Steady-state and laser flash photolysis study of the carbon-carbon bond fragmentation reactions of 2-arylsulfanyl alcohol radical cations

Article abstract of DOI:10.1021/jo0486544

The N-methylquinolinium tetrafluoroborate (NMQ⁺)-sensitized photolysis of the erythro-1,2-diphenyl-2-arylsulfanylethanols 1-3 (1, aryl = phenyl; 2, aryl = 4-methylphenyl; 3, aryl = 3-chlorophenyl) has been investigated in MeCN, under laser flash and steady-state photolysis. Under laser irradiation, the formation of sulfide radical cations of 1-3, in the monomeric (λ_max = 520-540 nm) and dimeric form (λ_max = 720-800 nm), was observed within the laser pulse. The radical cations decayed by first-order kinetics, and under nitrogen, the formation of ArSCH ^.Ph (λ_max = 350-360 nm) was clearly observed. This indicates that the decay of the radical cation is due to a fragmentation process involving the heterolytic C-C bond cleavage, a conclusion fully confirmed by steady-state photolysis experiments (formation of benzaldehyde and the dimer of the α-arylsulfanyl carbon radical). Whereas the fragmentation rate decreases as the C-C bond dissociation energy (BDE) increases, no rate change was observed by the replacement of OH by OD in the sulfide radical cation (k_OH/k_OD = 1). This suggests a transition state structure with partial C-C bond cleavage where the main effect of the OH group is the stabilization of the transition state by hydrogen bonding with the solvent. The fragmentation rate of 2-hydroxy sulfanyl radical cations turned out to be significantly slower than that of nitrogen analogues of comparable reduction potential, probably due to a more efficient overlap between the SOMO in the heteroatom and the C-C bond σ-orbital in the second case. The fragmentation rates of 1^+.-3^+. were found to increase by addition of a pyridine, and plots of k_base against base strength were linear, allowing calculation of the β Bronsted values, which were found to increase as the reduction potential of the radical cation decreases, β = 0.21 (3^+.), 0.34 (1^+.), and 0.48 (2 ^+.). The reactions of 1^+. exhibit a deuterium kinetic isotope effect with values that increase as the base strength increases: k _OH/k_OD = 1.3 (pyridine), 1.9 (4-ethylpyridine), and 2.3 (4-methoxypyridine). This finding and the observation that with the above three bases the rate decreases in the order 3^+. > 1^+. > 2^+., i.e., as the C-C BDE increases, suggest that C-C and O-H bond cleavages are concerted but not synchronous, with the role of OH bond breaking increasing as the base becomes stronger (variable transition state). It is probable that, with the much stronger base, 4-(dimethylamino)pyridine, a change to a stepwise mechanism may occur where the slow step is the formation of a radical zwitterion that then rapidly fragmentates to products.

Full text of DOI:10.1021/jo0486544

Steady-State and Laser Flash Photolysis Study of the
Carbon-Carbon Bond Fragmentation Reactions of 2-Arylsulfanyl
Alcohol Radical Cations
Enrico Baciocchi,*,† Tiziana Del Giacco,*,‡ Fausto Elisei,‡ Maria Francesca Gerini,†
Andrea Lapi,† Prisca Liberali,† and Barbara Uzzoli‡
Dipartimento di Chimica, Universita` “La Sapienza”, P.le A. Moro 5, 00185 Rome, Italy, and
Dipartimento di Chimica and Centro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN),
Universita` di Perugia, V. Elce di Sotto 8, 06123, Perugia, Italy
enrico.baciocchi@uniroma1.it; dgiacco@unipg.it
Received August 4, 2004
The N-methylquinolinium tetrafluoroborate (NMQ+)-sensitized photolysis of the erythro-1,2-
diphenyl-2-arylsulfanylethanols 1-3 (1, aryl ) phenyl; 2, aryl ) 4-methylphenyl; 3, aryl )
3-chlorophenyl) has been investigated in MeCN, under laser flash and steady-state photolysis. Under
laser irradiation, the formation of sulfide radical cations of 1-3, in the monomeric (λmax ) 520-
540 nm) and dimeric form (λmax ) 720f800 nm), was observed within the laser pulse. The radical
cations decayed by first-order kinetics, and under nitrogen, the formation of ArSCH•Ph (λmax
)
350-360 nm) was clearly observed. This indicates that the decay of the radical cation is due to a
fragmentation process involving the heterolytic C-C bond cleavage, a conclusion fully confirmed
by steady-state photolysis experiments (formation of benzaldehyde and the dimer of the R-aryl-
sulfanyl carbon radical). Whereas the fragmentation rate decreases as the C-C bond dissociation
energy (BDE) increases, no rate change was observed by the replacement of OH by OD in the
sulfide radical cation (kOH/kOD ) 1). This suggests a transition state structure with partial C-C
bond cleavage where the main effect of the OH group is the stabilization of the transition state by
hydrogen bonding with the solvent. The fragmentation rate of 2-hydroxy sulfanyl radical cations
turned out to be significantly slower than that of nitrogen analogues of comparable reduction
potential, probably due to a more efficient overlap between the SOMO in the heteroatom and the
C-C bond σ-orbital in the second case. The fragmentation rates of 1+•-3+• were found to increase
by addition of a pyridine, and plots of kbase against base strength were linear, allowing calculation
of the â Brønsted values, which were found to increase as the reduction potential of the radical
cation decreases, â ) 0.21 (3+•), 0.34 (1+•), and 0.48 (2+•). The reactions of 1+• exhibit a deuterium
kinetic isotope effect with values that increase as the base strength increases: kOH/kOD ) 1.3
(pyridine), 1.9 (4-ethylpyridine), and 2.3 (4-methoxypyridine). This finding and the observation
that with the above three bases the rate decreases in the order 3+• > 1+• > 2+•, i.e., as the C-C
BDE increases, suggest that C-C and O-H bond cleavages are concerted but not synchronous,
with the role of OH bond breaking increasing as the base becomes stronger (variable transition
state). It is probable that, with the much stronger base, 4-(dimethylamino)pyridine, a change to a
stepwise mechanism may occur where the slow step is the formation of a radical zwitterion that
then rapidly fragmentates to products.
