Chemistry Reference
In-Depth Information
in order to isolate them as stable species in air, kinetic stabilization by steric protection at the appropriate
positions is a key issue. From this viewpoint,
6OPO
system with methyl substituents at the 2, 5-positions
was first designed. However, this neutral radical could not be isolated due to the generation of benzyl-type
radical and subsequent
dimerization.
42
The
tert
-butyl group, sterically more congested group than methyl
group, greatly improves the stabilities of
4OPO
and
6OPO
, enabling them to be handled in air.
35,43
Taking
advantage of this strategy, 7-hydroxy-
TB6OPO
was also designed and synthesized as the first neutral
radical having an intramolecular hydrogen bond coupled with an extremely delocalized spin system.
44
These neutral radicals were prepared by the methods depicted in Scheme 3.10. Among the synthetic
steps,
tert
-butylation of 2,7-dimethoxynaphthlene, the first step of the method, is the most important
key reaction. The Friedel - Crafts type introduction of
tert
-butyl groups into the electron rich aromatic
compounds is not easy, because one-electron oxidation reaction of the substrate easily occurs and thus
inhibits the Friedel - Crafts reaction. This difficulty is overcome by the use of
t
-butyl alcohol and trifluo-
roacetic acid (CF
3
COOH) as solvents. Thus, tri-
tert
-butylated 4,9-dimethoxyphenalanone (“phenalanone”
in Scheme 3.10), a common synthetic intermediate of three kinds of neutral radicals, is prepared in five
steps. Reduction of the carbonyl group followed by demethylation and aerobic oxidation gives the 6-
hydroxyphenalenone. Treatment of this compound with lead(IV) oxide affords
TB6OPO
as a deep green
solid.
TB4OPO
, a topological isomer of
TB6OPO
, is also obtained as a green solid in four steps from
the phenalanone derivative. The key reaction step is the regioselective reductive demethoxylation reaction
of the dimethoxyphenalenone derivative. This reaction is furnished with lithium aluminum hydride in the
presence of copper(I) iodide. 7-Hydroxy-
TB6OPO
is also obtained as dark grayish green powder from
σ
CHO
t
-BuOH
conc H
2
SO
4
1. LDA-
t
-BuCH
2
CO
2
Bn
2. Et
3
SiH, CF
3
CO
2
H
3. H
2
,Pd-C
POCl
3
DMF
MeO
OMe
MeO
OMe
MeO
OMe
CF
3
COOH
O
CO
2
H
a. (COCl)
2
b. AlCl
3
1. LiAlH
4
PbO
2
TB6OPO
MeO
OMe
MeO
OMe
O
OH
2. LiI
phenalanone
6-hydroxyphenalenone
DDQ
O
O
LiAlH
4
CuI
O
O
LiI
LiI
HO
OH
MeO
HO
MeO
OMe
4-hydroxyphenalenone
PbO
2
PbO
2
7-hydroxy-TB6OPO
TB4OPO
Scheme 3.10
Synthetic methods for
TB4OPO
,
TB6OPO
, and 7-hydroxy-
TB6OPO
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