Transition-metal halides, homogeneous catalysis

Halide exchange reactions ( transhalogenation or Finkelstein reaction) have been, until recently, of particular synthetic importance only in the synthesis of alkyl (and strongly activated aryl) fluorides or iodides, where thermodynamic stability (for the fluorides) and solubility differences (for the iodides) shift the reversible exchange processes in the desired direction. With the advent of phase transfer catalysis (PTC) and transition metal catalysis (mainly homogeneous) this important family of reactions has been extended to practically all aromatic and aliphatic halides. 

J. F. Hartwig, Transition Metal Catalyzed Synthesis of Arylamines and Aryl Ethers from Aryl Halides and Triflates Scope and Mechanism, Angew. Chem. Int. Ed. 1998,37,2047. P. G. Jessop, T. Ikariya, and R. Noyori, Homogeneous Catalysis in Supercritical C02, Chem. Rev. 1995, 95, 259. 

Kappe and Stadler have developed an MW procedure for rapid production of triaryl phosphines by coupling diphenylphospine with aryl halides and triflates [134]. Taking into account the importance of phosphine ligands in a variety of transition metal-catalyzed reactions, convenient procedures for their production is valuable. Both homogeneous Pd-Ni and heterogeneous Pd catalysts were explored and the more unusual substrate phenyl triflate could also be coupled swiftly by use of nickel catalysis (Scheme 15.68). Couplings with other aryl halides proceeded in 26-85% yield after 3-30 min microwave irradiation at 180-200 °C. 

Other relevant material is contained in reviews on homogeneous catalysis of organic reactions by transition metal complexes, highly reduced metal carbonyl complexes, phosphine complexes of the early transition metal complexes and the lanthanoids, metal NMR spectra of transition metal organometallic systems, the coordination chemistry of C-Nitroso-compounds, mono-Cp halide complexes of the d- and/-block, and organometallic chemistry in the gas phase. o... 

Nevertheless, the electroreduction of not easily reducible organic compounds (FG)3C-X is possible at a more positive potential (-0.8 to -1.5 V/ECS) when homogeneous catalysis by a transition metal is associated with the consumable anode process. The low-valent transition metal M, " L (L= ligand) that reacts with the organic halide is generally formed from its corresponding halide by electroreduction. The electroreductive reaction can be achieved by the following sequence (Scheme 15.4) ... 

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