Mechanism of N-Fluorobenzenesulfonimide Promoted Diamination and Carboamination Reactions: Divergent Reactivity of a Pd(IV) Species

摘要

The mechanism of the Pd-catalyzed diamination and carboamination of alkenes promoted by N-fIuorobenzenesulfonimide (NFBS) was investigated. Stereochemical labeling experiments established that the diamination reaction proceeds via overall syn addition of the two nitrogen groups, whereas carboamination is the result of an anti addition of arene and nitrogen to the alkene. The intermediate Pd-alkyl complex arising from aminopalladation was observed, and an X-ray crystal structure of its 2,2'-bipyridine (bipy) complex was obtained, revealing strong chelation of the amide protecting group to palladium. Aminopalladation was shown to be an anti-selective process in both the presence and the absence of added ligands, proceeding via external attack of the nitrogen on a Pd-coordinated alkene. The intermediate Pd-alkyl complex was converted to diamination product upon exposure to NFBS with inversion of configuration via oxidative addition followed by dissociation of the benzenesulfonimide anion and S_N2 displacement of the Pd-C bond. Conversely, arylation of the Pd-alkyl complex proceeds via retention of stereochemistry, consistent with C-H activation of the arene at the Pd(IV) center. A small intermolecular isotope effect (k_H/K_D =1.1) and a large intramolecular isotope effect (k_H/k_D = 4) were measured for this process, indicating that C-H activation occurs via a poorly selective product-determining coordination of the arene followed by a highly selective C-H activation. Competition between arenes reveals an unusual reactivity order of toluene > benzene > bromobenzene > anisole.