Thermodynamics and kinetics of homolytic cleavage of carbon-oxygen bonds in radical anions obtained by electrochemical reduction of alkyl aryl ethers

Article abstract of DOI:10.1039/b110994d

The properties and the reactivity of the radical anions of 4-cyanophenyl alkyl ethers and naphthyl alkyl ethers have been determined by electrochemical methods. Under electrochemical conditions homolytic dissociation is the only observed process. Cyclic voltammetry studies lead to the conclusion that this process is a stepwise one, the initially produced radical anion cleaving by a slow first order reaction followed by a second electron transfer in a DISP1 mechanism. A Marcus type relationship between the cleavage rate constants and the standard free energy of the reaction leads to an intrinsic barrier in the range of 0.7 to 0.8 eV. The analysis of the intrinsic barrier values indicates that solvent organisation represents a modest contribution, the bond dissociation energy of the radical anion (structural contribution I being the main factor in the total barrier. Previously unknown bond dissociation energies of naphthyl ethers have been estimated using the correlations established in this work.

Full text of DOI:10.1039/b110994d

View Article Online / Journal Homepage / Table of Contents for this issue
Thermodynamics and kinetics of homolytic cleavage of carbon–
oxygen bonds in radical anions obtained by electrochemical
reduction of alkyl aryl ethers
2
Claude P. Andrieux,a Maria Farriol,b Iluminada Gallardo*b and Jordi Marquetb
a Laboratoire d’Electrochimie Moléculaire, Université Paris 7 Denis Diderot 2, place Jussieu,
75251 Paris Cedex 05, France
b Departament de Química, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Barcelona,
Spain. E-mail: lluminada.Gallardo@uab.es; Fax: ϩ 34 93-581-2920
Received (in Cambridge, UK) 4th December 2001, Accepted 20th February 2002
First published as an Advance Article on the web 20th March 2002
The properties and the reactivity of the radical anions of 4-cyanophenyl alkyl ethers and naphthyl alkyl ethers have
been determined by electrochemical methods. Under electrochemical conditions homolytic dissociation is the only
observed process. Cyclic voltammetry studies lead to the conclusion that this process is a stepwise one, the initially
produced radical anion cleaving by a slow first order reaction followed by a second electron transfer in a DISP1
mechanism. A Marcus type relationship between the cleavage rate constants and the standard free energy of the
reaction leads to an intrinsic barrier in the range of 0.7 to 0.8 eV. The analysis of the intrinsic barrier values indicates
that solvent organisation represents a modest contribution, the bond dissociation energy of the radical anion
(structural contribution) being the main factor in the total barrier. Previously unknown bond dissociation energies
of naphthyl ethers have been estimated using the correlations established in this work.
similar radical anions, those of 3-nitrobenzyl chloride and
Introduction
3-chloroacetophenone,11 may likewise be interpreted as the
There are extensive reports on the mechanisms of carbon–
outcome of a competition between the Lewis acid solvation of
halogen bond cleavage by electrochemical means.1,2 The initial
the developing halide ion and of the negatively charged oxygen
electron transfer to a molecule leads either to a radical anion
atom in the initial state. The kinetics of cleavage of radical
that cleaves to produce a radical and an anion in a stepwise
anions of α-substituted acetophenones3d,12 is governed by
process, or directly to a radical and an anion in a concerted
solvent reorganization and in this case the contribution of
process. Both the strength of the carbon–halogen bond and the
molecular structure (related to bond breaking) is negligible.
leaving group ability of the resultant anion play important roles
There are extensive reports on the mechanisms of carbon–
in determining the preferred process.3
Particularly revealing is the possibility of passing, in border-
halogen bond cleavage by electrochemical means,1,2 and signifi-
cant contributions to the mechanistic knowledge on mesolytic
line cases, from one mechanism to the other.1,2,4 The dynamics
cleavage of a C–C bond in diphenyletane derivatives,7a–d cation
of concerted electron transfer–bond breaking reactions (called
radicals of tert-butylated NADH analogues,7c the C–S bonds in
dissociative electron transfer) may be modelled on the frame-
sulfides,7f,13 the S–S bond in diaryl sulfides,14 the C–O bond in
work of Marcus’s theory,5 as proposed by Savéant.6
aryloxyacetophenones12,15 and the O–O bond in perbenzoates.16
The cleavage reaction of radical ions can be classified as
homolytic [eqn. (1a)] when the unpaired electron is located on
A rather different situation exists for the corresponding C–O
bond fragmentations in ethers. Thus, even though preparatively
the leaving group and the cleavage leaves the charge mainly
useful examples of carbon–oxygen bond cleavage in ethers have
in the same region it was in the radical ion;7 or heterolytic
been reported,17 very few chemical18 or electrochemical,19
[eqn. (1b)] in which there is “regioconservation” of the spin
mechanistic studies exist and the available kinetic data are
density.8 Savéant has described the heterolytic bond cleavage
(where the unpaired electron initially resides in an orbital
very scarce. This is probably due to the inertness of the
carbon–oxygen bond in ether vs. the carbon–halogen bond in
organic halides. It is sometimes necessary to insert an electron-
attracting group on the organic moiety to make the reduction
that does not belong to the leaving group) as an intramolecular
electron transfer1,8 in which the electron in an orbital centred on
A is transferred to B with concerted bond breaking.
potential of ethers more positive; however, this in turns stabil-
izes the resulting radical anion to preclude rapid unimolecular
fragmentation.20
(1a)
The two kinds of ion fragmentations reported in the
literature7,9–19,21 and, on the other hand, the radical anion coup-
ling reactions (that are components of SRN1 reactions21,22) are
(1b)
formally heterogenic processes [the reverse of eqn. (1b)]. Alkyl
aryl ethers can, in principle, show both types of cleavage, and in
In the particular case of bifunctional systems,9 passage from
one mechanism to the other is possible.
