Synthesis and structure of a helical diindenophenanthrene with four congested phenyl substituents as a molecular spiral staircase

Article abstract of DOI:10.1021/ol0481434

(Chemical Equation Presented) Treatment of 9 with potassium tert-butoxide produced 10 having a helical twist in a single operation. The X-ray structure of 10 shows that the four phenyl substituents are essentially parallel to one another but are virtually

Full text of DOI:10.1021/ol0481434

ORGANIC
LETTERS
2004
Vol. 6, No. 23
4355-4357
Synthesis and Structure of a Helical
Diindenophenanthrene with Four
Congested Phenyl Substituents as a
Molecular Spiral Staircase†
Weixiang Dai, Jeffrey L. Petersen,‡ and Kung K. Wang*
C. Eugene Bennett Department of Chemistry, West Virginia UniVersity,
Morgantown, West Virginia 26506-6045
kung.wang@mail.wVu.edu
Received September 14, 2004
ABSTRACT
Treatment of 9 with potassium tert-butoxide produced 10 having a helical twist in a single operation. The X-ray structure of 10 shows that the
four phenyl substituents are essentially parallel to one another but are virtually perpendicular to the helical axis of the diindenophenanthrene
ring system. Viewing from the direction perpendicular to the helical axis, the structure of 10 resembles that of a segment of a spiral staircase
having four parallel steps.
The distorted structure of 4,5-disubstituted phenanthrenes1
and related compounds2 has long been a subject of interest.
The nonbonded steric interactions between the two substit-
uents at the C4 and C5 positions cause them to bend away
from the mean plane of the aromatic system and exert a
torsional force that produces a helical twist of the aromatic
framework. The X-ray crystallographic structure of 4,5-
dimethylphenanthrene (1) shows a 27.9° twist between the
mean planes of the two outer aromatic rings (planes 1-2-
3-4-4a-10a and 4b-5-6-7-8-8a).1b The twist of the
corresponding aromatic rings of 2 (rings B and C), a
diindeno-fused 4,5-diphenylphenanthrene, was more pro-
nounced at 46.1°.1c In addition, the two phenyl substituents
are oriented essentially parallel to each other (within 1.3°),
but are at a 53.1° angle from the diindenophenanthrene
system as indicated by X-ray structure analysis. The presence
1
of a helical twist in 2 could also be discerned from its H
NMR spectrum with a set of AB quartet signals (J ) 21.0
Hz) from the diastereotopic methylene hydrogens on the five-
membered rings.
† Dedicated to Professor Denis W. H. MacDowell on the occasion of
his 80th birthday.
‡ To whom correspondence concerning the X-ray structures should be
addressed. Phone: (304) 293-3435, ext 6423. Fax: (304) 293-4904.
E-mail: jeffrey.petersen@mail.wvu.edu.
(1) (a) Newman, M. S.; Hussey, A. S. J. Am. Chem. Soc. 1947, 69, 3023-
3027. (b) Armstrong, R. N.; Ammon, H. L.; Darnow, J. N. J. Am. Chem.
Soc. 1987, 109, 2077-2082. (c) Li, H.; Petersen, J. L.; Wang, K. K. J.
Org. Chem. 2001, 66, 7804-7810.
(2) Tinnemans, A. H. A.; Laarhoven, W. H. Tetrahedron 1979, 35, 1537-
1541.
Our continued interest in nonplanar polycyclic aromatic
compounds led us to apply the synthetic sequence developed
for 2 to the construction of 10 having two additional phenyl
10.1021/ol0481434 CCC: $27.50
© 2004 American Chemical Society
Published on Web 10/20/2004

