Nucleophilic reactions at the sulfur of thiiranium and thiirenium ions. New insight in the electrophilic additions to alkenes and alkynes. Evidence for an episulfurane intermediate

Article abstract of DOI:10.1021/ja984304l

The thiiranium hexachloroantimonates 1a, 3, and 5a-c and the thiirenium hexachloroantimonates 6a-c and 7a with exocyclic S-R substituent (R = Me, Et, i-Pr) react at sulfur with dialkyl disulfides R″SSR″ (R″ = Me, Et and R″ ≠ R) in CD₂Cl₂ at 25°C to give S-R″ substituted ions. The reaction rates are affected by the steric hindrance of the substituents at sulfur and at ring carbons. t-2,t-3-Di-tert-butyl-r-1-methylthiiranium hexachloroantimonate (2) does not react, and the t-2-tert-butyl-c-3-phenyl-r-1-methylthiiranium (5a) reacts about 100 times faster than the c-2,t-3-di-tert-butyl-r-1-methylthiiranium ion (1a). The analysis of the kinetic data suggests that the sulfonium sulfur undergoes attack by the disulfide in the ring plane from a direction that is parallel to the C-C ring bond. This is also the direction which ensures the maximum overlap with the LUMO of thiiranium or thiirenium ions (determined at the RHF/3-21G*//RHF/3-21G* level). The combined consideration of the approach modality and of the maximum orbital overlap suggests that the nucleophilic substitution at sulfonium sulfur is not an S_N2-like reaction but occurs via an intermediate with episulfurane-like structure. The principle of microscopic reversibility will dictate that this is also the first intermediate in the electrophilic sulfenylation of unsaturated C-C bonds.

Full text of DOI:10.1021/ja984304l

3944
J. Am. Chem. Soc. 1999, 121, 3944-3950
Nucleophilic Reactions at the Sulfur of Thiiranium and Thiirenium
Ions. New Insight in the Electrophilic Additions to Alkenes and
Alkynes. Evidence for an Episulfurane Intermediate
Marco Fachini,† Vittorio Lucchini,*,‡ Giorgio Modena,† Manuela Pasi,† and
Lucia Pasquato*,†
Contribution from the Centro CNR Meccanismi di Reazioni Organiche (CMRO) and Dipartimento di
Chimica Organica, UniVersita` di PadoVa, Via Marzolo 1, 35131 PadoVa, Italy, and Dipartimento di
Scienze Ambientali, UniVersita` di Venezia, Dorsoduro 2137, 30123 Venezia, Italy
ReceiVed December 14, 1998
Abstract: The thiiranium hexachloroantimonates 1a, 3, and 5a-c and the thiirenium hexachloroantimonates
6a-c and 7a with exocyclic S-R substituent (R ) Me, Et, i-Pr) react at sulfur with dialkyl disulfides R′′SSR′′
(R′′ ) Me, Et and R′′ * R) in CD2Cl2 at 25 °C to give S-R′′ substituted ions. The reaction rates are affected
by the steric hindrance of the substituents at sulfur and at ring carbons. t-2,t-3-Di-tert-butyl-r-1-methylthiiranium
hexachloroantimonate (2) does not react, and the t-2-tert-butyl-c-3-phenyl-r-1-methylthiiranium (5a) reacts
about 100 times faster than the c-2,t-3-di-tert-butyl-r-1-methylthiiranium ion (1a). The analysis of the kinetic
data suggests that the sulfonium sulfur undergoes attack by the disulfide in the ring plane from a direction that
is parallel to the C-C ring bond. This is also the direction which ensures the maximum overlap with the
LUMO of thiiranium or thiirenium ions (determined at the RHF/3-21G*//RHF/3-21G* level). The combined
consideration of the approach modality and of the maximum orbital overlap suggests that the nucleophilic
substitution at sulfonium sulfur is not an SN2-like reaction but occurs via an intermediate with episulfurane-
like structure. The principle of microscopic reversibility will dictate that this is also the first intermediate in
the electrophilic sulfenylation of unsaturated C-C bonds.
The electrophilic additions of halogens and sulfenyl halides
Scheme 1
to alkenes and alkynes have been extensively studied.1,2
However, many facets of their mechanisms still wait for a
complete elucidation. The main feature of these very similar
processes is the stereospecific formation of anti addition
products. This stereochemical course was rationalized by the
intermediacy of cyclic cationic intermediates.3 This hypothesis
was then substantiated by the actual observation and isolation
of haloiranium,4 thiiranium,5,6 and thiirenium ions.7,8 The final
anti addition products are obtained by a SN2 mechanism, where
the nucleophile attacks the ring carbons of the thiiranium or
thiirenium intermediate in the ring plane and from the opposite
side of the leaving group. It was also pointed out8,9 that the
opening of irenium ions by nucleophiles is a rarely observed
case of nucleophilic substitution at the vinylic carbon atom
occurring with inversion of configuration and with a SN2-Vin
mechanism.
The mechanism of formation of thiiranium or thiirenium ions
from reagents is not, as yet, fully understood, and the presence
of an episulfuranic addition complex is matter of speculation
(Scheme 1).10 An old theoretical investigation suggests that in
the gas phase the chlorosulfurane structure is more stable than
the thiiranium chloride system,11 whereas an isolated claim for
the detection of a chloroselenurane12 has been disproved.13 The
spectroscopic description of isolated fluorosulfuranes has not
been followed by structural determinations.14
† CMRO-CNR, University of Padova.
