Photosensitized oxidation of alkyl phenyl sulfoxides. C-S bond cleavage in alkyl phenyl sulfoxide radical cations

Article abstract of DOI:10.1021/jo801088n

(Chemical Equation Presented) The 3-cyano-N-methylquinolinium Perchlorate (3-CN-NMQ₊ ClO4_-)-photosensitized oxidation of phenyl alkyl sulfoxides (PhSOCR₁R₂R₃,1, R₁ = R₂ = H, R₃ = Ph; 2, R₁ = H, R₂ = Me, R₃ = Ph; 3, R₁ = R₂ = Ph, R₃ = H; 4, R₁ = R₂ = Me, R₃ = Ph; 5, R₁ = R₂ = R₃ = Me) has been investigated by steady-state irradiation and nanosecond laser flash photolysis (LFP) under nitrogen in MeCN. Steady-state photolysis showed the formation of products deriving from the heterolytic C-S bond cleavage in the sulfoxide radical cations (alcohols, R ₁R₂R₃COH, and acetamides, R₁R ₂R₃CNHCOCH₃) accompanied by sulfur-containing products (phenyl benzenethiosulfinate, diphenyl disulfide, and phenyl benzenethiosulfonate). By laser irradiation, the formation of 3-CN-NMQ ^? (λ_max = 390 nm) and sulfoxide radical cations 1^?+, 2^?+, and 5^?+ (λ_max = 550 nm) was observed within the laser pulse. The radical cations decayed by first-order kinetics with a process attributable to the heterolytic C-S bond cleavage leading to the sulfinyl radical and an alkyl carbocation. The radical cations 3^?+ and 4^?+ fragment too rapidly, decaying within the laser pulse. The absorption band of the cation Ph₂CH⁺ (λ_max = 440 nm) was observed with 3 while the absorption bands of 3-CN-NMQ^? and PhSO^? (λ_max = 460 nm) were observed just after the laser pulse in the LFP experiment with 4. No competitive β-C-H bond cleavage has been observed in the radical cations from 1-3. The C-S bond cleavage rates were measured for 1^?+, 2^?+, and 5^?+. For 3^?+ and 4^?+, only a lower limit (ca. >3 × 10⁷ s^-1) could be given. Quantum yields (Φ) and fragmentation first-order rate constants (k) appear to depend on the structure of the alkyl group and on the bond dissociation free energy (BDFE) of the C-S bond of the radical cations determined by a thermochemical cycle using the C-S BDEs for the neutral sulfoxides 1-5 obtained by DFT calculations. Namely, Φ and k increase as the C-S BDFE becomes more negative, that is in the order 1 < 5 < 2 < 3, 4, which is also the stability order of the alkyl carbocations formed in the cleavage. An estimate of the difference in the C-S bond cleavage rate between sulfoxide and sulfide radical cations was possible by comparing the fragmentation rate of 5 ^?+ (1.4 × 10⁶ s^-1) with the upper limit (10⁴ s^-1) given for fert-butyl phenyl sulfide radical cation (Baciocchi, E.; Del Giacco, T.; Gerini, M. F.; Lanzalunga, O. Org. Lett. 2006, 8, 641-644). It turns out that sulfoxide radical cations undergo C-S bond breaking at a rate at least 2 orders of magnitude faster than that of corresponding sulfide radical cations.

Full text of DOI:10.1021/jo801088n

Photosensitized Oxidation of Alkyl Phenyl Sulfoxides. C-S Bond
Cleavage in Alkyl Phenyl Sulfoxide Radical Cations
Enrico Baciocchi,*,§ Tiziana Del Giacco,*,‡ Osvaldo Lanzalunga,*,† Paolo Mencarelli,*,†
and Barbara Procacci‡
Dipartimento di Chimica and Istituto CNR di Metodologie Chimiche-IMC, Sezione Meccanismi di
Reazione c/o Dipartimento di Chimica, Sapienza UniVersita` di Roma, P.le A. Moro 5, 00185 Rome, Italy,
and Dipartimento di Chimica and Centro di Eccellenza Materiali InnoVatiVi Nanostrutturati, UniVersita` di
Perugia, Via Elce di sotto 8, 06123 Perugia, Italy
osValdo.lanzalunga@uniroma1.it
ReceiVed May 20, 2008
The 3-cyano-N-methylquinolinium perchlorate (3-CN-NMQ+ ClO4-)-photosensitized oxidation of phenyl alkyl
sulfoxides (PhSOCR1R2R3, 1, R1 ) R2 ) H, R3 ) Ph; 2, R1 ) H, R2 ) Me, R3 ) Ph; 3, R1 ) R2 ) Ph, R3
) H; 4, R1 ) R2 ) Me, R3 ) Ph; 5, R1 ) R2 ) R3 ) Me) has been investigated by steady-state irradiation
and nanosecond laser flash photolysis (LFP) under nitrogen in MeCN. Steady-state photolysis showed the
formation of products deriving from the heterolytic C-S bond cleavage in the sulfoxide radical cations (alcohols,
R1R2R3COH, and acetamides, R1R2R3CNHCOCH3) accompanied by sulfur-containing products (phenyl
benzenethiosulfinate, diphenyl disulfide, and phenyl benzenethiosulfonate). By laser irradiation, the formation
of 3-CN-NMQ• (λmax ) 390 nm) and sulfoxide radical cations 1•+, 2•+, and 5•+ (λmax ) 550 nm) was observed
within the laser pulse. The radical cations decayed by first-order kinetics with a process attributable to the
heterolytic C-S bond cleavage leading to the sulfinyl radical and an alkyl carbocation. The radical cations
3
•+ and 4•+ fragment too rapidly, decaying within the laser pulse. The absorption band of the cation Ph2CH+
(λmax ) 440 nm) was observed with 3 while the absorption bands of 3-CN-NMQ• and PhSO• (λmax ) 460
nm) were observed just after the laser pulse in the LFP experiment with 4. No competitive ꢀ-C-H bond
cleavage has been observed in the radical cations from 1-3. The C-S bond cleavage rates were measured
for 1•+, 2•+, and 5•+. For 3•+ and 4•+, only a lower limit (ca. >3 × 107 s-1) could be given. Quantum yields
(Φ) and fragmentation first-order rate constants (k) appear to depend on the structure of the alkyl group and
on the bond dissociation free energy (BDFE) of the C-S bond of the radical cations determined by a
thermochemical cycle using the C-S BDEs for the neutral sulfoxides 1-5 obtained by DFT calculations.
