C-S bond cleavage in aromatic sulfide radical cations

Article abstract of DOI:10.1080/10426507.2012.736108

The results of our recent studies of the structural effects on the C-S bond fragmentation process of aromatic sulfur radical cations are reported.

Full text of DOI:10.1080/10426507.2012.736108

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C–S Bond Cleavage in Aromatic Sulfide
Radical Cations
Osvaldo Lanzalunga a
a Dipartimento di Chimica, Università di Roma “La Sapienza”
and Istituto CNR di Metodologie Chimiche (IMC-CNR) , Sezione
Meccanismi di Reazione , P.le Aldo Moro 5, Rome , Italy
Accepted author version posted online: 24 Oct 2012.Published
online: 29 May 2013.
To cite this article: Osvaldo Lanzalunga (2013) C–S Bond Cleavage in Aromatic Sulfide Radical
Cations, Phosphorus, Sulfur, and Silicon and the Related Elements, 188:4, 322-330, DOI:
10.1080/10426507.2012.736108
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Phosphorus, Sulfur, and Silicon, 188:322–330, 2013
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2012.736108
C–S BOND CLEAVAGE IN AROMATIC SULFIDE RADICAL
CATIONS
Osvaldo Lanzalunga
Dipartimento di Chimica, Universita` di Roma “La Sapienza” and Istituto CNR di
Metodologie Chimiche (IMC-CNR), Sezione Meccanismi di Reazione, P.le Aldo
Moro 5, Rome, Italy
GRAPHICAL ABSTRACT
Abstract The results of our recent studies of the structural effects on the C–S bond fragmen-
tation process of aromatic sulfur radical cations are reported.
Keywords Sulfur radical cation; fragmentation; electron transfer
The removal of an electron from sulfides leads to the formation of sulfide radical
cations (Scheme 1). These reactive species have attracted a considerable interest in recent
years for the practical and theoretical aspects1 and for their involvement in important
biological redox processes.2
Sulfide radical cations undergo a variety of transformations and are characterized by
a reactivity pattern that is very different from that of the corresponding neutral precursors.
A number of processes that are unavailable for the neutral sulfides are instead possible for
the corresponding radical cations.
Fragmentation reactions are among the most important transformations of sulfide
radical cations and have recently found interesting applications.3 These processes lead to
the formation of a cation and a radical and are much easier than in the neutral parent
substrate. Fragmentation generally involve a bond in a beta position with respect to the
sulfur atom that is the center of positive charge (Scheme 2), as observed in alkylaromatic
radical cations fragmentation reactions.4
C–S bond cleavage is one of the most important fragmentation pathways of sulfur
radical cations.5 It has to be remarked that detection of C–S fragmentation products have
provided clean-cut information on whether radical cations are formed as intermediates in
chemical and biological oxidation of sulfur compounds.6 This process is somewhat different
Received 26 September 2012; accepted 27 September 2012.
Address correspondence to Osvaldo Lanzalunga, Dipartimento di Chimica, Universita` di Roma “La
Sapienza” and Istituto CNR di Metodologie Chimiche (IMC-CNR), Sezione Meccanismi di Reazione, P.le Aldo
Moro 5, 00185, Rome, Italy. E-mail: osvaldo.lanzalunga@uniroma1.it
322

SULFIDE RADICAL CATION FRAGMENTATION
323
Scheme 1
from the β-fragmentation reactions since the C–S bond is in the α position with respect to
the center of the positive charge and it is more appropriate to consider this process as an
α-fragmentation. In aromatic sulfide radical cations, this process is generally heterolytic
leading to an alkyl cation and an arylsulfenyl radical (Scheme 3).
Scheme 2
Sulfoxide radical cations represent another interesting class of reactive intermedi-
ates.7 They can also undergo fragmentation reactions involving the Cα-S bond leading to
an alkyl cation and an arylsulfinyl radical (Scheme 4) with very high fragmentation rate
constants.8
Scheme 3
Our continuous interest in the chemistry of sulfide radical cations led us to analyze
the quantitative aspects of the C–S bond cleavage in aromatic sulfur radical cations. To this
purpose, we have carried out a series of steady-state and laser flash photolysis investigation
of radical cations of t-Alkyl aryl sulfides generated by photoinduced electron transfer via
the excited state of a sensitizer (Scheme 5). t-Alkyl aryl sulfides have been chosen since
with these substrates there is no competition with β-C–H bond cleavage.
Scheme 4
Kinetic studies of C–S bond cleavage in aromatic sulfide radical cations generated
using the most common sensitizers such as N-methylquinolinium (NMQ+), dicyanoan-
thracene, or tetracyanobenzene are problematic since the C–S bond cleavage rates might
be slower than the back-electron transfer (BET) from the reduced sensitizer. For example,

