Pyridazines. XLV . on the mechanism of an unusual 1,2‐diazine → 1,2‐diazole ring contraction

Starting with succinic acid‐2,3‐13C, phenyl(4‐pyridazinyl)methanol‐4,5‐13C was prepared in order to gain insight into the mechanism of an unusual pyridazine into pyrazole rearrangement reported previously. According to the 13C‐nmr spectrum of labeled 1‐phenyl‐2(4‐pyrazolyl)ethanone obtained upon p‐toluenesulfonic acid mediated ring contraction, the methylene and the pyrazole C‐4 carbon atoms of the reaction product originate from the pyridazine carbon atoms 4 and 5 of the educt. Thus, a mechanism for this ring‐transformation is proposed involving well stabilized carbocations as intermediates.