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ORGANIC
ION RADICALS
Chemistry and Applications

Zory V. Todres
Columbus, Ohio, U.S.A.

Marcel Dekker, Inc.
TM

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To my wife, Irina
The cloudless beauty of her heart, the profundity of her mind, and
the depth of her feelings have always provided reliable
support for me. For my children, Vladimir and Ellen, their
mother represents a supreme example.

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Foreword

This volume presents an overview of the widely varied subject matter that is relevant to understanding the chemistry of organic ion radicals. Detailed consideration is given to all aspects of ion radical solution chemistry, from discussing how ion radical structures are described theoretically to presenting the reagents commonly used to generate them, methods
for their characterization, and synthetic methods based on ion radicals. Less usual topics

are covered in depth in chapters on how to identify and study reactions proceeding through
ion radicals, and how to optimize yields for such reactions. A chapter is devoted to solvent
effects, which are especially important for such reactions.
Examination of the practical applications of ion radicals includes coverage of their
roles in biological systems as well as in material chemistry, ranging from optoelectronics,
organic metals, and magnets to lubricants and the manufacture of paper.
This book gathers into a single place the widely scattered material needed to consider
ion radicals from an organic chemist’s point of view. A highly useful aspect of this book is
its incorporation of relevant studies from the Russian literature, a significant amount of it
originating from Todres’ own group in Moscow. Little of this material has previously received proper emphasis, or even citation, in the English-language literature.
Stephen F. Nelsen, Ph.D.
Professor
Department of Chemistry
University of Wisconsin
Madison, Wisconsin

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Preface

Contemporary organic chemistry puts great emphasis on investigations of the structure and
reactivity of intermediate species, originating in the pathway from the starting compounds
to the end products. Knowledge of properties of the intermediate species and penetration
into the mechanism of reactions open new routes to increase the rate of formation and the
yield of the desired final products. Until recently, chemists focused their attention on radicals or charged species of the cationic/anionic type. Species of an intermediate nature combining ionic and radical properties—ion radicals—remained outside the scope of their investigations. Perfected instrumental techniques markedly advanced fine experiments. As a
result, the species, which were little (if at all) known to the chemists of previous decades,
now came to the forefront.

The behavior of organic ion radicals has become an area of current interest. Ion radicals arise by one-electron oxidation or one-electron reduction of organic compounds in
isolated redox processes and as intermediates along the pathways of reactions. A conversion of an organic molecule into an ion radical brings about a significant change in its electron structure and a corresponding alteration in its reactivity. This conversion allows necessary products to be obtained under mild conditions with high yields and improved
selectivity of transformation. In addition, there are several reactions that can proceed only
by the ion radical mechanism and lead to products otherwise inaccessible.
The theme of the book is the formation, transformation, and application of ion radicals in typical conditions of organic synthesis. The book presents an overview of organic ion
radical reactions and explains the principles of ion radical organic chemistry. Methods of
determining ion-radical mechanisms and controlling ion radical reactions are also reviewed.
When applicable, issues relating to ecology and biology are addressed. The inorganic
participants in the ion radical organic reactions are also considered. One of the chapters
gives representative synthetic procedures and considers the background of related synthetic
approaches.

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The book also provides a review of current practical applications as well as an outlook on those predicted to be important in the near future. The reader will learn of the
progress that has been made in technical developments utilizing the organic ion radicals.
Electronic and optoelectronic devices, organic magnets and conductors, lubricants, and
other applications are considered.
The behavior of organic ion radicals is still not completely understood. Thus, new interpretations of scientific data appear frequently in the literature. I have attempted to synthesize the ideas from various references that complement one another, although the connections among them may not be immediately obvious. (An Author Index is included to
help readers find such connections in the book.)
Science is a collective affair, and its main task is to produce trustworthy and general
knowledge. My apologies are due to any authors who have contributed significantly to the
development of this vast field but, for various reasons, have not been cited. The many contributors who are cited certainly do not reflect my preferences; their publications have been
selected as illustrative examples that may allow the reader to follow evolution of the corresponding topics.
Every new branch of science passes through several stages in its progress, including
the latent phase and the phase of increased interest. These initial phases have apparently already passed in organic ion radical chemistry; they spawned decades of heated debates. In
recent years, however, the boil has cooled to a simmer. The development of organic ion radical chemistry, especially with respect to its practical applications, has allowed chemists to

nail down ion radicals more confidently. Having become a regular division of general
knowledge, organic chemistry of ion radicals is now entering the third stage of development, namely, putting the ideas elaborated into general operation, including them in the
common property of organic chemistry. It is now necessary to generalize the obtained data
and to treat them comprehensively. Grafting the new branch to the organic chemistry tree
is the aim of this book.
I have worked in the field of organic ion radicals and their applications for several
decades and have become more and more fascinated by the beauty of this area and the diversity it presents. Understanding the role of ion radicals is as difficult as it is interesting. I
hope that this attempt to graft this branch to the organic chemistry tree will be useful both
for further advancing basic research and for facilitating new practical applications.
During my entire working life, I, like other researchers, have felt the pressure of the
scientific community’s appraisal. Criticism is crucial! The writing of this book was aided
by discussions with my colleagues and friends. I am indebted to all of them for their corrections and polemics. At the same time, their support was a great incentive. The scientific
atmosphere that surrounded me during my tenure at the Research and Educational Institute
of the Cleveland Clinic Foundation, the time when I was writing the book, also stimulated
my creative work.
The book is meant for researchers and technologists who are carrying out syntheses
and studying principles governing the choice of optimal organic reaction conditions. It will
be useful for physical organic chemists, ecologists, biologists, and specialists in electronics of organic materials, as well as professors, researchers, and students. As for students, I
have assumed that the reader is not acquainted with the field itself but possesses background knowledge in chemistry, both general and organic, a knowledge usually provided
during undergraduate studies in most, if not in all, countries.
Zory V. Todres

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Contents

Foreword

Preface
Chapter 1. Nature of Organic Ion Radicals and Their Ground-State Electron
Structure
1.1 Introduction
1.2 Unusual Features
1.3 Acid–Base Properties of Organic Ion Radicals
1.4 Metallocomplex Ion Radicals
1.5 Organic Ion Radicals with Several Unpaired Electrons and/or Charges
1.6 Polymeric Ion Radicals
1.7 Inorganic Ion Radicals in Reactions with Organic Substrates
1.8 Conclusion
References
Chapter 2. Formation of Organic Ion Radicals
2.1 Introduction
2.2 Chemical Methods of Organic Ion Radical Preparation
2.3 Equilibria in Liquid-Phase Electron-Transfer Reactions
2.4 Electrochemical Methods Versus Chemical Methods
2.5 Formation of Organic Ion Radicals in Living Organisms
2.6 Organic Ion Radicals in Solid Phases
2.7 Isotope-Containing Organic Compounds as Ion Radical Precursors
2.8 Conclusion
References

