Photoinduced One-Electron Oxidation of Benzyl Methyl Sulfides in Acetonitrile: Time-Resolved Spectroscopic Evidence for a Thionium Ion Intermediate

Article abstract of DOI:10.1021/acs.joc.5b01052

The photo-oxidation of 4-methoxybenzyl methyl sulfide (1a), benzyl methyl sulfide (1b), and 4-cyanobenzyl methyl sulfide (1c) has been investigated in the presence of N-methoxy phenanthridinium hexafluorophosphate (MeOP⁺PF₆^-) under nitrogen in CH₃CN. The steady-state photolysis experiments showed for the investigated sulfides exclusively the formation of the corresponding benzaldehyde as the oxidation product, reasonably due to a deprotonation of the sulfide radical cations. Photo-oxidation of 1a-1c occurs through an electron transfer process. Indeed, laser flash photolysis measurements showed an efficient formation of sulfide radical cations, detected in their dimeric form [(4-X-C₆H₄CH₂SCH₃)₂+] at ≈520 nm. At longer delay times, the absorption of the dimer radical cation was replaced by an absorption band assigned to the (α-thio)benzyl cation (thionium ion, γ_max = 420-400 nm), formed by oxidation of the benzyl radical and not by that of the (α-thiomethyl)benzyl radical, as expected if a C_α-H bond cleavage is operative. This finding highlights a particular stability of this kind of cation never reported before, even though its involvement in one-electron oxidation mechanisms of various sulfides has already been invoked. Density functional theory calculations allowed identification of a significant charge and spin delocalization involving both the phenyl ring and the sulfur atom of the radical cations.

Full text of DOI:10.1021/acs.joc.5b01052

Page 1 of 25
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Photoinduced OneꢀElectron Oxidation of Benzyl
Methyl Sulfides in Acetonitrile. TimeꢀResolved
Spectroscopic Evidence for a Thionium Ion
Intermediate
Marta Bettoni, § Tiziana Del Giacco,*,†‡ Marina Stradiotto,† Fausto Elisei†‡
†Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, Via Elce di Sotto 8, 06123
Perugia, Italy
‡Centro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Università di Perugia, Via Elce di
Sotto 8, 06123 Perugia, Italy
§Dipartimento di Ingegneria Civile ed Ambientale, Università di Perugia, Via G. Duranti 93, 06125
Perugia, Italy
Email: tiziana.delgiacco@unipg.it
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
ABSTRACT: The photooxidation of 4ꢀmethoxybenzyl methyl sulfide (1a), benzyl methyl sulfide (1b)
and 4ꢀcyanobenzyl methyl sulfide (1c) has been investigated in the presence of Nꢀmethoxy
−
phenanthridinium hexafluorophosphate (MeOP+PF6 ) under nitrogen in CH3CN. The steady state
1
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 2 of 25
1
2
3
4
5
6
7
8
9
photolysis experiments showed for the investigated sulfides exclusively the formation of the
corresponding benzaldehyde as oxidation product, reasonably due to a deprotonation of the sulfide
radical cations. Photooxidation of 1aꢀ1c occurs through an electron transfer process. Indeed, laser flash
photolysis measurements showed an efficient formation of sulfide radical cations, detected in their
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
+•
dimeric form [(4ꢀXꢀC6H4CH2SCH3)2 ] at ca. 520 nm. At longer delay times, the absorption of the
dimer radical cation was replaced by an absorption band assigned to (αꢀthio)benzyl cation (thionium
ion, λmax = 420ꢀ400 nm), formed by oxidation of the benzyl radical, and not by that of the (αꢀ
thiomethyl)benzyl radical, as expected if a CαꢀH bond cleavage is operative. This finding highlights a
particular stability of this kind of cation never reported before, even though its involvement in oneꢀ
electron oxidation mechanisms of various sulfides has already been invoked. DFT calculations allowed
to identify a significant charge and spin delocalization involving both the phenyl ring and the sulfur
atom of the radical cations.
Introduction
Organic sulfur radical cations are important intermediates in a great variety of chemical processes,
extending from those of industrial importance to the synthetic and biological ones.1 Most studies have
been carried out on the main reaction pathways of dialkyl sulfide radical cations, only more recently
radical cations from aromatic sulfides (aryl sulfides having general formula ArSR) have attracted
considerable attention.2 These intermediates, investigated after generation by photoinduced electron
transfer, radiolysis or chemical and electrochemical oxidation of sulfides, can decay through
competitive pathways whose relative rate constants depend strongly on the structure of the substrate
and the experimental conditions (solvent polarity, additives, etc.). Generally, sulfide radical cations
undergo the most common deprotonation reaction, that involves the cleavage of a CꢀH bond in β
position with respect to the sulfur atom (center of positive charge), together with the breaking of the Cꢀ
2
ACS Paragon Plus Environment

Page 3 of 25
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
S bond (αꢀcleavage), which leads to the formation of a sulfenyl radical and a carbocation (Scheme 1,
path a and b, respectively). Moreover, these intermediates can be attached at the sulfur by nucleophilic
species (Scheme 1, path c), as oxygen and superoxide anion to form sulfoxides.3
Scheme 1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
In particular, nucleophiles such as sulfides, can quickly attack sulfur radical cations forming dimeric
threeꢀelectron bonded complexes (two bonding σꢀ and one antibonding σ*ꢀelectrons), which are known
to establish an equilibrium with the molecular radical cation as formulated in eq. 1.
(1)
Asmus et al. have reported numerous examples which substantiated, by experimental data and
theoretical considerations, that the radical cations of aliphatic sulfides are stabilized by the formation of
a sulfurꢀsulfur threeꢀelectron bond.4 Sawaki et al. have argued the formation of dimers also from
aromatic sulfides (with a sulfur atom bound to an aromatic ring).5 They have investigated radical
cations of thioanisoles by laser flash photolysis and reported that both the σꢀ and πꢀtype dimers are
formed depending on the structure and concentration. More recent studies have further confirmed these
results.2h.,3d,6
Along this line, we have considered worthwhile to extend these investigations to the oneꢀelectron
photooxidation of benzyl methyl sulfides 1aꢀ1c, with different substituents in para position on the ring,
3
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 4 of 25
1
2
−
by MeOP+PF6 in CH3CN under nitrogen. These reactions have been studied by nanosecond laser flash
photolysis (LFP) and steadyꢀstate photolysis. The transient species produced, their decay pathways, and
the final photoproducts have been identified. The results of this study, reported herewith, have been
integrated by DFT calculations, in order to get information on the geometry of the radical cations and,
more importantly, the charge and spin distribution. This information was considered to be of great
utility to better understand on the dynamics of the radical cation fragmentation process and, in
particular, on the SOMO position.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Results
SteadyꢀState Photolysis. Steady state photolysis experiments were carried out by irradiating at 355 nm
−
a N2ꢀsaturated CH3CN solution of sulfides 1aꢀ1c in the presence of MeOP+PF6 . As already reported,7
in these experimental conditions the singlet excited state of Nꢀmethoxyphenanthridinium was
produced, which undergoes breaking of the NꢀO bond forming the phenanthridinium radical cation
(P+•). This species is a quite powerful oxidant (the estimated reduction potential is 2.0 V vs SCE in
CH3CN)7a able to accept one electron from the investigated sulfides because they have lower oxidation
potentials (Table 1), thus forming the corresponding sulfide radical cations (Scheme 2), highlighted by
LFP studies.
The photolysis of sulfides 1aꢀ1c produced benzaldehydes (2aꢀc) as only product (Scheme 3).8 The
1
products were identified with HꢀNMR and GCꢀMS techniques by comparison with authentic
specimens.
4
ACS Paragon Plus Environment

