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S
S
Br
Br
HS
SH
t -BuOK, THF
40%
36
105
(CH 2 ) 5
S
S
1) KSeCN, EtOH
HS(CH 2 ) 5 SH
37
Se
Se
2) 37 ,NaBH 4
20%
Cs 2 CO 3 ,THF
75%
106
107
Scheme 31 Synthesis of corannulene cyclophanes [ 32 , 129 ]
2.6 Other Corannulene Derivatives
2.6.1 Corannulene Cyclophane
Corannulene cyclophane 105 was prepared by treating 36 with a mixture of 1,4-bis
(mercaptomethyl)benzene and potassium tert -butoxide (Scheme 31 )[ 32 ]. Similarly,
1,5-pentadithiol/1,6-bis(bromomethyl)corannulene cyclophane (106) was prepared
from 37 and 1,5-pentadithiol in 75% yield [ 129 ]. Synthesis of [3,3]seleno/1,6-
dimethylcorannulene cyclophane (107), a cyclophane from two corannulenes,
was not straightforward [ 129 ]. The key step is the in situ transformation of 37 to
the bis(selenide anion), which reacts with the additional 37 to afford the seleno-
bridged dimer 107.
The bowl depths for 105 [ 32 ] and 106 [ 129 ] in the most stable form were
determined to be 0.87 and 0.96
, respectively, based on theoretical analysis. The
deep bowl depth of 106 is consistent with the experimental results showing that its
solution structure indicates a very rigid conformation. Although no significant
amount of strain is introduced by forming 105, the molecule is conformationally
restricted. The variable temperature proton NMR study sets the minimum limit for
the barrier to benzene ring flip at 18
Å
1 kcal/mol. Thus, construction of the
cyclophane effectively locks the structure into only one bowl form [ 129 ]. Two
isomers - such as a head-to-tail form and a head-to-head form of 107 - are possible.
The former is more stable than the latter by ca. 1.5 kcal/mol, and its calculated bowl
depth is 0.86
Å
, slightly flattened with respect to that of corannulene.
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