Structural and solvent effects on the C-S bond cleavage in aryl triphenylmethyl sulfide radical cations

Article abstract of DOI:10.1021/jo202418d

Steady-state and laser flash photolysis (LFP) studies of a series of aryl triphenylmethyl sulfides [1, 3,4-(CH₃O)₂-C ₆H₃SC(C₆H₅)₃; 2, 4-CH₃O-C₆H₄SC(C₆H₅) ₃; 3, 4-CH₃-C₆H₄SC(C ₆H₅)₃; 4, C₆H₅SC(C ₆H₅)₃; and 5, 4-Br-C₆H ₄SC(C₆H₅)₃] has been carried out in the presence of N-methoxyphenanthridinium hexafluorophosphate in CH ₃CN, CH₂Cl₂, CH₂Cl ₂/CH₃CN, and CH₂Cl₂/CH₃OH mixtures. Products deriving from the C-S bond cleavage in the radical cations 1^?+-5^?+ have been observed in the steady-state photolysis experiments. Time-resolved LFP showed first-order decay of the radical cations accompanied by formation of the triphenylmethyl cation. A significant decrease of the C-S bond cleavage rate constants was observed by increasing the electron-donating power of the arylsulfenyl substituent, that is, by increasing the stability of the radical cations. DFT calculations showed that, in 2^?+ and 3^?+, charge and spin densities are mainly localized in the ArS group. In the TS of the C-S bond cleavage an increase of the positive charge in the trityl moiety and of the spin density on the ArS group is observed. The higher delocalization of the charge in the TS as compared to the initial state is probably at the origin of the observation that the C-S bond cleavage rates decrease by increasing the polarity of the solvent.

Full text of DOI:10.1021/jo202418d

Article
pubs.acs.org/joc
Structural and Solvent Effects on the C−S Bond Cleavage in Aryl
Triphenylmethyl Sulfide Radical Cations
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, Sapienza Universita di Roma, P.le A. Moro 5, 00185 Rome, Italy
‡Dipartimento di Chimica and Centro di Eccellenza Materiali Innovativi Nanostrutturati, Universita
06123 Perugia, Italy
̀
di Perugia, via Elce di Sotto 8,
S
* Supporting Information
ABSTRACT: Steady-state and laser flash photolysis (LFP)
studies of a series of aryl triphenylmethyl sulfides [1, 3,4-
(CH3O)2-C6H3SC(C6H5)3; 2, 4-CH3O-C6H4SC(C6H5)3; 3, 4-
CH3-C6H4SC(C6H5)3; 4, C6H5SC(C6H5)3; and 5, 4-Br-
C6H4SC(C6H5)3] has been carried out in the presence of N-
methoxyphenanthridinium hexafluorophosphate in CH3CN,
CH2Cl2, CH2Cl2/CH3CN, and CH2Cl2/CH3OH mixtures.
Products deriving from the C−S bond cleavage in the radical
cations 1•+−5•+ have been observed in the steady-state
photolysis experiments. Time-resolved LFP showed first-
order decay of the radical cations accompanied by formation of the triphenylmethyl cation. A significant decrease of the C−
S bond cleavage rate constants was observed by increasing the electron-donating power of the arylsulfenyl substituent, that is, by
increasing the stability of the radical cations. DFT calculations showed that, in 2•+ and 3•+, charge and spin densities are mainly
localized in the ArS group. In the TS of the C−S bond cleavage an increase of the positive charge in the trityl moiety and of the
spin density on the ArS group is observed. The higher delocalization of the charge in the TS as compared to the initial state is
probably at the origin of the observation that the C−S bond cleavage rates decrease by increasing the polarity of the solvent.
bond is β with respect to the SOMO which is delocalized in the
aromatic ring (β-fragmentation).6
INTRODUCTION
■
Among the different classes of radical cations, sulfur radical
cations are of particular importance not only for practical and
theoretical aspects1 but also for their involvement in biological
oxidation processes.2
Depending on the structure and the experimental conditions,
sulfide radical cations might undergo several reactions. C−S
fragmentation to form an alkyl cation and a sulfenyl radical (eq
1) is certainly one of the most studied decomposition pathways
of these species.3
The quantitative aspects of the C−S bond cleavage in
aromatic sulfide radical cations have been analyzed in detail by
our research group by steady-state and laser photolysis studies
of tert-alkyl aryl sulfide radical cations (ArSCR3•+) where no
competition with β-C−H bond cleavage reaction is possible.
The effect of the structure of the tert-alkyl group on the rate of
the C−S bond cleavage was analyzed using a series of phenyl
sulfides: (CH3)3CSPh, PhMe2CSPh, Ph2MeCSPh, and
Ph3CSPh.7 It was found that the fragmentation rates were
not particularly fast, depending to a very limited extent on the
strength of the C−S bond in the radical cation, ranging from
6.6 × 104 s−1 for PhMe2CSPh•+ to 9.5 × 106 s−1 for Ph3CSPh•+.
The results of this investigation suggested that the C−S bond
scission is characterized by a large reorganization energy;
moreover, steric effects might play a significant role on the rate
of this process.
