NH2 as a directing group: from the unexpected cyclopalladation of aminoesters to the preparation of benzolactams by Pd(II)-catalyzed carbonylation of N-unprotected arylethylamines

Abstract

An unusual NH2-directed Pd(II)-catalytic 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 cyclometallation 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 to 6-membered lactams over the 5-membered analogues, which can be explained by a greater reactivity of the six-membered palladacycles.