A Computational Study of Heteroatom Analogues of Selenoxide and Selenone syn Eliminations.

Abstract

Selenoxide syn elimination is a widely used method for the synthesis of alkenes because it proceeds under exceptionally mild conditions, typically with excellent regio- and stereoselectivity. Surprisingly, hetero-selenoxide eliminations, where one or both olefinic carbon atoms are replaced with heteroatoms, have been little investigated, and their selenonyl counterparts even less so. A variety of such reactions, where the heteroatoms included combinations of O, N and S, as well as C, were investigated computationally. Selenoxides typically have lower activation energies and are slightly endothermic, while the corresponding selenones display higher activation energies and are exothermic in the gas state. The results are consistent with concerted, five-centre processes, leading to the formation of dioxygen, aldehydes, diazenes and imines from seleninyl or selenonyl peroxides, esters, hydrazines and amines, respectively. The more acidic selenenyl hydrodisulfide analogue undergoes proton transfer to the basic selenoxide oxygen atom instead of concerted elimination, resulting in the formation of a zwitterion. However, the formation of the corresponding selenonyl zwitterion is disfavoured compared to concerted syn elimination. The effects of solvents were also computed along with changes in enthalpy, entropy and free energy. Solvent effects were variable, while free energy calculations indicated overall ΔG values ranging between 3.60 and -32.12 kcal mol-1 for the syn eliminations of methyl methanethioseleninate and methaneperoxyselenonic acid, respectively. These computations suggest that the olefin-forming selenoxide syn elimination may be more general than currently understood and that replacement of the two carbon atoms with heteroatoms can lead to viable processes.