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Indirect Electroorganic SynthesesЧA Modern Chapter of Organic Electrochemistry [New Synthetic Methods (59)].

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Indirect Electroorganic SynthesesA Modern Chapter of Organic Electrochemistry
New Synthetic
Methods (59)
By Eberhard Steckhan*
The electrochemical formation and regeneration of redox agents for organic syntheses (indirect electrolysis) widens the potential of electrochemistry, as higher o r totally different
selectivities can often be obtained while at the same time the energy input can be lowered
significantly. Higher current densities can also be obtained by preventing otherwise often
encountered electrode inhibition. New types of redox catalysts can be formed in-situ and
can be regenerated after reaction with the substrates. This principle is of increasing importance also for the application of already known redox agents with regard to environmental
protection, since large amounts of a product can be generated in a closed circuit using only
relatively small amounts of the redox reagent. Consequently the operation of such a process
can be greatly simplified, and the release of ecologically objectionable spent reagents into
the environment can be prevented. The broad spectrum of redox catalysts currently in use
includes, inter alia, metal salts in very low or high oxidation states, halogens in various
oxidation states, and, in particular, a wide variety of transition-metal complexes. A great
deal of progress has recently been made in the application of organic electron transfer
agents, since compounds have been found that are sufficiently stable in both the reduced as
well as the oxidized state.
1. Introduction
In organic syntheses electrochemical reductions and oxidations offer numerous advantages over conventional
methods: mild reaction conditions, which often guarantee
a high selectivity; independence of a chemical redox reagent, which leads to a drastic reduction of environmental
problems; better use of raw materials; easier isolation of
products; simplification of continuous processing. One
method that occupies a position between direct electrochemical and homogeneous-chemical redox processes and
which under ideal conditions combines the advantages of
both procedures is indirect electrolysis. Although at one
stage almost forgotten this method has a long tradition,”]
and in some cases has found industrial a p p l i c a t i ~ n . [ ~For
,~]
example, one of the earliest electroorganic syntheses to b e
employed on a n industrial scale was the oxidation of glucose to gluconic acid with bromide as redox catalyst [Eq.
(l)]. In this process the bromine that is released at the
anode is the oxidizing agent.[”
ing energy consumption, and, last but not least, by its ability to make extremely high selectivities possible. Moreover,
the inhibition of the heterogeneous electron transfer that is
often encountered in direct electrochemical processes can
also be prevented by mediator-catalyzed electrochemical
reactions, thus making useful current densities possible.
The purpose of this article is not to summarize the scope
and limitations of a n already exhausted reaction principle,
but rather to outline the changes and prospects of a field
that is still developing and to encourage participation in its
evolution and in the exploitation of its potential.
‘Y
homogeneous
redox reaction
2C,H,,O,
+ CaCO, + H 2 0
B+
Ca(C6H,,O,),
Fig. I . The principle of indirect electrolysis exemplified by an oxidation.
+ CO, + 2 H2
(1)
As shown in Figure 1, the indirect electrolysis combines
a heterogeneous step (the production and regeneration of
the redox catalyst (Med = mediator) in its active form) with
the homogeneous redox reaction of Med and the substrate
RH.
This electrochemical method has recently experienced a
sudden revived interest,’4-’01stimulated in particular by its
advantages in diminishing environmental hazards, lower-
[*] Prof. Dr. E. Steckhan
Institut fur Organische Chemie und Biochemie der Universitzt
Gerhard-Domagk-Str. I , D-5300 Bonn 1 (FRG)
Angew Chrm Inr Ed Engl 28 (1986) 683-701
2. Basic Principles
2.1. The Principle of Indirect Electrolysis
Indirect electrochemical processes are, in a certain
sense, hybrids: They combine an electrochemical and
therefore heterogeneous electron transfer reaction with a
homogeneous redox process. The redox agent reacts with
the substrate in a homogeneous reaction and is subsequently regenerated in its active form at the electrode (see
Fig. 1).
The electrochemical regeneration can be performed in
three different ways:[3,6,111
The most well
consists
0 VCH Verlagsgesellschafl rnbH. 0-6940 Wernheirn. 1986
0870-0833/86/0808-0683 $ 02 50/0
683
in separating and isolating the redox agent, which has been
converted in the chemical reactor, and in regenerating it
externally in an electrolysis cell. This so called “ex-cell”
m e t h ~ d [ ~(see
. ~ I Fig. 2) possesses the following advant a g e ~ : [ ~chemical
. ’ ~ ~ and electrochemical steps can be optimized independently of each other and neither substrates
nor products can adversely affect the electrode reaction,
nor can the electrode interfere with the homogeneous reaction. Despite this method being technically complicated
most of the processes used on an industrial scale are twostage processes of this kind, like, e.g. the external regeneration of chromic acid.
Fig. 2. Principle of indirect electrochemical synthesis with external regeneration of the redox catalyst (Med‘”-”’@)exemplified by a reduction.
A second way of electrochemical regeneration consists
in the continuous internal electrochemical retransformation of the electron transfer agent into its active form without isolating it, i.e. within the reaction vessel (“in-cell”
process, Fig. 3).[3.61The electron transfer agent, often also
called mediator, that is inserted between electrode and
substrate undergoes a homogeneous chemical reaction
with the substrate and is subsequently regenerated at the
electrode. Thus, it serves as a catalyst for the electron
transfer between electrode and substrate.
Electrolysis cell
RHO,
Fig. 3. Principle of indirect electrochemical synthesis with internal regeneration of the redox catalyst (Med‘”-”’”) exemplified by a reduction.
Because of the coupling of two very different reactions
in one reactor, however, the one-step process is much more
difficult to realize than the two-step process, since conditions have to be found under which the organic substrates,
reactive intermediates, and products d o not hinder the
electrochemical regeneration of the reagent or they themselves are not attacked electrochemically. That is why this
method has only recently found more successful application. Aside from the advantage of lower investment costs,
684
it is of great importance for the industrial application of
this method that a continuous reaction processing is facilitated, since the products can be separated from catalytic
amounts of the redox reagent far more easily than from an
excess of reagent. It is, however, more advantageous if the
process can be carried out in a two-phase system in which
the product concentrates in the substrate p h a ~ e . I ’ ~ - ’ ~ ]
In the third type of indirect electrolysis the redox agent
that is formed at the electrode does not react homogeneously in the solution, but remains fixed at the electrode
surface, where it is continuously regenerated. Such an activation of the electrode surface can, on the one hand, take
place “in-situ” by the continuous regeneration of the redox agent in its active form on the electrode surface during
the electrolysis. This is the situation, e.g., both in the case
of the nickel(111) oxide-hydroxide electrode, which is
formed in alkaline medium during anodic polarization of a
nickel electrode, and which is especially suited for the oxidation of primary alcohols to carboxylic acids and of primary amines to nit rile^,^'^] and in the case of the amalgam
electrode, which is formed in the reduction of sodium o r
tetraalkylammonium salts at mercury cathodes and can,
for example, be employed for the hydrodimerization of activated ole fin^.[^.'^^ On the other hand, the electrode can be
chemically modified first by fixation of the redox catalyst
to the electrode surface by adsorption, polymeric coating,
or covalent binding. As the stability of such electrodes is
not yet satisfactory enough for preparative purposes, and
as this aspect of indirect electrochemical processes has already been extensively reviewed e l s e ~ h e r e , ~ ~it~ -will
~ ’ lnot
be discussed any further here.
2.2. The Redox Catalyst
The redox catalyst holds a key position in indirect electrolysis since it is involved in both the heterogeneous as
well as the homogeneous redox reaction. To be suitable for
both types of reactions the mediators must fulfill the following conditions:
1. Both reduced and oxidized forms must be chemically
stable. Even a slight side-reaction to give compounds no
longer convertible into the active form leads to a rapid
loss of catalytic activity.
2. Electron exchange with the electrode and reaction with
the substrate must be fast and as reversible as possible,
otherwise larger and therefore more expensive electrode
surfaces are necessary. In addition, side reactions are
often favored.
3. Redox reactions with other than the desired compounds, for instance with the solvent, must not take
place or must be suppressed.
4. Both redox states must be sufficiently soluble in the
chosen supporting electrolyte (exception: two-phase
systems).
Inorganic ions, metal salts and some metal complexes
usually fulfill the condition of chemical stability more easily than organic mediators, however the electron transfer is
often too slow, as it is in many cases connected with a
change in the complex structure. On the other hand, metal
Angew. Chem. Inr. Ed. Engl. 25 (1986) 683-701
ions often have the advantage of moderating the reactivity
of intermediates that are formed during the electron transfer by coordination or complexation and thereby enable
specific and selective reactions. Moreover, in the case of
transition metal complexes, the redox potential can easily
be influenced by choice of the central atom or the ligands.
Organic mediators in their active form, e.g. as radical
ions, are sometimes not sufficiently stable in many media.