Introduction
for a variety of practical applications. From the latter
point of view, the fragmentation reactions of radical
cations can find use in the initiation of radical polymer-
izations,1 the enhancing of silver halide photography
efficiency,2 and organic synthesis.3 Moreover, these reac-
tions can also be exploited as mechanistic probes to detect
One of the most important reactions of organic radical
cations is the cleavage of a â bond (â with respect to the
atom or aromatic ring bearing the positive charge) to
produce a cationic species and a free radical (eq 1).
Z+•-X-Y f ZX+/• + Y•/+
(1)
(1) Wrzyszczyn´ski, A.; Filipiak, P.; Hug, G. L.; Marciniak, B.;
Pa¸czkowski, J. Macromolecules 2000, 33, 1577-1582.
(2) (a) Gould, I. R.; Godleski, S. A.; Zielinski P. A.; Farid S. Can. J.
Chem. 2003, 81, 777-788. (b) Gould, I. R.; Lenhard, J. R.; Muenter,
A. A.; Godleski, S. A.; Farid, S. Pure Appl. Chem. 2001, 73, 455-458.
(c) Gould, I. R.; Lenhard, J. R.; Muenter, A. A.; Godleski, S. A.; Farid,
S. J. Am. Chem. Soc. 2000, 122, 11934-11943.
This process has been intensively investigated for some
time for its many theoretical aspects and its potential
* Corresponding authors. (E. Baciocchi) Tel: +39-06-49913711.
Fax: +39-06-490421. (T. Del Giacco) Tel. +39-075-5855559.
† Universita` “La Sapienza”.
(3) Albini, A.; Fagnoni, M.; Mella, M. Pure Appl. Chem. 2000, 72,
1321-1326.
‡ Universita` di Perugia.
10.1021/jo0486544 CCC: $27.50 © 2004 American Chemical Society
Published on Web 11/02/2004
J. Org. Chem. 2004, 69, 8323-8330
8323

Baciocchi et al.
electron-transfer processes in chemical and biochemical
oxidations.4,5
A class of radical cation fragmentations that has
attracted much attention is that involving the cleavage
of a C-C bond assisted by a OH group â with respect to
the positively charged moiety (eq 2).
Apart from furnishing interesting information per se, it
should also allow the comparison between nitrogen and
sulfur radical cations, thus providing further insight into
the energetics and dynamics of a process that is far from
being fully understood. Moreover, 2-arylsulfanyl alcohols
might be used as clocks to quantitatively assess the role
of electron-transfer steps in chemical and biochemical
oxidations of sulfides.
In this paper, we report the results of a laser flash and
steady-state photolysis study of the erythro-1,2-diphenyl-
2-arylsulfanylethanols 1-3 in MeCN.
One of the first and more thoughtfully investigated
examples of this process concerns the fragmentation
reactions of 2-amino alcohol radical cations (i.e., eq 2, Z
) R2N or ArNH), which was first investigated by Whitten
and his associates and then by the Schanze and Mariano
groups.6-9 It was found that the reaction rate is acceler-
ated by bases, with a quite large Brønsted â value (0.62)
measured against a series of pyridine bases.9 The pro-
posed mechanism was that the fragmentation of the
radical cation is assisted by an attack of the base at the
OH group concerted with the C-C bond cleavage. The
possibility of a stepwise mechanism was also suggested
where OH deprotonation might precede C-C bond cleav-
age.
In collaboration with Steen Steenken in Muelheim, our
group recently carried out a pulse radiolysis study of the
fragmentation reactions of 2-arylethanol radical cations
(eq 2, Z ) Ar) in water.10 In basic media, a concerted
mechanism similar to that suggested for the amino
alcohol radical cations was considered possible, but, since
the reaction rate was close to the diffusion-controlled
limit, the data were also consistent with the initial
formation of an intermediate radical zwitterion.
In the literature, there are a number of reports
indicating that the fragmentation described in eq 2 can
also be observed in 2-arylsulfanyl alcohol radical cations
(eq 2, Z ) ArS).11-14 However, no detailed kinetic study
has so far been carried out for these systems and no
quantitative information upon their reactivity is pres-
ently available.
The radical cations were generated by sensitized
photolysis in MeCN with N-methylquinolinium tetrafluo-
roborate (NMQ+) as the sensitizer. NMQ+ is a very
efficient sensitizer, whose excited state is characterized
by a very high reduction potential (2.7 V vs SCE).15
Moreover, by reacting with the substrate, it forms a
radical cation/radical couple whose separation is facili-
tated by the lack of any electrostatic barrier. To increase
the yields in separated radical ions, the laser photolysis
experiments were cosensitized by toluene.16 Some experi-
ments have also been carried out with threo-1,2-diphenyl-
2-phenylsulfanylethanol (4) and with 1-phenyl-2-phenyl-
sulfanylethanol (5).
Results
Steady-State Photolysis. Steady-state photolysis
sensitized by NMQ+ was carried out in N2-saturated
MeCN as described in Experimental Section. No reaction
was observed in the absence of sensitizer. Short reaction
times (10 min) and an excess of substrate on the
sensitizer were used in order to minimize any possible
overoxidation. In all cases, the reaction products (com-
parison with authentic specimens) were determined and
In view of our continuous interest in the chemistry of
radical cations and in the structural factors that deter-
mine reactivity and reaction pathways, it was felt that a
quantitative investigation of the C-C bond cleavage in
2-arylsulfanyl alcohol radical cations could be warranted.
