Stereoelectronic effects in the reactivity of electrogenerated cation radicals of arylselenides

Article abstract of DOI:10.1016/S0022-328X(00)00529-5

The role of stereoelectronic effects in the electrochemical oxidation of arylselenides Se-substituted by a trimethylsilylmethyl group and those with substituents bearing a carbonyl group have been considered. Although the HOMO of these compounds is formed of p_z-type electrons of the ArSe moiety, the predominant contribution from the lone pair of Se makes the heteroatom more susceptible to direct electronic effects than to effects transmitted through the aromatic ring. The σ-p hyperconjugation and the interaction through space were shown to lower the charge localization on the reaction center and to stabilize cation radicals of these compounds, thus changing the control of the potential-determining reaction and promoting second order reactions.

Full text of DOI:10.1016/S0022-328X(00)00529-5

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 613 (2000) 220–230
Stereoelectronic effects in the reactivity of electrogenerated cation
radicals of arylselenides
V.V. Jouikov *, D.S. Fattakhova
Physical Chemistry Department, Kazan State Uni6ersity, 420008 Kazan, Russia
Received 9 March 2000; received in revised form 19 June 2000
Abstract
The role of stereoelectronic effects in the electrochemical oxidation of arylselenides Se-substituted by a trimethylsilylmethyl
group and those with substituents bearing a carbonyl group have been considered. Although the HOMO of these compounds is
formed of pz-type electrons of the ArSe moiety, the predominant contribution from the lone pair of Se makes the heteroatom
more susceptible to direct electronic effects than to effects transmitted through the aromatic ring. The s-p hyperconjugation and
the interaction through space were shown to lower the charge localization on the reaction center and to stabilize cation radicals
of these compounds, thus changing the control of the potential-determining reaction and promoting second order reactions.
© 2000 Elsevier Science B.V. All rights reserved.
Keywords: Electrooxidation; Cation radicals; Arylselenides; Silicon organic compounds; Reactivity; Stereoelectronic effects
reactions is determined by the nature of their HOMO
formed with the lone pair of selenium and p-electrons
of one aromatic ring [17–19]. In other words, the ArSe
moiety is the fragment which is mainly responsible for
the primary redox properties of arylselenides in oxida-
tion processes. Obviously, it is not the electronic struc-
ture of the second aryl which determines the occurrence
of second order reactions of diarylselenides because
there is not any significant conjugation between the
orbitals of selenium and the p-system of the second
aromatic ring [20]. The reason why the second order
potential-determining reactions are inherent to diarylse-
lenides is higher dissociation energy of the Se–Csp2
bond compared to the Se–Csp3 bond strength [21] and
the fact that the deprotonation of an aromatic ring is
very unfavorable.
The possibility for cation radicals of arylselenides
other than Ar2Se to react in second order reactions was
only shown for arylethynylselenides ArSeCꢀCAr [14]
having no acid a-protons, and for arylmethylselenides
ArSeMe, whose methyl group is not stable enough as a
leaving group [22] to enable direct cleavage of the
cation radical. In the last case, the rupture of the
Se–Csp3 bond in the cation radicals is very unfavorable,
and the only possible potential-determining reaction is
1. Introduction
Electrochemical oxidation of a large number of or-
ganic selenides has been studied to date [1–7]. When
the reversibility of electron transfer was proved electro-
chemically, electrogenerated cation radicals were shown
to react in first (rupture either of Se–Csp3 or Csp3–H
bond [8–11]) or second (disproportionation, dimeriza-
tion [1,12–14]) kinetic order ensuing bulk reactions. A
necessary and sufficient condition for the second order
reactions to occur is the presence in the molecule of
diorganylselenide of two aromatic groups which are
immediately bound to the selenium atom [5,12,13,15]. If
only one or no aromatic fragment is linked to selenium,
first order reactions of cation radicals (if not dissocia-
tive electron transfer at all) take place [8–10,16]. The
feature which accounts for this discrimination is a
specific combination of two factors: the electronic struc-
ture of the frontier orbitals of these compounds and the
energy of the weakest bond adjacent to selenium. The
ability of arylselenides to give electrons in oxidation
* Corresponding author. Present address: UMR 6510 Synthe`se et
Electrosynthe`se Organiques, Universite´ de Rennes I, Campus de
Beaulieu, 35042 Rennes, France.
E-mail address: jouikov@ksu.ru (V.V. Jouikov).
0022-328X/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00529-5

V.V. Jouiko6, D.S. Fattakho6a / Journal of Organometallic Chemistry 613 (2000) 220–230
221
the elimination of an acid methyl proton. Suppression
2. Results and discussion
of this reaction by introducing a strong proton donor
enables the cation radicals to develop their second
order reactions [9,16,22].
2.1. i-Effect
The role of steric and inductive effects of alkyls
attached to selenium [8,23], as well as the effect of
substituents in the aromatic ring [24] with respect to the
behavior of alkylarylselenides upon oxidation were
studied in terms of multiparameter correlation of their
Eox with −ES, RS, |* and |+ constants of sub-
stituents. However, the potential-determining reaction
in all these cases was always of first kinetic order.
