Activation of organic reactions

Westaway, K. and Gedye, R.N., The question of specific activation of organic reactions by microwaves, J. Microw. Power Electromagn. Energy., 1995, 30, 219. 

This section is not concerned with transition metal catalysis. Catalytic activation of organic reactions by Lewis acids is a wide field of investigation [6]. It is usually preferred to other activation modes merely for commodity reasons. The idea of simultaneous use of pressure and traditional Lewis acid catalysts has been recognized for some time [7]. Curiously, the method was only developed in the last decade [8]. It was observed that coupling of both activation methods was beneficial in all [4 + 2] cycloadditions examined. However, most classical hard Lewis acids (AICI3, TiCU, SnCU, ZnCl2. ..) present a number of inherent problems such as... 

Microwave activation of organic reactions on clay-adsorbed compounds Rearrangement of pinacol to pinacolone intercalated in montmoriUonite Gutierrez et al. (137)... 

Corrosion associated with the action of micro-organisms present in the corrosion system. The biological action of organisms which is responsible for the enliancement of corrosion can be, for instance, to produce aggressive metabolites to render the environment corrosive, or they may be able to participate directly in the electrochemical reactions. In many cases microbial corrosion is closely associated with biofouling, which is caused by the activity of organisms that produce deposits on the metal surface. 

Complexes of DMAC and many inorganic haHdes have been reported (20). These complexes are of iaterest because they catalyze a number of organic reactions. Complexes of DMAC and such heavy metal salts as NiBr2 exert a greater catalytic activity than the simple salts (21). The crystalline complex of SO and dimethylacetamide has been suggested for moderating the reaction conditions ia sulfation of leuco vat dyestuffs (22). 

Aldol Addition and Related Reactions. Procedures that involve the formation and subsequent reaction of anions derived from active methylene compounds constitute a very important and synthetically useful class of organic reactions. Perhaps the most common are those reactions in which the anion, usually called an enolate, is formed by removal of a proton from the carbon atom alpha to the carbonyl group. Addition of this enolate to another carbonyl of an aldehyde or ketone, followed by protonation, constitutes aldol addition, for example... 

The second use of activation parameters is as criteria for mechanistic interpretation. In this application the activation parameters of a single reaction are, by themselves, of little use such quantities acquire meaning primarily by comparison with other values. Thus, the trend of activation parameters in a reaction series may be suggestive. For example, many linear correlations have been reported between AT/ and A5 within a reaction series such behavior is called an isokinetic relationship, and its significance is discussed in Chapter 7. In Section 5.3 we commented on the use of AS to determine the molecularity of a reaction. Carpenter has described examples of mechanistic deductions from activation parameters of organic reactions. 

Ideally, those molecules that are involved in the catalytic reaction should be the best characterizers of catalytic sites. Indeed, the path of the development of organic reaction mechanisms is paved with clever examples of stereochemistry and isotopic substitution that reveal the nature of activated complexes and intermediates and allow the unambiguous interpretation of the stereorelations... 

From what has been said, our first priority is, therefore, to understand equilibrium, which, as will be seen in Chapter 3, can be discussed in terms of the balance between order and disorder, and then to enquire about (organised) change when we consider kinetics and barriers to reaction. These problems are all related to the energy content of materials and the impact of external energy upon them. In the synthesis and activities of organisms an over-riding concern is with the overall energy uptake. Is it restricted to a limit ... 

The aim of this chapter is to draw the attention of experimental organic and catalytic chemists to a new field of catalysis, especially to catalytic methods which use micro-wave irradiation as a new means of activation of chemical reactions, called micro-wave catalysis . It is intended to advise synthetic organic chemists about the choice of catalytic steps which might be more efficient than conventional synthetic methods. 

Source Compiled from S.R. Montgomery, Functional group activity of promoted Raney nickel catalysts, in Catalysis of Organic Reactions, W.R. Moser, Ed., Marcel Dekker, New York, 1981, pp. 383-409. 

The activation of various reactions by Lewis acids is now an everyday practice in synthetic organic chemistry. In contrast, solvent effects on Lewis acid catalysed Diels-Alder reactions have received much less attention. A change in the solvent can affect the association step leading to the transition structure. Ab initio calculations on the Diels-Alder reaction of cyclopentadiene and methyl vinyl ketone in aqueous media showed that there is a complex of the reactants which also involves one water molecule119. In an extreme case solvents can even impede catalysis120. The use of inert solvents such as dichloromethane and chloroform for synthetic applications of Lewis acid catalysed Diels-Alder reactions is thus well justified. General solvent effects, in particular those of water, will be discussed in the following section. 

Hydrogen-bond donors have the ability to enhance the selectivities and rates of organic reactions. Examples of catalytic active hydrogen-bond donor additives are urea derivatives, thiourea derivatives (Scheme 10, Tables 12 and 13) as well as diols (Table 14). The urea derivative 7 (Scheme 9) increases the stereoselectivity in radical allylation reactions of several sulphoxides (Scheme 10)171. The modest increase in selectivity was comparable to the effects exerted by protic solvents (such as CF3CH2OH) or traditional Lewis acids like ZnBr2172. It was mentioned that the major component of the catalytic effect may be the steric shielding of one face of the intermediate radical by the complex-bound urea derivative. 

R. A. Yeates, H. Laufen, M. Leitold, The Reaction between Organic Nitrates and Sulf-hydryl Compounds. A Possible Model System for the Activation of Organic Nitrates , Mol. Pharmacol. 1985, 28, 555-559. 

This chapter describes the application of iridium-catalyzed reactions in the synthesis of molecular and macromolecular organosilicon compounds and related sihcon derivatives. Some mechanistic implications are introduced which illustrate the specific catalytic activation of organic and silicon compounds by iridium... 

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