Page 5 of 25
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
Table 1. Chemical Yields (%) of the Photoproducts, Relative Rate (kX/kH) and Oxidation
Potentials (Ep), for the MeOP+ꢀSensitized Photooxidation Reaction of Benzyl Methyl Sulfides 1aꢀ
1c in N2–Saturated CH3CN
b
sulfide
1a (X=OCH3)
1b (X=H)
unreacted sulfide,a (%) XꢀC6H4CHO,a(%)
kX/kH
2.25
1
EP,c V
1.55
77
83
88
17
16
12
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1.63
1c (X=CF3)
0.47
1.73
−
a [Sulfide] = 1.0 × 10−2 M, [MeOP+PF6 ] = 5.0 × 10−3 M. Yields are referred to the initial amount of substrate. b Determined
−
by kinetic competitive experiments (GC analysis of unreacted substrates); [Sulfide] = 5.0 × 10−3 M, [MeOP+PF6 ] = 5.0 ×
10−3 M. c Oxidation peak potential vs SCE measured by cyclic voltammetry in airꢀequilibrated CH3CN; sweep rate 100 mVsꢀ
1
.
Scheme 2
Quantitative analysis, performed by GC using the internal standard method, showed a similar amount
of benzaldehyde for each sulfide (17, 16 and 12% for 1a, 1b and 1c, respectively) after 15 min of
irradiation (Table 1). The material balance was satisfactory (>94%) in all experiments.
The structure of photoproducts formed can reasonably be rationalized on the basis of the formation
of the radical cation followed by benzylic deprotonation (Scheme 4), as already suggested by Albini et
al.9 for the oxidation of analogous ethyl sulfides (PhCH2SCH2CH3) photosensitized by 9,10ꢀ
−
dicyanoanthracene (DCA) and 2,4,6ꢀtriphenylpyrylium tetrafluoroborate (TPP+BF4 ).
5
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 6 of 25
1
2
3
4
Scheme 3
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 4
The resulting (αꢀthiomethyl)benzyl radical (4ꢀXC6H4ꢀCH•SCH3) is then oxidized, likely by the groundꢀ
state Nꢀmethoxyphenanthridinium (vide infra), to the corresponding cation, which forms the final
product (benzaldehyde) by reaction with adventitious water. The disappearance of Nꢀ
methoxyphenanthridinium was checked by recording the UVꢀvisible absorption spectra before and
after irradiation; this analysis endorsed the consumption of two MeOP+PF6 molecules for each
−
molecule of sulfide. The sulfurꢀcontaining product CH3SH was not identified experimentally because
of its high volatility. Other detected products were those deriving from the NꢀO fragmentation of
MeOP+ (phenanthridine, P, and methanol).
The relative reaction rates, expressed as ratio of the photooxidation rate constant of 4ꢀXꢀ
C6H4CH2SCH3 (kX) and that of the unsubstituted sulfide (kH), were determined by the kinetic
competitive method (Table 1). As shown in Figure S1 (see Supporting Information), log(kX/kH) values
decrease linearly as the sulfide oxidation potential (Ep, Table 1) increases; this result is in line with a
6
ACS Paragon Plus Environment