The influence of the reaction medium on the kinetics of
radical anion cleavage has been the object of several investi-
gations.10 The results of a recent investigation on the depend-
ence of the cleavage rate constant upon the solvent of two
fact, depending on the conditions, both can be observed.17,18
Thus, chemical reduction with alkali metals in apolar solvents
leads to dealkoxylation (intramolecular electron transfer),
whereas in more polar solvents, dealkylation is observed
(homolytic dissociation, the unpaired electron initially resides
in an orbital that belongs to the leaving group). Interestingly
DOI: 10.1039/b110994d
J. Chem. Soc., Perkin Trans. 2, 2002, 985–990
This journal is © The Royal Society of Chemistry 2002
985

View Article Online
enough, typical electrochemical conditions (DMF, tetra-
alkylammonium salts as supporting electrolyte) always produce
the homolytic dissociation.23 In the present paper, to carry out a
complete mechanistic study on the electrochemical homolytic
dissociation of alkyl aryl ethers establishing their mechanistic
details (concerted vs. stepwise; main factors in the kinetic
barrier) and the usefulness of Savéant’s model6,8 in homolytic
dissociations, this study has been carried out with two series
of ethers, alkyl 4-cyanophenyl ethers and alkyl naphthyl ethers;
as explained before, these substrates are convenient for the
electrochemical study.
However, when increasing the scan rate, the oxidation peak Ox2
disappears and another oxidation wave, Ox1, is obtained. At
high scan rates, only the reversible couple, R1/Ox1, appears (Fig.
1b). For 1j, in addition to the waves corresponding to reduction
R1 and oxidation Ox2, it is possible to observe—at any scan
rate—another irreversible oxidation wave at ϩ1.1 V, attribut-
able to the oxidation of the piperidine25 present in the aliphatic
part of the initial compound. Similar cyclic voltammetric
behaviour is obtained for the alkyl naphthyl ethers 2a–2j (Figs.
1c and 1d). At low scan rates, the oxidation peak Ox2Ј appears
approximately at ϩ0.02 V.26
Analysis of the peak intensity, at low and high sweep rates,
shows a two-electron process for low sweep rates and a one-
electron process at high sweep rates (by comparison with the
reduction of p-toluonitrile in the same medium). The shape of
voltammograms (peak width) suggests a mixed kinetic control
by electron transfer and chemical reaction for the R1 reduction
curve.27 The peak potential is not concentration dependent (in
the range 1–10 mM) and the variation of the peak potential
with the scan rate is in the range 30–60 mV by unit log scan rate,
for low scan rates.27 For example, for 1a in the range 0.1 to 1 V
sϪ1 the slope is 30 mV. Therefore, we can conclude that the
initially produced radical anion reacts following a slow first
order reaction pathway leading to a second electron transfer
following a DISP1 mechanism.28
The cyclic voltammetric data are summarised in Table 1. The
values of EЊ (standard potential of reversible couple R1/Ox1),
k (first order rate constant) and ks (heterogeneous electron
transfer rate constant) are determined by simulation of the
experimental curves using DIGISIM® software.29
Results and discussion
Cyclic voltammetry
Fig. 1 shows voltammograms typical of 1a–1j in DMF–0.1 M
Controlled potential electrolysis
Electrolysis was carried out until 2F at a potential slightly more
negative than the peak potential for each compound solution.
The electrolysis was carried out at potentials close to electrolyte
background reduction, therefore the initial compound is not
totally electrolysed. The current efficiency was less than 100%
due to partial reduction of nBu4NBF4 to tributylamine. In a
blank experiment with electrolysis at Ϫ2.6 V of DMF–0.1 M
nBu4NBF4 solution, tributylamine is recovered after passage of
2F. 4-Cyanophenol or naphthol and tributylamine (from the
supporting electrolyte nBu4NBF4) were the only products
obtained in each case. Analysis of the electrolysed solutions
by electrochemical and GC ϩ MS techniques indicated that
no benzonitrile, 4,4Ј-dicyanobiphenyl or naphthalene were
formed30 (Table 2 summarises the results obtained), so the
cleavage appears to be selective at the level of the aliphatic
carbon–oxygen bond, leaving the oxygen atom attached to the
aromatic part of the initial molecule.
In our cases, the effect of solvation and ion-pairing seems
negligible. Using different solvents (DMF, ACN or THF) and
different salts (nBu4NBF4 and CsClO4) as supporting electro-
lytes, no significant changes in the cleavage rates, standard
potentials, and nature and yields of products were observed.
The results for 1a are summarized in Table 3. In ACN ϩ 0.1 M
nBu4NBF4, the wave was slightly broad, its peak potential was
more negative and ks was the same order of magnitude as in
DMF ϩ 0.1 M nBu4NBF4. In contrast, in DMF ϩ 0.1 M
CsClO4 and in THF ϩ 0.4 M nBu4NBF4 the waves were quite
broad and ks was smaller than for DMF ϩ 0.1M nBu4NBF4.
The reversible wave was expected in the same range of scan
rate values, in all the cases. The 4-cyanophenol was the only
electrolysis product but with minor current efficiency (5 to
10%). This was possible due to great negative values of reduc-
tion applied potential and a large reduction of the supporting
electrolyte (ACN and THF cases).
Fig. 1 a. Experimental (—) and simulated (DIGISIM®: ᭺) cyclic
voltammograms of 1d (10.3 mM) in DMF–0.1 M nBu4NBF4 at 20 ЊC.
Scan rate 0.1 V sϪ1. Glassy carbon disk electrode (3 mm diameter).
The scan is in the potential range: 0.0/Ϫ2.75/ϩ1.0/0.0 V. b.
