C O M M U N I C A T I O N S
Table 2. N-Oxyl/Pd Double-Mediatory Electrooxidation of 2a
Using Various Supporting Electrolytes
the reaction using 1-tetradecene (2d) gave tetradecan-2-one (3d)
in 91% yield (entry 3). We next examined the tolerance of functional
groups under these conditions, and found that a variety of functional
groups could endure the reaction conditions. For instance, the
reaction of substrates bearing ester or phenyl groups gave the
corresponding ketones in moderate to good yields (entries 4 and
5). Notably, a formyl group was not oxidized under these conditions.
The oxidation of undec-10-enal (2g) formed 10-oxoundecanal (3g)
in 75% yield (entry 6).
entry
electrolyte
conversion (%)
yield (%)a
1
2
3
4
5
6
7
8
Et4NBF4
Et4NPF6
Et4NClO4
Et4NNTf2
Et4NOAc
LiClO4
92
95
100
85
69
66
77
48
4
83
82
80
In summary, we have developed an electrochemical method for
generating cationic palladium complexes [Pd(CH CN) ][X] , and
3 4 2
then used them for the in situ generation of the reagent for
accomplishing the electrooxidative Wacker-type reaction. Further
investigation of this strategy is in progress in our laboratory.
19
100
100
100
NaClO4
KClO4
Acknowledgment. We thank the SC-NMR Laboratory of
1
13
Okayama University for H and C NMR analyses.
a
Isolated yield.
Supporting Information Available: Experimental procedures and
characterization data. This material is available free of charge via the
Internet at http://pubs.acs.org.
Table 3. N-Oxyl/Pd Double-Mediatory Electrooxidation of Various
Olefins
References
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(
2
S. Electrochemistry 2006, 74, 656. (c) Mitsudo, K.; Matsuda, W.;
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(
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a
(4) Without electrolysis, the cationic complex was not obtained, and Pd(OAc)
2
Isolated yield.
was recovered.
(
5) For reviews concerning the Kolbe reaction, see: (a) Torii, S.; Tanaka, H.
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Wacker-type reaction did not proceed smoothly, and 81% of starting
material 2a was recovered (entry 5). On the basis of these results,
-
ClO
4
was determined to be the most effective anion moiety for
(6) Torii and co-workers reported that the isomerized ketones were obtained
as by-products from higher molecular weight alkenes in the electrooxi-
the reaction. Next, we investigated the effect of the cationic part
of the electrolyte. The cationic part did not have as great influence
6 4 3
dative Wacker-type reactions using (p-Br-C H ) N or hydroquinone as a
mediator.3b
(
7) Hayashi and Uozumi reported the high efficiency of cationic Pd, catalysts
in a Wacker-type reaction, see: Uozumi, Y.; Kato, K.; Hayashi, T. J.
Org. Chem. 1998, 63, 5071.
as the anionic moiety. With LiClO
4 4
or NaClO as the electrolyte,
the yield of 3a increased slightly to 83% or 82%, respectively
(
entries 6 and 7).
Finally, we carried out the Wacker-type oxidation using other
(8) In the presence of triphenylamine and Bu
Pd(OAc) gave cationic Pd complex [Pd(CH
when Pd(OAc) was electrooxidized in the presence of TEMPO, the Pd
4
NX, the electrooxidation of
2
3
CN) ][X] . On the other hand,
4
2
2
olefins in a similar electrooxidation system (Table 3). Higher
molecular weight terminal alkenes, which are hardly oxidizable
substrates for the Wacker reaction because of their low solubility
in polar solvents, could be used, and the corresponding ketones
were obtained in moderate to high yields (entries 1-3). Especially,
complex bearing TEMPO was also obtained. The complex also might
work as an active catalyst for the Wacker-type reaction. While the exact
structure has not been clear yet, elemental analyses suggested that the
complex might be [Pd(CH CN) (tempo)][X]. The identification of the
3 4
exact structure is undertaken.
JA069043R
J. AM. CHEM. SOC.
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VOL. 129, NO. 8, 2007 2247