C-H bond activation, by metals

This structural variation notwithstanding, only a few cationic transition-metal ions react efficiently with molecular oxygen under gas-phase conditions (see below). In contrast, many anionic metal complexes and clusters are readily oxidized by O2 to afford various metal-oxide anions [19]. From a conceptual point of view, however, anionic species appear to be inadequate reagents for the activation of hydrocarbons, because they generally require electrophilic attack. At present, only a few oxidations by transition-metal oxide anions have been reported to occur in the gas phase, and they are mostly limited to relatively polar substrates, such as the CH3OH CH2O conversion [20]. Because of the lower reactivity of hydrocarbons, their C-H bond activation by metal-oxide anions is likely to be limited to radical pathways driven purely thermodynamically, i.e., when Z)(0-H) exceeds Z)(C-H) of the substrate [21]. As radical-type pathways are prone to create selectivity problems, and over-oxidation is particularly difficult to control, the anionic route appears less attractive as far as partial oxidation of alkanes is concerned. 

As Shilov proposed [11], C—H bond activation by metal species can be classified into three categories based on their mechanisms. The first type is called organo-metallic activation where the reaction takes place in the first coordinatimi sphere of metal center and leads to the formation of M-C a-bond. This mechanism includes various derivatives, such as oxidative additimi, a-bond metathesis, 1, 2 addition, and electrophilic substitution, as shown in Fig. 1. Interestingly, recent theoretical studies also revealed that there exist a few alternatives to these classic types, namely, metal-assisted a-bond metathesis [18], oxidatively added... 

On the other hand, methane activation by homogeneous catalysts is a very difficult subject because (1) the C—H bond of methane is too strong to break under moderate conditions where catalyst molecules can survive, (2) common organic solvents cannot survive under methane activation conditions, and (3) the concentration of methane in solution is usually too low. In this short article, recent progress in methane activation by metal complexes is summarized according to the following classification. Related reactions of other hydrocarbons are also described to some extent for reference. (For comprehensive review of C—H bond activation by metal-complexes, see Ref (1).)... 

We begin our journey with elementary events. Our first example will be a simple chemical transformation, a hydrogen atom transfer between two atoms, in the gas phase. There are numerous systems that are chemically more exciting say, the mechanism of C—H bond activation by metal complexes, in solution, or how does an enzyme transfer chemical energy liberated at a localized site to a functionally relevant receptor site We adopt a bottom-up approach that almost all of chemistry is local in character even a complex process is a sequence of elementary steps, each involving only a few atoms. Just as organic chemists break a complete synthesis into its essential steps (Corey, 1991), so we want to chart what are the possibly few atom events that, played in rapid succession, make up a complex reaction. 

Facile C-H bond activation by Pt(II) metal centers seems to require at least one labile ligand in the coordination sphere of platinum. One of the earliest intermolecular examples of this is the activation of C-D bonds in benzene-f/, by 0 an.S -(PAIe .) Pt(neopentyl)(OTf) at 133 °C, where trifluoromethanesulfonate (triflate, OTf) provides the labile group (Scheme 7, A) (26). 

In fact, the C-H bond activation by the zirconium or tantalum hydride on 2,2-dimethylbutane can occur in three different positions (Scheme 3.5) from which only isobutane and isopentane can be obtained via a P-alkyl transfer process the formation of neopentane from these various metal-alkyl structures necessarily requires a one-carbon-atom transfer process like an a-alkyl transfer or carbene deinsertion. This one-carbon-atom process does not preclude the formation of isopentane but neopentane is largely preferred in the case of tantalum hydride. 

Another type of Cinchona alkaloid catalyzed reactions that employs azodicarbo-xylates includes enantioselective allylic amination. Jprgensen [51-53] investigated the enantioselective electrophilic addition to aUyhc C-H bonds activated by a chiral Brpnsted base. Using Cinchona alkaloids, the first enantioselective, metal-free aUyhc amination was reported using alkylidene cyanoacetates with dialkyl azodi-carboxylates (Scheme 12). The product was further functionalized and used in subsequent tandem reactions to generate useful chiral building blocks (52, 53). Subsequent work was applied to other types of allylic nitriles in the addition to a,P-unsaturated aldehydes and P-substituted nitro-olefins (Scheme 13). 

This then was the first report of a compound in which alkyl C—H bond activation by a transition metal had occurred. In the solid state, this equilibrium is also in favor of the hydrido complex (V), and its crystal structure has recently been determined (15). It shows compound V to be a dimer (VI), the oxidative addition of the methyl group of a ligand on each ruthenium atom being to a second ruthenium atom. Presumably one reason why this occurs is because the product of intramolecular ring closure would contain a highly strained three-membered Ru—P—C ring (i.e., in monomer V). 

