Selection regioselectivity

Swem dimer, hydrogen bonding, 104, 105 Synchronous transition states, peracid alkene epoxidation, 50-8 syn selectivity, regioselective aUyUc... 

The hydrogenation of dienes to monoenes introduces several problems of selectivity. Regioselective saturation of one of the double bonds is governed basically by the same effects that determine the relative reactivities of monoalkenes in a binary mixture that is, a terminal double bond is reduced preferentially to other, more substituted double bonds. During the reduction of a diene, a new competition also emerges since the newly formed monoene and the unreacted diene compete for... 

One of the most profound influences a solvent system can have on a reaction is a change of selectivity. Enzymes in organic solvents have been discovered in many cases to feature altered selectivity, including substrate specificity, enantioselectivity, prochiral selectivity, regioselectivity, and chemoselectivity (Wescott, 1994 Hirose, 1995). 

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 use of a class of pentafluorophenyl Pt(ll) complexes as catalysts allows the efficient epoxidation of simple terminal alkenes with environmentally benign hydrogen peroxide as the oxidant. Key features of this system are very high substrate selectivity, regioselectivity, and enantioselectivity, at least for this class of substrates. These properties are related to the soft Lewis acid character of the metal center that makes it relatively insensitive to water but, at the same time, capable of increasing the electrophilicity of the substrate by coordination. The reversal of the traditional electrophile/nucleophile roles in epoxidation helps explain the unprecedented reactivity observed. 

Biocatalysis usually exploits advantageous features of enzymes such as chemo-selectivity, regioselectivity, enantioselecivity and substrate spectrum of a certain broadness as depicted in Fig. 16.2-1. These points are addressed in examples in the following chapters. 

In Chapter 10 the routes to carbonyl compounds were based on oxidation of alcohols. This chapter groups together a number of direct approaches to carbonyl compounds by C-C disconnections, and examines the thinking behind them. A new type of selectivity, regioselectivity, will be introduced here and treated more fully in the next chapter. 

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. 

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. 

When using aliphatic alkynes the lactam products are also preferentially obtained in good yields. Through modification of the phosphoramidite ligand, high levels of enantio-selectivity, regioselectivity and product selectivities are obtained The uncatalyzed reaction of heterocyclic formamidines 201 with two equivalents of phenyl isocyanate affords the [2+2+2] cycloadduct 202 in 48 % yield... 

Aryl and heteroaryl amidines underwent oxidative C-H bond functionalization with alkynes with the aid of ruthenium catalysis [48]. In the presence of [RuCl2(p-cymene)]2/KPF5 as catalyst, the cyclization reactions led to the formation of a variety of 1-iminoisoindolines with high site selectivity, regioselectivity, and... 

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