Inhibitory effect of the nucleophile in Ullmann condensation: Theoretical and experimental investigation

Abstract

This article reports a study on amino acid chelated Cu(I) catalyzed coupling of benzylamine and bromobenzene. Experiments were conducted at 80°C with Cu(I) chelated to methionine, proline, hydroxyproline, tryptophan and tyrosine in dimethylsulfoxide (DMSO). In order to investigate kinetic effects, reaction time was restricted to 90min. Kinetically controlled product yields over amino acid ligated Cu(I) were in the order proline>hydroxyproline>methionine>tryptophan>tyrosine. Theoretical investigation of the reaction was carried out using density functional theory method, B3LYP/6-31+G(d,p) employing the solvation model SCRF=CPCM. The effective core potential of LANL2DZ was used to model copper and bromine. Geometry optimization of the structures of reactants, intermediates and activated complexes were carried out in the solvent DMSO. It was found that the reaction over amino acid Cu(I) complexes that gave low product yields are associated with high energy barriers for aryl halide bond activation. Further, the theoretical investigation revealed that benzylamine coordinates from nitrogen and phenyl ring and it hinders the pi-cordination of bromobenzene. Therefore, in order for the reaction to progress, formation of amioacid chelated copper bromobenzene complex and subsequent activation of CBr bond must occur before the coordination of benzylamine. As the stability of the aminoacid ligated copper benzyl amine complex is much higher than the stability of the corresponding bromobenzene complex, the resting state can be taken as the amine complex. Therefore, it is proposed that benzylamine must be replaced by bromobenzene in order for the formation of aminoacid chelated copper bromobenzene complex.