Recent Advances in Organochalcogen Synthesis Based on Reactions of Chalcogen Halides with Alkynes and Alkenes

The review describes reactions of selenium and tellurium halides with alkenes and alkynes leading to useful organochalcogen compounds with main focus on the literature of last decade. The reactions of selenium dichloride and dibromide - new electrophilic reagents, which have been recently introduced in organic synthesis, along with the reactions of "classic" electrophilic reagents selenium and tellurium tetrahalides are discussed. The application of selenium dihalides in the last decade allows carrying out a series of previously unknown efficient reactions, which provide a variety of useful products including novel heterocyclic compounds. As a rule, reactions of selenium dihalides with alkynes proceed as anti-addition to give anti-Markovnikov products, whereas reactions of tellurium tetrahalides with alkynes occur in most cases as syn-addition to give Markovnikov products. However, there are exceptions to the rules, which are discussed in the review. For example, reactions of tellurium tetrahalides with acetylene proceed as anti-addition. Addition of selenium dihalides to alkenes gives Markovnikov products or a mixture of Markovnikov and anti-Markovnikov products. The formation of Markovnikov thermodynamic products in this case is the result of rearrangement of initial formed anti-Markovnikov kinetic products. The trends of reactions of selenium dihalides with alkenes and alkynes can be rationalized by assuming the formation of seleniranium and selenirenium intermediates. Stereoselective syn-addition of tellurium tetrachloride to alkynes apparently proceeds via a mechanism involving the 4-membered transition state.