| The concept of Lewis base activation of selenium Lewis acids has been effectively reduced to practice in the Lewis base catalyzed selenofunctionalization of unactivated olefins. In this reaction, the weakly acidic species, N-phenylselenyl succinimide, is cooperatively activated by the addition of a "soft" Lewis base donor (phosphine sulfides, phosphine selenides, and phosphines), and a weak Bronsted acid activator. Through spectroscopic studies, this form of activation has been shown to lead to the formation of a cationic Lewis base-selenium(II) complex, which is a competent catalyst for the selenolactonization and selenoetherification reaction.;A variety of chiral, "soft" Lewis base donors were prepared and tested in the selenofunctionalization reaction. An enantioselective process was never realized. Mechanistic investigations have revealed that the seleniranium ion intermediate (which is formed after delivery of the electrophilic selenium source to the olefin, and is the stereodetermining event in this transformation) is configurationally unstable under the reaction conditions. In particular, rapid deselenylation and subsequent racemization of the seleniranium ion has been shown to occur in the presence of alcohol and olefin nucleophiles. Investigations into retarding the rate of racemization of the seleniranium ion intermediate have revealed that electron-poor and sterically encumbered arylselenides are less prone to racemize in the presence of nucleophiles. |
