Benzo[a]acecorannulene: Surprising formation of a new bowl-shaped aromatic hydrocarbon from an attempted synthesis of 1,2-diazadibenzo[d,m]corannulene

Article abstract of DOI:10.1021/ol061554v

Flash vacuum pyrolysis of 7,10-bis(2-bromophenyl)acenaphtho[1,2-d] pyridazine (C₂₆H₁₄Br₂N₂) has resulted in a surprising transformation, including dinitrogen loss, to give benzo[a]acecorannulene, a novel C₂

Full text of DOI:10.1021/ol061554v

ORGANIC
LETTERS
2006
Vol. 8, No. 23
5195-5198
Benzo[a]acecorannulene: Surprising
Formation of a New Bowl-Shaped
Aromatic Hydrocarbon from an
Attempted Synthesis of
1,2-Diazadibenzo[d,m]corannulene
Vikki M. Tsefrikas,† Steve Arns,‡ Patrick M. Merner,‡ C. Chad Warford,‡
Bradley L. Merner,‡ Lawrence T. Scott,*,† and Graham J. Bodwell*,‡
Department of Chemistry, Memorial UniVersity of Newfoundland,
St. John’s, Newfoundland, Canada AlB 3X7, and Department of Chemistry,
Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467-3860
gbodwell@mun.ca; lawrence.scott@bc.edu
Received June 23, 2006
ABSTRACT
Flash vacuum pyrolysis of 7,10-bis(2-bromophenyl)acenaphtho[1,2-d]pyridazine (C26H14Br2N2) has resulted in a surprising transformation, including
dinitrogen loss, to give benzo[a]acecorannulene, a novel C26H12 bowl-shaped fullerene fragment.
Since the introduction of flash vacuum pyrolysis1 (FVP) as
a new strategy for the synthesis of corannulene in 1991,2
many geodesic polyarenes have been successfully synthe-
sized using this versatile method.3 Little success has been
achieved, however, with the inclusion of heteroatoms in these
fullerene fragments. The heterofullerene C59N has been
studied extensively,4 but smaller aza-buckybowls remain
completely unknown, except to theoreticians.5 The lone
heterobuckybowl that has been successfully prepared contains
sulfur atoms;6 no nitrogen-containing aromatic bowl has been
synthesized to date. Accordingly, we decided to examine
(3) (a) Scott, L. T.; Bronstein, H. E.; Preda, D. V.; Ansems, R. B. M.;
Bratcher, M. S.; Hagen, S. Pure Appl. Chem. 1999, 71, 209. (b) Scott, L.
T. Angew. Chem., Int. Ed. 2004, 43, 4994. (c) Tsefrikas, V. M.; Scott, L.
T. Chem. ReV., in press. (d) Rabideau, P. W.; Sygula, A.
Acc. Chem. Res. 1996, 29, 235. (e) Mehta, G.; Rao, H. S. P. Tetrahedron
1998, 54, 13325.
(4) (a) Hirsch, A.; Nuber, B. Acc. Chem. Res. 1999, 32, 795. (b)
Hummelen, J. C.; Bellavia-Lund, C.; Wudl, F. Top. Curr. Chem. 1999,
199, 93. (c) Nuber, B.; Hirsch, A. Chem. Commun. 1996, 1421. (d)
Hummelen, J. C.; Knight, B.; Pavlovich, J.; Gonzalez, R.; Wudl, F. Science
1995, 269, 1554.
(5) (a) Priyakumar, U. D.; Sastry, G. N. J. Mol. Graphics Modell. 2001,
19, 266. (b) Priyakumar, U. D.; Sastry, G. N. J. Org. Chem. 2001, 66,
6523. (c) Sastry, G. N.; Priyakumar, U. D. J. Chem. Soc., Perkin Trans. 2
2001, 30.
(6) Imamura, K.; Takimiya, K.; Otsubo, T.; Aso, Y. Chem. Commun.
1999, 1859.
† Boston College.
‡ Memorial University of Newfoundland.
(1) (a) Brown, R. F. C. Pyrolytic Methods in Organic Chemistry:
Application of Flow and Flash Vacuum Pyrolytic Techniques; Academic
Press: New York, 1980. (b) Brown, R. F. C. Pure Appl. Chem. 1990, 62,
1981. (c) McNab, H. Aldrichimica Acta 2004, 37, 19.
(2) (a) Scott, L. T.; Hashemi, M. M.; Meyer, D. T.; Warren, H. B. J.
