Enhancing catalytic activity of pyridines via para-iminophosphorano substituents.

Four pyridines decorated with π-donating iminophosphorano substituents (R3PN-) in the 4-position were assessed as acylation catalysts. These catalysts display high sensitivity to the groups at phosphorus, with activities that are well correlated to the corresponding Hammett-type substituent constants (σp+), and can achieve superior activity over the most active dialkylamino-substituted pyridines. Iminophosphorano-substituted pyridines represent an easily accessible, tunable, and highly active class of nucleophilic organocatalysts.