Structure and C-S bond cleavage in aryl 1-methyl-1-arylethyl sulfide radical cations

Article abstract of DOI:10.1021/jo102086f

Steady state and laser flash photolysis (LFP) of a series of p-X-cumyl phenyl sulfides (4-X-C₆H₄C(CH₃) ₂SC₆H₅: 1, X = Br; 2, X = H; 3, X = CH ₃; 4, X = OCH₃) and p-X-cumyl p-methoxyphenyl sulfides (4-X-C₆H₄C(CH₃)₂SC₆H ₄OCH₃: 5, X = H; 6, X = CH₃; 7, X = OCH ₃) has been carried out in the presence of N-methoxy phenanthridinium hexafluorophosphate (MeOP⁺PF₆^-) under nitrogen in MeCN. Steady state photolysis showed the formation of products deriving from the C-S bond cleavage in the radical cations 1^+?-7 ^+? (2-aryl-2-propanols and diaryl disulfides). Formation of 1^+?-7^+? was also demonstrated by LFP experiments evidencing the absorption bands of the radical cations 1^+?- 3^+? (λ_max = 530 nm) and 5^+?- 7^+? (λ_max = 570 nm) mainly localized in the arylsulfenyl group and radical cation 4^+? (λ _max = 410, 700 nm) probably mainly localized in the cumyl ring. The radical cations decayed by first-order kinetics with a process attributable to the C-S bond cleavage. On the basis of DFT calculations it has been suggested that the conformations most suitable for C-S bond cleavage in 1 ^+?-4^+? and 7^+? are characterized by having the C-S bond almost collinear with the π system of the cumyl ring and by a significant charge and spin delocalization from the ArS ring to the cumyl ring. Such a delocalization is probably at the origin of the observation that the rates of C-S bond cleavage result in very little sensitivity to changes in the C-S bond dissociation free energy (BDFE). A quite large reorganization energy value (λ = 43.7 kcal mol^-1) has been calculated for the C-S bond scission reaction in the radical cation. This value is much larger than that (λ = 12 kcal mol^-1) found for the C-C bond cleavage in bicumyl radical cations, a reaction that also leads to cumyl carbocations.

Full text of DOI:10.1021/jo102086f

pubs.acs.org/joc
Structure and C-S Bond Cleavage in Aryl 1-Methyl-1-arylethyl Sulfide
Radical Cations
Enrico Baciocchi,† Marta Bettoni,‡ Tiziana Del Giacco,‡ Osvaldo Lanzalunga,*,†
Marco Mazzonna,† and Paolo Mencarelli†
†Dipartimento di Chimica and Istituto CNR di Metodologie Chimiche-IMC, Sezione Meccanismi di
Reazione c/o Dipartimento di Chimica, Universitaꢀdi Roma La Sapienza, P.le A. Moro 5, 00185 Rome, Italy,
and ‡Dipartimento di Chimica and Centro di Eccellenza Materiali Innovativi Nanostrutturati,
Universitaꢀ di Perugia, via Elce di Sotto 8, 06123 Perugia, Italy
osvaldo.lanzalunga@uniroma1.it
Received October 21, 2010
Steady state and laser flash photolysis (LFP) of a series of p-X-cumyl phenyl sulfides (4-X-C6H4C-
(CH3)2SC6H5: 1, X = Br; 2, X = H; 3, X = CH3; 4, X = OCH3) and p-X-cumyl p-methoxyphenyl
sulfides (4-X-C6H4C(CH3)2SC6H4OCH3: 5, X = H; 6, X = CH3; 7, X = OCH3) has been carried out in the
presence of N-methoxy phenanthridinium hexafluorophosphate (MeOPþPF6-) under nitrogen in MeCN.
Steady state photolysis showed the formation of products deriving from the C-S bond cleavage in the radical
cations 1þ•-7þ• (2-aryl-2-propanols and diaryl disulfides). Formation of 1þ•-7þ• was also demonstrated by
LFP experiments evidencing the absorption bands of the radical cations 1þ•-3þ• (λmax = 530 nm) and
5þ•-7þ• (λmax = 570 nm) mainly localized in the arylsulfenyl group and radical cation 4þ• (λmax = 410,
700 nm) probably mainly localized in the cumyl ring. The radical cations decayed by first-order kinetics with a
process attributable to the C-S bond cleavage. On the basis of DFT calculations it has been suggested that the
conformations most suitable for C-S bond cleavage in 1þ•-4þ• and 7þ• are characterized by having the C-S
bond almost collinear with the π system of the cumyl ring and by a significant charge and spin delocalization
from the ArS ring to the cumyl ring. Such a delocalization is probably at the origin of the observation that the
rates of C-S bond cleavage result in very little sensitivity to changes in the C-S bond dissociation free energy
(BDFE). A quite large reorganization energy value (λ = 43.7 kcal mol-1) has been calculated for the C-S
bond scission reaction in the radical cation. This value is much larger than that (λ = 12 kcal mol-1) found for
the C-C bond cleavage in bicumyl radical cations, a reaction that also leads to cumyl carbocations.
Introduction
theoretical aspects as well as for the important biological
implications.1 Among the reactions of sulfur radical cations,
C-S fragmentation to form an alkyl cation and a sulfenyl
radical (eq 1, for the most common case of aromatic sulfides)
is one of the most studied decomposition pathways of these
species.2,3 These reactions represent also an important tool
as the detection of fragmentation products can provide
The chemistry of sulfur radical cations has continued to
attract considerable interest, both for the practical and
(1) (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.;
€
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Biochem. Biophys. 2002, 397, 370. (e) Butterfield, D. A.; Kanski, J. Peptides 2002,
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~ꢂ~
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€
~ꢂ~
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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.
ꢁ ꢂ
€
J. Phys. Chem. A 2000, 104, 11877. (h) Bonifacic, M.; Hug, G. L.; Schoneich, C.
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€
12728. (k) Miller, B. L.; Kuczera, K.; Schoneich, C. J. Am. Chem. Soc. 1998, 120,
3345. (l) Fasani, E.; Freccero, M.; Mella, M.; Albini, A. Tetrahedron 1997, 53,
2219. (m) Mohan, H.; Mittal, J. P. J. Phys. Chem. A 1997, 101, 10012. (n) Miller,
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(o) Goez, M.; Rozwadowski, J.; Marciniak, B. J. Am. Chem. Soc. 1996, 118, 2882.
(3) A sulfide radical cation can also undergo C-S bond cleavage giving a
sulfenyl cation and an alkyl radical, but as far as we know, there is no report
dealing with this type of cleavage that generally is less thermodynamically
favored with respect to that reported in eq 1.
DOI: 10.1021/jo102086f
r
Published on Web 12/16/2010
J. Org. Chem. 2011, 76, 573–582 573
2010 American Chemical Society

Baciocchi et al.
