Solvent reorganization as a governing factor in the kinetics of intramolecular dissociative electron transfers. Cleavage of anion radicals of α-substituted acetophenones

Full text of DOI:10.1021/ja9622564

9788
J. Am. Chem. Soc. 1996, 118, 9788-9789
Solvent Reorganization as a Governing Factor in
the Kinetics of Intramolecular Dissociative Electron
Transfers. Cleavage of Anion Radicals of
r-Substituted Acetophenones
DRX•- + λ°
q
∆
G )
(2)
0
4
where the first term may be derived from accessible molecular
parameters according to
1
a
,1a
Claude P. Andrieux, Jean-Michel Sav e´ ant,*
Andr e´ Tallec,1b Robert Tardivel, and Caroline Tardy
1b
1a
DRX•- ) DRX + E°RX/RX•- - E°R•/R- + T(S - SRX) +
R•
T(SRX•- - SR ) (3)
-
Laboratoire d’Electrochimie Mol e´ culaire
de l’UniVersit e´ Denis Diderot, Unit e´ Associ e´ e
(
the D, E°, and S values are the bond dissociation energies, the
au CNRS No. 438, 2 place Jussieu, 75251 Paris Cedex 05
Laboratoire d’Electrochimie de l’UniVersit e´ de Rennes
Unit e´ Associ e´ e au CNRS No. 439, Campus de Beaulieu
standard potentials, and the molar entropies of the subscript
species, respectively). The solvent reorganization energy arises
from the fact that, during the reaction, the charge moves from
one portion of the molecule to another. λ° may thus be
estimated from
3
5042 Rennes Cedex, France
ReceiVed July 3, 1996
Cleavage of ion radicals2 may be viewed as an intra-
molecular electron transfer coupled with bond breaking in cases
where the unpaired electron initially resides in an orbital that
-6
2
1
1
1
1
1
D s
λ° ) e0
+
-
-
(4)
(
)(
)
2
aRX•-
2a
•
R ,X
-
d D
op
7
does not belong to the leaving group:
aRX•- is the radius of the sphere approximating the region of
the starting anion radical where the charge is initially located
and aR•,X-, the radius of the sphere approximating the location
of the charge on the leaving group when the bond is broken. d
is the distance between the centers of charge at the transition
state. The D values are the optical and static dielectric
constants, respectively. The standard free energy of anion
radical cleavage may be obtained from eq 5, where ∆SRX is the
entropy for the cleavage of R-X.
-•
•
-
R-X h R + X
The unpaired electron jumps from the initial orbital to the σ*
orbital of the R-X bond which breaks simultaneously. Hence,
a model of the dynamics of such reactions has been proposed
where the two main reaction coordinates are the bond distance
and a fictitious charge representing solvent reorganization in
the Marcus way. The activation free energy, ∆G , is thus
related quadratically to the standard free energy of the reaction,
8
9
q
∆
G° ) DRX + E°RX/RX•- - E°
•
-
- T∆SRX
(5)
X /X
∆G°:
q
q
0
q 2
0
So far, only the influence of the intramolecular factors
∆
G ) ∆G (1 + ∆G°/4∆G )
(1)
(
through eqs 1-3) have been investigated experimentally (see
refs 6d and 8) in reactions where they are largely predominant
q
The standard free energy of activation, ∆G0 , is the sum of
two contributions, one related to bond breaking and the other
to solvent reorganization:
1
0
over possible solvent reorganization effects.
In order to
maximize the effect of solvent reorganization and minimize the
contribution of intramolecular factors, we have selected a series
of anion radicals generated by electrochemical reduction of the
following R-substituted acetophenones.
(
1) (a) Universit e´ Denis Diderot. (b) Universit e´ de Rennes.
2) (a) Neta, P.; Behar, D. J. Am. Chem. Soc. 1980, 102, 4798. (b) Behar,
(
D.; Neta, P. J. Phys. Chem. 1981, 85, 690. (c) Behar, D.; Neta, P. J. Am.
