Nature of the chain propagation in the photostimulated reaction of 1-bromonaphthalene with sulfur-centered nucleophiles

Article abstract of DOI:10.1021/jo062569+

(Chemical Equation Presented) The reactivity of ^-SC(NH)NH ₂ (1), MeCOS^- (2), and PhCOS^- (3) toward 1-naphthyl radicals was studied in DMSO. The photostimulated reaction of anions 1, 2, and 3 with 1-bromonaphthalene (4) after quenching with MeI renders 1-(methylthio)naphthalene (6) as a main product together with bis(1-naphthyl) sulfide (7) and naphthalene (5). The thioacetate ion (2) and thiobenzoate ion (3) were unreactive toward 4 as electron-donor under photostimulation; however, in the presence of potassium tert-butoxide anion (entrainment conditions), they gave the mentioned products 5, 6, and 7, after the addition of MeI. Quenching of the triplet state of 4 was assigned as the photoinduced initiation step, with a rate constant value of (4.6 ± 0.5) × 10⁸ M^-1 s^-1 for tert-butoxide anion and a rough estimated value of (8 ± 7) × 10⁷ M^-1 s^-1 for anion 1. By using hydrogen abstraction from DMSO as the competitive reaction, the absolute rate constants for the addition of anions 1, 2, and 3 to 1-naphthyl radicals have been determined to be 1.0 × 10⁹, 1.2 × 10 ⁹, and 3.5 × 10⁹ M^-1 s^-1, respectively. This reactivity order is in agreement with the stability of the resulting radical anions (ArNu)^?- (10-12)^?-. The inhibition experiments of the photoinduced substitution reaction in the presence of radical scavengers and the global quantum yield higher than the unity are evidence of a radical chain mechanism for these substitution reactions by anions 1 and 2. Anion 3 adds to the 1-naphthyl radical, but is neither able to initiate nor to keep the propagation cycle. Evaluation of the electron-transfer driving forces for the reaction between (ArNu) ^?- and 4 together with the absence of a chain reaction for the anion 3 indicate that the propagation in the proposed mechanism is given by an acid-base reaction between the radical ^.C(O)Me or ^.C(NH)NH₂ (13) and a base.

Full text of DOI:10.1021/jo062569+

Nature of the Chain Propagation in the Photostimulated Reaction of
1-Bromonaphthalene with Sulfur-Centered Nucleophiles
Luciana C. Schmidt, Juan E. Arg u¨ ello,* and Alicia B. Pe n˜ e´ n˜ ory*
INFIQC, Departamento de Qu ´ı mica Org a´ nica, Facultad de Ciencias Qu ´ı micas, UniVersidad Nacional
de C o´ rdoba, Ciudad UniVersitaria, 5000 C o´ rdoba, Argentina
penenory@fcq.unc.edu.ar; jea@fcq.unc.edu.ar
ReceiVed December 14, 2006
-
-
-
2
The reactivity of SC(NH)NH (1), MeCOS (2), and PhCOS (3) toward 1-naphthyl radicals was studied
in DMSO. The photostimulated reaction of anions 1, 2, and 3 with 1-bromonaphthalene (4) after quenching
with MeI renders 1-(methylthio)naphthalene (6) as a main product together with bis(1-naphthyl) sulfide
(
7) and naphthalene (5). The thioacetate ion (2) and thiobenzoate ion (3) were unreactive toward 4 as
electron-donor under photostimulation; however, in the presence of potassium tert-butoxide anion
entrainment conditions), they gave the mentioned products 5, 6, and 7, after the addition of MeI. Quenching
(
of the triplet state of 4 was assigned as the photoinduced initiation step, with a rate constant value of (4.6
8
-1 -1
7
-1 -1
(
0.5) × 10 M
s
for tert-butoxide anion and a rough estimated value of (8 ( 7) × 10 M
s
for
anion 1. By using hydrogen abstraction from DMSO as the competitive reaction, the absolute rate constants
9
for the addition of anions 1, 2, and 3 to 1-naphthyl radicals have been determined to be 1.0 × 10 , 1.2
9
9
-1 -1
×
10 , and 3.5 × 10 M s , respectively. This reactivity order is in agreement with the stability of the
•- •-
resulting radical anions (ArNu) (10-12) . The inhibition experiments of the photoinduced substitution
reaction in the presence of radical scavengers and the global quantum yield higher than the unity are
evidence of a radical chain mechanism for these substitution reactions by anions 1 and 2. Anion 3 adds
to the 1-naphthyl radical, but is neither able to initiate nor to keep the propagation cycle. Evaluation of
•
-
the electron-transfer driving forces for the reaction between (ArNu) and 4 together with the absence of
a chain reaction for the anion 3 indicate that the propagation in the proposed mechanism is given by an
acid-base reaction between the radical C(O)Me or C(NH)NH
‚
‚
2
(13) and a base.
1
Introduction
been widely reviewed. The mechanism of this reaction is a
chain process with radicals and radical anions as intermediates,
whose initiation step involves an ET to the substrate. The general
accepted pathways for the propagation cycle are outlined in
Scheme 1.
Radical nucleophilic substitution involving electron-transfer
(ET) steps or SRN1 mechanism has proven to be a versatile
mechanism for replacing an adequate leaving group at the ipso
position by a nucleophile. This process has a considerably broad
scope in relation to substrates and nucleophiles, and the most
relevant results as well as synthetic applications have
Although the most extensively used initiation method is a
photoinduced ET (PET) from a charged nucleophile to the
substrate, there are not many studies on its mechanism. These
PETs can be accomplished in one of the following pathways:
(1) photoexcitation of a charge-transfer complex (CTC) formed
*
Corresponding authors. Phone: (+54) 351-4334170/73. Fax: (+54) 351-
4
333030/4334174.
10.1021/jo062569+ CCC: $37.00 © 2007 American Chemical Society
2
936
J. Org. Chem. 2007, 72, 2936-2944
Published on Web 03/10/2007

Chain Propagation in the Reaction of 1-Bromonaphthalene
SCHEME 1
SCHEME 2
FIGURE 1. UV-vis spectra of 4 and anion 1 in DMSO.
2
-6
between the nucleophile and the substrate followed by ET;
3
(
2) homolytic cleavage of the C-X bond; (3) ET from an
excited nucleophile to the substrate; (4) ET from a nucleophile
to an excited substrate; and (5) photoejection of an electron
yield of 0.127 was determined for substitution of neophyl iodide
7,8
4
by the enolate anion of acetophenone. The quantum yield of
9
initiation, evaluated by the suppression of the propagation steps
from an excited nucleophile.10
with the presence of the radical trap di-tert-butylnitroxide
For nucleophiles that are unreactive at initiation but quite
reactive at propagation, the addition of minute amounts of
another nucleophile able to initiate the reaction increases the
generation of intermediates. This allows the less reactive
initiation-nucleophile to start its own propagation. The process
thus can afford the substitution of a poor electron-donor
nucleophile through an entrainment reaction, which is also a
support for the chain nature of the reaction.
