Comparative reactivity of substituted 1,3- and 1,4-thiasilinane S-oxides in the sila-Pummerer rearrangement and inversion of the thiocarbonyl group

Article abstract of DOI:10.1134/S1070363207020119

The thermal sila-Pummerer rearrangement of diastereomeric 2,3,3-trimethyl-1,3-thiasilinane S-oxides was studied. Introduction of the methyl group in the 2 position of 3,3-trimethyl-3-thiasilinane S-oxide slows down the rearrangement. When heated in CCl₄, the trans isomer (2-Me_eq, SO_eq) converts into the cis isomer (2-Me _eq, SO_ax) which rapidly rearranges into 2,2,7-trimethyl-1,6,2-oxathiasilepane. On the contrary, the isomeric 2,4,4-trimethyl-1,4-thiasilinane S-oxide is thermally stable up to 160°C in DMSO. The inversion at the sulfur atom in 2,3,3-trimethyl-1,3-thiasilinane S-oxides and 2,4,4-trimethyl-1,4-thiasilinane S-oxides under the action of triethyloxonium tetrafluoroborate was studied. The trans isomer of 2,3,3-trimethyl-1,3-thiasilinane S-oxide (2-Me_eq, SO_eq) forms with Et₃O⁺BF ₄ ^- a salt which decomposes in two ways. The first involves recovery of the starting sulfoxide due to Sn2 substitution at the carbon atom of the ethoxy group, and the second, convertion into the cis isomer (2-Me_eq, SO_ax) which rearranges into 2,2,7-trimethyl-1,6,2-oxathiasilepane. Under the same conditions, the cis isomer of 2,3,3-trimethyl-1,3-thiasilinane S-oxide (2-Me_eq, SO_eq) decomposes to form siloxanes. trans-2,4,4-Trimethyl-4-thiasilinane S-oxide (2-Me_eq, SO _eq) under the action of Et₃O⁺BF ₄ ^- convers into the cis isomer (2-Me_eq, SO_ax). The B3LYP/6-311G(d,p) theoretical analysis showed that the thermal inversion at the sulfur atom in the compounds studied has a high energy barrier.

Full text of DOI:10.1134/S1070363207020119

ISSN 1070-3632, Russian Journal of General Chemistry, 2007, Vol. 77, No. 2, pp. 253 263.
Pleiades Publishing, Ltd., 2007.
Original Russian Text E.N. Suslova, I.A. Ushakov, B.A. Shainyan, 2007, published in Zhurnal Obshchei Khimii, 2007, Vol. 77, No. 2, pp. 280 290.
Comparative Reactivity of Substituted 1,3- and 1,4-Thiasilinane
S-Oxides in the Sila-Pummerer Rearrangement and Inversion
of the Thiocarbonyl Group
E. N. Suslova, I. A. Ushakov, and B. A. Shainyan
Favorskii Irkutsk Institute of Chemistry, Siberian Branch, Russian Academy of Sciences,
ul. Favorskogo 1, Irkutsk, 664033 Russia
e-mail: bagrat@irioch.irk.ru
Received September 14, 2006
Abstract The thermal sila-Pummerer rearrangement of diastereomeric 2,3,3-trimethyl-1,3-thiasilinane
S-oxides was studied. Introduction of the methyl group in the 2 position of 3,3-trimethyl-3-thiasilinane
S-oxide slows down the rearrangement. When heated in CCl4, the trans isomer (2-Meeq, SOeq) converts into
the cis isomer (2-Meeq, SOax) which rapidly rearranges into 2,2,7-trimethyl-1,6,2-oxathiasilepane. On the
contrary, the isomeric 2,4,4-trimethyl-1,4-thiasilinane S-oxide is thermally stable up to 160 C in DMSO. The
inversion at the sulfur atom in 2,3,3-trimethyl-1,3-thiasilinane S-oxides and 2,4,4-trimethyl-1,4-thiasilinane
S-oxides under the action of triethyloxonium tetrafluoroborate was studied. The trans isomer of 2,3,3-tri-
methyl-1,3-thiasilinane S-oxide (2-Meeq, SOeq) forms with Et3O+BF4 a salt which decomposes in two ways.
The first involves recovery of the starting sulfoxide due to SN2 substitution at the carbon atom of the ethoxy
group, and the second, convertion into the cis isomer (2-Meeq, SOax) which rearranges into 2,2,7-trimethyl-
1,6,2-oxathiasilepane. Under the same conditions, the cis isomer of 2,3,3-trimethyl-1,3-thiasilinane S-oxide
(2-Meeq, SOeq) decomposes to form siloxanes. trans-2,4,4-Trimethyl-4-thiasilinane S-oxide (2-Meeq, SOeq)
under the action of Et3O+BF4 convers into the cis isomer (2-Meeq, SOax). The B3LYP/6-311G(d,p) theoretical
analysis showed that the thermal inversion at the sulfur atom in the compounds studied has a high energy
barrier.
