Sulfur radical cations. Kinetic and product study of the photoinduced fragmentation reactions of (phenylsulfanylalkyl)trimethylsilanes and phenylsulfanylacetic acid radical cations

Article abstract of DOI:10.1021/jo051145x

Laser and steady-state photolysis, sensitized by NMQ⁺, of PhSCH(R)X 1-4 (R = H, Ph; X =SiMe₃, CO₂H) was carried out in CH₃CN. The formation of 1^+.-4^+. was clearly shown. All radical cations undergo a fast first-order fragmentation reaction involving C-Si bond cleavage with 1^+. and 2^+. and C-C bond cleavage with 3^+. and 4^+.. The desilylation reaction of 1^+. and 2^+. was nucleophilically assisted, and the decarboxylation rates of 3^+. and 4^+. increased in the presence of H₂O. A deuterium kinetic isotope effect of 2.0 was observed when H₂O was replaced by D₂O. Pyridines too were found to accelerate the decarboxylation rate of 3^+. and 4 ^+.. The rate increase, however, was not a linear function of the base concentration, but a plateau was reached. A fast and reversible formation of a H-bonded complex between the radical cation and the base is suggested, which undergoes C-C bond cleavage. It is probable that the H-bond complex undergoes first a rate determining proton-coupled electron transfer forming a carboxyl radical that then loses CO₂. The steady-state photolysis study showed that PhSCH₃ was the exclusive product formed from 1 and 3 whereas [PhS(Ph)CH-]₂ was the only product with 3 and 4.

Full text of DOI:10.1021/jo051145x

VOLUME 71, NUMBER 3
FEBRUARY 3, 2006
© Copyright 2006 by the American Chemical Society
Sulfur Radical Cations. Kinetic and Product Study of the
Photoinduced Fragmentation Reactions of
(Phenylsulfanylalkyl)trimethylsilanes and Phenylsulfanylacetic Acid
Radical Cations
Enrico Baciocchi,*,† Tiziana Del Giacco,*,‡ Fausto Elisei,‡ and Andrea Lapi†
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, Via Elce di sotto 8, 06123 Perugia, Italy
enrico.baciocchi@uniroma1.it; dgiacco@unipg.it
ReceiVed June 7, 2005
Laser and steady-state photolysis, sensitized by NMQ+, of PhSCH(R)X 1-4 (R ) H, Ph; X dSiMe3,
CO2H) was carried out in CH3CN. The formation of 1+•-4+• was clearly shown. All radical cations
undergo a fast first-order fragmentation reaction involving C-Si bond cleavage with 1+• and 2+• and
C-C bond cleavage with 3+• and 4+•. The desilylation reaction of 1+• and 2+• was nucleophilically
assisted, and the decarboxylation rates of 3+• and 4+• increased in the presence of H2O. A deuterium
kinetic isotope effect of 2.0 was observed when H2O was replaced by D2O. Pyridines too were found to
accelerate the decarboxylation rate of 3+• and 4+•. The rate increase, however, was not a linear function
of the base concentration, but a plateau was reached. A fast and reversible formation of a H-bonded
complex between the radical cation and the base is suggested, which undergoes C-C bond cleavage. It
is probable that the H-bond complex undergoes first a rate determining proton-coupled electron transfer
forming a carboxyl radical that then loses CO2. The steady-state photolysis study showed that PhSCH3
was the exclusive product formed from 1 and 3 whereas [PhS(Ph)CH-]2 was the only product with 3
and 4.
Introduction
Like most radical cations, sulfide radical cations can undergo
a fragmentation process involving the cleavage of a bond â to
the positive charge (Scheme 1, where X can be H, CR3, SiR3,
CO2H, etc.). This reaction, in general, has interesting implica-
tions concerning the structural effects upon the fragmentation
rate and mechanism.2 Moreover, it generally leads to carbon
radicals and can also have practical applications.3 In this respect,
it can be mentioned that the process depicted in Scheme 1 can
effectively be employed for the photoinitiation of free-radical
In the past decade, the chemistry of sulfide radical cations
has continued to attract considerable interest for the theoretical
and practical aspects as well as the important biological
implications.1
* To whom correspondence should be addressed. (E.B.) Tel: +39-06-
49913711. Fax: +39-06-490421. (T.D.G.) Tel: +39-075-5855559.
† Universita` “La Sapienza”.
‡ Universita` di Perugia.
10.1021/jo051145x CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/29/2005
J. Org. Chem. 2006, 71, 853-860
853

Baciocchi et al.
SCHEME 1
polymerization.3e Moreover, it has also been exploited as a
mechanistic probe in the enzymatic oxidations of sulfides.1t,4
In this context, studies of the fragmentation reactions of
sulfide radical cations aimed at acquiring information on the
structural effects upon kinetics and mechanism are certainly
warranted.
Recently, we have reported a very detailed kinetic study of
the fragmentation reactions of 2-arylsulfanyl alcohol radical
cations [Scheme 1, X ) CH(OH)Ph] that involve the OH-
assisted cleavage of a C-C bond.1c We have now carried out
a kinetic and products study of the fragmentation reactions of
the radical cations of silyl sulfides 1 and 2 and the thioacetic
acids 3 and 4.
FIGURE 1. Time-resolved absorption spectra of the NMQ+ (1.1 ×
10-3 M)/toluene (1 M)/3 (1.0 × 10-2 M) system in CH3CN: (a) N2-
saturated, recorded 0.016 (4), 0.35 (2), 1.4 (O), and 3.2 (b) µs after
the laser pulse and (b) O2-saturated, recorded 0.016 (4), 0.35 (2), 1.4
(O), and 3.2 (b) µs after the laser pulse. Insets: decay kinetics recorded
at 530 nm (1) and 340 nm (2). λecc ) 355 nm.