Namely, Φ and k increase as the C-S BDFE becomes more negative, that is in the order 1 < 5 < 2 < 3, 4,
which is also the stability order of the alkyl carbocations formed in the cleavage. An estimate of the difference
in the C-S bond cleavage rate between sulfoxide and sulfide radical cations was possible by comparing the
fragmentation rate of 5•+ (1.4 × 106 s-1) with the upper limit (104 s-1) given for tert-butyl phenyl sulfide
radical cation (Baciocchi, E.; Del Giacco, T.; Gerini, M. F.; Lanzalunga, O. Org. Lett. 2006, 8, 641-644). It
turns out that sulfoxide radical cations undergo C-S bond breaking at a rate at least 2 orders of magnitude
faster than that of corresponding sulfide radical cations.
Introduction
as well as for the variety of possible applications (e.g., photography
and polymerization initiations)1 and the possible role in many
biological and chemical oxidation processes.2
Fragmentation of radical cations to form a cation and a radical
(eq 1) is one of the most interesting reactions of these species which
has attracted considerable attention for the theoretical implications
+•
A-B-C f A-B• + C+
(1)
† Sapienza Universita` di Roma.
‡ Universita` di Perugia.
The feasibility of reaction (1) is due to the extra energy that
in the radical cation is associated with the removal of an electron
§ IMC - Sapienza Università di Roma.
10.1021/jo801088n CCC: $40.75
Published on Web 06/26/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 5675–5682 5675

Baciocchi et al.
from the HOMO of the parent substrate (A-B-C). In most
cases, the extra energy is predominantly located in the bond ꢀ
to the center of positive charge as it is the one that can overlap
most conveniently with the SOMO of the radical cation. The
BDE of this bond significantly decreases, and therefore, the bond
can be broken with relative ease. So one generally speaks of ꢀ
fragmentations of radical cations. However, somewhat different
situations can arise in alkyl aryl sulfide radical cations.3 In this
case, the positive charge is mainly located on the sulfur and
the cleavage can involve the sulfur carbon bond (an R bond
with respect to the positive charge) as shown in eq 2.
the photosensitized one-electron oxidation of alkyl phenyl
sulfoxides 1-5 in MeCN.
Ar+S•-CR1R2R3 f ArS• + +CR1R2R3
(2)
Given the relatively high oxidation potential of sulfoxides
(∼2 V vs SCE),6 3-cyano-N-methylquinolinium perchlorate (3-
CN-NMQ+) was used as the sensitizer9 (E° for 3-CN-NMQ+*/
3-CN-NMQ• ) 2.72 V vs SCE).11 DFT calculations were also
performed to obtain the values of bond dissociation energy
(BDE) for the C-S bond in sulfoxides 1-5 and from them the
values of the bond dissociation free energy (BDFE) in the
corresponding radical cations.
Our long-standing interest in the fragmentation reactions of
radical cations has recently led us to focus our attention on the
quantitative aspects of reaction (2) through a laser photolysis
study.4 The main observations were that the fragmentation rate
depends to a very limited extent on the strength of the C-S
bond in the radical cation and that the reorganization energy of
the process was influenced by the structure of the alkyl groups,
increasing with the number of phenyl groups bonded to the
carbon atom.
Results
Like sulfide radical cations, sulfoxide radical cations can also
undergo fragmentation reactions (eq 3) involving the cleavage
of the C-S bond. In fact, since sulfoxides are characterized by
significantly higher oxidation potentials than that of the corre-
sponding sulfides5 and the phenylsulfinyl radical is more stable
than the phenylthiyl radical,8 reaction (3) is expected to be much
faster than reaction (2). Thus, sulfoxide radical cations should
represent an interesting class of radical cations characterized
by a very high fragmentation rate.
Quenching of Fluorescence. All the sulfoxides examined
(1-5) efficiently quenched the fluorescence emission of 3-CN-
NMQ+ in accordance with the Stern-Volmer equation (4)
where I0 and I are the emission intensities in the absence and
in the presence of increasing concentrations of sulfoxides ([Q]),
respectively. The quenching rate constants (kq) were calculated
from the slopes of the linear Stern-Volmer plots (KSV) divided
1
by the lifetime of 3-CN-NMQ+* measured in N2-saturated
CH3CN (45 ns).11 All the kq values are very high and close to
the diffusion limit (1.1-1.5 × 1010 M-1 s-1). The efficient
quenching process involves an electron-transfer from the
sulfoxides to 13-CN-NMQ+* as confirmed by laser flash
photolysis experiments (see below) and in agreement with the
reduction potential of 13-CN-NMQ+*(2.72 V vs SCE in
CH3CN),11 which is much higher than that of the sulfoxides
examined (ca. 2.0 V vs SCE in CH3CN).6
Aspects concerning generation, spectral properties, and
structure of sulfoxide radical cations have been recently
investigated in our laboratory,6 but practically nothing is known
about their fragmentation process with respect to both products
and reactivity. Thus, we have felt it worthwhile to fill this gap,
and we now report in this paper a steady state and laser
photolysis study of the fragmentation reactions of a number of
alkyl phenyl sulfoxide radical cations (1•+-5•+) generated by
I0/I ) 1 + KSV[Q]
(4)
Steady-State Photolysis. Steady-state photolysis experiments
were carried out by irradiating a solution of sulfoxides 1-5
(0.01 M) in N2-saturated CD3CN at around 355 nm in the
presence of 3-CN-NMQ+ ClO4 (0.001 M). The amount of
-
(1) (a) Baciocchi, E.; Bietti, M.; Lanzalunga, O. Acc. Chem. Res. 2000, 33,
243. (b) Gaillard, E. R.; Whitten, D. G. Acc. Chem. Res. 1996, 29, 292. (c)
Maslak, P. Top. Curr. Chem. 1993, 168, 1. (d) Albini, A.; Fasani, E.;
d’Alessandro, N. Coord. Chem. ReV. 1993, 125, 269.
sulfoxide used was able to quench ca. 85% of the quinolinium
singlet state. The products were identified and quantitated by
1H NMR (products from the alkyl moiety) and HPLC (sulfur
containing products) analysis by comparison with authentic
specimens. No products were detected without irradiation or in
the absence of the sensitizer. UV and 1H NMR analysis of the
reaction mixture after photolysis indicated that the sensitizer is
not consumed during the irradiation.
(2) (a) Filipiak, P.; Hug, G. L.; Carmichael, I.; Korzeniowska-Sobczuk, A.;
Bobrowski, K.; Marciniak, B. J. Phys. Chem. A 2004, 108, 6503. (b) Huang,
M. L.; Rauk, A. J. Phys. Chem. A 2004, 108, 6222. (c) Baciocchi, E.; Del Giacco,
T.; Elisei, F.; Gerini, M. F.; Lapi, A.; Liberali, P.; Uzzoli, B. J. Org. Chem.
2004, 69, 8323. (c) Scho¨neich, C.; Pogocki, D.; Hug, G. L.; Bobrowski, K. J. Am.
