HATU(1-[Bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxid hexafluorophosphate) is a reagent used inpeptide couplingchemistry to generate an active ester from a carboxylic acid. HATU can be used along with Hünig's base(N,N-diisopropylethylamine, DIPEA) to form amide bonds. The general solvent is DMF [1].

HATU has been commonly used in alcohol and amine acylation reactions such as amide and ester formation. Such reactions are commonly performed in two distinct reaction steps: (1) reaction of a carboxylic acid with HATU to form the OAt-active ester; then (2) addition of the nucleophile to the active ester solution to form the acylated product such as alcohol or amine [2, 3].

References:
[1].  Carpino L A. 1-Hydroxy-7-azabenzotriazole. An efficient peptide coupling additive[J]. Journal of the American Chemical Society, 1993, 115(10): 4397-4398.
[2].  Carpino L A, Imazumi H, El‐Faham A, et al. The uronium/guanidinium peptide coupling reagents: Finally the true uroniumsalts[J]. AngewandteChemie International Edition, 2002, 41(3): 441-445.
[3].  Carpino L A, Imazumi H, Foxman B M, et al. Comparison of the Effects of 5-and 6-HOAt on Model Peptide Coupling Reactions Relative to the Cases for the 4-and 7-Isomers[J]. Organic letters, 2000, 2(15): 2253-2256.