The fragmentation reaction involves the cleavage of the C-Si
bond in 1+• and 2+• and the cleavage of the C-C bond, with
CO2 loss, in 3+• and 4+•. Some information on the decarboxy-
lation of 3+• has already been reported,1h but no kinetic study
was carried out. The radical cations 1+•-4+• were generated
by sensitized photolysis, mostly with N-methylquinolinium
SCHEME 2
tetrafluoroborate (NMQ+) as the sensitizer. Fragmentation
kinetics were studied by the laser flash photolysis techniques,
and products were determined in steady-state photolysis experi-
ments. The results of this study are reported herein.
(1) (a) Filipiak, P.; Hug, G. L.; Carmichael, I.; Korzeniowska-Sobczuk,
A.; Bobrowski, K.; Marciniak, B. J. Phys. Chem. A 2004, 108, 6503. (b)
Huang, M. L.; Rauk, A. J. Phys. Chem. A 2004, 108, 6222. (c) Baciocchi,
E.; Del Giacco, T.; Elisei, F.; Gerini, M. F.; Lapi, A.; Liberali, P.; Uzzoli,
B. J. Org. Chem. 2004, 69, 8323. (d) Baciocchi E.; Gerini, M. F. J. Phys.
Chem. A 2004, 108, 2332. (e) Scho¨neich, C.; Pogocki, D.; Hug, G. L.;
Bobrowski K. J. Am. Chem. Soc. 2003, 125, 13700. (f) Scho¨neich, C. Arch.
Biochem. Biophys. 2002, 397, 370. (g) Butterfield D. A.; Kanski, J. Peptides
2002, 23, 1299 (h) Korzeniowska-Sobczuk, A.; Hug, G. L.; Carmichael,
I.; Bobrowski, K. J. Phys. Chem. A 2002, 106, 9251. (i) Gawandi, V. B.;
Mohan, H.; Mittal, J. P. J. Phys. Chem. A 2000, 104, 11877. (j) Bonifae`iæ,
M.; Hug, G. L.; Scho¨neich, C. J. Phys. Chem. A 2000, 104, 1240. (k) Glass,
R. S. Topics Curr. Chem. 1999, 205, 1. (l) Yokoi, H.; Hatta, A.; Ishiguro,
K.; Sawaki Y. J. Am. Chem. Soc. 1998, 120, 12728. (m) Miller, B. L.;
Kuczera, K.; Scho¨neich, C. J. Am. Chem. Soc. 1998, 120, 3345. (n) Adam,
W.; Argu¨ello, J. E.; Pen˜e´n˜ory, A. B. J. Org. Chem. 1998, 63, 3905. (o)
Ioele, M.; Steenken, S.; Baciocchi, E. J. Phys. Chem. A 1997, 101, 2979.
(p) Fasani, E.; Freccero, M.; Mella, M.; Albini, A. Tetrahedron 1997, 53,
2219. (q) Mohan, H.; Mittal, J. P. J. Phys. Chem. A 1997, 101, 10012. (r)
Miller, B. L.; Williams, T. D.; Scho¨neich, C. J. Am. Chem. Soc. 1996, 118,
11014. (s) Goez, M.; Rozwadowski, J.; Marciniak, B. J. Am. Chem. Soc.
1996, 118, 2882. (t) Baciocchi, E.; Lanzalunga, O.; Malandrucco, S.; Ioele,
M.; Steenken, S. J. Am. Chem. Soc. 1996, 118, 8973.
Results
Laser Flash Photolysis Studies. The laser photolysis experi-
ments were carried out in the presence of 1 M toluene as
cosensitizer. Under these conditions, the sequence of reactions
a and b, in Scheme 2, takes place (T ) toluene and S ) sulfide),
which leads to a significant increase in the yield of separated
radical cations.5
Upon laser excitation (λexc ) 308 and/or 355 nm) of CH3CN
solutions of NMQ+/toluene/1-4, time-resolved absorption
spectra and decay kinetics were recorded under N2- and O2-
saturated conditions. Since the results were very similar for all
sulfides examined, only those concerning the sulfides 3 and 4
will be illustrated in detail. Absorption maxima and transients
observed with the other substrates are reported in the Supporting
Information (Table S1).
(2) (a) Gould, I. R.; Lenhard, J. R.; Farid, S. J. Phys. Chem. A 2004,
108, 10949 and references therein. (b) Shukla, D.; Liu, G.; Dinnocenzo, J.
P.; Farid, S. Can. J. Chem. 2003, 81, 744. (c) Carra, C.; Ghigo, G.;
Tonachini, G. J. Org. Chem. 2003, 68, 6083. (d) Freccero, M.; Pratt, A.;
Albini, A.; Long, C. J. Am. Chem. Soc. 1998, 120, 284. (e) Zhang, X.;
Yeh, S.-R.; Hong, S.; Freccero, M.; Albini, A.; Falvey, D. E.; Mariano, P.
S. J. Am. Chem. Soc. 1994, 116, 4211.
By laser photolysis of 3 in N2-saturated solutions, three
absorption bands were detected just after the laser pulse in the
400, 530, and 750 nm regions (Figure 1a). They were assigned
to NMQ• (λmax) 400 and 550 nm)6 and the monomer (λmax
530 nm) and the π-type dimer sulfide radical cations (λmax
)
≈
(3) (a) Gould, I. R.; Godleski, S. A.; Zielinski, P. A.; Farid, S. Can. J.
Chem. 2003, 81, 777. (b) Gould, I. R.; Lenhard, J. R.; Muenter, A. A.;
Godleski, S. A.; Farid, S. Pure Appl. Chem. 2001, 73, 455. (c) Gould, I.
R.; Lenhard, J. R.; Muenter, A. A.; Godleski, S. A.; Farid, S. J. Am. Chem.
