Stereoelectronic effects in cyclic sulfoxides, sulfones, and sulfilimines: Application of the perlin effect to conformational analysis

Article abstract of DOI:10.1002/chem.200601468

The ¹J_C-H coupling constants in conformationally constrained sulfoxides, bissulfoxides, sulfoxide-sulfones, and sulfilimines derived from 2-benzylidene-1,3-dithiane and 2-(2,2-dimethylpropylidene)-1,3- dithiolane were measured by mea

Full text of DOI:10.1002/chem.200601468

FULL PAPER
DOI: 10.1002/chem.200601468
Stereoelectronic Effects in Cyclic Sulfoxides,Sulfones,and Sulfilimines:
Application of the Perlin Effect to Conformational Analysis
Tobias Wedel,[a] Monika Müller,[a] Joachim Podlech,*[a] Helmut Goesmann,[b] and
Claus Feldmann[b]
Dedicated to Professor Günter Szeimies on the occasion of his 70th birthday
1
ꢀ
Abstract: The JCꢀ coupling constants
have a strong influence on the magni-
tude of coupling constants for axial and
C H bonds. The resultant weakening
H
ꢀ
in conformationally constrained sulfox-
ides, bissulfoxides, sulfoxide-sulfones,
and sulfilimines derived from 2-benzyl-
idene-1,3-dithiane and 2-(2,2-dimethyl-
propylidene)-1,3-dithiolane were mea-
sured by means of HMQC and HSQC
NMR experiments and the Perlin ef-
fects were calculated. The type and the
relative configuration of S=X groups
(X= O, NTos) in these compounds
of the respective C H bonds leads to a
ꢀ
equatorial C H bonds, respectively.
smaller coupling constant than for a re-
spective equatorial C H bond. Equato-
rial S=O groups have an influence on
b-C H bonds through a homoanomeric
ꢀ
Axial S=O bonds give rise to a stereo-
electronic effect on antiperiplanar axial
ꢀ
ꢀ
effect. Here, the axial C H bond is
Keywords: C-H coupling constants ·
heterocycles · hyperconjugation ·
NMR spectroscopy · Perlin effect ·
sulfur
weakened and a smaller coupling con-
stant is measured. Sulfilimine groups
show similar effects to sulfoxide
groups.
Introduction
decrease of the bond order (as can be visualized by a
double-bond–no-bond resonance)[5] is accompanied by a de-
1
Coupling constants give valuable information about the con-
formation and configuration especially in constrained mole-
cules with fixed geometry. First to mention are JHꢀ cou-
crease of the JCꢀ coupling constant.[6] This observation has
H
later been referred to as the Perlin effect.[7]
3
The nature of the proximate heteroatoms or functional
groups has a strong influence on which of the geminal hy-
drogen atoms shows a higher C-H coupling constant.[8] In
1,3-dioxane, for example, the axial lone pair is in resonance
with axial hydrogen atoms in the 2-, 4-, and 6-position lead-
ing to smaller coupling constants as for the respective equa-
torial hydrogens (normal Perlin effect, Figure 1, top).[9–12] In
H
pling constants which are strongly dependent on the torsion
angle of the respective vicinal hydrogens.[1] Further informa-
1
tion can be obtained from JCꢀ coupling constants though
H
these are significantly less frequently measured or used for
structure elucidation.[2] Perlin and Casu recognized that
1JCꢀ coupling constants of geminal hydrogens differ when
H
they are in the vicinity of heteroatoms when conformational
other words, an nO!s*ax-C hyperconjugation leads to a
ꢀ
H
[13]
fluctuation is inhibited.[3] This has been explained by stereo-
weakening of the axial C H bond.
ꢀ
electronic effects in which orbitals (bonds or lone pairs) are
An anomalous pattern in the magnitude of the coupling
constants has been found for similar systems containing
[4]
ꢀ
interacting with antiperiplanar C H bonds. The resultant
sulfur atoms.[7,10–12,14] Here, a hyperconjugation from the sCꢀ
S
bond into the s*eq-C is responsible for smaller coupling
ꢀ
H
[a] Dr. T. Wedel, Dipl.-Chem. M. Müller, Prof. Dr. J. Podlech
Institut für Organische Chemie der Universität Karlsruhe (TH)
Fritz-Haber-Weg 6, 76131 Karlsruhe (Germany)
Fax : (+49)721-608-7652
ꢀ
constants of the adjacent equatorial C H bonds (reverse
Perlin effect, Figure 1, bottom).[11,14–16] Alabugin calculated
the influence of the converse seq-C H!s*Cꢀ interaction and
ꢀ
S
E-mail: joachim.podlech@ioc.uka.de
found its contribution to be much smaller.[10]
[b] Dr. H. Goesmann, Prof. Dr. C. Feldmann
Perlin effects in cyclic sulfoxides and sulfones have been
Institut für Anorganische Chemie der Universität Karlsruhe (TH)
Engesserstrasse 15, 76131 Karlsruhe (Germany)
investigated occasionally (Figure 2).[17–19] A sax-C H!s*S
ꢀ
=
O
hyperconjugation[20,21] should lead to a normal Perlin effect.
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
However, in cyclic substrates elongation of the respective
Chem. Eur. J. 2007, 13, 4273 – 4281
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4273

bearing an equatorial S=O group, the coupling constant
here is significantly higher (1Jax-C-6 =138.8 Hz) than that for
the isomeric compound 2 (1Jax-C-6 =133.8 Hz). Nevertheless,
it has to be kept in mind, that interfering effects of the sili-
con might occur in these compounds. A normal Perlin effect
in thiane-1,1-dioxide (4) and 1,3-dithiane-1,1-dioxide (5) was
deduced from a calculated +2.8 and +2.6 Hz difference, re-
spectively, for the coupling constants a to the sulfone
group.[18] Stereoelectronic effects and especially Perlin ef-
fects have to the best of our knowledge never been studied
in sulfilimines.[22]
In the course of our investigations on nucleophilic addi-
tions to alkylidenebissulfoxides[23] we were able to synthe-
size several cyclic sulfoxides and sulfilimines which should
give further insights in the stereoelectronic effects occurring
in these compounds. These compounds are conformationally
constrained due to allylic strain without an interfering effect
arising from a bulky substituent in positions C-4, C-5, or C-
6, respectively. In this paper we wish to report on these ef-
fects.
