Homocoupling of arylboronic acids (Fig. 1) was successfully carried out by Pd/C in water/2-propanol (9:1 in volume ratio) under air, to obtain symmetric biaryls in good yield. Symmetric biaryls are important precursors for advanced pharmaceuticals, such as glucocorticoid receptor antagonists [1]. The base-free, ligand-free system and the heterogeneous nature of the catalysts allow a practical and environmentally friendly operation. The influence of the co-solvent ratio was studied. Pd/C showed an increasing activity when the solvent fraction of water was increased from 10% to 90%. The influence of the reaction temperature was also investigated. It was shown that a higher reaction temperature leads to higher yields of desired biaryls. Furthermore, the effect of functional groups at para position was explored. DFT calculations indicate that the presence of sulfur in the function group hinders Pd catalysis and results in a slow reaction. DFT calculations also suggest that the first oxidative addition step in the proposed mechanism is not the rate-limiting step. Reference: [1] Bioorganic & Medicinal Chemistry Letters 16 (2006) 6086-6090.