Soc. 2000, 122, 11934. (d) Albini, A.; Fagnoni, M.; Mella, M. Pure Appl.
Chem. 2000, 72, 1321. (e) Wrzszczyn˜ski, A.; Filipiak, P.; Hug, G. L.;
Marciniak, B.; Pc¸zkowski, J. Macromolecules 2000, 33, 1577.
750 nm).1l The time-evolution of the absorption spectra shows
that the signal decay recorded at 530 nm is coupled with the
growth of a further maximum at 340 nm region assigned to the
(5) Gould, I. R.; Ege, D.; Mosh, J. E.; Farid, S. J. Am. Chem. Soc. 1990,
112, 4290.
(6) Bockman, T. M.; Kochi, J. K. J. Am. Chem. Soc. 1989, 111, 4669.
(4) Pen˜e´n˜ory, A. B.; Argu¨ello, J. E.; Puiatti, M. Eur. J. Org. Chem. 2005,
10, 114.
854 J. Org. Chem., Vol. 71, No. 3, 2006

Sulfur Radical Cations
TABLE 1. Decay Rate Constants for Fragmentation of the Sulfide
Radical Cations [PhSCH(R)X]+• in CH3CN
radical cation
additives
ka (106 s-1
)
R ) H, X ) SiMe3 (1+•
)
1.2
36
R ) Ph, X ) SiMe3 (2+•
)
R ) H, X ) CO2H (3+•
)
0.31
1.6
4.1
1.1
9.3
4.5
24
H2O (2%)
H2O (5%)
R ) Ph, X ) CO2H (4+•
)
H2O (2%)
D2O (2%)
H2O (5%)
a Experimental error of (10%.
FIGURE 2. Time-resolved absorption spectra of the NMQ+ (1.1 ×
10-3 M)/toluene (1 M)/4 (9.2 × 10-3 M) system in CH3CN: (a) N2-
saturated, recorded 0.032 (4), 1.0 (2), 3.6 (O) and 6.4 (b) µs after the
laser pulse and (b) O2-saturated, recorded 0.032 (4), 0.75 (2), 2.7 (O)
and 6.4 (b) µs after the laser pulse. Insets: decay kinetics recorded at
520 nm (1) and 360 nm (2). λecc ) 355 nm.
absorption of the radical PhSCH2• (Figure 1a, inset).1o Accord-
ingly, in O2-saturated solutions, the time-resolved absorption
spectra are modified by the fast decay of PhSCH2 and NMQ•
•
FIGURE 3. Observed rate constant for the fragmentation of 1+• vs
which are efficiently quenched by molecular oxygen; thus, the
absorption in the 330-400 nm region decreases and the only
recorded transient concerns the radical cation in its monomeric
(λmax ) 530 nm) and dimeric (λmax ≈ 750 nm, Figure 1b) form.7
The laser photolysis of 4 gives similar results in terms of
absorption bands, time-evolution, and corresponding assign-
ments, both in oxygen- and nitrogen-saturated solutions (Figure
2). Small differences are only in the absorption maximum of
the monomer radical cation (λmax ) 520 nm) and the radical
PhSC•H(Ph) (λmax ) 360 nm).
CH3OH concentration in CH3CN.
Interestingly, it was observed that also for the carboxylic acids
the first-order decay of the radical cations is significantly
accelerated by addition of water. As shown in Table 1, the
addition of 5% H2O (v/v) increases the rate by more than 10
times with 3+• and more than 20 times with 4+•. When D2O
replaced H2O, the decay rate of 4+• decreased by a factor of
ca. 2.
Pyridine too accelerated the decarboxylation rate of 3+• and
4+•. Very interestingly, in both cases the plot of the observed
rate constants against pyridine concentrations was not linear but
reached a plateau at higher concentrations of pyridine as shown
in Figure 4. The kobs value at the plateau was almost the same
(ca. 2 × 107 s-1) for 3+• and 4+•. The same phenomenon was
observed with 4-cyanopyridine, but the rate constant corre-
sponding to the plateau was lower than with pyridine (ca. 4 ×
106 s-1, Figure S1, Supporting Information).
Laser photolysis experiments were also carried out with the
tetramethylammonium salts PhSCH2CO2NMe4 and PhSCH-
(Ph)CO2NMe4 formed from 3 and 4, respectively. The sensitizer
was 1,4-dicyanonaphthalene (DCN) since with NMQ+ there
were problems in the laser photolysis experiments, probably
due to some interaction between the cationic sensitizer and the
carboxylate. In these experiments, however, no transient at-
tributable to a zwitterionic radical cation was observed. Only
signals corresponding to the carbon radical were detected
together with those of DCN-• (λmax ) 390 nm)9 and the triplet
state of DCN (400-500 nm)9 (Figure S2, Supporting Informa-
tion). Probably, the decarboxylation of the zwitterionic radical
The decay rate of radical cations 1+•-4+• was determined
by following, in the presence of nitrogen, the decay of the dimer
radical cation around 750 nm and, in the presence of oxygen,
the absorption decay at about 530 nm (due to the monomer
radical cation). In both cases, the decay kinetics followed first-
order laws and provided almost identical values for the decay
rate constants. With compounds 2 and 4, in the presence of
nitrogen, it was also possible to obtain the decay rate constant
of the radical cation from the absorption rise due to the formation
of the carbon radical. All values of the first-order rate constants
(kobs) are reported in Table 1.
As already observed for the desilylation of aryltrimethylsilane
radical cations,8 addition of nucleophiles, e.g., MeOH, increased
the decay rate of 1+• and 2+•. For 1+•, a plot of kobs against
methanol concentration (Figure 3) was linear affording a second-
order rate constant of 3.5 × 106 M-1 s-1 for the MeOH-induced
desilylation. With 2+•, the MeOH-induced decay was too fast
to be measured.
