Conformational preferences of Si-Ph,H and Si-Ph,Me silacyclohexanes and 1,3-thiasilacyclohexanes. Additivity of conformational energies in 1,1-disubstituted heterocyclohexanes

Article abstract of DOI:10.1016/j.tet.2011.10.082

The conformational equilibria of 1-phenyl-1-silacyclohexane 1, 3-phenyl-1,3-thiasilacyclohexane 2, 1-methyl-1-phenyl-1-silacyclohexane 3, and 3-methyl-3-phenyl-1,3-thiasilacyclohexane 4 have been studied for the first time by low temperature ¹³

Full text of DOI:10.1016/j.tet.2011.10.082

Tetrahedron 68 (2012) 114e125
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Conformational preferences of SiePh,H and SiePh,Me silacyclohexanes
and 1,3-thiasilacyclohexanes. Additivity of conformational energies
in 1,1-disubstituted heterocyclohexanes
,
b
Bagrat A. Shainyan a , Erich Kleinpeter
*
a A.E. Favorsky Irkutsk Institute of Chemistry, Siberian Division of the Russian Academy of Science, 1 Favorsky Street, 664033 Irkutsk, Russian Federation
b Chemisches Institut der Universitat Potsdam, Karl-Liebknecht-Str. 24-25, D-14476 Potsdam (Golm), Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 August 2011
Received in revised form 6 October 2011
Accepted 24 October 2011
Available online 2 November 2011
The conformational equilibria of 1-phenyl-1-silacyclohexane 1, 3-phenyl-1,3-thiasilacyclohexane 2, 1-
methyl-1-phenyl-1-silacyclohexane 3, and 3-methyl-3-phenyl-1,3-thiasilacyclohexane
4 have been
studied for the first time by low temperature 13C NMR spectroscopy at 103 K. Predominance of the
equatorial conformer of compound 1 (Pheq/Phax¼78%:22%) is much less than in its carbon analog,
phenylcyclohexane (nearly 100% of Pheq). And in contrast to 1-methyl-1-phenylcyclohexane, the con-
formers with the equatorial Ph group are predominant for compounds 3 and 4: at 103 K, Pheq/Phax ratios
are 63%:37% (3) and 68%:32% (4). As the SieC bonds are elongated with respect to CeC bonds, the
barriers to ring inversion are only between 5.2e6.0 (ax/eq) and 5.4e6.0 (eq/ax) kcal molꢀ1. Parallel
calculations at the DFT and MP2 level of theory (as well as the G2 calculations for compound 1) show
qualitative agreement with the experiment. The additivity/nonadditivity of conformational energies of
substituents on cyclohexane and silacyclohexane derivatives is analyzed. The geminally disubstituted
cyclohexanes containing a phenyl group show large deviations from additivity, whereas in 1-methyl-1-
phenyl-1-silacyclohexane and 3-methyl-3-phenyl-1,3-thiasilacyclohexane the effects of the methyl and
phenyl groups are almost additive. The reasons for the different conformational preferences in carbo-
cyclic and heterocyclic compounds are analyzed using the homodesmotic reactions approach.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Conformational analysis
Heterocycles
Dynamic NMR
Theoretical calculations
Additivity of conformational energies
1. Introduction
decreases from 1.78 to 0.23 kcal molꢀ1 on going from methyl-
cyclohexane10 to 1-methyl-1-silacyclohexane.3 Moreover, the con-
There has been a considerable interest in medium-sized sili-
conecarbon heterocycles over many years from a theoretical and
synthetic point of view, and especially for stereochemical and
mechanistic studies of the reactions at silicon.1,2 Therefore, struc-
tural studies of silacyclohexanes remain a state-of-the-art problem
of conformational analysis. In the last few years Arnason et al. in-
tensively investigated the conformational equilibria, steric effects,
and stereoelectronic interactions in 1-X-1-silacyclohexanes
(X¼Me,3,4 F,5,6 CF3,7,8 SiH39) by various physico-chemical methods
such as gas-phase electron diffraction, dynamic nuclear magnetic
resonance, microwave spectroscopy, temperature-dependent
Raman spectroscopy, and theoretical calculations. Replacement of
carbon with silicon in the monosubstituted six-membered cycles
drastically alters the conformational energies of the substituents.
Thus, the conformational free energy (A value) for the methyl group
formational preferences may even invert. Thus, in contrast to
fluorocyclohexane11 and trifluoromethylcyclohexane,12 the axial
conformers are predominant for 1-fluoro-1-silacyclohexane6 and
1-trifluoromethyl-1-silacyclohexane.7,8 The increase of the relative
axial preference of substituents at silicon in silacyclohexanes with
ꢀ
respect to cyclohexanes is favored by the longer CeSi (1.892 A) as
ꢀ
compared to the CeC bond (1.540 A).
However, the influence of stereoelectronic effects cannot be
ruled out either.13e15 The latter become even more important upon
introduction of a second heteroatom into the heterocyclic system.
For example, the population of the axial conformer is increased
from fluorocyclohexane to 1-fluoro-1-silacyclohexane but is sub-
stantially decreased with the further inclusion of ring heteroatoms,
e.g., by introduction of the sulfur atom, that is, in 3-fluoro-3-sila-1-
thiacyclohexane.16
The substituents at the silicon atom, for which the conforma-
tional preferences were studied experimentally, are confined to the
above mentioned groups (Me, F, CF3, SiH3). Besides, the confor-
mational preferences for a large number of substituents in 1-X-1-
* Corresponding author. E-mail addresses: bagrat@irioch.irk.ru (B.A. Shainyan),
ekleinp@uni-potsdam.de (E. Kleinpeter).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.10.082

B.A. Shainyan, E. Kleinpeter / Tetrahedron 68 (2012) 114e125
115
silacyclohexanes were calculated theoretically.14,15 The studied
2. Results and discussion
compounds, however, did not include SiePh-substituted silacy-
clohexanes in spite of a wide application of the SiePh-substituted
silanes in synthetic organic and organosilicon chemistry.17e21 For
example, the electrophilic cleavage of SieAr bonds is an effective
method for the preparation of a variety of Si-functionalized acy-
clic and cyclic compounds.19,20 Recently, the SieX-derivatives
of 1,3-thiasilacyclohexanes (X¼H, F, OR), and 1-Me-1-F-1-
silacyclohexane have been synthesized in this way.16 So far, no
conformational studies were reported for these Si-phenyl
substituted silacyclohexanes or other heterosilacyclohexanes, or
even for any heterocyclohexanes with a phenyleheteroatom bond
except for a computational study of the PePh-substituted phos-
phorinane P-oxides.22 Apart from never being conformationally
studied, these compounds are of special interest because they are
qualitatively different from the methyl- or halogen-substituted
analogs16 since the phenyl group is an asymmetric rotor and its
rotation about the SiePh bond may (and does) cause notable var-
iations of nonbonded intramolecular interactions, as was shown by
molecular mechanics23e25 and quantum chemical26 calculations of
phenylcyclohexane. Another feature of the phenyl group is its
ability to act as a conformational anchor without notable distortion
of the cyclohexane ring (as is the case, e.g., for the tert-butyl
group).12,27
2.1. Dynamic NMR measurements
As follows from the low temperature 13C NMR study, hetero-
cycles 1e4 are conformationally flexible. On lowering the temper-
ature down to 103 K, their 13C NMR spectra show changes typical of
dynamic exchange processes, namely, broadening, decoalescence,
and splitting of the 13C signals. The conformational equilibria are
depicted in Schemes 1 and 2.
