Sp2 C—H bonds

The chelation-directed oxidation of sp2- as well as sp2-C-H bonds using catalytic amounts of palladium has been reported (Equation (Sb)).56 56 1... 

A ruthenium dihydrogen complex G or a ruthenacycle D, which was proposed as a potential intermediate, catalyzed the insertion of ethylene into sp2-C-H bonds, with TONs reaching 19 after 48 h of reaction and under very mild conditions (room temperature as opposed to the usual 135 °C) (Equation (96)).91,91a91c... 

Table 1.7 gives E values for various bonds. The values given are averaged over a large series of compounds. The literature contains charts that take account of hybridization (thus an sp C—H bond does not have the same energy as an sp2 C—H bond).87... 

This too satisfies our isodesmic criterion, because on both sides of the equation we have nine sp2-sp2 C-C bonds and 18 sp2 C-H bonds. This equation gives ... 

Activation of vinyl C-H bonds with RuH2(CO)(PPh3)3 catalyst has allowed the formal insertion of a,/l-unsaturated ketones or esters into the C-H bond of vinylsilanes and led to a regioselective C-C coupling at the -position [9] (Eq. 6). Activation of the sp2 C-H bond occurred with the aid of chelation of a coordinating functional group and provided vinylruthenium hydride 14. Insertion of olefin afforded the tetrasubstituted alkene 13. The ruthenium activation of a variety of inert C-H bonds has now been performed by Murai [10]. 

The sp2 C-H bond of aldehydes, formamides, or formate esters undergoes oxidative addition to ruthenium complexes to generate acylruthenium hydride, which can insert alkenes leading to the overall H-COR addition to alkenes [122] (Eq. 91). 

Li, Z. and Li, C.-J. 2005. CuBr-catalyzed direct indolation of tetrahydroisoquinolines via cross-dehydrogenative coupling between sp3 C-H and sp2 C-H bonds. Journal of the American Chemical Society, 127 6968-69. 

Furthermore, a novel electrophilic sp2 C-H bond activation and its application for the preparation of a-diazomethylzirconium complexes were also recently reported. Treatment of complexes 53 with ethyldiazoacetate or with trimethylsilyldiazo methane affords the C-a-metallated diazo alkane complexes 63-66 (Scheme 15). When 53a,b were allowed to react with the non terminal diazo derivative 67, the pyrazoles 70 and 71 were obtained in good yield. Formation of the pyrazoles could be rationalized by O-H bond activation of the enol tautomer of 67, leading to the zirconocene diazo intermediates 68 and 69 which... 

Palladium and copper-catalyzed arylation of C-H bonds by aryl halide reagents is reviewed. The emphasis of the review is on directing-group-containing arene and alkane arylation catalyzed by palladium and on sp2 C-H bond arylation catalyzed by copper. Literature up to early 2009 is covered. 

To date, palladium migration to vinylic positions remains unknown. Thus, palladium migration to sp2 carbons has been largely limited to aryl carbons. However, in addition to vinylic and aryl carbons, aldehydic and imidoyl carbons also possess sp2 hybridization and carry a C-H bond. This special type of sp2 C-H bond has also been subjected to palladium migration studies and preliminary success has been achieved. Currently, aryl to imidoyl [27] and aryl/alkyl to acyl [72] palladium migrations are known. Migrations from other types of carbons to imidoyl and acyl positions are... 

Besides electron-rich sp2 C-H bonds, Li and co-workers demonstrated that electron-deficient alkenes 24 were shown to undergo oxidative coupling reactions... 

The concept of chemistry beyond functional group transformation concerns mainly reaction participants, and this chapter is aimed at providing a summary of some recent development in that respect. The contents of this chapter include (1) sp3 C—H and sp2 C—H bond activation, (2) C—C bond activation, (3) C—O bond activation, (4) C—F bond activation, (5) C—N bond activation, and (6) small molecule activation (H2, 02, and CH4). More detailed information can be found in the cited references. 

Palladium Catalysts Palladium catalysts are effective and powerful for C—H bond functionalization. Carbene precursors and directing groups are commonly used strategies. Generally, sp3 C—H bond activation is more difficult than sp2 C—H bond activation due to instability of potential alkylpalladium intermediates. By choosing specific substrates, such as these with allylic C—H bonds, palladium catalytic systems have been successful. Both intramolecular and intermolecular allylic alkylation have been developed (Scheme 11.3) [18]. This methodology has presented another alternative way to achieve the traditional Tsuji-Trost reactions. 

Iron Catalysts Iron catalysts have been used in various reactions of sp3 C—H bond functionalization as shown above however, there are few examples of direct sp2 C—H bonds functionalization. Significantly, the first cross-coupling of an arylzinc reagent and 2-arylpyridine has been reported recently (Equation 11.29) [67]. This reaction represents an excellent example of synthetically viable iron-catalyzed C—C bond formation through C—H bond activation. The combination of iron, zinc, magnesium, 1,10-phenanthroline, TMEDA, and l,2-dichloro-2-methylpropane is important for the success of the reaction. It has been speculated that the phenanthroline coordinates to the iron and TMEDA coordinates to the zinc. The reactions proceed via a redox cycle of iron with the l,2-dichloro-2-methylpropane acting as an electron acceptor. 

Compared with C—C n and C—N bond formation, there are fewer examples of C—O bond formation reactions via direct sp2 C—H bond activation. Dong and co-workers reported a novel approach to form chiral lactones (Equation 11.42) [81]. This C—H bond functionalization strategy involves an unprecedented Rh-catalyzed hydroacylation of ketones. The basicity of the phosphine ligand plays a critical role in promoting hydroacylation over competitive decarbonylation. 

Li, B. Dixneuf, P. H. sp2 C-H bond activation in water and catalytic cross-coupling reactions. Chem. Soc. Rev. 2013,42, 5744-5767. 

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