Synthesis of Sterically Hindered Polychlorinated Biphenyl Derivatives.

A series of sterically hindered (methoxylated) polychlorinated biphenyl derivatives was synthesized using the Suzuki and the Ullmann coupling reaction. The Suzuki coupling with Pd(dba)(2)/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.