Tellurium ylides epoxidation

In this chapter, we will review the use of ylides as enantioselective organocata-lysts. Three main types of asymmetric reaction have been achieved using ylides as catalysts, namely epoxidation, aziridination, and cyclopropanation. Each of these will be dealt with in turn. The use of an ylide to achieve these transformations involves the construction of a C-C bond, a three-membered ring, and two new adjacent stereocenters with control of absolute and relative stereochemistry in one step. These are potentially very efficient transformations in the synthetic chemist s arsenal, but they are also challenging ones to control, as we shall see. Sulfur ylides dominate in these types of transformations because they show the best combination of ylide stability [1] with leaving group ability [2] of the onium ion in the intermediate betaine. In addition, the use of nitrogen, selenium and tellurium ylides as catalysts will also be described. 

Selenium and tellurium ylides take part in chemistry which is analogous to the reactions discussed thus far, and the subject has been well reviewed. Both alkenes and epoxides are formed in their reactions. 

The use of ylides derived from tellurium analogues of sulfides 2a and 2b for asymmetric epoxidation has been described [68, 69]. Preliminary experiments using 15 as catalyst gave high enantio- and diastereoselectivities, but poor yield (see Scheme 10.13). 

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