-3
(
DTBN), has a value of 10 . On this basis, the chain length or
Φpropagation obtained for the reaction is ca. 127. This is the first
report of Φpropagation for an SRN1 process, demonstrating that
overall quantum yields below 1 cannot be taken as a criterion
against a chain.
We have described for the first time the reactivity of sulfur-
11
centered nucleophiles such as thiourea anion and thioacetate
12
anion in photoinduced aromatic SRN1 as a “one-pot” method
Because the SRN1 is a chain mechanism, the overall reactivity
depends on the efficiency of the initiation, propagation, and
termination steps. The initiation step may be slow; nevertheless,
for the process to work efficiently, the chain propagation must
be fast and feasible to allow for long chains to build up.
The magnitude of the chain length of any SRN1 reaction can
easily be derived from the overall quantum yield by determining
the initiation quantum yield. The latter could be obtained when
the propagation cycle is eliminated or reduced significantly in
comparison with the initiation step. For instance, a quantum
for the synthesis of several sulfur aromatic compounds from
moderate to good yields. The use of commercially available,
easily handled thiourea, potassium tert-butoxide, and thioacetate
salts, and the mild conditions (room temperature under nitrogen
atmosphere in DMSO), make this process a very convenient
methodology for obtaining aryl thiols, alkyl aryl sulfides,
symmetrical or unsymmetrical diaryl sulfides, and diaryl dis-
ulfides (Scheme 2).
Moreover, with hydrogen abstraction from DMSO as a clock
2
-
reaction, the absolute rate constants for the addition of S ,
thiourea, and benzene thiolate anions to 1-naphthyl radicals have
9
9
been measured. The values obtained are 0.5 × 10 , 1.0 × 10 ,
(
1) (a) Rossi, R. A.; Pierini, A. B.; Pe n˜ e´ n˜ ory, A. B. Chem. ReV. 2003,
9
-1 -1
13
1
03, 71-167. (b) Rossi, R. A.; Pierini, A. B.; Santiago, A. N. In Organic
and 5.1 × 10 M s , respectively.
Reactions; Paquette, L. A., Bittman, R., Eds.; John Wiley & Sons, Inc.:
New York, 1999; Vol. 54, Chapter 1, pp 1-271. (c) Rossi, R. A.; Pierini,
A. B.; Pe n˜ e´ n˜ ory, A. B. Recent Advances in the SRN1 Reaction of Organic
Halides. In The Chemistry of Functional Group; Patai, S., Rappoport, Z.,
Eds.; Wiley: Chichester, 1995; Suppl. D2, Chapter 24, pp 1395-1485. (d)
Rossi, R. A.; Pe n˜ e´ n˜ ory, A. B. In CRC Handbook of Organic Photochemistry
and Photobiology, 2nd ed.; Horspool, W. M., Lenci, F., Eds.; CRC Press
Inc.: Boca Raton, FL, 2003; Chapter 47, pp 47-1-47-24. (e) Sav e´ ant, J.
M. Acc. Chem. Res. 1993, 26, 455-461. (f) Kornblum, N. Aldrichimica
Acta 1990, 23, 71-78. (g) Rossi, R. A.; Pe n˜ e´ n˜ ory, A. B. Curr. Org. Synth.
Even though the reactivity of thiourea and thioacetate anions
in the reactions with aryl halides is quite similar, these
nucleophiles show a very different behavior at the initiation
11,12
step.
The advantage of these reactions for the synthesis of
sulfur compounds together with the scarce information available
on the photoinduced initiation and the propagation steps have
prompted us to study the mechanism of these photostimulated
reactions. Therefore, we report herein the reactivity of thiourea
2
006, 3, 121-158.
2) Fox, M. A.; Younathan, J.; Fryxell, G. E. J. Org. Chem. 1983, 48,
(1), thioacetate (2), and thiobenzoate (3) anions versus 1-naph-
(
3
109-3112.
thyl radicals and a comparative and detailed study of the
photoinduced electron-transfer pathway at the initiation level
and their reactivity in the propagation steps. We have also
measured the quantum yields and the chain length for the
propagation cycle of these SRN1 reactions and provided evidence
for the identity of the chain carriers.
(
3) Hoz, S.; Bunnett, J. F. J. Am. Chem. Soc. 1977, 99, 4690-4688.
(4) Arg u¨ ello, J. E.; Pe n˜ e´ n˜ ory, A. B.; Rossi, R. A. J. Org. Chem. 2000,
6
5, 7175-7182.
5) Wu, B.; Zeng, F.; Ge, M.; Cheng, X.; Wu, G. Sci. China 1991, 34,
77; Chem Abstr. 1992, 116, 58463h.
(
7
(
6) Cheng, C.; Stock, L. M. J. Org. Chem. 1991, 56, 2436-2443.
(7) Arg u¨ ello, J. E.; Pe n˜ e´ n˜ ory, A. B. J. Org. Chem. 2003, 68, 2362-
2
1
2
368.
(
8) Tolbert, L. M.; Siddiqui, S. J. Org. Chem. 1984, 49, 1744-
(11) Arg u¨ ello, J. E.; Schmidt, L. C.; Pe n˜ e´ n˜ ory, A. B. Org. Lett. 2003, 5,
751.
4133-4136.
(9) Ivanov, V. L.; Eggert, L.; Kuzmin, M. G. High Energy Chem. 1987,
(12) Schmidt, L. C.; Rey, V.; Pe n˜ e´ n˜ ory, A. B. Eur. J. Org. Chem. 2006,
1, 284; Chem. Abstr. 1988, 109, 169594b.
2210-2214.
(10) Ahbala, M.; Hapiot, P.; Houmam, A.; Jouini, M.; Pinson, J.; Sav e´ ant,
(13) Arg u¨ ello, J. E.; Schmidt, L. C.; Pe n˜ e´ n˜ ory, A. B. ArkiVoc 2003, Part
(x), 411-419.
J.-M. J. Am. Chem. Soc. 1995, 117, 11488-11498.
J. Org. Chem, Vol. 72, No. 8, 2007 2937

Schmidt et al.
