Catalytic functionalization of methane and light alkanes in supercritical carbon dioxide

Article abstract of DOI:10.1002/chem.201403217

The development of catalytic methods for the effective functionalization of methane yet remains a challenge. The best system known to date is the so-called Catalytica Process based on the use of platinum catalysts to convert methane into methyl bisulfate with a TOF rate of 10^-3 s. In this contribution, we report a series of silver complexes containing perfluorinated tris(indazolyl)borate ligands that catalyze the functionalization of methane into ethyl propionate upon reaction with ethyl diazoacetate (EDA) by using supercritical carbon dioxide (scCO₂) as the reaction medium. The employment of this reaction medium has also allowed the functionalization of ethane, propane, butane, and isobutane.

Full text of DOI:10.1002/chem.201403217

DOI: 10.1002/chem.201403217
Full Paper
&
Synthetic Chemistry
Catalytic Functionalization of Methane and Light Alkanes in
Supercritical Carbon Dioxide
M. ꢀngeles Fuentes,[a] Andrea Olmos,[a] Bianca K. MuÇoz,[b, c] Kane Jacob,[b, c] M.
Elena Gonzꢁlez-NfflÇez,[d] Rossella Mello,[d] Gregorio Asensio,*[d] Ana Caballero,*[a]
Michel Etienne,*[b, c] and Pedro J. PØrez*[a]
Abstract: The development of catalytic methods for the ef-
fective functionalization of methane yet remains a challenge.
The best system known to date is the so-called Catalytica
Process based on the use of platinum catalysts to convert
methane into methyl bisulfate with a TOF rate of 10À3 s. In
this contribution, we report a series of silver complexes con-
taining perfluorinated tris(indazolyl)borate ligands that cata-
lyze the functionalization of methane into ethyl propionate
upon reaction with ethyl diazoacetate (EDA) by using super-
critical carbon dioxide (scCO2) as the reaction medium. The
employment of this reaction medium has also allowed the
functionalization of ethane, propane, butane, and isobutane.
Introduction
Methane is available in the earth’s crust either in natural or
shale gas and in ocean beds as clathrates in such amounts
that it could be considered the ideal raw material for C1
chemistry.[1,2] The main current use of methane from an indus-
trial point of view consists of the preparation of syngas (syn-
thesis gas), a mixture of carbon monoxide and hydrogen that
can be further employed in the synthesis of methanol in the
Fischer–Tropsch process for synthetic gasoline or in reactions
requiring carbon monoxide and/or hydrogen (hydroformyla-
tion, hydrogenation, ammonia synthesis, or carbonylation
among others).[3] Alternatively, the direct, selective and low-
energy conversion of methane into more valuable chemicals in
homogeneous media yet constitutes a long-term challenge.
Controlled oxidation of methane has been a major goal, given
the importance of methanol as a fuel and an oxygenated inter-
[a] Dr. M. ꢀ. Fuentes, Dr. A. Olmos, Dr. A. Caballero, Prof. Dr. P. J. Pꢁrez
Laboratorio de Catꢂlisis Homogꢁnea, Unidad Asociada al CSIC
CIQSO-Centro de Investigaciꢃn en Quꢄmica Sostenible and
Departamento de Quꢄmica y Ciencia de Materiales
Universidad de Huelva, 21007 Huelva (Spain)
Fax: (+34)959219942
Scheme 1. Different catalytic systems described to date based on the elec-
trophilic activation of methane.
