Stable Zwitterion Research Articles

Heterodiene syntheses. Part 23. Zwitterions as intermediates in the formation of Michael adducts or as stable products of the reaction between 2-oxoindolin-3-ylidene derivatives with enamines or aminals

1-t-Butyl-1-aminoethylenes [(E1)–(E3)] react with N-substituted 2-oxoindolin-3-ylidene derivatives (1a–c) to give Michael-type adducts but the reactivity of diaminoethylenes [(A1)–(A3)] depends on the nature of substituents. 1,1-Dimorpholinoethylene (A1) undergoes Michael reaction with all the substrates tested but 1,1-dipiperidinoethylene (A2) only with the N-methyl- and N-benzyl-darivatives (1a and b). However, stable zwitterions are formed by (A2) with the N-acetyloxoindoline derivative (1c) and by 1.1 -dipyrrolidinoethylene (A3) with all substrates. Their structure, determined by chemical and spectroscopic methods and by X-ray analysis, is the same as that of the intermediates proposed for the reaction between αβ-unsaturated carbonyl compounds and enamines. The reason for the alternative products, i.e. Michael adducts or stable zwitterions, is discussed in terms of the effect of substituents in stabilizing the charges of the zwitterion and also in terms of frontier-orbital interaction.