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Chapter 3. Basic Principles of Organic Ion Radical Reactivity
3.1 Introduction

3.2 Principle of the “Detained” Electron That Controls Ion Radical Reactivity
3.3 Principle of the “Released” Electron That Controls Ion Radical Reactivity
3.4 Behavior of Organic Ion Radicals in Living Organisms
3.5 Conclusion
References
Chapter 4. How to Discern Ion Radical Mechanisms of Organic Reactions
4.1 Introduction
4.2 Why Does a Reaction Choose the Ion Radical Mechanism?
4.3 Chemical Approaches to the Identification of Ion Radical Reactions
4.4 Physical Approaches to the Identification of Ion Radical Reactions
4.5 Examples of Complex Approaches to the Discernment of the Ion Radical
Mechanism of Particular Reactions
4.6 Conclusion
References
Chapter 5. How to Optimize Organic Ion Radical Reactions
5.1 Introduction
5.2 Physical Effects
5.3 Effect of Chemical Additives
5.4 Solvent Role
5.5 Salt Effects
5.6 Conclusion
References
Chapter 6. Organic Ion Radicals in Synthesis
6.1 Introduction
6.2 Reductive Reactions
6.3 Ion Radical Polymerization
6.4 Cyclization
6.5 Ring Opening
6.6 Fragmentation
6.7 Bond Formation

6.8 Conclusion
References
Chapter 7. Practical Applications of Organic Ion Radicals
7.1 Introduction
7.2 Organic Ion Radicals in Optoelectronics
7.3 Organic Metals
7.4 Organic Magnets
7.5 Organic Lubricants
7.6 Ion Radical Routes to Lignin Treatment
7.7 Conclusion
References

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Chapter 8. Concluding Remarks
8.1 Introduction
8.2 SRN1 Reaction
8.3 Stereochemical Aspects of Ion Radical Reactivity
8.4 Conclusion
References

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1

Nature of Organic Ion Radicals and
Their Ground-State Electron Structure

1.1 INTRODUCTION
Organic chemistry represents an extensive body of facts, from which the contemporary
doctrine of reactivity is built. The most important basis of this doctrine is the idea of intermediate species that arise along the way from starting material to final product. Depending
on the nature of the chemical transformation, cations, anions, and radicals are created during an intermediate stage. These species form mainly as a result of bond rupture. Bond rupture may proceed heterolytically or homolytically (Scheme 1-1):
RϪ ϩ Xϩ ← R–X → Rϩ ϩ XϪ

or

.

.

R–X → R ϩ X

Ions or radicals formed from a substrate further react, with other ions or radicals acting as reactants. Such changes in chemical bonds can be accompanied by a one-electron
shift. The concept of the one-electron shift (Pross 1985) can be illustrated by Scheme 1-2
for nucleophilic substitution:
NuϪ ϩ R↑↓Z → Nu↑↓R ϩ ZϪ
The species Nu↑↓R of Scheme 1-2 is not a radical pair; it is a covalent molecule of
the product resulting from the SN2 reaction. The process of transfer of the R group to a NuϪ
reactant proceeds in synchronicity with a one-electron shift and R–Z bond disruption. At
.
.
that time, two radical particles, Nu and R (formed in the course of reaction) remain immediately close and therefore unite rapidly. A one-electron shift may or may not lead to the
formation of radical particles. There are many reactions that consist not of a one-electron
shift, but of a one-electron transfer. The initial results of such one-electron transfers involves the formation of ion radicals.
.

This book concentrates on species of the type (RX)Ϯ , i.e., on cation and anion radicals. These species are formed during reaction, when an organic molecule either loses one
electron from the action of an electron acceptor or acquires one electron from the action of

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an electron donor, Scheme 1-3:
.

R–X ϩ e → (R–X)Ϫ

or

.

R–X Ϫ e → (R–X)ϩ

Ion radicals differ from their starting molecules only in a change of the total number
of electrons; no bond rupture or bond formation occurs. As will be seen from the following
chapters, after ion radical formation, cleavage and association reactions often occur. Geometry changes upon electron loss or gain can take place. Reactions with the participation
of ion radicals bring their own specific opportunities.
The concept of molecular orbital helps explain the electron structure of ion radicals.
When one electron abandons the highest occupied molecular orbital (HOMO), a cation radical is formed. If one electron is introduced externally, it takes up the lowest unoccupied
molecular orbital (LUMO), and the molecule becomes an anion radical.
These ion radicals have a dual character. They contain an unpaired electron and are,
therefore, close to radicals. At the same time, they bear a charge and, naturally, are close to
ions. Being radicals, the ion radicals are ready to react with radicals. Like all other radicals,
they can dismutate and recombine. Being ions, the ion radicals are able to react with particles of the opposite charge and are prone to form ionic aggregates. In contrast with radicals,

the ion radicals are especially sensitive to medium effects.
As long as an unpaired electron of an ion radical occupies an orbital covering all
atoms of a molecule, a definite distribution of spin density occurs between individual
atoms. The distribution determines the activity of one or another position of an ion radical
species. From the point of view of organic synthesis, such properties of ion radicals as stability, resistance to active medium components, capacity to disintegrate in the needed direction, and the possibility of participating in electron exchange are especially important.
All these properties become understandable (or predictable in cases of unknown examples)
from the organic ion radical electron structure. Therefore, our account will be based on the
analysis of connections between the structure of ion radicals and their reactivity or physical properties. This chapter concerns the peculiarities of conjugation in aromatic ion radicals, electron structures, and acid–base properties of ion radicals that have originated from
molecules of different chemical classes.
1.2 UNUSUAL FEATURES
1.2.1 Substituent Effects
The aim of this section is to show that substituent effects for organic ion radicals are quite
different from those of their parent, neutral molecules. Amino and nitro compounds are
good examples.
N,N-Dimethylaniline is a molecule with a lone electron pair on the nitrogen atom. Of
course, there is a strong interaction between this pair and the ␲-electron system of the ben.
zene ring. We often mark the symbol of the cation radical, i.e. ϩ , on the nitrogen atom.
However, according to the ab initio Hartree–Fock molecular orbital calculations (Zhang,
R., et al. 2000), this nitrogen atom is in fact negatively charged (Ϫ0.708), and the positive
charge is distributed on the carbon atoms, especially on the two methyl groups (ϩ0.482 on
each). Influenced by the positive-charge delocalization along the cation radical, the benzene ring becomes an electron-deficient unit with a positive charge of ϩ0.744. Summation
yields the total charge of ϩ1,000 for the N,N-dimethylamine cation radical. As seen, conventional ideas may not be applicable to the chemistry of ion radicals.