They often react irreversibly with the solvent, with the
electrolyte, or with the intermediates that are formed during the redox reaction. The number of stable organic electron transfer agents is therefore limited and their availability is often restricted to certain electrolytes. Nevertheless,
great progress has been made in this field recently.
In principle the redox catalysts can be classified into
two groups according to the reaction mechanisms they undergo in the homogeneous redox step.
Mechanism A : The homogeneous redox reaction consists of a pure electron transfer. Such a process is called
“redox catalysis” in its more stricter sense by Saveant et
al.,[2x1while Shonol’] has coined the expression “homomediatory system”. This mechanism is represented schematically in equations (2) to (4). The radical anions and cations
of aromatics, heteroaromatics and triaryl amines, for example, fall into this category of redox catalysts.
sulfur bond in p-methoxybenzyl sulfides with tris@-bromopheny1)amine as mediator [Eq. (5)].”x91
The reductive and oxidative power, respectively, of the
mediators and acceptors generated at low potentials can be
enhanced by photochemical excitation during the electrochemical g e n e r a t i ~ n , ~since
~ ~ . a~ ~higher
]
molecular orbital
is occupied thereby, and a n electron gap is generated in a
lower orbital. The principle of this type of reaction is outlined in the following equations for a n oxidation.
[Medoe]*
+ RH
RHOG
Med
3Medo”
Medoo
RHO@
+ RH
~~
+ RHO”
Med
Ro
eg
(7)
products
(4)
(2)
Med
lH0
Ro
4
+ RHO@
(3)
+products
(4)
ET=electron transfer
Electrode reactions that are often characterized by irreversible, i.e. slow electron transfer, can in this way be accelerated, and the more or less large overvoltages, that occur especially in conversions on a preparative scale, can be
removed.[251Hence, not only is the energy consumption
thereby reduced but also the selectivity increased. With
this type of reaction, electron transfer reactions in solution
can often be carried out even with redox reagents whose
potentials are u p to 600 mV lower than the electrode potentials of the substrates. These redox reactions “opposite
to the standard potential gradient”125-271
can take place if a
thermodynamically unfavorable electron transfer equilibrium [Eq. ( 3 ) ] is followed by a fast and irreversible reaction step [Eq. (4)]that removes the product of the electron
transfer from the equilibrium. In this case the overall reaction rate is determined by the equilibrium constant K , i.e.
by the difference between the standard potentials of the
partners of the redox step, and by the rate constant k of the
follow-up reaction. In addition to the advantage of lowering the potential, which is accompanied by a gain in energy and selectivity, the number of the transferred electrons can be controlled by choice of the redox catalyst.
Decisive for the selectivity of the reaction are the potential differences between the various functional groups of
the substrate and the mediator, as are also the rates of the
respective chemical follow-up reactions. A typical example
of such a reaction is the oxidative cleavage of a carbonAnyew. Chem. Int. Ed. Engl. 25 (1986) 683-701
Mechanism B : The homogeneous redox reaction is combined with a chemical reaction, for instance a hydrogen or
hydride atom abstraction. In the terminology of Saueant et
a1.[2x1this process is referred to as “chemical catalysis”,
while Shono[*]speaks of a “heteromediatory system.” The
following equations illustrate the principle of this type of
reaction for a hydrogen abstraction (oxidation).
Med
Medo”
+ RH
MedH”
+B
Ro
B
=
3Med’”
- +
- +
-MedH”
Med
Ro
HB@
products
base
If transition metal compounds are employed as redox
catalysts, mechanism A can be regarded as outer-sphere
and mechanism B as inner-sphere electron transfer. Here,
the chemical step [Eq. (S)] determines the selectivity of the
reaction. In addition, substantially higher potential differences can be overcome, as the chemical step [Eq. @)]is not
directly dependent upon the potential of the mediator.
The problem is, however, to find a chemical follow-up
reaction in accord with equation (9) that can make the reagent electrochemically regenerable. In the case of organometallic reagents this is usually not easy to achieve, as
mostly a ligand exchange has to occur f i r ~ t . ~Some
~ ’ . ~of~ ~
the most frequently used redox catalysts in this category
besides transition metal ions are halogen cations, hypohal685
ite ions, and molecular halogen, which are electrochemically generated from halide ions.'81 One of the oldest reactions, which is also used on an industrial scale, is the
anodic methoxylation of furan in the presence of bromide
ions [Eq. ( 1
Table I. Standard redox potentials of some inorganic redox agents vs. the
normal hydrogen electrode (NHE) [34].
Reduction
CO"
+
e0
Ce4" + e o
Mn3G e Q
MnO?+8Hs+5eo
RuO, 4H" + 4e0
Cr20:e
12H" + 6 e 0
M n 0 2 4 H " + 2e"
TI(OH)3 3 H" 2e0
Fe3' + e Q
Os04 2 e G
Fe(CN)iQ + e 0
Sn4e + e G
TiJe ee
V3@+ e 0
Cr'" + e 0
Co20
Ce'"
Mn2"
Mn2" 4 H 2 0
Ru02 2 H 2 0
2Cr(OH)2" 5 H 2 0
Mn2" 2 H 2 0
TI" 3 H 2 0
Fe2"
+
+
+
+
+
+
+
+
+
+
3. Indirect Electrochemical Syntheses with
Inorganic and Organometallic Redox Catalysts
The largest group of inorganic redox catalysts comprises
metal ions, especially transition metal ions, and their complexes. These reagents are often capable of undergoing selective redox reactions with a large number of organic substrates. By simply varying the reaction conditions a large
variety of products can be obtained, since such redox processes are usually not simple electron transfer reactions.
Rather, the substrates, reactive intermediates, or intermediate products are generally linked to the metal atom by
complexation, coordination, or even by bond formation, so
that their reactivity is reduced, and the selectivity thereby
enhanced (mechanism B, Section 2.2).
In most cases the use of these reagents is reasonabIe
from an economic and ecological point of view only if the
redox catalysts can be regenerated. Regeneration by means
of chemical reducing or oxidizing agents such as LiAIH, o r
[Fe(CN),]3Q is no solution, since it merely shifts the problem to another stage. Consequently, attempts were already
made at a relatively early date to find ways of electrochemically regenerating the redox agents. In principle, it should
be possible in all cases to use indirect electrolysis instead
of stoichiometric amounts of inorganic redox agents. Prerequisite, however, is that the number of regenerative cycles
is large and the current yield for the generation of the reagent remains constant at an acceptable order of magnitude
over a long period of time.1'3.241Otherwise, the energy consumption is too high. But even in those cases where indirect electrolysis is more expensive than the comparable
non-electrochemical process, the electrochemical regeneration can still be economically attractive, that is when a recycling of the reagent solution is necessary for ecological
reasons, e.g. in the electrochemical recovery of chromic
In the meantime, results of electrochemical regeneration
have been documented for the following inorganic oxidizing agents: CrV'-, Ce"'-, Mn"'-, Vv-, Fell'-, Hg"-, Pd"-,
TI"'-, Ag"-, Osv"'-compounds, [Fe(CN),]'O, RuIV-complexes, RuO,, as well as the nonmetallic ions BrO", ClO',
IO:, I@, Bra, CI@,and the halogens 12, Br2 and CI2. The
following electrochemically regenerated reducing agents
have also been studied: Fell-, Ti"-, Sn"'-, Sn"-, V"'-, Cr"compounds, PdO-, NiO-, Nil-, Co'-, Rh'- and FeO-complexes, and the nonmetallic reagent 0:". In addition, the
alkali metal and alkaline earth metal amalgam^[^,'^] and
solvated electrons fa11 into this category. An idea of the
oxidative and reductive power of some of the redox agents
686
Fe(C N)do
Sn2@
Ti2"
V*S
Cr2"
+
C12 (sol ) 2 e0
ClO" + 2H" + 2 e 0
C102 4 H R + 5e0
HClO H' + 2e0
CIOQ H,O 2e0
BrL(sol.) 2 e Q
BrO'
2H0 + 2eQ
HBrO H" + 2e"
H,O 2e"
BrO'
I2 + 2e0
I" + 2 e 0
IOQ + 2 H G + 2 e R
3 H 1 0 3 H " 4e"
10' + H 2 0 2e0
+
+
+
+
+
+
2CIQ
CIQ+H 2 0
CIS 2 H 2 0
CI'
H20
CI'
20H0
2 Br"
Br"
H20
Br" H 2 0
Br' + 2 0 H o
210
+
+
+
+
+
+
+
+
+
oso:"
+
+
+
+
10
I" H 2 0
1:
3H20
1'
2OH"
+
+
+
+
1.83
1.61
1.54
1.51
1.39
1.26
1.22
1.19
0.71
0.43
0.36
0.15
-0.37
- 0.26
-0.41
I .40
1.70
1.51
I .48
0.84
1.09
1.59
1.33
0.76
0.62
0.95
1.31
1.21
0.49
can be gleaned from their standard potentials listed in Table 1.
