Alkane C-H bond activation

A topic that has attracted considerable attenhon in the last 20 years is alkane activahon [105]. The achvation product is of interest in itself and is the hrst subject of this sechon but the ulhmate goal is funchonalizahon of the alkane to give alcohol or alkene. 

The earliest examples involved cyclometalahon, where a CH bond, often of an arene, is held in the vicinity of the metal. Creahon of a 2e vacancy at the metal often results in the formahon of the cyclometalahon product, a reachon that may be reversible or not. Eq. 2.34 shows a typical cyclometalahon. 

Eor the intermolecular achvahon of a C-H bond, a number of different situations can arise. Most often, the reaction of Eq. 2.35 is thermodynamically uphill. The oxidative addihon of RH is in general less favorable than that of H2 because of the rather weak M-R bond formed from an alkane. In contrast, arenes are much easier to achvate in this way, the M-Ar bond being much stronger this is true even 

Activation of Substrates with Non-Polar Single Bonds 

The selectivities of such reactions are very different from those found for traditional electrophilic and radical pathways, both of which are highly selective for tertiary over seconday CH bonds, with primary bonds being unreactive. In oxidative addition, in contrast, both primary and secondary CH bonds react and tertiary CH bonds do not. Depending on the system, the selectivity may favor primary or secondary bonds, depending on the intrinsic reactivity and steric encumbrance of the system. 

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The fact that the final product 3-Tp Rh(CO)(H)(R) does not appear on the ultrafast time scale (<1 ns,) (Fig. 4) indicates a free energy barrier greater than 5.2 kcal/mol for the alkane C-H bond activation. Nanosecond step-scan FTIR experiments on the 3-Tp Rh(CO)2/cyclohexane system show that the remnant of the 2-Tp Rh(CO)(S) peak persists for 280 ns after photoexcitation, while the product CO stretch at 2032 cm-1 rises with a... 

Figure 8 A proposed reaction mechanism for the alkane C-H bond activation by j3-Tp Rh(CO)2 covering the ultrafast dynamics to nanosecond kinetics. 

Lian T, Bromberg SE, Yang H, Proulx G, Bergman RG, Harris CB. Femtosecond IR studies of alkane C-H bond activation by organometallic compounds — direct observation of reactive intermediates in room temperature solutions. J Am Chem Soc 1996 118(15) 3769—3770. 

Asbury JB, Ghosh HN, Yeston JS, Bergman RG, Lian TQ. Sub-picosecond IR study of the reactive intermediate in an alkane C-H bond activation reaction by CpRh(CO)2. Organometallics 1998 17(16) 3417-3419. 

Lian, T., Bromherg, S. E., Yang, H., Proulx, G., Bergman, R. G., Harris, C. B., Femtosecond IR Studies of Alkane C H Bond Activation by Organometallic Compounds Direct Observation of Reactive Intermediates in Room Temperature Solutions, J. Am. Chem. Soc. 1996, 118, 3769 3770. 

Truitt Ml, Toporek SS, Rovira-Truitt R, White JL (2006) Alkane C-H bond activation in zeolites evidence for direct notium exchange. J Am Chem Soc 128(6) 1847-1852. doi 10.1021/ ja0558802... 

Note that the main difference between zirconium hydride and tantalum hydride is that tantalum hydride is formally a d 8-electron Ta complex. On the one hand, a direct oxidative addition of the carbon-carbon bond of ethane or other alkanes could explain the products such a type of elementary step is rare and is usually a high energy process. On the other hand, formation of tantalum alkyl intermediates via C - H bond activation, a process already ob-... 

Considerable interest in the subject of C-H bond activation at transition-metal centers has developed in the past several years (2), stimulated by the observation that even saturated hydrocarbons can react with little or no activation energy under appropriate conditions. Interestingly, gas phase studies of the reactions of saturated hydrocarbons at transition-metal centers were reported as early as 1973 (3). More recently, ion cyclotron resonance and ion beam experiments have provided many examples of the activation of both C-H and C-C bonds of alkanes by transition-metal ions in the gas phase (4). These gas phase studies have provided a plethora of highly speculative reaction mechanisms. Conventional mechanistic probes, such as isotopic labeling, have served mainly to indicate the complexity of "simple" processes such as the dehydrogenation of alkanes (5). More sophisticated techniques, such as multiphoton infrared laser activation (6) and the determination of kinetic energy release distributions (7), have revealed important features of the potential energy surfaces associated with the reactions of small molecules at transition metal centers. 

