Fluoradenes via palladium-catalyzed intramolecular arylation

Article abstract of DOI:10.1039/c0cc05004k

A novel approach towards 7b-aryl-indeno[1,2,3-jk]fluorene based on a nitrogen-containing core is reported. The acid-promoted Friedel-Crafts reaction of 9-(2-bromophenyl)-9-fluorenol with carbazole, triphenylamine or triindole afforded 9-(2-bromophenyl)fluorenyl-carbazole, -triphenylamine and -triindole derivatives, which were subsequently converted to 7b-aryl-fluoradenes via palladium-catalyzed intramolecular C-H direct arylation as a key step.

Full text of DOI:10.1039/c0cc05004k

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Fluoradenes via palladium-catalyzed intramolecular arylationw
Lingling Rong, Qiancai Liu,* Yingbo Shi and Jie Tang
Received 17th November 2010, Accepted 10th December 2010
DOI: 10.1039/c0cc05004k
A
novel approach towards 7b-aryl-indeno[1,2,3-jk]fluorene
based on a nitrogen-containing core is reported. The acid-
promoted Friedel–Crafts reaction of 9-(2-bromophenyl)-9-fluor-
enol with carbazole, triphenylamine or triindole afforded
9-(2-bromophenyl)fluorenyl-carbazole, -triphenylamine and
-triindole derivatives, which were subsequently converted to
7b-aryl-fluoradenes via palladium-catalyzed intramolecular
C–H direct arylation as a key step.
Scheme
1 Palladium-catalyzed intramolecular arylation towards
arenes and hetero-arenes.
Highly strained aromatic molecules such as fenestranes are
theoretically interesting but experimentally challenging targets
for synthetic organic chemists. It is worthwhile pursuing
well-defined methods to synthesize highly strained molecules
especially those bearing benzo nuclei fused across the bridgehead
positions.1 Fluoradene, namely 7b-H-indeno-[1,2,3-jk]fluorene (a)],
is an example of highly acidic curved hydrocarbon with a
bridgehead carbon shared by an indane and a fluorene, which
can also be considered as a product of C–C bond opening of
aromatic rings from the unknown hydrocarbon C19H10 (b) and
is also a unique indenofluorene.2 Only a few reports on
fluoradene chemistry have been published because of their
difficult synthesis.3 The distinct endo and exo faces of a make it
a suitable ligand to coordinate with metal centers to form
sterically coordinated complexes.4 However, the chemistry
of 7b-substituted derivatives of fluoradene has seldom been
explored and only two examples, 7b-triisopropylsilylfluoradene
and 7b-(2,4-dinitrophenyl)fluoradene, have been reported, and
these reports lack synthetic details.5 Other aryl derivatives of
fluoradene such as phenylfluoradene (c) and its derivatives,
which could allow access to tetrabenzo[5.5.5.5]fenestrane (d),
remain unknown (Fig. 1).
Palladium-catalyzed direct C–H arylation of arenes is one of
the simplest and most efficient methods for the synthesis of
carbo- and hetero-cycles from aryl precursors (Scheme 1).6
Generally this reaction is carried out with a palladium catalyst
such as Pd(OAc)2 or Pd(PPh3)2Cl2 in polar solvents (DMF or
DMA) with a base to trap HX. Several groups have conducted
elegant studies on constructing interesting and/or strained
poly- and hetero-arenes by using this methodology.7
As a continuation of the construction of polyarenes via
palladium-catalyzed arylation,7q we investigated the arylation
of 9-(2-bromoaryl)-9-arylfluorene, because it might undergo
unconventional intramolecular arylation in the presence of
catalytic palladium species to afford benzoannelated products.
Considering the effect of substituents on the reactivity of
arenes, nitrogen-containing activating groups are always
ortho–para directors, so carbazole, triphenylamine, and triindole
derivatives were employed. Such derivatives have been
incorporated in fluorene-based derivatives to improve the hole
injection and transport abilities and the resulting OLEDs show
enhanced efficiency and stability.8 Here, we wish to demon-
strate the formation of structurally interesting compounds
using palladium-catalyzed arylation as a prelude to the
synthesis of more complex polycyclic aromatic hydrocarbons.
The successful application of palladium-catalyzed arylation
for the construction of highly strained 7b-arylfluoradene
derivatives is reported.
9-(2-Bromophenyl)-9-fluorenol (2) was chosen as a key
intermediate for the strategy described above because it has
been used as a subunit to synthesize heterocycle-fused dispiro
organic compounds.9 2 could easily be prepared by metal–
halogen exchange of 2-bromo-iodobenzene with iPrMgBr at
À40 1C or 1,2-dibromobenzene with BuLi at À120 1C and
subsequently quenched with fluorenone following literature
methods.10 The synthetic route towards different fluoradenes
is outlined in Scheme 2. Initially the reaction of 2 with different
arenes or aryl ether was performed under acidic conditions.
Fig. 1 Several strained polycyclic hydrocarbons.
Department of Chemistry, East China Normal University,
3663 Zhongshan Rd(N), Shanghai 200062, China.