Experimental (—) and simulated (DIGISIM®: ᭺) cyclic voltammo-
grams of 1d (6.0 mM) in DMF–0.1 M nBu4NBF4 at 20 ЊC. Scan rate
420 V sϪ1. Gold UME disk electrode (25µm diameter). The scan is in the
potential range: Ϫ1.75/Ϫ2.75/Ϫ1.75 V, c. Cyclic voltammetry of 2b (4.6
mM) in DMF–0.1 M nBu4NBF4 at 20 ЊC. Scan rate 0.5 V sϪ1. Glassy
carbon disk electrode (3 mm diameter). The scan is in the potential
range: 0.0/Ϫ2.75/ϩ1.0/0.0 V. d. Cyclic voltammetry of 2b (5.0 mM) in
DMF–0.1 M nBu4NBF4 at 20 ЊC. Scan rate 3850 V sϪ1. Gold UME disk
electrode (25 µm diameter). The scan is in the potential range: Ϫ1.75/
Ϫ2.75/Ϫ1.75 V
nBu4NBF4. At low scan rates the reduction wave for each com-
pound, R1, is chemically irreversible (Fig. 1a). In the anodic
scan, and only after firstly carrying out one reduction scan,
an oxidation wave appears, Ox2, at approximately 0.60 V.24
These results show that the electrochemical reduction for
studied alkyl aryl ethers, in aprotic solvents, follows a mechan-
ism such as the one described in Scheme 1. The anion radical
986
J. Chem. Soc., Perkin Trans. 2, 2002, 985–990

View Article Online
Table 1 Cyclic voltammetric data of 1a–1j and 2a–2j in DMF ϩ 0.1 M nBu4NBF4 at 20.0 ЊC
b
Compound
Epc(R1)a (0.1 V sϪ1
)
∆Ep/mV (0.1 V sϪ1
)
Epa(Ox2)a (0.1 V sϪ1
)
ks /cm sϪ1
EЊ(R1/Ox1)a, b
[log (kb/sϪ1)] 0.2
1a
1b
1c
1d
1e
1f
Ϫ2.49
Ϫ2.355
Ϫ2.51
Ϫ2.425
Ϫ2.505
Ϫ2.36
Ϫ2.53
Ϫ2.62
Ϫ2.45
Ϫ2.455
Ϫ2.595
60
60
66
73
59
60
60
76
82
66
60
0.59
0.62
0.59
0.62
0.56
0.61
0.61
0.01
0.02
0.03
0.02
0.07
0.20
0.02
0.02
0.03
0.03
0.02
0.01
0.02
0.03
0.02
Ϫ2.47
Ϫ2.39
Ϫ2.47
Ϫ2.42
Ϫ2.47
Ϫ2.36
Ϫ2.51
Ϫ2.57
Ϫ2.45
Ϫ2.46
Ϫ2.55
1.0
2.9
0.5
3.1
0.7
2.3
1.6
0.7
3.7
2.7
0.15
1j
2a
2b
2d
2j
a Potentials are in V vs. SCE. b The values of ks, EЊ, and k are determined by simulation of the experimental cases using DIGISIM® software.
Table 2 Electrolysis of 1a–1j in DMF–0.1 M nBu4NBF4 at 20.0 ЊC
b
Compounda
Eapp
% productc
% alkyl phenyl ether recoveredd
1a
1b
1c
1d
1j
2a
2b
2d
2j
Ϫ2.55
Ϫ2.50
Ϫ2.53
Ϫ2.50
Ϫ2.55
Ϫ2.64
Ϫ2.55
Ϫ2.48
Ϫ2.60
34, (4-cyanophenol)
74, (4-cyanophenol)
23, (4-cyanophenol)
37, (4-cyanophenol)
6, (4-cyanophenol)
9, (1-naphthol)
26, (1-naphthol)
90, (1-naphthol)
20, (1-naphthol)
37
23
32
15
75
69
60
4
80
a All concentrations were 10 mM. b Potentials are in V vs. SCE. Working electrode, fibre carbon. c The yield of products was determined by GC ϩ MS
spectroscopy. d The initial compound was not totally electrolysed. The current efficiency was less than 100% due to the slow reduction of the
nBu4NBF4 electrolyte to tributylamine.
Table 3 Cyclic voltammetric data of 1a at 20 ЊC in different solvents and electrolytes
b
a, b
Compound
Epc(R1)a (0.1 V sϪ1
)
∆Ep/mV (0.1 V sϪ1
)
Epa(Ox2)a (0.1 V sϪ1
)
ks /cm sϪ1
EЊ(R1/Ox1
[log (kb/sϪ1)] 0.2
1a DMF–0.1 M nBu4BF4
1a DMF–0.1 M CsClO4
1a ACN–0.1 M nBu4BF4
1a THF–0.4 M nBu4BF4
Ϫ2.49
Ϫ2.605
Ϫ2.575
Ϫ2.565
60
80
50
80
0.59
0.64
0.58
0.63
0.07
0.004
0.07
Ϫ2.46
Ϫ2.52
Ϫ2.55
Ϫ2.48
1.0
0.9
0.9
0.6
0.004
a Potentials are in V vs. SCE. b The values of ks, EЊ, and k are determined by simulation of the experimental cases using DIGISIM® software.
