Effects of Base Nature and Concentration

To synthesize pyrroles from ketoximes and acetylene, alkali metal hydroxides (LiOH, NaOH, KOH, CsOH, RbOH) are employed as strong bases, key component of the superbase system. 

Dependence of the catalytic systan activity upon cation nature has been studied on the example of the reactions between cyclohexanone and acetophenone oximes and acetylene [159,160]. The catalyst activity is shown to increase with growth of the cation atomic number Li Na K Rb Cs [160,161]. 

The previous sequence, valid for many oximes of aliphatic and cycloaliphatic ketones, is not however absolute and can be varied depending on the reaction conditions and ketoxime type. 

The reaction of acetophenone oxime with acetylene (100°C, 3 h) is well catalyzed by all alkali metal hydroxides (taken in 10%-30% from the oxime weight), but Ca(OH)2 is inactive nnder these conditions [160]. Tetrabutylammonium hydroxide exerts weak catalytic action on the reaction only at harsher conditions (120 C). Potassium, zinc, and cadmium acetates as well as zinc, copper (I and II), and cobalt chlorides do not show catalytic activity in this reaction (starting acetophenone oxime is recovered almost completely [160]), though several cations of the aforementioned salts are known [95,162] to be catalysts of direct vinylation of NH-heterocycles with acetylene. 

Alkali metal hydroxides differ not only in reactivity but also in selectivity of action. For example, LiOH selectively catalyzes the reaction of the pyrrole ring construction from alkyl aryl ketoximes [160,163-165], while it is almost inactive at the stage of vinylation of the pyrrole formed. At the same time, LiOH is ineffective for cycloaliphatic ketoximes at both stages [159] the pyrrole ring formation is accelerated in this case by rubidium and tetrabutylammonium hydroxides [159,161]. 

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