E-mail: qcliu@chem.ecnu.edu.cn; Tel: +86 21 62233490
w Electronic supplementary information (ESI) available: Synthetic
procedures, characterizations, crystal data. CCDC 801484–801488.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c0cc05004k
c
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Table 1 Synthetic conditions used to produce fluoradenes
Entry Ar–H (1)
Product 3
Product 4
Scheme 2 Synthetic route to substituted fluoradenes.
1
However, in most cases, it failed to afford target bromide,
9-(2-bromophenyl)fluorene was isolated instead. An acid-
promoted Friedel–Crafts reaction10 was then employed
to react compound
2 with triphenylamine, carbazole,
9-hexylcarbazole, and 5,10,15-trihexyltriindole to afford
3a–3d successfully. The initial reaction was conducted in a
ratio of 1 : 2 (1a to 2); however, the resulting yield was low
(15%) and the solubility of 3a is limited. Ring closure was then
conducted using Pd(OAc)2/PPh3/BnEt3NCl/DMA/Na2CO3 to
form product 4a (55%) (Table 1, entry 1). Surprisingly, the
X-ray structure of 4a revealed that C–H activation occurred at
the C-1 position of the fluorenyl group. Although C–H
activation of fluorene to form six-membered carbocycles and
heterocycles has been reported several times,7k,l activation to
form highly strained five-membered carbocycles related to
aromatic compounds such as fluoradenes has never been
reported.
2
3
To improve solubility, N-hexylcarbazole (1b) was then
reacted with 2 in different ratios; monobromide 3ba and
dibromide 3bb were isolated in moderate yield (51% and
43%, respectively) (entries 2 and 3). Ring closures of 3ba
and 3bb under the conditions described for 4a gave
compounds 4ba and 4bb in excellent yield (82% and 71%).
As for 1c, the desired bromide intermediates 3ca, 3cb and 3cc
were isolated with yields of 61%, 34% and 83%, respectively,
compounds 3ca and 3cb were successfully converted into 4ca
(37%) and 4cb (71%). However in the case of 3cc, an insoluble
material 4cc was obtained which might be crosslinked
compounds or polymers. The insolubility of 4cc prevented
its characterization (entry 6). Compound 4d was obtained by
reaction of 2 with 5,10,15-trihexyl-triindole (1d), followed by
ring closure of 3d under the conditions described above with
an overall yield of 38% for the two steps. Remarkably, the
isolated yields of the C–H arylation steps are good to excellent
(from 37% (4ca) to 83% (4d)). In the case of 4d, the three C–H
arylation steps occur with 94% each (entry 7).
4
5
6
4cc insoluble materials
The compounds described above were characterized by
UV-vis and NMR spectroscopy, fluorimetry, mass spectro-
metry, and elemental analysis. The 13C NMR reveal that the
chemical shifts of bridge-head carbon appear between 65 ppm
and 67 ppm. In addition, the structures of 4a, 4ba, 4bb, 4ca,
4cb were confirmed by X-ray structure determination (Fig. 2)
(see also ESIw). UV-Vis spectra of compounds 4a, 4ba, 4bb,
4ca, 4cb and 4d exhibited maxima between 270–340 nm, which
were assigned to p - p* transitions. The emission maxima of
4a, 4ba, 4bb, 4ca and 4cb in solution appeared in the visible
region at 388, 396, 446, 449 and 406 nm, respectively. In
contrast, 4d was not emissive, which might be caused by its
triindole core.
7
The decomposition temperatures (Td) with 5% weight loss
under N2 atmosphere of 4a, 4ba, 4bb, 4ca, 4cb and 4d ranged
from 359 1C (4ca) to 458 1C (4bb). The DSC curves of the
above compounds 4a, 4ba, 4bb, 4ca, 4cb and 4d showed
obviously crystalline melting temperatures ranging from
208 1C (4ba) to 358 1C (4a). However, no melting peak for
The thermal properties of these compounds were investigated
by thermal gravimetric analysis (TGA) and differential scanning
calorimetry (DSC). They all exhibited high thermal stabilities.
c
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indicates that 4d might have lower crystallized ability than 4a,
4ba, 4bb, 4ca and 4cb.
In conclusion, the first 7b-aryl-indeno[1,2,3-jk]fluorenes
containing carbazole, triphenylamine or triindole cores were
successfully synthesized. This research demonstrates that
palladium-catalyzed intramolecular C–H activation can be
applied as an efficient and straightforward synthetic method
for the formation of highly strained arenes using well-defined
synthetic routes. The reaction proceeds readily using aryl
bromides based on carbazole, triphenylamine and triindole
functionalities. The ready construction of fluoradene derivatives
should allow for the synthesis of more complex polycyclic
aromatic hydrocarbons with highly strained structures.
Studies are underway to establish the scope and generality of
this strategy for the synthesis of more complex products.
We thank Shanghai Natural Science Foundation (Grant
No. 09ZR1409400), Shanghai Committee of Science and
Technology of China (Grant No. 10520710100) and the
French Institute of ECNU for financial support.
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2155–2157 2157