Origin of Stereoselectivities in Asymmetric Alkoxyselenenylations

Abstract

Asymmetric alkoxyselenenylations of alkenes have been studied by B3LYP/3-21G, B3LYP/6-31G*, and SCI-PCM calculations. Stereoselectivities can be rationalized by the relative stabilities of transition states for attack of nucleophiles on seleniranium intermediates. A model to explain and predict stereoselectivities has been developed.

Organoselenium Chemistry in Stereoselective Reactions

“…The mechanistic course of the oxyselenenylation reaction with the chiral reagents 22 and 25 has been investigated in detail. [31,41] The presence of a chiral moiety in these reagents results in a differentiation between the two faces of unsymmetrically substituted alkenes. The attack of the alkene double bond from either the Re-or the Siside is different from the steric and electronic point of view, and the resulting seleniranium ions 18 a and 18 b are diastereomers (Scheme 10).…”

“…It was found that the cationic species involved in the reaction are stabilized by solvent molecules, although the solvent ± selenium interaction is relatively weak and reversible. [41] Furthermore, alkenes with aromatic substituents can p-stack with the aryl surface of the reagents, leading to seleniranium ions with higher stabilities than alkenes with alkyl substituents. The independent synthesis of the diastereomeric seleniranium ions allowed a detail study of the different stabilities of these intermediates.…”

Computational Insights into the Role of the Frontiers Orbital in the Chemistry of Tridentate Ligands

“…Fo r instance, studies with 69 additional published crystal structures of organic, inorganic, and organometallic co mpounds containing divalent sulfur (S bonded to two ligands, Y and Z, different fro m H) revealed that, in the lattice, electrophiles tend to approach the sulfur atom roughly 20° fro m a line perpendicular to the Y-S-Z ato ms plane, whereas nucleophiles tend to approach approximately along the extension of one of the sulfur covalent bonds [57,58]. We believe that these regularities portray features of the electron distribution and indicate the preferred d irection of appro ximation of electrophiles and nucleophiles, as well as the preferred direction for non-bonded interactions.…”

Organoselenchemie in der stereoselektiven Synthese

“…Der mechanistische Verlauf der Oxyselenylierung mit den chiralen Reagentien 22 und 25 wurde detailliert untersucht. [31,41] Die Anwesenheit der chiralen Einheit in diesen Reagentien bedingt die Differenzierung zwischen den beiden Seiten unsymmetrisch substituierter Alkene. Der Angriff der Alken-Doppelbindung entweder von der Re-oder von der SiSeite ist sowohl aus sterischen als auch aus elektronischen Gründen verschieden, und die resultierenden SeleniraniumIonen 18 a und 18 b sind Diastereomere (Schema 10).…”

“…Die dabei involvierten kationischen Spezies werden durch Lö-sungsmittelmoleküle stabilisiert, wenngleich die Wechselwirkung zwischen Lösungsmittel und Selen relativ schwach und reversibel ist. [41] Zudem haben Alkene mit aromatischen Substituenten die Möglichkeit einer p,p-Wechselwirkung mit der aromatischen Einheit der Reagentien, was zu stabileren Seleniranium-Ionen als mit aliphatischen Alkenen führt. Eine unabhängige Synthese der diastereomeren SeleniraniumIonen erlaubte zudem eine detaillierte Untersuchung der Stabilitäten dieser Intermediate.…”