Figure 1. Stereoelectronic effects in 1,3-dioxanes and 1,3-dithianes visual-
ized by a double-bond–no-bond resonance.
Results and Discussion
(RS,RS)-2-Benzylidene-1,3-dithiane-1,3-dioxide (6) and rac-
(RS,RS,2Z)-2-benzylidene-3-(4-methylphenylsulfonylimino)-
1,3-dithian-1-oxide (7): Structure and conformation of di-
thiane-derived alkylidenebissulfoxide 6 were determined by
X-ray crystallographic analysis.[23a] This compound is confor-
mationally not very flexible (flipping of the chair is equiva-
lent to an interchange of substituents at the alkylidene
moiety), the conformation bearing the phenyl substituent in
the vicinity of the axial S=O group should be significantly
more stable than the flipped-chair conformation. Though
the alkylidene moiety might have a (presumably small)
effect on the stereoelectronic interactions at C-4, C-5, and
C-6, it is mandatory to achieve a conformationally rigid sub-
1
strate. JCꢀ coupling constants were determined by means
H
of coupled HMQC[24,25,26] and HSQC[25,27] NMR experiments
Figure 2. Normal Perlin effects in sulfoxides and sulfones with an axial
(Table 1). A small negative difference in the coupling con-
1
S=O bond (D1J=1Jeqꢀ Jax).[17–19]
stants (DJC-4 H =ꢀ0.7 Hz) at position C-4 of bissulfoxide 6
ꢀ
indicates a reverse Perlin effect which is explained by
C H bond is possible only with an axial, that is, antiperipla-
sC-2 S!s*eq-C-4 hyperconjugation (Figure 3, top row).[7,21]
ꢀ
ꢀ
ꢀ
ꢀH
nar S=O bond, whereas equatorial S=O groups could lead
to a reverse Perlin effect as observed for 1,3-dithianes. Ex-
perimental NMR spectroscopic data have been supplied for
oxidized 1,4-thiasilinanes 1–3, allocating a normal Perlin
effect for compounds 1 and 2 in position C-6.[17] Though the
Perlin effect is not reversed at position C-6 for compound 3
Because a C S(=O) bond is a poor donor (as compared
with a C S bond), this effect is only small. Nevertheless,
X-ray crystallographic analysis revealed that S3 C2
(179.6 pm) is slightly longer than S1 C2 (178.6 pm).
[18]
ꢀ
ꢀ
[23]
ꢀ
The normal Perlin effect for position C-5 is quite unex-
pected (DJC-5 H =+2.1 Hz). In 1,3-dithiane a reverse Perlin
ꢀ
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Chem. Eur. J. 2007, 13, 4273 – 4281

Stereoelectronic Effects in Cyclic Sulfur Compounds
FULL PAPER
Table 1. 1JCꢀH coupling constants in dithiane-derived bissulfoxide 6.
effects are often small they are additive in magnitude. The
axial C-5–H is weakened, either by interaction with the
axial S=O bond (syn effect, Figure 3, third row) or with the
equatorial S=O bond (plough effect, second row). If the
latter effect was working to a significant extent, the Perlin
effect should be even larger in the sulfoxide-sulfone 10 bear-
ing two axial S=O groups (see below). Because it is about
the same, we assume in fact that interaction with the equa-
torial S=O group is the dominant effect. Nevertheless, fur-
ther theoretical work on this topic seems to be mandatory
for a conclusive argumentation. It has to be noted that
Carbon
d
A
Proton
d
A
1JCꢀ
Perlin effect
H
[Hz]
DJ[Hz]
4-Heq
4-Hax
5-Heq
5-Hax
6-Heq
6-Hax
3.75
2.93
2.44
3.19
3.31
2.85
141.1
141.8
134.0
131.9
140.6
137.4
C-4
C-5
C-6
56.1
14.9
48.9
ꢀ0.7
+2.1
+3.2
though a normal Perlin effect is observed for position C-5
1
ꢀ
the equatorial C H is also significantly weakened ( J=
134.0 Hz), which might be due to a sCꢀS!s*ax-C interac-
ꢀ
H
tion.
A significant normal Perlin effect is observed for C-6
(DJC-6 H =+3.2 Hz). Hyperconjugation with the axial S=O
ꢀ
ꢀ
bond leads to a weakening of the axial C H bond at posi-
tion C-6 (Figure 3, fourth row). The concomitant lengthen-
ing of this S=O bond is confirmed by X-ray crystallographic
analysis and this bond is 1.4 pm longer than the equatorial
S=O bond at position C-6. The competing sC-2 ! s*eq-C-6
ꢀ
S
ꢀ
H
interaction which is occurring at position C-4 seems to be
too weak to impair the normal Perlin effect.
Sulfoxide-sulfilimine 7 was prepared from the correspond-
ing sulfilimine 9 (see below) by oxidation with meta-chloro-
1
perbenzoic acid (mCPBA). Inspection of the JCꢀ coupling
H
constants revealed that stereoelectronic effects arising from
the sulfilimine group are quite similar to those observed for
1
the corresponding bissulfoxide. While the very high Jax-C-4
ꢀ
H
coupling constant at the axial C-4–H (149.9 Hz) virtually ex-
cludes any stereoelectronic effect working at this bond, the
Figure 3. Explanation of Perlin effects in bissulfoxide 6.
1Jeq-C-4 coupling constant is somewhat reduced (145.6 Hz).