Ϫ
ϪArO–R
ؒ
ArO ϩ R
(2)
ؒ
To obtain the thermodynamic and kinetic details of this pro-
cess we will follow the theoretical model for cleavage of radical
anions described by Savéant.6a,b,8
Scheme 1
The standard free energy, ∆GЊ, of the overall reaction
[eqn. (2)] is:
resulting from single electron transfer to aromatic ether is a
frangible species, which decomposes in a stepwise process. In
the radical anion, the unpaired electron must be located initially
on the Ar portion of the molecule31 since its standard potential
is very close to that of benzonitrile or naphthalene for the 1a–1j
and 2a–2j series respectively. However it is always slightly more
Ϫ
ؒ
Ϫ
∆GЊ = DArO–R ϩ EЊArO–R/
ArO–R Ϫ EЊArO /ArO Ϫ
ؒ
ϪArO–R
T (SArOؒ ϩ SRؒ Ϫ Sؒ
(3)
Ϫ
ؒ
negative than EЊ(benzonitrile/ benzonitrile) or EЊ(naphthal-
the D, EЊ and S values are the bond dissociation energies, the
standard potentials and the molar entropies of the subscript
species, respectively.
Ϫ
ؒ
ene/ naphthalene), as expected from the effect of the R group.
The cleavage of the bond leads to ArOϪ and R and thus
ؒ
The activation free energy, ∆G‡, is related quadratically to
standard free energy of the reaction, ∆GЊ:
involves an homolytic dissociation of the Caliphatic–O bond in the
radical anion ArO–R1,8 since only the products arising from
ؒ
this reaction were recovered. The final reduction of R takes
place in solution by another radical anion32 due to the low
value of the cleavage rate constant (DISP 1 mechanism27,28).
∆G‡ = ∆GЊ‡ (1 ϩ ∆GЊ/4∆GЊ‡)2
(4)
The standard activation free energy (intrinsic barrier), ∆GЊ‡,
is the sum of two contributions, one related to bond breaking
(D) and the other to solvent reorganization (λo):
Thermodynamics and kinetics of homolytic C–O bond cleavage
in compounds 1a to 1j
We have just demonstrated that the bond cleavage between
the oxygen and the aliphatic carbon, in the reduction of
compounds 1a–1j, leads to the initial homolytic dissociation:
‡
Ϫ
∆GЊ = (Dؒ ArO–R ϩ λo)/4
(5)
987
J. Chem. Soc., Perkin Trans. 2, 2002, 985–990

View Article Online
Table 4 Reactivity data for the homolytic cleavage of aryl alkyl ethersa
b
Log (k/sϪ1
)
∆GЊ‡
Contribution, intra
Ϫ
ϪArOR
Compound
DArO–R
ϪEЊArOR/
ؒ
EЊArO /ArO
ؒ
Ϫ∆GЊ
1a
1b
1c
1d
1e
1f
3.19 (73.6)
2.52 (58.2)
3.31 (76.4)
2.71 (62.5)
3.22 (74.3)
3.06 (70.6)
3.31 (76.4)
2.47
2.39
2.47
2.42
2.47
2.36
2.51
0.56
0.56
0.56
0.56
0.56
0.56
0.56
0.13
0.72
0.01
0.59
0.10
0.15
0.05
1.0
2.9
0.48
3.08
0.7
2.3
1.6
0.745
0.895
0.775
0.83
0.75
0.68
0.67
0.74
0.60
0.775
0.64
0.75
0.74
0.765
1j
a Energies in eV (kcal molϪ1), potentials in V vs. SCE. b From ref. 24b.
where the first term may be derived from accessible molecular
parameters when, as explained before, the electron is initially
accommodated in an orbital belonging to the Ar group
leaves the charge mainly in the same region, as in the radical
anion.
Finally, we will consider the factors that govern the driving
force [Table 4 and eqn. (3)] and the rate constants for the cleav-
Ϫ
ؒ
Dؒ ArO–R = DArO–R ϩ E o
ؒ
ArO–R Ϫ E o
ؒ Ϫ
ArO / (ArO )
age reaction (Table 4). The EЊ values of ArOR/ ArOR are
practically constant (differences of 0.150 eV). EЊ values of
Ϫ
Ϫ
Ϫ
ؒ ؒ
ArO–R/
Ϫ
Ϫ
T (SArOؒ Ϫ Sؒ
ؒ Ϫ SArO–R ϩ Sؒ
)
ArO–R
(6)
(ArO )
Ϫ
Ϫ
ؒ
ؒ ؒ
ؒ
ArO /ArO and ArO / (ArO ) are the same for each series of
compounds, therefore the bond dissociation energy DArO–R must
be the main factor dictating the observed differences. The log
(k/sϪ1) values were in the range 0.48 to 3.08, the greater values
of log k and the smaller DArO–R values correspond to 1b and 1d
compounds, with R being benzyl and allyl, respectively. For
these compounds the values of ∆GЊ are the more negative in
good concordance with Marcus’s law. In this series of alkyl aryl
ethers, the bond dissociation energy is quite large. Due to the
bond cleavage leaving the oxygen atom in the aromatic part, the
Ϫ
(the D, EЊ and S values are the bond dissociation energies,
the standard potentials and the molar entropies of the subscript
Ϫ
ؒ
ؒ
species, respectively). The notation (ArO ) represents from
ؒ
the radical ArO , the injection of one electron into the π*
orbital. (ArO ) is thus an excited state of the carbanion ArO
Ϫ
Ϫ
ؒ
ؒ
where one electron of the pair located in a σ orbital has been
transferred to the π* orbital.8
The cleavage rate constant of the radical anion and the
standard potential for the formation of the radical anions of 1a
to 1j were derived from the cyclic voltammetric data. The bond
dissociation energy, DArO–R, was approximated to be the same as
DPhO–R and it is calculated from comparison with the values for
α-substituted acetophenones:12
standard potential value of the leaving group [EЊArOؒ/ؒ
ؒ ]
(ArO )
is very negative and close to the value of the standard potential
of the reduction of the initial compound. It must be noticed
that the ∆GЊ value for the cleavage reaction in the 1a to 1j
compounds is in the same order of magnitude as the C–C bond
cleavage7 or the C–O bond cleavage in α-substituted aceto-
phenones12 but the lower rate constant obtained here is due to
the very negative EЊ value of the leaving group.
