Tschitschibabin reaction, mechanism

Finally, the elimination-addition mechanism has been proposed to explain the course of the weU-known Tschitschibabin reaction, but this extension of hetaryne chemistry has been rightly objected to by several authors. 

As regards the mechanism of the Tschitschibabin reaction, little is definitely known. It has been represented as a direct bimolecular nucleophilic replacement of hydride by amide anion . However, when the situation in related reactions (see Alkylations, p. 202), and evidence from Tschitschibabin reactions in other heterocyclic series are considered, it seems likely that an addition compound is an intermediate . Barnes argued that the resulting structure would probably dissociate again, but would be stabilized by conversion into (93), and formulated the conversion of the latter into the reaction products as a bimolecular process ... 

Although the exact mechanism of the Tschitschibabin cyclisation has not been elucidated, it is reasonable, as shown in Scheme 4, to assume a series of reversible steps from the vinylogous ylide (or methylide) to a methine and an enol-betaine intermediate and then finally an irreversible dehydration to the indolizine nucleus. The reaction might be related to the modern electrocyclic 1,5 dipolar cyclization. 

The Tschitschibabin reaction285,266 of alkali metal amides with pyridine bases has been the subject of much recent discussion. While there is yet no agreement concerning the detailed mechanism (due to the lack of experimental information) there is no doubt that the overall reaction proceeds by an SN2 type addition-elimination pathway. 

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