DOI: 10.1134/S1070363207020119
Cyclic organosilicon sulfoxides containing two
heteroatoms of different nature in the ring are interest-
ing objects for investigation of chemical properties
and stereoelectronic interactions, as well as for con-
formational analysis. The first representatives of
thiasilinane S-oxides, 3,3-dimethyl-1,3-thiasilinane
S-oxide and 2,3,3-trimethyl-1,3-thiasilinane S-oxide,
were synthesized by us about 10 years ago [1, 2]. Later
4,4-dimethyl-1,4-thiasilinane S-oxide [3], 2,2,3,3-
tetramethyl-1,3-thiasilinane S-oxide, 2,4,4-trimethyl-
1,4-thiasilinane S-oxide, and 2,4,4,6-tetramethyl-1,4-
thiasilinane S-oxide were obtained [4], some of their
diastereomers were separated and isolated individual,
and their stereochemistry was studied [5, 6]. A series
of works on the conformational analysis of various
Si-substituted 1,4-thiasilinane S-oxides was also
performed [7 10].
in acyclic sulfoxides is well known, and factors affect-
ing its facility are thoroughly studied [11 19]. The
main factors are the steric effects of substituents at the
-carbon atom, electronic effects of substituent at the
sulfur atom, and sulfoxide stereochemistry. Substi-
tuents at the -carbon atom, shielding the S=O group
slow down [15, 16] or completely suppress [17] the
rearrangement. Increase of the sulfoxide basicity
facilitates silyl migration, so that alkyl sulfoxides
(except for R = t-C4H9) rearrange even in the cold
[18, 19], whereas aryl sulfoxides rearrange only upon
heating [14, 16].
Stereochemistry of the Si C S=O fragment in the
starting sulfoxide plays the key role [13, 18 20]. A
necessary condition for successful rearrangement is
syn-planar arrangement of the Si C and S=O bonds,
which favors interaction between silicon and oxygen
[19]. The effect is the most pronounced in cyclic
sulfoxides with an exocyclic silicon atom, which are
thermally stable only in the E configuration, that is, in
the case of anti-planar arrangement of the Si C and
S=O bonds [20 22].
Of special interest among reactions of -silylated
sulfoxides R3SiCH2S(O)R is the sila-Pummerer rear-
rangement, that is, their conversion into O-silylated
O,S-acetals R3SiOCH2SR via 1,3-migration of
silicon to oxygen. The sila-Pummerer rearrangement
253

254
SUSLOVA et al.
The information on the sila-Pummerer rearrangement
equatorial form Ie [5] into 2,2-dimethyl-1,6,2-oxa-
thiasilepane (II) via axial isomer Ia upon heating at
60 70 C in THF or CCl4. The mechanism of this
rearrangement we recently studied theoretically [24].
of cyclic organosilicon sulfoxides are restricted to our
work [23] on the conversion of 3,3-dimethyl-1,3-thia-
silinane S-oxide (I) existing preferentially (>95%) in
Me
Si
O
S
Me
(1)
S
Me
Si
Me
Si
O
S
O
Me
Me
Ie
Ia
II
Proceeding with this research, in the present paper
we have studied the sila-Pummerer rearrangement of
diastereomeric 2,3,3-trimethyl-1,3-thiasilinane S-
oxides and compared the resulting data with those for
-unsubstituted sulfoxide I. The rearrangement of
sulfoxide I slowly proceeds even at room temperature:
When a pure compound is kept for 3 5 days, ca.
5 7% of rearrangement product II forms (1H NMR
data). After longer keeping, formation of siloxane
[MeS(O)(CH2)3Si(Me2)]2O (III) is observed. In chlo-
roform, sulfoxide I fails to rearrange within 2 weeks
at room temperature, but suffers decomposition to
siloxane III. The latter was isolated by column chro-
matography and fully characterized. Probably, the
rearrangement in chloroform at room temperature is
prevented by S=O H CCl3 hydrogen bonding.
Diastereomeric 2,3,3-trimethyl-1,3-thiasilinane S-
oxides were prepared by two protocols: lithiation
methylation of 3,3-dimethyl-1,3-thiasilinane (IV) to
2,3,3-trimethyl-1,3-thiasilinane (V) [25] followed by
its oxidation to a mixture of the trans (VIee) (major)
and cis (VIae) (minor) isomers of 2,3,3-trimethyl-1,3-
thiasilinane S-oxide [5] or by oxidation of compound
IV to sulfoxide I whose lithiation methylation selec-
tively provides trans isomer VIee [4] (Scheme 1).
Scheme 1.
O
mCPBA
S
+
S
Me
S
Me
Me
Me
Si
Si
Me
Si
Me
O
BuLi
Me
Me
Me
MeI
V
VIee
VIae
S
Me
Si
mCPBA
Me
BuLi
VIee
IV
S
>
O
Me
Si
<
>
<
Me
I
mCPBA = m-ClC6H4CO3H.
The diastereomeric mixture was separated by
column chromatography to obtain individual trans
isomer VIee, cis isomer VIae, and a fraction en-
riched with cis isomer VIae. To compare the reac-
tivity of isomers VIae and VIee, their sila-Pummerer
rearrangement was studied by the method of com-
petitive reactions using the examples of pure trans
isomer VIee and a 2:1 mixture of isomers VIae and
VIee. It should be emphasized that the sila-Pummerer
rearrangement of axial cyclic sulfoxides was studied
here for the first time; so far such compounds could
not be obtained, since the axial isomers rearranged in
statu nascendi upon oxidation of the corresponding
sulfides [13, 20, 22].
It is noteworthy for subsequent analysis that trans
isomer VIee and cis isomer VIae with an equatorial
2-methyl group, while being major conformers, exist,
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 2 2007

COMPARATIVE REACTIVITY OF SUBSTITUTED 1,3- AND 1,4-THIASILINANE
255
according to [5], in equilibrium with minor con-
formers VIaa and VIea [Eqs. (2) and (3)].
atom in -silylated sulfoxides [15 17]. The rearran-
gement product was detected neither sulfoxide VIee
was kept in chloroform for one month at 18 C nor
even after it was heated in CCl4 at 65 C for 1.5 h.
