Chemistry Reference
In-Depth Information
R
2
R
3
R
3
R
2
1
:R
1
=R
2
=R
4
=H, R
3
=Me
9a
:R
1
=CH
2
CO
2
H, R
2
=H, R
3
=R
4
=Me
9b
:R
1
=R
4
=H, R
2
=R
3
=Me
9c
:R
1
=R
2
=R
3
=R
4
=Me
9d
:R
1
=R
2
=R
4
=H, R
3
=CF
3
R
1
R
4
Fig. 4
Structures of 4,5-disubstituted phenanthrenes
I
2
h
n
10
11
Scheme 2
Photodehydrocyclization for 2,4-di(
tert
-butyl)-5,7-dimethylphenanthrene [
25
]
of the enantiomers by HPLC on silica coated with (+)-poly(tritylmethacrylate) was
also achieved. By monitoring the helix inversion with a polarimetry, the activation
energy (
11
C[
23
]. It
corresponds to a half-life of enantiomerization of 3 s at 24.8
C. The methylene
hydrogen atoms in
9a
are diastereotopic, providing the opportunity to determine the
activation energy of racemization by variable-temperature NMR studies and giving
the activation energy of 18.6 kcal/mol at 78
C[
23
]. The higher activation energy of
9a
, compared to that of
1
, was attributed to the steric interactions of the substituents
at the C1 and C8 positions with the hydrogens on C10 and C9, respectively, which
contribute significantly to the helicity of
9a
.
The activation energy of racemization of
9b
(
{
) of racemization was determined to be 18.7 kcal/mol at
DG
22.9 kcal/mol at 30
C) is
also significantly higher than that of
1
[
25
]. The higher activation barrier was
attributed to the buttressing effect of the methyl groups at the C3 and C6 positions
in increasing steric crowing of the methyl groups at the C4 and C5 positions [
5
]. As a
result, the torsional angle
y
is increased to 32.9
. The combination of the buttressing
effect and the presence of the two methyl groups at the C1 and C8 positions increases
the activation barrier of
9c
to 25.1 kcal/mol at 49
C[
23
,
26
]. With a sterically
hindered
tert
-butyl group at the C4 position and a methyl group at the C5 position
as depicted in 2,4-di-
tert
-butyl-5,7-dimethylphenanthrene (
11
), the activation barrier
further increases to 26.6 kcal/mol at 80
C[
25
]. Phenanthrene
11
was synthesized by
photochemically-induced dehydrocyclization reaction of the corresponding stilbene
precursor
10
using I
2
as a dehydrogenation agent (Scheme
2
).
Replacing the two methyl groups in
1
with two trifluoromethyl groups in
9d
increases the torsional angle
y
to 34.0
[
6
]. With a sterically hindered
tert
-butyl
group at the C4 position and methyl groups at C1, C5, and C8 positions as depicted
in
12
(Fig.
5
), the torsional angle
y
further increases to 36.6
[
27
]. The total strain
energy in
12
was estimated to be 34.0 kcal/mol at the ab initio HF-SCF level.
DG
{
ΒΌ
