Natural catalyst

Over the years, thousands of compounds have been tried as cracking catalysts. These compounds fall into two general categories natural and synthetic. Natural catalyst, as the name denotes, is a naturally occurring clay that is given relatively mild treating and screening before use. The synthetic catalysts are of more importance because of their widespread use. Of the synthetic catalysts, two main types are amorphous and zeolitic. 

Figure 1. Fragment of the structure of natural catalysts, N4-metal complexes.

Thus for the first volume in this series we have performed a selection of oxidation and reduction reactions, arguably some of the most important transformations of these two types, mainly employing non-natural catalysts. In other volumes of this work other catalysts for oxidation and reduction will be featured and, of equal importance, the use of preferred catalysts for carbon-carbon bond formation will be described. In the first phase, therefore, this series will seek to explore the pros and cons of using many, if not most, well-documented catalysts and we will endeavour to report our findings in a nonpartisan manner. 

On moving away from carbohydrate chemistry one finds that non-natural catalysts are the materials of choice for the promotion of the classical aldol reaction and more recently-discovered variants. A wide range of methods are available and a small selection of these is described below. 

The library of natural catalysts has very little to offer for the catalysis of Diels-Alder (and the reverse) reactions (Diels-Alderases)[139]. For this reason one of the intriguing areas of biomimickry, namely the formation and use of antibodies exhibiting catalytic activity, has focused on [4 + 2] reactions to try to furnish proteins possessing useful catalytic properties. Thus in early studies a polyclonal catalytic antibody raised to hapten (57)[140] showed a modest rate enhancement for the reaction depicted in Scheme 48. 

In most other areas, especially in the field of carbon-carbon bond formation reactions, non-natural catalysts reign supreme. 

So, in the final analysis, biocatalysis should not be considered in a separate sector available only to the specialist bioorganic chemist. It is one method, in the portfolio of catalytic techniques, that is available to all chemists for the solution of present and future problems in organic synthesis. To erect a Chinese wall between the natural and non-natural catalysts is totally illogical and prevents some creative thinking, particularly in the area of coupled natural/ non-natural catalysts11611 and biomimetic systems11621. 

Cyclohexene and simple derivatives may be oxidized in the allylic position with a fair degree of stereocontrol using non-natural catalysts, see for example Schulz, M., Kluge, R. and Gelacha, F.G. Tetrahedron Asymmetry, 1998, 9, 4341. 

Perhaps less clear to a newcomer to a particular area of chemistry is when to use biocatalysis as a key step in a synthesis, and when it is better to use one of the alternative non-natural catalysts that may be available. Therefore we set out to extend the objective of Preparative Biotransformations, so as to cover the whole panoply of catalytic methods available to the synthetic chemist, incorporating biocatalytic procedures where appropriate. 

A REVIEW OF NATURAL AND NON-NATURAL CATALYSTS IN SYNTHETIC ORGANIC CHEMISTRY PRACTICAL TIPS FOR SOME IMPORTANT OXIDATION AND REDUCTION REACTIONS... 

In this volume we indicate some of the different natural and non-natural catalysts for hydrolysis, oxidation, reduction and carbon-carbon bond forming reactions leading to optically active products. Literature references are given to assist the reader to pertinent reviews. The list of references is not in the least comprehensive and is meant to be an indicator rather than an exhaustive compilation. It includes references up to mid-1999 together with a handful of more recent reports. 

The later sections of the book deal with the actual laboratory use of catalysts for asymmetric reduction and oxidation reactions. Most of the protocols describe non-natural catalysts principally because many of the corresponding biological procedures were featured in the sister volume Preparative Biotransformations. As in this earlier book, we have spelt out the procedures in great detail, giving where necessary, helpful tips and, where appropriate, clear warnings of toxicity, fire hazards, etc. 

The development of catalytic asymmetric reactions is one of the major areas of research in the field of organic chemistry. So far, a number of chiral catalysts have been reported, and some of them have exhibited a much higher catalytic efficiency than enzymes, which are natural catalysts.111 Most of the synthetic asymmetric catalysts, however, show limited activity in terms of either enantioselectivity or chemical yields. The major difference between synthetic asymmetric catalysts and enzymes is that the former activate only one side of the substrate in an intermolecular reaction, whereas the latter can not only activate both sides of the substrate but can also control the orientation of the substrate. If this kind of synergistic cooperation can be realized in synthetic asymmetric catalysis, the concept will open up a new field in asymmetric synthesis, and a wide range of applications may well ensure. In this review we would like to discuss two types of asymmetric two-center catalysis promoted by complexes showing Lewis acidity and Bronsted basicity and/or Lewis acidity and Lewis basicity.121... 