3.1. Oxidations with Metal Salts as Redox Catalysts
3.1.1. Oxidation of Aromatics in the Side-Chain
The electrochemical recovery of inorganic oxidizing
agents such as CrV'-, Vv-, CeIV- and Co"'-, and, to a
smaller extent, Ag"-compounds for the side-chain oxidation of aromatics has been the subject of very intensive
studies.
Synthesis of benzaldehydes a n d substituted benzaldehydes: The use of electrochemically regenerated Mn"'and Ce"'-compounds as "oxygen transfer agents" for the
formation of benzaldehydes from the corresponding methyl substituted aromatics was proposed at a very early
date and later reviewed by Fichter"' [Eq. (12)]. With the
~ [36.39.44.461
3
@ c 4 @ 17.37.38.41-431
ions ~
or Co3@as redox
e ,
Anode
n
catalysts for the oxidation of alkyl aromatics, only the twostep process with recycling of the electrolyte has so far led
to acceptable results, whereas in the catalysis by Ag2@ions
internal regeneration is especially favored, since the curAngew. Chem. Int. Ed. Engl. 25 (1986) 683-701
rent yield is reduced drastically when higher concentrations of Ag*@ions are produced.1401In a fundamentally
different way alkyl-substituted aromatics can also be oxidized in the benzylic position with Fenton's reagent that is
formed in situ [Eq. (13)-(19)].[451
generated cobalt'" acetate to give monoacetoxylated products was already patented in 1969[551[Eq. (22)]. Further patents have recently been granted for procedures using Cu-,
Fe-, Co-, Mn- and Pb-acetates as c a t a l y s t ~ . ~ ~ ' . ~ ~ ~
Anode
+ ee
Cathode Fe3e
-+
-
Fe2@
(13)
+ 2e' + 2 H @ H 2 0 2
Fe" + H Z 0 2-+ Fe3@+ OOH + 'OH
Ph-CH, + OOH
Ph-Ce + H20
Cathode: O2
Ph-CHq
+ O2
Ph-CH,OOO
(15)
KOAc/HOAc/HzO
C HzO A c
'X
(17)
(18)
Ph-8H-OOH
4
+Ph-CHO
X
(16)
Ph-CH2000
+ Fe"
GCH3 0
L 9
(14)
4
Ph-8H-OOH
n
co3@ c o 2 @
+ Fe3@+ 'OH
(19)
The redox pair Fe2@/.Fe3@
can also be replaced by V40/V3@
o r Cu2@/Cu@.
Synthesis of aromatic carboxylic acids: The oxidation of
arylalkanes to arylcarboxylic acids with electrochemically
regenerated chromic
has been studied intensively
by Kuhn et al. in the case of the transformation of otoluenesulfonamide to saccharin [Eq. (20)].124.281
The yields
are about 80%. The main problems encountered are the incomplete separation of the organic material and the poisoning of the lead dioxide electrode, which leads to a decrease in the current yields. These problems are not only
met with in this particular case but are characteristic for all
processes in which an external electrochemical regeneration of chromic acid takes part.
Anode
n
The indirect electrochemical oxidation of p-nitrotoland 2,4-dinitrotol~ene'~']
to the corresponding
nitrobenzoic acids and the transformations of toluene as
well as p-xylene, p-toluic acid and p-tolualdehyde into benzoic acid and terephthalic acid" 1,52-541 , respectively, by
electrochemically generated chromic acid have also been
studied.
Recently, great hopes have been put on the use of RuiVcomplexes as mediators for the oxidation of arylalkanes to
carboxylic acids with internal electrochemical regeneration. Thus, with the electrochemically produced complex
[(trpy)(bpy)RuO]'@ (trpy = terpyridine, bpy =bipyridine) it
is possible to convert p-toluic acid and p-xylene into terephthalic acid [Eq. (21)] and toluene into benzoic
The current yield is almost quantitative. After 100 cycles,
75% of the redox catalyst can still be recovered, which corresponds to a n estimated number of 400 possible cycles.
3.1.2. Oxidations at the Aromatic Nucleus
The oxidation of arenes to quinones, especially the oxidation of anthracene to anthraquinone, is usually performed with electrochemically regenerated CeIv or Cr",
even on a n industrial scale,12~3.58-601
but this method has
lost importance since the development of catalytic processes. With regard to a hydroquinone synthesis, the oxidation
of benzene to benzoquinone could be of industrial importance but the methods developed so far140,6'1are not yet
competitive enough. Aside from the above-mentioned redox systems, Mn"' and Ag" salts are also being investigated as catalysts.1621
Another interesting application is the selective oxidation
of benzene to phenol in the presence of iron salts, whereby
the "in-situ" formation of Fenton's reagent is presumed to
take place. Not only have reactions been studied in which
hydrogen peroxide is added and the concentration of Fez@
ions kept constant electrochemicallyr151but also systems in
which H 2 0 2 is produced simultaneously at the ~ a t h 0 d e . I ~ ~ The addition of Cu" salts is favorable, as Cu2@ions are
especially effective for the oxidation of carbon radicals
[Eq. (23)-(29)]. A similar reaction sequence can also be
achieved with the Cu@/Cu*@redox pair a l ~ n e . ~ ~ " . ~ ' ~
ene el^^.
C a t h o d e : Fe3@ + e@ + FeZ0
C a t h o d e : O2 + 2 e@ + 2 HQ*
(23)
H202
Fez@ + HzOz + Fe3@+ @OH + @OH
(24)
(25)
Pt- or C-Anode
I
CH3
COOH
Side-chain acetoxyfations: The selective side-chain acetoxylation of arylalkanes with internal electrochemically
Anyew. Chem. Int.
Ed. Engl. 25 (1986) 683-701
Phenol is obtained with a current yield of 60Y0,'~~~
and a
material yield of 64% with respect to hydrogen peroxide,[l5]
whereby it is especially favorable to perform the oxidation
in a two-phase benzenelwater system.['51 Chlorobenzene,
fluorobenzene and benzonitrile have been hydroxylated in
this way as
Fluorophenol is formed in particularly
high yields (80% material yield; o / p = 85 : 15).
687
C a t h o d e (Hg): CuZo
4 HQ
+
+
2 eQ
2 VOZQ + C u
NH20H
+
V3@
-
Cu
+2
-
(30)
V3@ + 2 H 2 0
0
NH2
+
OH'
+
+
Cu2@ (31)
V4@
(32)
(33)
+ e N H 2 + Ho + Cue (34)
CUB
+
-
v4Q
+
(35)
v3Q
Aminations of aromatic compounds are possible in an
analogous way by indirect electrochemical reduction of
hydroxyiamine [Eq. (30)-(35)].["81
In a sulfuric acid/water/dioxane system, aniline is obtained from benzene with a 70% current yield and toluidines from toluene with a 50% current yield.
Electrochemically produced and regenerated Ce'"-sulfate
is employed for the cleavage of D-glUCOniC acid to D-arabinose.[741 The electrochemically generated complex
[(trpy)(bpy)RuO]'@ already mentioned in Section 3.1.1 is
capable of converting 2-propanol into acetone (current
yield 100%) as well as of oxidizing ethanol to acetaldehyde
o r acetic
Torii et al. recently reported on the internal electrochemical production and regeneration of
R U O , . [ ~A
~ ]double mediator system is used in this case.
RuO, is not formed at the anode directly, but via reaction
with "C1@" ions derived electrochemically from C1' ions
[Eq. (45)]. The reaction takes place in neutral medium in a
two-phase system. Secondary alcohols furnish ketones
(%)YO),1,2-diols are cleaved to carboxylic acids (75%), 1,4and 1,5-diols afford lactones (75%), and amines are oxidized to amides.
OH
3.1.3. Oxidations with Electrochemically Generated
Mn"'-, CeiV-or Rev"'-Compounds
The conversion of carbonyl compounds[691and nitroalkanes[701into carboxymethyl- [Eq. (37)] and nitromethylradicals [Eq. (42)], respectively, with Mn"'-salts is a preparatively very valuable method. These radicals are trapped
by olefins such as 1,3-butadiene or by aromatics to give
synthetically interesting products, whereby the in-situ regeneration of the Mn3@ions is especially favorable. On the
basis of this procedure a method was developed by Monsanto Co. for the synthesis of sorbic acid via y-vinyl-y-but y r ~ l a c t o n e . [ ' ~ .The
~ ~ ] decisive steps are summarized in
equations (36)-(40). The nitromethylation of benzene can
be carried out similarly [Eq. (41)-(44)]["l (current yield
78%).
Anode: HOAc
+
M n ( O A c ) z -+ Mn(OAc)3 + He
+
e0
(36)
phase
boundary
3.1.4. Olefin Oxidations with Electrochemically Generated
Metal Salts as Redox Catalysts
Electrochemically produced TI1"-, Pd"-, Hg"- and
-compounds are especially suitable for indirect electrochemical olefin oxidations; these preferentially lead to
formation of carbonyl compounds (T13@-,(77-80i
Pd2@-,[8'-831
Hg2 @.ions[84.851), diols (TI3@-,Os8@-ions[x"1),
or carboxylic
acids (Hg2@_
[84.851 Ru4@-ions['51).
