Structural effects on the C-S bond cleavage in aryl tert -butyl sulfoxide radical cations

Article abstract of DOI:10.1021/jo400461c

The oxidation of a series of aryl tert-butyl sulfoxides (4-X-C ₆H₄SOC(CH₃)₃: 1, X = OCH ₃; 2, X = CH₃; 3, X = H; 4, X = Br) photosensitized by 3-cyano-N-methylquinolinium perchlorate (3-CN-NMQ⁺) has been investigated by steady-state irradiation and nanosecond laser flash photolysis (LFP) under nitrogen in MeCN. Products deriving from the C-S bond cleavage in the radical cations 1^+?-4^+? have been observed in the steady-state photolysis experiments. By laser irradiation, the formation of 3-CN-NMQ^? (λ_max = 390 nm) and 1 ^+?-4^+? (λ_max = 500-620 nm) was observed. A first-order decay of the sulfoxide radical cations, attributable to C-S bond cleavage, was observed with fragmentation rate constants (k _f) that decrease by increasing the electron donating power of the arylsulfinyl substituent from 1.8 × 10⁶ s^-1 (4 ^+?) to 2.3 × 10⁵ s^-1 (1 ^+?). DFT calculations showed that a significant fraction of the charge is delocalized in the tert-butyl group of the radical cations, thus explaining the small substituent effect on the C-S bond cleavage rate constants. Via application of the Marcus equation to the kinetic data, a very large value for the reorganization energy (λ = 62 kcal mol^-1) has been calculated for the C-S bond scission reaction in 1^+?-4 ^+?.

Full text of DOI:10.1021/jo400461c

Article
pubs.acs.org/joc
Structural Effects on the C−S Bond Cleavage in Aryl tert-Butyl
Sulfoxide Radical Cations
Tullio Cavattoni,† 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: The oxidation of a series of aryl tert-butyl sulfoxides (4-X-C6H4SOC(CH3)3: 1, X = OCH3; 2, X = CH3; 3, X = H;
4, X = Br) photosensitized by 3-cyano-N-methylquinolinium perchlorate (3-CN-NMQ+) has been investigated by steady-state
irradiation and nanosecond laser flash photolysis (LFP) under nitrogen in MeCN. Products deriving from the C−S bond
cleavage in the radical cations 1+•−4+• have been observed in the steady-state photolysis experiments. By laser irradiation, the
formation of 3-CN-NMQ• (λmax = 390 nm) and 1+•−4+• (λmax = 500−620 nm) was observed. A first-order decay of the sulfoxide
radical cations, attributable to C−S bond cleavage, was observed with fragmentation rate constants (kf) that decrease by
increasing the electron donating power of the arylsulfinyl substituent from 1.8 × 106 s−1 (4+•) to 2.3 × 105 s−1 (1+•). DFT
calculations showed that a significant fraction of the charge is delocalized in the tert-butyl group of the radical cations, thus
explaining the small substituent effect on the C−S bond cleavage rate constants. Via application of the Marcus equation to the
kinetic data, a very large value for the reorganization energy (λ = 62 kcal mol−1) has been calculated for the C−S bond scission
reaction in 1+•−4+•.
INTRODUCTION
■
The chemistry of sulfoxides has attracted continuous interest
for its relevance in organic synthesis;1 moreover, sulfoxide
functional groups are present in a number of biologically active
molecules and are often involved in the metabolism of sulfides.2
One-electron oxidation of sulfoxides represents one of the
reactivity aspects of these species that is still little investigated
as a consequence of the relatively high oxidation potential of
these species.3 Thus, only few studies are presently available for
the reactive intermediates produced in this process, that is,
sulfoxide radical cations.4
In recent years, our group investigated some aspects
concerning the structure and reactivity of aromatic sulfoxide
radical cations.5−8 In particular, we have focused our attention
on the C−S bond fragmentation process that represents one of
the major decomposition pathways available for both aromatic
sulfoxide7,8 and aromatic sulfide9 radical cations.
The analysis of the fragmentation reactions of tertiary and
secondary alkyl phenyl sulfoxide radical cations generated by
TiO2 photocatalyzed7 or 3-cyano-N-methylquinolinium per-
chlorate (3-CN-NMQ+) photosensitized oxidation8 indicates
that the heterolytic C−S bond cleavage, leading to the
phenylsulfinyl radical C6H5SO• and the alkyl carbocation R+
(eq 1), is a unimolecular process characterized by very high
fragmentation rate constants. Interestingly, we found that the
rates of C−S bond cleavage in aromatic sulfoxide radical cations
are at least 2 orders of magnitude higher than those observed
for the same process occurring in the corresponding sulfide
radical cations.8,10 This result is in accordance with the
significantly higher oxidation potentials of sulfoxides5,11 and
with the higher stability of the phenylsulfinyl radical with
respect to the phenylthiyl radical.12
The effect of the structure of the alkyl group on the rate of
C−S bond cleavage has been analyzed in detail by steady-state
and laser photolysis studies of a series of alkyl phenyl sulfoxide
radical cations (in eq 1, R = PhCH2, Ph(CH3)CH, Ph2CH,
Ph(CH3)2C, or (CH3)3C).8 It was found that the rates of C−S
bond cleavage were very sensitive to the structure of the alkyl
group and the strength of the C−S bond in the radical cation,
increasing with the stability of the carbocation formed.
