Reaction Classifications

Various types of organic reactions can be conducted using biocatalysts. Typical enzymatic reactions used for organic synthesis are shown in Fig. 10.1. In particular, hydrolytic enzymes for kinetic resolutions of racemates have been utilized widely because of their high level of stability, wide substrate specificities, lack of cofactor requirements and high level of availability. 

The selection of reactor type in the traditionally continuous bulk chemicals industry has always been dominated by considering the number and type of phases present, the relative importance of transport processes (both heat and mass transfer) and reaction kinetics plus the reaction network relating to required and undesired reactions and any aspects of catalyst deactivation. The opportunity for economic 

Reaction class Batch or fed-batch reactor characteristics Representative reaction time Continuous reactor requirements/benefits 

A Near isothermal Slow 10 min Long residence time. Plug flow or CSTR depending on kinetics and reaction network. Improved QC, lower inventory 

B Some accumulation of energy 1 s to 10 min kinetic influence Some benefits (selectivity and process volume) from improved temperature control and better mixing/transport 

C 70% accumulation of energy Very fast 1 s mixing controlled High quality mixing and mass transfer. High heat transfer area gives temperature control. Intensified and/or structured reactors. 

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To recognize reaction classification as an important step in learning from reaction instances... 

To be able to follow some algorithmic approaches to reaction classification... 

A more detailed classification of chemical reactions will give specifications on the mechanism of a reaction electrophilic aromatic substitution, nucleophilic aliphatic substitution, etc. Details on this mechanism can be included to various degrees thus, nucleophilic aliphatic substitutions can further be classified into Sf l and reactions. However, as reaction conditions such as a change in solvent can shift a mechanism from one type to another, such details are of interest in the discussion of reaction mechanism but less so in reaction classification. 

Since 1970 a variety of reaction classification schemes have been developed to allow a more systematic processing of the huge variety of chemical reaction instances (see Chapter III, Section 1 in the Handbook). Reaction classification serves to combine several reaction instances into one reaction type. In this way, the vast number of observed chemical reactions is reduced to a manageable number of reaction types. Apphcation to specific starting materials of the bond and electron changes inherent in such a reaction type then generates a specific reaction instance. 

From among the many reaction classification schemes, only a few are mentioned here. The first model concentrates initially on the atoms of the reaction center and the next approach looks first at the bonds involved in the reaction center. These are followed by systems that have actually been implemented, and whose performance is demonstrated. 

Concentration on the types of bonds broken or made in a reaction provides a basis for reaction classification. We first show this only for one bond (Figure 3-14). On the first level of a hierarchy, a bond can be distinguished by whether it is a single, double, or triple. Then, on the next level, a further distinction can be made on the basis of the atoms that comprise the bond. 

This reaction classification allows users to browse through hits of reaction searches, thus enabling them to focus on the types of reaction in which they are interested. 

The method that was developed builds on computed values of physicochemical effects and uses neural networks for classification. Therefore, for a deeper understanding of this form of reaction classification, later chapters should be consulted on topics such as methods for the calculation of physicochemical effects (Section 7.1) and artificial neural networks (Section 9.4). 

The next question is how to represent the reacting bonds of the reaction center. We wanted to develop a method for reaction classification that can be used for knowledge extraction from reaction databases for the prediction of the products of a reaction. Thus, we could only use physicochemical values of the reactants, because these should tell us what products we obtain. 

A wider variety of reaction types involving reactions at bonds to oxygen atom bearing functional groups was investigated by the same kind of methodology [30]. Reaction classification is an essential step in knowledge extraction from reaction databases. This topic is discussed in Section 10.3.1 of this book. 

Reaction classification is an essential step in knowledge acquisition from reaction databases. 

There are two fundamental approaches to automatic reaction classification model-driven and data-driven methods. 

L. Chen, Reaction classification and knowledge acquisition, in Handbook of Chemoinfyrmatics - From Data to Knowledge, ]. Gasteiger (Ed.), Wiley-VCH, Weinheim, 2003. 

The main characteristics of the method, developed in our group for reaction classification arc 1) the representation of a reaction by physicochemical values calculated for the bonds being broken and made during the reaction, and 2 use of the unsupervised learning method of a self-organi2ing neural network for the perception of similarity of chemical reactions [3, 4],... 

Til most cases, only one of the two regioisomers is preferentially formed. Wc will show here how reaction classification by a self-organizing neural network can be used for the prediction of the preferred regioisomer in a pyrazole synthesis. 

Only then can the full arsenal of processing reaction information, such as reaction center searching, reaction similarity perception, or reaction classification (see Section 3.5) be invoked. Figure 10.3-19 shows such a full-fledged reaction represen tation. 

Table 4.1 lists all published electrochemical promotion studies of 58 catalytic reactions on the basis of the type of electrolyte used. Each of these reactions is discussed in Chapters 8 to 10 which follow the same reaction classification scheme. 

C.G. Vayenas, S. Brosda, and C. Pliangos, Rules and Mathematical Modeling of Electrochemical and Chemical Promotion 1. Reaction Classification and Promotional Rules,/. Catal., in press (2001). 

Ring J, Behrendt H Anaphylaxis and anaphylactoid reactions - classification and pathophysiology. CUn Rev Allergy Immunol 1999 17 387-399. 

Chen L. Reaction classification and knowledge acqnisition. In Gasteiger J, editor, Handbook of chemoinformatics. Weinheim Wiley-VCFI, 2003 348-88. 

REACTION CLASSIFICATION Cartx)n-cartx>n bond forming ... 

Table 4.5 Summary of atom economical efficiency trends for various reaction classifications.

TABLE 51-1. Reaction Classification, Clinical Symptoms, and Potential Causative Drugs5,6... 

Building on a recently introduced reaction classification system that considers electronic effects, a descriptor for steric hindrance has been added.The expanded classification hierarchy has been applied to a range of representative reactions, including additions to carbonyl compounds, and enolate formation. 

A modern valence-bond description of the Diels-Alder reaction has been presented. The method of reaction classification by similarity has been expanded to include the effect of steric congestion in the classification of cycloaddition reactions. ... 

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