Theoretical evidence for the nucleophilic addition of sulfur dioxide to 1,2-bridged chloronium and bromonium ions

(Chemical Equation Presented) The nucleophilic addition of SO₂, SO₂ClF, and SO₂F₂ to carbenium ions and the nucleophilic addition of SO₂ to 1,2-bridged halonium ions are theoretically investigated by using B3LYP/6-311+G(d,p). On the basis of geometric changes in ion-solvent complexes compared to isolated molecules, the theoretical data for the addition of solvent to carbenium ions uniformly agree with experiments by Olah and Donovan. The relative reactivity of carbenium ions (methyl > ethyl > iso-propyl > tert-butyl) follows the familiar trend based on electron demand at the carbenium center. The theoretical data for the addition of SO₂ to 1,2-bridged halonium ions with use of similar methods indicate that this addition exhibits a reversed trend on the basis of the electron demand; SO₂ adds to 2,2-dimethylethylene chloronium and bromonium ions but does not add to the fluoronium analogue. Furthermore, the addition depends on the stereochemistry of the approaching SO₂. When SO₂approaches syn to the halogen atom on the halonium ion, addition is observed. When SO₂ is anti, addition is not observed. The reversed reactivity and stereochemistry of the addition of SO₂ to halonium ions can be explained by electron donation from the halogen atom to the sulfur atom of the approaching SO₂. This sulfur-halogen interaction activates the nucleophilicity of the approaching SO₂ and makes a normally unreactive tertiary carbenium carbon susceptible to addition. The theoretical evidence for covalently bound halonium ion-SO₂ complexes is discussed in the context of previously reported experimental evidence for the existence of equilibria involving β-halocarbenium ions.