TABLE 1. Reactions of Sulfur-Centered Nucleophiles with 1-Bromonaphthalene (4) in DMSOa
product yield (%)c
conditions
time (min)
comp.
entry
“S”-
addedb
convn
5
6
7
d,e
1
1
180, dark
15, hν
15, hν
180, hν
15, hν
15, hν
15, hν
15, hν
90, hν
90, hν
180, hν
d,e
2
91
25
9
2
48
9
19
8
d,e
3
DTBN
4
2
f
5
92
3
48
34
f
6
7
p-DNB
DTBN
TEMPO
f
22
39
<2
12
8.5
11
5
39
f
8
9
3
f
1
1
0
1
72
100
3
4
39
43
34
31
f
a
ArX ) 4, 0.05 M; “S” sulfur-centered nucleophiles, 0.25 M. After irradiation under nitrogen atmosphere, the reaction was quenched by MeI. b Compounds
-
c
added 0.01 M. Determined by GC using the internal standard method, error 5%. The conversion (convn) was determined by quantification of the recovered
substrate. Nucleophile/4 ratio of 10:1. From ref 11. f Together with 0.1 M t-BuOK.
d
e
Results
Reactions of the Sulfur-Centered Nucleophiles 1, 2, and
3
with 1-Bromonaphthalene (4). Table 1 summarizes the
results obtained for anions 1, 2, and 3 in their photoinduced
reactions with bromide 4.
The photostimulated reaction of anion 1 with 4 after quench-
ing with MeI renders naphthalene (5, 9%), 1-(methylthio)-
naphthalene (6, 48%), and bis(1-naphthyl) sulfide (7, 19%) (eq
4
). This reaction does not occur in the dark and is inhibited in
the presence of a very efficient radical trap like DTBN (Table
1
, entries 1, 2, and 3).
FIGURE 2. Transient absorption spectrum of 4 0.07 mM in DMSO:
(
9) 1 µs, (b) 3 µs, (2) 7.5 µs, and (1) 15 µs after the laser pulse.
Inset: Quenching effect of oxygen measured at 410 nm.
Bromide 4 absorbs maximally at 290 nm, whereas nucleophile
1
shows absorption below 280 nm (Figure 1). No charge
complex interaction could be detected by the UV-vis absorption
spectrum, 4 being mainly responsible for the photon absorption
The thioacetate anion (2) was unreactive toward 4 as electron-
1
4
in the preparative experiments. As a result, we have focused
on the photophysical properties of 4 to understand the nature
of the photoinduced initiation step of the reaction mechanism.
The photophysics of naphthalene derivatives is well-known, and
in the case of bromonaphthalenes they are not significantly
donor under photostimulation, but in the presence of potassium
tert-butoxide (t-BuOK) and under irradiation (entrainment
conditions), they gave 5 (3%), and sulfides 6 (48%) and 7 (34%)
after quenching with MeI (Table 1, entries 4 and 5).
This photoinduced reaction is completely suppressed by the
addition of a good electron-acceptor such as p-dinitrobenzene
1
5
dependent on the solvent employed. As it is expected,
-bromonaphthalene shows very weak fluorescence with a
1
(
p-DNB) (Table 1, entry 6) and is significantly inhibited by the
quantum yield value of 0.002 in DMSO. A strong absorption
band with a maximum at 410 nm and a lifetime (τ) of 8 µs is
observed by laser flash photolysis (LFP) using Nd:YAG laser
operating at 266 nm (Figure 2). The reported value for the
triplet-triplet (T-T) absorption of 4 in acetonitrile is 420 nm.15
This would indicate that the transient observed can be assigned
as a T-T band for the triplet excited state of 4 in DMSO; the
latter is confirmed by the strong quenching effect showed by
the presence of molecular oxygen (inset in Figure 2). A value
close to unity is estimated for the triplet quantum yield of 4 in
DMSO as compared to naphthalene, considering the reported
presence of radical scavengers like DTBN and TEMPO (Table
1, entries 7 and 8).
Nucleophile 3 fails to react with 4 even after 1.5 h of
irradiation (Table 1, entry 9). Although the expected products
, 6, and 7 are obtained under entrainment conditions, the global
5
reactivity of anion 3 is lower as compared to those of anions 1
and 2 (Table 1, entries 10 and 11).
Photoinduced Initiation Step and Laser Flash Photolysis
Measurements. To gain a better understanding of the photo-
induced initiation step, the photophysical properties of 4 are
studied in the presence and absence of anion 1. Nucleophiles 2
and 3 were not considered because they are not able to initiate
the reaction. By contrast, the base used for the formation of the
nucleophiles, t-BuOK, also the entrainment reactant in the
reactions of anions 2 and 3, is taken into consideration in this
photophysical study.
1
6
extinction coefficient value.
(
14) Foster, R. Organic Charge-Transfer Complexes; Academic Press:
London and New York, 1969.
15) Murov, S. L.; Carmichael, I.; Hung, G. L. Handbook of Photo-
chemistry, 2nd ed.; Marcel Decker Inc.: New York, 1993.
(
2938 J. Org. Chem., Vol. 72, No. 8, 2007

Chain Propagation in the Reaction of 1-Bromonaphthalene
SCHEME 3
TABLE 2. Rate Constant (kc) for the Reaction of 1-Naphthyl
a
Radical with Sulfur-Centered Nucleophiles in DMSO at 25 °C
“
(M)
S”-
kcb
10 (M s-1
)
9
-1
entry
kc9/kc“S”
1
2
3
1, 0.50
2, 0.25
2, 0.25
3, 0.25
3, 0.25
5
1.0
1.42
0.98
4.56
5.19
2.40
2.51
c
4
5
c
3.5
a
Photostimulation at 300 nm in 10 mL of DMSO, irradiation time 60
min. Anion benzene thiolate (9), 0.125 M; 4, 0.05 M. Product yield
b
determined by GC using the internal standard method. From duplicated
It was not possible to detect any transient absorption when
the nucleophiles studied were added to a solution of 4. On the
other hand, a dynamic quenching of the triplet state of 4 is
achieved by addition of t-BuOK in concentration over 0.10 mM.
experiments: average error e10%. c Together with 0.1 M t-BuOK. d Not
quantified.
