Iridium Porphyrin Catalyzed N–H Insertion Reactions: Scope and Mechanism

Ir(TTP)CH3 catalyzed N–H insertion reactions between ethyl diazoacetate (EDA) or methyl phenyldiazoacetate (MPDA) and a variety of aryl, aliphatic, primary, and secondary amines to generate substituted glycine esters with modest to high yields. Aniline substrates generally gave yields above 80%, with up to 105 catalyst turnovers, and without slow addition of the diazo reagent. Good yields were also achieved with aliphatic amines, though higher catalyst loadings and slow addition of the amine were necessary in some cases. Primary amines reacted with EDA to generate both single- and double-insertion products, either of which could be produced selectively in high yield with the proper choice of stoichiometric ratios and reaction temperature. Notably, mixed trisubstituted amines, RN(CH2CO2Et)(CHPhCO2Me), were generated from the insertion of 1 equiv of EDA and 1 equiv of MPDA into primary amines. The N–H insertion mechanism was examined using substrate competition studies, trapping experiments, and multiple spectroscopic techniques. Substrate competition studies using pairs of amines with EDA or MPDA revealed Hammett correlations with respective slopes of ρ = 0.15 and ρ+ = −0.56 as well as kinetic isotope ratios of kH/kD = 1.0 ± 0.2 and 2.7 ± 0.2. Competitive amine binding to the iridium center was demonstrated by kinetics and equilibrium binding studies. Equilibrium binding constants ranged from 102 to 105. Monitoring the reaction by absorption spectroscopy revealed a transient metalloporphyrin complex. The lifetime of this species was dependent on the nature of the amine substrate, which suggests that the catalytic cycle proceeds through a metal–ylide intermediate.