Due to the larger size of selenium and relatively low
conjugation of its lone pair with the aromatic system
compared to analogous sulfur compounds, the elec-
tronic effects transmitted through the aromatic ring
(mesomeric and direct polar conjugation) do not
change the electrochemical behavior of arylselenides
dramatically. Since the selenium atom is seemingly
more affected by direct electronic interactions, the role
of stereoelectronic effects in the electrochemical reactiv-
ity of arylselenides and of their cation radicals has been
considered in this paper.
It is known that substituents containing silicon atom
at the b-position to a heteroatom (N, O, S) lower the
oxidation potential of a molecule by 0.05–0.7 V de-
pending on the energy difference between the interact-
ing orbitals [25–30]. This effect does not need the
assistance of any group conducting electronic effects
and affects the heteroatom immediately. Since the mag-
nitude of the b-effect is not affected by the nature of the
substitution at the aromatic ring1 but only by a relative
level of thus modified HOMO of the ArSe moiety, the
oxidation of the following b-silylated arylselenides with
different para-substituents was studied:
p-RꢁC6H4ꢁSeCH2SiMe3
1a R=MeO
1b R=Me
1c R=H
1d R=Cl
1e R=Br
Upon oxidation at an oxidized platinum rotating
disk electrode in CH3CN–0.01
M
Et4NClO4,
aryl(trimethylsilylmethyl)selenides 1a–1e behave like
aryl(trimethylsilylmethyl)sulfides [30]. Their voltam-
mograms (Fig. 1) show two oxidation steps (for non-
silylated ArSeAlk, there is a single two-electron wave).
The limiting current of the first wave is controlled by
diffusion (il/C=const; il/ꢀ1/2=const; n$1; RDlg(il)/
D(1/T)=6.2 kJ mol−1 which corresponds to the tem-
perature coefficient of the viscosity of the solvent;
DE/Dlg((id−i )/id)=60 mV). Using cyclic voltamme-
try, no signals of reduction of cation radicals were
detected up to the sweep rate 6=50 V s−1. However,
using commutative voltammetry at the rotating disk
electrode [31] at the switch frequency f=100 Hz, the
signals of reduction of cation radicals have been
recorded. These reduction currents disappear at about
1.5 V indicating further oxidation of the cation radicals
at these potentials. Characteristic dependences of E1/2
on lg(ꢀ) and on lg(C) (DE1/2/Dlg(ꢀ)$20 mV and
DE1/2/Dlg(C)$20 mV (Table 1)) indicate that the po-
tential-determining reaction (PDR) of cation radicals of
aryl(trimethylsilylmethyl)selenides 1a–1e is of second
kinetic order, in contrast to first order reactions of
cation radicals of non-silylated alkylarylselenides.
Fig. 1. Oxidation of arylselenide 1c (C=10−3 mol l−1, T=20°C) in
The second oxidation wave is not as well shaped as
the first one and tends do disappear when increasing
CH3CN–0.01 M Et4NClO4. (1) Cyclic voltammogram, w=0.5 V s−1
;
(2) and (3) are curves registered at a Pt rotating disk electrode, ꢀ=20
s
−1, (2) classical curve, (3) commutated curve with the connection
according to the second scheme; switch frequency f=100 Hz, capac-
itive current reduced.
1 Only if the substituent does not bring any sterical hindrance
which can affect this stereoelectronic effect.

222
V.V. Jouiko6, D.S. Fattakho6a / Journal of Organometallic Chemistry 613 (2000) 220–230
Table 1
Characteristics of the first wave of oxidation of silylated arylselenides RC6H4SeCH2SiMe3 at a Pt oxidized RDE in CH3CN–0.01 M Et4NClO4;
C=10−3 mol l−1, T=20°C, ꢀ=20 s−1
+
R
E1/2 (mV) a
n
DE/Dlg((id−i )/id) (mV)
DE/Dlg(ꢀ) (mV) b
DE/DlgC (mV) c
|
R
1a
1b
1c
1d
1e
690
790
890
920
910
1.2
0.9
1.0
0.9
1.1
60
60
60
70
75
18
20
20
20
22
22
20
24
19
20
−0.778
−0.311
0
+0.114
+0.15
a Versus Ag/0.01 M AgNO3.
b
93 mV.
95 mV.
c
either the rate of rotation of the disk electrode or the
potential sweep rate. The process responsible for this
wave was shown to be mainly the oxidation of a
diaryldiselenide resulting from the process at the poten-
tials of the first wave.
negative kinetic shift with respect to E0 [23] than that of
1c2.
A much more spectacular effect of these specific
interactions is seen through the alteration of the nature
of ensuing reactions of electrogenerated cation radicals.
The behavior of these species can be considered from
the following standpoint.
So the whole feature of the oxidation of b-silylated
compounds (E+C2+C1 scheme, Eqs. (1–3)) is quite
different from the oxidation of simple alkylarylselenides
which occurs according to a E+C1+E scheme and
involves two electrons in the overall process [16].
