Palladium N-heterocyclic carbene catalyzed expected and unexpected C–C and C–N functionalization reactions of 1-aryl-3-methyl-1H-pyrazol-5(4H)-ones

Abstract

A palladium N-heterocyclic carbene [Pd(NHC)Cl₂] complex of vitamin B₁ developed earlier in our laboratory was successfully employed as an efficient catalyst for the regioselective C–C and C–N functionalization reactions of 1-aryl-3-methyl-1H-pyrazol-5(4H)-ones 1a–b. The catalyst was attempted for the C–H arylation, acylation and alkoxylation of 1a–b using the respective coupling substrates such as aryl iodides 2a–c, benzylic alcohols 3a–d and methanol/ethanol 4. It was surprisingly noted that the acylation and alkoxylation reactions underwent a diverse pathway to yield some unexpected products rather than the targeted ones. In the case of the arylation reactions only the targeted products have been observed. This has made the protocol very interesting compared to the conventional coupling reactions.