Studies in the cycloproparene series: Approaches to cyclopropa[b]tetracenes

Article abstract of DOI:10.1071/CH99136

Cyclopropa[b]naphthalene-3,6-dione (2) fails to add furan across the enedione olefinic bond in a Diels-Alder cycloaddition even at 14×10⁵ kPa. In contrast, isobenzofuran (4) adds efficiently at ambient temperature and pressure. The epoxytetracenedione (5) that is formed is air-sensitive and decomposes under conditions employed for dehydration. Aromatization of (5) to cyclopropatetracenedione (6) is not observed despite anthracene-l,4-dione (8) being obtained from its analogous tetrahydro precursor (7) under the same conditions. CSIRO 1999.

Full text of DOI:10.1071/CH99136

C S I R O P U B L I S H I N G
Australian Journal
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Volume 52, 1999
© CSIRO Australia 1999
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Aust. J. Chem., 1999, 52, 1123–1126
Studies in the Cycloproparene Series:
Approaches to Cyclopropa[b]tetracenes
Brian Halton,A David A. C. EvansB and Ronald N. WarrenerB
A School of Chemical and Physical Sciences, Victoria University of Wellington,
P.O. Box 600, Wellington, New Zealand.
Author to whom correspondence may be addressed.
B Centre for Molecular Architecture, Central Queensland University,
Rockhampton, Qld. 4702.
Cyclopropa[b]naphthalene-3,6-dione (2) fails to add furan across the enedione olefinic bond in a Diels–Alder
cycloaddition even at 14×105 kPa. In contrast, isobenzofuran (4) adds efficiently at ambient temperature and pres-
sure. The epoxytetracenedione (5) that is formed is air-sensitive and decomposes under conditions employed for
dehydration. Aromatization of (5) to cyclopropatetracenedione (6) is not observed despite anthracene-1,4-dione (8)
being obtained from its analogous tetrahydro precursor (7) under the same conditions.
Despite wide ranging studies of the cycloproparene
hydrocarbons,1,2 the limit of cyclopropa-fusion into a linear
aromatic acene hydrocarbon appears to be anthracene.3 To
the best of our knowledge there are no accounts describing
attempts to provide cyclopropatetracenes or higher homo-
logues. However, the recent availability of cyclopropanaph-
thalenedione (2) as a stable crystalline compound4 provides
a substrate eminently suitable for ring homologation by way
of Diels–Alder chemistry at the enedione carbon–carbon
double bond. This can allow for sequential or simultaneous
extension of the ring system from naphthalene to anthracene
to tetracene. We provide here the details of our approach to
cyclopropa[b]tetracene-3,10-dione (6).
The addition of furan to a quinone frequently is not the sim-
plest reaction to effect because of the instability of the product,
and use of high pressure is often needed.7 This has been found
true for its reaction with (2) as no evidence for product forma-
tion was obtained even when (2) and furan were subjected to a
pressure of 14 kbar for 65 h at temperatures up to 45°; cyclo-
propa quinone and furan were returned unchanged.At the same
high pressure but 65°, the quinone decomposed to an insoluble
polymer that provided no spectroscopic evidence for the
cycloaddition to have occurred. In contrast to the behaviour of
furan, its 10 π-electron homologue isobenzofuran (4)8 is
markedly more reactive and adds to benzoquinone in good
(77%) yield at ambient temperature (Experimental section).
The involvement of cycloproparenes and their derivatives
in cycloadditions dates to the first isolation of parent cyclo-
propabenzene and involves competition between the strained
C 1a–C 5a bridge bond and the lateral three-membered ring
C 1–C 1a σ bond.2 Cyclopropa quinone (2) is now easily
available from benzoquinone (1) (Scheme 1),4 and it carries
a recognized dienophile in the form of the enedione
carbon–carbon double bond in addition to the distorted
C 1a–C 7a bridge bond; both of these sites are contenders for
Diels–Alder cycloadditions.4 Furthermore, opening of the
three-membered ring by [σ2+π2] or [σ2+π4] addition to the
strained σ bond is well documented and might also be
expected to occur.5 Competition between the three reaction
centres, and the dominance of any one of them in a given
reaction, will depend upon the nature of the diene, the nature
of the cycloproparene, and the conditions employed for the
reaction. In this regard quinone (2) competitively adds buta-
1,3-diene across the olefinic bond and the strained σ bond in
a reaction that is temperature-dependent but excludes the
strained bridge bond.6
Manuscript received 12 October 1999
© CSIRO 1999
10.1071/CH99136 0004-9425/99/121123