Ph
Si
H
Ph
X
X
Si
H
1: X = CH2
2: X = S
Scheme 1. Conformational equilibrium of compounds 1 and 2.
Ph
Si
Me
Ph
X
X
Si
Me
3: X = CH2
4: X = S
In the case of geminally disubstituted cyclohexanes or hetero-
cyclohexanes the problem of additivity of their conformational free
energies arises. This problem, which until recently was confined to
cyclohexane derivatives, can be traced back to the 60s of the last
century28 but is still the subject of research including new objects
and methods.29e31 For cyclohexanes, the lack of additivity was
concluded to be the rule.32 The most vivid example of non-
additivity is 1-methyl-1-phenylcyclohexane, investigated both
experimentally33 and theoretically.23e26 The principal conclusion
was that the predominance of the axial phenyl conformer over the
equatorial in spite of a larger conformational free energy for the
Scheme 2. Conformational equilibrium of compounds 3 and 4.
The spectral data at room temperature and at the lowest
reached temperature of 103 K are given in Table 1. Variable tem-
perature spectra for compounds 2 and 3 are given in Figs. 2 and 4,
and the frozen spectra are shown in Figs. 1, 3, 5, and 6. As follows
from Table 1, upon lowering the temperature, the aliphatic signals
from the room temperature 13C NMR spectrum of 1-phenyl-1-
silacyclohexane 1 are shifted upfield and at w115 K the former
two signals decoalesce as shown in the spectrum at 103 K (Fig. 1).
The assignment of the closely spaced 13C resonances of C-2 and
C-4 in 3-phenyl-1,3-thiasilacyclohexane 2 was proved by the HMQC
spectrum, which shows the correlations of the 13C signal at
10.48 ppm with the two diastereotopic SiCH2C protons, and the
signal at 11.18 ppm with the two diastereotopic SiCH2S protons (see
Fig. SI-1 in Supplementary data). Lowering of the temperature
causes continuous upfield shifts, finally broadening of all four sig-
nals (Fig. 2) and at the lowest reached temperature (103 K) small
signals of the second conformer at ca. 7.3 and 25.5 ppm appear
(Fig. 3).
phenyl group (2.87 kcal molꢀ1 33,12
)
than for the methyl group
(1.78 kcal mol
)
ꢀ1 10 is due to destabilizing nonvalent interactions of
the phenyl ring with the 2,6-Heq and Me protons in the Ph-
equatorial conformer.26 Still, in the similarly substituted cyclo-
hexanes the additivity can be fulfilled for a limited range of the
substituents.32 Recently, based on the CCSD(T)/CBS calculations of
the monosubstituted silacyclohexanes with substituents F, CH3,
CF3, and mono- and disubstituted cyclohexanes with the same
substituents, it was concluded that for the equally disubstituted
rings the addition model ‘works only moderately well for the cy-
clohexanes’ and ‘remarkably well for the silacyclohexanes within
the limited selection of substituents’.34
The conformational analysis of silathiacyclohexanes (sila-
thianes) and their derivatives is our continuing interest.16,35e41 The
subject of the present study is to examine the conformational
preferences of the phenyl group in the mono- and disubstituted
silacyclohexanes and 1,3-thiasilacyclohexanes using the methods
of low temperature NMR spectroscopy and quantum chemical
calculations, and to compare them with those in the carbocyclic
analogs. As a result of this dynamic NMR study the general prob-
lem of the applicability of the concept of additivity of conforma-
tional effects of geminal substituents to the cyclohexane and
silaheterocyclohexane series is discussed. As the objects for the
experimental study we chose 1-phenyl-1-silacyclohexane 1, 3-
phenyl-1,3-thiasilacyclohexane 2, 1-methyl-1-phenyl-1-silacyclo-
hexane 3, and 3-methyl-3-phenyl-1,3-thiasilacyclohexane 4. Phe-
nylcyclohexane 5 and 1-methyl-1-phenylcyclohexane 6 were used
as the reference carbocyclic compounds, and 3-phenylthiane 7
and 3-methyl-3-phenylthiane 8 were calculated theoretically as
additional reference molecules for the silathianes 3 and 4,
respectively.
Room temperature NMR spectra of 1-methyl-1-phenyl-1-
silacyclohexane 3 were described in our previous paper;16 at low
temperatures the signals of carbons attached to silicon decoalesce
(Fig. 4) and the ratio of the conformers was determined from the
frozen spectrum (Fig. 5).
For 3-methyl-3-phenyl-1,3-thiasilacyclohexane 442 the assign-
ment of the closely resonating C-2 and C-4 was made from HMBC as
in case of 2. Both SiMe and C-2/C-4 signals decoalesce below 110 K,
however only the SiMe signal could be resiliently evaluated with
respect to free energy differences; the frozen spectrum at 103 K is
given in Fig. 6.
Note that the decoalescence is most pronounced for carbon
atoms directly attached to silicon. This seems reasonable since it is
the silicon atom, which bears two different substituents (H, Ph in 1,
2 or Me, Ph in 3, 4) changing their positions upon the ring inversion
(Schemes 1 and 2).
The principal question for determination of the ratio of the
conformers is the assignment of the decoalesced signals of the axial
and equatorial conformers of 1e4. For the SieMe,Ph-substituted
compounds 3 and 4, the more intense high-field SieMe signals in

116
B.A. Shainyan, E. Kleinpeter / Tetrahedron 68 (2012) 114e125
Table 1
13C chemical shifts for compounds 1e4 at 298 K (CDCl3) and 103 K (CD2Cl2/CHFCl2/CHF2Cl 1:1:3)
No.
T, ꢁK
MeSi
C-2/6
C-3/5
C-4
No.
