Dynamical properties of different models of elastic polymer rings: Confirming the link between deformation and fragility

Onset of glassiness in two-dimensional ring polymers: Interplay of stiffness and crowding

The effect of ring stiffness and pressure on the glassy dynamics of a thermal assembly of two-dimensional ring polymers is investigated using extensive coarse-grained molecular dynamics simulations. In all cases, dynamical slowing down is observed with increasing pressure, and thereby, a phase space for equilibrium dynamics is identified in the plane of the obtained monomer density and ring stiffness. When the rings are highly flexible, i.e., have low ring stiffness, glassiness sets in via the crowding of crumpled polymers, which take on a globular form. In contrast, at large ring stiffness, when the rings tend to have large asphericity under compaction, we observe the emergence of local domains having orientational ordering at high pressures. Therefore, our simulations highlight how varying the deformability of rings leads to contrasting mechanisms in driving the system toward the glassy regime.

Catalytic asymmetric nucleophilic fluorination using BF3·Et2O as fluorine source and activating reagent

Fluorination using chiral catalytic methods could result in a direct access to asymmetric fluorine chemistry. However, challenges in catalytic asymmetric fluorinations, especially the longstanding stereochemical challenges existed in BF₃·Et₂O-based fluorinations, have not yet been addressed. Here we report the catalytic asymmetric nucleophilic fluorination using BF₃·Et₂O as the fluorine reagent in the presence of chiral iodine catalyst. Various chiral fluorinated oxazine products were obtained with good to excellent enantioselectivities (up to >99% ee) and diastereoselectivities (up to >20:1 dr). Control experiments (the desired fluoro-oxazines could not be obtained when Py·HF or Et₃N·3HF were employed as the fluorine source) indicated that BF₃·Et₂O acted not only as a fluorine reagent but also as the activating reagent for activation of iodosylbenzene.

Catalytic asymmetric fluorination remains elusive, especially the longstanding stereochemical challenges which exist in BF₃Et₂O-based fluorinations. Here the authors show a catalytic asymmetric nucleophilic fluorination using BF₃·Et₂O as the fluorine reagent in the presence of chiral iodine catalyst.