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Electroorganic Reduction Synthesis. Vols. 1 & 2. By Sigeru Torii

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Electroorganic Reduction
Vols. 1 & 2. By
Sigeru Torii. WileyVCH, Weinheim
2006. 830 pp.,
E 299.00.—ISBN
In organic electrosynthesis, a wide variety of C C bond-forming reactions and
functional group conversions is accessible by electron transfer between the
substrate and the electrode. By this
method one can carry out reactions at
ambient temperature and pressure with
a low-cost reagent that produces no
waste. The conversions are potentialselective, can save reaction steps by
redox-“umpolung”, and can easily be
scaled up.
S. Torii has for decades been one of
the leaders in developing new electrosyntheses. In 1985 his outstanding
monograph on electroorganic anodic
synthesis appeared, and that is now
followed by a second book on reactions
at the cathode. It is organized in 13
chapters and an appendix.
In the first chapter, general schemes
of cathodic conversions, with competing
and succeeding reaction sequences, are
presented and illustrated by examples.
The roles of the solvent/supporting electrolyte, the electrode material, and other
conditions in influencing the product
selectivity are discussed.
Chapter 2 addresses the important
topic of carbonyl group reductions. It
starts with a comprehensive table on the
Angew. Chem. Int. Ed. 2006, 45, 6247 – 6249
reduction of carbon dioxide to give
products that range from formic acid to
ethylene. The reduction reactions of
aldehydes and ketones to alcohols, as
well as inter- and intramolecular hydrodimerizations, are described with an
enormous number of examples. That is
followed by carboxylations and intramolecular cyclizations with alkenes,
nitriles, enones, alkynes, and allenes.
For carboxylic acids, esters, amides,
and anhydrides, the chapter describes
cathodic hydrogenations, dimerizations,
and couplings with electrophiles.
Chapter 3 deals with the reduction
of olefins and alkynes. Double and triple
bonds are hydrogenated by electrogenerated hydrogen, either directly or catalytically, in reactions that are chemoselective, and in many cases stereoselective. The anionic intermediates can
be reacted with electrophiles (carbon
dioxide, alkyl halides) in substitutions
and additions. The chapter describes a
wealth of examples of inter- and intramolecular coupling of activated olefins
with electron-attracting groups (pyridine, aryl, carbonyl, amide, nitrile, sulfone, nitro).
The electroreduction of aromatic
nuclei is described in Chapter 4. Electrocatalytic hydrogenation of substituted aromatic compounds under ambient conditions leads to partly or fully
hydrogenated products in good yields.
Many cathodic Birch-type reductions of
substituted benzenes, fused aromatic
compounds, or aromatic steroids, in
water, tetrahydrofuran, ammonia, or
methylamine, are described.
Chapter 5 deals with the reduction
of groups containing nitrogen. Aliphatic
nitro compounds are converted selectively into oximes, hydroxylamines, or
amines, depending on the reaction conditions. A wealth of reactions is displayed in a table with 120 entries on the
selective reduction of aromatic nitro
compounds to nitroso compounds,
hydroxylamines, or amines. Another
table (60 entries) lists reactions of
nitro-aromatic compounds with additional substituents that lead to heterocycles by redox-“umpolung”.
The reduction of sulfur, selenium,
and tellurium compounds is treated in
Chapter 6. Divalent sulfur compounds
such as thiols can be reductively alkylated or activated to give Michael
donors. The reductive cleavage of disulfides opens up remarkable preparative
applications. In tetravalent sulfur compounds such as sulfonium salts and
sulfoxides, the sulfur-bearing group can
be reductively removed. For hexavalent
sulfur compounds, protocols for the
selective cleavage of unsymmetrical sulfones are presented. Phenylsulfonates of
alcohols and phenylsulfonamides of
amines, amino acids, and peptides can
be deprotected in reactions that are
potential-selective. Selenium and tellurium can be reductively converted into
selenides and tellurides by reaction with
alkyl halides.
The large Chapter 7 discusses the
important reductions of halogenated
compounds. Alkyl halides can be dimerized or dehalogenated to give alkanes
and alkenes. With 1,n-dibromides one
can obtain cyclopropanes, bicyclobutanes, and cyclopentanes. Vic-dihalides
afford olefins, and open up an easy
access to compounds with strained
double bonds. For aryl halides, one
finds many conditions for the replacement of a halogen by hydrogen. Reactions with electrogenerated nickel(0)
complexes give a wide variety of aryl–
aryl coupling products in high yields.
Electrogenerated aryl radicals can be
added inter- and intramolecularly to
double bonds. The chapter also
describes a wide variety of electrocarboxylations or electro-initiated SRN1
reactions with different nucleophiles.
Polyhalogen compounds can be selectively dehalogenated. Organometallic
compounds are obtained upon reduction of halides at active cathodes.