‡ University of Venezia.
(1) (a) De la Mare, P. B. D.; Bolton, R. Electrophilic Additions to
Unsaturated Systems; Elsevier: New York, 1982. (b) Ruasse, M.-F. AdV.
Phys. Org. Chem. 1993, 28, 207.
(2) (a) Capozzi, G.; Modena, G.; Pasquato, L. The Chemistry of Sulphenyl
Halides and Sulphenamides; Patai, S., Ed.; Wiley: Chichester, U.K., 1990;
p 403. (b) Lucchini, V.; Modena, G.; Pasquato, L. Gazz. Chim. Ital. 1997,
127, 177.
(3) (a) Roberts, I.; Kimball, G. E. J. Am. Chem. Soc. 1937, 59, 947. (b)
Kharasch, N.; Yiannios, C. N. J. Org. Chem. 1964, 29, 1190. (c) Modena,
G.; Scorrano, G. Int. J. Sulfur. Chem. 1968, 3, 115.
(4) (a) Olah, G. A.; Bollinger J. J. Am. Chem. Soc. 1968, 90, 947. (b)
Olah, G. A.; Bollinger J. J. Am. Chem. Soc. 1967, 89, 4744. (c) Olah, G.
A.; Westerman, P. W.; Melby, E. G.; Mo, Y. K. J. Am. Chem. Soc. 1974,
96, 3565.
(5) (a) Pettitt, D, J.; Helmkamp, G. K. J. Org. Chem. 1963, 28, 2932.
(b) Raynolds, P.; Zonnebelt, S.; Bakker, S.; Kellogg, R. M. J. Am. Chem.
Soc. 1974, 96, 3146.
We have recently investigated the reaction with water of the
c-2,t-3-di-tert-butyl-r-1-methylthiiranium ion (1a) and of the
(9) (a) Rappoport, Z. Tetrahedron Lett. 1978, 1073. (b) Capozzi, G.;
Lucchini, V.; Modena, G. ReV. Chem. Intermed. 1979, 4, 347. (c) Lucchini,
V.; Modena, G.; Pasquato, L. J. Am. Chem. Soc. 1995, 117, 2297.
(10) (a) Rayner C. M. Organosulfur Chemistry. Synthetic Aspects; Page,
P., Ed.; Academic Press: London, 1995; p 89. (b) Harring, S. R.; Edstrom,
E. D.; Livinghouse, T. AdV. Heterocycl. Nat. Prod. Synth. 1992, 2, 319.
(11) Csizmadia, V. M.; Schmid, G. H.; Mezey, P. G.; Csizmadia, I. G.
J. Chem. Soc., Perkin Trans. 2 1977, 1019.
(6) (a) Lucchini, V.; Modena, G.; Pasquato, J. Am. Chem. Soc. 1988,
110, 6900. (b) Lucchini, V.; Modena, G.; Pasquato, J. Am. Chem. Soc. 1991,
113, 6600.
(7) Capozzi, G.; De Lucchi, O.; Lucchini, V.; Modena, G. J. Chem. Soc.,
Chem. Commun. 1975, 248.
(8) Lucchini, V.; Modena, G.; Pasquato, L. J. Am. Chem. Soc. 1993,
115, 4527.
(12) Garratt, D. G.; Schmid, G. H. Can. J. Chem. 1974, 52, 1027.
(13) Reich, H. J.; Trend, J. E. Can. J. Chem. 1975, 53, 1922.
(14) Carretero, J. C.; Garcia Ruano, J. L.; Rodriguez, J. H. Tetrahedron
Lett. 1987, 28, 4593.
10.1021/ja984304l CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/09/1999

Nucleophilic Reactions at S of Thiiranium and Thiirenium
J. Am. Chem. Soc., Vol. 121, No. 16, 1999 3945
Chart 1
thiiranium and thiirenium ions with disulfide amounts to the
replacement of the substituent at sulfur. Therefore, to monitor
the reaction course, the alkyl substituents at thiiranium or
thiirenium sulfur and at disulfide must be different. One
exception is given by thiiranium ion 3, with tert-butyl and
adamantyl groups respectively cis and trans to the S-methyl
group. The reaction with dimethyl disulfide can be followed
by monitoring the formation of the isomer 4, with cis adamantyl
and trans tert-butyl groups. The two isomers can be easily
separated and have been demonstrated not to interconvert in
the absence of disulfides.6 The reactions have been carried out
in CD2Cl2 at 25 °C and monitored by 1H NMR at regular time
intervals.
The complete reaction system is reported in Scheme 2. The
R′′SSR′′ disulfide attacks the S-R sulfonium sulfur of the
thiiranium or thiirenium ion, generating the free olefin and the
mixed R′′S+(SR)SR′′ thiosulfonium salt. The latter can transfer
to the olefin one of the “other” SR′′ electrophilic group, giving
the thiiranium or thiirenium ion with the S-R′′ substituent and
the mixed disulfide RSSR”. This species is again able to carry
out the substitution at the original thiiranium or thiirenium ion,
giving rise to the disulfide RSSR. Furthermore, the fast
reequilibration among the three different disulfides catalyzed
by the thiosulfonium salt18 and the competitive rearrangement
of tert-butyl-substituted thiiranium ions to thietanium ions 8 or
9 and of tert-butyl-substituted thiirenium ions to thietium ions
10 have also to be considered.6,8 Under the adopted reaction
conditions (relatively high concentration of disulfide with respect
to that of the substrate), only the rearrangements of trans
thiiranium ions 1a,b and 3 (with a tert-butyl group cis to the
substituent at sulfur) to thietanium ions 8a,b and 9 have been
observed.
