Proton, Electron, and Hydrogen Atom Transfers from Ions, Radical, and Radical Ions Derived from Substituted Urazoles and Triazolediones

Article abstract of DOI:10.1021/jo00030a017

In dimethyl sulfoxide (DMSO) solution, pK_a's for the monoanion and radical derived from 4-phenylurazole have been determined to be 24.8 and 9 +/- 2, respectively.The acidity constant for the 4-phenylurazolyl radical has been determined via a thermochemical cycle that incorporates proton- and electron-transfer data for ions and radicals derived from 4-phenylurazole and 4-phenyl-1,2,4-triazoline-3,5-dione.The acidity data indicate that (a) the 4-phenylurazolide monoanion is ca. 14 pK_a units less acidic than 4-phenylurazole (pK_a = 11.0) and (b) the 4-phenylurazolyl radical is slightly more acidic than 4-phenylurazole.The estimated pK_a for the 4-phenylurazolyl radical is reasonable in light of the reversible cyclic voltammetric reduction observed for 4-phenyl-1,2,4-triazoline-3,5-dione.Also in DMSO solution, the homolytic strengths of hydrazyl N-H bonds present in 4-phenylurazole as well as for the monoanion and radical derived from 4-phenylurazole, are within 3 kcal/mol of each other.These data suggest that the 4-phenylurazolyl radical disproportionation raction (forming 4-phenylurazole and 4-phenyl-1,2,4-triazoline-3,5-dione) is approximately thermoneutral.Similar relationships are found for ions, radicals, and radical ions derived from 4-methylurazole and 4-methyl-1,2,4-triazoline-3,5-dione.