Diels-Alder cycloaddition of acetylene gas to a polycyclic aromatic hydrocarbon bay region

Article abstract of DOI:10.1039/c2cc33885h

The direct conversion of a polycyclic aromatic hydrocarbon bay region to a new, unsubstituted benzene ring by Diels-Alder cycloaddition of acetylene gas is reported for the first time. At 140 °C in dimethylformamide, under 1.8 atm pressure of acetylene ga

Full text of DOI:10.1039/c2cc33885h

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Diels–Alder cycloaddition of acetylene gas to a polycyclic aromatic
hydrocarbon bay regionw
Eric H. Fort, Matthew S. Jeffreys and Lawrence T. Scott*
Received 30th May 2012, Accepted 19th June 2012
DOI: 10.1039/c2cc33885h
The direct conversion of a polycyclic aromatic hydrocarbon bay
region to a new, unsubstituted benzene ring by Diels–Alder
cycloaddition of acetylene gas is reported for the first time. At
140 8C in dimethylformamide, under 1.8 atm pressure of acetylene
gas, 7,14-dimesitylbisanthene is slowly converted to 7,14-dimesityl-
benzo[ghi]bisanthene (21% conversion in 48 h).
years ago by Paquette and Magnus.6 Though significantly less
reactive than nitroethylene,4b phenyl vinyl sulfoxide does
transform 1 into 7,14-dimesitylbenzo[ghi]bisanthene (2) when
the two reaction partners are heated together without solvent.4b
In agreement with theory,3 benzannulation at the second bay
region occurs significantly more slowly, producing 7,14-dimesityl-
ovalene (3) in 17% yield only after heating at 155 1C without
solvent for 6 days, by which time much of the phenyl vinyl
sulfoxide polymerizes.4b
The large thermodynamic stabilization (‘‘aromaticity’’) of poly-
cyclic aromatic hydrocarbons provides them with a certain level of
protection from chemical reactions that would cause disruption,
even just temporarily, to the cyclic delocalization of electrons in
their conjugated p-systems.1 Diels–Alder cycloadditions in the bay
region of phenanthrene, for example, are completely unknown,
even with extraordinarily potent dienophiles.2 As we have
shown, however, Diels–Alder cycloadditions become progres-
sively easier in the bay regions of larger polycyclic aromatic
hydrocarbons, wherein progressively greater fractions of the
aromaticity are preserved (Fig. 1).3 Recognition of this
trend stimulated us to propose a Diels–Alder cycloaddition/
rearomatization strategy for growing uniform carbon nanotubes
from hemispherical polyarenes, aromatic belts, and other small
hydrocarbon templates.3,4
Molecular orbital calculations (B3LYP/6-31G*) predict
that the dienophilicity of phenyl vinyl sulfoxide ought to be
improved by replacement of the phenyl group with C6F5
or CF3 or by substitution of selenium for sulphur,7 but nitro-
ethylene emerged as the strongest dienophile of those that we
thought might serve our needs as a masked acetylene. In
the course of these computational studies, we were intrigued
to discover that acetylene itself is predicted to be almost
as reactive as phenyl vinyl sulfoxide for Diels–Alder cyclo-
addition to the bay region of bisanthene (DEact = 24.3 and
23.3 kcal molÀ1, respectively). Could acetylene possibly add in
Acetylene, HCRCH, would be the ideal carbon feedstock to
use for elongating hydrogen-terminated nanotubes and templates,
because it is cheap, and the only by-product would be hydrogen
gas, a desirable commodity in its own right. The poor reputation
of acetylene as a dienophile for Diels–Alder cycloadditions,5
however, caused us to search for a more powerful dienophile that
could ultimately yield the same product as a formal acetylene
cycloaddition/rearomatization and could thereby serve as a
‘‘masked acetylene’’. This search led us to nitroethylene, which
nicely transforms both bay regions of 7,14-dimesitylbisanthene (1)
into new, unsubstituted benzene rings, presumably by way of the
intermediates illustrated in Fig. 2.4b The bay regions of 1 represent
good models, in our opinion, for bay regions on the rims of all but
the smallest diameter hydrogen-terminated carbon nanotubes.3,4b,d
Before experimenting with nitroethylene, we first examined
phenyl vinyl sulfoxide, a masked acetylene introduced many
Merkert Chemistry Center, Department of Chemistry, Boston College,
Chestnut Hill, MA 02467-3860, USA. E-mail: lawrence.scott@bc.edu;
Fax: 617-552-6454; Tel: 617-552-8024
w Electronic supplementary information (ESI) available: Experimental
and computational details. See DOI: 10.1039/c2cc33885h
Fig. 1 Calculated activation energies (B3LYP/6-31G*) for Diels–
Alder cycloadditions of acetylene to the bay regions of progressively
larger polycyclic aromatic hydrocarbons. Bold lines indicate the aromatic
portions of the molecules.
c
8102 Chem. Commun., 2012, 48, 8102–8104
This journal is The Royal Society of Chemistry 2012

