HATU (Hexafluorophosphate Azabenzotriazole Tetramethyl Uronium) is a reagent used in peptide coupling chemistry to generate an active ester from a carboxylic acid.

[1] HATU was first reported by Louis A. Carpino in 1993 as an efficient means of preparing active esters derived from 1-hydroxy-7-azabenzotriazole (HOAt).

[4] It is, however, possible to obtain the uronium isomer by preparing HATU using KOAt in place of HOAt and working up the reaction mixture quickly to prevent isomerisation.

The OAt anion rapidly attacks the isouronium salt, affording the OAt-active ester and liberating a stoichiometric quantity of tetramethylurea.

[9] Thermal hazard analysis by differential scanning calorimetry (DSC) shows HATU is potentially explosive.