JOCArticle
cleancut information on whether radical cations are formed
as intermediates in chemical and biological oxidations of
sulfur compounds.4
radical cations 1þ•-7þ•. DFT calculations at the B3LYP/
6-311G(d,p) level of theory have been also carried out for radical
cations 1þ•-4þ• and 7þ• in order to get information on the
geometry of the radical cations and more importantly on their
charge and spin distribution. This information was deemed to be
necessary in order to reach a deeper insight on the dynamics of the
fragmentation process and particularly on the SOMO location,
þ •
A rS - R f ArS• þ þ
R
ð1Þ
When the SOMO is mostly located on sulfur (eq 1), the
cleavage of the C-S bond in a sulfide radical cation presents
quite peculiar characteristics with respect to the electronic
reorganization connected with the cleavage itself, which
differ somewhat from those exhibited by the more intensively
investigated C-H, C-C, and C-Si bond cleavages in aro-
matic radical cations.5 In the latter cases the scissile bond is β
with respect to the SOMO, which is delocalized in the
aromatic ring (β-fragmentation), whereas in the sulfide
radical cation the cleaved C-S bond is R to the SOMO on
the sulfur atom (R-fragmentation).
Our research group has been long involved in the chem-
istry of sulfide radical cations, and recently we have focused
our attention on the quantitative aspects of reaction (1) in
tert-alkyl phenyl sulfide radical cations (PhSCR3þ•) through
a laser photolysis study.6 An interesting observation was 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 is influenced by the
structure of the alkyl groups. However, this study involved
CR3 groups with different steric hindrance, and the results
suggested that steric effects might play a significant role on
the rate of the fragmentation process.
Thus, we have felt that the investigation of a series of
sulfide radical cations presenting the same steric situation at
the scissile bond might give clearer information on the
electronic effects on the rate of C-S bond cleavage, and
accordingly we now report on a steady state and laser
photolysis study of the C-S bond cleavage of the radical
cations of a series of aryl cumyl sulfides para-substituted in
the cumyl ring (1-7). This study was integrated by DFT
calculations at the B3P86/6-311þG(d,p) level of theory for
sulfides 1-7, which provided us the C-S bond dissociation
energy values (BDEs) that have been used to determine the
bond dissociation free energies (BDFEs) in the corresponding
Results
The method used for the photochemical generation of
radical cations 1þ•-7þ• was the same reported previously for
the generation of 2þ•6 and is based on the light-induced N-O
bond cleavage in the N-methoxyphenanthridinium cation
(MeOPþ).7 The phenanthridinium radical cation (Pþ•
)
formed is a quite powerful oxidant (E° = 1.9 V vs SCE) able
to oxidize the aromatic sulfides, which are characterized by
oxidation potentials around or lower than 1.6 V vs SCE2e,8
(Scheme 1).
SCHEME 1. Photochemical Generation of Radical Cations
1
þ•-7þ• by MeOPþ
(4) (a) Baciocchi, E.; Del Giacco, T.; Elisei, F.; Lapi, A. J. Org. Chem.
2006, 71, 853–860. (b) Baciocchi, E.; Del Giacco, T.; Elisei, F.; Gerini, M. F.;
Lapi, A.; Liberali, P.; Uzzoli, B. J. Org. Chem. 2004, 69, 8323. (c) Baciocchi,
E.; Lanzalunga, O.; Pirozzi, B. Tetrahedron 1997, 53, 12287–12298.
(d) Baciocchi, E.; Lanzalunga, O.; Marconi, F. Tetrahedron Lett. 1994, 35,
9771–9774.
Steady-State Photolysis. Steady state photolysis experi-
ments were carried out by irradiating a solution of sulfides
(5) (a) Baciocchi, E.; Bietti, M.; Lanzalunga, O. J. Phys. Org. Chem. 2006,
19, 467–478. (b) Wang, L.; Seiders, J. R.; Floreancig, P. E. J. Am. Chem. Soc.
2004, 126, 12596–12603. (c) Seiders, J. R.; Wang, L.; Floreancig, P. E. J. Am.
Chem. Soc. 2003, 125, 2406–2407. (d) Kumar, V. S.; Floreancig, P. E. J. Am.
Chem. Soc. 2001, 123, 3842–3843. (e) Baciocchi, E.; Bietti, M.; Lanzalunga,
O. Acc. Chem. Res. 2000, 33, 243. (f) Schmittel, M.; Burghart, A. Angew.
Chem., Int. Ed. Engl. 1997, 36, 2550–2589. (g) Dinnocenzo, J. P.; Simpson,
T. R.; Zuilhof, H.; Todd, W. P.; Heinrich, T. J. Am. Chem. Soc. 1997, 119,
987–993. (h) 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.
(i) Schepp, N. P.; Shukla, D.; Johnston, L. J.; Sarker, H.; Bauld, N. L.
J. Am. Chem. Soc. 1997, 119, 10325–10334. (j) Gaillard, E. R.; Whitten, D. G.
Acc. Chem. Res. 1996, 29, 292. (k) Maslak, P. Top. Curr. Chem. 1993, 168, 1.
(l) Todd, W. P.; Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.; Gould, I. R.
Tetrahedron Lett. 1993, 34, 2863–2866. (m) Albini, A.; Fasani, E.; d’Alessandro,
N. Coord. Chem. Rev. 1993, 125, 269. (n) Popielarz, R.; Arnold, D. R. J. Am.
Chem. Soc. 1990, 112, 3068–3082. (o) Dinnocenzo, J. P.; Farid, S.; Goodman,
J. L.; Gould, I. R.; Todd, W. P.; Mattes, S. L. J. Am. Chem. Soc. 1989, 111,
8973–8975.
1-7 (0.01 M) in N2-saturated CD3CN at around 355 nm, in
-
the presence of MeOPþPF6 (5 ꢀ 10-3 M). Reaction prod-
ucts were identified and quantitated by 1H NMR (products from
the alkyl moiety) and GC (sulfur-containing products) analysis,
by comparison with authentic specimens. No products were
detected without irradiation or in the absence of the sensitizer.
The photolysis of sulfides 1-7 produced in substantial
amounts the fragmentation products diphenyl disulfide
(photooxidation of 1-4) and bis-4-methoxyphenyl disulfide
(photooxidation of 5-7) from the sulfide moiety and 2-aryl-
2-propanols from the cumyl group. Other products observed
(7) Shukla, D.; Guanghua, L.; Dinnocenzo, J. P.; Farid, S. Can. J. Chem.
2003, 81, 744.
(8) Baciocchi, E.; Crescenzi, C.; Lanzalunga, O. Tetrahedron 1997, 53,
(6) Baciocchi, E.; Del Giacco, T.; Gerini, M. F.; Lanzalunga, O. Org.
Lett. 2006, 8, 641–644.
4469.
574 J. Org. Chem. Vol. 76, No. 2, 2011

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SCHEME 2. Fragmentation Products in the Photooxidation of Sulfides 1-7 by MeOPþ
were those deriving from the N-O fragmentation of the
MeOPþ (protonated phenanthridine, PHþ and methanol)
(Scheme 2).
of the LFP experiment with the 3/MeOPþ system are
reported in Figure 1. The results of the LFP experiments with
the MeOPþ/2 system (already reported in a previous study6)
and with the 1/MeOPþ system are shown in Figures S1-S2
in the Supporting Information.
