Metal-carbon bonds electrophilic cleavage

Metal alkyl reagents react with the acidic OH groups of silica, probably by the electrophilic cleavage of the metal-carbon bond. For example, the electrophilic cleavage of the metal-carbon bond occurs when organometallic reagents react with the electrophilic OH groups of the silica surface (Scheme... 

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]. 

Scheme 20 Proposed mechanistic pathways of reaction of an organometallic compound with an electrophilic halogen source (a) Oxidative addition-reductive elimination pathway, (h) Electrophilic Metal-Carbon bond cleavage...

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... 

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. ... 

Cleavage of metal-alkyl bonds in d° metal complexes, therefore, occur by an 5 2 mechanism involving direct attack on the metal-alkyl bond. The product of this process is usually formed with retention of stereochemistry at carbon. This stereochemistry implies that the electrophile reacts by frontside attack at the M-C bond, rather than backside attack at the a carbon. Equation 12.21 shows an example of electrophilic attack on a (P metal complex that occurs with retention of configuration at the metal-carbon bond. The coordina-tive unsaturation of the 16-electron Zr(IV) complex may facilitate reaction with retention of configuration because it allows coordination of the incipient Br during the reaction, as depicted in Figure 12.1. 

The reaction of HgCl" with metal-alkyl complexes led to the cleavage of the metal-carbon bond. This electrophile can attack the carbon atom or the metal atom depending on the electron richness of the metal. If the metal is electron poor (case of Mn below), the attack occurs on the carbon atom, which leads to inversion of configuration at carbon. On the other hand, if the metal is sufficiently basic (case of Fe below), the attack occurs at the metal or on the metal-carbon bond, which leads to retention of configuration at carbon. 

Finally, some electrophiles can behave as monoelectronic oxidants towards 18-electron metal-alkyl complexes, which leads to decomposition of these complexes by heterolytic cleavage of metal-carbon bond in the resulting 17-electron species ... 

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 first transformation is the electrophilic cleavage of the carbon-metal bonds, which allows the functionalization of the substrate. This is more frequently observed with the group 4 metals (Ti, Zr, see Chapter 10.06). Carbonylation... 

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... 

Reaction with Further Electrophiles of Group IVA (Sl,Ge,Sn). IV-Silylated aziridines can be prepared from ethyleneimine by amination of chlorosilanes in the presence of an HC1 acceptor, by dehydrocondensation with an organosilicon hydride or by cleavage of a silicon—carbon bond in 2-furyl-, 2-thienyl-, benzyl-, or allylsilanes in the presence of an alkali metal catalyst (262—266). N-Silylated aziridines can react with carboxylic anhydrides to give acylated aziridines, eg, A/-acetylaziridine [460-07-1] in high yields (267). At high temperatures, A/-silylaziridines can be dimerized to piperazines (268). Aldehydes can be inserted... 

This chapter illustrates that electron-rich transition metal-diene complexes can couple with carbon electrophiles and, thereby, provide unusual methods for carbon-carbon bond formation. These procedures are of interest from a synthetic viewpoint since normally uncomplexed dienes or polyenes are not reactive toward weak carbon electrophiles or, with strong electrophiles, undesirable reactions such as polymerization occur. Furthermore, the metal-mediated route often results in desirable regio- and/or stereo-selectivity. Important to the utility of these methods is the ability to free the organic ligand from the metal. In most instances efficient oxidative procedures have been developed for such cleavage reactions. 

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. 

There are a number of modes by which carbon-metal bonds can be cleaved. Conceptually, they can be represented by the microscopic reverse of each of the processes in Equations 2-5 which lead to the alkylation of the metal center. Thus, the reverse of Equation 2 is represented by the well-known electrophilic cleavage of organometals.(5)... 

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. 

The trifluoromethyl group provides a convenient model for the behavior of larger perfluorocarbons. Thus it is appropriate to mention a system [66] where a ligand designed to promote C-F cleavage instead resulted in carbon-carbon bond activation at Rh(I). No evidence for the anticipated C-F activation product, which is known for the hydrocarbon analogue [67], was detected. In contrast to the known reactions of the CF3 group with electrophiles (Sect. 2.2), the electron rich metal center is unable to effect C-F activation in this instance [66]. 

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