More recently, we have investigated in detail the C−S
fragmentation process of aryl triphenylmethyl sulfide radical
cations. A significant variation of the C−S bond cleavage rates
was observed by changing the arylsulfenyl ring substituent.13 In
Received: March 4, 2013
© XXXX American Chemical Society
A
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this context, it has to be noted that previous theoretical studies
carried out with a series of aryl-substituted thioanisole radical
cations (in 4-X-C6H4-SCH3+•, X = H, CN, OMe, or N(CH3)2)
showed remarkable differences in the charge and spin
distribution on the aromatic ring depending on the nature of
the substituent.14
Along this line, we considered it worthwhile to extend the
analysis of the structural effects on the C−S bond cleavage in
radical cations of aromatic sulfoxides by changing the
substituents in the arylsulfinyl ring, taking into account that
also in this case previous theoretical studies performed on aryl
methyl sulfoxide radical cations5 reported a remarkable effect of
the aryl substituent on the charge and spin delocalization. In
particular, it was found that in all the radical cations most of the
charge and spin density are localized on the SOMe group, but a
substantial delocalization was also observed on the aromatic
ring (almost 50% for the methoxy derivative and 30% for the
other radical cations).
from the alkyl moiety) analysis, by HPLC (sulfur containing
products) analysis, and by comparison with authentic speci-
mens (1H NMR spectrum and HPLC chromatogram of the 3-
CN-NMQ+ photosensitized oxidation of tert-butyl 4-methox-
yphenyl sulfoxide (1) are reported in the Supporting
Information). No products were detected without irradiation
or in the absence of the sensitizer.
As reported in the previous study of the oxidation of 3,8 the
photolysis of sulfoxides 1−4 produced tert-butyl alcohol and
tert-butyl acetamide as reaction products from the tert-butyl
group and diaryl disulfide (ArSSAr), aryl arenethiosulfinate
(ArSOSAr), and aryl arenethiosulfonate (ArSO2SAr) as sulfur
containing fragmentation products (Scheme 1). Small amounts
of aryl tert-butyl sulfones were also formed.16 The yields of
fragmentation products, referring to the initial amount of
substrate, are reported in Table 1.
The total yields exceed the value expected on the basis of the
3-CN-NMQ+ amount; thus, the sensitizer is regenerated during
1
In order to analyze how the electronic effect of the aryl
substituents is reflected in the reactivity (C−S fragmentation
process) of tert-alkyl aryl sulfoxide radical cations, we have
carried out a steady-state and laser photolysis study of a series
of aryl tert-butyl sulfoxides (1−4) in the presence of 3-cyano-N-
the photolysis. Accordingly, UV and H NMR analysis of the
reaction mixtures after photolysis indicated that less than 20%
of the initial amount of the sensitizer is consumed during the
irradiation.
Laser Flash Photolysis Studies. The laser photolysis
experiments (λexc = 355 nm) were performed under nitrogen in
the presence of 1 M toluene as a cosensitizer in order to
increase the yields of radical cation.17 Under these conditions,
the radical cation of the cosensitizer, PhCH3+•, is first formed
and is able to oxidize the sulfoxide to the corresponding radical
cation (Ered (PhCH3+•) = 2.35 V vs SCE).18 In the LFP
experiments with sulfoxides 1, 2, and 4, three main absorption
bands were detected just after the laser pulse at 360−400, 450−
530, and 550−700 nm. Only the first two absorption bands
were observed in the LFP experiments with sulfoxide 3, as has
already been reported in a previous study.8 As an example, the
time-resolved spectra of the LFP experiment with the 3-CN-
NMQ+/toluene/1 system are reported in Figure 1. Time-
resolved spectra for the 3-CN-NMQ+/toluene/2 and 3-CN-
NMQ+/toluene/4 systems are shown in Figures S4 and S5 in
the Supporting Information, respectively.