(7) Actually, in the experiments under O2, a transient absorbing below
400 nm is also observed. This transient was not identified, but clearly, there
is no link between its formation and the decay of the radical cation that
occurs at the same rate than under nitrogen.
(8) Dockery, K. P.; Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.; Gould,
I. R.; Todd, W. P. J. Am. Chem. Soc. 1997, 119, 1876.
(9) Das, P. K.; Muller, A. J.; Griffin, G. W. J. Org. Chem. 1984, 49,
1977.
J. Org. Chem, Vol. 71, No. 3, 2006 855

Baciocchi et al.
TABLE 2. Steady-State Photosensitized Oxidations of 1-4 in
Deareated CH3CNa
sensitizer
additivesb
products (yield, %)c
PhSCH2SiMe3 (1)
NMQ+
PhSCH3 (27)
PhSCH3 (72)
PhSCH2D (70)
PhSCH3 (38)
PhSCH3 (11)
PhSCH3 (41)
H2O
D2O
TPP
TCB
DCN
H2O
PhSCH2CO2H (3)
NMQ+
H2O
pyridine
H2O
PhSCH3 (16)
PhSCH3 (55)
PhSCH3 (6.0)
TCB
PhSCH(Ph)SiMe3 (2)
NMQ+
TPP
TCB
DCN
H2O
H2O
[PhSC(Ph)H-]2 (6)
[PhSC(Ph)H-]2 (5)
[PhSC(Ph)H-]2 (2)d
PhSCH2Ph (92)
PhSCH(Ph)CO2H (4)
NMQ+
H2O
pyridine
[PhSC(Ph)H-]2 (5)
[PhSC(Ph)H-]2 (4)
a General conditions: substrate 10-2 M, sensitizer 10-3 M, T ) 25 °C,
irradiation time ) 30 min. b When present, H2O ) 5% v/v, pyridine 2 ×
10-2 M, D2O ) 5% v/v. c Referred vs the substrate. Determined by GC or
1H NMR in the case of 5. d Adducts of the PhSC•H(Ph) radical with the
sensitizer were also detected.
FIGURE 4. Observed rate constant for the decay of 3+• (a) and 4+•
(b) vs pyridine concentration in CH3CN.
cation occurs immediately after the laser pulse as also observed
by Filipiak et al.1a
SCHEME 3
Steady-State Experiments. Photooxidation reactions were
carried out in a photoreactor equipped with 10 lamps (360 nm;
14 W each) and thermostated at 25 °C. The substrate concentra-
tion was 1 × 10-2 M and that of the sensitizer 1 × 10-3 M in
Ar-saturated CH3CN. Generally, the sensitizer was NMQ+, as
in the laser photolysis experiments. However, 2,4,6-tri-
phenylpyrylium tetrafluoroborate (TPP), 1,2,4,5-tetracyanoben-
zene (TCB), and 1,4-dicyanonaphthalene (DCN) were also used
for reasons which will be discussed below. Since the kinetic
investigation had shown that the decay of all radical cations is
accelerated by the presence of H2O, most photolyses were
performed in CH3CN containing 5% of H2O in order to have
more efficient and reproducible reactions. The reaction time was
30 min in all cases. All results are collected in Table 2.
The silyl derivative 1 and the carboxylic acid 3 exhibited
very similar behaviors. With NMQ+ as the sensitizer and in
CH3CN-5% H2O, both compounds produced thioanisole as the
only reaction product in a process which did not consume the
sensitizer (Scheme 3). The same outcome was observed when
the photolysis of 1 was sensitized by TPP or TCB. When 1 and
3 were photolyzed in CH3CN-5% D2O a selective incorporation
of a single deuterium atom into the methyl group of thioanisole
was observed. However, no incorporation of deuterium in the
product was observed when the photolysis of 1 was carried out
in CD3CN.
obtained with TPP and TCB as the sensitizers. However, with
DCN, 2 afforded a high yield of benzyl phenyl sulfide and no
consume of sensitizer was observed. All results are collected
in Table 2.
Discussion
Kinetic Results. The laser photolysis experiments together
with the steady state ones have clearly shown that the photolysis
of 1-4 sensitized by NMQ+ leads to the formation of the
corresponding radical cations and that these species decay,
forming an R-thiophenyl carbon radical (Scheme 4).
The C-Si bond cleavage in 1+• and 2+• (reaction a in Scheme
4) is nucleophilically assisted. In the absence of added nucleo-
philes, the assistance to C-Si bond cleavage can be provided
by the solvent itself or more probably by the residual water
present in CH3CN. A quantitative assessment of such a
nucleophilic assistance was determined for MeOH (Table 1),
and it is of interest to compare the second-order rate constant
for the MeOH-assisted fragmentation of 1+• (3.5 × 106 M-1
s-1) with the value of 7 × 105 M-1s-1 found for the MeOH-
assisted desilylation of the radical cation of PhN(Me)CH2SiMe3
(6) in CH3CN.11 Since 6+• is much more stable than 1+•,12
A completely different situation was found with 2 and 4. In
this case, with NMQ+, both substrates produced 1,2-bis-
(phenylsulfanyl)-1,2-diphenylethane (a meso/d,l mixture) (5) as
the only product (Scheme 3).
These reactions were much less efficient than those of 1 and
3, simply because in this case the sensitizer is consumed and
only 10% of sensitizer with respect to the substrate was used.10
On the other hand, if the reaction time is lengthened or more
catalyst is used some photolysis of 5 also occurred, which
complicated the product analysis. The same results were
(10) No effort was made to determine the fate of the quinoline radical
that probably undergoes a dimerization process.
(11) Su, Z.; Mariano, P. S.; Falvey, D. E.; Yoon, U. C.; Oh, S. W. J.