+•
•
+
(1)
R′S−R → R′S + R
These reactions are of particular interest since they can have
important applications as in coal desulfurization processes.4 In
addition, the detection of fragmentation products can provide
information on whether radical cations are formed as
intermediates in chemical and biological oxidation of sulfur
compounds.5
An interesting characteristic of the C−S bond cleavage in
aromatic sulfide radical cations (eq 1, R′ = aryl) is represented
by the fact that the cleaved C−S bond is α to the SOMO which
is mostly located on the sulfur atom (α-fragmentation). This
behavior differ from what observed in C−H, C−C, and C−Si
bond cleavages in aromatic radical cations where the scissile
More recently, information on the electronic effects of
substituents in the alkyl group on the rates of C−S bond
cleavage have been obtained by the analysis of a series of aryl
cumyl sulfide radical cations (4-X-C6H4C(CH3)2SC6H5•+
)
presenting the same steric situation at the scissile bond.8 The
Received: November 30, 2011
Published: January 12, 2012
© 2012 American Chemical Society
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rates of C−S bond cleavage were relatively low and very little
sensitive to the nature of the substituent. Only a modest
increase was observed from the lowest (3.6 × 104 s−1 for X =
Br) to the highest (4.5 × 105 s−1 for X = OCH3) fragmentation
rate. On the basis of the results of DFT calculations it was
suggested that the significant delocalization of charge and spin
densities in the thiophenolic group and the cumyl ring is
probably at the origin of the small effect of the substituents on
the rates of C−S bond cleavage. By applying the Marcus
equation a large reorganization energy could be estimated for
this process, which is intrinsically much slower than C−C bond
cleavage in bicumyl radical cations.9
distribution and relative energies of the transition states for the
C−S bond cleavage process of the two radical cations.
RESULTS
■
The method used for the photochemical generation of radical
cations 1•+−5•+ was the same as reported previously7,8 and is
based on the light-induced N−O bond cleavage in the N-
methoxyphenanthridinium cation (MeOP+).12 The phenan-
thridinium radical cation (P•+) 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 SCE13 (Scheme 1).
Along this line, we have considered it worthwhile to extend
the analysis of the substituents effect on the C−S bond cleavage
in radical cations of aromatic sulfides by changing the
substituents in the arylsulfenyl ring where the radical cation
should be mostly localized.10 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 triphenylmethyl sulfides (1−5).
These compounds are particularly suitable for the inves-
tigation of the C−S bond-cleavage processes by LFP studies.
Accordingly, the decay of the radical cation is accompanied by
the formation of the trityl cation, which is characterized by an
intense absorption band at 400−430 nm.7,11 Thus, the C−S
fragmentation of the radical cations 1•+−5•+ can be easily
monitored by following either the decay of 1•+−5•+ or the
buildup of (C6H5)3C+. The effect of the solvent polarity has
been also investigated by determining the C−S fragmentation
rates in CH3CN and CH2Cl2 and in the solvent mixtures
CH2Cl2/CH3CN and CH2Cl2/CH3OH.
Steady-State Photolysis. Steady-state photolysis experi-
ments were carried out by irradiating a solution of sulfides 1−5
(2.5−25 × 10−3 M) in N2-saturated CH3CN, CH2Cl2, and
CH2Cl2/CH3OH 1:1 at around 355 nm, in the presence of
MeOP+PF6− (0.5−2.5 × 10−3 M). The molar ratio sulfides/
MeOP+ was 5:1 in all the experiments. Reaction products were
identified and quantified by HPLC analysis by comparison with
authentic specimens. No products were detected without
irradiation or in the absence of the sensitizer.
The photolysis of sulfides 1−5 produced in substantial
amounts the fragmentation products diaryl disulfide from the
sulfide moiety and triphenylmethanol, in CH3CN and CH2Cl2,
or methyl triphenylmethyl ether, in the solvent mixture
CH2Cl2/CH3OH, from the alkyl group. Other products
observed were those deriving from the N−O fragmentation
of the MeOP+ (protonated phenanthridine, PH+ and
methanol) (Scheme 2).
The structure of photoproducts formed can reasonably be
rationalized on the basis of the formation of the radical cations
1•+−5•+ followed by the unimolecular cleavage of the C−S
bond (eq 1).14 This process leads to the trityl cation and the
arylsulfenyl radical (Scheme 3). The cations can then react with
adventitious water in CH3CN and CH2Cl2 to form the
alcohols15 or with CH3OH to give methyl triphenylmethyl
ether in CH2Cl2/CH3OH. Dimerization of arylsulfenyl radicals
leads to diaryl disulfides.
The yields of the fragmentation products, referred to the
initial amount of substrate, are reported in Table 1. The
sensitizer is completely consumed during the photolysis; thus,
the yields of the fragmentation products indicate an almost
quantitative oxidation of the sulfides by the phenanthridinium
radical cation (reaction b in Scheme 1). As expected, the yields
of diaryl disulfides (Table 1) are about half of that of the
triphenylmethanol or methyl triphenylmethyl ether.
This study has been integrated by DFT calculations at the
B3P86/6-311+G(d,p) level of theory for sulfides 2 and 3, 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 radical cations 2•+−3•+.