The structure of photoproducts formed can reasonably be
rationalized on the basis of the formation of the radical cations
1
þ•-7þ• followed by the unimolecular cleavage of the C-S
The intense absorption band at 530 nm can be assigned to
the radical cations 1þ•-3þ•6,13 formed by oxidation of
bond (eq 1). This process leads to the 1-methyl-1-arylethyl
cations and the arylsulfenyl radicals (Scheme 3).9 The cations
can then react with water11 to form the alcohols. Dimeriza-
tion of arylsulfenyl radicals leads to diaryl disulfides.
sulfides 1-3 by the phenanthridine radical cation (Pþ•
)
produced after N-O bond fragmentation in the MeOPþ
excited state.6 Time-resolved absorption spectra are not
modified by the presence of oxygen, thus confirming the
cationic nature of the transients.
SCHEME 3. C-S Bond Cleavage of Radical Cations 1þ•-7þ•
The time-evolution of the absorption spectra showed a
slow first-order decay of the signal at 530 nm (see inset of
Figure 1) substantially coupled with the growth of the
absorption at around 380-390 nm that can reasoþnably
attributed to the formation of the 4-X-C6H4C(CH3)2 cat-
ions (λmax = 380-410 nm).14 This observation confirms the
results of steady-state photolysis suggesting a decay of
1
þ•-3þ• due to the C-S bond cleavage with formation of
In the photooxidation of 4 and 7, 2-(4-methoxyphenyl)-
2-propanol was accompanied by significant amounts of
two dimeric products: 2,4-bis(4-methoxyphenyl)-4-methyl-2-
pentene (8) and 2,4-bis(4-methoxyphenyl)-4-methyl-1-pentene
(9). As reported in the literature,12 these products likely form
because the 2-(4-methoxyphenyl)-2-propyl carbocation produced
after C-S bond cleavage in 4þ• and 7þ• is relatively stable, and
thus the reaction with water to give 2-(4-methoxyphenyl)-2-
propanol occurs in competition with other processes (depro-
tonation to give 2-(4-methoxyphenyl)-propene followed by
coupling with a second molecule of carbocation) leading to
the two dimeric products as shown in Scheme 4.
1-methyl-1-arylethyl cations and the phenylsulfenyl radical
C6H5S•. The latter species does not emerge in the spectra
because its low extinction coefficient.15
While results of the time-resolved spectra of the MeOPþ/
1-3 systems were very similar, different features were ob-
served in the time-resolved spectra of the MeOPþ/4 system.
Upon laser excitation of a solution of MeOPþ (1.6 ꢀ 10-4 M)
and 4 (1.0 ꢀ 10-2 M) in N2-saturated CH3CN, an absorption
band at 410 nm was detected just after the laser pulse
accompanied by a broad band in the near-infrared (NIR)
region of the spectrum (λ > 650 nm) (Figure 2).
The 410 nm band is significantly blue-shifted with respect
to the absorption bands of radical cations 1þ•-3þ•, and its
assignment is more in accordance with a radical cation
mostly localized in the methoxylated cumyl ring. This hy-
pothesis is supported by the presence of the band in the NIR
region of the spectrum that can be assigned to an intramo-
lecular charge resonance interaction between the neutral
phenylsulfenyl ring and the charged acceptor 4-methoxy-
lated cumyl ring.16 In agreement, theoretical calculations
The quantum yields (Φ) measured for the products are
reported in Table 1. As expected, the quantum yields (Φ) of
diaryl disulfides (Table 1) are about half of that of the
products deriving from the cumyl moiety.
Laser Flash Photolysis Studies. By laser photolysis (λexc
=
355 nm) of N2-saturated CH3CN solutions of sulfides 1-3
(0.01 M) and MeOPþ (1.6 ꢀ 10-4 M), a broad and intense
absorption with maximum around 530 nm, accompanied by
a less intense absorption centered at 390 nm were detected
after the laser pulse. As an example, the time-resolved spectra
(13) Baciocchi, E.; Del Giacco, T.; Giombolini, P.; Lanzalunga, O.
Tetrahedron 2006, 62, 6566–6573.
(14) McClelland, R. A.; Chan, C.; Cozens, F. L.; Modro, A.; Steenken, S.
Angew. Chem., Int. Ed. Engl. 1991, 30, 1337.
(9) The C-S cleavage occurs by a unimolecular pathway as demonstrated
by the study of the stereochemistry of the C-S fragmentation in 1-pheny-
lethyl phenyl sulfide radical cation.10
(10) Baciocchi, E.; Fasella, E.; Lanzalunga, O.; Mattioli, M. Angew.
Chem., Int. Ed. Engl. 1993, 32, 1071–1073.
(11) The water content was ca. 0.01 M in the spectrophotometric grade
CH3CN used in all of the experiments
(12) Del Giacco, T.; Faltoni, A.; Elisei, F. Phys. Chem. Chem. Phys. 2008,
10, 200–210.
(15) λmax (C6H5S•) = 460 nm, ε = 2.6 ꢀ 103 M-1 s-1. Tripathi, G. N. R.;
Sun, Q.; Armstrong, D. A.; Chipman, D. M.; Schuler, R. H. J. Phys. Chem.
1992, 96, 5344.
(16) Intramolecular charge resonance interaction between a charged
acceptor methoxylated aromatic radical cation and neutral donor aromatic
rings in diarylmethanol radical cations17a and in trimeric lignin model
compounds17b led to absorption bands in the NIR region of the spectrum
similar to that observed in 4þ•
.
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SCHEME 4. Formation of Dimeric Products in the Photooxidation of Sulfides 4 and 7 by MeOPþ
TABLE 1. Quantum Yields (Φ) of Products Formed in the Photooxida-
tion of Aryl 1-Methyl-1-arylethyl Sulfides (1-7) Sensitized by MeOPþ in
N2-Saturated CD3CNa
FIGURE 2. Time-resolved absorption spectra of the MeOPþ (1.6 ꢀ
10-4 M)/ 4-CH3OC6H4C(CH3)2SC6H5 (4) (1.0 ꢀ 10-2 M) system in
N2-saturated CH3CN recorded 96 ns (b), 0.9 μs (O), 2.0 μs (1), and
5.9 μs (r) after the laser pulse. Insets: (a) buildup and decay kinetics
recorded at 360 nm; (b) decay kinetics recorded at 420 nm; (c) decay
kinetics recorded at 700 nm.
a[sulfide] = 1.0 ꢀ 10-2 M, [MeOPþ] = 5.0 ꢀ 10-3 M under nitrogen.
The error is (10%.
absorption band centered at 360 nm (Figure 2, inset a). This
band can be reasonably attributed to the 4-methoxycumyl
cation 4-CH3OC6H4C(CH3)2þ in accordance with the spec-
trum reported in the literature (λmax = 360 nm).14 The nature
of this transient is confirmed by the fact that addition of
small amounts of water leads to a significant increase of its
decay rate.18
LFP experiments with the MeOPþ/5-7 systems led to
results similar to those found with the MeOPþ/1-3 systems.