Since electron transfer in the course of electrooxida-
tion of b-silylated arylselenides follows a stepwise
mechanism, two points related to the peculiarities of the
electronic structure of these compounds arise here: first,
with respect to the oxidation potentials, and second,
with respect to the role of these factors in the stability
of the cation radicals. At the level of the effective
oxidation potential E1/2, these two factors partially
compensate each other. Indeed, an additional splitting
of the HOMO of the ArSe moiety due to the conjuga-
tion of the lone pair of selenium with the s C–Si bond
raises the level of the HOMO of ArSeCH2SiMe3 and
lowers the ionization potential of the molecule. This
effect is to be reflected in a decrease of the oxidation
potential. On the other side, reduced reactivity of the
cation radicals, caused by their stabilization by the
b-effect, will diminish a negative kinetic shift of the
effective potential E1/2 with respect to E0. Thus the total
effect of these intramolecular electronic interactions on
E1/2 will not be very large.
Some quantitative estimations can be made from the
consideration of oxidation of a non-silylated analog of
phenyl(trimethylsilylmethyl)selenide, iso-amylphenylse-
lenide. The electrochemical behavior of this compound
(one wave, n=2; Ep=1.089 V; DE1/2/Dlg((il−i )/il)=
120 mV; DE1/2/Dlg(ꢀ)=33 mV) is quite different from
the behavior of its silylated counterpart. The difference
in effective oxidation potentials of 1c and of iso-
amylphenylselenide (DEp=0.20 V) is consistent with
the difference in the HOMO levels of these compounds
which is 0.23 eV, especially taking into account that Ep
of iso-amylphenylselenide was subjected to a larger
Applying the principle of microscopic reversibility to
the potential-determining cleavage of a cation radical
AB +, and considering this cation radical as the transi-
’
tion state of the back reaction (formation of the cation
’
radical from the radical and the cationic fragments, B
and A+, Fig. 2), one can apply the general expression
of perturbation theory for a two-center interaction (ad-
mitting that kinetic shifts do not affect remarkably the
relative energy profiles of the components on the reac-
tion coordinate). The energy of interaction in this case
includes contributions from two terms, covalent (or-
bital) and coulombic (charge), and their ratio deter-
Fig. 2. Correlation of frontier orbitals in first and second order
potential-determining reactions of cation radicals AB +. Left: charge-
’
controlled cleavage of the cation radical; right: fragmentation of a
dication AB2+ preceded by an orbital-controlled disproportionation
of cation radicals. The orbital energy corresponds to the direction of
ordinate axes so the presentation of orbitals is turned upside down.
Horizontal dotted line designates the energy level which delimits
charge- and orbital-controlled interactions of cation radicals.
2 Apparently, at the concentration C=1 mmol l−1 the rate of the
second order potential determining reaction is lower than that of the
first order reaction, as was shown for the oxidation of arylmethylse-
lenides [11]; so the total kinetic shift of the effective potential Ep is
less for the second order reaction.

V.V. Jouiko6, D.S. Fattakho6a / Journal of Organometallic Chemistry 613 (2000) 220–230
223
Table 2
Vertical ionization potentials (IP) of b-trimethylsilylsubstituted
arylselenides p-R–C6H4SeCH2SiMe3 (eV)
(3)
Compound
IP1
IP2
IP3
The reaction with the rate constant k1 is current-deter-
mining (CDR) for the second wave of the oxidation of
selenides 1a–1e.
The oxidation potentials of aryl(trimethylsilyl-
methyl)selenides are generally lower than those of se-
lenides without a b-trimethylsilyl substituent, but the
difference is not so remarkable as in the case of ho-
mologous sulfides and ethers [26]. The reason for this is
a less effective overlapping of diffuse p-type lone pair of
selenium with the C–Si s-bond.
Half wave oxidation potentials E1/2 of 1a–1e were
found to correlate well with the |+R constants of sub-
stituents in the aromatic ring (see Eq. (3) in ref. [30]). The
positive sign of the polarographic reaction constant
(z=0.253 V) is in agreement with the nature of the
effects of substituents: donor substituents raise the level
of the HOMO thus facilitating the removal of an electron
upon oxidation. The absolute value of the constant z is
close to values of the constants of reaction series of
compounds with similar types of HOMO [32]. Since the
effective reaction constant z has a complex nature and
besides a thermodynamic term (zp) might contain a
contribution from the potential-determining chemical
reaction [33], it is necessary to elucidate corresponding
thermodynamic and kinetic terms separately by some
independent physicochemical method. In order to obtain
characteristics of frontier orbitals determining redox
properties of selenides 1a–1e and to characterize in-
tramolecular electronic effects in these compounds, their
photoelectron spectra were studied. The experimental
data are collected in Table 2.
1a
1b
1c
1e
7.58
7.68
7.77
7.81
9.12
9.02
9.03
9.04
10.00
10.62
10.06
mines the type of control of the potential-determining
reaction. The harder the reaction center, the larger the
charge term and more likely the charge-controlled reac-
tions are. Usually, these reactions are of first or pseudo-
first order (cleavage or the reaction with macro
components of the solution). When the separation of the
orbital energy levels is too large to control the process
efficiently (Fig. 2), another pathway becomes preferable.
