Abstract:
Chirality is an important fundamental property of molecular systems that is crucial in various fields, including chemistry, pharmacology, biology, and materials science. Research activities involving chiral recognition and separation play a major role in solving the complexities of homochirality and are instructive in terms of asymmetric catalysis and molecular recognition. Consequently, methods using supramolecular systems as host materials for the chiral recognition and separation of small organic molecules have garnered significant attention in recent years. Unlike purely covalent molecules with defined configurations and conformations, supramolecular assemblies can adopt chiral structures based on both the intrinsic chirality and spatial arrangement of components that are held together by non-covalent interactions. This thesis will describe the synthesis and structures of a series of chiral systems derived from P-N based scaffolds possessing acyclic, cyclic, and polyhedral cage topologies and their chiral recognition capabilities. For the acyclic system, in Chapter 2, a simple phosphoramide ligand, a formula [POPh2(R*)], where R* corresponds to an amide substituent (R)- ((C6H4)(CO)(NH)(CH(CH3)Ph)), has been synthesized and studied for chiral recognition capabilities. A family of chiral epoxides was screened for enantio-selective recognition capabilities. The highest binding selectivity of ~3000 was observed for the guest R-2- ((benzyloxy)methyl)oxirane, employing the R-phosphoramide ligand as a chiral host. Secondly, in Chapter 3, a cyclic P-N ring containing compounds, hexakis(α- methylbenzylamino)cyclotriphosphazene of formula, [P3N3(NHR*)6], [R* = (R)-(CH(CH3)Ph) and (S)-(CH(CH3)Ph) ], with six chiral alpha-methyl benzylamine groups, have been synthesized. The chiral recognition studies with various organic guest molecules indicated that the R-enantiomer of the phosphazene host exhibited the highest binding selectivity of ~2000, with the guest R-binol. Subsequently, in Chapters 4 and 5, chiral cages built from trinuclear Pd(II) ions, cis-protected by imido-P(V) anions, as vertices were synthesized using a range of wide-angle chelating linkers and shown to exhibit chiral recognition and separation activity. These cages are characterized by distinct portals and well-defined intrinsic cavities, making them highly suitable for host-guest interactions with exceptional affinity and selectivity, particularly in the context of chiral recognition of gest molecules with a wide variety of functional groups. Notably, the R-enantiomer of these cages, upon careful modulations on their portal functionalities, could successfully separate S enantiomer of epichlorohydrin up to 93% (ee) and R enantiomer of styrene oxides up to 98%(ee), showcasing the potential of these metalorganic cages for enantioselective applications.
