C-H activation formed

Rearrangement of allene on an OS3 cluster also produces a M3(/u-3- -HCCHCH) species (Scheme 12). The reaction of the very labile Os3(CO)n(NCMe) with allene gives as the initial product Os3(/u.- -C(CH2)2)(CO)ii. Photolysis of this product causes CO loss and C-H activation, forming HOs3(/u-3- -CH=C=CH2)(CO)9. Finally, thermolysis of the allenyl cluster at 125 °C produces the more stable isomer HOs3(/u,3- -CHCHCH)(CO)9. ... 

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). 

The partial oxidation of propylene occurs via a similar mechanism, although the surface structure of the bismuth-molybdenum oxide is much more complicated than in Fig. 9.17. As Fig. 9.18 shows, crystallographically different oxygen atoms play different roles. Bridging O atoms between Bi and Mo are believed to be responsible for C-H activation and H abstraction from the methyl group, after which the propylene adsorbs in the form of an allyl group (H2C=CH-CH2). This is most likely the rate-determining step of the mechanism. Terminal O atoms bound to Mo are considered to be those that insert in the hydrocarbon. Sites located on bismuth activate and dissociate the O2 which fills the vacancies left in the coordination of molybdenum after acrolein desorption. 

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

The ruthenium NHC complex 14 readily donates hydrogen to suitable acceptors and forms the C-H activated complex 15 [6]. The reversible nature of the C-H activation is established by the return of the hydrogen to restore the original... 

A similar approach to the one described above was utilized for the formation of quinone methide derivatives of osmium.14 Reaction of OsCl2(PPh3)3 with a phenolic diphosphine ligand in the presence of Et3N resulted in phosphine exchange followed by C—H activation and deprotonation by the base to form the two isomeric QM... 

Carretero and coworkers [58] encountered three C-H-activations after a first Heck reaction using a,(5-unsaturated sulfones 6/1-102 and iodobenzene. Under normal conditions, the expected Heck product 6/1-103 is formed however, if an excess of phenyliodide is used, then 6/1-104 is obtained in high yield. In this transformation three molecules of phenyliodide are incorporated into the final product (Scheme 6/1.27). 

The existence of tr-complex intermediates in C-H activation chemistry has been suggested to explain inverse kinetic isotope effects in reductive elimination processes whereby alkanes are formed from alkyl metal hydrides (Scheme 3).9... 

Acetic acid analogs can also be formed from a one-step C-H activation process using a palladium sulfate catalyst.15 A free radical process was ruled out for this formal eight-electron oxidation due to the high selectivities observed (90% based on methane converted) (Equation (7)). 

Acetic acid can be synthesized from methane using an aqueous-phase homogeneous system comprising RhCI as catalyst, CO and 02.17 Side-products included methanol and formic acid, although yields of acetic acid increased upon addition of either Pd/C or iodide ions. The active species is thought to be a CH3-Rh(l) derivative, formed from the C-H activation of methane. The activation of ethane was also achieved, although selectivities were lower, with products including acetic and propionic acids and ethanol (Equation (9)). 

Not all C-H activation chemistry is mediated by transition metal catalysts. Many of the research groups involved in transition metal catalysis for C-H activation have opted for alternative means of catalysis. The activation of methane and ethane in water by the hexaoxo-/i-peroxodisulfate(2—) ion (S2O82) was studied and proceeds by hydrogen abstraction via an oxo radical. Methane gave rise to acetic acid in the absence of external carbon monoxide, suggesting a reaction of a methyl radical with CO formed in situ. Moreover, the addition of (external) CO to the reaction mixture led to an increase in yield of the acid product (Equation (ll)).20... 

Homogeneous iridium(m) catalysts mediate arene C-H activations to form anti-Markovnikov products as in the hydroarylation of propene (Equation (631).64... 

The combined C-H activation/Cope rearrangement generates a new C-H bond in a highly stereoselective manner and, therefore, has the potential to be a strategic reaction in synthesis. An example of this is the enantiose-lective synthesis of (+)-sertraline as shown in Scheme l.91 The C-H insertion step proceeded smoothly to form 17 with 99% ee. The conversion of 17 to (+)-sertraline could be readily achieved using conventional steps. 

Dihydronaphthalenes are remarkable substrates for the combined C-H activation/Cope rearrangement, but under certain circumstances, further cascade reactions can occur. This was seen in the Rh -DOSP -catalyzed reaction of vinyldiazoacetate 26 with dihydronaphthalene 25 (Equation (35)).96 In this case, the isolated product was the formal C-H insertion product. The reaction proceeded through a combined C-H activation/Cope rearrangement to form 27, followed by the reverse Cope rearrangement. As both steps were highly stereoselective, the formal C-H insertion product 28 was produced with very high stereoselectivity (>98% de, 99.6% ee).96... 

The reaction with the siloxy derivative 29 is an interesting example because the product 30 is a 1,5-dicarbonyl derivative (Equation (36)).96 1,5-Dicarbonyls are classically prepared by a Michael addition, but the synthesis of 30 by a Michael addition is not possible because it would require addition to the keto form of 1-naphthol. The acetoxy derivative 31 resulted in a different outcome, leading to the direct synthesis of the naphthalene derivative 32 (Equation (37)).96 In this case, the combined C-H activation/Cope rearrangement intermediate was aromatized by elimination of acetic acid before undergoing a reverse Cope rearrangement. 

In certain cases, the C-H activation/Cope rearrangement is so favorable that double C-H functionalization can occur as illustrated in Equation (38). The product 33 was formed in 99% ee with excellent control of stereochemistry at four centers due to the involvement of a cascade process rather than a direct C-H insertion. 

The intramolecular G-H insertion of aryldiazoacetates is a very powerful strategy for synthesizing heterocyclic rings such as dihydrobenzofurans and dihydrobenzopyrans. The first effective asymmetric copper-catalyzed C-H activation reaction was reported by the group of Sulikowski in the synthesis of the anti-tumor antibiotic FR-66979 93 (Scheme 7)123 219 The G-H insertion of the aryldiazoacetate 90 formed four diastereomeric products 91a, 91b, 92a,... 

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