Steric effects of nucleophile-radical coupling reaction. Determination of rate constants for the reaction of aryl radicals with 2-naphthoxide anion

Article abstract of DOI:10.1039/b820374a

The absolute rate constants for the reaction of different aryl radicals with 2-naphthoxide anion were determined using an indirect method, a competition of the coupling reaction with the H-atom abstraction. We here show that the radical-ambident nucleophi

Full text of DOI:10.1039/b820374a

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Steric effects of nucleophile-radical coupling reaction. Determination of
rate constants for the reaction of aryl radicals with 2-naphthoxide anionw
Tomas C. Tempesti, Adriana B. Pierini and Maria T. Baumgartner*
Received (in Gainesville, FL, USA) 19th November 2008, Accepted 11th February 2009
First published as an Advance Article on the web 23rd March 2009
DOI: 10.1039/b820374a
The absolute rate constants for the reaction of different aryl radicals with 2-naphthoxide anion
were determined using an indirect method, a competition of the coupling reaction with the
H-atom abstraction. We here show that the radical-ambident nucleophile reactions are sensitive
to steric hindrance. A lower reactivity is found for 2-anisyl radical with respect to 4-anisyl and for
2-methoxy-1-naphthyl radical with respect to 1-naphthyl (k2b o k2a and k2d o k2c). The ortho
substitution to the radical centre decreases the rate constant. The reactivity is in agreement with
energy barriers determined by theoretical calculations.
The absolute value of the rate constant for the reaction of
Introduction
aryl radicals with some nucleophiles has been determined
The last decades have witnessed an explosive growth in the use
electrochemically. The phenyl radical appears to be less
of radical reactions in organic synthesis. Aryl radicals are
reactive than other aryl radicals. The rate constants for its
reaction with nucleophiles (PhSꢁ, (EtO)2POꢁ and
important reactive intermediates in many reactions of
–CH2COMe) are in the range 107ꢁ8
M
ꢁ1 sꢁ1 in liquid ammonia
synthetic relevance. For example, they can react with
at ꢁ33 1C (2.6 ꢂ 107, 3.8 ꢂ 108 and 2.7 ꢂ 108 Mꢁ1 sꢁ1
,
nucleophiles by the SRN1 mechanism to form a substitution
product (eqn (1)).1
respectively). Other radicals, such as 1-naphthyl, 3-pyridyl and
3- or 4-quinolyl radicals, react with rates close to the diffusion
s
limit (109 to 1010 Mꢁ1 ꢁ1).2
Arꢀ + Nuꢁ -- ArNu
(1)
In the SRN1 process,1 a photostimulated electron transfer from
the nucleophile anion to the substrate (aryl halide, ArX)
generated a radical anion of the precursor. The fragmentation
of the C–halogen bond originates the aryl radical. The radicals
thus formed can couple with the nucleophile to give the radical
anion of the product (eqn (2)–(4)).
The rate constants can be obtained by an indirect
method using a radical probe (eqn (6)). In this case the
substitution competes with the intramolecular cyclization.
The phenyl type radicals react with PhSꢁ and (EtO)2POꢁ
with a rate of 2.6 ꢂ 108 and 2.5 ꢂ 109 Mꢁ1 sꢁ1 in DMSO
at 25 1C.3,4
hn
ArX þ Nuꢁ ꢁ! ArXꢀꢁ þ Nuꢀ
ð2Þ
ꢁ
ArXꢀ - Arꢀ + Xꢁ
(3)
(4)
(5)
ð6Þ
Arꢀ + Nuꢁ - ArNuꢀ
ꢁ
ꢁ
ꢁ
ArNuꢀ + ArX - ArNu + ArXꢀ
Nevertheless, not many kinetic data are available for the
fundamental steps in which the aryl radicals are involved.
The determination of the rate constant of the radical coupling
reactions (eqn (4)) can contribute to the understanding of the
course of processes as well as to the achievement of valuable
synthetic results.
Another alternative to determining the coupling rate by
two competitive processes of the radical intermediate is by
employing the coupling with the nucleophile (eqn (4)) vs.