OsO, can be generated internally at the anode most favorably by a double mediator system [Eq. (46)].[861With the
redox catalyst [(trpy)(bpy)RuO]'@, allylic methylene
groups are oxidized to keto-groups while allylic methyl
groups are converted into carboxylic
osvill
O H i)H
94-9970
3.2. Oxidations with Inorganic Anions as Redox Catalysts
Anode: HOAc
Mn(OAc)3
+
+
M n ( O A c ) z ~ M n ( 0 A c +) HQ
~
CH3NOZ -Mn(OAc)2
+
e0
+ HOAc + @CHzNOz
(0) + @ C H z N O z - ~ C H 2 N 0 2
+ Mn(OAc)3-
@
&
N
H
O
,,
688
(OFCH2N02
(41)
(42)
(43)
+
+
+
Mn(OAc)z
HOAc
Of the various mediator systems the halide ions are of
special importance, for they have a broad spectrum of applicability. The molecular halogens, hypohalite ions, and
halide cations formulated as oxidizing agents can be generated without problem by the "in-cell" method. As the redox reaction with these mediator systems is generally coupled with a chemical reaction (mechanism B, Section 2.2),
very high potential differences between mediator- and substrate-redox systems can be overcome. Thus, secondary alcohols with oxidation potentials of about 2.8 V (vs. SCE)
can be oxidized to ketones at +0.6 to +0.8 (vs. SCE) via
(44)
Angew. Chem. Int. Ed. Engl. 2S (1986) 683-701
anodically generated I@-ions,which corresponds to a potential gain of about 2.0 V.'lnxl
The dimethoxylation of furan and its derivatives to 2,5dimethoxy-2,5-dihydrofuran [see Eq. (1 1)J[33.87.8x1 and the
oxidation of aldoses to saccharic acids [see Eq. ( 1)]12.9n.9'1
have achieved industrial importance. The anodic coupling
of activated methylene components like malonic esters and
j3-keto esters in the presence of 0.01 mol-% potassium iodide,IxY1in which the cathodic generation of metallic potassium can be used for the anionization of the methylene
component, has also proven very successful [Eq. (47)( 5 111.
Cathode: 2 K Q
Anode:
+ 2ee
2 I G + I2
+2 K
+ 2CHZX2
2 'CHX2 +
2CHIXz
+ 2'CHX2 + HZ
XZCH-CHX, + 2 I'
(50)
+ H2
(51)
2KQ
XZCH-CHX,
+
-
2 Br'
Brz
+
2 eo
(52)
Solution: Hr,
+ H 2 0 + HOBr + H B r
(54)
H3C-CH=CJTz
+
(55)
HORr
---ir
7 2 - 95%
(49)
The combination of this anode reaction with the cathodic hydrodimerization of acrylic acid esters'891has also
been the subject of numerous studies.
The synthesis of propylene oxide by way of heterogeneous catalysis has so far never been realized in satisfactory
yield. Therefore the indirect electrochemical synthesis via
intermediate formation of propylene chloro- o r bromohydrin by anodically produced hypochlorite or hypobromite,
respectively, was studied in great detail. The propylene halohydrins are saponified either externally or internally to
propylene oxide by the sodium hydroxide formed at the
[Eq. (52)-(57)]. This procedure, however,
has not as yet been used on an industrial scale.
Anode:
R
"'
R20
(48)
+
12
Anode
(47)
+ 2eQ
Solution: 2 K
't/
H3C-CH-CHz-Br
I
OH
I~~C-CH-CH~-BI-+ OH'+H3C-CH-CH2
+
BrG +
HzO (56)
Polyisoprenoids can be epoxidized regioselectively in
the o-position to functional groups in a similar way; in
this case "Br@" ions are formulated as the oxidizing specie~.''~]Polymer bound mediators such as anion exchange
resins in their bromide form can also be employed for the
synthesis of e p o ~ i d e s .The
~ ~ ~bromide
]
ion-catalyzed anodic oxyselenation of olefins with diphenyl diselenide [Eq.
(58)] made it possible to dispense with the use of positively
charged selenating reagents like PhSeBr.'971Under certain
conditions even the oxyselenation-deselenation sequence
can be carried out in one step [Eq. (59)].[98.991
Angew Chem. In!. Ed Engl. 25 (1986) 683-701
For example, a n effective synthesis of D,L-rose oxide is
based on this method [Eq. (60)].19s1
In a similar way carbonyl compounds can also be selenized in the a-position.'Ioo1
Indirect electrochemical oxidation with sodium bromide
or iodide not only enables the formation of carbon-hetero
atom bonds, but also of hetero-hetero atom bonds, e.g. the
nitrogen-sulfur bond in sulfenimides,"O'l the phosphorussulfur bond in phosphoric acid monothiol
and
the phosphorus-nitrogen bond in N-substituted phosphoramid ate^,["^^ as well as the sulfur-nitrogen bond in sulfenimines. The latter bond formation is of special importance
for the sulfenylation of penicillin and cephalosporin [Eq.
(61)].[Io2I
Since the direct electrochemical conversion of simple aliphatic alcohols occurs only at potentials of far more than
2.0 V (vs. SCE), indirect electrolysis is of great importance here. With sodium bromide or potassium iodide as
mediator, oxidations can already be performed at +0.6 V
(vs. SCE). Thereby, secondary alcohols produce ketones,
while primary alcohols mainly furnish esters [Eq.
(62)].116, 10s-I091
+
With the polymeric mediator poly-Cvinylpytidine hydrobromide it is possible to oxidize secondary hydroxi1
groups selectively in the presence of primary ones.[1071
The previously mentioned oxidation of alcohols with
689
radical is preferably trapped with alkenes and aromatic
compounds. Examples are the synthesis of aspartic acid
from maleic acid["71 and of diamines with elongated
chains from 1,3-butadiene [Eq. (65)-(67)].[' '61
Ru04/ClQ[Section 3.1.3, Eq. (45)][761is also possible with
the double mediator system tetraethylammonium bromide
and n-octylmethyl sulfide [Eq. (63)].11091Primary amines
can be converted into nitriles in an undivided cell with the
system NaBr/CH30H.["01
Methoxylation at the a-position to the nitrogen of a n
amide function can only be performed electrochemically
in good yields, and affords valuable synthetic building
blocks. This type of reaction is also possible by indirect
electrolysis via catalysis by NaCl, but a decisively different
regioselectivity is observed. This can be demonstrated
quite neatly with a derivative of lysine methyl ester as example [Eq. (64)].["']
OMe
1
COzMe
NaCI, CH,OH, - 2 e'
indnecl OXldalim
NHCOzMe
NHCOzMe
HN+CO,Me
I
COzMe
OMe
Indirect electrochemical oxidations with the nitrate ion
proceed via the NO3-radical and are suitable for the oxidation of secondary OH-groups"121 and of alkyl aromatic
compounds in the side-chain." 13] The periodate regeneration has also been used o n a n industrial scale for the production of dialdehyde starch from starch[1141and has recently been employed for the generation of acetaldehyde
from 2,3-butanediol."
3.3. Reductions with Metal Salts and Metal Complexes
as Redox Catalysts
3.3. I . Reductions by Electrochemically Produced
Low-Valent Metal Ions and Base Metals
Low-valent metal ions such as Ti30, V30, Sn20 and Cr2@
are common reducing agents for many organic comTheir use in stoichiometric amounts, however, gives rise to problems, especially if they are to be
used on an industrial scale. The most important problem is
the removal of spent metal salts in such a way that the environment is not endangered. But also the chemical formation of the low-valent ions gives rise to problems, since the
reducing agents have to be separated after conversion, thus
making a continuous reaction impossible. One way of circumventing these problems is resort to the indirect electrochemical reaction, and the ever increasing number of publications relating to this approach bear witness of the great
interest now being shown in this method.
Thus, the reductive cleavage of hydroxylamine and its
derivatives by electrochemically generated Ti30 - or V3@ions with formation of aminyl radicals and hydroxide ions
has been the subject of detailed investigations. The aminyl
690
The reduction of aromatic nitro groups with electrochemically generated Ti30 ions has also been investigated
intensively, especially in India. Aniline derivatives can be
generated in this way in high yields.ri'8-'201Because of the
structure of the chemical industry in India this method evidently has some advantages compared with the otherwise
conventional catalytic hydrogenation. Electrochemically
produced Sn2@ions are also used for the reduction of nitro
groups. In the case of nitrobenzene the reduction can be
stopped at the stage of phenylhydroxylamine,['2'1which
rearranges to p-aminophenol.
