title: Theoretical study to explain how chirality is stored and evolves throughout the radical cascade rearrangement of enyne-allenes.
abstract: This article reports a theoretical study to explain how the intrinsic property of chirality is retained throughout the radical cascade rearrangement of an enantiopure chiral enyne-allene (bearing one stereogenic center) selected as a model for this family of reactions. Calculations at the MRPT2/6-31G(d)//CASSCF(10,10)/6-31G(d) level of theory were used to determine the entire reaction pathway which includes singlet state diradicals and closed-shell species. The cascade process involves three elementary steps, i.e., by chronological order: Myers-Saito cycloaromatization (M-S), intramolecular hydrogen atom transfer (HAT), and recombination of the resulting biradical. The enantiospecificity of the reaction results from a double transmission of the stereochemical information, from the original center to an axis and eventually from this axis to the final center. The first two steps lead to a transient diradical intermediate which retains the chirality via the conversion of the original static chirogenic element into a dynamic one, i.e., a center into an axis. The only available routes to the final closed-shell tetracyclic product imply rotations around two σ bonds (σ(C-C) and σ(C-N), bonds β and α respectively). The theoretical calculations confirmed that the formation of the enantiomerically pure product proceeds via the nonracemizing rotation around the σ(C-C) pivot. They ruled out any rotation around the second σ(C-N) pivot. The high level of configurational memory in this rearrangement relies on the steric impediment to the rotation around the C-N bond in the chiral native conformation of the diradical intermediate produced from tandem M-S/1,5-HAT.
authors.affiliation: Aix-Marseille Université , CNRS, Institut de Chimie Radicalaire (UMR-7273), 13397 Marseille Cedex 20, France.
authors.forename: Anouk
authors.initials: A
authors.lastname: Gaudel-Siri
authors.forename: Damien
authors.initials: D
authors.lastname: Campolo
authors.forename: Shovan
authors.initials: S
authors.lastname: Mondal
authors.forename: Malek
authors.initials: M
authors.lastname: Nechab
authors.forename: Didier
authors.initials: D
authors.lastname: Siri
authors.forename: Michèle P
authors.initials: MP
authors.lastname: Bertrand
mesh_terms: D000466:Alkadienes / Q000737:chemistry*; D000480:Alkynes / Q000737:chemistry*; D003500:Cyclization; D008962:Models, Theoretical; D015394:Molecular Structure; D013237:Stereoisomerism
chemical_list: D000466:Alkadienes; D000480:Alkynes; C025947:propadiene