Diastereoselective [2 + 2] photocycloaddition of cyclohexenone derivative with olefins in supercritical carbon dioxide

We performed diastereoselective [2 + 2] photocycloaddition of the cyclohexenone derivative with olefins in supercritical carbon dioxide (scCO ₂). A dramatic enhancement of conversion was observed at around critical density when ethylene gas was employed as a coupling partner. Furthermore, we elucidated that the pressure dependence of the diastereomeric excess (de) was discontinuous at critical density owing to the difference of substrate solubility between near-critical carbon dioxide (ncCO₂) and scCO₂; both the reaction conversion and de values obtained were lower than those in conventional organic solvent. On the contrary, when cyclopentene (liquid) was utilized as a coupling partner, moderate conversion was always observed irrespective of CO₂ pressure, because the substrate could be dissolved in cyclopentene. Furthermore, we could accomplish de enhancement in scCO₂, in particular around critical density, compared to that in organic solvent. This highly selective photoreaction is due to the unique solvent property of scCO₂, which is called the clustering effect.