Electrophilic additions to a,P-unsaturated sulfoxides

Independently, Okamura and colleagues generated racemic a-lithiated-(x,P-unsaturated sulfoxides in the same way as Posner s group and reported their reactions with methyl iodide, carbonyl compounds, and epoxides [48.49]. They also noted the configurational instability of such intermediates. Alkylation of the intermediates using butyl iodide or benzyl bromide gave low yields of products. 

The treatment of nonracemic a-lithiated-a,P-imsaturated sulfoxides with aldehydes has received attention because of the possibility of high diastereoselection on creation of the new asymmetric centre. Posner added these nonracemic vinyllithium species to various aldehydes under different conditions of solvent, reaction temperature, base used for deprotonation, structure of aldehyde, and various metal additives, but obtained no more than 25% asymmetric induction [52]. 

Nucleophilic addition to the P-position of satisfactorily unsaturated sulfoxides has been known for many years [62-64]. Both carbon- and hetero-nucleophiles have been successfully added in such a manner, often followed by sulfoxide pyrolysis, the synthetic equivalent of addition of a vinyl group to a nucleophile. 

Much recent work in this area has been devoted to stereochemical control at the P-position during addition [11,65] and this is the main focus of this review. Comparatively little work has been devoted to control of stereochemistry at the a-position during the reaction. In many cases, however, this stereochemical control becomes unnecessary since the required product is achiral at the a-position (often after removal of the sulfoxide functionality). 

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The first report of asymmetric induction in the electrophilic addition to a,P-unsaturated sulfoxides was made by Abbott and Stirling [12,41] who found that treatment of nonracemic (5)-(-i-)-p-tolyl vinyl sulfoxide (26b) with bromine in acetic acid gave the dibromide (30) with ee (a-induction) of 32% and 92% overall chemical yield (Scheme 5.10). (+)-a,P-Methylvinyl p-tolyl sulfoxide gave a higher... 

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