Knorr pyrazole synthesis

Schematic of the Knorr pyrazole synthesis. Reagents: hydrazine or derivatives, 1,3-dicarbonyl compound, acid catalyst. Product: pyrazoles.

The Knorr pyrazole synthesis is an organic reaction used to convert a hydrazine or its derivatives and a 1,3-dicarbonyl compound to a pyrazole using an acid catalyst. The mechanism begins with an acid catalyzed imine formation, where in the case of hydrazine derivatives the attack can happen on either carbonyl carbon and result in two possible products. The other nitrogen of the hydrazine derivative then attacks the other carbonyl group which has also been protonated by the acid and forms a second imine group. This diimine compound gets deprotonated to regenerate the acid catalyst and provide the final pyrazole product.[1]

Mechanism

Mechanism of the Knorr pyrazole synthesis. This attack leads to one of the possible products while attack on the other carbonyl would results in the other pyrazole product.

References:

1.
Knorr, L. Ber. Dtsch. Chem. Ges. 1883, 16, 2597–2599.