Am. Chem. Soc. 1998, 120, 10676.
856 J. Org. Chem., Vol. 71, No. 3, 2006

Sulfur Radical Cations
SCHEME 4
SCHEME 5
SCHEME 6
process.2b Evidently, in the reaction of silyl derivatives such a
kinetic factor is insufficient to overcome the very large driving
force for the formation of the phenyl-substituted carbon radical.
Passing to discuss the reactivity data of the carboxylic acids,
it can be first noted that the difference in reactivity between
3+• and 4+• (ca. 3 times) is much smaller than that between 1+•
and 2+•. It seems therefore that in this case the stability of the
formed carbon radical has little influence on the fragmentation
rate of the carbon radical. It is also noteworthy that with both
whereas the formed carbon radicals have very close stability,13
the observation of a similar desilylation rate is noteworthy.
Accordingly a much slower desilylation of 6+• would have been
expected on purely thermodynamic grounds. An intrinsic larger
reactivity of a nitrogen radical cation with respect to that of a
structurally similar sulfur radical cation was already observed
by comparing the C-C bond cleavage reactions of â-amino
alcohol and â-sulfide alcohol radical cations.1c The phenomenon
was attributed to a more favorable orbital overlap between the
SOMO on the heteroatom and the σ orbital of the scissile bond
in the former than in the second radical cation. Thus, in a
nitrogen radical cation, the intrinsic barrier for the intramolecular
electron transfer leading to the cleavage should be lower than
in a sulfur radical cation. It is reasonable to suggest that this
explanation also holds for the C-Si bond cleavage reaction.
Of some interest is also the comparison between the MeOH-
assisted decay rate of 1+• and that (5.5 × 106 M-1 s-1) of the
radical cation of 4-methoxybenzyltrimethylsilane (MBTM+•).14
The rate constants are very similar, although the reduction
potential of 1+• is significantly lower than that of MBTM (1.35
V vs SCE)2d and the benzyl radical is significantly more stable
3
+• and 4+•, the decay rate constants are significantly increased
by addition of H2O. With 5% H2O, there is an increase in rate
of at least an order of magnitude. A possible suggestion is that
in the radical cation C-C bond cleavage is concerted with CO2H
deprotonation. Thus, as shown in Scheme 5, the favorable effect
of H2O might be due to its capacity of assisting O-H bond
breaking by hydrogen bond formation.
A mechanism where C-C bond cleavage is concerted with
O-H proton transfer also accounts for the deuterium kinetic
isotope effect (kH/kD ) 2) which was observed when the reaction
was carried out in the presence of D2O, which indicates that
the transfer of the proton is part of the reaction coordinate.
Pyridines too had a favorable effect on the decay rate of 3+•
and 4+• but exhibited the peculiar behavior shown in Figure 4.
Such behavior is typical of a reaction which involves first the
reversible and fast formation of an intermediate between the
radical cation and the base that then decomposes to products.16
In this case, the intermediate might reasonably be a H-bonded
complex between the radical cation and pyridine since the
possibility that it is the radical zwitterion formed by deproto-
nation of the radical cation can be excluded since the rate
constants corresponding to the plateau are different for pyridine
and 4-cyanopyridine.17 Thus, the reaction mechanism may be
that depicted in Scheme 6.
• 15
than PhSCH2 . Thus, again on thermodynamic grounds, we
would expect a larger rate for MBTM+•. A plausible explanation
is that in 1+• the positive charge, mostly located on sulfur, is
very close to the C-Si bond, whereas in MBTM+• it is largely
localized in the aromatic ring where it is stabilized by the
methoxy group. Therefore, the intrinsic barrier for intramolecular
electron transfer should be smaller with 1+• than with MBTM+•.
An additional observation concerning the reactivity of silyl
derivatives (Table 1) is that the desilylation rate is ca. 30 times
faster for 2+• than for 1+•. This difference may be reasonably
attributed to the much higher stability of the more delocalized
phenyl substituted carbon radical formed in the former case.
However, it has been recently suggested that in fragmentation
reactions of radical cation the possibility of delocalization of
spin or charge in one of the fragments may be an unfavorable
factor since it should increase the intrinsic barrier of the
When the reverse reaction rate of the H-bonded complex to
reactants, kr, is much faster than its fragmentation rate, kd, eq 1
holds, where K is the formation constant of the complex and
[Py] is the initial concentration of pyridine (in strong excess
with respect to that of the radical cation).16,18
(12) (a) The oxidation potential of 6 should be significantly lower than
that of N,N-dimethylaniline (0.77 V vs SCE)12b and, hence, much lower
than that of 1 that is 1.15 V vs SCE.1k (b) Parker, V. D.; Tilset, M. J. Am.
Chem. Soc. 1991, 113, 8778.
(13) (a) This is indicated by the similar bond energies of PhSCH2-H
and PhN(CH3)CH2-H, 9313b and 91.7 kcal/mol,13c respectively. (b) Zhang,
X. M.; Bordwell, F. G.; Van Der Puy, M.; Fried, H. E. J. Org. Chem. 1993,
58, 3060. (c) Dombrowski, G. W.; Dinnocenzo, J. P.; Farid, S.; Goodman,
J. L.; Gould, I. R. J. Org. Chem. 1999, 64, 427.
(14) Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.; Gould, I. R.; Mattes,
S. L.; Todd, W. P. J. Am. Chem. Soc. 1989, 111, 8973.
(15) (a) This is indicated by the different bond energies of PhSCH2-H
and PhSCH(Ph)-H, 9313b and 82.2 kcal/mol,15b respectively. (b) Measured
in DMSO. Bordwell, F. G.; Liu, W.-Z. J. Phys. Org. Chem. 1998, 11, 397.