The thiosulfonium salts and the olefins could not be detected
in the 1H NMR spectra. The steady-state approximation applied
to these species leads to the equations reported in Scheme 3.
The differential equations related to Scheme 3 have been
numerically integrated19 and fitted to the experimental points
with the Simplex procedure.20 From the set of optimized
constants provided by the Simplex procedure, we report in Table
1 the second-order rate constants kS, describing the attack of
the “symmetric” nucleophile R′′SSR′′.
t-2,t-3 isomer 2 (Chart 1),15 which will be named in this paper
with the trivial names of trans and cis thiiranium ions. While
water attacks the cis isomer 2 at the ring carbons, the attack to
the trans isomer 1a occurs exclusively at sulfonium sulfur, with
the formation of (E)-di-tert-butylethylene and of protonated
methanesulfenic acid. The reaction with 1a may be considered
as the reverse reaction of the electrophilic addition of the same
reagent (and of a generic electrophile) to the E alkene. Thus,
from the principle of microscopic reversibility, the study of the
reactivity toward nucleophiles of stable thiiranium ions may give
insights on the early steps of the electrophilic addition to alkenes.
This concept may be extended to the reaction of thiirenium ions
with nucleophiles and to the electrophilic addition to alkynes.
The understanding of the mechanism and of the structures
of transition states and intermediates involved in the reaction
shown in Scheme 1 may help to rationalize the steric and
electronic parameters that direct the face selectivity in the
asymmetric electrophilic addition to prochiral olefins. A possible
application of this concept is offered by the recent asymmetric
syntheses of sulfur- and selenium-based enantiopure electrophilic
reagents.16,17
For the sake of the following discussion, the shapes and
energies of the LUMO and NLUMO with σ symmetry of
thiiranium ions 1a and 2 and of thiirenium ion 6a, as well
as the natural atomic charges at sulfur of the thiiranium and
thiirenium ions listed in Chart 1, have been determined
computationally ab initio at the RHF/3-21G* level, on geom-
etries optimized at the same level.21-23 The shapes and the
energies are reported in Figure 2, while the natural atomic
charges are given in Table 1.
We present in this paper a case of nucleophilic substitution
by dialkyl disulfides at the sulfonium sulfur of stable thiiranium
and thiirenium ions. The reaction has been investigated both
kinetically and theoretically on the basis of frontier orbital
interactions to determine the most likely mechanism.
Results
Discussion
We have selected for our study the reaction of weak
nucleophiles, such as dialkyl disulfides, with some thiiranium
and thiirenium ions carrying, at the carbon atom, bulky groups,
which at the same time increase the stability of the ions and
depress the possibility of nucleophilic addition to the carbon
centers.
Effects of Substituents at Carbon and Sulfur. The analysis
of the kinetic constants presented in Table 1 offers good hints
(18) Nelander, B.; Sunner, S. J. Am. Chem. Soc. 1972, 94, 3576.
(19) Press: W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T.
Numerical Recipes. The Art of Scientific Computing; Cambridge U. P.:
Cambridge, U.K., 1986; p 547.
(20) Nash, J. C. Compact Numerical Methods for Computers; Adam
Hilger Ltd.: Bristol, U.K., 1979; p 141.
The reagents used for this research and the reaction products
are listed in Chart 1. As shown in Scheme 2, the reaction of
(15) Lucchini, V.; Modena, G.; Pasi, M.; Pasquato, L. J. Org. Chem.
1997, 62, 7018.
(16) Lucchini, V.; Modena, G.; Pasquato, L. J. Chem. Soc., Chem.
Commun. 1994, 1565 and reference cited therein.
(17) Wirth, T.; Fragale, G.; Spichty, M. J. Am. Chem. Soc. 1998, 120,
3376.
(21) Spartan 4.0 program package, 1995, distributed by Wavefunction,
Inc., Irvine, CA 92715.
(22) The 3-21G* basis set includes d functions for second and higher
row elements. For sulfur: Gordon, M. S.; Binkley, J. S.; Pople, J. A.; Pietro,
W. J.; Hehre, W. J. J. Am. Chem. Soc. 1982, 104, 2797.
(23) The optimized geometries will be reported elsewere.

3946 J. Am. Chem. Soc., Vol. 121, No. 16, 1999
Fachini et al.
Scheme 2
Scheme 3
the lowering must be attributed to the greater steric hindrance
exerted by exocyclic isopropyl.24
The most significant comparison is between the reactivity of
diethyl disulfide with the trans di-tert-butylthiiranium ion 1a
and with the cis ion 2 (entries 1 and 2): to a measurable rate
for the substitution at sulfur in trans ion 1a, there corresponds
no observed substitution in cis isomer 2. This behavior is
paralleled by the nucleophilic attack of water.15 While water
reacts with sulfur in 1a, such reaction could not be observed in
2. The relevance of the substituents at ring carbons is also
evident by the rate constant increase observed in trans thiiranium
ion 5a, with a cis-oriented phenyl substituent at carbon, when
compared to the rate constant in trans thiiranium ion 1a, with
a cis tert-butyl group (entries 1 and 4).