mediate. Significant advances are summarized in Scheme 1.[4]
In the late 1960s, Shilov and co-workers described the poten-
tial of platinum salts to promote the electrophilic activation of
methane, first stoichiometrically (Scheme 1, route A) then cata-
lytically (Scheme 1, route B).[5] The main drawback of using
platinum as a stoichiometric oxidant was addressed by Periana
and co-workers who described the conversion of methane into
methyl bisulfate in sulphuric acid as the reaction medium first
with mercury (Scheme 1, route C),[6]then with platinum salts as
catalyst precursors in the so-called Catalytica process
(route D).[7] Very recently, main-group metals (Tl, Pb) were
shown to oxidise the CÀH bonds of methane and light alkanes
E-mail: perez@dqcm.uhu.es
[b] Dr. B. K. MuÇoz, K. Jacob, Prof. Dr. M. Etienne
CNRS; LCC (Laboratoire de Chimie de Coordination)
205, route de Narbonne, BP 44099
31077 Toulouse (France)
[c] Dr. B. K. MuÇoz, K. Jacob, Prof. Dr. M. Etienne
Universitꢁ de Toulouse; UPS, INPT
LCC; 31077 Toulouse (France)
[d] Dr. M. E. Gonzꢂlez-NfflÇez, Dr. R. Mello, Prof. Dr. G. Asensio
Departamento de Quꢄmica Orgꢂnica
Facultad de Farmacia, Universidad de Valencia
Burjassot 46100-Valencia (Spain)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403217.
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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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Complexes 1–5 have been tested as catalysts for the reac-
tion of EDA with methane in scCO2 as the reaction medium. In
a high-pressure vessel, EDA (0.35 mmol) was placed in a poly-
propylene open container, whereas the catalyst (0.007 mmol of
compounds 1–5) was introduced in a polypropylene cylinder
with two parallel filters. The reactor was then heated and
maintained at 408C before pressurizing with methane at
155 atm. Carbon dioxide was then pumped in that tempera-
ture to reach a final pressure of 250 atm. This mixture corre-
sponded to a 0.96 molar ratio of CH4/CO2 (experimentally de-
termined by weight). Under these conditions of temperature
and pressure, a homogeneous fluid resulted within a few mi-
nutes. The mixture was stirred for 14 h before being analyzed
with the aid of a GC connected on-line to the reactor. The GC
trace showed the formation of ethyl propionate [Eq (1)] as the
result of the silver-mediated insertion of the CHCO2Et unit
from EDA into the CÀH bond of methane. As shown in Table 1,
trap to condense reactants, products, and some dissolved cata-
lyst. In addition, the residue inside the reactor (and outside the
catalyst container) was collected with methylene chloride. Both
samples (from the trap and the reactor) were investigated by
ICP-MS, showing the presence of silver in the supercritical
phase, with most of it remaining inside the reactor after de-
pressurization. A plot of the total amount of silver dissolved in
the reaction mixture as a function of the percentage of fluorine
in the catalyst is shown in Figure 1. In spite of initially adding
Table 1. Methane catalytic functionalization with the silver catalysts 1–
5.[a]
R
Cat.
Ethyl propionate
[mmol]
Yield [%]
(EDA-based)[b]
CF3
CF2CF3
CF2CF2CF3
CF2CF2CF2CF3
CF2CF2CF2CF2CF2CF3
1
2
3
4
5
0.103
0.080
0.067
0.063
0.064
29
22
19
18
18
Figure 1. Solubility of complexes 1–5 in a 155:95 atm. mixture of methane/
carbon dioxide at 408C as a function of the percentage of fluorine.
the same quantity of the different catalysts (7”10À3 mmol),
their concentration in solution varied within the range 1”10À4
to 7”10À4 mmol. Also, the homogeneous nature of the catalyt-
ic reaction under supercritical conditions was assessed with
pentane as the substrate and 2 as the catalyst [Eq. (2)]. It is
seen that very similar regioselectivity for carbene insertion into
the various CH bonds of pentane is observed either in pure
pentane acting as a solvent (and a substrate) or in scCO2,
which indicates an identical mechanism for both processes.
Additional evidence has been obtained with a competition ex-
periment between methane and ethane [Eq. (3)]. A respective
2.81:1 molar ratio of both gases in scCO2 was reacted with
EDA in the presence of 2 as the catalyst was fully soluble
under these conditions. Ethyl propionate and butyrate were
obtained leading to a corrected relative reactivity CÀHethane/CÀ
Hmethane of 14.[21] On the basis of the available data, we believe
that the homogeneous nature of this transformation is well es-
tablished.
