Conformational behavior of 3-borabicyclo[3.3.1]nonanes: 1. Study of molecular dynamics in 3-methoxy-7α-phenyl-1,5-dimethyl-3-borabicyclo[3.3.1]nonane

Article abstract of DOI:10.1016/S0022-328X(99)00462-3

3-Methoxy-7α-phenyl-1,5-dimethyl-3-borabicyclo[3.3.1]nonane 5 in solution at room temperature exists in the double chair conformation, as shown by NMR studies. Increasing the temperature leads to an increase in the population of the chair-boat conformation. At decreased temperature hindered rotation around the B-O bond is observed for 5. Dissolving 5 in deuteropyridine leads to the reversible formation of complex 6, which exists in the chair-boat conformation. The chair-boat conformation is also the most stable one for chelate compound 7 with a tetracoordinated boron atom.

Full text of DOI:10.1016/S0022-328X(99)00462-3

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Journal of Organometallic Chemistry 590 (1999) 227–233
Conformational behavior of 3-borabicyclo[3.3.1]nonanes
1. Study of molecular dynamics in
3-methoxy-7a-phenyl-1,5-dimethyl-3-borabicyclo[3.3.1]nonane
Mikhail E. Gurskii a, Ilya D. Gridnev b, Yuri N. Bubnov a,b,*, Andrew Pelter c,
Paul Rademacher d
a N.D. Zelinsky Institute of Organic Chemistry, Leninsky prospect 47, 117913 Moscow B-334, Russia
b A.N. Nesmeyano6 Institute of Organoelement Compounds of Russian Academy of Sciences, Va6ilo6a Str., 117813 Moscow, Russia
c Uni6ersity of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK
d Institute of Organic Chemistry, Uni6ersity of Essen, D-45117, Essen, Germany
Received 9 November 1998; accepted 20 July 1999
Abstract
3-Methoxy-7a-phenyl-1,5-dimethyl-3-borabicyclo[3.3.1]nonane 5 in solution at room temperature exists in the double chair
conformation, as shown by NMR studies. Increasing the temperature leads to an increase in the population of the chair–boat
conformation. At decreased temperature hindered rotation around the BꢀO bond is observed for 5. Dissolving 5 in deutero-
pyridine leads to the reversible formation of complex 6, which exists in the chair–boat conformation. The chair–boat
conformation is also the most stable one for chelate compound 7 with a tetracoordinated boron atom. © 1999 Elsevier Science
S.A. All rights reserved.
Keywords: 3-Borabicyclo[3.3.1]nonanes; Conformation; Molecular dynamics
1. Introduction
The 3b,7b-substituted compounds occur predomi-
Detailed conformational analyses has been carried
out for a considerable number of bicyclo[3.3.1]nonanes
[1–8]. Three conformations free from bond-angle strain
can be envisaged for bicyclo[3.3.1]nonanes, viz. the
rigid chair–chair (cc), 1, the rigid chair–boats (cb, 2
and bc), and the flexible double-boat (bb), 3 conforma-
tions [1–3]. However, the real molecules have a more
complicated geometry, which is mostly controlled by
strong H-3aꢀH-7a interactions.
nantly in a flattened (cc) conformation. A substituent in
either the 3a or 7a position favors the boat conforma-
tion of the corresponding ring and this results in either
a (bc) or (cb) conformation. Bulky substituents in both
the 3a and 7a positions make a (bb) conformation the
main one while, with smaller substituents, (cb) and (bc)
conformations may also be populated [1–3].
Insertion of a heteroatom into a carbocycle changes
the spatial characteristics for all conformations, and
therefore the conformational behavior of hetero-ana-
logues of bicyclo[3.3.1]nonanes is an interesting prob-
lem [9–12]. In particular, the introduction of a trigonal
boron atom into the 3-position of the bicy-
clo[3.3.1]nonane skeleton substantially alters the non-
bonding interactions in the molecule and therefore the
character of the conformational equilibria should
change compared to the carbon analogues.
Previously we have carried out a conformational
analysis of a series of 3,7a-disubstituted 3-borabicy-
* Corresponding author. Fax: +7-95-135-5085.
E-mail address: dir@ineos.ac.ru (Y.N. Bubnov)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00462-3