T, ꢁK
MeSi
C-2
C-4
C-5
C-6
1
298
103
103
298
103
103
10.67
8.39
10.44
13.8
11.00
12.06
24.89
23.52
24.95
25.6
23.99
23.99
29.86
29.11
29.11
31.2
29.35
29.35
2
298
103
103
298
103
103
11.18
7.4
10.2
13.40
10.48
7.3
9.33
12.69
11.44
10.50
27.58
25.4
27.85
26.99
26.72
26.72
32.28
25.5
31.05
32.30
31.44
31.44
1-Phax
1-Pheq
3a
3-Phax
3-Pheq
2-Phax
2-Pheq
4
4-Phax
4-Pheq
ꢀ3.8
ꢀ4.86
ꢀ1.74
ꢀ8.10
ꢀ1.84
ꢀ7.59
12.61
b
a
In the mixture CD2Cl2/CHFCl2/CHF2Cl (1:1:3).
Overlaps with the signal at 11.44 or 10.50 ppm.
b
Fig. 1. 13C NMR spectrum of the frozen conformational equilibrium of 1-phenyl-1-silacyclohexane 1 at 103 K.
Fig. 2. Variable temperature 13C NMR spectra of 3-phenyl-1,3-thiasilacyclohexane 2.

B.A. Shainyan, E. Kleinpeter / Tetrahedron 68 (2012) 114e125
117
Fig. 3. 13C NMR spectrum of the frozen conformational equilibrium of 3-phenyl-1,3-thiasilacyclohexane 2 at 103 K.
Fig. 4. Variable temperature 13C NMR spectra of 1-methyl-1-phenyl-1-silacyclohexane 3.

118
B.A. Shainyan, E. Kleinpeter / Tetrahedron 68 (2012) 114e125
Fig. 5. 13C NMR spectrum of the frozen conformational equilibrium of 1-methyl-1-phenyl-1-silacyclohexane 3 at 103 K.
the low temperature spectra (Figs. 5 and 6) were assigned to the
axial methyl groups by analogy with the assignment made for 1-
methyl-1-phenylcyclohexane,33 1-methyl-1-silacyclohexane de-
rivatives3 and are in accordance with general rules of cyclohexane
stereochemistry.43 For 1-phenyl-1-silacyclohexane 1, the assign-
ment of the signals to the axial or equatorial conformer (Fig. 1) was
made by analogy with that for its SieMe analog 3 since the posi-
tions of the C-2,6 and C-3,5 signals in compounds 1 and 3 are very
similar, and is consistent with that for 1-phenyl-1-cyclohexane44
and its derivatives.33 The same spectral pattern observed for 3-
phenyl-1,3-thiasilacyclohexane 2 (Fig. 3) allowed us to assign the
appearing signals at 7.3 and 25.5 ppm to the minor axial conformer.
Fig. 6. 13C NMR spectrum of the frozen conformational equilibrium of 3-methyl-3-phenyl-3-sila-1-thiacyclohexane 4 at 103 K.

B.A. Shainyan, E. Kleinpeter / Tetrahedron 68 (2012) 114e125
119
Therefore, compounds 1e4 exist predominantly as Pheq-con-
formers. The ratio of the conformers and other conformational
the 2-equatorial conformation. Employing the corresponding
3
3
3
values (3J4ax,5ax¼13.15, J4ax,5eq¼4.8, J4eq,5ax¼3.25, J4eq,5eq¼5.2,
3
3
3
characteristics are given in Table
2
together with phenyl-
3J5ax,6ax¼11.9, J5ax,6eq¼2.85, J5eq,6ax¼2.3, J5eq,6eq¼4.85 Hz) as ref-
3
3
3
cyclohexane 5 and 3-phenylthiane 7, which were shown to be
erences16 and adopting J4,5trans¼10.4, J4,5cis¼6.75, J5,6trans¼11.0,
conformationally homogeneous and existing as single Pheq
conformers.
-
3J5,6cis¼7.1 Hz for 2 as experimental values (all data
d and J of 2
are given in Supplementary data) from the spectrum simulation
Table 2
Conformational characteristics of 1-phenyl-1-silacyclohexane 1, 3-phenyl-1,3-thiasilacyclohexane 2, 1-methyl-1-phenyl-1-silacyclohexane 3, 3-methyl-3-phenyl-1,3-
thiasilacyclohexane 4, and reference compounds phenylcyclohexane 5 and 3-phenylthiane 7
Compound
Pheq/Phax, %
K
A¼GaxꢀGeq, kcal molꢀ1
Tc, K (pairs of signals)
D
Gs, (eq/ax)/(ax/eq),
kcal molꢀ1
1
2
3
78:22
79:21
95:5
75:25
62:38
3.35
3.00
19.0
0.25
0.22
0.60 (at 103 K)
0.64 (at 296 K)
0.12
118 (C-2/6)
113 (C-3/5)
115e125
5.39/5.71
5.17/5.42
d
3.0
1.78
1.63
2.12
130 (SiMe)
120 (C-2/6)
120 (SiMe)
d
5.96/6.01
5.50/5.63
5.47/5.67
d
64:36
68:32
0.10
4
5
7
0.15
w100:0
w100:0
d
2.87a
d
3.30b
d
d
a
Ref. 34.
b
D
G298K calculated at the B3LYP/6-311G** level of theory.
Because the conformational equilibrium of 3-phenyl-1,3-
thiasilacyclohexane 2 is so highly biased at 103 K we also simu-
lated the room temperature 1H NMR spectrum of this compound.
The comparison of experimental and simulated spectra is given in
Fig. 7 (spread parts, for emphasizing the excellent agreement, and
chemical shifts/H,H-coupling constants thus obtained are given in
Supplementary data).
after K¼[3Jax,axꢀ3Jtrans]/[3Jax,axꢀ3Jeq,eq] and K¼[3Jax,axꢀ3Jcis]/[3Jax,-
axꢀ3Jeq,eq], respectively, the following mean results were obtained:
K¼0.25 for the minor (25%) and 0.75 for the major conformer (75%).
This gives the conformational equilibrium constant K¼3.0 (Scheme
1), and (GaxꢀGeq)¼0.64 kcal molꢀ1 at room temperature very sim-
ilar to that at 103 K (0.60 kcal molꢀ1) obtained from the low tem-
perature study (Table 2). These simulation results at room
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000
800
600
Fig. 7. Simulated (above) and experimental (below) 1H NMR spectrum of 3-phenyl-1,3-thiasilacyclohexane 2.
In order to detect the room temperature conformational equi-
librium of 2 we used the vicinal H,H-coupling constants of the
protons at C-4 to C-6 and compared them to those in 2-Me3Si-3,3-
dimethyl-3-silathiane,16 which was proved to exist exclusively in
temperature are really significant because it proves (i) the preferred
conformer of 2 to be the conformer with the equatorial phenyl
substituent at silicon in 2 (from the 3J3ax,4ax¼5.8 Hz compared with
3J3ax,4eq¼1.3 Hz) and (ii) that the high-field signals at 103 K in the

120
B.A. Shainyan, E. Kleinpeter / Tetrahedron 68 (2012) 114e125
13C NMR spectrum (at 7.3 and 25.5 ppm) are really generated by the
second, the minor conformer with the axial phenyl substituent at
silicon.