Chem. Soc. 2003, 125, 13700. (d) Korzeniowska-Sobczuk, A.; Hug, G. L.;
Carmichael, I.; Bobrowski, K. J. Phys. Chem. A 2002, 106, 9251. (e) Gawandi,
V. B.; Mohan, H.; Mittal, J. P. J. Phys.Chem. A 2000, 104, 11877. (f) Glass,
R. S. Top. Curr. Chem. 1999, 205, 1.
(3) (a) Pen˜e´n˜ory, A. B.; Argu¨ello, J. E.; Puiatti, M. Eur. J. Org. Chem. 2005,
10, 114. (b) Adam, W.; Argu¨ello, J. E.; Pen˜e´n˜ory, A. B. J. Org. Chem. 1998,
63, 3905. (c) Ioele, M.; Steenken, S.; Baciocchi, E. J. Phys. Chem. A 1997, 101,
2979. (d) Baciocchi, E.; Lanzalunga, O.; Malandrucco, S.; Ioele, M.; Steenken,
S. J. Am. Chem. Soc. 1996, 118, 8973. (e) Baciocchi, E.; Rol, C.; Scamosci, E.;
Sebastiani, G. V. J. Org. Chem. 1991, 56, 5498.
(7) Baciocchi, E.; Intini, D.; Piermattei, C.; Rol, C.; Ruzziconi, R. Gazz.
Chim. Ital. 1989, 119, 649–652.
(8) Kice, J. L. In Free Radicals; Kochi, J. K., Ed.; John Wiley & Sons: New
York, 1973; Chapter 24.
(9) 3-Cyano-N-methylquinolinium (3-CN-NMQ+) was preferred to N-me-
thylquinolinium (NMQ+) because the absorption band of the reduced quinolinium
radical NMQ• (λmax ) 550 nm)10 overlaps with the absorption bands of sulfoxide
radical cations (vide infra).
(4) Baciocchi, E.; Del Giacco, T.; Gerini, M. F.; Lanzalunga, O. Org. Lett.
2006, 8, 641–644.
(5) For example the peak potential of methyl phenyl sulfoxide in CH3CN
(2.01 V vs SCE)6 is significantly higher than that of thioanisole (1.47 V vs SCE).7
(6) Baciocchi, E.; Del Giacco, T.; Gerini, M. F.; Lanzalunga, O. J. Phys.
Chem. A 2006, 110, 9940–9948.
(10) Bockman, T. M.; Kochi, J. K. J. Am. Chem. Soc. 1989, 111, 4669–
4683.
(11) Kitaguchi, H.; Ohkubo, K.; Ogo, S.; Fukuzumi, S. J. Phys. Chem. A
2006, 110, 1718–1725.
5676 J. Org. Chem. Vol. 73, No. 15, 2008

Photosensitized Oxidation of Alkyl Phenyl Sulfoxides
SCHEME 1
SCHEME 2
The photolysis of sulfoxides 1, 2, and 5 produced alcohols
(benzyl, R-methylbenzyl, and tert-butyl alcohol, respectively)
and acetamides (benzyl, R-methylbenzyl, and tert-butyl aceta-
mide, respectively) as main reaction products from the alkyl
fragments. With 3, the main reaction product observed was
diphenylmethanol, whereas 2-phenyl-2-propanol (with traces of
R-methylstyrene) was formed in the photolysis of 4. Small
amounts of carbonyl derivatives (generally less than 10% of
the total products) were also observed with primary and
secondary alkyl phenyl sulfoxides 1, 2, and 3 (benzaldehyde,
acetophenone, and benzophenone, respectively), which were
attributed to partial photooxidation of the alcohols under the
reaction conditions. Accordingly, the steady-state photolysis of
sulfoxides 1 and 3 sensitized by 3-CN-NMQ+ in N2-saturated
CH3CN at several irradiation times showed that benzyl alcohol
(from 1) and diphenyl methanol (from 3) are progressively
converted into benzaldehyde and benzophenone, respectively,
as the reaction proceeds (for details, see the Supporting
Information). For the products coming from the alkyl moiety,
the material balance was very satisfactory (>90%) in all
experiments.
TABLE 1. Quantum Yields (Φ) of Some Photoproducts Formed
-
in the 3-CN-NMQ+ ClO4 Photosensitized Oxidation of Alkyl
Phenyl Sulfoxides (1-5) in N2-Saturated CD3CNa
sulfoxide
Φ(a)b
Φ(b)c
Φtot
1
2
3
4
5
0.089d
0.21d
0.66d
1.7
0.12
0.21
0.21
0.42
0.66
1.7
0.18
0.10
0.28
a [sulfoxide] ) 1.0 × 10-2 M, [3-CN-NMQ+ ClO4-] ) 1.0 × 10-3
M; quantum yields of the products from the alkyl moiety. b Quantum
yields of alcohols, R1R2R3COH. c Quantum yields of acetamides,
R1R2R3CNHCOCH3. d The alcohol yield also includes the small amount
of carbonyl compound formed by oxidation of the alcohol (see text).
The actual Φ of the alcohol is therefore slightly higher than that in the
table as photons have been also used to photoxidize the alcohol.
The sulfur-containing products identified in all cases were
as follows: phenyl benzenethiosulfinate (PhSOSPh), diphenyl
disulfide (PhSSPh), and phenyl benzenethiosulfonate
(PhSO2SPh). Quantitative HPLC analysis showed that the
photolysis of 3 produced, after 15 min of irradiation, PhSOSPh
(0.5%, referred to the initial amount of substrate), PhSSPh
(1.3%), and PhS(O)2SPh (0.8%), together with diphenyl metha-
nol (8.3%) and benzophenone (1.0%). The material balance for
the sulfurated products is only around 60%, thus indicating that
other unidentified sulfur-containing products are formed.
On the basis of the above results, the outcome of the steady-
state photolysis of 1-5 sensitized by 3-CN-NMQ+ can be
substantially described by Scheme 1.
Clearly, the carbocation formed by C-S bond cleavage in
the intermediate radical cation (eq 3 and vide infra) is the
precursor of alcohols and acetamides. The sulfur-containing
products certainly derive from the phenylsulfinyl radical which
forms together with the carbocation from the C-S bond scission
(eq 3).
(5) would nicely explain the otherwise inexplicable observation
that 3-CN-NMQ+ is not consumed during the irradiation.
The formation of the sulfenate anion may reasonably account
for the sulfur-containing products formed in the steady-state
photolysis. Accordingly, this species and/or its protonated form
are characterized by a very rich chemistry and can promote a
number of reactions8(some are reported in Scheme 2) leading
to PhSOSPh, PhSSPh, and PhSO2SPh. Moreover, sulfinic acid
can also be formed, which might get lost during workup, thus
explaining the low overall quantum yield of sulfur-containing
products as compared to that of the products from the alkyl
moiety.
