Investigations into the chemistry of aromatic sulfines (Thioketone-S-oxides)

Abstract

It has been found that aromatic sulfines react with diazomethane via a 1,3-dipolar cycloaddition reaction to give Delta3-1,3,4-thiadiazoline-S-oxides as the initial product. This highly unstable heterocyclic system may be isolated in some cases at low temperatures. At higher temperatures these cycloadducts decompose by loss of nitrogen to form the unstable episulfoxides which then lose sulfur monoxide to form olefin type products. The cycloadducts can also decompose via a retrocycloaddition reaction to regenerate the starting sulfine and diazo compound. Aromatic sulfines in general react with nucleophilic reagents by initial attack of the nucleophile at the electron deficient sulfinyl sulfur. In attempts to react sulfines with dichlorocarbene, it was found that the sulfines were inert to this electrophilic carbene. Rather, the sulfines react with the trichloromethyl anion prior to its decomposition into dichlorocarbene. The initial reaction intermediate then loses chloride ion to form a dichloroepisulfoxide which then loses sulfur monoxide to form the gem-dichloro-olefin. The reactions of aromatic sulfines with alkoxide ions generate a variety of products depending on the alkoxide used and the structure of the sulfine. For example, 9-thiofluorenone- S-oxide reacts rapidly with alkoxides whereas di(p-anisyl) sulfine or diphenylsulfine react not at all or only at elevated temperatures. The initial reaction of an alkoxide with a sulfine is an equilibrium step between sulfine and the corresponding sulfinate ester formed by attack of alkoxide at sulfinyl sulfur. There are several different pathways for product formation after the initial equilibrium step which are highly dependent on -the structure of the alkoxide ion. Although mechanisms describing product formation are presented, further work must be done in this area in order to more clearly understand the processes involved in the reactions of sulfines with alkoxides. Verification of the initial equilibrium step was obtained by synthesizing methyl 9-fluorenesulfinate and methyl diphenylmethanesulfinate and subjecting these esters to methoxide ion under the nor mal reaction conditions. Pure diphenylsulfine was isolated from the reaction of methyl diphenylmethanesulfinate with methoxide in high yield. It is felt that this procedure for preparing diphenylsulfine is the method of choice because of its convenience, simplicity, and high yield.