Selenosulfonation and related processes: Methods, mechanisms and synthetic applications

Selenosulfonates (RSO 2 SeR’) are versatile reagents that undergo free-radical or electrophilic additions to alkenes, alkynes, allenes and other unsaturated compounds with complementary regioselectivity. Such “selenosulfonations”, thus introduce two orthogonal functionalities that lend themselves to further useful transformations, such as selenoxide eliminations to form unsaturated sulfones and addition-elimination processes with a variety of organometallic reagents and other nucleophiles. The sulfone moiety is capable of stabilizing α-anions for further modification with electrophiles, as well as promoting conjugate additions and facilitating various cycloadditions when adjacent to a double or triple bond. Selenosulfonations of dienes and other polyunsaturated compounds can be employed in radical cyclizations. Furthermore, selenosulfonates serve as selenylating agents capable of introducing selenium substituents, including fluorinated alkylseleno groups, into a wide variety of substrates. The unique properties of selenosulfonates have led to a number of useful photoredox and polymer-supported processes, and have provided unsaturated sulfones for key steps in several total syntheses. While the emphasis of this review is on synthetic methodology, mechanistic explanations and applications to total synthesis are also covered. The selenosulfonation “chemistree” produced a bountiful harvest!