Electrophiles cleavage of metal-carbon and

Electrophilic Cleavage of Metal-Carbon and Metal-Hydride cr-Bonds... 

ELECTROPHILIC CLEAVAGE OF METAL-CARBON AND METAL-HYDRIDE cr-BONDS... 

Cleavage of Platinum-Carbon o-Bonds.—Electrophilic cleavage of metal-carbon bonds may take place either by a direct attack on the bond or by an oxidative addition of the central metal followed by reductive elimination, cf. a recent review. Romeo and co-workers have studied reaction (8), which is first order... 

RELATIVE RATES OF ELECTROPHILIC CLEAVAGE OF THE CARBON-METAL BOND IN SOME /7-SUBSTITUTED BENZYL-METAL AND... 

Sequence 16 clearly demonstrates electrophilic attack by ozone in these reactions. As noted by Jensen and Rickborn (2), the rate of electrophilic cleavage of a carbon-metal bond increases as the polarization of that bond increases. This, in turn, is a direct consequence of the electronegativity of the second atom attached to mercury. The following representations, based on Pauling electronegativity values, illustrate this relationship. 

The anionic complex M2 [IrCl6] (M = Na and K) are commercially available and have been used as outer-sphere single-electron oxidants in mechanistic studies of the cleavage of metal-carbon bonds [218]. The isostructural oxidant [PtCle] is also known, especially its ability to oxidize saturated hydrocarbon by electrophilic C-H activation (inner-sphere two-electron oxidant) [219]. 

In the search for a reactive functional group which could be substituted on the acetylacetonate ring, chloromethylation of these chelates was attempted. The initially formed products were too reactive to be characterized directly. Treatment of rhodium acetylacetonate with chloromethyl methyl ether in the presence of boron trifluoride etherate afforded a solution of a very reactive species, apparently the chloromethyl chelate (XXX) (26). Hydrolytic workup of this intermediate yielded a polymeric mixture of rhodium chelates, but these did not contain chlorine On the basis of evidence discussed later on electrophilic cleavage of carbon from metal chelate rings and on the basis of their NMR spectra, these polymers may be of the type shown below. Reaction of the intermediate with dry ethanol afforded an impure chelate which is apparently the trisethyl ether (XXXI). Treatment of the reactive intermediate with other nucleophiles gave intractable mixtures. 

In fact, cleavage of the carbon-zirconium bond occurs classically with various electrophiles (see Electrophile), generally under mild conditions. In addition, the other carbon-metal bond in gem metaUozirconocenes undergoes a broad range of transformations. The different reactivities of the carbon-metal and carbon-zirconium bonds are the results of the difference of electronegativity of the two metal centers and the two different bond polarities. 

The protic cleavage of the carbon-metal a-bond ranks among the simplest of all electrophilic substitution processes. As organomercurials are readily prepared in high purity, can be manipulated with ease and are monomeric in solution, most mechanistic studies of the protonolysis of carbon-metal o-bonds have focused on the protic cleavage of organomercurials. Reviews and a book have been published on this subject. 

This chapter presents developments in the activation and functionalization of carbon-hydrogen bonds that have been discovered since 1993. Major breakthroughs in hydrocarbon activation appeared in the early 1980s, and in the following decade, an explosion of discoveries was seen in new examples of metal complexes that could activate C-H bonds. Mechanisms for cleavage included oxidative addition, electrophilic cleavage, radical H-abstraction, and metal atom reactions, and several texts are available that summarize the first decade of this work. " ... 

The rate expression (Equation 12.15) for electrophilic cleavage of the metal-carbon bond in CpFe(CO)jR (Equation 12.12) is consistent with a mechanism in which the equilibrium in Equation 12.14 precedes removal of the alkyl ligand from the iron. The rate of the reaction increases with ionic strength and solvent polarity, and these data are consistent with an ionic intermediate. The second-order dependence of the rate on HgX shows that the coordinated electrophile E in the intermediate is likely to be HgX with an HgX " counterion. ... 

Many rhodium(II) complexes are excellent catalysts for metal-carbenoid-mediated enantioselective C-H insertion reactions [101]. In 2002, computational studies by Nakamura and co-workers suggested the dirhodium tetracarboxylate catalyzed diazo compounds insertion reaction to alkanes C-H bonds proceed through a three-centered hydride-transfer-like transition state (Fig. 25) [102]. Only one rhodium atom of the catalyst is involved in the formation of rhodium carbene intermediate, while the other rhodium atom served as a mobile ligand, which enhanced the electrophilicity of the first one and facilitate the cleavage of rhodium-carbon bond. In this case, the metal-metal bond constitutes a special example of Lewis acid activation of Lewis acidic transition-metal catalyst. 

Hydrometallation and carbometallation of alkynylsilanes proceeds regio-and stereospecifically, the metal becoming attached to the silicon-bearing carbon atom in what is normally a co-addition process (hydrostannylation, however, shows the opposite regioselectivity). Electrophilic cleavage, with retention, of the carbon-metal bond then leads to vinylsilanes of various types. 

The skeletal rearrangements are cycloisomerization processes which involve carbon-carbon bond cleavage. These reactions have witnessed a tremendous development in the last decade, and this chemistry has been recently reviewed.283 This section will be devoted to 7T-Lewis acid-catalyzed processes and will not deal, for instance, with genuine enyne metathesis processes involving carbene complex-catalyzed processes pioneered by Katz284 and intensely used nowadays with Ru-based catalysts.285 By the catalysis of 7r-Lewis acids, all these reactions generally start with a metal-promoted electrophilic activation of the alkyne moiety, a process well known for organoplatinum... 

The impetus for the development of gem-bimetallics was initially to discover alkylidene-transfer reagents akin to Tebbe s reagent [14]. Schwartz prepared bimetallic aluminum—zirconocene derivatives by the hydrometallation of various vinyl metallic compounds [15—17]. Knochel has developed zinc—zirconium gem-bimetallics by hydrozircona-tion of vinylzincs and has used them as alkylidene-transfer reagents [18], More recently, other gem-bimetallics have been developed that exhibit different reactivities of the two carbon—metal bonds. Thus, Normant and Marek have reported the allylmetallation of vinyl metals to afford zinc—magnesium and zinc—lithium gem-bimetallics, which react selectively with various electrophiles such as ClSnBu3, H20, etc. [19, and references cited therein]. However, selective and sequential cleavage of the two carbon—metal bonds... 

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