LETTER
Electrochemical Oxidation of Alcohols
2177
OH
O
Pd(OAc)2 (10 mol%)
p-benzoquinone (10 mol%)
Ph
H
Ph
H
Ph
OH
K2CO3, DMF, 80 °C
3i
4i
+ HPdOAc
4'i
+ HPdOAc
– HPdOAc
OAc
Pd(OAc)2 (10 mol%)
p-benzoquinone (10 mol%)
H
H
O
– HPdOAc
+ HPdOAc
Pd
Ph
OH
Ph
H
OH
Ph
H
K2CO3, DMF, 80 °C
H
3h
4h
H
5
Scheme 8
(4) (a) Nishimura, T.; Onoue, T.; Ohe, K.; Uemura, S.
mediate complex 5. The allylic alcohol 3h is formed after
a reversible syn-b-hydride elimination. Its further oxida-
tion affords the a,b-unsaturated aldehyde 4h, as observed
independently (Scheme 7, top reaction). The fact that nei-
ther the saturated aldehyde 4i nor the allylic alcohol 3h
were observed during the oxidation of 3i (only 4i at traces
level) suggests that they were present at stationary con-
centration in the successive equilibria which are shifted by
the oxidation of 3h to 4h.21 Interestingly, the oxidation of
3i led to the a,b-unsaturated aldehyde 4h after activation
of two C–H bonds by a Pd(II) moiety.
Tetrahedron Lett. 1998, 39, 6011. (b) Nishimura, T.;
Onoue, T.; Ohe, K.; Uemura, S. J. Org. Chem. 1999, 64,
6750.
(5) (a) Schultz, M. J.; Park, C. C.; Sigman, M. S. Chem.
Commun. 2002, 3034. (b) Jensen, D. R.; Schultz, M. J.;
Mueller, J. A.; Sigman, M. S. Angew. Chem. Int. Ed. 2003,
42, 3810.
(6) ten Brink, G.-J.; Arends, I. W. C. E.; Sheldon, R. A. Science
2000, 287, 1636.
(7) Jensen, D. R.; Pugsley, M. S.; Sigman, M. S. J. Am. Chem.
Soc. 2001, 123, 7475.
(8) Ferreira, E. M.; Stoltz, B. M. J. Am. Chem. Soc. 2001, 123,
7725.
The Pd/BQ-catalyzed oxidation of 2-nonanol (3f) gave
ketone 4f as the major product (Table 2). The a,b-unsatur-
ated ketone (E)-n-pentyl-CH=CHCOMe (6f) was also
formed in 6% yield by a mechanism similar to that pro-
posed in Scheme 8.22
(9) Hallman, K.; Moberg, C. Adv. Synth. Catal. 2001, 343, 260.
(10) Popp, B. V.; Stahl, S. S. J. Am. Chem. Soc. 2006, 128, 2804.
(11) Adamo, C.; Amatore, C.; Ciofini, I.; Jutand, A.; Lakmini, H.
J. Am. Chem. Soc. 2006, 128, 6829.
(12) Aît-Mohand, S.; Hénin, F.; Muzart, J. Tetrahedon Lett.
1995, 36, 2473.
(13) Grennberg, H.; Gogoll, A.; Bäckvall, J.-E. Organometallics
1993, 12, 1790.
(14) Amatore, C.; Cammoun, C.; Jutand, A. Adv. Synth. Catal.
2007, 349, 292.
In conclusion, the oxidation of alcohols to aldehydes or
ketones is catalyzed by Pd(OAc)2 associated to catalytic
p-benzoquinone. The reaction is made catalytic because
the Pd(0) generated in the reaction is oxidized back to
Pd(II) by p-benzoquinone. p-Benzoquinone may be also
used in catalytic amount because the p-hydroquinone
formed in the oxidation process is recycled back to p-ben-
zoquinone by oxidation at an anode. This anaerobic pro-
cess appears to be quite general and selective and avoids
the formation of H2O2, as formed in aerobic reactions.
Work is in progress to optimize the reactions.
(15) For anaerobic electrochemical oxidation of alcohols
catalyzed by [(trpy)(bpy)Ru(IV)=O]2+ in H2O, see:
(a) Moyer, B. A.; Thompson, M. S.; Meyer, T. J. J. Am.
Chem. Soc. 1980, 102, 2310. (b) Thompson, M. S.; De
Giovani, W. F.; Moyer, B. A.; Meyer, T. J. J. Org. Chem.
1984, 49, 4972. (c) De Giovani, W. F.; Deronzier, A.
J. Electroanal. Chem. 1992, 337, 285. (d) Geneste, F.;
Moinet, C. New J. Chem. 2004, 28, 722. (e) Geneste, F.;
Moinet, C. J. Electroanal. Chem. 2006, 594, 105.
(16) General Procedure for Preparative Electrolyses
The electrosynthesis of 2d (entry 14 in Table 1) was carried
out in a two-compartment air-tight three-electrode cell under
argon at 80 °C. The two compartments were separated by a
sintered glass disk. The anode was a carbon cloth (ca. 4 cm2
surface area). The cathode was a nickel foam (ca. 1 cm2
surface area). The reference was a sat. calomel electrode
separated from the solution by a bridge filled with a solution
of n-Bu4NBF4 (0.3 M) in DMF (2 mL). The anodic and
cathodic compartments were, respectively, filled with 10 mL
of DMF and 2 mL of DMF containing n-Bu4NBF4 (0.3 M).
Then 250 mL (2 mmol) of alcohol 1d was added to the anodic
compartment followed by 276 mg (2 mmol) of K2CO3, 21.6
mg (0.2 mmol) of sublimated p-benzoquinone, and 45 mg
(0.2 mmol) of Pd(OAc)2. Afterwards, 343 mL (6 mmol) of
AcOH was introduced into the cathodic compartment. The
electrolysis was conducted at a controlled potential of +0.75
V, using a Tacussel PJT 35-2 potentiostat. The electrolysis
Acknowledgment
This work has been supported in part by the Centre National de la
Recherche Scientifique (UMR CNRS-ENS-UPMC 8640) and the
Ministère de la Recherche (Ecole Normale Supérieure). We thank
Johnson Matthey for a loan of palladium salt.
References and Notes
(1) For recent reviews, see: (a) Sheldon, R. A.; Arends, I. W. C.
E.; ten Brink, G.-J.; Dijksman, A. Acc. Chem. Res. 2002, 35,
774. (b) Stahl, S. S. Angew. Chem. Int. Ed. 2004, 43, 3400.
(2) Peterson, K. P.; Larock, R. C. J. Org. Chem. 1998, 63, 3185.
(3) (a) Steinhoff, B. A.; Fix, S. R.; Stahl, S. S. J. Am. Chem. Soc.
2002, 124, 766. (b) Steinhoff, B. A.; Stahl, S. S. J. Am.
Chem. Soc. 2006, 128, 4348.
Synlett 2007, No. 14, 2173–2178 © Thieme Stuttgart · New York