P-Elimination and reduction

Oxidative addition of the aldehyde C—H bond to Rh is followed by C=C insertion into the M—H to give a metallacycle this gives the product shown after reductive elimination. Oxidative addition of the strained C—O bond is followed by p-elimination and reductive elimination to give the enol which tautomerizes to acetone. 

Oxidative coupling, followed by p elimination and reductive elimination. If the p elimination were suppressed by avoiding p-H substituents the metallacycle might be isolable. A 1,6-heptadiene is another possibility, where the bicyclic structure of the oxidative coupling product might make the metallacycle isolable. 

Initial oxidative coupling of the two ethylenes would have to be followed by p elimination and reductive elimination. The resulting 1-butene would have to resist displacement by ethylene (unlikely) but give an oxidative coupling of butene with ethylene, with the Et group always in the 1-position of the metalacycle and the p-elimination would have to occur only at the former ethylene end of the metalacycle. 

Palladium phosphines served as improved catalysts for additions across the C-C bond of biphenylene. Olefins could be incorporated by a Heck-type vinylation, presumably involving insertion into the aryl-Pd bond of a biphenylyl metallacycle followed by P-elimination and reductive elimination of product (9). Suzuki-type additions could be made using arylboronic acids (10). It was also found that weakly acidic C-H bonds could serve as addition partners, such as methyl ketones or benzylic nitriles (11, 12) [21]. In these reactions, Pd(0) was proposed to insert first into the biphenylene C-C bond generating L2Pd(2,2 -biphenylyl), which was then protonated by p-cresol to leave a Pd-aryl bond that went on to couple with the conjugate base of the substrate to give the product. 

As a mechanistic hypothesis, the authors assumed a reduction of the Fe(+2) by magnesium and subsequent coordination of the substrates, followed by oxidative coupling to form alkyl allyl complex 112a. A ti—c rearrangement, followed by a syn p-hydride elimination and reductive elimination, yields the linear product 114 with the 1,2-disubstituted ( )-double bond (Scheme 29). This hypothesis has been supported by deuterium labeling experiments, whereas the influence of the ligand on the regioselectivity still remains unclear. 

The proposed mechanism of the above cycloisomerizations are depicted in Scheme 11.30. The oxidative coupling of a metal to an enyne yields a bicyclic metaUacyclopentene, which is a common intermediate. The reductive elimination and subsequent retro-[2+2] cycloaddition gave vinylcyclopentene derivatives, while the two patterns of P-elimination and subsequent reductive eUmination gave cychc 1,3- and 1,4-dienes, respectively. The existence of a carbene complex intermediate might explain the isomerization of the olefinic moiety. 

Catalysis. Iridium compounds do not have industrial applications as catalysts. However, these compounds have been studied to model fundamental catalytic steps (174), such as substrate binding of unsaturated molecules and dioxygen oxidative addition of hydrogen, alkyl halides, and the carbon—hydrogen bond reductive elimination and important metal-centered transformations such as carbonylation, p-elimination, CO reduction, and... 

The reduction of alkyl halides with Sml2 has been used to trigger p-elimination and the formation of alkenes.7 For example, Concellon reported the reduction of 2-acetoxy-l,l-dihaloalkanes8 and 2-acetoxy-l-chlorosilanes9 to give vinyl halides and vinylsilanes, respectively (Scheme 4.3). In the reduction of diha-loalkanes bearing two different halides, only the more reactive halide (I > Br > Cl) is reduced.7... 

Tosjflkydrazones. The condensation of a C3-, C7-, C,-, or Cao-ketosteroid with p-toluenesulfonylhydrazine and reduction of the resulting tosylhydrazone with excess NaBH in dioxane provides a means for elimination of the carbonyl oxygen... 

The combination of Ni(COD)2 and l,3-di-ferf-butylimidazol-2-ylidene (ItBu) was demonstrated to effectively catalyse an intramolecular alkene hydroacylation to construct five- and six-membered benzocyclic ketones in good to excellent yields (Scheme 14.71). Al-aryl-substituted NHCs are not effective for this transformation. The reaction is proposed to proceed via a process as shown in Scheme 14.72. Thus, 2-allylbenzaldehyde coordinates to Ni(0)/ItBu to form complex A. The oxidative cyclisation of A gives a nickelacycle intermediate B. B undergoes p-hydride elimination and reductive elimination to produce final product C, along with regeneration of the catalyst. ... 

Some activated esters also undergo oxidative addition to Ni(COD)(PPh3>2 (COD = czs,cw-l,5-cyclooctadiene) followed by decarbonylation to yield allqrlnickel(II) complexes, which undergo P-hydride elimination and reductive elimination (Scheme 1). 

A proposed mechanism for the formation of cycloheptanone 228 is illustrated in (Scheme 102). The reaction is believed to proceed through precoordination of the metal to the double bond of the allene that is proximal to the cyclobutane. Insertion and C—C bond cleavage leads to metallacycle P-II. CO insertion gives either intermediate P-III or P-IV, and reductive elimination provides the [6-1-1] cycloadduct 228. 

Readditlon, P-hydrIde elimination and reductive elimination sequences... 

HI) p-adduct and reductive elimination would then result in transfer of the methyl group... 

Murakami and co-workers further developed this reaction to a [4+2+2] cycloaddition [46, 47]. Cyclobutanone (52) can be effectively coupled with 1,6-diyne (53b) to afford bicyclo[6,3,0]undecadienone 54b in excellent yield (91%). Two possible mechanisms were proposed for this transformation (Scheme 10). Either pathway leads to intermediate 57c, which upon p-C elimination and reductive elimination of the four-membered ring furnishes the final product. 

Olefination of unactivated sp bonds via C—H activation was demonstrated by Baudoin via oxidative addition to aryl bromides with Pd(0) followed by successive P-elimination and concomitant reductive elimination to afford functionalized ole-finic arenes. Intra-molecular olefination of unactivated sp C—H bonds through the corresponding cyclohexenylbromide provided entry into the hexahydroindole core of the aeruginosin family of natural products (Table 3.9). 

Although the central bond of bicyclo [2.1.0] pentane (housane) has the highest strain energy (47.4kcalmol ), bicyclo[2.1.0]pentane possesses remarkable thermal stability. A temperature greater than 300 C was required for the thermal isomerization of bicyclo[2.1.0]pentane to cyclopentene. However, isomerization of bicyclopentane occurred at ambient temperature in the presence of transition metal salts or complexes (Table 2.1) [33]. In contrast to the ewiio-5-methyl isomer that formed 1-methylcyclopentene by oxidative addition of the central C-C bond and subsequent P-hydrogen elimination and reductive elimination (Scheme 2.22), the exo-isomer remained unreacted because of the absence of... 

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