[a] 0.007 mmol of catalyst. [b] By GC analysis using calibration curves. See
the Supporting Information.
the amount of ethyl propionate slightly decreased when
moving from 1 to 5, corresponding to yields within the range
of 29–18% (EDA-based). The remaining of the initial EDA was
converted into a mixture of diethyl fumarate and maleate
(from metal-mediated diazo coupling),[25] along with some
ethyl 2-hydroxyacetate (from adventitious water functionaliza-
tion).
Once the amounts of dissolved catalysts in scCO2 were de-
termined, we could estimate the turnover number (TON) of
the series of catalysts for the functionalization of methane, on
the basis of the amount of ethyl propionate formed (Table 1)
and of silver available in solution (data from Figure 1). Table 2
shows the corresponding data, from which the highest TON of
734 was obtained for complex 1, a remarkable number for the
functionalization of methane with a metal-based catalyst in ho-
mogeneous medium. The TON was observed to decrease
when the amount of dissolved silver was increased (Figure 2),
again complex 1 being the more active of the series under
Interestingly, we observed through the sapphire windows of
the reactor (see the Supporting Information) that most of the
initial solid remained in the polypropylene vessel at the end of
the reaction. A control experiment in which only scCO2 was in-
troduced in the reactor showed that catalysts completely dis-
solved in the supercritical fluid, which indicated that methane
is a very poor solvent for these compounds. To evaluate the
actual concentration of the catalyst dissolved in the reaction
medium, the reactor was slowly depressurized through a cold
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the CÀH bonds of methane, we decided to explore the next
members of the homologous series CnH2n+2 with n=2-4,
which have been scarcely employed with this methodology.[26]
The hydrocarbon (ethane, propane, n-butane, or iso-butane,
large excess, see the Supporting Information for exact
amounts) was reacted with EDA (0.5 mmol) in scCO2 for 4 h
Table 2. TON and TOF values for the reaction of methane and EDA with
1–5 as the catalysts.[a]
1
2
3
4
5
TON[b]
TOF[c]
734
0.014
242
0.004
204
0.003
87
0.001
112
0.002
with [Ag(F27ÀTp4Bo,3CF CF )(thf)] (5”10À3 mmol) as the catalyst to
give the insertion products shown in Scheme 4. This time was
enough to achieve full consumption of EDA (>98%). It is
worth mentioning that the catalyst was completely soluble in
the C2–C4 alkane/scCO2 mixtures. The yields in functionalized
products were 84% for ethane and >95% (EDA-based) for
propane and both linear and branched butanes. This contrasts
with the functionalization of methane that only consumed
31% of EDA to give ethyl propionate with 7% yield under
these conditions (different from those employed in the previ-
ous section). Propane, n-butane and iso-butane each led to
a mixture of two products as the result of the existence of two
distinct reaction sites. The relative reactivity (per CH bond)
found for primary versus secondary CÀH bonds in propane
and n-butane was 1.0:3.6 and 1.0:2.1, respectively. This differ-
ence must be related to the distinct steric hindrance of both
secondary CÀH bonds, since their BDE values are nearly identi-
cal. For iso-butane the ratio for primary versus tertiary sites in-
creased to 1.0:4.0.
2
3
[a] From the data in Table 1 and Figure 1 (see the Supporting Informa-
tion). [b] As mmol product/mmol catalyst. [c] As mmol product/(mmol
catalystsxh).
Figure 2. TON and TOF values of the reaction of methane with EDA with 1–
5 as the catalysts.
Conclusion
We have developed a family of
perfluorinated tris(indazolyl)bo-
rate silver complexes that are ca-
pable of catalyzing the transfer
of carbene units from ethyl di-
azoacetate to the very poorly
nucleophilic carbon–hydrogen
bond of methane. TON and TOF
values of 734 and 1.4”10À2 sÀ1
,
respectively, have been achieved
with the most active catalyst.
