Synthetic processes mechanisms

The mechanistic aspects of nucleophilic substitution reactions were treated in detail in Chapter 4 of Part A. That mechanistic understanding has contributed to the development of nucleophilic substitution reactions as important synthetic processes. Owing to its stereospecificity and avoidance of carbocation intermediates, the Sw2 mechanism is advantageous from a synthetic point of view. In this section we discuss... 

The methods available for synthesis have advanced dramatically in the past half-century. Improvements have been made in selectivity of conditions, versatility of transformations, stereochemical control, and the efficiency of synthetic processes. The range of available reagents has expanded. Many reactions involve compounds of boron, silicon, sulfur, selenium, phosphorus, and tin. Catalysis, particularly by transition metal complexes, has also become a key part of organic synthesis. The mechanisms of catalytic reactions are characterized by catalytic cycles and require an understanding not only of the ultimate bond-forming and bond-breaking steps, but also of the mechanism for regeneration of the active catalytic species and the effect of products, by-products, and other reaction components in the catalytic cycle. 

In view of the commercial importance of the reaction, considerable eifort has been expended on elucidating its mechanism and extending the reaction to a variety of other synthetic processes based on olefins. 

The photoreductive synthetic process that promotes the assimilation of carbon dioxide into carbohydrates, other reduced metabolites, as well as ATP (synthesis of the latter is termed photophosphorylation). Photosynthesis is the primary mechanism for transducing solar energy into biomass, and green plants utilize chlorophyll a to capture a broad spectrum of solar radiant energy reaching the Earth s surface. Photosynthetic bacteria typically produce NADPH, the reductive energy of which is converted to ATP. 

The presence of a specific organolithium compound in a synthetic process is sometimes assumed without isolation or further evidence other than having attained the expected product by a rationalized mechanism. For example, the synthesis of the alcohol depicted... 

The reaction-transform approach is based on a computer-readable representation of the reaction mechanism that describes the transformation of the atoms in the reactants to the product. The transform is applied to the input reactants themselves to generate the products. The reaction-transform approach thus more closely mimics the actual synthetic process however, it can be difficult to construct efficient transforms. This is the approach used in the ADEPT software (14). 

Unlike proteins, polysaccharides generally do not have definite molecular weights. This difference is a consequence of the mechanisms of assembly of the two types of polymers. As we shall see in Chapter 27, proteins are synthesized on a template (messenger RNA) of defined sequence and length, by enzymes that follow the template exactly. For polysaccharide synthesis there is no template rather, the program for polysaccharide synthesis is intrinsic to the enzymes that catalyze the polymerization of the monomeric units, and there is no specific stopping point in the synthetic process. 

The most extensively studied reactions of coordinated amino acid derivatives are those involving nucleophilic attack at the carbonyl group. These aspects, as well as some of those already covered in the previous section, have been reviewed.335-337 Mechanistic aspects of these reactions have also been discussed in Chapter 7.4. The emphasis in this section will be on the synthetic value of stoichiometric reactions of this type. The two most important synthetic processes are peptide hydrolysis and peptide synthesis, both involving the same mechanism. 

The stepwise electron reduction of C02, whether direct or indirect, catalyzed, or by direct transfer on an apparently inert conductive surface, has been the object of considerable attention since the first concise reports of formate anion production. Since then, the list of possible derivatives has grown from formates to carbon monoxide, methane, ethylene, and short-chain saturated hydrocarbons. As noted in Section 12.1, this area of research has been expanded in recent years [8, 80, 83], with information relating to increased yields, to the effect of electrode materials on selectivity, as well as further speculations on possible reaction mechanisms, having been obtained on a continuous basis. Yet, the key to these synthetic processes-an understanding of the relationship between the surface of the electrode and the synthetic behavior of the system-seems no closer to being identified. 

Biological N2 fixation (1), i.e., the reduction of N2 to NH3 catalyzed by FeMo, FeV, or FeFe nitrogenases, is one of the fundamental synthetic processes of nature (2-4). In spite of intense efforts over the last decades, its molecular mechanism is poorly understood, in particular because the pivotal chemical question has remained unanswered how do nitrogenases achieve to activate and convert the inert N2 molecule to ammonia under ambient conditions and mild redox potentials. 

A number of important synthetic processes involving mediated reactions of halides rather than direct electrochemical reduction have been reported. The reduction of aryl halides in the presence of alcohols with or without NH3 as a cosolvent leads to the oxidation of the alcohols to the carbonyl compounds. The reaction involves an electrocatalytic process mediated by electron transfer from the initially reduced aryl halide. Alcohols such as benzhydrol and 2-propanol are converted to their respective ketones, on a preparative scale163. The proposed mechanism is shown in Schemes 14 and 15. 

The coenzyme thiamine pyrophosphate (1) plays a central role in many parts of metabolism (Fig. 1). Its mechanism of action involves the formation of a thiazolium zwitterion 2 that was stabilized by a carbene resonance form 3 (10). This discovery opened up studies of the chemistry that such stabilized carbenes could catalyze, as chemists realized that the otherwise impossible chemistry that thiamine pyrophosphate catalyzes in nature could be generalized and adapted for useful synthetic processes. 

This chapter will attempt to survey critically the major areas of experimentally determined kinetic data which are available on polycondensation reactions and their mechanisms, and to emphasize the mechanistic similarities of many of the reactions. The statistics of polycondensation reactions will be touched on only to the extent that it helps understand the reactions and their kinetics. The general approach to the subject of kinetics is designed to be of primary use and interest to those polymer chemists who are concerned with the synthesis of products having desired properties, and with the understanding of their synthetic processes. 

The mechanism of the reactions involved in the synthetic process for preparing this compound has been explained by Heymann (Berl. Ber. 1891, p. 3066). The yellow solution resulting from the action of the sulphuric anhydride on phenyl-glycine contains the sulphonic acid of indoxyl-sulphuric ester. On dilution of this solution, oxidation is effected by the sulphuric anhydride. 

The dehydration of an alcohol usually competes with its dehydrogenation the proportion of each type of product obtained depends on the particular catalyst and the reaction conditions used. The selectivity of the catalyst is dictated by its acid-base and redox properties [7,8], which are acquired during the synthetic process. In fact, the transformation of alcohols is a test reaction for determining catalyst acidity and basicity. The dehydration is believed to be catalysed by acid sites and the dehydrogenation by both acid and basic sites, via a concerted mechanism [9] which, however, has been questioned [10]. 

Over the past several decades, substantial progress has been made elucidating the details and mechanisms of metal-mediated C—H activation. While a greater understanding of these reactions has been achieved, with each development, a number of new questions arise. The ability to rationally develop catalysts for C—H functionalization and control synthetic processes of hydrocarbons and C—H bonds of functionalized materials depends on an increasingly sophisticated understanding of these fundamental details. 

Results from the literature suggest that direct phenol acylation and the Fries rearrangement are frequently competitive processes and difficult to characterize by the mechanistic point of view. Consequently, in Section 5.1, we include the synthetic process where the phenol substrate, the acylating agent, and the catalyst are mixed together in the starting reaction mixture aside from the specific reaction mechanism, whereas in Section 5.2 we include reactions involving phenyl esters. 

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