Reactions of acetophenonmonomethyl- and -dimethylhydrazones with the Vilsmeier-Reagent; Formation of pyrazol-4-carbiminium salts: A contribution to the mechanism

Kira discovered the formation of (4e) N,N-Dimethyl-N-[1,3-diphenylpyrazol-4 -ylmethylen] ammonium perchlorat by the reaction from acetophenonphenylhydr azone le with the Vilsmeier-Reagent 2. On the example of the Acetophenonmon omethylhydrazone la we represent the mechanism of this reaction. 1a reacts with the Vilsmeier-Reagent 2 firstly to 3a (scheme 1). 3a is formylated in the secondary step by 2 on the methyl group of the acetophenon part followe d of a ring conclusion to the pyrazol 4a containing a carbiminium group in position 4 as substituent. 3a doesn't react to 6a directly. Hydrolysis of 4 a gives 5a. We found moreover, that acetophenondimethylhydrazones 8 react with 2 to 4, too (scheme 2, eq 2). But here the proton of the methylamino group in 1 is occupied by the methyl group in 8, Therefore, the reaction starts in this c ase with an attack on the methyl group of the acetophenon part followed of the ringconclusion to the intermediate 6 accompanied of the elimination of the methyl group. In the last step the not isolated pyrazol 6 is formylated by 2 in 4-position to 4. In a secondary reaction from 8 with 2 the compoun ds 9 are formed (scheme 2, eq. 3). 9 are intermediates in the reaction from 8 to 4 only in a small extent.