Azaphosphatranes as Hydrogen-Bonding Organocatalysts for the Activation of Carbonyl Groups: Investigation of Lactide Ring-Opening Polymerization

The hydrogen-bonding activation of C=O bonds by azaphosphatranes was explored in a model reaction, i. e., the ring-opening polymerization of lactide. The polymerization process was controlled, and allowed the preparation of polylactides with narrow dispersity under mild conditions (20 degrees C, 24 h, 10 mol-% catalyst loading). Interestingly, the steric hindrance of azaphosphatranes, as globular rigid structures, prevents any undesired interaction with the tertiary amine cocatalysts, as shown by X-ray analysis and semi-empirical calculations. In contrast to their organocatalytic activity in the CO2/epoxide reaction, all of the phosphonium derivatives tested were found to be efficient catalysts in this ROP benchmark reaction.