Solutions of the methoxylated sulfides 5-7 (0.01 M) and
MeOPþ (1.6 ꢀ 10-4 M), after the laser pulse, exhibited
a broad and intense absorption with maximum around 570 nm,
accompanied by a less intense absorption centered at 390 nm.
The time-resolved spectra are reported in Figures S3-S5
in Supporting Information. The intense absorption band at
570 nm can be assigned to the radical cations 5þ•-7þ•19
formed by oxidation of sulfides 5-7 by the phenanthridine
radical cation (Pþ•).
The time-evolution of the absorption spectra showed a
first-order decay of the signal at 570 nm (see inset of Figures
S3-S5 in Supporting Information) slower than that ob-
served in the LFP experiment with the corresponding non-
methoxylated cumyl sulfides 2-4. Again the decay was
FIGURE 1. Time-resolved absorption spectra of the MeOPþ (1.6 ꢀ
10-4 M)/4-CH3C6H4C(CH3)2SC6H5 (3) (1.0 ꢀ 10-2 M) system in
N2-saturated CH3CN recorded 0.2 (4), 2.4 (2), 7.4 (O) and 16 (b) μs
after the laser pulse. Inset: decay kinetics recorded at 530 nm.
have shown that in 4þ• the charge and spin densities are more
localized in the cumyl ring than in the ArS ring (vide infra).
The time-evolution of the absorption spectra shows that
the signal decay recorded at 420 nm (Figure 2, inset b) and
700 nm (Figure 2, inset c) is coupled with the growth of an
(18) The decay rate increase from 8.6 ꢀ 104 s-1 in the absence of water to
7.2 ꢀ 105 s-1 in the presence of 0.4% (v:v) of water. The rate of decay of
4-CH3OC6H4C(CH3)2þ in 2:1 (v:v) water/CH3CN is 4 ꢀ 107 s-1 14
.
(19) A similar λmax value (580 nm) has been reported for the radical cation
ꢂ
of 4-methoxythioanisole in water. Jonsson, M.; Lind, J.; Merenyi, G.;
Eriksen, T. E. J. Chem. Soc., Perkin Trans. 2 1995, 67–70.
(17) (a)Bietti, M.;Lanzalunga, O.J. Org. Chem. 2002, 67, 2632. (b) Baciocchi,
E.; Fabbri, C.; Lanzalunga, O. J. Org. Chem. 2003, 68, 9061–9069.
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TABLE 2. Decay Rate Constants (kf) of Aryl 1-Methyl-1-arylethyl
Sulfide Radical Cations (1þ•-7þ•) Generated by Photooxidation of 1-7
Sensitized by MeOPþPF6- (λecc = 355 nm), Oxidation Peak Potentials
(Ep), and C-S Bond Dissociation Energies (BDEs) for the Sulfides 1-7
and C-S Bond Dissociation Free Energies (BDFEs) for Radical Cations
to know the C-S bond dissociation free energies (BDFEs)
for the sulfide radical cations 1þ•-7þ•. These values were
estimated by the usual thermochemical cycle6 (details in
Supporting Information) using the C-S BDEs for the
neutral sulfides 1-7 obtained by DFT calculations, carried
out by using the Gaussian 03 package,23 at the B3P86/6-
311þG(d,p)//B3P86/6-311þG(d,p) level of theory. The
B3P86 functional was chosen because it is reported to be
applied with reasonable success to the calculations of BDE
values for a variety of C-X bonds,24,25 including C-S
bonds.25 It should also be noted that even though DFT
methods may underestimate absolute BDE values, this
should not affect the relative BDE values,25c which are those
we are mostly concerned with. The calculated C-S BDEs of
1-7 are reported in Table 2 together with the BDFEs of the
corresponding radical cations obtained by the thermochem-
ical cycle using the C-S BDEs for the neutral sulfides 1-7
corrected for the entropic factor, the peak oxidation poten-
tials of the sulfides reported in Table 2, and the reduction
potentials of the leaving carbocations available from the
literature.26
Since we are dealing with conformationally flexible mole-
cules, before starting the BDEs calculation, all 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,27 by using the Conformer
Search Module available in the Spartan 5.01 package.28 All
of the conformers found were optimized again, first at the
B3LYP/6-31G level of theory and then at the higher B3P86/
6-311þG(d,p) level of theory. The number of conformers
found for sulfides 1-7 were two for 1, two for 2, two for 3,
four for 4, four for 5, four for 6, and eight for 7. Only one
conformer was found for each radical fragment.
1
þ•-7þ•
aFrom LFP experiments in N2-saturated CH3CN. [sulfide] = 1.0 ꢀ
10-2 M, [MeOPþPF6-] = 8.8 ꢀ 10-4 M. bOxidation peak potential in V
vs SCE in CH3CN. cFrom DFT calculations, see text. dkcal mol-1. eAt
298 K. fDetails of calculations in Supporting Information
accompanied by a growth of absorption at 380-400 nm due
to the formation of 1-methyl-1-arylethyl cations. In the time-
resolved spectra of the MeOPþ/7 system at longer delay
times an absorption peak centered at 525 nm is observed (see
Figure S5 in Supporting Information) which can be assigned
to 4-CH3OC6H4S• according to the literature.20 In this case
the decay rate of 7þ• is fast enough (vide infra) to allow the
observation of the sulfenyl radical.21
It can be noted that in the MeOPþ/7 LFP experiment the
band at 360 nm attributed to 4-CH3OC6H4C(CH3)2þ is less
intense than that observed in the LFP experiment with
MeOPþ/4. This is likely due to the fact that when produced
from fragmentation of 4þ• the formation of 4-CH3OC6H4C-
(CH3)2þ (kbuildup ≈ 1.6 ꢀ 106 s-1, see inset a of Figure 2) is
faster than its decay (kdecay ≈ 8.6 ꢀ 104 s-1), whereas the
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 multiplic-
ity higher than the doublet state was negligible since the
þ
opposite occurs when 4-CH3OC6H4C(CH3)2 is produced
(23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, Jr, J. A; 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.
(24) (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. J. Mol. Struct. (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.
(25) (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.
from C-S bond cleavage in 7þ•
The decay rates of the radical cations were determined by
.
following the kinetics at 530 nm for 1þ•-3þ•, at 570 nm for
5
þ•-7þ• and at 700 nm for 4þ• (insets of Figures 1 and 2 and
S1-S5 in Supporting Information). In all cases the decay
kinetics followed first order laws in accordance with the
unimolecular fragmentation process.9,22 The rate of frag-
mentation of 5þ• and 6þ• were too low, and only an upper
limit rate constant (kf < 1 ꢀ 104 s-1) has been given for these
processes. The rate constants (kf) measured at 25 °C are
reported in Table 2.
Theoretical Calculations. Neutral Sulfides. For a mean-
ingful discussion of the experimental results, it was necessary
(20) Darmanyan, A. P.; Gregory, D. D.; Guo, Y.; Jencks, W. S. J. Phys.
Chem. A 1997, 101, 6855–6863.
