Protonated Phosphazenes: Structures and Hydrogen‐Bonding Organocatalysts for Carbonyl Bond Activation

The synthesis and application of protonated phosphazeniums as hydrogen-bond donor groups was demonstrated in the solid state and in solution. In particular, their catalytic activity was shown in the activation of the carbonyl group within cyclic esters, using a benchmark reaction, that is, in the ring-opening polymerization of lactide and valerolactone, in the presence of a basic co-catalyst. The reactions proceed differently depending on monomers and/or phosphazenium salts. The impact of catalysts and reactants steric hindrance upon the outcome of the reaction is then highlighted and discussed.