Page 7 of 25
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
rateꢀdetermining electron transfer step from the benzyl methyl sulfide to the phenanthridinium radical
cation (Scheme 2). The slope of ꢀ0.16 mol/kcal is characteristic of a substrateꢀlike transition state in a
slightly exergonic electronꢀtransfer.10
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Laser Flash Photolysis Studies. Upon laser excitation (λexc = 355 nm) of N2ꢀsaturated CH3CN
solutions of sulfides 1aꢀ1c (1.0×10ꢀ2 M) and MeOP+ (1.4×10ꢀ4 M), a broad and intense absorption with
maximum around 520 nm was detected just after the laser pulse. As an example, the time resolved
spectra of the 1b/MeOP+ system are reported in Figure 1, while those of 1a and 1c are shown in
Figures S2 and S3 (Supporting Information). Then it seems to be reasonable the attribution of this
absorption to the threeꢀelectron bonded dimer radical cations, [(ArCH2,CH3)S∴S(ArCH2,CH3)]+ꢀtype,
formed according to eq. 1. Indeed the corresponding [(CH3)2S∴S(CH3)2]+ structure, having a hydrogen
atom in place of the aryl group, is reported to absorb at shorter wavelength (465 nm in aqueous
solution)11 but it is known that the electron induction, as could be that exerted by the aryl group, into
the threeꢀelectron (2σ, σ*) bond can affect the energy of the σ → σ* transition, which will result in a
decrease in bond strength and then to a red shift in the absorption spectrum.4b The absorption bands of
the molecular radical cations 1a+•ꢀc+•, produced by fast oxidation of sulfides 1aꢀ1c due to the
phenanthridine radical cation, which should be in rapid equilibrium with the dimer radical cations,
should be around 300 nm,12 an undetectable spectral position with our experimental setꢀup. The decay
rates of the dimer radical cations, thus corresponding to those of monomer radical cation 1a+•ꢀ1c+•,5,6a
were determined in the presence of nitrogen by following the kinetics at 600 nm. This wavelength,
higher than that corresponding to the λmax, was chosen in order to reduce the overlap of the dimer
absorption with that of other transients (vide infra). In all cases, the decay kinetics followed clean first
7
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 8 of 25
1
2
3
4
5
6
order laws, in accordance with a unimolecular fragmentation process. The lifetimes (
C are reported in Table 2.
τ) measured at 25°
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0.06
ꢀ
A
B
A
ꢀA
0.04
0
10
20
30
time, ꢁs
0.02
0.00
300
400
500
600
700
λ
,nm
Figure 1. Timeꢀresolved absorption spectra of the MeOP+ (1.4×10ꢀ4 M)/1b (1.0×10ꢀ2 M) system in N2ꢀ
saturated CH3CN recorded 0.88 (ꢀ), 2.5 (ꢀ), 7.4 (ꢀ), 18 (ꢀ) and 32 (ꢁ) ꢁs after the laser pulse. λexc
= 355 nm. Inset: decay kinetics recorded at 400 (A) and 530 (B) nm.
Table 2. Lifetimes of the Dimer Radical Cations Determined without (τ) and with the presence of
NO3 Base (τbase), and of the Cations (τ
C) Formed in the MeOP+ꢀSensitized Photooxidation
−
Reaction of Benzyl Methyl Sulfides 1aꢀ1c in N2ꢀSaturated CH3CNa
[(ArCH2,CH3)S∴S(ArCH2,CH3)]+
sulfide
4ꢀXꢀC6H4CH SCH3
b
τC
τbase
0.84
0.92
0.91
τ
1a (X=OCH3)
1b (X=H)
7.8
7.7
7.3
9.8
16
12
1c (X=CF3)
a Values in ꢁs. Experimental error of 10%. b [Bu4N+NO3 ] = 2.0 × 10−2 M.
−
The timeꢀevolution of the absorption spectra showed a slow decay of the ꢀA signal at 520ꢀ550 nm
(see insets of Figure 1 and of Figures S2ꢀS3) followed by the growth of the absorption in the 400ꢀ450
8
ACS Paragon Plus Environment

Page 9 of 25
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
nm region. The assignment of the absorbance at 400ꢀ450 nm to the (αꢀthiomethyl)benzyl radical
formed by deprotonation of the radical cations (Scheme 4) can be excluded on the basis of the
following considerations. Firstly, the timeꢀresolved spectra in the 400ꢀ450 nm region (see Figure S4)
were not influenced by oxygen, that rules out the uncharged nature of the transients. In addition, the
absorption maximum for this kind of radicals is expected at shorter wavelengths.12 Actually, the
experimental absorption spectra of C6H5CH•SCH3 radical was obtained by photolysis of the dicumyl
peroxide/1b system in CH3CN, which produced the absorption spectra shown in Figure S5. As already
reported,13 irradiation of dicumyl peroxide with UV light promotes the formation of cumyloxyl radical
[C6H5C(CH3)2O•] within the laser pulse (eq 2) which displays a broad band at 490 nm. The timeꢀ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
evolution of the absorption spectra shows that this signal decay (t1/2 = 0.6 ꢁs) is coupled with the
growth of the absorption at 340 nm (t1/2 = 0.4 ꢁs) that can be reasonably attributed to the formation of
C6H5CH•SCH3 (eq 3).14
(2)
(3)
−
On the other hand, on addition of NO3 base (as tetraꢀnꢀbutylammonium salt, TBAN, 2.0×10ꢀ2 M),
the lifetimes of the cation radicals 1a+•ꢀ c+• were significantly shortened, as expected. In fact the decay
rates of the radical cation dimers, recorded in the presence of TBAN at higher wavelengths (580ꢀ600
nm) than λmax (in order to reduce the overlap of these transients with that absorbing at 400 nm),
followed clean firstꢀorder kinetics (see insets C of Figure 2 and Figures S6 and S7) with lifetime values
−
(see Table 2) about an order of magnitude lower than those obtained in the absence of NO3 . According
9
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 10 of 25
1
2
3
4
5
6
to these results there is a poor effect of the sulfide structure on the deprotonation rate of the radical
cation dimers.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ꢀ
A
B
A
0.03
ꢀ
A
C
0.02
0
2
4
6
time, ꢁs
0.01
0.00
300
400
500
600
λ
, nm
Figure 2. Timeꢀresolved absorption spectra of the MeOP+ (2.4×10ꢀ4 M)/1b (1.0×10ꢀ2 M) system in the
presence of 2.0×10ꢀ2 M TBAN recorded 0.18 (ꢀ), 0.22 (ꢁ), 1.8 (ꢀ), 3.0 (ꢀ) and 6.4 (ꢂ)
s after the
laser pulse in N2ꢀsaturated CH3CN. λexc = 355 nm. Inset: decay kinetics recorded at 400 (A), 450 (B)
ꢁ
and 580 (C) nm.
−
As observed, in the presence of NO3 , the radical cations generated a transient fully developed at
about 0.7, 1.8 and 2.7 ꢁs after the flash with λmax at 420, 410 and 400 nm for 1a, 1b and 1c,
respectively. The absorption spectra recorded just after the laser pulse appear as negative signal until
ca. 450 nm owing to the bleaching of the MeOP+PF6 ground state. From the insets it can be observed
−
that the decay of the radical cations at λ = 580 nm for 1b+• (inset C of Figure 2) and at 600 nm for 1a+•
and 1c+• (insets C of Figures S6 and S7, respectively) is practically synchronous with the buildup of the
−
transient at 400ꢀ420 nm (insets A of the corresponding Figures). The effect of NO3 can be understood
in terms of this anion acting as a Brönsted base with respect to the radical cation, endorsing the
hypothesis of the deprotonation as main reaction of the radical cation, responsible of the aldehyde
formation (Scheme 4). These results leads us to attribute the bands centered around 400ꢀ420 nm to the
10
ACS Paragon Plus Environment