ꢀ
H
Nevertheless, it is still significantly higher than that observed
effect is observed for this position resulting from seq-C-H
!
for the corresponding bond in sulfoxide 8 giving evidence
ꢀ
ꢀ
s*Cꢀ interaction. By contrast, the axial C H bond in posi-
that an S
A
S
ꢀ
tion C-5 is significantly weakened in bissulfoxide 6. The pos-
sibility and the quality of stereoelectronic effects towards a
bond in the b position of heteroatoms (homoanomeric
effect) has been calculated and discussed repeatedly.[9,11,28,29]
Alabugin and co-workers pointed out that hyperconjugation
S(=O) C bond (Table 2).
rac-(E)-2-Benzylidene-1,3-dithiane-1-oxide (8) and rac-(E)-
2-benzylidene-1-(4-methylphenylsulfonylimino)-1,3-dithiane
(9): Compound 8 was prepared by oxidation of 2-benzylid-
U
is possible from equatorial and axial lone pairs to equatorial
C
[29]
ꢀ
and axial C H bonds. W effects (interaction of two equa-
torial orbitals, one being the donor, one the acceptor) and
syn effects (interaction of two axial orbitals) are not neces-
sarily stronger than the so-called plough effects (interaction
between equatorial and axial orbitals).[30] The magnitude of
the interaction is dependent on the donor and the acceptor
orbital and, thus, from the heteroatoms and the atoms at-
tached in the b position. Unfortunately, Alabugin and co-
workers examined only substrates with lone pairs at the het-
eroatoms which unambiguously act as donors. The hyper-
ꢀ
conjugation occurring here from C H bonds in position C-5
with acceptor bonds like the b-S=O bonds have never been
studied. Nevertheless, the NMR spectroscopic data, that is,
1
the JCꢀ coupling constants can supply some information.
H
Alabugin and co-workers found that though homoanomeric
Chem. Eur. J. 2007, 13, 4273 – 4281
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4275

J. Podlech et al.
with 81% yield after chromatography. The signals in the
1H NMR spectrum are partly superimposed which made
a detailed analysis impossible (Table 3). Nevertheless, the
1JCꢀ coupling constants were measured and the Perlin ef-
H
fects were calculated wherever the signals were sufficiently
separated and were able to be assigned.
Table 3. 1JCꢀH coupling constants in dithiane-derived sulfoxide 8.
Carbon
d [ppm]
Proton
d [ppm]
1JCꢀ
Perlin effect
H
[Hz]
DJ[Hz]
4-Heq
4-Hax
5-Heq
5-Hax
6-Heq
6-Hax
2.65
139.1
C-4
C-5
C-6
31.8
27.2
55.0
ꢀ3.3
ꢁ0.0
ꢀ1.4
2.82–2.91
2.46–2.57
2.46–2.57
3.43
142.4
ꢁ132[a]
ꢁ132[a]
139.9
2.82–2.91
141.3
Figure 4. Explanation of Perlin effects in monosulfoxide 8.
[a] Signals partly obscured. A shallow analysis of the spectra revealed
that DJ is close to zero.
(ꢁ132 Hz), meaning that a stereoelectronic effect is occur-
ꢀ
ring for both C H bonds. We assume that a homoanomeric
Steric constraints and evaluation of the 1JCꢀ coupling
effect from the axial sulfur lone pair to the equatorial C H
ꢀ
H
constants (see above) confirmed that this compound bears
an equatorial sulfoxide group trans to the phenyl substitu-
ent. Equatorial oxidation at the other sulfur would lead to a
significant steric hindrance between the oxygen and the
phenyl group and oxygen in one of the axial positions would
lead to differing Perlin effects. Further evidence for the pro-
posed configuration arises from the g-gauche effect in this
substrate (see below, Figure 7) and from the chemical shift
of the vinylic proton (d=7.52 ppm), which is exactly identi-
cal for the bissulfoxide 6 and the monosulfoxide 8. Coupling
constants for the equatorial hydrogen next to the equatorial
sulfoxide are in the same range for both compounds (see
Supporting Information). We think that oxidation is in fact
occurring in the equatorial position, though axial oxidation
with subsequent flipping of the chair cannot generally be ex-
(nax S!s*eq-C H) similar to that calculated for thiane[29] is re-
ꢀ
ꢀ
sponsible for a short equatorial bond. It is noteworthy that
for this axial sulfur lone pair a 100% p character was calcu-
lated, resulting in more distinct donor ability.[29] The axial
ꢀ
C H bond might be stabilized by sax-C H!s*eq-S as it has
ꢀ
=
O
ꢀ
been proposed for the axial C H in bissulfoxide 6.
Again a negative Perlin effect (DJC-6 H =ꢀ1.4 Hz) next to
ꢀ
the equatorial sulfoxide group at position C-6 can be ex-
ꢀ
plained by sC-2 S!s*eq-C-4 hyperconjugation. Because a C
ꢀ
ꢀ
H
(S=O) bond will not be as effective a donor relative to the
ꢀ
C S bond, the effect is smaller than that observed for posi-
tion C-4.
Sulfilimine 9 was prepared from 2-benzylidene-1,3-di-
thiane by sulfilimination with chloramine T. The structure of
this substrate could unambiguously be confirmed by X-ray
crystallographic analysis (Figure 5). The NMR spectra con-
cluded. Interaction of the axial sulfur lone pairs in alk
A
A
firmed that the conformation present in the crystal is pre-
1
substantial reduction of the donor ability of these lone pairs
as the equatorial lone pairs are available for oxidation.
ferred similarly in solution. Evaluation of the JCꢀ coupling
H
constants again revealed that sulfilimines and sulfoxides
give rise to similar stereoelectronic effects. As has already
A negative Perlin effect (DJC-4 H =ꢀ3.3 Hz) was deter-
ꢀ
1
mined for position C-4 next to the sulfide moiety. A hyper-
been noted for sulfilimine 7 the axial JCꢀH coupling constant
conjugation sCꢀS!s*eq-C should be responsible for this ob-
at position C-6 is very high and participation of the equato-
rial C-6-H in hyperconjugation is less effective than that of
the corresponding sulfoxide 8 giving rise for a more distinct
negative Perlin effect (Table 4).
ꢀ
H
servation as it has been calculated for the parent 1,3-di-
thiane (Figure 4). The measured negative Perlin effect of
DJ=ꢀ6.7 Hz for position C-4 in 2-tert-butyl-1,3-dithiane
should be due to similar effects.[14b]
Discussion of the effects working at position C-5 is not
C
1
possible because the H NMR signals at this position are not
resolved and the 1JCꢀH coupling constants are not as accurate
as for the other protons. Nevertheless, the difference seems
to be close to zero and both coupling constants are small
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Stereoelectronic Effects in Cyclic Sulfur Compounds
FULL PAPER
Figure 5. Structure of sulfilimine 9 in the crystal.[23]
Table 4. 1JCꢀH coupling constants in dithiane-derived sulfilimine 9.