Now, it first involves an orbital-controlled dispropor-
tionation of two cation radicals resulting in the starting
molecule AB and a dication AB2+ (the covalent term of
the energy of interaction is maximal since the SOMOs of
cation radicals are degenerated). A large gain in energy
in the ensuing reaction of the dication is a driving force
of the entire process. Therefore, one can imagine an
arbitrary energy barrier separating charge and orbital-
controlled interactions. The cation radicals with the
energy below this barrier disproportionate whereas those
species, whose energy is higher undergo direct cleavage.
So for the direct cleavage, electrogenerated cationic
species must have enough energy to assure the separation
of orbitals of the species resulting from the cleavage to
be not too large to be ineffective. The cleavage of the
cation radicals of the reaction series 1a–1e and of
alkylarylselenides gives ArSe+ fragments (A+), which
First ionization potentials IP1 of these compounds
show a fairly good correlation (even with such a small
number of points) with the same type of constants of
substituents as the oxidation potentials Ep:
’
are similar for both reaction series, and alkyl radicals (B )
which have approximately similar energy. Obviously, the
stabilization of the cation radicals of 1a–1e by the
b-effect, due to an additional conjugation of the reaction
center with electrons of the bonding C–Si s-orbital,
lowers the energy of these species thus making their direct
cleavage unfavorable. The process now follows an E+
C2+C1 scheme. Although the main products of an
exhaustive electrooxidation (Q$1.1 F mol−1) of se-
lenides 1a–1e are corresponding diaryldiselenides, i.e.
formally products of the Se–Csp3 bond cleavage, the
mechanism is more complex and includes orbital-con-
trolled dimerization of electrogenerated cation radicals
giving a dimeric dication which then cleaves to give di-
aryldiselenide and two a-silylcarbocations, Me3SiCH+2 :
(1)
IP1=(0.24990.013)|+R +7.76890.005
(4)
n=4, r=0.997, sd=0.009, P=0.0027
The obtained reaction constant of electron transfer
(zIP=0.249 eV) reflects the susceptibility of the energy
of HOMO of the reaction series 1a–1e to electronic
effects of para-substituents in the aromatic ring. The
value of this constant is practically the same as that of
the polarographic reaction constant. It means that the
latter does not contain any kinetic contribution and
characterizes only the electron transfer step or, more
likely, that the kinetic shift of the effective potential
(DEk=E1/2−E0) is practically constant and independent
of the electronic effects transmitted through the aromatic
ring from para-substituents. It is to be noted
(2)

224
V.V. Jouiko6, D.S. Fattakho6a / Journal of Organometallic Chemistry 613 (2000) 220–230
that the complex polarographic reaction constant of
the oxidation of methylarylselenides ArSeMe (z=
0.282 V) does contain some kinetic term (the reaction
constant of the potential-determining reaction zk=1.1
[11]). This fact is in agreement with the charge con-
trolled deprotonation of the cation radicals being
more sensitive to the electronic effects of para-sub-
stituents than the orbital controlled dimerization of
the cation radicals of the reaction series 1a–1e.
Analysis of the electronic structure of these com-
pounds reveals several types of intramolecular interac-
tions which can affect the HOMO responsible for the
first ionization potential IP1 (Fig. 3). The main factor
is a conjugation of one of two lone pairs of selenium
(evidently, that of pz-type) with the p-system of the
aromatic ring. This interaction raises the level of the
HOMO compared to the HOMO level of benzene,
like in the molecules of alkylaryl and diaryl selenides
[20]. On the other side, the lone pair of selenium can
interact with the filled orbital of the Si–C s-bond of
the CH2SiMe3 group according to a a-p conjugation
mechanism. This interaction also results in an increase
in the HOMO level.
Fig. 3. Diagram of the correlation of HOMOs of molecular fragments
contributing to the HOMO of PhSeCH2SiMe3. (a) [38], (b) [39], (c)
[40,41].
Considering the PE spectrum of 1b, one can see
that the second ionization band is not simple (Fig. 4)
which might be caused by some intramolecular elec-
tronic interactions involving the second lone pair of
selenium. In order to clarify this point, the electronic
structure of this compound has been modeled using
both semi-empirical PM3 calculations [34,35] and ab
initio calculations with a 6-31G* basis set [36,37].
These calculations showed that the minimum of
potential energy of the system corresponds to a
conformation with the torsion angle ÚCsp2(2)–
Csp2(1)–Se–Csp3 (€) equal to 20°, which provides the
optimal overlapping of the pz orbital of Se (np) with
the p-electrons of the benzene ring. Plotting a 2D
diagram of potential energy as a function of two tor-
sion angles of rotation about the Csp2–Se and the
Se–Csp3 bonds (Fig. 5) revealed a steady minimum
with the orthogonal arrangement of the HOMO-form-
ing components (np electrons of Se and pz-orbitals of
the aromatic ring) and the ÚCsp2(1)–Se–Csp3–Si an-
gle (ƒ) equal to 67°. Obviously, these parameters are
most favorable for the effective s-p conjugation of
the secondary HOMO-forming fragment (CH2SiMe3
group) with the orbitals of the ArSe moiety. Unfavor-
able configuration is characterized by the values
€$90° and ƒ$127° and corresponds to the overlap
of the s-type lower-lying lone pair of selenium with
the p-system of the aromatic ring (Fig. 5).