Page 11 of 25
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
(αꢀthio)benzyl cations, 4ꢀXꢀC6H4CH+SCH3, obtained by oxidation of the corresponding (αꢀthio)benzyl
radicals, formed by CαꢀH cleavage of the radical cations. On the other hand, the bathochromic shift of
the cation absorption bands of about 80 nm compared to that of the corresponding radicals is in line
with what has already been observed with similar species.16
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The kinetic decay recorded at 420 nm is not modified by the presence of oxygen, thus confirming
the cationic nature of the transients (in Figure S8 the oxygen effect on the decay rate recorded at 420
nm is shown for 1a taken as an example). The missing evidence of 4ꢀXꢀC6H4C•HSCH3 in the timeꢀ
resolved spectra suggests that rate of its formation is slower than that of its disappearance. This aspect
will be discussed in the Discussion Section. The decay kinetics recorded at 400ꢀ410 nm were well
fitted by first order laws (lifetime values are collected in Table 2). The decay rate should be due to the
reaction of the cation with the residual water present in CH3CN to produce the only product observed
by us in steadyꢀstate photolysis, 4ꢀXC6H4CHO (Scheme 4). Indeed the lifetimes did not increase by
addition of water, probably because the amount of water in our medium (ca. 2×10ꢀ2 M) is sufficiently
high to trap completely the cation.17
Figure 3. Definition of geometrical features of radical cations 1a+● ꢀ1c+● reported in Table 3.
11
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 12 of 25
1
2
3
4
5
Table 3. Most Significant Dihedral Angle (deg) and Bond Lengths (Å) of Radical Cations 1a+● ꢀ
1c+● in AcN (CPCM model) Optimised by B3LYP/6ꢀ311G(d,p)
6
7
Φ
d1
d2
d3
d4
d5
d6
d7
8
9
1a+●
1b+●
1c+●
81
82
78
1.820
1.811
1.804
1.849
1.839
1.836
1.489
1.500
1.505
1.416
1.411
1.406
1.370
1.383
1.383
1.425
1.405
1.404
1.321
1.083
1.508
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Quantumꢀmechanical Calculations. Sulfide Radical Cations.
The most significant geometrical features (dihedral angles and bond length as defined in Figure 3) of
radical cations 1a+•ꢀ1c+• are reported in Table 3. From these data it can be observed that the
conformations for the radical cations have very similar geometries. In all of the energy minimum
conformations the CH2ꢀS bond is almost perpendicular to the phenyl ring (the dihedral angle Φ is
around 80°). The conformer at the minimum of energy of 1b+• is shown in Figure 4 (1a+• and 1c+•
conformers are reported in Figure S9 in Supporting Information).
Figure 4. Conformer at the minimum of energy of C6H5CH2SCH3+• (1b+•).
12
ACS Paragon Plus Environment

Page 13 of 25
The Journal of Organic Chemistry
1
2
3
4
5
6
Table 4. NPA Charges (qNPA) and Mulliken Spin Densities for the Most Stable Conformers of
Radical Cations 1a+● ꢀ1c+●
X
C6H4
CH2
S
CH3
7
8
1a+• (X=OCH3)
1b+• (X=H)
9
qNPA
0.042
0.538
0.064
0.265
0.089
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
spin
0.140
0.511
0.311
ꢀ0.019
0.097
0.361
0.450
0.007
0.141
qNPA
spin
0.369
0.103
0.017
0.098
0.640
0.524
ꢀ0.025
0.165
1c+• (X=CF3)
qNPA
0.110
0.002
spin
0.243
ꢀ0.019
0.749
0.025
The atomic charges were obtained by natural population analysis (NPA).18 Unpaired electron spin
densities were calculated using the Mulliken population analysis. From the NPA charges and spin
density data obtained for radical cations 1a+•ꢀ1c+• and reported in Table 4, it can be noted that the
charge and spin density of the radical cation are mainly localized on the Ar ring and sulfur. In
particular, charge and spin density increase significantly on the sulfur atom (from 0.265 to 0.524 and
from 0.361 to 0.749, respectively) at the expense of that on the Ar ring (from 0.538 to 0.103 and from
0.511 to 0.243, respectively) on going from the radical cation 1a+• to 1c+•. A reasonable explanation of
this distribution is that in the radical cation a throughꢀspace interaction between the aromatic ring and
the sulfur p orbital takes place, favored from the conformational geometry. This specific interaction is
well visualized in the SOMOꢀ2 representation for 1b+• shown in Figure 5 (1a+• and 1c+• conformers
are reported in Figure S10 in Supporting Information). The distribution of charge and spin between the
ring and sulfur can also be represented as a contribution of the resonance structures I and II to the
conformation of the radical cation (Scheme 5). Obviously, the higher the electron donating power of
13
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 14 of 25
1
2
3
4
5
6
the X substituent, the more the resonance structure II contributes to the resonance, as evidenced from
the data of Table 4.19
7
8
9
Some hyperconjugative interaction could be supposed between the CαꢀS bond and the π system of
the aromatic ring, which can be represented by a contribution of structure III (Scheme 5). The very low
charge on the CH2 group (Table 4) suggests, however, that structure III should be a much less
important contributor to the resonance hybrid of the radical cation conformations.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
LUMO
SOMO
SOMOꢀ1
SOMOꢀ2
Figure 5. Molecular orbitals of the energy minimum conformer of C6H5CH2SCH3+• (1b+•).
Scheme 5
H
.
+
.
.
+
X
X
CH2 SCH3
X
C
H
CH2 SCH3
+
SCH3
I
III
II
14
ACS Paragon Plus Environment

