Selective hydroxylation of methane by dioxiranes under mild conditions

Article abstract of DOI:10.1021/ol2004676

The direct conversion of methane to methanol at low temperatures was achieved selectively using dioxiranes 1a,b either in the isolated form or generated in situ from aqueous potassium caroate and the parent ketone at a pH close to neutrality. Results suggest that the more powerful dioxirane TFDO (1b) should be the oxidant of choice.

Full text of DOI:10.1021/ol2004676

ORGANIC
LETTERS
2
011
Vol. 13, No. 8
142–2144
Selective Hydroxylation of Methane by
Dioxiranes under Mild Conditions
2
†
†
†
Cosimo Annese, Lucia D’Accolti, Caterina Fusco, and Ruggero Curci*
,†,‡
Dipartimento Chimica, CNR-ICCOM, Universit aꢀ di Bari “A. Moro”, v. Amendola 173,
0126 Bari, Italy, and Department of Chemistry, Brown University, Providence,
Rhode Island 02912, United States
7
curci@ba.iccom.cnr.it
Received February 21, 2011
ABSTRACT
The direct conversion of methane to methanol at low temperatures was achieved selectively using dioxiranes 1a,b either in the isolated form or
generated in situ from aqueous potassium caroate and the parent ketone at a pH close to neutrality. Results suggest that the more powerful
dioxirane TFDO (1b) should be the oxidant of choice.
The direct oxyfunctionalization of methane under mild
conditions continues to pose a major challenge to the
chemical community. A highly efficient and selective oxida-
tion of methane to methanol is difficult, primarily because
Recently, using a solid triazine-based covalent platinum
complex catalyst, the efficient conversion of methane to
methanol in concentrated H SO at ca. 200 °C, via CH O-
2
4
3
5
SO H, was reported. The major drawback of this novel
3
1
of the ease of overoxidation to HCHdO and to CO2.
method is the use of fuming sulfuric acid, which poses
significant limitations regarding materials and costs.
The catalytic cycle using methane monooxygenase en-
zymes (MMO) to achieve the oxidation of methane has
attracted the interest of scientists engaged in the develop-
ment of biological methods for the production of commer-
6
The remarkable chemical reactivity of dimethyldioxirane
7
(DMDO) 1a and of methyl(trifluoromethyl)dioxirane
(TFDO) 1b (Figure 1) has been previously demonstrated.
1
,2
cial chemicals and for the degradation of toxic materials.
On the other hand, the development of metalÀcatalyst
systems for the direct oxidation of methane to methanol
3
has been one of the major goals in catalysis. Considerable
progress has been made in understanding the chemical
requirements for transition metal “activation” of car-
4
bonÀhydrogen bonds. However, in most cases the cata-
Figure 1. Common dioxiranes in isolated form.
lysts typically sufferedfrom irreversiblereduction and bulk
3
,4
metal formation, with consequent poor selectivity.
Using dioxiranes, hydrocarbons can be selectively oxy-
genated in what appears to be a highly stereoselective
8
concerted process. In the hydroxylation of “unactivated”
†
‡
Universit ꢀa di Bari “A. Moro”.
Brown University.
1) For example, see: Murray, L. J.; Lippard, S. J. Acc. Chem. Res.
(
2007, 40, 466. See also references cited therein.
(
(
2) Crabtree, H. Chem. Rev. 1995, 95, 987.
3) (a) De Vos, D. E.; Sels, B. F. Angew. Chem., Int. Ed. 2005, 44, 30.
(5) Palkovits, R.; Antonietti, M.; Khun, P.; Thomas, A.; Sch u€ th, F.
Angew. Chem., Int. Ed. 2009, 48, 6909.
(
4
b) Jones, C. J.; Taube, D.; Periana, R. A. Angew. Chem., Int. Ed. 2004,
3, 4626. (c) Shilov, A. E.; Shullpin, G. B. Chem. Rev. 1997, 97, 2879.
4) (a) Wolf, D. Angew. Chem., Int. Ed. 1998, 37, 3351. (b) Baerns, M.;
(6) Murray, R. W.; Jeyaraman, R. J. Org. Chem. 1985, 50, 2847.
(7) Mello, R.; Fiorentino, M.; Fusco, C.; Curci, R. J. Am. Chem. Soc.
1989, 111, 6749.
(
Ross, J. R. H. In Perspectives in Catalysis; Thomas, J. A., Zamaraev, K. I.,
Eds.; Blackwell: Oxford, 1992.
(8) For a recent review of dioxiranes and CÀH bond oxy-functionaliza-
tion, see: Curci, R.; D’Accolti, L.; Fusco, C. Acc. Chem. Res. 2006, 39, 1.
1
0.1021/ol2004676 r 2011 American Chemical Society
Published on Web 03/21/2011

