Quantum Chemical Characterization of Cycloaddition Reactions between 1,3-Butadiene and Oxyallyl Cations of Varying Electrophilicity
Cramer, C. J.; Barrows, S. E.
J. Phys. Org. Chem. 2000, 13, 176.
Hydroxyallyl cation and lithium and sodium oxyallyl cations are predicted to react with 1,3-butadiene both in a stepwise fashion and via concerted [4+3] cycloaddition with so-called extended stereochemistry. With hydroxyallyl cation, the stepwise process is preferred and subsequent second bond closures generate products equivalent to those that would arise from concerted [4+3] or [3+2] cycloadditions. For lithium and sodium oxyallyl cations, concerted, asynchronous processes are predicted to be preferred over stepwise ones, with [3+2] cycloaddition to generate a 3H-dihydrofuran followed by Claisen rearrangement of that intermediate being the lowest energy pathway for formation of a 7-membered ring. In the case of uncharged 2-oxyallyl, only transition state structures for concerted cycloadditions appear to exist. We infer that for [4+3] cycloadditions, concerted pathways are preferred over stepwise ones provided the separation between the electrophilicity of the allyl component and the electrofugacity of the 4p component is not too large. The Hammond postulate is shown to rationalize variations in free energies of activation for different processes as a function of allyl electrophilicity. Factors influencing the stereochemical outcome of different cycloadditions are discussed.
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