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Olefination of 6-bromo-1H-indole-2,3-dione with 1H-indol-3-yl acetate via the 2-chloroindoleninone

SyntheticPage 465

DOI: 

Submitted: August 17, 2010, published: August 24, 2010

Authors

Christopher Cooksey (rsc@chriscooksey.demon.co.uk)

A contribution from 

Reaction Scheme

Chemicals

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 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

Keywords

nucleophilic

Comments

monobromoindigo yields
Recently, others have repeated this synthesis and obtained much higher yields: Ajiki H, Pozzi F, Huang L, Massa L, Leona M and Lombardi JR, Raman spectrum of monobromoindigo, J Raman Spectr, 2011; doi: 10.1002/jrs.3066 (40%) Irina Mironova, Reflectance Analysis and Thermochromicity of Fabrics Dyed with Indigo, 6-Bromoindigo, and 6,6’- Dibromoindigo, BA thesis, Baruch College of the City University of New York 2013 (53%)
By Chris Cooksey on June 15, 2013