Study of the reactivity of aromatic C-H bonds in the presence of transition metal compounds began in the 1960s despite the quite early discovery of Friedel-Crafts alkylation and acylation reactions with Lewis acid catalysts. In 1967, we reported Pd(II)-mediated coupling of arenes with olefins in acetic acid under reflux [1], The reaction involves the electrophilic substitution of aromatic C-H bonds by a Pd(II) species, as shown in Scheme 2, and this is one of the earliest examples of aromatic C-H bond activation by transition metal compounds. Al-... 

Heterocyclizations using C-H bond activation by transition metals 92PAC335. 

Over the past 30 years there has been a massive effort to achieve selective C-H bond activation by transition metal catalysis and there now exists a variety of mechanisms for the activation of C-H bonds using a range of transition metal catalysts (Scheme 4) [8-18]. 

The suitability of in situ trapping of intermediate transition-metal hydrides formed by C—H bond activation by a coordinated alkene to form a cr-alkyl has been demonstrated to be feasible with > -allyl hydridoiridium complexes. However, insertion was also shown to occur in an intramolecular sense to yield methyl-substituted iridacyclopentanes ... 

C-H Bond Activation by High-Valence Transition Metal Oxide Clusters... 

Figure 26. Calculated reaction coordinate of C-C and C-H bond activation by various transition metals (1) M + C2H6 = CH3-M-CH3 (2) M + CH4 = H-M-CH3 (after Blomberg et al. 1991).

Figure 6 Activation of C-H bonds in aldehydes by an Mdium complex within a tetrahedral metal-organic host in aqueous solution, (a) Crystal structure of the host, viewed down the C2-axis of S5munetry (b) simplified view, showing the structure of one of the six identical ligands that comprise the edges of the tetrahedral host (c) schematic representation of host and (d) proposed mechanism for C-H bond activation by an encapsulated Ir complex.

Exploitation of Synthetic Reactions Via C—H Bond. Activation by Transition Metal Catalysts. Car-boxylation and Aminomethylation of Alkanes or Arenes. 

Most of the mechanisms for heteroaromatic C—H bond activation by a transition-metal catalyst fall into one of these four categories (i) electrophilic aromatic metala-tion, (ii) carboxylate-ligand-promoted concerted metalation-deprotonation (CMD), (iii) base-assisted metalation and (iv) oxidative addition of C—H to the metal center. The type of mechanism operating in the cleavage of the C—bond depends on the electronics of the heterocycle (and therefore its substituents) and the reaction conditions being employed. 

The present review is organized in the following way in the first part, C—H bond activation mediated by rare-earth metals and actinides following traditional reaction pathways is summarized in the second part, non-traditional C—H bond activation reactivity will be discussed in detail in order to understand the underlying mechanisms. The scope of the review is limited to rare-earth metals and actinides, but, in some cases, closely related reactivity of group 4 metals wiU be included for comparison. The purpose of the review is not only to provide an overview of C—H bond activation by f-elements but also to bring to attention unusual reactivity following mechanisms different from a-bond metathesis and 1,2-addition. 

In spite of the absence of C—H bond activation by an oxidative addition mechanism mediated by f-elements, these two approaches, use of a bimetallic system or a redox noninnocent ligand, bring new opportunities to discover novel types of C—H bond activation utilizing f-element metal complexes, as discussed below. It is interesting to note that, in most cases, strong reducing conditions are appHed. 

Pertinent reviews published during 1980 cover cyclometallation of P-donor ligands, CO insertion into metal-carbon o-bonds, mechanistic features of catalytic CO hydrogenation reactions, and stoicheiometric reactions of transition-metal carbene complexes. Other articles of interest deal with the stability of metal-carbon bonds, " transition-state geometry for insertion of metals into C-H bonds, organic synthesis using Group VIII metal complexes, and C-H bond activation by transition metals. " Molecular orbital calculations on the interconversion of metal bis(olefin) and metallocyclo-pentane complexes have been reported. ... 

Commercially available and inexpensive y-Fe203 magnetic nanoparticles (particle size 58 nm) also efficiently activate B2pin2 and promote a direct borylation of alkenes. " The mechanism of this unusual nano-Fc203-catalyzed aromatic borylation reaction is not clear. The kinetic isotope effect was measured to be 1.3, indicating that a C—H bond activation by oxidative addition to the iron catalyst is not likely. An electrophUic metalation by Fe—B species, followed by reductive ehmination, seems conceivable. 

Metallocycles as intermediates in synthesis of heterocycles by transition metal-catalyzed coupling reactions under C—H bond activation 99AG(E)1698. 

SPECTROSCOPY OF THE POTENTIAL ENERGY SURFACES FOR C-H AND C-O BOND ACTIVATION BY TRANSITION METAL AND METAL OXIDE CATIONS... 

Spectroscopy of the Potential Energy Surfaces for C-H AND C-O Bond Activation by Transition Metal and Metal Oxide Cations 331 By R. B. Metz... 

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