Equilibrium between 2-oxomorpholin-3-yl radicals and viologen radicals. Determination of reduction potentials

Article abstract of DOI:10.1021/ja00027a026

Bi(3,5,5-trimethyl-2-oxomorpholin-3-yl) (TM-3 dimer) undergoes bond homolysis to yield 3,5,5-trimethyl-2-oxomorpholin-3-yl (TM-3), which reduces propyldiquat (PDQ2⁺) to its radical cation PDQ⁺. The byproduct is 5,6-dihydro-3,5,5-trimethyl-1,4-oxazin-2-one (8). Similarly, bi(5,5-dimethyl-4-ethyl-2-oxomorpholin-3-yl) (DEM-3 dimer) cleaves to 5,5-dimethyl-4-ethyl-2-oxomorpholin-3-yl (DEM-3), which reduces paraquat (PQ²⁺) to its radical cation PQ⁺. The byproduct, 5,5-dimethyl-4-ethyl-3-methoxy-2-oxomorpholine (10), results from rapid addition of methanol solvent to the transient 5,6-dihydro-4-ethyl-5,5-dimethyl-1,4-oxazin-2-onium cation (11). Concentration versus time data for the respective viologen radical cations together with reduction potentials for the viologens place the reduction potentials for TM-3 dimer and DEM-3 dimer at -0.56 and -0.33 V versus NHE, respectively, in Tris/Tris·H⁺ buffered methanol. The kinetics of reduction are analyzed using numerical integration, and the two reducing agents are compared with dithionite.