Chemistry Reference
In-Depth Information
Table 1 Synthesis of [N]heliphenes (
n
¼
6-9). Reaction conditions: CpCo(CO)
2
,
m
-xylene, h
n
,
D
[
30
]
Precursor
[N]Heliphene
Name and yield
[6]Heliphene
(12), 12%
[7]Heliphene
(13), 8% or 2%
or
[8]Heliphene
(14), 2%
[9]Heliphene
(15), 2%
NICS (Nucleus Independent Chemical Shifts) values from 2a and 7a:
the
diatropicity of ring B increases from NICS
¼
6.4 to NICS
¼
7.5 in going
from 2a to 7a, whereas for ring C NICS
2.9 in each case. In 10b there is
even greater shielding of the central ring protons, which is again consistent with
decreasing diatropicity of NICS
¼
2.5 for that ring [
29
].
The longer angular [N]phenylenes are of both experimental and theoretical
interest, becoming helical with
n
¼
5 and referred to as [N]heliphenes [
23
].
Syntheses of angular [6]-[9]phenylenes were reported in 2002 [
30
,
31
]. In each
case the [N]heliphene was accessed by a two- or threefold [2
>
2] cycloaddi-
tion of the appropriate ethynyl precursor as outlined in Table
1
. Heliphenes 12 and
13 are stable in the solid state while 14 and 15 are only moderately so.
þ
2
þ
