Dimethylsulfonium trimethylsilylmethylid

The reaction of ylides with alkenes proceeds in two steps according to the MIRC reaction mechanism. Dimethylsulfonium trimethylsilylmethylide (2), generated in situ from the corresponding sulfonium salt 1 with ec-butyllithium, reacts with various a,/l-unsaturated ketones to cyclopropanate the activated C-C double bond (see Table 2). The resulting l-acyl-2-trimethylsilylcyclopropanes 3 are isolated as mixtures of diastereomers. 

In contrast to the above described reactivity of dimethylsulfonium trimethylsilylmethylide (2), triphenylarsonium trimethylsilylmethylide (5) is able to cyclopropanate the C-C double bond of chalcone [( )-l,3-diphenylpropenone] and some of its derivatives. The arsonium ylide is prepared in situ from (trimethylsilylmethyl)triphenylarsonium tetraphenylborate (4) by deprotonation with phenyllithium. The reactions with various substituted chalcones were carried out at room temperature and at — 70 "C. Although the yields achieved for cyclopropanes 6 are almost equal, the formation of the -isomer predominates at lower temperature whereas at room temperature the isomer ratio is nearly 1 1. 

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