324
O. LANZALUNGA
Scheme 5
using NMQ+ as the sensitizer the cumyl phenyl sulfide radical cation decayed by second-
order kinetics and very small amounts of photoproducts were obtained thus suggesting that
the sulfide radical cation mainly undergo a back electron transfer process with the reduced
N-methylquinolinium radical (NMQ·) (Scheme 6).9
Scheme 6
The competition between the BET and the C–S fragmentation processes in the ox-
idation photosensitized by NMQ+ can be easily visualized in the laser flash photolysis
(LFP) experiments with aryl triphenylmethyl sulfides. Phenyl triphenylmethyl sulfide rad-
ical cation is characterized by a very fast C–S fragmentation (ca. 107 s−1, see later), much
faster than the back electron transfer, thus in the LFP experiment the first order decay
of the radical cation (λMAX = 530 nm) is accompanied by the buildup of the intense
absorption of the triphenyl methyl cation at 400–430 nm (Figure 1a). With the more sta-
ble 3,4-dimethoxyphenyl triphenylmethyl sulfide radical cation the C–S fragmentation rate
constant is lower than that of the BET process thus the radical cation (λMAX = 540 nm)
decays by a second-order process by reaction with NMQ· and its decay is not accompanied
by the formation of the triphenyl methyl cation (Figure 1b).
Scheme 7

SULFIDE RADICAL CATION FRAGMENTATION
325
Figure 1 (a) Time-resolved absorption spectra of the NMQ+ (1.0 × 10−3 M)/toluene (1 M)/phenyl triphenyl-
methyl sulfide (1.0×10−2 M) system in N2-saturated CH3CN recorded 0.02 (ꢀ), 0.068 (ꢀ), 0.21 (◦), and 1.6
μs (ꢁ) after the laser pulse (λexc = 355 nm). (b) Time-resolved absorption spectra of the NMQ+ (1.0×10−3
M)/toluene (1 M)/3,4-dimethoxyphenyl triphenylmethyl sulfide (1.0 × 10−2 M) system in N2-saturated CH3CN
recorded 0.16 (ꢀ), 2.8 (ꢀ), 10 (◦), and 32 μs (ꢁ) after the laser pulse.
In order to analyze the C–S bond cleavage process avoiding the competition of the
BET process, the sulfide radical cations have been generated by the method proposed by
Dinnocenzo and Farid,10 based on the very fast light-induced breaking of the N–O bond in
N-methoxyphenanthridinium cation (MeOP+). This process leads to the methoxyl radical
and the phenanthridine radical cation (Scheme 7, path a). The latter species is a quite
powerful oxidant with a redox potential of 1.9 V versus saturated calomel electrode (SCE)
and can form the radical cations of aryl sulfides with exergonic processes (Scheme 7,
path b).
Scheme 8
In the LFP experiments with the t-alkyl aryl sulfides/MeOP+ systems, the broad ab-
sorption bands (530 nm) of the aromatic sulfide radical cations in the monomeric form were
observed.11 The decays of the radical cations follow a first-order process in accordance with
the unimolecular C–S fragmentation reaction12 with no competition of the BET process.

