Halides metal—carbon bonds

Stabilization of intermediates by strong adsorption will frequently be a necessary precondition for synthesis. Thus, in the case of the Kolbe reaction, further oxidation of the radicals is prevented the formation of metal-carbon bonds in the reduction of alkyl halides (Fleischmann et al., 1971a Galli and Olivani, 1970) or oxidation of Grignard reagents (Fleischmann et al., 1972c) is shown by the isolation of organometallic... 

He did not think of it then, and when he did, he showed that apparently the formation of a metal-carbon bond was unlikely on thermochemical grounds [68] in other words, he was taken in by his own propaganda but in a Note added in Proof in that same work he indicated that electrochemical factors (solvation and Coulombic energies) could make this initiation exo-energetic. However, at that time, 1960-1970, such a suggestion would have been no more plausible than when it was made by others. It was only the painstaking and detailed exploration of the nature of the solutions of A1X3 in alkyl halides [104, 112] that provided the basis of fact which was required to make the theory plausible. 

In general, carbonylation proceeds via activation of a C-H or a C-X bond in the olefins and halides or alcohols, respectively, followed by CO-insertion into the metal-carbon bond. In order to form the final product there is a need for a nucleophile, Nu". Reaction of an R-X compound leads to production of equivalent amounts of X", the accumulation of which can be a serious problem in case of halides. In many cases the catalyst is based on palladium but cobalt, nickel, rhodium and mthenium complexes are also widely used. 

The chemical reactivities of such titanium homoenolates are similar to those of ordinary titanium alkyls (Scheme 2). Oxidation of the metal-carbon bond with bromine or oxygen occurs readily. Transmetalations with other metal halides such as SnCl4, SbClj, TeCl4, and NbCls proceed cleanly. Reaction with benzaldehyde gives a 4-chloroester as the result of carbon-carbon bond formation followed by chlorination [9]. Acetone forms an addition complex. No reaction takes place with acid chloride and tm-alkyl chlorides. 

Asymmetric hydrometallation of ketones and imines with H-M (M = Si, B, Al) catalyzed by chiral transition-metal complexes followed by hydrolysis provides an effective route to optically active alcohols and amines, respectively. Asymmetric addition of metal hydrides to olefins provides an alternative and attractive route to optically active alcohols or halides via subsequent oxidation of the resulting metal-carbon bonds (Scheme 2.1). 

There are also catalysts that lack any apparent source of metal-carbon bonds. These catalysts include the aforementioned alumina- and silica-sup-ported transition metal oxides (which, in principle, do not demand any activation by organometallic compounds), and also several group 6-8 transition metal chlorides (soluble in hydrocarbons or chlorohydrocarbons), most typically RuC13. Some of these transition metal halides require activation by a cocatalyst of the Lewis acid type (e.g. A1C13, GaBr3, TiCU) [66,67], Noble metal chlorides may be used in alcoholic solvents or in water containing emulsifiers [68]. 

Complexes of the type [Rh(TPP)(RX)] [RX = C H X (n = 3-5, X = Cl or Br n = 3-6, X = I) TPP = dianion of tetraphenylporphyrin] were prepared by Anderson et al. (179). The nature of RX was found to determine the overall electrochemical behavior for the reduction of [Rh(TTP)(RX)]. For some complexes, specifically those where X = Br and I, the bound alkyl halide could be reduced without cleavage of the metal-carbon bond. This resulted in the electrochemically initiated conversion of [Rh(TPP)(RX)] to a [Rh(TPP)(R)] complex. The E. value for this reduction was dependent on the chain length and halide of the RX group and followed the trend predicted for alkyl halides. The reduction of the bound RX occured at Ei values significantly less negative that those for reduction of free RX under the same solution conditions. 

All these ligands have extensive chemistry here we note only a few points that are of interest from the point of view of catalysis. The relatively easy formation of metal alkyls by two reactions—insertion of an alkene into a metal-hydrogen or an existing metal-carbon bond, and by addition of alkyl halides to unsaturated metal centers—are of special importance. The reactivity of metal alkyls, especially their kinetic instability towards conversion to metal hydrides and alkenes by the so-called /3-hydride elimination, plays a crucial role in catalytic alkene polymerization and isomerization reactions. These reactions are schematically shown in Fig. 2.5 and are discussed in greater detail in the next section. 

