Regioselectivity in the reductive bond cleavage of diarylalkylsulfonium salts: Variation with driving force and structure of sulfuranyl radical intermediates

Article abstract of DOI:10.1021/ja809918k

This investigation was stimulated by reports that one-electron reductions of monoaryldialkylsulfonium salts never give aryl bond cleavage whereas reductions of diarylmonoalkylsulfonium salts preferentially give aryl bond cleavage. We studied the product ratios from the reductive cleavage of di-4-tolylethylsulfonium and di-4-tolyl-2-phenylethylsulfonium salts by a variety of one-electron reducing agents ranging in potential from -0.77 to +2.5 ev (vs SCE) and including thermal reductants, indirect electrolyses mediated by a series of cyanoaromatics, and excited singlet states. We report that the cleavage products vary from regiospecific alkyl cleavage to predominant aryl cleavage as a function of the potential of the reducing agent. We conclude that differences between the reductive cleavages of mono- and diarylsulfonium salts are direct consequences of the structures of the sulfuranyl radical intermediates and the bond dissociation energies of the alkyl and aryl bonds. Competitions between the rates of cleavage and oxidation of the intermediate sulfuranyl radicals and between concerted and stepwise mechanisms are discussed to explain the variations in bond cleavage products as a function of the driving forces for the reductions. Density functional theory investigations of the nature of the antibonding S-alkyl and S-aryl orbitals of the starting sulfonium salts provide additional insight.

Full text of DOI:10.1021/ja809918k

Published on Web 07/06/2009
Regioselectivity in the Reductive Bond Cleavage of
Diarylalkylsulfonium Salts: Variation with Driving Force and
Structure of Sulfuranyl Radical Intermediates
,
†
†
†
Jack A. Kampmeier,* AKM Mansurul Hoque, Franklin D. Saeva,
†
‡
‡
‡
Donald K. Wedegaertner, Pia Thomsen, Saif Ullah, Jacob Krake, and
,
‡
Torben Lund*
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627, and Department
of Science, Systems and Models, Roskilde UniVersity, P.O. Box 260,
DK-4000 Roskilde, Denmark
Received December 19, 2008; E-mail: kamp@chem.rochester.edu; tlund@ruc.dk
Abstract: This investigation was stimulated by reports that one-electron reductions of monoaryldialkylsul-
fonium salts never give aryl bond cleavage whereas reductions of diarylmonoalkylsulfonium salts
preferentially give aryl bond cleavage. We studied the product ratios from the reductive cleavage of di-4-
tolylethylsulfonium and di-4-tolyl-2-phenylethylsulfonium salts by a variety of one-electron reducing agents
ranging in potential from -0.77 to +2.5 eV (vs SCE) and including thermal reductants, indirect electrolyses
mediated by a series of cyanoaromatics, and excited singlet states. We report that the cleavage products
vary from regiospecific alkyl cleavage to predominant aryl cleavage as a function of the potential of the
reducing agent. We conclude that differences between the reductive cleavages of mono- and diarylsulfonium
salts are direct consequences of the structures of the sulfuranyl radical intermediates and the bond
dissociation energies of the alkyl and aryl bonds. Competitions between the rates of cleavage and oxidation
of the intermediate sulfuranyl radicals and between concerted and stepwise mechanisms are discussed to
explain the variations in bond cleavage products as a function of the driving forces for the reductions.
Density functional theory investigations of the nature of the antibonding S-alkyl and S-aryl orbitals of the
starting sulfonium salts provide additional insight.
Introduction
We have been intrigued by a puzzle first identified by Hall
1
2
and Horner and confirmed by Beak and Sullivan. One-electron
reduction of phenyldialkylsulfonium salts gives cleavage of the
weaker S-alkyl bonds in the approximate order of the stability
of the resulting radicals (primary < secondary < benzyl); the
2
,3
phenyl group is not cleaved. In contrast, the stronger S-aryl
bond is preferentially cleaved in one-electron reduction of
The ethyl group is important for suppressing the rate of
competing nucleophilic displacements that occur at the methyl
group in diarylmethylsulfonium salts. A variety of excited- and
ground-state reductants are effective in the cleavage reaction,
which is shown in eq 1:
1
,2,4
diphenylmethylsulfonium salts.
This paradox implies that
the reductions proceed by way of 9-S-3 sulfuranyl radical
intermediates whose properties (e.g., relative rates of C-S bond
cleavages) vary with structure. We now report that the ratio of
aryl to alkyl bond cleavage in some diarylalkylsulfonium salts
is a function of the potential of the reductant.
5
-
e
1
f 4-CH C H SC H + (4-CH C H ) S
(1)
3
6
4
2
5
3
6
4 2
Results
We studied the reductive cleavages of di-4-tolylethylsulfo-
Both tolyl and ethyl cleavage products from 1-PF are observed,
6
nium hexafluorophosphate (1-PF
6
):
with the tolyl cleavage product favored by a factor of 2-3. The
data are expressed in terms of F, the ratio of tolyl cleavage
product to ethyl cleavage product:
†
University of Rochester.
‡
Roskilde University.
(
(
1) Hall, E. A. H.; Horner, L. Phosphorus Sulfur Relat. Elem. 1981, 9,
73–280.
1
2
(5) For example, the cleavage ratios reported by Hall and Horner are
2) Beak, P.; Sullivan, T. A. J. Am. Chem. Soc. 1982, 104, 4450–4457.
compromised by nucleophilic displacement because they used meth-
ylsulfonium salts and tetrabutylammonium bromide as the supporting
electrolyte in acetonitrile. As the temperature was increased in their
electrolytic experiments, the amount of methyl cleavage increased, as
(3) Grimshaw, J. In The Chemistry of the Sulfonium Group; Stirling, C. M.,
Ed.; Wiley: New York, 1981; Chapter 7.
(
4) Eriksson, P.; Engman, L.; Lind, J.; Merenyi, G. Eur. J. Org. Chem.
005, 701–705.
2
N
expected for a competing S 2 reaction.