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M.E. Gurskii et al. / Journal of Organometallic Chemistry 590 (1999) 227–233
clo[3.3.1]nonanes by NMR spectroscopy [13a] and
photoelectron spectroscopy [13b,c]. It was found that
a flattened (cc) conformation is surprisingly stable for
these compounds even in those cases in which the
7a-substituent is a phenyl group. This phenomenon
deserves further attention because even in a cyclohex-
ane ring the phenyl group has been found to have a
strong preference for the equatorial orientation [14]
(the conformational energy of a phenyl group in
phenylcyclohexane was estimated to be from 11.29
[14a] to 12.29 [14b] kJ mol−1). In this paper we
present a detailed study of the conformational behav-
2.1. Analysis of the coupling constants
1
The coupling network in the H-NMR spectrum of
5 (Fig. 1) was established by 1Hꢀ1H COSY experi-
ments. Data on the chemical shifts and coupling con-
stants of 5 in four different solvents are collected in
Table 1.
H-7 has two vicinal couplings, viz. one with H-6a
(6.0–6.3 Hz) and another with H-6b, which is 2.8–3.3
Hz. These values are characteristic for ea and ee cou-
plings respectively [16]. They clearly indicate an equa-
torial orientation of H-7 and therefore the phenyl
group is axial in a cyclohexane ring in a chair confor-
mation.
The long-range W-coupling constants between H-6a
and one of the H-9 protons, between another H-9 and
one of the H-4 protons, and between H-6b and an-
other H-4 proton allowed us to assign unambiguously
the signals H-9syn, H-9anti and H-4a, H-4b because a
W-configuration required for the existence of the long-
range coupling constants can be achieved in each case
in one way only. Furthermore, the analysis of the
relative values of W-couplings 4J(H-4a, H-6b) (2.8
Hz), 4J(H-6a, H-9syn) (1.3 Hz), and 4J(H-4b, H-
9anti ) (0.6 Hz) indicates a chair conformation for the
boron ring, because for a boat conformation the
largest of the three values would be expected for the
latter coupling. Thus, the analysis of the coupling con-
stants in the 1H-NMR spectrum of 5 allows us to
conclude that in neutral solvents at room temperature
5, exists in a (cc) conformation.
ior of
a new representative of the 3-borabicy-
clo[3.3.1]nonane series, viz. 3-methoxy-7a-phenyl-1,5-
dimethyl-3-borabicyclo[3.3.1]nonane 5.
2. Results and discussion
Compound 5 was prepared by allylboron-alkyne
condensation [15] of phenylethyne and tris(methal-
lyl)borane followed by methanolysis and catalytic hy-
drogenation of the bicyclic compound 4 thus obtained
(Scheme 1).
2.2. Analysis of the NOESY spectrum
Scheme 1. Reagents and conditions: (i) 130–140°C; (ii) MeOH; (iii)
H2, Pd/SrCO3.
Independent confirmation of the predominance of a
(cc) conformation for 5 at room temperature was ob-
tained from the 2D NOESY experiment (see Table 2).
Most informative are the NOEs between H-2b and
H-9syn, H-4a and H-6a, and H-9anti and H-6b (see
Fig. 2).
2.3. Study of the molecular dynamics in 5 at 6arious
temperatures
It was found that NMR spectra of 5 are tempera-
ture dependent. Fig. 3 shows a notable change in the
appearance of the signal of H-6a when the tempera-
ture was raised from 293 to 343K. The chemical shift
of the signal changes from 2.27 to 2.12 ppm and the
3
value of the coupling J(H-6a, H-7) increases from 3.3
to 6.0 Hz. Such a change in the coupling constant
indicates the increase at high temperatures of the pop-
ulation of a conformation in which H-7 is axial, i.e. in
the boat conformation of the carbocyclic ring of 5
Fig. 1. 1H-NMR spectrum (400 MHz, CDCl3, 298 K) of compound
5. At the top: the proton signal H-9syn and H-9anti (a) signal of
(H-6a,H-8a) decoupled, (b) signal of (H-2a, H-4a) decoupled.

