Activation of C-H bonds by metal complexes

Shilov AE, Shulpin GB (1997) Activation of C—H bonds by metal complexes. Chem Rev 97 2879-2932... 

J. Halpem - Activation of C-H Bonds by Metal Complexes Mechanistic, Kinetic and Thermodynamic Considerations, Inorg. Chim. Acta 100,41,1985. 

The use of supercritical fluids as reaction media for organometallic species is also investigated. Reactions include photochemical replacement of carbon monoxide with N2 and H2 in metal carbonyls, where the reaction medium is supercritical xenon. Also, photochemical activation of C-H bonds by organometallic complexes in supercritical carbon dioxide is investigated. More recent studies on photochemical reactions also include laser flash photolysis of metal carbonyls in supercritical carbon dioxide and ethane and laser flash photolysis of hydrogen abstraction reaction of triplet benzophenone in supercritical ethane and... 

Functionalization of C-H bonds by metal carbenoid or nitrenoid insertions represents a promising alternative to the more traditional approach of direct activation by a metal center. Carbenoids and nitrenoids show unusual regio- and stereoselectivity in insertions into C-H bonds, and unlike insertions of metal centers, these are intrinsically functionalizations rather than activations, which must be followed by functionalization (although in either case, loss of the functionalized group, to regenerate the active metal complex, is still required for catalysis) [129]. The use of dimeric Rh(n) complexes in this area has been extensively developed [129]. 

The interaction of saturated C—H and C—C bonds with heterogeneous metal catalysts forms the basis of widely applied reactions such as isomerization, cracking, and re-forming of alkanes. In recent years, much attention has been devoted to the selective activation of C—H bonds by transition metal complexes in homogeneous solution under mild conditions.601 604 In principle, an alkane can undergo oxidative addition to a noble metal complex according to... 

Even the activation of C-H bonds by transition metal carbene complexes has parallels in phosphane chemistry [215]. However, the reasons for the C-H activation by transition metal carbene complexes are still poorly understood and the reaction cannot as yet be predicted, although interesting applications in catalysis might be envisaged. 

The activation of C-H bonds by d° metal centers can be investigated very effectively with complexes of the type t-Bu3SiNH)3ZrR, where R is an alkyl or aryl group, as these compounds undergo reversible elimination of hydrocarbon. In these examples, concerted mechanisms involving R-H bond interaction at d° metal imido complexes, either isolable or transient, give metathesis products via R-H elimination. ... 

The ability to activate a specific inert C-H bond and utilize it as a more versatile functional group is an emerging area in chemistry [8], Activation of C-H bonds with metal catalysts and further functionalization of these intermediates to valuable products has the potential to replicate the utility of reactions such as the Suzuki or Heck whilst also satisfying the high atom and step efficiency demanded by environmental constraints. This would result in a more efficient transformation and also allow us to look at new approaches in the synthesis of complex molecules that are free from the restraints of functional group manipulation. 

ACTIVATION OF C-H BONDS BY LOW-VALENT METAL COMPLEXES ( THE ORGANOMETALLIC CHEMISTRY )... 

Scheme IV. 12 (part 1). Activation of vinylic C-H bonds by metal complexes in a low oxidation state via an oxidative addition mechanism. Reference numbers are in brackets.

Various other examples of the activation of C-H bonds by conventional, low-valent metal complexes have been described [27]. Not all of these reactions proceed via an unambiguous, simple oxidative addition mechanism. 

The effects of ligands in the activation of C-H bonds by transition metal complexes were discussed in certain works. It has been suggested [66a] that a combination of hydride and lone-pair ligands with a minimum of 7c-bonding should be an optimal combination for the reaction between methane and some model Rh(I) and Ru(II) complexes. For example, it should be advantageous to... 

Chapter IV. Activation of C-H Bonds by Low-Valent Metal Complexes... 

We are not aware of any industrial application that uses metal activation of C-H bonds to obtain functionalised molecules. We have included this topic, because it is potentially of great importance by providing a short-cut for the conversion of hydrocarbons to its functionalised derivatives. Two extreme cases will be discussed, reactions with electron-rich metal complexes and reactions with electrophilic metal complexes. As always in organometallic chemistry there are cases in between that utilise both bonding interactions. 

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. 

In connection with the activation of C—H bonds the cyclometalation has become a very general reaction, intensively investigated by Kaesz and coworkers4 and reviewed by Bruce.5 In transition metal complexes an organic ligand reacts with the transition metal resulting in the formation of a metal-... 

A few decades later in 1968, Halpem formulated the need for new approaches to the activation of C-H bonds with a particular focus on saturated hydrocarbons. C-H bond activation, equated with dissociation of carbon-hydrogen bonds by metal complexes , was identified as one of the most important challenges in catalysis [4]. Perhaps the most influential discovery in this area was made in the late 1960s by Hodges and Garnett, who demonstrated that a homogeneous aqueous so-... 

The activation of C—H bonds in hydrocarbons by oxidative addition to low-valent platinum group metal complexes is also feasible. This problem is discussed in more detail in Section III.D.3). 

Kagan H, Namy JL (1999) Influence of Solvents or Additives on the Organic Chemistry Mediated by Diiodosamarium. 2 155-198 Kakiuchi F, Murai S (1999) Activation of C-H Bonds Catalytic Reactions. 3 47-79 Kakiuchi F, Chatani N (2004) Activation of C-H Inert Bonds. 11 45-79 Kant J (2004) Stereospecific Introduction of Cephalosporin Side Chains Employing Transition Metal Complexes. 6 247-262 Keen SP see Gibson SE (nde Thomas) (1998) / 155-181 Kiessling LL, Strong LE (1998) Bioactive Polymers. 1 199-231 Kim DD see Beak P (2003) 5 139-176... 

The activation of C-H bonds is currently one of the most fascinating challenges encountered for the construction of organic molecules. Two recent reviews pointed out how important this process is from an economical point of view [46, 47]. In particular C-H activation initiated by transition metal complexes has... 

Whereas there are numerous examples of the oxidative addition of aromatic C—H bonds to metal complexes [e.g., equations (h) and (p)] attempts to activate paraffinic hydrocarbons by C—H oxidative addition are generally unsuccessful. Since the reverse... 

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