Organolithium reagents, reaction with sulfoxides

Alkyltriphenylphosphonium halides are only weakly acidic, and a strong base must be used for deprotonation. Possibilities include organolithium reagents, the anion of dimethyl sulfoxide, and amide ion or substituted amide anions, such as LDA or NaHMDS. The ylides are not normally isolated, so the reaction is carried out either with the carbonyl compound present or with it added immediately after ylide formation. Ylides with nonpolar substituents, e.g., R = H, alkyl, aryl, are quite reactive toward both ketones and aldehydes. Ylides having an a-EWG substituent, such as alkoxycarbonyl or acyl, are less reactive and are called stabilized ylides. 

This rapid ligand exchange reaction was observed not only in simple aryl sulfoxides but also bulky sulfoxides with racemization of the recovered sulfoxide. The reaction is initiated by attack of organolithium reagent on the sulfinyl sul-... 

Furukawa, N. Ogawa, S. Matsumura, K. Fuji-hara, H. Extremely facile ligand-exchange and disproportionation reactions of diaryl sulfoxides, selenoxides, and triarylphosphine oxides with organolithium and Grignard reagents. 

Sulfur-lithium exchange is easier and has much greater potential (much of it still unrealised) when the sulfur is at the sulfoxide oxidation level. It has long been known that organolithiums, like Grignard reagents, will attack a sulfoxide, displacing with inversion at sulfur the substituent best able to support an anion. The reaction has been commonly used to form sulfoxides with defined stereochemistry 152157... 

Sulfoxides react with organolithium and Grignard reagents by four different reaction pathways. One involves nucleophilic substitution with Walden inversion at the sulfinyl sulfur atom. A second involves a concomitant ligand exchange and disproportionation reactions. A third process, recently discovered, comprises ligand coupling reactions, and the fourth is the formation of a-sulfinyl car-banions by proton abstraction. The four reactions are summarized in Scheme 4. 

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