The catalysts must be stable to physical impact loading and thermal shocks and must withstand the action of carbon dioxide, air, nitrogen compounds, and steam. They should also be resistant to sulfur compounds the synthetic catalysts and certain selected clays appear to be better in this regard than average untreated natural catalysts. 

Enzymes are an important group of proteins. They serve as natural catalysts immobilizing various components that will be later joined or degraded. 

Enzymatic polymerizations are an emerging research area with not only enormous scientific and technological promise, but also a tremendous impact on environmental issues. Biocatalytic synthetic pathways are very attractive as they have many advantages, such as mild reaction conditions, high enantio-, regio- and chemoselectivity, and the use of nontoxic natural catalysts. 

In the same year Jacob Berzelius introduced the concept of catalysis, which he developed as a result of studies of the effects of acids and bases in promoting the hydrolysis of starch and of the effects of metals on the decomposition of hydrogen peroxide. Berzelius proposed the term catalyst from the Greek "katalysis," meaning "dissolution." Although he had been concerned primarily with inorganic catalysts, Berzelius recognized that a natural catalyst,... 

The particularly remarkable features displayed in this respect by the natural catalysts, the enzymes, has provided major stimulus and inspiration for the development of novel catalysts by either manipulating the natural versions or by trying to devise entirely artificial catalysts that would nevertheless display similar high efficiencies and selectivities. In his Nobel award lecture in 1902, Emil Fischer has shown remarkable prescience when he said / can foresee a time in which physiological chemistry will not only make greater use of natural enzymes but will actually resort to creating synthetic ones [5.1]. 

Enzymes are very efficient natural catalysts present in plants and animals. They do not require high temperatures to break down the starch to maltose. In humans, a salivary amylase breaks down the starch in our food. If you chew on a piece of bread for several minutes, you will notice a sweet taste in your mouth. The above hydrolysis reactions are summarised in Figure 15.21. 

Enzymatic ester hydrolysis is a common and widespread biochemical reaction. Since simple procedures are available to follow the kinetics of hydrolytic reactions, great efforts have been made during the last years to explain this form of catalysis in chemical terms, i.e., in analogy to known non-enzymatic reactions, and to define the components of the active sites. The ultimate aim of this research is the synthesis of an artificial enzyme with the same substrate specificity and comparable speeds of reaction as the natural catalyst. 

Though the investigation of photocatalytic oxygenations performed of the laboratory scale are often motivated by attempts to understand and mimic the catalytic cycle of cytochrome P450 (a natural catalyst of monooxygenation reactions), the results obtained [159, 253, 266] could be applied to industrial processes as well. 

Clay minerals and their substituent oxides will be emphasized here. Gays are among the most important natural catalysts (63-64, 84-86). Zeolites, clays and a few substituent binary oxides of clays, namely Si02, Al CL, MgO, CaO, TiO, and Fe-oxides and hydroxides are among the best characterized mineral catalysts, in spite of... 

Some caution should be exercised however when working with synthetic enzymes. A similarly designed and expressed de novo enzyme to mimic the naturally occurring triose phosphate isomerase was reported in 2004 only for the papers to be retracted when it appeared that contamination by unmodified Escherichia coli had been responsible for the observed enzymic activity. For this reason it seems preferential to target reactions that have no known natural catalysts. 

There is significant interest in enzymes as they have proven to be powerful and environment-friendly natural catalysts for the polymerization of water-soluble monomers that can function under milder reaction conditions than those used in traditional free radical polymerization techniques. Hence, the combination of SCCO2 and water as reaction medium is a significant advancement made by Villarroya et al. 

Another example involves complex oxidation with the participation of natural catalysts such as goethite (ct-FeOOH), a redox-reactive solid ... 

Enzymes are ideal natural catalysts compared to chemical catalysts because they are more environmentally benign than heavy metal catalysts, and they can utilize benign substrates, work under mild reaction conditions, and provide high structural selectivity in products while cutting down byproducts. Other advantages of enzymes are particularly apparent in the synthesis and modification of complex polymers, including those that are chiral, electrochemically active, biodegradable, or... 

The rates of cyclic photophosphorylation around PS I catalysed by the natural catalysts are rather low, about one order of magnitude lower than those of linear electron transport [59], while they are very high when artificial electron carriers, such as phenazine methosulfate, are added to the system. Cyclic photophosphorylation has been shown to occur in intact leaves [65] and algae [66]. 

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