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TABLE 1-1 Nitrogen HFC Constants (aN ) from Experimental ESR Spectra of Nitro
Compounds

Compounds

Constant
aN, mT

Nitroalkanes

2.4–2.5

Nitrobenzene
2-Chloronitrobenzene
2,6-Dichloronitrobenzene
Nitrodurene

1.0
0.9
1.4
2.0

Reference
Stone & Maki (1962),
McKinney & Geske (1967)
Geske & Maki (1960)
Starichenko et al. (2000)
Starichenko et al. (2000)
Geske & Ragle (1961)

In anion radicals of nitro compounds, an unpaired electron is localized on the nitro
group, and this localization depends on the nature of the core molecule bearing this nitro
substituent. The value of the hyperfine coupling (HFC) constant in the ESR spectrum reflects the extent of localization of the unpaired electron; aN values of several nitro compounds are given in Table 1-1.

Let us compare HFC data from Table 1-1. Aliphatic nitro compounds produce anion
radicals, in which an unpaired electron spends its time on the nitro group completely. In the
nitrobenzene anion radical, an unpaired electron is partially delocalized over the aromatic
ring due to conjugation, and the nitrogen hyperfine coupling constant decreases by one half
as compared with the aliphatic counterparts. Diminution of the ␲-conjugation in the PhNO2
system as a result of the nitro group distortion intensifies the localization of the unpaired
electron on the nitro group. In the nitrobenzene anion radical, however, an unpaired electron is not evenly spread between the nitro group and the benzene ring. The calculation
(Stone & Maki 1962) of spin distribution in the nitrobenzene anion radical showed that the
nitro group retains about 0.65–0.70 of the unit spin density (i.e., an unpaired electron resides mainly on the nitro group). These calculations are based upon HFC constants aN and
aH taken from the nitrobenzene anion radical ESR spectrum. The same result arises from
the unpaired electron distribution by the use of the MO Hueckel approximation: 0.31 of the
unit spin density over the phenyl nucleus and 0.69 on the nitro group (Todres 1981).
Of course, it is the entire molecule that receives the electron upon reduction. However, the nitro group is the part where the excess electron spends the majority of its time.
Consideration of quantum chemical features of the nitrobenzene anion radical is of particular interest. The model for the calculation includes a combination of fragment orbitals for
Ph and NO2, and the results are represented in Scheme 1-4. The left part of the scheme
.
refers to the neutral PhNO2, and the right part refers to the anion radical, PhNOϪ
2 (Todres
1981).
Some changes in all the orbital energies accompany the placing of an electron on the
lowest unoccupied molecular orbital. According to the calculations, relative energy gaps
remain unchanged for the orbitals in the nitrobenzene anion radical compared with those of
the parent nitrobenzene. For the sake of graphic clearness, Scheme 1-4 disregards the difference mentioned, keeping the main feature of the equality in the energy gaps.
The nitro group in the parent nitrobenzene evidently acts as ␲-acceptor, which pulls
the electron density out of the aromatic ring. An unpaired electron will obviously occupy
the first vacant ␲-orbital of the nitro fragment (i.e., the lowest-energy-fragment orbital). Interaction between occupied and vacant orbitals is the most favorable. In the nitrobenzene

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SCHEME 1-4
.

anion radical, the one-electron populated fragment orbital of NOϪ
2 will pump spin density
in the ring. Such interaction should be very advantageous due to the level proximity of the
.
lowest vacant ring orbital and the highest occupied orbital of NOϪ
2 . Therefore, the nitro
group can, in fact, act as ␲-donor in the nitrobenzene anion radical. Such prediction is not
self-evident, since the nitro group in neutral aromatic nitro compounds is recognized as a
strong ␲-acceptor and, in principle, even as a reservoir for four to six additional electrons.
Comparing half-wave potentials of reversible one-electron reduction of meta-dinitrobenzene and other benzene derivatives, one can determine the Hammett constant for the nitro
.
group in the nitrobenzene anion radical. When the NO2 group is transformed into the NOϪ
2
group, a change in both the sign and the value of the correlation constant is observed (Todres, Pozdeeva et al. 1972a,b). A formal comparison of the Hammett constants for the NO2,
.
Ϫ.
NO Ϫ
2 , and NH2 groups shows NO 2 to be close to NH2 in terms of donating ability:
Ϫ.
␴m(NO2) ϭ ϩ0.71, ␴m(NO 2 ) ϭ Ϫ0.17, ␴m(NH2) ϭ Ϫ0.16.
Having captured the single electron, the nitro group then acts as a negatively charged
substituent. Similarly, the stable anion radical resulting from aryl diazocyanides
.
[(ArNBNCN)Ϫ] contains a substituent [(NBNCN)Ϫ ] that interacts with the aryl ring as
a donor (Kachkurova et al. 1987). Using other nitro derivatives of an aromatic heterocyclic

.
series, the generality and statistical relevance of the observed ␴m(NOϪ
2 ) constant were established (Todres, Zhdanov et al. 1968; Todres, Pozdeeva et al. 1972a). The sign and absolute magnitude of the Hammett constant are invariant regardless of which cation (Kϩ,
Naϩ, or Alk4Nϩ) in the anion radical salts of nitro compounds was studied. Such invariance is caused by the linear dependence between electrochemical reduction potentials of
substituted nitrobenzenes and the contribution of the lowest vacant ␲*-orbital of the nitro
group to the ␲-orbital of this anion radical, which is occupied by the single electron (Koptyug et al. 1988).
Experiments on the reactivity of the anion radicals under consideration are set out
later. The ability of nitrobenzene anion radicals to undergo coupling with benzenediazo
cations has been studied (Todres, Hovsepyan et al. 1988). This reaction is known to proceed for aromatic compounds having donor-type substituents (NH2, OH). Aromatic compounds containing only the nitro group do not participate in azo-coupling. It is also worth
noting that benzenediazo cations are strong electron acceptors. For instance, the interaction
between benzene- or substituted benzene-diazonium fluoroborates and the sodium salt of
the naphthalene anion radical results in electron transfer only (Singh et al. 1977). The products were naphthalene (from its anion radical) and benzene or its derivatives (from ben-