DFT calculations at the B3LYP/6-311G(d,p) level of theory
have been also carried out for radical cations 2•+−3•+ in order
to get information on the geometry, charge, and spin
distribution of the radical cations. Calculations allowed, for
the first time, to identify the structure, charge, and spin
Laser Flash Photolysis Studies. By laser photolysis (λexc
=
355 nm) of N2 saturated CH3CN solutions of sulfides 1−4
(0.01 M) and MeOP+ (1.6 × 10−4 M), broad and intense
absorption bands with maxima in the 530−630 nm region of
the spectrum depending on the substrate were detected after
the laser pulse. The same bands were also observed after laser
photolysis of N2 saturated solutions of sulfides 2 and 3 and
Scheme 1. Photochemical Generation of Radical Cations 1•+−5•+ by MeOP+
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Scheme 2. Fragmentation Products in the Photooxidation of Sulfides 1−5 by MeOP+
Scheme 3. C−S Bond Cleavage of Radical Cations 1•+−5•+
Table 1. Products and Yields Formed in the Photooxidation
of Aryl Triphenylmethyl Sulfides (1-5) Sensitized by
Table 2. Maximum Absorption Wavelengths (λmax) and
Decay Rate Constants (kf) of Aryl Triphenylmethyl Sulfide
Radical Cations (1•+−5•+) and Rate Constants for the
Buildup of Triphenylmethyl Cation (kb) Determined by LFP
in the Photooxidation of 1−5 with MeOP+PF6− (λexc = 355
nm) in N2-Saturated CH3CN
a
MeOP+
a
From LFP experiments (λexc = 355 nm) in N2-saturated CH3CN.
b
[Sulfide] = 1.0 × 10−2 M, [MeOP+PF6−] = 8.8 × 10−4 M. Decay rate
constants recorded at the maximum of absorption of the radical cation.
c
Buildup rate constants of trityl cations recorded at 430 nm.
a
[Sulfide] = 2.5−25 × 10−3 M, [MeOP+] = 0.5−2.5 × 10−3 M under
b
nitrogen. Molar ratio sulfides/MeOP+ = 5:1. Yields are referred to the
The time-evolution of the absorption spectra showed a slow
first-order decay of the radical cations coupled with the growth
of the absorption at 400−430 nm assigned to the formation of
the trityl cation (C6H5)3C+.11 In the LFP experiment with the
MeOP+/2 system a residual absorption is also observed at 520
nm which can be assigned to 4-CH3OC6H4S•.18
As an example, the time-resolved spectra of the LFP
experiment with the 2/MeOP+ system in CH3CN are reported
in Figure 1. Time resolved spectra for the 1/MeOP+, 3/
MeOP+, and 4/MeOP+ systems are shown in Figures S1−S3 in
the Supporting Information. The results of the LFP experi-
initial amount of substrate. The error is 5%.
MeOP+ in CH2Cl2 or CH2Cl2/CH3OH 1:1. These bands can
be assigned to the radical cations 1•+−4•+
7,8,16
formed by
oxidation of sulfides 1−4 by the phenanthridine radical cation
(P•+) produced after N−O bond fragmentation in the MeOP+
excited state.12 The λmax values for the radical cations 1•+−4•+
are reported in Table 2. Absorption spectra are not modified by
the presence of oxygen, thus confirming the cationic nature of
the transients.
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While the time-resolved spectra of the MeOP+/1−4 systems
resulted similar, different features were observed in the time-
resolved spectra of the MeOP+/5 system. Upon laser excitation
of a solution of MeOP+ (1.6 × 10−4 M) and 5 (1.0 × 10−2 M)
in N2-saturated CH3CN, an intense absorption band at 410 nm
and a less intense band at 510 nm were detected just after the
laser pulse (Figure S4 in the Supporting Information). The two
bands can be assigned respectively to the trityl cation and the
19
sulfenyl radical 4-BrC6H4S• produced after C−S bond
cleavage in 5•+. Clearly, the fragmentation rate of the radical
cation 5•+ is so fast that the process takes place within the laser
pulse. Thus, the radical cation is not observed but only the
species produced in the fragmentation.
In conclusion, LFP experiments confirm the results of the
steady-state photolysis suggesting that the decay of 1•+−5•+ is
due to the C−S bond cleavage with formation of trityl cations
and the arylsulfenyl radicals ArS•. The latter species can be only
observed in the LFP experiment with the MeOP+/2 system
(see Figure 1, 12 μs after the laser pulse) due to the relatively
strong absorption of 4-CH3OC6H4S• (λmax = 520 nm, ε = 6.3 ×
103 M−1 s−1)18 and with the MeOP+/5 system where it is
formed immediately after the laser pulse. The other arylsulfenyl
radicals are characterized by low extinction coefficients, and
their absorptions do not emerge in the spectra.20
The rates of fragmentation of the aryl triphenylmethyl sulfide
radical cations 1•+−4•+ were determined by following either the
decay kinetics (kf) at the λmax of the radical cations (Table 2) or
the increase of absorbance at 430 nm due to the formation of
the triphenylmethyl cation (kb). In all cases, the decay kinetics
followed first-order laws in accordance with an unimolecular
fragmentation process. The rate of fragmentation for 5•+ was
too high, and only a lower limit rate constant (kf > 3 × 107 s−1)
Figure 1. Time-resolved absorption spectra of the MeOP+ (1.6 × 10−4
M)/4-CH3OC6H4SC(C6H5)3 (2) (1.0 × 10−2 M) system in N2-
saturated CH3CN recorded 0.16 (Δ), 1.8 ( ), 12 ( ), and 32 ( ) μs
after the laser pulse. Inset: (a) decay kinetics recorded at 610 nm; (b)
buildup kinetics recorded at 420 nm.