1/kobs ) 1/kd + 1/kdK[Py] (1)
Thus, a plot of 1/kobs vs 1/[Py] should give a linear trend
observed with both 3+• and 4+• (Figures 5 and S1, Supporting
Information).
(16) Cox, B. G. Modern Liquid-Phase Kinetics; Compton, R. G., Davies,
S. G., Evans, J., Eds.; Oxford University Press: Oxford, U.K., 1994; pp
31-32.
(17) Moreover, in the laser photolysis experiments carried out with the
carboxylate anions it was not possible to observe the radical cation as it
decays immediately after the laser pulse.
J. Org. Chem, Vol. 71, No. 3, 2006 857

Baciocchi et al.
SCHEME 8
pyridine (pKa ) 12.3 in CH3CN)20 and 4-cyanopyridine (pKa
) 8.0 in CH3CN),20 2.8 × 107 s-1 vs 4.3 × 106 s-1, is rather
small, indicating that step a in Scheme 7 is almost insensitive
(â ∼0.19) to the strength of the base.
Product Formation. There is no doubt that, for all substrates,
the products observed in the steady-state photolyses derive from
the carbon radicals formed in the fragmentation reactions
described in Scheme 4. The results reported in Table 2 clearly
indicate that both for the acids and the silanes the fate of the
carbon radical appears to depend on the nature of the group R.
Accordingly, when R ) H (compounds 1 and 3), the carbon
•
radical is PhSCH2 and the exclusive reaction product is
thioanisole, while, when R ) Ph (compounds 2 and 4), the
product is the 1,2-bis(phenylsulfanyl)-1,2-diphenylethane (5),
that is the dimer of the carbon radical PhSC•H(Ph) (Scheme
3).
FIGURE 5. 1/kobs vs 1/[pyridine] for the decay of 3+• (a) and 4+• (b)
in CH3CN.
In the reactions leading to thioanisole the sensitizer acted as
a catalyst and significant yields of product were obtained. In
the reactions leading to 5, the sensitizer was consumed and the
yield of product (corresponding to the initial amount of
sensitizer) was much lower.
SCHEME 7
Further investigation of the reaction leading to thioanisole
was deemed necessary, as it represents an interesting case of a
nonoxidatiVe desilylation with 1 and a nonoxidatiVe decarboxy-
lation with 3, obtained under oxidizing conditions. Thus, to
determine the origin of hydrogen which converts PhSCH2• into
PhSCH3, experiments were carried out in CD3CN and in CH3-
CN in the presence of D2O. No incorporation of D into
thioanisole was observed in the former whereas, when the
photolyses of 1 and 3 were run in CH3CN with 5% of D2O,
PhSCH2D was the reaction product. These experiments exclude
a free radical hydrogen abstraction for the conversion of
The reciprocal of the line intercept provides, kd values of 2.8
× 107 s-1 and 2.2 × 107 s-1 for the H-bonded complex from
3+• and 4+•, respectively, that are very close to the values of
kobs corresponding to the plateau in Figure 4. With 3+• and
4-cyanopyridine (Figure S1, Supporting Information), kd is 4.3
× 106 s-1, which compares with the value of 4 × 106 s-1 for
the kobs corresponding to the plateau in Figure S1 (Supporting
Information).
The observation of similar values of kd for 3+• and 4+• is
very remarkable since it indicates that there is no influence of
the stability of the formed radical upon the fragmentation rate
of the H-bonded complex. This suggests that the fragmentation
may occur in two steps. In the first step, the hydrogen-bonded
radical cation undergoes a proton-coupled intramolecular elec-
tron transfer leading to a carboxyl radical (Scheme 7, path a).
In the second step, the carboxyl radical loses CO2 and produces
the carbon radical (Scheme 7, path b).
If the first step is rate determining, no effect of the stability
of the formed radical upon the fragmentation rate of the complex
is expected. That an electron-transfer process be coupled to
proton transfer is a mechanistic possibility that has received
great attention and support in recent times.19 Thus, the sugges-
tion that also an intramolecular electron transfer can be coupled
to a proton transfer as in path a of Scheme 7 seems reasonable.
A final notation is that, with 3+•, the difference in kd between
•
PhSCH2 into PhSCH3 and suggest instead an ionic process.
•
A reasonable hypothesis might be that PhSCH2 is reduced
by NMQ•, with regeneration of NMQ+. A carbanion is formed
that reacts with H2O/H+ to produce thioanisole (Scheme 8). This
hypothesis, however, seems unlikely on the following grounds.
•
First, the reduction potential of PhSCH2 that can be estimated
to be around -1.13 V vs SCE21 is more negative than that of
•
NMQ+ (-0.85V vs SCE).22 Thus, the reduction of PhSCH2
by NMQ• is endergonic. Second, PhSC•H(Ph) is expected to
•
be more easily reducible than PhSCH2 , as its reduction potential
•
(-0.926 V vs SCE)15b is less negative than that of PhSCH2
and it is therefore difficult to explain why this radical is not
reduced under the same reaction conditions. Third and more
significantly, conversion of 1 and 3 into PhSCH3 was also
obtained when photosensitizers with much less negative reduc-
tion potential than NMQ+ like TCB (Ered ) -0.66 vs SCE)23
(20) Schlesener, C. J.; Amatore, C.; Kochi, J. K. J. Am. Chem. Soc. 1984,
106, 6, 7472.
(18) The treatment neglects the decay rate of the radical cation in the
absence of pyridine. This approximation is justified as this rate is
significantly less than that of the H-bonded complex and the constant for
the formation of the complex should be quite large. From the slope and
intercept of the plots in Figure 5, values of 124 and 125 M-1 are obtained
for 3+• and 4+•, respectively.