Me
O
The rearrangement product, 2,2,7-trimethyl-1,6,2-
oxathiasilepane (VII), appears in appreciable amounts
( 10%) only after a solution of trans isomer VIee in
CCl4 has been heated at 50 75 C for 3 4 h. Its struc-
ture is proved by the appearance in the 1H NMR spec-
trum of a new doublet of the CH3 group at 1.5 ppm
and the corresponding quartet of the methine proton at
5 ppm. Such position of the methine proton signal is
characteristic of the SCHOSi fragment and virtually
coincides with the position of the SCH2O group signal
(2)
(3)
S
S
Me
Si
Me
Si
Me
O
Me
Me
VIee
VIaa
Me
O
S
S
Me
Si
Me
Si
Me
O
Me
Me
1
in rearrangement product II (4.9 ppm) [23]. H NMR
VIae
VIea
monitoring showed that only on further heating of the
solution for 5 h at 80 C the conversion of VIee reaches
90%. Therewith, the reaction is complicated by the
decomposition of sulfoxide VIee to siloxane [EtS(O)
(CH2)3Si(Me2)]2O (VIII), as well as by the decom-
position of rearrangement product VII, involving, by
analogy with [23], elimination of acetaldehyde. Sil-
oxane VIII was isolated by column chromatography
and fully characterized. In view of Eq. (2), the general
scheme of transformations can be represented as
follows (Scheme 2).
Unlike unsubstituted sulfoxide I, a pure trans
isomer of 2-methyl-substituted sulfoxide VIee, ac-
1
cording to H NMR data, does not rearrange at room
temperature but fairly slowly decomposes to siloxanes
under the action of air moisture (by 75% after
2 weeks). The higher stability of sulfoxide VIee com-
pared to sulfoxide I is consistent with the known fact
that the sila-Pummerer rearrangement slows down
with increasing number of substituents at the -carbon
Scheme 2.
Me
Me
O
S
Me
Si
O
Me
Si
S
Me
Me
VIee
VII
H2O
H2O
[EtS(O)(CH2)3Si(Me2)]2O MeCHO + [HS(CH2)3Si(Me2)]2O
VIII
IX
The formation of mercaptosiloxane IX is evidenced
almost completely, with concurrent decomposition to
siloxanes. Even in a chloroform solution at room
temperature, up to 12% of rearrangement product VII
and 24% of siloxanes formed within 6 days.
1
by the presence in the H NMR spectrum of signals
which coincide with those of the pure sample isolated
in [23]. The content of rearrangement product VII
reaches ca. 40%, and the VII:VIe:siloxanes ratio is
3.5:1:5. We failed to isolate pure compound VII by
column chromatography, since it partially decom-
posed on silica due to the lability of the Si O C bond.
1
By H NMR spectroscopy we studied the dynamics
of the rearrangement at various temperatures by the
method of competitive reactions with a 2:1 mixture
of VIae and VIee in CCl4. The relative contents of
VIae, VIee, and VII in the solution were determined
from the integral intensities of the most characteristic
signals: doublets of the 2-Me groups at 1.30, 1.35,
The rearrangement of cis isomer VIae with an axial
S=O group proceeds much easier. Even at room
temperature after 1 day it rearranges to product VII
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 2 2007

256
SUSLOVA et al.
and 1.45 ppm in VIae, VIee, and VII, respectively, a
quartet of the methine proton at 5 ppm in VII, as well
as 6-Heq signals in isomers VIae and VIee and a low-
field signal of the 7-CH2 group in VII with character-
istic multiplet splittings. The rearrangement proceeds
to an appreciable extent only at 55 C: after 2.5 h, ca.
7% of product VII forms. At 65 C after 1 h, its
content reaches 26%, and the VIae:VIee ratio varies
from 2:1 to 1.6:1. Taking into account that trans
isomer VIee scarcely rearranges at this temperature,
we can conclude that under these conditions rearran-
gement product VII is formed exclusively from cis
isomer VIae. Upon further heating to 70 C, the
content of product VII reaches 54%, and the VIae:
VIee ratio changes to 1:1. The decrease of the content
of isomer VIee in the reaction mixture is caused by
its partial decomposition to siloxane.
lone pair of the heteroatom with the system or d
orbitals of the substituent, as well as enhanced angular
strain lower the barrier to inversion [27]. In the ab-
sence of factors significantly stabilizing the transition
state for sulfur inversion, racemization of sulfoxides
with alkyl and/or aryl substituents is characterized by
1
a high activation enthalpy H 35 42 kcal mol and
nearly zero activation entropy S and proceeds only
under severe conditions ( 200 C) [26]. Arenethiol-
sulfinates ArS(O)SAr [28], vinyl sulfoxides with elec-
tron-acceptor groups at the double bond [29], and allyl
sulfoxides ArS(O)CH2CH=CH2 [29] racemize much
easier; in these cases, the transition state is stabilized
by the interaction of the lone electron pair of the
sulfinyl sulfur atom with vacant orbitals of the sul-
fenyl sulfur atom [28], conjugation of the lone elec-
tron pair with the electron-deficient double bond [29],
or interaction of the sulfoxide oxygen atom with the
C=C bond to form O-allyl arenesulfenate ArS
OCH2CH=CH2 [30]. The racemization of benzyl
sulfoxides, too, proceeds much easier, but it follows
another mechanism: ratedetermining homolytic dis-
sociation of the S CH2 bond and fast intracage re-
combination of the radicals formed, which is eviden-
ced by the high positive activation entropy [31]. A
DFT theoretical analysis of the racemization of me-
thanesulfinyl chloride MeS(O)Cl showed that the
configuration inversion at the sulfur atom occurs
easily when catalyzed by triethylamine, and this
process can be a key step in the synthesis of optically
active sulfoxides from diacetone-Dglucose [32].
Analysis of integral intensities in the course of
heating of the CCl4 solution of the mixture of isomers
VIae and VIee at 80 C showed that the initial
VIII:VIae:VIee ratio of 0:70:30, becomes
10:45:45, 22:26:52, 31:15:54, and 45:9:46 after
10 min, 1 h, 1.5 h, and 2.5 h, respectively. These data
are indicative of the fact that rearrangement product
VIII forms almost exclusively from cis isomer VIae.