Am. Chem. Soc. 1991, 113, 7082. (b) Scott, L. T.; Cheng, P.-C.; Hashemi,
M. M.; Bratcher, M. S.; Meyer, D. T.; Warren, H. B. J. Am. Chem. Soc.
1997, 119, 10963.
10.1021/ol061554v CCC: $33.50
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potential synthetic routes to the geodesic aza-polyarene 1,2-
unreacted 2, which suggests that 3 is stable under the reaction
diazadibenzo[d,m]corannulene (1).
conditions and simply sequesters the catalyst.
The choice of aza-bowl 1 as a synthetic target was strongly
influenced by the success of two prior syntheses of the
corresponding hydrocarbon, dibenzo[a,g]corannulene (eq 1).7
In 2000, Scott et al. reported that both flash vacuum pyrolysis
in the gas phase and palladium-catalyzed intramolecular
arylation reactions in solution can be used to synthesize the
bowl-shaped fullerene fragment.7
In light of the failure of the palladium-catalyzed intramo-
lecular arylation approach, flash vacuum pyrolysis of precur-
sor 2 was investigated as a means to effect homolytic
cleavage of the aryl-bromine bonds and bring about radical
cyclizations to give the desired aza-bowl (1). In the gas-
phase FVP synthesis of dibenzo[a,g]corannulene (eq 1),
pyrolysis of 7,10-bis(2-bromophenyl)fluoranthene at 1050
°C gives a 38% isolated yield of the C28H14 bowl.9 Consider-
ing the very harsh conditions, little hope was held for the
retention of nitrogen during this process. Not surprisingly,
exposure of precursor 2 to these conditions failed to give
any detectable quantities of aza-bowl 1. Interestingly,
however, a single, unexpected major product was formed
that could be isolated and characterized.
Analysis of the crude pyrolysate by mass spectrometry
revealed a molecular ion at m/z 324. This mass corresponds
to the molecular formula C26H12, which was confirmed by
HRMS and differs from the molecular formula of the desired
target compound (1, C26H12N2) by two nitrogen atoms.
Apparently, the harsh pyrolysis conditions not only induce
the loss of two HBr molecules from the starting material
but also cause dinitrogen expulsion to produce the hydro-
carbon, C26H12. Loss of N2 from pyridazines under pyrolysis
conditions has previously been observed on many occa-
sions.10
In light of this precedent, we have now synthesized the
nitrogen-containing compound 2, a potential precursor to 1,
which is analogous to the precursor used in the hydrocarbon
route (eq 1), but with aza functionality built into the core.
Details on the preparation of 2 are summarized at the
conclusion of this letter.
NMR spectroscopy provides all the additional information
necessary to assign the structure of the hydrocarbon product
as benzo[a]acecorannulene (4). The 1H NMR spectrum
(Figure 1) contains diagnostic signals for structural features
For the synthesis of many corannulene derivatives, solu-
tion-phase reactions represent useful alternatives to flash
vacuum pyrolysis.7,8 Despite these advances, however, all
attempts to close 7,10-bis(2-bromophenyl)acenaphtho[1,2-
d]pyridazine (2) by the palladium-catalyzed intramolecular
arylation method proved futile; unreacted starting material
was recovered in all cases.
It is not known with certainty why the nitrogen-containing
analogue (2) fails to react while its hydrocarbon counterpart
can be successfully cyclized in 55% isolated yield.7 One
plausible explanation is that donation of the nitrogen lone
pair into a vacant d orbital on the palladium locks the
conformation as shown in 3, thereby preventing the rotation
required for cyclization across the bay region. Increasing the
mole % of palladium catalyst leads to lower recovery of
Figure 1. 1H NMR spectrum of benzo[a]acecorannulene (4).
(7) Reisch, H. A.; Bratcher, M. S.; Scott, L. T. Org. Lett. 2000, 2,
1427.
that have previously been identified in the closely related
bowl-shaped aromatic hydrocarbons acecorannulene (5)11 and
benzo[a]corannulene (6).12
(8) (a) Seiders, T. J.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc.
1996, 118, 2754. (b) Sygula, A.; Rabideau, P. W. J. Am. Chem. Soc. 2000,
122, 6323.
