Reaction classification nucleophilic additions

Base catalysis of ligand substitutional processes of metal carbonyl complexes in the presence of oxygen donor bases may be apportioned into two distinct classifications. The first category of reactions involves nucleophilic addition of oxygen bases at the carbon center in metal carbonyls with subsequent oxidation of CO to C02, eqns. 1 and 2 (l, 2). Secondly, there are... 

Attack of nucleophiles at the a-position of the enaminone predominates, leading to Michael addition which mostly results in substituted, mainly cyclic end-products. Also observed are subsequent amine elimination and reactions at the carbonyl. Some initial reactions of nucleophilic reagents at the enaminone carbonyl carbon are known. Enaminones are often better starting materials for several reactions than the corresponding dicarbonyls. As a result, a-aminomethylene ketones act as 1,3-biselectrophiles. Due to their combined electrophilic and nucleophilic properties, enaminones act as 1,3-bisnucleophiles as well. The assumed first step in the following reactions is the one used for classification of the reactions. In addition, enaminones are used as heterodienes in 4 + 2-cycloaddition mostly with electron-deficient dienophiles. 

Polyynes have served as starting materials for the synthesis of a wide variety of heterocyclic ring systems. The reactions used involve addition to triple bonds, and any of the common mechanistic pathways may be followed, i.e. nucleophilic, electrophilic or free radical attack as well as concerted cycloadditions. Although the evidence does not permit unequivocal classification of many of the reactions into one of these categories, the ones considered here are those which most likely involve nucleophilic attack at some stage. In a formal sense the reactions amount to successive additions of a divalent nucleophile to two triple bonds the first involves intermolecular and the second intramolecular attack, as illustrated in equation (19) for the addition of HoS to a diyne. 

The classification within this section is based on the structural (rather than the mechanistic) relationship between the starting materials and products. Mechanistically, all of the reactions considered in this section involve nucleophilic substitution as the first step, except for aromatic substitution via the aryne mechanism, which involves elimination followed by nucleophilic addition. 

The mechanism classification and the overall transformation classification are orthogonal to each other. For example, substitution reactions can occur by a polar acidic, polar basic, free-radical, pericyclic, or metal-catalyzed mechanism, and a reaction under polar basic conditions can produce an addition, a substitution, an elimination, or a rearrangement. Both classification schemes are important for determining the mechanism of a reaction, because knowing the class of mechanism and the overall transformation rales out certain mechanisms and suggests others. For example, under basic conditions, aromatic substitution reactions take place by one of three mechanisms nucleophilic addition-elimination, elimination-addition, or SrnL If you know the class of the overall transformation and the class of mechanism, your choices are narrowed considerably. 

As we have seen (Section 4, p. 191) the range of effective molarities associated with ring-closure reactions is very much greater than that characteristic of intramolecular general acid-base catalysis the main classification is therefore in terms of mechanism. By far the largest section (I, Tables A-D) gives EM s for intramolecular nucleophilic reactions. These can be concerted displacements (mostly at tetrahedral carbon), stepwise displacements (mostly addition-elimination reactions at trigonal carbon), or additions, and they have been classified in terms of the nucleophilic and electrophilic centres. 

We will begin with a brief discussion of the physical and spectroscopic properties of alkenes and alkynes. But the major emphasis in the chapter is on two main types of reactions, ionic addition and radical-chain addition. For ionic additions we will make extensive use of the classification of reagents as electrophiles and nucleophiles, as described in Chapter 8. 

Electrode processes are conveniently classified according to the nature of the final product1 and its formal mode of formation, since then the interplay between nucleophile(s) or electrophile(s), substrate, and loss or addition of electron(s) is best expressed. It is upon our ingenuity to choose the correct combination of electrolyte components that the practical success of an electrochemical reaction rests, and therefore the rather formalized classification system to be outlined and exemplified below is the logical point of departure into the maze of mechanistic intricacies of electrode processes. 

Polar effects in radical reactions seem also not quite well understood. There are some attempts to divide radicals into electrophilic and nucleophilic classes (Pryor, 1966 Pryor et al., 1969 Johnston et al., 1966) resembling the pattern of ionic reactants, generally on the basis of the Hammett equation. This classification, however, seems to be alien to the nature of most carbon radicals. In addition reactions (2) the Hammett />-value is usually positive (radical addition to substituted nitrobenzenes Bartlett and Kwart, 1950, 1952 Sinitsyna and... 

Because of the difficulties discussed above, our classification of the reactions of HNCC is based on the products obtained from those reactions. They are discussed in six separate sections A. Oxidation Reactions, B. Protonation and Deprotonation Reactions, C. Reduction Reactions, D. Electron-Transfer Reactions and Electrochemical Studies, E. Reactions with Soft Nucleophiles, and F. Oxidative Addition of the Small Molecules H2,12, and HX. 

Our UNDERSTANDING OF THE REACTIONS of popular reagents dates back to the early 1920s, when Lewis, Lowry, and Br nsted began developing their acid-base theories. Shortly thereafter, Lap worth, who had pioneered the study of carbonyl addition reaction mechanisms in the early 1900s, proposed the classification of polar reagents into the classes we know today as electrophiles and nucleophiles. 

The mechanisms of the usual organic reactions are now clearly established, and the reactions are classified as ionic, radical, and molecular. More detailed classifications have also been made. The mechanisms of many reactions involving non-transition metal compounds are clear enough for example, in the Grig-nard or Reformatsky reaction, the first step is the irreversible oxidative addition of alkyl halides to form Mg-carbon or Zn-carbon bonds, in which the carbon is considered to be a nucleophilic center or carbanion which reacts with various electrophiles. 

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