10.1021/ja809918k CCC: $40.75  2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 10015–10022 9 10015

A R T I C L E S
Kampmeier et al.
a
[
4-CH C H SC H ]
Table 1. Reductive Cleavage of 1-PF
6
3
6
4
2
5
F )
b
c
[
(4-CH C H ) S]
expt
reductant
F
-∆GET (eV)
3
6
4 2
1
1
1
d
1
2
3
4
5
6
7
8
anthracene
2.8 (2.5)
2.8 (2.7)
2.8 (3.3)
2.7 (2.3)
2.8
∼0.4
∼0.6
∼0.9
∼0.7
0.6
The results are reported in Table 1.
Sulfonium salt 1-PF is photocleaved by direct irradiation at
00 nm. This direct photolysis favors ethyl cleavage (F ) 0.36),
in contrast to the reductive cleavages (F ≈ 2-3) reported in
Table 1. The peak reduction potential of 1-PF is E ) -1.7 V
d
PA
DMA
6
e
3
•-
f
(CD ) CO
Na(Hg)
K(C)
3 2
g
h
8
(∼1)
-
6
p
i
4-tolyltriphenylphosphoranyl radicalj
2.1 (2.5)
3.1 (4.0)
∼0.1
versus SCE in dimethylformamide (DMF) at a scan rate of 100
mV/s; the sensitizers and experimental conditions in expts 1-4
were chosen to ensure that electron transfer would be exothermic
and direct photolysis or energy transfer would be unfavorable.
One specific photosensitized reduction was studied in detail;
the fluorescence of 9-phenylanthracene (PA) and the total
quantum yield for the formation of sulfides in acetonitrile varies
4-tolyltriphenylphosphoranyl radical
∼0.1
a
In
a
typical experiment, [1]
)
0.03 Mb in acetone-d
6
at rt;
[sensitizer] ) 0.045-0.060 M in expts 1-4. Determined by NMR
integration; values in parentheses were determined by gas chromato-
c
6
graphy. For expts 1-3, ∆GET ) (Eox - Ered) - E*. Values are
approximate because the pertinent data are in different solvents. For
d
7
8
e
anthracene and 9-phenylanthracene (PA),
λ
ex
) 300 nm. For
9
f
as a function of [1-PF
correlations and comparable values for the bimolecular rate
6
]. Stern-Volmer analyses give linear
1,4-dimethoxyanthracene (DMA), λex ) 385 nm. (4-Tolyl) N was the
3
1
0
sensitizer, with λex ) 300 nm; acetone radical anion was the reductant.
g
11
h
i
9
-1 -1
In acetonitrile.
See refs 2 and 12. Initiated by the photolysis of
constant for quenching (3 × 10 M s ) and product formation
9
-1 -1
14,15
phenylazoisobutyronitrile (PAIBN) at 385 nm, with [PAIBN] ) 0.01 M
(
8 × 10 M s ). The limiting quantum yield is 0.5.
Thermal reactions are also effective, as reported in expts 4-8.
The reaction of triphenylphosphine with 1-PF (expts 5 and 6)
is a free-radical chain reaction involving ground-state phospho-
j
and [Ph
0
3
P] ) 0.24 M. Initiated by photolysis at 300 nm, with [Ph P] )
3
13
.12 M; 4-tolyltriphenylphosphoranyl radical was the reducing agent.
6
1
6
ranyl radicals as the reductant. When the chain was initiated
by irradiation at 300 nm (expt 5), a 1:1 correspondence between
the yields of 4-tolylethyl sulfide and 4-tolyltriphenylphospho-
nium ion was observed, providing quantitative evidence of
cleavage to give 4-tolyl radicals.
The data in Table 1 show that F is approximately independent
of the nature of the reductant. The interpretation of these results
seemed straightforward: the reductions give a common sulfura-
nyl radical intermediate, and the product ratio is a property of
R
that intermediate that is determined by kAr/k , the ratio of rate
constants for the unimolecular fragmentation of the sulfuranyl
radical to give tolylethyl sulfide and ditolyl sulfide, respectively.
We were quite surprised, therefore, to discover other one-
electron reducing agents that give very different results: both
4
Figure 1. Sulfide ratio F for the indirect electrolysis of 1-BF in DMF by
mediators of varying potential (DCA ) 9,10-dicyanoanthracene; DCN )
1,4-dicyanonaphthalene; DCB ) 1,4-dicyanobenzene).
1
7
cobaltocene (E° ) -1.04 V vs SCE in acetonitrile) and
1
8
Kosower’s 1-ethyl-4-(methoxycarbonyl)pyridinyl radical (E°
-0.82 V vs SCE in acetontrile) reduce 1-PF to give only
ethyl cleavage (F < 0.04).
)
6
We followed up on our initial observations using cobaltocene
and Kosower’s radical with a series of indirect electrochemical
reductions of 1-BF
reductant was systematically varied. The results for the indirect
electrolyses of 1-BF are shown in Figure 1. Although there is
4
in which the potential of the mediating
(
6) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259–271.
(
7) Lund, H. Acta Chem. Scand. 1957, 11, 1323–1330. Absorption Spectra
in the UltraViolet and Visible Region; Lang, L., Ed.; Academic Press:
New York, 1961; Vol. 1, p 228.
4
considerable scatter in the data, the unmistakable observation
is that the ratio of cleavage products, F, is not constant; rather,
the ratio increases with the reducing potential of the reductant
over the range -0.77 to -1.61 V (vs SCE). At the lowest
potentials [9,10-dicyanoanthracene (DCA) and benzil], only
ditolyl sulfide was observed, corresponding to exclusive ethyl
cleavage and the reductions by cobaltocene and the pyridinyl
radical. At the highest potentials (perylene and quinoxaline),
tolyl cleavage to give tolylethyl sulfide predominates, reminis-
cent of the data in Table 1.
(
8) Parker, V. D.; Eberson, L. Tetrahedron Lett. 1969, 10, 2843–2846.
Asher, S. A. Anal. Chem. 1984, 56, 720–724.
