Nucleophilic substitution reactions described

Many of the nucleophilic substitution reactions described are halogen displacements which take place most readily when the halogen is activated by an electron-withdrawing group in the molecule. Thus, attempts to prepare... 

In addition to undergoing the nucleophilic substitution reactions described in Chapter 10, alkyl halides undergo elimination reactions. In an elimination reaction, groups are eliminated from a reactant. For example, when an alkyl halide undergoes an elimination reaction, the halogen (X) is removed from one carbon and a proton is removed from an adjacent carbon. 

The term nucleophilicity refers to the effect of a Lewis base on the rate of a nucleophilic substitution reaction and may be contrasted with basicity, which is defined in terms of the position of an equilibrium reaction with a proton or some other acid. Nucleophilicity is used to describe trends in the kinetic aspects of substitution reactions. The relative nucleophilicity of a given species may be different toward various reactants, and it has not been possible to devise an absolute scale of nucleophilicity. We need to gain some impression of the structural features that govern nucleophilicity and to understand the relationship between nucleophilicity and basicity. ... 

These types of compounds are expected to be produced by utilizing nucleophile substitution reaction at the 2 position of l-methoxyindole-3-carbaldehyde (115a) and 3-acetyl-1-methoxyindole (107). In practice, after conversion of 115a to 195a (53%) as described in Section IV.J, 195a is allowed to react with various amines. Consequently, many derivatives of 271 are obtained. Typical examples (271a-c) are shown in the scheme (99H1157). 

Classical trajectory calculations, performed on the PES1 and PESl(Br) potential energy surfaces described above, have provided a detailed picture of the microscopic dynamics of the Cl- + CH3Clb and Cl" + CH3Br SN2 nucleophilic substitution reactions.6,8,35-38 In the sections below, different aspects of these trajectory studies and their relation to experimental results and statistical theories are reviewed. 

Brand-new results show the existence of heptacoordinated silicon as described in some of the following papers of this chapter, which also contribute to the discussion of mechanistical pathways in the course of nucleophilic substitution reactions at silicon. From these results one may speculate whether compounds with octa- and nonacoordinated silicon may be characterised in the near future. Although it is a problem to assign coordination numbers in -w-bound systems, it is worthwhile to note Jutzi s dccamethylsilicocene with a formal Si-coordination number ten in the oxidation state +2 in this context. With respect to Si(U)-compounds it should be stated that there are further derivatives with the... 

Molecular transport junctions differ from traditional chemical kinetics in that they are fundamentally electronic rather than nuclear - in chemical kinetics one talks about nucleophilic substitution reactions, isomerization processes, catalytic insertions, crystal forming, lattice changes - nearly always these are describing nuclear motion (although the electronic behavior underlies it). In general the areas of both electron transfer and electron transport focus directly on the charge motion arising from electrons, and are therefore intrinsically quantum mechanical. 

The formation of C-O, C-S, C-N and C-C bonds by nucleophilic substitution is described in subsequent chapters. In this section the synthesis of haloalkanes by halogen-halogen exchange and related reactions are presented. 

Historically, the thermal transesterification of (-)-ethyl p-toluene-sulfinate 224 with n-butanol affording (+)-n-butyl p-toluenesulfinate 225 described by Phillips in 1925 (100) is the first nucleophilic substitution reaction at chiral sulfur involving a Walden-type inversion. The evidence for inversion of configuration in this reaction was based on the assumption that both (-)-esters 224 and 225 obtained from the kinetic resolution have the same configuration. 

Types of Nucleophilic Substitution Reactions. A reaction described as SnI, short for substitution, nucleophilic (un-imolecular), comprises two steps ... 

Recently, Lewis-acid-catalyzed nucleophilic substitution reactions of propargyl alcohols have been described. In general, costly transition metals such as Ru, Re, Pd or Au are used in these transformations. At this point Bi(III) salts are believed to be a cheap and environmentally benign alternative. 

This approach can be illustrated by describing the preparation of the polymer rhodium catalyst II (Sec. 9-lg). The synthesis is based on a nucleophilic substitution reaction of chlor-omethylated polystyrene [Grubbs and Kroll, 1971] ... 

The first step in the addition of alkoxides to Cgg is, consistently, the formation of the alkoxy Cgg anion. The subsequent process is strongly dependent on the presence of oxygen. In the presence of oxygen, 1,3-dioxolane derivatives of Cgg are formed [110]. In the absence of oxygen the oligo alkoxy fullerenide anions can be formed [111, 112]. Reaction of alkoxides with Cgg usually results in complex mixtures. This may be why only a few reactions of Cgg with alkoxides have been described [113]. Nevertheless, defined alkoxy fullerenes can be obtained by nucleophilic substitution reactions of alkoxides with halogenofullerenes (Chapter 9) [113]. 

The introduction of substituents into position 7 of a 2,4-disubstituted pteridine can be effected very cleanly by the use of acyl radicals typically and has been known for many years. Treatment of aldehydes with /-butyl hydroperoxide and iron(ll) generates acyl radicals which add selectively to the 7-position. A recent exploitation of this chemistry has provided a large number of new examples including both aryl and alkyl acyl radicals as reagents <2004PTR129> pA , data have been compiled (Section 10.18.4) and many nucleophilic substitution reactions of the 7-acylated pteridines and functional group modifications have been described (Section 10.18.7.2). 

Reaction of 2-chloropyridine gives 2-chloro-6-fluoropyridine as the major product which arises from the preferential substitution of hydrogen over chlorine and would be unexpected on the basis of the nucleophilic substitution mechanism described above. The product obtained was suggested, therefore, to arise from the addition of fluorine to the most electron rich carbon-nitrogen double bond, followed by elimination of HF [155]. 

The synthetic utility of many of the substitution reactions described so far is limited because there are well-established thermal routes to the same products. However, a third group of photochemical nucleophilic substitutions involves aryl halides and nucleophiles based on sulfur, phosphorus or, of particular importance, carbon. Two examples are the reaction of bromobenzene with the anion of t-butyl methyl ketone 13.12), and the replacement of bromine by cyanomethyl in 2-bromopyridine (3.13). This type of reaction offers a clear advantage over lengthy thermal alternatives, and intramolecular versions have been used in the synthesis of indoles (e.g. 3.14) or benzofurans from o-iodoaniline or o-iodoanisole respectively. 

The useful ring closure providing benzothiepines involved the formation of both S-C bonds by reaction of diaryllithium or divinyllithium organometallic species with S2+ equivalents. Another method for formation of both S-C bonds toward dihydro- and tetrahydrothiepines and thiepines is the nucleophilic substitution reaction of S2 equivalents (Na2S or Li2S) with dibromides. Synthesis of thiepines by the related stepwise formation of two S-C bonds was also described. 

This rate law is consistent with mechanism (3), in which the bond to the leaving group (chloride) is broken and the bond to the nucleophile (hydroxide) is formed simultaneously, in the same step. A reaction that occurs in one step is termed a concerted reaction. Because two species (hydroxide ion and chloroethane) are involved in this step, the step is said to be bimolecular. This reaction is therefore described as a bimolecular nucleophilic substitution reaction, or an SN2 reaction. 

The fact that the rate law depends only on the concentration of tert-butyl chloride means that only tert-butyl chloride is present in the transition state that determines the rate of the reaction. There must be more than one step in the mechanism because the acetate ion must not be involved until after the step with this transition state. Because only one molecule pert-butyl chloride) is present in the step involving the transition state that determines the rate of the reaction, this step is said to be unimolecular. The reaction is therefore described as a unimolecular nucleophilic substitution reaction, or an SN1 reaction. 

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