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, I 2
benzene
O
h
ν
FeCl 3 / MeNO 2
DCM
57%
77%
134
135
133
Scheme 33 Mallory-Scholl reaction sequence for the synthesis of tetrabenzocoronene 135 [ 88 ]
I
TMS
Pd(dba) 2 ( 5 mol %)
P( o -tolyl) 3 (5 mol %)
MeCN/toluene, 110 ºC, 24 h
OTf
136
137
138
92%
Scheme 34 Pd-catalyzed cyclization reaction of an aryne intermediate [ 89 ]
2.5.2 Transition Metal Mediated Cyclizations
In the last two decades transition metal catalyzed cyclization/annulation reactions
have also been reported. In most cases alkynes are cyclized within a biphenyl-
containing substrate. Different modes of action for the transition metal can be
distinguished. In the first case the metal undergoes an oxidative addition into an
aryl-halogen bond and then performs the carbometallation of the alkyne. Here, an
example by Larock and coworkers is presented, in which an in situ formed aryne
species is cyclized with an iodo biaryl substrate 136 (Scheme 34 )[ 89 ].
In the second case the metal acts as a
-Lewis acid activating the alkyne for a
Friedel-Crafts-type cyclization [ 90 ]. With this approach small graphene-type
structures can also be efficiently assembled (Scheme 35 )[ 91 ].
Additionally, a mechanism via a metal-carbene intermediate has been observed.
In the example shown in Scheme 36 a Ru catalyst was used to form four C-C bonds
in one step for the synthesis of coronene 16 (Scheme 36 )[ 92 ].
A different disconnection was recently presented by Wang and coworkers. In his
approach bis(
π
-tosylhydrazone)s were cyclized using an Rh-catalyst [ 93 ]. The
starting materials are easily accessible via Suzuki cross-coupling reaction
(Scheme 37 ).
In all cases presented above, annulation reactions were limited to intramolecular
arrangements. There are also transformations involving two independent reaction
partners. In 2005 a Cr-mediated synthesis of PAHs from halobiaryls was published
[ 94 ]. The 2,2 0 -dihalobiaryl starting material 150 is first treated with BuLi before
addition of the Cr reagent. Final treatment with an acetylenic reaction partner
delivers the phenanthrene core 151 (Scheme 38 ). Recently, Wu and coworkers
presented a Pd-catalyzed version starting from 2,2 0 -diiodobiphenyls to access
substituted phenanthrenes [ 95 ].
N
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