(
9) Quast, H.; Fuchsbauer, H. L. Chem. Ber. 1986, 119, 1016–1038.
Wartini, A. R.; Staab, H. A.; Neugebauer, F. A. Eur. J. Org. Chem.
1
998, 1161–1170.
(
(
(
(
(
10) Bukhtiarov, A. V.; Kudryavtsev, Y. G.; Lebedev, A. V.; Golyshin,
V. N.; Kuz’min, O. V. J. Gen. Chem. USSR 1989, 59, 1505–1506.
11) Lahiri, G. K.; Stolzenburg, A. M. Angew. Chem., Int. Ed. 2003, 32,
29–432.
12) Bergbreiter, D. E.; Killough, J. M. J. Am. Chem. Soc. 1978, 100, 2126–
133.
13) Horner, L.; Rottger, F.; Fuchs, H. Chem. Ber. 1963, 96, 3141–3147.
Sav e´ ant, J.-M.; Binh, S. K. Electrochim. Acta 1975, 20, 21–26.
14) Nichol, S. L.; Kampmeier, J, A. J. Am. Chem. Soc. 1973, 95, 1908–
4
2
4
The electrochemical reduction of 1-BF mediated by 1,4-
dicyanonaphthalene (DCN; E° ) -1.17 V vs SCE) in DMF
was studied in detail. The concentration of sulfonium salt could
be followed by capilliary electrophoresis; quantitative analysis
showed a 1:1 correspondence of sulfonium salt consumed to
sulfides formed. As reported in Table 2, expts 1-5, F is 0.47
( 0.04 (n ) 5); that is, both tolyl and ethyl cleavage were
observed, with ethyl cleavage predominating by a factor of 2.
Coulometry gave n ≈ 1 for the number of electrons required to
consume one molecule of sulfonium salt.
1
915. (a) Wang, X.; Saeva, F. D.; Kampmeier, J. A. J. Am. Chem.
Soc. 1999, 121, 4364–4368.
(
15) Robert, M.; Sav e´ ant, J.-M. J. Am. Chem. Soc. 2000, 122, 514–517.
Constentin, C.; Robert, M.; Sav e´ ant, J.-M. Chem. Phys. 2006, 324,
4
0–56. Sav e´ ant, J.-M. Elements of Molecular and Biomolecular
Electrochemistry; Wiley: Hoboken, NJ, 2006; Chapter 3.
16) Kampmeier, J. A.; Nalli, T. W. J. Org. Chem. 1993, 58, 943–949.
17) Herberich, G. E.; Schwarzer, J. J. J. Organomet. Chem. 1972, 34,
C43-C47.
(
(
(
18) Kosower, E. M.; Pozioniek, E. J. J. Am. Chem. Soc. 1964, 86, 5517–
5
527.
1
0016 J. AM. CHEM. SOC. 9 VOL. 131, NO. 29, 2009

Reductive Bond Cleavage of Diarylalkylsulfonium Salts
A R T I C L E S
Table 2. Product Yields from Indirect Electrolysis of 1-BF
4
in DMF
Mediated by DCN
a
% yield
TolSEt
b
F
expt
TolH
Tol
2
S
1
2
3
4
5
6
18
16
18
17
19
13
5
26
23
24
22
32
28
24
50
45
53
50
76
80
47
0.51
0.51
0.45
0.45
0.43
0.35
0.51
c
d
e
7
a
b
Based on complete electrolysis of 1-BF
4
.
F ) [TolSEt]/[Tol
2
S].
).
Acetic acid was added to the electrolysis medium. Acetonitrile
c
Ethylcyanonaphthalenes were observed (8% yield based on 1-BF
4
4
Figure 2. Cyclic voltammogram for the reduction of 2-BF in DMF at a
scan rate of 100 mV/s.
d
e
solvent.
Table 3. Products from Indirect Electrolysis of 2-BF
4
in DMF
a
Approximately 70% of the putative tolyl fragments (as judged
Mediated by DCN
by the yield of TolSEt) were recovered as toluene. Ethylcy-
anonaphthalenes, the coupling products of the radical anion of
DCN and ethyl radical were observed, albeit in yields
insufficient to account for the mediator consumed or the putative
b
c
d
%
reduction
[styrene] (mM)
[Tol
2
S] (mM)
[TolSR] (mM)
F
F
corr
1
2
4
8
0.00
0.29
0.36
0.41
0.47
0.61
0.31
1.00
1.89
2.08
2.47
2.89
0.18
0.39
0.61
0.81
1.03
1.19
0.58
0.39
0.33
0.39
0.42
0.41
0.58
0.55
0.40
0.48
0.51
0.52
1
9
43
57
2
ethyl fragments (19%, as judged by the yield of Tol S). The
7
1
value of F was not significantly affected by the addition of acetic
acid to the reduction medium (Table 2, expt 6) or changing the
solvent from DMF to acetonitrile (Table 2, expt 7).
1
00
a
b
The initial concentrations of 2-BF
reduction ) 100[(mmol of electrons)/(initial mmol of 2-BF )]. F )
[TolSR]/[Tol
[styrene]).
4
and DCN were 4.26 mM.
%
c
In general, these results can be rationalized in terms of
reduction of DCN to give the radical anion (eq 2) followed by
4
d
2
S];
R
)
2-phenylethyl.
F
corr
)
2
[TolSR]/([Tol S] -
•
-
reduction of the sulfonium salt by DCN to give the observed
sulfides and the corresponding tolyl and ethyl radicals (eqs 3
and 4):
of the coupling product of ethyl radical and DCN• is consistent
-
with the observed value n ≈ 1, since eq 7 consumes electrons
•-
-
•-
(DCN ) but not sulfonium salt. Clearly, ethyl groups are
disappearing by other, unidentified, pathway(s).
