Crystal structure of and conformational equilibria in 1,1′-dihydroxybis(4H-cyclopenta[def]phenanthrene) and related derivatives

Article abstract of DOI:10.1002/mrc.839

The aromatic protons and carbons in the NMR spectra of certain clamped 1,1,2,2-tetraarylethane-1,2-diols undergo intermediate exchange at ambient temperature as the result of conformational equilibrium between two enantiomeric gauche forms. The crystal structures for two of these derivatives show the conformational preference for this geometry in the solid state in contrast to other reported undamped 1,1,2,2-tetraarylethane-1,2-diols. The conformational preference of these vicinal diols parallels that for the 1,1,2,2-tetraarylethanes. However, in the asymmetrically substituted diphenylmethylfluorene series, the vicinal diol prefers to adopt a gauche conformation in contrast to the hydrocarbon which prefers the anti geometry attributed to hydrogen bonding effects in the former. The results of AM1 calculations for the conformational preferences of this series of diols and ethanes are included and agree with experimental observations. Copyright

Full text of DOI:10.1002/mrc.839

MAGNETIC RESONANCE IN CHEMISTRY
Magn. Reson. Chem. 2001; 39: 294–298
Note
Crystal structure of and conformational equilibria in
1, 1ꢀ-dihydroxybis(4H-cyclopenta[def]phenanthrene)
and related derivatives
Mary Hoang, Gabriela Mladenova, Alan C. Hopkinson, Jailal Ramnauth and
Edward Lee-Ruff∗
Department of Chemistry, York University, Toronto, Ontario M3J 1P3, Canada
Received 27 November 2000; Revised 30 January 2001; Accepted 30 January 2001
The aromatic protons and carbons in the NMR spectra of certain clamped 1,1,2,2-tetraarylethane-1,2-diols
undergo intermediate exchange at ambient temperature as the result of conformational equilibrium
between two enantiomeric gauche forms. The crystal structures for two of these derivatives show the
conformational preference for this geometry in the solid state in contrast to other reported unclamped
1,1,2,2-tetraarylethane-1,2-diols. The conformational preference of these vicinal diols parallels that for the
1,1,2,2-tetraarylethanes. However, in the asymmetrically substituted diphenylmethylfluorene series, the
vicinal diol prefers to adopt a gauche conformation in contrast to the hydrocarbon which prefers the anti
geometry attributed to hydrogen bonding effects in the former. The results of AM1 calculations for the
conformational preferences of this series of diols and ethanes are included and agree with experimental
observations. Copyright  2001 John Wiley & Sons, Ltd.
KEYWORDS: NMR; ethane-1,2-diols; conformation; hydrogen bonding
1, 10-dihydroxybis(9H-fluorene) (2). We also report on the
INTRODUCTION
crystal structure of 1, which concurs with the solution data
Steric crowding in ethanes can result in an increase in the
for its conformation.
rotational barrier and the exhibition of stable diastereomeric
and enantiomeric rotamers. Examples of this class of
compounds include the triptycenes which are substituted
with a bulky tertiary or secondary carbon at the bridgehead
RESULTS AND DISCUSSION
Diols 1 and 29 were prepared by reductive coupling, in the
presence of zinc, of 4H-cyclopenta[def]phenanthrene-4-one
and fluorenone, respectively, according to the procedures
described by Tanaka et al.9 The room temperature 1H NMR
spectrum of 1 showed a signal at υ 7.30 ppm and a broad
peak at 7.62 ppm for all 16 aromatic protons [Fig. 1(a)]. Upon
position,1,2 and other tetrasubstituted ethanes carrying bulky
substituents.3,4 Electronic effects such as intramolecular
hydrogen bonding in vicinal diols can further restrict rotation
about carbon–carbon ꢀ-bonds, although such interactions
are not as energetically strong as hydrogen bonding in 1,3-
diols where the more ideal six-centered arrangement can
be achieved.5 Nevertheless vicinal diols contain the highest
proportion of intramolecular H-bonded conformers.6 This
has been explained in terms of the magnitude of the entropy
factor.7 According to Buckley and Giguere,8 ethanediol is
present in the gauche form only in the gas phase as well as in
dilute solution. In our investigations of the photochemistry
of some 1,1,2,2-tetraarylethane-1,2-diols, we observed some
unusual exchange phenomena in the NMR spectra of
1, 10-dihydroxybis(4H-cyclopenta[def]phenanthrene) (1) and
°
cooling to ꢀ40 C, eight distinct aromatic signals could be
observed with the expected multiplicity associated with a
C2 symmetry [Fig. 1(b)]. The 13C NMR spectrum of diol
1 showed a similar temperature dependence where seven
aromatic carbon signals (the aliphatic carbon signal appears
at υ 89.7 ppm) are observed under ambient conditions but on
°
cooling to ꢀ40 C these were resolved into 14 signals.
What is interesting is the large changes in chemical
shifts of the aromatic protons and carbon signals at the
extreme temperatures. The aromatic proton signals range
from υ 5.9 to 8.3 ppm in the low-temperature spectrum,
suggesting a conformation involving very deshielded and
shielded hydrogen nuclei consistent with a gauche geometry
expected from intramolecular hydrogen bonding of a vicinal
ŁCorrespondence to: Lee-Ruff, Department of Chemistry, York
University, Toronto, Ontario M3J 1P3, Canada.
Contract grant sponsor: Natural Science and Engineering Research
Council of Canada (NSERC).
DOI: 10.1002/mrc.839
Copyright  2001 John Wiley & Sons, Ltd.