(21) (a) Value estimated in DMSO using the Bordwell equation (BDE
) 1.37pKa + Ered(radical, vs NHE)23.1 + 56)21b where BDE of PhSCH2-H
is 93 kcal/mol,13b and the pKa of PhSCH3 in DMSO is 42.21c (b) Bordwell,
F. G.; Cheng, J.; Ji, G. Z.; Satish, A. V.; Zhang, X. J. Am. Chem. Soc.
1991, 113, 9790. (c) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456.
(22) Yoon, U. C.; Quillen, S. L.; Mariano, P. S.; Swanson, R.; Stavinoha,
J. L.; Bay, E. J. Am. Chem. Soc. 1983, 105, 1204-1218.
(19) Shukla, D.; Young, R. H.; Farid, S. J. Phys. Chem. A 2004, 108,
10386 and references therein.
858 J. Org. Chem., Vol. 71, No. 3, 2006

Sulfur Radical Cations
with NMQ+ as the sensitizer, lead to the corresponding radical
cations that undergo a fragmentation reaction involving C-Si
bond cleavage (1+• and 2+•) and C-C bond cleavage (3+• and
4+•). An R-thiophenyl carbon radical is produced, namely
SCHEME 9
and even TPP (-0.24 vs SCE)24 were used. On the other hand,
when 2 was photolyzed using a sensitizer, DCN, with a
reduction potential of -1.26V vs SCE,25 more negative of that
of PhSC•H(Ph), the reduction product PhSCH2Ph was obtained
in high yield (no consume of the sensitizer). Clearly, Scheme
8, with DCN in the place of NMQ+, holds in this case.
An alternative hypothesis to account for the non-oxidative
desilylation is that the carbon radical is protonated to form a
radical cation which then is reduced by NMQ• with regeneration
of NMQ+(Scheme 9).
The proton for reaction a in Scheme 9 may be provided by
the acid formed in the fragmentation reaction of the radical
cation (Scheme 4, paths a and b) and accordingly no thioanisole
was formed when the photolysis of 1 sensitized by TCB was
performed in the presence of a small amount of Na2CO3.26 Other
products were observed, mostly diphenyl disulfide and thiophe-
nol, probably coming from further photolysis of addition
PhSCH2 from 1+• and 3+• and PhSC•H(Ph) from 2+• and 4+•.
•
The products study showed that the two radicals undergo a
•
different fate: PhSCH2 is converted into PhSCH3 whereas
PhSC•H(Ph) dimerizes to [PhSC(Ph)H-]2. Evidence is presented
•
suggesting that PhSCH3 is formed by protonation of PhSCH2 .
A radical cation is produced that is then reduced to PhSCH3 by
NMQ•. Protonation of PhSC•H(Ph) may be much slower since
the basicity of this radical (pKa )-5.6) is much lower than that
PhSCH2• (pKa ) 3.6). Thus, the preferred reaction of PhSC•H-
(Ph) is the dimerization.
The kinetic study of the fragmentation reactions allowed a
number of conclusions. The rate of C-Si bond cleavage is very
sensitive to the stability of the formed carbon radical as 2+•
resulted 30 times more reactive than 1+•. This contrasts with
the decarboxylation reaction where 4+• was only 3-6 times
more reactive than 3+•, depending on the reaction conditions.
The rates of the decarboxylation reactions are significantly
accelerated by the presence of water, and the phenomenon
suggests that the C-C bond scission in the radical cation is
concerted with H2O-assisted CO2H deprotonation. Pyridine too
increases the fragmentation rates of 3+• and 4+•; the increase,
however, is not a linear function of base concentration, but a
plateau is reached at higher base concentrations. This result is
interpreted on the basis of a mechanism involving the fast and
reversible formation of a H-bonded complex between the radical
cation and the base. The following fragmentation of the
H-bonded complex probably to occurs in two steps. In the first
step (that is rate determining), a proton-coupled intramolecular
electron transfer takes place leading to a carboxyl radical. Fast
loss of CO2 from the carboxyl radical occurs in the second step.
•
products between PhSCH2 and the sensisitizer and/or its radical
anion.
In the framework of the mechanism in Scheme 9, the
drastically different behaviors of PhSCH2 and PhSC•H(Ph) may
•
be justified by the great difference in their pKa. Accordingly, it
can be calculated that in CH3CN the pKa of (PhSCH3)+• is 3.627
whereas that of (PhSCH2Ph)+• is -5.6.28 Thus, the protonation
of PhSCH2• may occur several orders of magnitude faster than
that of PhSC•H(Ph) so that only with the former radical it can
overcome the competitive dimerization reaction. In this respect,
it is noteworthy that pulse radiolysis experiments have shown
that the deprotonation of (PhSCH3)+• in H2O is too slow to be
measured (k < 30 s-1). In contrast, the deprotonation rate of
(PhSCH2Ph)+• is 2.5 × 103 s-1 1o
.
With respect to the above hypothesis, it can also be noted
that very recently Hanzlik and his associates suggested the
Experimental Section
•
protonation of the carbon radical Ph(Me)NCH2 to explain the
Materials. (Phenylsulfanylmethyl)trimethylsilane (1) and phen-
ylsulfanylacetic (3) acid were purchased and used as received.
Phenyl(phenylsulfanyl)acetic acid (4),31 [phenyl(phenylsulfanyl)-
methyl]trimethylsilane (2),32 1,2-bis(phenylsulfanyl)-1,2-diphenyl-
ethane (a 1:1 meso/d,l mixture) (5),33 and N-methylquinolinium
tetrafluoroborate34 were prepared according to literature procedures.
