Pyrolysis of a tubular aromatic compound

Article abstract of DOI:10.1021/ol034352x

(Matrix presented) In the attempt to close the walls of a small tubular system that is a substructure of a [4,4] armchair nanotube a very unusual rearrangement reaction was observed.

Full text of DOI:10.1021/ol034352x

ORGANIC
LETTERS
2003
Vol. 5, No. 8
1269-1271
Pyrolysis of a Tubular Aromatic
Compound
M. Deichmann, C. Na1ther, and Rainer Herges*
Institut fuer Organische Chemie, UniVersitaet Kiel, Otto-Hahn-Platz 4,
D-24098 Kiel, Germany
rherges@oc.uni-kiel.de
Received February 27, 2003
ABSTRACT
In the attempt to close the walls of a small tubular system that is a substructure of a [4,4] armchair nanotube a very unusual rearrangement
reaction was observed.
1
After the recent achievement of the rational synthesis of C60
the conventional synthesis of carbon nanotubes or short
pieces thereof remains as one of the most interesting targets
in nonnatural product synthesis. Aiming at this target we
pursue a strategy of ring-enlargement metathesis and sub-
sequent dehydrocyclization.2 The first step of this strategy
was realized by metathetic dimerization of tetradehydrodi-
anthracene to the cyclic tetraanthraceneylidene (picotube) 1,
which is the first rationally synthesized fully conjugated
tubular aromatic compound. Eightfold cyclodehydration at
the ortho positions of the benzene rings should lead to the
complete closure of the tube walls and to the formation of a
short piece of a [4,4] armchair carbon nanotube.
increase of the strain energy the first step is endothermic by
only 8.5 kcal mol-1 and the enthalpy of formation of the
most stable isomers of the following steps is predicted to be
only 3.2, 3.4, and 1.4 kcal mol-1. After the fourth step
yielding a D2h symmetric compound the strain energy
increases dramatically to 60.2. The subsequent steps are
endothermic with 48.2, 43.0, and 38.2 kcal mol-1.
In preliminary studies we applied the frequently used
oxidation procedures for the condensation of aromatic
compounds3 in solution such as Scholl4 or Kovacic5 condi-
tions or irradiation in the presence of iodine,6 however, we
only obtained polymeric material. Flash vaccum pyrolysis
(FVP) induced dehydrocyclizations which are known to be
able to build up a considerable amount of strain, e.g. in the
synthesis of buckybowls,7 also did not lead to the expected
Semiempirical (AM1) calculations predicted the overall
elimination of eight hydrogen molecules to be endothermic
by 205.9 kcal mol-1, on average 25.7 kcal mol-1 for each
dehydrocyclization step. However, since there is a stepwise
(3) Review: (a) Hagen, S.; Hopf, H. Top. Curr. Chem. 1998, 196, 47-
82. (b) Balaban, A. T.; Nenitzescu In Friedel-Crafts and Related Reactions;
Olah, G. A., Ed.; Wiley-Interscience: New York, 1964; p 979.
(4) Scholl, R.; Meyer, K. Chem. Ber. 1934, 67, 1229-1238.
(5) (a) Kovacic, P.; Koch, F. W. J. Org. Chem. 1965, 30, 3176-3181.
(b) Vivekanantan, S. I.; Wehmeier, M.; Brand, J. D.; Keegstra, M. A.;
Mu¨llen, K. Angew. Chem. 1997, 109, 1676-1679.
(6) Mallory, F. B.; Butler, K. E.; Evans, A. C.; Brondyke, E. J.; Mallory,
C. W.; Yang, C.; Ellenstein, A. J. Am. Chem. Soc. 1997, 119, 2119-2124.
(7) E.g.: Scott, L. T.; Bratcher, M. S.; Hagen, S. J. Am. Chem. Soc.
1996, 118, 8743-8744 (AM1: 47.0 kcal mol-1 per dehydrocyclization step).
(1) Scott, L. T.; Boorum, M. M.; McMahon, B. J.; Hagen, S.; Mack, J.;
Blank, J.; Wegner, H.; de Meijere, A. Science 2002, 295, 1500-1503.
(2) (a) Kammermeier, S.; Herges, R. Angew. Chem. 1996, 108, 470-
472; Angew. Chem., Int. Ed. Engl. 1996, 35, 417-419. (b) Kammermeier,
S.; Jones, P. G.; Herges, R. Angew. Chem. 1996, 108, 2834-2836; Angew.
Chem., Int. Ed. Engl. 1996, 35, 2669-2671. (c) Kammermeier, S.; Jones,
P. G.; Herges, R. Angew. Chem. 1997, 109, 2317; Angew. Chem., Int. Ed.
Engl. 1997, 36, 2200-2202. (d) Kammermeier, S.; Jones, P. G.; Herges,
R. Angew. Chem. 1997, 109, 1825-1828; Angew. Chem., Int. Ed. Engl.
1997, 36, 1757-1760.
10.1021/ol034352x CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/28/2003