Other light alkanes (ethane, pro-
these reaction conditions. This TON corresponds to a TOF of
0.014 sÀ1. As a general trend, the catalytic functionalization of
methane is favoured by high pressures of the hydrocarbon
and by low catalyst concentrations. The latter is the result of
a decrease in the deleterious EDA dimerization that gives di-
ethyl fumarate and maleate, as demonstrated from our previ-
ous work.[25]
pane, n-butane, and iso-butane) have also been functionalized
and the relative intramolecular reactivity values for the CÀH
bonds in those alkanes have been obtained. This contribution
has also demonstrated the potential of using an inert phase,
such as scCO2, as a reaction medium to isolate the CÀH bonds
of methane from other more reactive bonds, favoring the
aforementioned catalytic functionalization.
Overall, elimination of other more reactive CÀH bonds from
the reaction medium by using scCO2 as the solvent allows the
functionalization of methane following a methodology previ-
ously described for more reactive CÀH bonds.
Experimental Section
General methods
All substrates were purchased from Aldrich. Substrates and sol-
vents were rigorously dried prior to their use. The complexes [Ag-
Catalytic functionalization of ethane, propane, and butanes:
Relative reactivity toward methane
[27]
[21]
(acetone)(F21-Tp4Bo,3CF )] (1),
[Ag(F27-Tp4Bo,3CF CF )(thf)] (2),
[Ag-
3
2
3
(acetone)(F33-Tp4Bo,3(CF ) CF )] (3),[22] [Ag(acetone)(F39-Tp4Bo,3(CF ) CF )]
2
2
3
2
3
3
After having demonstrated the catalytic capabilities of the
[22]
2
5
3
silver complexes 1–5 to transfer CHCO2Et units from EDA to
(4)[22] and [Ag(acetone)(F51-Tp4Bo,3(CF ) CF )] (5), were prepared ac-
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Scheme 4. Functionalization of C1–C4 alkanes with EDA by using complex 2 as the catalyst in scCO2.
cording to the literature from the respective hydrotris(indazolyl)bo-
rate salts.[28] The supercritical experiments were carried out in
a commercial THAR Technologies R100SYS supercritical plant or in
Phenomenex empty 33 mL HPLC columns 21.2 mm ID”1 in OD”
100 mmL connected to a HIP valve through 1/16“ stainless steel
tubes. A Varian GC 450 was directly connected to the supercritical
reactor vessels for online product analysis. Ag was analyzed by in-
ductively coupled plasma mass spectrometry (ICP-MS) with an
Quantification of the amount of silver complex dissolved
After analysis of the reaction mixture by gas chromatography, the
reactor was depressurized slowly through a trap cooled at À788C.
Once atmospheric pressure was reached, the reactor was opened
and, after removing the catalyst container, it was washed with di-
chloromethane (3”10 mL). The trap was allowed to warm to room
temperature and it was washed with dichloromethane in the same
manner. All solutions were collected and the solvent was removed
and the residue was dissolved in 2 mL of aqueous HNO3 (5%) and
diluted to a final volume of 10 mL with Milli-Q water. The samples
were analyzed by ICP-MS. See the Supporting Information for data
collected.
HP4500 instrument with detection limit of 0.01 mgLÀ1
.
Functionalization of methane in supercritical carbon dioxide
with [Ag(acetone)(F21-Tp4Bo,3CF )] to [Ag(acetone)(F51-
3
Tp4Bo,3(CF ) CF )] series
2
5
3
Functionalization of light alkanes C1–C4 in supercritical
carbon dioxide with [Ag(F27-Tp4Bo,3CF CF )(thf)] (2)
2
3
These reactions were performed in a 100 mL stainless steel reactor
mechanically stirred by paddles. The corresponding complex (1–5,
0.007 mmol) was placed into a polypropylene tube, and both sides
were closed with a permeable cap. A second polypropylene con-
tainer was charged with ethyl diazoacetate (0.35 mmol). Both con-
tainers were fixed to the mixer axis of the reactor (see the Support-
ing Information for details). The reactor was closed and pressurized
first with 155 atm. of methane and then with carbon dioxide up to
a total pressure of 250 atm. at a constant temperature of 408C
(computer controlled). Through a sapphire window we could ob-
serve differences in the solubility of the reaction components, EDA
completely dissolved in the reaction medium while the major part
of the catalyst seemed to remain in the polypropylene container
(see the Supporting Information). After 14 h of stirring at 408C, the
reaction was analyzed by gas chromatography. The gas mixture
was directly injected into the GC column by opening the valve
connecting the reactor and the GC analysis. The exact amounts of
the products were determined using calibration curves previously
recorded with commercial samples.