DCN + e f DCN
(2)
(3)
+
•-
2
The possibility that Tol S might be formed by an elimination
reaction (eq 6) in the electrochemical reductions would confound
Tol S Et + DCN f TolSEt + Tol· + DCN
2
+
•-
the F values reported in Figure 1 and Table 2 and our
Tol S Et + DCN f Tol S + Et· + DCN
(4)
2
2
understanding of the reductive cleavage of 1-BF
we prepared and studied the indirect electrolysis of di-4-tolyl-
-phenylethylsulfonium fluoroborate (2-BF ). Our purpose was
4
. Accordingly,
The radicals are subsequently consumed in a complex variety
of ways. On the basis of previous studies of mediated reductions
2
4
2
0
to reveal the elimination reaction and facilitate the measurement
of the resulting alkene.
of aryl and alkyl halides, we anticipated that the tolyl radical
might be reduced to tolyl anion (eq 5), which might then react
with the sulfonium salt (eq 6). In contrast, we expected the alkyl
radical to be scavenged by the radical anion of DCN to give
coupling products (eq 7):
•
-
-
Tol· + DCN f Tol + DCN
(5)
(6)
-
+
Tol + Tol S Et f TolH + Tol S + CH dCH
2
2
2
2
The E
potentials of 2-BF
p
(-1.67 V vs SCE) and E1/2 (-1.60 V vs SCE)
in DMF were obtained by cyclic voltam-
The formation of toluene and ethylcyanonaphthalenes provide
evidence for the formation of tolyl and ethyl radicals, respec-
tively (eqs 3 and 4). Although toluene accounts for ∼70% of
the tolyl fragments, it is not clear whether tolyl radical or tolyl
anion is the immediate precursor to the toluene. The low yield
4
metry, as shown in Figure 2. In the indirect reduction experi-
ments, sulfonium salt and mediator were present in approxi-
mately equal concentrations, and the potential was set at a value
00 mV more cathodic than the reduction potential of the
mediator.
In fact, the mediated reduction of 2-BF
gives easily measured yields of styrene, the expected elimination
product (Table 3). The yield of Tol S was corrected for the
elimination reaction ([Tol S] - [styrene]) to give Fcorr ) 0.51
1
by DCN• in DMF
-
(
(
19) Lan, J. Y.; Schuster, G. B. J. Am. Chem. Soc. 1985, 107, 6710–6711.
Lan, J. Y.; Schuster, G. B. Tetrahedron Lett. 1986, 27, 4261–4264.
Saeva, F. Top. Curr. Chem. 1990, 156, 59–92.
4
20) Simonet, J.; Michel, M.-A.; Lund, H. Acta Chem. Scand. 1975, B29,
2
4
89–498. Sav e´ ant, J.-M. AdV. Phys. Org. Chem. 1990, 26, 39.
2
J. AM. CHEM. SOC. 9 VOL. 131, NO. 29, 2009 10017

A R T I C L E S
Kampmeier et al.
4
Table 4. Products from Indirect Electrolysis of 2-BF in Acetonitrile
Mediated by DCN
a
reduction [DCN] (mM) [styrene] (mM) [Tol
b
b
F
c
%
2
S] (mM) [TolSR] (mM)
F
corr
0
1
3
4
6
8
9
3.82
3.87
3.60
3.48
3.17
2.99
2.85
0.00
0.00
0.05
0.10
0.18
0.31
0.46
0.00
0.25
0.66
1.03
1.37
1.83
2.27
0.00
7
3
9
6
3
8
0.16
0.34
0.49
0.68
0.86
1.05
0.65 -
0.52 0.56
0.48 0.53
0.49 0.57
0.47 0.57
0.46 0.58
a
b
%
reduction )100[(mmol of electrons)/(mmol of 2-BF
4
)] F )
c
[
[
TolSR]/[Tol2S]; R ) 2-phenylethyl.
styrene]).
F
corr ) [TolSR]/([Tol S] -
2
Figure 3. Average values of corrected product ratios Fcorr for the indirect
electrolyses of 2-BF in DMF as a function of mediator potential.
4
Table 5. Cleavage Product Ratios for the Indirect Electrolyses of
a
2
-BF
4
in DMF Mediated by Cyanoaromatic Reductants
b
F
c
Table 6. Cleavage Product Ratios for the Indirect Electrolyses of
F
corr
2-BF
4
in Acetonitrile Mediated by Cyanoaromatic Reductants
E°
V vs SCE)
mediator
GC-MS
HPLC
GC-MS HPLC
a
F
b
(
mediator
E° (V vs SCE)
F
corr
d
d
c
9
1
9
1
9
1
,10-dicyanoanthracene -0.77
,4-dicyanonaphthalene -1.17
-
-
0
0
9,10-dicyanoanthracene
1,4-dicyanonaphtalene
9-cyanoanthracene
-0.77
-1.17
-1.30
-1.49
-
0
0.25 0.42 ( 0.03 (6) 0.29
0.61 0.85 ( 0.22 (6) 0.71
1.13 0.94
0.86 1.03
0.87 0.89
0.52
0.99
1.38
1.68
1.66
0.48
0.58
0.72
0.56 ( 0.02
0.68
1.02
-cyanoanthracene
,4-dicyanobenzene
-cyanophenanthrene
-cyanonaphthalene
-1.30
-1.49
-1.50
-1.77
1.54
1.30
1.65
1,4-dicyanobenzene
a
b
F ) [TolSR]/ [Tol
2
2
S]; R ) 2-phenylethyl.
S] - [styrene]). 2-Phenylethyl-4-tolyl sulfide was not detected.
F
corr ) [TolSR]/
c
(
[Tol
a
Electrolysis was carried to completion; products were measured at
b
intervals throughout the electrolysis, as in Tables 3 and 4. F )
and the variation of the ratio of aryl to alkyl cleavage as a
function of the potential of the reducing agent (Tables 5 and 6
and Figure 3) require sulfuranyl radical intermediates in the
pathways to cleavage products. In fact, fast-scan cyclic volta-
mmetry investigations by Sav e´ ant gave direct evidence for
an intermediate in the reductive cleavage of 9-anthryldimeth-
ylsulfonium cation (eq 8):
c
[
TolSR]/[Tol
2
S];
R
)
2-phenylethyl.