The more stable transient absorption band at around 380 nm
can be assigned to the radical 3-CN-NMQ• formed after the
electron transfer from sulfoxides 1−4 to 3-CN-NMQ+*.8
The other two absorption bands can be assigned to the
radical cation of the sulfoxides.5,8 It is interesting to note that
the absorption spectra of the radical cations 1+•, 2+•, and 4+•
are different from those of the corresponding aryl methyl
sulfoxide radical cations5 where only one band was observed
significantly red-shifted with respect to the long wavelength
absorption band of 1+•, 2+•, and 4+•.
methylquinolinium perchlorate (3-CN-NMQ+ClO4 ) as a
−
sensitizer in MeCN.8,15 We have chosen this class of sulfoxides
because the aryl triphenylmethyl sulfoxide radical cations,
which would represent a better comparison with the
corresponding sulfides, are not stable species and display a
reactivity too high to be investigated.
DFT calculations at the B3P86/6-311+G(d,p) level of theory
for sulfoxides 1−4 have been carried out to provide 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 1+•−4+•. DFT calculations at the
B3LYP/6-311G(d,p) level of theory have been also carried out
in order to obtain information about the geometry, charge, and
spin distribution of the radical cations and of the transition
states for the C−S bond cleavage process.
RESULTS
■
Steady-State Photolysis. Steady-state photolysis experi-
ments were carried out by irradiating a solution of sulfoxides
1−4 (1 × 10−2 M) in N2-saturated CD3CN at around 355 nm
The ΔA recorded at 450−500 nm showed a fast decay of the
radical cations, followed by a slower decay which should be
assigned to the decay of the radical ArSO• that absorbs at these
in the presence of 3-CN-NMQ+ClO4 (1 × 10−3 M). The
−
products were identified and quantitated by 1H NMR (products
Scheme 1
B
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Table 1. C−S Fragmentation Products and Yields in the 3-CN-NMQ+ Photosensitized Oxidation of Alkyl Phenyl Sulfoxides (1−
a
4) in N2-Saturated CD3CN
a
b
[Sulfoxide] = 1.0 × 10−2 M; [3-CN-NMQ+] = 1.0 × 10−3 M. Yields refer to the initial amount of substrate. The error is 5%.
decay kinetics at the λmax of the radical cations (Table 2). In all
cases, the decay kinetics followed first-order laws in accordance
with a unimolecular fragmentation process. Given the high
fragmentation rates, back electron transfer should not be a
problem with aryl tert-butyl sulfoxide radical cations, as was
previously reported for aryl sulfide radical cations.21 All the rate
constants measured in CH3CN at 25 °C are reported in Table
2.
Theoretical Calculations. Neutral Sulfoxides. For a
meaningful discussion of the experimental results, it was
necessary to know the C−S bond dissociation free energies
(BDFEs) for the aryl tert-butyl sulfoxide radical cations 1+•, 2+•,
and 4+• (the C−S BFDE for 3+• was reported in a previous
paper).8 These values were estimated using the C−S BDEs for
the neutral aryl tert-butyl sulfoxides 1, 2, and 4 and were
obtained by DFT calculations carried out using the Gaussian 03
package22 at the B3P86/6-311+G(d,p)//B3P86/6-311+G(d,p)
level of theory. The B3P86 functional was chosen because it is
reported to be applied with reasonable success to the
calculations of BDE values for a variety of C−X bonds,23,24
including C−S bonds.24−27 The calculated C−S BDEs of 1, 2,
and 428 are reported in Table 2 with the BDFEs of the
corresponding radical cations. C−S BDFEs were calculated in
CH3CN using the thermochemical cycle reported in Figure 2,
the C−S BDEs for the neutral sulfides corrected for the
entropic factor, the peak oxidation potentials of the sulfides
reported in Table S1 in the Supporting Information, and the
reduction potential of the leaving tert-butyl cation (0.09 V vs
SCE) available from the literature.29
Figure 1. Time-resolved absorption spectra of the 3-CN-NMQ+ (8.0
× 10−4 M)/4-CH3OC6H4SOC(CH3)3 (1) (1.0 × 10−2 M) system in
●
○
▼
△
N2-saturated CH3CN recorded 0.79 ( ), 2.1 ( ), 5.0 ( ), 32 ( ),
■
and 50 ( ) μs after the laser pulse. Inset: (a) decay kinetics recorded
at 500 nm, and (b) decay kinetics recorded at 620 nm.
wavelengths (see insets of Figure 1 and Figures S4 and S5 in
the Supporting Information).19 The two consecutive decays are
more evident with 3+• and 4+• because the decay of the radical
cations occurs at a rate much faster than the rate of
recombination of ArSO•.20
In conclusion, LFP experiments confirm the results of the
steady-state photolysis, suggesting that the decay of 1+•−4+• is
due to the C−S bond cleavage with formation of the tert-butyl
cation and the arylsulfinyl radicals ArSO•.