Carbon
d [ppm]
Proton
d [ppm]
1JCꢀ
Perlin effect
H
[Hz]
DJ[Hz]
Figure 6. Explanation of Perlin effects dithiane-derived sulfoxide-sulfone
10.
4-Heq
4-Hax
5-Heq
5-Hax
6-Heq
6-Hax
2.72
2.86
2.54
2.69
3.34
3.24
140.7
144.1
130.5
131.1
142.9
146.1
C-4
C-5
C-6
31.1
26.5
51.6
ꢀ3.4
ꢀ0.6
ꢀ3.2
ꢀ
and C
A
a plough b-interaction (sax-C H!s*eq-S O), which has already
been discussed for bissulfoxide 6 and monosulfoxide 8. Nev-
ertheless, it has to be noted that also the equatorial C-5–H
dation in the less sterically hindered position S-3 (S-1 in sul-
is weakened significantly (1Jeq-C-5 H =136.2 Hz), most proba-
ꢀ
fone 10) can be concluded for the following reasons: 1) The
vinylic hydrogen is shifted in the H NMR spectra from d=
bly due to a double sCꢀS!s*eq-C-5 interaction with both an-
ꢀ
H
1
ꢀ
tiperiplanar C S bonds.
7.52 ppm for sulfoxide 6 to d=8.19 ppm for monosulfone
10; 2) The axial 6-H (numbering as in sulfone 10) is shifted
from d=2.93 ppm in sulfoxide 6 to d=3.31 ppm; 3) The
axial 5-H reaching in the direction of the new S=O bond is
shifted from d=3.19 ppm to d=3.48 ppm (Table 5).
The positive Perlin effect (DJC-6 H =+0.6 Hz) at position
ꢀ
C-6 might be explained by a sax-C H!s*ax-S interaction.
ꢀ
=
O
Shainyan and co-workers already observed that an axial S=
O group part of a sulfone is a less effective acceptor com-
pared with an S=O group which is part of a sulfoxide (see
compounds 1 and 2).[17] The competing sCꢀS!s*eq-C-5 inter-
ꢀ
H
action, weakening the equatorial C-6–H bond consequently
is in the same range giving rise to a negligible Perlin effect
of only +0.6 Hz. Nevertheless, thiane-1,1-dioxide (4) and
1,3-dithiane-1,1-dioxide (5), the coupling constants of which
have been calculated by Juaristi and co-workers,[18] show sig-
Table 5. 1JCꢀ coupling constants in dithiane-derived sulfoxide-sulfone
H
10.
Carbon
d [ppm]
Proton
d [ppm]
1JCꢀ
Perlin effect
H
[Hz]
DJ[Hz]
4-Heq
4-Hax
5-Heq
5-Hax
6-Heq
6-Hax
3.35
2.90
2.51
3.48
3.55
3.31
140.3
137.1
136.2
134.4
139.0
138.4
C-4
C-5
C-6
48.6
16.5
54.3
+3.2
+1.8
+0.6
nificantly higher Perlin effects (4: DJC-2 H =+2.8 Hz, 5:
ꢀ
DJC-6 H =+2.6 Hz, Figure 2).
ꢀ
The conformation of monosulfone 10 avoids severe allylic
strain which would arise from a flipped-chair conformation.
This is supported by stereoelectronic effects described
above and by evaluation of the 13C NMR spectra: 13C shifts
in position C-5 of thiane- and 1,3-dithiane-derived S-oxides
are strongly dependent from the number of b-sulfoxides
with S=O groups in axial position (g-gauche effect).[32] Axial
S=O groups which are part of a sulfone moiety do not con-
tribute to this effect and compounds bearing a sulfilimine
group behave inconsistently in that context.[33] The 13C shift
of C-5 in monosulfone 10 is d=16.5 ppm and is therefore
comparable to the shift measured for trans-1,3-dithiane-1,3-
The following Perlin effects resulted from the respective
A
1JCꢀ coupling constants (Table 5): For position C-4 again a
H
normal Perlin effect (DJC-4 H =+3.2 Hz) was determined,
ꢀ
which should again be due to a sax-C-4 H!s*ax-S interaction
ꢀ
=
O
(Figure 6). The coupling constants are exactly the same as
for C-6 in bissulfoxide 6.
A normal Perlin effect was observed for position C-5
ꢀ
G
G
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J. Podlech et al.
dioxide (12, d=16.6 ppm)[34] and for bissulfoxide 6 (d=
14.9 ppm) giving further evidence for the presence of a sulf-
oxide with an axial S=O group (Figure 7). On the other
mation to be most stable in which the best donor lone pair
(or the best donor bond) is antiperiplanar to the best accept-
1
or bond. The H NMR signals are fully resolved for the cor-
responding tert-butyl-substituted derivative 15[36] and the
1JCꢀH coupling constants were determined (Table 6).
Table 6. 1JCꢀH coupling constants in dithiolane-derived bissulfoxide 14
Carbon
d [ppm]
Proton
d [ppm]
1JCꢀ
Perlin effect
H
[Hz]
DJ[Hz]
4-Heq
4-Hax
5-Heq
5-Hax
3.52
3.63
3.73
2.82
146.2
140.0
147.9
140.3
C-4
C-5
31.1
26.5
+6.2
+7.6
In summary, all coupling constants are above 140 Hz and
the Perlin effects are much larger than for the six-membered
heterocycles discussed above. The weakening of the axial
ꢀ
C H bonds can be explained by a sax-C H!s*ax-S hyper-
ꢀ
=
O
conjugation. Obviously no appreciable stereoelectronic
ꢀ
effect is able to weaken the equatorial C H bonds. The
146.2 and 147.9 Hz coupling constants are among the highest
5
measured in the systems presented here. Remarkably, a J
Figure 7. The g-gauche effect. 13C shifts are given in ppm.
long-range coupling of 1.1 Hz between the vinylic hydrogen
and the eq-5-C–H could be resolved for this compound.
hand, monosulfoxide 8 is present in a conformation without
an axial S=O group as the chemical shift of C-5 (d=
27.2 ppm) is comparable to that of 1,3-dithiane-1-oxide (11)
with its S=O group in an equatorial orientation (d=
28.5 ppm).[35] Two axial S=O groups lead to a chemical shift
of d=7.9 ppm as measured for compound 13.[35] Sulfilimines
7 and 9 fit in that scheme though it has to be kept in mind,
that this empirical rule on the g-gauche effect is not general-
ly valid for this class of compounds.[34]
Conclusion
Orbital interactions of sulfoxides and sulfilimines have been
elucidated for the first time by measuring 1JCꢀ coupling
H
constants in conformationally constrained substrates without
interfering heteroatoms other than sulfur. The strongest ef-
fects arise from the interaction of axial S=O bonds with an-
ꢀ
tiperiplanar C H bonds, but other consistent effects, for ex-
A
ample, homoanomeric effects can be rationalized similarly.
