Conformational analysis of 3-methyl-3-silathiane and 3-fluoro-3-methyl-3- silathiane

Article abstract of DOI:10.1002/poc.1758

The conformational equilibria of 3-methyl-3-silathiane 5, 3-fluoro-3-methyl-3-silathiane 6 and 1-fluoro-1-methyl-1-silacyclohexane 7 have been studied using low temperature ¹³C NMR spectroscopy and theoretical calculations. The conformer ratio

Full text of DOI:10.1002/poc.1758

Research Article
Received: 29 April 2010,
Revised: 23 May 2010,
Accepted: 26 May 2010,
Published online 21 July 2010 in Wiley Online Library: 2011
(wileyonlinelibrary.com) DOI 10.1002/poc.1758
Conformational Analysis of 3-Methyl-3-
Silathiane and 3-Fluoro-3-Methyl-3-Silathiane
a
b
a
*
Svetlana V. Kirpichenko , Erich Kleinpeter , Igor A. Ushakov
and Bagrat A. Shainyana
The conformational equilibria of 3-methyl-3-silathiane 5, 3-fluoro-3-methyl-3-silathiane 6 and 1-fluoro-1-methyl-
1-silacyclohexane 7 have been studied using low temperature 13C NMR spectroscopy and theoretical calculations. The
conformer ratio at 103 K was measured to be about 5ax:5eq ¼ 15:85, 6ax:6eq ¼ 50:50 and 7ax:7eq ¼ 25:75. The equatorial
preference of the methyl group in 5 (0.35 kcal molS1) is much less than in 3-methylthiane 9 (1.40 kcal molS1) but
somewhat greater than in 1-methyl-1-silacyclohexane 1 (0.23 kcal molS1). Compounds 5–7 have low barriers to ring
inversion: 5.65 (ax !eq) and 6.0 kcal molS1 (eq ! ax) (5), 4.6 kcal molS1 (6), 5.1 kcal molS1 (Meax ! Meeq), and
5.4 kcal molS1 (Meeq ! Meax) (7). Steric effects cannot explain the observed conformational preferences, like equal
population of the two conformers of 6, or different conformer ratio for 5 and 7. Actually, by employing the NBO
analysis, in particular, considering the second order perturbation energies, vicinal stereoelectronic interactions
between the Si–X and adjacent C–H, C–S, and C–C bonds proved responsible. Copyright ß 2010 John Wiley & Sons,
Ltd.
Supporting information may be found in the online version of this article.
Keywords: conformational analysis; low-temperature NMR spectroscopy; NBO analysis; quantum chemical calculations;
3-silathianes
the decrease in the ring inversion barriers in silacyclohexanes
relative to the corresponding cyclohexanes (5–6 kcal molꢀ1 vs.
INTRODUCTION
The conformational behavior of diheterocyclohexanes having the
sulfur atom and a second heteroatom in the 1,3-position of the
ring systems (1,3-dithianes, 1,3-oxathianes) was the subject of
intensive experimental and theoretical studies.[1–6] However, only
limited data have been reported on the conformational proper-
ties of the related heterocycles having the endocyclic silicon
atom, in particular, 1,3-thiasilacyclohexanes (3-silathianes). Over
many years there has been a considerable interest in the
chemistry of these heterocycles most significant results of which
are given in a recent review.[7] Due to the presence of two
heteroatoms of different nature, 3-silathianes exhibit the peculiar
and complementary reactivity. However, much less is known
about the conformational behavior of these compounds as
compared to the related monoheterocycles containing only
the sulfur or silicon atom. In the last few years Arnason et al.
intensively investigated the conformational equilibria, steric
10–11 kcal molꢀ1) was explained originally by the longer endocy-
[9,15]
˚
˚
clic Si–C bond (1.904 A) compared to the C–C bond (1.534 A)
although stereoelectronic effects were also suggested to play an
important role.[16–19]
As to silathianes, some years ago the route of the chair-to-chair
inversion of the unsubstituted 3-silathiane was calculated. Accor-
ding to these calculations, the free energy of activation was found
to be 5.5 kcal molꢀ1 [20]
Recently, we reported the results of
.
experimental and computational investigation of the confor-
mational behavior of 2-X-3,3-dimethyl-3-silathiane (X ¼ H, Me,
SiMe3) by low-temperature 13C NMR spectroscopy.[21] In conti-
nuation of these studies, we present here the first data on the
effect of the substituent at the silicon atom and the joint effect of
the two ring heteroatoms on the conformational properties of
these compounds.
effects of substituents and stereoelectronic interactions in
[14]
1-X-1-silacyclohexanes (X ¼ H,[8] Me,[9,10] F,[11,12] CF3,[13] SiH3
,
cf. Scheme 1) by various physico-chemical methods such as
gas-phase electron diffraction, dynamic nuclear magnetic reson-
ance, microwave spectroscopy, temperature-dependent Raman
spectroscopy and quantum chemical calculations.