H-atom abstraction from the solvent. The rate constants for
the reaction between phenyl, 1-naphthyl or 9-anthryl radicals
and pinacolone anion in DMSO have been reported using this
methodology (3.3 ꢂ109, 2.9 ꢂ 109 and 4.4 ꢂ108 Mꢁ1 sꢁ1
,
respectively).5
´
INFIQC, Departamento de Quımica Organica, Facultad de Ciencias
Quımicas, Universidad Nacional de Cordoba, Ciudad Universitaria,
´
´
´
On the other hand, it is known that the nucleophile–radical
coupling reaction is less sensitive to steric hindrance than
expected. The ortho substituted aryl radicals react quite
well with many nucleophiles even when they have an
important steric demand. In the reaction of the enolate
anion of acetone with 2,4,6-triethylphenyl radical, a 70%
yield of coupling has been determined. However, the yield
´
Cordoba, 5000, Argentina. E-mail: tere@fcq.unc.edu.ar;
Fax: 54-351-4333030/4334170; Tel: 54-351-4334170/73
w Electronic supplementary information (ESI) available: 1H and 13C
NMR spectra of compound 3e. The energy of PES relevant points of
the reaction of the anion of 1 with different aryl radicals and solvents.
The hybridization change (sp2 to sp3) in the reaction path for the
coupling of 2-naphthoxide anion and 2-methoxy-1-naphthyl radical.
See DOI: 10.1039/b820374a
ꢃc
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decreases considerably (16%), accompanied by an increase in
the reduction product (ArH) when two i-Pr groups are
adjacent to the radical centre.6 The reaction of pinacolone
anion with 2,6-dimethylphenyl radical in DMSO afforded
reduction and substitution products in a ratio as high as
We studied the photoinitiated reactions of the anion of 1
with different aryl halides (2) (eqn (9)).
1
:
4. In contrast, the reaction with halobenzene
(phenyl radical) provides less than 1% of the H-abstraction
product.7 These results suggest that the approach of the
enolate ion is more difficult when the hindrance at the radical
centre increases.
ð9Þ
Recently, we have studied the reaction of the anion
of 2-naphthol (1) with the 2-methoxy-1-naphthyl radical
to obtain a disubstituted binaphthyl system (eqn (7)). Thus
the reaction offers an alternative route to synthesize
BINOL. We showed qualitative experimental evidence
of the steric effects.8 We observed a decrease in product
yield compared with the reaction of this anion with
1-naphthyl radical in similar conditions (35 and 52%,
respectively), accompanied by an increase in the reduction
(56 and 20%).
We began our studies by examining the reaction of the
anion of 1 with the 4-anisyl radical (2a). The substitution
product 3a and anisole are obtained in 54.0 and 8.6% yield,
respectively (see Scheme 1).
When the methoxy group is ortho to radical centre, the
percentage of substitution product decreased and that of
anisole increased (39.9 and 14.9%) (Scheme 1).
Similar results were observed in the reactions of 1-naphthyl
(2c) and 2-methoxy-1-naphthyl (2d) radicals. The first gave
65.1% of substitution and 10.1% of naphthalene. For 2d the
reduction increased to 45.9% and the substitution decreased
to 34.9% (see Scheme 1). These values confirm the effect of
ortho substitution on the radical centre.
The anthracenyl radical reacts with 2-naphthoxide anion
affording 55.1% of 3e and 39.9% of anthracene.
ð7Þ
Kinetic analyses of the reactions of aryl radicals with the
anion of 1 were performed. All the reactions were carried out
in DMSO at 25 1C. The yield of the products (ArNu and ArH)
formed were then accurately determined by gas chromatography.
In all cases, blank assays to verify the complete recovery of
reduced products were performed.10 The substituted aryl
radicals bear the same substituent (the OMe group) to enforce
the same electronic effect by the substituent. The concentration
of nucleophile, by far larger than that of 2a–e, remained
sufficiently constant throughout the experiments. Under
these conditions both competing processes (eqn (8)) were
pseudo-first order events. The yield of the products formed
was determined and kNu/kH calculated.
In order to determine the importance of the steric
hindrance, in this paper we report a first quantitative study
of the steric effect in the coupling reactions (eqn (4)).
The coupling rate constants for the reaction of aryl radicals
with the anion of 2-naphthol (1) were determined in
DMSO using hydrogen abstraction from the solvent as a
competitive reaction. Theoretical calculations were performed
in order to provide support for the experimental results
reported here.