TABLE 3. Quantum Yields and Chain Lengths of the PET
Reactions of 4 with Sulfur-Centered Nucleophilesa
8
-1
The quenching constant rate value was (4.6 ( 0.5) × 10 M
-1
-1
s
from the slope in a plot of τ against t-BuOK concentra-
b
φproducts
tion. Neutral thiourea has no effect on the triplet lifetime of 4,
and the reactivity of the anion 1 was difficult to measure because
of the interference of t-BuOK used as a base for the anion
formation. Contrary to our expectation, the addition of thiourea
to a solution of 4 with an excess of t-BuOK results in an increase
in the lifetime of the triplet state of 4. This unusual observation
is ascribed to the diminution of the concentration of t-BuOK
concomitant with the addition of thiourea, as t-BuOK is more
reactive than anion 1 toward the triplet state of 4. The value of
b
entry
1
“S”-
5
6
7
φglobal
chain length
1
1
1
2
3
0.22
0.25
0.44
0.19
0.10
0.27
1.68
2.01
0.91
1.02
0.07
0.37
0.39
2.27
2.65
1.35
1.33
0.17
0.27
7.7 ( 1.4e
c
2
d
3
4c
4.9 ( 0.5f
no chain
0.12
c
5
c
6
a
Performed under nitrogen atmosphere, using 0.05 M solutions of
substrate and 0.50 M solutions of the nucleophiles. b Determined at λ max
c
-
1
300 nm, from duplicated experiments: average error e10%. In the
the slope from a plot of τ versus the concentration of anion
is the same, but negative, as that determined for t-BuOK,
confirming the reactivity observed. A rough value of (8 ( 7)
d
presence of t-BuOK 0.1 M. In the presence of 0.25 M of 1,4-cyclohexa-
1
diene. e Calculated from the intercept of Figure 3 (using equation chain
-
f
length ) kNu[Nu ]/(kH[DMSO] + kH′[C6H8]). Calculated by eq 7 from
7
-1 -1
×
10 M
s
for the triplet state quenching by anion 1 could
the Discussion.
be estimated using NaOMe as a base.
Determination of the Rate Constant for Nucleophilic
Addition. From the reaction in Table 1, it is clear that anions
Once the yields from the substitution (6 and PhSAr) and
reduction (5) products were determined in the photoinduced
reactions of 1-bromonaphthalene with anions 1, 2, 3, and 9 in
excess, it was possible to calculate the kc using eq 5 and the
1
and 2 have similar reactivity toward 1-naphthyl radicals. Even
though the whole reaction for anion 3 is slower, the product
distributions are similar to those observed for anions 1 and 2.
Considering that the SRN1 process comprises initiation, propaga-
tion, and termination steps, it was interesting to evaluate the
relative reactivity of anions 1, 2, and 3 toward 1-naphthyl
radicals in competitive experiments. Because the three nucleo-
philes render the same product, 1-naphthalene thiolate ion (8),
we choose benzene thiolate anion (9) as a partner to carry out
separated competitive reactions, and to avoid the further
competition for 1-naphthyl radicals by anion 8, which would
finally give product 7.
18
relative reactivities using eq 6 (Table 2). For ion 9, we obtained
from the competitive experiments with 1, 2, or 3 similar values
9
-1 -1
for kc9, with an average value of (6 ( 2) × 10 M s .
kc [ArNu] [DMSO]
t
)
(5)
(6)
-
kH
[ArH] [Nu ]
t
o
-
kc9
[ArSPh] [“S” ]
t
o
)
kc1,2or3
[ArSMe] [9]
t
o
In addition, the absolute rate constants for the coupling (kc)
of 1-naphthyl radicals with anions 1, 2, 3, and 9 can be
determined in DMSO using hydrogen abstraction from the
solvent as competitive reaction, whose rate constant kH has a
Quantum Yield Measurements. The quantum yields (λ )
00 nm) for the reactions between 4 and anions 1, 2, and 3
were determined between 0.17 and 2.65. A total quantum yield
of 2.27 was obtained in the reaction with 1, which can be
improved to 2.65 with the addition of t-BuOK as initiator. Using
,4-cyclohexadiene (C6H8) as a radical scavenger, the product
is not observed, and the quantum yield drops to 1.35 (Table
). With increasing amounts of 1,4-cyclohexadiene from 0.1 to
.4 M, the ratio Φ5/Φ6 increases linearly with the concentration
of the radical scavenger with a slope ) (1.85 ( 0.09) and the
intercept ) (0.13 ( 0.02) (Figure 3). The slope of this plot
represents the ratio between the hydrogen abstraction rate
3
6
-1 -1
17
value of 7.1 × 10 M
s
at 25 °C. Three competitive
reaction pathways can take place for the intermediate aryl
radicals as outlined in Scheme 3: pathway (a), coupling with
anions 1, 2, or 3 to yield the substitution product 1-naphthalene
thiolate anion (8) after electron-transfer or fragmentation;
pathway (b), hydrogen abstraction from the solvent DMSO to
render the reduction product naphthalene; and pathway (c),
coupling with the benzene thiolate anion (9) to afford the
substitution product (1-naphthyl) phenyl sulfide after electron-
transfer.
1
7
3
0
-
(
18) [Nu ]o is the initial concentration of the nucleophile; ArH and ArNu
(
16) Carmichael, I.; Helman, W. P.; Hug, G. L. J. Phys. Chem. Ref. Data
987, 16, 239-260.
17) Andrieux, C. P.; Sav e´ ant, J.-M.; Su, K. B. J. Phys. Chem. 1986,
0, 3815-3823.
are the concentrations of the reduction and substitution products at time t.
These equations are based on the assumption that the reactions of the aryl
radicals with the nucleophiles and the solvent are first order in the latter
species, and their concentrations are constant during the experiments.
1
(
9
J. Org. Chem, Vol. 72, No. 8, 2007 2939

Schmidt et al.
CHART 1
the intermediacy of the anion radical, it can be inferred that the
•
-
standard potential of the 10b/10b couple must be more
negative than the effective reduction potential. Accordingly, the
0
Ep value represents an upper limit for E .
FIGURE 3. Quantum yield ratio determined at λmax 300 nm for the
reaction of 1 (0.5 M) and 4 (0.05 M) under nitrogen atmosphere with
Discussion
6 8
variable concentrations of 1,4-cyclohexadiene (C H ).
A detailed mechanistic study of the photochemical substitu-
tion reaction between thiourea (1), thioacetate (2), and thioben-
zoate anions (3) with 4 was performed in DMSO. We evaluated
the PET initiation, the addition rate constants of the nucleophiles
to the 1-naphthyl radical, the efficiency of the initiation and
propagation steps (the chain length), as well as the nature of
the chain carrier in the propagation steps.
Nucleophilic Substitution Reactions of 1-Bromonaphtha-
lene with Anions 1, 2, and 3. Mechanism for the PET
Reductions of 4 and Rate Constant for Nucleophilic Addi-
tion. Nucleophiles 1, 2, and 3 react under photostimulation with
constant of 1-naphthyl radical from 1,4-cyclohexadiene and the
addition rate constant to 1 (see discussion below, Figure 3).
1
9
For anions 2 and 3, the global quantum yields were 1.33 and
.17, respectively, both in the presence of t-BuOK as electron-
0
transfer donor (Table 3). The PET reaction between t-BuOK
and bromide 4 only yields naphthalene as product with a
quantum yield of 0.27 (Table 3).