Conformation of substituted ethane-1,2-diols
295
1
°
Figure 1. H NMR spectra (600 MHz) of diol 1 at (a) 25 and (b) ꢀ40 C.
diol. Such a geometry would place one of the peri-hydrogens
directly in the shielding cone of the other aromatic moiety.
The x-ray crystal structure of diol 1 was determined
and shows very clearly the C2 gauche conformation and the
proximity of one of the peri-hydrogens to the other aromatic
ring (Fig. 2). The low-temperature 1H NMR spectrum of 1 is
remarkably similar to that of a Diels–Alder adduct 810 (Fig. 3)
in which bond rotation about the C—C bond between the
two aromatic moieties is completely restricted. Since such
intramolecular hydrogen bonding forces are relatively weak
for 1,2-diols, increasing the temperature should overcome
the rotational barrier and give a 1H NMR spectrum for 1
averaging C2v geometry. A higher boiling solvent (DMSO)
was used for the high-temperature NMR spectra. The room
temperature 1H NMR spectrum of 1 in this solvent is similar
°
to that in CDCl3. The high-temperature spectrum of 1 at 82 C
gave three signals in the aromatic region (υ 7.0–7.8 ppm)
with relative intensities of 4 : 2 : 2. In addition, the OH signal
Copyright  2001 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2001; 39: 294–298

296
M. Hoang et al.
phenyl substituents in 6 to adopt a twisted geometry enabling
these to stack or nest by ꢃ-interaction which would diminish
geminal repulsion and result in a decrease in the valence
angle. Such a decrease in the valence angle would minimize
vicinal interactions, which leads to an anti preference
for conventional reasons. By contrast, the planar fluorene
substituents in 5 would favor the gauche conformer because
of the proximity of the peri-hydrogens (H-1 and H-8) between
the two fluorene groups in the anti conformer leading to a
destabilization of 6.6 kcal molꢀ1 (1 kcal D 4.184 kJ) relative
to the gauche form.
We performed semi-empirical (AM1) calculations for
diols 1–4 in order to establish the importance of intramolec-
ular hydrogen bonding in determining conformational pref-
erence in these systems. The energy differences between
the gauche and anti conformers were determined and com-
pared with the values for the corresponding tetrasubstituted
ethanes. The results indicate that the conformational pref-
erence for both the diols and the hydrocarbons correlate
(see Table 1). For the bisfluorene series, the gauche form
is preferred by 4.4 and 5.2 kcal molꢀ1 for the diol 2 and
hydrocarbon 5, respectively. On the other hand, the anti con-
formation is preferred in the tetraphenyl-substituted series
3 and 6. For the asymmetric series 4 and 7, the gauche
form is slightly preferred over the anti form in the diol but
the reverse preference is observed for the hydrocarbon 7.
Figure 2. X-ray crystal structure of diol 1. Element key:
carbon, partially hatched circles; oxygen, dotted circles;
hydrogen, small open circles.
1
The room temperature H NMR spectrum of diol 4 shows
appearing at υ 6.3 ppm at room temperature broadens and
°
a complex pattern of signals associated with the fluorene
ring intermediate between a C2v and C2 structure. Such a
spectrum may be rationalized in terms of an intermediate
exchange between the two enantiomeric gauche conform-
shifts to higher field (υ 5.5 ppm) at 82 C. The C2v averaged
structure for 1 would exhibit four distinct aromatic signals.
It is possible that two of the signals overlap under these
conditions, as is evident from the relative intensities of the
three observed signals.
1
°
ers. The low-temperature H NMR spectrum ꢁꢀ60 Cꢂ of
Similar behavior was seen in the 1H NMR spectrum
4 shows a more complicated spectrum which is probably
associated with the freezing out of and resultant asymme-
try of the twisted phenyl groups. The crystal structure of
4 clearly shows the gauche conformation (Fig. 5). It must be
noted, however, that crystal structures do not necessarily
correlate with those in solution. The contrasting behavior
between 4 and 7 as determined by semi-empirical calcula-
tions may be ascribed to intramolecular hydrogen bonding
effects in the case of 4, although the same effects should
increase the conformational preference in the case of diols 2
and 3 relative to the corresponding hydrocarbons 5 and 6,
respectively. The calculated conformational energies for this
series point to a greater preference towards the anti form in
the diols, indicating that intramolecular hydrogen bonding
may not be a significant factor in determining conformations
in these derivatives. Electronic and steric effects associated
with the aryl substituents may be the predominant factors in
establishing their geometry.
°
for diol 2. The low-temperature ꢁꢀ63 Cꢂ spectrum of 2 in
CD2Cl2 showed eight signals in the aromatic region with
partial overlap between two of these. A very shielded signal
was observed at υ 5.80 ppm which we attribute to the pair of
peri aromatic hydrogens in proximity to the shielding cone
of the other fluorene moiety, in a similar fashion to diol 1.
Pascal et al.11 reported broad signals for 2 at 20 C which
become resolved into four signals at 60 C. They interpreted
°
°
this behavior to conformational equilibria between the anti
and gauche forms of this diol (Fig. 4). Such an equilibrium
would not give the low-temperature spectrum which we
observe for diol 2. The anti conformation of this diol would
have C2v symmetry and show four aromatic proton signals.
With the expected eight aromatic proton signals of the gauche
conformer, the low-temperature H NMR spectrum for the
proposed anti–gauche equilibrium for 2 would be expected
to be much more complex.
1
Unlike diols 1 and 2, the crystal structure of 1,1,2,2-
tetraphenylethane-1,2-diol (3) is exclusively in the anti
conformation.12 This contrasting behavior between diols
1 and 2 and diol 3 in their conformational preferences
is similar to that between 9, 90-bifluorenyl (5) and 1,1,2,2-
tetraphenylethane (6) in which the former exists as the
gauche conformer and the latter as the anti conformer.13
The explanation provided for the contrasting conformational
preference in the hydrocarbons is the ability for the geminal
Supporting information available
13C NMR spectra for diol 1 at ambient temperature and
ꢀ40 C and the 1H spectra in DMSO-d6 at 80 C are
available. Variable-temperature proton spectra for diol 2
are also included. The x-ray crystallographic data including
geometric coordinates for diol 1 are available. A summary of
selected crystallographic data for diol 4 is also included.
°
°
Copyright  2001 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2001; 39: 294–298

