H. Y. Cho, L.T. Scott / Tetrahedron Letters 56 (2015) 3458–3462
3459
is molybdenum (V). The Waldvogel group has demonstrated that
an intramolecular Scholl reaction can be promoted by molybde-
num (V) chloride to afford a desired eight-membered ring system
commonly used Scholl oxidation conditions; however, no alkenes
had ever been cyclotrimerized. Considering our projects in polycy-
clic aromatic hydrocarbon (PAH) synthesis, we were particularly
interested in cyclic alkenes, such as acenaphthylenes.
1
0
in 89% yield.
Scholl-type oxidation reactions involving antimony (V) have
been demonstrated by several research groups. The Kochi group
has been particularly interested in a masked form of SbCl as trial-
5
The initial experiments were carried out with 1 equiv of 4,7-di-
tert-butylacenaphthylene (1), 3 equiv of DDQ, and 3 equiv (1.4% v/
v) of trifluoromethanesulfonic acid in dichloromethane at ambient
temperature. To our great pleasure, the desired product
(2,5,8,11,14,17-hexa-tert-butyldecacyclene, 2) was afforded from
4,7-di-tert-butylacenaphthylene (1) under the oxidation conditions
in good yield (48% yield, Scheme 3). The product was soluble
enough in common organic solvents to be purified and to be fully
characterized.
kyloxonium hexachloroantimonate (V) salts. In the presence of this
antimony (V) reagent, 1-methoxynaphthalene is readily oxidized
to the corresponding dehydrogenated dimer in quantitative
yield.1 It has been observed that a thallium (III) reagent can like-
wise transform a variety of aromatic compounds into dehydroge-
nated coupling products. McKillop et al. have demonstrated that
1
1
,3-bis(3,4-dimethoxyphenyl)propane can be converted into the
The effects of the concentration of the substrate and the
reagents on the efficiency of the reaction were examined. When
3 equiv of DDQ and 3 equiv (1.4% v/v) of triflic acid were employed,
the reaction having 0.02 M concentration (with regard to the sub-
strate) afforded the desired product in 48% yield. When a more
concentrated condition (0.10 M) was used, the efficiency of the
reaction did not change significantly, giving the trimer in 40% yield.
However, the yield of the reaction was more dependent on the con-
centration of the reagents (both the oxidant and the acid). When
1 equiv of DDQ and 1 equiv (0.46% v/v) of triflic acid were
employed, the reaction with 0.02 M substrate concentration fur-
nished the product in an improved yield of 68%. The cyclized tri-
meric product was not obtained, however, when 7 equiv of DDQ
and triflic acid were employed in a 0.02 M solution of 1. To the best
of our knowledge, this is the first example of an intermolecular oxi-
dative cyclotrimerization of an alkene to form a triply fused ben-
zene ring.
corresponding bridged biphenyl in the presence of thallium (III)
trifluoroacetate (TTFA) in trifluoroacetic acid (81% yield).
1
2
Scholl-type coupling with DDQ/acid
Intramolecular oxidative CÀC bond forming reactions involving
non-metallic reagents have been demonstrated by the Rathore
1
3
group. In this process, the DDQ/methanesulfonic acid system
promotes six new intramolecular carbonÀcarbon bond formations,
leading to a hexa-peri-hexabenzocoronene (HBC) in excellent yield.
In addition, Chen and Liu demonstrated that a dibenzochrysene
derivative can be transformed to a planar polycyclic aromatic
hydrocarbon. In their system, a dibenzochrysene derivative pos-
sesses two non-planar fjord regions, which are transformed to
closed ring systems under the DDQ/methanesulfonic acid system
in 91% yield.
1
4
A recent report by the Johnson group demonstrated that qua-
terrylene can be prepared by a Scholl-type oxidative dimerization
using triflic acid and DDQ or molecular oxygen.15 The authors
reported an efficient synthetic route to quaterrylene from perylene
utilizing DDQ/acid as well as a solution-phase H NMR spectrum of
the quaterrylene dication.
Conventional routes to triphenylene cores
Conjugated materials have attracted significant interest in
material sciences owing to their utilities in various types of
1
1
7
devices. Throughout the history of organic semiconductors and
electronics, conjugated polymers have been the main types of the
materials in those fields. Among these, star-shaped oligomers bear-
ing triphenylene cores are the most relevant to our studies; thus,
they will be briefly introduced in this section.
Oxidative cyclotrimerization of an alkene
During our investigations on circumtrindene, we have con-
ducted triple aldol condensation reactions to obtain the FVP pre-
cursors to circumtrindene (Scheme 2).16 These reactions usually
require harsh reaction conditions and are conducted in high-
boiling organic solvents. Moreover, the precursors to the aldol con-
densation usually require multiple synthetic steps to install the
ketone functionality. So far, very few synthetic methods to access
the carbon skeletons (i.e., triply-annulated central benzene rings)
that are required for circumtrindene precursors have been
reported. In this regard, we were interested in developing a new
methodology to construct a central benzene ring by intermolecular
trimerization of simple hydrocarbons.
We envisioned that alkenes might be trimerized to give a cen-
tral benzene system under suitable oxidative reaction conditions.
Notably, this method would eliminate the necessity to functional-
ize the substrate (e.g., preparing ketones); simple hydrocarbons
would be directly oxidized to form new CÀC bonds. Under the
One of the classic methods for the synthesis of triphenylene is
the oxidative cyclotrimerization of benzene. Several oxidants can
be employed for this process; iron (III) chloride is one of the most
1
8
commonly used (Scheme 4). In addition, molybdenum (V) chlo-
1
9
20
ride or vanadium (V) chloride can also participate in this pro-
cess as an oxidant. Furthermore, Sato et al. demonstrated that
photocyclization (with UV light) of ortho-terphenyl can afford tri-
2
1
phenylene in the presence of iodine.
Transition-metal catalyzed reactions also have been developed
to prepare the triphenylene skeleton from benzyne. Pérez and
Guitián have shown that triphenylene can be synthesized by the
cyclotrimerization of benzyne generated from 2-(trimethylsilyl)
DDQ
TfOH
O
DCM, rt
Lewis acid
1
2
Scheme 2. Triple aldol condensation.
Scheme 3. Oxidative cyclization of an acenaphthylene.