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zene- or substituted benzene-diazonium fluoroborates). The potassium nitrobenzene anion
radical appears to react with diazonium cations according to the electron transfer scheme,
too (as the naphthalene anion radical does). Products of azo-coupling were not found (Todres, Hovsepyan et al. 1988).
The potassium salt of the phthalodinitrile (ortho-dicyanobenzene) anion radical also
reacts with an electrophile according to the electron transfer scheme. If the electrophile is
tert-butyl halide, the reaction proceeds via the mechanism, including at the first-stage dissociative electron transfer from the anion radical to alkyl halide, followed by recombination of the generated tertiary butyl radical with another molecule of the phthalodinitrile
anion radical. The product mixture resulting in the reaction includes 4-tert-butyl-1,2-dicyanobenzene, 2-tert-bytylbenzonitrile, and 2,5-di(tert-butyl)benzonitrile (Panteleeva and
co-authors 1998).
An attempt to detain an unpaired electron was made by means of the second nitro
group in the anion radical of nitrobenzene (Todres, Hovsepyan et al. 1988). The potassium
salt of the anion radical of ortho-dinitrobenzene did yield an azo-coupled product according to Scheme 1-5 (the nitrogen oxide evolved was detected).
The reaction leads to a para-substituted product, entirely in accordance with the

calculated distribution of spin density in the anion radical of ortho-dinitrobenzene (Todres 1990). It was established, by means of labeled-atom experiments and analysis of the
gas produced, that azo-coupling is accompanied by conversion of one of the nitro groups
into the hydroxy group and liberation of nitric oxide. In other words, the initial radical
product of azo-coupling is stabilizing by elimination of the small nitrogen monoxide
radical to give the stable nonradical final product (Todres, Hovsepyan et al. 1988),
Scheme 1-5.

SCHEME 1-5

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During transformation of the parent molecule to the corresponding anion radical,
changes in substituent effects appear to be possible not only for the nitro group, but also for
other substituents. We have just observed the opportunity to use the nitro group as a donor
(not an acceptor) in the anion radicals of aromatic nitro compounds. In the case of AlkO
and AlkS substituents, we have a chance to encounter the donor-to-acceptor transformation
of the thioalkyl group after one-electron capture by thioalkylbenzenes (Bernardi et al.
1979). Both groups, AlkO and AlkS, are commonly known as electron donors. However,
in the anion radical form, these groups exert nonidentical effects. The methoxy group keeps
its donor properties, while the methylthio group appears to be an acceptor. This is evident
from a comparison of the ESR spectra of the nitrobenzene anion radical and its derivatives,
in particular the MeO- and MeS-substituted ones. The introduction of substituents into nitrobenzene in general affects the value of aN arising from the splitting of an unpaired electron by the nitrogen atom in the anion radical. If the group introduced is a donor, the
aN(NO2) value increases; if it is an acceptor, the aN(NO2) value is reduced. As follows from
such a comparison of aN constants, the MeO and EtO groups act similarly to the Me and SϪ
groups (donors). At the same time, the MeS and EtS groups act similarly to CN, SO2Me,
and SO2Et groups (acceptors) (Ioffe et al. 1970, Alberti, Martelli, & Pedulli 1977, Alberti,
Guerra et al. 1979, Bernardi et al. 1979).

The sharp contrast between the electronic effects exerted by the oxyalkyl and
thioalkyl groups in anion radicals was explained by means of group orbital-energy diagrams. The usual mechanism involving n,␲-conjugation requires the MeO or MeS group
to be situated in the same plane as the aromatic ring of the parent (neutral) molecules. According to calculations (Bernardi et al. 1979), “the most stable conformation is the planar”
for the anion radical of anisol. In the case of the anion radical of thioanisol, however, “the
preferred conformation is orthogonal.” The planar conformation is stabilized by the usual
n,␲-conjugation between the ring and oxygen or sulfur. Such n,␲-conjugation is impossible in the orthogonal arrangement, and only the ␴-electrons of the sulfur or oxygen appear
to be involved. Only the ␴-orbitals of these atoms are symmetrically available for overlapping with the aromatic ␲-orbitals when fragments of the molecule are oriented perpendicularly. However, interaction between the ␲-electrons of the nucleus and the vacant
␴-orbitals of the substituent is also possible in principle, because this interaction is symmetrically allowed. In practice, ␴,␲ and ␴*,␲ interactions are not too important in the case
of uncharged molecules, since the gap between the aromatic ␲-orbitals and ␴/␴*-orbitals
of the substituents is too wide. This is obvious from the left part of Scheme 1-6.
Conversion of a neutral molecule into an anion radical leads to occupation of the lowest-energy vacant orbital. The latter is the ␲-orbital of the benzene ring in both anisole and
thioanisole. Charge transfer is possible only by means of an interaction between vacant
and occupied orbitals and only if an energy gap between them is not too wide. As the
␴*-orbitals of the anisole MeO group are too far away from the ring ␲-orbital occupied by
the single electron, the conjugation conditions in the anion radical compared with the neutral molecule remain unchanged. This is evident from the right part of Scheme 1-6.
The thioanisole MeS group differs from the anisole MeO group in the fact that the ␴*
is at a lower energy level (Alberti, Guerra et al. 1979). In this case, population of the lowest vacant aromatic ␲-orbital by a single electron changes the conjugation conditions.
Namely, the ␴*, ␲ interaction becomes more favorable than the n,␲ interaction, because
the energy gap between the ␴*- and the ␲-orbitals is narrower. In other words, conditions
created in the anion radical promote charge transfer from the ring to the substituent rather
than from the substituent to the ring, as is the case in the neutral molecule. That is why the

