Ethynylbenziodoxolones (EBX) as Reagents for the Ethynylation of Stabilized Enolates
Herein, we report a detailed study on the electrophilic alkynylation of cyclic keto esters and amides with ethynylbenziodoxolone (EBX) reagents. The structure and stability of this class of reagents is first described more in details. Differential scanning calorimetry (DSC) experiments showed a strong exothermic decomposition with EBX reagents, leading to guidelines for the safe use of these compounds. The extension of the method to aromatic alkynes and a broad range of benziodoxol(on)e reagents is then reported. Based on our preliminary results using Cinchona-based phase-transfer catalysts, the enantioselective alkynylation of cyclic keto esters could be achieved. Binaphthyl-derived ammonium catalysts developed by Maruoka and co-workers gave the highest asymmetric induction with up to 79% ee for an indanone-derived keto ester. Throughout this work, asymmetric induction was observed only in the case of benziodoxolone reagents, demonstrating their superiority over conventional alkynyliodonium salts. The deeper understanding gained about the factors leading to higher asymmetric induction will be very useful in the future to develop a truly general and highly enantioselective alkynylation method.