326
O. LANZALUNGA
Scheme 9
Steady-state photolysis experiments supported the occurrence of C–S bond cleavage
showing the formation of diphenyl disulfide, tertiary alcohols, and acetamides together with
products deriving from the N–O fragmentation of MeOP+ (protonated phenanthridine and
methanol). C–S bond cleavage leads to the tertiary alkyl cation and the arylthiyl radical.
The cations can then react with the solvent acetonitrile or with traces of water to form the
acetamides and tertiary alcohols. Dimerization of arylthiyl radical leads to diaryl disulfide
(Scheme 8).
The effect of the structure of the t-alkyl group on the rates of C–S bond cleavage was
determined by LFP following the decay of the absorption bands of the radical cations of
a series of t-alkyl phenyl sulfides.11 The data reported in Table 1 indicates that the rates
of C–S bond cleavage are not particularly fast and increase by increasing the stability of
the carbocation leaving group in the order Me3C+ < PhMe2C+ < Ph2MeC+ < Ph3C+.
However, the influence of the C–S bond dissociation free energies (BDFEs) of the radical
cations, determined by theoretical calculation (Table 1), on the fragmentation rate constants
was rather low. In order to explain these results, it was suggested that the intrinsic barrier
of the C–S fragmentation process increases as the charge in the carbocation is increasingly
delocalized, that is as the phenyl groups progressively replace the methyl groups.
Steady-state and laser flash photolysis study of a series of aryl cumyl sulfides para-
substituted in the cumyl ring has been carried out in order to have information on the
electronic effects of the substituents on the t-alkyl groups on the rates of C–S bond cleavages
(Scheme 9).13
The decay-rate constants, determined by LFP experiments following the decay kinet-
ics of the radical cations at their maximum absorption wavelengths (Table 2), are not very
sensitive to the nature of the cumyl substituent. As expected, the rates of C–S fragmentation
increased with the electron donating power of the substituent and concomitant increase in
the stability of the carbocation, but the magnitude of the observed increase is modest.
By density functional theory (DFT) calculations two energy minimum conformations
(a and b), shown in Figure 2 for the cumyl phenyl sulfide radical cation, were identified.
Table 1 Decay rate constants (kf) of t-alkyl phenyl sulfide radical cations generated by photooxidation sensitized
by MeOP+PF6− (λecc = 355 nm) and C–S BDFEs of t-alkyl phenyl sulfide radical cations
kf (s−1
)
BDFE (kcal mol−1
)
R1 = R2 = R3 = CH3
R1 = Ph, R2 = R3 = CH3
R1 = R2 = Ph, R3 = CH3
R1 = R2 = R3 = Ph
< 1 × 104
6.6 × 104
2 × 105
10.7
1.1
−2.2
9.5 × 106
−12.3

SULFIDE RADICAL CATION FRAGMENTATION
327
Table 2 Decay rate constants (kf) of 4-X-cumyl phenyl sulfide radical cations generated by photooxidation
sensitized by MeOP+PF6− (λecc = 355 nm) and C–S BDFEs for 4-X-cumyl phenyl sulfide radical cations
kf (s−1
)
BDFE (kcal mol−1
)
X = Br
3.6 × 104
4.5 × 104
5.8 × 104
4.5 × 105
0.4
0.1
X = H
X = CH3
X = OCH3
−2.9
−5.1
The two conformations are characterized by different charge and spin distribution.
In the b type conformations the C–S bond is almost perpendicular to the ring of the
cumyl moiety and this 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 efficiently
overlap with the π system in the cumyl carbocation. In the a conformers, charges and spin
populations are mainly localized on the arylsulfenyl moiety. On going to conformers b
charge and spin density increase significantly in the cumyl ring. Thus, the observation that
the rates of C–S bond cleavages are almost insensitive to changes in the C–S BDFE can
be explained by the significant delocalization of charge and spin density in the cumyl ring
and the little further accumulation of positive charge at the cumyl carbon occurring at the
transition state of the C–S fragmentation reaction.
By applying the Marcus equation to the kinetic data, a large reorganization energy
(λ = 44 kcal/mol) was determined. The fact that most of the charge is localized in the ArS
moiety of the radical cation and that such a charge has to be transferred to the t-alkyl moiety
in the final products may be responsible for the large reorganization energy.
In order to have information on the electronic effects of the arylsulfenyl substituents
on the rates of C–S bond cleavages a steady state and laser flash photolysis study of a series
of aryl triphenylmethyl sulfides has been carried out (Scheme 10).14
In the time-resolved spectra of the LFP experiment the broad absorption bands of
the radical cations (λMAX = 530–600 nm) were detected after the laser pulse. With these
systems the time evolution of the absorption spectra showed a first-order decay of the
Figure 2 Energy minimum conformations calculated for the cumyl phenyl sulfide radical cation. (Color figure
available online).