As a possible route to the sulfurated products, a possible
suggestion, which will receive some support from the laser
photolysis experiments (vide infra), is that the sulfinyl radical
is first reduced to phenyl sulfenate by 3-CN-NMQ• (eq 5).
(12) E°(PhSO•/PhSO-) ) ∆G°(PhSO-H)/23.06 + E°(H•/H+)-0.059pKa,
where ∆G°(PhSO-H) is the Gibbs energy for the homolytic fragmentation of
the PhSO-H bond (78 kcal mol-1), given by the PhSO-H bond dissociation
energy (which can be assumed identical to that of CH3SOH)13 minus the entropy
factor (T∆S°, taken as 8 kcal mol-1 at 298 K),14 E°(H+/H•) is the reduction
potential of the proton (-2.0 V),15 and pKa is related to the dissociation of the
phenyl sulfenic acid PhSOH (pKa ) 5).16
3-CN-NMQ• + PhSO• f 3-CN-NMQ+ + PhSO- (5)
This reaction is thermodynamically feasible as the reduction
potential of PhSO• (1.08 V vs SCE in CH3CN, calculated at 25
°C on the basis of a thermochemical cycle)12 is much higher
than that of 3-CN-NMQ+ (-0.60 V vs SCE in CH3CN)17 and
should therefore be very fast (vide infra). Moreover, reaction
(13) Gollnick, K.; Strake, H.-U. Pure Appl. Chem. 1973, 33, 217–245.
(14) Baciocchi, E.; Del Giacco, T.; Elisei, F. J. Am. Chem. Soc. 1993, 115,
12290–12295.
(15) Wayner, D. D. M.; Parker, V. D. Acc. Chem. Res. 1993, 26, 287–294.
(16) Marshall, D. R.; Thomas, P. J.; Stirling, C. J. M. J. Chem. Soc., Chem.
Commun. 1975, 940–941.
J. Org. Chem. Vol. 73, No. 15, 2008 5677

Baciocchi et al.
SCHEME 3
The quantum yields (Φ) measured for the products (alcohols
and acetamides) coming from the alkyl moiety of sulfoxides
are reported in Table 1. Given the excellent material balance
observed with these compounds, the sum of quantum yields of
alcohols and acetamides (Φtot) also reported in Table 1 can be
considered a measure of the photoefficiency of the fragmentation
process.
An apparently intriguing observation is that 4 exhibits a
quantum yield >1. This observation, however, can be reasonably
explained by a thermal acid-catalyzed partial decomposition of
the sulfoxide which occurs under the photolytic conditions.
Accordingly, in the photolysis of 4, protons are formed (deriving
from the reaction of cumyl cation with adventitious water) which
can be captured by the neutral sulfoxide. C-S bond cleavage
in the protonated sulfoxide leads to the very stable cumyl cation
and eventually to 2-phenyl-2-propanol (Scheme 3), thus appar-
ently increasing the quantum yield.
very similar, whereas different features were observed in the
time-resolved spectra of 3 and 4. Upon laser excitation of the
first three sulfoxides, two absorption bands were detected just
after the laser pulse at 380-400 and 500-550 nm (Figure 1,
where the time-resolved spectra of 5 are reported; those of 1
and 2 are in Figures S1 and S2 in the Supporting Information).
Very likely, the absorption at around 390 nm is mostly due to
3-CN-NMQ•27 and that at around 520 nm is due to the sulfoxide
radical cation 5•+.6 The ∆A recorded at 520 nm within 30 µs
(Figure 1, inset b) indicate a fast decay which should belong to
the radical cation, followed by a slower decay which should be
due to the radical PhSO•, that absorbs weakly at this wavelength
(the absorption maximum is around 460 nm).29
At longer delay times (16 µs), the absorption at 390 nm is
replaced by a residual absorption centered at 375 nm which
decayed on a slower time scale (Figure 1, inset a) and might
tentatively be attributed to the phenyl sulfenate formed according
to eq 5.
As already mentioned, the time-resolved spectra obtained with
3 and 4 were different from those of 1, 2,and 5. In the LFP
experiments with 3-CN-NMQ+/toluene/3, after the laser pulse
only a large and intense band at about 440 nm was observed
(Figure 2). This band can be assigned to the cation Ph2CH+
according to the literature.30 The nature of this transient is
confirmed by the fact that the decay of this absorption band
(see Figure 2, inset) is not influenced by the presence of
dioxygen.
In agreement with this explanation, we found that addition
of HClO4 (2.5 × 104 M) to a solution of 4 (0.01 M) in CD3CN
led, after 5 min, to the decomposition of 58% of sulfoxide with
formation of cumyl alcohol (1H NMR analysis) as the main
product.18
Since the complication due to the acid catalyzed C-S
cleavage in protonated sulfoxide should be related to the stability
of the formed carbocation, the same experiment as above was
also carried out with sulfoxide 3, whose thermal decomposition
should lead to a carbocation of stability similar to that of cumyl
cation.20 In this case, however, the substrate was recovered to
a large extent (almost 90%) and only 10% of diphenyl methanol
was formed. Thus, with 3, the thermal decomposition should
not significantly affect the quantum yield values in Table 1.
Probably, the factor which makes 4 particularly susceptible to
acid catalyzed C-S bond cleavage is the large steric hindrance
in this tertiary system.21 All the other sulfoxides examined were
found stable under acidic conditions.
The decay at 440 nm is very fast (k ) 3.2 × 107 s-1) and is
very likely due to the reaction of the carbocation with adventi-
tious water,31 responsible for the alcohol formation observed
in the steady-state photolysis. Clearly, the fragmentation rate
of the radical cation 3•+ is so fast that the process takes place
within the laser pulse. Thus, the radical cation is not observed,
but only the carbocation produced in the fragmentation. The
30
intense absorption band of Ph2CH+(ε = 50000 M-1 cm-1
)
Laser Flash Photolysis Studies. The laser photolysis experi-
ments (λexc ) 355 nm) were performed under nitrogen in the
presence of 1 M toluene as cosensitizer in order to increase the
yields of radical cation.25 Under these conditions, the couple
overlaps and covers the less intense absorption bands of 3-CN-
27
29
NMQ• (ε = 3500 M-1 cm-1
)
and PhSO• fragments.
PhCH3•+/3-CN-NMQ• is first formed, and it is PhCH3 (Ered
•+
) 2.35 V vs SCE)26 that converts the sulfoxide into the radical
cation. The time-resolved spectra of sulfoxides 1, 2, and 5 were
(17) Ohkubo, K.; Suga, K.; Morikawa, K.; Fukuzumi, S. J. Am. Chem. Soc.
2003, 125, 12850–12859.
(18) We also tried to carry out photolysis of 4 in the presence of pyridine
which is a good base in CH3CN.19 However, this attempt failed because pyridine
was a good quencher of the 3-CN-NMQ+ excited state.