Electrochemical Reduction of 1- and 2-Naphthalenethiol
Acetate and Benzoate. Electrochemical investigations have
been carried out in DMSO at room temperature; for preparative
purposes, 2-naphthalenethiol, acetate (10b), and 2-naphthale-
nethiol, benzoate (11b), were used instead of 1-substituted
derivatives. In the electrochemical reduction of 1- and 2-sub-
stituted naphthalene derivatives, it has been reported that neither
the reduction potential nor the fragmentation rate constant is
significantly dependent on the substituted position. For this
reason, results from compounds 10b and 11b are considered
equivalent to 10a and 11a (Chart 1).
In cyclic voltammetry, the two naphthalenethioesters show
one reduction peak. In the case of 11b, the peak measured at a
sweep rate faster than 5 V/s can be assigned to a one-electron
diffusion-controlled reversible process, with a standard reduction
potential value of -1.64 V.
4
to afford products 5, 6, and 7 after addition of MeI. Anion 1
is able to initiate the reaction and add to the 1-naphthyl radical,
while anions 2 and 3 are not reactive at the PET initiation step
toward 4 and t-BuOK was necessary as entrainment reactant.
The presence of naphthalene as a side product and its enhance-
ment by the addition of a well-known hydrogen donor such as
1
,4-cyclohexadiene are evidence for 1-naphthyl radicals as
2
0
reactive intermediates in these reactions.
All of these results together with the inhibition experiments
suggest that the reactions studied proceed by a radical chain
SRN1 mechanism (Scheme 1). This process comprises a pho-
toinduced initiation step, a radical addition of the 1-naphthyl
radical to the nucleophiles 1, 2, or 3, propagation, and
1
termination steps.
The three sulfur nucleophiles studied showed different
reactivity in the initiation and propagation steps of the SRN1
process. The PET from the anion to 4 depends mainly on the
oxidation potential of the anion, which is its ability as electron-
donor.
A different behavior was observed for compound 10b, whose
cyclic voltammetry shows a single one-electron irreversible
reduction peak in the available potential window, with a peak
potential Ep ) -2.08 V vs SCE at 0.2 V/s. The irreversibility
remained the same up to the maximum sweep rate accessible
Bordwell et al. have developed linear correlations of rates of
ET reactions with pKHA values. For example, linear plots of
2
1
(
100 V/s). The Ep for 10b, even after 21 mV correction, does
0
not represent the E value because of the irreversibility of the
peak. However, because the cyclic voltammetric data are
consistent with an electrochemical reductive cleavage involving
-
Eox (A ) versus pKHA with slopes near unity establish the
presence of an intrinsic relationship between oxidation potentials
of carbanions and their equilibrium acidities in DMSO. They
-
showed that an increase in the ion basicity there is an Eox (A )
(19) Because product 7 is not observed in these reactions and assuming
shift to a more negative potential. In general, it has been
observed that the higher is the pKHA, the higher is the electron-
donor capability of its conjugated base and so the higher is the
steady state for the intermediates in the SRN1 mechanism, (ΦArH)/
(
ΦArSMe) ) (kH[DMSO])/(kc[“S”]) + (kH′[C6H8])/(kc[“S”]).
20) Costentin, C.; Robert, M.; Sav e` ant, J. M. J. Am. Chem. Soc. 2004,
26, 16051-16057.
(
1
22
probability of a spontaneous ET reaction.
(21) At a low and moderate sweep rate (0.05-1 V/s), Ep varied linearly
Our experimental data clearly show that the weaker bases
thioacetate and thiobenzoate anions are not able to transfer one-
electron to the acceptor 1-bromonaphthalene even under pho-
with the logarithm of the sweep rate by 45 mV per unit on average with a
peak width Ep/2 - Ep ) 65 mV on average. These values are typical of an
“
EC” mechanism with a mixed kinetic control by E (electron-transfer
processes with the formation of the anion radical) and C (first-order cleavage
of the anion radical) steps. From these electrochemical experiments, the
reductive cleavage of 10b would follow a stepwise mechanism involving
the intermediacy of the anion radical. For references, see: Andrieux, C.
P.; Le Gorande, A.; Sav e` ant, J. M. J. Am. Chem. Soc. 1992, 114, 6892-
(22) (a) Bordwell, F. G.; Zhang, X. Acc. Chem. Res. 1993, 26, 510-
517. (b) Bordwell, F. G.; Zhang, X.; Filler, R. J. Org. Chem. 1993, 58,
6067-6071. (c) Bordwell, F. G.; Clemens, A. H.; Smith, D. E.; Begemann,
J. J. Org. Chem. 1985, 50, 1151-1156.
6
904 and references cited therein.
2940 J. Org. Chem., Vol. 72, No. 8, 2007

Chain Propagation in the Reaction of 1-Bromonaphthalene
SCHEME 4
t-BuOK and 4 only affords the reduction product naphthalene.
For this anion, which is only reactive at the initiation step, the
quantum yield for the formation of naphthalene can be taken
as an approximate value for the efficiency of the initiation step.4
The preparative photolysis experiments show a similar overall
reactivity for anions 1 and 2. Anion 3 also gives a product ratio
comparable to that of anions 1 and 2, although the reaction is
slower, requiring much longer irradiation time for a quantitative
conversion of 4. This distinct behavior indicates differences
between the nature of the chain propagation steps. For nonac-
tivated aromatic halides, the coupling reaction between the
nucleophile and the radical is the most important step in
determining whether an SRN1 process takes place. If this reaction
cannot compete efficiently with the termination steps, the chain
will be short, or even nonexistent.
toestimulation and need an additional source of electrons to
initiate the reaction, while thiourea anion behaves as a good
electron-donor under the same experimental conditions. (The
23
pKa values in DMSO are PhCOSH, 5.2; MeCOSH, undeter-
mined;24 and thiourea, 21.1. ) Furthermore, other thiolate
anions such as alkanethiolate anions (pKa from 17.9 to 10.3 for
23b
t-BuSH to PhCH2SH, respectively),2 sulfide anion (pKa HS :
3c
-
1
3), and benzene thiolate anion (pKa PhSH: 10.3) are able to
transfer one-electron to aryl halides and react by the SRN1
mechanism.1 On the other hand, thiocyanate anion (pKa
NCSH: 4) is not reactive both at initiation and at propagation
a,b
The rate constants for the addition of the nucleophiles studied
to the 1-naphthyl radical were determined by means of the
hydrogen atom abstraction from DMSO as competitive reaction.
Hence, the rate constant measured for thiourea anion (1.0 ×
13
steps. Thus, the behavior observed for sulfur-centered nucleo-
philes resembles the reactivity of ketone enolate carbanions in
comparison with nitronate anions in SRN1 reactions. While
ketone enolates anions are able to transfer one-electron to ArX
to initiate an SRN1 mechanism, nitromethane anions require
entrainment conditions to react under irradiation.2 (The pKa
values in DMSO are acetone, 26.5; acetophenone, 24.7; and
9
-1 -1
1
0 M s ) is comparable to the value obtained in liquid
9
-1 -1 29
ammonia toward 4-cyanophenyl radical (4.2 × 10 M s ).