DR–OC H Ϫ DR–Br = DC H COCH –OC H Ϫ DC H COCH –Br (7)
6
5
6
5
2
6
5
6
5
2
the DR–Br values are obtained from the literature (tBuBr,33
C6H5CH2Br,12 and 4-CNC6H4CH2Br12) or from the reduction
peak potential of the alkyl halide and the approximate linear
Thermodynamics and kinetics of homolytic C–O bond cleavage
in compounds 2a to 2j
2
3b
–
Ϫ
correlation: DR–Br = Ϫ3(Ep Ϫ EЊX /X ) ϩ constant. The stand-
ؒ
ard free energy of radical anion cleavage ∆GЊ (the driving force
for bond cleavage) may be obtained from eqn. (3), with ∆S (the
entropy cleavage ArO–R) approximately equal to 1 meV KϪ1;
∆S corresponding to the formation of two molecules from one
is small and does not vary significantly in the series. The acti-
vation free energy ∆G‡ may be obtained from k = Aexp(Ϫ∆G‡/
RT) in the application of eqn. (4); the pre-exponential factors
are taken to be equal to 5 × 1012 sϪ1.12 The intrinsic barrier free
energy ∆GЊ‡ was calculated according to the Marcus equation
[eqn. (4)]. These values are of the same order of magnitude 0.7–
0.8 eV for all compounds. We may calculate the contribution
For these compounds no bond dissociation energy data, DArO–R
,
are available. We will demonstrate that it is possible to estimate
reasonable values for DArO–R in naphthyl ethers in a very simple
way. As discussed before, neglecting the quadratic character of
eqn. (4)3b and introducing the values of ∆GЊ‡ [eqn. (5) and (6)]
with λo = 0 and ∆GЊ [eqn. (3)],
∆G‡ = ∆GЊ‡ ϩ ∆GЊ/2
(8)
∆G‡ = 3DNaphO–R/4 ϩ 3EЊNaphO–R/
ؒ
NaphO–R/4 Ϫ
ؒ/4 Ϫ EЊNaphO /NapHO
Ϫ
Ϫ
of the intramolecular factor to the bond cleavage: Dؒ ArO–R/4
E o
ؒ ؒ
ؒ
/2 (9)
Ϫ
Ϫ
NaphO / NaphO
Ϫ
by application of eqn. (6). In eqn. (6), EЊArOؒ/ؒ
ؒ
was
(ArO )
leading to:35
Ϫbenzonitrile
ؒ
approximated to EЊbenzonitrile/
considering that the
radical part located on the oxygen atom is not involved in
the injection of one elctron into the π* orbital of the ArO .
ؒ
DNaphO–R = 4[0.123 Ϫ 0.058 log k Ϫ
3E o
Ϫ
ؒ
NaphO–R/4]/3 (10)
Ϫ
Therefore Dؒ
must be considered as a superior limit.
NaphO–R/
ArO–R
The results are summarized in Table 4. In any case, as can be
Ϫ
seen in Table 4, Dؒ
represents the main factor of the
The results are presented in Table 5. The bond dissociation
energy for naphthyl ethers is lower than for the corresponding
4-cyanophenyl ethers in good accordance with available
thermodynamic data. For example, DArO–R for compound 2a is
2.67 and for compound 1a is 3.19 eV. This variation is in good
accord with available thermodynamic data on C–H bond values
for analogous compounds,36 DH-1-naphthylmethyl ≅ 3.69 eV and
DH–CH C H ≅ 3.84 eV. Therefore, one last outcome of the present
study could be the estimation of reliable values for the bond dis-
sociation energy of naphthyl ethers, since data for simple ethers
are currently available but no such values exist in the literature for
naphthyl ethers.
ArO–R
total barrier, the solvent reorganisation providing a modest
contribution for the intrinsic barrier in homolytic dissociations
of radical anions. This is in strong contrast with the reported
behaviour of α-substituted acetophenones, typical examples of
intramolecular dissociative electron transfer.34 Our observation
confirms the previously proposed fact that solvent reorganis-
ation is very much linked to charge reorganisation during the
reaction.34 In intramolecular dissociative electron transfer the
solvent is organised around a negative charge, which develops
on the leaving group during the reaction [eqn. (1b)]. In homo-
lytic dissociations (the examples studied here), the cleavage
2
6
5
988
J. Chem. Soc., Perkin Trans. 2, 2002, 985–990

View Article Online
Table 5 Bond dissociation energy for alkyl naphthyl ethersa 2a–2j
b
c
Log (k/sϪ1
)
DArO–R
Ϫ
ϪArOR
Compound
ϪEЊArOR/
ؒ
ϪEЊArO /ArO
ؒ
2a
2b
2d
2j
2.57
2.45
2.46
2.55
0.03
0.03
0.03
0.03
0.7
3.7
2.7
0.15
2.67 (61.6)
2.33 (53.8)
2.41 (55.6)
2.70 (62.3)
a Energies in eV (kcal molϪ1), potentials in V vs. SCE. b From ref. 20b. c See text.
a basic medium, butanone–K2CO3 as previously described, 1c.
1H NMR(CDCl3) δ (ppm) 7.55 (d, 2H), 6.92 (d, 2H), 5.86 (m,
1H), 5.17 (dd, 2H), 4.05 (t, 2H), 2.54 (dd, 2H).
