydrogen bonds, including nonac-
tivated CÀH bonds such as those of alkanes, by this methodol-
ogy is also known. Catalytic systems are based on rhodium,
copper, silver, and gold, with
to the presence of the fluorine substituents. Actually, the corre-
sponding [Ag(CO)(FNÀTp4Bo,3C F )] complexes display n(CO)
values within the very narrow range of 2166–2167 cmÀ1, which
indicates not only an electron-deficient metal centre,[24] but
also a very limited electronic effect of the length of the fluori-
nated chain at the 3-position.
n
2n+1
a couple of examples on iron
and ruthenium.[19,20]
Diazo reagents of general for-
mula N2 =CR1CO2R2 were em-
ployed for CÀH insertion reac-
tions allowing the direct conver-
sion of alkanes into esters upon
chain homologation (Scheme
2b). This transformation has Scheme 3. Silver complexes employed as catalysts in this work.
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Chem. Eur. J. 2014, 20, 1 – 7
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