Because we are dealing with conformationally flexible
molecules, before starting the BDE calculations, all the available
conformations for the molecule and the radicals formed in the
C−S scission process have to be found. To this end, a
The rates of fragmentation of the aryl tert-butyl sulfoxide
radical cations 1+•−4+• (kf) were determined by following the
Table 2. Maximum Absorption Wavelengths (λmax), Decay Rate Constants (kf) of Aryl tert-Butyl Sulfoxide Radical Cations
(1+•−4+•) Generated by Photooxidation of 1−4 Sensitized by 3-CN-NMQ+ (λecc = 355 nm), C−S Bond Dissociation Energies
(BDEs) for the Aryl tert-Butyl Sulfoxides 1−4, and C−S Bond Dissociation Free Energies (BDFEs) for Radical Cations 1+•−4+•
a
a
bc
,
cde
, ,
4-XC6H4SOC(CH3)3
λmax (nm)
kf (105 s−1
)
C−S BDE (neutral substrates)
C−S BDFE (radical cations)
1+• X = MeO
2+• X = Me
3+• X = H
620
570
1.9 0.5
4.1 0.5
35.80
35.94
−12.5
−16.9
−18.1
f
f
f
f
520
14
36.2
4+• X = Br
590
18
2
36.39
−19.9
a
b
c
From LFP experiments in N2-saturated CH3CN. [Sulfide] = 1.0 × 10−2 M; [3-CN-NMQ+] = 8 × 10−4 M. From DFT calculations; see text. In
d
e
f
kilocalories per mole. At 298 K. Details of calculations in the Supporting Information. From ref 8.
C
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natural population analysis (NPA).34 Unpaired electron spin
densities were calculated using the Mulliken population
analysis.
The two energy minimum conformations for 1+• and 2+•
differ only for the dihedral angle of the substituent and have
very similar energies: 1a+• is 0.14 kcal/mol lower in energy than
1b+•, whereas 2a+• is 0.04 kcal/mol lower in energy than 2b+•.
In Figure 3 are displayed the two most stable conformers of
1+•. Energy minimum conformations for 2+•, 3+•, and 4+• are
reported in Figures S7 and S8 in the Supporting Information.
Figure 2. Thermochemical cycle for the calculation of C−S BDFE
values of sulfoxide radical cations 1+•−4+•.
systematic conformational search was carried out, at the
semiempirical PM3 level of theory,30 using the Conformer
Search Module available in the Spartan 5.01 package.31 All 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 numbers of conformers
found for sulfoxides were two for 1, two for 2, and one for 4.
The numbers of conformers found for the arylsulfinyl radicals
were two for 4-MeOC6H4SO•, two for 4-MeC6H4SO•, and one
for 4-BrC6H4SO•.
In all 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 because the expectation value ⟨S2⟩
of the total spin operator S2 was, in all cases, within 5% of the
expectation value for a doublet (0.75). Harmonic vibrational
frequencies were calculated at the B3P86/6-311+G(d,p) level
of theory to confirm that the stationary points found
correspond to local minima and to obtain the zero-point
vibrational energy (ZPVE) corrections. For the ZPVE, a scale
factor of 0.9845 was used.32 When more than one conformer
was found for a given compound, its energy (Eel + EZPVE) to be
used in the BDE 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.
+•
Figure 3. Most stable conformers for 4-MeOC6H4SOC(CH3)3
(1+•).
In Table 3 are displayed NPA charges and spin density values
obtained for the conformers of radical cations 1+•−4+•,
Table 3. NPA Charges (qNPA) and Spin Densities for Radical
Cations 1+•−4+•
X
Ar
S
O
tert-butyl
1a+•
1a+•
1b+•
1b+•
2a+•
2a+•
2b+•
2b+•
3+•
qNPA
spin
qNPA
spin
qNPA
spin
qNPA
spin
qNPA
spin
qNPA
spin
−0.078
0.080
−0.078
0.081
0.087
0.007
0.087
0.007
−
0.314
0.238
0.314
0.236
0.027
0.158
0.029
0.160
0.049
0.104
−0.102
0.117
1.275
0.191
1.274
0.189
1.231
0.219
1.236
0.220
1.139
0.230
1.157
0.209
−0.747
0.297
0.236
0.194
0.234
0.194
0.355
0.244
0.349
0.240
0.510
0.282
0.471
0.283
−0.744
0.301
−0.701
0.372
−0.700
0.373
−0.698
0.384
Sulfoxide Radical Cations. DFT calculations were per-
formed for sulfoxide radical cations 1+•−4+• 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 used in previous works on aryl sulfide radical
cations.13,33
3+•
−
0.189
4+•
4+•
−0.713
0.353
0.038
partitioned for the different structural components of the
radical cations defined in Figure 4.
The starting geometries for the energy minimization of each
radical cation were those of the corresponding neutral sulfoxide.
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
because 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). The number of conformers found for sulfoxide
radical cations 1+•−4+• were two for 1+•, two for 2+•, one for
3+•, and one for 4+•. The atomic charges were obtained by
In the attempt to identify a possible transition state for the
C−S bond cleavage in the radical cations, we stepwise
elongated the C−S bond in the radical cations 1+•−4+• starting
Figure 4. Structural components of aryl tert-butyl sulfoxide radical
cations.