(21) The absorption spectrum of 4-CH3OC6H4S•, obtained by laser
photolysis of tert-butylperoxide (0.4 M) in the presence of 4-CH3OC6H4SH
(2.7 ꢀ 10-2 M) in MeCN, shows a broad band centered at 520 nm (shoulder
at 470 nm). This transient decays by second-order kinetics with t1/2 = 7.3 μs.
(22) The radical cation decays were somewhat complicated by the pre-
sence of a transient species as already found by Dinnocenzo et al.7 The decay
rate constant for this species was the same in all samples (ca. 1 ꢀ 106 s-1), and
the nature of this transient has not been identified even though it is likely due
to the photolysis of MeOPþ.
(26) Sim, B. A.; Milne, H.; Griller, D.; Wayner, D. D. M. J. Am. Chem.
Soc. 1990, 112, 6635–6638.
(27) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209.
(28) Spartan 5.01; Wavefunction, Inc.: Irvine, CA.
J. Org. Chem. Vol. 76, No. 2, 2011 577

Baciocchi et al.
JOCArticle
FIGURE 3. 4-BrC6H4C(CH3)2SC6H5 (1þ•) conformers at the
minimum of energy.
þ•
FIGURE 4. Definition of geometrical features of radical cations
1
þ•-4þ•, 7þ• reported in Table 3.
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 B3P86/6-311þG(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.29 When
more than one conformer was found for a given compound,
its energy (Eel þ EZPVE) to be used in the BDEs calculation,
TABLE 3. Most Significant Dihedral Angles (deg) and Bond Lengths
(A) of Radical Cations 1þ•-4þ•, 7þ•
˚
Φ1 Φ2
1a -8
1b 58
d1
d2
d3
<d4> <d5> <d6>
d7
55 1.735 1.908 1.521 1.405 1.389 1.396 1.896
90 1.757 1.945 1.512 1.416 1.381 1.402 1.884
54 1.729 1.916 1.522 1.403 1.392 1.394
92 1.751 1.949 1.515 1.415 1.384 1.398
2a -8
2b
3a
53
8
-55 1.736 1.910 1.519 1.405 1.388
1.402 1.503
3b -60
4a -69
90 1.759 1.947 1.510 1.418 1.380 1.409 1.498
92 1.770 1.939 1.505 1.421 1.374 1.415 1.325
90 1.770 1.939 1.506 1.422 1.373 1.416 1.325
-55 1.733 1.913 1.519 1.405 1.386 1.404 1.343
88 1.747 1.960 1.505 1.415 1.378 1.411 1.333
was obtained by Boltzmann averaging the energy (Eel
þ
4b
7a
68
7
EZPVE) of all of the corresponding minima. In the Supporting
Information are reported the Cartesian coordinates, the
electronic energy, and the zero-point vibrational energy of
all of the minima found.
Sulfide Radical Cations. DFT calculations were performed
for those sulfide radical cations characterized by measurable
fragmentation rate constants (1þ•-4þ• and 7þ•) with the aim
of obtaining the geometry of the minima, charges, and spin
distributions. The calculations were carried out at the
B3LYP/6-311G(d,p)//B3LYP/6-311G(d,p) level of theory,
the same that was already used in a previous work on similar
sulfide radical cations.30
The starting geometries for the energy minimization of
each radical cation were those of the corresponding neutral
sulfide. Harmonic vibrational frequencies were calculated at
the B3LYP/6-311G(d,p) level of theory to confirm that the
stationary points found correspond to local minima. For all
of the radical cations, 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). Two minima were found for 1þ•-4þ•, and
eight minima were instead found for 7þ•. The atomic charges
were obtained by Natural Population Analysis (NPA).31
Unpaired electron spin densities were calculated using the
Mulliken population analysis.
7b -57
and 2aþ• is 1.45 kcal/mol lower in energy than 2bþ•, whereas
3aþ• and 3bþ• have approximately the same energy.
The two energy minimum conformations for 4þ• (Figure
S10 in Supporting Information) have a similar geometry
differing only for the relative orientation of the methoxy
group with respect to the phenylsulfenyl group. Both con-
formations resemble those found for conformers 1bþ•-3bþ•
(vide infra).
For radical cation 7þ• eight energy minimum conforma-
tions were found. These conformations can be divided, by
the geometrical point of view, in two families resembling the
above-mentioned a type (four conformations due to the four
possible relative orientations of the two methoxy groups)
and b type (four conformations) for the radical cations
1
þ•-3þ• (vide infra). The relative amount of the two types
a and b can be calculated as 74% and 26%, respectively. The
most stable of each type of conformation is shown in Figure
S11 in the Supporting Information.
The most significant geometrical features (dihedral angles
and bond length as defined in Figure 4) of radical cations
1
þ•-4þ•, 7þ• are reported in Table 3.
In Table 4 are displayed NPA Charges and spin density
values obtained for radical cations in 1þ•-4þ•, 7þ•, parti-
tioned for the different structural components of the radical
cations defined in Figure 5.
The two energy minimum conformations for 1þ•-3þ•
(1aþ•-3aþ• and 1bþ•-3bþ•) exhibit quite different geometry,
(see later for further details). In Figure 3 are reported the
1aþ• and 1bþ• conformers (2aþ•-2bþ• and 3aþ•-3bþ• con-
formers are reported in Figures S8 and S9 in the Supporting
Information). 1aþ• is 0.42 kcal/mol lower in energy than 1bþ•
From the data reported in Table 3, it can be observed that
the a conformations for radical cations 1þ•-3þ•and 7þ• have
very similar geometries, and the same can be said for the b
conformations, a and b being however characterized by
distinct geometrical features. In particular the dihedral angle
Φ1 is around 8° for the a conformers (the fragmenting C-S
bond is almost coplanar with the phenylsulfenyl ring as
already observed in the minimum energy conformers of alkyl
(29) Merrick, J. P.; Moran, D.; Radom, L. J. Phys. Chem. A 2007, 111,
11683.
(30) Baciocchi, E.; Gerini, M. F. J. Phys. Chem. A 2004, 108, 2332–2338.
(31) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899.
578 J. Org. Chem. Vol. 76, No. 2, 2011

Baciocchi et al.
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TABLE 4. NPA Charges (qNPA) and Spin Densities for Radical Cations
1
þ•-4þ•, 7þ•
Arcum
Alkcum
0.060 0.595 0.194
0.112 -0.005 0.492 0.362
0.118 0.071 0.534 0.078
0.300 -0.012 0.459 0.175
0.095 0.068 0.617 0.221
0.077 -0.002 0.528 0.397
0.251 0.079 0.565 0.105
0.293 -0.011 0.503 0.215
0.091 0.060 0.598 0.181
0.147 -0.006 0.505 0.349
0.246 0.072 0.535 0.064
0.372 -0.011 0.468 0.159
0.538
0.420 -0.015 0.392 0.090
0.538 0.053 0.476 0.010 -0.076
0.421 -0.016 0.394 0.090 0.111
0.048 0.517 0.372 -0.146 -0.079
0.002 0.396 0.371 0.028 0.109
0.058 0.450 0.270 -0.107 -0.110
0.079 0.065
S
ArS
X
Y
1a qNPA -0.003
0.154
0.038
0.200
0.077
1a spin
1b qNPA
1b spin
2a qNPA
2a spin
2b qNPA
2b spin
3a qNPA
3a spin
3b qNPA
3b spin
4a qNPA
4a spin
4b qNPA
4b spin
7a qNPA
7a spin
7b qNPA
7b spin
0.070
0.005
0.084
0.013
FIGURE 6. SOMO of the two energy minimum conformers of 3þ•
.
cumyl sulfide radical cations 1þ•-3þ•. If, for example, we
consider the most stable conformer of cumyl phenyl sulfide
radical cation 2aþ•, it can be noted that NPA charges and
spin populations are mainly localized on the ArS ring and on
the sulfur atom. On going to conformer 2bþ• we note that the
charge and spin density increase significantly in the Arcum
ring and almost correspondingly decrease in the ArS ring,
whereas the sulfur atom is little affected.