1 ꢀ ax : Eð2Þ½
s
ðSieHÞ/all
s
ꢂ ꢃ þ Eð2Þ½
s
ðSieCPhÞ/all
s
ꢂ ꢃ
¼ 10:39 þ 18:89 ¼ 29:28 kcal molꢀ1
For further discussion, it was important to obtain the so far
unknown conformational free energy for the phenyl group at-
tached to silicon. The value of
ꢂ ꢃ þ Eð2Þ½
s
ðSieCPhÞ/all
s
ꢂ ꢃ
D
Go measured by us from the low
1 ꢀ eq : Eð2Þ½
s
ðSieHÞ/all
s
temperature 13C NMR spectra of 1-phenylsilacyclohexane 1, is
0.22e0.25 kcal molꢀ1 (Table 2), which is one order of magnitude
lower than the conformational energy of the Ph group in phe-
nylcyclohexane (2.87 kcal molꢀ1). Note, however, that conforma-
tional preferences of the Ph group depend not only on the nature
of heteroatom it is attached to but also on the presence (and,
apparently, the nature) of another heteroatoms in the ring, as
clearly demonstrated by larger predominance of the Pheq-con-
former of compound 2 versus compound 1 (Table 2).
¼ 10:58 þ 17:64 ¼ 28:22 kcalmolꢀ1
2 ꢀ ax : Eð2Þ½
s
ðSieHÞ/all
s
ꢂ ꢃ þ Eð2Þ½
s
ðSieCPhÞ/all
s
ꢂ ꢃ
¼ 11:11 þ 18:71 ¼ 29:82 kcal molꢀ1
2 ꢀ eq : Eð2Þ½
s
ðSieHÞ/all
s
ꢂ ꢃ þ Eð2Þ½
s
ðSieCPhÞ/all
s
ꢂ ꢃ
2.2. Theoretical calculations
¼ 10:72 þ 19:02 ¼ 29:74 kcalmolꢀ1
To get a deeper insight into the effect of heteroatoms (Si and S)
on the conformational equilibrium of heterocyclohexanes we have
performed DFT and MP2 calculations of the conformers of the
monophenyl compounds 5, 1, 2, and the Me,Ph-geminally di-
substituted compounds 6, 3, 4. For 1-phenyl-1-silacyclohexane 1
the G2 calculations were also performed. The results are summa-
rized in Table 3.
These results allow one to conclude that (i) interaction with the
heterocycle of higher acceptor activity 2 is expectedly larger than
with 1; (ii) this effect is small for the axial conformer
Table 3
MP2/6-311G(d,p), B3LYP/6-311G(d,p) (in italics) and G2 (in bold) calculated total energies (E, hartrees) and thermodynamic parameters (
D
E¼E(Phax)ꢀE(Pheq), ZPE,
Ho Go
D
298
D
Ho
,
298
D
Go298, kcal molꢀ1, So, cal molꢀ1 Kꢀ1) for the conformational equilibria in compounds 1e6
No.
Phax
Pheq
D
E
ꢀD
ꢀ
298
E
ZPE
So
E
ZPE
So
5a
1
ꢀ465.570121
158.39
157.42
151.56
150.59
101.14
100.62
108.79
108.62
ꢀ465.576233
158.03
157.16
151.36
150.53
100.52
99.49
109.72
109.44
3.84
4.35
0.55
0.92
0.72
4.14
4.58
0.67
0.92
0.72
0.08
0.97
ꢀ2.00
ꢀ0.59
ꢀ0.67
ꢀ0.46
0.38
3.95
4.25
0.95
1.17
0.68
0.43
1.16
ꢀ3.35
ꢀ1.02
ꢀ0.82
1.69
ꢀ467.040766
ꢀ716.596008
ꢀ718.459153
ꢀ717.986477
ꢀ1075.065105
ꢀ1077.350025
ꢀ504.774572
ꢀ506.365209
ꢀ755.809324
ꢀ757.798740
ꢀ1114.276815
ꢀ1116.688918
ꢀ467.047692
ꢀ716.596885
ꢀ718.460616
ꢀ717.987632
ꢀ1075.065057
ꢀ1077.350913
ꢀ504.771484
ꢀ506.364306
ꢀ755.808109
ꢀ757.798801
ꢀ1114.277252
ꢀ1116.689815
2
134.31
133.48
175.81
174.60
169.58
168.44
152.32
151.38
110.71
111.16
108.58
105.58
117.67
109.59
119.73
118.10
134.12
133.39
175.95
174.61
169.45
168.32
152.18
151.21
111.90
110.96
104.04
104.14
117.18
116.81
119.70
119.79
ꢀ0.03
0.92
6a
3
ꢀ1.94
ꢀ0.57b
ꢀ0.76
0.04
0.27
0.56
4
0.37
1.17
0.67
a
A detailed theoretical conformational analysis of compounds 5 and 6 was reported in the literature.26 The results for these compounds in Table 2 are given for uniformity of
comparison.
b
Experimental value determined from low temperature 13C NMR spectra is ꢀ0.32 kcal molꢀ1 33
.
For the monophenyl substituted species 1, 2, 5 the conforma-
tional equilibrium is shifted to the Pheq-conformers, being maxi-
mum for phenylcyclohexane 5, in full agreement with the
experiment. The appearance of more than 20% of the axial con-
former for 1-phenyl-1-silacyclohexane 1 (Table 2) is reasonably
rationalized considering the ring enlargement due to the longer
CeSi versus CeC bonds (vide supra). However, this effect cannot
account for a reverse shift of the conformational equilibrium to-
ward the equatorial conformer by further enlargement of the ring
size by introduction of the sulfur atom, that is, on going from 1 to 2
(Table 2). A tentative explanation can be suggested based on the
recent hypothesis of partitioning the total energy of substituted
cyclohexanoids into ring and substituent contributions.45 For this,
we have performed the NBO analysis of the conformers of 1 and 2
and compared the second order perturbation energies of the donor
(0.54 kcal molꢀ1) but substantially larger for the equatorial con-
former (1.52 kcal molꢀ1). Therefore, on going from 1 to 2, the
conformational equilibrium should shift in favor of the equatorial
conformer, which is the case (Table 2).