1124
B. Halton, D. A. C. Evans and R. N. Warrener
Isobenzofuran (4) is conveniently generated in situ and
this is best achieved by way of Diels–Alder addition of 3,6-
di(2-pyridyl)-1,2,4,5-tetrazine to 1,4-dihydro-1,4-epoxy-
naphthalene (Scheme 2).9 The initial [4+2] cycloaddition is
followed by rapid loss of molecular nitrogen with formation
of the thermally sensitive diazine (3) that undergoes retro-
Diels–Alder cleavage (either spontaneously from the heat
liberated in the first sequence or by gentle warming), and
isobenzofuran (4) is formed together with 3,6-di(2-pyridyl)-
pyridazine. Generated in this way in the presence of quinone
(2), diene (4) adds to the C 4–C 5 double bond to give the
epoxytetracene (5) in yields of 50–70% (Scheme 3). The
conversion can be made quantitative by employing an excess
of diene (4) but it is more desirable to repeat the reaction
sequence for a second time using c. 30% of the reagents
(Experimental section). There is no detectable reaction
between (2) and the tetrazine, and isobenzofuran (4) is not
intercepted by the quinone (5) across either the C 1a–C 11a
bridge bond or the C 1–C 1a σ bond.
3.21 ppm, respectively, and H 2/11 appear as a singlet at
7.58 ppm with the remaining four aromatic protons coupled
as an AA´BB´ pattern. The molecular symmetry is evident
from the appearance of 10 resonances for the 19 carbon
atoms, and the shielding expected2 for C 2/11 of the cyclo-
proparene (δ 111.6) is clearly evident. The C 4/9 and C 3a/9a
bridgehead and bridge carbons resonate at 81.8 and 48.8
ppm, respectively.
Removal of the oxygen bridge and aromatization of an
epoxy dione such as (5) can be achieved with comparative
ease under mild reaction conditions in a procedure that has
been referred to but not previously published.10 Thus, the
endo adduct (7) derived from 1,4-benzoquinone (1) and
isobenzofuran (4) (Scheme 4) is dehydrated with pyridine at
While the n.m.r. spectra recorded confirm the formation
of epoxytetracene (5) all attempts to isolate the pure com-
pound have been frustrated by its ease of decomposition.
Conventional workup leads to the removal of the organic
bases but the significant discoloration is accompanied by
the formation of insoluble materials; subsequent attempts to
purify (5) by radial chromatography resulted in the loss of
the compound. Rapid filtration through a short column of
silica gel, however, provided a small quantity of (5) as a
colourless gum (c. 12%) that was free of contaminants and
had an orange fluorescence on irradiation at 350 nm.
Structure (5) is assigned to this compound with confidence
from the observation of a molecular ion in the electron
impact mass spectrum, and a protonated molecular ion in
the chemical ionization mass spectrum, respectively. The 1H
n.m.r. spectrum shows that the cycloaddition is across the
C 4–C 5 π bond of (2) to give the endo adduct (5) as the
C 4/9 bridgehead protons (δ 5.87) are coupled to the C 3a/9a
bridge protons (δ 3.83); there is no evidence for the forma-
tion of exo adduct. The non-equivalent methylene protons
of (5) are a closely spaced AB system (J 7.4 Hz) at 3.18 and

