title: Stereoselective rhodium-catalysed [2+2+2] cycloaddition of linear allene-ene/yne-allene substrates: reactivity and theoretical mechanistic studies.
abstract: Allene-ene-allene (2 and 5) and allene-yne-allene (3 and 7) N-tosyl and O-linked substrates were satisfactorily synthesised. The [2+2+2] cycloaddition reaction catalysed by the Wilkinson catalyst [RhCl(PPh3 )3 ] was evaluated. Substrates 2 and 5, which bear a double bond in the central position, gave a tricyclic structure in a reaction in which four contiguous stereogenic centres were formed as a single diastereomer. The reaction of substrates 3 and 7, which bear a triple bond in the central position, gave a tricyclic structure with a cyclohexenic ring core, again in a diastereoselective manner. All cycloadducts were formed by a regioselective reaction of the inner allene double bond and, therefore, feature an exocyclic diene motif. A Diels-Alder reaction on N-tosyl linked cycloadducts 8 and 10 allowed pentacyclic scaffolds to be diastereoselectively constructed. The reactivity of the allenes on [2+2+2] cycloaddition reactions was studied for the first time by density functional theory calculations. This mechanistic study rationalizes the order in which the unsaturations take part in the catalytic cycle, the reactivity of the two double bonds of the allene towards the [2+2+2] cycloaddition reaction, and the diastereoselectivity of the reaction.
authors.affiliation: Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona (UdG), Facultat de Ciències, Campus de Montilivi, s/n, 17071 Girona (Spain), Fax: (+34) 972-41-81-50.
authors.forename: Ewelina
authors.initials: E
authors.lastname: Haraburda
authors.forename: Óscar
authors.initials: Ó
authors.lastname: Torres
authors.forename: Teodor
authors.initials: T
authors.lastname: Parella
authors.forename: Miquel
authors.initials: M
authors.lastname: Solà
authors.forename: Anna
authors.initials: A
authors.lastname: Pla-Quintana
keywords: allenes; cycloaddition; density functional calculations; reaction mechanisms; rhodium