Re ceived: Oc to ber 10, 1995 O mecanismo de redução do composto N-fenil-4-nitrobenzenosulfonamida foi investigado em meio aquoso por polarografia de corrente direta e pulso diferencial, voltametria cíclica e coulometria a potencial controlado. Em pH < 10, o composto é reduzido em uma única etapa de 4 elétrons. Entretanto duas etapas de redução são verificadas em elevados valores de pH (pH > 11). A redução do grupo ni tro à hidroxilamina na molécula é sempre o processo preponderante no mecanismo eletródico.The elec tro chem i cal be hav ior of an i line pro tected by a nitrobenzene sulphonyl group in aque ous so lu tion at a mer cury elec trode is re ported. At pH < 10 the com pound was re duced in a sin gle well-defined step. Re duc tion of the ni tro group in volv ing a pre ced ing protonation st ep was pos tu lated. Two re duc tion steps are pres ent at higher pH (pH > 11). Con trolled po ten tia l elec trol y sis con firms that the re duc tion of the ni tro group in a four-electron step to N-phenyl-4-hydroxylamine sulphonamide is al ways the pre pon der ant pro cess. Key words: nitrobenzenesulphonamide, cath odic cleav age, amine unprotection In tro duc tionElec tro chem i cal cleav age method us ing con trolled poten tial elec trol y sis at a cath ode un der milder con di tions as a se lec tive way of re mov ing re duc ible pro tect ing groups has been in ves ti gated as a pos si ble al ter na tive to the dras tic meth ods of ten used, such as al kali metal so lu tions, high tem per a ture, and very acidic so lu tions, which can lead to deg ra da tion or racemisation in the mol e cule 1,2 . The cath odic cleav age of amines and amino-acids protected by a nitrobenzenesulphonyl group has been elec trochem i cally in ves ti gated in re cent years with the aim of study ing a sim ple method of unprotection of the amino func tional group 3-5 . In these stud ies, the re duc tion of N-butyl-2-, 3-or 4-nitro-benzenesulphonamide 3 ; N,N-di-n-butyl-4-nitrobenzenesulphonamide; and N-phenyl -2-, 3-, or 4-nitrobenzenesulphonamide 5 , and 4-nitrobenzenesulphonyl-alanine and 4-nitrobenzenesulphonyl-phenylalanine in N,N-dimethyl-formamide 4 has been in ves ti gated. In pre vi ous pa pers it is pos si ble to as cer tain that the elec trochem i cal re moval of the nitrobenzenesulphonyl group in an aprotic sys tem oc curs by S-N bond cleav age (es sen tial for unprotection of amine or aminoacid), but it is greatly affected by the pro ton avail abil ity and fea tures of the R group of the re sult ing amidic group. In gen eral, a re duc tion mecha nism of nitrobenzenesulphonyl de riv a tives of aliphatic amines and amino ac ids in N,N-dimethylformamide (DMF) lead ing to cleav age of the S-N bond af ter a two-electron trans fer is al ways pre pon der ant. How ever, the com pet i tive N-H bond cleav age makes the elec trochem i cal unprotection method un suit able for ar o matic amines, as cleav age of the S-N bond is not the ma jor process.The pro tec tion of an amine func tion is usu ally car ried out by a re ac tion un der s...