Chem. Soc. 1981, 103, 103. (d) Behar, D.; Neta, P. J. Am. Chem. Soc. 1981,
1
03, 2280. (e) Bays, J. P.; Blumer, S. T.; Baral-Tosh, S.; Behar, D.; Neta,
P. J. Am. Chem. Soc. 1983, 105, 320. (f) Norris, R. K.; Barker, S. D.; Neta,
P. J. Am. Chem. Soc. 1984, 106, 3140. (g) Meot-Ner, M.; Neta, P.; Norris,
R. K.; Wilson, K. J. Phys. Chem. 1986, 90, 168.
Z
CH2X
O
(
3) (a) Saeva, F. D. Top. Curr. Chem. 1990, 156, 61. (b) Saeva, F. D.
Intramolecular Photochemical Electron Transfer (PET) - Induced Bond
Cleavage Reactions in some Sulfonium Salts Derivatives. In AdVances in
Electron Transfer Chemistry; Mariano, P. S., Ed.; JAI Press: New York,
X
Z
X
Z
Br
4-NO2
1a
OCH3
1
994; Vol. 4, pp 1-25. (c) Arnold, B. R.; Scaiano, J. C.; McGimpsey, W.
4-CN
H
1b
2a
O
H
3a
3b
G. J. Am. Chem. Soc. 1992, 114, 9978. (d) Chen, L.; Farahat, M. S.; Gan,
H.; Farid, S.; Whitten, D. G. J. Am. Chem. Soc. 1995, 117, 6399.
O
C
O
Ph
4-NO2
2b
2c
(
4) (a) Tanner, D. D.; Chen, J. J.; Chen, L.; Luelo, C. J. Am. Chem. Soc.
4
-OCH3
1
991, 113, 8074. (b) Tanko; J. M.; Drumright, R. E.; Sulemen, N.; Brammer,
4
-OCH3
L. E. J. Am. Chem. Soc. 1994, 116, 1585. (c) Andersen, M. L.; Mathivanan,
N.; Wayner, D. D. M. J. Am. Chem. Soc. 1996, 118, 4871.
4a
SC2H5
H
(
5) Severin, M. G.; Arevalo, M. C.; Maran, F.; Vianello, E. J. Phys.
Chem. 1993, 97, 150.
6) (a) Sav e´ ant, J.-M. Single Electron Transfer and Nucleophilic Substitu-
Over the whole series of anion radicals generated by cyclic
•
-
(
voltammetry, only 2b was found to be stable down to the
tion. In AdVances in Physical Organic Chemistry; Bethel, D., Ed.; Academic
Press: New York, 1990; Vol. 26, pp 1-130. (b) Sav e´ ant, J.-M. Acc. Chem.
Res. 1993, 26, 455. (c) Sav e´ ant, J.-M. Dissociative Electron Transfer. In
AdVances in Electron Transfer Chemistry; Mariano, P. S., Ed.; JAI Press:
New York, 1994; Vol. 4, pp 53-116. (d) Sav e´ ant, J.-M. Tetrahedron 1994,
11
lowest scan rate (0.1 V/s). 4a exhibts an irreversible wave at
low scan rate which becomes reversible around 10 V/s. The
cleavage rate constant and the standard potential for the
formation of the anion radical of 4a were derived from these
cyclic voltammetric data (see the supporting information). All
the other compounds give rise to an irreversible wave between
5
0, 10117.
(
7) In the opposite case,7
and the intramolecular (small) reorganization pertains essentially to
b-j
the reaction is better viewed as a homolysis
7
b
rehybridization. The solvent reorganization energy is also expected to
be small since the charge stays on the same portion of the molecule during
bond cleavage. (b) Anne, A.; Fraoua, S.; Moiroux, J.; Sav e´ ant, J.-M. J.
Am. Chem. Soc. 1996, 118, 3938. (c) Maslak, P.; Vallombroso, T. M.;
Chapman, W. H.; Narvaez, J. N. Angew. Chem., Int. Ed. Engl. 1994, 33,
(10) (a) The influence of orbital symmetry restrictions has also been
examined.1 (b) Adcock, W.; Andrieux, C. P.; Clark, C. I.; Neudeck, A.;
Sav e´ ant, J.-M.; Tardy, C. J. Am. Chem. Soc. 1995, 117, 8285.
0b
(11) It is interesting to note that charge transfer to 2b is relatively slow.
-
1 -1
7
1
3. (d) Maslak, P.; Narvaez, J. N.; Vallombroso, T. M. J. Am. Chem. Soc.
