A NMR study on the ring contraction of N‐methylpyrimidinium salts into pyrazoles

Abstract 1H‐NMR evidence is presented for the covalent hydrazination of 1‐methyl‐, 1,2‐dimethyl‐, 1,4,6‐trimethyl‐ and l‐methyl‐4‐phenyl‐pyrimidinium iodide (1a‐d) at C(6). With fully deuterated hydrazine hydrate the salts 1a‐d undergo ring contraction into pyrazole and pyrazole derivatives (8a‐d) containing deuterium at position 4. Evidence is presented for the intermediacy of a tautomeric mixture of open‐chain compounds, in which H/D exchange occurs before cyclisation takes place.