Restricted rotation in 9-phenyl-anthracenes: A prediction fulfilled

Article abstract of DOI:10.1021/ol102665y

The calculated phenyl rotation barrier in 9-phenylanthracene has been reported as ～21 kcal mol^-1, but experimental verification of this barrier is limited by its intrinsic symmetry. V-T NMR indicated the barrier to interconversion of the syn (C

Full text of DOI:10.1021/ol102665y

ORGANIC
LETTERS
2011
Vol. 13, No. 2
256-259
Restricted Rotation in
9-Phenyl-anthracenes: A Prediction
Fulfilled
Kirill Nikitin,* Helge Mu¨ller-Bunz, Yannick Ortin, Jimmy Muldoon, and
Michael J. McGlinchey*
School of Chemistry and Chemical Biology, UniVersity College Dublin,
Belfield, Dublin 4, Ireland
kirill.nikitin@ucd.ie; michael.mcglinchey@ucd.ie
Received November 5, 2010
ABSTRACT
The calculated phenyl rotation barrier in 9-phenylanthracene has been reported as ∼21 kcal mol-1, but experimental verification of this barrier
is limited by its intrinsic symmetry. V-T NMR indicated the barrier to interconversion of the syn (C2v) and anti (C2h) rotamers of 9,10-bis(3-
fluorophenyl)anthracene to be ∼21 kcal mol-1. Likewise, the V-T NMR spectra of 9-(1-naphthyl)-10-phenylanthracene reveal that the rotational
barrier of the unsubstituted phenyl ring is at least 21 kcal mol-1
.
The phenomenon of atropisomerism, whereby slowed rota-
tion about carbon-carbon single bonds can give rise to
separable different structures, has a long and distinguished
history. The first successful isolation of enantiomeric 2,2′,6,6′-
tetrasubstituted biphenyls was reported by Christie and
Kenner in 1922,1a and the field was developed significantly
in now classic studies from the groups of Adams1b,c and of
Oki.1d As emphasized by Mislow, for rotations in 2,2′-
disubstituted biphenyls the transoid transition state is mark-
edly favored relative to its cisoid analogue.2a,b Moreover,
very considerable salt effects have been reported for the
racemization of biphenyls having cationic groups in the 2,2′-
positions.2c
of a wide range of 2,2′-disubstituted biaryls revealed
quantitative correlations between the rotational barriers and
the van der Waals radii of the relevant substituents.3
Moreover, it was proposed that pairwise contributions to the
rotational barriers were additive; however, this suggestion
has recently been refuted.4
(1) (a) Christie, H.; Kenner, J. J. J. Chem. Soc. 1922, 121, 614–620. (b)
Adams, R.; Yuan, H. C. Chem. ReV. 1933, 12, 261–338. (c) Adams, R.;
Teeter, H. M. J. Am. Chem. Soc. 1940, 62, 2188–2190. (d) Oki, M. Top.
Stereochem. 1965, 14, 1–81.
(2) (a) Mislow, K. Introduction to Stereochemistry; W. A. Benjamin,
Inc.: Reading, MA, 1965; pp 78-81. (b) Mislow, K.; Hyden, S.; Schaefer,
H. J. Am. Chem. Soc. 1962, 84, 1449–55. (c) Mullen, K.; Heinz, W.; Klarner,
F. G.; Roth, W. R.; Kindermann, I.; Adamczak, O.; Wette, M.; Lex, J.
Chem. Ber. 1990, 123, 2349–2371.
It soon became evident that the barrier height was heavily
dependent on the steric bulk of the substituents, with
hydrogen, of course, being the least demanding. The
comprehensive pioneering study by Bott, Field, and Sternhell
(3) Bott, G.; Field, L. D.; Sternhell, S. J. Am. Chem. Soc. 1980, 102,
5618–5626.