5
Taking anion 1 as a reference, the following reactivity order is
-
-
-
obtained: SC(NH)NH2 (1) 1.00, MeCOS (2) 1.2, PhCOS
3) 3.5, and PhS- (9) 5. This order is in agreement with the
nitromethane, 17.2.23b
)
(
Finally, both thioacetate and thiobenzoate anions are reactive
by thermal ET when a better electron-acceptor such as the
expected one from the stability of the radical anion formed in
the addition step. The reversible reduction of 11b by cyclic
voltammetry indicates that the electrochemical radical anion
26
diazonium tetrafluoroborates derivative is used.
The nature of the PET (initiation step) was studied by means
of UV spectroscopy and LFP. There is no evidence of the
formation of a CTC between the nucleophile 1 and the bromide
•-
formed (11b ) is stable at the time scale of the voltammetric
experiments. On the other hand, an irreversible process is
observed for the reduction of 10b at the maximum sweep rate
accessible (100 V/s). Extra delocalization of the unpaired
electron over the phenyl ring leads to stabilization of the
corresponding radical anion for 11b.
4
, which allows us to disregard this possibility as a mechanism
for the ET reaction. Furthermore, quenching studies by LFP
clearly indicate that the triplet state of 4 is responsible for the
•
-
ET reaction with 1 and t-BuOK, giving 4 as a reactive
With a similar strategy, Galli and co-workers have measured
the rate constant for the addition of the pinacolone enolate anion
intermediate.2 Fragmentation of the latter renders the 1-naph-
7
28
thyl radical and bromide ion (Scheme 4).
8
to 9-anthracenyl and 1-naphthyl radicals (4.4 × 10 and 2.9 ×
The quenching rate constant values determined for anion 1
9
-1 -1
30a
1
0 M s , respectively), and, more recently, the coupling
8
8
-1 -1
and t-BuOK (0.8 × 10 and 4.6 × 10 M s , respectively)
are evidence for a quite efficient process with a quantum yield
of approximately 0.3. The photoinduced reaction between
30b
of various nucleophiles to vinyl radicals has been investigated.
In general, the addition of nucleophiles to aryl radicals, other
than the simplest phenyl radicals, is a very fast reaction ranging
8
10
-1 -1 1
from 10 to 10
M
s .
(
23) (a) Courtot-Coupez, J.; Le Demezet, M. Bull. Soc. Chim. Fr. 1969,
033-1039. (b) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456-463. (c)
Bordwell pKa table: www.chem.wisc.edu/areas/reich/pkatable/.
24) The pKa of MeCOSH should be 1 or 2 pK units higher than the
value of PhCOSH.
25) Borosky, G. L.; Pierini, A. B.; Rossi, R. A. J. Org. Chem. 1992,
7, 247-252.
26) (a) Petrillo, G.; Novi, M.; Garbarino, G.; Filiberti, M. Tetrahedron
989, 45, 7411-7420. (b) Petrillo, G.; Novi, M.; Garbarino, G.; Filiberti,
The differences in the global reactivity found for anions 1,
2, and 3 cannot be explained in terms of the coupling between
1-naphthyl radical and the corresponding anion given that these
anions show similar addition rate constants (between 1 and 3.5
1
(
(
9
-1 -1
×
10 M s ). Moreover, anion 3, which shows the minor
5
(
overall reactivity, adds faster to the naphthyl radicals. Therefore,
the differences in reactivity are given by their ability of building
up a chain, which is strongly dependent on the anion employed.
1
M. Tetrahedron Lett. 1988, 29, 4185-4188.
(27) In general, a stepwise process with consecutive ET and fragmentation
of the C-X bond is proposed for aromatic halides in which the radical
anion is an intermediate. By quenching studies of the fluorescent excited
state of 2-naphthoxide ion by aromatic halides, we have proposed a
concerted ET mechanism (C-X, BDE control) for bromo- and iodobenzene,
whereas 1-bromonaphthalene exhibits a stepwise process (π LUMO
Chain Propagation Steps: Efficiency and Chain Carrier.
The quantum yield measured for substitution (Φglobal) depends
both on the efficiency of initiation and on the turnover in the
propagation steps. A nucleophile may be reactive both at the
initiation step and in the coupling reaction with the radical (anion
7
control). Whereas a change from concerted to stepwise was observed by
changing the scan rate for PhI, PhBr and 1-iodonaphthalene follow a
stepwise mechanism over the whole range of scan rate under electrochemical
initiation. (a) Pause, L.; Robert, M.; Saveant, J.-M. J. Am. Chem. Soc. 1999,
1
), only as initiator (t-BuOK), at the propagation cycle (anion
2), or at the addition step (anion 3). A good approximation for
calculating the magnitude of the chain length (Φpropagation) of
1
21, 7158-7159. On the other hand, despite the lower driving force exerted
under homogeneous relative to that under electrochemical reduction, a
stepwise mechanism has been proposed for the reduction of PhI by different
radical anions. (b) Enemaerke, R. J.; Christensen, T. B.; Jensen, H.;
Daasbjerg, K. J. Chem. Soc., Perkin Trans. 2001, 2, 1620-1630.
(29) Combellas, C.; Dellerue, S.; Mathey, G.; Thi e´ bault, A. Tetrahedron
Lett. 1997, 38, 539-42.
(30) (a) Annunziata, A.; Galli, C.; Marinelli, M.; Pau, T. Eur. J. Org.
Chem. 2001, 1323-1329. (b) Branchi, B.; Galli, C.; Gentili, P. Eur. J. Org.
Chem. 2002, 2844-2854.
(
28) The cleavage rate constants for ArX have been determined in
8
-1
DMSO, and for 1-bromonaphthalene it is around 2 × 10 s . M’Halla, F.;
Pinson, J.; Sav e´ ant, J.-M. J. Am. Chem. Soc. 1980, 102, 4120-4127.
J. Org. Chem, Vol. 72, No. 8, 2007 2941

Schmidt et al.