A frequent theme in alcohol carbonylations by transition metals is the use of a halide or pseudo-halide promoters or cocatalysts. Despite major problems of corrosion associated with its use, iodide is almost always found to be most effective in this capacity. This is because the halide serves several purposes, for each of which iodide is ideally suited. One of the most important roles of these anion promoters can be that of facilitating the formation of metal-carbon bonds via the formation of intermediate alkyl halides. Under typical catalytic conditions for a variety of systems, at least some portion of the added halide is converted to the corresponding strong halo-acid. In fact, conditions are generally set so that this event is maximized. 

One of the characteristic features of the metal-bis-acetylide complexes in chemical reactions is that they undergo a ligand exchange reaction with metal dihalides in amines in the presence of a cuprous halide catalyst to produce a monoalkynyl-metal-monohalide complex (Eq. IS) which results from selective cleavage of the metal-carbon bond weakened by the /ram-alkynyl group in the bis-acetylide... 

Transmetallation is probably the most frequently employed preparative procedure in organometallic chemistry, and perhaps the commonest variant on this route to new metal-carbon bonds is the interaction of a transition-metal halide MX and a hydrocarbyl derivative of a (usually) more electropositive metal M R ... 

One-electron oxidations of transition metals by organic halides can take two forms, which result in either metal-carbon bond formation ... 

One-electron oxidations can give stable ty -ff-metal-carbon bonds accompanied by loss of donor ligand. The reactions, which proceed according to Eq. (e), arc not presented here unless formation of an > -T-metal-carbon bond with the carbon of RX is established. One-electron oxidation can often compete with the two-electron oxidative addition with coupled alkyl cis coproducts. The reaction of an anionic transition-metal complex with an organic halide is an oxidative addition ... 

The stabilities of the metal-carbon bond formed from oxidative additions are as varied as their mechanistic pathways. Metal-carbon bond strengths increase going down a triad in an isostructural series of complexes. Alkyl migration to CO ligands on the metal to form acyl derivatives is more facile in first-row transition metals because of their lower metal-carbon bond energies. The thermal stability of alkyls vs. acyls does not follow any pattern, except that the availability of a sixth coordination site in ML (acyl) complexes favors the alkyl carbonyl isomer. The corresponding acyl, which can be made by running the reaction of the alkyl or aryl halide in CO (at 1-3 atm), is more stable by... 

The imino complexes are prepared by (i) insertion into complexes containing a metal-carbon bond, (ii) oxidative addition of RX to zero-valent isonitrile complexes and subsequent insertion of the coordinated isonitrile, (iii) nucleophilic reaction of R with halogeno-metal isonitrile complexes and (iv) reaction of anionic isonitrile complexes with alkyl halides. 

Unequivocal demonstration of the formation of macrozwitterions is confined to anionic polymerization. Ironically zwitterions were first postulated as intermediates in cationic vinyl polymerizations initiated by Friedel-Crafts halides4. Friedel-Crafts halides, probably the most widely used cationic initiators, are molecules, not ions. However, formation of an anion with a metal-carbon bond seems to be energetically unfavourable and initiation is thought to occur by self ionization or involve a co-catalyst. 

Electrochemical reactions serve as efficient and convenient methods for the synthesis of organoelemental compounds. There are four major methods for the formation of element (metal)-carbon bonds. The first method utilizes the anodic oxidation of organometallic compounds using reactive metal anodes. In the second method, the organic compounds are reduced using reactive metal cathodes. The third method involves the cathodic reduction of organic compounds in the presence of metal halides. The fourth one utilizes both the cathodic and the anodic processes. 

The combination of organic halides and metal halides is also effective for the reductive formation of the metal-carbon bond. This type of reaction proceeds by a mechanism involving the cathodic reduction of organic halides [Eq. (9)] because reduction potentials... 

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