(4) Ruzziconi, R.; Spizzichino, S.; Lunazzi, L.; Mazzanti, A.; Schlosser,
M. Chem.sEur. J. 2009, 15, 2645–2652.
10.1021/ol102665y  2011 American Chemical Society
Published on Web 12/13/2010

Concomitantly, it was demonstrated that interconnected,
fused unsubstituted aromatic systems, e.g. binaphthyls, are
atropisomeric thus opening up routes to chiral catalysts and
switches.5 In many cases the barrier exceeds 25 kcal mol-1
and the atropisomers are separable on the laboratory time
scale. Slowed rotation of substituted phenyl groups attached
to an aliphatic or aromatic framework is well-known.6a
Although reports of restricted rotation of unsubstituted
phenyls are scarce, they have been observed in 1,4-adducts
of 9-phenylanthracene.6b
ground state, the phenyl would be oriented orthogonal to
the anthracene ring plane, whereas in the transition state the
phenyl and anthracenyl fragments would adopt a nonplanar,
stepped, ladder-like arrangement, as in Figure 2.
Furthermore, we have very recently shown that phenyl
rotation is only minimally restricted in the 9,10-diphenyl-
dibenzo-dihydrobarrelene, 1, and cannot be readily observed
by variable-temperature NMR spectroscopy; in contrast,
when the bridge length is increased to four atoms, as in the
9,10-diphenylbicyclo[4.2.2]dioxadecadiene, 2, this barrier
exceeds 23 kcal mol-1.7 In the latter case, both ortho-
hydrogens of the potentially rotating phenyls would engage
in simultaneous steric repulsion with the peripheral benzo
rings. In 1 the dihedral angle between the exterior benzo
blades is 134°, whereas it increases to 166° in 2.7 This raises
the question as to whether rotation of an unsubstituted phenyl
group in 9-phenylanthracene, 3, would be significantly
hampered in the case where the interplanar angle of the
peripheral blades has increased to 180° (Figure 1).
Figure 2. Views of the DFT-calculated transition state for phenyl
rotation in 9-phenylanthracene, 3.
The intrinsic C2V or D2h symmetry of 9-phenylanthracene, 3,
or 9,10-diphenylanthracene, 4, respectively, renders the pairs
of ortho and meta CH positions equivalent and so does not
allow the rotational process to be monitored by variable-
temperature NMR spectroscopy. Hence, it is necessary to break
the symmetry, but to do so in such a fashion so as not to perturb
the molecular geometry significantly. In previous work, we have
used this approach to study the independent rotation of ethyl
or phenyl substituents as well as of the tripods in
[C6Et5C(O)Me)]Cr(CO)3, [C6Et6]Cr(CO)(CS)(NO)]+, or
(C5Ph5)Fe(CO)(CHO)PR3, respectively.9
To this end, we here report the syntheses, structures, and
dynamic behavior of 9,10-bis(3-chlorophenyl)anthracene, 5a,
9,10-bis(3-fluorophenyl)anthracene, 5b, and 9-(1-naphthyl)-
10-phenylanthracene, 6. Diphenylanthracenes 5, previously
reported in conjunction with their photophysical properties,
can be conveniently prepared by palladium-catalyzed cross-
coupling of 9,10-dibromoanthracene and the corresponding
arylboronic acids.10 Although we are unaware of any direct
claims of atropisomerism in molecules such as 5, there are
cases in which more than ten aromatic resonances are present
in their 13C spectra, indicating restricted rotation on the NMR
time-scale.11
Figure 1. Rotational barriers in molecules 1-3 are markedly
dependent on the interplanar angle between the exterior blades: <8
kcal mol-1 (1), >23 kcal mol-1 (2), and (calculated) ∼21 kcal mol-1
(3).
The barrier to phenyl rotation in 9-phenylanthracene, 3,
has been computed at the DFT level, and it was shown that
rotation of the phenyl group would require a substantial
energy cost of 20.2-20.8 kcal mol-1, dependent on the basis
set used.8 Moreover, these data also indicated that, in the
The 125 MHz 13C NMR spectrum of 9,10-bis(3-chlo-
rophenyl)anthracene, 5a, at 298 K exhibited more than ten
individual resonances. Full assignment of all peaks revealed
that, as expected, the peaks of C(12), C(13), C(15), and C(16)
were in fact narrowly spaced twin signals, with separations
of 4, 4, 2, and 5 Hz, respectively. Moreover, as the
temperature was gradually raised to 363 K, the peaks did
not broaden or coalesce. These observations not only suggest
(5) (a) Che, C. M.; Huang, J. S. Coord. Chem. ReV. 2003, 242, 97–113.
