Nucleophilic substitution type 4 reactions

Attempts at replacing Br by nucleophilic substitution-type reactions usually resulted in Br elimination if a reaction actually occurred. This is presumably due to steric effects at the polymer surface. A clean reaction for near-totaldehydrobromination leading to the production of unsaturation was found to be the reaction with n-butylamine in tetrahydrofuran (THF). Theoretically rebromination (in the dark) of this unsaturated surface should double the surface Br concentration compared with the initial bromination reaction, and this situation is more-or-less achieved in the case of HDPE, as shown in Table 5. It is interesting to note the significant increase in adhesion of the unsaturated surface compared with the original polyolefin and the fact that similar surface behaviour of LDPE and HDPE, as monitored by contact angle, is not necessarily reflected in similar surface composition measured by ESCA. These results will be the subject of further study. The unsaturation centres introduced by... 

Knowledge of the order of basicities of cyclic and linear ethers is important for understanding certain phenomena in cyclic ether polymerization. As indicated earlier, chain transfer to polymer is a general feature of the cationic polymerization of cyclic ethers because the nucleophilic site of the monomer molecule (oxygen atom) is transferred to the polymer unit. To what extent chain transfer to polymer competes with propagation depends on the relative nucleophilicity of monomer and polymer unit. Thus, for five-membered THF, the polymer unit is a weaker base than the monomer. This makes the polymer less reactive than the monomer in nucleophilic substitution type reactions. Consequently, for this monomer, chain transfer to polymer is slow as compared to propagation. In contrast, in the polymerization of three-membered EO, the polymer unit is more basic than monomer. Therefore, reactions involving the polymer chain are important in this system. Practical consequences will be discussed in the subsequent sections devoted to polymerization of different classes of cyclic ethers. 

First-order reaction kinetics is frequently observed in organic chemistry in the form of an SnI reaction, indicating it is a first-order nucleophilic substitution type. An example is the solvolysis of tcrt-butylbromide at alkaline pH to form t rt-butanol and bromide ion. The reaction probably proceeds in two steps ... 

Aldol reactions using a quaternary chinchona alkaloid-based ammonium salt as orga-nocatalyst Several quaternary ammonium salts derived from cinchona alkaloids have proven to be excellent organocatalysts for asymmetric nucleophilic substitutions, Michael reactions and other syntheses. As described in more detail in, e.g., Chapters 3 and 4, those salts act as chiral phase-transfer catalysts. It is, therefore, not surprising that catalysts of type 31 have been also applied in the asymmetric aldol reaction [65, 66], The aldol reactions were performed with the aromatic enolate 30a and benzaldehyde in the presence of ammonium fluoride salts derived from cinchonidine and cinchonine, respectively, as a phase-transfer catalyst (10 mol%). For example, in the presence of the cinchonine-derived catalyst 31 the desired product (S)-32a was formed in 65% yield (Scheme 6.16). The enantioselectivity, however, was low (39% ee) [65],... 

According to Bunnett and his co-workers [49,50,50a] the reaction is of the nucleophilic substitution type and may be represented by the following scheme ... 

The common types of chemical reactions found in living cells are nucleophilic substitution, elimination reactions, addition reactions, isomerization reactions, and oxidadon-reduction reacdons. 21. In addition to being an important energy source, carbohydrates are important structural molecules in organisms and have a role in intracellular and intercellular communication. 

The substitution type reaction is not restricted to substitution at a carbon atom, either aliphatic or aromatic, but includes the main group and transition elements. Several examples will be given of the varieties of the use of thiolates as nucleophiles, and although most of these reactions are... 

A type of reaction of considerable mechanistic interest in the kinetics of electrochemical and solvolytic reactions is the reduction of alkyl halides RX where a nucleophilic substitution type of reaction occurs with the electrode acting as the nucleophile. The anion X and the hydrocarbon RH are the main products. In some cases, organometallic intermediates are produced, e.g., in the reduction of alkyl iodides or bromides at Pb. Here the process which leads to lead tetraalkyls is of considerable commercial significance. 

Photonucleophilic aromatic substitution reactions of phenyl selenide and telluride with haloarenes have also been proven to involve the S jlAr mechanism, with the formation of anion radical intermediates. Another photonucleophihc substitution, cyanomethylation, proves the presence of radical cations in the reaction mechanism. Liu and Weiss have reported that hydroxy and cyano substitution competes with photo substitution of fluorinated anisoles in aqueous solutions, where cation and anion radical intermediates have been shown to be the key factors for the nucleophilic substitution type. Rossi et al. have proposed the S j lAr mechanism for photonucleophihc substitution of carbanions and naphthox-ides to halo anisoles and l-iodonaphthalene. > An anion radical intermediate photonucleophilic substitution mechanism has been shown for the reactions of triphenyl(methyl)stannyl anion with halo arenes in liquid ammonia. Trimethylstannyl anion has been found to be more reactive than triphenylstannyl anion in the photostimulated electron- transfer initiation step. 