We calculated values of kNu from kNu/kH and knowledge of
the kH value in DMSO obtained by radical clock and electro-
chemical experiments. The kH used is 2.8 ꢂ 106 Mꢁ1 sꢁ1 for
anisyl radicals,5 6.4 ꢂ 106 Mꢁ1 sꢁ1 for naphthyl radicals5 and
6.0 ꢂ 105 Mꢁ1 sꢁ1 for anthracenyl radicals.11 The results are
summarized in Table 1.
Results
In an SRN1 process,1 the radicals formed (eqn (2) and (3)) can
couple with the nucleophile to form the radical anion of the
substitution product or can abstract a H-atom from the
solvent (eqn (8)).
According to Table 1, kNu for 2a (5.1 ꢂ 108 Mꢁ1
s
ꢁ1) is
2.5 times higher than that for 2b (1.9 ꢂ 108 Mꢁ1
s
ꢁ1).
The naphthyl radicals 2c and 2d provide a further example
of the effect of ortho substitution to a radical centre. The value
þNuꢁ
kNu
Sꢀ þ ArH þ ꢁSH Arꢀ ꢁ!ArNuꢀꢁ
ð8Þ
kH
The kNu value can be determined by product analysis
according to the approximation previously reported,9,5
whenever the reduced products (ArH) can be detected
accurately and the rate constant kH is known.
Scheme 1
ꢃc
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Table 1 Values of kNu obtained for the reaction of the anion of 1 with aryl halidesa
Substrate
[NuH]0/M
[ArX]0/M
[ArH]b/M
[ArNu]b/M
kNu/kH
kNu
2a, X = I
0.509
0.474
0.519
0.346
0.491
0.510
0.505
0.489
0.510
0.503
0.0520
0.0551
0.0676
0.0564
0.0476
0.0462
0.0544
0.0488
0.0526
0.0510
0.0045
0.0039
0.0101
0.0147
0.0057
0.0051
0.0250
0.0241
0.0210
0.0201
0.028
0.024
0.027
0.021
0.031
0.028
0.019
0.019
0.029
0.029
177.1
187.7
74.5
(5.1 ꢄ 0.1) ꢂ 108
(1.9 ꢄ 0.2) ꢂ 108
(10.2 ꢄ 0.2) ꢂ 108
(1.41 ꢄ 0.05) ꢂ 108
(2.42 ꢄ 0.07) ꢂ 107
2b, X = I
2c, X = I
2d, X = I
2e, X = Br
60.0
161.2
156.0
21.6
23.1
39.2
41.6
a
b
Photoinitiated reactions carried out under nitrogen at 25 1C; reaction time = 120 min. Determined by GLC and the internal standard method.
of kNu for 2d is sevenfold lower than that for 2c. In this case
the effect of substitute is higher than that observed for anisyl
radicals.
substituents is more severe for 2d. This radical has the lowest
reactivity of the series (2c 4 2a 4 2b 4 2d).
In order to complement the experimental results, the
potential energy surfaces (PES) for the coupling step were
calculated. We have demonstrated that the reactions of the
ambident nucleophiles and the radical are kinetically
controlled12 and that the TSs are located early along the
reaction coordinates. The rehybridization at the C1 of the
naphthyl moiety takes place at shorter C–C distances than
those of the localized TSs (see Fig. ESI 1w).
The PESs were inspected from the most stable conformer of
the radical anions formed by AM1 and B3LYP methods.13
Table 2 shows the difference in energy between the critical
structures located along the PESs. The energy information for
the main structures is provided in the ESI.w
Similar PES profiles were obtained with AM1 and B3LYP
for the coupling reaction of the anion of 1 and radicals 2a–e.
The radicals combined exothermically with the anion to form
the radical anion of the product. In the PES, radicals 2b and 2d
combined with the anion of 1 to form an electrostatic
encounter complex (IT) (Fig. 1) in which the hydrogen atoms
of the methoxy group interact with the oxygen centre of the
nucleophile. In the IT for the radicals 2a and 2c, the hydrogen
ortho to the radical centre interacts with this oxygen.