(b) Mori, K.; Katoh, T.; Suzuki, T.; Noji, T.; Yamanaka, M.; Akiyama, T.
Angew. Chem., Int. Ed. 2009, 48, 1–4. (c) Sandoval, C. A.; Ohkuma, T.;
Muniz, K.; Noyori, R. J. Am. Chem. Soc. 2003, 125, 13490–13503. (d)
Mikami, K.; Aikawa, K.; Yusa, Y.; Jodry, J. J.; Yamanaka, M. Synlett 2002,
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Organometallics 1994, 13, 682–689.
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M. J. Eur. J. Org. Chem. 2010, 5203–5216.
(10) (a) Langer, V.; Becker, H. D. Z. Kristallogr. 1992, 199, 313–315.
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Chen, C. H. J. Mater. Chem. 2008, 18, 4529–4536.
(8) (a) Nori-shargha, D.; Asadzadeha, S.; Ghanizadehb, F.-R.; Deyhimic,
F.; Aminic, M. M.; Jameh-Bozorghi, S. THEOCHEM 2005, 717, 41–51.
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Org. Lett., Vol. 13, No. 2, 2011
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the concurrent existence of syn (C2V) and anti (C2h) isomers
of 5a (Figure 3) but also indicate an aryl rotation barrier
Figure 3. Slowed rotation of the phenyls cannot be observed
experimentally in the D2h-symmetric system 4; however, the
symmetry can be lowered, to C2V in syn-5, to C2h in anti-5, and to
Cs in 6.
greater than 21 kcal mol-1. DFT optimization of the energy
profile of 5a at the B3LYP level furnished a barrier of ∼22
kcal mol-1; in addition, it revealed that 5a adopts a ladder-
like conformation in the transition state, as previously
reported for the unsubstituted parent molecule, 3. Interest-
ingly, attempts to separate the syn and anti isomers of 5a by
chromatographic or crystallization techniques failed, perhaps
suggesting that the barrier is unlikely to exceed 24 kcal
mol-1.
Figure 4. Molecular structure of 9,10-di(3-fluorophenyl)anthracene,
5b (anti isomer shown), and sections of the 125 MHz 13C NMR
spectrum of syn/anti-5b.
indicated an exchange rate of 2 s-1 and a rotational barrier
of ∼21 kcal mol-1 for the 3-fluorophenyl moiety, in
reasonably good agreement with the DFT results.
This work was extended to include 9,10-bis(3-fluorophe-
nyl)anthracene, 5b, in which the smaller steric size of fluorine
relative to chlorine might lead to a somewhat lower rotational
barrier if buttressing effects play a significant role.12
Fortunately, the fluoro-compound 5b furnished X-ray quality
crystals, and the structure of the anti isomer appears in Figure
4. It transpired that, in the solid state, syn- and anti-
atropisomers are present in equal amounts, and the fluorine
occupancies of both meta-positions of the phenyl rings are
50%. The fluorophenyls adopt an angle of 85° with the plane
of the anthracene thus deviating only slightly from the
perpendicular orientation. Nonoptimized virtual rotation of
the phenyls indicates that very significant steric strain would
occur in the planar conformation as the nominal closest
approach distance (CAD) of H(1) and H(12) would be only
0.72 A. As for 3,8a DFT optimization at the B3LYP level
indicated a nonplanar transition state and indicated that
rotation of one of the fluorophenyl moieties would evoke
an energy penalty of ∼22 kcal mol-1.
Having obtained both theoretical and experimental evi-
dence that the rotation of a meta-substituted phenyl linked
to the 9-position of anthracene is slow on the NMR time
scale, and may be somewhat sensitive to the buttressing effect
of the meta-substituent, it was deemed appropriate to
investigate the rotational barrier of an unsubstituted phenyl.