*
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
It was shown that the conformational behavior of silacyclo-
hexanes differs significantly from that of the related cyclohex-
anes. The equatorial preference of the substituents drastically
diminishes (for 1) or even inverts (for 2–4) on going from
monosubstituted cyclohexanes to the corresponding silacyclo-
hexanes. The increasing population of the axial conformer and
a
S. V. Kirpichenko, I. A. Ushakov, 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
J. Phys. Org. Chem. 2011, 24 320–326
Copyright ß 2010 John Wiley & Sons, Ltd.

CONFORMATIONAL ANALYSIS
between the 13C chemical shifts for C-2 and C-4 in the 13C NMR
spectra and slightly different low-temperature high field shifts,
the splitting of these two signals at 103 K cannot be examined
with respect to the conformational equilibrium; the 13C signal of
the Si–Me and C-5 are residual and were examined. The signal set
at high field in the 13C NMR spectra was assigned to the 5ax
conformer by analogy with those for 1-methyl-1-silacyclohexa-
ne[9] and in line with known criteria for methylcyclohexane.[22–24]
The axial–equatorial conformer ratio of 15:85 determined from
area measurements of the Si–Me and C-5 signals in the spectrum
of 5 gave a K value of 5.67 corresponding to a free energy
difference (A value) of 0.35 kcal molꢀ1 in favor of the 5eq con-
former at 103 K (cf. Scheme 2).
X
Si
H
X
Si
H
eq
ax
1: X = Me
2: X = F
3: X = CF3
4: X = SiH3
Scheme 1. Conformational equilibrium in 1-X-1-silacyclohexanes.
The low-temperature 13C NMR spectrum of 6 (Fig. 2) shows
broad but already separated signals for C-5 and the Si–methyl
carbon atoms of the axial and equatorial conformers at 103K;
integration is difficult because the decoalesced signals are still
very broad but a conformer ratio of 6ax:6eq ca. 1:1 can be
unequivocally concluded (cf. Fig. 2).
To compare the conformational characteristic of 6 with those
of related silacyclohexane, we prepared 1-fluoro-1-methyl-1-
silacyclohexane 7 and studied its conformational behavior. The
low-temperature 13C NMR spectra of 7 are shown in Fig. 3 and the
spectral data are given in Table 1. Determination of a 7ax:7eq
ratio at low temperature was only possible from integration of
the Me-Si signals (Fig. 3) that led to a value of ca. 75% for the
7eq conformer in the equilibrium at 103 K (considering the
impurity doublet at the 7eq signal).[25] This gives an A value of
0.28 kcal molꢀ1 in favor of 7eq at this temperature.
The decoalescence of signals of the axial and equatorial
conformers of 5–7 was clearly observed and the corresponding
coalescences temperatures Tc for all compounds are well-defined
within a range of about 10 K. The following Gibbs free energies of
activation DGz for the ring inversion were determined by the
method of Shanan-Atidi and Bar-Eli[26] from the coalescence
temperatures considering the different population of participat-
ing conformers: 5.65 kcal molꢀ1 (ax ! eq) and 6.0 kcal molꢀ1
(eq ! ax) (123 K) for 5; ca. 4.6 kcal molꢀ1 (106 K) for 6;
5.1 kcal molꢀ1 (Meax ! Meeq) and 5.4 kcal molꢀ1 (Meeq ! Meax)
(112 K) for 7. These ring inversion barriers are lower than that in
3,3-dimethyl-3-silathiane (6.3 kcal molꢀ1).[21]
RESULTS AND DISCUSSION
The changes observed in the 13C NMR spectra of 3-methyl-
3-silathiane 5 and 3-fluoro-3-methyl-3-silathiane 6 upon lowering
the temperature down to 103 K reveal the ring interchange
process (Scheme 2).
