LIGAND EFFECTS IN THE PALLADIUM-CATALYZED REDUCTIVE CARBONYLATION OF NITROBENZENE

Reductive carbonylation of nitrobenzene in methanol in the presence of a palladium catalyst yields mainly methyl N-phenylcarbamate. Diphenyl urea is formed as the most important byproduct. A series of 4,4'-disub stituted-2,2'-bipyridyl ligands (R = F3C, Cl, H, Me, MeO, and Me(2)N) has been used to study the influence of the donating capacity of the l igand on the catalytic activity and selectivity. By way of two differe nt types of complexes, Pd(ligand)(2)-(OTf)(2) and Pd(ligand)Cl(OTf), t he influence of the anions in the catalytic system has also been studi ed. Electron-withdrawing substituents on the bipyridyl ligand turned o ut to completely deactivate the catalyst, while only small differences were found between the ligands with electron-donating substituents. C hloride anions showed an inhibiting effect. The presence of water redu ced the selectivity toward carbamate. At prolonged reaction times the urea side product was catalytically converted into the desired carbama te. Under more severe conditions carbamates, urea side products, and a nilines were found with methoxy substituents on their phenyl rings. X- ray structures were elucidated for Pd(bpy)(2)(OTf)(2) and Pd(Me-bpy)(2 )(OTf)(2). The Pd(bpy)(2)(OTf)(2) crystals were monoclinic, space grou p P2(1)/n, a = 8.0186(8) Angstrom, b = 28.459(4) Angstrom, c = 11.315( 3) Angstrom, beta = 97.59(3)degrees, Z = 8, and final R = 0.046 for 31 94 observed reflections. The Pd(Me-bpy)(2)(OTf)(2) crystals were tricl inic, space group P1, a = 11.726(6) Angstrom, b = 11.870(3) Angstrom, c = 13.871(2) Angstrom, alpha = 103.63(6)degrees, beta = 107.78(2)degr ees, gamma = 113.32(3)degrees, Z = 2, and final R = 0.147 for 5462 obs erved reflections. The CF3SO3- moieties were not stable during refinem ent.