Subtle Balance between Various Phenanthroline Ligands and Anions in the Palladium-Catalyzed Reductive Carbonylation of Nitrobenzene

Abstract

Palladium-phenanthroline catalyst systems for the reductive carbonylation of nitrobenzene in methanol yield methyl N-phenylcarbamate as the major product next to small amounts of N,N'-diphenylurea, aniline, and azoxybenzene. The influence of a series of 4,7disubstituted 1,lO-phenanthroline ligands (R = C1, H, Me, MeO, and Me2N) in close correlation with two different noncoordinating anions (triflate or tetrafluoroborate) on the catalytic activity and selectivity was studied. Though all the rigid phenanthroline ligands afford stable catalyst precursors, no conversion into carbamate was obtained under the influence of the electron-withdrawing chloride substituents on the ligand. With the mildly electron-donating substituents H, Me, and MeO, high activities up to 311 moV(moVh) could be measured. A very subtle balance between the donating capacity of the ligand and the particular noncoordinating anion used was found. A cyclic voltammetric study established that the reduction of the originally Pd species into a PdO intermediate becomes more difficult with increasing donating capacity of the phenanthroline ligand. An X-ray structure was elucidated for Pd(phen)~(OTf)~. The Pd(phen)~(OTf)~ crystals were triclinic, space group Pi, a = 10.387(1) A, b = 11.539(2) A, c = 13.449(3) A, a = 70.53(1), /3 = 67.42(2), y = 81.51(l)', 2 = 2, and final R = 0.045 for 6448 observed reflections.