Conformational analysis of 3,3-dimethyl-3-silathiane, 2,3,3-trimethyl-3- silathiane and 2-trimethylsilyl-3,3-dimethyl-3-silathiane-preferred conformers, barriers to ring inversion and substituent effects

Article abstract of DOI:10.1002/poc.1677

The first conformational analysis of 3-silathiane and its C-substituted derivatives, namely, 3,3-dimethyl-3-silathiane 1, 2,3,3-trimethyl-3-silathiane 2, and 2-trimethylsilyl-3,3-dimethyl-3-silathiane 3 was performed by using dynamic NMR spectroscopy and B3LYP/6-311G(d,p) quantum chemical calculations. From coalescence temperatures, ring inversion barriers δG^≠ for 1 and 2 were estimated to be 6.3 and 6.8 kcal/mol, respectively. These values are considerably lower than that of thiacyclohexane (9.4 kcal/mol) but slightly higher than the one of 1,1-dimethylsilacyclohexane (5.5 kcal/mol). The conformational free energy for the methyl group in 2 (-°G⁰ - 0.35 kcal/mol) derived from low-temperature¹³C NMR data is fairly consistent with the calculated value. For compound 2, theoretical calculations give δE value close to zero for the equilibrium between the 2-Me _ax and 2-Me_eq conformers. The calculated equatorial preference of the trimethylsilyl group in 3 is much more pronounced (-δG⁰ - 1.8 kcal/mol) and the predominance of the 3-SiMe _{3 eq} conformer at room temperature was confirmed by the simulated¹H NMR and 2D NOESY spectra. The effect of the 2-substituent on the structural parameters of 2 and 3 is discussed. Copyright

Full text of DOI:10.1002/poc.1677

Research Article
Received: 5 October 2009,
Revised: 7 December 2009,
Accepted: 13 December 2009,
Published online in Wiley InterScience: 2 March 2010
(www.interscience.wiley.com) DOI 10.1002/poc.1677
Conformational analysis of
3,3-dimethyl-3-silathiane,
2,3,3-trimethyl-3-silathiane and
2-trimethylsilyl-3,3-dimethyl-3-silathiane—
preferred conformers, barriers to ring
inversion and substituent effects
a
b
a
*
Svetlana V. Kirpichenko , Erich Kleinpeter and Bagrat A. Shainyan
The first conformational analysis of 3-silathiane and its C-substituted derivatives, namely, 3,3-dimethyl-3-silathiane 1,
2,3,3-trimethyl-3-silathiane 2, and 2-trimethylsilyl-3,3-dimethyl-3-silathiane 3 was performed by using dynamic NMR
spectroscopy and B3LYP/6-311G(d,p) quantum chemical calculations. From coalescence temperatures, ring inversion
¼
barriers DG for 1 and 2 were estimated to be 6.3 and 6.8 kcal/mol, respectively. These values are considerably lower
than that of thiacyclohexane (9.4 kcal/mol) but slightly higher than the one of 1,1-dimethylsilacyclohexane (5.5 kcal/
mol). The conformational free energy for the methyl group in 2 (SDG- ¼ 0.35 kcal/mol) derived from low-temperature
13C NMR data is fairly consistent with the calculated value. For compound 2, theoretical calculations give DE value
close to zero for the equilibrium between the 2-Meax and 2-Meeq conformers. The calculated equatorial preference of
the trimethylsilyl group in 3 is much more pronounced (SDG- ¼ 1.8 kcal/mol) and the predominance of the 3-SiMe3 eq
1
conformer at room temperature was confirmed by the simulated H NMR and 2D NOESY spectra. The effect of the
2-substituent on the structural parameters of 2 and 3 is discussed. Copyright ß 2010 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper.
Keywords: 3-silathianes; conformational analysis; substituent effects; dynamic NMR; quantum chemical calculations
INTRODUCTION
RESULTS AND DISCUSSION
3,3-Dimethyl-3-silathiane 1
Theoretical and experimental studies of the conformational
behavior of a large number of mono and diheterocyclohexanes
containing the silicon[1–12] or sulfur atoms[2,13–24] comprise a
topic of current interest in organic and organoelement chemistry.