D
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from the energy minimum conformations. For radical cations
1+• and 2+•, the most stable conformer was chosen. We
partially optimized the geometry for each bond length. In the
optimization process, the C−S bond length was kept fixed,
whereas all the other degrees of freedom were allowed to relax.
By increasing the C−S bond, we observed an initial increase of
the energy of the system followed by a decrease at longer C−S
distances (Figure 5).
DISCUSSION
■
The results of the steady-state photolysis experiments
supported the existence of a heterolytic C−S bond cleavage
in aryl tert-butyl sulfoxide radical cations 1+•−4+•. In all cases,
tert-butyl acetamide and tert-butyl alcohol are formed as the
primary reaction products from the tert-butyl cation, produced
by the C−S bond cleavage, which reacts with the solvent
(Ritter reaction) or adventitious water (Scheme 2).
The sulfur-containing products (diaryl disulfide (ArSSAr),
aryl arenethiosulfinate (ArSOSAr), and aryl arenethiosulfonate
(ArSO2SAr)) derive instead from the arylsulfinyl radical.
Dimerization of ArSO• might lead to the aryl arenethiosulfo-
nates.7,36 Diaryl disulfides and aryl arenethiosulfinates likely
derive from the aryl sulfenates formed after reduction of ArSO•
by 3-CN-NMQ• (eq 2). This exergonic process8 can explain
the observation that 3-CN-NMQ+ is not entirely consumed
during the irradiation.
3‐CN‐NMQ• + 4‐XC6H4SO• → 3‐CN‐NMQ+ + 4‐XC6H6SO−
(2)
From the sulfenate anion or its protonated sulfenic acid form,
ArSOSAr and ArSSAr can be produced according to the
reactions displayed in Scheme 3. Small amounts of aryl sulfinic
acids that might get lost during workup can also be formed.
This can explain the slightly lower overall quantum yield of
sulfur-containing products as compared to that of the products
from the tert-butyl moiety.
The kinetic analysis of the C−S bond fragmentation in aryl
tert-butyl sulfoxide radical cations 1+•−4+•, carried out by LFP
experiments in CH3CN following the unimolecular decay of the
radical cations at their maximum absorption wavelengths, has
provided important information on the quantitative aspects of
the electronic effects of arylsulfinyl ring substituents on C−S
bond cleavage. From the fragmentation rate constants (kf) of
1+•−4+• reported in Table 2, it can be immediately noted that,
as expected, the rates increase by decreasing the electron
donating power of the substituent, that is by decreasing the
stability of the radical cations. However, the substituent effect
on the C−S bond cleavage rates is rather small because an
increase in the fragmentation rate of less than 10 times is
observed when going from the most stable 4-MeO-substituted
radical cation 1+• to the less stable 4-Br-substituted radical
cation 4+•. The 7.4 kcal/mol difference in BDFE between 1+•
and 4+• (Table 2) is associated with a very small difference in
ΔG# (1.4 kcal/mol).
Figure 5. Plot of Erel vs the C−S bond distances (d(C−S)) for radical
+•
+•
+•
cations 1+•
(
), 2 ( ), 3 ( ), and 4
(
□
●
▲
△
).
For each radical cation, the 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 using the visualization
program Molden.35 In this way it was verified that the
displacements composing the mode correspond to the C−S
bond fragmentation process.
In Table 4 are displayed the C−S bond distances, the relative
energies of the transition states, the NPA charges (qNPA), and
spin density values obtained for the optimized structures of
radical cations 1+•−4+• and for the TS, partitioned for the
different structural components defined in Figure 4.
Theoretical calculations have been carried out in order to
have information on the energetics of the C−S bond cleavage
Table 4. C−S Bond Distances (d(C−S)), Relative Energies (Erel) of the Transition States (TS) for the C−S Bond Fragmentation
Process in Radical Cations 1+•−4+•, NPA Charges (qNPA), and Spin Densities for Radical Cations 1+•−4+• and for the
Corresponding Transition States
qNPA
spin
d(C−S) (Å)
Erel (kcal/mol)
X
Ar
SO
t-Bu
X
Ar
SO
t-Bu
1+•
1+• (TS)
2+•
2+• (TS)
3+•
3+• (TS)
4+•
4+• (TS)
2.070
3.064
2.209
2.983
2.450
2.966
2.394
2.956
0
−0.078
−0.112
0.087
0.078
−
0.314
0.223
0.528
0.208
0.530
0.250
0.441
0.260
0.443
0.253
0.236
0.681
0.355
0.700
0.510
0.723
0.471
0.710
0.080
0.045
0.007
0.004
0.238
0.150
0.158
0.128
0.104
0.109
0.117
0.108
0.488
0.539
0.591
0.616
0.614
0.662
0.562
0.630
0.194
0.266
0.244
0.252
0.282
0.229
0.283
0.238
2.61
0
0.027
1.32
0
−0.028
0.049
−
−
0.827
0
−
0.188
0.017
−0.102
−0.131
0.038
0.792
0.168
0.024
E
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Scheme 2
explanation of this fact can be based on the results of DFT
calculations showing that a significant fraction of the charge is
delocalized in the tert-butyl group of the radical cations. Such a
delocalization is probably at the origin of the observation that
rates of C−S bond cleavage showed little sensitivity to changes
in the C−S bond dissociation free energy (BDFE). On the
contrary, for aryl triphenylmethyl sulfide radical cations, the
same type of calculations indicated that most of the charge is
localized in the ArS moiety of the radical cation and a
significant delocalization of the charge and spin density when
going from the reactant to the TS of the C−S fragmentation
process should occur, explaining the greater substituent effects
on the C−S bond cleavage rates.
