Quantum chemical characterization of cycloaddition reactions between 1,3-butadiene and oxyallyl cations of varying electrophilicity

Hydroxyallyl cation and lithium and sodium oxyallyl cations are predicted t o react with 1,3-butadiene both in a stepwise fashion and via concerted [4 + 3] cycloaddition with so-called extended stereochemistry. With hydroxyall yl cation, the stepwise process is preferred and subsequent second bond clo sures 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 an predicted to be preferred over stepw ise processes, with [3 + 2] cycloaddition to generate a 3H-dihydrofuran fol lowed by Claisen rearrangement of that intermediate being the lowest energy pathway for formation of a seven-membered ring. In the case of uncharged 2 -oxyallyl, only transition state structures for concerted cycloadditions ap pear to exist. We infer that for [3 + 3] cycloadditions, concerted pathways are preferred over stepwise pathways provided that the separation between the electrophilicity of the allyl component and the electrofugacity of the 4 pi component is not too large. The Hammond postulate is shown to rational ize 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. Copyright (C) 2000 John Wiley & Sons, Ltd.