NH₂ As a Directing Group: From the Cyclopalladation of Amino Esters to the Preparation of Benzolactams by Palladium(II)-Catalyzed Carbonylation of N-Unprotected Arylethylamines

Abstract

An unusual NH2-directed Pd(II)-catalyzed carbonylation of quaternary aromatic α-amino esters to yield benzolactams has been developed. The steric hindrance around the amino group is pivotal for the success of the process. The stoichiometric cyclometalation of a variety amino esters has been studied in order to evaluate the influence of the different variables (size of the metallacycle, aromatic ring substituents, and steric bulk) in the process, and a complete kinetico-mechanistic study of the cyclopalladation process has been carried out. The experimental results indicate that the full substitution of the carbon in the α position of the amino esters plays an important role in their cyclopalladation reaction. The reaction shows a strong bias toward six-membered lactams over the five-membered analogues, which can be explained by a greater reactivity of the six-membered palladacycles.