In connection with our continuing interest in the stereo-
chemistry of the Si,S-containing heterocycles, in particular, 1,3-
and 1,4-silathiacyclohexanes (silathianes) and their S-functional
derivatives[25–33] we became interested in the conformational
properties of these compounds which have been studied much
less than the related monoheterocyclohexanes containing only
one sulfur or silicon atom. To fill in this gap and to study the ring
inversion barriers and the influence of the 2-substituent in
3,3-dimethyl-3-silathianes on the conformational equilibrium, in
this work the conformational analysis of 3,3-dimethyl-3-silathiane
1, 2,3,3-trimethyl-3-silathiane 2 and 2-trimethylsilyl-3,3-dimethyl-
3-silathiane 3 is reported. Almost all of our previous structural and
conformational studies refer to S-functional silathianes. The
present paper is the first on the experimental and theoretical
conformational analysis of silathianes. The only previous report
concerning this problem was our theoretical analysis of
Si-unsubstituted 1,2-, 1,3-, and 1,4-silathianes.[27]
The gas phase electron diffraction data showed that 3,3-
dimethyl-3-silathiane 1 exists in a distorted chair conformation
of C2v symmetry[34] with structural parameters similar to those of
thiane (thiacyclohexane),[35] silacyclohexane[36] and methylsila-
cyclohexane.[7]
First attempts to measure the ring inversion barrier of
compound 1 by low temperature 1H NMR spectroscopy were
*
Correspondence to: S. V. Kirpichenko, A.E. Favorsky Irkutsk Institute of Chem-
istry, Siberian Branch of the Russian Academy of Sciences, 1 Favorsky Street,
Irkutsk 664033, Russia.
E-mail: svk@irioch.irk.ru
a
S. V. Kirpichenko, B. A. Shainyan
A.E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian
Academy of Sciences, 1 Favorsky Street, Irkutsk 664033, Russia
b
E. Kleinpeter
Department of Chemistry, University of Potsdam, Karl-Liebknecht-Str. 24-25,
D-14476 Potsdam(Golm), Germany
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S. V. KIRPICHENKO, E. KLEINPETER AND B. A. SHAINYAN
undertaken as early as 30 years ago but failed since separate
signals of the axial and equatorial methylene or MeSi protons
were not observed even at 133 K; nevertheless, the authors
estimated the upper limit of the ring inversion barrier to be
6.7 kcal/mol.[37]
the original A values measured for substituted cyclohexanes may
substantially vary on going to polysubstituted cyclohexanes
or heterocyclohexanes. For example, both the experimental
(1.42 kcal/mol)[40] and the calculated (1.00 kcal/mol)[20] free
energy difference ꢀDG8 between the 2-Meeq and 2-Meax thianes
is lower than that in methylcyclohexane of 1.7–1.8 kcal/
mol.[1,41–44] Molecular mechanics calculations showed the
equatorial isomer of 2-methylsilacyclohexane to be 1.27 kcal/
mol more stable than the axial,[45] that is, the effect of sulfur and
silicon on the equatorial predominance of the 2-Me group is
similar. Also, the equatorial isomer of 1,1,2-trimethylcyclohexane
is 1.53 kcal/mol more stable than the axial (from the measure-
ments on the model ‘counterpoised’ compound 1,1,2,4-
tetramethylcyclohexane), that gives the conformational energy
of the 2-Me group to be 0.21 kcal/mol less than in methylcy-
clohexane.[46]
There are no literature data on the conformational analysis of
2,3,3-trialkylheterocyclohexanes, which could be compared with
compound 2; the closest conformationally studied system is
cis-2,3-dimethylthiane.[40] In spite of nearly equal conformational
energies A for the 2-Me and 3-Me groups in thiane (1.42 vs.