M.E. Gurskii et al. / Journal of Organometallic Chemistry 590 (1999) 227–233
229
Table 1
Chemical shifts (ppm) and coupling constants (Hz) in the 1H-NMR spectra of compounds 5 and 7 a
Proton
CDCl3
C6D6
LT mixture b
C5D5N
7 in CDCl3
lH-4a
1.20
0.56
2.47
1.88
3.38
1.57
1.42
1.33
3.35
7.70
7.56
7.41
16.9
14.3
12.7
2.8
1.22
0.57
2.27
1.80
3.18
1.54
1.28
1.30
3.34
7.49
7.39
7.28
16.8
13.8
12.7
3.2
0.91
0.28
2.13
1.60
3.10
1.29
1.14
1.04
3.10
7.41
7.27
7.13
17.4
13.7
12.8
3.3
1.08
0.35
2.26
1.62
3.12
1.38
1.13
1.16
3.20
7.55
7.42
7.28
16.1
14.3
12.5
5.5
1.22
0.44
2.00
1.74
3.00
1.77
1.11
1.21
-
7.41
7.56
7.73
13.7
13.4
12.7
13.6
4.8
lH-4b
lH-6a
lH-6b
lH-7
lH-9anti
lH-9syn
lMe
l(OMe)
l(o-Ph)
l(m-Ph)
l(p-Ph)
2J(H-4a, H-4b)
2J(H-6a, H-6b)
2J(H-9syn, H-9anti )
3J(H-6a, H-7)
3J(H-6b, H-7)
4J-6a, H-9syn)
4J(H-6b, H-4b)
4J(H-4a, H-9anti )
6.3
1.8
0.7
2.7
6.0
1.5
1.2
2.8
6.2
1.8
0.8
2.9
6.3
1.2
0.7
2.6
B0.2
B0.2
2.4
a All spectra run at 303 K.
b CD2Cl2ꢀCDCl3ꢀCCl4 in the ratio 60:27:13. LT=low temperature.
Table 2
NOEs observed in 2D NOESY experiments on compound 5 a
H-4a
H-4b
H-6a
H-6b
H-7
H-9syn
H-9anti
Me
OMe
o-Ph
m-Ph
p-Ph
H-4a
H-4a
H-6a
H-6b
H-7
x
NOE
x
noe
noe
noe
NOE
noe
noe
noe
noe
noe
x
NOE
x
NOE
NOE
NOE
x
noe
NOE
NOE
noe
H-9syn
H-9anti
Me
OMe
o-Ph
m-Ph
p-Ph
noe
x
NOE
x
noe
noe
noe
x
noe
noe
NOE
noe
noe
noe
noe
noe
noe
x
x
x
x
a NOEs written in capitals are obvious cross-peaks, i.e. between the signals which have cross-peaks in the COSY spectrum.
(see Scheme 2). This effect is probably caused by the
increased steric requirements of the phenyl group at
higher temperatures.
Fig. 4 demonstrates spectral dynamics in the 13C-
NMR spectra of 5. One can see that four signals, viz. of
C-2, C-6, C-1, and CH3, observed at room temperature
as single peaks, are split at low temperatures into two
signals with equal intensity.
This effect is induced by hindered rotation about the
Fig. 2. NOEs indicative of the (cc) conformation of 5 in CDCl3 at
298 K.
BꢀO bond due to the overlap of the sp3-hybridized lone