Scheme 1
Figure 1. Molecular structure of 10. Its solid structure is
constrained by a crystallographic 2-fold rotation axis.
conformation. Sighting along the line joining C1 and C16,
the dihedral angle between the bonds attaching the two
phenyl substituents to C1 and C16 is 53.6°, whereas viewing
along the C15-C16 line the dihedral angle between the
bonds connecting the two inner phenyl groups to C15 and
C16 is even more pronounced at 63.5°. As a result, the two
outer phenyl substituents are pointed essentially in opposite
directions. Viewing along the C1-C14 line and following
the direction of rotation of the helical twist, the bond
connecting a phenyl substituent to C14 is rotated 184.3° from
the bond connecting a phenyl substituent to C1. The
diindenophenanthrene system is also severely distorted with
a 29.3° twist angle between the mean planes of rings A and
B and a more pronounced 59.2° twist angle between the
mean planes of rings B and C, which is substantially larger
than that of 2 at 46.1°. Viewing from the direction perpen-
dicular to the helical axis of the diindenophenanthrene ring
system, the structure of 10 resembles that of a segment of a
spiral staircase having four parallel steps or that of a portion
of one strand of DNA containing four bases.6
substituents at the two outermost aromatic rings (Scheme
1). Conversion of 3,3 prepared from 1,1′-biphenyl-2-ol, to
triflate 4 followed by coupling with phenylacetylene pro-
duced 5, which was then treated with dimethyl (1-diazo-2-
4
oxopropyl)phosphonate and K2CO3 to afford diyne 6. The
transformations from 6 to 10 are similar to what were
reported previously for 2. Condensation between 2 equiv of
6 and 7 produced 8, which was then reduced to furnish 9.
Treatment of 9 with potassium tert-butoxide under refluxing
p-xylene then produced 10 in a single operation. The
transformation from 9 to 10 presumably involved two
prototropic acetylene to allene rearrangements of the two
internal acetylenic moieties followed by two formal intramo-
lecular Diels-Alder reactions and subsequent prototropic
rearrangements of the resulting Diels-Alder adduct as
described previously for 2.
The X-ray structure of 10 (Figure 1) shows that the four
phenyl substituents are essentially parallel to one another
but are virtually perpendicular to the helical axis of the
diindenophenanthrene system. Because the perpendicular
distances of ca. 3.1 Å between the neighboring phenyl
substituents are shorter than the optimal π-system van der
Waals contact distance of ca. 3.4 Å,5 the phenyl substituents
are not directly on top of one another but are in a skewed
Due to additional buttressing effect, the rate of racemiza-
tion of 10 could be expected to be even slower than that of
2, which is most likely to be conformationally stable at 25
1
°C.1c The H NMR spectrum (600 MHz) of 10 in CDCl3
also exhibited a set of AB quartet at δ 4.34 and 4.39 (J )
21.0 Hz) from the diastereotopic methylene hydrogens,
manifesting the presence of a helical twist.
(3) Casiraghi, G.; Casnati, G.; Puglia, G.; Sartori, G.; Terenghi, G. J.
Chem. Soc., Perkin Trans. 1 1980, 1862-1865.
(4) Mu¨ller, S.; Liepold, B.; Roth, G. J.; Bestmann, H. J. Synlett 1996,
521-522.
(5) Robertson, J. M. Organic Crystals and Molecules; Cornell University
Press: Ithaca, NY, 1953; pp 206-214.
The two most upfield signals (doublet) of the aromatic
hydrogens at δ 5.34 and 5.68 are attributable to the ortho
(6) Schmuck, C. Angew. Chem., Int. Ed. 2003, 42, 2448-2452 and
references therein.
4356
Org. Lett., Vol. 6, No. 23, 2004

