HBTU (2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexaﬂuorophosphate) is a coupling reagent used in solid phase peptide synthesis. It was introduced in 1978 and shows resistance against racemization. It is used because of its mild activating properties.
The product obtained by reaction of HOBt with tetramethyl chloro uronium salt (TMUCl) was assigned to a uronium type structure, presumably by analogy with the corresponding phosphonium salts, which bear a positive carbon atom instead of the phosphonium residue. Later, it was shown by X-ray analysis that salts crystallize as aminium rather than the corresponding uronium salts .