Versatile oxidizing agent. Prepn: C. Willgerodt, Ber. 26, 357 (1893). Synthesis: H. Saltzman, J. G. Sharefkin, Org. Synth. 5, 658 (1973). Mechanistic study: R. M. Moriarty et al., J. Am. Chem. Soc. 103, 686 (1981). Use as oxidant: S. Mukerjee et al., ibid. 119, 8097 (1997); G. Mielniczak, A. Lopusinski, Synlett 2001, 505; as mediator: P. Dauban et al., J. Am. Chem. Soc. 123, 7707 (2001); C.-C. Guo et al., Appl. Catal. A 230, 53 (2002). Brief review: A. Minatti, Synlett 2003, 140-141.