Cr" reagents are known for their selectivity in organic
reductions. With the in-situ formation and regeneration of
Cr"-chloride, allylic and benzylic halides could be coupled
in anhydrous dimethylformamide (DMF) in yields comparable with those obtained by other coupling metho d ~ . ~The
'~~
dehalogenation
]
of a-hydroxyhalides and their
ethers can be realized in a similar way. The reagent, the
ethylenediamine complex of Cr(C104)2, is electrochemically generated in moist dimethylformamide. Butanethiol
acts as hydrogen donor for the trapping of the intermediate radicals.11251
To avoid elimination reactions, the a-hydroxyl group has to be protected. This method has proved
to be a good alternative to the tributyltin hydride method
for the synthesis of deoxynucleotides [Eq. (68)].
Me
I
CHOEt
0
Cathode
+ BUSH,
- 112 BuSSBu
OH Br
EtOTHO
Me
Ye
Br
(68)
CHOEt
I
Angew. Chern. Ini. Ed. Engl. 25 11986) 683-701
The a-trichloromethyl-substituted alcohols or ethers,
which are readily accessible in a great structural variety,
can be reduced to preparatively interesting products by
CrClz in water or water/DMF as electrolyte. In the case of
secondary hydroxyl groups, the reaction leads to formation of (a-configurated alkenyl chlorides in one step [Eq.
(69), (70)].1'261Should the alcohols have a hydroxyl group
Cr2Q Cr3Q
Suspensions of base metals can be generated at the cathode at high current densities (>0.5 A c m P 2 ) in twophase systems. Since very small particles with oxygen-free
surfaces are generated, the reactivity of these metal suspensions clearly surpasses that of the metals that are
usually employed."31. 13'] Thus, zinc, copper, iron, and tin
powders have been generated in high current yields and
have been used for the reduction of nitro compounds and
halogenated hydrocarbons [Eq. (73)).
CHZCl-CHCl,
I
C1
n
CH2=CHC1
+
2 Cl?
(73)
Zn Zn2@ 84% c u r r e n t y i e l d
u
Cathode
attached to a tertiary C-atom, then, depending on the reaction conditions, either formation of alkylidene dichlorides
or rearrangement with formation of carbonyl compounds
is
3.3.2. Indirect Electrochemical Reductions with
Transition Metal Complexes
Transition metal complexes are finding increasing application as redox catalysts, especially for reductions. The cathodically generated active forms such as Nio-, Nil-, Cot-,
Sno-, PdO-, or Rh'-complexes react with alkylating agents
such as halides via oxidative addition. By electrochemical
reduction of the alkyl complex the metal-carbon bond can
be cleaved with regeneration of the active form of the complex. Such a catalytic cycle is presented in Scheme 1 for
RX
-
0 c1
II
&3
+
R'-CO-CH,-R~
I
RI-C-CH-R~
- 2 Cr3@
Geminal dibromocyclopropanes can be reduced to allenes in anhydrous DMF with externally generated and regenerated C r Z Oions by an inner-sphere electron transfer
If the reaction is carried out with internal
regeneration, only monobromocyclopropanes are obtained
via an outer-sphere electron transfer mechanism after the
first regenerative cycle. This means that the Cr"' complex
that is formed after the first conversion must contain different ligands than the originally employed chromium(m)halogen dimethylformamide complex. Such problems also
arise with other metal complexes as redox catalysts if the
redox reaction proceeds via an inner-sphere electron transfer (mechanism B, Section 2.2).[31,32,1281
Electrochemically generated CrZe ions can even effect
the pinacolization of otherwise unsatisfactorily dimerizing
carbonyl compound^.['^^.'^^^ Here, the Cr20 ions d o not act
as redox agent but catalyze the formation of a Crl'l-complex of the carbonyl compound, which is directly hydrodimerized at the cathode with formation of the pinacol.
Thus, for instance, fi-j~nylideneacetaldehyde['~~'
and flavanone [Eq. (72)]['301can be pinacolized in 60% and 78%
yield, respectively.
(meso dl
=
Anyew. Chem. In! Ed. Engl. 25 (1986) 683-701
16 : 6 2 )
Scheme I . Principle of the indirect electrochemical reduction of'alkyl halides
by a Co'/Co"'-complex system.
cobalt complexes, such as vitamin Bt2.[1331
In the case of
the alkylcobalt(Ir1) complexes the reduction potential can
be lowered to a value of -0.9 V (vs. SCE) by irridiation
with visible light, so that the generally highly selective and
specific reaction can proceed under very mild conditions.[133-
136. I391
711
H
65 Yo
90 %
A particularly suitable catalyst is vitamin B12,which not
only catalyzes ally1 halide coupling"4t1and the dehalogenation of alkyl halide^,['^'.'^*.^^^^ but is also excellently
69 I
suited for the alkylation and acylation of Michael systems
[Eq. (74)].['33.134.1371 Th e latter proceeds via photochemically supported electrochemical cleavage of the intermediate acylcobalt(r~i)complex (Scheme 2).1'35.1391
yields with cycle numbers of 20 to 40 for the catalyst [Eq.
(75)].['"'1
Aryl-CH(CH,)-Cl
+
NiC12iPh2P(CH?)3PPh2/COD
C02
THFIHMPTiBu4NBF4, + 2 ev
>
A ryl-CH (CH,)-COOH
(7 5)
COD = 1,5-cyclooctadiene, T H F = tetrahydrofuran,
H M P T = hexamethylphosphoric acid triamide
8 1"/o
8 07"
7 6q5,
Aryl iodides and bromides can not only be reductively
cleaved under catalysis by Nio-complexes but also by electrochemically generated and regenerated triphenylphosphanepalladium(0) complexes. Thus, bisaryls are obtained in
material
yields of 50-99Y0,['~~'
or, in presence of unsatuScheme 2. Nucleophilic acylation by photochemically supported electrorated compounds (styrene, 1,3-butadiene derivatives, phechemical cleavage of acylcobalt(i1r) complexes (indirect electrochemical acylation).
nyl acetylene), the products of ArH addition are obtained
in yields of 20-78%.[1501The mechanism is in accordance
with the one presented in Scheme 3, with palladium being
This technique can be used for the synthesis of natural
substituted for nickel. Cycle numbers of 15 have been reproducts such as optically active pheromones, 3-substiported.
tuted steroids, cyclopentanoids, sugars, e t ~ . [ ~ ~ ~ , ~ ~ ~ ~
Besides phosphanenickel(0) complexes,['441 Nil-comNio-complexes can be used in a similar way as the Colplexes are also suitable for the indirect electrochemical recomplexes. Thus, e.g., the formation of bisaryl by reducduction of alkyl halides. They react via an organonicktion of aryl halides with electrochemically generated Nioel(111) intermediate. The Nil-complexes are electrogenercomplexes has been reported on in detail. The reaction
ated from the square planar Nil'-complexes of macrocyclic
mechanism, which closely resembles that presented for the
quadridentate l i g a n d ~ " ~I 5.l 1(see Scheme 4).
cobalt system (Scheme l), is reproduced in Scheme 3. The
reaction involves the intermediates [ArylNiXL2], which
react at the electrodes. Phosphanes have proved to be useRCH2CHZ
ee
ful ligands.[3'. 143,1441
RCHzFHZ
HY
1 rlril11L1
INi'lLl
RCH2CH2Z + INi"L1
A
:
+
I
------)
i
I
"electroinactive"
b, CH2=CHZ
RX
4
[Ni'LI'
'LF+
"eiectro inactwe"
'.
-RCH2CH,Z
1 Ni "L1*@
ArylX'
'Rf?+ W + RkH) *
Scheme 3. Bisaryl formation via electrochemically generated and regenerated
Ni"-complexes ( L = PR+
Poly-para-phenylenes are obtained on a preparative scale
when aromatic dihalides are used as starting comp o u n d ~ . ~The
' ~ ~electrochemical
l
generation and reduction
of [ArylNiXL2]in the presence of C 0 2 or H,C=CH, leads
to the formation of benzoic
o r 1,l-diaryletha n e ~ ~ ' ~and
' ] styrene derivative^."^"^ Nio-complexes also
catalyze the reductive carboxylation of 1-arylethyl halides
to 2-arylpropionic
In this way, the pharmaceutically effective 2-arylpropionic acids are accessible in high
692
Scheme 4. Reduction of alkyl halides in presence (a2. b,) and absence (a,, b,)
of activated olefins by electrochemically produced and regenerated Ni'-complexes of macrocyclic quadridentate ligands.
The stability of the newly formed nickel-carbon bond is
decisive for the subsequent course of the reaction: If it is
not very stable, the complex decomposes into an alkyl radical and the original Nil'-complex (path a); the result is a
large number of cycles. This is the case, e.g., with secondary and tertiary alkyl bromides. If the bond is relatively
stable, however, as for instance in the case of primary alkyl
bromides, the alkylnickel(i~r)complex is reduced to an alkylnickel(1r) complex, which loses an alkyl anion and is
Angew Chem Inr. Ed Engl 25 (1986) 683- 701
converted into an electroinactiue Nil1-complex (path b); the
cycle number is accordingly low. The structure of the ligand also influences the lifetime of the organonickel(tI1)
complex. Thus, a less stable complex is formed with [N,N'ethylenebis(salicylideneiminato)]nickel(~~) ([Ni(salen)])
than with (5,5,7,12,12,14-hexamethyl1,4,8,1 l-tetraazacyclotetradecane)nickel(lt) ([Ni(teta)]'"); hence, [Ni(salen)]
favors the reaction via the radical path with high cycle
numbers, even with primary alkyl bromides (path a). Primary halides essentially afford dimers (R-R), whereas disproportionation (R-H
R(-H)) predominates in the case
of tertiary halides.