As a final point, the inspection of the data in Table 1 reveals
that thiirenium ions react about 2 orders of magnitude faster
than thiiranium ions with the same substitution pattern (cf.
entries 1 and 7 and entries 3 and 10). No correlation is found
with the natural atomic charges at sulfur. The reactivity
difference is explained neither by steric nor by stereoelectronic
effects, but rather by the fact that the two ring systems are
basically different, so that other factors may interfere. We will
not further discuss this point.
Direction of Attack to Sulfonium Sulfur. The sulfonium
sulfur of thiiranium and thiirenium ions can in principle be
approached by the nucleophile from any direction. For the sake
of this discussion we select the reciprocally orthogonal directions
labeled x, y, and z in Figure 1.
regarding the preferred approaching direction of the nucleophile.
We will show that this preference is a decisive argument for
the selection of the most likely reaction mechanism.
This discussion is based on the assumption that the reaction
rates are mainly determined by the bulkiness of the substituents
at sulfur and ring carbons of thiiranium and thiirenium ions.
To rule out the possibility that the electrophilicity of the sulfur
atom may affect the kinetic constants, the natural atomic charges
at sulfur in thiiranium and thiirenium ions have been calculated
ab initio at the RHF/3-21G* level. The values, reported in Table
1, do not show any meaningful correlation with the reaction
rates, suggesting that electronic effects are negligible.
The x and y approaches are located in the plane described by
the three-member ring respectively along the direction desig-
nated by the bisector of the CSC angle and along a direction
orthogonal to it. In thiiranium ions with identical and equally
oriented substituents at ring carbons (as cis thiiranium ion 2),
the +y approach and the -y approach are equivalent, as
illustrated in Figure 1a. In thiiranium ions with different
substituents at ring carbons, or with differently oriented sub-
stituents, the +y approach and the -y approach are no longer
equivalent, as evidenced in Figure 1b. A closer inspection
reveals that the nucleophile is less hindered when approaching
Because of the relatively high error in the evaluation of the
kinetic constants, only differences of at least 1 order of
magnitude are considered significant and will be discussed.
The steric effects exerted by the methyl and ethyl substituents
at the sulfur of thiiranium ions are balanced by the reverse
substitution pattern of the dialkyl disulfides, so that the rate
constants are similar (cf. entries 4 and 5 and entries 7 and 8).
The presence of an isopropyl group at sulfur is more effective,
leading to a significant reactivity lowering both in thiiranium
ions (cf. entries 5 and 6) and in thiirenium ions (entries 8 and
9). Because the natural charges at sulfur show that the isopropyl
and the ethyl substituents exert similar stereoelectronic effects,
(24) This is in agreement with other observations: (a) Reetz, M. T., Seitz,
T. Angew. Chem., Int. Ed. Engl. 1987, 26, 1028. (b) Toshimitsu, A.; Nakano,
K.; Mukai, T.; Tamao, K. J. Am. Chem. Soc. 1996, 118, 2756.

Nucleophilic Reactions at S of Thiiranium and Thiirenium
J. Am. Chem. Soc., Vol. 121, No. 16, 1999 3947
Table 1. Calculated Natural Atomic Chargesa at Sulfur, Initial Concentrations, and Second-Order Rate Constantsb for the Reaction of
Disulfides with Some tert-Butyl-Substituted Thiiranium and Thiirenium Ions in CD2Cl2 at 25 °C
thiiranium ion
disulfide
molar concn
natural atomic
charge at sulfur
R
molar concn
kS, M-1 s-1
1
2
3
4
5
6
1a
2
3
5a
5b
5c
Me
Me
Me
Me
Et
0.68
0.71
0.74
0.64
0.70
0.68
2.0 × 10-2
2.3 × 10-2
4.9 × 10-3
1.7 × 10-2
1.7 × 10-2
1.8 × 10-2
EtSSEt
EtSSEt
MeSSMe
EtSSEt
MeSSMe
MeSSMe
7.4 × 10-1
8.8 × 10-1
3.7 × 10-1
1.0 × 10-1
1.0 × 10-1
4.6 × 10-1
4.5 (( 0.2) × 10-5
no obsd reaction
2.3 (( 0.2) × 10-5
4.4 (( 0.1) × 10-3
1.4 (( 0.1) × 10-3
5.9 (( 0.2) × 10-5
i-Pr
thiirenium ion
disulfide
natural atomic
charge at sulfur
R
molar concn
molar concn
kS, M-1 s-1
7
8
9
6a
Me
Et
i-Pr
Me
0.71
0.78
0.72
0.70
1.7 × 10-2
2.2 × 10-2
3.0 × 10-2
1.8 × 10-2
EtSSEt
3.3 × 10-2
3.9 × 10-2
1.2 × 10-1
3.8 × 10-2
1.5 (( 0.2) × 10-3
3.8 (( 0.4) × 10-3
7.8 (( 0.3) × 10-5
4.4 (( 0.4) × 10-3
6b
6c
7a
MeSSMe
MeSSMe
EtSSEt
10
a Calculated ab initio at the 3-21G*//3-21G* level. b Average and standard deviation from three experiments.