SCHEME 5
any SRN1 reaction is by the ratio of the overall quantum yield
and the quantum yield of the initiation step (eq 7). The latter
could be obtained provided that the propagation cycle is
eliminated or reduced significantly in comparison with the
initiation step.
mechanism outlined in Scheme 5 is proposed. Addition of
4
1-naphthyl radical to anions 1, 2, or 3 yields the corresponding
•-
radical anion (10-12) . Two competitive reactions are possible
•
-
for the latter: in pathway (a), (10-12) transfer their odd
electron to bromide 4, which is the common step to continue
the propagation cycle of an SRN1 mechanism; in pathway (b),
chain length ) Φpropagation ) Φglobal/Φinitiation
(7)
•-
fragmentation of (10-12) gives the 1-naphthalenethiolate
anion (8) and the radical 13 (i.e., formamidinyl, acetyl, or
benzoyl depending on the nucleophile). For the first two radicals,
deprotonation mainly by the t-BuOK or anion 1 can afford the
radical anion 14. This radical anion is able to transfer the odd
Table 2 shows quantum yields higher than the unity for the
reactions of anions 1 and 2. These results indicate that the
substitution reactions occur by a radical chain reaction and for
anion 1 the addition of 1,4-cyclohexadiene quenches the
formation of product 7 but 6 remains present, although in lower
yield. The chain lengths for the reaction of bromide 4 with theses
anions can be estimated using different approaches. In the case
of anion 1, because suppression of the substitution product 6
cannot be achieved even at concentration of the radical
scavenger up to 0.4 M, the chain length for the reaction with
this anion can be estimated from the intercept in Figure 3. The
31
electron to 4 to allow for a chain to build up. In the case of
anion 3, the fragmentation pathway (b) leads to the benzoyl
radical (13, Y ) Ph, X ) O), which has no acidic hydrogen.
As a consequence, this fragmentation affords a further unreactive
benzoyl radical and becomes a termination step.
Pathway (a) has been suggested for the reactions of anions 2
and 3 with the arenediazonium tetrafluoroborates in aprotic
solvent, leading to the corresponding aryl thioesters as final
-
reciprocal of the intercept would represent the ratio kNu[Nu ]/
26
product in moderate to high yields. Therefore, by treatment
(kH[DMSO] + kH′[C6H8]) that accounts for the ratio between
of the potassium thioacetate or sodium thiobenzoate salts with
the propagation and the termination steps in the SRN1 mecha-
nism. Considering this approximation, an estimated value around
+
1
-naphthyl-N2 , the corresponding thioesters could be isolated
in up to 60% and 70% yield, respectively. In these reactions,
8
can be calculated for the chain length of the reaction between
•-
once the naphthalene thioester radical anion (10-11) is
4
and anion 1. The case of anion 2 is simpler. Because this
formed, ET to the arenediazonium salts is considerably fast
anion is not able to initiate the reaction, eq 7 can be used to
calculate the chain length. In consequence, with the overall
quantum yield of 1.33 and assuming the value of 0.27 as the
initiation quantum yield (t-BuOK alone), a chain length value
around 5 can be estimated for the reaction with anion 2.
The low quantum yield determined for the reaction between
anion 3 and 4 may suggest that the process is not a chain reaction
or that the addition step with 1-naphthyl radical is very slow or
that the initiation is not efficient. These last two assumptions
can be easily ruled out. First, good conversion is achieved at
longer irradiation time and the reduction product naphthalene
is formed in very low yields (Table 1), which is stated by the
measurements of the addition rate constants of 1-naphthyl radical
toward anions 1, 2, and 3 in competitive experiments (Table
(
pathway a) so that the fragmentation is not a competitive
reaction. This mechanism is consistent with the electrochemical
properties of the arenediazonium salts, which are powerful
electron-acceptors. For example, most of them have positive
reduction potential and are employed for electrode surface
modifications.
32
33
For the bromide derivative, the reaction follows pathway (b)
3
4
of the mechanism in Scheme 5. The detection of the
fragmentation product rather than the thioester derivatives and
the reaction quantum yield over the unity strongly support the
occurrence of pathway (b). This step involves fragmentation of
•
-
the radical anions (10-12) and the possibility of building up
(31) Another possibility would be the formation of new radical anions
2
). Second, the use of an entrainment reagent such as t-BuOK
by coupling of anions 1, 2, or 3 with radical 13, which are able to continue
with the propagation cycle. The absence of chain propagation and the fact
that not even traces of PhCOSCOPh were observed for anion 3 allow us to
disregard this possibility.
provides a quite efficient initiation step.
On this basis, the chain length or rather the existence of a
chain for these reactions depends on the efficiency of the
propagation steps in relation to the possible termination
pathways. To account for the results obtained herein, the
(32) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaumme, O.; Hitmi,
R.; Pinson, J.; Sav e´ ant, J.-M. J. Am. Chem. Soc. 1997, 119, 201-207.
(33) Pinson, J.; Podvorica, F. Chem. Soc. ReV. 2005, 34, 429-439.
2942 J. Org. Chem., Vol. 72, No. 8, 2007

Chain Propagation in the Reaction of 1-Bromonaphthalene
TABLE 4. Calculated ∆GET0 (kcal mol-1) for the ET Reaction
a main product together with bis(1-naphthyl) sulfide (7) and
naphthalene (5) after quenching with MeI. The aryl thioesters
are not intermediate products in these reactions because the
radical anion fragments into two reactive species: 1-naphthalene
thiolate anion and a new radical. In the case of anions 1 and 2,
the latter radical is able to continue with the propagation
accounting for a chain length; see below.
+
between 4, 1-Naphtahlene Diazonium Salt (1-NaphN2 ), and Radical
Anions 10b and 11b
thioester
4
1-naphN2+
•
•
-
-
1
1
0b
1b
6.2
11.5
-41.3
-36.0
Quenching of the triplet state of 4 was assigned as the
photoinduced initiation step, where the triplet excited state of
a chain when deprotonation of the radical 13 is likely. However,
the feasibility of process (b) is worth verifying on thermody-
namic grounds by evaluating the relative driving force of the
ET reaction between the donor radical anions (10a and 11a )
and the acceptor 1-bromonaphthalene. The free energy change
4
acts as a better electron-acceptor toward anion 1 and the
entrainment reactant t-BuOK. Anions 2 and 3 need an entrain-
ment reagent in agreement with the expected reducing power
and the pKa values of the conjugated acids. For their corre-
sponding bases, the lower is the pKa, the more positive is the
oxidation potential.
•-
•-
0
∆
GET for the ET from an electron-donor D to an electron-
0
•+
0
•-
acceptor A is given by eq 8 when E (D /D) and E (A/A )
are the standard potentials of the appropriate redox couples,
respectively, and ∆Ecoul is the Coulombic interaction energy
for the two singly charged radical ions formed in the SET
process.
The absolute rate constants determined for the addition of
9
anions 1, 2, and 3 to 1-naphthyl radicals are 1.0 × 10 , 1.2 ×
9
9
-1 -1
1
0 , and 3.5 × 10 M s , respectively, and the reactivity
order, kNu3 > kNu2 = kNu1, is in agreement with the stability of
the resulting radical anions (ArNu) .
•
-
0
-1
∆
GET (kcal mol ) )
Even anion 3 is the most reactive in the addition step; the
global reaction is the least efficient as compared to anions 1 or
0
•+
0
•-
2
3.06 [E (D /D) - E (A/A )] - ∆E
(8)
coul
2
. This can be explained taking into account the nature of the
chain propagation of the reaction.
