Organic reactions mechanistic components

The photochemistry of carbonyl compounds has been extensively studied, both in solution and in the gas phase. It is not surprising that there are major differences between the photochemical reactions in the two phases. In the gas phase, the energy transferred by excitation cannot be lost rapidly by collision, whereas in the liquid phase the excess energy is rapidly transferred to the solvent or to other components of the solution. Solution photochemistry will be emphasized here, since both mechanistic study and preparative applications of organic reactions usually involve solution processes. 

While the fundamental mechanistic components of organic chemistry can be combined to describe complex mechanisms associated with complex reactions, the individual mechanistic components fall into a relatively small and well-defined group of four. These are SN1, SN2, El, and E2 reactions. In this chapter, the fundamentals associated with Sn2 reactions are presented. 

The most detailed modelling approach summarized in Figure 1 is found at the mechanistic level. These models are explicit accounts of the chemistry of elementary steps. Thus the hierarchy of the levels, i.e., reaction models in Figure 1, now becomes quite clear. Mechanistic models, which provide the temporal and many times spatial variation of the composition of each component and reaction intermediate, are based at the lowest modelling level. Their output, however, is typically phrased in terms of ensembles of stable molecular constituents which is more characteristic of the intermediate level molecular models. The molecular models, in turn, require subsequent organization in order to connect to the global reaction models and relevant product fractions at the top or global level. 

This review article deals with addition and cycloaddition reactions of organic compounds via photoinduced electron transfer. Various reactive species such as exdplex, triplex, radical ion pair and free radical ions are generated via photoinduced electron transfer reactions. These reactive species have their characteristic reactivities and discrimination among these species provides selective photoreactions. The solvent and salt effects and also the effects of electron transfer sensitizers on photoinduced electron transfer reactions can be applied to the selective generation of the reactive species. Examples and mechanistic features of photoaddition and photocycloaddition reactions that proceed via the following steps are given reactions of radical cations with nucleophiles reactions of radical anions with electrophiles reactions of radical cations and radical anions with neutral radicals radical-radical coupling reactions addition and cycloaddition reactions via triplexes three-component addition reactions. 

The most logical organization of the mechanistic and stereochemical features of the allylmetal addition reaction is by metal. These two most important components of the reaction are inexorably bound and critically dependent on the nature of the metal. In turn, the metal also carries with it the types of ligands and activators that are often also integral to a discussion of the mechanism and moreover influential in the stereochemical outcome of the process. Thus, each subsection, defined by metal or metalloid class, will contain a discussion of the current structure of understanding of the reaction mechanism followed by the stereochemical consequences at all levels of stereoselection outlined above. 

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