Fig. 3 Converting an aromatic hydrocarbon bay region into a new
benzene ring by Diels–Alder cycloaddition of acetylene and thermal
rearomatization.
deterioration of the O-rings as a result of their prolonged
exposure to hot DMF vapours, but this is a minor technical
problem.10
In none of our experiments with acetylene as the dienophile
did we ever detect any of the 2-fold Diels–Alder cycloaddition
product 3. Even phenyl vinyl sulfoxide is barely able to effect
the second, more difficult, benzannulation (2 - 3), and the
dienophilicity of acetylene seems to be slightly lower than that
of phenyl vinyl sulfoxide, as predicted.
Acetylene should not be dismissed as a hopeless dienophile.
The preliminary experiments reported here reveal that Diels–
Alder cycloadditions of acetylene can be extended even
to the bay region of a polycyclic aromatic hydrocarbon
(Fig. 3). Pushing the reaction to high conversion is a chemical
engineering problem that has not yet been undertaken, but it
should be solvable. Our results offer hope that acetylene itself
may one day be used as the sole carbon source with which to
grow carbon nanotubes from small hydrocarbon templates
under metal-free, reagent-free (possibly even solvent-free11)
conditions.
Fig. 2 Converting aromatic hydrocarbon bay regions into new
benzene rings using nitroethylene as a ‘‘masked acetylene’’ for the
Diels–Alder cycloaddition/rearomatization strategy. Mes = mesityl
(2,4,6-trimethylphenyl).
Diels–Alder fashion to the bay region of a polycyclic aromatic
hydrocarbon?
Our experiments were all performed in a glass-lined steel
reactor that permits real-time control of temperature and the
acetylene gas pressure. For safety reasons,8 the acetylene gas
pressure was held constant at 1.8 atm for the duration of every
reaction. To our delight, small but measurable amounts of
7,14-dimesitylbenzo[ghi]bisanthene (2) were produced when
the reaction was run in toluene at 105 1C for 24 h. Integration
of the NMR spectrum of the material recovered from the
reaction revealed a 6 : 94 ratio of Diels–Alder product (2) to
unchanged starting material (1), i.e., a 6.0% conversion of
1 to 2. Running the reaction at 180 1C in ortho-dichloroben-
zene for 24 h increased the conversion to 8.5%.
We thank Prof. Kian Tan and Dr. Thomas Lightburn for
access to and assistance with their controlled pressure reactor.
This research was supported by the National Science Foundation
and the Department of Energy.
Notes and references
1 R. G. Harvey, Polycyclic Aromatic Hydrocarbons, Wiley-VCH,
New York, N.Y., 1997.
2 S. R. Wang and Z. Xie, Organometallics, 2012, 31, 3316–3323.
3 E. H. Fort, P. M. Donovan and L. T. Scott, J. Am. Chem. Soc.,
2009, 131, 16006–16007.
These results prove unequivocally that acetylene is capable
of adding in Diels–Alder fashion to the bay region of a
polycyclic aromatic hydrocarbon, albeit quite slowly under
our reaction conditions. In an effort to speed up the reaction
by increasing the concentration of acetylene dissolved in the
reaction medium, we turned to N,N-dimethylformamide
(DMF), a solvent in which the hydrogen bonding capability
of acetylene greatly increases its solubility.9 After 48 h at
140 1C in DMF, a 21% conversion of 1 to 2 was achieved
(Fig. 3). Attempts to push the conversion higher by running
the same reaction for seven days were complicated by
4 (a) T. J. Hill, R. K. Hughes and L. T. Scott, Tetrahedron, 2008, 64,
11360–11369; (b) E. H. Fort and L. T. Scott, Angew. Chem., Int.
Ed., 2010, 49, 6626–6628; (c) L. T. Scott, Polycyclic Aromat.
Compd., 2010, 30, 247–259; (d) E. H. Fort and L. T. Scott,
J. Mater. Chem., 2011, 21, 1373–1381; (e) A. P. Belanger,
K. A. Mirica, J. Mack and L. T. Scott, in Fragments of
Fullerenes and Carbon Nanotubes: Designed Synthesis, Unusual
Reactions, and Coordination Chemistry, Wiley, Hoboken, 2012,
pp. 235–258; (f) L. T. Scott, E. A. Jackson, Q. Zhang,
B. D. Steinberg, M. Bancu and B. Li, J. Am. Chem. Soc., 2012,
134, 107–110.
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Chem. Commun., 2012, 48, 8102–8104 8103

5 R. Walsh and J. M. Wells, Int. J. Chem. Kinet., 1975, 7, 319–329.
6 L. A. Paquette, R. E. Morck, B. Harirchian and P. D. Magnus,
¨
J. Am. Chem. Soc., 1978, 100, 1597–1599.
7 (a) E. H. Fort, Ph.D. dissertation, Boston College, Chestnut Hill,
Massachusetts, 2010; (b) J. Moise, R. Goumont, E. Magnier and
C. Wakselman, Synthesis, 2004, 2297, 2302.
8 (a) C. M. Detz and H. B. Sargent, in Kirk-Othmer Encycl. Chem.
Technol, Wiley, New York, NY, 3rd edn, 1978, pp. 192–203;
(b) M. E. Chiddix, Ann. N. Y. Acad. Sci., 1967, 145, 150–160.
9 A. C. McKinnis, Ind. Eng. Chem., 1955, 47, 850–853.
10 Details in the supplementary information.
11 E. H. Fort and L. T. Scott, Tetrahedron Lett., 2011, 52, 2051–2053.
c
8104 Chem. Commun., 2012, 48, 8102–8104
This journal is The Royal Society of Chemistry 2012