title: Diastereoselective hydrogen-transfer reactions: an experimental and DFT study.
abstract: Radical reductions of halogenated precursors bearing a heterocycle exo (α) to the carbon-centered radical proceed with enhanced anti-selectivity, a phenomenon that we termed "exocyclic effect". New experimental data and DFT calculations at the BHandHLYP/TZVP level demonstrate that the origin of the exocyclic effect is linked to the strain energy required for a radical intermediate to reach its reactive conformation at the transition state (ΔE(≠)(strain)). Furthermore, radical reductions of constrained THP systems indicate that high 2,3-anti inductions are reached only when the radical chain occupies an equatorial orientation. Hydride deliveries to different acyclic substrates and calculations also suggest that the higher anti-selectivities obtained with borinate intermediates are not related to the formation of a complex mimicking an exocycle. From a broader standpoint, this study reveals important conformational factors for reactions taking place at a center vicinal to a heterocycle or an α-alkoxy group.
authors.affiliation: Département de Chimie, Université de Montréal, C.P. 6128, Succursale Centre-Ville, Montréal, Québec, H3C 3J7, Canada.
authors.forename: François
authors.initials: F
authors.lastname: Godin
authors.forename: Michel
authors.initials: M
authors.lastname: Prévost
authors.forename: Serge I
authors.initials: SI
authors.lastname: Gorelsky
authors.forename: Philippe
authors.initials: P
authors.lastname: Mochirian
authors.forename: Maud
authors.initials: M
authors.lastname: Nguyen
authors.forename: Frédérick
authors.initials: F
authors.lastname: Viens
authors.forename: Yvan
authors.initials: Y
authors.lastname: Guindon
keywords: borinate; conformational analysis; density-functional calculations; diastereoselectivity; radical reactions