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 (scCO2). 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.