Sulfoxide-magnesium exchange

B. Generation of Magnesium Carbenoids by Sulfoxide-Magnesium Exchange Reaction... 

Satoh has developed a related sulfoxide-magnesium exchange reaction between aryl a-chloroalkyl- and a-chlorovinylsulfoxides, and has extensively studied the chemistry of the resulting magnesium carbenoids and magnesium alkylidene carbenoids. ... 

This sulfoxide-magnesium exchange reaction could be successfully applied to a new synthesis of allenes (Scheme 3.12) [6]. The procedure is a novel method for synthesis of allenes from three components, ketones, chloromethyl p-tolyl sulfoxide, and sulfones, in relatively short steps. A key step is an attack of the lithium a-sulfonyl carbanion on the electron-deficient carbene carbon. /1-Elimination of the sulfoxyl group then occurs to give the allene. 

Sulfoxide-magnesium exchange on the a-chloroalkyl sulfoxide 42 was performed with ethylmagnesium bromide followed by trapping with benzaldehyde-dimethyl-aluminum chloride, to furnish 99% of the sulfoxide 43 with 99% ee. Chlorohydrins 44 were obtained in 56% yield as a 94 6 diastereomer mixture. After conversion to the epoxide 45, the latter had an ee of >98% (Scheme 3.18). 

In addition to halides, sulfoxides can also serve as organomagnesium precursors. Importantly, the sulfoxide-magnesium exchange is a very convenient method for the preparation of configurationally well-defined magnesium carbenoids such as 25 (Scheme 5.3). 

The recently introduced sulfoxide/magnesium-exchange gives rise to the formation of alkylmagnesium halides as well. Thus, the reaction of the sulfoxide 257 with iPrMgBr at -78 C furnished the desired magnesium carbenoid 258 that reacts with PhCHO with an excellent diastereoselectivity providing the mono-protected 1,2-diol 259 in 61% yield (dr= 93 7) (Scheme 4.54) [133]. 

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