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SCHEME 1-6

orthogonal conformation is stabilized instead of the planar one. The conversion of

thioanisole into the anion radical causes the change in the orientation of the MeS group relative to the aromatic ring plane. This is depicted on the right part of Scheme 1-6. Once
again, not the energy levels but the relative energy gaps remain unchanged for the anion
radicals as compared to the parent neutral molecules.
Change in the substituent nature after transformation of neutral aromatics into the
corresponding ion radicals may be used intuitively for preparation of some unusual derivatives. One may transform an organic molecule into its anion radical, change the substituent
effect, perform the desired substitution, and, after that, take a surplus electron off by means
of a soft oxidant and eventually obtain the desired unusual derivative in its stable form.
Studies along this line are intriguing in the cases of both anion and cation radicals.
1.2.2 Connection Between Ion Radical Reactivity and Electron
Structure of Ion Radical Products
The reaction of aryl and hetaryl halides with the nitrile-stabilized carbanions (RCϪ–CN)
leads to derivatives of the type ArCH(R)CN. Sometimes, however, dimeric products of the
type ArCH(R)CH(R)Ar are formed (Moon et al. 1983). As observed, 1-naphthyl, 2pyridyl, and 2-quinolyl halides give the nitrile-substituted products, while phenyl halides,
as a rule, form dimers. The reason consists of the manner of a surplus electron localization
in the anion radical that arises upon replacing halogen with the nitrile-containing carbanion. If the resultant anion radical contains an unpaired electron within LUMO covering
mainly the aromatic ring, such an anion radical is stable, with no inclination to split up. It
is oxidized by the initial substrate and gives the final product in the neutral form, Scheme
1-7:
.

[Ar]Ϫ CH(R)CN Ϫ e → ArCH(R)CN
If the anion radical formed acquires an unpaired electron on the CN group orbital, this
group easily splits off in the form of the cyanide ion. So the dimer is formed as the final

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product, Scheme 1-8:

.

2PhCH(R)[CN]Ϫ → 2CNϪ ϩ PhCH(R)CH(R)Ph
One-electron reduction of organyl halides often results in the spontaneous elimination of halide and the formation of organyl radicals according to Scheme 1-9:
.

.

RX ϩ e → RXϪ → R ϩ XϪ
The organyl radicals resulting in this cleavage can combine with the nucleophile anion, Scheme 1-10:
.

.

R ϩ YϪ → RYϪ

The anion radical of this substituted product initiates a chain reaction network,
Scheme 1-11:
.

.

RYϪ ϩ RX → RY ϩ RXϪ

According to Saveant (1994), an important contribution to the overall efficiency of
.
this substitution reaction is given by the step in which the RYϪ anion radical is formed. In
this step, an intramolecular electron-transfer/bond-forming process occurs when the nu.
cleophile YϪ attacking the radical R begins to form the new species, characterized by an
elongated two-center three-electron CІY bond. An unpaired electron in this anion radical

is at first allocated on a “low-energy” ␴C-Y* molecular orbital. With the progress of the formation of the C–Y bond, the energy of the ␴* molecular orbital increases sharply until a
changeover occurs. If R is Ar, the ␲* molecular orbital of the molecule becomes the
LUMO. An internal transfer of the odd electron to the LUMO then takes place. So it follows that the substitution under consideration will be easier when the energy of the ␲*
.
molecular orbital available in the ArYϪ species is lower. Papers by Rossi et al. (1994),
Galli with co-workers (1995), and Borosky et al. (2000) have again underlined the rule: The
lower the energy of the LUMO of the RYϪ (ArYϪ) species, the easier (faster) the reaction
.
.
between R (Ar ) and YϪ.
It is worth noting, however, that primary halide-containing anion radicals may be somewhat stable if an aromatic molecule has another electron acceptor group as a substituent—
such as the nitro, cyano (Lawless et al. 1969), carbonyl (Bartak et al. 1973), or pyridinyl group
(Neta & Behar 1981). In these cases, dehalogenation reactions proceed as intramolecular
.
Ϫ.
electron transfers from the groups NOϪ
through the conjugated ␲ system to the car2 , CN
bon–halogen fragment orbital. After that, the halide ion is eliminated. The splitting rate depends on the nature of the halogen (I Ͼ Br Ͼ CI) and on the position of the halogen with respect to another substituent (ortho Ͼ para Ͼ meta) (Alwair & Grimshaw 1973; Neta & Behar
1981; Behar & Neta 1981; Galli 1988). The cleavage proceeds more easily at those positions
that bear maximal spin density. Change of the nitro group to the nitrile or carbomethyl group
leads to some facilitation of halogen elimination: A greater portion of spin density reaches
the carbon–halogen orbital and the rate of dehalogenation increases. For instance, the anion
radical of 4-fluoronitrobenzene is characterized by the aF HFC constant of 0.855 mT and high
stability (Starichenko et al. 1981). In contrast, the anion radical of 4-fluorobenzonitrile has a
significantly larger aF HFC constant of 2,296 mT and does readily dissociate into the benzonitrile ␴-radical and the fluoride ion (Buick et al. 1969).
It should be emphasized that the cause of halide mobility in aromatic anion radicals
is quite opposite to that in heterolytic aromatic substitution at the carbon–halogen bond. In
anion radicals, the carbon–halogen bond is enriched with electron density and, after halide

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ion expulsion, an aromatic ␴-radical is formed. In neutral molecules, the carbon–halogen
bond conjugated with an acceptor group becomes poor with respect to its electron density;
a nucleophile attacks a carbon atom bearing a partial positive charge. Some kind of ␲-binding was established between the nitro group and chlorine through the benzene ring in 4-nitrochlorobenzene (Geer & Byker 1982). As a result, the inductive effect of chlorine becomes suppressed in the neutral molecule. In the anion radical, LUMO populated by one
electron comes into operation. The HOMO role turns out to be insignificant. In anion radicals, this orbital can cause only a slight disturbance. The negative charge to a significant
degree moves into the benzene ring, and this movement is enforced at the expense of the
chlorine-inductive effect. The carbon–chlorine bond is enriched with an electron. Eventually, ClϪ leaves the anion radical species. This event is quite simple, and its simplicity is
based on the ␲-electron character of HOMO and LUMO.
However, there are some cases when an unpaired electron is localized not on the
␲-orbital but on the ␴-orbital of an anion radical. Of course, in such cases a simple molecular-orbital consideration based on the ␲ approach does not coincide with experimental data.
Chlorobenzothiadiazole may serve as a representative example (Gul’maliev et al. 1975). Although the thiadiazole ring is a weaker acceptor than the nitro group, the elimination of the
chloride ion from the 5-chlorobenzothiadiazole anion radical does not take place
(Solodovnikov & Todres 1968). At the same time, the anion radical of 7-chloroquinoline
loses the chlorine anion (Fujinaga et al. 1968). Notably, 7-chloroquinoline is very close to
5-chlorobenzothiadiazole in terms of structure and electrophilicity of the heterocycle. To explain this difference, calculations are needed that can clearly take into account the ␴-electron framework of the molecules compared. Solvation of intermediate states on the way to
a final product should be involved in the calculations as well (Parker 1981).
The alkyl halide anion radicals do not have ␲-systems entirely. Nevertheless, they are
able to exist in solutions. The potential barrier for the C–Cl cleavage is estimated to be ca.
70 kJиmolϪ1 (Abeywickrema & Della 1981; Eberson 1982). The carbon–halogen bond
may capture one electron directly (Casado et al. 1987; Boorshtein & Gherman 1988).
It is interesting to compare SCl and SCN in relation to NO2 as the reference group.
Aryl sulfenyl chlorides and thiocyanates were subjected to two independent model reductive cleavage reactions by treatment with (a) cyclooctatetraene dipotassium (C8H8K2) in
tetrahydrofurane (THF) or (b) HSiCl3 ϩ R3N (R ϭ alkyl) in benzene (Todres & Avagyan
1972, 1978). As shown, aromatic sulfenyl chlorides under conditions (a) and (b) produce
disulfides or thiols; the presence of the nitro group in the ring does not affect the reaction.
Aryl thiocyanates without the nitro group behave in a similar way. However, aryl thiocyanates containing the nitro group in the ring are converted into anion radicals, with the
SCN remaining unchanged. This pathway is represented by Scheme 1-12:

