Expeditious synthesis of contorted hexabenzocoronenes

Article abstract of DOI:10.1021/ol9001834

Contorted hexabenzocoronenes (HBCs) have been synthesized in an expedited manner utilizing a double Barton-Kellogg olefination reaction and a subsequent Scholl cyclization. The scope of both transformations was investigated using a series of pentacene qui

Full text of DOI:10.1021/ol9001834

ORGANIC
LETTERS
2009
Vol. 11, No. 11
2225-2228
Expeditious Synthesis of Contorted
Hexabenzocoronenes
Kyle N. Plunkett, Kamil Godula, Colin Nuckolls,* Noah Tremblay,
Adam C. Whalley, and Shengxiong Xiao
Department of Chemistry and The Center for Electronics of Molecular
Nanosctructures, Columbia UniVersity, New York, New York 10027
cn37@columbia.edu
Received February 17, 2009
ABSTRACT
Contorted hexabenzocoronenes (HBCs) have been synthesized in an expedited manner utilizing a double Barton-Kellogg olefination reaction
and a subsequent Scholl cyclization. The scope of both transformations was investigated using a series of pentacene quinones and double
olefin precursors. The utility of these reactions to help create functionalized and oligomeric HBCs in a rapid manner is demonstrated.
Here we detail a new methodology for the synthesis of contorted
hexabenzocoronenes (HBCs) that proceeds in high yield and
produces highly functionalized molecules capable of further
modification. The HBCs (1, Scheme 1) investigated here have
structures,1,2 they have unusual self-assembly/reactivity on
the surface of crystalline metals,3 and they are precursors
for the synthesis of hemispheric forms of carbon.4
Until now the syntheses of these HBCs have been limited
for two primary reasons. One, the final step in the synthesis
is a photocyclization/oxidation of a double olefin (2), shown
in Scheme 1. This reaction is intolerant of some functional
groups and also ineffective with substituents in the most
sterically hindered positions. Two, the double olefin is
laborious to form because it requires two sequential olefi-
nations starting from a thioketone (3). Here we reveal a
method that circumvents these problems by using previously
unknown dithioquinones, which are derived from pentacene
quinones, to rapidly build up the double olefin skeleton.
Scheme 1
.
Previously Employed Retrosynthetic Pathway to
HBC
(1) Xiao, S.; Tang, J.; Beetz, T.; Guo, X.; Tremblay, N.; Siegrist, T.;
Zhu, Y.; Steigerwald, M. L.; Nuckolls, C. J. Am. Chem. Soc. 2006, 128,
10700–10701
.
(2) Xiao, S.; Myers, M.; Miao, Q.; Sanaur, S.; Pang, K.; Steigerwald,
M. L.; Nuckolls, C. Angew. Chem., Int. Ed. 2005, 44, 7390–7394
(3) Rim, K. T.; Siaj, M.; Xiao, S.; Myers, M.; Carpentier, V. D.; Liu,
L.; Su, C.; Steigerwald, M. L.; Hybertsen, M. S.; McBreen, P. H.; Flynn,
G. W.; Nuckolls, C. Angew. Chem., Int. Ed. 2007, 46, 7891–7895.
(4) Whalley, A. C.; Plunkett, K. N.; Steigerwald, M. L.; Nuckolls, C.
Manuscript in preparation.
four carbons (red) annulated around the coronone core (black),
which produces a structure that is severely distorted from
planarity. We have previously shown that these HBCs are useful
in several respects: they behave as semiconductors with
relatively high hole mobility when assembled into columnar
.
10.1021/ol9001834 CCC: $40.75
Published on Web 04/24/2009
 2009 American Chemical Society