This requires that the symmetry of the enVironment of the
phenyl has to be broken rather than the symmetry of the
phenyl itself. To this end, 9-(1-naphthyl)-10-phenylan-
thracene, 6, was prepared and characterized by X-ray
crystallography. A previous DFT computation on 1,9′-
naphthylanthracene predicted a structure in which the two
aromatic moieties would be orthogonal with a naphthyl
rotation barrier of 38 kcal mol-1.8a As shown in Figure 5,
the phenyl and naphthyl fragments in 6 adopt dihedral angles
of 80° and 88°, respectively, with the plane of the anthracene
and a dihedral angle between themselves of 12°. Assuming
time-averaged mirror symmetry in solution, and a very high
barrier to naphthyl rotation, the ortho and meta positions of
the phenyl ring should be nonequivalent unless rotation about
the C(9)-C(11) bond were to become rapid on the NMR
time scale.
The 125 MHz 13C NMR spectrum of 5b was recorded at
298 K without fluorine decoupling (Figure 4); complete
assignment of all peaks revealed that C(12), C(15), and C(16)
gave rise to twin signals separated by 3.6, 2.0, and 2.5 Hz,
respectively. As the temperature was allowed to rise, the
twinned peaks of C(12) and C(16) gradually broaden but do
not reach coalescence, while the peaks of C(15) coalesce at
353 K. An intermediate rate analysis of this exchange process
The 600 MHz 1H NMR spectrum of 6 at 303 K is shown
in Figure 5. The anthracene protons H(1)/H(8) were unam-
biguously distinguished from H(4)/H(5) by their NOE
interactions with the ortho phenyl protons H(12)/H(16) which
appear at 7.57 and 7.52 ppm, once again revealing restricted
rotation of the phenyl ring. The peak at 7.51 ppm corresponds
to that of the ortho phenyl protons in 9,10-diphenylan-
(12) Rieger, M.; Westheimer, F. H. J. Am. Chem. Soc. 1950, 72, 19–
28.
258
Org. Lett., Vol. 13, No. 2, 2011

tuted phenyl moiety exhibits slow rotation on the NMR time
scale even at this temperature. One can therefore justify a
minimum barrier of ∼21 kcal mol-1, in agreement with the
previously reported theoretical value for phenylanthracene,
3, (20.8 kcal/mol-1) and our own DFT B3LYP level
calculations for 6 (∼22 kcal mol-1).
To conclude, the previously calculated prediction of a
rotational barrier of ∼21 kcal mol-1 for 9-phenylanthracene
has been fulfilled experimentally by examining a close
structural analogue in which the symmetry has been lowered
from C2V to Cs, thus rendering the edges of the unsubstituted
phenyl moiety nonequivalent. While the magnitude of this
rotational barrier might not have been anticipated intuitively,
it provides an archetypical example whereby it may be
rationalized on the basis of simultaneous double repulsions
experienced by the two ortho-positions of the phenyl ring
with hydrogens on the anthracene. Although the barrier of
∼21 kcal mol-1 is not quite large enough to engender
conformationally stable and separable isomers at room
temperature, the angular rigidity of the 9-phenylanthracene
system has important stereochemical consequences. These
factors may be usefully considered when designing new
generations of enantioselective catalysts that contain biaryl
and, in particular, phenylanthracene fragments.13
Acknowledgment. We thank Science Foundation Ireland
and University College Dublin for generous financial support
and the Higher Education Authority through PRTLI Cycle
3 for instrumentation funding.
Figure 5. Molecular structure of 9-(1-naphthyl)-10-phenylan-
Supporting Information Available: Experimental pro-
cedures and NMR characterization for compounds 5a, 5b,
and 6 (COSY, TOCSY, HSQC, HMBC, NOE), as well as
X-ray crystallographic collection, refinement details, and cifs.
This material is available free of charge via the Internet at
http://pubs.acs.org.
thracene, 6, and its 2D 1H-1H 600 MHz COSY spectrum showing
four individual color-coded spin systems; benzene is marked with
an asterisk (*).
thracene and so is assigned to H(16). The other ortho phenyl
proton, H(12) which is proximate to the benzo ring of the
naphthalene moiety, experiences a slightly different environ-
ment and is somewhat deshielded. However, these peaks
begin to broaden at 363 K demonstrating that the unsubsti-
OL102665Y
(13) Ma, L.; Jia, P.; Zhang, Q.; Du, D.-M.; Xu, J. Tetrahedron:
Asymmetry 2007, 18, 878–884.
Org. Lett., Vol. 13, No. 2, 2011
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