The room-temperature 13C NMR parameters for 5, 6 and (for
comparison purposes) 1-fluoro-1-methyl-1-silacyclohexane 7 are
listed in Table 1 as are the low-temperature spectral data for the
axial and equatorial conformers at 103 K as far as possible (e.g. 5ax
and 5eq in Scheme 2). The well-defined changes in the 13C NMR
spectra for 5 with temperature are shown in Fig. 1.
On lowering the temperature down to 103 K the 13C NMR
spectrum of 5 reveals line broadening and following splitting
(excluding the C-6 signal) into a set of main (at lower field)
and smaller (at higher field) signals, that indicate a mixture of a
major and a minor conformer. Because of the small discrepancy
Me
Si
X
Me
S
Si
X
S
ax
eq
5: X = H
6: X = F
Scheme 2. Conformational equilibrium in 3-X-3-Me-3-silathianes.
Table 1. 13C NMR parameters for compounds 5-7 in CDCl3 solution at 298 K and in the mixture CD2Cl2/CHFCl2/CHF2Cl (1:1:3) at low
temperatures
Chemical shifts d, ppm (J, Hz)
Compound
T (K)
C-2
C-4
C-5
C-6
MeSi
5
298
103
103
298
103
103
298
103
103
11.97
11.35
27.17
25.4
28.0
32.21
31.0
ꢀ5.59
ꢀ4.62
5ax
5eq
6
6ax
6eq
7
10.7, 9.4
ꢀ–8.87
14.54 d (14.78)
ca. 12.9
14.07 d (12.20)
ca. 12.9
28.79 d (2.15)
ca. 26.8
ca. 28.9
24.04 da (12.53)
23.5
Not identified
32.15
31.1
ꢀ3.93 d (14.86)
ca. ꢀ5.8
ca. ꢀ3.3
14.60 d (14.74)
13.5
Not identified
29.54
28.8
Not identified
14.60 d (14.74)
Not identified
ꢀ2.55 d (14.86)
ca. ꢀ4.6
7ax
7eq
ca. ꢀ2.5
a C-5 and C-3.
J. Phys. Org. Chem. 2011, 24 320–326
Copyright ß 2010 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

S. V. KIRPICHENKO ET AL.
Figure 1. Low-temperature 13C NMR spectra of 3-methyl-3-silathiane 5.
The room-temperature NMR spectra of compounds 5 and 6 are
time-averaged due to the fast ring inversion (Scheme 2). Still, the
diastereotopic protons of the 2-CH2 and 6-CH2 methylene groups
give separate signals, that rises the question of their assignment
to the protons cis or trans to the SiMe group. Such an assignment
was made by using the 2D HSQC and HMBC techniques. The H-2
protons in 5 appear as two doublets at 1.67 and 1.86 ppm with
To calculate the weight-averaged room-temperature coupling
constants, one should take into account that from the equation
DG8 ¼ –RT ln K, the ratio 5ax:5eq ¼ 15:85 at 103 K becomes 35:65 at
room temperature. The so calculated weight-averaged trans
coupling constants are equal to 4.2 Hz, and the corresponding cis
coupling constants are 1.4 Hz, in excellent coincidence with the
3
3
experimental values of JH-2-SiH (4.1 and 2.0 Hz) and JH-4-SiH (4.3
and 2.4 Hz).
1
2J 14.6 Hz and have equal JCH of 129.6 Hz. The upfield signal is
further split by long-range H–H couplings. The H-4 protons
appear as two multiplets at 0.66 and 0.88 ppm with 2J 15.0 Hz and
have JCH constants of 118.8 and 119.8 Hz, respectively.
The H-2 protons in 6 appear as two doublets at 1.75 and
2.00 ppm with 2J 14.9 Hz. The upfield signal is further split by
coupling with the fluorine atom and long-range H–H couplings.
The 1H–19F coupling of the upfield signal disappears in the fluoro
1
The 2D-HMBC spectra of 5 show cross peaks of the SiMe
carbon only with the downfield signals of the H-2/4 protons
suggesting that these signals belong to protons trans to the SiMe
1
decoupled H NMR spectrum. Since only the ax,ax coupling is
large enough to be found in usual 2D NMR spectra the signal at
3
group. The same conclusion follows from analysis of the JHH
1.75 ppm must belong to the proton trans to fluorine. The 19F
1
couplings with the SiH proton.
signal in the H coupled 19F NMR spectrum is a sextet (due to
Figure 2. Low temperature 13C NMR spectra of 3-fluoro-3-methyl-3-silathiane 6.
