The first constrained phosphenium cations

Changing the geometrical arrangement around an atom in a molecule can markedly change its reactive profile. Notably, geometrically constrained P(III) species A, react with various small molecules, i.e. NH₃, H₂O, ROH, and HBPin, while their classical counterparts exhibit no such reactivity. Herein we present the pyridine based constrained phosphenium cation 1, which is the first geometrically constrained phosphenium cation (GCPC). 1 reacts with H₂O, ROH, R₂NH and NH₃ to give the formal oxidative addition products as the appropriate phsophonium cations. Remarkably, the reaction of 1 with NH₃ is reversible, and as such opens the possibility to use 1 as a catalyst for hydroamination reactions. Unfortunately, thus far, the activation of CO, CO₂, silanes, and especially H₂ remains elusive for species of type A as well as for 1. Therefore, we now develop the NHC-based GCPC 2, the progress of which is also presented here.