Concerted and stepwise mechanisms in metal-free and metal-assisted [4+3] cycloadditions involving allyl cations

Abstract

The thermal [4+3] cycloaddition reaction between allenes and tethered dienes (1,3-butadiene and furan) assisted by transition metals (AuI, AuIII, PdII, and PtII) was studied computationally within the density functional theory framework and compared to the analogous non-organometallic process in terms of activation barriers, synchronicity and aromaticity of the corresponding transition states. It was found that the metal-mediated cycloaddition reaction is concerted and takes place via transition structures that can be even more synchronous and more aromatic than their non-organometallic analogues. However, the processes exhibit slightly to moderately higher activation barriers than the parent cycloaddition involving the hydroxyallylic cation. The bond polarization induced by the metal moiety is clearly related to the interaction of the transition metal with the allylic π* molecular orbital, which constitutes the LUMO of the initial reactant. Finally, replacement of the 1,3-butadiene by furan caused the transformation to occur stepwise in both the non-organometallic and metal-assisted processes. Concerted or stepwise? We present a comparison between classical oxyallyl cation-diene cycloadditions and the analogous reaction involving a metal moiety (see scheme) in terms of activation barriers, synchronicity, and aromaticity of the corresponding transition states. The factors that control the concertedness of the process are analyzed theoretically and discussed. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.