▲
○
●
ments with the 4/MeOP+ system have already been reported in
a previous study.7 Similar time-resolved spectra have been
observed after laser photolysis of N2 saturated solutions of
sulfides 2−3 and MeOP+ in CH2Cl2 or CH2Cl2/CH3OH 1:1
(Figures S5−S8 in the Supporting Information). In the latter
solvent, the buildup of the trityl cation is followed by its fast
decay within 3 μs because of the reaction with the nucleophilic
CH3OH solvent (see insets b of Figures S6 and S8, Supporting
Information).
Table 3. Decay Rate Constants of the Aryl Triphenylmethyl Sulfide Radical Cations 2•+−3•+ (kf) and Buildup Rate Constants for
the Triphenylmethyl Cation (kb) in Different Solvents at 25 °C
a
Accurate rate determination is prevented by the fast decay of the cation in the presence of CH3OH.
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Table 4. Peak Oxidation Potentials (Ep) and C−S BDE of Sulfides 2−4 and C−S BDFEs of Sulfide Radical Cations 2•+−4•+
a
b
c
d
Peak oxidation potentials Ep (V vs SCE in CH3CN) from cyclic voltammetry. kcal mol−1. Values from ref 7. The value reported in ref 7 was
recalculated using a more recent value (0.21 V vs SCE) for the reduction potential of triphenylmethyl cation (ref 24).
can be given. All the rate constants measured in CH3CN at 25
°C are reported in Table 2.
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 + EZPVE) to be
used in the BDEs calculation, was obtained by Boltzmann
averaging the energy (Eel + EZPVE) of all the corresponding
minima. In the Supporting Information are reported the
Cartesian coordinates, the electronic energy, and the zero-point
vibrational energy of all the minima found.
The solvent effect on the C−S fragmentation process in 2•+
and 3•+ has been investigated by measuring the decay rate
constants for the radical cations 2•+ and 3•+ (kf) and the rate
constants for the formation of the trityl cation (kb) in CH3CN
and CH2Cl2 and in the solvent mixtures CH3CN/CH2Cl2 and
CH3OH/CH2Cl2. All of the rate constants are reported in
Table 3.
Theoretical Calculations. Neutral Sulfides. DFT calcu-
lations, carried out by using the Gaussian 03 package,22 at the
B3P86/6-311+G(d,p)//B3P86/6-311+G(d,p) level of theory
have been carried out for the neutral sulfides 2 and 3 in order
to determine the C−S BDEs (the C−S BDE of 4 is available in
the literature7). From these values, it is possible to estimate the
C−S bond dissociation free energies (BDFEs) for the sulfide
radical cations 2•+ to 3•+ by a thermochemical cycle7,8,23
(details in the Supporting Information).
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,25,26 including C−S bonds.26
It should also be noted that even though DFT methods may
underestimate absolute BDE values, this should not affect the
relative BDE values,26c which are those for which we are mostly
concerned. The calculated C−S BDEs of 2−4 are reported in
Table 4, together with the BDFEs of the corresponding radical
cations obtained by the thermochemical cycle using the C−S
BDEs for the neutral sulfides 2−4 corrected for the entropic
factor, the peak oxidation potentials of the sulfides reported in
Table 4, and the reduction potential of the leaving
triphenylmethyl carbocation (0.21 V vs SCE) available from
the literature.24
Since we are dealing with conformationally flexible
molecules, before starting the BDEs calculation, 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,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-
311+G(d,p) level of theory. The number of conformers found
was four for 2 and two for 3. Only one conformer was found
for each radical fragment.
Sulfide Radical Cations. DFT calculations were performed
for sulfide radical cations 2•+−3•+ 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 one that was
already used in a previous work on similar sulfide radical
cations.8,30
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
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 for 2•+ and only a minimum for 3•+ were
found. The atomic charges were obtained by natural population
analysis (NPA).31 Unpaired electron spin densities were
calculated using the Mulliken population analysis.
In all of the energy minimum conformations of 2•+ and 3•+
the three phenyl rings of the trityl group are not coplanar. The
geometry of the two conformers found for radical cation 2•+ are
very similar, differing only for the dihedral angle of the methoxy
group (180° and 0°). The less stable conformer is only 0.26
kcal/mol above the most stable one. The most stable
conformer of 2•+ and the conformer at the minimum of
energy of 3•+ are shown in Figure 2.
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
Figure 2. Most stable conformer for 4-MeOC6H4SC(C6H5)3•+ (2•+)
and conformer at the minimum of energy of 4-MeC6H4SC(C6H5)3•+
(3•+).
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In Table 5 are displayed NPA charges and spin density values
obtained for the most stable conformers of radical cations 2•+
and 3•+, partitioned for the different structural components of
the radical cations defined in Figure 3.
Table 7. Energy Values (E and Erel) Calculated for the
•+
Conformer of Radical Cation 4-MeC6H4SC(C6H5)3 (3•+)
as a Function of the C−S Bond Distance
C−S distance, Å
E, au
Erel, kcal/mol
1.98407
2.08407
2.18407
−1402.2043135
−1402.2039104
−1402.2034192
−1402.2033518
−1402.2034536
−1402.2042226
−1402.2056275
−1402.2074183
0.0000
0.2529
0.5612
a
a
a
2.22968
0.6035
2.28407
2.38407
2.48407
2.58407
0.5396
0.0571
−0.8245
−1.9483
a
Transition state.
Figure 3. Structural components of aryl triphenylmethyl sulfide radical
cations.