Scheme 2. General Strategy for Barton-Kellogg Modification of Pentacene Quinones
Furthermore, we have employed a Scholl reaction to supple-
ment, and in some cases completely eliminate, the need for
the photocyclization. These findings are significant because
they allow quick access to HBC structures that can be further
elaborated into oligomers and other useful structures.
The Barton-Kellogg reaction is a powerful tool for creating
tetra-substituted alkenes by reaction of a thioketone with a
diazomethane and subsequent reduction with a triarylphos-
phine.5-7 To our knowledge, there are no previous examples
of a double Barton-Kellogg on a dithioquinone substrate.
This is presumably because the sulfur analogs of benzo-
quinone,8 naphthoquinone,8 and anthraquinone9 are unstable
and form polymeric species through disulfide formation.
Remarkably, we could form dithioquinones (5a-e) from
pentacene quinones (4a-e) via reaction with Lawesson’s
reagent in toluene at 80 °C (Scheme 2). Unfortunately, we were
unable to isolate the dithioquinones in pure form and therefore
opted to prepare them in situ and react them directly with two
molar equivalents of the diphenyldiazomethane derived from
hydrazones 6a-d to produce a double thioepoxide. Reduction
of the thioepoxide functionality with triphenylphosphine pro-
duced the desired double olefins (7a-g).
as symmetrically brominated pentacene quinone derivatives 9
and 10, were found to be too insoluble to be functionalized
with Lawesson’s reagent and therefore were not able to undergo
the Barton-Kellogg reactions. To combat the insolubility, we
installed dodecyloxy chains on the pentacene quinone core to
yield 4a and 4b. Both compounds were soluble during the
reaction with Lawesson’s reagent; however, only minor amounts
of the bis-thioepoxide products were obtained (<5%). The
majority of the mass balance was intractable material that was
not characterizable. The low yields may be due to the electron-
rich pentacene quinone core that could stimulate poly disulfide
formation similar to what has been observed with other
quinones.9 When bromine groups were added to dodecyloxy-
containing pentacene quinone cores (4c and 4d), yields rose to
as much as 60% for the double olefins 7a, 7b, 7d, and 7e. (Table
1). This yield refers to the isolated yield of the double olefin
Table 1. Double Olefin Yieldsa
double olefin
% yield
7a
7b
7c
7d
7e
7f
45
57
38
32
29
36
45
The proper choice of starting pentacene quinone derivative
was crucial for the success of the double Barton-Kellogg
olefination due to the combination of the starting materials’
solubility and electronic structure. These parameters are sum-
marized in Figure 1. The parent pentacene-6,13-dione 8, as well
7g
a Structures shown in Figure 2.
from the Barton-Kellogg three-step process in Scheme 2. We
suggest that the bromines deactivate the pentacene quinone core
and stabilize the dithioquinone toward side reactions.
Desymmetrized 2-bromopentacene-6,13-dione (4e) was
appreciably more soluble than the symmetric analogs 8, 9,
and 10 and could be successfully modified with Lawesson’s
reagent without the requirement for solubilizing chains.
Reactions ran smoothly with all diphenyldiazomethanes
tested to give yields of ∼40% for 7c, 7f, and 7g (Table 1).
In general, a successfully designed pentacene quinone
precursor incorporates either solubilizing substituents that
are balanced with withdrawing groups or substituents that
desymmetrize the pentacene quinone to help solubilize them.
Figure 1. Pentacene quinones that were subjected to Barton-Kellogg
reaction conditions.
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Org. Lett., Vol. 11, No. 11, 2009

