Linear polyethylene propagation starting from Pd phosphine-sulfonate complexes, Pd(CH(3))(L)(Ar(2)PC(6)H(4)SO(3)) (L = 2,6-lutidine, Ar = o-MeOC(6)H(4) (2a) and L = pyridine, Ar = Ph (2b)), was studied both experimentally and theoretically. Experimentally, highly linear polyethylene was obtained with Pd(CH(3))(L)(Ar(2)PC(6)H(4)SO(3)) complexes 2a and 2b. Formation of a long alkyl-substituted palladium complex (3) was detected as a result of ethylene oligomerization on a palladium center starting from methylpalladium complex. Additionally, well-defined ethyl and propyl complexes (6(Et) and 6(Pr)) were synthesized as stable n-alkyl palladium complexes. In spite of the existence of beta-hydrogens, the beta-hydride elimination to give 1-alkenes was very slow or negligible in all cases. On the other hand, isomerization of 1-hexene in the presence of a methylpalladium/phosphine-sulfonate complex 2a indicated that this catalyst system actually undergoes beta-hydride elimination and reinsertion to release internal alkenes. On the theoretical side, the relative energies were calculated for intermediates and transition states for chain-growth, chain-walking, and chain-transfer on the basis of the starting model complex Pd(n-C(3)H(7))(pyridine)(o-Me(2)PC(6)H(4)SO(3)) (8). First, cis/trans isomerization process via the Berry's pseudorotation was proposed for the Pd/phosphine-sulfonate system. The second oxygen atom of sulfonate group is involved in the isomerization process as the associative ligand, which is one of the most unique natures of the sulfonate group. Chain propagation was suggested to take place from the less stable alkylPd(ethylene) complex 10' with the TS of 27.4/27.7 ((E+ZPC)/G) kcal/mol. Possible beta-hydride elimination was suggested to occur under low concentration of ethylene: the highest-energy transition state to override for beta-hydride elimination was either >37.4/25.3 kcal/mol (TS(9-12)) or 29.1/27.4 kcal/mol (TS(8'-9') to reach 12'). The ethylene insertion to the iso-alkylpalladium species (14') is allowed via a TS of 28.6/29.1 kcal/mol (TS(14'-15')), slightly higher in energy than that for the normal-alkylpalladium species (TS(10'-11')). Easy chain transfer was suggested to proceed from the more stable PdH(olefin) complex 12' if beta-hydride elimination to 12' does take place. Thus, the production of linear polyethylene with high molecular weight under ethylene pressure suggests that the cis and trans PdH(alkene)(phosphine-sulfonate) complexes (12 and 12') are merely accessible in the presence of excess amount of ethylene.

Linear copolymers of ethylene and acrylonitrile were prepared using palladium complexes bearing phosphine-sulfonate bidentate ligands. Acrylonitrile units located in the linear polyethylene backbones were detected for the first time by 13C NMR spectroscopy. Catalyst systems employing isolated palladium complexes such as 3 showed much higher activity for the copolymerization than the in situ generation procedures, and molecular weight of the copolymers and acrylonitrile incorporation were dependent on the palladium complexes. Obtained linear copolymers of ethylene and acrylonitrile melt at higher temperature than branched copolymers.

A new strategy for catalytic functionalization of C-H bonds by means of electrochemical oxidation is described. Combination of palladium-catalyzed aromatic C-H bond cleavage and halogenation with electrochemically generated halonium ions enables highly efficient, selective halogenations of aromatic compounds in a green-sustainable manner. The required reagents for this reaction are an arene and an aqueous hydrogen halide as substrates, a palladium salt as a catalyst, and an organic solvent. No further additives such as electrolytes, oxidants, or ligands are necessary to achieve effective catalytic activity. Several remarkable advantages of the use of the electrochemical method are also described.

Catalytic functionalization of unreactive C-H bond has become one of the most attractive research subjects in modern organic chemistry. To date, a variety of catalytic reactions involving C-H bond cleavage have been reported. In this review, we briefly survey the reported research with respect to efficient and selective transition-metal-catalyzed CC bond formation via C-H bond cleavage.