Anthracenyl radicals show a lower reactivity toward the
anion of 1 than that of radicals 2a and 2b. Similar results for
this radical were obtained with pinacolone anion.5
Discussion
In these reactions, the aryl radicals (Arꢀ) react regioselectivity
with the anion of 1 at the C1 of the naphthyl moiety. A change
in hybridization from sp2 to sp3 occurs in this position in the
formation of the radical anion of the substitution products
(eqn (10)). This radical anion has a rigid structure with
substitutes in both positions adjacent to the coupling
centre (C1).
ð10Þ
In the gas phase, the reactions take place without energy
barriers (negative DE) evaluated as the energy difference
between of the TS and the reactive. Different results are
obtained considering the IT (see Table 2). The DETS–IT
calculated by AM1 and B3LYP shows a similar tendency.
The 1-naphthyl radical 2c is the most highly reactive radical
and its energy barrier calculated is the smallest. The ortho
substitution at the radical centre increased the barrier. In
The lower reactivity of radicals 2b and 2d compared with 2a
and 2c, respectively, can be explained in terms of the steric
hindrance to the coupling at the radical centre to form the
radical anion intermediates. In the radical 2d, the two
hydrogens near the radical centre are replaced by two
substitutes, the OMe group, and the aryl moiety. In
consequence the steric compression exerted by the ortho
Table 2 Energy differences between PES relevant points of the reaction of the 2-naphthoxide anion with different aryl radicals derived from ArX
AM1
B3LYP (6-31G*)
B3LYP (6-31G*) Continuum solventd
DEb
DETS–IT
DEra
DEb
DETS–IT
DEr
DEb
DETS–IT
c
c
c
Ar
DEra
2a
2b
2c
2d
ꢁ46.69
ꢁ46.40
ꢁ43.99
ꢁ43.55
ꢁ0.72
ꢁ0.91
ꢁ1.68
ꢁ0.66
3.17
3.40
1.86
4.37
ꢁ36.56
ꢁ32.98
ꢁ34.97
ꢁ29.82
ꢁ4.50
ꢁ1.92
ꢁ5.84
ꢁ1.31
1.60
0.13
0.02
1.68
ꢁ33.03
ꢁ30.55
ꢁ30.52
ꢁ28.05
ꢁ1.28
1.52
0.47
2.00
0.35
2.92
ꢁ1.05
2.23
a
DEr (kcal molꢁ1) = reaction energy (energy difference between radical anion and reactants (anion of 1 + radical, distance = 10 A for AM1 and
15 A for B3LYP). DE (kcal molꢁ1) = energy difference between transition states and reactants (anion of 1 + radical, distance = 10 A for AM1
b
and 15 A for B3LYP). DETS–IT (kcal molꢁ1) = energy difference between transition state and intermediate (IT). Continuum solvent model for
DMSO, without geometry optimization. Differences in the total solution phase energies informed.
c
d
ꢃc
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the results obtained we conclude that the radical-ambident
nucleophile reactions are sensitive to steric hindrance.
The ortho substitution to the radical centre decreases the rate
constant (k2b o k2a and k2d o k2c). This effect is more
significant when the aryl system is a highly congested radical
centre (1-naphthyl 4 phenyl) k2d o k2b.
The theoretical calculations show that the order of the energy
barriers is 2-methoxy-1-naphthyl 4 2-anisyl 4 4-anisyl 4
1-naphthyl, in agreement with the experimental results.
These results are of great interest to show the limitation of
this reaction type in the syntheses of disubstituted biaryl
compounds.
Experimental
Fig.
1 Reaction path, in the gas phase, for the coupling of
2-naphthoxide anion and aryl radical.
General experimental methods
1H and 13C NMR spectra were recorded on a nuclear magnetic
resonance spectrometer with CDCl3 as solvent. Gas chromato-
graphic analyses were performed on
a chromatograph
(Hewlett Packard 5890 Series II) with a flame-ionization
detector and using a HP-1 capillary column (methyl silicone,
10 m ꢂ 0.53 mm ꢂ 2.65 mm film thickness). The GS/MS
analyses were carried out on a Shimadzu GC-MS QP 5050
spectrometer, employing a 30 m ꢂ 0.12 mm DB-5 MS column.
The distillation at reduced pressure was performed with a
Kugelrohl. Irradiation was conducted in a reactor equipped
¨
with two 400-W lamps emitting maximally at 350 nm
((Philips Model HPT, air and water refrigerated). Potentio-
metric titration of halide ions was performed in a pH meter
using an Ag/Ag+ electrode. Melting points were not
corrected. Column chromatography was performed on silica
gel (70–270 mesh ASTM).