Page 15 of 25
The Journal of Organic Chemistry
1
2
3
4
5
6
7
Discussion
8
9
The results of the steadyꢀstate photolysis experiments have clearly indicated that in all cases
benzaldehydes are formed as the major primary reaction products. The carbonyl compounds are
supposed to come from oxidation followed by reaction with adventitious water of the (αꢀ
thiomethyl)benzyl carbon radical, which is formed by CαꢀH bond cleavage in the radical cation
(Scheme 4), the intermediate detected by laser flash photolysis experiments within the laser pulse. The
possibility of cleavage of the Cα-S bond (to give a benzyl cation and a methylthiyl radical) of 1a+•ꢀ1c+•,
together with the deprotonation from the benzylic position, as observed with benzyl phenyl sulfide
radical cations,3dꢀe,22 can be experimentally excluded. This is in line with a deprotonation reaction much
more exergonic than the CαꢀS bond cleavage.23
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Peculiarity of this study has been to point out, by flash photolysis experiments, that the intermediate
cation (4ꢀXꢀC6H4CH+SCH3), and not its precursor (4ꢀXC6H4ꢀCH•SCH3), is accumulated in the 1aꢀ1c
photooxidation. In fact, Asmus et al. observed, by pulse radiolysis investigation, RSR(ꢀH)• radical as
intermediate in the oxidation of dimethylsulfide and its derivatives by hydroxyl radical, formed by
deprotonation of ether monomer radical cation or of the dimer in equilibrium with it (eq 1).4a,f This
radical, probably present as hybrid of mesomeric forms (−CH•−S− ↔ −CH=S•−), absorbed at 280 nm
in aqueous solution. The formation of the corresponding cation RSR(ꢀH)+, as product of
disproportionation of the radical RSR(ꢀH)•, was detected not with absorption measurements (no distinct
band was identified on the spectrum), but by conductivity recording.4f The exclusive observation of the
cation in our case may be traced back to a different stability of the thionium ions, as supported by a
MNDO study performed on this kind of cation.25 In particular, this theoretical treatment has shown that
(αꢀthio)benzyl cation (PhCH+SCH3) is significantly more stable than the corresponding ion with
15
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 16 of 25
1
2
3
+
hydrogen in place of phenyl (CH3SCH2 ). The effect of the phenyl substituent on the energetics of
thionium ion formation was estimated through ꢀꢀH value, namely the difference between the enthalpy
change for the reactions shown in Scheme 6 and 7, quantified as ꢀ28 kcal/mol. The greater stability of
PhCH+SCH3 is due to the fact that its structure can be described in terms of various mesomeric forms
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
+
(Scheme 6), more numerous than those of CH3SCH2 (Scheme 7), as indicated by the values obtained
by calculation of the πꢀbond orders for the two cation structures, equal to 0.60 for CH−S and 0.61 for
C−Ph bonds in the benzyl cation, 0.86 for C−S and 0 for C−H bonds in the methylthiomethyl cation.25
Another interesting comparison is with the behavior of the corresponding aromatic thioether cations,
PhCH+SPh, intermediate species whose formation have been invoked in the oxidation of aryl benzyl
sulfides studied by pulse radiolysis and laser flash photolysis, but never experimentally observed.3d,e,h
This different behavior could be explained on the basis of the lower stability of PhCH+SPh with respect
to PhCH+SCH3, as the feasible conjugation between the sulfur atom and the near aromatic
π system
could prevent an efficient intervention of the sulfur to the delocalization of the positive charge.
Scheme 6
Scheme 7
16
ACS Paragon Plus Environment

Page 17 of 25
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
Another interesting observation is that the (αꢀthiomethyl)benzyl radical, to be oxidized by the
ground state sensitizer (see Scheme 4), should have an oxidation potential more negative than that of
MeOP+ (ꢀ0.81 V vs SCE in MeCN, obtained from the corresponding Nꢀethyl structure)7b and this is in
agreement with a very stable (αꢀthiomethyl)benzyl cation.26 The oxidation potentials of similar
radicals, such as C6H5ꢀCH•OCH3 and C6H5ꢀCH•OH (ꢀ0.33 V and ꢀ0.30 V vs SCE in CH3CN,
respectively)28,29 point out a significant lower stability of the corresponding cations and would not
make oxidation practicable.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
From the decay kinetic data of radical cations 1a+•ꢀ1c+• reported in Table 2, it can be noted that the
deprotonation rates are practically not affected by the substituent type, both in the absence and the
presence of the base (τ=7÷8 and ca. 0.9 ꢁs, respectively).30 On the contrary, the deprotonation rate
+•
constant of toluene radical cation (PhCH3 , k = 1×107 sꢀ1)31 in water is nearly five orders of magnitude
higher than that of 4ꢀmethoxytoluene (4ꢀCH3OꢀC6H4ꢀCH3 , k = 4×102 sꢀ1)32, taken as a reference. This
+•
+•
remarkable dependence on the substituent is explainable considering that 4ꢀCH3OꢀC6H4ꢀCH3
+•
becomes a weaker acid with respect to PhCH3 because the methoxy group (an electronꢀdonating
substituent) decreases its reduction potential (1.71 V, to compare with 2.35 V of toluene, both vs SCE
in CH3CN).33 In other words, on the basis of these data it can be stated that for radical cations of such
structure the kinetic acidity parallels thermodynamic acidity. In the case of benzyl methylꢀ and 4ꢀ
methoxybenzyl methyl sulfide radical cations, the significant amount of spin and positive charge
delocalized not only on the ring but also on the sulfur atom (as indicated by DFT calculations), makes
weaker the substituent effect. This is reflected by the difference in oxidation potential between 1a and
1b (0.08 V, against 0.64 V of the corresponding benzyl derivatives) and then on the kinetic acidity.
17
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 18 of 25
1
2
3
4
Conclusions
5
6
7
8
Radical cations 1a+•ꢀ1c+•, photochemically generated by MeOP+, were able to deprotonate and then to
form the corresponding aldehyde compounds. Benzyl radicals, namely the intermediates generally
observed after the CαꢀH bond cleavage, were not sufficiently accumulated to be detected by LFP
experiments, but the corresponding (αꢀthio)benzyl cations (thionium ions) were highlighted, coming
from their oxidation. In line with this assignment, the decay rate of these transients (a first order
process) were unaffected by oxygen, while their formation reactions were considerably accelerated by
the intervention of a base (TBAN). This result has been explained by invoking the stability of the
thionium ions, due to a delocalization of the positive charge extended to the whole structure. Until now
thionium ions had been pointed out by pulse radiolysis conductivity experiments as intermediates in the
oxidation of dialkyl sulfides, but their absorption spectra were never identified.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The lifetimes of the radical cations were very slightly sensitive to the nature of the substituent, both
with and without base. These results have been rationalized by considering (DFT calculations) that
charge and spin are delocalized among the ring and the sulfur atom, which lead to a resulting leveling
of the acid strength of the benzylic position.
Experimental Section
Starting Materials. 4ꢀMethoxybenzyl methyl sulfide (1a), benzyl methyl sulfide (1b) and 4ꢀ
trifluoromethylbenzyl methyl sulfide (1c) were prepared by reaction of the corresponding benzyl
mercaptans with CH3I in ethyl alcohol according to the procedure described in literature.34 Substrates
1a and 1b were characterized as already described.35 The purity of 1c was checked by NMR and GCꢀ
1
MS analysis. . HꢀNMR (400 MHz, CDCl3) of 1c:
δ 7.68 (d, 2H), 7.53 (d, 2H), 3.81 (s, 2H), 2.01 (s,
3H). 13C NMR (400 MHz, CDCl3): δ (ppm) 129.2, 128.8, 125.5, 124.2, 37.9; 14.9. GCꢀMS (70 eV, EI)
of 1c: m/z 206 (M+, 46), 187 (5), 159 (100), 139 (4), 109 (21), 89 (5), 63 (5). Benzaldehydes 2aꢀc and
18
ACS Paragon Plus Environment

