Dehydrochlorination of Carboxylic Acid Chlorides by an Aluminum/Phosphorus-Based Frustrated Lewis Pair and the Formation of Ketene Adducts

The Al/P-based frustrated Lewis pair (FLP)Mes(2)P-C(=CH-Ph)-Al(CMe3)(2) (1) reacted with carboxylic acid chlorides [Cl-C(=O)Me, Cl-C(=O)Et, Cl-C(=O)CH2CMe3] by dehydrochlorination. The released hydrogen chloride molecules were captured by one equivalent of the FLP 1 to yield the previously published HCl adduct, Mes(2)P(H)(+)-C(=CH-Ph)-Al(Cl)(CMe3)(2)(-) (3). This remarkable reaction resulted in the formal generation of ketenes, R(H)C=C=O, which were trapped by a second equivalent of 1 via the coordination of the C=O groups to the Lewis acidic aluminum and the Lewis basic phosphorus atoms (4 to 6). Five-membered AlC2PO heterocycles were formed, which had an exocyclic C=C double bond. Quantum chemical DFT calculations were performed to study the thermodynamic driving forces of the adduct formation and to identify reactive intermediates with respect to a mutual activation of this process by phosphorus and aluminum atoms.