Conformation of substituted ethane-1,2-diols
297
Figure 3. 1H NMR spectrum (400 MHz) of Diels–Alder adduct 8.
Table 1. Conformational energies of 1,1,2,2-tetraaryldiols
and-ethanes
Calculated
dihedral
angle, Âꢁ ꢂ
Experimental
dihedral
Egauche ꢀ Eanti
Compound (kcal molꢀ1
a
°
°
)
angle, Âꢁ ꢂ
Diol 1
Diol 2
Diol 3
Diol 4
Ethane 5
Ethane 6
Ethane 7
ꢀ3.4
ꢀ4.4
C9.6
ꢀ1.4
63.7
64.1
180.0
46.6
73.7 (65.8)b
179.1
179.1
55.8
—
—
47.1
59.7c
—
ꢀ5.2ꢁꢀ6.6ꢂb
C5.8ꢁC5.0ꢂb
C1.3ꢁC0.6ꢂb
Figure 4. Conformations of diols 1 and 2.
—
a Dihedral angle for the low-energy conformer.
b Calculated from EFF molecular mechanics.13
EXPERIMENTAL
c Ref.13
.
Spectra
All solvents were purified and dried by standard procedures.
Compounds
Infrared (IR) spectra were recorded on an HP-2000 FTIR
Diol 2 was prepared by a modified literature method14
involving reductive coupling of fluorenone as described
below for the preparation of diol 1. Diol 4 was prepared
by a literature procedure15 involving phenylation of methyl
9-hydroxy-9-fluorene carboxylate.
1
spectrometer in KBr pellet form. H and 13C NMR spectra
were obtained on Bruker ARX-400 and AVANCE-600
NMR spectrometers using CDCl3 and DMSO-d6 as solvent,
respectively. These spectra were obtained using the zg30
pulse program with processing on a Silicon Graphics O2
workstation using the Bruker XWIN-NMR software. Mass
spectra were measured on a VG Micromass 16F instrument
at 28 eV or a Kratos Profile spectrometer.
Diol 4
15
°
°
M.p. 138–140 C (lit. m.p. 138–160 C, depending on the
solvent used for crystallization. 1H NMR: υ 7.49 (d, 6H), 7.25
Copyright  2001 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2001; 39: 294–298

