3-alkylidene indolines

Grigg discovered that a 5-exo-dig Pd-catalyzed cyclization of IV-acetylenic-o-haloanilines 291 to give 3-exo-alkylidene indolines 292 occurs in the presence of a hydride source, such as formic acid [346]. The reaction is stereoselective and regiospecific. 

Only alkyl groups at indole a-positions show any special reactions. Many related observations confirm that in a series of equilibria, / -protonation can lead to 2-alkylidene-indolines, and hence reactivity towards electrophiles at an a-, but not a /3-alkyl group, for example in DCl at 100 °C 2,3-dimethylindole exchanges H for D only at the 2-methyl. 

The Fischer s bases, 2-alkylidene-l,3,3-trisubstituted indolines, which may be further substituted in the indoline ring, are the most versatile and useful bases for preparing spiropyrans. For convenience, the name Fischer s base will be used hereafter in the general sense indicated above, as well as for the specific compound 2-methylene-l,3,3-trimethylindoline. Substituents in the 1- and 3-positions are methyl unless otherwise specified thus 3-phenylFischer s base denotes the 1,3-dimethyl-3-phenyl compound. 

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