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 320–326

CONFORMATIONAL ANALYSIS
Figure 3. Low-temperature 13C NMR spectra of 1-fluoro-1-methyl-1-silacyclohexane 7.
1
similar H,19F coupling constants to two vicinal axial and three
cyclohexanes, the equatorial position of a substituent is usually
favored.[34] It was explained by 1,3-diaxial repulsion effects
destabilizing the axial conformers. In compliance with this
˚
classical postulate the longer S–C (1.82 A) bond compared to the
˚
C–C bond (1.54 A) would decrease steric hindrance of the axial
forms in thianes with respect to the corresponding cyclohexanes.
Indeed, the equatorial preference for the methyl group in 3-
methylthiane 9 is smaller than that in methylcyclohexane 12.
Further decrease in the A value on going from 3-methylthiane 9
to 3-methyl-3-silathiane 5 (Table 2) can be due to the longer C–Si
geminal methyl protons).
The 2D-HMBC spectra of 6 show a strong cross peak of the
signal at 2.00 ppm with the SiMe carbon, whereas the signal at
1.75 ppm gives weak cross peaks with the C-4/6 ring carbon
atoms and no cross peak with the SiMe carbon. This agrees well
with the calculated JHC coupling constants. The calculated avera-
ged trans coupling constant JH-2-SiMe is 1.25 Hz, and the cis
coupling constant JH-2-SiMe is only 0.17 Hz, that proves their
correct assignment made above.
3
3
˚
bond (1.90 A).
The presence of cross peaks of the SiMe group protons with
both protons of the SiCH2S methylene group in the NOESY
spectrum agrees well with about the equal population of the two
conformers proved by low-temperature NMR.
As for monosubstituted halocyclohexanes, all halogen atoms
prefer the equatorial position. However, the conformational free
energy of the fluorine atom (0.28 kcal molꢀ1) is much smaller
than for other halogen atoms.[29,35–37] In 3-X-halothianes (X ¼Cl,
Br), the equatorial isomers are 1.8 kcal molꢀ1 favored over the
axial isomers.[38] Although there are no data about 3-fluorothiane
The 1JCH couplings for the SiCH2S group in 6 are different and
equal to 128.5 Hz for the signal at 1.75 ppm and 131.2 Hz for that
1
at 2.00 ppm. The calculated values for 6-ax are JCHax ¼ 133.6,
1JCHeq ¼ 129.4 Hz, and for 6-eq JCHax ¼ 129.9, JCHeq ¼ 128.5 Hz.
10, the calculations show a significant preference (1.38kcalmolꢀ1
)
1
1
1
When averaged, they give JCH ¼ 131.0 Hz for the proton cis to
of its equatorial conformer, which is 0.46kcalmolꢀ1 smaller than
that of 3-methylthiane 9. The increase of the calculated ratio in
favor of the axial conformer of 3-fluoro-3-silathiane 8 (16:84) as
compared to that of 3-fluorothiane 10 (9:91) is reasonable
because of the longer C–Si bond. However, in 3-fluoro-3-
methylthiane 11, the conformational energies of Me and F having
the same sign are subtracted in a nonadditive manner, as if the
smaller effect of F in 10 outweighs the larger effect of Me in 9 and
the 11-MeaxFeq conformer becomes energetically more favorable.
This is indicative of other than steric effects operating in the
systems containing both fluorine and sulfur atoms. Indeed, in
the absence of the sulfur atom in the ring, the conformational
equilibrium of the geminal 1-Me-1-X-cyclohexanes (X ¼Cl, Br) is
dictated by the methyl group having the A value much greater
than that of the halogen atoms.[37] The experimental DG8 values
for both halogen compounds show only minor deviations from
the values calculated with the assumption of additivity.[37]
According to the data of Table 2, both 1-fluoro-1-methylcyclohe-
xane 14 and 1-fluoro-1-methyl-1-silacyclohexane 7 also show
a significant preference for the Me-equatorial conformers
1
fluorine, and JCH ¼ 129.6 Hz for that trans to the fluorine. As
distinct of that, the experimental values of 1JCH for the two SiCH2S
protons in 5 are equal, 129.6 Hz. Calculations give 1JCHax ¼ 132.2,
1JCHeq ¼ 128.7 Hz for 5ax, and 1JCHax ¼ 131.1, 1JCHeq ¼ 128.8 Hz for
1
5eq. After weight-averaging they give the same JCH of 130.0
and 130.2 Hz for the protons trans and cis to Me, in excellent
agreement with their experimental coincidence. Note, that
SiCH2S group in both conformers of 5 and 6 shows the reverse
1
Perlin effect, JCHax > 1JCHeq
.