(19) Baciocchi, E.; Del Giacco, T.; Elisei, F.; Lapi, A. J. Org. Chem. 2006,
71, 853–860.
(20) Arnett, E. M.; Hofelich, T. C. J. Am. Chem. Soc. 1983, 105, 2889.
(21) The steric hindrance in 3•+, should be comparable or slightly lower than
that in 5•+ [Es for CHPh2 (-1.43)22 is less negative than Es for CMe3 (-1.54)23].
Since the steric requirements of the methyl group are less than those of the phenyl
group,24 compound 4 should be characterized by the largest steric hindrance.
(22) Bowden, K.; Chapman, N. B.; Shorter, J. J. Chem. Soc. 1964, 3370.
(23) Taft, R. W. In Steric Effects in Organic Chemistry; Newman, M. S.,
Ed.; Wiley: New York, 1956; Chapter 13.
(24) MacPhee, J. A.; Panaye, A.; Dubois, J.-E. Tetrahedron 1978, 34, 3553–
3562. Charton, M. J. Am. Chem. Soc. 1975, 97, 1552–1556.
(25) Dockery, K. P.; Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.; Gould,
I. R.; Todd, W. P. J. Am. Chem. Soc. 1997, 119, 1876–1883.
FIGURE 1. Time-resolved absorption spectra of the 3-CN-NMQ+ (1.0
× 10-2 M)/toluene (1 M)/5 (1.0 × 10-2 M) system in N2-saturated
CH3CN recorded 0.32 (4), 2 (2), 7.2 (O) and (9) 16 µs after the laser
pulse (λexc ) 355 nm). Inset: (a) decay kinetics recorded at 390 nm,
(b) 520 nm.
5678 J. Org. Chem. Vol. 73, No. 15, 2008

Photosensitized Oxidation of Alkyl Phenyl Sulfoxides
FIGURE 3. Time-resolved absorption spectra of the 3-CN-NMQ+ (8.8
× 10-4 M)/toluene (1 M)/4 (1.0 × 10-2 M) system in N2-saturated
CH3CN recorded 0.08 (4), 1.4 (2), 6.3 (O) µs after the laser pulse.
λexc ) 355 nm.
FIGURE 2. Time-resolved absorption spectra of the 3-CN-NMQ+ (1.0
× 10-2 M)/toluene (1 M)/3 (1.0 × 10-2 M) system in N2-saturated
CH3CN recorded 0.040 (4), 0.064 (2) and 3.2 (O) µs after the laser
pulse (λexc ) 355 nm). Inset: decay kinetics recorded at 440 nm.
TABLE 2. Total Quantum Yields (Φ) of Photoproducts Formed
in the 3-CN-NMQ+ ClO4--Photosensitized Oxidation of Alkyl
Phenyl Sulfoxides (1-5) and Decay Rate Constants of the
Corresponding Radical Cations, C-S Bond Dissociation Energies
(BDEs) for the Alkyl Phenyl Sulfoxides 1-5, and tert-Butyl Phenyl
Sulfide and C-S Bond Dissociation Free Energies (BDFEs) For
Their Corresponding Radical Cations
A still different situation is found with the 3-CN-NMQ+/
toluene/4 system. In this case, too, the absorption band of 4•+
was not visible after the laser pulse, suggesting a very fast
fragmentation. Thus, since the cumyl carbocation lifetime should
be less than the limit of detection of the LFP apparatus,32 the
absorptions due to 3-CN-NMQ• (390 nm) and PhSO• (460 nm)
can be clearly detected (Figure 3).
C-S BDE
(neutral
C-S BDFE
(radical
a
b
substrate
Φtot
k (106 s-1
)
substrates)c,d
cations)d,e,f
At longer delay times, a residual absorption was again
observed at 380 nm which might tentatively be attributed to
the phenyl sulfenate. The decay of the phenylsulfinyl radical
follows second-order kinetics, with a rate constant of 1.9 × 1010
M-1 s-1 (see Figure S3 in the Supporting Information)34 which
is consistent with our previous suggestion that PhSO• decays
by an electron transfer reaction with 3-CN-NMQ• (eq 5) to form
the sensitizer 3-CN-NMQ+ and the phenylsulfenate (PhSO-).
Some support for this hypothesis can be found in the observation
that the half-life of PhSO• recorded at 470 nm (1.0 µs) is similar
to that of 3-CN-NMQ• (1.2 µs) which has been determined from
the initial fast decay of the absorption at 420 nm.35 Moreover,
the fact that the PhSO• decay is diffusion controlled (k ) 1.9 ×
1
2
3
4
5
0.21
0.42
0.66
1.7
1.1
2.6
29.9
28.2
21.4
26.5
36.2
53.6
-9.6
-19.6
-27.0
-26.2
-18.1
7.6h
>30
>30
1.4
0.28
C6H5SC(CH3)3
<0.01g
a In N2-saturated CD3CN. [sulfoxide] ) 1.0 × 10-2 M, [3-CN-NMQ+
ClO4-] ) 1.0 × 10-3 M; quantum yields of the products from the alkyl
moiety. b In N2-saturated CH3CN. [sulfoxide]
)
1.0
×
10-2 M,
[3-CN-NMQ+ ClO4-] ) 8.8 × 10-4 M, [toluene] ) 1.0 M, c From DFT
d
calculations, see text. kcal mol-1
.
e At 298 K. f Details of calculations
in the Supporting Information. g From ref
4 using N-methoxy-
phenanthridinium as sensitizer. h Reference 39.
1010 M-1 s-1) may be considered consistent with the already
mentioned very large exergonicity (∆G° = -2 eV) of reaction
(5).
(26) Schlesener, C. J.; Amatore, C.; Kochi, J. K. J. Phys. Chem. 1986, 90,
3747–3756.
(27) The absorption spectrum of 3-CN-NMQ• (λmax ) 390 nm) was produced
together with that of thioanisole (TA) radical cation (λmax ) 520 nm)28 after
laser flash photolysis of the 3-CN-NMQ+ /thioanisole/toluene system in CH3CN.
The extinction coefficient of 3-CN-NMQ• (ε = 3500 M-1 cm-1) was obtained
from the following equation: ε390(3-CN-NMQ•) ) ∆A390(3-CN-NMQ•) ×
The decay rates of the radical cations 1•+, 2•+, and 5•+ were
determined in the presence of nitrogen by following the kinetics
at 560 nm (Figure S4-S6 in the Supporting Information). This
wavelength, higher than that corresponding to the λmax of the
radical cations, was chosen in order to reduce the overlap of
the radical cation absorption with that of PhSO•. In all cases,
the decay kinetics followed first order laws, which shows that,
given the high fragmentation rates, back-electron transfer is not
a problem with sulfoxide radical cations, as previously reported
for sulfide radical cations.36 Thus, the C-S bond cleavage rates
were conveniently measured for 1•+, 2•+, and 5•+. For 3•+ and
4•+, however, as already said, fragmentation rates were too fast
for our apparatus (the radical cations were not observed) and
only a lower limit (ca. >3 × 107 s-1) could be given. The rate
constants are reported in Table 2, along with the quantum yields
(Φtot).
