ANIONIC OXIDATION OF THIOLS AND CO-OXIDATION OF THIOLS WITH OLEFINS

Abstract The anionic oxidation of thiols to disulfides by molecular oxygen is commonly used for the sweetening of hydrocarbon fuels to increase fuel stability. In truly anionic oxidations, the rate determining step is the electron transfer reaction of the thiolate anion with oxygen. Since the more stable anions react at a slower rate, aliphatic thiols are oxidized faster than aromatic thiols. The rate of anionic oxidations is greatly enhanced by dipolar solvents since they complex with the metal (sodium) cation associated with the thiolate anion. Some anionic thiol oxidations are catalyzed. In these, the thiolate anion reacts with the oxidized form of the catalyst and not directly with the oxygen. Such catalysts are the N,N'-dialkyl-p-phenylene diamines, quinones, transition metal-chelates and -salts. Aliphatic amines alone also catalyze the air oxidation of thiols, probably by forming alkylammonium thiolates. The co-oxidation of thiols and olefins to hydroperoxides is a rapid autoxidation reaction closely related to the radical addition of thiols to olefins. The substituted 2-mercaptoethyl hydroperoxides from thiol-mono-olefin co-oxidations spontaneously rearrange to the corresponding 2-sulfinylethanols. In the absence of alkylamines, however, the same hydroperoxides are reduced by excess thiol to the 2-mercaptoethanols. The order of reactivity of anions, thiols, and olefins in co-oxidations is similar to that observed in their other radical reactions. The more reactive double bond of diolefins with isolated double bonds, such as dicyclopentadiene and Aldrin, can be selectively co-oxidized. The intermediate allylic radical from conjugated diolefin-thiol co-oxidations reacts with oxygen at the more highly substituted allylic carbon. The products are derived from the more important resonance form of the allylic radical, since its combination with the oxygen diradical does not require any significant activation energy. Another consequence of the rapid radical combination with oxygen is the complete preservation of cis- or trans-character in the co-oxidation reaction of cis- and trans-piperylene.

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