Synthesis of 5‑(Trifluoromethyl)pyrazolines by Formal [4 + 1]-Annulation of Fluorinated Sulfur Ylides and Azoalkenes

In situ formed 1,2-diaza-1,3-dienes were used in formal [4 + 1]-annulation reactions with fluorinated sulfur ylides. This protocol provides a novel and efficient method for the synthesis of 5-(trifluoromethyl)­pyrazolines in moderate to excellent yields.