Figure 1. Orthogonal approaching directions of the nucleophile to the
sulfonium sulfur of (a) cis substituted and (b) trans-substituted-
thiiranium ions.
along the -y direction, where the substituents at carbon and at
sulfur are on the same side of the ring, than along the +y
direction, where the substituents are on the opposite sides of
the ring.
The approach along the z direction occurs from a direction
orthogonal to the molecular plane and from the opposite side
to the exocyclic substituent at sulfur. It should be considered
that in these ions the exocyclic C-S bond is almost perpen-
dicular to the plane containing the ring.25
The results outlined in the preceding paragraph show that
the nucleophile attack is sensitive to the substitution pattern both
at sulfur and at ring carbons. We can therefore rule out the z
approach, which cannot be influenced by the hindrance of the
substituent at sulfur, and the x approach, which is insensitive
to the substituent pattern at the ring carbons. Only the +y and
the -y approaches may be affected by the hindrances of the
complete substitution pattern. If we assume these as the
exclusive approaching directions (and electronic factors, il-
Figure 2. Shapes and eigenvalues (hartree) of LUMO and LUMO+1
of thiiranium ions 1a and 2 and of LUMO and LUMO+2 of thiirenium
ion 6a, calculated ab initio at the 3-21G*//3-21G* level.
lustrated in the next paragraph, will substantiate this assump-
tion), then the set of kinetic constants in Table 1 can be
comprehensively rationalized. With reference to Figure 1, the
two +y and -y approaches are both sterically hindered in the
case of cis thiiranium ion 2, and this explains why this ion is
unreactive toward disulfide. On the other hand, the -y approach
of trans thiiranium ion 1a is decidedly less hindered than the
(25) (a) Huang, X.; Batchelor, R. J.; Einstein, F. W. B.; Bennet, A. J. J.
Org. Chem. 1994, 59, 7108. (b) Destro, R.; Pilati, T.; Simonetta, M. J.
Chem. Soc., Chem. Commun. 1977, 576. (c) Destro, R.; Pilati, T.; Simonetta,
M. NouV. J. Chim. 1979, 3, 533.

3948 J. Am. Chem. Soc., Vol. 121, No. 16, 1999
Fachini et al.
Scheme 4
+y approach, so that the attack of disulfide may occur along
the -y direction. Only the approach along the -y direction
explains why the attack to thiiranium ions 5a and 5b (with a
cis-oriented phenyl substituent at one ring carbon) is so much
faster than the attack to thiiranium ions 1a and 3 (with a cis-
oriented tert-butyl substituent). A nucleophile approaching along
the +y, x, or z direction of the four thiiranium ions 1a, 3, and
5a,b would be subjected to steric effects almost to the same
degree.
Symmetry Properties of LUMO and NLUMO of Thiira-
nium and Thiirenium Ions. The LUMOs of di-tert-butyl-
substituted thiiranium ions 1a and 2 and thiirenium ion 6a,
calculated ab initio at the 3-21G*//3-21G* level and reported
in Figure 2, are Walsh-type orbitals of b2 symmetry (apart from
small local perturbations in 1a and 2, due to the different
orientations of the tert-butyls). These orbitals present the greatest
extension around the sulfur atom and along the y direction and
offer the greatest interaction with an occupied orbital of the
nucleophile when this latter approaches exactly along this
direction. This interaction, between an occupied orbital of the
nucleophile with the LUMO of the electrophile, is energy-gap
controlled.
The next vacant orbital with σ symmetry is the NLUMO
(LUMO+1) in the case of thiiranium ions 1a and 2 and the
LUMO+2 in the case of thiirenium ion 6a. These orbitals, which
are of a1 symmetry (with local perturbations originating from
the substituents at sulfur and ring carbons), display the greatest
extension along the x direction. The interaction with a nucleo-
phile approaching along this direction will be predominant
(orbital-overlap control) only if a particularly favorable orbital
overlap overcomes the LUMO interaction with the nucleophile
in the y direction. Our kinetic data, which point to an approach
along the y direction, indicate that the nucleophilic attack to
sulfur of thiiranium and thiirenium ions is dominated by the
energy-gap control.
Mechanism of Electrophilic Addition to Unsaturated C-C
Bond. The first step of the electrophilic addition of sulfenyl
chlorides to alkenes or alkynes may also be regarded as the
nucleophilic attack of the unsaturated C-C bond to the sulfur
atom of sulfenyl chloride. As such, the principle of maximum
overlap between the HOMO of the nucleophile, i.e., the
symmetric π orbital, and the LUMO of the electrophile will
dictate the symmetrical approach of the electrophile (i.e.,
equidistant from the two unsaturated C atoms). In analogy with
the nucleophilic substitution at the sp3 carbon, the three classical
mechanisms, SN1, SN2, and AdN-E will be discussed in
connection with the substitution at sulfur.26 The three mecha-
nisms are shown in Scheme 4 and may be outlined as follows:
(i) SN1, with rate-determining unimolecular heterolysis of the
S-Cl bond; (ii) SN2, with concerted approach of the π cloud
of the unsaturated C-C bond and detachment of the chloride
ion; (iii) AdN-E, with the intermediacy of sulfuranic species,
which may be considered the mechanistic analogue of the
Meisenheimer intermediate27 (for the nucleophilic addition to
aromatic rings) or of the anionic tetrahedral intermediate28 (for
the nucleophilic addition to C-C double bonds).