The organonickel(lI1) complex that is formed from alkyl
bromides and the electrochemically generated [Ni(teta)]@
complex can undergo insertion reactions with added activated olefins via path bz (Scheme 4) thereby forming a new
nickel-carbon bond. Further reduction leads to cleavage of
the complex with release of the Michael adduct and a Nil1complex that is no longer "electroactive". The more labile
Ni(salen) complex, on the other hand, yields the Michael
adducts via the radical path a, with regeneration of the original Ni"-complex. In this case the reaction is catalytic,[3Y.IS21
Indirect electrolysis is very useful for the in-situ generation of transition metal catalysts. Following the basic studies carried out by Lehrnkuhl et a1.,[Is6]some very interesting
results have emerged recently. Thus, dinitrosyliron(0) complexes can be generated electrochemically and used for the
cyclodimerization of conjugated dienes (Scheme 6).[15'. Isx1
FeCI,
+ NO or[Fe(NO),CI]L
+
The electrochemical allylation of carbonyl compounds
by electroreductive regeneration of a diallyltin reagent
from ally1 bromide and an Sno compound leads in methanol or methanollwater to formation of homoallylic alcohols in yields of 7 0 - 9 0 Y 0 . ' ~ ~ ~ ~
Using a cathodically generated and regenerated bis(bipyridine)rhodium(l) complex as two-electron transfer
agent it was possible for the first time to achieve the nonenzymatically coupled selective regeneration of NADH
from NAD@.The direct cathodic reduction leads mainly to
NAD dimers while the indirect electrochemical reduction
by the rhodium complex as two-electron transfer agent
provides almost exclusively enzymatically active NADH,
and only traces of dimers. If the reaction is carried out in
presence of alcohol dehydrogenase, carbonyl compounds
can be transformed into the respective alcohols in-situ
(Scheme 5).[1541With 2,2'-bipyridine-5-sulfonic acid as ligand, the electrode covering that would be otherwise observed can be avoided and the reduction potential can be
shifted in an positive direction by 200 mV to a value of
- 720 mV vs. Ag/AgCl. The result is complete prevention
of NAD-dimer
propylene carbonate
ay
9 0 - lOOQ/"
Scheme 6. Electrochemical generation and application of the dinitrosyliron(0) complex as catalyst for the cyclodimerization of conjugated dienes.
The C,-fraction obtained in petroleum cracking can thus
be used directly as substrate. In the case of butadiene,
turnover rates of 20000 per hour can be achieved with
complete selectivity regarding the formation of 4-vinylcyclohexene.
Highly active catalysts for olefin metathesis can be obtained in a similar way by electrochemical reduction of
WCI6 or MoCI, [Eq. (76)].'1591Since the oxidation state of
the catalyst generated in this way can be stabilized by potential control, these reactions are very highly selective.
Aluminum is used as anode material because it furnishes
the required Lewis acid with formation of aluminum trichloride. Hydroformylation reagents can also be generated
electrochemically.[1601
&Cathode, -0.9 V
eo + wc16
Al-anode,CH2C12
VF
SCE
n
CP +
1
W C ~ ( ~ - ~ ~
I
2-pentene
(76)
2 - b u t p n e + '3 - h e x e n e
2 160 cycles/h
4.. Indirect Electrochemical Syntheses with
Organic Redox Catalysts
Scheme 5 . Regeneration of NADH from NAD' by electrochemically generated [ R h ( b ~ y ) ~ ]aO
s electron transfer agent.
Angew. Chum. i n t . Ed. Engl. 25 (1986) 683-701
The most important prerequisite for the suitability of a
compound as mediator is its stability in all the oxidation
states being passed through, otherwise a rapid loss in catalytic activity will take place. This condition is rarily fulfilled by organic molecules, as their active forms, mostly
radical cations o r anions, react irreversibly in many media.
Only in recent years has there been a distinct increase in
the number of known stable organic mediators. Thus, radical anions and dianions of mostly aromatic compounds
have been used for reductions in aprotic solvents, whereas
693
viologens and violens are suitable as electron transfer
agents for reductions in protic solvents; however, they can
only be used at potentials of u p to about - 1.0 V.
For oxidations, the radical cations of compounds like
9,10-diphenylanthracene,thianthrene, phenoxathiine o r
dibenzodioxine are likely candidates as one-electron acceptors; however, because of their reactivity they can only
be used in media of low nucleophilicity. Triarylamines behave differently, if their reactive para-positions are
blocked by substituents from attack by nucleophiles. Thus,
many of these triarylamine radical cations show an excellent stability even in solvents like methanol. Aside from
pure electron transfer agents, other mediators are being
employed that react by way of a hydrogen or hydride abstraction (mechanism B, Section 2.2). Methods for the theoretical treatment of the homogeneous redox catalysis of
electrochemical reactions were mainly developed by
Saviant et a1.['"l
4.1. Reductions with Radical Anions and Dianions of
Organic Compounds
Radical anions and dianions of a great number of organic compounds, mostly aromatics, have been tested as
mediators for reductions. Table 2 gives an overview of the
potentials of some organic mediators frequently used for
reductions.
Cathode Med
Medoe
(77)
+ RX _i Med + RXoQ
ET
Medoe
RXoe -R'+X'
MedgQ
+ Ro
2R'
+ SolvH
Re + S o h H
MedoG + R o
R'
MedR'
L
Re
+ Med
R - R or RH
L
+ Solvg
RH + Solve
L
MedRQ
uRH
+ SolvH
+HMedR
+ RX
RMedR
MedR'
+ R(-H)
-
+ Solv'
+ Xe
R ~ + R X- R R + X Q
HMedR= monoalkylation product,
RMedR= dialkylation product of the mediator
tions. The coupling of the radicals RO with the radical anions of the mediators [Eq.(84)] results in the alkylation
products MRQ, which are either protonated (HMedR) or
form the bisalkylation product RMedR. With naphthalene
as mediator the following alkylation products could be
identified :[1611
R H
R H
R H
R H
Table 2. Reduction potentials of common mediator systems.
Ref.
Compound
-E,,2[VJ
(vs. SCE)
la1
Methylbenzoate
2,2'-Binaphthyl
Benzonitrile
Chrysene
m-Toluonitrile
p-Toluonitrile
2-Methylnaphthalene
Phenanthrene
1,l'-Binaphthyl
Naphthalene
Biphenyl
2.17
2.21
2.24
2.25
2.27
2.34
Compound
-E,,*[VJ
(vs. SCE)
Perylene
Phthalonitrile
4-Methoxybenzophenone
9,IO-Diphenylanthracene
Anthracene
Phenanthridine
Benzomquinoline
Pyrene
Benzo[h]quinoline
1.67
1.69
I1651
1.75
[I651
1.85
1.96
2.00
2.08
2.09
2.12
la1
[a]
[I651
[I651
[a]
I1651
Ref.
2.46
2.45
2.45
2.50
2.70
[a] C. K. Mann, K. K. Barnes: Electrochemical Reactions in Nonaqueous Sys(ems. Marcel Dekker, New York 1970.
Alkyl and aryl halides, sulfonates, sulfonamides, sulfones, sulfides, onium compounds and epoxides can be
cleaved by electrochemical reduction with organic mediators at potentials that can be much more positive than
those of the substrates. The driving force of this reaction
opposite to the standard potential gradientIz6I is generally
the fast and irreversible cleavage of the carbon-hetero
atom bond. The essential reaction steps (77)-(87) correspond to those that are also observed with sodium naphthalenide.
As products, either the hydrocarbons RH or the coupling products RR are obtained via the carbanions R Q or
via the radicals RO. Second order reactions of the radicals
such as dimerizations or disproportionations [Eq. (Sl)] are,
however, unlikely under the conditions of indirect electrolysis, since the radicals are formed in homogeneous solution and therefore occur only in low stationary concentra694
H H
H R
The indirect electrochemical cleavage of halides has
been studied analytically in detail, especially by Lund and
Simonet et a1.['61-'691The course of the reaction strongly
depends on the structure of the substrates. Thus, aryl and
benzyl halides d o not form alkylation products of the mediators, whereas these products dominate with aliphatic
halides. In the latter case primary halides favor the formation of monoalkylation products, while tertiary halides favor the formation of dialkylation products. As a result of
its alkylation, the mediator is naturally withdrawn from
further reaction, so that stoichiometric amounts are required and the reaction sequence cannot be referred to as
an "indirect electrolysis'' in the fullest sense of the word.
