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List reported a ring‐opening reaction of meso‐aziridine using 6e and carboxylic acid to afford the ring‐opening products with 85–99% ee (Scheme 2.85). They obtained the X‐ray crystallographic analysis of heterodimer 36 between CPA 6e and acetic acid, and proposed the heterodimeric association of a carboxylic acid with CPA as a new activation principle for organocatalysis. The carboxylic acid plays a dual role in the reaction by both accomplishing the ring‐opening reaction and protecting the catalyst from degradation [172].

Scheme 2.85. Ring‐opening of meso‐aziridines.

List reported an enantioselective oxidation of sulfide catalyzed by confined Brønsted acid 10a to furnish sulfoxides with 85–98% ee (Scheme 2.86) [173].

Scheme 2.86. Enantioselective oxidation of sulfides to sulfoxides.

Source: Based on [173].

List reported an enantioselective transformation of oxirane to thiiranes using chiral phosphoric acid 6e in the presence of thioamide. Both oxiranes and thiiranes were obtained with 85–99% ee and 76–95% ee, respectively, by the kinetic resolution (Scheme 2.87). Formation of the initial heterodimer 37 between CPA 6e and the sulfur donor was proposed [174].

Scheme 2.87. Kinetic resolution of oxiranes by the transformation of oxirane into thiiranes.