Conclusions
The initially produced radical anion of alkyl aryl ethers reacts
by a slow first order reaction. The cleavage of the bond leads to
ArOϪ and R and thus involves an homolytic dissociation of
ؒ
Allyl naphthyl ether,45 2d. Compound 2d was prepared by the
reaction of 1-naphthol with allyl bromide in a basic medium,
butanone–K2CO3 as previously described, 1c. 1H NMR-
(CDCl3) δ (ppm) 8.22–6.53 (m, 7H), 5.97 (m, 1H), 6.0 (m, 1H),
5.3–5.15 (dd, 2H), 4.45 (d, 2H).
the Caliphatic–O bond in the molecule ϪArO–R.1,8 The final
ؒ
ؒ
reduction of R takes place in solution by the radical anion due
to the low value of the cleavage rate constant (DISP1 mechan-
ism23,24). A Marcus type relationship between the cleavage rate
constants and the standard free energy of the reaction leads to
an intrinsic barrier in the range of 0.7 to 0.8 eV. In our
Ϫ
Instruments and procedures
approximation, Dؒ
represents the main factor of the
ArO–R
Instruments and procedures were the same as previously
described for cyclic voltammetry and electrolysis.46
total barrier, the solvent reorganisation being a modest contri-
bution to the intrinsic barrier for homolytic dissociations of
radical anions. This observation confirms the early proposal
that solvent reorganisation is very much linked to charge
reorganisation during the reaction. In intramolecular dissoci-
ative electron transfer the solvent, which is organised around a
negative charge in the initial state of the radical anion, has to
reorganise around a negative charge on the leaving group dur-
ing the reaction [eqn. (1)]. In homolytic dissociations (the
examples studied here), the charge is placed in the leaving group
from the initial electron transfer beginning and therefore, little
solvent reorganisation is needed during the reaction.
References
1 J.-M. Savéant, Adv. Phys. Org. Chem., 2000, 35, 117.
2 C. P. Andrieux, Organic Electrochemical Mechanisms, in
Encyclopedia of Analytical Chemistry, Wiley, Chichester, 2000,
p. 9983.
3 (a) J.-M. Savéant, Adv. Phys. Org. Chem., 1990, 26, 1–130; (b) C. P.
Andrieux, A. Le Gorande and J. M. Savéant, J. Am. Chem. Soc.,
1992, 114, 6892; (c) C. P. Andrieux, E. Differding, M. Robert and
J.-M. Savéant, J. Am. Chem. Soc., 1993, 115, 6592; (d ) C. P.
Andrieux, J.-M. Savéant, A. Tallec, R. Tardivel and C. Tardy, J. Am.
Chem. Soc., 1997, 119, 2420; (e) C. P. Andrieux, C. Combellas,
F. Kanoufi, J.-M. Savéant and A. Thiébault, J. Am. Chem. Soc.,
1997, 119, 9527; ( f ) S. Antonello and F. J. Maran, J. Am. Chem.
Soc., 1998, 120, 5713; (g) S. Jakobsen, H. Jensen, S. U. Pendersen
and K. Daasbjerg, J. Phys. Chem. A, 1999, 103, 4141; (h) J. M.
Tanko and J. P. Phillips, J. Am. Chem. Soc., 1999, 121, 6078; (i) R. J.
Enemaerke, T. B. Christensen, H. Jensen and K. Daasbjerg, J. Chem.
Soc., Perkin Trans. 2, 2001, 1620.
4 C. P. Andrieux, M. Robert, F. D. Saeva and J.-M. Savéant, J. Am.
Chem. Soc., 1992, 116, 7864.
5 (a) R. A. Marcus, J. Chem. Phys., 1965, 43, 679; (b) L. Eberson,
Electron Transfer Reactions in Organic Chemistry, Springer, Berlin,
1987; (c) H. Kojima and A. J. Bard, J. Am. Chem. Soc., 1975, 97,
6317.
From our approach, it is also possible to determine reliable
values for the bond dissociation energy of naphthyl ethers (no
such values exist in the literature).
Experimental
Chemicals
4-Cyanophenyl benzyl ether37 1b, tert-butyl 4-cyanophenyl
ether38 1f, 1-piperidino-2-(4-cyanophenoxy)ethane39 1j, benzyl
naphthyl ether40 2b, and 1-piperidino-2-(1-naphthoxy)ethane41
2j were prepared and identified following previously described
procedures.
All products were identified by comparison of their
spectroscopy behaviour with those reported in the literature.
All chemicals were obtained from Aldrich and were of the
highest purity available. They were used as received.
6 (a) J.-M. Savéant, J. Am. Chem. Soc., 1987, 109, 6755; (b) J.-M.
Savéant, Adv. Electron Transfer Chem., 1994, 4, 53–116.
7 (a) P. Maslak and J. N. Narvaez, Angew. Chem., Int. Ed. Engl., 1990,
29, 283; (b) P. Maslak, T. M. Vallombroso, W. H. Chapman
and J. N. Narvaez, Angew. Chem., Int. Ed. Engl., 1994, 33, 73;
(c) P. Maslak, J. N. Narvaez and T. M. Vallombroso, J. Am. Chem.
Soc., 1995, 117, 12373; (d ) P. Maslak, W. H. Chapman and
T. M. Vallombroso, J. Am. Chem. Soc., 1995, 117, 12380; (e) A.
Anne, S. Fraoua, J. Moiroux and J.-M. Savéant, J. Am. Chem. Soc.,
1996, 118, 3938; ( f ) P. Maslak and J. Theroff, J. Am. Chem. Soc.,
1996, 118, 7235.
8 J.-M. Savéant, J. Phys. Chem., 1994, 98, 3716 and references therein.
9 (a) Z.-R. Zheng and D. H. Evans, J. Am. Chem. Soc., 1999, 121,
2941; (b) Z.-R. Zheng, D. H. Evans, E. Soazara Chan-Shing and
J. Lessard, J. Am. Chem. Soc., 1999, 121, 9429.
10 C. P. Andrieux, M. Robert and J.-M. Savéant, J. Am. Chem. Soc.,
1995, 117, 9340 and references therein.
