Rearrangement of the active ester intermediate during HOBt/EDC amide coupling

3-{[1′-(tert-Butyloxycarbonylamino)ferrocen-1-yl]carbonyl} -benzotriazole 1-oxide (3) has been successfully separated during the synthesis of benzotriazol-1-yl 1′-(tert-butyloxycarbonylamino)ferrocene-1- carboxylate (2) as an active ester for peptide coupling. The yield of 3 increased by using polar, rather than nonpolar solvents. The two compounds have been fully characterized and studied by X-ray crystallography and spectroscopic methods. The active ester derivative 2 formed a urethane bond with the glycine ethyl ester while the N-oxide 3 did not react. The X-ray structural analysis of 3 shows strong intermolecular hydrogen bonding involving the urethane group and the N-oxide of an adjacent molecule [N-O⋯H-N = 2.859(2) Å]. No hydrogen bonding is present in the solid state for compound 2, while solution studies indicate the presence of intramolecular hydrogen bonding. Both complexes display a quasi-reversible single one-electron oxidation, the halfwave potentials E_1/2 for 2 and 3 were 672 ± 5 and 591 ± 5 mV, respectively.