.

O2NC6H4SCN ϩ 1⁄2C8H8K2 → 1⁄2C8H8 ϩ NCSC6H4NO2Ϫ Kϩ
Splitting of the SCN group is not observed and, after the one-electron oxidation, the
.
initial NCSC6H4NOϪ
2 anion radical produces NCSC6H4NO2. The recoveries are close to
quantitative; disulfides and thiols are not observed. The thiocyanate group (SCN) thus competes less successfully with the nitro group (NO2) for the extra electron than the sulfenyl
chloride group (SCl).
The conclusion just outlined was entirely confirmed by quantum chemical calculations. The results of the calculations are shown on the Table 1-2 (LCAO MO CNDO/2 approach, Todres, Tomilin, & Stankevich 1982). As seen in the table, the SCl group charge
depends slightly on whether or not the NO2 group is present in the benzene ring. In the case

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TABLE 1-2 Effective Charges (qi ) on R and SX
Groups in p-RC6H4SXϪ Anion Radicalsa
R

SX

ϪqR

Ϫqsx

H
NO2
H

NO2

SCI
SCI
SCN
SCN

0.05
0.20
0.06
0.48

0.78
0.76
0.54
0.27

a

The rest of the charge, up to unity, is in the benzene rings.

of thiocyanate anion radicals, the charge on the SCN group is diminished by 50% if the NO2
group is present in the molecule.
Thus, SCl and SCN have different electron-attraction properties. That conclusion
was not predictable a priori. Until recently, the extent of polarization in the SCl group has
been considered to be comparable to that in the SCN group, according to the S␦ϩ–X␦Ϫ
scheme. For instance, Kharash et al. (1953), have pointed out that nitroaryl thiocyanates as
well as nitroaryl sulfenyl chlorides, when dissolved in concentrated sulfuric acid, are converted into the same nitroaryl sulfenium ions, O2NArSϩ. However, the preceding findings
indicate otherwise.
Other pertinent examples include splitting of the anion radicals from p-nitrophenyl

metyl sulfone and p-cyanophenyl methyl sulfone (Pilard et al. 2001). The nitrophenyl
species undergoes the preferential cleavage of the Ar–S bond, whereas the cyanophenyl
species expels both CN and CH3SO2 groups in the two parallel cleavage reactions.
All the examples show that foreseeing a splitting direction and extending it from one
parent compound to another is risky, especially in the organic chemistry of radical ions.
1.2.3 Bridge Effect Peculiarities
Let us consider ion radicals of paracyclophanes. As the basis for our consideration, we will
choose the following species, depicted in Scheme 1-13 (a)–(e).
(a) The anion radical of pseudogeminal-[2.2]paracyclophane-4,7,12,15-tetrone, in
which the 1,4-benzoquinone units lie one underneath the other
(b) The anion radical of syn-[2.2](1,4)naphthalenophane-4,7,14,17-tetrone, in
which there is the same spatial situation
(c) The anion radical of anti-[2.2](1,4)naphtalenophane-4,7,12,15-tetrone, in
which the naphthoquinone units are further apart
(d) The cation radical of syn-[2.2](1,4)naphthalenophane-4,714,17-tetramethoxy
derivative, which is close to case (b)
(e) The cation radical of anti-[2.2](1,4)naphthalenophane-4,7,14,17-tetramethoxy
derivative, which resembles the case (c) structure
As seen, cyclophane structures (b)–(e) have the unique feature that the through-bond
distance within the paracyclophane fragment is held constant while the spatial distance between the ion radicalized and neutral moieties is changed. So the relative importance of
through-bond and through-space mechanisms for electron transfer can be learned directly
from experimental data on these molecules.
All the depicted compounds (Scheme 1-13) were studied in electrochemical reduction
[cases (a) to (c)] or oxidation [cases (d) and (e)]; two one-electron peak potentials were re-

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vealed, with differences significantly higher than 20 mV (Wartini, Valenzuela et al. 1998;
Wartini, Staab et al. 1998). A large difference between the first two reductions or oxidation
potentials is indicative of the delocalization of the first (unpaired) electron (Rak & Miller
1992). In other words, the two electrochemically active fragments can accept or lose a single
electron, the second one-electron transfer being markedly hampered. In their turns, ESR and
ENDOR spectra of the anion radicals under investigation gave evidence of delocalization of
an unpaired electron over the whole molecule in each case. Because of the close spatial contact of the quinone units [0.31 nm between the centers of the 1,4-benzoquinone rings, Scheme
1-13(a)], one may suppose that the unpaired electron simply jumps over this narrow gap. If
so, the whole-molecule delocalization would be impossible in the case of the mutual anti-arranged 1,4-naphthoquinone units [see structure (c) in Scheme 1-13]. However, this anti-arranged anion radical shows the full spin electron delocalization. Consequently, ␴,␲-conjugation is realized in the anion radicals of the paracyclophanes considered.
In the same way, the displacement of the unpaired electron over the whole molecules
was observed for (d) and (e) cation radicals from Scheme 1-13, in which 1,4-dimethoxynaphthalene units are syn- or anti-annelated to [2.2]paracyclophane. Taken together, the
experimental results considered provide direct evidence for the through-bond mechanism
of electron transfer in these paracyclophane systems. In a recent study, the electron transfer between 1,4-dimethoxybenzene and 7,7-dicyanobenzoquinone methide moieties in synor anti-cyclophane systems reached the same conclusion: The through-bond mechanism