The SN1 process implies the generation of the sulfenium
cation. However, species that can be assigned to the structure
of a RS+ ion have been detected only in the gas phase and only
in the case of aromatic sulfenium ions.29 Any attempt to generate
these ions in a condensed medium has always led to species
where the RS+ moiety is associated with a carrier. Thus the
action of strong Lewis acids (AgBF4 or SbCl5) on sulfenyl
halides generates chlorosulfonium ions, which may be described
as sulfenium ions carried by sulfenyl chloride molecules.30 For
these reasons we are inclined to exclude the possibility of a
nonconcerted SN1 process.31
As in the case of nucleophilic substitution at the sp3 carbon,
the SN2 mechanism requires strict limitations to the entering
direction of the nucleophile, the π electrons of the double
bond: the approach must occur along the direction of the S-Cl
bond and from the side opposite to the leaving group. If the
addition of sulfenyl chloride to an unsaturated C-C bond
follows this mechanism, and because the “product” is generated
in one step, the principle of microscopic reversibility requires
(26) IUPAC recommends the designators DN+AN, ANDN, and AN+DN
for the SN1, SN2, and AdN-E mechanisms, respectively. Guthrie, R. D.;
Jencks, W. P. Acc. Chem. Res. 1989, 22, 343.
(27) Bunnett, J. F.; Zahler, R. E. Chem. ReV. 1951, 49, 273. Manderville,
R. A.; Buncel, E. J. Am. Chem. Soc. 1993, 115, 8985.
(28) Rappoport, Z. Acc. Chem. Res. 1992, 25, 474 and references therein.
(29) Paradisi, C.; Hamdan, M.; Traldi, P. Org. Mass Spectrom. 1990,
25, 296.
(30) Capozzi, G.; Lucchini, V.; Modena, G.; Rivetti, F. J. Chem. Soc.,
Perkin Trans. 2 1975, 361.
(31) A stable sulfenium ion, with a sulfur atom protected toward
nucleophiles by a cage, has been recently isolated and characterized:
Kobayashi, K.; Sato, S.; Horn, E.; Furukawa, N. Tetrahedron Lett. 1998,
39, 2593.

Nucleophilic Reactions at S of Thiiranium and Thiirenium
J. Am. Chem. Soc., Vol. 121, No. 16, 1999 3949
and known compounds used in this research were either purchased from
standard chemical suppliers or prepared according to literature proce-
dures and purified to match the reported physical and spectral data.
Solvents were purified according to standard procedures.
that the nucleophilic substitution by chloride ion to the sulfonium
sulfur of the “product” (thiiranium or thiirenium ion) will occur
along the same direction traveled by the leaving group (the
chloride ion) in the forward reaction. Thus, according to this
mechanism, the entering direction of chloride ion, or of any
other nucleophile, to the sulfur atom of thiiranium and thiirenium
ions must be the x direction shown in Figure 1. The kinetic
data in Table 1 and the considerations based on orbital
interaction exclude the probability of this approaching direction.
The exclusion of the x approach is particularly important, as it
rules out the SN2 mechanism in the nucleophilic substitution to
sulfonium sulfur and hence, for the principle of microscopic
reversibility, also the SN2 mechanism for the nucleophilic
substitution at the sulfenyl halide by the C-C unsaturated bond.
The AdN-E mechanism requires an intermediate, which may
be described as an episulfurane species. We have already
outlined in the Introduction that, although episulfuranes have
been proposed as “first” intermediates in the addition of sulfenyl
chlorides to alkenes and alkynes,10 their existence has never
been demonstrated.11-14 The addition of a nucleophile to
thiiranium or thiirenium ions along the less hindered y direction
may generate an episulfurane structure, which has the charac-
teristics of an intermediate and cannot be considered a SN2
transition state, because the direction of the entering group and
the detachment direction of the leaving group (the C-C moiety)
are far from collinearity (the two directions are approximately
orthogonal). Thus, the y approach requires the intermediacy of
a discrete species and is therefore compatible with the AdN-E
mechanism only.
Kinetic Measurements. The concentrations of the reagents in CD2-
Cl2 have been determined by comparison of the integrated areas of
appropriate resonances with that of the proton impurity of the solvent.
The concentration of the impurity has been previously measured by
comparison with the signal of 1,4-dinitrobenzene, at a concentration
determined by weighing. The reactions have been monitored, at regular
intervals of time, using NMR tubes equipped with airtight screw caps.
To compensate for the varying spectrometer conditions, the monitored
intensities were normalized against their sum. The differential equations
in Scheme 3 have been numerically integrated19 and fitted to the
normalized concentrations with the Simplex procedure.20
General Synthesis of Reagents. The hexachloroantimonates of ethyl
and isopropyl alkylbis(alkylthio)sulfonium salts have been prepared
similarly to the methyl analogue.32 The thiiranium and thiirenium ions
have been prepared from the corresponding alkenes or alkynes and the
appropriate thiosulfonium hexachloroantimonates following the already
reported procedures.6,8 The spectral characteristics of thiiranium and
thiirenium hexachloroantimonates 1a-4 and 6a and of thietanium
hexachloroantimonates 8a and 9 are reported elsewhere.6,8
Ethylbis(ethylthio)sulfonium Hexachloroantimonate. Prepared in
75% yield as a viscous oil. 1H NMR (200 MHz, CD2Cl2): δ 1.61 (9H
t, J ) 7.3); 3.52 (6H, broad q, J ) 7.3).
