a Synthesis towards PR ligands 1, 3, 5. Reaction conditions: (i) Pd(PPh3)4, K2CO3, EtOH, toluene, aryl boronic acid, 85 °C, 16 h. b Synthesis of ligands 2, 4, 6. Reaction conditions: (ii) Pd(PPh3)2Cl2, Na2CO3, MeCN:H2O (1:1), μW 150 °C, 6 min; (iii) TCDI, THF, 50 °C, 16 h