Sulfur Dioxide

(A) Alkenes are isomerized by sulfur dioxide. The process is generally explained by invoking an ene reaction between an SO2 molecule and the alkene. Recently, Vogel et al. revised this mechanism and showed that polysulfones resulting from the co-polymerization of alkenes and SO2 catalyze the reaction. This has led to the development of a new method for the selective cleavage of methyl-substituted allyl ethers under neutral conditions. (B) Methane is transformed selectively to methanesulfonic acid at low temperature by liquid-phase sulfonation of methane with SO2 and O2 in the presence of Pd salts and Cu salts as the catalysts or of CaO as promoter. (C) Irradiation of N-hydroxy-2-thiopyridone esters with visible light in the presence of sulfur dioxide gives the corresponding thiosulfonates. These can be converted into sulfones by treatment with KOH and an alkylating agent. (D) 1,3-Dienes undergo hetero Diels-Alder reactions with SO2 at low temperature in the presence of an acid promoter, such as CF3CO2H or BF3·Et2O, leading to sultines. (E) Allyl tin derivatives undergo ene reactions with sulfur dioxide, furnishing tin allyl sulfinates. Recently, it was shown that allylsilanes and enoxysilanes derived from aldehydes, ketones and esters also produce ene adducts with SO2. The silyl sulfinates from ene reactions can be brominated (Br2 or NBS) or chlorinated (Cl2 or NCS) to produce the corresponding sulfonyl halides. The latter react with primary or secondary amines or alcohols to give the corresponding sulfonamides or sulfonic esters (one-pot, three-component syntheses). (F) Sultines 2, arising from HDA reactions between dienes 1 and SO2, can be quenched with carbon nucleophiles such as enoxysilanes or allylsilanes to give silylsulfinates 4. Desilylation of 4 and reaction with carbon electrophiles generates polyfunctional sulfones in one-pot, four-component operations. Alternatively, 4 can be oxidized with NCS and reacted with amines or alcohols to produce polyfunctional sulfonamides 6 or sulfonic esters 7, containing up to three stereogenic centers. Hydrolysis of silyl sulfinates 4 and subsequent desulfitation generates polyketides and analogs 8, also in one-pot operations.