Ligands coordinated hydrocarbons

The air-sensitive violet complex thus formed was treated with methyl-magnesium bromide and then hydrolyzed to give /rans-Ph(Me3Si)C= CPh(SiMe3) (90, 91). Apparently, this is the first example of transfer of silyl groups from a metal to a coordinated hydrocarbon ligand. It seems that a similar step is involved in the dehydrogenative cis double silylation of acetylenes catalyzed by diethyl(bipyridyl)nickel(II) (156). 

A comparison of this scheme with those described above shows their similarity. In both cases there are three distinct steps necessary for and specific to the transfer of 7r-bonded hydrocarbon ligands coordination of an attacking agent by a substrate molecule, a 7r,<7-rearrangement of the ligand, and finally ligand transfer itself proceeding as cis insertion. 

Complexes with cr-coordinated heteroatoms and rr-coordinated hydrocarbons have been synthesized and studied extensively, but they are not included in this article. On the other hand, complexes with cr-bonded carbon atoms and 77-bonded CX groups (X = heteroatom), with the exception of species with metal-metal bonds and Tj rTj -coordinated carbonyl or isocyanide ligands, have been very rarely reported in the literature. In most cases heteroatoms preferentially coordinate by lone pair electrons of the heteroatom and not through the adjacent unsaturated bond. 

Aromatic hydrocarbons can be oxidized to the corresponding phenols by transition metal peroxo complexes and, in particular, vanadium(V) peroxo complexes, which act either as electrophilic oxygen transfer reagents or as radical oxidants -, depending on the nature of the ligands coordinated to the metal and on the experimental conditions. Vanadium picolinato peroxo complex (V0(02)PIC(H20)2) (39) (PIC = picoUnic acid anion) has been reported to be particularly effective in the hydroxylation of benzene and substituted benzenes (equation 50) . Accordingly, 39 smoothly oxidizes substituted benzenes 38 to the corresponding monophenols 40 in acetonitrile at room temperature. 

Several examples have been given of the reactions of nucleophiles with coordinated organic ligands. Unsaturated hydrocarbons such as ethene, butadiene and benzene are not normally attacked by nucleophiles, but on coordination to a transition metal their susceptibility to such attack is often greatly enhanced, particularly when the complex is cationic. On the other hand many otherwise reactive species such as the cyclohexadienyl cation, protonated benzene (p. 303) can be stabilized by coordination, so that controlled reactions can be performed at ligand sites. In a complex there may be several potential sites at which a nucleophile might attack. Often different nucleophiles prefer different sites. This is illustrated with reference to fy -arenetricarbonylmanganese salts. 

New Transformations of Unsaturated Hydrocarbon-Ligands Coordinated with Mono- and Dinuclear Palladium Units. 

This chapter reviews mechanistic studies on the stoichiometric reactions of coordinated hydrocarbons with nucleophiles and electrophiles, together with some related processes. Kinetic investigations of such reactions, especially nucleophilic additions, have increased considerably in recent years, greatly increasing our knowledge of the intimate mechanisms involved and of the factors important in controlling ligand reactivity. 

Iron carbonyl complexes containing 77 -alkyl-77 -allyl coordinated hydrocarbon ligands are obtained in several ways. Nucleophilic addition to cationic iron complexes containing 77 -pentadienyl ligands yields (pentenediyl)iron complexes. Oxidatively-induced reductive elimination of these complexes can be utilized as a means to generate 1,2,3-trisubstituted cyclopropanes.The reaction of cationic cycloheptadienyl complexes (Scheme 22) with appropriate nucleophiles also yields the alkyl-allyliron carbonyl complexes. Fe(CO)s also reacts with a- or /3-pincnc in refluxing dioxane (Scheme 22) to produce an alkyl-allyliron complex. Recently, 1,2- and 1,4-disubstituted [(pentadienyl)Fe(CO)3] cations were shown to react with carbon nucleophiles, such as sodium dimethylmalonate, to yield 77 77 -allyl complexes as products. 

Related catalytic processes are discussed elsewhere (Chapter 14) as are intramolecular ligand rearrangements (Chapter 13). Quantitative studies of nucleophilic attack on carbonyl ligands are also generally outside the scope of this chapter, and are discussed only where attack at carbon monoxide occurs competitively with reaction at a coordinated hydrocarbon. A short section on nucleophilic attack at coordinated isocyanides is included for the first time. 

A surprising result was the first report of stereospecific kinetic endo-hydride addition to a coordinated hydrocarbon ligand/ The NMR... 

Multipalladium assemblies sandwiched by unsaturated hydrocarbon ligands have been developed by Kurosawa and Murahashi. A general synthetic scheme involves the reaction of 2 equivalents of Pd and (ra 2) equivalents of Pd , with two linear polyenes having (n+1) C = C bonds to construct a linear Pd skeleton (Scheme 39). A homoleptic solvated Pd dimer Pd2(CH3CN)6f has been employed as a monovalent palladium precursor. The polyene ligand coordinate to the Pd chain in r fashion. For polyene with n... 

Vanadium(V) Oxytrichloride. Vanadium(V) oxytrichloride (VOCl ) is readily hydrolyzed and forms coordination compounds with simple donor molecules, eg, ethers, but is reduced by reaction with sulflir-containing ligands and molecules. It is completely miscible with many hydrocarbons and nonpolar metal hahdes, eg, TiCl, and it dissolves sulfur. 

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