Selectivity Regioselectivity Stereoselectivity

Keywords Orbital mixing. Orbital amplitude. Orbital phase. Orbital polarization. Orbital deformation, Regioselectivity, Stereoselectivity, n Facial selectivity... 

The reactive species generated by the photoexcitation of organic molecules in the electron-donor-acceptor systems are well established in last three decades as shown in Scheme 1. The reactivity of an exciplex and radical ion species is discussed in the following sections. The structure-reactivity relationship for the exciplexes, which possess infinite lifetimes and often emit their own fluorescence, has been shown in some selected regioselective and stereoselective photocycloadditions. However, the exciplex emission is often absent or too weak to be identified although the exciplexes are postulated in many photocycloadditions [11,12], The different reactivities among the contact radical ion pairs (polar exciplexes), solvent-separated radical ion pairs, and free-radical ions as ionic species... 

The following sections describe the regioselective, site-selective, and stereoselective synthesis of oxetanes. 

In this chapter, recent developments in the regioselective, site-selective, and stereoselective preparation of oxetanes have been summarized. The relative nudeophilicity of the alkene carbons was seen to be important for regioselectivity, in addition to the well-known radical stability rule. Likewise, the three-dimensional structures of the triplet 1,4-biradicals were seen to play an important role in stereoselectivity. For photochemical reactions that proceed via radical ion pairs, the spin and charge distributions are crucial determinants of regioselectivity. It follows that the concepts used in selective oxetane synthesis should stimulate future investigations into the mechanistically and synthetically fascinating Paterno-Bitchi-type reactions. 

Chemoselective methods for oxidation and reduction, and protecting groups to help control chemoselectivity, will appear throughout this book, and we shall return in detail to peptides and their biological functions in Chapter 42. Before then we will address in detail stereoselectivity (in Chapters 32,33, and 41) but the very next chapter will deal with the other aspect of selectivity—regioselectivity. 

For example, MPc complexes are synthesized in the zeolite framework by subjecting the zeolite to metal ion exchange and then treating it with molten dicyanobenzene. These "ship-in-a-bottle complexes cannot leave the zeolite without destroying the framework. Such zeolite catalysts, whose super-cages serve as a sort of reaction flask with molecular dimensions, continue to possess shape selectivity, reactant selectivity, regioselectivity and stereoselectivity. 

Selectivity A term used to describe reactivity according to a specific region or group or structural motif in a given molecular structure (see chemoselectivity, regioselectivity, stereoselectivity). 

Specific Reversible Interactions in Life Processes Antenna Function of Membranes and Tbeir Regioselective, Conformation Selective and Stereoselective Interactions with Regulatory Peptides... 

Enzymes generally show four kinds of selectivity substrate selectivity, reaction selectivity, regioselectivity, and stereoselectivity. An enzyme selects one substrate out of a mixture in solution and performs a desired chemical reaction on it. It also directs that reaction to a particular region of the substrate even though there may be others of equal or greater intrinsic chemical reactivity, and achieves the transformation with good stereoselectivity. This can include the formation of a single enantiomer, as in the synthesis of natural amino acids. 

One potential application for PCPs is catalytic reaction because of their functional cavity surface, high regioselectivity, stereoselectivity, and shape or size selectivity. The activation of small gas molecules is a quite big challenge and an important target in the field of PCPs. However, there are few reports about this area and the topic is currently being explored [238,239]. Today, most PCPs with catalytic properties have been used only for organic reactions [54,93,96,134,192,240-260]. 

In addition to being regioselective alcohol dehydrations are stereoselective A stereo selective reaction is one m which a single starting material can yield two or more stereoisomeric products but gives one of them m greater amounts than any other Alcohol dehydrations tend to produce the more stable stereoisomer of an alkene Dehydration of 3 pentanol for example yields a mixture of trans 2 pentene and cis 2 pentene m which the more stable trans stereoisomer predominates... 

Examples of the use of lithium aluminum hydride in stereoselective [75] (equation 9), regioselective [16] (equation 10), and product-selective [77](equation 11) displacements of fluonne are available... 

For a successful application in synthesis, several problems have to be solved regioselectivity, whether the C-C bond is formed with the 1- or 3-position in an unsymmetrical ambident anion, EjZ selectivity in the formation of the double bond, and simple diastereoselectivity, since two new stereogenic centers are created from prostereogenic compounds. Further, different types of induced stereoselectivity or enantioselectivity may be required. Allylmetals with a wide choice of substituents are accessible by various methods (Sections D. 1.3.3.3.1.-10.). 

A further improvement utilizes the compatibility of hindered lithium dialkylamides with TMSC1 at —78 °C. Deprotonation of ketones and esters with lithium dialkylamides in the presence of TMSC1 leads to enhanced selectivity (3) for the kinetically generated enolate. Lithium t-octyl-t-butyl-amide (4) appears to be superior to LDA for the regioselective generation of enolates and in the stereoselective formation of (E) enolates. 

The discovery that Lewis acids can promote Diels-Alder reactions has become a powerful tool in synthetic organic chemistry. Yates and Eaton [4] first reported the remarkable acceleration of the reactions of anthracene with maleic anhydride, 1,4-benzoquinone and dimethyl fumarate catalyzed by aluminum chloride. The presence of the Lewis-acid catalyst allows the cycloadditions to be carried out under mild conditions, reactions with low reactive dienes and dienophiles are made possible, and the stereoselectivity, regioselectivity and site selectivity of the cycloaddition reaction can be modified [5]. Consequently, increasing attention has been given to these catalysts in order to develop new regio- and stereoselective synthetic routes based on the Diels-Alder reaction. 

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