SCHEME 1-13

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can remain the dominant reaction pathway at short donor–acceptor distances as well
(Pullen et al. 1997).
Other examples of the specified bridge effect deal with anion radicals of aryl derivatives of the tri-coordinated boron or tri- and four-coordinated phosphorus. In the
tris(pentafluorophenyl)boran anion radical, spin density is effectively transferred from the
boron p-orbital to an antibonding ␲-molecular orbital of the phenyl rings (Kwaan et al.
2001). Studies of the phosphorus-containing aromatic anion radicals have been more informative. As known, phosphorus interrupts conjugation between aryl fragments in the corresponding neutral compounds. In contrast to the neutral molecules, the phosphorus atom
does transmit conjugation in Ar3P and Ar3P(O) anion radicals. At least formally, the P atom
appears to be a bridge, not a barrier. Spin delocalization takes place along the whole
molecule in each case. Perhaps, the phosphorus unfilled p- or d-orbitals take part in this

transmission effect (Il’yasov et al. 1980).
The cation radicals of triaryl phosphine and trialkyl phosphites have also been probed
to hyperconjugation; photoelectron, ESR spectra, and reactions with aromatic radicals were
studied. As it turned out, the cation radicals of Ar3P are characterized by strong hypercon.
jugation (Egorochkin and others 2001). The cation radical [P(OMe/OEt)3]ϩ contains an
unpaired electron predominantly on the phosphorus atom, which facilitates the radical
.
coupling of this cation radical with the Ar radical from aryl diazonium salts. In [PhP(OMe/
.
ϩ.
ϩ.
OEt)2] , and [Ph2P(OMe/OEt)] , and [Ph3P]ϩ cation radicals, an unpaired electron is increasingly shifted from the phosphorus atom to the phenyl ring(s). This reduces the spin
density at the central phosphorus atoms, making the reaction of the mentioned cation radi.
cals with Ar slower and eventually preventing it altogether (Yasui, Fujii et al. 1994; Yasui,
Shioji, Ohno 1994). Similarly to the cation radicals, the phosphoranyl radicals with and
without the aryl ligand(s) exhibit small and large values of phosphorus HFC constants, respectively, in the ESR spectra (Boekenstein et al. 1974; Davies, Parrot, & Roberts 1974;
Davies, Griller, & Roberts 1976). This means that the unpaired electron is located mainly
on the aryl ligands of aryl phosphoranyl radicals or entirely on the central phosphorus in
the case of alkyl phosphoranyl radicals.
Let us direct our attention now to the PBC bond in phosphaalkene ion radicals. The
literature contains data on two such anion radicals in which a furan and a thiophene ring are
bound to the carbon atom, and the 2,4,6-tri(tert-butyl)phenyl group is bound to the phosphorus atom. According to ESR spectra of the anion radicals, an unpaired electron is delocalized on a ␲*-orbital built from the five-membered ring (furanyl or thienyl) and the PBC
bond. The participation of the phosphaalkene moiety in this molecular orbital was estimated as about 60%. Although the cumbersome tris(tert-butyl)phenyl group is led out of
the conjugation sterically, some moderate (but sufficient enough) transmission of the spin
density does take place through the PBC bridge (Jouaiti et al. 1997). Scheme 1-14 depicts
the structures under discussion.
The same situation was revealed for the case of the anion radical of the phosphaalkene containing the phenyl ring linked to the carbon atom and the 2,4,6-tri(tertbutyl)phenyl group linked to the phosphorus atom of the PBC bond. The unpaired electron is delocalized in this anion radical on both the PBC bond and the phenyl ring
(Geoffroy et al. 1992). In the benzene and furane derivatives containing two conjugated
ArPBC bonds, an unpaired electron is delocalized on the whole bonds of the anion radicals, resulting in one-electron reduction. Neither localized structures nor diradical species
were observed (Al Badri et al. 1999). However, if the carbon atom of this PBC bond is an

integral part of the cyclopentadiene ring, the unpaired electron distribution proceeds ac-

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SCHEME 1-14

cording to a scheme of spin-charge scattering (Al Badri et al. 1997). Scheme 1-15 illustrates
this special case.
Hence, the possibility of acquiring aromaticity conferred by the presence of six
␲-electrons in the five-carbon-membered ring considerably increases the electron affinity
of this ring. As a consequence, one of the two ␲-electrons of the PBC bond remains on the
phosphorus atom, and another one combines with the excess electron to create the cyclopentadienyl ␲-electron sextet. The situation is analogous to that in the diphenylfulvene
anion radical, as analyzed in Chapter 3 (see Section 3.2.2).
In the case of p-phosphaquinone, the 2,4,6-tri(tert-butyl)phenyl group is bound to the
phosphorus atom, and the carbon atom of the PBC bridge is an integral part of the 3,5di(tert-butyl)-4-oxocyclohexa-2,5-dien-1-ylidene moiety. The corresponding anion radical
(a structure of the p-benzosemiquinone type) is very similar to diarylphosphoranyl radicals,
in the sense of the unpaired electron delocalization (Sasaki et al. 1999). In the phosphoric
analog of biphenyl, namely, 4,4Ј,5,5Ј-tetramethyl-2,2Ј-phosphinine, the corresponding anion radical was generated on a potassium mirror. ESR/ENDOR spectra and density functional theory calculations show that, in contrast with the neutral species, this anion radical
is planar and that the unpaired electron is delocalized mainly on the phosphorus–carbon–carbon–phosphorus fragment of the two linked six-membered rings. The phosphorus
p␲-orbitals play a large part in this delocalization (Choua et al. 2000).
Diphosphaallene derivatives ArPBCBPAr are peculiar compounds because of the
presence of the two orthogonal carbon–phosphorus double bonds. Those compounds were
transformed into cation radicals upon electrochemical or chemical one-electron oxidation.
As found, the unpaired electron is located on a molecular orbital constituted mainly of a
p-orbital of each phosphorus atom and a p-orbital of the carbon atom (Chentit et al. 1997;
Alberti, Benaglia et al. 1999). Upon electrochemical or chemical reduction, aromatic phosphaallene derivatives yield anion radicals. These species have two equivalent phosphorus


SCHEME 1-15

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nuclei. The unpaired electron oscillates between the two phosphorus atoms according to
Scheme 1-16 (Sidorenkova et al. 1998, Alberti, Benaglia et al. 1999):
.