Isopropylbis(isopropylthio)sulfonium Hexachloroantimonate. Ob-
1
tained in 78% yield as an oil. H NMR (250 MHz, CD2Cl2): δ 1.60
(18H, d, J ) 6.6), 3.89 (3H, broad m).
c-2,t-3-Di-tert-butyl-r-1-ethylthiiranium Hexachloroantimonate
(1b). 1H NMR (250 MHz, CD2Cl2): δ 1.18 (9H, 3-t-Bu, s), 1.35 (9H,
2-t-Bu, s), 1.68 (3H, 1-Me, broad t, J ) 7.3), 3.16 (1H, 1-CH2, m),
3.33 (1H, 1-CH2, m), 3.80 (1H, 2-H, d, J ) 13.4), 4.04, (1H, 3-H, d).
13C NMR (62.9 MHz, CD2Cl2): δ 14.26, 27.26, 29.32, 31.81, 33.03,
35.09, 72.27, 72.83. Anal. Calcd for C12H25Cl6SSb: C, 26.9; H, 4.7;
S, 6.0. Found: C, 26.8; H, 4.4; S, 5.7.
t-2-tert-Butyl-c-3-phenyl-r-1-methylthiiranium Hexachloroanti-
monate (5a). 1H NMR (250 MHz, CD2Cl2): δ 1.29 (9H, s, t-Bu), 2.16
(3H, s, Me), 4.38 (1H, 2-H d, J ) 12.0), 5.56 (1H, 3-H, d); 7.60-7.76
(5H, Ph, m). The orientation of the tert-butyl and phenyl groups and
the assignment of the ring protons have been determined with
differential NOE spectroscopy.33 The results are reported as follows:
observed nucleus: {perturbed nucleus}, % enhancement. 1H NOE (200
MHz, CD2Cl2): t-Bu: {3-H}, 0.8; Me: {2-H}, 1.5; {Ph, o-H}, 0.9;
2-H: {t-Bu}, 10.8; {Me}, 3.7; {Ph, o-H}, 8.6; 3-H: {t-Bu}, 18.0; {Me},
0.6; {2-H}, 1.4; {Ph, o-H}; Ph, o-H: {Me}, 0.9; {2-H}, 2.9; {3-H},
1.7. 13C NMR (62.9 MHz, CD2Cl2): δ 16.67, 27.15, 33.59, 66.05, 71.37,
122.80, 130.65, 131.62, 133.94. Anal. Calcd for C13H19Cl6SSb: C, 28.8;
H, 3.5; S, 5.9. Found: C, 27.8; H, 3.5; S, 5.6.
t-2-tert-Butyl-c-3-phenyl-r-1-ethylthiiranium Hexachloroanti-
monate (5b). 1H NMR (250 MHz, CD2Cl2): δ 1.28 (9H, t-Bu, s), 1.33
(3H, Me, t, J ) 7.5), 2.47 (2H, CH2, m), 4.36 (1H, 2-H, d, J ) 11.8),
5.58 (1H, 3-H, d), 7.60-7.75 (5H, Ph, m). 13C NMR (62.9 MHz, CD2-
Cl2): δ 12.08, 27.19, 29.68, 33.25, 66.50, 69.11, 123.22, 130.62, 131.33,
133.79. Anal. Calcd for C14H21Cl6SSb: C, 30.2; H, 3.8; S, 5.8. Found:
C, 29.8; H, 3.7; S, 5.5.
t-2-tert-Butyl-c-3-phenyl-r-1-isopropylthiiranium Hexachloroan-
timonate (5c). 1H NMR (250 MHz, CD2Cl2): δ 1.13 (3H, 1-Me, d, J
) 6.9), 1.27 (9H, t-Bu, s), 1.60 (3H, 1-Me, d, J ) 6.9), 2.44 (1H, 1-H,
heptet), 4.35 (1H, 2-H, d, J ) 11.8), 5.59 (1H, 3-H, d), 7.60-7.76
(5H, Ph, m). 13C NMR (62.9 MHz, CD2Cl2): δ 20.70, 22.08, 27.26,
33.06, 44.26, 66.62, 68.46, 123.28, 130.71, 131.17, 133.78. Anal. Calcd
for C15H23Cl6SSb: C, 31.6; H, 4.1; S, 5.6. Found: C, 30.0; H, 4.0; S,
6.0.
Conclusions
The LUMO of thiiranium and thiirenium ions, with b2
symmetry, enjoys the greatest interaction (under energy-gap
control) with an occupied orbital of the disulfide nucleophile
when this latter approaches along the y direction (Figure 1).
The interaction leads to the formation of an episulfurane
intermediate, implying the occurrence of the AdN-E mechanism.
