Chemistry Reference
In-Depth Information
Scheme 20 Nickel-
catalyzed synthesis of DAPs
from 2-bromo-1-
ethynylarenes: (a)
dibenzopentalenes 66 [
94
],
(b) angular 68 and (c) linear
dinaphthopentalenes 70 [
95
],
(d) di-(3,4)-thienopentalene
72 [
96
]
a
R
3
R
3
R
2
R
1
NiCl
2
(PPh
3
)
2
-Zn
PhCH
3
/DME 80 C
R
1
R
1
R
2
Br
R
2
R
3
65a
:R
1
=H,R
2
=Me,R
3
=TMS
65b
:R
1
,R
2
=H,R
3
=TMS
65c
:R
1
,R
2
=OMe,R
3
=TMS
65d
:R
1
=CO
2
Me, R
2
=H,R
3
=TMS
65e
:R
1
=Me,R
2
=H,R
3
=Tol
65f
:R
1
=Me,R
2
=H,R
3
=C
6
H
4
OMe
65g
:R
1
=Me,R
2
=H,R
3
=C
6
H
4
CO
2
Me
66a
,41%
66b
, 35%
66c
,13%
66d
, 24%
66e
,16%
66f
,13%
66g
, 13%
b
R
R
NiCl
2
(PPh
3
)
2
-Zn
PhCH
3
/DME 80 C
Br
R
67a
:R=Ph
67b
:R=Tol
68a
,15%
68b
,13%
c
R
R
NiCl
2
(PPh
3
)
2
-Zn
PhCH
3
/DME 80 C
Br
R
70a
,20%
70b
,11%
69a
:R=Ph
69b
:R=TMS
d
R
Br
NiCl
2
(PPh
3
)
2
-Zn
PhCH
3
,110 C
S
S
S
R
R
71a
:R=Ph
71b
:R=TMS
72a
,21%
72b
,18%
Work reported in 1999 found that a palladium catalyst could transform a
substituted 2-iodo-1-ethynylbenzene into a DAP [
93
]. However, the reaction
appeared to be an isolated case as substrates of that nature more often form
phenylene-ethynylene macrocycles under the given conditions. Despite the lack of
generalizability, the synthetic accessibility of starting materials and the ability to
form three bonds in one pot made this transformation intriguing for further investi-
gation [
94
]. Optimized conditions using a variety of 2-bromo-1-ethynylarenes (65)
and a nickel catalyst in the absence of additives successfully suppressed the compet-
ing biaryl coupling to give the desired DAPs (66)inmoderateyields(Scheme
20
a).
This new methodology allowed access to DAPs with a wide variety
of substituents. The same conditions were used to synthesize successfully
extensions of the PAH to dinaphthopentalenes of different connectivities (68/70,
Scheme
20
b, c) [
95
]. Additionally, it has been shown to provide access to di-(3,4)-
thieno[
a,e
]pentalene 72 (Scheme
20
d) [
96
].
DAPs 66a, 68a, and 70a were all analyzed by X-ray crystallography and, as with
the [N]phenylenes,
the degrees of bond alternation can be an indication of
