Synthesis of [2‐(Trimethylsilyl)ethynyl]pyrazoles Based on Bis(trimethylsilyl)acetylene and Arylacetyl Chlorides
A practical approach to the synthesis of [2-(trimethylsilyl)ethynyl]pyrazoles was developed through condensation of hydrazines with enynones that are easily accessible frombis(trimethylsilyl)acetylene and arylacetyl chlorides. Optimized conditions for reactions with monoarylhydrazines permit the chemoselective formation of 5-alkynylpyrazoles in high yields (48–94 %). When monoalkylhydrazines or the parent hydrazine is used, mixtures of 3- and 5-alkynylpyrazoles are formed in 73–79 % yields. The method uses inexpensive starting materials and can be applied to a variety of substrates, which makes it convenient for the synthesis of alkynylpyrazoles.