Because stereoelectronic effects have not only an effect on
coupling constants, but also on conformation and especially
on chemical reactivity, this knowledge should be of substan-
tial interest for determining the chemistry of sulfoxides and
related compounds.
thiolane-derived bissulfoxide 14 exhibits a C-2 half-chair
Experimental Section
The synthesis of (RS,RS)-2-benzylidene-1,3-dithiane-1,3-dioxide (6),[23a]
(RS,RS)-2-ethylidene-1,3-dithiolane-1,3-dioxide (14),[23b] and 2-benzylid-
A
reactions were carried out under oxygen-free argon by using oven-dried
glassware and a vacuum line. Flash column chromatography was carried
out by using Merck silica gel 60 (230–400 mesh), and thin-layer chroma-
tography was carried out by using commercially available Merck F254 pre-
coated sheets. 1H and 13C NMR spectra were recorded on Bruker AV-
400, DRX 500, and AV-600 instruments, respectively. Chemical shifts are
given in ppm downfield of tetramethylsilane. 13C NMR spectra were re-
corded with broad-band proton decoupling and were assigned by using
conformation with the sulfur atoms and the C=C double
bond adopting a plane. This could be proven by X-ray crys-
tallographic analysis[23b] and by NMR spectroscopic investi-
gations. Both sulfoxide groups are oriented, almost perfectly,
in opposite directions. Hydrogens in position C-4 and C-5
are virtually axial and equatorial, respectively. This is in
agreement with Kirbyꢁs rule,[35] which designates that confor-
DEPT experiments. 1JCꢀ coupling constants were measured by using
H
coupled HMQC experiments[24,25] or coupled HSQC experiments (Bruker
4278
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 4273 – 4281

Stereoelectronic Effects in Cyclic Sulfur Compounds
FULL PAPER
pulse program hsqcetgpi2; power level pl12 set to 120 dB to prevent de-
coupling).[25, 27] Melting points were measured on a Büchi apparatus and
were not corrected. IR spectra were recorded on a Bruker IFS-88 spec-
trometer. Elemental analyses were performed on a Heraeus CHN-O-
rapid machine. EI, FAB, and high-resolution mass spectra were recorded
on a Finnigan MAT-90 spectrometer. Optical rotations were recorded on
a Perkin–Elmer 241 polarimeter.
1758C; Rf =0.06 (hexane/ethyl acetate 1:4); 1H NMR (400 MHz, CDCl3):
d=2.37 (s, 3H; CH3), 2.40–2.48 (m, 1H; H-5eq), 2.89 (ddd, 2J=13.5, 3J=
12.2, 3J=3.1 Hz, 1H; H-6ax), 3.11–3.33 (m, 3H; H-6eq, H-5ax, H-4ax), 3.51
(ddddd, 2J=11.7, 3J=2.9, 3J=2.9, 3J=2.7, 3J=1.2 Hz, 1H; H-4eq), 7.15–
7.20 (m, 2H; aromTol), 7.21–7.27 (m, 2H; aromPh), 7.35–7.45 (m, 3H;
aromPh), 7.53 (s, 1H; =CH), 7.77–7.82 ppm (m, 2H; aromTol); 13C NMR
(100 MHz, CDCl3): d=15.6 (t; C-5), 21.5 (q), 47.8 (t; C-4), 52.4 (t; C-6),
126.3 (d), 128.9 (d), 129.6 (d), 131.0 (d), 131.2 (s), 136.5 (s), 140.2 (d),
rac-(E)-2-Benzylidene-1,3-dithiane-1-oxide (8): 2-Benzylidene-1,3-di-
thiane (210 mg, 1.00 mmol) was dissolved in fresh molten PhOH
(12.0 mmol, 1.13 g) and H2O2 (30%, 4 mmol, 450 mL) was slowly added
at room temperature. The mixture was quenched after 10 min with satu-
rated Na2SO3 solution (1 mL) and was washed with aqueous NaOH solu-
tion (10%, 12 mmol, 4.35 mL). The aqueous phase was extracted with
CH2Cl2 (3”) and dried (Na2SO4). The solvent was removed and the resi-
due was purified by chromatography on SiO2 (hexane/ethyl acetate 2:1!
1:4) yielding ketene dithioacetal 8 (182 mg, 0.811 mmol, 81%) as a color-
less waxy solid: m.p. 708C (ethyl acetate/hexane); Rf =0.19 (hexane/ethyl
141.1 (s), 142.4 ppm (s); selected 1J
600 MHz, CDCl3): 1J(C–H-5ax)=137.6, 1J
141.1, 1J(C–H-6ax)=148.4, 1J
(C–H-6eq)=144.2 Hz; IR (DRIFT): n˜ =1146
ACHTREUNG
A
N
ACHTREUNG
A
ACHTREUNG
(s), 969 cmꢀ1 (s); MS (FAB): m/z (%): 306 [(M+1)+]; elemental analysis
calcd (%) for C11H15NO3S3 (393.5): C 54.93, H 4.87, N 3.56; found: C
54.72, H 5.04, N 3.67.