1.40 kcal/mol, respectively)[40] the 2-Meeq-3-Meax 2-Meax-3-Meeq
equilibrium is shifted to the left; at 178 K the ratio of the
conformers is 61:39%, and the ꢀDG8 value is 0.16 kcal/mol.[40]
The authors assigned the observed nonadditivity to the
difference in the interaction of the 2- and 3-Me groups due to
different dihedral angles Me—C—C—Me in the two conformers,
57 and 528, respectively.[40] In our opinion, the factor of at least
not less importance is the presence of only one 1,3-syn-axial
repulsive interaction between the 3-Meax and 5-Hax in the
2-Meeq-3-Meax conformer vs. two such interactions between the
2-Meax and 4-Hax and 6-Hax in the 2-Meax-3-Meeq conformer.
The only earlier studied 2-substituted 3-silathiane for which a
conclusion about its conformational preference can be made is
2-phenyl-3,3-dimethyl-3-silathiane;[37] from the large free con-
formational energy A for the phenyl group (2.8 kcal/mol[47]) and
substantial difference between the Jtrans and Jcis H—H coupling
it was considered anancomeric (i.e., the conformational equi-
librium is biased to one conformer) with the equatorial phenyl
group.[37]
Using very low temperature 13C NMR spectroscopy, we were
now able to study experimentally the chair-to-chair inversion in
compound 1 (Eqn 1).
ð1Þ
Below 183 K the SiMe singlet at ꢀ3.5 ppm broadens and at 133 K
decoalesces into two signals of 1:1 ratio at ꢀ5.50 and ꢀ1.75 ppm,
respectively (Fig. SI-1). They were assigned to the axial and
equatorial methyl groups, respectively, in accordance with the
general rule that axial substituents in cyclohexane derivatives
resonate at a higher field than their equatorial analogs.[7,38] All
other carbon signals suffer an upfield shift of 1.4–2.4 ppm on
going from room temperature to 133 K.
¼
The Gibbs free energy of activation DG for the ring inversion
calculated from the low-temperature experiment was found to be
¼
6.3 kcal/mol (hereinafter the entropy of activation DS is
assumed to be close to zero). This is considerably lower than
¼
in thiane (DG ¼ 9.4 kcal/mol)[16] due to the more flexible chair
conformation as a result of the longer Si—C as compared to the
C—C bonds. Lower CSiC as compared to CCC bending force
constants also contribute to a more flexible ring thus facilitating
its inversion, although we do not consider it to be the main factor.
This factor is more important in acyclic compounds whereas
bending of the endocyclic CSiC angle is restricted by the ring
structure. On the other hand, more puckered (from the ‘sulfur
side’) ring conformation of 1 relative to 1,1-dimethylsila-
cyclohexane results in a higher barrier to ring inversion for the
¼
former (DG ¼ 6.3 and 5.5 kcal/mol,[3] respectively).
We have calculated the barrier to the degenerate chair-to-chair
ring inversion of compound 1. Two transition states were located
along the reaction coordinate: the first one, [TS1]z, having a
half-chair/sofa structure similar to that of silacyclohexane found
by Arnason et al.,[39] is characterized by one imaginary frequency
of ꢀ77 cmꢀ1 and is 4.7 kcal/mol above the chair conformer; the
second one, [TS2]z, having a distorted 2,5-boat structure and an
imaginary frequency of ꢀ95 cmꢀ1, is 5.1 kcal/mol above the chair
conformer.
For equilibrium in 2 (Eqn 2), we calculated the energy
difference DE between the two conformers to be virtually
negligible, 0.08 kcal/mol in favor of the equatorial conformer. The
calculated conformational free energy ꢀDG8 of the 2-methyl
group in 2 is 0.36 kcal/mol.
ð2Þ
With the aforesaid taken into account, a near-equalization of
population of the equatorial and axial conformers of compound 2
is due to: (i) the presence of two heteroatoms (Si and S) in the
molecule; (ii) the effect of the two methyl groups in the SiMe2
fragment (although it should be less important than of the CMe2
group[46]); and (iii) flattening of the ‘Si-part’ of the ring[3,31,37] that
lowers the difference in interaction of the 2-Meax or 2-Meeq in
compound 2 with the H-6ax. It must be noted that the above
effects are rather small, but their combined action has a
synergistic effect, since simple sum of these effects is not enough
to provide the observed near-equalization of the populations of
the conformers.
