Diaminomethylation of Pyrazoles

We studied the reaction of N ‐alkyl(aryl)pyrazoles with silylformamidine 1 that exists in an equilibrium with its carbene form via migration of the trimethylsilyl group from the carbon to nitrogen atom. The carbene insertion proceeded at the C−H group in 5 th position. N ‐alkyl(aryl)pyrazoles featuring substituents such as Br, CN and NO 2 group reacted readily affording the corresponding aminals. This approach allowed us to synthesize pyrazole dialdehyde and phosphine featuring a vicinal aldehyde group. The reaction does not require any catalyst. The insertion position was unambiguously confirmed by X‐ray analysis of an aminal and a set of derivatives. Aminals hydrolysis afforded the corresponding pyrazole carbaldehydes. Methanolysis of the aminals gave N‐methylimines of the pyrazolecarbaldehydes. Highly acidic nitro pyrazole instead of the insertion underwent silylation in 5 th position.