Table 5. NPA Charges (qNPA) and Spin Densities for the
Most Stable Conformers of Radical Cations 2•+ and 3•+
Ph3C
S
Ar
X
2•+
2•+
3•+
3•+
qNPA
spin
qNPA
spin
0.1400
0.0455
0.1936
0.0774
0.5437
0.4307
0.5803
0.5013
0.3903
0.4079
0.1385
0.4070
−0.074
0.1160
0.0877
0.0143
From the data reported in Table 5, it can be noted that NPA
charges and spin populations in both 2•+ and 3•+ are mainly
localized on the sulfur atom. The spin density is also relatively
high on the arylsulfenyl ring and comparable for the two radical
cations. Charge delocalization in the arylsulfenyl ring is much
higher in 2•+ than in 3•+. This can be attributed to the methoxy
substituent which is able to stabilize the positive charge by an
electron releasing effect more efficiently than the methyl group.
In the attempt to identify a possible transition state for the
C−S bond cleavage in the radical cations we have stepwise
elongated the C−S bond in the radical cations starting from the
energy minimum conformations. For each bond length we have
partially optimized the geometry. In the optimization process
the C−S bond length was kept fixed at a given value, whereas
all the other degrees of freedom were allowed to relax. By
increasing the C−S bond distance by 0.1 Å steps, we observed
an initial increase of the energy of the system followed by a
decrease at longer C−S distances (Table 6 and Table 7, for 2•+
and 3•+, respectively, and Figure 4). For each radical cation, the
Figure 4. Plot of Erel vs the C−S bond distances (d(C−S)) for 2•+
•+
▲
(
) and 3 (Δ).
geometry corresponding to the maximum energy value was
optimized (opt=TS) to the nearest transition state. Each
stationary point found was characterized by a frequency
calculation and one imaginary frequency was found, thus
confirming that the stationary point found is a saddle point.
Furthermore, the normal mode corresponding to the imaginary
frequency was animated by using the visualization program
Molden.32 In this way it was verified that the displacements
composing the mode corresponds to the C−S bond
fragmentation process.
In Tables 8 and 9 are displayed the NPA charges and spin
density values obtained for the optimized structures of radical
cations 2•+ and 3•+ obtained by increasing the C−S bond
distance and for the TS, partitioned for the different structural
components defined in Figure 3.
Table 6. Energy Values (E and Erel) Calculated for the Most
Stable Conformer of Radical Cation
•+
4-MeOC6H4SC(C6H5)3 (2•+) as a Function of the C−S
DISCUSSION
■
Bond Distance
The study of the C−S bond fragmentation in aryl
triphenylmethyl sulfide radical cations has provided important
information on the quantitative aspects of the electronic effects
of arylsulfenyl ring substituents on the C−S bond cleavage in
tert-alkyl aryl sulfide radical cations.
C−S distance, Å
E, au
Erel, kcal/mol
1.96954
2.06954
2.16954
2.26954
−1477.4396609
−1477.4391159
−1477.4382580
−1477.4378033
−1477.4377912
−1477.4380284
−1477.4402328
0.0000
0.3420
0.8804
1.1657
Theoretical calculations, carried out for 2•+ and 3•+, indicated
for 2•+ the presence of two energy minimum conformations
very similar from both geometric and energetic points of view,
while only one conformer at the minimum of energy was found
for 3•+. As shown in Figure 2, in both 2•+ and 3•+, due to the
steric encumbrance of the trityl group, the C−S bond cannot be
a
a
a
2.28890
1.1733
2.36954
2.56954
1.0244
−0.3589
a
Transition state.
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Table 8. NPA Charges (qNPA) and Spin Densities for the Optimized Structures of Radical Cation 2•+
NPA charges
spin population
C−S distance, Å
Ph3C
S
Ar
OCH3
Ph3C
S
Ar
OCH3
1.96954
2.06954
2.16954
2.26954
0.1400
0.2066
0.2834
0.3678
0.3848
0.4570
0.6285
0.5437
0.4943
0.4396
0.3797
0.3676
0.3162
0.1927
0.3903
0.3769
0.3599
0.3410
0.3373
0.3216
0.2859
−0.0740
−0.0778
−0.0828
−0.0885
−0.0897
−0.0948
−0.1071
0.0455
0.0338
0.4307
0.4510
0.4764
0.5042
0.5098
0.5320
0.5619
0.4079
0.4021
0.3947
0.3863
0.3847
0.3771
0.3510
0.1160
0.1131
0.1093
0.1051
0.1043
0.1007
0.0903
0.0195
0.0044
a
2.28890
0.0011
2.36954
2.56954
−0.0097
−0.0032
a
Transition state.
Table 9. NPA Charges (qNPA) and Spin Densities for the Optimized Structures of Radical Cation 3•+
NPA charges
spin population
C−S distance, Å
Ph3C
S
Ar
CH3
Ph3C
S
Ar
CH3
1.98407
2.08407
2.18407
0.1936
0.2576
0.3316
0.3683
0.4137
0.5020
0.5921
0.6726
0.5803
0.5292
0.4723
0.4444
0.4100
0.3438
0.2768
0.2162
0.1385
0.1270
0.1118
0.1040
0.0943
0.0751
0.0549
0.0377
0.0877
0.0863
0.0843
0.0833
0.0819
0.0791
0.0762
0.0735
0.0774
0.0570
0.5013
0.5242
0.5520
0.5653
0.5812
0.6071
0.6211
0.6307
0.4070
0.4049
0.4010
0.3986
0.3953
0.3859
0.3702
0.3569
0.0143
0.0138
0.0132
0.0128
0.0124
0.0115
0.0104
0.0096
0.0339
a
2.22968
0.0233
2.28407
2.38407
2.48407
2.58407
0.0111
−0.0045
−0.0018
0.0029
a
Transition state.
