Synthesis of sterically hindered polychlorinated biphenyl derivatives

A series of sterically hindered (methoxylated) polychlorinated biphenyl derivatives were synthesized using the Suzuki and the Ullmann coupling reactions. The Suzuki coupling with Pd(dba)2-dicyclohexylphosphino-2,6- dimethoxybiphenyl (DPDB) gave better yields (65-98%) compared to the classic Ullmann coupling reaction (20-38%). Despite the reactive catalyst system, no significant coupling with aromatic chlorine substituents was observed. Crystal structure analysis of four PCB derivatives revealed solid state dihedral angles ranging from 69.7 to 81.0, which indicates that these highly ortho-substituted PCB derivatives have some conformational flexibility.