230
M.E. Gurskii et al. / Journal of Organometallic Chemistry 590 (1999) 227–233
complex 6. The most dramatic change is observed for
3
the coupling J(H-6a, H-7), from approximately 12 Hz
at 246 K (Fig. 4(a)) to 5.5 Hz at room temperature
(Fig. 4(d)). The former value is characteristic for aa
couplings in cyclohexane rings [17], which means that
at 246 K the boat conformation of the carbocyclic
ring of 6 is the most stable one.
In the low-temperature 13C-NMR spectrum of 5 in
deuteropyridine, no splittings of signals similar to
those observed for solution of 5 in a neutral solvent
were observed. This is not surprising because in 6 the
p-orbital of boron is already occupied by the lone pair
of pyridine, and overlap with the lone pair on oxygen
does not occur.
Thus, it can be concluded that it is the formation of
complex 6 that leads to a change in the conforma-
tional preferences of compound 5, making the boat
conformation of the carbocyclic ring the most stable
one. This is caused by the transformation of the trigo-
nal sp2-hybridized boron atom in 5 to the tetragonal
sp3-hybridized boron atom in 6. The spatial character-
istics of an sp3-hybridized boron atom should be sub-
stantially the same as those for a carbon atom, so the
stability of the (cb) conformation found for 6 is in a
good accord with the conformational properties of
3a,7a-disubstituted bicyclo[3.3.1]nonanes.
Fig. 3. Signal of the proton H-6a, H-8a in the 1H-NMR spectrum
(C6D6, 400 MHz). (a) Of 5 at 293°K; (b) at 343°K.
Scheme 2.
The latter statement is further supported by the
conformational analysis of a stable compound with
a
tetracoordinated boron atom, viz. dibenzoyl-
pair on oxygen with the vacant p-orbital of boron.
Similar effects were earlier observed for MeOBMe2
and (Me2B)2O [16]. An account of a quantitative study
of the activation barriers for hindered rotation around
the BꢀO bond in different 3-borabicylo[3.3.1]nonanes
will be presented in a separate paper.
methanochelate 7, which we prepared from 5 (Scheme
4).
The chemical shifts and coupling constants observed
1
in the H-NMR spectrum of 7 are given in Table 1.
The coupling 3J(H-6a, H-7) is 13.4 Hz, which indi-
cates the axial location of H-7, and therefore the equa-
torial orientation of the phenyl group. Thus the
carbocyclic ring of 7 is in a boat conformation. This
conclusion is strongly supported by the results of 2D
NOESY exper-iments (see Table 4, Fig. 6), in which a
long-range (five bonds) NOE between H-7 and H-9anti
was observed. The chair conformation of the bora-
cyclic ring follows, from arguments similar to those in
2.4. The influence of complexation on the
conformational equilibrium
In Table 3 the temperature dependence of the 11B
chemical shift of a solution of 5 in deuteropyridine is
shown. The chemical shift of boron changes from 19.4
ppm at 246 K to 53.1 ppm at 348 K. The latter value
is similar to that for parent compound 5, the chemical
shift of which in CDCl3 is 53.4 ppm and is almost
independent of the temperature. The change in 11B
chemical shift with decreasing temperature reflects the
shift of the equilibrium between 5 and 6 in favor of
the latter.
4
the previous discussion, from the W-coupling J(H-4a,
H-9anti )=2.4 Hz and the NOE between H-4b and
H-9syn (see Fig. 6).
3. Conclusions
Fig. 5 displays the appearance of the signal of H-6a
The conformational analysis of 3-methoxy-7-phenyl-
1,5-dimethyl-3-borabicyclo[3.3.1]nonane 5 shows that
at room temperature the (cc) conformation is the most
stable one for this compound. An increase in tempera-
ture leads to a shift of the conformational equilibrium,
which increases the population of the (cb) conforma-
1
in the H-NMR spectrum of 5 in deuteropyridine at
different temperatures. According to the previous dis-
cussion, the changes in the spectrum taking place
when the temperature is decreased reflect the shift of
the equilibrium (Scheme 3) in favor of the pyridine

