Chemistry Reference
In-Depth Information
This synthetic approach has been widely used for preparing numerous fluor-
anthenes by chemists interested in tailor-made corannulenes (see below, Methods A-
D). Notably, norbornadiene in the Diels-Alder reaction described above can be
replaced with an alkyne to give 8,9-disubstituted fluoranthenes [
25
-
29
].
Treating 9 (R
1
Ac, R
2
¼
¼
H) with PCl
5
allowed transformation of the two methyl
ketones in the side chains to chlorovinyl groups which occurred in 85% yield.
Pyrolysis of 10 thus generated at high temperature furnished corannulene (1),
presumably via a diethynyl intermediate produced by loss of hydrogen chlorides.
Scott envisioned that terminal acetylenes would rearrange to vinylidene carbenes
under these conditions and the carbenes would insert into the C-H bonds of the
naphthalene fragment to close the two six-membered rings [
21
].
Although the obvious success of the preparation of corannulene by the FVP
strategy comes from the small number of steps, the drawbacks, especially low
functional group tolerance and small-scale runs, have to be improved. Automated
or complete flow chemistry-based synthesis using the FVP technology has not yet
been achieved but has good potential [
30
]; in contrast, greater advances have been
made by finding mild scalable synthetic conditions in solution phase (see below,
Methods A-D).
Shortly after the publication of the pyrolysis protocol, an alternative synthetic
approach for synthesizing corannulene (1) in solution phase appeared. The key
point of this strategy was a reductive coupling reaction to close the carbon-carbon
bond across the 1,10 and 6,7 positions of 1,6,7,10-tetrakis(bromomethyl)-
fluoranthene followed by dehydrogenation, but Wurtz type couplings were not
initially observed [
31
], and standard pyrolysis was attempted to finish the synthesis.
The successful example of a corannulene derivative made by a wholly solution
phase synthesis was reported by the same group a few years later using a
reductive coupling based on low-valent titanium chemistry [
32
]. Therein,
2,5-dimethylcorannulene (13) was prepared en route to a corannulene cyclophane.
Siegel and coworkers originally introduced the key step, i.e., Method A, on the
basis of reductive benzylic coupling chemistry pioneered by Prakash and Olah [
33
]
(Scheme
3
). The synthetic approach started with 7,10-diethyl-1,6-dimethyl-
fluoranthene (11), which was prepared according to Scheme
2
[
32
]. Bromination
of 11 efficiently generated tetrabromide 12. Under titanium-mediated conditions,
12 was converted to tetrahydrocorannulene 14, oxidation of which furnished the
desired 2,5-dimethylcorannulene (13). In addition to providing access to
corannulene, this synthesis showed that solution phase methods could introduce
substituents regioselectively, as in 13, 15, and 16, which was not possible by means
of pyrolysis methods [
34
].
Siegel's methodology for producing 2,5-dimethylcorannulene (13) was applied
later independently by his group [
34
] and Rabiedau [
35
] for the synthesis of the
corannulene parent compound (Scheme
4
). Radical bromination of 1,6,7,10-
tetramethylfluoranthene with NBS under forcing conditions yielded octabromide
17, which underwent the ring closure by treating with reduced metals under
anhydrous condition (Method B). Dichlorocorannulene 19 was synthesized in the
same manner by the titanium-mediated protocol [
34
].
