Bond-selective activation

The bond dissociation energy of fluoromethane is 115 kcal mol , which is much higher than the other halides (C-Cl, C-Br and C-1, respectively 84, 72 and 58 kcal mol ) [6], Due to its strength, the carbon-fluorine (C-F) bond is one of the most challenging bonds to activate [7], A variety of C-F bond activation reactions have been carried out with different organometallic complexes [8], Among them, nickel [9] and ruthenium complexes have proven to proceed selectively under mild conditions [10],... 

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... 

Example Selective activation of C-H bonds is rarely observed in saturated alkyl groups, but the iridium complex 1 does react by C-H insertion of the metal into a ligand bond upon treatment with LiBr in solution. The reaction can be tracked by LT-FAB-MS (Fig. 9.17). A decreasing intensity of the molecular ion of 1, m/z 812.4, and increasing of 2, m/z 856.4, indicate the progress of this reaction. Furthermore, the halogen exchange is indicated by the isotopic pattern. 

One may also resort here to organotransition metal complexes. For example, benzene rings can be selectively activated to nucleophilic attack by complexation to chromium tricarbonyl (Scheme 12.8) [21]. Similarly, an allylic acetate can also be selectively activated in the presence of a bromide (29 versus 3Q) by addition of a palladium(O) catalyst in THF, which coordinates with the double bond [22] (Scheme 12.9). 

Most recently, Baltzer and co-workers have incorporated a lysine-bound nicotinamide into a more complex peptide scaffold [75]. This approach takes advantage of the augmented reactivity of a lysine residue contained in a helix-turn-helix scaffold (as described previously [76]). An adjacent histidine is able to selectively catalyze the formation of an amide bond between activated esters and the lysine c-amino group under aqueous conditions. Thus, reaction of the 42-residue peptide LA-42 withp-nitrophenyl hT-methylnicotinate in an aqueous solution at pH 5.9 yields the nicotinoyl-functionalized peptide (Fig. 27). 

The adsorption and reaction of methanol on metal surfaces has been widely studied (18-34). Methanol has C-0, C-H, and 0-H bonds, serving as one of the simplest systems for the selective activation of chemical bonds. The methoxyl (CH30(a)) species has been considered as an intermediate of the methanol decomposition. On many transition metal surfaces, adsorbed methanol molecules are usually decomposed to H2 and CO, although Ag and Cu are used as catalysts for the conversion of methanol to formaldehyde. The adsorption and reaction of alcohol molecules on Mo surfaces has been studied on the (100) (4) and (110) (35) surfaces. Alcohol molecules are decomposed effectively also on these surfaces. 

There are however certain cases where the selective activation of a carbon-hydrogen bond is viable. 

In the similar structure of l.l,1,2,4,4,4-heptafluoro-3-phenylbut-2-ene, the reduction with sodium borohydride takes place readily under mild conditions and both products of vinylic and allylic Sn2 displacement are obtained the relative amounts of the products can be controlled by adding triphenylphosphane to the mixture80 (Table 2). The nonfluorinated chain in Hpenta-fluorophenyl)-2-phenylethane (12) selectively activates the para C-F bond to reduction by lithium aluminum hydride, giving the monoreduced product 13 in high yield.81... 

Diacetyl (DA) is used as a flavour enhancer in the food industry and is currently manufactured from methyl ethyl ketone (MEK) in homogeneous systems via an oxime intermediate (ref.1). In principle, DA can also be manufactured by the selective oxidation of MEK and several reports have appeared in the literature which apply heterogeneous catalysts to this task (refs. 2-4). A number of reports have specified the importance of basic or weakly acidic sites on the catalyst surface for a selectively catalysed reaction and high selectivities to DA at moderate conversions of MEK have been reported for catalysts based on C03O4 as a pure oxide and with basic oxides added conversely scission reactions have been associated with acidic oxide additives (refs. 2-4). Other approaches to this problem have included the application of vanadium phosphorus oxide (VPO) catalysts. Ai (ref. 5) has shown that these catalysts also catalyse the selective oxidation of MEK to DA. Indeed this catalyst system, used commercially for the selective oxidation of n-butane to maleic anhydride (ref.6), possesses many of the desired functionalities for DA formation from MEK, namely the ability to selectively activate methylene C-H bonds without excessive C-C bond scission. 

In this paper, we will report the electronic and catalytic reactivities of the model VC/V(110) surface, and our attempt to extend them to VC powder catalysts. By using high-resolution electron energy loss spectroscopy (HREELS) and NEXAFS techniques, we observed that the surface properties of V(110) could be significantly modified by the formation of vanadium carbide some of the experimental results on these model surfaces were published previously.3-5 We will discuss the selective activation of the C-H bond of isobutane and the C=C bond of isobutene on V(110) and on VC/V(110) model systems. These results will be compared to the catalytic performances of vanadium and vanadium carbide powder materials in the dehydrogenation of isobutane. 

Due to their electronic properties, early transition metal carbides should in principle have some advantages in controlling the selective activation of C-H and C=C bonds. As discussed earlier, the electronic properties of early transition metal carbides are similar, up to the Fermi level, to those of Pt group metals (Group 8-10 metals in general). Therefore, by comparing with the parent metals, these carbides should be less active for C=C bond cleavage and more effective for C-H bond activation. Furthermore, the broadened unfilled portion of the d-band should make these carbides better electronic acceptors than both the parent metals and... 

---