With several double olefin analogs in hand, we investigated
methods to oxidatively close the HBC core. In our previous
studies, we utilized a Katz-modified Mallory photocyclization
of the double olefins to afford HBC analogs.10,11 This
reaction is problematic because some substrates do not fully
close. Moreover, some functionalities are incompatible with
the photocyclization conditions. As an example, the resulting
mixture obtained from the photocyclization of 7e contained
around 50% HBC 11 (determined by TLC) and 50% of its
half-cyclized analogs, even after 18 h of exposure time
(Scheme 3). We found that the addition of FeCl3 converted
is the first instance where the Scholl reaction has been
employed to close these contorted HBCs.14
Our attempts at complete closure of double olefin 7e with
the Scholl reaction alone were unsuccessful giving back
starting material and some uncharacterizable byproducts after
quenching with methanol. This result suggests that the Scholl
reaction may only work for systems that have already been
partially closed. We screened several other double olefins
to test this assumption. While 7d reacted in a manner similar
to 7e (i.e., undergoing the Scholl reaction only after half-
cyclization via the Katz-modified Mallory photocyclization),
7a was remarkably different. 7a reacted with FeCl3 for 30
min to give half-cyclized 12 after quenching with methanol
(Scheme 4a). The fully closed HBC 13 was only obtained
Scheme 3. Combining a Photocyclization and Scholl Reaction
Scheme 4. HBC Closures Solely with Scholl Reactions
this mixture to the fully closed HBC in less than 20 min.
This result builds off of the vast body of research that uses
the Scholl reaction to close flat aromatic systems.12,13 This
after further subjecting 12 to FeCl3 overnight. Interestingly,
when the initial Scholl reaction of double olefin 7a was
prolonged from 30 min to overnight, only a small amount
of the fully closed HBC 13 was observed. This suggests that
a stable radical cation is formed on the half-cyclized HBC
and may require quenching before the second half of the
HBC can be oxidatively closed to form HBC 13.15 Similar
reactivity was seen for double olefin 7b, with half-cyclized
HBC being initially isolated followed by overnight oxidation
to give the fully closed HBC. Double olefins based on
2-bromopentacene-6,13-dione (4e) were also tested for
compatibility with the Scholl reaction. Amazingly, 7c
undergoes complete ring closure to the HBC within 20 min
with FeCl3 (Scheme 4b). We assumed this increased reactiv-
ity was due to the presence of alkoxy groups para to the
ring closure sites, which have been shown in other systems
to accelerate ring closures.16 We therefore prepared double
olefins with alkoxy groups meta (7f) and para (7g) to the
closing bonds. Although we were not surprised that the meta
alkoxy-containing 7f did not undergo the Scholl reaction,
we were amazed that the para alkoxy-containing 7g did not
lead to closed HBC either. We can only assume double olefin
7c provides an optimal amount of directing ability and
electron density to complete the Scholl reaction. Through
these initial studies, there appears to be no hard set rules for
the success of the Scholl reaction in taking a double olefin
Figure 2. Double olefins that were successfully (top) and unsuc-
cessfully (bottom) closed via the Scholl reaction with FeCl3. Note:
Double olefins 7d, 7e, and 7f could be successfully closed by
combining the Katz-modified Mallory photocyclization and Scholl
reaction. Double olefin 7g led to an inseparable mixture of isomers
during photocyclization. Compound 7c can be fully closed using
photocyclization alone. Compounds 7a and 7b are half-closed
during photocyclization.
Org. Lett., Vol. 11, No. 11, 2009
2227

to its final closed state. However, the Scholl reaction in
conjunction with the Katz-modified Mallory photocyclization
is an effective scheme for a higher yielding synthesis of
contorted and highly functionalized HBCs.
The new expeditious synthesis of HBC derivatives allowed
for the rapid construction of higher ordered electronic
structures such as HBC oligomers (Scheme 5). Yamamoto
In summary, this study details an expedited, versatile
synthesis of several HBC analogs. First, we utilize soluble
and stable pentacene bis-thioquinone derivatives to prepare
double olefin precursors. Second, we reveal the feasibility
of the Scholl reaction to help form many of the contorted
HBC derivatives. The HBC derivatives included in this
publication have high-yielding syntheses and are suitable for
fuctionalization through metal-catalyzed reactions to afford
many potentially useful materials including oligomeric
species.
Scheme 5. Oligomerization of HBC
Acknowledgment. We acknowledge financial support
from the Nanoscale Science and Engineering Initiative of
the National Science Foundation under NSF Award Number
CHE-0641523 and by the New York State Office of Science,
Technology, and Academic Research (NYSTAR). We ac-
knowledge support from the Chemical Sciences, Geosciences
and Biosciences Division, Office of Basic Energy Sciences,
US DOE (#DE-FG02-01ER15264).
Supporting Information Available: Detailed experimen-
tal procedures and spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
OL9001834
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yield.17 To create even longer oligomers, an iridium-
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