0.052 0.473 0.012 -0.075
0.113
0.289
0.093
0.438
0.292 -0.007 0.356 0.215
An increase of charge and spin density on the cumyl ring at
the expense of that on the ArS ring on going from the a to the
b conformer, is also observed in radical cations 1þ•-3þ• and
7
þ•. A reasonable explanation is that in the b conformations
a through-space interaction between the aromatic rings of
the phenylsulfenyl and the cumyl moieties takes place. In
other words, the charge and spin density of the radical cation
which are mainly localized on the ArS ring and sulfur in the a
conformation (mostly on sulfur) are still mainly localized on
sulfur in the b conformation, but now there is a significant
delocalization between the Arcum and the ArS rings. This
specific interaction between the two aromatic rings is well
visualized in the SOMO representation for 3bþ• shown in
Figure 6.
This delocalization occurs only in the b conformers as in
this case the two aromatic systems are almost facing each
other (the dihedral angle between the planes of the two rings
is 40-53°). In the a conformers instead the dihedral angle
between the planes of the two aromatic rings (around 90°)
does not allow an analogous spin and charge delocalization
as shown by the SOMO of 3aþ• reported in Figure 6. This
shift of charge and spin between the two rings can also be
represented as a contribution of the resonance structure II to
the b conformation of the radical cation (Figure 7) more
important of that of structure III (the reverse holds in the a
conformations). It can be noted that the relative stability of
the b with respect to the a conformer in 1þ•-3þ• increases by
increasing the þR electron donating power of the X sub-
stituent (H < Br < CH3, higher contribution of the reso-
nance structure II in Figure 7).
A further observation is that in b conformations the C-S
bond (d2 in Figure 6) is significantly longer than in a
conformations and is almost exactly perpendicular to the
cumyl ring. These two observations lead us to suggest some
hyperconjugative interaction between the d2 C-S bond and
the π system of the cumyl ring, which can be represented by a
contribution of structure IV (Figure 7). In view of the very
low charge on the Alkcum group (Table 4) we feel, however,
that structure IV should be a much less important contribu-
tor to the resonance hybrid of b conformations of radical
cations 1þ•-3þ• and 7þ• than structure II.
FIGURE 5
phenyl sulfide radical cations30) and 53-60° for the b con-
formers. A difference in the dihedral angle Φ2 is also
observed being around 55° for the a conformers and around
90° for the b conformers (where the fragmenting C-S bond is
almost perpendicular to the cumyl ring). Significant differ-
ences in the bond lengths are also observed between the a
and b conformers. In particular the d2 fragmenting C-S
bond and the C-C bonds d4 and d6 in the cumyl ring are
always longer in the b conformers of 1þ•-3þ•, 7þ•. On the
opposite the C-S bond d3 and the C-C bond d5 are always
longer in the a conformers of 1þ•-3þ•, 7þ•. In other words,
the cumyl ring in the b conformers has a more quinoid-like
structure.32
Interestingly, for 4þ• the two conformers are instead
characterized by a similar geometry resembling that of
radical cations 1bþ•-3bþ•. The dihedral angles Φ1 and Φ2
are in both the conformers close to 70° and 90°, respectively.
The bond distances are also practically the same in the two
conformations and similar to those found for the b confor-
mers of 1þ•-3þ•
From the data reported in Table 4, it can be noted that like
the geometries, the distribution of charge and spin density
too is significantly different in the a and b conformations of
.
(32) The degree of planarity of the cumyl carbon bonded to the S atom,
defined by the difference between 360° and the sum of the CH3-C-CH3,
CH3-C-C(Arcum), and C(Arcum)-C-CH3 angles, was always higher for
the b conformers of 1þ•-3þ• with respect to the a conformers (see Table S2 in
Supporting Information).
J. Org. Chem. Vol. 76, No. 2, 2011 579

Baciocchi et al.
JOCArticle
cumyl moiety, it can be immediately noted that the C-S
bond cleavage rates are very little sensitive to the nature of
the substituent. As expected, the rates increase by increasing
the electron-donating power of the substituent, that is,
increasing the stability of the formed carbocation, but
on going from bromo to methoxy the increase in rate is only
by a factor of 12.5. In term of BDFE, the 5.5 kcal/mol
difference in BDFE between the most reactive 4þ• and the
least reactive 1þ• is associated to a very small difference in
ΔG# (1.5 kcal/mol).
FIGURE 7. Resonance structures for radical cations 1þ•-4þ•
.
It has been already said that, differently from the other
radical cations, with 4þ• the two conformers 4aþ• and 4bþ•
have very similar geometries. Analogously, similar values of
charge and spin distribution in the Arcum and ArS rings are
exhibited by the two conformers. It is noticeable that in both
conformations, the presence of the methoxy substituent in
the cumyl ring of 4þ• determines a >0.5 charge distribution
in the Arcum, indicating a predominating role of structure II
with this radical cation, whereas that of structure III seems
almost negligible. This is in nice agreement with the pre-
viously discussed UV-vis spectrum of 4þ•, which has sug-
gested an anisole type radical cation. For radical cation 4þ•
both conformers a and b show the possibility of a through-
space interaction of the phenylsulfenyl ring with the spin and
charge densities in the cumyl system that appears signifi-
cantly enhanced by the presence of the methoxy group.33 We
remind that for radical cations 1þ•-3þ• and 7þ•, instead,
UV-vis spectra have suggested a sulfur type radical cation,
which is in line with the finding that for these radical cations
(particularly for 1þ•-3þ•) most of charge and spin are
localized on sulfur (structures I and III in Figure 7).
Looking for more substantial effects, we also studied the
p-X-cumyl p-methoxyphenyl sulfide radical cations 5þ•-7þ•
hoping that with a more stable sulfur radical cation, the
effect of the X substituent in the cumyl moiety on the C-S
bond cleavage rate would be larger than that observed with
1
•þ-4•þ. As expected, slower rates were observed with
respect to 1•þ-4•þ, but unfortunately the decrease was such
that it was possible to obtain reliable rates only for 7•þ, which
precluded any comparison.
