Chemistry Reference
In-Depth Information
H
H
EtO
R
N
N
O
O
EtO
H
N
N
R
R
27a
:R=H
27b
:R=Br
2 ClO
4
26
28a
:R = H
28b
:R = Br
Scheme 5
Synthesis of 4,5-bis(2-pyridyl)phenanthrene-3,6-diols [
16
]
Br
N
O
O
N
Br
Fig. 7
View of 2,7-dibromo-4,5-bis(2-pyridyl)phenanthrene-3,6-diol (
28b
) along the two fold
symmetry axis [
16
,
34
]. Reprinted with permission from [
16
]. Copyright (1971) American
Chemical Society
of
28b
shows that each pyridine is intramolecularly hydrogen bonded to the neigh-
boring hydroxyl group, creating a twofold axis of symmetry (Fig.
7
)[
34
]. The two
pyridyl groups are oriented essentially parallel to each other and are very close with a
nonbonded intramolecular contact distance of 2.81
˚
, which is shorted than the
usual
-system van der Waals contact distance of ca. 3.4
˚
between parallel
aromatic hydrocarbons in crystals [
35
] or the graphite layer distance of 3.35
˚
.
Similarly, 13,14-bis(2-pyridyl)pentaphene (
29
) (Fig.
8
) was synthesized from
condensation of
26
with benzyne, generated in situ from anthranilic acid and isoamyl
nitrite [
36
]. Variable-temperature NMR studies of the mono- (
30
) and dimethylated
pyridinium salts (
31
) indicated rapid rates of racemization on the NMR time scales
below 79
C with the monomethylated
30
having an activation barrier of 16.6 kcal/
mol at 30
C. A synchronous rotation with the pyridyl rings remaining parallel to
each other was proposed as the molecular movement for racemization.
More recently, a three-step synthetic sequence for the 4,5-diarylphenanthrenyl
derivatives
37a-c
was reported (Scheme
6
)
37
]. Condensation between
benzannulated enediyne
32a
and diketone
33
produced propargylic diol
34a
.
Reduction of
34a
with triethylsilane in the presence of trifluoroacetic acid furnished
35a
. Treatment of
35a
with potassium
tert
-butoxide promoted 1,3-prototropic shifts
to generate, in situ, the benzannulated enyne-allene
36a
, which in turn underwent
p
