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hv
Method A
NBS
Br
Br
1) TiCl 3 /LiAlH 4 or
TiCl 4 /Cu
~ quant.
Br
Br
Zn DME
2) DDQ
11
12
23%
13
Cl
Cl
15 (48%)
14
16 (45%)
Scheme 3 Synthesis of corannulene derivatives in solution phase by Method A [ 32 , 34 ]
hv
NBS
Br 2 HC
CHBr 2
B or C
70
85%
Br 2 HC
CHBr 2
quant.
1
17
Br
Br
Method B : TiCl 4 , Zn/Cu, DME
or VCl 4 , LiAlH 4 , DME
Br
Br
Method C : 1) NaOH, dioxane, H 2 O
2) Zn, NaI, EtOH or DMF
Cl
Cl
18 (83%)
19 (85%)
Scheme 4 Synthesis of corannulene derivatives in solution phase by Methods B and C [ 34 - 38 ]
In a further refinement to the synthesis, Sygula and Rabideau discovered an
appealing alternative method (Method C, Scheme 4 ) to prepare 1,2,5,6-tetrabromo-
corannulene 18 from 17 by the use of sodium hydroxide to deprotonate the
remaining benzylic hydrogen and initiate carbon-carbon bond formation [ 36 - 38 ].
The desired molecule 1 was formed (90%) by treatment of 18 with zinc and
potassium iodide [ 37 , 38 ].
Under forcing bromination conditions, octabromide 17 forms from 1,6,7,10-
tetramethyl fluoranthene bearing no flanking ortho substituents. Hexabromides 20
are obtained due to the steric hindrance of the flanking functional groups. Extension
of the alkyl substituents (for example to ethyl) at the 1,6,7,10 positions also limits
the degree of bromination. Nonetheless, ring closure of hexabromides 20
regioselectively to dibromocorannulenes 21 can be achieved by the base promoted
procedure (Method C, Scheme 5 )[ 25 , 39 , 40 ].
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