Tetramethylammonium salts of 3 and 4 were prepared by reacting
the corresponding acids with an equimolecular amount of tetram-
ethylammonium hydroxide in methanol and recrystallized from
dichloromethane/hexane. Thioanisole, 1,4-dicyanonaphthalene, 1,2,4,5-
tetracyanobenzene, 2,4,6-triphenylpyrilium tetrafluoroborate, py-
ridine, 4-cyanopyridine, acetonitrile (HPLC plus grade), and toluene
were purchased and used as received.
formation of N,N-dimethylaniline in the H2O2-induced oxidation
of N-methyl-N-phenylglycine catalyzed by horseradish peroxi-
dase (HRP).29 Our own observation that also the reaction of 3
with H2O2/HRP produces thioanisole30 shows that the process
may be general and to some extent lends support to the
mechanism proposed in Scheme 9.
Conclusions
Laser and steady-state photolysis in CH3CN of the silyl
sulfides 1 and 2 and the phenylthioacetic acids 3 and 4, mostly
(23) Mella, M.; Freccero, M.; Albini, A. Chem. Commun. 1995, 41.
(24) Dome´nech, A.; Garc`ıa, H.; Alvaro, M.; Carbonell, E. J. Phys. Chem.
B 2003, 107, 3040.
(25) Huang, Y.; Wayner, D. D. M. J. Am. Chem. Soc. 1994, 116, 2157.
(26) In this experiment, TCB was the sensitizer since with NMQ+ it
seems that there is some reaction of the cationic sensitizer with the basic
medium and the reaction was less efficient, anyway no thioanisole was
observed also in this case.
Laser Flash Photolysis. Excitation wavelengths of 308 nm (from
a XeCl excimer laser, 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.35
The transient spectra were obtained by a point-to-point technique,
monitoring the change of absorbance (∆A) after the laser flash at
(27) (a) Calculated on the basis of a thermochemical cycle by the equation
pKa ) 1/1.37[∆G°(PhSCH2-H) +23.06E°(H•/H+) -23.06E°(PhSCH3)]
where ∆G°(PhSCH2-H) is given by the BDE of the bond (93 kcal/mol)13b
minus the entropic contribution taken as 8 kcal/mol at 298 K, E°(H•/H+) is
-2.00 V vs SCE in CH3CN27b and E°(PhSCH3) is 1.47 V vs SCE.1k (b)
Wayner, D. D. M.; Parker, V. D. Acc. Chem. Res. 1993, 26, 287.
(28) As in ref 25 with BDE(PhSCPh-H) ) 82.2 kcal/mol15b and
E°(PhSCH2Ph) ) 1.55V vs SCE.1k
(31) Lehto, S.; Shirley, D. A. J. Org. Chem. 1957, 22, 989.
(32) Ager, D. J. J. Chem. Soc., Perkin Trans. 1 1986, 195
(33) (a) Alnajjar, M. S.; Franz, J. A. J. Am. Chem. Soc. 1992, 114,1052.
(b) Kauffman, T. Angew. Chem., Int. Ed. Engl. 1974, 13, 291-304.
(34) Donovan, P. F.; Conley, D. A. J. Chem. Eng. Data 1966, 11, 614.
(35) (a) Romani, A.; Elisei, F.; Masetti, F.; Favaro, G. J. Chem. Soc.,
Faraday Trans. 1992, 88, 2147. (b) Go¨rner, H.; Elisei, F.; Aloisi, G. G. J.
Chem. Soc., Faraday Trans. 1992, 88, 29.
(29) Totah, R. A.; Hanzlik, R. P. J. Am. Chem. Soc. 2002, 124, 10000.
(30) Baciocchi, E.; Gerini, M. F. To be published.
J. Org. Chem, Vol. 71, No. 3, 2006 859

Baciocchi et al.
intervals of 5-10 nm over the spectral range 300-800 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. Nitrogen or oxygen was bubbling through the solution.
All measurements were carried out at 22 ( 2 °C unless otherwise
indicated.
Determination of Deuterium Incorporation. The procedure
was identical as reported above. D2O (5% v/v, 99.9% d) was present
in the reaction medium. Deuterium incorporation was determined
by GC-MS analysis of thioanisole by the ratio of the intensity of
the molecular peaks m/z ) 124 and 125, corrected for the 13C
contribution.
Steady-State Photooxidation General Procedure. Photooxi-
dation reactions were carried out in a photoreactor equipped with
10 lamps (360 nm; 14 W each). A 4 mL solution containing the
substrate (1 × 10-2 M) and the photosensitizer (1 × 10-3 M) in
Ar-saturated CH3CN and, when present, H2O (5% v/v) was
irradiated in a rubber cap-sealed jacketed tube for 30 min,
thermostated at 25 °C by a Peltier apparatus. In some cases, pyridine
(2 × 10-2 M) was present. An internal standard (4-methylben-
zophenone) was added, and the mixture was analyzed by GC, GC-
Acknowledgment. MIUR, University “La Sapienza” of
Rome, and University of Perugia are thanked for the financial
support. We gratefully acknowledge Dr. Giuseppe Lapi and the
children of classroom 3aC of Istituto Comprensivo Don Milani
primary school (Latina) for their help in cover art design.
Supporting Information Available: Preparation and charac-
terization of 2 and 4. Table of absorption maxima and transients
produced by LFP in N2-saturated CH3CN. Plots of kobs vs [4-CN-
pyridine] and 1/kobs vs 1/[4-CN-pyridine]for the decay of 3+•. Time-
resolved absorption spectra of the 1,4-dicyanonaphthalene/PhSCH-
(Ph)CO2NMe4 system in N2-saturated CH3CN. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
MS, and H NMR. In photooxidation reactions carried out in the
presence of Na2CO3, the procedure is identical as reported above
except for the presence of 30 mg of the salt and for the solution
stirring during the irradiation. Blank experiments, carried out by
irradiating the solution in the absence of the sensitizer, did not show
in all cases product formation.
JO051145X
860 J. Org. Chem., Vol. 71, No. 3, 2006