1
quantified by GC, GC-MS, and H NMR.
Irradiation of 1 (1 × 10-2 M) and NMQ+ (1 × 10-3 M)
in N2-saturated MeCN gave benzaldehyde (40%) and 1,2-
bis(phenylsulfanyl)-1,2-diphenylethane (20%) as the ex-
clusive products (the yields were determined with respect
to NMQ+).17 The observed outcome can be reasonably
rationalized as described in Scheme 1. Formation of the
radical cation by photoinduced electron transfer is fol-
lowed by a heterolytic C-C bond cleavage leading to
benzaldehyde and an R-phenylsulfanyl radical. Dimer-
ization of the latter produces a mixture of meso- and d,l-
1,2-bis(phenylsulfanyl)-1,2-diphenylethane. Similar re-
sults were obtained in the steady-state photolysis of 2
(4) Baciocchi, E.; Lanzalunga, O.; Malandrucco, S.; Ioele, M.; Steen-
ken, S. J. Am. Chem. Soc. 1996, 118, 8973-8974.
(5) Baciocchi, E. In Free Radicals in Biology and Environment;
Minisci, F., Ed.; Kluwer Academic Publishers: Dordrecht, 1997; Nato
Asi Series.
(6) Gaillard, E. R.; Whitten, D. G. Acc. Chem. Res. 1996, 29, 292-
297.
(7) Zhuoyi, S.; Mariano, P. S.; Falvey, D. E.;Yoon, U. C.; Oh, S. W.
J. Am. Chem. Soc. 1998, 120, 10676-10686.
(8) Burton, R. D.; Bartberger, M. D.; Zhang, Y.; Eyler, J. R.; Schanze,
K. S. J. Am. Chem. Soc. 1996, 118, 5655-5664 and references therein.
(9) Lucia, L. A.; Burton, R. D.; Schanze, K. S. J. Phys. Chem. 1993,
97, 9078-9080.
(10) Baciocchi, E.; Bietti, M.; Manduchi, L.; Steenken, S. J. Am.
Chem. Soc. 1999, 121, 6624-6629.
(11) Gravel, D.; Farmer, L.; Ayotte, C. Tetrahedron Lett. 1990, 31,
63-66.
(15) Yoon, U. C.; Quillen, S. L.; Mariano, P. S.; Swanson, R.;
Stavinoha, J. L.; Bay, E. J. Am. Chem. Soc. 1983, 105, 1204-1218.
(16) Gould, I. R.; Ege, D.; Mosh, J. E.; Farid, S. J. Am. Chem. Soc.
1990, 112, 4290-4301.
(17) Same products were observed also when the photolysis was
carried out in the presence of O2.
(12) Vath, P.; Falvey, D. E.; Barnhurst, L. A.; Kutateladze, A. G. J.
Org. Chem. 2001, 66, 2887-2890.
(13) Li, Z.; Kutateladze, A. G. J. Org. Chem. 2003, 68, 8236-8239.
(14) Ci, X.; Whitten, D. G. In Photoinduced Electron Transfer; Fox,
M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part C, p 567
8324 J. Org. Chem., Vol. 69, No. 24, 2004

2-Arylsulfanyl Alcohol Radical Cations
SCHEME 1
TABLE 1. Quenching Rate Constants of NMQ+
Fluorescence (kq) and Photoreaction Quantum Yields (Φ)
of 1, 2, and 3 in MeCN
compound
substituent
kq (1010 M-1 s-1
)
Φ
2
1
3
-CH3
-H
-Cl
2.7
2.0
1.8
0.26
0.49
0.62
FIGURE 1. Time-resolved absorption spectra of the NMQ+
(8
10-5M)/toluene (1 M)/1 (5 10-3 M) system in
MeCN: (a) N2-saturated, recorded 0.032 (b), 0.46 (O), 1.8 (2),
and 6.4 (4) µs after the laser pulse and (b) O2-saturated,
recorded 0.08 (b), 0.4 (O), 0.9 (2), and 6.4 (4) µs after the laser
pulse. Insets: decay kinetics recorded at 520 nm (1) and 350
nm (2). λecc ) 308 nm.
×
×
and 3 and also when the reaction was carried out in the
presence of pyridine.
No products were observed, however, when 5 was
irradiated under identical conditions. In the presence of
pyridine, too, 5 was much less reactive than 1. Accord-
ingly, in the simultaneous irradiation (10 min) of two
separate solutions (1 × 10-2 M) of 1 and 5 with NMQ+
(5 × 10-3 M) and pyridine (1 × 10-2 M) in N2-saturated
MeCN, benzaldehyde was obtained from 1 in 16% yield
(16% conversion) with respect to the substrate, while
from 5, the yield in benzaldehyde was only 0.5%.
Quantum yields (Φ) for the photolysis of 1-3, under
the conditions listed above, were also determined on the
basis of the formation of benzaldehyde. The results,
reported in Table 1, indicate that the reaction is quite
efficient, with values of Φ between 0.26 (2) and 0.62 (3).
Fluorescence Quenching. Fluorescence-quenching
experiments were carried out in N2-saturated MeCN to
determine the efficiency by which substrates 1-3 quench
the lowest excited singlet state of NMQ+. The fluores-
cence intensity of NMQ+ was recorded by steady-state
experiments in the absence (IF°) and in the presence (IF)
of increasing concentrations of the substrates (S). Table
1 lists the second-order rate constants for NMQ+ fluo-
rescence quenching (kq) calculated from the slopes of the
linear Stern-Volmer plots (IF°/IF vs [S]) divided by the
Since the results were very similar for all sulfides
examined, only those concerning the unsubstituted sul-
fide 1 will be illustrated in detail.