5196
Org. Lett., Vol. 8, No. 23, 2006

Scheme 1
To begin with, the pair of narrow doublets appearing
upfield of chloroform at δ 6.56 and 6.51 (J ) 5.2 Hz)
signifies the presence of an unsaturated, external five-
membered ring; for comparison, the corresponding signal
in the 1H NMR spectrum of acecorannulene (5)11 appears at
δ 6.49 (s, 2H). The fact that two doublets are observed in
1
the H NMR spectrum of the isolated product, instead of a
singlet, further indicates a lack of symmetry. Furthermore,
the singlet at δ 7.53 appears in the same chemical shift range
as that for the singlet from the hydrogens R to the
five-membered ring on the core of acecorannulene (5, δ
7.38).11 Finally, the pair of two-hydrogen multiplets at δ 8.47
1
and 7.68 in the H NMR spectrum is similar in chemical
shift and fine structure to the AA′BB′ pattern (δ 8.68 and
7.76) in the spectrum of benzo[a]corannulene (6),12 suggest-
ing that the new C26H12 ring system has a benzo group
annulated to its core. On the basis of these spectral data, the
only plausible chemical structure for the major product
obtained from pyrolysis of 2 is benzo[a]acecorannulene (4).
Like acecorannulene (5), hydrocarbon 4 exhibits a high
sensitivity to alumina and silica gel, which imposes signifi-
cant losses of material with each chromatography. From the
1
material balance and the H NMR spectrum of the initial
pyrolysate, we estimate the actual yield of 4 to fall in the
range of 15-20%. Material purified to the level of that
shown in Figure 1 was obtained in low yield by semi-
preparative HPLC.
The 7,10-bis(2-bromophenyl)acenaphtho[1,2-d]pyridazine
(2) used for these experiments was synthesized using Boger’s
1,2,4,5-tetrazinef1,2-diazine strategy.16 This involved the
synthesis and inverse electron demand Diels-Alder (IEDDA)
reaction of the acenaphthenone-derived enamine 17 and 3,6-
bis(2-bromophenyl)-1,2,4,5-tetrazine (16) (Scheme 2).
The synthesis of 1,2,4,5-tetrazine 16 was achieved using
an approach originally developed by Stolle´.17 Thus, acylation
of acylhydrazine 10 with 2-bromobenzoyl chloride (11)
afforded diacylhydrazine 12. Reaction of 12 with PCl5 gave
a mixture of 1,3,4-oxadiazole 13 (31%) and bis(chloroimine)
14 (36%). The formation of two products is an interesting
contrast to analogous syntheses of diaryl-1,2,4,5-tetrazines
by Heuschmann, in which either the bis(chloroimine) or the
1,3,4-oxadiazole was obtained depending upon the steric
requirements of the aryl groups.18 This was expected to be
of little consequence in view of Heuschmann’s report that
Although the mechanism for formation of this unexpected
product is not immediately obvious, one possibility is
outlined in Scheme 1. This proposal suggests that the desired
aza-bowl (1) is, in fact, formed as the initial product but
that the molecule is too unstable at high temperatures to
survive.10 Cleavage of one aryl-nitrogen bond would
generate carbene 7, which could shuffle its carbon atoms in
a Jones rearrangement,13 via 8, to give the isomeric phenyl
carbene 9. A subsequent cyclization with loss of dinitrogen
would lead to hydrocarbon 10. From there, a well-prece-
dented five/six-ring swap rearrangement14 would separate the
five-membered rings from each other and ultimately produce
the stable product, benzo[a]acecorannulene (4). Variations
on this mechanism that involve earlier loss of dinitrogen
would also account for the observed results.15
(14) (a) Scott, L. T.; Roelofs, N. H. J. Am. Chem. Soc. 1987, 109, 5461.
(b) Scott, L. T.; Roelofs, N. H. Tetrahedron Lett. 1988, 29, 6857. (c) Scott,
L. T.; Hashemi, M. M.; Schultz, T. H.; Wallace, M. B. J. Am. Chem. Soc.
1991, 113, 9692. (d) Scott, L. T. Pure Appl. Chem. 1996, 68, 291. (e)
Necula, A.; Scott, L. T. J. Anal. Appl. Pyrolysis 2000, 54, 65.
(15) One such alternative has been suggested by a referee, and a slightly
modified version of this proposal is included in the Supporting Information.
We emphasize that both proposed mechanisms are speculative and that at
least one of them is incorrect.