A
mCPBA (553 mg, 3.20 mmol) in CHCl3 (8 mL) was added within 30 min
to a cooled (ꢀ208C) solution of bissulfoxide 6 (480 mg, 2.00 mmol) in
CHCl3 (10 mL). The solution was stored for 12 h at ꢀ238C (e.g., in a
fridge) and filtered without warming through a column filled with Alox
(pH 10). The column was rinsed with CH2Cl2 (50 mL) and the washings
were concentrated in vacuum. Chromatography of the residue on SiO2
(MPLC, CH2Cl2/MeOH 100:1) yielded monosulfone 10 (244 mg,
0.952 mmol, 48%) and recovered starting material (180 mg, 0.750 mmol,
38%): [a]2D0 =ꢀ189 (c=1.0 in CHCl3); Rf =0.68 (CH2Cl2/acetone 2:1);
1
acetate 1:4); H NMR (400 MHz, CDCl3): d=2.46–2.57 (m, 2H; 5-Heq, 5-
Hax), 2.65 [ddddd, 2J(4-Heq–4-Hax)=13.1, 3J(4-Heq–5-H)=4.3, 3J(4-Heq–5-
H)=3.7, 4J(4-Heq–6-Heq)=1.3, 4J(4-Heq–6-Hax)=0.8 Hz, 1H; 4-Heq], 2.87
(m, 1H; 4-Hax), 2.90 (mc, 1H; 6-Hax), 3.43 [dddd, 2J(6-Hax–6-Heq)=12.1,
3J(6-Heq–5-H)=5.4, 3J(6-Heq–5-H)=3.0, 4J(6-Heq–4-Heq)=1.3 Hz, 1H; 6-
Heq], 7.35–7.45 (m, 3H; Ph), 7.52 (s, 1H; =CH), 7.74–7.79 ppm (m, 2H;
Ph); 13C NMR (100 MHz, CDCl3): d=27.2 (t; C-5), 31.8 (t; C-4), 55.0 (t;
C-6), 128.5 (d), 129.0 (d), 130.2 (d), 133.7 (s), 134.6 (d), 136.7 ppm (s); se-
2
3
1H NMR (500 MHz, CDCl3): d=2.51 (ddddd, J(5-Heq–5-Hax)=15.5, J(5-
H
lected 1J
eq)=139.1, 1J
A
G
eq–6-Heq)=5.2, 3J(5-Heq–4-Heq)=4.9, 3J(5-Heq–6-Hax)=3.1, 3J(5-Heq–4-
H
G
G
ACHTREUNG
Hax)=2.5 Hz, 1H; 5-Heq), 2.90 (ddd, 2J(4-Hax–4-Heq)=14.5, 3J(4-Hax–5-
Hax)=12.7, 3J(4-Hax–5-Heq)=2.5 Hz, 1H; 4-Hax), 3.31 (ddd, 2J(6-Hax–6-
141.3 Hz; IR (DRIFT): n˜ =3067 (m), 3050 (m), 3023 (m), 2953 (m), 2921
(s), 2904 (m), 2843 (m), 1484 (s), 1445 (s), 1423 (s), 1339 (m), 1222 (s),
1204 (m), 1173 (m), 1153 (m), 1058 (s, S=O), 1045 cmꢀ1 (s); MS (EI,
808C): m/z (%): 224 (56) [M+], 135 (12), 134 (100), 122 (12), 118 (22),
108 (13), 106 (84), 102 (15), 90 (24), 89 (20); HRMS (EI) calcd for
C11H12OS2: 224.0330; found: 224.0326.
H
eq)=13.8, 3J(6-Hax–5-Hax)=12.1, 3J(6-Hax–5-Heq)=3.1 Hz, 1H; 6-Hax),
2
3
3
3.35 (dddd, J(4-Heq–4-Hax)=14.5, J(4-Heq–5-Heq)=4.9, J(4-Heq–5-Hax)=
2.4, 4J(4-Heq–6-Heq)=1.2 Hz, 1H; 4-Heq), 3.48 (ddddd, 2J(5-Hax–5-Heq)=
15.5, 3J(5-Hax–4-Hax)=12.7, 3J(5-Hax–6-Hax)=12.1, 3J(5-Hax–6-Heq)=2.9,
3J(5-Hax–4-Heq)=2.4 Hz, 1H; 5-Hax), 3.55 (dddd, 2J(6-Heq–6-Hax)=13.8,
3J(6-Heq–5-Heq)=5.2, 3J(6-Heq–5-Hax)=2.9, 4J(6-Heq–4-Heq)=1.2 Hz, 1H;
6-Heq), 7.48–7.55 (m, 3H; arom), 7.62–7.65 (m, 2H; arom), 8.19 ppm (s, =
CH); 13C NMR (125 MHz, CDCl3): d=16.5 (t; C-5), 48.6 (t; C-4), 54.3 (t;
C-6), 129.1 (d), 130.4 (s), 131.1 (d), 131.9 (d), 139.1 (s), 147.1 ppm (s); se-
rac-(E)-2-Benzylidene-1-(4-methylphenylsulfonylimino)-1,3-dithiane (9):
Sodium p-toluenesulfonylchloramide (chloramine T, 369 mg, 1.40 mmol)
was added to a stirred solution of 2-benzylidene-1,3-dithiane[37b] (244 mg,
1.17 mmol) in acetonitrile (4 mL) and stirring was continued for 2 h at
room temperature. Precipitated NaCl was removed by filtration and the
solvent was removed at reduced pressure. The residue was purified by
chromatography on SiO2 (CH2Cl2/MeOH 50:1) to yield sulfilimine 9 as
white crystals (343 mg, 0.910 mmol, 78%): m.p. 140–1428C; Rf =0.29
(CH2Cl2/MeOH 20:1); 1H NMR (400 MHz, CDCl3): d=2.31 (s, 3H;
CH3), 2.40–2.56 (m, 1H; H-5eq), 2.57–2.73 (m, 2H; H-4eq, H-5ax), 2.74–
lected 1JCꢀ coupling constants: 1J
G
ACHTREUNG
H
1J
A
G
N
ACHTREUNG
6ax)=138.4 Hz; IR (DRIFT): n˜ =3002 (m), 2922 (m), 1601 (m), 1443 (m),
1311 (s, SO2), 1179 (m), 1136 (s, SO2), 1112 (m), 1049 (s, S=O), 924 cmꢀ1
(m); MS (EI, 258C): m/z (%): 256 (14) [M+], 171 (23), 153 (54), 147
(18), 141 (13), 136 (31), 134 (24), 129 (27), 118 (16), 115 (13), 108 (20),
107 (50), 106 (31), 105 (56), 102 (30), 91 (23), 90 (23), 89 (52), 87 (18), 79
(28), 28 (32), 77 (100), 75 (69); UV/Vis (EtOH): lmax (e)=192 (12100),
201 (12000), 271 nm (11200 molꢀ1 dm3 cmꢀ1); HRMS (EI) calcd for
C11H12O3S2: 256.0228; found: 256.0219; elemental analysis calcd (%) for
C11H12O3S2 (256.3): C 51.54, H 4.72; found: C 51.74, H 4.75.