Page 19 of 25
The Journal of Organic Chemistry
1
2
3
tetraꢀnꢀbutylammonium nitrate (TBAN) were commercially available. NꢀMethoxyphenanthridinium
4
5
6
−
hexafluorophosphate (MeOP+PF6 ) was prepared according to a literature procedure.7a, 36 Acetonitrile
7
8
9
of HPLC grade for product analysis and acetonitrile of spectrophotometric grade for spectroscopic
measurements were used as received.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
−
SteadyꢀState Oxidation. A solution prepared by dissolving MeOP+PF6 (5.0×10ꢀ3 M) and benzylic
sulfide (1.0×10ꢀ2 M) in 10 ml MeCN, was irradiated in an Applied Photophysics multilamp apparatus
with six phosphorꢀcoated fluorescent lamps (15 W each) emitting at 355 nm (ꢀλ1/2 = 20 nm), at running
water temperature (15 °C), with argon bubbling through the solution. After irradiation, the reaction
1
mixture was analyzed after adding an internal standard (bibenzyl) by GC , GCꢀMS and HꢀNMR and
all products formed were identified by comparison with authentic specimens. The sensitizer analysis
was performed by optical density measurements on HPꢀ8451 diode array spectrophotometer. The
material balance was always satisfactory (> 94%).
Blank experiments, performed by keeping a solution of MeOP+PF6 and sulfide in the dark or by
−
irradiation of solutions in the absence of the sensitizer, showed no formation of products.
Competitive Experiments The kinetic experiments were performed at 20 °C by irradiating (as above)
10 ml of N2ꢀsaturated CH3CN solutions containing MeOP+PF6 (5.0×10ꢀ3 M) and the two substrates
−
(both 5.0×10ꢀ3 M). The amounts of the products (benzaldehydes) were determined by GC with respect
to an internal standard at different times and the values were inserted into a suitable kinetic equation.37
The reported krel values are the average of at least three determinations.
Laser Flash Photolysis. Excitation wavelength of 355 nm (from a Nd:YAG laser, Continuum, third
harmonic, pulse width ca. 7 ns and energy < 3 mJ per pulse) was used in nanosecond flash photolysis
experiments.38 The transient spectra were obtained by a pointꢀtoꢀpoint technique, monitoring the
change of absorbance (
ꢀ
A) after the laser flash at intervals of 5ꢀ10 nm over the spectral range 300ꢀ800
19
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 20 of 25
1
2
nm, averaging at least 10 decays at each wavelength. A 2 ml solution containing the substrate (1.0×10−2
3
4
5
6
7
−
M) and the sensitizer (MeOP+PF6 , 5.0×10ꢀ3 M) was flashed in a quartz photolysis cell while nitrogen
8
9
was bubbling through them. The experimental error was ± 10%.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Cyclic Voltammetry. Ep values were obtained by cyclic voltammetry experiments, conducted with an
AMEL 552 potentiostat controlled by a programmable AMEL 568 function generator (cyclic
voltammetry at 100 mVsꢀ1, 1mm diameter platinum disc anode and SCE as reference) in CH3CN–
LiClO4 (0.1 M).
Computational Methodology Quantum mechanical calculations were carried out by using the
Gaussian 09 package.18 Charge and spin density distribution of the radical cations were obtained by
using the B3LYP functional, after geometrical optimization performed with the same DFT model. All
calculations were performed with a 6ꢀ311G(d,p) basis set.39
Acknowledgements. The authors thank MIUR (Ministero dell’Università e della Ricerca, Rome, Italy)
and University of Perugia (PRIN 2010–2011, no. 2010FM738P) for funding.
Supporting Information Available: Timeꢀresolved absorption spectra after LFP of the MeOP+/1a, 1c
systems with and without TBAN, MeOP+/1b system in O2ꢀequilibrated CH3CN and dicumyl peroxide
peroxide in the presence of 1b; decay kinetics of 1a+• in the presence of TBAN in N2ꢀ and O2ꢀ
equilibrated CH3CN; most stable conformers for 1a+• and 1c+•; SOMO of the energy minimum
1
conformers of 1a+• and 1c+•; H NMR and 13C NMR spectra of 4ꢀCNꢀC6H4CH2SCH3 (1c). This
material is available free of charge via the Internet at http://pubs.acs.org.
20
ACS Paragon Plus Environment