328
O. LANZALUNGA
Scheme 10
radical cations coupled with the growth of a strong absorption at 400–430 nm assigned to
the formation of the trityl cation (C6H5)3C+ as already described for the oxidation of phenyl
triphenylmethyl sulfide photosensitized by NMQ+ (see Figure 1a). Thus, it was possible to
determine the C–S fragmentation rates either by following the decay kinetics at the λMAX
of the radical cations (kf) or by following the increase of absorbance at 430 nm due to the
formation of the triphenylmethyl cation (kb). The kinetic data, reported in Table 3, clearly
show that the C–S bond cleavage rates are very sensitive to the nature of the arylsulfenyl
substituent. As expected, the rates decrease by increasing the electron donating power of
the substituent, that is, by increasing the stability of the radical cations.
+•
Theoretical calculations, carried out for 4-CH3O-C6H4SC(C6H5)3 and 4-CH3-
C6H4SC(C6H5)3+• showed that charges and spin populations are mainly localized on the
sulfur atom. Transition states (TS) for the C–S bond cleavage in the two radical cations
were identified by stepwise elongating the C–S bond starting from the energy minimum
conformations and calculating the energy of the optimized structures. Accordingly, an ini-
tial increase of the energy of the system followed by a decrease at longer C–S distances was
observed. Interestingly, it was found that in the TS, the charge is more delocalized than in
the starting radical cation. A small fraction of charge is distributed in the triphenylmethyl
group and a significant accumulation of positive charge in this group should occur at the TS
of the C–S fragmentation process. Thus, a substantial decrease of the fragmentation rate
constants is observed by increasing the stability of the radical cation that is by introducing
electron donating substituents in the arylsulfenyl ring.
The increase of the C–S bond cleavage rate constants by decreasing the polarity of
the solvent has been also rationalized on the basis of the higher charge delocalization in the
TS of the C–S bond cleavage process. Accordingly, a more polar solvent should stabilize
the initial conformation more efficiently than the TS, increasing the barrier for the C–S
bond fragmentation process.
Table 3 Decay rate constants (kf) of aryl triphenylmethyl sulfide radical cations and rate constants for the
buildup of triphenylmethyl cation (kb) determined by LFP in the photooxidation of aryl triphenylmethyl sulfide
−
with MeOP+PF6 (λecc = 355 nm) in N2-saturated CH3CN and C–S BDFEs for aryl triphenylmethyl sulfide
radical cations
kf (s−1
)
kb (s−1
)
BDFE (kcal mol−1
)
X = 4-Br
> 3 × 107
9.5 × 106
3.1 × 106
2.0 × 105
4.4 × 104
>3 × 107
1.0 × 107
3.2 × 106
1.8 × 105
4.8 × 104
N.D.
−13.6
−13.2
−10.2
N.D.
X = H
X = 4-CH3
X = 4-OCH3
X = 3,4-(OCH3)2

SULFIDE RADICAL CATION FRAGMENTATION
329
High λ values were determined also for the C–S fragmentation of aryl triphenylmethyl
sulfide radical cations, thus indicating that the large reorganization energy is an intrinsic
characteristic of the C–S bond fragmentation process of aryl sulfide radical cations.
In conclusion, the results of our studies of the structural effects on the C–S frag-
mentation rate constants of t-alkyl aryl sulphide radical cations clearly indicates that it is
more convenient to introduce electron withdrawing substituents in the arylsulfenyl ring
rather than electron donating groups in the t-alkyl moiety in order to obtain higher C–S
bond cleavage rate constants and thus better mechanistic probes for the detection of sulfide
radical cations in chemical and biochemical processes.
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