Bond activation reactions

However, recently, a theoretical paper has been published, which provides interesting arguments for a conventional silylmetal hydride rather than for a 3c2e M(H)Si bond [132,133]. For the great implications of these compounds for Si —H bond activation reactions consult, e.g., on the work of Crabtree [129]. 

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],... 

Transition metal centered bond activation reactions for obvious reasons require metal complexes ML, with an electron count below 18 ("electronic unsaturation") and with at least one open coordination site. Reactive 16-electron intermediates are often formed in situ by some form of (thermal, photochemical, electrochemical, etc.) ligand dissociation process, allowing a potential substrate to enter the coordination sphere and to become subject to a metal mediated transformation. The term "bond activation" as often here simply refers to an oxidative addition of a C-X bond to the metal atom as displayed for I and 2 in Scheme 1. 

One of the remarkable features of these 14 electron fragments, which were developed experimentally on the basis of applied MO theory considerations, is their ability to attack selected C-H and, in particular, unactivated C-Si bonds of various organosilanes. Mechanism studies of these bond activation reactions at this point suggest a new type of a-complexation in a common transition state or intermediate for both C-H and C-Si activation, which has to be further investigated in detail through experiments and by theory. 

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... 

Studies of intramolecular C-H bond activation reactions at Pt(II) (and Pd(II)) leading to cyclometalated products have typically found that these reactions are facilitated by ancillary ligands that can easily dissociate (20,21). In the absence of any weakly bonded ligand, unusually... 

Isotope Effects in C-H Bond Activation Reactions by Transition Metals (225) were reviewed, and some pitfalls in interpreting kinetic isotope were pointed out. The interpretation of the kinetic isotope effects offered by the authors of the original reports (75,76,85) on the system shown in Schemes 15,16 was criticized. 

Discovered over a century ago, electrophilic mercuration is probably the oldest known C-H bond-activation reaction with a metal compound. The earliest examples of aromatic mercuration were reported by Volhard (mercuration of thiophene) [1], Pesci (mercuration of aromatic amines) [2], and Dimroth [3], who was the first to mercurate benzene and toluene, generalize the reaction, and assign the correct structures to the products originally observed by Pesci. Since the work of Dimroth electrophilic aromatic metalation reactions with compounds of other metals, for example Tl(III), Pb(IV), Sn(IV), Pt(IV), Au(III), Rh(III), and Pd(II), have been discovered [4], In this chapter, we will focus on intermolecular SEAr reactions involving main-group metal electrophiles and resulting in the formation of isolable metal aryls which find numerous important applications in synthesis [5], Well-known electrophilic cyclometalation reactions, for example cyclopalla-dation can be found in other chapters of this book and will not be reviewed here. 

The simplest oxidative addition reaction in organic chemistry is the bond activation reaction of, for example, a C—H by a carbenoid reagent X2E , for example, E = C, Si, Ge, Sn, Pb, while X = H or any other monovalent groups. Scheme 6.5a shows the R and R states for this bond activation process, using both electron-dot pairing schemes as well as FO—VB bond-pairing diagrams. 

These oxidative—addition reactions have been treated extensively by Su et al. (29-31), using the VBSCD model. In all cases, a good correlation was obtained between the computed barriers of the reaction and the respective AEst quantities (which enter into the expression of G), including the relative reactivity of carbenoids, and of PtL2 versus PdL2 (29-31). Another treatment led to the same reactivity patterns for C—F bond activation reactions by Rh(PR3)2X and Ir(PR3)2X d8 complexes, which are isolobal to carbenoids (30). A similar extended correlation was found recently for C—Cl activation by d10-PdL2 (32), and is dealt with in Exercise 6.9. 

Ruthenium is not an effective catalyst in many catalytic reactions however, it is becoming one of the most novel and promising metals with respect to organic synthesis. The recent discovery of C-H bond activation reactions [38] and alkene metathesis reactions [54] catalyzed by ruthenium complexes has had a significant impact on organic chemistry as well as other chemically related fields, such as natural product synthesis, polymer science, and material sciences. Similarly, carbonylation reactions catalyzed by ruthenium complexes have also been extensively developed. Compared with other transition-metal-catalyzed carbonylation reactions, ruthenium complexes are known to catalyze a few carbonylation reactions, such as hydroformylation or the reductive carbonylation of nitro compounds. In the last 10 years, a number of new carbonylation reactions have been discovered, as described in this chapter. We ex-... 

Probing Bond Activation Reactions with Femtosecond Infrared... 

Figure 10 Solvation of the coordinatively unsaturated j5-CpRe(CO)2 intermediate by a EtsSiH molecule partitions the silane Si-H bond activation reaction to two pathways.

Lees AJ, Purwoko AA. Photochemical mechanisms in intermolecular C-H bond activation reactions of organometallic complexes. Coord Chem Rev 1994 132 155-160. 

Bromberg SE, Yang H, Asplund MC, Lian T, McNamara BK, Kotz KT, Yeston JS, Wilkens M, Frei H, Bergman RG, Harris CB. The mechanism of a C-H bond activation reaction in room-temperature alkane solution. Science 1997 278 260-263. 

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. 

A less common method to give -alkyl or aryl complexes is the C H bond activation reactions. Elimination of HNMe2 from Ta(0-2-Ind-4,6-Bu 2-C6H3)(NMe2)4 yields (14). Activation of a C-H bond in a Ta- NMc2 group led I----------------------------1... 

Treatment of the imide hydride Cp 2Ta(=NBu )H with [Ph3C][B(C6F5)4] yields the cationic imide [Cp 2Ta-(=NBu )(THF)][B(C6F5)4] (22). (22) reacts cleanly with H2 to yield [Cp 2Ta(NHBu )H][B(C6F5)4], and undergoes C-H bond activation reactions with propyne or pheny-lacetylene to afford [Cp 2Ta(NHBuO(C=CR)][B(C6F5)4] (R = Me, Ph). The heterolytic cleavage reactions of (22) may be the result of the presence of both electrophilic and nucleophilic sites of reactivity in the same molecule. Intramolecular activation of a C-H bond of a Cp lig-I------------------------------1... 

The next section will describe the experimental techniques we use to synthesize and study clusters and present typical data. The following section will discuss the cluster size sensitive behavior observed in kinetic studies of H-H and C-H bond activation reactions as well as the size sensitive behavior of hydrogen uptake, and discusses the potential implications of these experiments in catalysis and chemisorption. The last section gives the highlights of recent studies of the electronic properties of mass selected, monodispersed, platinum clusters containing up to 6 Pt atoms supported on Si02. 

T) -C5H5)Rh(C2H4)CO. The same authors have also studied the C-H and Si-H bond activation reactions of (ii -C5H5)Rh(C2H4)CO in low temperature matrices. 

In this section, we will highlight the development in the use of metal alkox-ides for the synthesis of new and interesting organometallic compounds, many of these are either inaccessible or difficult to synthesize by common synthetic procedures. We will not discuss (a) the chemistry of organometallic compounds containing alkoxides as supporting ligands, for which excellent reviews by Chisholm and co-workers (154, 513, 514) are available and (b) intramolecular cyclometalation (i.e., C—H bond activation) reactions of metal aryloxides due to the availability of an excellent account of this topic in a review article by Rothwell (515). Furthermore, a brief mention of the use of a related metal derivative (i.e., metal aryloxide) will be made merely for comparison. 

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