Page 21 of 25
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
References and notes
1. (a) Bobrowski, K.; HouéeꢀLevin, C.; Marciniak, B. Chimia 2008, 62, 728ꢀ734. (b) Schöneich, C.;
Pogocki, D.; Hug, G. L.; Bobrowski, K. J. Am. Chem. Soc. 2003, 125, 13700ꢀ13713. (c)
Butterfield, D. A.; Kanski, J. Peptides 2002, 23, 1299ꢀ1309. (d) Schöneich, C. Arch. Biochem.
Biophys. 2002, 397, 370ꢀ376. (e) Tobien, T.; Cooper, W. J.; Nickelsen, M. G.; Pernas, E.; O’Shea,
K. E.; Asmus, K.ꢀD. Environ. Sci. Technol. 2000, 34, 1286ꢀ1291. (f) Goto, Y.; Matsui, T.; Ozaki,
S.; Watanabe, Y.; Fukuzumi, S. J. Am. Chem. Soc. 1999, 121, 9497ꢀ9502. (g) Miller, B. L.;
Kuczera, K.; Schöneich, C. J. Am. Chem. Soc. 1998, 120, 3345ꢀ3356. (h) Bobrowski, K.; Hug, G.
L.; Marciniak, B.; Miller, B. L.; Schöneich, C. J. Am. Chem. Soc. 1997, 119, 8000ꢀ8011. (i)
Baciocchi, E.; Lanzalunga, O.; Pirozzi, B. Tetrahedron 1997, 53, 12287ꢀ12298. (j) Marciniak, B.;
Hug, G. L.; Rozwadowski, J.; Bobrowski, K. J. Am. Chem. Soc. 1995, 117, 127ꢀ134. (k) Ozaki, S.;
Ortiz de Montellano, P. R. J. Am. Chem. Soc. 1995, 117, 7056ꢀ7064. (l) Kobayashi, S.; Nakano,
M.; Kimura, T.; Schaap, A. P. Biochemistry 1987, 26, 5019ꢀ5022.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2. (a) Baciocchi, E.; Bettoni, M.; Del Giacco, T.; Lanzalunga, O.; Mazzonna, M.; Mencarelli, P. J.
Org. Chem. 2011, 76, 573ꢀ582. (b) Del Giacco, T.; Lanzalunga, O.; Mazzonna, M.; Mencarelli, P.
J. Org. Chem. 2012, 77, 1843−1852. (c) Baciocchi, E.; Del Giacco, T.; Gerini, M. F.; Lanzalunga,
O. Org. Lett. 2006, 8, 641ꢀ644. (d) Filipiak, P.; Hug, G.L.; Bobrowski, K.; Marciniak, B. J.
Photochem. Photobiol. A Chem. 2006, 177, 295–306. (e) Memarian H. R.; Baltork I. M.; Bahrami
K. Bull. Korean Chem. Soc. 2006, 27, 106ꢀ110. (f) Latour, V.; Pigot, T.; Simon, M., Cardy H.;
Lacombe, S. Photochem. Photobiol. Sci. 2005, 4. 221ꢀ229. (g) Filipiak, P.; Hug, G.L.; Carmichael,
I.; KorzeniowskaꢀSobczuk, A.; Bobrowski, K.; Marciniak, B. J. Phys. Chem. A 2004, 118, 6503–
6512. (h) Baciocchi, E.; Del Giacco, T.; Elisei, F.; Gerini, M. F.; Guerra, M.; Lapi, A.; Liberali, P.
J. Am. Chem. Soc. 2003, 125, 16444ꢀ16454.
21
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 22 of 25
1
2
3
4
5
6
7
8
9
3. (a) Lanzalunga, O.; Lapi, A. J. Sulfur Chem. 2012, 23, 101ꢀ129. (b) Peñéñory, A. B.; Argüello, J.
E.; Puiatti, M. Eur. J. Org. Chem. 2005, 1, 114ꢀ122. (c) Baciocchi, E.; Gerini, M. F.; Lanzalunga,
O.; Lapi, A.; Lo Piparo, M. G. Org. Biomol. Chem. 2003, 1, 422ꢀ426. (d) Del Giacco, T.; Elisei,
F.;Lanzalunga O. Phys. Chem. Chem. Phys. 2000, 2, 1701ꢀ1708. (e) Baciocchi, E.; Lanzalunga, O.;
Malandrucco, S.; Ioele, M.; Steenken, S. J. Am. Chem. Soc. 1996, 118, 8973ꢀ8974. (f) Baciocchi,
E.; Crescenzi, C.; Lanzalunga, O. Tetrahedron 1997, 53, 4469ꢀ4478. (g) Baciocchi, E.; Del Giacco
T.; Ferrero, M. I.; Rol, C.; Sebastiani, G. V. J. Org. Chem. 1997, 62, 4015ꢀ4017. (h) Ioele, M.;
Steenken, S.; Baciocchi, E. J. Phys. Chem. A 1997, 101, 2979ꢀ2987.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4. (a) Chaudhri, S. A.; Mohan, H.; Anklam, E.; Asmus, K.ꢀD. J. Chem. Soc., Perkin Trans. 2, 1996,
383ꢀ390. (b) Goebl, M.; Bonifacic, M.; Asmus, K.ꢀD. J. Am. Chem. Soc. 1984, 106, 5984ꢀ5988. (c)
Chaudhri, S. A.; Goebl, M.; Freyholdt, T.; Asmus, K.ꢀD. J. Am. Chem. Soc. 1984, 106, 5988ꢀ5992.
(d) Asmus, K.ꢀD. Acc. Chem. Res. 1979, 12, 436ꢀ442. (e) Asmus, K.ꢀD.; Bahnemann, D.; Fischer,
C. H.; Veltwisch, D. J. Am. Chem. Soc. 1979, 101, 5322ꢀ5329. (f) Bonifačić, M.; Möckel, H.;
Bahnemann D.; Asmus, K.ꢀD. J. Chem. Soc., Perkin Trans 2, 1975, 675ꢀ685.
5. Yokoi, H.; Hatta, A.; Ishiguro, K.; Sawaki, Y. J. Am. Chem. Soc. 1998, 120, 12728ꢀ12733.
6. (a) Baciocchi, E.; Del Giacco, T.; Elisei, F.; Lapi A. J. Org. Chem. 2006, 71, 853ꢀ860. (b)
Baciocchi, E.; Del Giacco, T.; Elisei, F.; Gerini, M. F.; Lapi, A.; Liberali, P.; Uzzoli, B. J. Org.
Chem. 2004, 69, 8323ꢀ8330.
7. (a) Shukla, D.; Liu, G.; Dinnocenzo, J. P.; Farid, S. Can. J. Chem. 2003, 81, 744ꢀ757. (b)
Wosinska, Z. M.; Stump, F. L.; Ranjan, R.; Lorance, E. D.; Finley, G. N.; Patel, P. P.; Khawaja, M.
A.; Odom, K. L.; Kramer, W. H.; Gould, I. R. Photochemistry and Photobiology 2014, 90, 313–328
8. Only for 1c, traces (< 1%) of 1,2ꢀ(4ꢀcyanophenyl)ꢀ1,2ꢀbis(methylthio)ethane, [4ꢀCNꢀ
C6H4CH(SCH3)]2, were detected together with 4ꢀCF3ꢀC6H4CHO.
9. Bonesi, S. M.; Fagnoni, M.; Albini, A. Eur. J. Org. Chem. 2008, 15, 2612ꢀ2620.
22
ACS Paragon Plus Environment

