Very low-temperature dynamic 29Si NMR study of the conformational equilibrium of (1,1′-phenyl-1,1′-silacyclohex-1-yl)disiloxane

Full text of DOI:10.1002/mrc.4870

Received: 14 February 2019
DOI: 10.1002/mrc.4870
Revised: 12 March 2019
Accepted: 14 March 2019
L E T T E R ‐ C A S E R E P O R T S
29
Very low‐temperature dynamic Si NMR study of the
conformational equilibrium of (1,1′‐phenyl‐1,1′‐
silacyclohex‐1‐yl)disiloxane
1 | INTRODUCTION
311++G(d,p)//B3LYP/6‐311++G(d,p) level of theory. As
was stated in the literature, GIAO/B3LYP/6‐311++G(d,
p) calculations represent the best compromise between
accuracy and expense.[8,9]
Low‐temperature solution NMR spectroscopy is the most
enlightening method for the conformational analysis of
silacyclohexanes. In view of much higher flexibility of
silacyclohexanes (due to elongated C─Si bonds and
expanded C─Si─C angles) as compared with cyclohex-
anes, the temperatures of decoalescence of appropriate
NMR signals in the ring interconversion process are
much lower and fall in the range from 140 to 90 K.
Most commonly used nuclei for conformational analysis
by NMR spectroscopy are 1H, 13C, and often 19F (in
organofluorine compounds) nuclei.[1,2] To the best of
our knowledge, there are no examples of the use of 29Si
NMR spectroscopy at low temperature for the conforma-
tional analysis of silacyclohexanes and, more generally,
of any conformationally flexible organosilicon molecules.
In continuation of our ongoing research on the confor-
mational analysis of silacyclohexanes and sila(hetero)
cyclohexanes,[3–6] we synthesized (1,1′‐phenyl‐1,1′‐
silacyclohex‐1‐yl)disiloxane 1[7] and examined its low‐
temperature 1H and 13C NMR spectra; unfortunately,
neither were informative as far as the conformational
composition of compound 1 is concerned. Therefore, we
estimated the relative energies of the three possible
conformers (Figure 1) theoretically at the M062X/6‐
311G(d,p) and MP2/6‐311G(d,p) levels of theory and
showed the ax–ax conformer to be the most stable and
the eq–eq conformer the least stable one (cf. Figure 1 and
Table 1).
The calculated 29Si chemical shifts can be considered
as reliable for the following reasons: (a) the calculated
chemical shifts fall in the range of 2.3 ppm, which nicely
coincide with the experimental range of 2.6 ppm; (b) the
most intense signal in the spectrum (cf. Figure 2) appears
at highest field; the most stable and, hence, the most
abundant ax–ax conformer also gives the most upfield
signal (cf. Table 1); (c) the most downfield signal in the
29Si NMR spectrum in Figure 2 is the least intense one
and belongs to the least stable eq–eq conformer (cf.
Table 1); and (d) two signals of the same intensity lie in
between these two signals and according to the calculated
chemical shifts and relative stability (cf. Table 1) belong
to the ax–eq conformer.
The barrier to the six‐membered ring interconversion
could be estimated as well. In Figure 2, it can be readily
seen that at 113 K, there is still fast exchange of the
participating conformers on the NMR timescale, whereas
at 103 K, there is complete decoalescence of signals
of the participating conformer and the easy opportunity
to determine the ratio of the three conformers in the
present conformational equilibrium (vide supra). When
employing T = 110 K to be about the coalescence temper-
ature Tc and the chemical shift difference between the
signals of the ax–ax and eq–eq conformers Δν = 310 Hz
(at 119.2 MHz), the free energy of activation of the six‐
membered ring interconversion process ΔG# proves to
be ~4.8 kcal/mol (via ΔG# = 19.14 Tc [10.32 + log Tc/kc]
together with kc = π Δν/√2).[10] This is one of the lowest
barriers to six‐membered ring interconversion ever
measured.
In this paper, we present the results of the 29Si low‐
temperature NMR study of the siloxane 1, which allowed
to freeze the conformational equilibrium, to determine
the ratio of the three conformers and to assign them in
accordance with the relative stability by applying the the-
oretically calculated 29Si chemical shifts.
Siloxane 1 was prepared by alkaline hydrolysis of the
solution of 1‐phenyl‐1‐chloro‐1‐silacyclohexane in CH2Cl2
with equimolar amount of aqueous NaOH (the two‐phase
system) in the presence of equimolar amount of Et3N.
After staying overnight, the organic layer was separated,
29Si NMR chemical shifts of the conformers of
compound
1
and of the reference compound
hexamethyldisiloxane, which is most structurally close
to compound 1, were calculated at the GIAO/B3LYP/6‐
Magn Reson Chem. 2019;1–3.
wileyonlinelibrary.com/journal/mrc
© 2019 John Wiley & Sons, Ltd.
1