F
corr
2
) [TolSR]/([Tol S] -
d
[
styrene]). 2-Phenylethyl-4-tolyl sulfide was not detected.
(
0
0.6 (n ) 6). The uncorrected values in Table 2 (F ) 0.47 (
.04) and the corrected values in Table 3 (Fcorr ) 0.51 ( 0.06)
are in reasonable agreement, showing that the results in Table
for the ditolyethylsulfonium salt (1-BF ) are not significantly
biased by an elimination reaction.
In addition to toluene, styrene, and the sulfides, the GC-MS
and HPLC traces showed two 2-phenylethylcyanonaphthalenes
assigned to coupling reactions of DCN and 2-phenylethyl
radicals. Together, the two coupling products were formed in
7% yield, referenced to the corrected yield of ethyl fragments
2
1
2
4
+
-
•
anthrylS Me + e f [anthrylSMe ] f anthrylSMe + Me·
2
2
(
8)
•
-
At low scan rates, the cyclic voltammogram showed only the
reduction wave. However, at scan rates of 12 500 V/s and above,
an oxidation wave was detected, revealing the reversibility of
the electron transfer and the presence of a short-lived intermedi-
ate. Sulfuranyl radicals have also been proposed as intermedi-
ates in the displacement reactions of radicals with sulfides
eq 9)
2
([Tol
2
S] - [styrene]). Finally, trace amounts (∼3.5% yield based
21
on sulfonium salt consumed) of 1-cyano-2-phenylethane were
observed. Ethylbenzene was not detected.
2
2
•
-
The reduction of 2-BF
studied (Table 4). The corrected sulfide ratio, Fcorr ) 0.56 (
.02 (n ) 5), is in agreement with the results in DMF, showing
4
by DCN in acetonitrile was also
(
•
0
R′· + R S f [R SR′] f R· + RSR′
(9)
2
2
that the reductive cleavage of the sulfonium salt is unaffected
by solvent. In contrast, the solvent did affect the subsequent
postcleavage reactions. The styrene yield is significantly less
5-18%) in acetonitrile than in DMF (20-45%). Similar
observations are reported in Table 2 for the reduction of 1-BF
2
3,24
and in a variety of other reactions.
Similarly, a variety of
sulfuranyl radical structures have been proposed, some derived
from direct observation, some deduced from experimental
(
25
4
;
observations, and still others based on computational methods.
although the yield of toluene is significantly less in acetonitrile
than in DMF, the cleavage ratio F is unaffected.
For arylsulfonium salts with electron-delocalizing substituents
on the aryl ring, there is good evidence for a sufuranyl structure
Table 5 summarizes values of Fcorr for the reductive cleavages
of sulfonium salt 2-BF
4
in DMF by a family of cyanoaromatic
(21) Andrieux, C. P.; Robert, M.; Saeva, F. D.; Sav e´ ant, J.-M. J. Am. Chem.
Soc. 1994, 116, 7864–7871.
mediators covering a range of reduction potentials from -0.77
to -1.77 V vs SCE. The data were obtained by two different
experimental analytical techniques (GC-MS and HPLC);
average values of Fcorr are plotted in Figure 3. Table 6 gives
comparable results for the indirect reduction of 2-BF by selected
4
cyanoaromatic mediators in acetonitrile.
(
22) Kampmeier, J. A.; Evans, T. R. J. Am. Chem. Soc. 1966, 88, 4096–
4
097.
(
23) Anklam, E.; Margaretha, P. Res. Chem. Intermed. 1989, 1, 127–155.
(24) In one case, an intermediate sulfuranyl radical was captured by
oxidation to give the corresponding sulfonium salt. See: Leanardi, P.;
Tundo, A.; Zanardi, G. J. Chem. Soc., Chem. Commun. 1985, 1390–
1
391.
(
25) Franz, J. A.; Roberts, D. A.; Ferris, K. F. J. Org. Chem. 1987, 52,
Discussion
2
7
256–2262. Ferris, K. F.; Franz, J. A. J. Org. Chem. 1992, 57, 777–
78. Volatron, F.; Demalliens, A.; Lefour, J. M.; Eisenstein, O. E.
The paradoxical preference for breaking the stronger S-aryl
bond in the reduction of diarylalkylsulfonium salts (Table 1)
Chem. Phys. Lett. 1986, 130, 419–422. Pogocki, D.; Schoeneich, C.
J. Org. Chem. 2002, 67, 1526–1535.
1
0018 J. AM. CHEM. SOC. 9 VOL. 131, NO. 29, 2009

Reductive Bond Cleavage of Diarylalkylsulfonium Salts
A R T I C L E S
1
2
in which a π-aryl radical anion is substituted with a sulfonium
R · and R ·. In structures b and c, alkyl cleavage is faster than
2
6
group. There is, however, no direct evidence for this type of
structure when the aryl group is a simple phenyl. Most of the
proposed structures for sulfuranyl radicals involve reorganization
aryl cleavage, and the net result is that the ratio of products
[R ·]/[R ·] is determined by the ratio of the populations,
[b]/[c].
1
2
3
1
23
of the σ bonding of the starting sulfonium cation. The simplest
of these structures is a pyramidal σ* species with a two-
center-three-electron (2c-3e) bond to sulfur; dialkylalkoxy-
sulfuranyls and dialkylthioalkylsulfuranyls are assigned to this
category on the basis of electron spin resonance (ESR) evidence.
On the other hand, the most common structures invoke three-
center-four-electron (3c-4e) or three-center-three-electron
The three-center bonding description of sulfuranyl radical
intermediates resolves the paradox posed by aryl cleavage in
diarylalkyl sulfonium salts. The pertinent structures are j and
k:
2
7
(
3c-3e) bonding.
