One-electron reduction of methanesulfonyl chloride. The fate of (MeSO2Cl) and MeSO2 intermediates in oxygenated solutions and their role in the cis-trans isomerization of monounsaturated fatty acids.

The one-electron reduction of methanesulfonyl chloride (MeSO2Cl) leads, in the first instance, to an electron adduct MeSO2Cl¥- which lives long enough for direct detection and decays into sulfonyl radicals MeSO2 ¥ and Cl-, with k ) 1.5 106 s-1. Both, MeSO2Cl¥- and MeSO2 ¥ showed a similar absorption in the UV with ìmax of 320 nm. In the presence of oxygen, MeSO2Cl¥- transfers an electron to O2 and establishes an equilibrium with superoxide. The rate constant for the forward reaction was measured to 4.1 109 M-1 s-1, while for the back reaction only an interval of 1.7 105 to 1.7 106 M-1 s-1 could be estimated, with a somewhat higher degree of confidence for the lower value. This corresponds to an equilibrium constant in the range of 2.4 103 to 2.4 104. With reference to E° (O2/O2 ¥-) ) -155 mV, the redox potential of the sulfonyl chloride couple, E°(MeSO2Cl/MeSO2Cl¥-), thus results between being equal to -355 and -414 mV (vs NHE). MeSO2Cl¥- reduces (besides O2) 4-nitroacetophenone. The underlying electron transfer took place with k ) 1.5 109 M-1 s-1, corroborating an E° for the sulfonyl chloride couple significantly exceeding the above listed lower value. The MeSO2 ¥ radical added to oxygen with a rate constant of 1.1 109 M-1 s-1. Re-dissociation of O2 from MeSO2OO¥ occurred only very slowly, if at all, that is, with k , 105 s-1. MeSO2 ¥ radicals can act as the catalyst for the cis-trans isomerization of several Z- and E-mono-unsaturated fatty acid methyl esters in homogeneous solution. The effectiveness of the isomerization processes has been addressed, and in the presence of oxygen the isomerization is completely suppressed. Introduction Sulfur-