hydrogens of the two inner phenyl substituents connected
to C15 and C16. The fact that two distinct signals were
observed for these ortho hydrogens suggests that the rate of
rotation of the two inner phenyl groups is relatively slow on
the NMR time scale at 25 °C. Two distinct signals attribut-
able to the meta hydrogens were also observed at δ 6.03
and 6.24. This is in sharp contrast to the 1H NMR spectrum
(600 MHz) of 2 at 25 °C, which showed only one signal
(doublet) at δ 6.52 for the ortho hydrogens and one signal
(triplet) at δ 7.00 for the meta hydrogens. However,
temperature-dependent NMR study of 2 showed that at -40
°C two doublets at δ 6.46 and 6.51 for the ortho hydrogens
and two triplets at δ 6.93 and 7.05 for the meta hydrogens
appeared, and the coalescence temperatures were determined
to be -22 and -10 °C, respectively, corresponding to
rotational barriers of 12.5 and 12.6 kcal/mol at these two
temperatures.
Scheme 2
Interestingly, signals from the ortho and meta hydrogens
of the two outer phenyl substituents connected to C1 and
C14 of 10 were conspicuously missing. Instead, broad humps
could be discerned on the baseline due to restricted rotation
of the phenyl groups. On lowering the temperature from +25
to -20 °C, additional peaks started to appear, and at -20
°C two doublets at δ 6.21 and 6.36 attributable to the ortho
hydrogens and two triplets at δ 6.46 and 6.73 attributable to
the meta hydrogens could be identified. The coalescence
temperatures for the ortho and meta hydrogens are 25 and
30 °C, respectively, corresponding to rotational barriers of
14.3 and 14.2 kcal/mol at these two temperatures. At 55 °C
on a 270 MHz NMR spectrometer, a single doublet at δ 6.29
for the ortho hydrogens and a single triplet at δ 6.58 for the
meta hydrogens could be clearly discerned.
allowing a minor portion of 14 to undergo the Myers-Saito
cyclization reaction.9 On the other hand, the use of the diyne
without a phenyl substituent on the central benzene ring
produced the corresponding benzo[b]fluorene exclusively in
90% yield.10
Compared to the synthesis of 2, the use of diyne 6 bearing
a phenyl substituent on the central benzene ring resulted in
a lower yield for 10. A model study with 13 revealed that
the expected benzo[b]fluorene 17 was produced via a
sequence of reactions involving a prototropic rearrangement
to form the benzannulated enyne-allene 14 followed by a
Schmittel cyclization reaction7 to form biradical 15, which
in turn underwent an intramolecular radical-radical coupling
and a prototropic rearrangement (Scheme 2). However, two
additional adducts, 18 and 19, were also produced. A
competing intramolecular [2 + 2] cycloaddition reaction of
14 could account for 18. The generation of biradical 16 from
the Myers-Saito cyclization reaction8 of 14 followed by
trapping the aryl radical center with the phenyl substituent
and a prototropic rearrangement could furnish 19. Presum-
ably, the emergence of nonbonded steric interactions between
the two phenyl substituents along the pathway toward 17
could be responsible for an increase of its activation energy,
Acknowledgment. K.K.W. thanks the Petroleum Re-
search Fund (38169-AC1), administered by the American
Chemical Society, and the National Science Foundation
(CHE-0414063) for financial support. J.L.P. acknowledges
the support (CHE-9120098) provided by the National Science
Foundation for the acquisition of a Siemens P4 X-ray
diffractometer. The financial support of the NSF-EPSCoR
(1002165R) for the purchase of a 600 MHz NMR spectrom-
eter is gratefully acknowledged.
Supporting Information Available: Experimental pro-
1
cedures, spectroscopic data, and H and 13C NMR spectra
of 4-6, 8-10, 12, 13, and 17-19; ORTEP drawings of the
crystal structures of 10 and 19; and X-ray crystallographic
data of 10 and 19 (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
OL0481434
(9) (a) Engels, B.; Lennartz, C.; Hanrath, M.; Schmittel, M.; Strittmatter,
M. Angew. Chem., Int. Ed. 1998, 37, 1960-1963. (b) Engels, B.; Hanrath,
M. J. Am. Chem. Soc. 1998, 120, 6356-6361. (c) Schreiner, P. R.; Prall,
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M. A. J. Am. Chem. Soc. 2000, 122, 9265-9270. (e) Cramer, C. J.; Kormos,
B. L.; Seierstad, M.; Sherer, E. C.; Winget, P. Org. Lett. 2001, 3, 1881-
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(7) (a) Schmittel, M.; Strittmatter, M.; Kiau, S. Tetrahedron Lett. 1995,
36, 4975-4978. (b) Schmittel, M.; Strittmatter, M.; Vollmann, K.; Kiau,
S. Tetrahedron Lett. 1996, 37, 999-1002.
(8) (a) Myers, A. G.; Kuo, E. Y.; Finney, N. S. J. Am. Chem. Soc. 1989,
111, 8057-8059. (b) Myers, A. G.; Dragovich, P. S. J. Am. Chem. Soc.
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I. Tetrahedron Lett. 1989, 30, 4995-4998. (d) Nagata, R.; Yamanaka, H.;
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