perpendicular to one of the aromatic rings of the trityl moiety
in a conformation most suitable for the C−S bond cleavage
(the empty carbon p orbital formed in the cleavage is perfectly
suited for an efficiently overlap with the π system) like those
observed for aryl cumyl sulfide radical cations.8 The NPA
charges and spin populations in 2•+ and 3•+ are mainly localized
on the sulfur atom and on the arylsulfenyl ring (Table 5). It is
interesting to note that NPA charges and spin populations are
less localized on the trityl group of 2•+ and 3•+ with respect to
the cumyl group in the b conformations of cumyl phenyl sulfide
radical cations (4-X-C6H4C(CH3)2SC6H5•+, X = H, CH3,
OCH3).8 This difference can be reasonably attributed to the
electron releasing arylsulfenyl ring substituents in 2•+ and 3•+
which are able to stabilize the positive charge reducing the
electron density in the same ring.
From the kinetic data for the fragmentation of radical cations
1•+−5•+ in CH3CN reported in Table 2, it can be immediately
noted that the C−S bond cleavage rates are very sensitive to the
nature of the substituent. As expected, the rates increase by
decreasing the electron-donating power of the substituent, that
is by decreasing the stability of the radical cations. An increase
of about 200 times in fragmentation rate is observed on going
from the 3,4-dimethoxy-substituted radical cation 1•+ to the
unsubstituted one 4•+. The 3.4 kcal/mol difference in BDFE
between 4•+ and 2•+ (Table 4) is associated to a substantial
difference in ΔG⧧ (2.3 kcal/mol, see Table 10). This situation is
much different from what observed in aryl cumyl sulfide radical
cations where the C−S bond cleavage rates were insensitive to
the nature of the substituent.8 Thus, the effect of substituents
on the C−S bond cleavage process is significantly greater when
they are introduced in the arylsulfenyl ring, where most of the
charge and spin density is localized, than in an aromatic ring of
the tert-alkyl group.8 In this respect, a key role on the relative
magnitude of the fragmentation rate constants seems to be
played by the effect of the substituents on the stabilization of
the aryl sulfide radical cation. The stabilizing effect of electron-
Table 10. Activation Free Energies (ΔG⧧), Free Energies
(ΔG°), and Reorganization Energies for the C−S Bond
Cleavage Sulfide Radical Cations 2•+−4•+
a
b
In kcal mol−1. Calculated from the Eyring eq 3 using the kf values
reported in Table 2 and a collision frequency (Z) value of 6 × 1011
M−1 s−1.
donating arylsulfenyl ring substituents is much higher than that
exerted by the same substituents in the tert-alkyl group.
Accordingly, the decrease in the oxidation potential observed
by introducing a methoxy group in the arylsulfenyl ring (0.26 V,
see Table 4) is much higher than that (0.07 V) found when the
same substituent is introduced in the cumyl ring of aryl cumyl
sulfides.8
Important information on the energetics of the C−S bond
cleavage in aryl sulfide radical cations has been provided, for the
first time, by DFT calculations. We increased the C−S distance
by 0.1 Å steps in the radical cations 2•+ and 3•+ starting from
the energy minimum conformations and calculated the energy
of the optimized structures. It was found that the increase of
the C−S bond distance led to an initial increase of the energy of
the system followed by a decrease at longer C−S distances
(Tables 6-7).33 Thus, with this procedure (see above), we were
able to identify the transition states for the C−S fragmentation
process in the two radical cations. As predicted for the highly
exergonic C−S bond cleavage processes in 2•+ and 3•+ (see
Table 10), the transition states are reached quite early along the
reaction coordinate and a relatively small increase of the C−S
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Article
bond distance is observed. In line with the early TS, the C−S
bond cleavage rates are very sensitive to the ability of the
substituent to stabilize the positive charge in the starting radical
cations.
triphenylmethyl sulfide radical cations, we have analyzed how
this great extent of delocalization is reflected in the
reorganization energy value (λ) of the C−S bond cleavage.
To this purpose we have treated the kinetic data for the
fragmentation of radical cations 2•+−4•+ in terms of the Marcus
equation (eq 2),37 where ΔG⧧ is the activation free energy of
the reaction, calculated by the Eyring equation (eq 3), replacing
kf with the values reported in Table 2 and Z as 6 × 1011 M−1
s−1.38 ΔG° is the free energy variation in the same reaction
(C−S BDFE given in Table 4), and λ is the reorganization
energy required for the fragmentation process.
If we compare the TS of the two systems, it can be noted that
the C−S bond distance and the energy relative to the starting
conformer (Erel) for 4-MeOC6H4SC(C6H5)3•+ (2•+) are higher
than those calculated for 4-MeC6H4SC(C6H5)3•+ (3•+) (Figure
4). The degree of elongation of the C−S bond as measured by
the increase of the C−S bond in the TS is ca. 16% of the initial
bond length for 2•+ and ca. 12% for 3•+. The higher Erel value
found for 2•+ is in accordance with the lower rate of C−S
fragmentation found for this radical cation. Although the
difference in the Erel of the two TS (0.6 kcal/mol) is not high
enough to account for the observed differences in the C−S
bond cleavage rates (ΔG⧧ =1.6 kcal/mol in MeCN and 1.45
kcal/mol in CH2Cl2) it should be considered that calculations
are referred to vacuum and enthalpic and entropic solvent
effects might be responsible for the discrepancy.