ε
520(TA•+)/∆A520(TA•+), taking into account that the two transients 3-CN-NMQ•
28
and TA+• have the same concentration and ε520(TA•+) ) 6100 M-1 cm-1
.
(28) Yokoi, H.; Hatta, A.; Ishiguro, K.; Sawaki, Y. J. Am. Chem. Soc. 1998,
120, 12728–12733.
(29) Darmanyan, A. P.; Gregory, D. D.; Guo, Y.; Jencks, W. S. J. Phys.
Chem. A 1997, 101, 6855–6863.
(30) Bartl, J.; Steenken, S.; Mayr, H.; McClelland, R. A. J. Am. Chem. Soc.
1990, 112, 6918–6928.
(31) A lower value (2.5 × 106 s-1) for the decay rate constant of the
diphenylmethyl cation in CH3CN, attributed to the reaction with water, is reported
in the literature.30 The difference is likely due to the higher water content in the
CH3CN used by us with respect to that used by Steenken et al. (ca. 0.01 M vs
e0.002 M). Accordingly, when considering the second-order rate constants for
the reaction of Ph2CH+ with water, we can estimate a value (3.2 × 109 M-1
s-1) of the same order of magnitude as that determined by Steenken et al. (k2 e
1.3 × 109 M-1 s-1).
(32) The rate of decay of the cumyl cation in the low nucleophilic solvent
33
2,2,2-trifluoroethanol is >5 × 107 s-1
.
(33) Cozens, F. L.; Kanagasabapathy, V. M.; McClelland, R. A.; Steenken,
S. Can. J. Chem. 1999, 77, 2069–2082.
Theoretical Calculations. For a meaningful discussion of
the experimental results, it was necessary to know the C-S
(34) This value is not compatible with a self-recombination reaction (k )
3.0 × 109 M-1 s-1).29
(35) This wavelength has been chosen to reduce the influence of the residual
absorption.
(36) Baciocchi, E.; Del Giacco, T.; Giombolini, P.; Lanzalunga, O. Tetra-
hedron 2006, 62, 6566–6573.
J. Org. Chem. Vol. 73, No. 15, 2008 5679

Baciocchi et al.
SCHEME 4
SCHEME 5
it can be confidently concluded that C-S bond cleavage is the
almost exclusive fragmentation process even when a deproto-
nation pathway (Scheme 5) might take place.
Interestingly, sulfoxide radical cations behave quite differently
from the radical cations of the corresponding sulfides where
ꢀ-C-H bond cleavage competes quite efficiently with C-S
bond cleavage.3,36 This outcome is certainly due to the fact that
the breaking of the C-S bond is much easier in the sulfoxide
than in the sulfide radical cations due to the already mentioned
much higher stability of the phenylsulfinyl than phenylthiyl
radicals.8
Looking at the problem in a more quantitative way, however,
calculations based on the usual thermochemical cycle (details
in the Supporting Information) show that in the benzyl phenyl
sulfoxide radical cation, ꢀ-deprotonation still appears slightly
more exergonic (about 2 kcal mol-1) than the C-S bond
cleavage path (BDFEs, -11.4 and - 9.6 kcal mol-1, respec-
tively). Probably, stereoelectronic factors which may unfavor-
ably affect the deprotonation process are in play. Another
hypothesis is that the reorganization energy for the deprotonation
(ꢀ-cleavage) may be larger than for the breaking of the C-S
bond (R cleavage). This is an important point which should
deserve future attention from a theoretical point of view. For
the sulfoxide radical cations 2•+ and 3•+ leading to a carbocation
more stable than the benzyl cation,40 C-S bond cleavage is
always much more exergonic than deprotonation.
Both the Φtot and the k values, reported in Table 2, increase
in the order 1 < 5 < 2 < 3, 4; i.e., they increase as the stability
of the carbocation formed increases40 and the BDFE of the
scissile C-S bond in the sulfoxide radical cation (last column
in Table 2) becomes more negative. It can also be noted that
fragmentation rates parallel quantum yields almost quantita-
tively. Indeed, the reactivity ratios k(2+•)/k(1+•) ) 2.4, k(5•+)/
k(1•+) ) 1.3 and k(2•+)/k(5•+) ) 1.9 are very similar to the
quantum yields ratios, Φ(2)/Φ(1) ) 2.0, Φ(5)/Φ(1) ) 1.3 and
Φ(2)/Φ(5) ) 1.5, respectively. An intriguing observation,
however, is that quantum yields, as well as fragmentation rates,
are very similar for 1•+ and 5•+, in spite of the large difference
in driving force (8.5 kcal mol-1) between the two fragmentation
processes. Thus, it seems that the fragmentation rate of 1•+ is
significantly larger than that expected on the basis of the BDFE
of the C-S bond. At present, the most plausible hypothesis is
that the C-S bond cleavage in 1•+ involving a primary carbon
may benefit of nucleophilic assistance by the solvent or
adventitious water. Such assistance is of course impossible with
5•+ whose C-S bond involves a nucleophilically inaccessible
tertiary carbon.41 Further work to check this suggestion might
be of interest.
bond dissociation free energies (BDFEs) for the sulfoxide radical
cations and, for the purpose of comparison, for the tert-butyl
phenyl sulfide radical cation. These values were estimated by
the usual thermochemical cycle4 (details in the Supporting
Information) using the C-S BDEs for the neutral counterparts
(sulfoxides 1-5 and tert-butyl phenyl sulfide) obtained by DFT
calculations, at the B3P86/6-311+G(d,p)//B3P86/6-311+G(d,p)
level of theory. The B3P86 functional was chosen because it is
reported to be applied with reasonable success to the calculation
of BDE values for a variety of C-X bonds37,38 including C-S
bonds.38 It should also be noted that even though DFT methods
may underestimate absolute BDE values, this should not affect
the relative BDE values38c which are those we are mostly
concerned with in this paper. The calculated C-S BDEs of 1-5
and tert-butyl phenyl sulfide are reported in Table 2 together
with BDFEs of the corresponding radical cations obtained by
the thermochemical cycle. Further details of the calculations
are given in the Experimental Section.
Discussion
The results of the steady-state photolysis experiments have
clearly indicated that in all cases alcohols and (in the oxidation
of 1, 2, and 5) acetamides are formed, by far, as the major
primary reaction products. Clearly, as also confirmed by laser
photolysis, an alkyl cation is formed by C-S bond cleavage in
the radical cation, which reacts with the solvent and (or)
adventitious water to form the corresponding acetamide and
alcohol, respectively (Scheme 4).