Scheme 3
in aryl sulfoxide radical cations 1+•−4+• and to provide a
possible explanation of the relatively small substituent effects on
the C−S fragmentation rate constants. DFT calculations for 1+•
and 2+• indicated the presence of two energy minimum
conformations very similar from both geometric and energetic
points of view, whereas only one conformer at the minimum
energy was found for 3+• and 4+•. As shown in Figure 3 and
Figures S7 and S8 in the Supporting Information, the tert-butyl
group is almost perpendicular to the arylsulfinyl ring as
observed in our previous study of the most stable
conformations of aryl methyl sulfoxide radical cations.5
The NPA charges and spin populations in 1+•−4+• (Table 3)
are mainly localized on the sulfinyl group. The charge and spin
density are also relatively high on the tert-butyl group and
decrease by increasing the electron donating power of the X
substituent.
The increase of the C−S bond distance starting from the
energy minimum conformations led to an initial increase in the
energy of the system followed by a decrease at longer C−S
distances (Figure 5). Thus, the transition states for the C−S
fragmentation process have been identified. As predicted for the
highly exergonic C−S bond cleavage processes, the TS are
reached quite early along the reaction coordinate, and a
relatively small increase of the C−S bond distance is observed.
If we compare the TS of the C−S fragmentation of aryl tert-
butyl sulfoxide radical cations, it can be noted that the C−S
bond distance and the energy relative to the starting conformer
(Erel) increase regularly by increasing the electron releasing
power of the substituent (Table 4). The increase of the C−S
bond distance in the TS and the degree of elongation of the C−
S bond as measured by the increase of the C−S bond in the TS
(ca. 23% of the initial bond length for 4+• and ca. 48% for 1+•)
with the electron donating power of the substituents reflect a
later TS for the less exergonic processes. The highest and
lowest Erel values, found for 1+• and 4+•, respectively, are in
accordance with the lowest and highest rate of C−S
fragmentation found for these radical cations.
In our previous studies,8 we have suggested that the C−S
bond cleavage in aryl sulfoxide radical cations should be
characterized by a relatively high reorganization energy (λ). In
particular, the λ value for the fragmentation process should be
significantly larger for aromatic sulfoxide than for aromatic
sulfide radical cations. This hypothesis can account for the fact
that the difference in fragmentation rates between sulfide and
sulfoxide radical cations (at least 2 orders of magnitude)
appears small relative to the difference in the thermodynamic
driving force.8,10,37
In order to verify our hypothesis, we have treated the kinetic
data for the fragmentation of radical cations 1+•−4+• in terms of
the Marcus equation (eq 3),38 where ΔG# is the activation free
energy of the reaction calculated by the Eyring equation (eq 4),
replacing kf with the values reported in Table 2, and Z as 6 ×
1011 M−1 s−1.39 ΔG° is the free-energy variation in the same
reaction (C−S BDFE given in Table 2), and λ is the
reorganization energy required for the fragmentation process.
ΔG# = (λ/4)(1 + ΔG°/λ)2
(3)
ΔG# = RT ln(Z/kf)
(4)
A good fit of the experimental data to eq 3 was obtained and
is shown in Figure 6. The λ value obtained from the nonlinear
least-squares fitting of the data (62 kcal/mol) clearly indicates a
quite high intrinsic barrier for the C−S bond cleavage reaction.
The very high λ value obtained from the Marcus plot
confirms our hypothesis, clearly indicating that a large
reorganization energy is an intrinsic characteristic of the C−S
bond fragmentation process of aryl sulfoxide radical cations. It
has to be noted that the λ value is even higher than those
previously determined for the C−S bond cleavage in aryl
triphenylmethyl sulfide13 and aryl cumyl sulfide radical
cations33 (ca. 52 and 44 kcal mol−1, respectively).