A consequence of the three-center bonding is a structural
change from a pyramidal sulfonium ion to a planar (quasi-T-
shaped) sulfuranyl radical (3):
Structure j obviously fragments to give an alkyl radical in
3
2
preference to an aryl radical. However, structure k is unique
3
3
and can only fragment to give an aryl radical. A further
implication of this interpretation is that the fragmentation of
these sulfuranyl radicals is faster than their permutational
isomerization, j a k. Were the converse true, the equilibrating
radicals would leak out of the low-energy transition state to
give only alkyl cleavage, contrary to observations. The same
conclusion was suggested previously on the basis of studies of
free-radical displacement reactions at sulfur.
How then can we understand the different product ratios from
6 4
diarylalkylsulfonium salts 1-PF and 2-BF with different reducing
agents, as reported in Tables 1, 5, and 6 and Figures 1 and 3?
The salient feature of this structure is that the apical three-center
bonds are different from the equatorial two-center-two-electron
(
2c-2e) bond; ESR observations and computational methods
23
28,34
support this conclusion. More specifically, the 3c-3e structure
predicts that the apical bonds are the locus of bond making and
breaking in sulfuranyl radicals. From a reaction-chemistry
perspective, the 3c-3e description of sulfuranyl radicals is
especially appealing. For example, the reactions of radicals with
sulfides have been shown to prefer a linear arrangement of the
sulfur atom and the entering (bond-making) and leaving (bond-
There are three distinct experimental regimes. Regime I is
characterized by strongly exothermic electron transfers (∆GET
≈
-0.4 to -0.9 eV) and product ratios that are approximately constant
2
8,29
breaking) groups.
The 3c-3e structure of 3 clearly describes
and favor aryl cleavage (F ≈ 2-3). This regime includes both
excited- and ground-state reductants (Table 1). In stark contrast,
regime III is characterized by strongly endothermic electron
transfers (∆GET ≈ +0.7 to +0.9 eV) and exclusive alkyl cleavage
these experimental observations.
Most interestingly, the distinction between apical and equato-
rial bonds in three-center structures leads immediately to
isomeric sulfuranyl radicals. For example, there are three
possible T-shaped structures for the sulfuranyl radicals derived
from aryldialkyl- sulfonium salts:
(
F ≈ 0). This regime includes cobaltocene, Kosower’s pyridinyl
•-
radical, and DCA as reducing agents. In regime II, the F increases
from 0.3 to 1.7 as the endothermicities of the mediated electron
transfers decrease from approximately +0.5 eV to zero (Tables 5
and 6 and Figure 3). The phosphoranyl radical chain reductions
(Table 1, expts 7 and 8) also belong to regime II.
(
31) The structures of b and c clearly predict that cleavage ratios derived
2
from different sulfonium salts will not commute. Because the ratio
of populations of b and c varies with the structure of the sulfonium
salts, the apparent cleavage ratio is a composite of the direct
competition kR1/kR2 and the population ratio [b]/[c]. This “population
The structures a, b, and c clearly accommodate the experimental
result that the stronger S-aryl bond is never cleaved in the
reduction of aryldialkylsulfonium salts, since there is always
an alkyl group in the 3c-3e bonding system and therefore a
effect” is also seen quite clearly in the reduction of cis- and trans-1-
2
phenyl-2-methyltetrahydrothiophenium salts. Because of the ring,
3
0
there is no sulfuranyl radical structure in which primary and secondary
alkyl groups simultaneously occupy apical positions and compete
directly. Furthermore, there is no reason for the population ratio b/c
to be constant for the cis and trans isomers or to relate to the primary/
secondary cleavage ratio observed when acyclic phenyldialkylsulfo-
nium salts are reduced.
path for cleaving one of the weaker S-alkyl bonds. For
1
2
structure a, the ratio of products [R ·]/[R ·] equals kR1/kR2, the
ratio of the rate constants for cleavage of the apical bonds, and
is determined in part by the relative stability of the radicals
(
32) DFT calculations for 1 and 2 indicated that the reduction to give 4-tolyl
radical rather than the corresponding alkyl radical is thermodynamically
disfavored by 11.3 and 10.5 kcal/mol, respectively.
(
(
26) Ghanimi, A.; Simonet, J. New J. Chem. 1997, 21, 257–265.
27) Perkins, C. W.; Martin, J. C.; Arduengo, A. J.; Lau, W.; Laegria, A.;
Kochi, J. K. J. Am. Chem. Soc. 1980, 102, 7753–7759.
(33) The distinction between aryldialkyl and diarylalkylsulfonium salts is
2
(
(
(
28) Kampmeier, J. A.; Jordan, R. B.; Liu, M. S.; Yamanaka, H.; Bishop,
D. J. ACS Symp. Ser. 1978, 69, 275–289.
preserved in the reduction of cyclic sulfonium salts. The reduction
of 1-phenyltetrahydrothiophenium cation gives only ring-opened
product; phenyl cleavage is not observed. In contrast, the reduction
of the benzofused system, 1-phenylthiaindan hexafluorophosphate,
gives ∼15% aryl cleavage. The 1-phenylthiaindan cation is reduced
to give a mixture of two nonequilibrating sulfuranyl radicals, one of
which must give phenyl cleavage.
29) Cooper, J. W.; Roberts, B. P. J. Chem. Soc., Chem. Commun. 1977,
2
28–230.
30) DFT calculations indicated that the reduction of phenyldiethylsulfo-
nium ion to ethyl radical and phenylethylsulfide is thermodynamically
favored by 13 kcal/mol over the alternative reduction to phenyl radical
and diethylsulfide.
(34) Tada, M.; Nakagini, H. Tetrahedron. Lett. 1992, 33, 6657–6660.
J. AM. CHEM. SOC. 9 VOL. 131, NO. 29, 2009 10019

A R T I C L E S
Kampmeier et al.
The weaker S-alkyl bond is the one that breaks in an
unconstrained competition between aryl and alkyl cleavage in a
sulfuranyl radical intermediate, as observed in the reductive
for the photochemical reactions, the competitions for the intermedi-
ates are now between the rapid product-forming steps (k and k )
3 4
and the exothermic back electron transfers to the ground state (kBET
and k′BET). For expts 1-3 in Table 1, the return to the ground state
is exothermic by 2-3 eV. Nevertheless, products are formed,
indicating that the rates of back electron transfer and product
formation must be roughly comparable for the photochemical
30
cleavage of phenyldialkylsulfonium ions. The paradoxical prefer-
ence for aryl cleavage in the reduction of diarylalkylsulfonium ions
implies a sulfuranyl radical that can give only aryl cleavage (k).
