Alpha-lithiation of n-alkyl groups in pyrazoles

Abstract

1,3,5-Trimethylpyrazole and l-ethyl-3,5-dimethylpyrazole undergo lithiation exclusively at the α -position of the N-alkyl group. 1-Benzylpyrazole is metallated under kinetic control at -78° at the CH2 group, but the metallorganic intermediate rearranges rapidly at 23° to give the thermodynamically more stable 5-lithio-l-benzylpyrazole. 1-Methylpyrazole gives mixtures of α- and 5-lithiation; 1-ethylpyrazole showed only 5-lithiation. The carbanions were trapped by a variety of electrophiles. © 1983.