Because reaction would occur between a radical anion donor
10a or 11a) and a neutral molecule (4) giving the corresponding
The inhibition experiments of the photoinduced substitution
reaction in the presence of radical scavengers and the global
quantum yield higher than the unity are evidence of a radical
chain mechanism for the substitution reactions by anions 1 and
2. Anion 3 adds to 1-naphthyl radical, but it is neither able to
initiate the reaction nor to keep the propagation cycle. Positive
values of the ET driving forces for the reaction between
(
neutral naphthalene thioester and 1-bromonaphthalene radical
anion, the Coulombic interaction (∆Ecoul) that is necessary to
separate opposite charged fragments is not considered.
0
The ∆GET value for the SET reaction can thus be calculated
using the redox potential of compounds 10b and 11b (used as
a model for compounds 10a and 11a, see the above results)
•
-
2
0
(ArNu) and 4 together with the absence of a chain reaction
for the anion 3 indicate that the propagation step is given by an
and the 1-bromonaphthalene available in the literature. Al-
0
though the E value for 10b is approximated, qualitative
‚
‚
acid-base reaction between the radical C(O)Me or C(NH)-
NH and a base to afford (OdCdCH ) or (HNdCdNH)
2 2
that transfer their odd electron to 4, continuing the chain.
information on the driving force of the reaction can be achieved.
•
-
•-
0
The ∆GET values calculated on this basis are reported in Table
3
5
0
4
. From the ∆GET values collected in Table 4, it results that
•
-
the reaction involving the more stable 11b is highly ender-
•
-
gonic, while for 10b its fragmentation is faster (see the
electrochemical results) and the reaction with 1-bromonaph-
thalene is also endergonic. These results are also in agree-
ment with a fragmentation process followed by proton-transfer
and electron-transfer as outlined in pathway (b), which is the
more plausible step to account for a chain reaction, while a direct
ET process (pathway a) is highly unlikely for bromide 4
Experimental Section
2 2
Chemicals. t-BuOK, SC(NH ) , MeCOSK and PhCOSH, 1-bro-
monaphthalene, naphthalene, thiophenol, p-dinitrobenzene, TEMPO,
and DTBN were all high-purity commercial samples, which were
used without further purification. DMSO was distilled under
vacuum and stored over molecular sieves (4 Å). Anions 1, 3, and
-
PhS were generated in situ by acid-base deprotonation using
(Scheme 5).
t-BuOK. 2- Naphthalenethiol, acetate (10b), and 2-naphthalenethiol,
benzoate (11b), were synthesized by reaction between 2-naphtha-
lenetiol and acetyl or benzoyl chloride.36
Conclusion
All products are known and exhibited physical properties
identical to those reported in the literature. Also, they were isolated
by radial chromatography from the reaction mixture and character-
1
-Bromonaphthalene (4) reacts with thiourea (1), thioacetate
(2), and thiobenzoate (3) anions by a photoinduced radical
1
13
ized by H and C NMR and mass spectrometry.
substitution reaction to afford 1-(methylthio)naphthalene (6) as
3
7
Registry no.: 1-(methylthio)naphthalene, [10075-72-6]; bis-
1-naphthyl) sulfide,38 [607-53-4]; 1-naphthyl phenyl sulfide,
39
(
[
(34) When the reaction is performed with an excess of 1-bromonaph-
36
7570-98-1]; 2-naphthalenethiol, acetate, [831-23-2]; 2-naphtha-
thalene, it was possible to observe 1-naphthalenethiol, acetate (10a) (see
ref 12). This clearly supports the competition between ET and fragmentation
steps as shown in Scheme 5. Furthermore, if ArSCOMe was an intermediate
in these reactions, we would be able to trap it by an intramolecular reaction
with an amino or hydroxy group ortho to the leaving group. In the
photoinduced reaction of o-amino-iodobenzene with thioacetate anion,
followed by addition of MeI, the only products observed were ArSMe with
the amino group mono- and di-methylated without any traces of the
benzenethiazole derivative.
40
lenethiol, benzoate, [10154-60-6].
Instrumentation. Time-Resolved Absorption Spectroscopy.
The laser flash photolysis system was based on a pulsed Nd:YAG
(36) Penn, J. H.; Liu, F. J. Org. Chem. 1994, 59, 2608-2612.
(37) Gilman, H.; Webb, F. J. J. Am. Chem. Soc. 1949, 71, 4062-4066.
(38) Hauptmann, H.; Wladislaw, B. J. Am. Chem. Soc. 1950, 72, 710-
712.
(35) It should be emphasized that the relative value to 10b must be
considered a lower limit in view of the preceding considerations. Therefore,
the actual DGET value for the ET reaction with 1-bromonaphthalene should
be larger than that reported in Table 4.
(39) Bunnett, J. F.; Creary, X. J. Org. Chem. 1974, 39, 3173-3174.
(40) Chen, C.-T.; Kuo, J.-H.; Pawar, V. D.; Munot, Y. S.; Weng, S.-S.;
Ku, C.-H.; Liu, C.-Y. J. Org. Chem. 2005, 70, 1188-1197.
0
J. Org. Chem, Vol. 72, No. 8, 2007 2943

Schmidt et al.
laser, using 266 nm radiation as the excitation wavelength. The
single pulses were of ca. 10 ns duration, and the energy was ca. 20
mJ/pulse. A Xe lamp was employed as the detecting light source.
The laser flash photolysis apparatus consisted of the pulsed laser,
the Xe lamp, a monochromator, a photomultiplier (PMT) system,
and oscilloscope. The output signal of the oscilloscope was
transferred to a personal computer for study.
Acknowledgment. We thank Prof. M. A. Miranda for LFP
facilities. This work was supported in part by the Consejo
Nacional de Investigaciones Cient ´ı ficas y T e´ cnicas (CONICET),
SECyT-Universidad Nacional de C o´ rdoba, and FONCyT,
Argentina. L.C.S. gratefully acknowledges receipt of a fellow-
ship from CONICET.
Electrochemistry. The working electrode was a 3 mm-diameter
glassy carbon electrode disk that was carefully polished and
ultrasonically rinsed in absolute ethanol before use. The counter
electrode was a platinum wire, and the reference electrode was an
aqueous SCE electrode. All experiments have been done at 20 °C.
Cyclic voltammetric data were recorded using a commercial
computer-controlled potentiostat.
Supporting Information Available: General methods, com-
pleted Table 2, quenching of the triplet of 4 by t-BuOK, and cyclic
voltammograms of 10b and 11b. This material is available free of
charge via the Internet at http://pubs.acs.org.
JO062569+
2944 J. Org. Chem., Vol. 72, No. 8, 2007