title: Influence of the acetylenic substituent on the intramolecular carbolithiation of alkynes: a DFT theoretical study.
abstract: A theoretical study of the intramolecular 5-exo-dig carbolithiation of substituted propargyl o-lithioaryl ethers, leading to dihydrobenzofurans, has been performed. The results show that a DFT description of the reaction (B3P86, 6-31G**) matches the experimental data provided that an explicit solvation by two molecules of THF is considered. To take place, the cyclization also implies that the acetylenic chain adopts a conformation in which a significant interaction arises between the lithium and the C≡C triple bond. Reaching the cyclization TS requires the passage of an activation barrier that should not be higher than 12-13 kcal mol(-1). From a thermodynamic point of view, the reaction is exothermic whatever the substituent R (from approximately -40 to -62 kcal mol(-1)). In the starting substrate, a supplementary interaction between the Li and a substituent at the propargylic position can develop, influencing the future double-bond configuration. Thus, derivatives exhibiting an R-Li interaction tend to provide E olefins. In contrast, when no coordination between the lithium cation and the terminal R occurs, syn carbolithiation takes place, and the configuration of the exocyclic olefin is likely to be Z. This hypothesis accounts for most of the experimental results published before.
authors.affiliation: Laboratoire COBRA, CNRS UMR 6014 & FR 3038, Université de Rouen and INSA de Rouen, 76821 Mont St Aignan Cedex, France.
authors.forename: Catherine
authors.initials: C
authors.lastname: Fressigné
authors.forename: Rudy
authors.initials: R
authors.lastname: Lhermet
authors.forename: Anne-Lise
authors.initials: AL
authors.lastname: Girard
authors.forename: Muriel
authors.initials: M
authors.lastname: Durandetti
authors.forename: Jacques
authors.initials: J
authors.lastname: Maddaluno