Although the activation and functionalization of C-H bonds of alkanes are the important, promising routes for synthesis of functionalized materials, it is difficult to achieve the functionalization of alkanes because they are unreactive due to the low reactivity of alkane C-H bonds. Carboxylation of alkanes to carboxylic acids is one of the interesting and important functionalization processes. 

Thus, a highly reactive species is needed to make this type of bond activation reaction feasible under mild conditions. In addition, to be useful, the C-H bond activation must occur with both high chemo- and regiose-lectivity. Over the past several decades, it has been shown that transition metal complexes are able to carry out alkane activation reactions (1-5). Many of these metal-mediated reactions operate under mild to moderate conditions and exhibit the desirable chemoselectivity and regioselectiv-ity. Thus, using transition metal complexes, alkane activation can be preferred over product activation, and the terminal positions of alkanes, which actually contain the stronger C-H bonds, can be selectively activated. The fact that a hydrocarbon C-H bond has been broken in a... 

Control of H-C(sp3) Bond Cleavage Stoichiometry Clean Reversible Alkyl Ligand Exchange with Alkane in [LPt(Alk)(H)2]+ (L=[2.1.1]-(2,6)-Pyridinophane) (226) this complex activates hydrocarbons RH to yield LPtRHjT. This is similar to the C-H bond activation shown in Scheme 17 but occurs without added acid. 

Keywords Alkane metathesis Borylation C-H bond activation Dehydrogenation Hydroarylation Iridium catalyst Silylation... 

For many years the activation of unfunctionalized alkanes has been the Holy Grail of organic synthesis and, indeed, it has only been during the past few years that catalysts have evolved which allow an alkane C—H bond to be selectively... 

Having set out the properties of tantalum and zirconium hydride toward C-H bond activation of alkanes we now describe the catalytic hydrogenolysis of C-C bonds. It was previously shown in the laboratory that supported-hydrides of group 4 metals, and particularly of zirconium, catalyze the hydrogenolysis of alkanes [21] and even polyethylene [5] into an ultimate composition of methane and ethane. However, to our initial surprise, these zirconium hydrides did not cleave ethane. (=SiO)2Ta-H also catalyzes the hydrogenolysis of acyclic alkanes such as propane, butane, isobutane and neopentane. But, unlike the group 4 metals, it can also cleave ethane [10], Figure 3.7 illustrates this difference of behavior between (=SiO)2Ta(H) and [(=SiO)(4.j,)Zr(H) ], x= or 2). With Ta, propane is completely transformed into methane by successive reactions, while with Zr only equimolar amounts of methane and ethane are obtained. 

Some of these intermediates are analogous to those proposed by Chauvin in olefin metathesis ( Chauvin s mechanism ) [36]. They can be transformed into new olefins and new carbene-hydrides. The subsequent step of the catalytic cycle is then hydride reinsertion into the carbene as well as olefin hydrogenation. The final alkane liberation proceeds via a cleavage of the Ta-alkyl compounds by hydrogen, a process already observed in the hydrogenolysis [10] or possibly via a displacement by the entering alkane by o-bond metathesis [11]. Notably, the catalyst has a triple functionality (i) C-H bond activation to produce a metallo-carbene and an olefin, (ii) olefin metathesis and (iii) hydrogenolysis of the metal-alkyl. 

In spite of significant fundamental studies and its significant economic potential as an alternate route to alkenes, the oxidative dehydrogenation of alkanes to alkenes is not currently practiced.383 The main reason is that the secondary oxidation of the primary alkene products limits severely alkene yields, which becomes more significant with increasing conversion. This is due mainly to the higher energies of the C—H bonds in the reactant alkanes compared to those of the product alkenes. This leads to the rapid combustion of alkenes, that is, the formation of carbon oxides, at the temperatures required for C—H bond activation in alkanes. 

The first activation of an alkane C-H bond was described in 1969 [29]. Three decades were to pass until the development of the current catalytic procedures for dehydrogenation and C-O, C-C, and C-B bond-forming reactions. Progress has been slow. Nevertheless, significant advances in catalyst research were achieved in the 1990s, aided by the development of improved metal ligands and the increased understanding of the mechanism of transition metal-catalyzed C-H activation reactions. Further improvements of catalytic cycles are nec-... 

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