In contrast to 1-methyl-1-phenylcyclohexane 6, the experi-
mentally determined preferable conformation for the geminally
disubstituted heterocycles 3 and 4 is the one with the equatorial
phenyl and axial methyl group. For the Si,S-heterocycle 4 this is
proved at both DFT and MP2 levels of theory, whereas for the
Si-heterocycle 3 only DFT calculations predict strong predominance
of the Pheq conformer.
As concluded by Wiberg et al.,26 the main reason for the un-
favorable 6-Pheq conformation is the presence of the axial geminal
methyl group, which does not allow the equatorial phenyl group to
adopt the conformation minimizing steric repulsive interactions
with the H-2,6eq atoms, that is, when the plane of the phenyl ring
bisects the C2eC1eC6 angle (Scheme 3).
orbitals
s(SieH) and s(SieCPh) with the proper orbitals of the
heterocycle:

B.A. Shainyan, E. Kleinpeter / Tetrahedron 68 (2012) 114e125
121
in cyclohexanes cannot be considered as a general rule although
the trend when comparing with silacyclohexanes is evident. The
next important issue is the aforementioned specific behavior of
substituents, which are asymmetric rotors, like Ph, whose rotation
about the CePh or SiePh bond is affected by the second
geminal substituent leading to substantial variations in energy, as
proved by the largest deviations from additivity (Table 4,
DDGo¼1.36e1.84). It may even reverse the positions of the two
substituents in the preferable conformer relative to the predicted
from their conformational energies, like in 1-methyl-1-
phenylcyclohexane, 1-methyl-1-phenylcyclohexanol, or 1-(dime-
thylamino)-1-phenylcyclohexane. Note also that the presence of
a heteroatom in a position remote from the endocyclic carbon atom
bearing two substituents practically does not alter the situation
observed for cyclohexanes: deviations for 3-methyl-3-phenylthiane
and 3-methyl-3-fluorothiane are large, being maximum for the for-
mer (Table 4).
To gain a better insight into the origin of these differences and to
estimate the energetic consequences of the nonbonded interactions,
we have calculated the homodesmotic reactions53,54 of the con-
formers of 3-methyl-3-phenyl-1,3-thiasilacyclohexane 4 with its
unsubstituted analog, 1,3-thiasilacyclohexane, with retention of the
positions of the substituents, as depicted in Eqs. 1 and 2, and the
corresponding reactions of the conformers of 1-methyl-1-phenyl-1-
silacyclohexane 3 (Eqs. 3 and 4) and 3-methyl-3-phenylthiane 8
with their unsubstituted analogs (Eqs. 5 and 6).
Me
Me
Scheme 3. Conformational equilibrium of 1-methyl-1-phenylcyclohexane 6.
As a result, depending on the method of calculation, the 6-Phax
conformer is 0.52e1.85 kcal molꢀ1 more stable than the 1-Pheq
conformer (see Ref. 26 and our data in Table 3). Evidently, the
ꢀ
ꢀ
longer SieC (1.904 A) and SeC (1.82 A) bonds compared to the CeC
ꢀ
bond (1.534 A) should attenuate such a destabilizing effect of the
geminal methyl group. Indeed, the H/H distances between the
phenyl ortho hydrogens (Ho) and the methyl and methylene hy-
drogens in the axial and equatorial conformers of compounds 3 and
4 are larger than the sum of the van der Waals (vdW) radii of hy-
ꢀ
drogen atoms (2.4 A). Note, that the calculated structure of 3-Phax
excellently coincides with the X-ray structure of trans-1-(p-bro-
mophenyl)-4-tert-butyl-1-methyl-1-silacyclohexane.46
As was shown in Introduction, the additivity of the conforma-
tional energies of the substituents is rather an exception in the
cyclohexane series, but a rule in the silacyclohexane series. To make
more valid conclusions on additivity or nonadditivity, we have
analyzed the up to now available data on the conformational
equilibria for the geminally disubstituted compounds of the two
series. The following A values from the most recent compilation of
Bushweller47 were used: Me (1.74), CF3 (2.50), Ph (2.87), F (0.36), Cl
(0.54), Br (0.48), Me2N (1.53), OH (1.01). The results are summarized
in Table 4.
Even a brief inspection of Table 4 reveals that, in general, de-
viations from additivity are much larger for the cyclohexane than
for the silacyclohexane series. This might seem to be fully consis-
tent with the conclusion made by Arnason et al.,34 unless there was
a good deal of evidence of additivity even in the cyclohexane series,
for example, in 1-halo-1-methylcyclohexanes,11,52 1-vinyl- and 1-
ethynyl-1-cyclohexanols, or 1-acetoxy-1-methylcyclohexanes11
omitted from Table 4 for brevity. Therefore, the lack of additivity
Reactions 1 and 2 are only slightly exothermic, the values of DE,
D
Ho
and
D
Go
being ꢀ0.43, ꢀ0.36 and ꢀ0.38 kcal molꢀ1 for
298
298
reaction 1, and ꢀ0.51, ꢀ0.52 and ꢀ1.19 kcal molꢀ1 for reaction 2.
Noteworthy, the similar reactions of the conformers of 1-methyl-1-
phenylcyclohexane 6 with cyclohexane calculated in Ref. 26 have
different signs of the thermal effect: the reaction of 6-Pheq is also
exothermic by ꢀ2.0 kcal molꢀ1, whereas that of 6-Phax is endo-
thermic by 0.8 kcal molꢀ1
.
The values of
D
E,
D
Ho
and
D
Go
are equal to ꢀ0.35, ꢀ0.21,
298
298
and ꢀ0.50 kcal molꢀ1 for reaction 3, and 0.09, 0.08, and
ꢀ0.14 kcal molꢀ1 for reaction (4), and are rather close to those for 1-
methyl-1-phenylcyclohexane 6. For reactions
5 and 6 the
Table 4
Additivity of substituent effects in the geminally disubstituted cyclohexanes and heterocyclohexanes, kcal molꢀ1
monosubstituted species)
(
D
Go
is the algebraic sum of D
Go for the corresponding
add
X
X
Si
Y
a
Y
Go
Z
a
X
Y
Fb
D
Go
D
DDGo
Ref.
34
Z
X
Y
F
D
Go
D
Go
add
DDGo
Ref.
axeeq
add
axeeq
Me
0.86
0.86
0.53
0.53
0.32
1.60
1.38
1.31
0.76
0.74
0.52
0.78
0.23
1.45
0.61
1.84
CH2
Me
0.26
0.28
0.39
0.36
0.52
0.63
0.10
0.24
0.24
34
16
34
b
Me
CF3
34
CH2
Me
CF3
Me
Me
Ph
Ph
NMe2
NMe2
ꢀ1.13
23,26
48
49
CH2
S
S
Me
Me
Me
Ph
Ph
Fc
ꢀ0.11
0.15
ꢀ0.78
0.02
0.25
0.09
0.10
0.01
This work
This work
16
ꢀ0.4
0.21
ꢀ0.5
1.34
ꢀ0.79
c
Ph
Me
OH
OH
0.31
0.73
1.86
0.42
1.36
50,51
32
0.10
1.46
1.36
0.59
This work
Me
S
c
Me
F
Ph
0.5
ꢀ0.13
0.46
16
S
a
‘ax’ refers to conformers with XaxYeq, ‘eq’dwith XeqYax
.
b
c
CCSD(T)/CBS calculated
D
E values are given.
