Activation of C - H Bonds

PaRshall, Homogeneous catalytic activation of C-H bonds, Acc. Chem. Res. 8, 113-7 (1975). 

The large amounts of natural gas (mainly methane) found worldwide have led to extentive research programs in the area of the direct conversion of methane [1-3]. Ihe oxidative transformation of methane (OTM) is an important route for the effective utilization of the abundant natural gas resources. How to increase catalyst activity is a common problem on the activation of methane. The oxidation of methane over transition m al oxides is always high active, but its main product is CO2, namely the product of deep oxidation. It is because transition metal oxides have high oxidative activity. So, they were usually used as the main corrqtonent of catalysts for the conqilete oxidation of alkane[4]. The strong oxidative activity of CH4 over tran on metal oxides such as NiO indicates that the activation of C-H bond over transition metal oxides is much easier than that over alkaline earth metal oxides and rare earth metal oxides. Furthermore, the activation of C-H bond is the key step of OTM reaction. It is the reason that we use transition metal oxides as the mam conq>onent of the OTM catalysts. However, we have to reahze that the selectivity of OTM over transition metal oxides is poor. 

The activation of C-H bonds of alkanes is one of the challenges of catalytic research. Sublimation of tetraneopentyl zirconium (ZrNp4) onto the surface of... 

In addition to coordination by heteroatoms, the 7r-bond of cyano group also participates the activation of C-H bonds (Equation (14)).14 The ruthenium-catalyzed alkylation of benzonitriles with triethylvinylsilane proceeds at the ortho-position predominantly. 

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 Kalck P, Urrutigoi ty M, Dechy-Cabaret O (2006) Hydroxy- and Alkoxycarbonylations of Alkenes and Alkynes. 18 97-123 Kanno K, see Takahashi T (2005) 8 217-236 Keen SP, see Gibson SE (n6e Thomas) (1998) 1 155-181 Kendall C, see Wipf P (2005) 8 1-25... 

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. 

These findings have stimulated enormously the search for intermolecular activation of C-H bonds, in particular those of unsubstituted arenes and alkanes. In 1982 Bergman [2] and Graham [3] reported on the reaction of well-defined complexes with alkanes and arenes in a controlled manner. It was realised that the oxidative addition of alkanes to electron-rich metal complexes could be thermodynamically forbidden as the loss of a ligand and rupture of the C-H bond might be as much as 480 kl.mol, and the gain in M-H and M-C... 

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. 

Recently, complex lrH2 C6H3-2,6-(CH2P Bu2)2 (25) has been used in the activation of C—H bonds of several substrates (Scheme 13.13) [32], including alkane polymers and oligomers (Scheme 13.14). 

Two models for the mechanism of the isomerization have been proposed. The first model was originally proposed because of similarities between enzymes involved in the activation of C-H bonds next to a carbonyl function (Fig. la). Stereochemical considerations were used to suggest that the ring structure of the sugar is opened to... 

ASYMMETRIC SYNTHESIS BASED ON CATALYTIC ACTIVATION OF C-H BONDS AND C-C BONDS... 

The activation of C—H bonds and C—C bonds has attracted much attention in both academic and industrial laboratories because of their potential economic and ecological advantages. In the field of asymmetric synthesis, enantioselective catalytic C—X bond formation via the activation of C—H bonds and/or C—C bonds should have a great impact on asymmetric synthesis in both theory and practice. In theory, it is interesting to see how these very unreactive bonds can react preferentially in the presence of more reactive bonds with asymmetric control. In a practical sense, such C—H and C—C bonds are equivalent to the C M bonds in organometaUic reactions and would turn the corresponding stoichiometric amounts of metal into catalytic amounts. Conceptually, there are two fundamental ways to... 

C—C bond formation plays a fundamental role in organic synthesis. In spite of great success in asymmetric epoxidations and hydrogenations, highly efficient asymmetric C—C bond formations are still in great demand. In this section, we will present the results of asymmetric C—C bond formation via the activation of C—H bonds according to the types of C—H bond in the substrates (sp C—H bonds, sp C—H bonds, and sp C—H bonds). 

In contrast to the asymmetric activation of C—H bonds in benzyl silyl ethers, the dirhodium tetraprolinate, Rh2(5-DOSP)2 (Figure 5.7), was found to be an efficient catalyst in an enantioselective C—H activation of acetals (Scheme 5.16). Interestingly, when the acetals had a methoxy substituent on the aromatic ring, the Stevens rearrangement was a main competing side reaction of the C—H activation of acetals. 

If vinylcarbenoids were used, highly diastereoselective (>98% de) and enantioselective (95.1-99.6% ee) activation of C—H bonds of 1,2-dihydronaphthalenes were... 

Scheme 5.15. Asymmetric activation of C—H bonds in benzyl silyl ethers.

Compared with the activation of C—H bonds, the activation of C—C bonds is rarely used in asymmetric synthesis. Two main factors contributed to this difficulty ... 

In Chapters 4-6, recent hndings on activation of C-H bonds, C-C bonds and small molecules (C=0, HCN, RN=C, and CO2) are covered. The latest developments on C-C bond reorganization such as metathesis (which earned the Nobel prize in chemistry, 2005) are also described. 

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