The latter value, which should be considered as the calculated
ring inversion barrier, is 1.2 kcal/mol lower than the experimental
value. Note that the B3LYP/6-311þG(d,p) calculated barrier
to the chair-to-chair inversion for 3-silathiacyclohexane (the
Si-unsubstituted analog of compound 1) is 5.6 kcal/mol.[27]
2,3,3-Trimethyl-3-silathiane 2
Alkyl substituents in alkylcyclohexanes show a large preference
for the equatorial orientation (large positive A values). However,
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CONFORMATIONAL ANALYSIS OF 3,3-DIMETHYL-3-SILATHIANE
NMR spectra even under fast (on the NMR timescale) exchange
conditions. Lowering the temperature should result in splitting of
all signals in the 13C NMR spectrum into two, provided that the
decoalescence temperature is reached and passed. With this,
the SiMe part of the spectrum must appear as two doublets in the
ratio equal to the ratio of the 2-Meax and 2-Meeq conformers of 2.
Experimentally, in the 13C NMR spectra of compound 2 in
conventional organic solvents up to 153 K, an upfield shift of all
signals by 1–2 ppm and their broadening (most pronounced for
the SiMe and C-6 signals) were observed as a result of not
completely finished freezing of the ring inversion and different
broadening of the 13C signals due to different values of Dn for
various carbon atoms. The conformational equilibrium was finally
frozen in the freon solvent mixture CD2Cl2/CHFCl2/CHF2Cl
(1:1:3).[5,7] The coalescence range is shown in Fig. 1 and the
frozen 13C NMR spectrum at 103 K in Fig. 2. The minor set of
signals with C-4 and C-6 more shielded was assigned to the
2-Meax conformer because of steric compression effects of 2-Meax
they experience.[40,52] The major set of the signals was assigned
to the 2-Meeq conformer. Note that smaller shielding of the ring
carbons in the Meax vs. Meeq isomer was also observed in the
Me-substituted thianes.[40]
Table 1. Vicinal coupling constants in 2-R-3,3-dimethyl-3-
silathianes
Vicinal coupling constants (Hz)
Compound
R
3J5ax6ax 3J5eq6eq 3J4ax5ax 3J4eq5eq
2
3
Me
8.97a
7.88a
4.85
5.40b
9.87a
13.15
12.25b
8.46a
5.2
5.80b
Me3Si 11.9
Ph
10.75b
a Averaged Jcis and Jtrans values.
b Values obtained from Reference [37].
From the data of Table 1 it is clearly seen that the ratio of the
coupling constants for compound 2 (ꢁ1.15) is principally
different from the Jax-ax/Jeq-eq ratio for its 2-SiMe3 analog 3
(ꢁ2.5) or 2-Ph analog (ꢁ2.1). Substantial difference in the vicinal
couplings is characteristic of anancomeric compounds whereas
their similarity points to a near-equalization of conformer
1
populations.[48] The simulated H NMR spectrum of compound
From the various spectra of the coalescence range, both the
coalescence temperatures Tc and (by extrapolation to Tc) the
chemical shift differences at Tc (Dnc/Hz) were determined. All
carbon signals decoalesced; thus the free energy of activation
could be determined on the basis of six signal pairs—only the
chemical shift difference of C-5 was so small that the coalescence
could be followed only by the signal broadening. The barrier to
2 at room temperature obtained by the use of the PERCH
program of Laatikainen[49–51] excellently coincides with the
experimental spectrum (Fig. SI-2).
Similarity of the experimental vicinal H—H coupling constants
for compound 2 (Table 1) is in agreement with a small energy
difference between the conformers (vide supra) and both are
indicative of a close to equal population of the equatorial and
axial conformers in equilibrium (Eqn 2). With this in mind, and
aiming at gaining quantitative information about the position of
the conformational equilibrium, we have undertaken a low
temperature 13C NMR study of compound 2.