G values are given.
B3LYP/6-311G** calculated
D

122
B.A. Shainyan, E. Kleinpeter / Tetrahedron 68 (2012) 114e125
Me
Si
Me
H
S
H2Si
H2Si
H2Si
S
Si
S
S
Si
S
S
Ph
Me
Ph
H
Me
H
Ph
(1)
(2)
(3)
+
+
+
+
Ph
Si
H
Ph
Si
S
S
Si
H
+
Me
Si
Me
Si
H
Si
Ph
+
Ph
Si
H
Ph
Si
H2Si
Si
Me
Ph
Me
H
(4)
(5)
+
+
+
Me
Me
S
S
S
S
Ph
+
Ph
Ph
S
S
S
S
Me
Me
(6)
+
+
values of
D
E,
D
Ho
and
D
Go
are much larger: ꢀ3.74, ꢀ3.63,
298
298
and ꢀ4.39 kcal molꢀ1 for reaction 5, and ꢀ2.28, ꢀ2.12, and
D
Go298 ¼ ð0:22 ꢄ 0:10Þ þ ð0:074 ꢄ 0:003ÞDa
¼ 6; so ¼ 0:2:
r ¼ 0:996; n
ꢀ3.02 kcal molꢀ1 for reaction 6.
In the case of additivity, the homodesmotic reactions similar to
those above occurring with retention of the positions of the sub-
stituents must be close to thermoneutral, and so they are for re-
actions 1e4. At the same time, reactions 5 and 6 of the conformers of
3-methyl-3-phenylthiane 8 are exothermic. The degree of exo-
thermicity is a measure of internal strain created by the two geminal
substituents in the molecule. Careful analysis of the geometry of the
3. Experimental
3.1. Synthesis
Synthetic manipulations were carried out using standard inert
atmosphere techniques. Solvents were dried and purified by stan-
dard procedures. Thin layer chromatography was performed on
20 mm precoated Merck silica gel plates (60 F-254) and visualized
by iodine vapors. Column chromatography was carried out using
silica gel 60 (0.063e0.200 mm, ICN Biomedical Inc.).
conformers, in particular, of the dihedral angle
of the phenyl ring and the plane bisecting the CSiC [in Eqs.1e4] or C-
2eC-3eC-4 [in Eqs. 5 and 6] angle revealed that
is close to 90ꢁ for
the Ph-eq conformers of compounds 1e4 and 7, and is close to
a between the plane
a
a
0ꢁ for the Ph-ax conformers of compounds 1 and 2. That means that
in reactions 1, 3, and 4 the nonbonded interactions of the phenyl
group with the heterocyclohexane ring are very similar and the re-
actions must be almost thermoneutral, which is just the case. The
rotation of the phenyl group caused by the geminal methyl group
Synthesis of 1-methyl-1-phenyl-1-silacyclohexane 3 and 3-
methyl-3-phenyl-1-thia-3-silacyclohexane
4
was described
earlier.16,42
and defined as Da
¼
a
(Ph,Me)ꢀ
a(Ph,H) amounts to 2.5, 9.0, 5.0, 1.0,
3.1.1. 1-Phenyl-1-silacyclohexane (1). 1-Phenyl-1-silacyclohexane
(1) was prepared by
modified procedure of West.55
57.0, 38.0ꢁ for the corresponding (Ph,Me) and (Ph,H) substituted
pairs of compounds in reactions 1e6. That means that the deviations
from thermoneutrality (characterizing the changes of the non-
bonded interactions) should increase in the order:
a
A
di-Grignard ethereal solution (350 mL) prepared from 1,5-
dibromopentane (14.81 g, 0.064 mol) and magnesium powder
(4.2 g, 0.175 g-a) was added dropwise to a stirred solution of PhSiCl3
(14.81 g, 0.07 mol) in diethyl ether (70 mL) at room temperature,
stirred at reflux for 6 h, cooled to room temperature, n-hexane
(125 mL) was added, and the mixture was stored for 2 days at
room temperature. The precipitate was separated by decantation
and the solution concentrated in vacuo to a volume of ca. 100 mL.
Addition of n-hexane and evaporation of the solvent was repeated
3 ꢀ Phax < 4 ꢀ Pheq < 3 ꢀ Pheq < 4 ꢀ Phax < 8 ꢀ Phax < 8 ꢀ Pheq
and, indeed, the Da values excellently correlate with the DFT cal-
culated values of
D
Go
for reactions 1e6:
298

B.A. Shainyan, E. Kleinpeter / Tetrahedron 68 (2012) 114e125
123
twice until a solid precipitated almost quantitatively. This solid
was filtered off, the solvent removed under reduced pressure, and
the residue distilled in vacuo to give crude 1-chloro-1-phenyl-1-
silacyclohexane 9 as colorless liquid (4.26 g, 0.02 mol, 29% yield,
bp 99e104 ꢁC/1 mmHg). dH (CDCl3): 1.12 (ddd, 2H, CHA-2/6, J¼15.0,
10.9, 4.8 Hz), 1.23 (m, 2H, CHB-2/6), 1.38 (m, 1H, CHA-4), 1.75 (m,
3H, CHB-4), 1.90 (m, 2H, CH-3/5), 7.45 (m, 3H, Hmþp), 7.66 (dd, 2H, J
7.3, 1.2 Hz). dC NMR (CDCl3): 15.52 (C-2/6), 23.47 (C-3/5), 29.16 (C-
4), 127.97 (Cp), 130.32 (Cm), 133.27 (Co), 134.73 (Ci). dSi (CDCl3):
15.71. This crude product was dissolved in diethyl ether (4 mL) and
added to a stirred suspension of LiAlH4 (0.51 g, 13 mmol) in diethyl
ether (10 mL). The resulting mixture was refluxed for 3 h, cooled to
room temperature, and n-pentane (20 mL) and a saturated aque-
ous NH4Cl solution were added. The organic layer was separated,
the aqueous phase extracted with n-pentane, and the combined
organic extracts dried over anhydrous MgSO4. The solvent was
removed under reduced pressure and the residue distilled in
vacuo to give (1) in 60% yield (2.10 g, 11.9 mmol) as a colorless
liquid, bp 99e103 ꢁC/6 mmHg. dH (CDCl3): 0.90 (ddd, 2H, CHA-2/6,
J¼19.5, 10.1, 4.9 Hz), 1.08 (m, 2H, CHB-2/6), 1.40 (m, 1H, CHA-4), 1.67
(m, 3H, CHB-4, CHA-3/5), 1.91 (m, 2H, CHB-3/5), 4.37 (t, 1H, SieH, J
