Carbon−Nitrogen-Bond-Forming Reductive Elimination of Arylamines from Palladium(II) Phosphine Complexes

Abstract

A series of monomeric palladium amido complexes of the form trans -(PPh 3 ) 2 Pd(Ar)(NAr‘ 2 ) and (DPPF)Pd(Ar)(NAr‘ 2 ) (DPPF = 1,1‘-bis(diphenylphosphino)ferrocene) and dimeric palladium amido complexes of the form {(PPh 3 )Pd(Ar)(μ-NHR)} 2 (R = Ph, t -Bu) have been prepared by the reaction of lithium and potassium amides with palladium aryl halide complexes. An X-ray crystal structure of (DPPF)Pd( p -NMe 2 C 6 H 4 )[N( p -CH 3 C 6 H 4 ) 2 ] was obtained. Upon thermolysis in the presence of PPh 3, serving as a trapping agent, both the monomeric and dimeric palladium amido complexes underwent C−N-bond-forming reductive elimination to form arylamines in high yields along with a Pd(0) species. Reductive elimination was also observed from azametallacycle (PPh 3 )Pd(η 2 -C 6 H 4 C 6 H 4 NH), to form carbazole and Pd(PPh 3 ) 4 at room temperature. Mechanistic studies on the reductive elimination reactions of the monomeric PPh 3 -ligated amido complexes indicated the presence of two competing pathways for the formation of amine. At low [PPh 3 ], reductive elimination occurs via phosphine dissociation to form a three-coordinate intermediate; however, as [PPh 3 ] is increased, a pathway of reductive elimination from a four-coordinate complex becomes dominant. The DPPF-ligated palladium amido complexes directly eliminated amine from the four-coordinate complex. The mechanism of the reductive elimination from dimeric palladium amido complexes was also studied. These complexes undergo reductive elimination of amine via dimer dissociation to generate a three-coordinate intermediate analogous to those formed by the PPh 3 -ligated monomeric amido complexes. The C−N-bond forming reductive elimination reactions were accelerated by electron-withdrawing groups on the Pd bound aryl group and by electron-donating groups on the amido ligand, suggesting that the aryl group acts as an electrophile and the amido ligand acts as a nucleophile.