By laser photolysis of 1 in N2-saturated solutions, three
absorption bands were detected just after the laser pulse
in the 400 (a shoulder), 520, and 700 nm regions (Figure
1). They were assigned to NMQ• (λmax) 400 and 550 nm)19
and the monomer (λmax) 520 nm) and the dimer sulfide
radical cations (λmax ≈ 720 nm).20 The time-evolution of
the absorption spectra shows that the signal decay
recorded at ca. 400 and 520 nm is coupled with the
growth of an additional maximum at 350 nm region
assigned to the absorption of the radical PhSCH•Ph21
(Figure 1a, inset); accordingly, in O2-saturated solutions,
the time-resolved absorption spectra are modified by the
fast decay of PhSCH•Ph and NMQ•, which are efficiently
quenched by molecular oxygen; thus, the shoulder at ca.
400 nm disappears and the only recorded transient
concerns the radical cation in its monomeric (λmax ) 520
nm) and dimeric (λmax ≈ 720 nm, Figure 1b) forms.
Clearly, the observation of PhSCH•Ph fully confirms the
results of the steady-state experiments, indicating that
the decay of the radical cation is due to a fragmentation
process involving the heterolytic C-C bond cleavage
(Scheme 1).
1
lifetime of NMQ+* (20 ns).18 The Stern-Volmer plots
show that each substrate efficiently quenches the emis-
sion of NMQ+, presumably via a diffusion-controlled
electron-transfer process. This behavior is in line with
the laser flash photolysis experiments where the radical
cations of 1-3 were produced within the laser pulse (see
below).
The decay rate of radical cations 1+•-4+• was deter-
mined by following, in the presence of nitrogen, the
absorption rise at 350 nm (due to the carbon radical) and,
in the presence of oxygen, the absorption decay at 520
Laser Flash Photolysis Studies. The laser photoly-
sis experiments were carried out in the presence of 1 M
toluene as a cosensitizer. Upon laser excitation (λexc
)
308 and/or 355 nm) of MeCN solutions of NMQ+/toluene/
1-4, time-resolved absorption spectra and decay kinetics
were recorded under N2- and O2-saturated conditions.
(19) Bockman, T. M.; Kochi, J. K. J. Am. Chem. Soc. 1989, 111,
4669-4683.
(20) Yokoi, H.; Hatta, A.; Ishiguro, K.; Sawaki Y. J. Am. Chem. Soc.
1998, 120, 12728-12733.
(21) Actual absorption spectrum of PhSCH•Ph was obtained by
photolysis of PhSCH2Ph (0.01 M) and di-tert-butyl peroxide (0.5 M) in
MeCN (λexc ) 308 nm).
(18) Fukuzumi, S.; Fujita, M.; Noura, S.; Ohkubo, K.; Suenobu, T.;
Araki, Y.; Ito, O. J. Phys. Chem. A 2001, 105, 1857-1868.
J. Org. Chem, Vol. 69, No. 24, 2004 8325

Baciocchi et al.
TABLE 2. Decay Rate Constants and Activation Energies for Fragmentation of the Sulfide Radical Cations in MeCN
radical cation
substituent
kfr (106 s-1
)
∆Hq (kcal mol-1 K-1
)
∆Sq (cal mol-1 K-1
)
E°red (V)a
2+•
1+•
3+•
4+•
-CH3
-H
-Cl
0.26
1.3
5.9 ( 0.8
6.2 ( 0.7
5.7 ( 0.8
-15 ( 4
-9 ( 3
-7 ( 3
1.43
1.55
1.67
11
0.71
a Versus SCE in MeCN, taken equal to those of the corresponding ArSCH2Ph+• 22
.
TABLE 3. Decay Rate Constants (kBase) and Brønsted Parameters (â) for the Base-Catalyzed Fragmentation of the
Sulfide Radical Cations in MeCN
kbase (108 M-1 s-1
)
radical
cation
pyridine
4-Et-pyridine
4-MeO-pyridine
4-Me2N-pyridine
substituent
(pKa ) 12.33)a
(pKa ) 13.35)a
(pKa ) 14.04)a
(pKa ) 17.74)a
âb
2+•
1+•
3+•
4+•
-CH3
-H
-Cl
0.13
1.1
0.25
1.6
5.7
0.43
3.0
6.6
44
64
46
0.48
0.34
0.21
3.6
0.55
a pKa in MeCN.23 b Slope of the log kbase vs pKa plot.
the two conditions, thus indicating the absence of any
significant kinetic isotope effect when OH is replaced by
OD.
Addition of pyridine or substituted pyridines acceler-
ates the decay rate of radical cations; according to eq 3,
linear plots were obtained by reporting the first-order
rate constants (kobs) versus base concentration ([base]),
which provided us with the second-order rate constants
(kbase) for the base-promoted reactions (Table 3).
kobs ) kfr + kbase[base]
(3)
The kbase values increase as the base strength increases,
in good accord with the Brønsted equation (Figure S2,
Supporting Information). The Brønsted â parameters,
which was observed to be significantly different for 1+•,
2+•, and 3+•, are also reported in Table 3.
The effect of replacing OH by OD in the sulfide was
also studied in the presence of pyridine. In this case, the
fragmentation of 1+• is faster in MeCN-1% H2O than in
MeCN-1% D2O (9.7 × 107 M-1 s-1 vs 5.0 × 107 M-1 s-1).
From these values, a deuterium kinetic isotope effect
(kOH/kOD) of 1.9 was calculated.
