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The origins of barriers to orbital symmetry/topology‐allowed pericyclic reactions: Conjugate addition of methylene to s ‐ cis ‐1,3‐butadiene

1981; Wiley; Volume: 2; Issue: 1 Linguagem: Inglês

10.1002/jcc.540020102

1096-987X

Nathan L. Bauld, David D. Wirth,

Advanced Chemical Physics Studies

Abstract The barriers on the MINDO /3 reaction paths for π and σ approaches to concerted cycloaddition of methylene to s ‐ cis ‐1,3‐butadiene are analyzed by a new energy decomposition scheme and compared to the results of published extended Hückel calculations. The relatively small (6 kcal) preference for the σ approach is essentially based on next‐nearest‐neighbor (1,3) resonance interactions, which are stabilizing in the σ but destabilizing in the π approach. Both routes are characterized by large distortion energies (mainly disrotation at the diene termini in the case of the π approach) and nonbonded repulsions. The excessive magnitude of both of these effects ultimately derives from the weak pericyclic binding which prevails at initial diene geometries, especially the pericyclic topology involving the methylene LUMO . Although previous work has emphasized the contribution of closed‐shell repulsions to the barrier, no such effect is explicitly included in MINDO . The closed‐shell repulsions to which the extended Hückel barrier has been solely attributed are shown to involve next‐nearest‐neighbor (antiaromatic) resonance interactions.