Solvent Control of Cheletropic Recombination in Benzosultine-Derived ortho -Quinodimethane Diels–Alder Reactions

Thermolysis of benzosultines generates ortho-quinodimethanes (o-QDMs) widely used in Diels-Alder (DA) chemistry, yet productive cycloaddition is often limited by rapid cheletropic SO2 recombination to form benzosulfolenes. Combined computational and experimental studies support a model in which benzosultine thermolysis proceeds through kinetically competent o-QDM/SO2 encounter complexes positioned at the boundary between pericyclic and open-shell reactivity. Although reversible C-S homolysis accounts for efficient TEMPO trapping, stereochemical probe experiments establish that productive cycloaddition proceeds through a concerted DA pathway. The absence of o-QDM dimerization indicates that free o-QDM does not accumulate under the reaction conditions, leading to an apparent mechanistic paradox: how can intermolecular cycloaddition compete with rapid cheletropic SO2 recombination? The combined computational and experimental data are most consistent with a model in which DA reactions occur within the same solvent cage as competing SO2 recombination. As a result, cheletropic SO2 recombination is highly medium sensitive: replacing aromatic solvents such as toluene with nonpolar aliphatic hydrocarbons markedly suppresses benzosulfolene formation and reverses product selectivity in favor of productive DA cycloaddition. These findings establish solvent-controlled encounter-complex partitioning as a general strategy for improving benzosultine-derived o-QDM cycloadditions.