Mechanism sulfur exchange

The rearrangement (automerization) of Dewar thiophene 5-oxide (61), observed by NMR, occurs so much more rapidly than that of the corresponding episulfide that special mechanisms have been invoked. The one which involves a zwitterionic intermediate (Scheme 108) is favored over a pseudopericyclic sulfur-walk mechanism in which the electrons of the carbon-sulfur o--bond and the pair of electrons on sulfur exchange places as the sulfur atom migrates around the ring (80JA2861). 

The behavior of chiral phenyl /-butyl sulfoxide 219 and a-phenyl-ethyl phenyl sulfoxide 220 is completely different in strongly acidic media and in the presence of halide ions. Two reactions were found (266) to occur in parallel. One results in the loss of optical activity, and the second leads to the decomposition of the sulfoxide. It was observed that the racemization process is not accompanied by [ 0] oxygen exchange. In the case of sulfoxide 220 the complete loss of optical activity at chiral sulfur is accompanied by partial racemization at the chiral carbon center. These results are consistent with a sulfenic acid-ion-pair mechanism formulated by Modena and co-workers (266) as follows (it is obvious that the formation of achiral sulfenic acid is responsible for racemization). 

This indirect proof of the appearance of CF3SF leads to the conclusion that fluorination of sulfenyl chlorides of the series CF Cl3 SCl (n = 0, 1, 2) with alkali metal fluorides follows the mechanism observed in the formation of sulfenyl fluorides the initial chlorine-fluorine exchange at the sulfur atom is followed by isomerization to the sulfenyl chloride containing the corresponding more highly fluorinated methyl group. 

The reaction temperatures and some of the activation energies cited above seem to be too low to support a radical-chain reaction mechanism. Guryanova found that exchange of radioactive elemental sulfur with the p sulfur atoms of bis-p-tolyl tetrasulfide proceeds at 80-130 °C with an activation energy of only 50 kJ/mol in the case of the corresponding trisulfide the activation energy was determined as 60 kJ/mol. These data sharply contrast with the observation that liquid sulfur has to be heated to more than 170 °C to detect free radicals by electron spin resonance spectroscopy and the activation energy for homolytic SS bond scission has been determined as 150 kJ/mol (see above). 

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