However, the VI VII conversion cannot be radi-
cally increased by elevated temperature, since under
these conditions the content of siloxanes in the reac-
tion mixture reaches 70%, that is, considerably
exceeds their amount in the rearrangement of pure
trans isomer VIee. This result provides evidence to
show that cis isomer VIae easier, due to its lower
stability, decomposes to siloxanes than trans isomer
VIee.
The thermal inversion of configuration at the sulfur
atom in sulfoxides I and VI is impossible to study,
since they rearrange and decompose to siloxanes,
unlike the situation with their isomeric 1,4-thiasi-
linane S-oxide derivatives which are much more stable
thermally.
The above analysis allows us to suggest that the
thiasilinane S-oxides studied undergo the sila-
Pummerer rearrangement either due to the axial S=O
group in starting cis isomer VIae, or, in the case of
trans isomer VIaa, the percentage of the minor axial
sulfoxide is increased by heating. The alternative
pathway which includes thermal configuration inver-
sion at the sulfur atom, should probably be ruled out,
the more so as the configurational stability of sulf-
oxides has long been known [26]. Theoretical analysis
of factors affecting the barrier to pyramidal inversion
at the heteroatom showed that introduction of bulky
and electronegative substituents, conjugation of the
Diastereomeric 2,4,4-trimethyl-1,4-thiasilinane
S-oxides (XI) were prepared by lithiation methylation
of 4,4-dimethyl-1,4-thiasilinane S-oxide (X) [4]. trans
Isomer XIee with equatorial thiocarbonyl and 2-me-
thyl groups is formed as the major diastereomer, and
cis isomer XIae with an axial thiocarbonyl group and
an equatorial 2-methyl group as the minor one. Iso-
mers XIee and XIae were isolated by column
chromatography [4, 6].
Me
Me
Me Si
Me
(1) BuLi
Me Si
+
Me Si
O
(4)
S
(2) MeI
S
S>
<
>
Me
Me
XIee, base
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 2 2007
O
O
X
XIae, minor

COMPARATIVE REACTIVITY OF SUBSTITUTED 1,3- AND 1,4-THIASILINANE
257
The XIee:XIae ratio in the mixture remains un-
changed after storage during half a year at room
temperature or after reflux in a CDCl3 solution for 2 h.
Unlike its isomer VIee, 2,4,4-trimethyl-1,4-thiasi-
linane S-oxide (XIee) undergoes no transformations
not only upon reflux in CCl4 but also upon heating at
165 C for 2 h in DMSO. Further heating at 180 C
during 7 h leads to its decomposition to cyclic sil-
oxanes [Si(Me2)2O]n, as evidenced by the presence in
sulfonium salts upon O-alkylation, for example, with
trialkyloxonium tetrafluoroborate, with subsequent
alkaline hydrolysis occurs much easier. The reaction
involves retention of configuration at the stage of
formation of alkoxysulfonium salt and with inversion
of configuration at the stage of hydrolysis by the
mechanism of SN2 substitution at the sulfur atom
[33]. Such approach allows one to accomplish inver-
sion of configuration in sulfoxides and was succes-
sfully realized both with acyclic and cyclic sulfoxides
[33 35].
1
the H NMR spectrum of a single Me2Si proton signal
at 0.08 ppm. This fact is provides independent evi-
dence for the suggestion that the easiness of the I II
and VI VII rearrangement is caused by the prox-
imity of the oxygen and silicon atoms in axial sul-
foxides Ia, VIaa, and VIae and for our earlier
suggested intramolecular mechanism of the sila-
Pummerer rearrangement [24].
We examined the possibility of inversion of con-
figuration at the sulfur atom in trans-2,4,4-trimethyl-
1,4-thiasilinane S-oxide (XIee). Upon reaction with
Et3O+BF4 at room temperature with subsequent trea-
tment with NaOH it easily affords the isomerization
product which was isolated by column chromato-
graphy and turned out to be identical to the minor
diastereomer XIae formed by reaction (4).
Compared to thermal isomerization of sulfoxides,
their chemical isomerization by formation of alkoxy-
Me
Me
Me Si
Me
(1) Et3O+BF4
Me Si
+
S
OEt
Me Si
O
(5)
S
(2) OH
S
Me
Me
O
BF4
XIee
XII
XIae
Intermediate salt XII was obtained as a crude pro-
(VIee) was of interest as a possible route to hardly
accessible cis isomer VIae, since trans isomers of
2-substituted 3,3-dimethyl-1,3-thiasilinane S-oxides
are prepared in good yields, as shown in Scheme 1.
However, the reaction of compound VI with
Et3O+BF4 in CH2Cl2 neither as a pure trans isomer
VIee nor as a mixture with cis isomer VIae involved
inversion at the sulfur atom. NMR monitoring of the
process showed that trans isomer VIee forms with
Et3O+BF4 salt XIII (Scheme 3), which is corroborated
by the downfield shift of characteristic signals in the
1H NMR spectrum. The largest shift (0.6 0.7 ppm) is
observed for H2ax and H6ax, which are most proxi-
mate to the cationic center, and the shift of the H6eq
signal is slightly smaller (0.4 ppm) Characteristic
multiplicities of the signals are therwith preserved.
The signal of the 2-CH3 group shifts downfield by
0.14 ppm.
duct and characterized by NMR spectroscopy. The
1
structure is proved by the presence in the H NMR
spectrum of signals of the ethoxy group (triplet of
CH3 protons at 1.43 ppm and two multiplets of dia-
stereotopic OCH2 protons at 4.42 and 4.52 ppm) and a
doublet of the CH3 group in the 2 position of the
heterocycle, shifted by 0.15 ppm downfield as
compared to sulfoxide XIee. The spectrum of salt XII
was performed by its comparison with the spectrum
of the parent compound XIee with account for signal
multiplicities and integral intensities.
