Summary Reactive Intermediates

Hammond postulate Related species (on a reaction-energy diagram) that are closer in energy are also closer in structure. In an exothermic reaction, the transition state is closer to the reactants in energy and in structure. In an endothermic reaction, the transition state is closer to the products in energy and in structure (pi 149) 

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Summary Reactive Intermediates 168 EssentialTerms 168 Study Problems 170... 

In summary, we have shown that stable cationic charge centers can significantly enhance the reactivities of adjacent electrophilic centers. Most of the studied systems involve reactive dicationic electrophiles. A number of the reactive dications have been directly observed by low temperature NMR. Along with their clear structural similarities to superelectrophiles, these dicationic systems are likewise capable of reacting with very weak nucleophiles. Utilization of these reactive intermediates has led to the development of several new synthetic methodologies, while studies of their reactivities have revealed interesting structure-activity relationships. Based on the results from our work and that of others, it seems likely that similar modes of activation will be discovered in biochemical systems (perhaps in biocatalytic roles) in the years to come. 

Besides their obvious role as reactive intermediates in a powerful synthetic approach the 4a,4b-dihydrophenanthrenes offer a fascinating combination of unusual chemical and physical properties. Over the past 15 years these topics were investigated at length at the Weizmann Institute in Rehovot and elsewhere, and the present review is intended to provide an up-to-date summary of the activity in this field. 

We intend Reactive Intermediate Chemistry to serve as a free-standing resource to be used by the entire chemical community. It should be especially useful for first or second year graduate students, for whom it could form the basis of a course in reactive intermediate chemistry. To that end, each chapter features a concluding section devoted to a summary of the current situation, as well as a roll call of near-term problems and probable research directions. A list of key reviews and suggestions for additional reading accompanies each chapter. 

This chapter begins with an introduction to the basic principles that are required to apply radical reactions in synthesis, with references to more detailed treatments. After a discussion of the effect of substituents on the rates of radical addition reactions, a new method to notate radical reactions in retrosynthetic analysis will be introduced. A summary of synthetically useful radical addition reactions will then follow. Emphasis will be placed on how the selection of an available method, either chain or non-chain, may affect the outcome of an addition reaction. The addition reactions of carbon radicals to multiple bonds and aromatic rings will be the major focus of the presentation, with a shorter section on the addition reactions of heteroatom-centered radicals. Intramolecular addition reactions, that is radical cyclizations, will be covered in the following chapter with a similar organizational pattern. This second chapter will also cover the use of sequential radical reactions. Reactions of diradicals (and related reactive intermediates) will not be discussed in either chapter. Photochemical [2 + 2] cycloadditions are covered in Volume 5, Chapter 3.1 and diyl cycloadditions are covered in Volume 5, Chapter 3.1. Related functional group transformations of radicals (that do not involve ir-bond additions) are treated in Volume 8, Chapter 4.2. 

We have seen in this chapter how carbenes can be formed from many other reactive intermediates such as carbanions and diazoalkanes and how they can react to give yet more reactive intermediates such as ylids. Here is a summary of the main relationships between carbenes and these other compounds. Note that not all the reactions are reversible. Diazoalkanes lose nitrogen to give carbenes but die addition of nitrogen to carbenes is not a serious reaction. 

These reactive intermediates were proposed first for the Lossen rearrangements in 1891 by Tiemann b and for the photochemical decomposition of hydrazoic acid in 1928 by Beckman and Dickinson 2>. The chemistry in this field, which attracted little interest for a long time, was stimulated by the development of carbene chemistry. In the last fifteen years, beginning with a summary by Kirmse 3> in 1959, excellent reviews on nitrene chemistry appeared 4-12),... 

Summary The formation of reactive intermediates via dehalogenation of chlorosilanes was investigated by using lithium powder and sonication. Whereas in the absence of a diene substrate mainly polysilanes are obtained, reactions with, e.g., dimethylbutadiene, yield the corresponding cycloaddition products, indicating silylenes and silaethenes as intermediates. 

Summary A number of isomers of composition CsHiSi and C2H4Si2 have been generated by pulsed flash pyrolysis of appropriate precursors and isolated in an argon matrix. Their photochemical interconversions were studied. Quantum chemical calculations have been performed at the BLYP/6-31G level of theory. They play a decisive role in the identification of the highly reactive intermediates. Although silacyclobutadienes were shown to be minima on their respective energy hypersurfaces, no experimental evidence for the existence of such compounds was found. Instead, silylenes were detected, which undergo a variety of mutual interconversions. 

Summary Reactions of dichlorodivinylsilane and LitBu in a molar ratio 1 1 and 1 2 lead to highly reactive intermediates, which can be trapped by suitable trapping agents. From that mono and double addition products are formed, which provide experimental hints for the intermediate formation of the neopentylsilene H2C=CH(Cl)Si=CHCH2/Bu 3 and the 2-silaallene tBuCH2CH=Si=CHCH2tBu 4. In particular, the formation of double cycloadducts from 2 with Lifflu is a preparatively facile route for the synthesis of silaspirocycles such as 12, 14, and 15, which could be characterized by single crystal X-ray structure analysis. 

In summary, the Coll(bpy)2 /HOOH/(4 1 MeCN/py) system forms a reactive intermediate (20) that selectively ketonizes methylenic carbon, and as such is closely similar to the intermediate of the Fe KPA)2/HOOH/(2 1 py/HOAc) system.36 The ability of FeP(DPAH)2 to activate O2 to an intermediate that has the same unique selectivity for hydrocarbon ketonization is further support for a common stabilized-dioxygen reactive complex (see Chapter 6), Several cobalt-dioxygen complexes exhibit oxygenase reactivity with organic substrates, 0,4l which is consistent with the dioxygen formulation for species 20. 

A new kind of reactive intermediate in organic chemistry. Also, a summary of nucleophilic displacement reactions. 

This chapter has discussed how carbenes can be formed and how they can react to give stable compounds or yet more reactive intermediates. Below is a summary of the main relationships between carbenes and other intermediates ... 

Summary Reacting 2-neopentyl substituted silacyclobutanes la,b with MeLi/HMPA (hexamethylphosphoric triamide) anionic polymerization to give polymers 3a,b plays only a minor role for product formation. Instead, the head-to-head dimers 2a,b are isolated as main products. Their formation is explained by a complex reaction mechanism, in which various carbanionic, highly reactive intermediates are discussed. Obviously, the bis-a-silyl substituted carbanions 10a,b are remarkably stable, as can be concluded from Si NMR spectroscopic investigations at low temperature and from the products formed by trapping reactions with alcohols. 

Many aminonaphthalenesulfonic acids are important in the manufacture of azo dyes (qv) or are used to make intermediates for azo acid dyes, direct, and fiber-reactive dyes (see Dyes, reactive). Usually, the aminonaphthalenesulfonic acids are made by either the sulfonation of naphthalenamines, the nitration—reduction of naphthalenesulfonic acids, the Bucherer-type amination of naphtholsulfonic acids, or the desulfonation of an aminonaphthalenedi-or ttisulfonic acid. Most of these processes produce by-products or mixtures which often are separated in subsequent purification steps. A summary of commercially important aminonaphthalenesulfonic acids is given in Table 4. 

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