Chemistry Reference
In-Depth Information
Scheme 21 Pd-catalyzed
syntheses of (a) DAPs 74,
(b) dithieno[
a,e
]pentalene 76,
and (c) hexa-aryl-substituted
pentalene 78 [
97
]
a
hydroquinone
Cs
2
CO
3
,CsF
Pd
2
(dba)
3
,P(
t
Bu)
3
1,4-dioxane, 135 C
R
4
R
1
R
4
R
2
R
3
R
3
R
3
R
2
Br
R
2
R
1
R
4
R
1
73a
:R
1
,R
2
,R
3
=H,R
4
=Ph
73b
:R
1
,R
2
,R
3
=H,R
4
=TMS
73c
:R
1
=CCPh,R
2
=H,
R
3
=CF
3
,R
4
=Ph
74a
,88%
74b
,83%
74c
,72%
b
hydroquinone
Cs
2
CO
3
,CsF
Pd
2
(dba)
3
,P(
t
Bu)
3
1,4-dioxane, 135 C
Ph
Ph
S
S
S
Br
Ph
55%
75
76
c
hydroquinone
Cs
2
CO
3
,CsF
Pd
2
(dba)
3
,P(
t
Bu)
3
1,4-dioxane, 135 C
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
I
Ar
60%
77
:Ar=Tol
78
Ph
Ar
Ph
Ph
Ar
Ar
Ph
Br
PPh
3
,Pd(OAc)
2
DIPEA, 135 C
FeCl
3
,MeNO
2
CH
2
Cl
2
,CS
2
,rt
36%
+
Br
Ar
Ar
Ph
Ph
Ph
89%
Ph
Ar
79
80
81
:Ar=C
6
H
4
OPr
Scheme 22 Synthesis of multiple pentalene-containing PAH 81 [
98
]
Very recently, a procedure for the synthesis of DAPs using gold catalysis was
reported [
99
]. Rearrangement of 1,2-diethynylarynes 85 allowed the authors to
produce DAPs without substituents at the C5/10 position (86), including
unsubstituted dibenzo[
a,e
]pentalene 56b (Scheme
23
a). The conditions were also
successful for thiophene-substituted 87, although the yield was lower as a result of
the stability of the product (Scheme
23
b). The authors propose a mechanism that
passes through a gold vinylidene intermediate.
In 2007, Saito and coworkers found that treatment of TIPS-protected phenyl-
acetylene with lithium followed by treatment with iodine resulted in a 7% yield of
DAP 91 and 28% 1,4-diiodo-1,3-butadiene 90 (Scheme
24
)[
100
] The dilithium
dibenzopentalenide 89 was eventually characterized by X-ray crystallography and
NMR, the first X-ray structure of a DAP dianion (Fig.
17
). Unlike the neutral DAP,
there is more bond alternation in the six-membered rings of 89 than the five-
membered rings.
The surprising formation of 91 inspired the authors to investigate further the
reactivity of this DAP derivative. When 91 was treated with methyllithium (MeLi)