⧧
2
ΔG = (λ/4)(1 + ΔG°/λ)
(2)
⧧
ΔG = RT ln(Z/k )
(3)
f
The values of ΔG⧧ and ΔG° (kcal mol−1) are reported in
Table 10. In the same table are also reported the λ values for
the same reactions obtained from eq 4, which is derived from
eq 2.
Interesting results have been obtained from the analysis of
the solvent effects on the C−S fragmentation process,
investigated by following both the decay kinetics at the λmax
for the radical cations 2•+ and 3•+ (kf) and the buildup rate
constants for the formation of the triphenylmethyl cation (kb)
in different solvents (CH3CN and CH2Cl2 and in the solvent
mixtures CH3CN/CH2Cl2 and CH3OH/CH2Cl2). From the
kinetic data reported in Table 3, it can be readily noted that the
rate of C−S bond cleavage regularly increase by decreasing the
polarity of the solvent. Accordingly, the C−S bond cleavage
rates increase from 2 × 105 s−1 to 8.6 × 105 s−1 for 2•+ and from
3 × 106 s−1 to ca. 1 × 107 s−1 for 3•+ on going from the more
polar CH3CN to the less polar CH2Cl2 solvent. In the solvent
mixtures CH3CN/CH2Cl2 and CH3OH/CH2Cl2 a regular
increase of the fragmentation rates can be observed by
increasing the relative amount of CH2Cl2. It is possible to
rationalize the effect of the solvent polarity on the C−S
fragmentation rates by considering that in aryl triphenylmethyl
sulfide radical cations most of the charge is localized in the ArS
moiety of the radical cation, as indicated by the results of DFT
calculations (Table 5). This implies that such charge has to be
transferred to the triphenylmethyl group in the final products.
In the TS of the C−S bond cleavage the charge is more
delocalized than in the starting radical cation as clearly shown
from the data of Tables 8 and 9. The NPA charge localized on
the triphenylmethyl group increase from 0.14 to 0.38 for 2•+
and from 0.19 to 0.37 for 3•+ on going from the starting radical
cation to the transition states. Thus, a more polar solvent
should stabilize the initial conformation more efficiently than
the TS, increasing the barrier for the C−S fragmentation
process. A decrease of the cleavage reactivity in radical cations
by increasing the solvent polarity has been already observed in
the fragmentation of tetraphenylethane radical cations.35
The results of the solvent effects on the C−S bond cleavage
of aromatic sulfide radical cations are instead different from
those found in the C−C bond cleavage of bicumyl radical
cations where the fragmentation rates are practically unaffected
by changing the solvent polarity.36 The absence of solvent
effects on the C−C bond cleavage rates was interpreted by
considering that all the charge is localized in a cumyl ring and
remains in that ring after the C−C bond cleavage.9
⧧
λ = 2ΔG − ΔG°
⧧2
⧧
1/2
+ 2[(ΔG − ΔG × ΔG°)]
(4)
Looking at the data in Table 10, it can be noted that the λ
values obtained from the three reactions are similar, providing
an average value of 52 2 kcal mol−1 that can be reasonably
taken as the reorganization energy for the C−S bond cleavage
in aryl triphenylmethyl sulfide radical cations. This result, taken
together with the λ value determined for the same process in
aryl cumyl sulfide radical cations (43.7 kcal/mol),8 clearly
indicates that a large reorganization energy is an intrinsic
characteristic of the C−S bond fragmentation process of aryl
sulfide radical cations. The higher λ value determined for the
C−S bond cleavage in aryl triphenylmethyl sulfide radical
cations suggests that a larger degree of charge transfer from ArS
to the tert-alkyl group (greater extent of internal and solvent
reorganization) should occur in the TS of the C−S bond
cleavage in aryl triphenylmethyl sulfide radical cations with
respect to aryl cumyl sulfide radical cations.
CONCLUSIONS
■
A significant variation of the C−S bond cleavage rates in aryl
triphenylmethyl sulfide radical cations can be produced by
changing the arylsulfenyl ring substituent or the reaction
medium. Fragmentation rates decrease by increasing the
electron-releasing properties of the arylsulfenyl ring substituent,
that is by increasing the stability of the radical cations. This
situation is much different from what observed in aryl cumyl
sulfide radical cations where the C−S bond cleavage rates were
very little sensitive to the nature of the substituent. These
results have been rationalized by considering that the effect of
the substituents on the stabilization of the aryl sulfide radical
cation are significantly higher when they are introduced in the
arylsulfenyl ring, where most of the charge and spin density is
localized, than in the tert-alkyl group. For the first time, the
structure, relative energies, spin and charge distribution of the
transition states for the C−S bond cleavage in aryl sulfide
radical cations have been calculated. The charge is more
localized in the starting radical cation than in the transition
state; therefore, a more polar solvent result in a decrease in the
C−S bond cleavage rates compared to a less polar solvent.
