Chemistry Reference
In-Depth Information
Stabilization of neutral radicals is another interesting property of bowl-shaped
corannulene. The buckybowl neutral radicals 39 [
52
] and 40 [
79
] are stable as solids
in air or as solutions in degassed toluene for long periods. Experimental results
suggest spin delocalization of the radical species onto the corannulene moiety in 40
is more significant than 39.
N
N
O
O
N
N
39
40
2.2 Highly Substituted Corannulenes
Highly substituted corannulenes are not only important building blocks for new
organic materials, they also provide the possibility to extend the aromatic system
of the corannulene core. Direct multiple electrophilic aromatic substitution on
corannulene has shown limited but important successes. In contrast, functional-
ization of multihalocorannulenes is a general route.
Two special chlorination procedures for pentachlorocorannulene and decachloro-
corannulene open new avenues for the study of this area (see below). Already
mentioned above, 1,2,5,6-tetrabromocorannulene (18) is a formal precursor of
corannulene when using the Rabideau closure, but it is of direct importance for
making various corannulene derivatives. The importance of multi-halocorannulenes
stems from the variety and reliability of nucleophilic aromatic substitutions and
related transition-metal catalyzed couplings with nucleophiles.
2.2.1 Synthesis
Already introduced in the chemistry of the 2,3-dichlorocorannulenes like 19 [
34
],
nucleophilic aromatic substitution and related transition-metal catalyzed coupling
chemistry transfers well with minor modifications to other halogenated
corannulenes. Preparations of tetraethynylcorannulenes 41 [
37
-
39
] and tetraaryl-
corannulenes 42 [
37
,
38
] were accessed by Pd-catalyzed Sonogashira and Suzuki
reactions, respectively, in good to excellent yields (Scheme
10
). Similarly, 1,2,5,6-
tetramethylcorannulene (43) was obtained in good yield from 18 by the
Ni-catalyzed protocol [
37
,
38
]. Tetrasubstituted corannulene 44 with two kinds of
functional groups was prepared from 21 by the Sonogashira reaction [
39
].
Limited alkyl groups can be regioselectively introduced on the corannulene core
by Friedel-Crafts alkylation. For example, five sterically congested
tert
-butyl
