COMMUNICATIONS
Synthesis of 4:
with phenylhydrazone
A
dry, argon-flushed Schlenk tube was charged
(1.0 equiv) in absolute dichloromethane
New Catalyst Systems for the Catalytic
Conversion of Methane into Methanol**
3
À1
(
10 mLmmol ) and with tetrafluoroboronic acid (1 equiv) in absolute
diethyl ether (54 wt%). The solution was stirred for 30 minutes at room
temperature, and then evaporated under reduced pressure. The highly
hygroscopic salt was dissolved in orthoformate:methanol (2:1, 20 equiv)
and transferred under argon into a pyrex tube; the sealed tube was then
heated in a sand bath at 808C for 12 h. The mixture was cooled to room
temperature and concentrated in vacuo. The crude product was dried under
high vacuum for 2 h. The residue was recrystallized from methanol to give
the triazolium salt 4 as an ochre crystalline solid in 65% yield.
Michael Muehlhofer, Thomas Strassner,* and
Wolfgang A. Herrmann
The catalytic conversion of methane into methanol is one of
the major challanges for chemists. Methane, as the major part
of natural gas, is currentlythe cheapest source for h yd ro-
carbons, and the need for methanol will increase in the near
future. Catalytic homogeneous oxidation at low temperatures
is economicallyinteresting, but also verydifficult to achieve as
a result of the high stabilityof C ÀH bonds. Metal centers
Synthesis of 6 (general procedure): The aromatic aldehyde (10 mmol) was
added to a solution of 4 (331 mg, 1 mmol, 10 mol%) in absolute THF
À1
(
0.7 mLmmol ) of at room temperature. The reaction mixture was
tempered for 5 min, then KOtBu (112 mg, 1 mmol, 10 mol%) in absolute
À1
THF (0.4 mLmmol ) was added dropwise. The reaction mixture was
which allow a direct oxidative addition are probablyneeded
for this approach to succeed.
Palladium and platinum compounds have been successfully
stirred for 16 h, poured into water, extracted twice with dichloromethane,
and dried (MgSO ). The solvent was evaporated and the residue was
4
purified bycolumn chromatography(silica gel, dieth yl ether:pentane 1:1)
or by crystallization to give the aromatic acyloins as colorless crystalline
solids or pale yellow oils.
[1, 2]
used for the functionalization of alkanes and arenes.
After
[3]
the pioneering work of Shilov and Shteinman, Periana
et al.[ and Fujiwara et al., in particular, reported interesting
results. Some of the ligands which have been used in
CÀH activation are shown in Figure 1.
2]
[1]
Received: December 5, 2001 [Z18335]
[
[
1] T. Ugai, S. Tanaka, S. Dokawa, J. Pharm. Soc. Jpn. 1943, 63, 269 ± 300;
Chem. Abstr. 1951, 45, 5148e].
2] a) R. Breslow, J. Am. Chem. Soc. 1958, 80, 3719 ± 3726; b) R. Breslow,
R. Kim, Tetrahedron Lett. 1994, 35, 699 ± 702.
[
[3] J. Sheehan, D. H. Hunneman, J. Am. Chem. Soc. 1966, 88, 3666 ± 3667.
[4] J. Sheehan, T. Hara, J. Org. Chem. 1974, 39, 1196 ± 1199.
[5] W. Tagaki, Y. Tamura, Y. Yano, Bull. Chem. Soc. Jpn. 1980, 53, 478 ±
4
80.
[
[
6] C. Zhao, S. Chen, P. Wu, Z. Wen, Huaxue Xuebao 1988, 46, 784 ± 790.
7] J. Marti, J. Castells, F. L o¬ pez-Calahorra, Tetrahedron Lett. 1993, 34,
5
21 ± 524.
[
[
8] R. L. Knight, F. J. Leeper, Tetrahedron Lett. 1997, 38, 3611 ± 3614.
9] A. U. Gerhard, F. J. Leeper, Tetrahedron Lett. 1997, 38, 3615 ± 3618.
[
[
10] C. A. Dvorak, V. H. Rawal, Tetrahedron Lett. 1998, 39, 2925 ± 2928.
11] a) D. Enders, K. Breuer, J. H. Teles, Helv. Chim. Acta 1996, 79, 1217 ±
Figure 1. Ligands which have been successfullyused in C ÀH activation
1
221; b) D. Enders, K. Breuer, Comprehensive Asymmetric Catalysis,
reactions.
Vol. 3, Springer, Heidelberg, 1999, pp. 1093 ± 1102.
[
[
12] R. L. Knight, F. J. Leeper, J. Chem. Soc. Perkin. Trans. 1 1998, 1891 ±
1
893.
Up to now the catalytic system described by Periana and co-
workers, a platinum complex with the bipyrimidine ligand 1,
has proven to be the most efficient and highlyselective system
providing methanol in yields of up to 72%.[ The major
drawback of the system is the reaction medium: oleum leads
to a large amount of diluted sulfuric acid when the formed
ester is hydrolized. Very recently an even higher activity was
reported for palladium and platinum complexes of ligand 3.[
This structural motif can also be found in other ligands, such
as 2, which are derived from 1,10-phenanthroline (Figure 1).[
In general, however, onlya small number of systems which are
capable of functionalizing methane catalytically has been
13] a) D. Enders, K. Breuer, G. Raabe, J. Runsink, J. H. Teles, J.-P. Melder,
K. Ebel, S. Brode, Angew. Chem. 1995, 107, 1119 ± 1122; Angew.
Chem. Int. Ed. Engl. 1995, 34, 1021 ± 1024; b) J. H. Teles, J.-P. Melder,
K. Ebel, R. Schneider, E. Gehrer, W. Harder, S. Brode, D. Enders, K.
Breuer, G. Raabe, Helv. Chim. Acta 1996, 79, 61 ± 83.
4]
[
[
14] For the synthesis from l-tert-leucine byreduction (a) and c yc lization
(b), see: a) D. A. Dickman, A. I. Meyers, G. A. Smith, R. E. Ganley,
Org. Synth. 1990, Coll. Vol. 7, 530 ± 533; b) M. S. Newman, A. Kutner,
J. Am. Chem. Soc. 1951, 73, 4199 ± 4204.
5]
15] H. G. Rule, J. Crawford, J. Chem. Soc. 1937, 138 ± 145.
6]
[
*] Dr. T. Strassner, M. Muehlhofer, Prof. Dr. W. A. Herrmann
Technische Universit‰t M¸nchen
Anorganisch-chemisches Institut
Lichtenbergstrasse 4, 85747 Garching (Germany)
Fax : (49)89-289-13473
E-mail: thomas.strassner@ch.tum.de
[
**] CÀH activation byN-heteroc cy lic carbenes, Part 1. This work was
supported byS¸d-Chemie AG.
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
Angew. Chem. Int. Ed. 2002, 41, No. 10
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
1433-7851/02/4110-1745 $ 20.00+.50/0
1745