Since DFT calculations have shown a significant delocaliza-
tion of the charge and spin density on going from the reactant
to the TS of the C−S fragmentation process of aryl
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Article
voltammetry; C−S BDFEs of radical cations 2•+−3•+, Cartesian
Coordinates, energies, ZPVE from DFT calculations for neutral
sulfides 2 and 3, for triphenylmethyl and arylsulfenyl radicals
and for radical cations 2•+ and 3•+ conformers at the minimum
of energy; NPA charges and Mulliken spin densities for radical
cations 2•+ and 3•+; Cartesian coordinates, energies, NPA
Charges and Mulliken spin densities for TS of the C−S bond
cleavage in the radical cations 2•+ and 3•+. This material is
available free of charge via the Internet at http://pubs.acs.org.
EXPERIMENTAL SECTION
■
Starting Materials. Aryl triphenylmethyl sulfides 1−5 were
prepared by acid-catalyzed reaction of the commercially available
aryl-substituted thiophenol with triphenylmethanol as reported in the
literature.39 Compounds 1−3 were characterized by H NMR, 13C
1
NMR and HRMS (ESI-TOF) analysis. Compound 4 was synthetized
and characterized in a previous study.7 Compound 5 was characterized
according to the spectral data reported in the literature.40 N-
Methoxyphenanthridinium hexafluorophosphate was prepared accord-
ing to a literature procedure.12 CH3CN (spectrophotometric grade)
was distilled over CaH2. CH2Cl2 (spectrophotometric grade) was
distilled over P2O5. CH3OH (spectrophotometric grade) was used as
received.
AUTHOR INFORMATION
■
Corresponding Author
Spectral Data of Substrates 1−3. 3,4-Dimethoxyphenyl
*E-mail: osvaldo.lanzalunga@uniroma1.it; paolo.mencarelli@
uniroma1.it; dgiacco@unipg.it.
1
Triphenylmethyl Sulfide (1). H NMR (300 MHz, CDCl3): δ 3.47
(s, 3H), 3.80 (s, 3H), 6.31 (d, J = 2 Hz, 1H), 6.58 (d, J = 8 Hz, 1H),
6.74 (dd, J = 8 Hz, 2 Hz, 1H), 7.18−7.39 (m, 15H). 13C NMR (300
MHz, CDCl3): δ 55.3, 55.8, 70.7, 110.6, 118.9, 124.6, 126.6, 127.6,
129.4, 130.1, 144.7, 148.1, 149.6. HRMS (ESI-TOF): calcd C27H24O2S
+ Na+ [M + Na]+ 435.1395, found 435.1458.
ACKNOWLEDGMENTS
■
Financial support from the Ministero dell’Istruzione, dell’Uni-
versita
̀
e della Ricerca (MIUR) is gratefully acknowledged. We
4-Methoxyphenyl Triphenylmethyl Sulfide (2). 1H NMR (300
MHz, CDCl3): δ 3.70 (s, 3H), 6.52−6.90 (m, 4H); 7.17−7.42 (m,
15H). 13C NMR (300 MHz, CDCl3): δ 55.2, 70.8, 113.7, 124.4, 126.6,
127.6, 130.0, 137.8, 144.7, 160.1. HRMS (ESI-TOF): calcd C26H22OS
+ Na+ [M + Na]+ 405.1289, found 405.1231.
thank Prof. Enrico Baciocchi for helpful discussions and Prof.
Ruggero Caminiti (Chemistry Department, Sapienza Universita
di Roma) for generously letting us use his computer time at the
supercomputing center “CASPUR”.
̀
4-Methylphenyl Triphenylmethyl Sulfide (3). 1H NMR (300 MHz,
CDCl3): δ 2.21 (s, 3H), 6.82−6.83 (m, 2H), 7.17−7.42 (m, 17H). 13
C
REFERENCES
■
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130.6, 135.1, 138.0, 144.7. HRMS (ESI-TOF): calcd C26H22S + Na+
[M + Na]+ 389.1340, found 389.1355.
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methoxyphenyl) disulfide, bis(4-methylphenyl) disulfide, diphenyl
disulfide, bis(4-bromophenyl) disulfide, protonated phenanthridine,
1
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ASSOCIATED CONTENT
■
S
* Supporting Information
Time-resolved absorption spectra after LFP of the MeOP+/1,
MeOP+/3, MeOP+/4, and MeOP+/5 systems in CH3CN; time-
resolved absorption spectra after LFP of the MeOP+/2,
MeOP+/3 systems in CH2Cl2 and CH2Cl2/CH3OH 1:1; cyclic
(4) Venimadhavan, S.; Amarnath, K.; Harvey, N. G.; Cheng, J.-P.;
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Piparo, M. G. Org. Biomol. Chem. 2003, 1, 422. (b) Baciocchi, E.;
1851
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Article
21a
•
Lanzalunga, O.; Pirozzi, B. Tetrahedron 1997, 53, 12287. (c) Baciocchi,
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(20) λmax (C6H5S ) = 460 nm, ε = 2.6 × 103 M−1 s−1. λmax (4-
CH3C6H4S ) = 485 nm, ε = 2.65 × 103 M−1 s−1.21b The ε values for 4-
•
•
•
BrC6H4S and 3,4-(CH3O)2C6H3S are not available in the literature.
(21) (a) Tripathi, G. N. R.; Sun, Q.; Armstrong, D. A.; Chipman, D.
M.; Schuler, R. H. J. Phys. Chem. 1992, 96, 5344. (b) Hermann, R.;
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Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
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less selective tert-butyl, 1-methylcyclopentyl, benzyl, and 1-phenylethyl
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