Chemistry Reference
In-Depth Information
D
I
R
steps
a
CpCo(CO)
2
m-
xylene, h
A
B
C
,
I
R
1
2a
:R=H
2b
:R=TMS
33%
CpCo(CO)
2
BTMSA
a
h
1) BuLi
Et
2
O, -78
Br
I
Br
Br
steps
b
C
,
2) I
2
93%
19%
Br
Br
Br
Br
3
D
R
R
Br
I
step
s
c
i
A
B
C
R
I
Br
R
5a
:R=H,7%
5b
:R=Pr,2%
4a
:R=H
4b
:R=Pr
D
I
I
steps
d
i
A
B
C
Br
Br
R
R
R
R
6a
:R=H
6b
:R=Pr
7a
:R=H,14%
7b
:R=Pr,1%
Scheme 2 Synthesis of (a) bent [4]phenylene (2a) and (b)(2b)[
27
]. Synthesis of (c)
anti
(5) and
(d)
syn
doublebent [5]phenylene (6)[
28
]. For 5a and 7a:(
i
) (1) CpCo(C
2
H
4
)
2
, THF,
25
C (2)
1,3-cyclohexadiene, THF, 110
C. For 5b and 7b:(
i
) CpCo(CO)
2
,
m-
xylene, h
n
,
D
.
a
BTMSA
¼
bis(trimethylsilyl)acetylene
reported (Scheme
2
b). In this case, 5-iodo-1,2,4-tribromobenzene was generated in
high yields by regioselective lithiation of 1,2,4,5-tetrabromobenzene followed by
quenching with iodine. Consecutive Sonogashira coupling reactions yielded com-
pound 3. This pentayne underwent a twofold intra- and intermolecular [2
2]
cyclization to yield 2b in an improved 7% yield over five steps. However, attempts
to perform the protodesilation of 2b to parent hydrocarbon 2a resulted in
decomposition.
In a fashion similar to that discussed above, Vollhardt and coworkers were also
able to access
anti
(5) and
syn
(7) doublebent [5]phenylenes by first constructing the
appropriate hexayne precursor (Scheme
2
c, d) [
28
]. Both isomers are accessible
through successive Sonogashira couplings of the appropriately substituted 1,2,4,5-
tetrahalobenzene, followed by intramolecular [2
þ
2
þ
2] cyclization.
The simpler singly bent [5]phenylene 10 is also available through a somewhat
different route (Scheme
3
)[
29
]. In this case, tetrayne 8 was synthesized via a
þ
2
þ
