Structure–Activity Relationship in the Iridium-Catalyzed Isomerization of Primary Allylic Alcohols

The evaluation of a series of cationic iridium complexes in the isomerization of primary allylic alcohols has revealed the unique ability of Crabtree catalysts [(Cy3P)(C6H5N)Ir(cod)]·X 1a (X = PF6) and 1b (X = BArF; cod = 1,5-cyclooctadiene) to catalyze efficiently this transformation. Emphasis has been placed on understanding the electronic factors that govern the catalytic process by using a range of para-substituted pyridine N-donor ligands. Excellent correlations between analytical data (13C NMR chemical shift; νCO) and established thermodynamic and kinetic parameters (pKa, Mayr nucleophilicity parameter N) have been elaborated. Overall, the results obtained point to the possibility of qualitatively predicting the activity of Crabtree catalyst analogues in the isomerization of primary allylic alcohols.