Potential step catalytic system

The potential application of this catalytic system was illustrated by Takemoto in the application to a tandem conjugate addition towards the asymmetric synthesis of (-)-epibatidine, a biologically active natural product [100, 101], The authors designed an enantioselective double Michael addition of an unsaturated functionalized P-ketoester to a p-aryl nitro-olefm. The asymmetric synthesis of the 4-nitro-cyclohexanones was achieved in both high diastereoselectivity and enantioselectivity, with the natural product precursor synthesized in 90% yield and 87.5 12.5 er (Scheme 49). The target (-)-epibatidine was subsequently achieved in six steps. 

These first two steps, electron transfer and CO2 binding, are also common to Fe, Co, and Ni macrocychc complexes. For these catalysts, one-electron reductions corresponding to Fe(I/0) [37-40], Fe(II/I) [41], Co(II/I) [42-44], and Ni(II/I) [45] couples are followed by reaction with CO2. However, for these catalytic systems, which operate at much more negative potentials than the Pd complexes, the ratedetermining step is not the reaction of CO2 with the reduced metal species. This reaction is very rapid. As shown in Table 1, the rate of reaction of Cod)(rac-Me6[14]4,ll-diene) with CO2 is 1.7 x 10 [14]. 

Another system of interest in connection with potential steps (and, see below, LSV) is the catalytic or EC system, described in simplified form by... 

Another case of interest with LSV is the catalytic system, described above in the section on potential steps. The equations are the same except that the potential here is not constant and very negative, following (2.29) in its dimensionless form. For small and intermediate rates of the homogeneous chemical reaction (dimensionless constant K), the same procedure as mentioned above, that is, convergence simulations, must be used. For large K, however, the LSV curves become sigmoid, with a plateau equal to the current for the potential step, G = y/K. This can be used to test methods. Figure 2.7 shows LSV curves for some A -values, where this effect is seen. 

A brief survey demonstrates that kinetic models taking into account both heterogeneous and homogeneous elementary steps potentially can serve as powerful tools for analysis of experimental data, development of catalytic systems, and selection of optimal reaction conditions. Despite a lack of information... 

There is no doubt that because of the greater atom efficiency and reduction in transformation steps, catalysis has the greater potential to provide the more economic and elegant solution. It is also tme that many catalytic systems currently remain limited due to narrow substrate specificity, while auxiliary approaches, with their somewhat greater maturity and predictability, may be applied more widely. 

Another case of interest with LSV is the catalytic EC system, described above in the section on potential steps. The equations are the same except that the potential here is not constant nor very negative, following (2.29) in its dimensionless form. For small and intermediate rates of the homogeneous chemical reaction... 

Molecular oxygen and a metalloporphyrin represent a potential catalytic system. Thus, it has been shown ° that [Rh (tetraphenylporphyrinato)Cl] is a very efficient catalyst for the reduction of a ketone to alcohol in the presence of oxygen and NaBH4 in tetrahydrofuran. The rate-determining step is reaction (8.6.1), in which the metalloporphyrin catalyst is formed, serving as a generator of borane (BH3), which can be then transferred to ketone (P = por-phyrinato) ... 

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