Oximes of Alkyl Aryl Ketones

SCHEME 1.45 Synthesis of 2-phenyl-3-alkyl(phenyl)pyrroles from alkyl phenyl ketoximes 

The structure of the starting alkylphenyl ketoximes does not affect substantially on N-vinylpyrrole yields, though the tendency to the yields increase with growth of carbon atoms in the alkyl radical is noticeable. 

The reaction smoothly proceeds also at 100°C in the presence of 30% (from ketoxime weight) KOH in DMSO in autoclave under initial acetylene pressure of 8-16 atm. The maximum pressure reached at the reaction temperature is 20-25 atm. Then, intensive consumption of acetylene begins, and pressure quickly decreases. As it was already noted, initially, N-unsubstituted pyrroles are formed, which further are vinylated in the presence of acetylene excess. If it is necessary to obtain the corresponding NH-pyrrole, the synthesis is carried out with calculated amount of acetylene or with its lack. LiOH appears to be a selective catalyst of the pyrrole ring construction, the application of which does not require strict dosing of acetylene. 

The yield of 2-aryl-N-vinylpyrroles essentially depends on the substituent in the benzene nucleus. Sharp drop of 4-bromophenyl-N-vinylpyrrole (from 4-bromoace-tophenone oxime) is logical it is caused by bromine saponification with strong base. From 4-nitroacetonenone oxime, the pyrrole is not formed under the conditions studied due to instability of the substituent in superbase medium. When the temperature decreases to 80°C, the reaction of 4-phenoxyacetophenone oxime and acetylene leads to the corresponding pyrrole in 43% yield [221]. 

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A review of synthetic methods for arylpyrroles focuses particularly on the Trofimov reaction of oximes of alkyl aryl ketones with acetylene (192 references). ... 

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