Fischers Base Salts by Alkylation of Indolenines

The N-alkylation can be carried out with or without a solvent, using alkyl halides, tosylates, sulfates, phosphates, and for difficult cases the very powerful but extremely toxic methyl fluorosulfonate. Excess methyl and ethyl iodides are convenient solvents longer chain alkyl halides often are used with chloroform or tetrachloroethane as solvents. Ketones such as acetone and butanone have been used, and higher acyclic and alicyclic ketones have been claimed18 to give high yields of substantially pure salts. 

The reaction of an indolenine with an alkyl halide having the alkyl group ethyl or longer can also lead to elimination rather than alkylation, with the formation of the hydrohalide salt of the indolenine. This salt can be mistaken for the expected quaternary salt. In this case, treatment with base will merely regenerate the starting indolenine, which is easily distinguished by IR spectrum, as indicated earlier, from the Fischer s base formed from the quaternary salt. Often the solid isolated is a mixture of the two salts, and both components can be identified in the spectmm of the corresponding mixture of bases. 

A detailed study of the N-methylation of 2,3,3-trimethylindolenine with methyl sulfate using triethylbenzylammonium chloride as a phase-transfer catalyst has appeared. The reported yields of high-purity (96-99%) Fischer s base were 90-92%.19 N-alkylation is the most practical method for preparing a Fischer s base with alkyl groups having, for example, thienyl,20 hydroxy, carboxy, or sulfonate substituents. 

Reaction between butanesultone and 2,3,3-trimethylindolenine gave a 74% yield of the water-soluble N-(n-butylsulfonate) inner salt of Fischer s base this salt and others from substituted indolenines were used as intermediates for cyanine and merocyanine dyes, but not specifically for spiropyrans. 21 

3-Trimethylindolenine can also be N-alkylated by a-haloketones such as bromoacetone (58% yield). For a series of l -acetonylBIPS, the thermal fade rate constant was several times larger than for the corresponding l -methylBIPS. The absorption spectra exhibited bathochromic shifts in ethanol, but negligible shifts in 

---