¼
ring inversion DG calculated by the Eyring equation at the
coalescence temperature is 6.7–6.9 kcal/mol, which is ꢁ0.5 kcal/
mol larger than that of the 2-unsubstituted analog 1. From the
:
integrated intensities of all signals, an average ratio 2-Meeq
2-Meax of (59 ꢂ 1):(41 ꢂ 1) was obtained, to give K of ꢁ1.5 that
corresponds the conformational free energy ꢀDG8 of 0.35 kcal/
mol. This value is in excellent agreement with the theoretically
calculated ꢀDG8 of 0.36 kcal/mol.
Unlike compound 1, the SiMe signals in 2,3,3-trimethyl-3-
silathiane 2 are diastereotopic due to the presence of the center
of chirality at the C-2 atom and give two signals in the 1H and 13
C
Figure 1. Low temperature 13C NMR spectra of 2,3,3-trimethyl-3-silathiane 2
J. Phys. Org. Chem. 2010, 23 859–865
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S. V. KIRPICHENKO, E. KLEINPETER AND B. A. SHAINYAN
Figure 2. Low temperature 13C NMR spectrum of 2,3,3-trimethyl-3-silathiane 2 at T ¼ 103 K. Less intense signals (defined as ‘primed’) refer to the 2-Meax
conformer
Calculations gave two transition states, from both the sides of
equilibrium (2). The one connecting the chair 2-Meeq and the
1,4-twist 2-Meax conformer, [TS1]z, has an imaginary frequency of
ꢀ58 cmꢀ1 and is 6.0 kcal/mol higher than the former conformer,
whereas another one connecting the chair 2-Meax and the
1,4-boat 2-Meax conformer, [TS2]z, has an imaginary frequency of
ꢀ101 cmꢀ1 and is 5.8 kcal/mol higher than the chair 2-Meax
conformer. Both transition states, [TS1]z and [TS2]z, have the
half-chair/sofa structure similar to that of compound 1 and of
silacyclohexane.[39]
Figure 3. Simulated (above) and experimental (below) 1H NMR spectrum of 2-trimethylsilyl-3,3-dimethyl-3-silathiane 3
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CONFORMATIONAL ANALYSIS OF 3,3-DIMETHYL-3-SILATHIANE
1
of the H NMR spectra. Since the conformational energy of the
trimethylsilyl group (2.5 kcal/mol)[53] is larger than that of the
methyl group, the energetic predominance of the equatorial
conformer of 3 can be expected to be much more pronounced
than that of 2 and equilibrium (Eqn 3) should be strongly biased
to the right.
ð3Þ
As in the case of compound 1, the calculated barrier is ꢁ1 kcal/
mol lower than the experimental value, but both experiment and
theory suggest that the ring inversion barrier for the 2-Me
substituted compound 2 is 0.5–1.0 kcal/mol higher than for its
2-unsubstituted analog 1.
However, the conformational energy of the substituents in
heterocyclohexanes is subject to the nature and position of the
heteroatoms (vide supra). Indeed, the conformational energy of
the Me3Si group in 2-trimethylsilylthiane calculated at the MP2/
6-31G(d)//HF/6-31G(d) level of theory is 2.05 kcal/mol,[20] which is
0.45 kcal/mol lower than in trimethylsilylcyclohexane. Introduc-
tion of the endocyclic silicon atom results in further decrease in
the energetic difference between the axial and equatorial
conformers due to longer Si—C vs. C—C bonds. Thus, the
energy difference DE between the 3-ax and 3-eq conformers
calculated at the B3LYP/6-311G(d,p) level of theory is 1.18 kcal/
mol and the free energy difference ꢀDG8 is 1.80 kcal/mol in favor
2-Trimethylsilyl-3,3-dimethyl-3-silathiane 3
Similar to 2,3,3-trimethyl-3-silathiane 2, 2-trimethylsilyl-3,3-
dimethyl-3-silathiane 3 has a center of chirality at the C-2 atom
and, therefore, its 1H and 13C NMR spectra also inherently contain
two SiMe signals since the two groups remain diastereotopic
even under fast exchange conditions; the splitting patterns of the
ring protons for the two compounds look similar in certain parts
Figure 4. The 2D 1H–1H NOESY spectrum of 2-trimethylsilyl-3,3-dimethyl-3-silathiane 3. The H-2ax–H-4ax and H-2ax–H-6ax cross peaks are marked with
circles
J. Phys. Org. Chem. 2010, 23 859–865
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S. V. KIRPICHENKO, E. KLEINPETER AND B. A. SHAINYAN
of the latter. Taking into account the fact that theoretical
calculations nicely reproduce the experimental conformational
energies (see above for compound 2), one may conclude that
they are low when compared with trimethylsilylcyclohexane or
even with 2-trimethylsilylthiane, they are large enough to provide
strong predominance of the 3-eq conformer. In fact, the free
energy difference of 1.8 kcal/mol results in >95.5:4.5% pre-
dominance at room temperature and >99.9:0.1% at 103 K.