995, 117, 12373. (e) Maslak, P.; Chapman, W. H.; Vallombroso, T. M.;
The standard rate constant, kS ) 0.22 cm
s , was determined from cyclic
voltammograms (see the supporting information). Charge transfer to
-1
-1
Watson, B. A. J. Am. Chem. Soc. 1995, 117, 12380.
acetophenone itself is even slower (kS ) 0.14 cm
s ), pointing to a strong
(
(
8) Sav e´ ant, J.-M. J. Phys. Chem. 1994, 98, 3716.
9) Marcus, R. A. J. Chem. Phys. 1965, 43, 679.
localization of the negative charge on the carbonyl oxygen of the anion
radical in both cases, albeit somewhat less with 2b than with acetophenone.
S0002-7863(96)02256-1 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 40, 1996 9789
Table 1. Reactivity Dataa
contributions
b
c
E°X /X-
•
-E°RX/RX•-
j
q k
intral
solvm
cmpd
log k (s-1)
D
RX
-∆G° (cleav)
∆G
0
e
1
1
2
2
3
3
4
a
b
a
c
a
b
a
6.6
7.8
6.4
7.6
5.4
5.4
1.2
1.85
1.90
2.53
2.53
1.78
1.78
2.14
1.44
1.44
1.24
1.24
0.24
0.24
0.06
0.83
1.04
1.81
1.97
1.81
1.88
1.87
0.71
0.87
0.82
0.98
0.56
0.63
0.08
0.65
0.66
0.72
0.7
0.68
0.69
0.71
0.26
0.22
0.18
0.14
0.00
0.00
0.07
0.39
0.44
0.54
0.56
0.68
0.69
0.64
c
e
d
d
d
d
d
f,g
f,g
f,h
f,h
f,i
a
Energies in eV, potentials in V vs SCE. b From the value DRX ) 2.05 eV obtained from the peak potential, E
p
, of the dissociative electron
(details on the electrochemistry of PhCOCH Br will
2
Br is corrected for the weakening of the bond by the para electron-withdrawing substituent as
Br by means of the equation: DRX ) (2/3)(E°X•/X- - E
p
)
14a,b
transfer reduction of PhCOCH
be published elsewhere). The value for PhCOCH
for the corresponding benzyl bromides.
2
2
c
1
4a,c
d
From DPhCOCH X ) DPhCOCH Br + DPhCH X - DPhCH Br; the DPhCH X values are obtained from ref 15.
2
2
2
2
2
e
f
g h i j
From ref 14a,b. Assumed to be the same as in acetonitrile. See ref 16. From ref 17. From ref 16b. From the application of eq 5, taking ∆S
-
1
k
)
1 meV K which corresponds to the formation of two molecules from one and is small and does not vary significantly in the series. The
12 l 18
-1 -1
•
-
preexponential factor is taken equal to kT/h ) 5 × 10
M
s . In the application of eq 3, E°R /R was taken as equal to 0.00 V vs SCE and the
m
molar entropies of the charged species (which are the predominant entropic terms) were assumed to compensate each other. From the difference
between the exprimental intrinsic barrier and the contribution of bond breaking (intra).
0
.1 and 100 V/s. The values of the cleavage rate constants and
of the standard potential for the formation of the anion radical
were derived from the variation of the peak potentials with scan
rate and from the value of the peak width according to a
procedure described in the supporting information.
12
The results (Table 1)13 are displayed in Figure 1 as a plot of
log k (cleavage) vs the standard free energy of the reaction.
The latter quantity was estimated as described in Table 1. The
standard potentials E°RX/RX•- were obtained from the same
experiments (see the supporting information). There is a clear
correlation between the rate constants and the standard free
energy of the reaction of the type depicted by eq 1 with a value
of the average symmetry factor close to 0.5 (0.45). The average
intrinsic barrier is 0.70 eV. Why is the intrinsic barrier so large,
and why does it depend so weakly upon molecular structure?
We may estimate the contribution of the intramolecular bond
cleavage by application of eq 3. As seen in Table 1, it represents
Figure 1. Variation of the cleavage rate constant with the standard
free energy of the reaction.
a modest-to-negligible percentage of the total barrier, particularly
for compounds that do not bear ring substituents.
(12) (a) Nadjo, L.; Sav e´ ant, J.-M. J. Electroanal. Chem. 1973, 48, 113.