Fig. 4. Photoelectron spectrum of p-MeC6H4SeCH2SiMe3. Clearly
seen is the complex nature of the second ionization band.
The complex character of the second band in the
PE spectrum of 1b might result from the superposi-
tion of two bands. A PM3 modeling of inner occu-
pied orbitals of this molecule allows us to shed some
light on the nature of this band (Fig. 6). The theoreti-
cal value of the IP1 of 1b (Table 3) is shifted with
respect to the experimental value by D=8.38−
7.68=0.7 eV. Taking this value into account, an ex-
perimental value of IP2 in the first approach must be
Fig. 5. Diagram of MMX energy of the molecule 1b as a function of
two torsion angles: (€) ÚCsp2(2)–Csp2(1)–Se–Csp3 and (ƒ)
ÚCsp2(1)–Se–Csp3–Si.

V.V. Jouiko6, D.S. Fattakho6a / Journal of Organometallic Chemistry 613 (2000) 220–230
225
9.53−0.7=8.83 eV, which is in good agreement with
the energy of the left shoulder of a complex second
band in the PE spectrum (Fig. 4). The ionization of the
ns orbital of Se in this case should occur at 9.83−
0.7=9.13 eV, which actually corresponds to the right
shoulder of this absorbance band. Thus the IP1 of 1b
corresponds to the ionization of the HOMO which is
built up with the p-electrons of the unsymmetric c2
orbital of the aromatic ring, np-electrons of Se and the
electrons of the bonding Csp3–Si s-orbital, whereas the
ionization of the unsymmetric c3 orbital and the ns
orbital of selenium occurs with the overlapping of the
absorbance bands giving a complex signal with an
average value of IP2=9.02 eV.
It is noteworthy that according to the PM3 calcula-
tions, the next after the HOMO bonding orbital of
methyl and bromo (Fig. 7) derivatives practically corre-
sponds to the unsymmetrical c3 orbital of the aromatic
ring whereas in the chloro compound there are two
close lying mixed orbitals (Fig. 8). Obviously, larger
conjugation of the lone pair of chlorine with the p-sys-
tem of the aromatic ring causes a redistribution of
contributions from corresponding fragments forming
this orbital.
Fig. 6. Orbital correlation of the molecular fragments upon the
formation of 1b and its lower-lying occupied MOs according to PM3
calculation. To fit all energies to the same scale, orbital levels of
toluene have been calculated too. For the MeSeCH2SiMe3 molecule,
the same geometry has been taken as in the corresponding fragment
in Me–C6H4SeCH2SiMe3.
2.2. Field effect (interaction through space)
Table 3
Energies of filled orbitals (IP, eV) for the molecules R–
C6H4SeCH2SiMe3 according to the PM3 calculations
Among different neighboring groups which can affect
the electrochemical behavior of organo-element com-
pounds (e.g. amino [25], methylthio [42,43], sulfur, sele-
nium [44] or tellurium [45]), the carbonyl group
presents particular interest because it does not undergo
oxidation within the range of electroactivity of arylse-
leno compounds. So the oxidation of some alkylarylse-
lenides containing a carbonyl group attached either to
the aromatic ring (2a, 2b) or to the alkyl group (3a–3e)
have been considered.
Compound
IP1
IP2
IP3
9.830
IP4
1b
1c
1d
1e
8.378
8.541
8.540
8.597
9.534
9.566
9.779
9.803
10.320
10.346
9.958
9.832
10.018
Being oxidized under the conditions described above,
these alkylarylselenides show two oxidation waves (see
Fig. 2 in ref. [22]), the first is controlled by diffusion
(il/ꢀ1/2=const, il/C=const; n$1; RDlg(il)/D(1/T)=
6.2 kJ mol−1; DE/Dlg((id−i )/id)=60 mV). Their half
wave potentials E1/2 shift towards more positive poten-
tials by 20 mV upon an increase in the rotation speed
by a factor of 10, except for the compound 3e, for
which this value is 30 mV. Thus the primary oxidation
steps of 2a, 2b and 3a–3d follow an E+C2 mechanism:
Fig. 7. Correlation of molecular orbitals upon the formation of
BrC6H4SeCH2SiMe3.

226
V.V. Jouiko6, D.S. Fattakho6a / Journal of Organometallic Chemistry 613 (2000) 220–230
a reversible electron transfer with a second order PDR
of cation radicals formed. By experiments with the
addition of pyridine this reaction was shown to be
disproportionation, as in the case of the oxidation of
diphenylselenide [46].