Anyway, we tried to treat the kinetic data for the frag-
mentation of radical cations 1þ•-4þ•, 7þ• in terms of the
Marcus equation, eq 2,35 where ΔG# is the activation free
energy of the reaction, obtained by the rate constants in
Table 2 via the Eyring equation (eq 3) with Z taken as 6 ꢀ
1011 M-1
s
.
-1 36 ΔG° is the free energy variation in the same
reaction (C-S BDFE given in Table 2), and λ is the reorga-
nization energy required for the fragmentation process.
2
ΔG# ¼ ðλ=4Þð1 þ ΔG°=λÞ
ð2Þ
ð3Þ
ΔG# ¼ RT lnðZ=kf Þ
Very rewardingly, a quite satisfactory fit of the experi-
mental data to eq 2 was obtained, which is shown in Figure 8.
From the nonlinear least-squares fitting a flat curve is
obtained in accordance with the small influence of the
BDFEs on the fragmentation rates described above. The λ
value obtained from the fit (43.7 ( 3.1 kcal mol-1, see also
Figure S7 in Supporting Information) clearly indicates a
quite high intrinsic barrier for the C-S bond cleavage
reaction.
An interesting observation is that the very small sensitivity
of the C-S bond cleavage rate to the stability of the formed
carbocation for the sulfide radical cations displayed in
Table 2 sharply contrasts with what is observed in the SN1
solvolysis of sulfonium salts (eq 4, R = tert-alkyl, arylalkyl,
or diarylalkyl)37,38 a reaction that formally strongly resem-
bles the C-S bond cleavage in sulfur radical cations (eq 1),
the only difference being that in eq 1 the leaving group is a
sulfur radical, whereas in eq 4 it is a neutral sulfide. In this
case, the stabilization of the incipient carbocation is the
dominant factor affecting the solvolysis rate.39 For ex-
ample, in the solvolysis of X-substituted benzhydryldi-
methylsulfonium salts, the solvolysis rate in MeOH is
Discussion
In the previous section, we have observed that two con-
formations (a and b), with different charge and spin distribu-
tion, are available for the radical cations 1þ•-3þ• and 7þ•. At
variance the two conformations for 4þ• are both of the b type.
The peculiarity of the b type conformations common to all
radical cations is that they present the C-S bond perpendi-
cular to the ring of the cumyl moiety and exhibit a substantial
delocalization of SOMO in both rings. It seems reasonable to
suggest that the b conformation is the most suitable one for
the C-S bond cleavage as the empty carbon p orbital formed
in the cleavage is perfectly suited for an efficient overlap with
the π system (stereoelectronic effect) in the formed cumyl
carbocation. Moreover, in this conformation the C-S bond
is significantly longer than in the a conformation. We
will therefore discuss the kinetic data in Table 2 by assum-
ing that the b conformation is that undergoing C-S bond
cleavage.34
By considering first the series 1þ•-4þ•, where the radical
cations differ only for the nature of the para substituent in the
(33) A methoxy group in the cumyl ring is also present in 7þ•, and
accordingly in this case too there is a significant enhancement of charge
density in Arcum of the b conformation with respect to 1þ•-3þ•
.
(35) Marcus, R. A. Annu. Rev. Phys. Chem. 1964, 15, 155.
(36) Eberson, L. Electron Transfer Reactions in Organic Chemistry;
Springler Verlag: Berlin Heidelberg, 1986; Chapter 3.
(34) In order to get an evaluation of the values of the interconversion
barriers between a and b conformations, a conformational transition state
was located (by using the keyword Opt=QST2) in the interconversion path
between a and b conformations in radical cation 2þ•. The two barriers were
found to be very low: 1.97 and 0.52 kcal/mol for the a f b and b f a
conversion, respectively. Therefore it should no matter if the radical cation is
predominantly formed in conformation a when this is the most stable
conformation.
(37) Hine, J. B.; Golinkin, H. S. Can. J. Chem. 1963, 41, 3139–3142.
ꢂ
(38) Juric, S.; Denegri, B.; Kronja, O. J. Org. Chem. 2010, 75, 3851–3854.
(39) The sensitivity of the solvolysis rate to the stability of the formed
carbocation37 was found to be very similar to that observed for the
corresponding chlorides (very large Fþ ≈ -4.5 for substituted tert-cumyl
chlorides40).
580 J. Org. Chem. Vol. 76, No. 2, 2011

Baciocchi et al.
JOCArticle
SCHEME 5. C-S and C-C Bond Cleavage of Radical Cations
4
þ• and 10þ•
TABLE 5. Rate Constants (kf), Activation Free Energies for Frag-
mentation Reactions (ΔG#), and Bond Dissociation Free Energies
(BDFEs) of Radical Cations 4þ• and 10þ•
radical cations
4þ•
kf
ΔG#a
BDFEa
4.5 ꢀ 105 s-1
8.4
6.0b
-5.1
10þ•
2.5 ꢀ 107 s-1b
4.8b
akcal mol-1. bref 41.
FIGURE 8. Diagram of ΔG# vs ΔG° for the fragmentation reac-
tions of radical cations 1þ•-4þ•,7þ•. The solid circles correspond to
the experimental values; the solid curve is calculated by nonlinear
least-squares fit to eq 2.
In Table 5 are reported the rate constants and the activa-
tion free energies of the fragmentation reactions in Scheme 5
together with the relative C-S and C-C bond dissociation
free energies (BDFEs) for the two radical cations.
accelerated by almost 30 times on passing from X = H to
X = Me.38
From the data reported in Table 5 it can be readily noted
that the rate of C-S bond cleavage in 4þ• is significantly
lower than the rate of C-C bond cleavage in 10þ• even
though the former reaction is exergonic while the second
process is endergonic. This leads to the conclusion that the
C-S bond cleavage in cumyl aryl sulfide radical cations is
intrinsically a much slower process than the C-C bond
cleavage in bicumyl radical cations. Indeed, a λ value as
low as 12 kcal mol-1 was determined for the C-C bond
cleavage in the bicumyl radical cation,41,42 which should be
compared with a λ for the C-S bond cleavage of 43.7 kcal
mol-1, obtained from the Marcus plot in Figure 8.
The kinetic barrier associated with the bond cleavage in an
aromatic radical cation, should derive from the intramolec-
ular nuclear and solvent reorganization attending the trans-
fer of an electron from the σ orbital of the scissile bond to the
aromatic π system. The above results indicate that the cost of
this reorganization is significantly higher for the C-S than
for the C-C bond cleavage. A tentative explanation might
consider that whereas in 10þ• all of the charge is localized in
the methoxylated ring and remains in that ring after the C-C
bond cleavage, in 4þ• a substantial amount (almost half) of
charge is also localized in the ArS moiety of the radical
cation, as indicated by DFT calculations. This implies that
such a charge has to be transferred to the cumyl moiety in
the final products. Thus, the extent of the reorganization
should be much larger for the C-S bond cleavage in 4þ• than
for the C-C bond cleavage in 10þ•, which certainly should
We feel that a rationalization of the above difference may
be traced back to the large differences in reactivity and in the
initial state charge distribution of the two systems. In
sulfonium salts, the solvolyses are very slow reactions, many
orders of magnitude slower than the fragmentation of sulfur
radical cations; moreover, in the initial state, the charge is
almost completely located on the sulfur atom.38 Thus, a late
transition state with a well advanced state of C-S bond
breaking can be predicted where a quite large amount of
positive charge has been transferred from sulfur to the carbon,
so determining a strong dependence of sulfonium salts solvo-
lysis rates on the stability of the formed carbocation.