The experimental and calculated characteristics of conformation-
al equilibria for compounds 5–7 are given in Table 2. A series of
reference molecules, 3-fluoro-3-silathiane 8, 3-methylthiane 9,
3-fluorothiane 10, 3-fluoro-3-methylthiane 11, 1-methylcyclohe-
xane 12, 1-fluorocyclohexane 13, 1-fluoro-1-methylcyclohexane 14
was also calculated for direct comparison (Table 2).
It is well known that the replacement of a CH2 group in
cyclohexane derivatives by heteroatom results in changes in
some structural parameters and affects the conformational
characteristics of the heterocycles.[1–6] In monosubstituted
J. Phys. Org. Chem. 2011, 24 320–326
Copyright ß 2010 John Wiley & Sons, Ltd.
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S. V. KIRPICHENKO ET AL.
Table 2. B3LYP/6-311G(d,p) calculated and experimental characteristics for conformational equilibria of compounds 1, 2, 5–14
(DE ¼ Eax ꢀ Eeq, A ¼Gax ꢀ Geq, kcal molꢀ1) (MP2 values are given in parentheses)
Molecule
E(Meax), au
E(Meeq), au
DE
A298 K
(ax:eq)calc
25:75
(ax:eq)exp
Aexp (K)
References
[4]
1
ꢀ526.684783
ꢀ526.685474
0.43
0.66
32:68
26:74
63:37
15:85
0.45 (293)
0.23 (110)
ꢀ0.31 (293)
0.35 (103)
2a
5
ꢀ586.684583
ꢀ885.575584
(ꢀ883.859490)
ꢀ984.917954
(ꢀ983.013448)
ꢀ626.027407
ꢀ945.572620
ꢀ634.155530
ꢀ694.096433
ꢀ733.426986
ꢀ275.266440
ꢀ335.211564
(ꢀ334.244228)
ꢀ374.539487
ꢀ586.684256
ꢀ885.575701
(ꢀ883.859244)
ꢀ984.916553
(ꢀ983.011509)
ꢀ626.028144
ꢀ945.574447
ꢀ634.158292
ꢀ694.098861
ꢀ733.426340
ꢀ275.269792
ꢀ335.211342
(ꢀ334.243845)
ꢀ374.541931
ꢀ0.20
0.07
(ꢀ0.15)
ꢀ0.88
(ꢀ1.22)
0.46
ꢀ0.24
0.19
(ꢀ0.04)
ꢀ0.78
(ꢀ1.22)
0.72
60:40
42:58
[7]
This work
6
79:21
50:50
0 (103)
This work
This work
[21]
7b
8a
23:77
16:84
4:96
25:75
n.a.
2:98
n.a.
n.a.
0.28 (103)
1.40 (190)
1.15
1.73
0.98
1.84
9
10a
11
12
13a,c
1.52
ꢀ0.41
1.38
ꢀ0.13
9:91
55:45
1.5:98.5
45:55
2.10
2.46
0.3:99.7
37:63
1.76 (157)
0.24 (218)
[22]
[23]
ꢀ0.14
ꢀ0.24
1.53
ꢀ0.11
ꢀ0.21
1.66
14
6:94
n.a.
a For compounds 2, 8, 10, 13, having no Me group, the first column of energy refers to the Fax conformers.
b When our manuscript was under preparation, Arnason and al. reported conformational analysis of compound 7 by low-temperature
19F NMR.[18] Our data on the low-temperature 13C NMR are in good agreement with his data.
c The calculated conformational energies A of the fluorine atom in fluorocyclohexane are small negative, ꢀ0.1 to ꢀ0.2 kcal molꢀ1, as
distinct from small positive experimental A value.[30]
– calculated for the reference compound 14 and both measured
and calculated for compound 7.
interactions cannot play an important role since even in 1-F-
cyclohexane and 3-F-thiane the corresponding calculated Fꢁ ꢁ ꢁH
˚
On going from 1-fluoro-1-methylcyclohexane 14 to 3-fluoro-
3-methylthiane 11 and then to 3-fluoro-3-methyl-3-silathiane 6,
the preference of the Meax conformers increases in the order
1.66 > ꢀ0.41 > ꢀ0.78 kcal molꢀ1 being inverted when the sulfur
atom appears in the ring. The alternative sequence 14 ! 7 ! 6
gives the order 1.66 > 0.72 > ꢀ0.78 kcal molꢀ1, also inverting the
sign with appearing the sulfur atom in the ring.