2
3
3
2.87 (m, 1H; H-4ax), 3.20 (ddd, J=12.6, J=11.5, J=2.8 Hz, 1H; H-6ax),
3.30 (dddd, 2J=12.6, 3J=5.7, 3J=3.1, 4J=1.3 Hz, 1H; H-6eq), 7.12–7.23
(m, 2H; aromPh), 7.28–7.37 (m, 3H; aromPh), 7.40 (s, 1H; =CH), 7.46–
7.55 (m, 2H; aromTol), 7.75–7.83 ppm (m, 2H; aromTol); 13C NMR
(100 MHz, CDCl3): d=21.4 (q), 26.4 (t; C-5), 31.1 (t; C-4), 51.5 (t; C-6),
126.3 (d, 2C), 127.4 (s), 128.5 (d, 2C), 129.1 (d), 129.4 (d, 2C), 130.0 (d),
130.4 (d), 132.6 (s), 139.4 (d), 141.5 (s), 141.9 ppm (s); selected 1J
coupling constants (HSQC, 600 MHz, CDCl3): 1J(C–H-5eq)=130.5, 1J
H-5ax)=131.1, 1J(C–H-4eq)=140.7, 1J(C–H-4ax)=144.1, 1J
(C–H-6eq)=
142.9, 1J
(C–H-6ax)=146.1 Hz; IR (DRIFT): n˜ =1279 (s), 1137 (s),
(C–H)
(C–
U
(15):
Ethane-1,2-dithiol (2.83 g, 30 mmol) was added dropwise at 08C to 3,3-di-
methylbutyric chloride (30 mmol) and stirring was continued for 30 min
at this temperature. Perchloric acid (70%, 3.1 mL, 36 mmol) was careful-
ly added dropwise. An exothermic reaction started after 0.5–5 min. The
mixture was stirred for 30 min at room temperature, cooled to 08C, and
freshly distilled Ac2O (15 mL) was carefully added dropwise. The dithiol-
G
ACHTREUNG
U
G
ACHTREUNG
ACHTREUNG
975 cmꢀ1 (s); MS (EI, 1908C): m/z (%): 378.1 [(M+1)+]; HRMS (FAB)
calcd for C18H20NO2S3: 378.0657; found: 378.0661; elemental analysis
calcd (%) for C18H19NO2S3 (337.5): C 57.26, H 5.07, N 3.71; found: C
57.01, H 5.00, N, 3.71.
anylium salt was precipitated with anhydrous Et2O (50 mL) and filtered
under argon. The red needles were washed with Et2O (3”20 mL) and
dissolved in anhydrous MeCN (30 mL). Et3N was added until the red
color disappeared and the solvents were removed under reduced pres-
sure. The resulting oil was dissolved in saturated aqueous NH4Cl solution
(40 mL) and the solution was extracted with EtOAc (3”20 mL). The
combined organic layers were dried (Na2SO4 and K2CO3), the solvents
were removed, and the residue was distilled by bulb-to-bulb distillation
yielding 2-(2,2-dimethylpropylidene)-1,3-dithiolane (2.09 g, 12.0 mmol,
40%) as a pale yellow oil: 1H NMR (250 MHz, CDCl3): d=1.11 (s, 9H;
tBu), 3.14–3.19, 3.37–3.42 (2 m, 2”2H; 4-H2, 5-H2), 5.61 ppm (s, 1H;
=CH).
rac-(RS,RS,2Z)-2-Benzylidene-3-(4-methylphenylsulfonylimino)-1,3-dithi-
an-1-oxide (7): Freshly purified mCPBA (129 mg, 0.75 mmol) in CH2Cl2
(6 mL) was transferred within 30 min through a syringe pump to a pre-
cooled (08C) solution of sulfilimine 9 (189 mg, 0.500 mmol) in CH2Cl2
(6 mL) whereupon
a white solid precipitated. Satd NH4Cl solution
(15 mL) was added and the organic layer was separated. The aqueous
layer was extracted with CH2Cl2 (3”15 mL) and the combined organic
layers were extracted with brine (25 mL) and dried (Na2SO4), and the
solvent was removed at reduced pressure. The oily residue (224 mg) was
purified by chromatography on SiO2 (hexane/ethyl acetate 1:4–0:1) to
yield sulfoxide 7 (132 mg, 0.336 mmol, 68%) as white crystals: m.p. 174–
Chem. Eur. J. 2007, 13, 4273 – 4281
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4279

J. Podlech et al.
(+)-Diethyl tartrate (24.0 mmol, traces of water were removed by azeo-
tropic distillation with toluene) and freshly distilled Ti(O-iPr)4
[11] a) G. Cuevas, E. Juaristi, A. Vela, J. Phys. Chem. A 1999, 103, 932–
937; b) E. Juaristi, G. Cuevas, A. Vela, J. Am. Chem. Soc. 1994, 116,
5796–5804.
AHCTREUNG
(6.00 mmol) were dissolved at room temperature under argon in anhy-
drous CH2Cl2 (120 mL) and the mixture was stirred for 30 min. 2-(2,2-Di-
methylpropylidene)-1,3-dithiolane (2.09 g, 12.0 mmol) in CH2Cl2 (12 mL)
was added, the mixture was cooled to ꢀ408C and was stirred for 2 h.