Studies in the Cycloproparene Series
1125
(4) and decomposition materials (n.m.r.). While attempts to purify the
80° to give anthracene-1,4-dione (8) in 58% yield; the reac-
tion sequence involved is as illustrated for (5) in Scheme 3.
With adduct (7) the use of sodium acetate in refluxing acetic
acid also brings about the dehydration efficiently. However,
as the three-membered ring of a cycloproparene is sensitive
to electrophiles2 the dehydration of (5) was attempted with
pyridine rather than acetate/acetic acid. Moreover, as the
three-membered ring σ bond of cyclopropabenzene is
cleaved11 upon heating at 80°, the thermal stability of (5) was
examined. Epoxytetracene (5) was found to be stable at 58°
over 95 h in chloroform, and thus attempts to remove the
C 4/C 9 oxygen bridge were made at temperatures no higher
than this. After heating cycloproparene (5) for 1 h with pyri-
dine decomposition was evident as the 1H n.m.r. spectrum no
longer displayed meaningful resonances; workup provided a
bright yellow glass that had no proton signals compatible
with the sought after tetracene (6). Alternative approaches to
the desired ring system form a part of our continuing studies.
product by radial chromatography (plate thickness
2
mm;
dichloromethane elution) provided none of the desired product, filtra-
tion through
a short column (1.5 cm by 10 cm) of SiO2
(dichloromethane elution) gave four fractions of which the third was the
title adduct (5) as an essentially pure colourless gum (12 mg, 12%) that
darkened and became brown on standing (Found: M+ 288.0787.
C19H12O3 requires 288.0786. Found (c.i.): [M+H]+ 289.0862.
[C19H13O3]+ requires 289.0865). H n.m.r. δ 3.18, d, J 7.4 Hz, 1H,
1
CHAHB; 3.21, d, J 7.4 Hz, 1H, CHAHB; 3.83, app. dd, J 2.0, 3.6 Hz, 2H,
H3a/9a; 5.87, app. dd, J 2.0, 3.6 Hz, 2H, H4/9; 6.82–6.89, AA´, 2H,
H6/7 or H 5/8; 7.03–7.09, BB´, 2H, H 5/8 or H 6/7; 7.58, s, 2H, H2/11.
13C n.m.r. δ 18.2, C 1; 48.8, C 3a/9a; 81.8, C 4/9; 111.6, C 2/11; 120.0,
C 5/8 or C 6/7; 126.5, C 6/7 or C 5/8; 131.7, C 1a/11a; 137.2 and 140.9,
C 2a/10 and C 4a/8a; 193.4, CO. All attempts to crystallize the gum
resulted in significant darkening of the sample without formation of
solid; aerial oxidation is presumed to have occurred. In the absence of
air and in (D)chloroform solution adduct (5) was found to be stable for
days and was held without change at 58o for a period of 95 h.
Attempted Aromatization of Adduct (5) by Dehydration
A sample of adduct (5) (c. 4 mg) dissolved in (D5)pyridine was
unchanged at ambient temperature over 1 h. 1H n.m.r. δ 2.83, d, J 7.3
Hz, 1H, CHAHB; 2.92, d, J 7.3 Hz, 1H, CHAHB; 3.97, br s, H 3a/9a;
6.00, br s, H4/9; 6.70–6.74, AA´, 2H; 7.47, s, H2/11 (the BB´ compo-
nent was overlapped by a solvent peak). Upon warming to 58° for 1 h
the 1H n.m.r. spectrum changed and showed no signals compatible with
(5), its dehydration product (6) or a dimeric species from either.
Experimental
General
Melting points were recorded on a Gallenkamp digital melting point
apparatus and are uncorrected. 1H n.m.r. spectra were recorded for
(D)chloroform solutions on Bruker AM300 or AVANCE DPX400 MHz
instruments with 13C spectra obtained from the latter; tetramethylsilane
was internal standard unless otherwise stated. The assignments made
(4aα,9α,9aα,10α)-4a,9,9a,10-Tetrahydro-9,10-epoxyanthracene-1,4-
1
have been confirmed with the aid of HETCOR H–13C correlations. High-
dione (7)
resolution mass spectra were recorded on a Micromass AutoSpec
instrument. Flash column filtrations and column and radial chromatog-
raphy employed silica gel as the stationary phase with dichloromethane
or acetone as the eluent.
To a mixture of 1,4-dihydro-1,4-epoxynaphthalene9 (225 mg, 1.56
mmol), 3,6-di(2´-pyridyl)-1,2,4,5-tetrazine9 (370 mg, 1.57 mmol) and
1,4-benzoquinone (1) (169 mg, 1.57 mmol) in a round bottomed flask
was added chloroform (5 ml). After the evolution of nitrogen ceased the
mixture was diluted with chloroform (20 ml), washed with HCl (0.5 M,
2×20 ml) and the organic phase separated, dried, and evaporated to
dryness. The residue was chromatographed on silica first with chloro-
form as eluent to remove unchanged benzoquinone and then acetone to
elute the title adduct (7) (271 mg, 77%) which crystallized (methanol)
as colourless needles, m.p. 148–150o (Found: C, 74.3; H, 4.7%; M+,
266.0626. C14H10O3 requires C, 74.3; H, 4.5%; M+, 226.0629). I.r. νmax
1674s, 1304m, 1291s, 1156w, 1134w, 1052m, 988m, 761m, 744w,
790m, 875m, 842m, 763s, 722w cm–1. U.v. λmax (EtOH) 228 (ε 10313),
262sh (992), 269 nm (728). 1H n.m.r. δ 3.55–3.62, m, H4a/9a;
