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

S'ha preparat satisfactòriament compostos contenint dues unitats d'allé i un doble (2 i 5) o un triple enllaç (3 i 7) amb unitats enllaçants N-tosil i O. Aquests compostos s'han emprat com a substrats en processos de cicloaddicio [2+2+2] catalitzats pel complex de Wilkinson [RhCl(PPh3)(3)]. Els substrats 2 i 5, contenint un doble enllac en posicio central, han donat compostos triciclics en una reaccio on s'han format de forma diastereoselectiva quatre centres estereogenics. La reaccio dels substrats 3 i 7 contenint un triple enllac en posicio central, han permes lobtencio de tricicles contenint un anell central ciclohexanic en una reaccio de nou diastereoselectiva. Tots els cicloadductes s'han format mitjancant la reaccio regioselectiva del doble enllac intern de lalle. Aixi s'han obtingut compostos contenint un die exociclic. Una reaccio de Diels-Alder en els substrats amb unio N-tosil 8 i 10, ha permes la construccio diastereoselectiva destructures pentacicliques. La reactivitat dels allens en la cicloaddicio [2+2+2] s'ha estudiat per primer cop mitjancant calculs terics basats en la teoria del funcional de la densitat. Aquest estudi mecanistic ha permes racionalitzar lordre en que les insaturacions participen en la reaccio, la reactivitat dels dos dobles enllacos de lalle en la cicloaddicio [2+2+2] i la diastereoselectivitat del proces

Financial support from the Spanish Ministry of Education and Science (MINECO) (projects nos.: CTQ2011-23121/BQU, CTQ2011-23156/BQU and CTQ2012-32436), the DIUE of the Generalitat de Catalunya (project no.: 2009SGR637), and the FEDER fund (European Fund for Regional Development) for the grant UNGI08-4E-003 is gratefully acknowledged. E. H. thanks Universitat de Girona for a predoctoral grant. Excellent service by the Centre de Serveis Cientifics i Academics de Catalunya (CESCA) is gratefully acknowledged. Support for the research of M. S. was received through the ICREA Academia 2009 prize for excellence in research funded by the DIUE of the Generalitat de Catalunya