Sirnonet et a1.[1641
therefore coined the term "perturbed redox-catal ysis".
In a number of cases the reductive halogen-hydrogen exchange has been studied on a preparative scale: If, for example, 6-chloro-1-hexene is reduced in the presence of
naphthalene as redox catalyst, besides I-hexene also methylcyclopentane is obtained as product of the radical cyclization [Eq. (SS)].
Cathode
Also the indirect reductive deblocking of tosylamide, tosyl ester, benzyloxycarbonyl, o-nitrobenzyloxycarbonyl,
benzyl ester, and benzyl ether groups have been studied in
Angew. Chem. Int Ed. Engl. 25 11986) 683-701
4.2. Reductions by Viologen Radical Cations
Tos
N-Tos
Tos-N
L
Py
Pyrene
=
10s = p-toluenesulfonyl
great detai1,[26.166-1681 Thus, Sirnonet et al."671used pyrene
as mediator for the release of polyaza-macrocycles from
poly-p-toluenesulfonamides [Eq. (89)l.
A very effective cleavage of a tosyl-nitrogen bond is apparently facilitated by the combined effect of the electrochemically generated anthracene radical anion as electron
transfer agent with ascorbic acid as proton donor and additional reducing agent [Eq. (90)]."681
PhCH2
0
1
Tos-Nu
I
CI3,Ph
Viologens, i.e. N . N-dialkyl-2,2'- or -4,4'-bipyridinium
salts, were already used some time ago as electron transfer
agents in the coulometric titration of r e d ~ x - e n z y m e s . " ~ ~ ~
Hence, it was a logical step to apply the same principle to
reductions on a preparative scale. The electrochemical reduction of NAD' to NADH presented itself as an especially interesting problem (see also Section 3.3.2). Since the
direct electrochemical reduction of NAD@ neveF affords
NADH exclusively, a number of authors chose the detour
via an enzyme-coupled indirect electrolysis with methyl
viologen (MV=N,N'-dimethyl-4,4'-bipyridinium dichloride) as mediator (see Scheme 7). The intercalated redox
enzyme must be able to take u p two electrons successively
from the electrochemically generated MVO and then to
transfer them at one and the same time to NAD@. It was
possible to carry out the NADH and NADPH regeneration with the enzyme systems ferredoxin r e d ~ c t a s e , " li~~]
poamide dehydrogena~e,"'~~
as well as 2-oxocarboxylate
reductase and enoate r e d u ~ t a s e . [ ' ~ ~ l
'
PhC<lz 0
84%
The indirect electrochemical cleavage of sulfones convincingly demonstrates that the selectivity of this method is
superior to that of the direct electrolysis [Eq. (91)J."691
Scheme 7. Indirect electrochemical reduction of NAD' assisted by a redox
enzyme.
DMFIBU~NI
1 0 % and n u m e r o u s
o t h e r products
The perturbed redox catalysis, i.e. the mono- or bisalkylation of the electron transfer agent by the intermediary alkyl radicals [Eq. (77)-(79) and (84)-(86)], has been exploited for the production of arylalkanes that are difficult
to obtain by Friedel-Crafts alkylation. The most suitable
alkylating agent is tert-butyl chloride (or bromide),['6', I7O1731 which can also be used for the alkylation of ketones.
Tertiary alcohols and the compounds that are alkylated exclusively or additionally in the aryl moiety are formed [Eq.
(92), (93)].[1741
With enoate reductase, enoates can also be hydrogenated stereospecifically without intermediate NADH formation. This reaction also succeeds without isolation of
the enzyme, by use of intact clostridium cells. Simon et al.
were even able to generate (R)-1,2-propanediol "electromicrobially" from hydroxyacetone via indirect electrochemical formation of NADH using whole yeast cells
[Eq. (94)-(97)].["']
2MVz@
cathode
+2e0
2MV"
2 ~ ~ +0H O0 + NAD@m
NADH
He
NAD@+ CH3CH(OH)CH20H
+
+
+
25%
+
50%
3070
Radicals which lead to the alkylation of the mediator
can also be released by electron transfer from the mediator
radical anion to sulfonium or ammonium ions.L1751
Angew. Chem. I n l . Ed. Engl. 25 11986) 683-701
+ 2 MV2@
+ NADH + CH,COCH,OH
sum:
CH3COCH20H 2 H@+ 2e'
60%
(94)
-
CH3CH(OH)CHZOH
(95)
(96)
(97)
4.3. Oxidations with Triarylamine Radical Cations
Oxidation reactions catalyzed by organic mediators attracted very little attention from chemists for some time,
for it was a foregone conclusion that organic molecules
would not be stable enough to reach an acceptable number
of cycles. Nelson et al., however, were able to prove that
the radical cations of triarylamines[1801and the related Nphenylcarbazoles[ls'lare very stable, if the para positions
of the phenyl rings are blocked by substituents from attack
695
by nucleophiles. The suitability of triarylamines as mediatprs has for instance been-demonstrated in the oxidation of
cyanide ions with electrochemically generated trianisylamine radical cations as electron transfer agents.['"]
and others1180. '84.1851 h ave in the meantime synthesized a
large variety of substituted triarylamines and have tested
their suitability as redox catalyst^.^'^^^ The compounds
cover a potential range from +0.76 V to 1.96 V (vs. NHE),
so that one should be able to find a suitable mediator for
every reaction (see Table 3). It should be pointed out, however, that the stability of the triarylamine radical cations
strongly depends on the reaction medium. Bromine- and
chlorine-substituted compounds have proven to be especially reliable. Tris(2,4-dibromophenyl)amine, for example, is extremely stable. In methanolic solution 2500 cycles
could be performed without any appreciable consumption
of the mediator.['861
Table 3. Standard potentials for the oxidation of triarylamines and related
compounds to the radical cations.
\
cleavage of carbon-sulfur bonds in sulfides also shifts the
electron transfer equilibrium in favor of the products. In
addition, in many cases the reaction between substrate and
triarylamine radical cation does not proceed as a simple
homogeneous electron transfer (mechanism A, Section
2.2), but presumably an interaction (complex formation or
even bond formation) between mediator and substrate facilitates the electron transfer, so that potential differences
higher than 600 mV can be overcome. Eberson came to the
same conclusion in an analysis on the basis of the Marcus
theory.11871
In this way oxidation reactions are possible under very
mild conditions and with high selectivity, which makes the
method especially useful for the oxidative cleavage of protective groups. Thus, carbonyl compounds could be deprotected in high yields from 1,3-dithianes and 1,3-dithiolanes
with l b or If in moist acetonitrile in the presence of sodium carbonate [Eq. (98), (99)]."881The cleavage conditions
are so mild that hydroxy groups and double bonds can be
tolerated without any problems.