The XIee XIae inversion in methylene chloride
at room temperature proceeds by 50% after 1 h and
by 70% after 5 h. Further increase of the duration of
the process does not increase of the content of cis
isomer XIae in the reaction mixture. After decomposi-
tion of salt XII, the XIee:XIae ratios after 1- and 5-h
reactions were 1:1 and 2.3:1, respectively. Isomers
XIee and XIae were isolated by column chromato-
graphy, and their NMR spectra were consistent with
those described earlier [4, 6].
Alkaline hydrolysis of salt XIII with aqueous
NaOH affords 46% of starting sulfoxide VIee, 28%
of rearrangement product VII, and 26% of decomposi-
tion products, viz. siloxanes. The major decomposi-
tion product and the only one isolated by column
chromatography was siloxane VII. Attempted decom-
Inversion at the sulfur atom in the equatorial iso-
mer of trans-2,3,3-trimethyl-3-thiasilinane S-oxide
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 2 2007

258
SUSLOVA et al.
position of salt XIII under the same conditions with
softer bases, K2CO3 or NH4HCO3, only slightly
changed the composition of the products, while the
content of siloxanes slightly decreased. As the de-
composition temperature was decreased to 8 C, the
content of siloxanes substantially, more than two
times, decreased.
result from fast sila-Pummerer rearrangement of salt
XIII formed initially from cis isomer VIae, due to the
proximity of the oxygen and silicon atom in its
molecule to each other.
Starting sulfoxide VIee is probably recovered
through base attack on the -carbon atom of the
ethoxy group in salt XIII [33] (in Scheme 3, siloxanes
are omitted for simplicity).
In view of our previous results [23, 24], seven-
membered ring VII can be tentatively considered to
Scheme 3.
Et3O+BF4
+
S
O
OEt
S
Me
Me Si
Me Si
Me
BF4
Me
Me
VIee
XIII
SN2C
SN2S
+
S ^
CH3
<
O
OEt
CH2
HO
S
Me
Me Si
Me
Me Si
Me
VII
VIae
VIee
Me
EtOH
EtOH
OH
Investigation of the reaction of Et3O+BF4 with 1:1
and 2:1 mixtures of isomers VIee and VIae provided
additional information concerning the inversion and
linear disiloxane salt {[EtS(OEt)(CH2)3SiMe2]2O}2+
2BF4 (XIV) (two signals of the SiMe2 group at 0.26
and 0.28 ppm and a triplet of the SCH2CH3 group at
1.5 ppm), but, because of the overlap with complex
multiplets belonging to protons of other groups, these
signals proved impossible to assign reliably. The rapid
disappearance of cis isomer VIae, the absence of its
ethoxysulfonium salt, as well as the doubled amount
of siloxanes as compared to the reaction with pure
trans isomer VIee suggest that cis isomer VIae easily
decomposes, already at the stage of reaction with
Et3O+BF4, to salt XIV whose alkaline hydrolysis
affords siloxane VII.
1
rearrangement processes. According to H NMR data,
already after 40 min cis isomer VIae is completely
consumed, but the only salt formed is salt XIII
produced from trans isomer VIee. No second set of
characteristic signals of the H2ax, H6ax, H6eq, and
2-CH3 protons, that would be expected in the case of
formation of the ethoxysulfonium salt from cis isomer
VIae, was observed in the spectrum. Moreover, much
1
siloxanes are formed. The H NMR spectrum contains
signals which could be tentatively assigned to the
Et3O+BF4
+
OH
S
[EtS(OEt)(CH2)3SiMe2]2O 2BF4
XIV
(6)
VIII.
Me Si
Me
O
Me
VIae
Therewith, 23% of starting trans isomer VIee is
recovered from the reaction mixture, and 27% of
rearrangement product VII and 50% of siloxanes are
formed. The major compound among the siloxanes,
like in reaction with pure transisomer VIee, is sil-
oxane VII. Under the conditions of alkaline hydro-
lysis, trans isomer VIee is so stable that can be re-
covered in the pure form. Rearrangement product VII
is less stable; however, it was unequivocally identified
in the crude product by NMR. These results are in
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 2 2007

COMPARATIVE REACTIVITY OF SUBSTITUTED 1,3- AND 1,4-THIASILINANE
qualitative agreement with published data which show
259
that O-silylated thioacetals withstand treatment with
5% aqueous alkali [36].
TS
We performed a theoretical assessment of the bar-
riers to thermal inversion of the S=O group in the
cyclic organosilicon sulfoxides studied, by means of
calculation of transition states for the transformation
of their axial isomers to equatorial ones:
H
eq
ax
H0
S
S
Me2Si
Me2Si
R
R
O
O
O
The weak endothermic effect for the SOax SOeq
inversion reflects a somewhat higher stability of the
axial conformer in the gas phase. In solution, due to
the large dipole moment, the equatorial sulfoxides are
more stable (cf. [38] and references therein).
O
S
(7)
,
Me2Si
R
O
S
S
The fact that the energetic parameters of the inver-
sion process, as well as the geometric and vibrational
parameters of the transition state are virtually in-
dependent on the structure of the sulfoxide suggests
that the obtained characteristics of the process are
general for all cyclic sixmembered sulfoxides and
confirm that in the absence of nucleophilic catalysis
or strong nonspecific interaction with solvent (due to
a higher dipole moment of the transition state as
compared to those of both conformers) the inversion
is hindered.
Me2Si
Me2Si
R
R
S
O
.
