Novel directed synthesis of functionalized pyrazole derivatives via regioselective solvent-free thiylation of 3-alkenylpyrazoles with arenethiols

Abstract

Unprecedented easy (3 min., room temperature) regioselective addition of thiols to the endocyclic double bond of 3-vinyl-, prop-1-enyl- and isopropenyl, including those with bulky substituents in the position 1 of the pyrazole ring, has been found for the reaction of arenethioles with 3-alkenyl-5-chloropyrazoles. The reaction proceeds under solvent-free and catalyst-free conditions to afford 3-[(2-arylsulfanyl)alkyl]-5-chloropyrazoles in excellent yields. High rate of the process is likely due to protonation of the "pyridine" nitrogen atom of the pyrazole ring to give a pyrazolium ion which activates the vinyl group towards nucleophilic attack by the simultaneously formed arylthiolate anions.