Solvent effects and other factors in carbenic and cationic pi cyclizations. I.
The susceptibility of the decomposition of the tosylhydrazones of simple carbonyl compounds to changes in the proton-donating ability (protonicity) of the solvent have long been known, but only recently has the mechaninistic path of this reaction been explored in detail. The nature of the products in tosylhydrazone decomposition (and thus by inference the nature of the intermediates) has been found to depend on (1) the protonicity of the solvent, (2) the concentration of the base, and (3) the nature of the cation association with the base. in recent deuterium-labelling studies on the base-catalyzed decomposition of the tosylhydrazones of cyclopropanecarboxaldehyde (I), norbornan-2-one (II), and camphor (III), evidence has accumulated for the existence of two distinct paths involving a "carbonic" and a "cationic" intermediate in this reaction depending on the reaction conditions. Present evidence indicates that the observed products from tosylchydrazone decomposition arise from either a carbene or a diazonium salt, generated from an intermediate diazo compound. In the absence of a Lewis acid, there is no present evidence to support a carbonium ion as an intermediate in the protonic decomposition of tosylhydrazones.