Formation of small amounts of carbonyl derivatives, benzal-
dehyde with 1 and ketones with 2 and 3, was also observed.
These products have been attributed (see the Results) to partial
oxidation of the alcohol under the reaction conditions. It might
also be suggested that the carbonyl compounds come, at least
in part, from oxidation followed by reaction with adventitious
water of the R-sulfoxyphenyl carbon radical formed by a
competitive R-C-H bond cleavage in the radical cation (Scheme
5). However, this possibility is unlikely, particularly with 2 and
3, as the lowest yield of carbonyl product is found with the
sulfoxide 1 which is the one where the competition by R-C-H
bond cleavage is expected to be more favored (vide infra). Thus,
(37) (a) Yao, X.-Q.; Hou, X.-J.; Jiao, H.; Wu, G.-S.; Xu, Y.-Y.; Xiang, H.-
W.; Jiao, H.; Li, Y.-W. J. Phys. Chem. A 2002, 106, 7184. (b) Zhao, J.; Cheng,
X.; Yang, X. THEOCHEM 2006, 766, 87. (c) Van Speybroeck, V.; Marin, G. B.;
Waroquier, M. ChemPhysChem 2006, 7, 2205. (d) Su, X.-F.; Cheng, X.; Liu,
Y.-G.; Li, Q. Int. J. Quantum Chem. 2007, 107, 515.
(39) Present work. In a previous paper,4 we have reported a value of 10.7
kcal mol-1 for the C-S BDFE, based on a C-S BDE of 56.7 kcal mol-1 for
tert-butyl phenyl sulfide, calculated by a different method.
(40) Abboud, J.-L. M.; Alkorta, I.; Davalos, J. Z.; Mu¨ller, P.; Quintanilla,
E.; Rossier, J.-C. J. Org. Chem. 2003, 68, 3786–3796.
(38) (a) Johnson, E. R.; Clarkin, O. J.; DiLabio, G. A. J. Phys. Chem. A
2003, 107, 9953. (b) Yao, X.-Q.; Hou, X.-J.; Jiao, H.; Xiang, H.-W.; Li, Y.-W.
J. Phys. Chem. A 2003, 107, 9991. (c) Feng, Y.; Liu, L.; Wang, J.-T.; Huang,
H.; Guo, Q.-X. J. Chem. Inf. Comput. Sci. 2003, 43, 2005.
5680 J. Org. Chem. Vol. 73, No. 15, 2008

Photosensitized Oxidation of Alkyl Phenyl Sulfoxides
by oxidation of PhSSPh with the H2O2/MoO2Cl2 system in
CH3CN at room temperature.49 Acetonitrile (spectrophotometric
grade) was used as received.
A precise comparison between the fragmentation (C-S bond
cleavage) rates of sulfoxide and sulfide radical cations would
have been of great interest, but this is not possible as the
fragmentation rate of the sulfoxide radical cation 4•+ which
might be compared with that known of the corresponding sulfide
radical cation4 was too fast to be measured. However, we can
estimate that sulfoxide radical cations should fragment at least
2 orders of magnitude faster than sulfide radical cations. This
consideration is based on the comparison of the fragmentation
rate of t-butyl phenyl sulfoxide radical cation (5•+) (1.4 × 106
M-1 s-1) with that of tert-butyl phenyl sulfide radical cation
which has been estimated as <104 M-1 s-1,4 (Table 2). Even
though this difference may seem very large, it should be noted
that it is associated with a really large change (almost 25 kcal
mol-1) in the C-S BDFE for the two radical cations, from 7.6
kcal mol-1 for the sulfide to -18.1 kcal mol-1 for the sulfoxide
(Table 2).42 Thus, the difference in fragmentation rate between
sulfide and sulfoxide radical cations appears small relative to
the difference in the thermodynamic driving force. This
observation would suggest a reorganization energy for the
fragmentation process significantly larger for sulfoxide than for
sulfide radical cations. This might not be surprising since in
the sulfoxide radical cation there is a very polar S-O bond,
presumably well solvated in the polar MeCN, whose polarity
and length change during the fragmentation process leading to
the sulfinyl radical. Accordingly, the negative charge on oxygen
is larger in the phenylsulfinyl radical29 than in the methyl phenyl
sulfoxide radical cation6 and the opposite occurs with respect
to the positive charge on sulfur. Moreover, comparing B3LYP
calculations, the S-O bond length is 0.04 Å shorter in the
sulfoxide radical cation than in the sulfinyl radical. Thus, both
the internal and the solvent reorganization energies might be
quite large.
Laser Flash Photolysis. Excitation wavelength of 355 nm (Nd:
YAG laser, Continuum, third harmonic, pulse width ca. 7 ns and
energy <3 mJ per pulse) was used in nanosecond flash photolysis
experiments.50 A 1 mL solution containing the substrate (1.0 ×
10-2 M), the sensitizer (3-CN-NMQ+ClO4-, 8.8 × 10-4 M), and
the cosensitizer (1 M toluene) was flashed in a quartz photolysis
cell while nitrogen or oxygen was bubbling through them. All
measurements were carried out at 22 ( 2 °C. The transient spectra
were obtained by a point-to-point technique, monitoring the change
of absorbance (∆A) after the laser flash at intervals of 5-10 nm
over the spectral range 320-650 nm, averaging at least 10 decays
at each wavelength. The error estimated on the rate constants was
(10%.
Fluorescence Quenching. Measurements were carried out on a
Spex Fluorolog F112AF spectrofluorometer. Relative emission
intensities at 427 nm (3-CN-NMQ+ClO4- emission maximum) were
-
measured irradiating at 329 nm (3-CN-NMQ+ClO4 absorption
maximum) a solution containing the sensitizer (<1.7 × 10-5 M)
with the substrate at different concentrations (from 9 × 10-3 to 1
× 10-2 M) in air-saturated CH2Cl2. The error estimated on the
Stern-Volmer constants (KSV) was ( 5%.
-
Quantum Yields. A 1 mL solution of 3-CN-NMQ+ClO4 (1.0
× 10-3 M) and 1-5 (1.0 × 10-2 M) in N2-saturated CD3CN was
placed in a quartz cell and irradiated with four phosphor-coated
fluorescent lamps emitting at 355 ( 15 nm. The light intensity (ca.
1 × 1015 photons s-1) was measured by potassium ferric oxalate
actinometry. The error estimated on the Φ values was (10%. The
1
photoproducts not containing sulfur were quantified by H NMR
analysis, by use of 1,4-dimethoxybenzene as internal standard.
Phenyl benzenethiosulfinate (PhSOSPh), diphenyl disulfide (Ph-
SSPh), and phenyl benzenethiosulfonate (PhSO2SPh) were identified
in the reaction of 3-CN-NMQ+ (1.0 × 10-3 M) and 3 (1.0 × 10-2
M) in N2-saturated CH3CN (1 mL) after 15 min of irradiation. The
photolyzed mixture was analyzed with HPLC by comparison with
authentic specimens and quantitatively determined by using the
direct calibration method.