FIGURE 2. Normalized decay kinetics of the NMQ+ (8 × 10-5
M)/toluene (1 M)/1 (5 × 10-3 M) system in MeCN recorded in
O2- and N2-saturated solutions at 520 and 350 nm, respec-
tively.
nm (due to the radical cation). In both cases, the decay
kinetics followed first order laws and provided almost
identical values of the decay rate constants (see Figure
2). In some cases, particularly for 1, the decay rate of
the radical cation was also measured by following the
absorption changes of the dimer radical cation. The decay
rate constants (kfr), for all the radical cations, measured
at 25 °C are reported in Table 2. The activation param-
eters (∆Hq and ∆Sq) for the fragmentation process of 1+•-
Discussion
3
+• (Table 2) were determined over the temperature range
Fragmentations in the Absence of Bases. NMQ+
has been confirmed to be an excellent sensitizer for the
photoinduced generation of aromatic sulfide radical
cations. Accordingly, the rates of fluorescence quenching
are diffusion-controlled (Table 1) and the quantum yields
for the photoreaction of 1-3 are also relatively large
(0.26-0.62, Table 1). As expected, the quantum yields
increase in the order 2 < 1 < 3, that is, as the
fragmentation rate of the radical cation increases (see
below), thus better competing with back-electron transfer.
Looking first at the effect of ring substituents on the
fragmentation rate constants, we note that kfr increases
in the order 2+• < 1+• < 3+•, that is as the E°red of the
radical cations increases (Table 2). This observation is
in line with expectations because increasing the E°red of
the radical cation leads to a corresponding decrease in
from 258 to 358 K by Eyring analysis (see Supporting
Information for the Eyring plots and further spectral data
of the investigated radical cations). It appears that bond
fragmentation in the radical cations 1+•-3+• is character-
ized by both low activation enthalpies and negative
activation entropies. Laser photolysis of 5 in the presence
of NMQ+ and 1 M toluene also produced 5+• (monomer
at 530 nm and dimer at 750 nm). However, in this case,
the decay of 5+•, significantly slower than that of 1+•,
exhibited mixed first-order/second-order kinetics, indicat-
ing that the decay is to some extent influenced by
radical-radical reactions. Thus, no reliable rate constant
for the fragmentation rate of 5+• could be determined.
Some experiments were carried out in MeCN with 1%
H2O and in MeCN with 1% D2O using 1 as the substrate.
Identical values of kfr (1.6 × 106 s-1) were found under
8326 J. Org. Chem., Vol. 69, No. 24, 2004

2-Arylsulfanyl Alcohol Radical Cations
It is certainly of interest to compare the reactivity of
2-arylsulfanyl alcohol radical cations with that of 2-hy-
droxy anilinium analogues. To make a meaningful com-
parison, however, radical cations of comparable stability
and studied in the same solvent should be considered.
These requirements are substantially fulfilled if we
compare the reactivity of 2+• (Ered ) 1.4 V vs SCE) with
FIGURE 3. Transition state structure for the fragmentation
of a 2-arylsulfanyl alcohol radical cation in the absence of base.
that of the 4-cyanoanilinium radical cation 6+• (Ered
)
1.3 V vs SCE) determined by Schanze in MeCN.8 The
result is that 6+• is much more reactive than 2+• (7.4 ×
106 s-1 vs 2.6 × 105 s-1). Since the C-C BDEs are very
similar for 2+• and 6+•, 36.9 and 34.6 kcal mol-1, respec-
tively (by DFT calculations), we attribute the higher
reactivity of 6+• with respect to 2+• to a more efficient
overlap between the SOMO of the radical cation and the
2p σ-orbital of the scissile C-C bond when the SOMO is
on nitrogen (2p orbital) than when it is on sulfur (3p
orbital), due to the better matching of the energies in the
former case. Such an overlap is essential for the intra-
molecular electron-transfer process, from the C-C bond
to the SOMO on the heteroatom, which has to accompany
the C-C bond cleavage.
the heterolytic bond dissociation energy (BDE)24 of the
C-C bond (Scheme 1). Accordingly, by DFT calculations
BDEs (kcal mol-1) were found to be: 36.9 for 2+•, 33.2
for 1+•, and 29.1 for 3+•.
The activation parameters displayed in Table 2 confirm
previous observations that the fragmentation reactions
of radical cations are characterized by a negative entropy
of activation, which has been attributed to the partial
formation of a double bond in the transition state6 and/
or to the stereoelectronic requirements of the process
(vide infra).8 In our case, it would seem that differences
in reactivity are mostly related to the entropic factor, but
this may be due to the considerable experimental errors,
inherent to this type of determination.
The observation that the fragmentation rates decrease
as the C-C BDE increases suggests that some degree of
C-C bond breaking must occur in the transition state of
the fragmentation process. This conclusion is fully sup-
ported also by the finding that the fragmentation of 5+•
must be significantly slower than that of 1+• since no
products were obtained in the steady-state photolysis of
5. In 5+•, the absence of the 2-phenyl group significantly
increases the C-C BDE by 9.3 kcal mol-1 with respect
to that in 1+•.
The same reactivity, however, has been observed when
the alcoholic OH group is replaced by OD, which suggests
that, in the absence of bases, the rupture of the O-H
bond is not associated with the cleavage of the C-C bond.
Thus, as previously suggested for the fragmentations of
arylalkanol radical cations in water10 and also in the case
of 2-arylsulfanyl alcohol radical cations, the favorable
effect of the OH group with respect to C-C bond
cleavage25 might be due to a stabilization of the transition
state by hydrogen bonding with the solvent.26 The transi-
tion state structure might be that represented in Figure
3,28 where there is a partial cleavage of the C-C bond
and a partial positive charge on oxygen, thus allowing
partial formation of the carbon oxygen double bond.29
Base-Promoted Fragmentations. As in the case of
2-amino alcohol and 2-phenylethanol radical cations, the
fragmentation rate of 2-sulfanyl alcohol radical cations
is accelerated by the presence of a base. The second-order
rate constants for the decay of 1+•-3+• in the presence of
pyridine bases (kbase) and the Brønsted â values are
displayed in Table 3.