Fig. 2 TSs of 2a–d with distance C1 of anion of 1 to the radical
Materials
centres indicated.
Potassium tert-butoxide, 4-iodoanisole, 2-iodoanisole, 1-iodo-
naphthalene and 9-bromoanthracene were commercially
available and used as received. 2-Naphthol was recrystallized
from ethanol–water.15 DMSO was stored under molecular
sieves (4 A). 1-iodo-2-methoxynaphthalene (2dI) was prepared
from reaction of 1-iodo-2-naphthol, potassium tert-butoxide
in DMSO with methyl iodide. 1-iodo-2-naphthol was prepared
from reaction of 2-naphthol, iodide and H2O2 in ethanol.16
consequence the reactivity decreases in agreement with the
experimental results.
The different results obtained by 2b and 2d (ortho substituted
radicals) are attributed to higher steric constraints (Fig. 2).
This effect increased in 2d and the DE correlated with this.
The solvent effect was taken into consideration in the DFT
methodology.14 In the presence of DMSO, the solvent of
the experimental reactions, the PESs are similar to those
calculated in the gas phase. The reactants are more stabilized
than all the other species, as expected from the higher charge
localization of the anion (see Tables in ESIw). The solvent
affects the TS of the reaction, but this effect is similar for all
the radicals and the barriers ordering determined in the gas
phase maintains under a continuum solvent; the DE for the
ortho substituted radicals being higher (2b 4 2a and 2d 4 2c).
The reactivity order is 2c 4 2a 4 2b 4 2d in agreement with
the experimental findings.
Photoinitiated reaction of the anion of 2-naphthol (1) with
haloaryls
The following procedure is representative of these reactions.
The reactions were carried out in a 25 mL three-neck
round-bottomed flask equipped with nitrogen inlet and
magnetic stirrer into the thermostatic bath. To 10 mL of dry
and degassed DMSO under nitrogen were added potassium
tert-butoxide (887 mg, 7.92 mmol) and then 2-naphthol
(733 mg, 5.09 mmol). After 15 min 4-iodoanisole (121.7 mg,
0.52 mmol) was added and the reaction mixture was irradiated
for 120 min. The reaction was quenched with an excess of
ammonium nitrate and water (30 mL). The mixture was
extracted twice with methylene chloride (20 mL); the organic
extract was washed twice with water, dried (MgSO4), and
quantified by GLC. The iodide ions in the aqueous solution
Conclusions
The absolute rate constant determined for the addition of the
anion of 2-naphthol to radicals 2a–e are 5.1 ꢂ 108, 1.9 ꢂ 108,
1.0 ꢂ 109, 1.4 ꢂ 108 and 2.4 ꢂ 107 Mꢁ1 sꢁ1, respectively. From
ꢃc
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were determined potentiometrically. The reduction products
(ArH) were compared by CGL with authentic commercial
samples.
The equilibrium geometries were obtained with complete
geometry optimization. The intermediates and transition
states were first located through the reaction coordinate
method and then refined with the appropriate procedure.
The stationary points were located by Hessian matrix
calculations: all positive eigenvalues for a minimum energy
species and one imaginary frequency for transition states. PEs
explorations within the B3LYP DFT functional and the
6-31G* basis set were carried out by the same procedure.
The solvent effect was modeled with a continuum model as
implemented in the G03 program. The solvent stabilization
was evaluated from single point calculations on the gas-phase
optimized geometries at the B3LYP/6-31G* theory level;
electrostatic and non-electrostatic contributions being considered.
All products are known and exhibited physical properties
identical to those reported in literature. Also, they were
isolated by column chromatography [petroleum ether–acetone
(9 : 1)] from the reaction mixture and characterized by 1H and
13C NNR and mass spectrometry.
1-(4-Anisyl)-2-naphthol (3a).17 m/z = 251 (17); 250 (100);
235 (22); 218 (9); 207 (22); 189 (12); 179 (15); 178 (23); 176 (10);
152 (13); 94 (11); 76 (11); 55 (10).