2
LETTER ‐ CASE REPORTS
FIGURE 1 M062X/6‐311G**‐optimized geometry of the conformers of siloxane 1 (“ax” and “eq” denote the location of the phenyl group)
TABLE 1 Relative energies ΔE (kcal/mol), free energies ΔG° (kcal/mol), calculated and experimental ratios, and calculated and experi-
mental 29Si chemical shifts of the conformers of 1 at 103 K
QC method
Conformer
ΔE
ΔG° (at 103 K)
Conformers
δ (29Si)
M062X/6‐311G(d,p)
ax–ax
ax–eq
eq–eq
0.00
0.10
0.16
0.00
0.07
0.10
46.6%
33.1%
20.3%
—
—
—
MP2/6‐311G(d,p)
ax–ax
ax–eq
eq–eq
0.00
0.59
1.10
—
—
—
—
—
—
—
—
—
B3LYP/6–311++G(d,p)
Experimental result
ax–ax
ax–eq
eq–eq
—
—
—
—
—
—
—
—
—
−7.7 ppma
−6.3 and −6.1 ppma
−5.4 ppma
ax–ax
ax–eq
eq–eq
—
—
—
—
—
—
61%
32%
7%
−5.8 ppma
−5.3 and −4.6 ppma
−3.3 ppma
aReference tetramethylsilane (external in capillary).
FIGURE 2 29Si NMR spectra of (1,1′‐phenyl‐1,1′‐silacyclohex‐1‐yl)disiloxane 1 at various temperatures (above 113 K and below 103 K)
washed with water, and dried over CaCl2. The solvent was
removed, and the crude product (44%, dense colorless oil)
was purified by column chromatography on silica by suc-
cessive elution with hexane and hexane–ether, 9:1.
Room temperature 1H, 13C, and 29Si NMR spectra
coincided with those reported in Shainyan et al.[7] Low‐
1
temperature H and 13C NMR spectra are reported in
Shainyan et al.[7] The low‐temperature 29Si NMR spectra

LETTER ‐ CASE REPORTS
3
[2] E. Kleinpeter, Adv. Heterocycl. Chem. 2004, 86, 41. Conforma-
tional Analysis of Saturated Heterocyclic Six‐Membered Rings
were recorded on a Bruker AVANCE 600 spectrometer at
119.2 MHz in a Freon mixture of CD2Cl2/CHFCl2/
CHF2Cl in a ratio of 1:1:3 (15‐mg siloxane in about 0.5
ml Freon mixture). Acquisition parameters are the
following: 1,024 scans, SW = 11,900 Hz, AQ = 2.75 s,
pulse duration: 21 μs, representative time: 5 s, inverse
gated, and direct observe probe. The probe temperature
was calibrated by means of a thermocouple PT 100
inserted into a dummy tube. The low‐temperature mea-
surements were estimated to be accurate to 1°.
[3] B. A. Shainyan, E. Kleinpeter, Tetrahedron 2012, 68, 114.
[4] B. A. Shainyan, E. Kleinpeter, Tetrahedron 2013, 69, 5927.
[5] B. A. Shainyan, S. V. Kirpichenko, N. N. Chipanina, L. P.
Oznobikhina, E. Kleinpeter, S. A. Shlykov, D. Y. Osadchiy,
J. Org. Chem. 2015, 80, 12492.
[6] B. A. Shainyan, S. V. Kirpichenko, E. Kleinpeter, J. Org. Chem.
2017, 82, 13414.
[7] B. A. Shainyan, E. Kleinpeter, E. N. Suslova, Russ. J. Gen.
Chem. 2019, 89, 580.
All geometric and vibrational calculations were per-
formed with no restrictions on the geometry by using
M062X correlation functional and at the MP2 level (with-
out frequency calculations) with 6‐311G(d,p) basis set
using Gaussian 09 program suite.[11] Belonging of station-
ary points to minima on the PES was confirmed by the
absence of imaginary frequencies. Theoretical chemical
shifts were calculated by gauge‐including atomic orbital
method[12,13] at B3LYP/6‐311++G(d,p) level using
Gaussian 09 program,[11] on the geometry optimized at
the same level of theory, with respect to.
[8] P. R. Rablen, S. A. Pearlman, J. Finkbiner, J. Phys. Chem. A
1999, 103, 7357.
[9] R. Jain, T. Bally, P. R. Rablen, J. Org. Chem. 2009, 74, 4017.
[10] K. Pihlaja, E. Kleinpeter, Carbon‐13 NMR Chemical Shifts
in Structural and Stereochemical Analysis, Wiley‐VCH, NY
1994 295.
[11] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci,
G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P.
Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F.
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N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
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O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski,
R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P.
Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O.
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ORCID
Erich Kleinpeter https://orcid.org/0000-0001-8993-0033
Erich Kleinpeter1
Bagrat A. Shainyan2
1Chemisches Institut, Universität Potsdam, Potsdam
(Golm), Germany
2A. E. Favorsky Irkutsk Institute of Chemistry, Siberian
Branch of the Russian Academy of Sciences, Irkutsk, Rus-
sian Federation
[12] R. Ditchfield, Mol. Phys. 1974, 27, 789.
[13] K. Wolinski, J. F. Himton, P. Pulay, J. Am. Chem. Soc. 1990,
112, 8251.
Correspondence
Erich Kleinpeter, Chemisches Institut, Universität Potsdam,
Karl‐Liebknecht‐Str. 24‐25, D‐14476 Potsdam (Golm),
Germany.
How to cite this article: Kleinpeter E, Shainyan
BA. Very low‐temperature dynamic 29Si NMR study
of the conformational equilibrium of (1,1′‐phenyl‐
1,1′‐silacyclohex‐1‐yl)disiloxane. Magn Reson
Email: ekleinp@uni‐potsdam.de
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Chem. 2019;1–3. https://doi.org/10.1002/mrc.4870