As proposed in the previous study,8 the higher λ value for the
C−S bond cleavage in aryl sulfoxide radical cations might be
associated with the presence of the very polar S−O bond that is
well-solvated in CH3CN. The quite large internal and solvent
reorganization energies mainly derive from the change in the
length and polarity of the S−O bond during the fragmentation
process. Accordingly, when going from the starting sulfoxide
radical cations 1+•−4+• to the TS of the C−S fragmentation
It is interesting to note that the electronic effects of the X
substituent on the C−S bond cleavage in aryl tert-butyl
sulfoxide radical cations are significantly lower than those
observed in the C−S fragmentation of aryl triphenylmethyl
sulfide radical cation, where the C−S bond cleavage rates were
very sensitive to the nature of the substituent.13 A possible
F
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oxidation of diaryl disulfides with the H2O2/MoO2Cl2 system in
CH3CN at room temperature.45 Aryl tert-butyl sulfones were
synthesized by oxidation of the corresponding sulfides with H2O2 in
glacial acetic acid.46 CH3CN (spectrophotometric grade) was distilled
over CaH2.
Spectral data of substrates 1 and 2 and of reaction products are in
accordance with those reported in the literature.47,48
4-Bromophenyl tert-Butyl Sulfoxide (4). To a solution of 4-
bromophenyl tert-butyl sulfide41 (1.03 g, 4.2 mmol) in 150 mL of
methanol/water (1:1) was added sodium periodate (0.9 g, 4.2 mmol),
and the reaction mixture was stirred overnight at room temperature
and then extracted with chloroform. The extract was dried over
anhydrous magnesium sulfate, and the solvent was removed under
reduced pressure. 4 was purified by column chromatography (hexane/
ethyl acetate, 3:1) and obtained as a white solid (0.6 g, 55%): mp 70−
1
72 °C; H NMR (300 MHz, CD3CN) δ 1.08 (s, 9H), 7.46−7.69 (m,
4H); 13C NMR (300 MHz, CD3CN) δ 21.5, 55.1, 124.6, 127.6, 131.2,
139.6; HRMS (ESI-TOF) calcd for C10H13SOBr + Na+ 282.9768,
found 282.9812.
Steady-State Photolysis. A 1 mL solution containing 3-CN-
NMQ+ (1.0 × 10−3 M) and 1−4 (1.0 × 10−2 M) in N2-saturated
CD3CN placed in a Schlenk tube was irradiated with four phosphor-
Figure 6. Diagram of ΔG# vs ΔG° for the fragmentation reactions of
radical cations 1+•−4+•. The solid circles correspond to the
experimental values; the curve is calculated by a nonlinear least-
squares fit to eq 3.
coated fluorescent lamps emitting at 355
15 nm. An internal
standard (methyl phenyl sulfoxide) was added, and the mixtures were
analyzed by 1H NMR and HPLC. The following products were
identified by comparison with authentic specimens: tert-butyl alcohol,
tert-butyl acetamide, diaryl disulfides, aryl arylthiosulfonates, aryl
arylthiosulfinates, and aryl tert-butyl sulfones. The photoproducts were
quantified by HPLC and 1H NMR. HPLC was performed with an HP
Agilent 1100 Series HPLC system with a UV/visible detector at 230
nm. An Alltima C18 column (5 μm, 4.6 × 250 mm) was equilibrated
with CH3OH/H2O (75:25), and compounds were eluted with an
ascending gradient of CH3OH (75−100%) at a flow rate of 0.7 mL/
min. The material balance was always satisfactory (>90%). A blank
experiment, carried out by irradiating the solutions in the absence of 3-
process, we observe an increase of the negative charge on
oxygen and a decrease of the positive charge on sulfur in
addition to an increase of the S−O bond length (see Table S2
in the Supporting Information).
CONCLUSIONS
■
The C−S bond cleavage rates in aryl tert-butyl sulfoxide radical
cations show little sensitivity to the nature of the substituent,
decreasing by increasing the electron donating properties of the
arylsulfenyl ring substituent, that is by increasing the stability of
the radical cations. These results have been explained by
considering that a significant fraction of the charge and spin
densities are delocalized in the tert-butyl group of the radical
cations, and little further accumulation of the charge and spin
density should occur when going from the reactant to the TS of
the C−S fragmentation process. A different situation was
observed in a previous study for the C−S fragmentation of aryl
triphenylmethyl sulfide radical cations13 where most of the
charge is localized in the ArS moiety and the aryl substituents
exerted a significant effect on the C−S bond cleavage rates.
C−S bond cleavage in aryl sulfoxide radical cations is
characterized by a particularly high reorganization energy (λ),
much higher than that found in the C−S fragmentation process
of aromatic sulfide radical cations, most likely because of the
presence of the very polar S−O bond that is well solvated in
CH3CN.
−
CN-NMQ+ClO4 , did not show in any case product formation.
