Olefination of 6-bromo-1H-indole-2,3-dione with 1H-indol-3-yl acetate via the 2-chloroindoleninone; 6-Bromoindigo
SyntheticPage 465
DOI:
10.1039/SP465
Submitted Aug 17, 2010, published Aug 24, 2010
Christopher Cooksey (
rsc@chriscooksey.demon.co.uk)
A contribution from
R J H Clark Group, UCL
Chemicals Used
6-Bromoisatin 464
Indoxyl acetate (Aldrich)
PCl5
Procedure
A mixture of 6-bromoisatin (1.0 g, 4.4 mmol) and PCl5 (1.0 g, 4.8 mmol) in chlorobenzene (30 cm3) was heated under N2 at 98 – 102 °C for 4 h. After cooling the brown solution, indoxyl acetate (716 mg, 4.2 mmol) was added and the mixture was allowed to stand overnight. The violet reaction mixture was diluted with ethanol (30 cm3), filtered, and the residue washed with ethanol (2 x 30 cm3) to give a dark blue solid (309 mg), which was recrystallized from ethyl benzoate (100 cm3), washing with ethanol (25 cm3) then ethanol – diethyl ether (1:1, 25 cm3), to give a black solid with a coppery lustre (282 mg, 20% yield).
Author's Comments
Isatins react with PCl5 to give (supposedly) 2-chloroindoleninones. 6-Bromoisatin is reported (Baker JT, Duke CC, Aust J Chem 1976, 29, 1023 – 1030) to be unreactive to PCl5 in refluxing toluene and decomposition occurred with higher boiling point solvents. Using chlorobenzene appeared to be satisfactory but yields were modest. But the reaction is convenient and the product is easily isolated because it is insoluble in nearly all cold solvents. The lack of solubility precludes chromatographic separation of useful amounts of the product, but recrystallisation can be achieved from nearly any solvent with b.p. >200 °C. Recrystallisation of 6-bromoindigo (18.1 mg) from ethyl benzoate (10 cm3) gave 14.5 mg. NMR characterisation was via the orange N,N'-trifluoroacetate derivative. 6-Bromoindigo, isolated from natural shellfish purple, has more recently been directly characterised by NMR (http://www.imog2007.org/files/Wednesday%20Posters/Wednesday%20Analytical%20Developments/P277-WE%20James.pdf).
Heating to 130 °C for 1 h gave a lower yield (16%); the reverse reaction using
isatin and
6-bromoindoxyl acetate gave an even lower yield (10%). Modest yields were obtained from the appropriate bromoisatin for other monobromoindigos: 4-bromo (28%), 5-bromo (38%), 7-bromo (27%).
Data
EI-MS m/z (%) 342 (100), 340 (100), 314 (14), 312 (14), 262 (5), 233 (9), 205 (28)
λmax (tetrachloroethane) 601 nm
N,N'-trifluoroacetate:
δH (CDCl3) 8.30d (1H, 1.2), 8.06d (1H, 8.4), 7.95d (1H, 7.7), 7.79td (1H, 7.4, 1.3), 7.75d (1H, 8.1), 7.56dd (1H, 8.1, 1.4), 7.42t (1H, 7.4)
Lead Reference
Clark RJH, Cooksey CJ, New J Chem, 1999, 3, 323–328;
DOI: 10.1039/a808562e
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Keywords: nucleophilic