(8)
Me2Si
R
The calculations of individual conformers and
transition states were performed at the B3LYP/6-
311G(d,p) level of theory with the Gaussian-03 suite
of programs (B.03 version) [37]. The geometry of
transitions states was optimized in the Tight mode;
the transition states all have a single imaginary vibra-
tion. Analysis of the shape of this vibration provides
evidence to show that the calculated transition states
connect the axial and equatorial conformers of the
sulfoxides. The energy profile of S=O inversion,
depicted in the figure below, practically does not
depend neither on the relative position of heteroatoms
nor on the presence or orientation of the substituent
in the ring.
The above assumption that the recovery of starting
sulfoxide VIee from salt XIII is the result of nucleo-
philic attack on the -carbon atom of the ethoxy
group (Scheme 3), is likely to be true only if the
formation of the salt does not reduce the barrier to
inversion at the sulfur atom to very small values,
when spontaneous racemization at room temperature
is possible. To verify this option, we calculated the
inversion in the cation of ethoxysulfonium salt XIII:
Some calculated characteristics for the conformers
and transition states are given in the table.
+
S
+
S
OEt
Me2Si
Me2Si
Me
Me
OEt
The S O bond in axial sulfoxides is 0.002 0.006
+
longer than in equatorial due to (C Hax)
*(S=O)
S
.
Me2Si
(9)
stereoelectronic interactions. This is proved, in par-
ticular, by the fact that the axial methyl group de-
creases the l(S O) value. In the transition state, there
is a substantial, by 0.04 , elongation of the S O
Me
OEt
The results are shown in the table. Noteworthy, the
salt formation substantially, by 0.033 for cis isomer
VIae and by 0.050 for trans isomer VIee, elongates
the Si C(2) bond, that is, drastically weakens it. The
ease of Si C(2) bond rupture in cyclic organosilicon
-sulfoxides, especially in isomers with an axial S=O
group (due to the proximity of the oxygen and silicon
atoms to each other), is responsible for the impossi-
bility of the interconversion of axial and equatorial
bond coplanar to the C S C plane (
table). The Si C(2) bond length in axial -sulfoxide
VIae is 1.920 that is 0.007 longer than in its
360o, see
equatorial isomer VIee. This is indicative of a more
facile decomposition of cis isomer VIae with rupture
of the Si C(2) bond, and is in line with the experi-
mental observations.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 2 2007

260
SUSLOVA et al.
Total energies (E), dipole moments ( ), S O bond lengths (lS O), sums of the angles at the sulfur atom ( ), thermal
effect ( H0) and barriers to S=O inversion ( H ) in 2-R-3,3-dimethyl-3-thiasilinane S-oxides, 2-R-4,4-dimethyl-4-thia-
silinane S-oxides, and ethoxysulfonium salts XIII
lS O
,
H0,
kJ mol
H ,
kJ mol
1
Structure
R
H
E, au
, D
,
, cm
1
1
1,3-SOax
1,3-SOeq
1,3-SO (TS )
1,3-SOax
1,3-SOeq
1,3-SO (TS )
1,3-SOax
1,3-SOeq
1,3-SO (TS )
1,4-SOax
1,4-SOeq
1,4-SO (TS )
1,4-SOax
1,4-SOeq
1,4-SO (TS )
1,4-SOax
1000.115855
1000.112297
1000.040000
1039.437155
1039.435557
1039.365395
1039.438361
1039.435008
1039.363240
1000.112324
1000.108797
1000.036266
1039.435541
1039.433822
1039.362066
1039.438734
1039.435954
1039.364146
1118.458265
1118.459340
1118.406829
3.86
5.02
5.97
3.93
4.78
5.65
3.81
4.82
5.71
3.92
5.07
6.01
4.00
4.83
6.00
3.85
4.85
5.74
1.518
1.512
1.557
1.520
1.516
1.558
1.521
1.515
1.558
1.518
1.514
1.560
1.519
1.517
1.558
1.519
1.516
1.561
1.642
1.645
1.639
310.5
304.6
359.9
312.8
312.7
359.9
311.8
310.7
359.8
310.4
309.3
359.9
311.2
311.0
359.9
311.1
309.5
359.9
308.1
305.6
359.7
9.3
4.2
8.8
9.2
4.5
7.3
2.8
199.2
188.4
197.2
199.7
192.9
195.8
135.1
567i
550i
555i
566i
544i
557i
369i
Meax
Meeq
H
Meax
Meeq
Meeq
1,4-SOeq
1,4-SO (TS )
1,3-+SOEtax
1,3-+SOEteq
1,3-+SOEt (TS )
sulfoxides via the stage of salt formation, as is the
case in their organic analogs [33].
analytical TLC, on silica plates (Merck 60 F-254,
visualization in iodine vapors). The melting points
were determined on a Boetius hot stage (VEB Ana-
lytik). All reactions were carried out in argon with
protection from air moisture. Solvents and reagents
were dried and purified by standard protocols directly
prior to use. Triethyloxonium tetrafluoroborate was
prepared according to [39] and reprecipitated from
methylene chloride into ether directly prior to use, mp
As follows from the table, the formation of salt
1
XIII from trans isomer VIee is 11.6 kJ mol more
energetically favorable than from cis isomer VIae,
which is apparently responsible for the absence of the
1
latter in the H NMR spectrum of the reaction mix-
ture in the reaction of salt Et3O+BF4 with a mixture
of the isomers. Salt formation substantially, by a
factor of 1.5, decreases the barrier to inversion, but it
still remains very high (see table), which corroborates
the assumption on the mechanism of the recovery of
sulfoxide VIee from salt XIII, depicted in Scheme 3.
1
88 91 C. H NMR spectrum (CDCl3), , ppm: 1.25 t
(3H, Me), 3.73 q (2H, CH2O).