CÀH bonds, high tertiary vs secondary selectivities
t 7À9
R from 15 to >250) can be routinely achieved.
s
(
We recently reported on the direct dioxirane oxyfunc-
tionalization of structurally complex alkanes, such as 2,4-
8
,9
didehydroadamantane, binor-S, and cubane.
The many facets of selective oxyfunctionalization of
8
alkanes using dioxiranes have been reviewed.
One of the remaining challenges in dioxirane chemistry
is the direct functionalization of methane to methanol.
Despite its high technological interest, this important
aspect of dioxirane reactivity has not yet been reported.
Herein we report on the application of dioxiranes to the
direct, remarkably low-temperature oxidation of methane
to methanol with high reactivity and selectivity. Results
presented herein provide a significant breakthrough for
this key transformation.
We have employed dioxiranes 1a and 1b either in the
isolated form or generated in situ. Solutions of isolated
dioxiranes 1a and 1b in acetone or 1,1,1-trifluoropropa-
none (TFP), which were 0.06À0.1 M and 0.6À1.0 M
respectively, were obtained adopting procedures already
1
13
Figure 2. { H} C NMR (100 MHz, acetone-d
reaction mixture resulting from oxidation of 99.5% enriched
) spectrum of the
6
1
3
[ C]methane with isolated TFDO (1b) at 2.5 atm, temp 0 °C,
solvent acetone/TFP 4:1 (0.5 mL of reaction mixture brought to
mL volume with acetone-d solvent).
6
6
,7
described in detail.
1
Based on the above findings, we were able to establish
Scheme 1. Methane Oxidation with Dioxiranes Generated in
Situ
that the transformation is also feasible at atmospheric
pressure. Accordingly, a dioxirane solution was placed in
a sealed glass flask and a known amount of CH gas slowly
4
bubbled through the solution via a side tube.
The solution was stirred and kept at subambient tem-
perature (from À10 to 3 °C). After an appropriate reaction
time, the amount of methanol formed and of residual
dioxirane were determined (Table 1, entries 2À4).
The data show that, using the powerful TFDO in
acetone, the CH OH yield is >95%, with 15% conversion
3
after a 24 h reaction time (entry 2). The methanol yield
was also >95% employing water as the main solvent,
although at a lower conversion (ca. 7%) (entry 3). As
expected, with the less effective DMDO oxidant in water, a
Alternatively, dioxiranes in situ were generated in buf-
fered aqueous solutions (pH 7.5À8) using potassium
1
peroxomonosulfate (KHSO )/ketonemixturesaccording
0
5
1
1
to an established protocol (Scheme 1).
Initially, oxidations were carried out in a pressure vessel
charged with a cold dioxirane solution and pressurized at
longer reaction time (72 h) is required to form CH OH
3
with similar methane conversion (ca. 5%) (entry 4). Note-
worthy, in none of the cases above were overoxidation
products detected. Therefore, the selectivity observed is
over 95% under the given conditions.
In addition, we found that methane oxidations can also
be carried out in buffered aqueous media by employing
dioxiranes generated in situ (Table 1, entries 5 and 6).
In this procedure, the apparatus and conditions were
quite similar to those above, except the reaction flask
was initially charged with a mixture of the parent ketone
2
.5 atm with methane gas (Table 1, entry 1). At a tempera-
ture of 0 °C, the vessel pressure gradually decreased with
time, signaling the progressive uptake of methane gas. In
parallel, a significant decrease in dioxirane concentration
was monitored (cf. note f, Table 1). After a 24 h reaction
time, the mixture was analyzed by NMR and the amount
of methanol product determined by calibrated GC. Start-
1
3
13
ing with 99.5% enriched [ C]methane, C NMR spectro-
scopic analysis of the final reaction mixtures confirmed
that methanol is the largely predominant liquid-phase
product generated (Figure 2).
(
i.e., TFP or acetone) and excess KHSO aqueous solu-
5
tion, buffered at pH 7.5. We found that, with a 24 h
reaction time at 2À5 °C, methane oxidation also affords
methanol as the only product detectable (Table 1, entries
(
9) Annese, C.; D’Accolti, L.; Fusco, C.; Gandolfi, R.; Eaton, P. E.;
5
and 6).
Control experiments showed that reactions carried out
Curci, R. Org. Lett. 2009, 11, 3574.
(
(
10) Oxone, Caroate, triple salt (2KHSO
5
KHSO
4
K
11) (a) Cassidei, L.; Fiorentino, M.; Mello, R.; Sciacovelli, O.; Curci,
2
SO
4
).
3
3
omitting the ketone precursor of dioxirane failed to oxidize
methane, even when the bulk oxidant KHSO was used in
large excess (entry 7).
R. J. Org. Chem. 1987, 52, 699. (b) Murray, R. W.; Singh, M. Organic
Syntheses, Coll. Vol. 9; Freeman, J. P., Ed.; Wiley: New York, 1998; p 288.
5
(c) Denmark, S. E.; Wu, Z. Synlett 1999, 847.
Org. Lett., Vol. 13, No. 8, 2011
2143