The LUMO+1 of thiiranium ions or the LUMO+2 of thiirenium
ions, with a1 symmetry, have the greatest interaction (under
orbital-overlap control) with the nucleophile approaching along
the x direction. This direction is collinear with the detachment
direction of the C-C unsaturated bond as leaving group. The
reaction will therefore occur with a SN2 mechanism.
The comprehensive consideration of the kinetic data reported
in Table 1 points to the y approaching direction and therefore
to the AdN-E mechanism and to the intermediacy of an
episulfurane species. It is of course conceivable that the
episulfurane intermediate, as a discrete species, can undergo
some degree of rearrangement along the reaction coordinate.
The principle of microscopic reversibility suggests the geometry
of the episulfurane structure directly derived from the addition
of chloride ion to thiiranium or thiirenium ion but cannot give
any hint regarding the eventual rearrangement of this structure
and the reciprocal orientation of alkene or alkyne and of sulfenyl
chloride during the first step of the electrophilic addition to the
unsaturated C-C bond (cf. Scheme 4).
Experimental Section
2,3-Di-tert-butyl-1-ethylthiirenium Hexachloroantimonate (6b).
1H NMR (250 MHz, liquid SO2): δ 1.48 (3H, 1-Me, t, J ) 7.6), 1.55
General. Melting points are uncorrected. 1H NMR spectra were
recorded at 200 MHz or at 250 MHz, 13C NMR at 62.9 MHz, using
CD2Cl2 or liquid SO2 as solvent. The NMR instruments are equipped
with a variable-temperature control unit, and all the kinetic measure-
ments were obtained at 25 °C. Commercial CD2Cl2 was carefully
anhydrified before use with A4 molecular sieves. Commercial reagents
(32) (a) Capozzi, G.; Lucchini, V.; Modena, G.; Rivetti, F. J. Chem.
Soc., Perkin Trans. 2 1975, 900. (b) Weiss, R.; Schlierf, C. Synthesis 1976,
323.
(33) Neuhaus, D.; Williamson, M. The Nuclear OVerhauser Effect in
Structural and Conformational Analysis; VCH Publishers: New York, 1989.

3950 J. Am. Chem. Soc., Vol. 121, No. 16, 1999
Fachini et al.
(18H, t-Bu, s), 2.95 (2H, 1-CH2, q). 13C NMR (62.9 MHz, CD2Cl2): δ
10.98, 28.16,33.28, 40.14, 114.98. Anal. Calcd for C12H23Cl6SSb: C,
27.0; H, 4.3; S, 6.0. Found: C, 26.9; H, 4.2; S, 5.9.
2-Adamantyl-3-tert-butyl-1-ethylthiirenium Hexachloroantimonate
(7b). H NMR (250 MHz, CD2Cl2): δ 1.46 (3H, 1-Me, t, J ) 7.6),
1
1.50 (9H, t-Bu, s), 1.7-2.3 (15H, m, adamantyl), 2.93 (2H, 1-CH2, q).
13C NMR (62.9 MHz, CD2Cl2): δ 11.04, 27.56, 28.23, 33.18, 34.81,
35.39, 40.20, 40.37, 113.94, 114.06. Anal. Calcd for C18H29Cl6SSb:
C, 35.3; H, 4.8; S, 5.2. Found: C, 34.2; H, 4.7; S, 4.9.
t-4-tert-Butyl-r-1-ethyl-2,2,c-3-trimethylthietanium Hexachloro-
antimonate (8b). 1H NMR (250 MHz, CD2Cl2): δ 1.12 (9H, t-Bu, s),
1.28 (3H, 3-Me, d, J ) 6.5), 1.56 (3H, 1-Me, m), 1.80 (6H, 2-Me2,
bs), 3.14 (1H, m), 3.39 (1H, m), 3.63 (1H, m), 3.76 (1H, 4-H, broad d,
J ) 11.3). 13C NMR (62.9 MHz, CD2Cl2): δ 11.62, 16.61, 19.21, 27.03,
30.24, 33.53, 36.53, 44.25, 66.22, 72.92. Anal. Calcd for C12H25Cl6-
SSb: C, 26.9; H, 4.7; S, 6.0. Found: C, 26.7; H, 4.5; S, 5.8.
2,3-Di-tert-butyl-1-isopropylthiirenium Hexachloroantimonate
1
(6c). H NMR (250 MHz, CD2Cl2): δ 1.47 (6H, 1-Me, d, J ) 6.9),
1.50 (18H, t-Bu, s), 3.25 (1H, 1-H, heptet). 13C NMR (62.9 MHz, CD2-
Cl2): δ 19.68, 28.58, 33.09, 47.33, 115.86. Anal. Calcd for C13H25-
Cl6SSb: C, 28.5; H, 4.6; S, 5.8. Found: C, 28.3; H, 4.2; S, 5.4.
2-Adamantyl-3-tert-butyl-1-methylthiirenium Hexachloroanti-
monate (7a).1H NMR (250 MHz, CD2Cl2): δ 1.51 (9H, t-Bu, s), 1.7-
2.3 (15H, adamantyl, m). 13C NMR (62.9 MHz, CD2Cl2): δ 27.47,
27.94, 30.39, 33.45, 35.09, 35.39, 39.89, 113.64, 113.87. Anal. Calcd
for C17H27Cl6SSb: C, 34.1; H, 4.5; S, 5.4. Found: C, 33.5; H, 4.5; S,
5.4.
JA984304L