Catalytic hydrogenation of 1 and 2-pyrazoline. Isolation of pyrazolidine and process schemes

The catalytic hydrogenation of both 1 and 2-pyrazoline was studied in order to obtain the pyrazolidine. Among the studied catalysts, the Pd/C type catalyst permitted to hydrogenate the 1-pyrazoline but did not hydrogenate 2-pyrazoline. A slow hydrogenation of 2-pyrazoline occurred with a Pt/C and a mixed Pd/Pt/C catalyst. We demonstrated that catalyst poisoning occurred, which is due to the presence of the NH link of the 2-pyrazoline, blocking the active centers of the catalyst. At least, the Rh/C catalyst permitted to hydrogenate the mixture of 1 and 2-pyrazolines with a quantitative yield. The experiments showed that it was possible to reduce the reaction time by using catalysts having different specific areas. The Raney nickel catalyst also gave good results but its use requires much more precautions. Two process schemes were then established, taking into account both the synthesis of the 1 and 2-pyrazolines and their hydrogenation.