The potentials of the second wave of alkylarylse-
lenides in the presence of CF3COOH (i.e. when electro-
generated cation radicals follow a second order reaction
and the oxidation of secondary radicals does not inter-
fere with the current of their oxidation) have practically
the same values as the potentials of disappearance of
the reduction signals on commutated voltammograms.
Therefore, the second wave of 2a, 2b and 3a–3e can be
attributed to the oxidation of cation radicals generated
at the first wave potentials.
Second kinetic order potential-determining reactions
Fig. 10. Cation radical o-MeSeC6H4COOMe+ according to 6-31G*
’
ab initio calculations.
are not typical for alkylarylselenides bearing no car-
bonyl group and even for selenide 3e which has a
carbonyl group. For these compounds, the cation radi-
cals undergo a first order reaction. Evidently, the main
feature which distinguishes selenides 2a, 2b and 3a–3d
from all the rest and determines the second order of
PDR is that Se and O atoms in these compounds are
separated by a chain comprising three or four other
atoms, allowing free rotation about included bonds.
This chain assures a maximal approach of the carbonyl
oxygen to selenium. In the cation radical, the latter
becomes positively charged and its attractive interac-
tion through space with the oxygen carrying partial
negative charge can provide an additional stabilization
to this species. Molecules 2a and 2b are planar and
rigid, whereas 3a–3e (Fig. 9) allow a free rotation
enabling cation radicals to arrange the distance
Se(l+)···O(l−) at a minimum.
Fig. 8. Correlation of molecular orbitals upon the formation of
ClC6H4SeCH2SiMe3.
In a first approach, the energy of the interaction
through space stabilizing cation radicals 2a, 2b and
3a–3e can be estimated in terms of a purely electro-
static model. For this estimation, charge density distri-
bution and geometry of the molecule of
o-methylselenobenzene acid 2a and its cation radical
have been calculated ab initio using the SCF/3-21G*
basis set [37].
Charge distribution on the SeMe fragment is practi-
cally the same in the neutral molecule and in the
molecule of selenoanisole: Se(+0.37)–C(−0.12) and Se(+
0.35)–C(−0.14) respectively. More than three quarters of
the total positive charge in the cation radical was
shown to be localized on the Se atom. In the cation
radical, the Se and O atoms are closer to each other
with respect to the neutral molecule (Fig. 10) and the
Se···O distance is comparable with the length of a
Fig. 9. Cation radical of 3a according to a PM3 calculation. A
trans-isomer has been studied.
,
stretched chemical bond (l=2.56 A). The participation

V.V. Jouiko6, D.S. Fattakho6a / Journal of Organometallic Chemistry 613 (2000) 220–230
227
Fig. 11. Rotation about Csp3–C(O) and the Se–Csp3 bonds favoring shortest Se···O interatomic distance l (torsion angle €) and frontal orientation
of interacting orbitals (torsion angle ƒ).
of the n-electrons of oxygen in the interaction with
positively charged Se atom results in the change of the
CꢂO bond multiplicity index and elongation of this
The ethyl ester of 2-phenylselenoacetic acid 3e also
has a carbonyl group which can approach selenium but
its voltammetric behavior is fairly different. The
voltammogram of this compound shows a single wave,
the current of which is controlled by diffusion and
corresponds to the transfer of two electrons. At low
concentrations, the wave slope is close to 60 mV, and
the value of DE1/2/Dlg(ꢀ) is 3093 mV. These data
allow us to conclude that a reversible electron transfer
takes place followed by a charge-controlled reaction of
first order (E+C1+E mechanism), as for the oxida-
tion of the majority of alkylarylselenides. Seemingly,
the interaction through space in 3e is not strong enough
to stabilize the cation radical and to change its reactiv-
ity. In addition, the a-proton in the cation radical of 3e
is more acid, than the corresponding proton in
PhSeMe, because of the proximity of an acceptor
COOEt group, so the deprotonation of the cation
radical is favored kinetically. The last reason seems to
be the major one because fixed orientation of the
oxygen lone pair along with the possibility of free
rotation about the Csp3–C(O) and the Se–Csp3 bonds
enable almost frontal and hence the most efficient
overlapping of interacting orbitals (Fig. 11) so the
through space stabilization of this species must be very
efficient.
An attempt to increase the energy of the interaction
through space and thus to stabilize the cation radical
has been made by decreasing the polarity of the media.
A 25:75 mixture of toluene and acetonitrile has a static
dielectric permittivity m=16 so in such a solvent, the
energy of interaction of two charges must be two times
higher than in acetonitrile alone (m=36). Indeed, being
oxidized in this binary solvent, 3e shows a different
slope of the dependence DE1/2−Dlg(ꢀ) which becomes
2295 mV indicating second order of the potential-de-
termining reaction. Apparently, the stabilization energy
is now large enough to bring the cation radical of 3e
below the range of charge-controlled cleavage (see Fig.
2).
,
bond (1.207 and 1.231 A in neutral molecule and in the
cation radical, respectively). Besides, the torsion
ÚCsp2–Csp2–Se–Csp3 and the diedral ÚCsp2–Csp2
–
CꢂO angles change (by +4.2° and −13.6°, respec-
tively) towards the flatening of the Se–Csp2–Csp2–CꢂO
fragment.
