Evaluation of Aromatic Amination Catalyzed by Palladium on Carbon: A Practical Synthesis of Triarylamines

A heterogeneous palladium on carbon (Pd/C)-catalyzed coupling between amines and aromatic halides including aromatic chlorides has been achieved using sodium tert-butoxide (NaO-t-Bu) and 1,1'-bis(diphenylphosphino)ferrocene (dppf) as a ligand in cyclopentyl methyl ether (CPME). The use of potassium tert-butoxide (KO-t-Bu) in place of NaO-t-Bu brought about the benzyne-mediated aromatic amination even without Pd/C and dppf, giving a mixture of regioisomers when 4-substituted bromobenzenes were employed as the substrate. The combination of Pd/C, dppf, NaO-t-Bu could be utilized for the syntheses of a broad range of triarylamines by replacing CPME with mesitylene which can provide a higher reaction temperature. The Pd/C could be quantitatively recovered and reused until at least the fourth cycle without any loss in catalytic activity. The quite low leaching of palladium (< 1.1%) was demonstrated by an inductively coupled plasmaatomic emission spectrometric analysis.