Sulfoxides occupy a useful and interesting part of the middle ground between sulfides and sulfones—they are weakly nucleophilic, like sulfides (and can be alkylated with methyliodide to give sulfoxonium salts, as we saw on), but at the same time they stabilize anions almost as well as sulfones. They are easily made by controlled oxidation of sulfides and the chart below gives the main ways to get from sulfides to the two oxidized functional groups.

Sulfoxides can be used to make alkenes stereoselectively because sulfoxides next to electron withdrawing or conjugating groups are unstable on heating, decomposing by an elimination process. The rather unstable phenylsulfenic acid (PhSOH) is eliminated and the reaction occurs partly because of the creation of conjugation and partly because PhSOH decomposes to volatile products. The starting material was made from cycloheptanone by sulfenylation of its enolate followed by oxidation.

The elimination follows a type of mechanism we call a pericyclic reaction. but for now you can just think of it as an elimination in which the proton is removed by the leaving group. The alkene product in this case is in a seven-membered ring—it has to be cis.

This reaction provides a useful way of introducing a double bond next to a carbonyl group. Here it is in a synthesis of the Queen Bee Substance (the compound fed by the workers to those bee larvae destined to become queens). The compound is also a pheromone of the termite and is used to trap these destructive pests. The sulfur is introduced by reacting the enolate of the ester with the sulfur electrophile MeSSMe. Next, the protecting group is removed with acid, and the sulfide is oxidized to the sulfoxide with sodium periodate (NaIO₄) ready for elimination. Heating to 110°C then gives the Queen Bee Substance in 86% yield.

This elimination takes place more easily still when sulfur is replaced by a selenium—PhSe groups can be introduced by the same method, and oxidized to selenoxides with m-CPBA at low temperature. The selenoxides are rarely isolated because the elimination takes place rapidly at room temperature.

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مواضيع ذات صلة

The Peterson reaction is stereospecific

The one-step Julia olefination

The Julia olefi nation is stereoselective

The Julia olefination is regiospecific and connective

Sulfoxide elimination—oxidation to enones

Addition of nucleophiles to alkynes

Z-selective reduction of alkynes using Lindlar’s catalyst

Predominantly E alkenes can be formed by stereoselective elimination reactions

Addition of nucleophiles to alkynes

E-selective reduction of alkynes using sodium in liquid ammonia

Using light to make Z alkenes from E alkenes

Equilibration of alkenes