Chiral counterion controlled asymmetric catalysis via an ion-pairing interaction has attracted immense attention in recent years. Despite a number of successful studies, the mechanistic elucidation of the stereocontrolling element in the ion-pairing interaction is rarely conducted and hence its nature is still far from being well understood. Herein we report an in-depth mechanistic case study of a newly developed enantioselective ring expansion reaction of 1,3-dithiane derivatives catalyzed by chiral phosphoric acid (CPA). An unprecedented enantioselective 1,2-sulfur rearrangement/stereospecific nucleophilic addition sequence was proven to be the stereoselective pathway. More importantly, by thorough investigation of the intrinsic nature of the stereospecific nucleophilic addition to the cationic thionium intermediate, we discovered that the key interaction in this process is the nonclassical C-H···O hydrogen bonds formed between the conjugate base of the CPA catalyst and the cationic intermediate. These C-H···O hydrogen bonds not only bind the catalyst to the substrates to form energetically favored states throughout the overall processes but also firmly maintain the relative positions of these fragments as the "fixed" contact ion pair to sustain the chiral information generated at the initial sulfur rearrangement step. This mechanistic case study provides a very clear understanding of the nature of the ion-pairing interaction in organocatalysis. The conclusion encourages the further development of the research field with the focus to design new organocatalysts and cultivate novel organocatalytic transformations.
A catalytic enantioselective synthesis of β‐amino secondary amides was achieved using vinyl azides as the enamine‐type nucleophile and chiral N‐Tf phosphoramide as the chiral Brønsted acid catalyst through a five‐step sequential transformation in one pot. The established sequential transformation involves an enantioselective [4+2] cycloaddition reaction of vinyl azides with N‐acyl imines as the key stereo‐determining step that is efficiently accelerated by a chiral N‐Tf phosphoramide catalyst in a highly enantioselective manner in most cases. Further generation of the iminodiazonium ion intermediate through ring opening of the cycloaddition product and subsequent skeletal rearrangement involving Schmidt‐type 1,2‐aryl group migration followed by recyclization of the resulting nitrilium ion were also initiated by the same acid catalyst. Final acid hydrolysis of the recyclized products in the same pot gave rise to enantioenriched β‐amino amides through C−C bond formation at the α‐position of the secondary amides.
Computational analyses have revealed that the distortion of the catalyst and the substrates and their interactions are key to determining the stability of the transition state. Hence, two strategies “distortion...
pH imaging and measurement in biological systems is important because changes in pH are closely related with physiological and pathological processes. In this paper, a pH‐responsive fluorescence change based on the dynamic exchange between emitting aggregate state and quenching monomer state is reported. At low pH, the aggregate bearing a neutral carboxylic acid group in its protonated form continues to emit red light. In contrast, at high pH, the aggregate dissociates to the fluorescence‐quenching monomer owing to electrostatic repulsion among the anionic carboxylate moieties generated from the deprotonation process, resulting in a pH‐responsive change.
The molecules containing a chiral sulfur center such as chiral sulfoxides are used as a toolbox for the synthesis of chiral compounds. Although the stereoselective reactions using chiral sulfur groups as a chiral directing group have been developed, the chirality transfer reactions arising from a sulfur atom have been less reported. In this short review, the recent advances in the development of chirality transfer from a sulfur to a carbon stereocenter via rearrangement reactions will be described.
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