a
Table 1. Methane Hydroxylation by Dimethyldioxirane (1a) and by Methyl(trifluoromethyl)dioxirane (1b)
c
d
[
CH
4
]
o
,
[oxidant]
mM
o
,
ox/CH
Ratio
4
b
,
temp,
reaction
conv,
%
3
[CH OH],
entry
oxidant
TFDO (1b)
mM
solvent
°C
time, h
mM
e
f
1
2
3
4
5
6
7
;
144
62
;
Acetone
Acetone
0
24
24
24
72
24
24
24
;
100
9
TFDO (1b)
62
30
26
6
1.00
2.5
2.0
20
À10
3
15.0
7.0
5.0
15.0
<2
TFDO (1b)
79
H
2
H
2
H
2
H
2
H
2
O
2.1
1.3
0.9
0.2
;
g
DMDO (1a)
TFDO (1b) in situ
53
O/Acetone
3
h
126
168
630
O
O
O
2À5
2À5
2À5
i
DMDO (1a) in situ
8
20
j
5
KHSO
13
48
;
a
4 4
Unless noted otherwise, reactions were carried out in a sealed flask under CH gas. The amount of CH introduced was established using a constant
b
pressure gas buret at ca. 1 atm. Initial peroxide concentrations were determined by iodometry and/or by a reported GC method (ref 7). Initial
c
concentration ratio of isolated dioxirane (or of KHSO
5
bulk oxidant) to CH
formed. Determined by calibrated GC. Methane gas 2.5 atm, 50 mL pressure vessel containing 3 mL of 0.7 M TFDO in 12 mL of acetone. In this
experiment, performed with CH gas excess, the percent conversion was ca. 70%, as determined on the basis of dioxirane limiting reagent; however,
concurrent peroxide decomposition over the long reaction time (ref 12) might contribute to an overestimate of the conversion magnitude. Solvent H
4
. Methane conversion was estimated based on of the amount of CH OH
3
d
e
f
4
g
2
O/
h
i
acetone 1.5:1. Dioxirane (1b) generated in situ by TFP/caroate 1:2, aqueous solution pH 7.5À8.0. DMDO (1a) generated in situ, acetone/caroate 1:1,
j
aqueous solution pH 7.5À8.0. Caroate salt and no ketone, aqueous solution pH 7.5À8.0.
1
4
In view of the high CÀH bond energy of methane (104
kcal/mol, 435 kJ/mol), its lower rate of oxidation with
respect to the ease of dioxirane hydroxylation of unactivated
tertiary and secondary carbonÀhydrogen bonds in alkanes
is hardly surprising. In fact, theoretical calculations provide
an estimate of the activation barriers in the gas phase for the
oxidation of methane, propane, and isobutane with DMDO
tone moiety to a solid matrix might provide the addi-
tional advantage of easy recycling of the catalyst. Thus,
further work seems appropriate to achieve this goal.
In our view, the direct conversion of methane to
methanol under mild conditions described herein mer-
its further exploration. Remarkably, it does not involve
costly catalysts and elaborate multistep procedures. It
is noteworthy since the rich deposits of methane natural
gas and methane hydrates represent a huge source of
13
as 44.2, 30.3, and 22.4 kcal/mol, respectively.
Concerning the in situ method, reactions carried out with
TFDO take place with adequate methane conversion. In
this case, the recyclability and stable activity of the ketone
catalyst could be assessed. In fact, we found that recharging
the spent reaction mixture with more methane and bulk
1
5
materials and energy.
Acknowledgment. Financial support by the Ministry of
Education of Italy (MIUR, Grant PRIN 2008) and by the
Italian National Research Council (CNR, Rome) is grate-
fully acknowledged. Thanks are also due to Dr. J. Ciabat-
toni for helpful discussions.
KHSO oxidant results in additional methanol production.
5
The potential of the caroate/ketone system appears
significant for its practical applications. Indeed, with this
method the ketone is not appreciably consumed, since it
simply acts as a catalyst to activate the inexpensive caroate
oxidant. Covalent attachment of the trifluoromethyl ke-
Supporting Information Available. Experimental pro-
1
13
cedures, sample GC runs, H and CNMR characteriza-
tion data of reaction mixtures. This material is available
free of charge via the Internet at http://pubs.acs.org.
(
12) Curci, R.; Dinoi, A.; Fusco, C.; Lillo, M. A. Tetrahedron Lett.
996, 37, 249.
13) Glukhovtsev, M. N.; Canepa, C.; Bach, R. D. J. Am. Chem. Soc.
998, 120, 10528.
14) For instance, see: Song, C. E.; Lim, J. S.; Kim, S. C.; Lu, K.-J.;
Chi, D. Y. Chem. Commun. 2000, 2415.
1
1
(
(15) (a) Sloan, E. D., Jr.; Koh, C. A. Clathrate Hydrates of Natural
Gases, 3rd ed.; CRC Press-Taylor and Francis: Boca Raton, FL, 2008. (b)
Milkov, A. V. Earth Sci. Rev. 2004, 66, 183.
(
2
144
Org. Lett., Vol. 13, No. 8, 2011