(
b) Andrieux, C. P.; Sav e´ ant, J.-M. In Electrochemical Reactions in
We are thus led to conclude that solvent reorganization is
the main factor dictating the height of the intrinsic barrier,
contributing 75-100% to the total in the acetophenone series.
This observation falls in line with the fact that, in the initial
state of the anion radical, the solvent is organized around a
negative charge which is mostly concentrated on the carbonyl
oxygen and has to reorganize around the negative charge which
develops on the leaving group during the reaction. Such values
of the solvent reorganization energy are compatible with
reasonable values of the two solvation spheres’ radii, on the
order of 2 Å each, and of the distance between the two centers
of charge, on the order of 6 Å. A radius of 2 Å for the solvation
sphere of the oxygen carbonyl is also compatible with the small
value, 0.14 cm/s, found for the standard rate constant of the
electrochemical reduction of acetophenone. Obviously, these
estimations are very approximate. Nevertheless, it is noteworthy
that the contribution of solvent reorganization to the intrinsic
barrier appears to be less with 1a and 1b, where charge
concentration on the oxygen carbonyl is expected to be less
than with the other compounds owing to charge delocalization
on the 4-NO2 and 4-CN groups, respectively.
InVestigation of Rates and Mechanisms of Reactions, Techniques of
Chemistry; Bernasconi, C. F., Ed.; Wiley: New York, 1986; Vol. VI/4E,
Part 2, pp 305-390.
(13) The value found for 3a is in good agreement with the values recently
4c
reported for an extended series of R-phenoxyacetophenones bearing various
substituents on the phenoxy ring, although not a 3-methyl group as in 3a.
(14) (a) Andrieux, C. P.; Le Gorande, A.; Sav e´ ant, J.-M. J. Am. Chem.
Soc. 1992, 114, 6892. (b) Andrieux, C. P.; Differding, E.; Robert, M.;
Sav e´ ant, J.-M. J. Am. Chem. Soc. 1993, 115, 6592. (c) Clark, K. B.; Wayner,
D. D. M. J. Am. Chem. Soc. 1991, 113, 9363.
(
15) (a) Benson, S. N. Thermochemical kinetics; Wiley: New York, 1976.
b) Handbook of Chemistry and Physics, 72nd ed.; CRC: Cleveland, 1991-
992; pp 9, 116-121.
(
1
-
16b
(
16) (a) A reliable value, 1.06 V vs SCE, is available for CH3CO2 ,
-
-
but not for PhCO2 . The value given in ref 16, 0.36 V vs SCE, is smaller
than for CH3CO2 , which seems counterintuitive in view of the fact that
-
-
CH3CO2 is a stronger base than PhCO2 (the pKa values in acetonitrile
16c,d
-
are 22.3 and 20.7, respectively ). Starting from the E° value of CH3CO2 ,
we estimated the value for PhCO2 from the equation
-
E°PhCO2
-
- E°
-
(V) ) D
- D
CH CO -H
3 2
+
CH CO
PhCO -H
3
2
2
0.06(pK
- pK
)
a,CH CO H
2
a,PhCO H
2
3
assuming that the entropic terms, ∆SRX/R•+H•, are about the same in both
cases. (b) Eberson, L. Acta Chem. Scand., B 1984, 439. (c) Kolthoff, I. M.;
Chantoni, M. K.; Bhownik, S. J. Am. Chem. Soc. 1968, 90, 23. (d) Coetzee,
J. F. Prog. Phys. Org. Chem. 1967, 4, 76.
(
17) Hapiot, P.; Pinson, J.; Yousfi, N. New J. Chem. 1992, 16, 877.
Supporting Information Available: Cyclic voltammetric data and
determination of the cleavage rate constant and standard potential for
the formation of the anion radicals from these data (4 pages). See any
current masthead page for ordering and Internet access instructions.
-
(
18) (a) The peak potential of PhCOCH2 in DMSO is -0.10 V vs SCE
1
8b
at 0.1 V/s. The negative shift caused by a diffusion-controlled followup
dimerization may be estimated as 90-100 mV, leading to an E° value of
1
8b,c
0
1
.00.
(b). Bordwell, F. G.; Harrelson, J. A. Can. J. Chem. 1990, 68,
714. (c) Andrieux, C. P.; Hapiot, P.; Pinson, J.; Sav e´ ant, J.-M. J. Am.
Chem. Soc. 1993, 115, 7783.
JA9622564