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A novel ion‐pairing strategy was demonstrated in 2008 by Toste and coworkers (Scheme 4.39) [128]. In this example, chiral phosphoric acid or silver phosphate salts would convert amines and sulfides featuring β‐leaving groups into the corresponding aziridinium or episulfonium. The chiral‐anion would cata lyze an asymmetric ring opening of these cationic three‐membered heterocycles with alcohols, to the corresponding 1,2‐amino‐alcohols and β‐alkoxy sulfides in high yields and enantioselectivities. A computational investigation of the mechanism revealed that a C–H•••O interaction between the three‐membered ring’s C–H and the phosphate oxygen is key for stabilizing the ion‐pair intermediate, and is essential for the enantioselectivity of the reaction [129].

Scheme 4.39. Enantioselective aziridinium and episulfonium ring‐opening.

Source: Based on [128].

Another novel application of chiral‐anion catalysis was demonstrated in 2018 by the Nicewicz group. In this report, a pyrillium/chiral phosphate salt was developed that was catalytically active for a photoredox catalyzed Diels‐Alder, providing bicyclic cyclohexene products in good yield and moderate enantioselectivity (Scheme 4.40) [130]. Impressively, the reaction could also be conducted intermolecularly, with a cyclic or acyclic diene. This represents a rare example in chiral ion‐pairing catalysis, where the reactivity of radical intermediates is leveraged in an asymmetric fashion.

Scheme 4.40. Asymmetric Diels Alder via photoredox/chiral anion dual catalysis.

Source: Based on [130].