Horner–Wadsworth–Emmons (HWE): E‑Selective Olefination (Mechanism + Rules)

Understand the HWE reaction: phosphonate carbanion addition, oxaphosphetane formation, elimination, and why E-alkenes are favored.

• Reaction: Aldehyde/ketone → alkene (olefination)
• What it does: Wittig-like but often E-selective with stabilized phosphonates
• Typical reagents: Phosphonate ester + strong base (NaH, KOtBu) + aldehyde
• Key intermediate: Betaine / oxaphosphetane (conceptually)
• Driving force: Formation of strong P=O bond in phosphate byproduct
• Exam type: Stereochemistry (E/Z)
• Exam type: Reagent selection