1-(2-Anisyl)-2-naphthol (3b).18 1H NMR: dH 3.67 (s, 3H);
5.18 (s, 1H); 7.04–7.12 (m, 2H); 7.17–7.25 (m, 5H); 7.38–7.47
(m, 1H); 7.71–7.75 (m, 2H). 13C NMR: dC 55.84; 112.01;
117.69; 117,85 (c); 121.55; 122.55 (c); 123.17; 124.81; 126.24;
128.02; 128.10 (c); 129.13; 129.50; 130.23; 133.38; 133.46 (c);
150.63 (c). m/z 251 (18); 250 (100); 235 (10); 219 (15); 218 (16);
207 (18); 191 (12); 190 (9); 189 (25); 179 (15); 178 (25); 176 (9);
152 (9); 95 (11); 94 (13); 76 (12).
Acknowledgements
This work was supported by the Agencia Co
(ACC), Consejo Nacional de Investigaciones Cientı
Tecnicas (CONICET) and Secretarıa de Ciencia y Tegnologı
(SECyT), Universidad Nacional de Cordoba, Argentina.
´
rdoba Ciencia
´
ficas y
´
´
´
a
´
1-Naphthyl-2-naphthol (3c).17 m/z = 271 (18); 270 (100); 269
(50); 268 (12); 255 (14); 253 (39); 252 (27); 251 (16); 250 (13);
241 (10); 240 (11); 239 (38); 135 (11); 126 (26); 125 (21);
120 (11); 119 (26); 113 (16).
T. C. T. thanks CONICET for the fellowship granted.
References
1 R. A. Rossi, A. B. Pierini and A. B. Penenory, Chem. Rev., 2003,
´
2-Methoxy-20-ol-1,10-binaphthalenyl (3d).8,19 1H NMR: dH
3.79 (3H, s); 4.90 (1H, s); 7.01–7.50 (8H, m); 7.83–7.92
(3H, m); 8.05 (1H, d). 13C NMR: dC 56.82; 113.90; 115.03 (c);
115.41 (c); 117.45; 123.22; 124.19; 124.81; 124.92; 126.40;
127.32; 128.13; 129.15 (c), 129.44 (c); 129.77; 131.06; 133.78 (c);
137.03, 151.25 (c); 156.02 (c). m/z 301 (16); 300 (100); 268 (8);
239 (7).
103, 71–167; R. A. Rossi, A. B. Pierini and A. N. Santiago, in
Organic Reactions, ed. L. A. Paquette and R. Bittman, Wiley,
New York, 1999, pp. 1–271.
2 C. Amatore, M. A. Oturan, J. Pinson, J. M. Saveant and
A. Thiebault, J. Am. Chem. Soc., 1985, 107, 3451–3459;
C. Amatore, C. Combellas, J. Pinson, M. A. Oturan,
S. Robveille, J. M. Saveant and A. Thiebault, J. Am. Chem.
Soc., 1985, 107, 4846–4853.
3 A. L. J. Beckwith and S. M. Palacios, J. Phys. Org. Chem., 1991, 4,
404–412.
4 A. Annunziata, C. Galli, M. Marinelli and T. Pau, Eur. J. Org.
Chem., 2001, 1323–1329.
5 B. Branchi, C. Galli and P. Gentili, Eur. J. Org. Chem., 2002,
2844–2854.
6 J. F. Bunnett and J. E. Sundberg, Chem. Pharm. Bull., 1975, 23,
2620–2628.
7 B. Branchi, C. Galli, P. Gentili, M. Marinelli and P. Mencarelli,
Eur. J. Org. Chem., 2000, 2663–2668.
8 M. T. Baumgartner, T. C. Tempesti and A. B. Pierini, Arkivoc,
2003, x, 420–433.
9 The equation used in the relative reactivity determination of
coupling reaction of nucleophiles vs. H-atom abstraction from
the solvent was:
1-(9-Anthracenyl)-2-naphthol (3e). Isolated by column
chromatography and eluted with petroleum ether: acetone
(90:10). M.p. 214–215 1C. 1H NMR: dH 4.67 (s, 1H);
6.80 (d, 1H); 7.10–7.18 (m, 1H); 7.27–7.54 (m, 8H); 7.92
(d, 1H); 8.02 (d, 1H); 8.13 (d, 2H); 8.67 (s, 1H). 13C NMR:
dC 116.62 (c); 117.54; 123.49; 125.06; 125.70; 160.00; 126.62;
126.75; 128.07; 128.42; 128.61; 128.75; 129.15; 130.26; 131.58
(c); 131.82 (c); 151.63 (c). m/z 321 (25); 320 (100); 319 (19):
317 (11); 316 (9); 303 (12); 302 (9); 289 (13); 252 (14); 250 (9);
158 (12); 151 (20); 150 (36); 145 (11); 144 (18); 143 (10); 138
(13); 132 (11); 131 (10). HRMS: calc. for C24H16O 320.12012;
found: 320.12033.