For the purely coulombic model of interaction, the
interaction energy stabilizing the cation radical of 2a in
acetonitrile and related to 1 mol is
qSeqOe2
E= −
36m0l
1.03(−0.67)(1.6×10−19)26.022×1023
36×8.85×10−12×2.56×10−10×4.184
= −
=31.17 kcal mol−1
(5)
The decrease of the cation radical total energy by this
value brings the system below the barrier delimiting
orbital and charge controlled potential-determining re-
actions of such species (Fig. 2). Of course, one has to be
careful when using this value because its derivation
presumes some assumptions (besides the very use of
data obtained by quantum chemical calculations for the
estimation of the interaction energy):
1. The covalent term of the total energy of the two-
center interaction [47] has not been taken into ac-
count though the interatomic distance l (Se···O) is of
the order of a stretched s-bond. In addition, even in
spite of becoming positively charged in the cation
radical, the Se atom still has a large volume, hence
a point charge assumption is not quite correct.
2. The polarization of electron shells of the interacting
atoms has not been taken into account.
3. Interactions with the supporting salt ions were not
considered.
In principle, the last factor must decrease the energy
of coulomb interactions, although this effect is partially
compensated by neglecting the contribution from the
covalent interactions.
It is interesting that oxidative behavior of selenochro-
manone, also carrying a carbonyl group at the ortho-
position to selenium but oriented in the opposite

228
V.V. Jouiko6, D.S. Fattakho6a / Journal of Organometallic Chemistry 613 (2000) 220–230
direction so as it cannot interact with the selenium
atom, is typical for alkylarylselenides (n=2, DE/
Dlg((il−i )/il)=100 mV, DE1/2/Dlg(ꢀ)=31 mV). It is
also seen that its oxidation potential (E1/2=1.155 V
[48]) is more positive than the potentials of 2a and 2b,
affected by field effect3.
When first order ensuing reactions of electrogener-
ated cation radicals are unfavorable for some reason,
dimerization or disproportionation takes place. These
two reactions are not kinetically competitive since they
have a common transition state [47]. In some cases
(b-silylated arylselenides) this transition state resolves
through the dimerization pathway, in others (arylse-
lenides bearing a substituent with the carbonyl group
interacting through space with the reaction center) it is
disproportionation which takes place. The charge term
of the orbital controlled formation of such a transition
state is negligibly small. Thus the action of both of
these stereoelectronic effects leads to similar results and
consists in stabilization of primary cation radicals, low-
ering electrostatic repulsion of these species and thus
favoring their latent second order reactions. Of course,
these effects, being not very strong, can intervene only
in the cases when the energy profiles of the two reaction
pathways are close on the reaction coordinate.
3. Conclusions
The electrochemical behavior of arylselenides can be
summarized as follows. Upon oxidation, primary redox
properties of these compounds are determined by a
frontier orbital, HOMO, which is built up with p-elec-
trons of the unsymmetric c2 orbital of the aromatic
ring and of n-electrons of the pz-orbital of the het-
eroatom (Fig. 3). For this reason the redox properties
of Ar2Se and ArSeAlk are very similar because it is the
same fragment ArSe which is responsible for them4. In
a cation radical, the localization of the positive charge
on Se is very high and this fact along with the Se–Csp3
and (Se)C–H bonds being weaker compared to the
Se–Csp2 bond make the cation radicals of Ar2Se and
ArSeAlk behave differently: those of Ar2Se undergo
orbital-controlled second order reactions whereas those
of ArSeAlk cleave in first order charge-controlled reac-
tions. For arylselenides, the match between p and n
levels is less favorable for an effective conjugation than
in corresponding sulfides so the HOMO of seleno
derivatives is less sensitive to electronic effects acting
through the aromatic ring. The decrease of the conjuga-
tion of the heteroatom with the aromatic ring and an
increased role of its lone pair in the formation of the
HOMO is clearly seen through the decrease in the
polarographic reaction constants z. As the transmission
of the electronic effects via the ring decreases along
with the increasing contribution from n-electrons of the
heteroatom, z decreases (z=0.346 [49] and 0.275 [50]
for ArSMe and ArSCH2SiMe3 and z=0.282 and 0.253
for ArSeMe and ArSeCH2SiMe3, respectively). So in
spite of the b-effect being the weakest for arylselenides,
the selenium atom is more sensitive to direct electronic
effects namely because of this weakened p-n con-
jugation.
Here, only intramolecular interactions were consid-
ered, but intermolecular interactions (with an external
nucleophile or with supporting electrolyte anions) must
also be an important factor in the electrochemical
reactivity of alkylarylselenides.
4. Experimental
Voltammetric measurements were carried out using a
polarographic complex including a PU-1 polarograph
with a microcomputer BK-0010 connected via a 1113-
PV1A analog-to-digital converter. Rotation velocity in
the rotating disk experiments was controlled by a FP-37
numeric counter. A Pt oxidized working electrode in a
three-electrode configuration was used. The auxiliary
electrode was a Pt wire and the reference electrode was
Ag/0.1 M AgNO3 in CH3CN. The working electrode
was carefully washed in acetonitrile and rinsed in di-
ethyl ether before each run.