298
M. Hoang et al.
from the difference map and refined isotropically using a
2
˚
˚
riding model (C—H 0.96 A, U11 D 0.08 A ) (see Supporting
information section). The final structural refinement gave
˚
˚
°
cell dimensions 18.919(4) A, 9.266(2) A, 24.768(5), ˛ D 90 ,
3
˚
°
°
ˇ D 107.03ꢁ3ꢂ , ꢄ D 90 , V D 4152ꢁ2ꢂ A , Z D 8, ꢅꢁMo K˛ꢂ D
˚
0.71069 A, Fꢁ000ꢂ D 1712, R D 0.0568 with data: restraints:
parameter ratio of 5391 : 0 : 557. Two independent molecules
are present within the unit cell.
Crystal structure determination for diol 4
Crystals of diol 4 were obtained by slow evaporation
from a hexane–diethylether–methanol solution. Data were
collected on a Nonius Kappa CCD diffractometer using
Figure 5. Crystal structure of diol 4. Element key: carbon,
partially hatched circles; oxygen, dotted circles; hydrogen,
small open circles.
˚
graphite monochromated Mo K˛ radiation ꢁꢅ D 0.71073 A).
°
A combination of 1 phi and omega (with kappa offsets)
scans were used.17 The structures were solved and refined
using the SHELXTLn PC V5.118 package. Refinement was
by full-matrix least-squares on F2 using all data (negative
intensities included). Hydrogen atoms were included in
calculated positions.
(t, 2H), 7.18 (m, 6H), 7.01 (t, 2H), 6.93 (d, 2H), 3.46 (br.s,
1H, OH), 3.10 (br.s, 1H, OH). 13C NMR: υ 146.6, 143.0, 141.0,
129.2, 127.8, 127.5, 127.4, 127.3, 126.0, 119.6, 87.6, 81.8.
1,10-Dihydroxybis(4H-cyclopenta[def]phenanthrene) (1)
Acknowledgement
A
mixture of 4H-cyclopentaphenanthren-4-one16 (1 g,
We thank the Natural Science and Engineering Research Council of
Canada (NSERC) for financial support. We thank Professor W. F.
Reynolds for useful suggestions in the interpretation of the variable-
temperature NMR spectra.
1.03 mmol), 1 g (15.3 mmol) of Zn powder and 0.4 g
(2.9 mmol) of zinc chloride in 10 ml of 10% aqueous THF
was stirred at room temperature for 4 h. The reaction mix-
ture was worked up with 1.5 ml of 3 M HCl and the Zn
was filtered off. The organic layer was taken up in toluene,
washed with 3 ð 5 ml of water and dried over anhydrous
magnesium sulfate. The solvent was removed under reduced
pressure. Diol 1 was isolated and obtained pure by prepar-
ative TLC (20% ethyl acetate –80% hexane) in 43% yield
(0.092 g). Some 4H-cyclopenta[def]phenanthren-4-ol16 was
also obtained from this reaction in 17% yield (0.037 g) as
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1
°
a side product. M.p. 235–237 C. H NMR: υ [CDCl3, room
temperature; see Fig. 1(a)] 7.62–7.30 (br. s, 16 H, aromatic H),
°
υ [CDCl3, ꢀ40 C; see Fig. 1(b)] 8.31 (d, 1H), 7.96 (d, 1H), 7.81
(t, 1H), 7.73 (d, 1H), 7.55 (d, 1H), 7.37 (d, 1H), 6.80 (t, 1H),
5.91 (d, 1H). 13C NMR: υ (CDCl3, room temperature) 136.5,
°
127.6, 127.4, 125.1, 125.3, 122.0 (br. peak), υꢁCDCl3, ꢀ40 Cꢂ
144.7, 141.7, 136.2, 135.9, 127.9, 127.2, 127.1, 126.7, 125.5, 125.0,
124.96, 124.8, 123.0, 120.5. IR (KBr): 3502 cmꢀ1 (OH). MS: m/z
410 ꢁMCꢂ, 392, 205, 189. Anal. Calculated for C30H18O2: C,
87.8; H, 4.39. Found: C, 87.5: H, 4.4%.
Crystal structure determination for 1, 1ꢀ-
dihydroxybis(4H-cyclopenta[def]phenanthrene) (1)
Crystals of diol 1 were grown by slow evaporation from a 1 : 1
solution of CHCl3 and hexane. X-ray diffraction data were
collected on a Siemens SMART CCD system at the University
of Windsor. A structural solution for the diol was refined
in crystal system P21/n using 5391 out of 26 237 observed
reflections. The structure was solved by Direct Methods
followed by Fourier Synthesis, using the SHELX 93 PC
package. Final refinement was done using full-matrix least-
squared procedures with anisotropic thermal parameters on
all non-hydrogen atoms. The hydrogen atoms were located
Copyright  2001 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2001; 39: 294–298