Reductive carbonylation of nitrobenzene in methanol in the presence of a palladium catalyst yields mainly methyl N-phenylcarbamate.Diphenylurea is formed as the most important byproduct.A series of 4,4'-disubstituted-2,2'-bipyridyl ligands (R = F3C, C1, H, Me, MeO, and Me2N) has been used to study the influence of the donating capacity of the ligand on the catalytic activity and selectivity.By way of two different types of complexes, Pd(1igand)z-(OTo2 and Pd(ligand)Cl(OTf), the influence of the anions in the catalytic system has also been studied.Electron-withdrawing substituents on the bipyridyl ligand turned out to completely deactivate the catalyst, while only small differences were found between the ligands with electron-donating substituents.Chloride anions showed a n inhibiting effect.The presence of water reduced the selectivity toward carbamate.At prolonged reaction times the urea side product was catalytically converted into the desired carbamate.Under more severe conditions carbamates, urea side products, and anilines were found with methoxy substituents on their phenyl rings.X-ray structures were elucidated for Pd(bpy)a(OTf)z and Pd(Me-bpy)z(OTf)z.The Pd(b~y)~(OTfl:! crystals were monoclinic, space group P21/n, a = 8.0186(8) A, b = 28.459(4)A, c = 11.315(3)A, /3 = 97.59(3)", 2 = 8, and final R = 0.046 for 3194 observed reflections.The Pd(Me-bpy)2(OTf)2 crystals were triclinic, space group P1, a = 11.726(6)A, b = 11.870(3)A, c = 13.871(2)A, a = 103.63(6)",/3 = 107.78(2)",y = 113.32(3)", 2 = 2, and final R = 0.147 for 5462 observed reflections.The CF3S03-moieties were not stable during refinement.
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Wehman et al. (1994) studied this question.