However, alkyl cleavage is also observed, requiring a second
intermediate that preferentially gives alkyl cleavage (j). One
interpretation of the observed values of F in the thermal reactions
involves competitive rates of formation and reaction of these
two intermediates, as shown in Scheme 1.
reactions (kBET ≈ k
3
and k′BET ≈ k
4
). In contrast, the return electron
transfer reactions in regime III are exothermic by only 0.7-0.9
eV. Even so, the kinetic analysis (Scheme 1) for the thermal
reactions of poor reducing agents such as DCA , cobaltocene, and
Kosower’s radical requires that the rates of return electron transfer
•-
Scheme 1
are much higher than the rates of product formation (k-1 . k
3
and
k
-2 . k
4
). Since the rates of product formation should be
approximately constant, the driving-force differences suggest that
the back electron transfers in the photochemical reactions should
be faster than the return electron transfers in the thermal reactions,
in disagreement with the kinetic proposal. This apparent contradic-
tion is resolved by Marcus inverted region behavior, which can
significantly decrease the rates for strongly exothermic return
36
to the ground state in the photochemical reactions. Consequently,
the kinetic analyses in Schemes 1 and 2 qualitatively accommodate
the full range of photochemical and thermal reactions reported in
Tables 1, 5, and 6.
The kinetic scheme in Scheme 1 proposes that sulfuranyl
intermediates k and j are the products of the forward electron
transfer and are populated in the ratio k
electron transfer is significantly exothermic (regime I; Table 1, expts
-6), the rapid product-forming steps (with rate constants k and
) will be faster than the endothermic return electron transfer
reactions, and the cleavage ratio will be approximated by the
1 2
/k . When the forward
37
Daasbjerg recently reported that the rate constant for product
4
k
4
3
8
-1
formation from diphenylethylsulfuranyl radical is 7 × 10 s .
Since we observed a quantum yield of 0.5 for product formation
for the reductive cleavage of the closely related ditolyethylsul-
fonium salt 1 photosensitized by PA, we set the back electron
transfer rate constant equal to the rate constant for product
3
5
population ratio: F ≈ k
1 2
/k . On the other hand, when the forward
electron transfers are significantly endothermic (regime III), the
exothermic back reactions will be faster than cleavage and provide
an indirect mechanism for equilibrating j and k. In the limit k-1
8
-1
formation, 7 × 10 s . This back electron transfer rate constant
•+
for the reaction of PA with ditolylethylsulfuranyl radical
.
k
3
4
and k-2 . k , the product ratio will be determined by the
3
8
corresponds to a Marcus reorganization energy of 1.77 eV.
In related fashion, we used the Marcus equation to calculate
rate constants for return electron transfer (k-1 and k-2 in Scheme
1) for different mediators reacting with the sulfuranyl radical
derived from 2. These rate constants were used in turn to
simulate the variation in Fcorr with mediator potential shown in
Figure 3. The best-fit simulation corresponded to a reorganiza-
tion energy of 1.2 eV; experimental and calculated results for
difference in the free energies of the transition states for the aryl
and alkyl cleavages. In this situation, alkyl cleavage will predomi-
nate, as observed for the weak reductants cobaltocene, pyridinyl
•-
radical, and DCA . Between these two limiting regimes I and III,
all six rate constants matter (regime II), and F will vary with the
35
structures and potentials of the reductants, as observed in Figures
1
and 3.
Although the preceding analysis of Scheme 1 is consistent with
the results of the thermal reactions, the kinetic analysis should also
accommodate the photochemical results. As shown in Scheme 2
F
corr are compared in Figure 4.
Scheme 2
Figure 4. Experimental (red 2) and calculated (black 9) product ratios
4
corr as a function of mediator potential for the indirect electrolyses of 2-BF .
F
An alternative interpretation of the data is provided by
Sav e´ ant’s proposal that the mechanism of reductive bond
(
35) The rates of the forward and back electron transfer reactions of the
sulfonium/sulfuranyl couple also depend on the nature of the redox
partner. In the limit of exothermic reductions, the nature of the redox
partner apparently does not have a significant effect on the relative
(36) Gould, I. R.; Farid, S. Acc. Chem. Res. 1996, 29, 522–528.
(37) Vase, K. H.; Holm, A. H.; Norrman, K.; Pedersen, S. U.; Daasbjerg,
K. Langmuir 2008, 24, 182–188.
rates k /k
1 2
and k-1/k-2, as judged by the small variations in F. On the
(38) Eberson, L. Electron Transfer Reactions in Organic Chemistry;
Reactivity and Structure Concepts in Organic Chemistry, Vol. 25;
Springer-Verlag: Berlin, 1987; p 25.
other hand, the nature of the redox partner may account for some of
the scatter in Figure 1.
1
0020 J. AM. CHEM. SOC. 9 VOL. 131, NO. 29, 2009

Reductive Bond Cleavage of Diarylalkylsulfonium Salts
A R T I C L E S
Figure 5. Coefficients on the atoms in the LUMO of compound 2.
Figure 6. Coefficients on the atoms in the LUMO+1 of compound 2.
cleavages can change from concerted to stepwise as the driving
force for the electron transfer increases. In previous studies of
the electrochemical reduction of sulfonium salts, Sav e´ ant
observed both stepwise and concerted mechanisms as a function
of the molecular structure of the sulfonium salt and identified
borderline structures for which modest changes in driving force
radical anion of 4-methoxycarbonylazobenzene (E° ) -1.02
V vs SCE) is presumably accompanied by an undetected
concerted pathway. The products of the mediated reduction were
not measured; based on Figure 3, we predict that the concerted
cleavage to ethyl radical and diphenylsulfide is the major
reaction.
