Carbophilic reactions of amines or methanol with thioaldehyde-S-oxides

Reaction of primary aliphatic amines with gamma-ethylenic thioaldehyde-S-oxides 1a,b affords the corresponding imines 2a,b via a carbophilic addition of amine. When treated with sodium methoxide (1 equiv) and an excess of a primary amine, some methyl sulfinates 3 or 6, bearing a benzylic or allylic chain on the sulfur, are converted into imines 4 or 7. Treatment of the methyl sulfinates 3 with methoxide anion in THF or methanol generally affords a mixture of aldehydes 9, the corresponding dimethylacetals 14, alpha,alpha'-dimethoxydisulfides 10, esters 11, thionoesters 12 and methyl alpha-methoxysulfinates 15, whose ratios depend on the conditions. The formation of these compounds is best explained by a deprotonation-elimination of the methyl sulfinates into the corresponding thioaldehyde-S-oxides, which then undergo a carbophilic addition of methoxide anion or methanol. Several possible reaction paths from the so-formed alpha-methoxysulfenate anions IX are discussed for their conversion into the final compounds.