Some other conclusions from analysis of the calculated data of
Table 2 can be summarized as follows. In 1-fluoro-1-methylcyc-
lohexane 14 the equatorial preference of the Me group
(1.66 kcal molꢀ1) is considerably reduced relative to 1-methylcy-
clohexane 12 (2.46 kcal molꢀ1) in spite of the close to zero A value
for the fluorine atom in 13. This is indicative of nonadditivity
of the effects of the two substituents. In 1-fluoro-1-methyl-1-
silacyclohexane 7, these effects are summed up since the Me
group and F atom at silicon have the A values of the opposite sign
(cf. with 1 and 2). The appearing of the sulfur atom in the ring
results in a moderate increase of the equatorial predominance of
the Si–Me group (cf. 5 and 1), whereas the calculated equatorial
preference of the Si–F substituent increases drastically (cf. 8 and
2). Accordingly, the relative stability of 6ax with respect to 7ax
increases.
distance (ꢂ2.7 A) exceeds the sum of the vdW radii to say nothing
˚
of 3-F-3-silathiane and compound 6, where it is larger than 3 A. If
the mentioned influences on the A values were the only factors
determining the relative stability of the conformers of the
compounds under consideration, the equilibrium in Scheme 2
would be similar for 6 and 7, and more shifted to the right for 6
than for 5 since the axial fluorine should impart an additional
stability to the 6-eq conformer. As can be seen from Table 2, this is
not the case for 3-silathianes: the 5ax:5eq ratio is 15:85, whereas
the 6ax:6eq ratio is ca. 1:1 and 7ax:7eq ratio is 25:75. Therefore,
the question is what are the factors stabilizing or destabilizing the
axial and equatorial conformers of 5–7. In order to rationalize
the observed differences we have performed the NBO analysis of
these conformers and compared the second order perturbation
energies E(2) for the donor–acceptor interaction between the
pertinent vicinal bonds. In 6ax, the interaction of the s(C2–S)
and s(C4–C5) orbitals with s (Si–F) orbital (2.06 kcal molꢀ1) is
*
0.5 kcal molꢀ1 stronger than the doubled s(C2–C3) ! s (Si–F)
*
*
interaction in 7ax, while the sum of the two s(C–Hax) ! s (Si–F)
interactions in 6eq (3.74 kcal molꢀ1) is 0.36 kcal molꢀ1 weaker
than in 7eq. This suggests that 6ax is less unfavorable with respect
to 6eq as compared to 7ax versus 7eq, and accounts for the
difference between the 6ax:6eq ratio of ca. 50:50 and the 7ax:7eq
ratio of 25:75. Similarly, the E(2) value for all orbital interactions
between the antiperiplanar Si–H and C–Hax bonds in 5eq
(5.24 kcal molꢀ1) is 1.5 kcal molꢀ1 stronger than in 6eq, whereas
Therefore, because of the dependence of the A values on the
presence, nature, and relative position of heteroatoms, the
situation is not as simple as if it were determined only by the
conformational energies of the substituents at the silicon atom.
First, the value of A for fluorine atom attached to Si is negative
(ca. ꢀ0.3 kcal molꢀ1).[12] Second, the value of A for the Me group
attached to Si remains positive although much less than in
carbon analogs (ca. 0.4 kcal molꢀ1).[9] Third, the 1,3-diaxial Fꢁ ꢁ ꢁH
the s(C2–S) ! s (Si–H) interaction in 6ax (0.97 kcal molꢀ1) is
*
weaker than the s(C2–S) ! s (Si–F) interaction (1.52 kcal molꢀ1).
*
This is responsible for the difference between the 6ax:6eq ratio of
ca. 50:50 and the one of 5ax:5eq equal to 15:85.