4.9 Hz), 7.39 (m, 3H, Hmþp), 7.58 (m, 2H, Ho). dC (CDCl3): 10.67 (C-2/
6), 24.89 (C-3/5), 29.86 (C-4), 127.96 (Cm), 129.37 (Cp), 134.49 (Co),
136.09 (Ci). dSi (CDCl3): ꢀ18.37. The 13C NMR data of (9) and (1) are
consistent with the reported data.56
thioacetic acid was removed under reduced pressure and the crude
product was purified by column chromatography (SiO2, hexane/
Et2O, increasing polarity from 50:1 to 1:1) to afford 10 (1.207 g,
4.4 mmol, 63%). dH (400 MHz; CDCl3; Me4Si): 1.14 (2H, m, SiCH2C),
1.74 (2H, m, CCH2C), 2.33 (3H, s, CH3CO), 2.93 (2H, t, 3J¼7.3 Hz,
CCH2S), 3.07 (1H, dd, 2J¼13.8 Hz, 3J¼2.9 Hz, SiCHACl), 3.11 (1H, dd,
SiCHBCl), 4.49 (1H, quint, 3J¼3.1 Hz, HSi), 7.42 (3H, m, Hmþp), 7.58
(2H, d, Ho). dC (100 MHz; CDCl3; Me4Si): 9.90 (SiCH2C), 24.50
(CCH2C), 26.79 (CH2Cl), 30.70 (CH3CO), 32.07 (CCH2S), 128.24 (Cm),
130.34 (Cp), 134.83 (Co), 134.47 (Ci), 195.80 (CO). dSi (100 MHz;
CDCl3; Me4Si): ꢀ12.47. Found:
C 52.90; H 6.36; Si 10.33%.
C12H17SiOSCl requires C, 52.82; H, 6.28; Si, 10.29%.
3.1.4. (Chloromethyl)allylphenylsilane
(12). (Chloromethyl)allyl-
phenylsilane (12) was prepared from phenyl(chloromethyl)chlor-
osilane (3.162 g, 16.5 mmol) in 43% yield (90% purity by 1H NMR
spectroscopy) by a procedure described for (chloromethyl)meth-
ylallylphenylsilane42 and used without further purification. dH
(400 MHz; CDCl3; Me4Si): 2.07 (2H, m, SiCH2C), 3.12 (2H, m,
SiCH2Cl), 4.51 (1H, quint, 3J¼2.9 Hz, HSi), 4.99 (1H, dd, 3J¼10.1 Hz,
C]CH2), 5.05 (1H, dd, 3J¼17.1 Hz, C]CH2), 5.86 (1H, ddt, 3J¼17.0
and 8.1 Hz, CH]C), 7.43 (3H, m, Hmþp), 7.62 (2H, m, Ho). dC
(100 MHz; CDCl3; Me4Si): 17.73 (SiCH2C), 26.37 (SiCH2Cl), 115.51 (]
CH2), 128.28 (Cm), 130.49 (Cp), 132.89 (CH]), 134.50 (Ci), 134.89 (Co).
dSi (100 MHz; CDCl3; Me4Si): ꢀ12.29.
3.1.2. 3-Phenyl-1-thia-3-silacyclohexane (2). To a suspension of
LiAlH4 (0.153 g, 4.0 mmol) in Et2O (3 mL) was added 3-[(chlor-
omethyl)phenylsilyl]propylthioacetate 10 (1.10 g, 4.0 mmol) in
Et2O/n-pentane (12 mL) at 0 ꢁC. The mixture was allowed to warm
to room temperature and stirred for 2 h at this temperature. Then it
was added to a stirred mixture of hydrochloric acid (10%, 10 mL)
and ether (5 mL) at 0 ꢁC, the aqueous layer separated and extracted
with n-pentane. The combined organic phase was dried over
MgSO4, filtered, and concentrated under reduced pressure. Chro-
matography of the residue (0.795 g) (SiO2, n-pentane/Et2O, gradi-
ent) yielded 3-phenyl-1-thia-3-silacyclohexane 2 (0.355 g, 90%
purity by 1H NMR spectroscopy, 41%) and 3-(methylphenylsilyl)
propanethiol 11 (0.5 mmol, 0.090 g, 11%) arising from the simulta-
neous reduction of the thioacetate and CH2Cl groups.
3.2. NMR measurements
1H, 13C, and 29Si NMR spectra were recorded on a Bruker DPX
400 spectrometer at working frequencies 400 (1H), 100 (13C), and
79 (29Si) MHz and the low temperature 13C NMR spectra on
a Bruker AV-600 (at 150.95 MHz). Chemical shifts were de-
termined relative to residual CHCl3 (1H,
d
7.27), internal CDCl3 (13C,
d 53.73) and are given in parts per
d
77.0), and internal CD2Cl2 (13C,
million downfield to TMS (for 1H, 13C). Analysis and assignment of
the 1H NMR data were supported by homonuclear (COSY) and
heteronuclear (HSQC and HMBC) correlation experiments. A sol-
vent mixture of CD2Cl2, CHFCl2, and CHF2Cl in a ratio of 1:1:3 was
used for the low temperature measurements. The probe temper-
ature was calibrated by means of a thermocouple PT 100 inserted
into a dummy tube. The low temperature measurements were
estimated to be accurate to ꢄ2 K. The equilibrium constants (K)
were determined by integration of the separated signals in the
frozen spectra at 103 K, and the free energy differences were cal-
Analytically pure product 2 was obtained by column chroma-
tography (hexane/Et2O, 100/1) as a colorless oil. nmax (liquid film)
3066, 3048, 3007, 2903, 2846, 2125, 1427, 1148, 962, 857, 834, 810,
724, 699 cmꢀ1
; dH (400 MHz; CDCl3; Me4Si): 1.01 (1H, ddt,
2J¼14.8 Hz, 3J¼10.2 and 5.0 Hz, CHA-4), 1.20 (1H, ddd, 2J¼14.8 Hz,
3J¼7.2 and 3.8 Hz, CHB-4), 2.06 (2H, m, CH2-2), 2.13 (m, 1H, CHA-5),
2.31 (1H, m, CHB-5), 2.58 (1H, ddd, 2J¼14.1 Hz, 3J¼9.2 and 2.7 Hz,
CHA-6), 2.62 (1H, ddd, 2J¼14.1 Hz, 3J¼10.1 and 3.9 Hz, CHB-6), 4.55
culated as
D
Go¼ꢀRT ln 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; due to population differences of the con-
formers, the method of Shanan-Atidi and Bar-Eli was employed.57
A complete line shape analysis was not performed because of very
few k/T pairs at low temperatures required for the Eyring corre-
2
3
(1H, tt, JSieH¼197.5 Hz, 3J¼5.2 Hz and JHeSieH ¼1.6 Hz, HSi), 7.42
(3H, m, Hmþp), 7.62 (2H, m, Ho). dC (100 MHz; CDCl3; Me4Si): 10.48
(C-4), 11.18 (C-2), 27.58 (C-5), 32.28 (C-6), 128.01 (Cm), 129.79 (Cp),
134.36 (Co), 134.94 (Ci). dSi (100 MHz; CD2Cl2; Me4Si): ꢀ26.29.
