Pyrazole catalytic reduction

Thoms and Schnupp741 reduced pyrazole itself to pyrazoline, with a palladium catalyst in acetic acid at 20° but this is the only reported catalytic reduction of a pyrazole ring unsubstituted on nitrogen. Under these conditions V-phenyl pyrazoles are converted to V-phenyl-pyrazolines and then at 80° to N-phenylpyrazolidines.741... 

Pyrazoles are very resistant to catalytic reduction, resisting hydrogenation over nickel at 150 °C and 100 atm (66AHC(6)347). Imidazoles are generally resistant to reduction. 

A -Pyrazolines are obtained by the reduction of pyrazoles with sodium and alcohol, by catalytic hydrogenation on palladium or by electrochemical means (B-76MI40402). In some cases the reduction proceeds further yielding pyrazolidines and open-chain compounds. 

Figure 12 shows the reaction profile for the hydrosilylation process involving the most stable fi3-sily 1-ally 1 complex, 10a-anti, calculated with model B. Examination of the reaction profile suggests that the rate determining step of the catalytic cycle is the reductive elimination. More specifically, the transfer of the silyl moiety to the (J-carbon of the styrene. Since recoordination of the pyrazole ligand occurs in this step, it is possible that enhancement of this ligands ability to recombined with the Pd center may lead to improved activities. 

They can be replaced with hydrogen atoms by catalytic (Pd, Ni, etc.) or chemical reduction (HI or Zn/H2S04). For example, halopyrazoles with HI and red phosphorus at 150C give pyrazoles, and 5-bromo-l,2,4-thiadiazole is reduced by Raney nickel to the parent heterocycle. 2-Bromothiazole can be reduced electrochemically. 

Pyrazole and 3,5-dimethylpyrazole were effective stoichiometric catalysts in the Baylis-Hillman reaction of cyclo-pentenone 892 with /i-nitrobenzaldehyde 893 in basic media to give adducts 894 in good yields (Equation 190) <2004TL5171>. An asymmetric borane reduction of ketones catalyzed by AT-hydroxyalkyl-Z-menthopyrazoles has been reported <2000JHC983>. 3-Aryl-/-menthopyrazoles 895 were assessed for their catalytic activity for asymmetric Diels-Alder reactions <2002JHC1235, 2003JHC773>. 

Pyrazole resistant to oxidation and reduction reaction due to loss of aromaticity, but may be hydrogenated catalytically, first to pyrazoline, and then to pyrazolidine. Both of these compounds are stronger bases than p3rrazole. 

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