Chemistry Reference
In-Depth Information
TIPS
TIPS
TMS
Br
TMS
TMSCCH, CuI
Pd(PPh
3
)
2
Cl
2
Et
3
N, 110
1) KF-H
2
O, 18-crown-6
DME
Br
C
2) TIPS-CC-CC-CC-TIPS
CpCo(CO)
2
,DME,
37%
Br
Br
TMS
68%
TMS
TIPS
8
9
R
TIPS
R
1) TBAF, THF
2) CpCo(CO)
2
BTMSA/THF,
R
65%
R
10a
:R=TMS
10b
:R=H
TFA, CHCl
3
, 43%
Scheme 3 Synthesis of singly bent [5]phenylene 10 [
29
]
H
H
TMS
TMS
H
H
2
,1atm
Pd/C, Et
2
O
TMS
TMS
H
2b
11
Scheme 4 Hexahydrogenation of ring C of 2b [
27
]
fourfold Sonogashira coupling of 1,2,3,4-tetrabromobenzene with trimethylsilyl-
acetylene. Compound 8 was then deprotected in situ followed by a twofold
[2
þ
2
þ
2] cyclization with bis(triisopropylsilyl)-protected 1,3,5-hexatriyne to
give angular [3]phenylene-containing tetrayne 9. A second twofold [2
2]
with
bis
(trimethylsilyl)acetylene (BTMSA) and deprotection yielded 10b. All the
bent [5]phenylenes were air-sensitive, and the parent hydrocarbons (5a, 7a, and
10a) were relatively insoluble.
The aromaticity of the reported [4]- and [5]phenylenes was examined using a
number of criteria. For 2, shielding of the protons bound to ring C relative to those
on ring B confirm the increased cyclohexatrienic character of the former and
benzenoid character of the latter [
27
]. As shown in Scheme
4
, ring C of compound
2b can be all
cis
-hexahydrogenated, proving it to be the most activated. This is
comparable to the reactivity of the central ring in angular [3]phenylene [
12
]. In the
case of 7a, deshielding of ring B protons is even more pronounced than in 2a
especially in the bay region; however, chemical shifts of the ring C protons are
comparable [
28
]. The same pattern is
þ
2
þ
seen in comparing the calculated