M.E. Gurskii et al. / Journal of Organometallic Chemistry 590 (1999) 227–233
231
Fig. 4. Section plots of 13C-NMR spectra (low-temperature mixture of solvents, 100 MHz) of 5 at different temperatures. (a) T=193°K; (b)
T=218°K; (c) T=230°K; (d) T=239°K; (e) T=261°K; (f) T=273°K; (g) T=298°K.
tion. The transformation of the boron atom into
a tetracoordinated state, either by complexation with
pyridine or by formation of a chelate compound
7, leads to a change in conformational preferen-
ces, making the (cb) conformation the most stable
one.
residue was distilled to give 4 (9.9 g, 73%), b.p. 122–
123°C (1 torr). Found, C, 80.67; H, 9.42; B, 4.28%.
Calculated, for C17H23BO is, C, 80.33; H, 9.12; B,
4.25%.
4. Experimental
Table 3
4.1. 3-Methoxy-7h-phenyl-1,5-dimethyl-3-borabicy-
clo[3.3.1]non-6-ene 4
Chemical shifts 11B (l, ppm) of 5 in deuteropyridine
T (K)
l
11B
Phenylethyne (5.2 g, 0.051 mol) was added dropwise
to tris(methallyl)borane (9.05 g, 0.05l mol) at 145–
155°C. The mixture was heated for 15 min, then cooled
to ambient temperature and methanol (5 ml) was
added. The mixture was heated under reflux for 20 min,
the excess of methanol was removed in vacuo and the
246
255
270
303
338
348
19.4
27.6
37.4
49.0
52.9
53.1

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M.E. Gurskii et al. / Journal of Organometallic Chemistry 590 (1999) 227–233
Fig. 5. Temperature dependence of the signal of H-6a, H-8a proton in the 1H-NMR spectrum of 5 (C5D5N, 400 MHz): (a) T=246 K; (b) T=255
K; (c) T=270 K; (d) T=303 K; (e) T=338 K; (f) T=348 K.
g). A total of 900 ml of hydrogen were absorbed. After
filtration, the solvent was removed and the residue was
distilled in vacuo to give 5, (8.44 g, 93%) b.p. 121–
122°C (1 torr), n2D0 1.5322. Found, C, 79.54; H, 9.89; B,
4.34%. Calculated for C17H25BO is C, 79.70; H, 9.84; B,
4.22%.
4.3. 7-Phenyl-1,5-dimethyl-3-borabicyclo[3.3.1]nonyl
benzoylacetophenoate 7
Scheme 3.
To a solution of dibenzoylmethane (0.95 g, 0.0047
mol) in ether (10 ml) was added 5 (1.21 g, 0.0047 mol).
After 1 h orange crystals precipitated. Ether was dis-
tilled off and the precipitate was washed with hexane to
give 7, (1.9 g, 100%) m.p. 198–200°C. Found, C, 83.05;
H, 7.40; B, 2.65%. Calculated for C31H33BO3, is C,
83.04; H, 7.42; B, 2.41%.
Scheme 4.
Acknowledgements
4.2. 3-Methoxy-7-phenyl-1,5-dimethyl-3-bora-
bicyclo[3.3.1]nonane 5
This work was carried out with the financial support
of the Russian Foundation for Basic Research (Project
nos. 98-03-32993a and 96-15-97289) and ‘The Integra-
tion’ program (grant no. 234).
A solution of 1 (9 g, 0.035 mol) in absolute methanol
(10 ml) was hydrogenated over 5% Pd on SrCO3 (0.50

M.E. Gurskii et al. / Journal of Organometallic Chemistry 590 (1999) 227–233
233
Table 4
NOEs observed in 2D NOESY experiments for compound 7 a
H-4a
H-4b
H-6a
H-6b
H-7
H-9syn
H-9anti
Me
o-Ph
m-Ph
p-Ph
H-4a
H-4b
H-6a
H-6b
H-7
H-9syn
H-9anti
Me
x
NOE
x
noe
NOE
noe
x
NOE
x
NOE
noe
noe
NOE
NOE
x
noe
x
noe
NOE
x
noe
noe
x
noe
noe
NOE
noe
o-Ph
m-Ph
p-Ph
noe
x
x
x
a NOEs written in capitals are obvious cross-peaks, i.e. between the signals which have cross-peaks in the COSY spectrum.
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