In view of the complex character of the multiplets of the H-4,
H-5, and H-6 signals in the 1H NMR spectrum of compound 3, it
was simulated (Fig. 3) and, after iteration, the geminal/vicinal
H,H-coupling constants of 3 were determined (Table 1). The
CONCLUSIONS
The conformational behavior of silathianes 1–3 was studied using
the room temperature 1H NMR, low temperature 13C NMR
spectroscopy, and quantum chemical calculations. For
¼
3,3-dimethyl-3-silathiane 1, the ring inversion barrier DG of
6.3 kcal/mol was determined, which is considerably lower than in
thiacyclohexane but slightly higher than in 1,1-dimethylsilacy-
clohexane. For 2,3,3-trimethyl-3-silathiane 2, small free energy
differences ꢀDG8 of the 2-Me group were determined both
experimentally and theoretically: in the 13C NMR spectrum of the
frozen at 103 K conformational equilibrium, separated signals of
the two conformers were observed slightly preferring the
equatorial conformer (ꢀDG8 ¼ 0.35 kcal/mol). The ring inversion
barrier in 2 (6.8 kcal/mol) is somewhat higher than in its less
congested analog 1. 2-Trimethylsilyl-3,3-dimethyl-3-silathiane 3
strongly prefers the 2-equatorial conformation. Conformational
energies of the 2-substituents in the studied heterocyclohexanes
are ꢁ1.4 kcal/mol lower than the corresponding A values
determined for the substituted cyclohexanes. This is the result
of a synergistic effect of the two heteroatoms in the ring
reinforced by the presence of two methyl groups at the silicon
atom and by flattening of the ‘Si-part’ of the ring. The
substituents in the 2-position of 3,3-dimethyl-3-silathiane 1 do
not change the chair conformation of the ring but elongate the
equatorial position of the 2-substituents in
2-phenyl-3,3-dimethyl-3-silathiane is preponderated as proved
3 and in
2
by similar Jgem H,H coupling constants and by substantial
difference in the vicinal coupling constants (Table 1), and as it was
observed for the Ph and SiMe3 substituents in the corresponding
substituted cyclohexanes.[47,54] In addition, the necessary NOESY
cross peaks of the H-2ax to the axial protons at C-4 and C-6 only,
were found in 2-trimethylsilyl-3,3-dimethyl-3-silathiane 3 (Fig. 4).
Effect of the 2-substituent on the structure of
3,3-dimethyl-3-silathianes
Comparison of the calculated geometries of the chair conformers
of compounds 1–3 allows one to follow the effect of the
2-substituent on their structure. For all compounds the chair
conformation is only slightly distorted, the C—C—C—S and
C—C—C—Si dihedral angles varying within the range of 3–58.
The S—C—Si bond angle and the C—S and C—Si bond distances
vary more distinctly (Table 2).
˚
C—S and C—Si bonds (up to 0.025 A) and reduce the S—C—Si
bond angle (up to 48) due to steric and electron-donating effects.