À
À
ÁÁ
ln ½Nuꢁꢅ0= ½Nuꢁꢅ0 ꢁ ½ArNuꢅt
kNu
kH
À
À
ÁÁ
¼
ln ½SHꢅ0= ½SHꢅ0 ꢁ ½ArHꢅt
where [Nuꢁ]0 and [SH]0 are initial concentrations and [ArNu]t and
[ArH]t are concentrations at time t of both products. This equation
is based on first-order reactions of the radicals, see J. F. Bunnett,
in Investigation of Rates and Mechanisms of Reactions,
ed. E. S. Lewis, Wiley-Interscience, New York, 3rd edn, 1974,
part 1, p. 159.
Computational procedure
All calculations were carried out with the semiempirical AM1
and B3LYP methods as implemented in Gaussian 03 within
the UHF (opened shell systems) or RHF (closed shell systems)
formalisms in order to properly account for the electronic
nature of the species under study. First, the most stable
conformers of the radical anions RNuꢁ. were evaluated
through an AM1 conformational search by scanning the two
or three main torsion angles. The conformers thus obtained
were then refined with complete geometry optimization. The
geometries thus found were used as starting points to study
the PEs corresponding to the C–C bond breaking/formation.
10 A photostimulated reaction of reduction products (anisole,
naphthalene and anthracene) was performed in standard
conditions and the product recuperated was determined by CG.
11 F. M’Halla, J. Pinson and J.-M. Saveant, J. Am. Chem. Soc., 1980,
102, 4120–4127.
12 M. T. Baumgartner, G. A. Blanco and A. B. Pierini, New J. Chem.,
2008, 32, 464–471.
13 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, T. Vreven Jr,
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi,
ꢃc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 1523–1528 | 1527

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V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma,
G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski,
S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas,
D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman,
J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski,
B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi,
R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson,
W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN 03,
Revision B.04, Gaussian, Inc., Pittsburgh, PA, 2003.
geometries at the B3LYP/6-31G* theory level, electrostatic and
non-electrostatic contributions being considered. In the PCM
model he solvent is represented as a polarizable continuum
(with dielectric constant e) surrounding the molecular complex at
an interface constructed by combining atomic van der Waal radii
with the effective probe radius of the solvent. Charges are allowed
to develop on this interface according to the electrostatic potential
of the solute and e, then the polarized reaction field of the solvent
acts back on the quantum mechanical description of the solute.
The wave function of the complex is relaxed self-consistently with
the reaction field to solve the Poisson–Boltzmann (PB) equations.
Solvent was represented with the following parameters: dielectric
constant and probe radius.
15 O. Perrin, L. Armarrego and D. Perrin, Purification of Laboratory
Chemicals, Pergamon, London, 2nd edn, 1980.
16 J. E. Marsh, J. Chem. Soc., 1927, 3164; L. Jurd, Aust. J. Sci. Res.
2A, 1949, 246.
17 M. T. Baumgartner, A. B. Pierini and R. A. Rossi, J. Org. Chem.,
1991, 56, 580.
18 R. Beugelmans and M. Chbani, New J. Chem., 1994, 18, 949.
19 C. Koy, M. Michalik, C. Dobler and G. Oehme, J. Prakt. Chem.,
1997, 660.
14 The solvent effect was modelled with Tomasi’s polarised
continuum model (PCM) [ S. Miertus, E. Scrocco and J. Tomasi,
Chem. Phys., 1981, 55, 117; S. Miertus and J. Tomasi, Chem. Phys.,
1982, 65, 239; M. Cossi, B. Barone, R. Camini and J. Tomasi,
Chem. Phys. Lett., 1996, 255, 327]. The solvent effect was evaluated
from single point PCM calculations on the gas-phase optimized
ꢃc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009
1528 | New J. Chem., 2009, 33, 1523–1528