HRMS: Mþ, 194.0598. C10H14SSi requires Mþ, 194.0586.
0
lation. Moreover,
as compared with
D
G
s is strongly preferred as a kinetic parameter
Hs and Ss since the latter values are less
D
D
Compound 11: dH (400 MHz; CDCl3; Me4Si): 0.38 (3H, d,
3J¼3.4 Hz, MeSi), 0.97 (2H, m, SiCH2C), 1.35 (1H, t, 3J¼7.8 Hz, SH),
1.71 (2H, quint, 3J¼7.8 Hz, CCH2C), 2.53 (2H, dd, 3J¼7.4 Hz, CCH2S),
4.39 (1H, q, 3J¼3.2 Hz, HSi), 7.40 (3H, m, Hmþp), 7.56 (2H, d, Ho). dC
(100 MHz; CDCl3; Me4Si): ꢀ5.66 (MeSi), 12.67 (SiCH2C), 29.21
(CCH2C), 32.24 (CCH2S). dSi (100 MHz; CDCl3; Me4Si): ꢀ13.59.
Found: C, 61.24; H 8.05; Si 14.06%. C10H16SiS requires C, 61.16; H,
8.21; Si, 14.30%.
reliable for discussing the kinetics of the studied dynamic
processes.58,59
3.3. Theoretical calculations
All calculations were performed with full optimization of all
variables at the DFT level of theory with the Becke’s three-
parameter hybrid method using the Lee, Yang, and Parr correla-
tional functional and the triple split valence basis set 6-311G(d,p), or
at the MP2 level (MøllerePlesset second order perturbation theory)
with the same basis set as implemented into the Gaussian 03
package.60 Vibrational frequencies were computed on the geometry
optimized structures at the same level of theory at 298.15 K and
3.1.3. 3-[(Chloromethyl)phenylsilyl]propylthioacetate (10). Freshly
distilled thioacetic acid (0.586 g, 7.7 mmol) was added dropwise to
(chloromethyl)allylphenylsilane 12 (1.377 g, 7.0 mmol) and irradi-
ated with a DRT-400 mercury lamp for 3 h at 40 ꢁC. Excess of

124
B.A. Shainyan, E. Kleinpeter / Tetrahedron 68 (2012) 114e125
7. Girichev, G. V.; Giricheva, N. I.; Bodi, A.; Gudnason, P. I.; Jonsdottir, S.; Kvaran,
A.; Arnason, I.; Oberhammer, H. Chem.dEur. J. 2007, 13, 1776e1783.
1 atm of pressure. Unscaled zero point vibrational energies (ZPVE)
were used for the calculation of thermodynamic parameters.
ꢁ
8. Girichev, G. V.; Giricheva, N. I.; Bodi, A.; Gudnason, P. I.; Jonsdottir, S.; Kvaran,
ꢁ
A.; Arnason, I.; Oberhammer, H. Chem.dEur. J. 2009, 15, 8929e8931.
ꢁ
ꢁ
9. WallevikO, S.; Bjornsson, R.; Kvaran, A.; Jonsdottir, S.; Arnason, I.; Belyakov, A. V.;
4. Conclusions
Baskakov, A. A.; Hassler, K.; Oberhammer, H. J. Phys. Chem.
2127e2135.
A 2010, 114,
The conformational equilibria of 1-phenyl-1-silacyclohexane
1, 3-phenyl-1,3-thiasila-cyclohexane 2, 1-methyl-1-phenyl-1-
silacyclohexane 3, and 3-methyl-3-phenyl-1,3-thiasilacyclohexane
4 were studied by low temperature 13C NMR spectroscopy down
to 103 K and theoretical computations. The predominant con-
formers of the Me,PheSi disubstituted compounds 3 and 4 are
those with the equatorial phenyl group, as distinct from 1-methyl-
1-phenylcyclohexane 6. This occurs due to the longer SieC bonds
that allows rotation of the equatorial phenyl group about the SieCi
bond to minimize its repulsive interactions with all SieCHa hy-
drogen atoms, which is impossible in phenylcyclohexane 6 because
of H.o Me repulsive interactions. The equilibrium constants for
compounds 1, 2, 3, and 4 are 3.00e3.35, 19, 1.63e1.78 and 2.12,
respectively. The barriers to direct ring inversion (from the less
populated to the more populated conformer) are 5.2e6.0 and the
reverse barriers 5.4e6.0 kcal molꢀ1. The ring inversion barrier of 1-
phenyl-1-silacyclohexane 1 (5.4e5.7 kcal molꢀ1) is much lower
than that of phenylcyclohexane (8.8 kcal molꢀ1).44
The analysis of the problem of additivity of conformational en-
ergies in the geminally substituted silacyclohexanes versus cyclo-
hexanes has shown that, in general, the conformational effects in
silacyclohexanes are much closer to additivity than in their car-
bocyclic analogs. In both series, maximum deviation from additiv-
ity is observed for the Ph-substituted species, since rotation of the
phenyl group, as an asymmetric rotor, leads to substantial varia-
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Acknowledgements
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The financial support of this work by the Russian Foundation for
Basic Research and Deutsche Forschungsgemeinschaft (Grant
RFBR-DFG No. 11-03-91334) is gratefully acknowledged. We thank
Dr. Matthias Heydenreich and Dipl.-Ing (FA) Angela Krtitschka
(University of Potsdam) for preparing the freon samples, recording
the low temperature spectra and PERCH simulation of the room
temperature 1H NMR spectrum, and Dr. Svetlana Kirpichenko
(Irkutsk Institute of Chemistry) for synthesis of compounds and
helpful discussion.
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Supplementary data
HMQC NMR spectrum of 1, simulated 1H NMR spectra of 2, re-
sults of calculations of compounds 1e8. Supplementary data as-
sociated with this article can be found in online version, at
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