Angewandte
Communications
Chemie
À
C C Coupling
Base-Selective Five- versus Six-Membered Ring Annulation in
Palladium-Catalyzed C–C Coupling Cascade Reactions: New Access to
Electron-Poor Polycyclic Aromatic Dicarboximides
[5]
À
Abstract: Palladium-catalyzed base-selective annulation of
dibromonaphthalimide to different aryl boronate esters by
been invested to explore the direct arylation of C H bonds.
À
Not only intermolecular, but also intramolecular C H
À
combined Suzuki–Miyaura cross-coupling and direct C H
arylation reactions have been extensively studied to develop
new p-conjugated polycycles.[4c] For instance, such direct
arylation procedures have been used for the core extension of
electron-rich[6] as well as electron-poor p-conjugated systems
like rylene dicarboximides[1d] and bowl-shaped PAHs.[7]
arylation afforded a series of new five- and six-membered ring
annulated electron-poor polycyclic aromatic hydrocarbons.
À
Cesium carbonate (Cs2CO3) as auxiliary base in these C C
coupling cascade reactions led exclusively to six-membered
ring annulation, while the use of organic base diazabicycloun-
decene (DBU) afforded the corresponding five-membered ring
annulated products. This base-dependent selective mode of
annulation is attributed to different mechanistic pathways
directed by the applied base. The selective annulation was
revealed by single crystal X-ray analysis of the respective five-
and six-membered ring annulated products. The optical and
redox properties of the new polycyclic aromatic dicarbox-
imides were characterized by UV/Vis absorption and fluores-
cence spectroscopy and cyclic voltammetry.
À
From the mechanistic perspective, the key C H bond
activation step has initially been discussed to proceed via
oxidative addition, s-bond metathesis or an electrophilic
activation depending on the nature of the substrate and
transition metal complex fragment.[8] However, in recent
years the mechanistic picture has become gradually more
complex as detailed experimental and computational studies
provided evidence for new metalation mechanisms, including
À
effects of bifunctional ligand systems and base-assisted C H
2
2
activations.[9] Numerous such base-assisted C(sp ) C(sp )
coupling reactions have been accomplished in the past using
organic bases like 1,8-diazabicyclo[5.4.0]undec-7-ene
À
A
romatic dicarboximides are an important class of com-
pounds, which inherit low lying frontier molecular orbitals
that are of significance for n-type semiconductors[1] and afford
materials of high resistance against thermal, chemical and
photooxidative degradation. Therefore, many dicarboximides
have been introduced as market products such as the
perinone and perylene bisimide color pigments.[2] However,
the number of innovative synthetic approaches toward
polycyclic aromatic dicarboximides (PADIs) is still limited,[3]
and most known compounds of this family were obtained by
conventional chemical methods that were introduced about
a century ago. This is rather surprising if we consider that
carbon-carbon coupling reactions of aromatic molecules have
been intensively investigated over the last decades and
broadly applied for the synthesis of diverse polycyclic
aromatic hydrocarbons (PAHs),[4] but have not been explored
as a method of general scope for PADIs. Besides the studies
on transition-metal-catalyzed cross-coupling reactions of two
activated aromatic components, initially developed by Stille,
Suzuki, Negishi, Hiyama and Kumada, much efforts have
(DBU),[6b,c,10] but inorganic carbonate bases have also been
[11]
À
applied for C H arylations, and the effect of carboxylate-
assisted metalation has recently been discussed.[12]
Despite this enormous development in the field of metal-
catalyzed base-assisted C H arylation, there is apparently no
À
À
report in the literature where the reaction mode of C C
coupling of individual coupling systems has been modulated
by the applied auxiliary base to achieve selectively either six-
or five-membered ring annulation.[4c] We have recently
reported that cesium carbonate (Cs2CO3) assisted palladium-
À
(Pd)-catalyzed C C coupling cascade reactions of pyrene
boronate esters with peri-dibromonaphthalimide led to the
formation of novel electron-poor PAHs by six-membered ring
annulation.[13] Here we report for the first time that the mode
À
of ring annulation in Pd-catalyzed C C coupling cascade
reactions of dibromonaphthalimide with different aryl bor-
onic acid pinacol esters can be directed by variation of the
base to achieve predominantly either six- or five-membered
ring annulated products. Thus, when inorganic Cs2CO3 base
À
was used in these C C coupling reactions six-membered ring
annulated polycyclic aromatics were formed, while the
organic base DBU led to preferential formation of the
respective five-membered ring annulated products.
We have investigated the Pd-catalyzed cross-coupling
reaction of dibrominated naphthalimide 10[3c] with aryl
boronic acid pinacol esters 9a–e using [Pd(dba)2] as Pd0
source and Cs2CO3 or DBU as an auxiliary base, otherwise
under very similar reaction conditions (Scheme 1). The
reaction of pyrene boronate ester 9a with 10 in the presence
of Cs2CO3 led exclusively to the formation of six-membered
[*] S. Seifert, Dr. K. Shoyama, Prof. Dr. F. Wꢀrthner
Universitꢁt Wꢀrzburg, Institut fꢀr Organische Chemie
Am Hubland, 97074 Wꢀrzburg (Germany)
E-mail: wuerthner@uni-wuerzburg.de
Dr. D. Schmidt, Prof. Dr. F. Wꢀrthner
Universitꢁt Wꢀrzburg, Center for Nanosystems Chemistry (CNC)
Theodor-Boveri-Weg, 97074 Wꢀrzburg (Germany)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
Angew. Chem. Int. Ed. 2017, 56, 1 – 7
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!