These reactions were performed in a supercritical reactor consist-
ing of an empty 33 mL HPLC column connected to a high-pressure
valve through a 1/16“ stainless steel tube. To ensure the homoge-
nization of the reaction mixture, the reactors were immersed in an
ultrasound bath thermostated at 408C. Two polypropylene recep-
tacles were placed inside the column containing EDA (0.5 mmol)
and the catalyst 2 (0.005 mmol). The first container was opened on
one side while the second one was closed with permeable caps on
both sides. The reactor was then closed and weighted. For reac-
tions with methane the column was connected to a methane
bottle and pressurized to 150 atm. For reactions with ethane, the
reactor was connected to an ethane bottle and pressurized to
30 atm. For reactions with propane, butane, and isobutane, the re-
actor was cooled to 08C and connected to a pressurized bottle of
the desired alkane for a period of time long enough to charge 3 to
5 g of the alkane. The exact amount of alkane was determined by
weight difference before and after pressurization (see the Support-
ing Information). The reactor was then placed in an ultrasonic bath
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[12] R. A. Periana, O. Mironov, D. Taube, G. Bhalla, C. J. Jones, Science 2003,
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[13] P. J. PØrez, Alkane C-H Activation by Single-Site Metal Catalysis, Springer,
2012.
[14] Y.-R. Luo, Comprehensive Handbook of Chemical Bond Energies, CRC,
2007.
[15] a) M. Hackett, G. M. Whitesides, J. Am. Chem. Soc. 1988, 110, 1449–
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thermostated at 408C. After temperature stabilization the reactor
was pressurized with CO2 until a total pressure of 250 atm. was
reached. The reactor was sonicated for 4 h before connecting it to
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tion).
Acknowledgements
[17] See reference [13], Chapter 6, Alkane catalytic functionalization by car-
bene or nitrene insertion reactions, by M. M. Díaz-Requejo, A. Caballero,
M. R. Fructos, P. J. PØrez.
Support for this work was provided by the MINECO (CTQ2011–
28942-CO2–01, CTQ2011–28942-CO2–02, and CTQ2010–21172),
the Consolider Ingenio 2010 (Grants. CSD2006–003 and
CSD2007–00006), the Institut de Chimie of the CNRS, the Junta
de Andalucía (P10-FQM-06292, P12-FQM-01765), the Generali-
tat Valenciana (ACOMP/2010/155 and ACOMP/2012/217), and
the European Research Area chemistry programme (2nd Call
“Chemical activation of carbon dioxide and methane”, contract
number 1736154). M.A.F. and A.O. thank MINECO for FPI and
Juan de la Cierva fellowships. LICAH (Universidad de Huelva) is
acknowledged for ICP-MS studies.
[18] K. P. Kornecki, J. F. Briones, V. Boyarskikh, F. Fullilove, J. Autschbach, K. E.
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[24] These compounds are classified as nonclassical metal-carbonyls, since
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Keywords: carbene insertion
compounds · methane activation · silver catalysts
·
CÀH activation
·
diazo
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Published online on && &&, 0000
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&
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
6
ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
&
Synthetic Chemistry
M. ꢀ. Fuentes, A. Olmos, B. K. MuÇoz,
K. Jacob, M. E. Gonzꢂlez-NfflÇez, R. Mello,
G. Asensio,* A. Caballero,* M. Etienne,*
P. J. Pꢁrez*
Methane activation: Methane can be
catalytically converted into ethyl propio-
nate upon reaction with ethyl diazoace-
tate in the presence of highly fluorinat-
ed silver complexes as catalysts and by
using supercritical carbon dioxide scCO2
as the reaction medium (see scheme).
&& – &&
Catalytic Functionalization of Methane
and Light Alkanes in Supercritical
Carbon Dioxide
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
7
ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