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Concerted and Stepwise Mechanisms in Metal‐Free and Metal‐Assisted [4+3] Cycloadditions Involving Allyl Cations
Author(s) -
Fernández Israel,
Cossío Fernando P.,
de Cózar Abel,
Lledós Agustí,
Mascareñas José Luis
Publication year - 2010
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201001714
Subject(s) - cycloaddition , furan , chemistry , transition metal , allylic rearrangement , organometallic chemistry , moiety , aromaticity , group 2 organometallic chemistry , density functional theory , computational chemistry , metal , photochemistry , transition state , stereochemistry , molecule , organic chemistry , catalysis
The thermal [4+3] cycloaddition reaction between allenes and tethered dienes (1,3‐butadiene and furan) assisted by transition metals (Au I , Au III , Pd II , and Pt II ) 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.