21,39
(
∼200 mV) led to changes in the mechanism.
Most recently,
It is clear that only the weaker S-alkyl bond will be broken
in the concerted mechanism (F < 0.04). Therefore, a different
mechanism (i.e., a stepwise mechanism) is required to break
the stronger S-aryl bond. The stepwise mechanism is an
essential component of both the kinetic and driving-force
interpretations of the results. In the kinetic proposal, the
mechanism is always stepwise. In the mixed-mechanism
Daasbjerg used kinetic methods to detect a stepwise mechanism
for the indirect electrolysis of diphenylethylsulfonium salt
mediated by 4-methoxycarbonylazobenzene. The experimental
data correspond to an intermediate sulfuranyl radical with a
3
7
-9
lifetime of 1.4 × 10 s, which is long enough for the radical
to escape the geminate-pair solvent cage.
3
9
In our thermal reactions, the structure of the sulfonium salt
is constant, but the driving force changes by ∼1 eV as the
structure of the reducing agent changes. In the limit of regime
I, the exothermic electron transfers correspond to large driving
forces and a stepwise mechanism. Conversely, in the limit of
regime III, the endothermic electron transfers correspond to
small driving forces and a concerted mechanism. In effect, the
reaction finds a concerted path because there is insufficient
driving force to access the sulfuranyl radical intermediates.
Between these two mechanistic limits, the concerted and
stepwise mechanisms will compete in a ratio that changes with
changing potential of the reducing agent; as the reduction
potential of the mediator becomes more negative, the driving
force for the electron transfer and the stepwise mechanism
become more favorable.
interpretation, stepwise and concerted mechanisms compete.
However, it cannot be the case that the stepwise mechanism
gives only aryl cleavage. Were this true, only aryl cleavage
would be observed in the limit of the strong driving forces in
regime I. In fact, the stepwise mechanism in the limit of regime
I gives a mixture of aryl and alkyl cleavage, with a preference
for aryl cleavage (F ≈ 2.5). As discussed previously, sulfuranyl
radical intermediates j and k rationalize the formation of alkyl
and aryl cleavage products, respectively, and resolve the paradox
that the presence of two aryl groups is a prerequisite for aryl
cleavage.
The formation of j involves a change in alkyl-sulfur bonding,
whereas the formation of k involves a change in the aryl-sulfur
bonding. We used density functional theory (DFT) calculations
to try to find the structures and energies of sulfuranyl radicals
j and k. Unfortunately, the calculations failed to converge to
minimized structures that did not contain a broken bond. The
lowest-energy conformation for 2 in the gas phase and the
coefficients on the atoms in the LUMO and LUMO+1 for that
ground-state structure were obtained by DFT calculations. The
coefficients on the atoms in the LUMO (Figure 5) clearly
indicate the predominantly antibonding nature of the S-alkyl
bond, with some involvement of the aryl π system. On the other
hand, the coefficients on the atoms in the LUMO+1 (Figure 6)
involve predominantly antibonding character in the S-aryl bond
(along with the π system). According to these DFT calculations,
the LUMO/LUMO+1 separation is modest (∼0.5 eV).
While recognizing that these calculations apply to the
starting sulfonium salt, we propose that the initial interaction
of the reducing agent with the antibonding S-alkyl orbital
The stepwise (regime I) and concerted (regime III) mecha-
nisms give different limiting ratios of aryl and alkyl cleavage
products (F ≈ 2.5 and 0, respectively). For any value of F, one
can solve the simultaneous equations to estimate the mix of
concerted and stepwise mechanisms. When F ≈ 0 (DCA, E° )
-
0.77 V), only alkyl cleavage is observed, corresponding to
00% concerted mechanism. When F ) 0.4 (DCN, E° ) -1.17
1
V), the mix of products and mechanisms corresponds to 60%
concerted and 40% stepwise reactions. When the reducing agent
is acetone radical anion (E° ) -2.4 V), F ) 2.5 and the
mechanism is 100% stepwise. The stepwise mechanism identi-
3
7
fied for the reduction of diphenylethylsulfonium ion by the
(
39) Pause, L.; Robert, M.; Sav e´ ant, J.-M. J. Am. Chem. Soc. 2001, 123,
886–4895.
4
J. AM. CHEM. SOC. 9 VOL. 131, NO. 29, 2009 10021

A R T I C L E S
Kampmeier et al.
leads ultimately to sulfuranyl radical j and that the initial
interaction with the antibonding S-aryl orbital leads ulti-
mately to intermediate k. Since the permutational intercon-
version of these sulfuranyl radicals is slower than their
fragmentation, the observed product ratio from the stepwise
mechanism (F ≈ 2.5) reflects the relative populations (k/j
experiments, Leif Olson to the DFT calculations, and Joseph P.
Dinnocenzo to many stimulating discussions. This work is
derived in part from the M.S. theses of Saif Ullah (2005) and
Pia Thomsen (2006), Roskilde University. Financial support was
provided by the National Science Foundation Center for Pho-
toinduced Charge Transfer (CHE-912001).
≈
2.5). Because there are two ways to perturb the S-aryl
bonding to form k but only one way to perturb the S-alkyl
bonding to form j, the observed product ratio should be
corrected for this statistical effect. Thus, the relative rate of
formation of k is ∼1.25 times that of j, indicating that the
activation energies leading to the two intermediates j and k
are similar.
Supporting Information Available: Characterization of the
sulfonium cations 1 and 2 and the reaction products;
experimental procedures for the photochemical, thermal, and
mediated electrochemical reductions; simulation of the
product ratios; and DFT calculations of the absolute energies
and optimized geometry of 2. This material is available free
of charge via the Internet at http://pubs.acs.org.
Acknowledgment. It is a pleasure to acknowledge the
contributions of Bo Svensmark to the capilliary electrophoresis
results, Xiuzhi Wang to the Stern-Volmer and quantum-yield
JA809918K
1
0022 J. AM. CHEM. SOC. 9 VOL. 131, NO. 29, 2009