Discovery of temperature-dependent, autoinductive reversal of enantioselectivity: palladium-mediated [3+3]-annulation of 4-hydroxycoumarins†
An unusual temperature-dependent autoinductive reversal of enantioselectivity (TARE) was discovered in an asymmetric palladium-mediated [3+3]-annulation of 4-hydroxycoumarin with Morita–Baylis–Hillman acetate. The absolute stereochemistry of the reaction product can be readily inversed by solely modifying the reaction temperature (from 10 °C to 60 °C), affording multicyclic adducts with the opposite configurations respectively in moderate to excellent enantiopurities. Furthermore, the first reported example of palladium-mediated bidirectional asymmetric autoinduction was identified to mainly contribute to the reversal of enantioselectivity, in which the corresponding adduct actively participated in the stereocontrol during the reaction. The correlation between reaction temperature and autoinduction was established, which might broaden the horizon of stereocontrol in asymmetric catalysis.