C—H bond, cleavage step

Catalytic Reactions Involving a C-H Bond Cleavage Step... 

Dimerization of acrylonitrile is a cheaper way to synthesize highly valuable hexamethylenediamine [30]. In some cases of dimerization of acrylic acid esters, acrylonitriles, and acroleins, the direct C-H bond cleavage step seems to be involved in the catalytic reaction. At an early stage of catalytic dimerization of acrylonitrile, czs-l,4-dicyanobut-l-ene is formed as the major product, not trans-isomer [31]. This high czs-selectivity is suggested to indicate selective cleavage of C-H cis to CN by the metal coordinated to the nitrile group in side-on fashion. 

The pivalate anion is a key component in the C-H bond cleaving step, lowering the energy of the C-H bond cleavage (Step 3) and acting as a catalytic proton shuttle from benzene to the stoichiometric carbonate base. Competition experiments... 

Direct oxidative activation of sp3 C—H bonds adjacent to a heteroatom is an ideal synthetic route to heteroatom-containing derivatives. Recently, our group has developed an efficient method for the alkylation of 1,3-dicarbonyl compounds using Fe2(CO)9 as a catalyst (Equation 11.2) [8]. The scope of this transformation is fairly broad and various heteroatom-containing compounds have been shown as effective alkylation reagents. Kinetic isotopic effect (KIE) experiments have shown that the reaction has a kH/kD value of 5.4 0.1, which supports a rate-determining C—H bond cleavage step for the overall transformation. 

Under conditions in which an intermolecular isotope effect is measured, if the rate law is a mixture of expressions that are dependent and independent of isotope effects, then the observed rate constants for deuterated and undeuterated substrates will have a smaller ratio than the product distribution measured for the intramolecular effect. In the case of malate synthase, the measured intermolecular isotope effect is only 1.36 whereas the intramolecular effect is 3.8 (25, 25). If proton abstraction occurred in the first irreversible step, there would be no difference between the measured intramolecular and intermolecular and isotope effects. The fact that the two isotope effects are different indicates that the C-H bond cleavage step is preceded by an isotopically insensitive, but kinetically significant, step. Therefore, if the C-H bond cleavage step is concerted with carbon-carbon bond formation and that step is partially rate limiting (which it must be, since an isotope effect is observed), that concerted step becomes more rate limiting on deuteration and the magnitude observed for (WAT) increases. 

Because there is an isotopically insensitive, kinetically significant step preceding the C-H bond cleavage step, the observation of an invariant (V/A() on deuteration is obviously not the result of a concerted mechanism, since that would require the two events to be in one step, and it was shown that the two processes are on either side of the rate-determining process in the reaction se-... 

Although a radical mechanism has been proposed,10 more recently a rate-limiting C-H bond cleavage step has been invoked,11 and it is generally accepted that the formic acid acts as a hydride donor.9 This is consistent with the observation of production of carbon dioxide and reduction product in equivalent quantities.10... 

One of the more gratifying aspects of the mechanism in Figure 20 concerns the stage at which the second electron from Cuh(I) enters into the reaction mechanism. The rate constant for the C—H bond cleavage step... 

Information on the mechanism of the borylation of alkanes and arenes has been reported by Hartwig. Catalytic cycles proposed for the functionalization of alkanes and arenes by the Cp Rh and (dfbpy)Ir complexes are shown in Schemes 18.15 and 18.16. Both cycles involve the formation of 16-electron metal-boryl intermediates and the reactions of these intermediates with C-H bonds, followed by the formation of the B-C bond in the final product. Details on the C-H bond cleavage step were gained by studies on isolated metal-boryl complexes. 

It is shown that the oxidative addition of Phi does not take place with the 18 electron intermediate A but with the cation B and is rate-determining. It requires higher temperature than the C-H bond cleavage step. This is also consistent with a weak Ru(II)-OAc bond observed in the formation and stability of related mthenacycles [90]. 

RhCp Cl2]2 as the catalyst and CHgCN as the solvent, the cyclization showed broad scope to both internal alkynes and arylphosphonic acid monoesters. The observed isotopic effects = 2.6) indicated that the C-H bond cleavage step... 

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