.

ArPBC(Ϫ)–P( )Ar ↔ ArP( )–C(Ϫ)BPAr
Consequently, the electron structures of the diphosphaallene ion radicals resemble those of
the allene ion radicals, where the allenic fragment works as a bridge for conjugation (see
Section 3.3.2).
One quite unusual effect deserves to be described as applied to the CBC bridge connecting two phenyl rings in the 4-nitrostilbene anion radical. This bridge is good at transmission of conjugation in neutral stilbenes, but in stilbene anion radicals it can operate as
a hollow on the conjugation route. At first glance, the distribution of unpaired electrons in
the nitrostilbene anion radical has to be similar to that in the nitrobenzene anion radical. The
consensus of opinion is that the LUMO of neutral aromatic nitro derivatives (the orbital that
accommodates the introduced electron) is essentially an orbital of the “free” nitro group.
The styryl fragment of neutral 4-nitrostilbene is conjugated with the nitro group and acts as
a weak donor. This is indicated by values of the Hammett constants: ␴␳(NO2C6H4–) is
ϩ0.23 and ␴␳(–CHBCHPh) is Ϫ0.07.
If the styryl substituent retained its donor nature in the anion radical state, an increase, not a decrease, in the value of the nitrogen HFC constant [a(N)] would have been
observed. Experiments show that a(N) values for anion radicals of nitrostilbenes decrease
(not increase) in comparison with the a(N) value for the anion radical of nitrobenzene
(Todres 1992). Both “naked” anion radicals and anion radicals involved in complexes with
the potassium cations obey such regularity. In cases of potassium complexes with THF as

a solvent, a(N) ϭ 0.980 mT for PhNO2 anion radical and a(N) ϭ 0.890 mT for
PhCBCHC6H4NO2-4 anion radical. In the presence of 18-crown-6-ether as a decomplexing agent in THF, a(N) ϭ 0.848 mT for PhNO2 anion radical and a(N) ϭ 0.680 mT for
PhCHBCHC6H4NO2-2 anion radical.
Reduction of nitrobenzene (Grant & Streitwieser 1978; Todres, Dyusengaliev et al.
1984) and 4-methoxynitrobenzene (Todres, Dyusengaliev et al. 1984) by uranium-, thorium-, and lanthanum-di(cyclooctatetraene) complexes leads to azo compounds. Scheme 117 illustrates these reductive reactions using uranium–(C8H8)2 complex as an example.
Under the same conditions, 4-styryl nitrobenzene (4-nitrostilbene) undergoes cis-totrans isomerization only, with no changes in the nitro group (Todres, Dyusengalier et al.
1984, 1985), Scheme 1-18.
Thus, it appears that the focal point of the reaction has transferred. The presence of a
styryl (not a methoxyl) group protects the nitro group from reduction. For some reason or
other, the styryl group causes a shift of excess electron density from the nitro to the ethylene fragment.

SCHEME 1-17

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SCHEME 1-18

This subtle difference between the anion radicals of nitrobenzene and nitrostilbene,
observed experimentally, is well reproduced by quantum chemical calculations (Todres et
al. 1984). Single-electron wave function analysis of the vacant orbitals in both molecules
shows that one-electron reduction of cis-4-nitrostilbene must be accompanied by the predominant localization of an upaired electron in the region of the ethylene moiety. Participation of the nitro-group atomic orbitals appears to be insignificant. The nitro-group atomic
coefficients in the molecular wave function for cis-4-nitrostilbene are half of those for nitrobenzene. The excess electron population (q) of the first vacant orbital for the nitro groups
is 0.3832 for the nitrobenzene anion radical and 0.0764 for the nitrostilbene anion radical.
The unpaired electron is localized largely on the ethylene fragment of the nitrostilbene
skeleton (q ϭ 0.2629). Moreover, the first vacant level of the cis-4-nitrostilbene molecule
has lower energy than that of the nitrobenzene molecule: 38 and 135 kJ, respectively.
This means that 4-nitrostilbene is a more effective electron acceptor than nitrobenzene. This theoretical conclusion is verified by experiments. The charge-transfer N,Ndimethylaniline complexes formed by nitrobenzene or 4-nitrostilbene have stability constants of 0.085 LиmolϪ1 and 0.296 LиmolϪ1, respectively. Moreover, the formation of the

charge-transfer complex between cis-4-nitrostilbene and N,N-dimethylaniline indeed results in cis-to-trans conversion (Dyusengaliev et al. 1995). This conversion proceeds
slowly in the complex but runs rapidly via the nitrostilbene anion radical (Todres 1992).
The cis–trans conversion of ion radicals will be considered in more details later (see Sections 3.2.5 and 7.2.1).
It is interesting to compare the fate of the CBC bridge in the anion radicals of 4-nitrostilbene (see earlier) and 4-acetyl-␣,␤-diphenylstilbene (Wolf et al. 1996). When treated
with potassium or sodium in THF and then with water, neutral 4-nitrostibene does not undergo a many-electron reduction of the nitro group or the CBC bridge. Under the same
conditions, 4-acetyl-␣,␤-diphenylstilbene produces a pinacol, [Ph2CBC(Ph)C6H4C
(OH)CH3]2. As calculations show, the carbonyl of the acetyl group in 4-acetyl-␣,␤diphenylstilbene is the site of significant reduction. The formal charge on the carbonyl carbon and oxygen atoms became significantly more negative upon addition of one electron,
whereas the olefinic carbons become only slightly more negative (Wolf et al. 1996). It is
worth pointing out that the phenylcarbonyl group is a stronger acceptor than the nitrophenyl
group: ␴␳(—COC6H5) ϭ ϩ0.46 [␴␳(—COCH3) ϭ ϩ0.52], whereas ␴␳(NO2C6H4–)
ϭ ϩ0.23 [␴␳(—NO2) ϭ ϩ0.78]. In addition, the phenylacetyl group in the molecule under
consideration is conjugated only slightly, if at all, with the CBC bridge, because this
molecule is propeller shaped (Hoekstra & Vos 1975). Although the acetyl group lies in the
plane of the phenyl ring attached to it, it remains separated from the CBC bridge.
In contrast, the nitro and ethylenic fragments in trans-4-nitrostilbene form the united
conjugation system. Such conjugation is a necessary condition for the whole-contour delocalization of an unpaired electron in arylethylene anion radicals. Whether this condition is
the only one or there is some interval of allowable strength for the acceptor is a question
left to future experiments.

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