Chapter 8 addresses the reduction of
alcohols, ethers, and esters. The cathodic
reduction of alcohols at the C OH bond
is only possible when strong electronwithdrawing groups are present. Tosylates and triflates are converted by C O
cleavage into anions that undergo protonation or 1,3-elimination to give
cyclopropanes. The reductive cleavage
of fluorenyl, tritylone, or phenyltetrazolyl ethers provides a method for the
cathodic deprotection of alcohols.
Chapter 9 deals with organic compounds connected to elements of
Groups IIIA, IVA, VA, IB, and IIB.
Cathodic reactions of these compounds
can be used to prepare various organometallic compounds, to convert them
) 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
into free radicals and anions, to use
these species as reducing agents, or to
derivatize them. Anions generated from
alkyl halides, arenes, and olefins are
silylated by reaction with silyl chlorides.
Furthermore, organotin halides, organotin-silicon compounds, and bis-stannanes can be prepared at the cathode.
Alkylation of phosphine followed by
cathodic cleavage of the phosphonium
salt leads to the synthesis of differently
substituted phosphines. Wittig reactions
of phosphonium salts can be initiated by
electrogenerated bases (EGBs), or
Wittig–Horner reactions by reduction
of halogenated alkyl phosphonates.
Chapter 10 describes the synthesis of
organometallic compounds and their
nickel(0) catalyzes the intramolecular
arylation of alkenes and alkynes, or
supports the selective carboxylation of
triple bonds. Cathodically-produced
cobalt(I) complexes promote allylic
coupling, intramolecular alkylation,
and arylation. Electrogenerated palladium(0) complexes cleave allyl acetates,
catalyze the Heck arylation, and promote cathodic coupling and carboxylation of aryl halides and allyl acetates.
Chapter 11 deals with the important
indirect electroreductions with metal
complexes and organic compounds as
electrocatalysts. Nickel(0) catalysts
afford aryl dimers from aryl halides,
substituted benzoic acids from aryl halides and carbon dioxide, or poly-pphenylene from 1,4-dihalobenzenes.
Cobalt(I) species from vitamin B12,
cobaloxime, or aquacobalamin make it
possible to generate alkyl radicals from
alkyl halides and to use them for synthetic purposes. Electrogenerated palladium(0) complexes allow the cyanation
of aryl halides, the allylation of CH
acids, the coupling of aryl halides to
biaryls, or carboxylation to give benzoic
acid derivatives. Other conversions are
described for iron, manganese (epoxidation), chromium, samarium (allyl coupling, pinacolization), titanium (reduction of the nitro group), tin, lead, and
zinc (allylation of carbonyl compounds
and imines). With methylviologen/
NADH as mediator, carbonyl groups
can be reduced stereoselectively to alcohols.
Conversions initiated by electrogenerated bases (EGBs) are described in
Chapter 12. A very useful table summarizes diverse applications of 17 different
EGBs. EGBs generated from quinone
methides, azo compounds, amides, and
oxygen as probases are employed for the
Wittig and Wittig–Horner olefinations,
base-catalyzed reactions, and the alkylation or carbonyl addition of CH-acidic
compounds. Carbanions at positions a
to a halogen substituent can be generated by cathodic C Hal cleavage and/or
deprotonation by an EGB. The electrogenerated trichloromethyl anion has
been added to numerous aldehydes.
Chapter 13 on electropolymerization deals with the synthesis of conducting polymers, electro-initiated polymerizations, and the electrochemical conversion of polymers. The preparation,
doping, and production of composites
and their potential applications are
described for the cases of polypyrrole,
polythiophene, poly-p-phenylene, polyazulene, polycarbazole, polyacetylene,
and polyaniline. Electroinitiated polymerization can be achieved at the anode
(aryl olefins, phenols, hydroquinone)
and cathode (isoprene, acrolein, acrylonitrile, or acrylamide). The conversion
of electroactive substituents in polymers
is also described.
The contents of the two volumes are
easily accessible with the help of an
index containing more than 1400 keywords. The very detailed appendix,
which lists 900 conversions, is extremely
useful. These are ordered according to
the starting compounds that lead to
different products by selective reactions,
for which the conditions, the chapter,
and the literature reference are given.
The comprehensive collection of
cathodic conversions in this work concentrates on literature that appeared
from 1965 to nearly 2000. The large
amount of information is well organized
and presented, in about 1600 schemes
and 107 tables, and is supported by
3400 literature references. The number
of errors and misprints is low. The
strength of the book lies in the presentation of reactions and their diversity,
rather than mechanistic discussions.
Another strength is the extensive coverage of Japanese and Chinese literature,
which contributes greatly to many
aspects of the topic and is generally
not easily accessible. The book can be
strongly recommended to everybody in
) 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
academia and industry who wants to
apply and evaluate reductive methods in
synthesis, and thereby to use the advantages of electrolytic reactions.
Hans J. Schfer
Organisch-Chemisches Institut
Universit3t M5nster (Germany)
DOI: 10.1002/anie.200685405
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Angew. Chem. Int. Ed. 2006, 45, 6247 – 6249
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