dehydrocyclization but yielded an interesting and completely
unexpected compound.
hybridized, but at least 4 bonds must have been made and
broken to form the complicated polycyclic ring system
including the seven-membered ring. According to DFT
calculations (B3LYP/6-31G*) the pyrolysis product is 52
kcal/mol (!) more stable than the picotube. We propose the
following mechanism for the radical isomerization (Scheme
2).
The FVP experiments were performed at 800 °C (quartz
tube, diameter 20 mm, heated length 30 cm, 15 mL Argon
min-1, 2.5 mbar). The overall conversion of the starting
material was 40%. At higher temperatures the material
decomposed to amorphous carbon at the hot quartz surface
and a large number of isomers were formed which were
difficult to separate. At 800 °C anthracene, bianthryl,8
tetraanthryl,9 and an unsymmetrical main product10 were
isolated (Scheme 1).11 Mass spectrometry proved that the
Scheme 2. Proposed Mechanism of the Isomerization
Scheme 1. Reaction Products of the Flash Pyrolysis of the
Picotube
latter was an isomer of the starting material. The structural
assigment finally was achieved by X-ray analysis (Figure
1).
There is still a beltlike 16-electron periphery as in the
starting compound and all carbon atoms are still sp2
A remarkable feature in the 1H NMR spectrum is the signal
at 4.65 ppm. According to an analysis of the NMR spectrum
the signal can be assigned to an ortho proton of the phenyl
group, which is located within the shielding region above
the center of a benzene ring on the opposite side of the
beltlike structure (Figure 2). The phenyl ring is forced to
Figure 1. Ortep plot of the crystal structure of the cyclic pyrolysis
Figure 2. Distances of the o-hydrogen atom of the phenyl group
product.
from the carbon atoms of the opposing benzene ring (in Å).
1270
Org. Lett., Vol. 5, No. 8, 2003

rotate to this sterically and electronically unfavorable position
by the steric interaction with the neighboring suberene unit.
Acknowledgment. We thank the Deutsche Forschungs-
gemeinschaft for support.
Supporting Information Available: X-ray structure data
in CIF format. This material is available free of charge via
the Internet at http://pubs.acs.org.
1
(8) Anthracene and bianthryl were identified by comparison of H and
13C spectra with commercially available and independently synthesized
samples: Liebermann, C.; Gimbel A., Chem. Ber. 1887, 20, 1855.
(9) Tetranthryl was independently synthesized from bromoanthracene,
dibromobianthryl, and butyllithium: M. Deichmann, R. Herges, unpublished.
Because of the low solubility tetraanthryl could only be identified by UV
and MS. The identity with the pyrolysis product was checked by UV, MS,
and co-injection to the HPLC.
OL034352X
(11) 250-mg (355 µmol) picotubes were used for pyrolysis in each run.
The recovery was 90%. 40% (142 µmol) thereof was starting material, 5%
(12 mg, 17 µmol) cyclic pyrolysis product, 8 mg of anthracene, 12 mg of
bianthryl, and about 3 mg of tetraanthryl were isolated.
(10) Elementary analysis: calcd C 95.5, H 4.5; exptl C 95.4, H 4.6.
Org. Lett., Vol. 5, No. 8, 2003
1271