The situation is different in sulfur radical cations where, as
already seen, a significant delocalization of spin and positive
charge in the cumyl moiety is already present in the radical
cation and the C-S BDFE is very low. It is therefore very
probable that the transition state for the cleavage of the C-S
bond is reached quite early along the reaction coordinate
with very little further transfer of positive charge to the
cumyl moiety. Hence, substituents that can stabilize the
positive charge in the cumyl group should have a very small
effect on the rate of C-S scission, as observed.
Another interesting comparison is also possible between
the scission of the C-S bond in 4þ• and the homolytic C-C
bond cleavage in a p-methoxy-substituted bicumyl radical
cation (10þ•), a reaction studied in detail by Maslak and his
group.41 As illustrated in Scheme 5, in both systems the
positive charge is mainly (exclusively for 10þ•) located in the
4-methoxycumyl ring and the same carbocation is formed by
C-S and C-C bond cleavage.
(42) It should be pointed out, however, that the difference in λ between
C-C and C-S bond cleavage might be somewhat lower than that resulting
from the above figures since it is known that DFT calculations may under-
estimate the values of the BDEs of the C-S bond in the neutral substrates
from which the BDFE energy for 4þ• has been obtained by the thermo-
chemical cycle. However, even with a C-S BDE for 4 that is 6-8 kcal mol-1
larger than that resulting from DFT calculations, the difference in the kinetic
barrier between C-C and C-S bond cleavage in aromatic radical cations
would remain very significant. For example the fit in Figure 8 using 8 kcal
mol-1 more positive ΔG° values leads to a lower λ value (27.0 kcal mol-1),
which is however still much higher than the 12 kcal mol-1 calculated for the
C-C bond cleavage reaction. On the other hand, C-C bond cleavage would
remain 2 kcal mol-1 more endergonic than C-S bond cleavage.
(40) Okamoto, Y.; Inukai, T.; Brown, H. C. J. Am. Chem. Soc. 1958, 80,
4972–4976.
(41) Maslak, P.; Vallombroso, T. M.; Chapman, W. H.; Narvaez, J. N.
Angew. Chem., Int. Ed. Engl. 1994, 33, 73–75.
J. Org. Chem. Vol. 76, No. 2, 2011 581

Baciocchi et al.
JOCArticle
1
was added and the mixtures were analyzed by H NMR, GC,
significantly contribute to making the kinetic barrier for the
former cleavage higher than for the second. In addition the
solvent reorganization too should be larger for the C-S than
the C-C bond cleavage as the former entails a larger degree
of charge transfer from ArS to the cumyl moiety.
and GC-MS. The following products were identified by com-
parison with authentic specimens: 2-phenyl-2-propanol, 2-(4-
methoxyphenyl)-2-propanol, 2-(4-methylphenyl)-2-propanol,
2-(4-bromophenyl)-2-propanol, bis(4-methoxyphenyl) disul-
fide, diphenyl disulfide, protonated phenanthridine, and metha-
nol. 2,4-Bis(4-methoxyphenyl)-4-methyl-1-pentene and (E)-2,4-
bis(4-methoxyphenyl)-4-methyl-2-pentene were identified by
comparison of the 1H NMR and GC-MS data with those
reported in the literature.12 The photoproducts were quantified
Conclusions
C-S Bond cleavage in aryl 1-methyl-1-arylethyl sulfide
radical cations 1þ•-7þ• is an exergonic or almost isergonic
process leading to cumyl cations and arylsulfenyl radicals,
1
by GC and H NMR. The material balance was always satis-
factory (>90%). Blank experiment, carried out by irradiating
the solutions in the absence of MeOPþ, did not show in all cases
product formation.
characterized by a relatively low reaction rate (104-105 s-1
)
and by only a modest increase of the fragmentation rate
constants by increasing the stability of the formed cumyl
carbocation, i.e., as the C-S bond dissociation free energies
(BDFEs) become more negative. These results have been
rationalized by considering (DFT calculations) that the
conformation most suitable for C-S bond cleavage (C-S
bond almost collinear with the π system of the cumyl ring)
allows that a significant fraction of charge and spin is
distributed in the ArS ring and in the cumyl ring. Thus, little
further accumulation of positive charge at the cumyl carbon
should occur at the transition state of the C-S scission
reaction. The fact that charge and spin are delocalized
among the two rings of the radical cations may also be
responsible for the large reorganization energy (λ = 43.7
kcal mol-1) estimated for the C-S bond scission by the
Marcus equation, which makes such a scission intrinsically
much slower than C-C bond cleavage in bicumyl radical
cations, a reaction that also leads to cumyl carbocations.
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. The transient spectra were obtained by
a point-to-point technique, monitoring the change of absor-
bance (ΔA) after the laser flash at intervals of 5-10 nm over the
spectral range 300-800 nm, averaging at least 10 decays at each
wavelength. Nitrogen was bubbling through the solution. All
measurements were carried out at 22 ( 2 °C unless otherwise
indicated. The experimental error was (10%.
Acknowledgment. Financial support from the Ministero
ꢀ
dell’Istruzione, dell’Universita e della Ricerca (MIUR) is
gratefully acknowledged.
Supporting Information Available: Starting materials, time-
resolved absorption spectra after LFP of the MeOPþ/1-2, 5-7
systems; cyclic voltammetry; C-S BDFEs of radical cations
1
þ•-7þ•; degree of planarity of the cumyl carbon bonded to the
Experimental Section
S atom in neutral sulfides 1-4, 7 and in sulfide radical cations
Steady State Photolysis. Photooxidation reactions were car-
ried out in a photoreactor equipped with 4 lamps (360 nm; 14 W
each). A 2 mL solution containing the substrate 1-7(1.0ꢀ 10-2 M)
and MeOPþ (5 ꢀ 10-3 M) in N2-saturated CD3CN was placed
in a quartz cell and irradiated with two phosphor-coated
fluorescent lamps emitting at 355 ( 15 nm. The light intensity
(ca. 3 ꢀ 1015 photons s-1) was measured by potassium
ferric oxalate actinometry. An internal standard (bibenzyl)
1
þ•-4þ•, 7þ•; diagram of ΔG# vs ΔG° for the fragmentation
reactions of radical cations 1þ•-4þ•, 7þ• with error limits;
Cartesian Coordinates, energies, ZPVE from DFT calculations
for neutral sulfides 1-7, for cumyl and arylsulfenyl radicals and
for radical cations 1þ•-4þ•, 7þ•; 2þ•-4þ•, 7þ• conformers at the
minimum of energy; NPA Charges and Mulliken spin densities
for radical cation 1þ•-4þ•, 7þ•. This material is available free of
charge via the Internet at http://pubs.acs.org.
582 J. Org. Chem. Vol. 76, No. 2, 2011