EXPERIMENTAL
Synthesis
The C—S distance increases upon substitution, the largest
˚
3-Silathianes 1–3 have been synthesized by the sequence of
reactions (Scheme SI-1) described in previous studies.[55,56]
elongation of 0.025 A being observed for the 2-SiMe3 axial
substituent. The C—Si distance increase is less pronounced,
˚
being 0.006 A for the equatorial Me and SiMe3 substituents and
NMR measurements
twice as that for the axial ones. Apparently, the observed
elongations are due to the combined steric and electron-
donating effects of the 2-Me and 2-SiMe3 groups. The S—C—Si
bond angle decreases on going from the unsubstituted
compound 1 to the Me-substituted 2 and, further, to the
SiMe3-substituted 3. This, in our opinion, is due to the fact that the
lower is the S—C—Si angle the weaker are nonvalent interactions
of the 2-substituent with the rest of the molecule. If so, the effect
must be more pronounced for the axially substituted species due
to 1,3-syn-axial repulsive interactions and, as follows from the
data of Table 2, this is the case.
NMR spectra were recorded at 400 (1H) and 100 (13C) MHz;
chemical shifts are reported in parts per million downfield to TMS.
The temperature of the probe was calibrated by means of a
thermocouple PT 100 inserted into a dummy tube. The low
temperature measurements were estimated to be accurate to
ꢂ2 K. The temperature was varied by the use of a BCU-05
variable temperature unit, with a temperature stability of ꢂ0.2 K.
Samples were allowed to equilibrate for a minimum 10 min at
each temperature before the spectrum was recorded. For
determination of the equilibrium constant K of the confor-
mational equilibria, the 13C NMR spectra of compound 2 were
recorded in a freon mixture (CD2Cl2/CHFCl2/CHF2Cl ¼ 1:1:3) at the
lowest temperatures where two sets of signals, one for each of
the conformers, were obtained. The equilibrium constant K was
evaluated by careful integration of the signals of the well
separated sets at lowest temperature T ¼ 103 K which sub-
sequently provided the free energy differences (ꢀDG8 ¼ RT lnK).
The chemical shift differences Dnc/Hz of the 13C signals of the
frozen conformers of 2 were determined by extrapolation to the
coalescence temperature Tc and were used (together with Tc) to
calculate the ring interconversion barrier by the Eyring equation
in the usual way.[52]
Table 2. Calculated C—S and C—Si bond distances and
S—C—Si bond angle in molecules 1–3
˚
C—S (A)
˚
C—Si (A)
2-Substituent
H
S—C—Si (8)
1.845
(1.826)a
1.857
1.900
(1.884)a
1.912
111.6
(111.0)a
108.6
Meax
Meeq
SiMe3ax
SiMe3eq
1.858
1.870
1.860
1.906
1.910
1.906
109.3
107.7
108.8
The refined coupling constants of compound 3 (the multiplets
of the H-5 protons in the 1H NMR spectrum were not resolved in
Reference [56] are given in Table 1. The 2D NOESY spectrum of 3
a GE data from Reference [34].
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CONFORMATIONAL ANALYSIS OF 3,3-DIMETHYL-3-SILATHIANE
was recorded with a mixing time of 500 ms and digital resolution
of 0.2 Hz.
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Theoretical calculations
The geometry optimization and the energy calculations were
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Vibrational frequencies and thermodynamic parameter were
computed at the same level of theory at 298.15 K and one
atmosphere of pressure. The zero point vibrational energies (ZPE)
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SUPPORTING INFORMATION
Low temperature 13C NMR spectrum of 1; simulated 1H NMR
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the B3LYP/6-311G(d,p) level of theory. This material is available in
the online version of the paper.
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Acknowledgements
Financial support from the Russian Foundation for Basic Research
and Deutsche Forschungsgemeinschaft (Grant RFBR-DFG No.
08-03-91954) is gratefully acknowledged. We thank Dr Igor A.
Ushakov (Irkutsk) and Dipl.-Ing. (FH) Angela Krtitschka (Potsdam)
for recording the NMR spectra and, especially, Professor Ingvar
Arnason (Reykjav´ık) for the low-temperature 13C NMR spectra of 1
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