Iron porphyrin catalyzed N-H insertion reactions
Is Version Of
A series of metalloporphyrin complexes was surveyed as catalysts using ethyl diazoacetate (EDA) as the carbene source for insertion into N-H bonds of amines. Iron(III) tetraphenylporphyrin chloride, Fe(TPP)Cl, was found to be an efficient catalyst for EDA insertion into N-H bonds of amines. A variety of aliphatic and aryl amines were studied, with most reactions reaching completion in 20 minutes or less giving insertion yields ranging from 68-97%. The reactions could be done in air with only a slight decrease in yield. Little dimerization of EDA was observed in these reactions. Primary amines were able to undergo a second insertion of EDA when done in a step-wise process. Imidazole underwent an N-H insertion, but other N-heterocyclic compounds which were not as basic, were poor substrates and afforded C-H insertion products or no reaction. To gain insight into the pathway of the insertion reaction, several mechanistic experiments were conducted including the decomposition of EDA and competition reactions. Decomposition of EDA occurred slowly in the presence of the catalyst and absence of amine, but was significantly enhanced when pyridine was present. These experiments suggested that an iron(III) porphyrin carbene complex is involved in the catalysis. Competition reactions were conducted with a series of para- substituted aniline derivatives and the relative rates correlated with [Sigma]+ on a Hammett plot. Electron donating groups enhanced the reaction as indicated by the negative value of [Rho] ([Rho]=-0.66 ± 0.05, R²=0.93). These results are consistent with a nucleophilic attack of the amine substrate at the carbene carbon.