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 320–326

CONFORMATIONAL ANALYSIS
dried (MgSO4), filtered and the solvents removed. Compound
7 was obtained a colorless liquid in 66% yield (0.88 g) by fast
vacuum distillation with a heat gun (140–1608C). 1H NMR (CDCl3):
CONCLUSIONS
The conformational equilibria of 3-methyl-3-silathiane 5 and
3-fluoro-3-methyl-3-silathiane 6, and, for comparison, of 1-fluoro-
1-methyl-1-silacyclohexane 7, were studied using low-tempera-
ture 13C NMR spectroscopy down to 103 K and by theoretical
calculations. The conformational behavior of 3-silathianes signifi-
cantly differs both from that of the related thianes and from that
of the related silacyclohexanes: the experimental ratio 5ax:5eq at
103 K is close to the 26:74 ratio measured for 1-methyl-1-silacy-
clohexane but drastically differs from the 2:98 ratio for 3-methy-
lthiane. At the same time, the experimental ratio 6ax:6eq of ca.
50:50 differs from the experimental ratio 7ax:7eq ¼ 25:75;
although there are no experimental data for 3-fluoro-3-methy-
lthiane, the calculations predict small predominance of the
MeaxFeq conformer. Simple steric reasoning can neither predict
nor even explain the observed conformational preferences, like
equal population of the two conformers of 6, or different ratio of
the conformers for 5 and 7. This was made possible by using the
NBO analysis, namely, considering the vicinal orbital interactions
between the Si–X and the adjacent C–H, C–S and C–C bonds.
3
d ¼ 0.26 (d, 3H, MeSi, JH-F ¼ 7.3 Hz), 0.61–0.71 (m, 2H, CH-2/6),
0.87–0.94 (m, 2H, CH-2/6), 1.34–1.43 (m, 1H, CH-4), 1.53–1.59 (m,
1H, CH-4), 1.73–1.78 (m, 4H, CH-3/5). 13C NMR (Table 1). 19F NMR
(CDCl3): d ¼ –138.18 (sext, JH-F ¼ 6.0 Hz). 29Si NMR (CDCl3): d ¼
27.66 (d, JSi–F ¼ 286.2 Hz). The 1H, 13C, 29Si NMR data are
consistent with the reported data.[18]
NMR experiments
The 1H, 13C, 19F, 29Si NMR spectra were recorded on a Bruker
DPX-400 spectrometer (1H, 400.1 MHz; 13C, 100.6 MHz; 19F,
376.3 MHz; 29Si, 79.5 MHz) and the low temperature 13C NMR
spectra on a Bruker AV-600 (at 150.95 MHz). Chemical shifts were
determined relative to residual CHCl3 (1H, d 7.27), internal CDCl3
(13C, d 77.0), internal CD2Cl2 (13C, d 53.73) and are given in ppm
downfield to TMS (for 1H, 13C, 29Si) or CFCl3 (for 19F). Analysis and
assignment of the 1H NMR data were supported by homonuclear
(COSY) and heteronuclear (HSQC 13C–1H, HMBC 13C–1H) 2D
correlation experiments. 1JC–H coupling constants were measured
by using coupled 2D HMBC or 2D HSQC experiments. 13C NMR
spectra were recorded with broad-band proton decoupling and
EXPERIMENTAL
1
were assigned by using DEPT experiments. The JCH coupling
constants were obtained from cross sections of the 2D{1H–13C}
HSQC spectra[42] recorded without wide band decoupling from
13C by GARP pulse sequence.
General
All starting materials and solvents were commercially available
and were dried and purified by conventional procedures prior to
use. Synthesis and room-temperature NMR spectra of 3-methyl-
3-silathiane 5 and 3-fluoro-3-methyl-3-silathiane 6 were descri-
bed in Ref.[39] 1-Methyl-1-phenyl-1-silacyclohexane was synthes-
ized from methylphenyldichlorosilane similar to the earlier
described procedure.[40]
From the 2D HSQC spectrum of 3-fluoro-3-methyl-3-silathiane
3
6 the values of JFH ¼ ꢀ1.2 Hz for the proton signal at 2.00 ppm
3
and JFH ¼ þ3.2 Hz for that at 1.75 ppm were determined. The
theoretically calculated averaged (1:1) trans and cis values are
þ5.15 and –0.61 Hz, respectively.