Differently than for the fragmentations in the absence
of bases, the reaction of 1+• promoted by pyridine,
4-ethylpyridine, and 4-methoxypyridine exhibits a deu-
terium kinetic isotope effect when the alcoholic OH is
replaced by OD, with values that increase as the base
strength increases: kOH/kOD ) 1.3 (pyridine), 1.9 (4-
ethylpyridine), and 2.3 (4-methoxypyridine).30 This find-
ing and the observation that, with the above three bases,
the rate decreases in the order expected on the basis of
C-C BDEs (i.e. 3+• > 1+• > 2+•), suggest a transition state
structure where C-C bond cleavage and the intramo-
lecular electron transfer from the scissile bond to the
sulfur atom are to some extent coupled with the base-
induced OH bond breaking. The more logical hypothesis
is that, in this case, the 2-arylsulfanyl alcohol radical
cations fragment by the same concerted mechanism
(Figure 4, Grob-type fragmentation) proposed by the
Whitten and Schanze groups for the corresponding reac-
tions of amino alcohol radical cations.6,8
(22) Baciocchi, E.; Intini, D.; Piermattei, A.; Rol, C.; Ruzziconi, R.
Gazz. Chim. Ital. 1989, 119, 649-652.
(23) (a) Kaliurand, I.; Rodima, T.; Leito, I.; Koppel, I.; Schwesinger,
R. J. Org. Chem. 2000, 65, 6202-6208. (b) Savelova, V. A.; Popov, A.
F.; Solomoichenko, T. N.; Sadovskii, Y. S.; Piskunova, Z. P.; Lobanova,
O. V. Russ. J. Org. Chem. 2000, 36, 1465-1473.
(24) This refers to the C-C bond cleavage in the radical cation
leading to R-sulfanyl-substituted radical and the protonated benz-
aldehyde.
(25) The OH group must certainly have a role, as no fragmentation
occurs in its absence.
(26) In the formation of hydrogen bonds, replacement of hydrogen
by deuterium does not significantly affect the equilibrium constant
between the free and the hydrogen bonded species.27
(27) (a) Vinogradov, S. N.; Linnell, R. H. In Hydrogen Bonding; Van
Nostrand Reinhold Company: New York, 1971; p 124. (b) Joesten, M.
D.; Schaad L. J. In Hydrogen Bonding; Marcel Dekker: New York,
1974; Appendix.
(28) Since the experiments to determine the kOH/kOD isotope effect
were carried out in MeCN containing 1% H2O or 1% D2O (see
Experimental Section), it might be that under these conditions “Solv”
in Figure 3 is H2O or D2O.
(29) As suggested by a referee, our observations might also be
consistent with a very early transition state with respect to OH bond
breaking.
(30) Addition of water induced an increase in the fragmentation rate
of 1+•, so that the rates became too fast to be measured with the
necessary precision.
J. Org. Chem, Vol. 69, No. 24, 2004 8327

Baciocchi et al.
SCHEME 2
FIGURE 4. Transition state structure for the base-promoted
fragmentation of a 2-arylsulfanyl alcohol radical cation.
However, the observed increase of kOH/kOD as the base
becomes stronger clearly indicates that the structure of
the transition state in Figure 4 has to be variable.
Namely, as we move to stronger bases, the role of the
cleavage of the OH bond in the transition state becomes
progressively more important than the cleavage of the
C-C bond. In other words, the bond cleavage in the
transition state is concerted but not synchronous. Fol-
lowing the language used in â-elimination reactions, to
which these fragmentation have been correctly assimi-
lated,8 these transition states might be indicated as
E1cb-like.
intermediate, but it should be formed in a fast step
following the formation of the radical zwitterion (Scheme
2, paths a, c, and e). A slow formation of the alkoxyl
radical (Scheme 2, paths d and e) should exhibit a rate
dependence on the stability of the radical cation, which
is not observed.33
In this respect, it is interesting to note that convincing
evidence for the formation of an alkoxyl intermediate in
a fragmentation of 2-hydroxy sulfanyl radical cation has
been presented recently by Li and Kutateladze for the
case of 2-(R-hydroxyneopentyl)-1,3-dithianyl radical cat-
ion (7+•).13 In this system, the sulfur atoms are part of a
six-member cycle and it is possible that there is a kinetic
barrier for the Grob fragmentation because of orbital
misalignment between the SOMO on sulfur and the
scissile C-C bond. This results in a stepwise fragmenta-
tion mechanism with the slow formation of the alkoxyl
radical.
As suggested for the reactions of anilinium radical
cations,8,9,31 the concerted mechanism reported in Figure
4 should be favored by an anti relationship between the
charged sulfur and the hydroxy oxygen and the C1 and
C2 phenyl groups (stereoelectronic effect). Indeed, we
found that with pyridine, the erythro form 1+• is about 2
times more reactive than the threo form 4+• (Table 3), in
agreement with the fact that the above-mentioned anti
relationship can be better obtained in the erythro than
in the threo diastereomer (vide infra). However, practi-
cally the same difference in the reactivity between the
two diastereomers has also been observed in the absence
of bases (Table 2), where the mechanism should not
involve O-H bond breaking in the transition state.
Furthermore, DFT calculations (Supporting Information)
show that in order to reach the conformation with anti
OH and phenylsulfanyl groups, starting from the con-
formation with the absolute minimum of energy (where
the OH and the phenylsulfanyl groups are gauche), 4+•
has to spend ca. 3 kcal mol-1 more than 1+•. Thus, were
stereoelectronic effects important, the difference in the
fragmentation rate of the two diastereomers would be
much higher than that actually observed. In conclusion,
stereoelectronic effects seem to play a very minor role in
the fragmentation of 2-phenylsulfanyl alcohol radical
cations, probably due to a process that, as said before, is
shifted to the E1cb side of the mechanistic spectrum.
