From traditional PTA to novel CAP: a comparison between two adamantane cage-type aminophosphines

The cage-like water-soluble monodentate phosphine 1,3,5-triaza-7-phosphatricyclo[3.3.1.1]decane (also known as 1,3,5-triaza-7-phosphadamantane, PTA) has been known for many years and has been widely used in organometallic chemistry due to its ability to solubilize and stabilize transition metal complexes in aqueous phase. Through synthetic modifications, many PTA derivatives have also been obtained and used as coordinating ligands. The numerous PTA metal compounds produced have found application in particular in catalysis, but also in medicinal chemistry and for photoluminescent purposes. More recently, the synthesis of 1,4,7-triaza-9-phosphatricyclo[5.3.2.1]tridecane, abbreviated as CAP, has been reported. This phosphine, that can be considered as a higher homologue of PTA, has already shown interesting properties, sometimes considerably different from PTA. This minireview summarizes and compares the properties and reactivity of the two ligands and the activity in catalysis and in medicinal chemistry of the transition metal complexes described so far in the literature.