Why Is the Concerted (2+2) Mechanism of the Reactions of SO₃ with Alkenes Favored over the (3+2) Mechanism? Density Functional and Correlated ab Initio Calculations and a Frontier MO Analysis

Abstract

The (2+2) cycloadditions of SO3 to ethene, propene, and 2-methylpropene were investigated with Hartree−Fock, MP2, QCISD(T), and hybrid Hartree−Fock/density functional theory (HF-DFT) methods. Experimental data support a (2+2) mechanism for these reactions. The thermally allowed (3+2) cycloadditions of SO3 to ethene and propene were also examined. With the exception of MP2 calculations, all levels of theory predict that SO3 reacts by a concerted (2+2) pathway. The transition structure has considerable zwitterionic character, and there is a strong preference for Markovnikov addition. The rearrangements of sulfites to sultones are disfavored. The origin of the large preference for the normally forbidden (2+2) process is attributed to frontier orbital interactions, and is contrasted to the (3+2) cycloaddition mechanism favored by OsO4.

Establishing the (3 + 2) Mechanism for the Permanganate Oxidation of Alkenes by Theory and Kinetic Isotope Effects

“…Because the (2 + 2) adduct will not be formed, we did not explore the rearrangement surface. DFT is capable of predicting a (2 + 2) mechanism when it is favored; the (2 + 2) transition state for the reaction of SO 3 with ethylene is predicted by B3LYP calculations to be lower in energy than the (3 + 2) transition state, in accord with experiment …”

Implicit Solvation Models: Equilibria, Structure, Spectra, and Dynamics

“…The SES approximation has served as the basis of a number of previous investigations ,,,,,,,− of solvation effects on barriers using conventional or variational transition state theory. Some recent examples employing implicit-solvent methods are given in Table , which uses the same conventions as Table .…”

Are There Metal Oxides That Prefer a [2 + 2] Addition over a [3 + 2] Addition to Olefins? Theoretical Study of the Reaction Mechanism of LReO3 Addition (L = O-, Cl, Cp) to Ethylene1

“…An explanation for the reversal of the activation barriers was given in terms of the strongly polarized frontier orbitals of SO 3 , which has the LUMO essentially localized at sulfur and the HOMO localized at the oxygen atoms. 7 The second important work was recently published by Rappe ét al. 8 These workers calculated the reaction energies for the addition of LReO 3 (L ) Cp*, Cp, Cl, CH 3 , OH, OCH 3 , O -) to ethylene, yielding either the dioxylate via [3 + 2] addition or the oxetane via [2 + 2] addition.…”

“…Two recent theoretical studies have been published that are relevant for the topic of this work. Houk et al 7 reported ab initio and DFT calculations that show that the [3 + 2] addition of SO 3 to ethylene, yielding ethylensulfite, has a higher barrier than the [2 + 2] addition giving the four-membered cyclic sultone. It follows that the formally symmetry-forbidden [2 + 2] addition can become more favorable than the [3 + 2] addition.…”