1,1,3,3-Tetramethyl-1,3-bis[3-(methylsulfinyl)-
propyl]disiloxane (III). Pure 3,3-dimethyl-1,3-thia-
silinane S-oxide (I) was kept in a closed vessel at
room temperature in the light for two weeks. From
1H NMR data, the mixture formed contains 50% of
starting compound I, 15% of rearrangement product
II, and 30% of siloxane III. The latter compound
was isolated as a light yellow oil by column chroma-
tography, eluents hexane ether (7:1), ether methanol
(from 20:1 to 1:1, MeOH). Rf 0.61 (ether methanol,
EXPERIMENTAL
The IR spectra were taken on a Specord IR-75 in-
strument in thin layer. The NMR spectra were regis-
tered on a Bruker DPX-400 spectrometer at 400 (1H),
100 (13C), and 80 MHz (29Si) for solutions in CDCl3,
reference TMS. The structure of all new compounds
was proved by the j-modulated 13C NMR spectra.
Column chromatography was performed on silica 60
[0.063 0.200 mm (Merck or ICN Biomedical)] and
1
8:1). IR spectrum, , cm : 2940, 1395, 1230,
1
1030, 1010, 820, 780. H NMR spectrum (CDCl3),
, ppm: 0.085 s (6H, SiMe2), 0.63 m (1H, SiCHA),
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 2 2007

COMPARATIVE REACTIVITY OF SUBSTITUTED 1,3- AND 1,4-THIASILINANE
261
0.72 m (1H, SiCHB), 1.80 m (2H, CCH2C), 2.55 s
(3H, MeS), 2.68 m (2H, CH2S). 13C NMR spectrum
(CDCl3), C, ppm: 0.087 (SiMe), 16.52 (SiC), 17.27
(CCC), 38.12 (SMe), 57.55 (SCH2). 29Si NMR spec-
trum (CDCl3), Si, ppm: 8.53. Found, %: C 42.69; H
9.03; S 18.52; Si 16.18. C12H30S2Si2O3. Calculated,
%: C 42.10; H 8.77; S 18.71; Si 16.37.
(1H, OCHB), 4.52 m (1H, OCHA). Salt XII was
decomposed by adding to the residue after removal of
the solvents 3 ml of water and 1 N aqueous NaOH
(dropwise) to pH 10 11. The mixture was then
diluted with saturated brine, extracted with CH2Cl2
(3 3 ml), and dried over anhydrous MgSO4. The
solvents were removed to obtain 0.05 g (80%) of a
crude product. Its 1H NMR spectrum showed the
presence of isomers XIee and XIae in a 1:1 ratio.
Column chromatography, eluents hexane, hexane
ether (21:3), ether-methanol (from 20:0.5 to 8:1)
gave 5 mg of cis isomer XIae, 4 mg of a mixture of
isomers XIee and XIae in a 1:7 ratio, and 8 mg of
pure crystalline isomer XIee, mp 54 55 C.
2,2,7-Trimethyl-1,6,2-oxathiasilepane (VII). A
5% CCl4 solution of 2,3,3-trimethyl-1,3-thiasilinane
S-oxide (VI) as a pure trans isomer VIee or as a mix-
ture of isomers VIee and VIae was heated at 50
80 C, periodically analyzing the composition of the
1
reaction mixture by H NMR spectroscopy. Below is
1
given the H NMR spectrum of compound VII ob-
tained for the reaction mixture VII:siloxanes:VIee
(40:50:10). 1H NMR spectrum (CDCl3), , ppm:
0.06 s (6H, SiMe2), 0.75 d.d.d.d (1H, H5A, J 14.8,
6.8, 3.4, 1.3 Hz), 1.49 d (3H, CH3, J 6.2 Hz), 2.10 m
(1H, H6A), 2.64 m (1H, H7B), 2.89 d d d d (1H, H7A,
J 14.6, 5.6, 3.1, 1.3 Hz), 5.02 q (1H, CH, J 6.2 Hz).
The H5B and H6B signals at 0.6 ppm and 1.7 ppm,
respectively, overlap with signals of siloxanes.
Reaction
of
2,3,3-trimethyl-1,3-thiasilinane
S-oxide with triethyloxonium tetrafluoroborate.
a. To a stirred solution of 0.32 g of triethyloxonium
tetrafluoroborate in 5 ml of CHCl3, 0.24 g of a solu-
tion of compound VIee in 1 ml of CHCl3 was added
1
dropwise. The H NMR spectrum taken after 40 min
showed the absence of starting sulfoxide VIee and
presence of 1-ethoxy-2,3,3-trimethyl-1,3-thiasilinan-
1-ium tetrafluoroborate (XIII) and a small amount
1,3-Bis[3-(ethylsulfinyl)propyl]-1,1,3,3-tetra-
methyldisiloxane (VIII) was isolated by column
chromatography of the above reaction mixture as a
reddish oily substance, eluents hexane, hexane ether
(20:1, 7:1), ether methanol (20:1, 10:1, 8:1), and
MeOH. Rf 0.83 (ether methanol, 8:1). IR spectrum,
1
( 15%) of siloxanes. H NMR spectrum of salt XIII
(CDCl3), , ppm: 0.23 s (3H, SiMe), 0.32 s (3H,
SiMe), 0.76 d d d (1H, H4ax, J4ax4eq 15.0, J4ax5ax 14.1,
J4ax5eq 4.9 Hz), 0.95 d.d.d.d (1H, H4eq, J 15.0, 4.5,
3.2, 1.3 Hz), 1.23 t (3H, CH3, J 7.0 Hz), 1.58 d (3H,
CH3, J 6.9 Hz), 1.88 m (1H, H5ax), 2.46 d d d d (1H,
1
1
, cm : 2950, 1405, 1235, 1050, 1020, 825, 790. H
NMR spectrum (CDCl3), , ppm: 0.09 s (6H, SiMe2),
H
5eq, J 15.9, 10.7, 5.1, 2.2 Hz), 2.91 q (1H, H2ax, J
0.62 d.d.d (1H, SiCHA), 0.73 d d d (1H, SiCHB, J
14.6, 10.8, 5.7 Hz), 1.34 t (3H, CH3, J 7.5 Hz),
1.81 m (2H, CH2), 2.69 m (4H, CH2SOCH2). 13C
NMR spectrum (CDCl3), C, ppm: 0.39 (SiMe2), 6.86
(Me), 17.09 (SiCH2), 17.91 (CH2), 45.75 (MeCH2SO),
6.9 Hz), 3.17 t d (1H, H6ax, J6ax6eq 13.0, J6ax5ax 13.0,
J6ax5eq 2.2 Hz), 3.51 q (2H, OCH2, J 7.0 Hz), 3.64 m
(1H, H6eq).
