Catalysis turnover number

As discussed before, very high turnover numbers of the catalytic site and a large active electrode area are the most important features for effective catalysis. In the following sections three relatively successful approaches are illustrated in detail, all of which make use of one or both of these parameters. A further section will deal with non-redox modified electrodes for selectivity enhancement of follow-up reactions. 

Borabenzene complexes of cobalt such as Co(C5H5BPh)(COD) (51) and its 5-ethyl analog show the same type of catalysis but improved activity and chemoselectivity (77). Thus, 51 as the catalyst precursor gave the hitherto best results in the catalytic synthesis of the valuable 2-vinylpyridine from C2H2 and CH2=CHCN (120°C, 51 bar, 2 hours, turnover number 2164) (77,101). Furthermore, this catalyst for the first time allowed the synthesis of pyridine from C2H2 and HCN under mild conditions (110°C, 23 bar, 60 minutes, turnover number 103) (77). 

The ratio of the rate of an enzyme- (or ribozyme-) catalyzed reaction to the rate of the reaction in the absence of catalysis. This ratio equals kcJknon, where Acat is the turnover number and Anon is the noncatalyzed rate constant. See Catalytic Proficiency... 

Homogeneous catalysis is, in the first instance, a synthetic approach. Therefore, a high turnover number is highly desirable for pragmatic and economic reasons. Nevertheless, experiments should be conducted at rather low substrate to precursor ratio as well as rather high substrate to precursor ratio. This type of approach is particularly well suited to semi-batch investigations. A wide variation in substrate to precursor ratio is readily performed with a minimum of resources. It may even be useful to provide perturbations in product, temperature and pressure. 

Heterogenization of homogeneous metal complex catalysts represents one way to improve the total turnover number for expensive or toxic catalysts. Two case studies in catalyst immobilization are presented here. Immobilization of Pd(II) SCS and PCP pincer complexes for use in Heck coupling reactions does not lead to stable, recyclable catalysts, as all catalysis is shown to be associated with leached palladium species. In contrast, when immobilizing Co(II) salen complexes for kinetic resolutions of epoxides, immobilization can lead to enhanced catalytic properties, including improved reaction rates while still obtaining excellent enantioselectivity and catalyst recyclability. 

The quest for new chiral ligands is of fundamental importance for progress in asymmetric catalysis [1], The ultimate goal is the development of a chiral ligand that is readily available in both enantiomeric forms, which imparts high turnover numbers and enantioselectivity to the catalyst. Although such ligands are known... 

The main task in technical application of asymmetric catalysis is to maximize catalytic efficiency, which can be expressed as the ttn (total turnover number, moles of product produced per moles of catalyst consumed) or biocatalyst consumption (grams of product per gram biocatalyst consumed, referring either to wet cell weight (wcw) or alternatively to cell dry weight (cdw)) [2]. One method of reducing the amount of catalyst consumed is to decouple the residence times of reactants and catalysts by means of retention or recycling of the precious catalyst. This leads to an increased exploitation of the catalyst in the synthesis reaction. 

At one extreme diffusivity may be so low that chemical reaction takes place only at suface active sites. In that case p is equal to the fraction of active sites on the surface of the catalyst. Such a polymer-supported phase transfer catalyst would have extremely low activity. At the other extreme when diffusion is much faster than chemical reaction p = 1. In that case the observed reaction rate equals the intrinsic reaction rate. Between the extremes a combination of intraparticle diffusion rates and intrinsic rates controls the observed reaction rates as shown in Fig. 2, which profiles the reactant concentration as a function of distance from the center of a spherical catalyst particle located at the right axis, When both diffusion and intrinsic reactivity control overall reaction rates, there is a gradient of reactant concentration from CAS at the surface, to a lower concentration at the center of the particle. The reactant is consumed as it diffuses into the particle. With diffusional limitations the active sites nearest the surface have the highest turnover numbers. The overall process of simultaneous diffusion and chemical reaction in a spherical particle has been described mathematically for the cases of ion exchange catalysis,63 65) and catalysis by enzymes immobilized in gels 66-67). Many experimental parameters influence the balance between intraparticle diffusional and intrinsic reactivity control of reaction rates with polymer-supported phase transfer catalysts, as shown in Fig. 1. 

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