| Abstract: | 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 acetophenonphenylhydrazone 1e with the Vilsmeier‐Reagent 2 . On the example of the Acetophenonmonomethylhydrazone 1a 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 followed 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 4a 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 case with an attack on the methyl group of the acetophenon part followed of the ringconclusion to the intermediate 6 accompanied of the eli‐mination 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 compounds 9 are formed (scheme 2, eq. 3). 9 are intermediates in the reaction from 8 to 4 only in a small extent. |