Photoelectron spectra were registered using a spec-
trometer ES-3201 with a hemispherical analyzer. The
photon source was a resonance line of He(I). The
precision of measurements of ionization potentials was
as good as 0.02–0.04 eV, depending on the overlapping
of absorbance bands.
b-Silylated arylselenides 1a–1e were synthesized ac-
cording to described procedures [55,56,50]. 2a was pre-
pared according to [57,58], its methyl ether 2b was
obtained by methylation of 2a by MeI in a methanol
solution of NaOH [50]. Selenides 3a–3c were prepared
by electrochemical oxidation of Ph2Se2 in acetonitrile in
the presence of cyclohexene, hexene-1 and heptene-1,
respectively, according to [59].
3 In principle, the presence of an electron withdrawing substituent
at the ortho-position to Se in 2a and 2b must increase their oxidation
potentials in two ways: by acceptor mesomeric effect and by dis-
turbing the conjugation of the heteroatom with the p-system due to
the ortho-repulsion of two neighboring groups.
4 This is proven by IP, dipole moments and NMR chemical shifts
[14,51,52], oxidation potentials E1/2 [1,9–14,17,24] and the correlation
of E1/2 of large numbers of alkylarylselenides and h6 of their charge
transfer complexes with electrophilic s+ constants [8,11,14,17,24]. In
addition, nucleophilic reactions of cationic intermediates electrogen-
erated from arylselenides occur at the heteroatom [1–5,16,18,53] or at
the a-carbon [9,54] to selenium.
1-Phenylseleno-2-acetamidocyclohexane, 3a. M.p.
148°C. C14H19NOSe. Anal. Calc.: C 56.77, H 6.42.
Found: C 56.85; H 6.4%. IR: w(N–H) s 3300 cm−1
;

V.V. Jouiko6, D.S. Fattakho6a / Journal of Organometallic Chemistry 613 (2000) 220–230
229
w(CꢂO) s 1625 cm−1, w(C–H) m 715 cm−1. NMR (100
MHz, CDCl3, TMS), l ppm: 7.6 (d 2H), 7.22 (m 3H),
5.95 (broad s 1H), 3.32 (m 1H, J=4.06 Hz), 2.8 (m 1H,
J=3.99 Hz), 2.2 (m 2H), 1.96–1.85 (m 2H), 1.76 (s
3H), 1.6–1.3 (m 4H). Mass, m/e (%): 297 (M+, 3), 238
(30), 236 (16), 157 (20), 140 (22), 129 (4), 98 (90), 81
(100), 60 (48).
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1952.
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E.G. Kataev, Zh. Obsch. Khim. 56 (1986) 822.
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6045.
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Zh. Obsch. Khim. 55 (1985) 2050.
1-Phenylseleno-2-acetamidohexane, 3b. M.p. 73°C.
C14H21NOSe. Anal. Calc.: C 56.19, H 7.02. Found: C
56.21; H 6.95%. IR: w(N–H) s 3300 cm−1; w(CꢂO) s
1625 cm−1, w(C–H) s 725 cm−1. NMR (100 MHz,
CDCl3, TMS), l ppm: 7.6 (d 2H), 7.2 (m 3H), 6.6 (d
1H), 3.98 (m 1H, J=4.01 Hz), 2.98 (d 2H, J=4.0 Hz),
1.75 (s 3H), 1.6–1.3 (m 6H), 1.12 (t 3H). Mass, m/e
(%): 299 (M+, 4), 240 (18), 238 (9), 197 (4), 157 (12),
142 (70), 100 (20), 86 (100).
1-Phenylseleno-2-acetamidoheptane, 3c. M.p. 81°C.
C15H23NOSe. Anal. Calc.: C 57.51; H 7.35. Found: C
57.56; H 7.28%. IR: w(N–H) s 3300 cm−1; w(CꢂO) s
1625 cm−1, w(C–H) s 725 cm−1. NMR (100 MHz,
CDCl3, TMS), l ppm: 7.6 (d 2H), 7.22 (m 3H), 6.1 (d
1H), 3.93 (m 1H, J=4.0 Hz), 2.8 (d 2H, J=3.98 Hz),
1.9 (s 3H), 1.72–1.56 (m 8H), 1.2 (t 3H). Mass, m/e
(%): 313 (M+, 4), 254 (14), 189 (4), 172 (6), 156 (64),
142 (10), 114 (16), 100 (100), 91 (22).
3d and 3e were prepared by reaction of PhSeH with
3-chloropropionic acid and ethyl ether of 2-chloroacetic
acid, respectively, according to Ref. [60].
Analytical grade acetonitrile was additionally
purified according to [61] and kept over molecular
sieves 4A. Et4NBF4 was obtained by neutralization of
40% aqueous solution of Et4NOH with 7.8 M HBF4
then recrystallized from EtOH, dried and kept over
P2O5.
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