A solvent mixture of CD2Cl2, CHFCl2, and CHF2Cl in a ratio of
1:1:3 was used for the low temperature measurements. The probe
temperature was calibrated by means of a thermocouple PT 100
inserted into a dummy tube. The low temperature measurements
were estimated to be accurate to ꢃ2 K. The chemical shifts
difference Dnc, Hz was determined by extrapolation to the
coalescence temperature Tc and used to calculate kc and the ring
inversion barriers by the Eyring equation at Tc in the usual way for
compound 6,[43] and employing the method of Shanan-Atidi and
Bar-Eli[26] for compounds 5 and 7, due to population differences
of the conformers.
1-Methyl-1-phenyl-1-silacyclohexane (15)
A solution of methylphenyldichlorosilane (7.6 g, 0.04 mol) in diethyl
ether (100 ml) was added dropwise to the di-Grignard reagent
prepared from 1,5-dibromopentane (9.2 g, 0.04 mol) and magnes-
ium powder (3.6g, 0.15 mol) in diethyl ether (250ml) at room
temperature. The reaction mixture was refluxed for 6 h, cooled and
a saturated aqueous NH4Cl solution was then added. The layers
were separated and the aqueous layer was extracted with Et2O. The
combined organic phase was dried (CaCl2), filtered and diethyl
ether removed. The residue was purified by vacuum distillation to
give 15 (2.69 g, 35% yield) as a colorless liquid, b.p. 96–978C/
Computational methods
1
The geometry optimization and the energy calculations were
carried out at the DFT/B3LYP/6-311G(d,p) level of theory for
the axial and equatorial conformers of compounds 1, 2, 5–14.
Vibrational frequencies and thermodynamic parameters were
computed at the same level of theory at 298.15 K and 1 atm of
pressure using the unscaled zero point vibrational energies
(ZPE). For silathianes 5, 6 and fluorocyclohexane 13 the MP2/
6-311G(d,p) calculations were also performed; thermodynamic
parameters were calculated using the MP2 energies and
the results of B3LYP vibrational calculations. All calculations
were performed with full optimization of all variables using the
Gaussian 03 package.[44] The chemical shieldings and coupling
constants were calculated using the GIAO[45,46] method at the
B3LYP/6-311G(d,p) level of theory. The NBO analysis[47,48] as
implemented in the Gaussian 03 package, was performed for the
axial and equatorial conformers of 5–7.
2 mmHg. H NMR (CDCl3): d ¼ 0.28 (s, 3H, MeSi), 0.80–0.86 (m, 2H,
CHA-2/6), 0.92–0.99 (m, 2H, CHB-2/6), 1.40–1.47 (m, 1H, CHA-4),
1.52–1.59 (m, 1H, CHB-4), 1.68–1.75 (m, 2H, CHA-3/5), 1.78–1.84 (m,
2H, CHB-3/5), 7.36–7.38 (m, 3H, Hmþp), 7.55–7.57 (m, 2H, Ho). 13C
NMR (CD2Cl2/CHCl2F/CHCl2): d¼ –3.8 (Si–Me), 13.8 (C-2/6), 25.6 (C-3/
5), 31.2 (C-4), 130.0 (Cp), 128.9 (Cm), 134.8 (Co), 140.6 (Ci). The 1H and
13C NMR data coincide with the published data.[41]
1-Fluoro-1-methyl-1-silacyclohexane (7)
BF3.2CH3COOH (1.59 ml, 11.6 mmol) was added to a stirred solution
of 1-methyl-1-phenyl-1-silacyclohexane (2.00 g, 10.5 mmol) in
CH2Cl2 (5 ml). The reaction mixture was refluxed for 5 h, cooled to
room temperature, diluted with n-pentane (5 ml), and neutralized
with a saturated solution of Na2CO3 (0.4 ml). The mixture was
extracted with n-pentane, the combined organic extracts were
J. Phys. Org. Chem. 2011, 24 320–326
Copyright ß 2010 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

S. V. KIRPICHENKO ET AL.
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Acknowledgements
The financial support of this work by the Russian Foundation for
Basic Research and Deutsche Forschungsgemeinschaft (Grant
RFBR-DFG No. 08-03-91954) is gratefully acknowledged. Record-
ing of the low-temperature spectra by Dr. Matthias Heydenreich
and Dipl.-Ing (FA) Angela Krtitschka (University of Potsdam) is
greatly acknowledged.
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J. Phys. Org. Chem. 2011, 24 320–326