Molecular Belts - Polyarenes - page 250

FVP, 800˚C
40%

3

5

Fig. 5 Flash vacuum pyrolysis rearrangement of 3 [ 8 ]

CHO

TiCl 4 , Z n 1) 8 eq. Br 2 , CHCl 3

DME 2) 10 eq. t -BuOK, THF, rt

CHO
6 7 8

55% overall

Fig. 6 Kawase's synthesis of [4]CMPA [ 15 ]

3.1 First Synthesis by Kawase

En route to the synthetically elusive CPPA belts, Kawase reported the synthesis of
strained [4]cyclometaphenyleneacetylene ([4]CMPA) in 1996 (8, Fig. 6 ). This
planar isomer of [4]CPPA was prepared through a McMurry coupling, bromination,
and elimination sequence [ 15 ].
Having proved this synthesis tractable for the formation of bent alkynes, a
similar approach was applied to synthesize [6]CPPA and [8]CPPA (12 and 13,
Fig. 7 ). The “shotgun”-style macrocyclization reaction of 4,4 0 -diformyl- Z -stilbene
under McMurry conditions provided moderate yields of macrocycles 9-11 (Fig. 7 ).
These macrocycles were then subjected as a mixture to the same bromination/
elimination sequence that provided [4]CMPA [ 15 ].
Macrocycles 10 and 11 were converted efficiently to yield [6]CPPA and [8]
CPPA for the first time (17% overall yield, 4:1 12:13)[ 16 ]. Owing to their strained,
electron-rich nature, these belts were observed to oxidize upon storage and [6]
CPPA was shown to decompose explosively upon heating in air. Interestingly, the
[4]CPPA precursor 9 was found to undergo only addition of tert -butoxide or THF
(when used as solvent) at the reactive site (Fig. 8 )[ 15 ].
This reactivity indicates that, while appropriate for the introduction of moderate
strain, smaller CPPAs are unobtainable by basic elimination [ 14 ].