Page 23 of 25
The Journal of Organic Chemistry
1
2
3
4
5
10. Julliard M. In Photoinduced Electron Transfer, Part B; Fox, M. A and Chanon M., Eds.; Elsevier:
Amsterdam, 1988; Chapther 2.5.
6
7
8
9
11. Chaudhri, S. A.; Gobl, M.; Freyholdt, T.; Asmus, K.ꢀD.J. Am. Chem. Soc. 1984, 106, 5988ꢀ5992.
12. Filipiak, P.; Hug, G. L.; Carmichael, I.; KorzeniowskaꢀSobczuk, A.; Bobrowski, K.; Marciniak, B.
J. Phys. Chem. A 2004, 108, 6503ꢀ6512.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
13. Avila, D. V.; Ingold, K. U.; Di Nardo, A. A.; Zerbetto, F.; Zgierski, M. Z.; Lusztyk, J. J. Am.
Chem. Soc. 1995, 117, 2711ꢀ2718.
14. Likewise DFT calculations indicate the radical C6H5CH•SCH2COO− to absorb mainly at 335 nm in
the gas phase.12 The substituent on the ring is expected to have little influence on the radical
absorption. Indeed, the corresponding radicals (4XꢀC6H5)2CH•, having an aryl in place of the
thiomethyl group, absorb at 350, 330 and 334 nm with X = OCH3, H and CF3, respectively.15
15. Bartl, J.; Steenken, S.; Mayr, H.; McClelland, R. A. J. Am. Chem. Soc. 1990, 112, 6918ꢀ6928.
16. For example, cations such as (4XꢀC6H5)2CH+ absorb at λmax = 500, 435 and 425 nm, while the
corresponding radicals (4XꢀC6H5)2CH•, absorb at 350, 330 and 334 nm with X = OCH3, H and CF3,
respectively.15
17. In more anhydrous MeCN, the cation turned out to decay with a longer lifetime.
18. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.;
Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.;
Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.:
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.;
Vreven, T.; Montgomery Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers,
E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.;
Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.;
23
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 24 of 25
1
2
3
4
5
6
7
8
9
Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.;
Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.;
Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. GAUSSIAN 09 (Revision B.01),
Gaussian, Inc., Wallingford, CT, 2010.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
19. Indeed, the oxidation potential value of 4ꢀmethoxybenzyl methyl sulfide (1.55 V, Table 1) is close
enough to that of 4ꢀmethoxytoluene (1.71 V)20, a structure for which charge and spin in the
corresponding radical cation can be exclusively on the aromatic ring. On the contrary, benzyl
methyl sulfide oxidation potential (1.63 V) is somewhat lower than that of the corresponding
toluene (2.35 V)21, in agreement with a greater contribution of structure I to the radical cation
structure when the stabilizing effect of the substituent is missing.
20. Fukuzumi S.; Kochi, J.K. J. Am. Chem. Soc. 1981,103, 7240ꢀ7252.
21. Dockery, K. P.; Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.; Gould, R.; Todd, W. P. J. Am. Chem.
Soc. 1997, 119, 1876–1883.
22. Baciocchi, E.; Rol, C.; Scamosci, E.; Sebastiani, G.V. J. Org. Chem. 1991, 56, 5498ꢀ5502.
+•
23. The bond dissociation free energy of the CαꢀS cleavage in the PhCH2SCH2CH3 (BDFE of 28
kcal/mol) was estimated to be much more endergonic than the CαꢀH bond fragmentation reaction
(BDFE of ꢀ8 kcal/mol). The breaking of the two bonds in the aromatic sulfide radical cations is
instead characterized by similar ꢀH° values.24
24. Baciocchi, E. Acta Chem. Scand. 1990, 44, 645ꢀ652.
25. Ginsburg, J. L.; Langler, R. F. Can. J. Chem. 1983, 61, 589ꢀ593.
26. On the other hand, Albini et al. suggested the oxidation of the analogous radical PhCH•SCH2CH3
by the ground state of DCA,9 although the sensitizer potential is even more negative (ꢀ0.89 V
vs.SCE in MeCN)27 than that of MeOP+.
27. Breslin, D. T.; Fox, M. A. J. Am. Chem. Soc. 1993, 115, 11716ꢀ11721.
24
ACS Paragon Plus Environment

Page 25 of 25
The Journal of Organic Chemistry
1
2
3
4
5
28. Lund, V T.; Wayner, D. D. M.; Jonsson, M.; Larsen, A. G.; Daasbjerg, K. J. Am. Chem. Soc. 2001,
123, 12590–12595.
6
7
8
9
29. Wayner, D. D.; Houmam, A. Acta Chem. Scand. 1998, 52, 377ꢀ384.
30. Since the relative rate does not vary in the presence of the base, it can be assumed that the radical
cation decay rate matchs with the deprotonation rate; hence this reaction is the rateꢀdetermining
step, in line with the limited accumulation of this intermediate.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
31.Sehested, K.; Holcman, J. J. Phys. Chem. 1978, 82, 651–653.
32. Baciocchi, E.; Bietti, M.; Manduchi, L.; Steenken, S. J. Am. Chem. Soc. 1999, 121, 6624–6629.
33.Baciocchi, E.; Bietti, M.; Lanzalunga, O. J. Phys. Org. Chem. 2006, 19, 467–478.
34. Labuschagne, A. J. H.; Malherbe, J. S.; Meyer, C. J.; Schneider, D. F. J. Chem. Soc., Perkin Trans.
1 1978, 955ꢀ961.
35. (a) Van EstꢀStammer, R.; Engberts, J. B. F. N. Can. J. Chem. 1973, 51, 1187ꢀ1192. (b) Adcock'l,
W.; Gupta, B. D. J. Org. Chem. 1976, 41, 1498ꢀ1504. (c) Zaraiskii, A. P.; Kachurin, O. I. Russ. J.
Org. Chem. 2003, 39, 1642ꢀ1645. (d) Holland, H. L.; Brown, F. M.; Kerridge, A.; Turner, C. D. J.
Mol. Cat. B: Enzymatic 1999, 6, 463–471. (d) Holland, H. L.; Brown, F. M.; Kerridge, A.; Turner,
C. D. J. Mol. Cat. B: Enzymatic 1999, 6, 463–471.
36. Lorance, E. D., Kramer, W. H.; Gould, I. R. J. Am. Chem. Soc. 2002, 124, 15225–15238.
37. Bunnett, J. F. In Investigation of Rates and Mechanism of Reactions; ed. E. S. Lewis, Wiley, New
York, 1974; part I, pp. 158ꢀ161.
38. Görner, H.; Elisei, F.; Aloisi, G. G. J. Chem. Soc., Faraday Trans. 1992, 88, 29ꢀ34.
39. Masunov, A. M.; Tretiak, S. J. Phys. Chem. B 2004, 108, 899–907.
25
ACS Paragon Plus Environment