11 H. Jensen and K. Daasbjerg, Acta Chem. Scand., 1998, 52, 1151.
12 C. P. Andrieux, J.-M. Savéant, A. Tallec, R. Tardivel and C. Tardy,
J. Am. Chem. Soc., 1996, 118, 9788.
13 (a) C. G. Screttas and M. M. Screttas, J. Org. Chem., 1978, 43,
1064; (b) T. Cohen and M. Buphaty, Acc. Chem. Res., 1989, 22,
152; (c) M. G. Severin, M. C. Arevalo, G. Farnia and E. Vianello,
J. Phys. Chem., 1987, 91, 466; (d ) M. C. Arevalo, G. Farnia,
M. G. Severin and E. Vianello, J. Electroanal. Chem., 1987, 220, 201;
(e) L. Griggio and M. G. Severin, J. Electroanal. Chem., 1987, 223,
185; ( f ) M. G. Severin, G. Farnia, E. Vianello and M. C. Arevalo,
4-Cyanophenyl propyl ether,42 1c. Compound 1c was prepared
by the reaction of 4-cyanophenol with alkyl bromide in a basic
medium, butanone–K2CO3. The solution was refluxed for 15 h.
After cooling, the solution was poured into water and extracted
with ether. After drying and recrystallizing the residue some
colorless crystals were obtained. 1H NMR(CDCl3) δ (ppm) 7.56
(d, 2H), 6.92 (d, 2H), 3.93 (t, 2H), 1.80 (m, 2H), 1.02 (t, 3H).
Allyl 4-cyanophenyl ether,43 1d. Compound 1d was prepared
by the reaction of 4-cyanophenol with alkenyl bromide in a
basic medium, butanone–K2CO3, as previously described, 1c.
The solution was refluxed for 8 h. After cooling, the solution
was poured into water and extracted with ether. After drying
and recrystallizing the residue some white crystals were
obtained. 1H NMR(CDCl3) δ (ppm) 7.56 (d, 2H), 6.94 (d, 2H),
6 (m, 1H), 5.32 (dd, 2H), 4.57 (d, 2H).
But-3-enyl 4-cyanophenyl ether,44 1e. Compound 1e was pre-
pared by the reaction of 4-cyanophenol with alkenyl bromide in
J. Chem. Soc., Perkin Trans. 2, 2002, 985–990
989

View Article Online
25 Addition of authentic piperidine (Aldrich, 99.5%) to a solution of 1j
causes an increase in intensity of the ϩ1.1 V oxidation peak.
26 This potential is identical to the one found for the oxidation of the
anion naphthoxide.24b
27 (a) L. Nadjo and J.-M. Savéant, J. Electroanal. Chem., 1973, 48, 113;
(b) C. P. Andrieux and J.-M. Savéant, Electrochemical Reactions, in
Investigation of Rates and Mechanism of Reactions, Techniques of
Chemistry, C. F. Bernasconi, Ed., Wiley, New York, 1986, vol. 6,
ch. 2.1, p. 305.
J. Electroanal. Chem., 1988, 251, 369; (g) M. G. Severin, M. C.
Arévalo, F. Maran and E. Vianello, J. Phys. Chem., 1997, 119, 6950;
(h) S. Antonello and F. Maran, J. Am. Chem. Soc., 1999, 121, 9668.
14 (a) R. Benasi and F. Taddei, J. Phys. Chem. A., 1998, 102, 6173;
(b) K. Daasbjerg, H. Jensen, R. Benassi, F. Taddei, S. Antonello,
A. Gennaro and F. Maran, J. Am. Chem. Soc., 1999, 121, 1750.
15 (a) M. L. Andersen, N. Mathivanan and D. D. M. Wayner,
J. Am. Chem. Soc., 1996, 118, 4871; (b) M. L. Andersen and D. D.
M. Wayner, J. Electroanal. Chem., 1996, 412, 53; (c) M. L. Andersen,
W. N. Long and D. D. M. Wayner, J. Am. Chem. Soc., 1997, 119,
6590.
16 S. Antonello and F. Maran, J. Am. Chem. Soc., 1997, 119, 12595.
17 (a) A. Maercker, Angew. Chem., Int. Ed. Engl., 1987, 26, 972 and
references therein; (b) U. Azzena, G. Melloni and L. Pisano,
Tetrahedron Lett., 1993, 34, 5635; (c) U. Azzena, G. Melloni,
L. Pisano and B. Sechi, Tetrahedron Lett., 1994, 35, 6759; (d ) U.
Azzena, S. Demartis, M. G. Fiori, G. Melloni and L. Pisano,
Tetrahedron Lett., 1995, 36, 8123; (e) J. Marquet, E. Cayón,
X. Martin, F. Casado, I. Gallardo, M. Moreno and J.-M. Lluch,
J. Org. Chem., 1995, 60, 3814; ( f ) R. González-Blanco, J. L.
Boudelande and J. Marquet, J. Org. Chem., 1997, 62, 6903.
18 (a) U. Azzena, T. Denurra, G. Melloni, E. Fenude and
G. Rassu, J. Org. Chem., 1992, 57, 1444; (b) U. Azzena, T. Denurra,
G. Melloni and E. Fenude, Gazz. Chim. Ital., 1996, 126, 141;
(c) U. Azzena, F. Casado, P. Fois, I. Gallardo, L. Pisano, J. Marquet
and G. Melloni, J. Chem. Soc., Perkin Trans. 2, 1996, 2563;
(d ) M. C. R. L. R. Lazana, M. L. T. M. B. Franco and B. J. Herold,
J. Am. Chem. Soc., 1989, 111, 8640; (e) J. M. Saá, P. Ballester,
P. M. Deyá, M. Capó and X. Garcías, J. Org. Chem., 1996, 61,
1035.