Experimental Section
Computational Details. All of the calculations were carried out
by using the Gaussian 03 package.51 Since we are dealing with
conformationally flexible molecules, before starting the BDEs
calculations, all of the available conformations for the molecule
and the radicals formed in the C-S scission process have to be
found. To this end, a systematic conformational search was carried
out, at the semiempirical PM3 level of theory,52 by using the
Conformer Search Module available in the Spartan 5.01 package.53
All of the conformers found were optimized again, first at the
Materials. Benzyl phenyl sulfoxide (1), 1-phenylethyl phenyl
sulfoxide (2), diphenylmethyl phenyl sulfoxide (3), 2-phenyl-2-
propyl phenyl sulfoxide (4), and tert-butyl phenyl sulfoxide (5) were
prepared by oxidation of the corresponding sulfides with sodium
periodate in aqueous ethanol.43 Benzyl phenyl sulfide is com-
mercially available. 1-Phenylethyl phenyl sulfide, diphenylmethyl
phenyl sulfide, and tert-butyl phenyl sulfide were prepared by acid-
catalyzed reaction of thiophenol with 1-phenylethanol, diphenyl-
methanol, and tert-butanol, respectively,44 and 2-phenyl-2-propyl
phenyl sulfide was prepared by acid-catalyzed addition of thiophenol
on R-methylstyrene.45 3-Cyano-N-methylquinolinium percholorate
was prepared by reaction of 3-cyanoquinoline with dimethyl sulfate;
the sulfate salt was then transformed to the perchlorate salt by using
perchloric acid.46Alcohols 1a-5a, carbonyl products (benzalde-
hyde, acetophenone, and benzophenone), and diphenyl disulfide
are commercially available. Acetamides 1b, 2b, and 5b and
phenyl benzenethiosulfonate were prepared according to litera-
ture procedures.3e,47,48 Phenyl benzenethiosulfinate was prepared
(47) Lopez-Serrano, P.; Jongejan, J. A.; van Rantwijk, F.; Sheldon, R. A.
Tetrahedron: Asymmetry 2001, 12, 219–228. Newcomb, M.; Varck, T. R.; Goh,
S.-H. J. Am. Chem. Soc. 1990, 112, 5186.
(48) Kice, J. L.; Rogers, T. E. J. Am. Chem. Soc. 1974, 96, 8015–8019.
(49) Jeyakumar, K.; Chand, D. K. Tetrahedron Lett. 2006, 47, 4573–4576.
(50) Romani, A.; Elisei, F.; Masetti, F.; Favaro, G. J. Chem. Soc., Faraday
Trans. 1992, 88, 2147–2154. Go¨rner, H.; Elisei, F.; Aloisi, G. G. J. Chem. Soc.,
Faraday Trans. 1992, 88, 29–34.
(51) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci,
B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada,
M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida M.; Nakajima, T.;
Honda, Y.; Kitao O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.;
Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.;
Dapprich S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck,
A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T., Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, ReVision B.05,
Gaussian, Inc.: Pittsburgh, PA, 2003.
(41) Probably no nucleophilic assistance takes place also with 3•+ and 4•+
involving C-S bonds with secondary carbons.
(42) This difference is in part due to the difference in oxidation potential
between sulfoxide and sulfide (ca. 0.5 V higher for sulfoxide).5 The rest (ca. 16
kcal mol-1) can be associated with the higher stability of sulfinyl than sulfenyl
radicals.
(43) Leonard, N. J.; Johnson, C. R. J. Org. Chem. 1962, 27, 282–284.
(44) Screttas, C. G.; Micha-Screttas, M. J. Org. Chem. 1977, 42, 1462.
Cutress, N. C.; Grindley, T. B.; Katritzky, A. R.; Topsom, R. D. J. Chem. Soc.,
Perkin Trans. 2 1974, 263.
(45) Screttas, C. G.; Micha-Screttas, M. J. Org. Chem. 1979, 44, 713.
(46) Kato, S.; Nakata, J.; Imoto, E. Bull. Chem. Soc. Jpn. 1971, 44, 1928–
1933.
(52) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209.
J. Org. Chem. Vol. 73, No. 15, 2008 5681

Baciocchi et al.
B3LYP/6-31G level of theory and then at the higher B3P86/6-
311+G(d,p) level of theory. The number of conformers found for
sulfoxides 1-5 were: two for 1, three for 2, four for 3, three for 4,
and one for 5. Only one conformer was found for tert-butyl phenyl
sulfide and for each radical fragment.
sponding minima. In the Supporting Information are reported the
Cartesian coordinates, the electronic energy, and the zero-point
vibrational energy of all the minima found.
Acknowledgment. Financial support from the Ministero
dell’Istruzione, dell’Universita` e della Ricerca (MIUR), is
gratefully acknowledged.
In all of the calculations, the keywords integral(grid)Ultrafine)
scf)tight were used. For open shell (radical) species, spin
contamination due to states of multiplicity higher than the doublet
state, was negligible since the expectation value <S2> of the total
spin operator S2 was, in all cases, within 5% of the expectation
value for a doublet (0.75). Harmonic vibrational frequencies were
calculated at the B3P86/6-311+G(d,p) level of theory to confirm
that the stationary points found correspond to local minima and to
obtain the zero-point vibrational energy (ZPVE) corrections. For
the ZPVE a scale factor of 0.9845 was used.54 When more than
one conformer was found for a given compound, its energy (Eel +
EZPVE) to be used in the BDEs calculation was obtained by
Boltzmann averaging the energy (Eel + EZPVE) of all the corre-
Supporting Information Available: Stability of photoprod-
ucts in the steady-state photolysis of the 3-CN-NMQ+/1 and
3-CN-NMQ+/3 systems, laser flash photolysis of the 3-CN-
NMQ+/toluene/1 and 3-CN-NMQ+/toluene/2 systems, decay
kinetics of PhSO• at 470 nm, decay kinetics of 1+•, 2+•, and
5+• at 560 nm, C-S BDFEs for the sulfoxide and sulfide radical
cations, C-H BDFE for benzyl phenyl sulfoxide radical cation,
Cartesian coordinates, electronic energies, and zero-point vi-
brational energies obtained from DFT calculations. This material
is available free of charge via the Internet at http://pubs.acs.org.
(53) Spartan 5.01, Wavefunction, Inc., 18401 Von Karman Ave., Suite 370,
Irvine, CA 92612.
(54) Merrick, J. P.; Moran, D.; Radom, L. J. Phys. Chem. A 2007, 111, 11683.
JO801088N
5682 J. Org. Chem. Vol. 73, No. 15, 2008