Cumene hydroperoxide (technical grade, 80%, 48 mmol, diluted with
CH2Cl2 to twice the volume) was added within 1 h. The solution was
warmed to ꢀ208C and stored for 15 h in a freezer (<ꢀ208C). H2O
(4.32 mL) was added and the mixture was stirred vigorously for 1 h at
room temperature. The slurry was kept for 1 h in an ultrasonication bath
and the resulting suspension was filtered through a glass sinter (G2) cov-
ered with a Celite pad (1.5 cm). The filter cake was washed repeatedly
with small amounts of CH2Cl2. The solvents were removed and chroma-
tography on SiO2 (CH2Cl2/acetone 2:1) yielded sulfoxide 15 (1.57 g,
7.61 mmol, 63%) as colorless crystals: m.p.: 165–1668C (CH2Cl2/hexane);
Rf =0.19 (CH2Cl2/acetone 2:1); [a]2D0 =ꢀ226 (c=1.0, CHCl3); 1H NMR
(400 MHz, CDCl3): d=1.39 (s, 9H; tBu), 3.52 (ddd, 2J(4-Heq–4-Hax)=
13.6, 3J(4-Heq–5-Hax)=4.5, 3J(4-Heq–5-Heq)=1.8 Hz, 1H; 4-Heq), 3.63
(ddd, 2J(4-Hax–4-Heq)=13.6, 3J(4-Hax–5-Hax)=13.3, 3J(4-Hax–5-Heq)=
4.3 Hz, 1H; 4-Hax), 3.72 (dddd, 2J(5-Heq–5-Hax)=14.2, 3J(5-Heq–4-Hax)=
4.3, 3J(5-Heq–4-Heq)=1.8, 5J(5-Heq–=CH)=1.1 Hz, 1H; 5-Heq), 3.82 (ddd,
2J(5-Hax–5-Heq)=14.2, 2J(5-Hax–4-Hax)=13.3, 3J(5-Hax–4-Heq)=4.5 Hz,
1H; 5-Hax), 7.28 ppm (s, 1H, =CH). Assignment of the protons was made
with the assumption that the 5J(5-Heq–=CH) coupling is a trans coupling;
13C NMR (100 MHz, CDCl3): d=29.6 (q, 3C), 37.8 (s), 48.4 (t), 51.9 (t),
[12] S. Wolfe, C.-K. Kim, Can. J. Chem. 1991, 69, 1408–1412.
[13] a) A. S. Cieplak, J. Am. Chem. Soc. 1981, 103, 4540–4552; b) A. S.
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5621–5624; b) E. Juaristi, G. Cuevas, Tetrahedron Lett. 1992, 33,
1847–1850; c) E. Juaristi, G. Cuevas, A. Flores-Vela, Tetrahedron
Lett. 1992, 33, 6927–6930.
[15] The sulfur lone pairs and especially a sulfoxide lone pair are hardly
effective as donors: a) E. Juaristi, M. OrdoÇez, Tetrahedron 1994,
50, 4937–4948; b) V. K. Aggarwal, J. M. Worrall, H. Adams, R.
Alexander, B. F. Taylor, J. Chem. Soc. Perkin Trans. 1 1997, 21–24.
[16] Participation of sulfur d orbitals has been considered repeatedly but
was found to have minor influence on stereoelectronic effects:
a) A. E. Reed, P. von R. Schleyer, J. Am. Chem. Soc. 1990, 112,
1434–1445; b) N. D. Epiotis, R. L. Yates, F. Bernardi, S. Wolfe, J.
Am. Chem. Soc. 1976, 98, 5435–5439.
[17] B. A. Shainyan, I. A. Ushakov, E. N. Suslova, J. Sulfur Chem. 2006,
27, 3–13.
[18] R. Notario, M. V. Roux, G. Cuevas, J. Cardenas, V. Leyva, E. Juaris-
ti, J. Phys. Chem. A 2006, 110, 7703–7712.
[19] B. A. Shainyan, S. V. Kirpichenko, Russ. J. Gen. Chem. 2003, 73,
1709–1714; B. A. Shainyan, S. V. Kirpichenko, Zh. Obshch. Khim.
2003, 73, 1806–1811.
[20] A sulfoxide S=O group is not properly described by an S=O double
bond. This picture is used herein anyway. a) D. B. Chesnut, L. D.
Quin, J. Comput. Chem. 2004, 26, 734–738; b) V. M. Bzhezovskii,
N. N. Ilꢁchenko, M. B. Chura, L. G. Gorb, L. M. Yagupolꢁskii, Russ.
J. Gen. Chem. 2005, 75, 86–93; V. M. Bzhezovskii, N. N. Ilꢁchenko,
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153.5 (s), 164.5 ppm (d); selected 1J
500 MHz, CDCl3): 1J(C–H-4eq)=146.2, 1J
147.9, 1J
(C–H-5ax)=140.3 Hz; IR (DRIFT): n˜ =2973 (m), 1593 (m), 1477
ACHTREUNG
A
N
ACHTREUNG
ACHTREUNG
(m), 1086 (m), 1027 cmꢀ1 (s, S=O); MS (EI, 308C): m/z (%): 206 (23)
[M+], 163 (100) [(MꢀC3H7)+], 113 (22), 112 (13), 104 (14), 103 (11), 77
(13), 76 (11), 57 (17); HRMS (EI) calcd for C8H14O2S2: 206.0435; found:
206.0426; elemental analysis calcd (%) for C8H14O2S2 (206.3): C 46.57, H
6.84; found: C 46.50, H 6.80.
[21] a) S. Wolfe, A. Stolow, L. A. LaJohn, Tetrahedron Lett. 1983, 24,
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[23] a) T. Wedel, J. Podlech, Org. Lett. 2005, 7, 4013–4015; b) T. Wedel,
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Acknowledgement
We thank Prof. Dr. W. Klopper (University of Karlsruhe) and Prof. Dr.
E. Juaristi (Mexico) for inspiring discussions, and Dr. Andreas Rapp, An-
nelie Kuiper, and Pia Lang for acquiring the NMR spectra. This work
was supported by the Deutsche Forschungsgemeinschaft (DFG).
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[26] Spectra and coupling constants are given in the Supporting Informa-
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[30] An explanation of this term is given in footnote 33 of reference [29]:
Plough is another name for the star constellation Ursa Major, also
known as the big dipper.
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72, 408–411.
[6] It has recently been shown that the 1JCꢀ value is not necessarily a
H
consequence of nO!s*Cꢀ delocalization: G. Cuevas, K. Martínez-
H
Mayorga, M. d. C. Fernµndez-Alonso, J. JimØnez-Barbero, C. L.
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2416; Angew. Chem. Int. Ed. 2005, 44, 2360–2364.
[31] W. L. Xu, Y. Z. Li, Q. S. Zhang, H. S. Zhu, Synthesis 2004, 227–232.
[32] Details on the crystallographic analysis of sulfilimine 9 are given in
the Supporting Information. CCDC-622411 contains the supplemen-
tary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
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