(16) (a) Boger, D. L. J. Heterocycl. Chem. 1996, 33, 1519. (b) Boger,
D. L. Bull. Soc. Chim. Belg. 1990, 99, 599. (c) Boger, D. L.; Weinreb, S.
M. Hetero Diels-Alder Methodology in Organic Synthesis; Academic
Press: New York, 1989. (d) Boger, D. L. Chem. ReV. 1986, 86, 781. (e)
Boger, D. L. Tetrahedron 1983, 39, 2869.
(9) (a) Bratcher, M. S. Ph.D. dissertation, Boston College, 1996. (b)
Bratcher, M. S.; Scott, L. T. Abstracts of Papers, 207th National Meeting
of the American Chemical Society; American Chemical Society: Wash-
ington, D. C.: San Diego, CA, March 1994; abstr ORGN 420.
(10) See, for example: (a) MacBride, J. A. H. J. Chem. Soc., Chem.
Commun. 1972, 1219. (b) Barton, J. W.; Rowe, D. J. Tetrahedron Lett.
1983, 24, 299. (c) Wilcox, C. F., Jr.; Lassila, K. R.; Kang, S. J. Org. Chem.
1988, 53, 4333.
(11) Sygula, A.; Abdourazak, A. H.; Rabideau, P. W. J. Am. Chem. Soc.
1996, 118, 339.
(12) (a) Mehta, G.; Srirama Sarma, P. V. V. Chem. Commun. 2000, 19.
(b) Peng, L.; Scott, L. T. J. Am. Chem. Soc. 2005, 127, 16518.
(13) Gaspar, P. P.; Hsut, J.-P.; Chari, S.; Jones, M., Jr. Tetrahedron 1985,
41, 1479.
(17) Stolle´, R. J. Prakt. Chem. 1906, 73, 277.
(18) Hartmann, K.-P.; Heuschmann, M. Tetrahedron 2000, 56, 4213.
Org. Lett., Vol. 8, No. 23, 2006
5197

gases cleanly afforded 3,6-bis(2-bromophenyl)-1,2,4,5-tet-
razine (16).20 Enamine 17 was generated upon reaction of
acenaphthenone21 with 5 equiv of pyrrolidine and reacted
immediately without purification with tetrazine 16. This
delivered 7,10-bis(2-bromophenyl)acenaphtho[1,2-d]pyridazine
(2) in 71% yield. Gram quantities of 2 could be prepared
comfortably using this approach.
Scheme 2
Herein, we have reported the formation of a new geodesic
polyarene, benzo[a]acecorannulene (4), as the major isolable
product from flash vacuum pyrolysis of 7,10-bis(2-bro-
mophenyl)acenaphtho[1,2-d]pyridazine (2) at 1050 °C. The
desired aromatic aza-bowl, 1,2-diazadibenzo[d,m]corannu-
lene (1), could not be found among the products but may
have been an intermediate on the pathway leading to 4.
Attempts to effect the conversion of 2 to 4 by palladium-
catalyzed intramolecular arylation reactions in solution gave
only recovered starting material.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council (NSERC) of Canada and the
U.S. National Science Foundation for financial support of
this work. We are grateful to one of the reviewers for his/
her proposed mechanism for the formation of 4, a slightly
modified version of which appears in the Supporting
Information.
Supporting Information Available: Experimental pro-
1
cedures and characterization data for new compounds. H
and 13C NMR spectra for compounds, 2, 4, and 12-16.
UV-vis spectrum for compound 2. An alternative proposed
mechanism for the formation of 4. This material is available
free of charge via the Internet at http://pubs.acs.org.
both species react smoothly with hydrazine hydrate to afford
the corresponding dihydro-1,2,4,5-tetrazines. However, 1,3,4-
oxadiazole 13 was found to be unreactive and bis(chlor-
oimine) 14 gave only a modest yield of the desired
dihydrotetrazine 1519 (38%). Oxidation of 15 with nitrous
OL061554V
(20) Brooker, P. J.; Parsons, J. H.; Reid, J.; West, P. J. Pestic. Sci. 1987,
18, 179.
(21) Synthesized from 2-(naphthalen-1-yl)ethanoic acid by reaction with
PPA or by conversion to the acid chloride followed by intramolecular
Friedel-Crafts acylation.
(19) Only one dihydro-1,2,4,5-tetrazine is formed, but we could not
determine whether this was the 1,2- or the 1,4-dihydro isomer. Heuschmann
(ref 17) reported the formation of 1,2-dihydro-1,2,4,5-tetrazines in similar
systems.
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