19 (a) E. Santiago and J. Simonet, Electrochim. Acta, 1975, 20, 853;
(b) M. D. Koppang, N. F. Woolsey and D. E. Bartak, J. Am. Chem.
Soc., 1984, 106, 2799; (c) M. D. Koppang, N. F. Woolsey and
D. E. Bartak, J. Am. Chem. Soc., 1985, 107, 4692; (d ) T. A.
Thornton, N. F. Woolsey and D. E. Bartak, J. Am. Chem. Soc., 1986,
108, 6497; (e) T. A. Thornton, G. A. Ross, D. Patil, K. Mukaida,
J. O. Warwick, N. F. Woolsey and D. E. Bartak, J. Am. Chem. Soc.,
1989, 111, 8640.
20 (a) The first reduction wave, at ≈ Ϫ1.0 V, in 4-nitrophenyl alkyl
ethers, 4-nitrophenyl phenyl ether, 4-nitrobenzyl alkyl ethers and
4-nitrobenzyl phenyl ether, remains reversible at any scan rate. That
is to say, within the time of cyclic voltammetric measurement the
reductive cleavage of the C–O bond in the anion radical does not
occur.20b (b) M. Farriol, PhD Thesis, Universitat Autonoma,
Barcelona, March 2000.
28 C. Amatore and J.-M. Savéant, J. Electroanal. Chem., 1980, 107,
353.
29 Notice that the peak potential measured at 0.1 V sϪ1 is slightly more
negative than the reported EЊ’s well in concordance with the ks value
(<0.1 cm sϪ1), except for 1b when the larger value of ks (0.2 cm sϪ1
leads to a peak potential value more positive than the EЊ value.
30 The RH products (volatile products) were not quantified.
)
31 (a) Theoretical calculations31b point to a significantly larger spin
distribution (the unpaired electron) on the cyanophenyl region and a
not Caliphatic–O bond stretching in the formation of radical anions in
contrast with what was inferred by scission of the S–S bond in diaryl
disulfides.14 (b) L. Pisano, M. Farriol, X. Asensio, I. Gallardo, A.
Gonzalez-Lafont, J. M. Lluch and J. Marquet, J. Am. Chem. Soc., in
the press.
32 Another reaction was possible leading to a two electron process:
ؒ
H-atom abstraction from DMF by the alkyl R , however no
reduction products of the DMF part were recovered.
33 C. P. Andrieux, I. Gallardo, J.-M. Savéant and K. B. Su, J. Am.
Chem. Soc., 1986, 108, 638.
34 In the acetophenone series, the solvent is organised around a
negative charge which is mostly concentrated on the carbonyl
oxygen and has to reorganise around the negative charge which
develops on the leaving group during the reaction. The contribution
of solvent reorganisation to the intrinsic barrier appears to be less in
4-NO2 and 4-CN substituted acetophenones, where the charge of
the radical anion is more delocalised.12
Ϫ
35 In the application of eqn. (9): EЊNaphOؒ/ؒ
ؒ
= Ϫ2.40 V and
(NaphO )
24b
Ϫ
EЊNaphO /NaphO
ؒ
= Ϫ0.03 V.
36 Handbook of Chemistry and Physics, 72nd edn., D. R. Lide (Editor
in Chief ), CRC, Cleveland, 1991–1992.
37 D. Tzeng and W. P. Weber, J. Org. Chem., 1981, 46, 265.
38 G. Mann and J. Phartwing, J. Org. Chem., 1997, 62, 5413.
39 J. Marquet, E. Cayón, X. Martin, F. Casado, I. Gallardo,
M. Moreno and J.-M. Lluch, J. Org. Chem., 1995, 60, 3814.
40 P. Maslak and R. D. Guthrie, J. Am. Chem. Soc., 1986, 108, 2637.
41 V. Dauksas and V. Paplaitis, Zh. Khim., 1972, 14, 121.
42 P. Reynaud, J. D. Brion, T. X. Nguyen, C. Davrinche, F. Pieri and
M. L. Arnould-Guerin, Eur. J. Med. Chem., 1989, 24, 427.
43 L. Capella, P. C. Montevecchi and M. L. Mavacchia, J. Org. Chem.,
1995, 60, 7424.
44 S. Y. Al-Qaradawi, K. B. Cosstick and A. Gilbert, J. Chem. Soc.,
Perkin Trans. 1, 1992, 1145.
45 A. El-Qisairi and P. M. Henry, J. Organomet. Chem., 2000, 603, 50.
46 (a) I. Gallardo, G. Guirado and J. Marquet, J. Electroanal. Chem.,
2000, 488, 64; (b) I. Gallardo, G. Guirado and J. Marquet, Chem.
Eur. J., 2001, 7, 1759.
21 (a) J.-M. Savéant, Acc. Chem. Res., 1993, 2, 455; (b) J.-M. Savéant,
Tetrahedron, 1994, 50, 10117; (c) M. Chanon, M. Rajzmann and
F. Chanon, Tetrahedron, 1990, 46, 6193.
22 (a) J. F. Bunett, Acc. Chem. Res., 1978, 11, 413; (b) N. Kornblum,
Angew. Chem., Int. Ed. Engl., 1975, 14, 734.
23 F. Casado, L. Pisano, M. Farriol, I. Gallardo, J. Marquet and
G. Melloni, J. Org. Chem., 2000, 65, 322.
24 (a) This potential is identical to the one found for the oxidation
of the anion 4-cyanophenoxide.24b (b) P. Hapiot, J. Pinson and
N. Yousfi, New. J. Chem., 1992, 16, 877.
990
J. Chem. Soc., Perkin Trans. 2, 2002, 985–990