the alkene is in a separate organic layer not in contact with
the anode.12 This avoids over-oxidation of the aldehyde to
carboxylic acid. But once again, applications were limited
to unsubstituted arenes or those bearing electron-donating
groups.
Concerns about toxicity and expense of metal catalysts,
their solubility in organic media, and the attraction of
carrying out such conversions in a simpler and more
conventional undivided cell led us to examine mediated
electron transfer as a possible method for effecting the
electrocatalytic equivalent of ozonolysis of electronegatively
substituted alkenes.
Figure 1. Electrocatalytic oxidation of a substrate Sub by
electrocatalyst Cat.
(Cat) whose oxidation potential is lower than that of the
substrate (Sub). The catalyst is chosen such that its oxidation
affords a stable cation radical intermediate (Cat+•). In
favorable cases, Cat+• may in turn abstract an electron from
the substrate, initiating a reaction cascade resulting ultimately
in conversion to products and regenerating Cat to be
reoxidized by the electrode to continue the catalytic cycle.
This can occur even though the oxidation potential of the
substrate may be as much as 500 mV or more higher than
that of the catalyst.8 For successful electrocatalysis to take
place, the thermodynamically unfavorable second step must
be compensated kinetically by a rapid following reaction of
Sub+•.
Steckhan and co-workers introduced the use of substituted
triphenylamines (1) as convenient electrocatalysts8a,13 and
pointed out their advantages for a variety of anodic chemical
conversions: they are soluble in a variety of common organic
solvents, the cation radicals are long-lived if all three para-
positions are substituted, and the oxidation potential of the
compound can be adjusted by variation in the ring substit-
uents. The triphenylamines most widely used as electrocata-
lysts have been 4,4′,4′′-tribromotriphenylamine (1a) and to
a much lesser extent its hexabromo analogue (1b). A
preliminary experiment established the fact that 1a is
completely ineffective at promoting the electrocatalytic
oxidation of electronegatively substituted stilbenes; its oxida-
tion potential is too low. The higher oxidation potential of
1b has sometimes been used to carry out conversions of
substances that are unaffected by 1a, but its synthesis requires
tedious separations from polybromo congeners.13a We re-
cently found a more convenient electrocatalyst, 4,4′,4′′-
trimethyl-2,2′,2′′-trinitrotriphenylamine (1c), which is easily
prepared in pure form by room temperature nitration of
commercially available tri-p-tolylamine by Cu(NO3)2 in
acetic anhydride.14 1c is readily soluble in organic solvents,
affords a stable cation radical upon one-electron oxidation,
and, most importantly, has an oxidation potential 0.5 V
higher than that of 1a (Figure 2).
Processes not involving mediated electron transfer but
rather reaction of the alkene with an electrochemically
generated reagent have been reported. Several metal ions,
e.g., Mn(III), Pd(II), and Os(VIII), have been shown to effect
the electrocatalytic oxidation of alkenes,9 but such reactions
have rarely been observed to proceed cleanly to a single
product. Scha¨fer and Ba¨umer reported an exception to this
generalization.10 They carried out the anodic oxidation of a
number of 1,2-disubstituted alkenes using a mixture of an
IO4- salt and RuCl3 in an acetonitrile-water-CCl4 mixture.
The alkenes were converted to carboxylic acids in good
yields by a complex sequence in which periodate first
oxidizes ruthenium to RuO4, which then effects bis-hydroxy-
lation of the alkene. Periodate then oxidatively cleaves the
diol to two molecules of aldehyde, which are oxidized further
to carboxylic acids by RuO4. In this cleverly designed
sequence, RuO4 is used for two separate conversions and
periodate also performs two functions: to oxidatively
regenerate RuO4 and to cleave the intermediate diol. The
overall sequence is rendered catalytic by regeneration of
periodate at the anode. Alkene cleavages to afford carboxylic
acids can also be carried out by using ozone generated
anodically at a PbO2 anode; yields are good but current
efficiencies were low (5-6%).11 There are few chemical
methods other than ozonolysis which can effect cleavage of
a carbon-carbon double bond into two molecules of alde-
hyde in this fashion, but the aldehydes formed in this process
are oxidized further to acids under these conditions. Anodic
cleavage of alkenes into aldehydes can be effected by a
Figure 2. Oxidation potentials of triarylamine electrocatalysts vs
Ag/AgNO3.
We report here the 1c-electrocatalyzed anodic oxidation
of a number of stilbenes (2a-f) bearing two or more
electron-withdrawing groups and suggest a possible mech-
anism for the observed conversions. Several possible reaction
intermediates (3a-c) were also subjected to the electrolysis
-
IO4 -ruthenium-tungsten system, using an unusual cell in
which the anode is immersed in an aqueous solvent while
(9) Simonet, J.; Pilard, J.-F. Organic Electrochemistry; Lund, H.,
Hammerich, O., Eds.; Dekker: New York, 2001; Chapter 29, pp 1183-
1185.
(10) Ba¨umer, U.-St.; Scha¨fer, H. J. Electrochim. Acta 2003, 48, 489.
(11) Ba¨umer, U.-St.; Scha¨fer, H. J. J. Appl. Electrochem. 2005, 35, 1283.
(12) Steckhan, E.; Kandzia, C. Synlett 1992, 139.
(13) (a) Schmidt, W.; Steckhan, E. Chem. Ber. 1980, 113, 577. (b)
Steckhan, E. Angew. Chem., Int. Ed. Engl. 1986, 25, 683.
(14) Wu, X.; Dube, M. A.; Fry, A. J. Tetrahedron Lett. 2006, 47, 7667.
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Org. Lett., Vol. 9, No. 26, 2007