CrownPhos: A Review of Crown Ether Derivatives Decorated with Phosphorus Donors

Crown ethers are cyclic polyethers of varied ring sizes. The ability to bind to cations of specific size and charge is based on the ring size, donor atoms present in the crown cavity, and geometry. The host-guest chemistry of crown ethers is further enriched by the introduction of other donor atoms into the crown cavity. For example, aza-crown ethers are crown ethers wherein one or more oxygen atoms are replaced by nitrogen donors. The replacement of donor atoms appreciably alters the ability of the crown cavity to host guest molecules. Phosphorus donors can be introduced to the crown ethers on the periphery to generate hybrid ligands. In these hybrid "CrownPhos" ligand systems, the crown cavity selectively binds to hard cations and phosphorus donors preferentially bind to soft transition metal centers. The presence of guest cations in the crown ether cavity can alter the steric and electronic properties of the phosphorus donors and thereby modulate catalytic ability. The current review focuses on various synthetic and catalytic aspects of crown ethers, aza-crown ethers, and thia-crown ethers appended with phosphorus donors on the periphery.