C-H addition

The iron complex Fe[P(OC8H5)3]2[(CgH40)P(OC6H5)2]2 has been synthesized by metal-atom evaporation-techniques (190). The complex is, formally, the result of two ortho-oxidative, C-H additions, accompanied by loss of a molecule of H2. 

Klingebiel32 observed a related insertion with the silene 131 (Eq. 47), which gave the product 132 under the thermal conditions of its formation. Surprisingly, the regiochemistry of the C—H addition to the Si=C bond was opposite to the case in Eq. (46). 

Intermolecular addition of activated methylenes to unsaturated systems has been investigated with silver,36 silver/ gold, and palladium catalysts. Thus, C-H addition of 2,4-pentandione to 1,3-cyclohexadiene occurs in THF at 0°C with 5mol% of palladium(ll) catalyst without base. Josiphos ligand 20 is used as a chirality source to induce... 

However, at elevated temperatures, the disilene (/-Uu (Si)(k)SiSi( R)(Si/-Bu () 721 undergoes isomerization to give the 1,2-disilyl benzene derivative 730, which can be rationalized in terms of a C-H addition to the Si-Si double bond (Scheme 97). 

Extensive studies of kinetics and isotope effects by Hartwig and coworkers support the mechanism shown in Scheme 5 for the lr(I)/dtbpy catalyzed borylation [81]. In particular, these studies indicate that the iridium(III) trisboryl bipyridine complex (10) is the species that activates the arene C-H bond this is in agreement with DFT calculations by Sakaki et al. predicting the key intermediacy of the trisboryl complex and the seven-coordinated Ir(V) species resulting from C-H addition [82]. C-H addition to Ir(III) was also proposed in the (Ind)Ir(COD)/ phosphine-catalyzed borylation by Smith et al. [76]. 

Initiating a remarkable series of papers in 2008-2009, Grubbs and Whited reported the formation of an Ir(I) carbene complex, 44, from the reaction of methyl f-butyl ether (MTBE) with Ozerov s (PNP)lrH2 (PNP = [N(2-P Pr2-4-Me-C6H3)2] ) and NBE [121-127]. C-H addition to (PNP)lr presumably occurs followed by... 

Probing C—H addition/elimination in Pt(ll)/Pt(IV) systems The importance of oxidative addition of aromatic and aliphatic C—H bonds to Pt(II) centers and its microscopic reverse, reductive elimination of C—H from Pt(IV) species, is ubiquitous in the context of both catalysis and synthesis. It is thus inevitable that the chemical, mechanistic, and kinetic facets of such reactions have become a prominent focus of group 10 poly(pyrazolyl)borate research, although this remains a relatively nascent area. 

THF at room temperature to produce substituted phenylallenes (231). Various arynes and terminal and internal alkynes can be used. The reaction of alkyne without propar-gylic hydrogen gave an acetylenic C-H addition product (a phenylalkyne) and a dehydro-Diels-Alder product (a phenanthrene).266... 

Light-induced Electron Transfer C-H Addition to Enones (MSMA to RSMA)... 

Table 1. Asymmetric catalyzed aromatic C-H addition of aromatics to C=X.

Table 2. Asymmetric F-C alkylations by conjugated C-H addition to electron-poor C-C double bonds.

This chapter covers the intramolecular cyclization of olefin-tethered benzimidazoles and the intermolecular C-H addition of azoles to olefins [8], These reactions... 

The formal addition of a C-H bond at activated methylenes and methynes (pronucleophiles) to activated alkenes in the presence of a base is well known as the Michael reaction (Scheme 1, Type A) [1]. In modem organic syntheses, the use of transition metal (TM) catalysts enables the C-H addition of activated methylenes and methynes to activated alkenes perfectly under neutral conditions (Scheme 1, Type B) [2]. In general, the nonfunctionalized carbon-carbon multiple bonds (for example, EWG2 = H in Scheme 1) are unreactive toward carbon nucleophiles because of their electron rich Jt-orbitals. The pioneering efforts by various research groups resulted in the development of transition metal-catalyzed addition of a C-H bond at active alkanes to such unactivated C-C multiple bonds. This reaction consists of the formal addition of a C-H bond across the C-C multiple bonds and is called a hydrocarbonation reaction. As a milestone in this hydro-carbonation area, early in the 1970s, Takahashi et al. reported the Pd-catalyzed addition of the C-H bond of pronucleophiles to 1,3-dienes [3], The first Pd-catalyzed reaction of activated methylenes with unsubstituted allenes was apparently reported by Coulson [4]. The synthetic applications of this reaction were very limited. In the last decade, the Pd-catalyzed addition of C-H bonds to various unacti-... 

Scheme 1. Formal C-H addition of activated methylenes and methynes to activated C-C multiple bonds.

Coordinatively unsaturated 14- or 16-electron fragments L M, where M has a d6, ds, or dw configuration, are capable of oxidatively adding C—H bonds of arenes and alkanes and have been studied in considerable detail. Calculations suggest that the reaction proceeds via an i -alkane complex.125 More electron-rich as well as heavier transition metal centers, i.e., 3rd-row metals, facilitate C—H oxidative addition. In the case of C—H addition to Pd and Pt phosphine complexes MI, high activation barriers ( 30 kcal mol-1) have been calculated for monodentate phosphines, whereas chelating phosphines lead to values as low as 4 kcal mol-1 (M = Pt).126... 

Zhou, S.L., Wang, S.W., Yang, G.S. et al. (2007) Synthesis, stractrrre, and diverse catalytic activities of [ethylenebis(indenyl)]lanthanide(III) amides on N-H and C-H addition to carbodumides and e-caprolactone polymerization. Organometallics, 26, 3755. 

The years 1982 and 1983 saw exciting new developments in direct intermolecular C-H activation when three research groups independently reported similar alkane C-H additions to electron-rich 16-electron Ir(I) and... 

Oxidative addition of a C-C bond (C-C bond activation) to a metal, like C-H addition, is potentially very important on both a laboratory and an industrial scale. If a metal complex were available that could react via OA with a specific alkane C-C bond, for example, the ultimate result would be exclusive functionalization of a normally unreactive carbon atom. Once that occurs, there are numerous transformations available to convert the M-C bond to other functionalities. Catalytic activation of C-C bonds in long-chain hydrocarbons found in petroleum could provide low-energy, efficient routes to production of compounds that are useful in gasoline refining. Unfortunately, unstrained C-C bonds—the type found in saturated hydrocarbons—typically do not readily undergo OA for the same reasons associated with lack of reactivity of C-H bonds. In fact, the situation ought to be worse with C-C bond activation, because OA now produces not one but two M-C bonds at the expense of breaking a robust C-C bond. There should, moreover, be more steric hindrance created when a C-C bond approaches... 

Bergman has also reported an example of C-H addition to a zirconium-nitrogen double bond. The complex Cp2Zr(NHR)Me loses methane to generate an imido complex that can either be trapped with THF or reacted with benzene (Eq. 19). No reactions with alkanes were reported [86]. 

Fig. 4. Relative stabilities of arene C-H addition adducts vs.r 2-arene complexes...

Examples of C-C bond activation, in contrast, are much less common, which can be attributed to several factors, mainly kinetic in nature (1) the metal center has an easier approach to C-H than to C-C bonds (2) statistical abundance of C-H bonds (3) substantially higher activation barrier for C-C oxidative addition compared with C-H addition due to the more directed nature of the C-C bond and (4) metal-carbon bonds tend to be weaker than metal-hydrogen bonds due to the steric repulsion of the surrounding ligands. 

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