\
Anode (1.0 V)
n
n,
2
1
4
3
Compound
number
U
v
Substituents
4a
-
la
H
4b
-
- -
Ib
lc
Id
le
If
Ig
lh
li
lk
I1
lm
In
lo
IS
H
H
H
H
H
C02H
C02CH3
Br
H
H
Br
H
H
H
H
H
Br
H
H
H
H
H
H
H
H
H
H
Br
H
H
H
H
H
Br
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
2a
-
-
-
It
lu
2h
Br
H
Br
H
H
H
-
-
-
Br
Br
Br
Br
H
Br
w x
EUIVl
vs.NHE
Y
2
- - H
-
-
0.75
H
OCH,
H
-
-
CH3
SiMe,
I
H~C=C(CFI)
Br
Br
Br
Br
COCF,
COCH,
Br
C F3
Br
C~FS
CN
COCF,
H2C=C(CF3)
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Br
0.76
0.91
100
-
-
Br
NO2
Br
H
-
-
-
Br
Br
COCF,
Br
Br
Br
Anode (I .3 V)
IP
Iq
Ir
3
IV
lw
H
OCH3
Br
CH3
SiMe,
I
Br
Br
Br
Br
Br
Br
COCH,
Br
C F3
NO2
C2F5
CN
COCF,
Br
H
Br
NO2
Br
n
2 If@@ 2 If
u *
CHEN/H20/Na2C03
1.08
1.27
1.28
1.30
1.41
1.42
1.42
I45
1.50
1.56
1.60
1.64
1.64
168
1.68
1.72
I .72
1.74
1.80
1.83
185
I86
1.96
The cleavage of the carbon-sulfur bond in p-methoxybenzyl thioethers with formation of disulfides and secondary products of the p-methoxybenzyl cations [Eq. (loo)]
has been exploited for the directed incorporation of disulfide bridges in poly-cystinyl peptides via S(4-methoxybenzyl)cysteine units in combination with S-tritylcysteine
gr0ups.f"~1
Substituted benzyl ethers"901 and benzyl
can
also be deblocked with formation of alcohols and carboxAnode (1.3 V)
Compounds with potentials that are several hundered
millivolts more positive than those of the mediators (electrochemical oxidation opposite to the standard potential
gradient; see Section 2.2) can be oxidized with electrochemically generated triarylamine radical cations, since a
fast and irreversible bond cleavage usually follows the primary electron transfer. Especially favorable follow-up
reactions are deprotonations in the benzyl- or allyl-position or in the a-position to hetero atoms. The irreversible
696
+
C H 3 O e C H O
Angew. Chem. Int. Ed. Engl. 25 (1986) 683-701
Anode (1.74 V)
ylic acids, respectively. If a suitable mediator is selected
these cleavages can be performed in a multifunctional molecule selectively and in a directed manner depending on
the substitution pattern of the protective groups. Thus, the
directed protection and selective deblocking of hydroxy
groups attached to primary and secondary C-atoms in polyhydroxy compounds can be realized without any problem [Eq. (lOl)].i'yOhl
I :t
I
Pr-C H-C: I l< H,-OH
I
1. Naidioxaane
2. amryl chloride
3. NaHiDMF
6-7
It@@ i t
\
R-CO-SR'
I
R-COOH
+
R'SSR'
75-9370
R'
=
P h , H&O-C,H,,
Ph-CH2, n-C,H,,
Starting from S-phenyl trans-2-hydroxycyclopentylthioacetate, this reaction can be used for the synthesis of
an otherwise difficultly accessible cyclopentano-y-lactone
with the trans-configuration [Eq. (104)].i'9'1 Rearrange-
Et
Pr-C €1-C €I<
CH3CN/H,0/NazC03
H2-OAn
Anode
Lt
I
P r i : H< II< H-B r
1 Ph3PBrz/DMF
2 CH3CN/NaHC03
I
Et
I
Pr-CII-CH-CH2-OH
ments can be initiated by the indirect anodic cleavage of
carbon-sulfur bonds [Eq. (105) and (106)].i'931
I
3 lt/Anode
OH
OL1Z
957"
A n = 4 - 1 n e t h o x y b e n z y 1 , Bz = b e n z y l
Anode
6 7
-
HO S P h
If@@ If
R1-C-CH-SPh
--T+
PhSSPh
I
Even substantially milder reaction conditions are possible,
if the anisyl ether function as permanent protective group
is combined with the 3,4-dimethoxybenzyl ether function
The selectivity of the
as intermediate protective
reaction can be demonstrated with the anisyl ether of
4-phenyl-3-buten-1-01 [Eq. ( 102)].[19"c1 The educt
contains two electrophores, whose potentials differ only
by 100 mV. The direct electrolysis therefore proceeds unselectively, while the indirect electrolysis leads only to attack at the easier oxidizable function. Non-activated OHgroups are stable towards triarylamine radical cations with
potentials u p to 1.6 V (vs. NHE).
I
I
R2
L2
F: ?*
R2-C-CH-SPh
If@@ I f
L?
(105)
0 R'
I/
I
R~-C-CH-OH
-
Anode
Anode
n
passivation
product mixture
Ph-C Ii=C H-(C H2)2-OCH2An
-1.1
E,
=
195 V
E,
v s NHI
= 185
V
P h < H=C H-(C H2)z-OH
/'y
2 If@@2 I f
uV)
80'0
Anode ( 1 . 3
The indirect electrochemical deblocking of benzyl esters
by triarylamine radical cations not only differentiates
highly selectively between unsubstituted benzyl esters and
4-methoxybenzyl, 2,4-dimethoxybenzyl o r benzhydryl esters, but, other than in previous methods, also between the
acid labile 2,4-dimethoxybenzyl esters and the likewise
acid labile 4-methoxybenzyl and benzhydryl esters.""] The
N-t-butoxycarbonyl(Boc)- and N-benzyloxycarbonyl(Z)protective groups are stable under these conditions. In
contrast, the N-p-methoxybenzyloxycarbonyl(M0Z)-protecting group can be cleaved smoothly by It '@.Thiol esters can also be cleaved by indirect electrolysis in good
yield with It as a mediator [Eq. (103)].1'9'b1
Angew. Chem. Int. Ed. Engl.
2s (1986) 683-701
Interesting from the industrial point of view are the indirect electrochemical oxidations of benzylic alcohols, benzaldehyde dimethyl acetals and arylalkanes with It as mediator. We were able to show that benzylic alcohols can be
oxidized with high selectivity to benzaldehydes or their dimethyl acetals not only in acetonitrile in a divided cell but
also in methanol in an undivided
Arylalkanes that are oxidizable only to the stage of the
benzaldehyde dimethyl acetals by direct anodic electrolysis in methanol are oxidized to methyl benzoates under
mildly acidic conditions and at very low potentials with It
as mediator [eq. ( 107)]."86.'94b1 Even toluene with a standard potential of 2.64 V (vs. NHE) can be transformed into
697
methyl benzoate in 95% material yield. If the reaction is
carried out in presence of N a 2 C 0 3 or small amounts of
NaOMe as base, trimethylorthobenzoates are obtained in
good selectivity [Eq. ( 107)].~'x6.
'94h1 Wh en benzaldehyde dimethyl acetals are used as starting compounds in a
CH,OH/LiCIO, electrolyte in presence of low concentrations of NaOMe, orthoesters are obtained almost exclusively (selectivity > 90%). The current yields are especially
good in all cases, if the substrate is used as a co-solvent.[lW 194hl
nally,['961that is after the isolation of the hydroquinone,
and interr~ally."~'~
TEMPO is oxidized in-situ in acetonitrile containing lutidine. Thus, primary alcohols can be
converted into aldehydes, while secondary alcohols are
scarcely attacked.['981Under these conditions, primary amines preferably form nitriles; in the presence of water, hydrolysis of the intermediate azomethines with generation
of carbonyl compounds is favored [Eq. (108)-( 112)].[1991
5. Outlook
Anode
(I 74 V)
Anode
(1 74 V)
4.4. Oxidations with Electrochemically Regenerable
Hydride or Hydrogen Atom Abstracting Reagents
As already demonstrated in Section 2.2, even those oxidation reagents that effect a hydrogen or hydride abstraction can be regenerated electrochemically. Hydrogen, for
example, can be abstracted by in-situ electrochemically oxidized N-hydr~xyphthalimide.['~'~
The presence of a base
like pyridine is necessary. In this way alcohols, arylalkanes, benzyl ethers and olefins can be oxidized. The cycle
numbers, however, are still very low. Electrochemically regenerable hydride abstracting agents are 2,3-dichloro-5,6dicyano-p-benzoquinone (DDQ), as well as the oxoammonium ion accessible from 2,2,6,6-tetramethylpiperidyl
oxide (TEMPO). DDQ has been regenerated both exter-
Q
0
I1
I
0H
0
The optimism sparked off by euphoric predictions on
the future development of electroorganic syntheses in the
early sixties was suddenly displaced by a certain disillusionment during the seventies. In the meantime, however,
there has been a gradual but distinct revival of interest in
electrosynthesis, both in the laboratory and in industry.
Responsible for this fresh impetus are new insights into
chemical mechanisms and the development of new materials and new techniques.[**'] One of these techniques,
namely indirect electrochemical synthesis, has become a
subject of intensive study and it is expected that a host of
new possibilities for its application will emerge in the future. The particular advantages of this technique are its
low burden on the environment, the favorable exploitation
of energy because of the acceleration of electrode reactions, and the high selectivity of many of the syntheses.
Special chances of development are foreseen for indirect
electrolysis with transition metal complexes as redox catalysts, since these mediators are accessible in large variety,
can easiiy be modified, and mostly undergo specific reactions. The in-situ regeneration of metal ions in low oxidation states opens up new possibilities for the use of reactive and highly sensitive reagents. Some organic mediators,
in particular triarylamines, have proved to be surprisingly
stable and even fulfill the prerequisites for use on an industrial scale, with cycle numbers of several thousands,
Even some present day environmental problems can be reduced by resort to indirect electrochemical processes.
Thus, the dehalogenation of polychlorinated biphenyls
(PCB's) could be carried out with 9,10-diphenylanthracene
as redox catalyst.r2001Entirely new reactions can be expected to emerge from the photochemical excitation of
electrochemically generated and regenerated mediators.
New insights into this field are also of great importance for
the photoelectrochemical conversion of light into electrical
or chemical energy.
My own results quoted in this article were only possible
through the great efforts of motivated co-workers. which are
mentioned in the references, and by generous support of the
Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, the Arbeitsgemeinschaft industrieller Forschungsuereinigungen, and the BASF Aktiengesellschaft. My
sincere thanks to ail of them!
I
OH
698
II
I
0
8
Received: January 2, 1985;
revised: October 2, 1985 [A 586 IE]
German version: Angew. Chem. 98 (1986) 681
Angew. Chem. Int. Ed. Engl. 25 (1986) 683-701
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;
Kokai Tokkyo
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71 (1969) 49605f.
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Abstr. 94 (1981) 129502k.
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70 1
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