Laser Flash Photolysis. An 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. A 1 mL solution containing the substrate (1.0 × 10−2 M),
the sensitizer (3-CN-NMQ+, 8.8 × 10−4 M), and the cosensitizer (1 M
toluene) was flashed in a quartz photolysis cell while nitrogen was
bubbling through them. All measurements were carried out at 22
2
°C. The transient spectra were obtained by a point-to-point technique,
monitoring the change of absorbance (ΔA) after the laser flash at
intervals of 5−10 nm over the spectral range of 350−700 nm,
averaging at least 10 decays at each wavelength. The error estimated
on the rate constants was 10%.
Theoretical Calculations. The calculations were run on the
22TFlops Linux Cluster “Matrix” consisting of 330 nodes dual-socket
quad-core Opteron CPU each with Infiniband DDR as the main
interconnect and a shared filesystem based on Luster 1.8.4 via IB.
Located at Rome supercomputing center CASPUR.
ASSOCIATED CONTENT
■
EXPERIMENTAL SECTION
■
S
* Supporting Information
Starting Materials. tert-Butyl phenyl sulfoxide (3) was synthetized
and characterized in a previous study.8 tert-Butyl 4-methoxyphenyl
sulfoxide (1), tert-butyl 4-methylphenyl sulfoxide (2), and 4-
bromophenyl tert-butyl sulfoxide (4) were prepared by oxidation of
the corresponding sulfides with sodium periodate in aqueous ethanol40
as previously reported for the synthesis of 3. tert-Butyl 4-
methoxyphenyl sulfide, tert-butyl 4-methylphenyl sulfide, and 4-
bromophenyl tert-butyl sulfide were prepared by acid-catalyzed
reaction of the corresponding thiophenols with tert-butyl alcohol.41
3-Cyano-N-methylquinolinium perchlorate was prepared by reaction
of 3-cyanoquinoline with dimethyl sulfate; the sulfate salt was then
transformed to the perchlorate salt using perchloric acid.42 tert-
Butylacetamide, aryl arylthiosulfonates were prepared according to
literature procedures.43,44 Aryl arylthiosulfinates were prepared by
1H NMR and 13C NMR spectra of 4-Br-C6H4SOC(CH3)3 (4);
1H NMR spectrum and HPLC chromatogram of the 3-CN-
NMQ+ photosensitized oxidation of tert-butyl 4-methoxyphenyl
sulfoxide (1); time-resolved absorption spectra after LFP of the
3-CN-NMQ+/toluene/2 and 3-CN-NMQ+/toluene/4 systems
in CH3CN; cyclic voltammetry; C−S BDFEs of radical cations
1+•−4+•, conformers at the minimum of energy of 2+•, 3+•, and
4+•; Cartesian coordinates, energies, and ZPVE from DFT
calculations for neutral sulfides 1, 2, and 4, for arylsulfinyl
radicals, and for radical cations 1+•−4+•; Cartesian coordinates
and energies for TS of the C−S bond cleavage in the radical
cations 1+•−4+•; and S−O bond distances and NPA charges on
G
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Article
sulfur and oxygen in radical cations 1+•−4+• and in the TS for
the C−S bond fragmentation process. This material is available
free of charge via the Internet at http://pubs.acs.org.
(16) Formation of aryl tert-butyl sulfones (<5% referred to the
starting material) probably derives from the presence of traces of
oxygen in the reaction mixture. Accordingly, their amounts are
significantly reduced by a more extended nitrogen saturation of the
reaction mixtures. Aryl sulfones are likely formed by reaction of the
sulfoxide radical cations 1+•−4+• with superoxide anion produced after
reduction of oxygen by the radical 3-CN-NMQ•.
AUTHOR INFORMATION
■
Corresponding Author
(17) 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.
(18) Schlesener, C. J.; Amatore, C.; Kochi, J. K. J. Phys. Chem. 1986,
90, 3747−3756.
*O.L.: osvaldo.lanzalunga@uniroma1.it. T.D.: dgiacco@unipg.
it. P.M.: paolo.mencarelli@uniroma1.it.
Notes
The authors declare no competing financial interest.
(19) Darmanyan, A. P.; Gregory, D. D.; Guo, Y.; Jencks, W. S. J. Phys.
Chem. A 1997, 101, 6855−6863.
(20) The rate of the self-recombination reaction for PhSO• is k = 3.0
× 109 M−1 s−1.19
ACKNOWLEDGMENTS
■
Thanks are due to the Ministero dell’Istruzione, dell’Universita
e della Ricerca (MIUR) for financial support, PRIN 2010-2011
(2010PFLRJR) project (PROxi project). We also thank Prof.
Enrico Baciocchi for helpful discussion.
̀
(21) Baciocchi, E.; Del Giacco, T.; Giombolini, P.; Lanzalunga, O.
Tetrahedron 2006, 62, 6566−6573.
(22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J.
B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision
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