H activation

The C-H activation reaction is a reaction that cleaves a caihon-hydrogen bond. Here the carbon-hydrogen bond is mostly referred to unactivated caibon-hydrogen bonds. 

Selective or/Ao-alkylation and -aiylation of aryl iodides can be achieved by the cooperative catalytic action of palladium and noibomene. The first reported case was the or/Ao-dialkylation of aryl iodides, followed by Heck reaction. Here an aiyl iodide with free o-positions reacts with an aliphatic iodide and a terminal olefin in the presence of palladium/noibomene as catalyst and a base, to give a 2,6-substituted virtylarene. Analogously, an aryl iodide with one substituted o-position leads to a virtylarene containing two different ortho groups.  

Example 1, A three-component reaction allowing the constmction of three adjacent C-C bonds through C-1 and C-H activation.  

Mechanism for the reaction of an o-substitued aryl iodide Pd(0), Pd(ll) and Pd(IV) intermediates and catalytic role of palladium and norbomene.  

Name Reactions A Collection of Detailed Mechanisms and Synthetic Applications, DOI 10.1007/978-3-319-03979-4 54, Springer International Publishing Switzerland 2014 

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The phosphorodithioates DNA derivatives have been shown to bind specifically to complementary DNA or ENA sequences to form stable adducts. Because they are also highly resistant to degradation by cellular exonucleases, these derivatives can be useful both for appHcations in research and as therapeutic dmgs. Phosphorodithioate DNA has been shown to stimulate Rnase H activity in nuclear cell extracts and is a potent inhibitor of HIV type-1 reverse transcriptase (56). 

Dimethylthiophene reacts photochemically with rran.s-Rh(PMe3)2(CO)Cl to yield the isomeric products 218 (R = Me) and 219 because of C—H activation of thiophene (960M872) as well as species 220 (R = Me). Unsubstituted thiophene in similar conditions gives five products, the 3-thienyl activated isomers 218 (R = H) and 219 (R = H), the 2-thienyl isomers, 221 and 222, and 2-thienyl analogs of 220 (R = H). 

The pentamethylcyclopentadienyl derivatives of rhodium Cp RhL (L = PMe3, C2H4) oxidatively add thiophene preferentially via the C—S activation route compared to that based on the C—H activation [880M1171,94JOM(472)311]. The Tp derivatives by contrast yield mainly the latter. Tp Rh(PEt3) acts almost selectively and forms exclusively 225 (R = Et), whereas Tp Rh(PMc3) forms a major amount of 225 (R = Me) and minor amount of 226 (960M2678). 

Complex 105 in an atmosphere of carbon monoxide experiences rearrangement to 106, which through C-H activation gives the final product 107 with i -coordiantion of the 3,4-diphosphacyclopentadienone hgand (97JCS(CC)1539). 

Similar pyrimidine-to-pyridine conversions were also reported for purine and 8-azapurine with C-H active acetonitriles, ethyl acetoacetate, acetylacetone with dimedone 8-azapurine is converted into triazolotetra-hydroquinoline (Scheme 15) (73JCS(P1)1620, S(Pl)1625, S(P1)1794). 

It appeared to be a logical consequence to transfer this synthetic principle to more suitable metals like ruthenium and introduce bulky, kinetically stabilizing ligands at the metal. An interesting example for this approach is the complex 78. The latter is synthesized from Cp RuCl(PR3)2 with ClMgCH2SiMe2H through 77 by a thermal Si — H activation reaction. 

Similarly, when both the Cp and arene ligands are permethylated, the reaction of 02 with the Fe1 complex leads to C-H activation of the more acidic benzyl bond [57]. When no benzylic hydrogen is present, superoxide reacts as a nucleophile and adds onto the benzene ligand of the FeCp(arene)+ cation to give a peroxocyclohexadienyl radical which couples with a Fe Cp(arene) radical. A symmetrical bridging peroxo complex [(Fe"Cp)2(r 5-C6H60)2] is obtained. The C-H activation reactions of the 19e Fe1 radicals BH can be summarized as follows... 

The special salt effect is a constant feature of the activation of substrates in cages subsequent to ET from electron-reservoir complexes. In the present case, the salt effect inhibits the C-H activation process [59], but in other cases, the result of the special effect can be favorable. For instance, when the reduction of a substrate is expected, one wishes to avoid the cage reaction with the sandwich. An example is the reduction of alkynes and of aldehydes or ketones [60], These reductions follow a pathway which is comparable to the one observed in the reaction with 02. In the absence of Na + PFg, coupling of the substrate with the sandwich is observed. Thus one equiv. Na+PFg is used to avoid this cage coupling and, in the presence of ethanol as a proton donor, hydrogenation is obtained (Scheme VII). 

Scheme 23 Formation of tetrahydroazepinones 113 and methylenepyrrolidines 111 by a formal [5+2] cycloaddition with C-H activation [85]...

In this case the use of the Sm(II) "ate" complex Na[Sm N(SiMe3)2 3] as starting material afforded yet another novel C-substituted amidinate complex resulting from y C-H activation of a N(SiMe3)2 ligand (Scheme 194). All new... 

Very few reports concerning transformations of ligands with other donor atoms exist (Table V). P—H activation at secondary phosphines is the most common motif, with the metal-metal bonds at the heterometallic faces stabilizing the resulting fragments in each case (Figs. 31. 32, 33).In the formation of both... 

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