Representative nucleophilic substitution reactions

Representative Functional Group Transformations by Nucleophilic Substitution Reactions of Alkyl Halides... 

Formation, transamination, and mutual transitions of aminomethyl derivatives of phosphines, representing nucleophilic substitution at the carbon atom in the P—C—O(N) fragment, take place under milder conditions than similar reactions of alcohols and amines. The reason is likely the participation of a phosphino group in intramolecular interactions (86MI1). 

The most frequently encountered reactions in organic sulfur chemistry are nucleophilic displacement reactions. The mechanism and steric course of reactions have been the main points of interest of research groups all over the world, in particular, Andersen, Cram, Johnson, and Mislow in the United States Kobayashi and Oae in Japan Kjaer in Denmark and Fava and Montanari in Italy. The results of these investigators have been discussed exhaustively in many reviews on sulfur stereochemistry. In a recent report on nucleophilic substitution at tricoordinate sulfur, the literature was covered by Tillett (10) to the end of 1975. Therefore only some representative examples of nucleophilic substitution reactions at chiral sulfur are discussed here. However, recent results obtained in the authors laboratory are included. 

Nucleophilic substitution reactions, and others involving attack by nucleophilic species on fully and partially saturated systems, form a very heterogeneous set of reactions, from which we select only a few representative examples to illustrate special points. For the most part, the reactions involved are of the kind familiar from aliphatic chemistry. 

The same result can be achieved in one step with m-chloroperoxybenzoic acid and water.719 Overall anti addition can also be achieved by the method of Prevost. In this method the olefin is treated with iodine and silver benzoate in a 1 2 molar ratio. The initial addition is anti and results in a 3-halo benzoate (71). These can be isolated, and this represents a method of addition of IOCOPh. However, under the normal reaction conditions, the iodine is replaced by a second PhCOO group. This is a nucleophilic substitution reaction, and it operates by the neighboring-group mechanism (p. 308), so the groups are still anti ... 

Depending on the relative nucleophilicities, [Nu]50% ranges from micromolar to molar concentrations (Table 13.5). Although these values represent only order-of-magnitude estimates, they allow some important conclusions. First, in uncontaminated freshwa-ters (where bicarbonate typically occurs at about 10"3 M, chloride and sulfate occur at about 10 4 M, and hydroxide is micromolar or less, Stumm and Morgan, 1996), the concentrations of nucleophiles are usually too small to compete successfully with water in SN2 reactions involving aliphatic halides. Hence the major reaction will be the displacement of the halide by water molecules. In salty or contaminated waters, however, nucleophilic substitution reactions other than hydrolysis may occur Zafiriou (1975), for example, has demonstrated that in seawater ([CL] 0.5 M) an important sink for methyl iodide is transformation to methyl chloride ... 

Based on these differences, it is possible to represent the VBCMDs for typical nucleophilic substitution reactions on Si versus C as shown in Figs. 6.16a and b. In Fig. 6.16a, the ionic curve X SiL3 + X is very stable in the pentacoordinated geometry due to the electrostatic energy of the triple ion structure, much like the case of the (FIIF) species discussed before. Consequently, the pentacoordinated (XSiL3X) species will generally be a... 

Nucleophiles often participate in nucleophilic substitution reactions. The general form of these reactions may be represented by the following equation where Nuj and Nu2 are nucleophiles ... 

A Lewis base is a species that interacts with a center of positive charge (or deficiency of electrons) because of its unshared pair of electrons. Therefore, a Lewis base is known as a nucleophile, and Eq. (5.30) represents a nucleophilic substitution reaction. 

The nitroazoles are widely used in the reaction of vicarious nucleophilic substitution of hydrogen. Vicarious nucleophilic C-amination is, practically, the single method of direct introduction of the amino group into nitro compounds. Using the vicarious nucleophilic substitution reaction we have successfully carried out the C-amination of some representatives of nitrobenzazoles, nitroazoles, and model compounds thereof and studied the structure of aminated products and the C-amination mechanism [673-678],... 

Even though fragmentation of radical anions represents a key step in SrnI reactions (Scheme 76) and in aliphatic nucleophilic substitution reactions (Sn2) proceeding via single electron transfer (Scheme 77), such processes and their mechanistic implications will not be discussed in this section (several reviews are available [271-277]). 

After this work, a further elegant experiment was carried out by Hughes et al.,2 with the measurement of the second-order rate constant for a concerted nucleophilic substitution reaction, and this was done in two ways. The substrate was one enantiomer of 2-iodooctane (7) and the nucleophile was radioactive iodide anion, 1, in propanone (acetone). The nature of the reaction is outlined in Scheme 7.4. Firstly, the rate constant was determined polarimetrically to give a rate constant ka then the rate constant for exchange of I was determined this is represented by kex. The ratio of these rate constants within experimental error was 2 1. 

Bimolecular nucleophilic substitution reactions (Sn2) involve pentacoordinate carbon in their transition state. Whereas pentacoordinate CHs -like intermediates of Se2 reactions represent only an eight-electron system around the carbocationic center involving 3c-2c bonding (octet rule is obeyed), an Sn2 reaction intermediate would represents a 10-electron system involving three-center, four electron (3c-4e) bonding (a lO-C-5 species). This, however, is an unstable situation since carbon cannot accommodate 10 electrons in its valence shell. A simple picture of typical molecular orbitals in 3c-4c bonding is shown in Figure 6.9. 

In contrast to the normal amino function which in nucleophilic substitution reactions is a rather poor leaving group, azolyl substituents represent a special case. Thus, the triazolyl group in 6-bromo-2-(pivaloylamino)-4-(l,2,4-triazoI-l-yl)pyrido[2,3-c/]pyrimidine (30) is substituted by a methoxy substituent.331... 

Many reactions can usefully be described in terms of two-dimensional energy diagrams in which change in two different bonds are represented. For example, a nucleophilic substitution reaction at some element X (e.g., silicon) might occur by any of three general mechanisms ... 

The organophosphorus esters represent another class of environmental chemicals that are hydrolyzed by nucleophilic substitution reactions. These chemicals comprise one of the most important classes of agrochemicals with insecticidal activity (Fest and Schmidt, 1983). The organophosphorus esters also have important industrial uses such as oil additives, flame retardants, and plasticizers (Muir, 1988). The wide range of biological activity that this class of chemicals exhibits is due to the variety of substituents that can be attached to the central phosphorus atom. 

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