Organic Reactions of Sulfur Dioxide. Reactions with Nucleophilic Duoble Bonds Leading to the Isomerization, Aromatization, Selective Hydrogen-Deuterium Exchange, and Electron-Transfer Processes

Article abstract of DOI:10.1021/jo00173a028

Sulfur dioxide reacts readily with 1-alkoxycyclohexenes to give unstable 1,2,3,4,5a,6,7,8,9,9b-decahydro-4a,9a-dialkoxydibenzothiophene 5,5-dioxides, which, after elimination of alcohol, afford crystalline 1,2,3,4,5a,6,7,8,9-nonahydro-4a-alkoxydibenzothiophene 5,5-dioxides.These tricyclic sulfones undergo sulfur dioxide extrusion to give a 1:1 mixture of 2- and 6-cyclohex-1-enyl-1-alkoxy-1-cyclohexenes.This mixture of the 1,3- and 1,4-dienes reacts quantitatively with sulfur dioxide to give back the starting tricyclic sulfones.The apparent isomerization of the 1,4-diene to 1,3-isomer is an example of a quite general isomerization of a wide variety of olefins in sulfur dioxide.It appears that these reactions involve the allylic sulfinic acids as transient intermediates, which are capable of a very facile 1,3-thiaallylic rearrangement.When the reactions of those olefins that do undergo isomerization in liquid sulfur dioxide are carried out in the presence of deuterium oxide, a highly stereospecific hydrogen-deuterium exchange is observed.For example, optically active 1-methyl-4-isopropyl-cyclohexene affords 6,6-dideuterio-1-(trideuteroimethyl)-4-isopropylcyclohexene without appreciable racemization. 1,4-Cyclohexadienes react with liquid sulfur dioxide at room temperature to give quantitatively the aromatic hydrocarbon, elemental sulfur, and water. 1,4-Disubstituted 1,3-cyclohexadienes, on the other hand, under similar conditions provide only a small amount of the aromatic hydrocarbon, the main product being a polymeric hydrocarbon(s).Surprisingly, 1-methyl-4-isopropyl-1,3-cyclohexadiene in the presence of oxygen provided 1-methyl-4-isopropylbenzene exclusively.This reaction involves ascaridole as the intermediate, which then reacts with sulfur dioxide to give the aromatic hydrocarbon and sulfuric acid.When the reaction with oxygen is conducted at a low temperature, the ascaridole can be obtained in a quantitative yield.The mechanisms of olefin isomerization, hydrogen-deuterium exchange, and aromatization of cyclohexadienes are discussed.