5.72–5.79, m. H9/10; 6.00, s, H2/3; 7.15, s, 4H, aromatic. Mass spec-
trum m/z 266 (M+, 3%), 119 (10), 118 (100), 115 (8), 90 (6), 89 (7).
Attempted Cycloaddition of 1H-Cyclopropa[b]naphthalene-3,6-dione
(2) to Furan
To
a
solution of dione (2)4 (20 mg, 0.1176 mmol) in
dichloromethane (1.5 ml) in a pressure cell was added an excess of
furan (3 drops). The vessel was closed and pressurized to 14 kbar
(14×105 kPa). After 65 h the pressure was released and 1 drop of the
1
solution added to (D)chloroform. The H n.m.r. spectrum showed the
presence of starting materials only. After adding 1 drop extra of furan,
the pressure cell was resealed, repressurized (14 kbar) and heated at 45°
for 65 h; the outcome was unchanged.
Resealing the vessel and heating at 67o and 14 kbar for 90 h led to a
solution that contained furan together with an insoluble deposit. The 1H
n.m.r. spectrum of the deposit [suspension in (D)chloroform] showed
no evidence for the desired adduct and its insolubility in all common
solvents (chloroform, dichloromethane, methanol, ethyl acetate, and
acetone) implies polymer formation.
Anthracene-1,4-dione (8)
Method A. The tetrahydro epoxy quinone (7) (25 mg, 0.84 mmol)
was heated in pyridine (3 ml) at 80o overnight. The solution was poured
into hydrochloric acid (0.5 M, 50 ml) and extracted with chloroform
(2×25 ml). The combined organic extracts were dried, concentrated in
vacuum and the residue was chromatographed over silica (chloroform
elution) to afford yellow needles of anthracene-1,4-dione (8) (13 mg,
58%) which was purified by vacuum sublimation (150o/0.2 mmHg),
m.p. c. 160o (sublim.) (lit.12 m.p. 225o). 1H n.m.r. δ 7.06, s, 2H, H2, H3;
7.78–7.57, m 2H, aromatic; 7.97–8.12, m, 2H, aromatic; 8.61, s, 2H,
H9, H 10. Mass spectrum m/z 210 (5%), 209 (17), 208 (M+, 93), 180
(21), 152 (51), 126 (38), 118 (100), 97 (25), 83 (21), 71 (27), 63 (21),
56 (40), 43 (50), 41 (30), all other peaks less than 20%.
Cycloaddition of 1H-Cyclopropa[b]naphthalene-3,6-dione (2) to
Isobenzofuran (4)—(3aα,4α,9α,9aα)-3a,4,9,9a-Tetrahydro-4,9-
epoxy-1H-cyclopropatetracene-3,10-dione (5)
To a mixture of quinone (2) (60 mg, 0.353 mmol),4 1,4-dihydro-1,4-
epoxynaphthalene9 (51 mg, 0.354 mmol) and 3,6-(di-2-pyridyl)-1,2,4,5-
tetrazine9 (84 mg, 0.355 mmol) in a round bottomed flask (5 ml) was
added chloroform (1.5 ml). Nitrogen evolution from the purple mixture
began almost immediately, the solution warmed to c. 40° and the colour
was discharged to brown over c. 1 h. After 90 min, the solution was
poured into dichloromethane (10 ml), washed with HCl (1 M, 10 ml),
H2O (2×5 ml), and the organic phase dried (MgSO4) and concentrated in
vacuum to a brown solid. The 1H n.m.r. spectrum of the crude product
showed adduct (5) and unchanged quinone (2) in a ratio of c. 7:3.
Resubjection of the mixture to isobenzofuran (4) [from dihydro-
epoxynaphthalene (17 mg, 0.118 mmol) and the tetrazine (28 mg,
0.1186 mmol) in chloroform (1 ml)] and workup as before provided a
brown solid free from unchanged (2) but containing side-products from
Method B. Tetrahydro epoxy dione (7) (13 mg, 0.56 mmol) and
anhydrous sodium acetate (34 mg, 0.41 mmol) in glacial acetic acid (5
ml) were refluxed under a nitrogen atmosphere for 1 h.After cooling the
solution was poured into water (50 ml) and the aqueous mixture
extracted with chloroform (50 ml). The organic extract was separated,
dried, and the solvent removed to afford anthracene-1,4-dione (8) that
was purified by vacuum sublimation, m.p. c. 160° (sublim.), identical to
the sample from above.

1126
B. Halton, D. A. C. Evans and R. N. Warrener
4
5
Acknowledgments
Halton, B., Kay, A. J., Zhi-mei, Z., Boese, R., and Haumann, T., J.
Chem. Soc., Perkin Trans. 1, 1996, 1445; Halton, B., Kay, A. J., and
Zha, Z. M., J. Chem. Soc., Perkin Trans. 1, 1993, 2239.
Ando, S., Imamura, J., Tajitsu, M., and Saito, K., Heterocycles,
1998, 48, 1769; Laue, J., Seitz, G., and Wassmuth, H., Z.
Naturforsch., Teil B, 1996, 51, 348; Saracoglu, N., Durucasu, I., and
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N., and Balci, M., Tetrahedron Lett., 1991, 32, 7097; Neidlein, R.,
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Ms C. S. Jones (Victoria University) kindly provided
the sample of quinone (2) that enabled these studies to be
performed at the Centre for Molecular Architecture
(CMA) of Central Queensland University. A visiting pro-
fessorship (to B.H.) with financial assistance from the
CMA and Victoria University is gratefully acknowledged.
B.H. thanks the members of the CMA for their generous
and genial hospitality.
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