Given this situation, it is conceivable that by further
increasing the strength of the base, a limit can be reached
where we pass from a concerted to a stepwise mechanism
where the slow step is the formation of a radical zwitter-
ion (and accordingly the rate is almost diffusion con-
trolled) that then undergoes fast C-C bond cleavage to
the reaction products (E1cb mechanism, Scheme 2, paths
a and b). This probably occurs with the much stronger
base, 4-(dimethylamino)pyridine (DMAP), since with this
base, 1+•, 2+•, and 3+• react at practically the same rate,32
as predicted by the stepwise mechanism (Scheme 2, paths
a and b). An alkoxyl radical that undergoes a fast
â-fragmentation might also be involved as a reaction
Summary and Conclusions
By laser flash and steady-state photolysis studies, the
formation of 2-arylsulfanyl alcohol radical cations 1+•-
4
+• and their C-C bond fragmentation, a heterolytic-type
process affording an R-phenylsulfanyl-substituted carbon
radical and benzaldehyde, has been shown. The frag-
mentation reactions exhibit first-order kinetics with rates
that increase as the C-C bond dissociation energy (BDE)
in the radical cation decreases. Since the fragmentation
rate remains the same when the alcoholic OH is replaced
by OD (kOH/kOD ) 1), a transition state structure is
suggested with partial C-C bond cleavage and an intact
(33) One might also suggest that the mechanism is stepwise (Scheme
2, paths a, c, and e) with all bases and that the slow step is the
formation of the zwitterion (path a) with the stronger base, 4-(di-
methylamino)pyridine, whereas it is the formation of the alkoxyl
radical (step c) with the other bases. However, in the latter case the
formation of the zwitterion should be reversible and no clean first-
order kinetics should have been observed due to a retarding effect of
the protonated base.
(31) Ci, X.; Kellett, M. A.; Whitten, D. G. J. Am. Chem. Soc. 1991,
113, 3893-3894.
(32) With bases stronger than DMAP, the fragmentation rate was
too fast to be measured with our laser flash photolysis apparatus.
8328 J. Org. Chem., Vol. 69, No. 24, 2004

2-Arylsulfanyl Alcohol Radical Cations
diphenyl-2-arylsulfanylethanols. 1-Phenyl-2-phenylsulfanyle-
thanol and 2-phenyl-2-phenylsulfanylethanol were both present
in the crude reaction mixture. 1-Phenyl-2-phenylsulfanyl-
ethanol (48% yield) was obtained as a colorless oil.
O-H bond. Such a transition state can be stabilized by
formation of a hydrogen bond between the partially
positive OH group and the solvent. It was found that the
fragmentation rate of a 2-sulfanyl alcohol radical cation
is significantly lower than that of a structurally similar
2-hydroxy anilinium radical cation, which has similar
C-C BDE values. The difference is attributed to a less
efficient overlap between the SOMO in the heteroatom
and the s-orbital of the scissile C-C bond in the sulfide
radical cation than in the nitrogen counterpart.
Steady-State Photooxidation: General Procedure.
Photooxidation reactions were carried out in a Rayonet reactor
equipped with 16 lamps (3500 Å; 24 W each). A solution
containing the 2-arylsulfanyl alcohol (1 × 10-2 M) and NMQ+
(1 × 10-3 M) in N2-saturated MeCN was irradiated in a rubber
cap-sealed, jacketed tube for 10 min, thermostated at 25 °C
by a Peltier apparatus. An internal standard (4-methylbenzo-
phenone) was added, and product analysis (comparison with
The fragmentation rates of 1+•-4+• increase in the
presence of pyridine, and the second-order rate constants
for the base-induced process correlate with the base
strength, providing â Brønsted values that increase as
the E°red of the radical cation decreases. In the base-
1
authentic specimens) was carried out by GC, GC-MS, and H
NMR. In the case of photooxidations in the presence of pyridine
(1 × 10-2 M), the procedures were the same with the exception
that more NMQ+ (5 × 10-3 M) was used.
Fluorescence Quenching. Measurements were carried
out on spectrofluorometer. Relative emission intensities at 390
nm (NMQ+ emission maximum) were measured irradiating
at 315 nm (NMQ+ absorption maximum) and 25 °C a solution
containing NMQ+ (5 × 10-5 M) with the substrate at different
concentrations (from 0 to 5 × 10-4 M) in MeCN degassed by
argon.
assisted process, a deuterium kinetic isotope effect, kOH
/
kOD, is observed that increases with the strength of the
base: pyridine (1.3), 4-ethylpyridine (1.9), and 4-meth-
oxypyridine (2.3). This finding and the observation that
with the above three bases the rate decreases as the C-C
BDE increases suggest that C-C and O-H bond cleav-
ages are concerted but not synchronous, with the role of
OH bond breaking increasing as the base becomes
stronger (variable transition state). Accordingly, this
reaction appears to exhibit very small stereoelectronic
requirements (anti relationship between the sulfanyl and
OH groups), as 1+• (erythro) is only 2 times more reactive
than the threo diastereomer 4+•. With the much stronger
base 4-(dimethylamino)pyridine, a stepwise mechanism,
involving the formation of a zwitterion that then under-
goes fragmentation, seems more probable. Accordingly,
with this base the fragmentation rate turns out to be
independent of the C-C BDE.
Laser Flash Photolysis. Excitation wavelengths of 308 nm
(from a XeCl excimer laser, Lambda Physik, pulse width
ca. 15 ns and energy <3 mJ per pulse) and 355 nm (from a
Nd:YAG laser, Continuum, third harmonic, pulse width ca.