b. To a stirred solution of 0.05 g of a mixture of
sulfoxides VIee and VIae in a 2:1 ratio in 1 ml
CHCl3, a solution of 0.06 g of triethyloxonium tetra-
fluoroborate in 3 ml of CH2Cl2 was slowly added. The
mixture was stirred for 2 h, the solvents removed
under reduced pressure, and the residue was dried in
55.13 (OSCH2). 29Si NMR spectrum (CDCl3),
,
ppm: 7.12. Found, %: C 44.82; H 9.30; S 17.3S7i;
Si 15.50. C14H34S2Si2O3. Calculated, %: C 45.40;
H 9.19; S 17.28; Si 15.19.
1
Reaction of trans-2,4,4-trimethyl-1,4-thiasi-
linane S-oxide (XIee) with triethyloxonium tetra-
fluoroborate. A mixture of 0.057 g of compound
XIee and 0.1 g of triethyloxonium tetrafluoroborate in
1.5 ml of CH2Cl2 was stirred for 1 h at room tem-
perature, and the solvents were then evacuated. The
1H NMR spectrum of the oily residue showed the
presence of 1-ethoxy-2,4,4-trimethyl-1,4-thiasilinan-
1-ium tetrafluoroborate (XII) mixed with siloxanes.
1H NMR spectrum of salt XII (CDCl3), , ppm: 0.14 s
(3H, SiMe), 0.21 s (3H, SiMe), 1.13 d.d (1H, H3a, J
a vacuum (1 mm Hg). The H NMR spectrum of the
residue showed the absence of the starting sulfoxides
and presence of two ethoxysulfoxonium salts in a
1.7:1 ratio, mixed with 30% of siloxanes.
Decomposition of 1-ethoxy-2,3,3-trimethyl-1,3-
thiasilinan-1-ium tetrafluoroborate (XIII). After
treatment with ether in the cold to remove all ether-
soluble impurities, the suspension of 0.19 g of salt
XIII in a mixture with 1.67 ml of H2O and 6.67 ml
of MeOH was cooled to +10 C, and a 0.1N solution
of K2CO3 (23 ml) was added dropwise until a sus-
tainable pink color (by phenolphthalein) appeared.
The reaction mixture was extracted with CH2Cl2, the
extract was dried over MgSO4, and the solvent was
15.6, 12.1 Hz), 1.34 m (3H, H5a + H5e + H3e), 1.43 t
(3H, CH3, J 7.0 Hz), 1.61 d (3H, CH3, J 6.6 Hz),
3.66 m (2H, H6a + H2a), 4.05 m (1H, H6e), 4.42 m
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 2 2007

262
SUSLOVA et al.
1
removed. According to H NMR data, the residue
contains 51% of sulfoxide VIee, 27% of rearrange-
ment product VII, and 22% of siloxanes. Column
chromatography on silica, eluents hexane, hexane
ether (20:1, 7:1), ether methanol (from 20:1 to 8:1),
starting sulfoxide VIee (Rf 0.48, ether methanol, 8:1),
13. Block, E. and Aslam, M., Tetrahedron., 1988, vol. 44,
no. 2, p. 281.
14. Brook, A.G. and Anderson, D.G., Can. J. Chem., 1968,
vol. 46, no. 12, p. 2115.
15. Carey F. A. Hernandez O., J. Org. Chem., 1973,
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and siloxane VII (Rf 0.18 in the same system). The
16. Ager D. J., J. Chem. Soc., Perkin Trans. 1, 1983,
reaction mixture obtained from sulfoxides VIee and
VIae was decomposed in a similar way. Column
chromatography under the same conditions gave
sulfoxide VIee and siloxane VII.
no. 4, p. 1131.
17. Kocienski, P.J., Tetrahedron Lett., 1980, vol. 21,
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ACKNOWLEDGMENTS
19. Barbaro, G., Battaglia, A., Giogianni, P., Maccagna-
ni, G., Macciantelli, D., Bonini, B.F., Mazzanti, G.,
and Zani, P., J. Chem. Soc., Perkin Trans. 1, 1986,
no. 2, p. 381.
The authors are highly grateful to S.V. Kirpichenko
for fruitful discussions of the results of the present
work.
20. Carey, F.A., Dailey, O.D., Hernandez, O., and Tu-
cker, J.R., J. Org. Chem., 1976, vol. 41, no. 25,
p. 3975.
The work was financially supported by the Russian
Foundation for Basic Research (project no. 04-03-
32131).
21. Bonini, B.F., Lenzi, A., Maccagnani, G., Barbaro, G.,
Giogianni, P., and Macciantelli, D., J. Chem. Soc.,
Perkin Trans. 1, 1987, no. 9, p. 2643.
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