Three-Membered Rings With One Selenium or Tellurium Atom

Electronic and Steric Effects on the Reactivity of Seleniranium Ions with Alkenes in the Gas Phase

“…Seleniranium ions have the general structure 1 and can be formed through the displacement of a suitable β-leaving group or by the addition of a selenium electrophile to a carbon–carbon double bond. , When enantioenriched, ring-opening of these intermediates by nucleophiles in principle affords products 2 or 2 ′ with retention of stereochemical integrity, noting that intramolecular nucleophiles furnish cyclized products ( Path A , Scheme ). This pathway has been exploited by many groups to functionalize alkenes with the aim to develop methods that are both enantioselective and catalytic as highlighted in several recent reviews, − where Denmark and co-workers have established Lewis base activation of the intermediate seleniranium ion as a particularly useful approach for several transformations. − However, there are two plausible racemization pathways available to chalcogen iranium ions that must be controlled to minimize loss of enantiopurity. , The first is ring-opening to give a carbocation 1 ′ followed by bond rotation, which can be minimized by judicious choice of the substituents on the ring carbons ( Path B , Scheme ). The second is preventing nucleophilic attack at the electrophilic selenium, which is more challenging as illustrated by numerous reports in the literature of configurational scrambling by deselenylation from either inter- (including counterions or solvent) or intramolecular (e.g., pendant alcohols or carboxylic acids) nucleophiles generating electrophilic reagent 3 , which can be recaptured by the alkene leading to racemization of the initial seleniranium ion 1 ( Path C , Scheme ).…”