A classical reaction leading to 1,4-difunctional compounds is the nucleophilic substitution of the bromine of cf-bromo carbonyl compounds (a -synthons) with enolate type anions (d -synthons). Regio- and stereoselectivities, which can be achieved by an appropiate choice of the enol component, are similar to those described in the previous section. Just one example of a highly functionalized product (W.L. Meyer, 1963) is given. 

Section 8 1 Nucleophilic substitution is an important reaction type m synthetic organic chemistry because it is one of the mam methods for functional group transformations Examples of synthetically useful nucleophilic sub stitutions were given m Table 8 1 It is a good idea to return to that table and review its entries now that the details of nucleophilic substitution have been covered... 

Alkynes of the type RC CH may be prepared by nucleophilic substitution reactions in which one of the starting matenals is sodium acetyhde (Na" C=CH)... 

Acylium ion (Section 12 7) The cation R—C=0 Acyl transfer (Section 20 3) A nucleophilic acyl substitution A reaction in which one type of carboxylic acid derivative IS converted to another... 

Nucleophilic Substitutions of Benzene Derivatives. Benzene itself does not normally react with nucleophiles such as haUde ions, cyanide, hydroxide, or alkoxides (7). However, aromatic rings containing one or more electron-withdrawing groups, usually halogen, react with nucleophiles to give substitution products. An example of this type of reaction is the industrial conversion of chlorobenzene to phenol with sodium hydroxide at 400°C (8). 

Broadly speaking, nucleophilic substitution may be divided into (a) the direct displacement of hydrogen and (b) the displacement of other substituents. Displacements of type (a) are rare and are typified by the Tschitschibabin reaction. Pyrazine reacts with NaNHa/NHs to yield 2-aminopyrazine, but no yield has been quoted (46USP2394963). Generally, the synthesis of aminopyrazines, aminoquinoxalines and aminophenazines is more readily accomplished by alternative methods, particularly displacement of halogen from the corresponding halo derivatives, which are themselves readily available. 

Nucleophilic substitution of the chlorine atom in 2-chloropyrazine and 2-chloroquinoxa-lines has been effected with a variety of nucleophiles, including ammonia and amines, oxygen nucleophiles such as alkoxides, sodium azide, hydrazine, sulfur containing nucleophiles, cyanide, etc., and reactions of this type are typical of the group (see Chapter 2.02). 

In contrast to electrophilic reagents, the highly -tt-deficient character of the pteridine nucleus is responsible for its vulnerability towards nucleophilic attack by a wide variety of reagents. The direct nucleophilic substitution of pteridine itself in a Chichibabin-type reaction with sodamide in diethylaniline, however, was unsuccessful (51JCS474). Pteridin-6-one, on the other hand, yielded pteridine-6,7-dione under the same conditions, via a still unknown reaction mechanism. 

Halogen atoms in the 2-position of imidazoles, thiazoles and oxazoles (542) undergo nucleophilic substitution reactions. The conditions required are more vigorous than those used, for example, for a- and y-halogenopyridines, but much less severe than those required for chlorobenzene. Thus in compounds of type (542 X = Cl, Br) the halogen atom can be replaced by the groups NHR, OR, SH and OH (in the last two instances, the products tautomerize see Sections 4.02.3.7 and 4.02.3.8.1). 

Although different reaction types exhibit large quantitative differences, and there are exceptions, the order 5>6>3>7>4> 8-10 is a rough guide of relative reactivity for many systems. Some quantitative data on typical reactions involving nucleophilic substitution or participation are shown in Scheme 3.4. 

The first three chapters discuss fundamental bonding theory, stereochemistry, and conformation, respectively. Chapter 4 discusses the means of study and description of reaction mechanisms. Chapter 9 focuses on aromaticity and aromatic stabilization and can be used at an earlier stage of a course if an instructor desires to do so. The other chapters discuss specific mechanistic types, including nucleophilic substitution, polar additions and eliminations, carbon acids and enolates, carbonyl chemistry, aromatic substitution, concerted reactions, free-radical reactions, and photochemistry. 

It was noted early by Smid and his coworkers that open-chained polyethylene glycol type compounds bind alkali metals much as the crowns do, but with considerably lower binding constants. This suggested that such materials could be substituted for crown ethers in phase transfer catalytic reactions where a larger amount of the more economical material could effect the transformation just as effectively as more expensive cyclic ethers. Knbchel and coworkers demonstrated the application of open-chained crown ether equivalents in 1975 . Recently, a number of applications have been published in which simple polyethylene glycols are substituted for crowns . These include nucleophilic substitution reactions, as well as solubilization of arenediazonium cations . Glymes have also been bound into polymer backbones for use as catalysts " " . 

Sulfides, compounds of the type RSR, are prepared by nucleophilic substitution reactions. Treatment of a primary or secondary alkyl halide with an alkanethiolate ion (RS ) gives a sulfide ... 

Nucleophilic substitution by cyanide ion (Sections 8.1, 8.13) Cyanide ion is a good nucleophile and reacts with alkyl halides to give nitriles. The reaction is of the S m2 type and is limited to primary and secondary alkyl halides. Tertiary alkyl halides undergo elimination aryl and vinyl halides do not react. 

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