7 ns and energy <3 mJ per pulse) were used in nanosecond
flash photolysis experiments.37,38
The transient spectra were obtained by a point-to-point
technique, monitoring the change of absorbance (∆A) after the
laser flash at intervals of 5-10 nm over the spectral range
300-900 nm, averaging at least 10 decays at each wavelength.
The lifetime values (the time at which the initial signal is
reduced to 1/e, experimental error of (10%) are reported for
transients showing first-order decay kinetics. All solutions
were flowed through a quartz photolysis cell while nitrogen
or oxygen was bubbling through them. All measurements were
carried out at 22 ( 2 °C unless otherwise indicated.
Quantum Yields. A 3 mL solution of NMQ+ (8.8 × 10-4
M) and 1-3 (1.0 × 10-2 M) in N2-saturated MeCN was placed
in a quartz cell and irradiated at 313 nm, selected with a
Balzer interference filter by a high-pressure Hg lamp. The
photoproducts were quantified by GC analysis by use of
4-methyl-benzophenone as an internal standard. The substrate
conversion was held below 10% to avoid secondary reactions.
The light intensity (ca. 4 × 1014 photons s-1) was measured
by potassium ferric oxalate actinometry.
DFT Calculation of Radical Cation BDE Values. The
C-C heterolytic bond dissociation energies (BDEs) for the
radical cations were determined by quantum chemical calcula-
tions accomplished by the Gaussian 98 package.39 All geom-
etries were optimized at the B3LYP/6-31G(d) level.40,41 Single-
Experimental Section
Materials. 2-(Phenylsulfanyl)ethanol was purchased and
used as received. 1,2-Bis(phenylsulfanyl)-1,2-diphenylethane
(a 1:1 meso/d,l mixture) was prepared according to literature
procedure.34,35 N-Methylquinolinium tetrafluoroborate was
prepared according to a literature procedure.36 Acetonitrile and
toluene were used as received.
erythro-1,2-Diphenyl-2-arylsulfanylethanols (1-3). trans-
Stilbene oxide (20 mmol), potassium carbonate (21 mmol), and
the corresponding substituted thiophenol in ethanol (95%, 30
mL) were refluxed for 2.5 h, after which the mixture was cooled
and poured in water (20 mL) and extracted three times with
diethyl ether (3 × 20 mL). The collected organic layers were
then washed with diluted NaOH (100 mL) and with water,
dried over anhydrous Na2SO4, evaporated, and purified by
silica gel chromatography (petroleum ether-ethyl acetate,
gradient 1:0 to 4:1).
threo-1,2-Diphenyl-2-phenylsulfanylethanol (4). The
synthetic procedure is identical to that reported above for the
corresponding erythro isomer with the exception that cis-
stilbene was used. After purification, a colorless waxy solid
was obtained (68% yield).
1-Phenyl-2-phenylsulfanylethanol (5). Styrene oxide (42
mmol) was slowly added to a stirred solution of thiophenol (38
mmol) and potassium carbonate (58 mmol). The mixture was
refluxed for 2.5 h. The workup and purification procedures
were identical to those reported above for the erythro-1,2-
(37) Romani, A.; Elisei, F.; Masetti, F.; Favaro, G. J. Chem. Soc.,
Faraday Trans. 1992, 88, 2147-2154.
(38) Go¨rner, H.; Elisei, F.; Aloisi, G. G. J. Chem. Soc., Faraday
Trans. 1992, 88, 29-34.
(39) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.,
Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen,
W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.;
Pople, J. A. Gaussian 98, revision A.7; Gaussian, Inc.: Pittsburgh, PA,
1998.
(34) Alnajjar, M. S.; Franz, J. A. J. Am. Chem. Soc. 1992, 114,1052-
1058.
(35) Kauffman, T. Angew. Chem., Int. Ed. Engl. 1974, 13, 291-304.
(36) Donovan, P. F.; Conley, D. A. J. Chem. Eng. Data 1966, 11,
614.
(40) Parr, R. G.; Yang, W. In Density-Functional Theory of Atoms
and Molecules; Oxford University Press: New York, 1989.
J. Org. Chem, Vol. 69, No. 24, 2004 8329

Baciocchi et al.
point energies for the optimized structures were computed
using the MPW1P86 functional42 and the same basis set.
Conformational Analysis. For all species, optimized
geometries were determined at the B3LYP/6-31G(d) level40,41
by using a variety of different starting geometries in order to
minimize the possibility of finding structures that correspond
to a local but not a global energy minimum. In the energy-
minimized structure of the erythro radical cation (1+•) the
phenylsulfanyl and the OH groups are gauche, while the anti
conformer is only a local minimum, located 2.2 kcal mol-1
above the absolute minimum. A qualitatively similar situation
has been found even for the threo radical cation (4+•), but with
this diastereomer the energy difference between the gauche
and the anti conformations is higher (5.7 kcal mol-1).
Acknowledgment. MIUR, University “La Sapienza”
of Rome, and University of Perugia are thanked for the
financial support.
Supporting Information Available: Absorption maxima
and transients produced by LFP of NMQ+/toluene/sulfide in
N2-saturated MeCN; temperature effect on the decay rate
constant (kfr) of 2+•, 1+•, and 3+• in MeCN; Brønsted plot (log
kbase vs pKa) for fragmentation of 2+•, 1+•, and 3+• in MeCN;
analytical and spectroscopic data for compounds 1-5; and
optimized geometries (Cartesian coordinates) and calculated
energies for 1+•-5+• and their fragmentation products. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(41) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio
Molecular Orbital Theory; J. Wiley & Sons: New York, 1987.
(42) Yao, X.-Q.; Hou, X.-J-; Jiao, H.; Xiang, H.-W.; Li, Y.-W. J. Phys.
Chem. A 2003, 107, 9991.
JO0486544
8330 J. Org. Chem., Vol. 69, No. 24, 2004