Nucleophilic substitution, cellulose

A similar approach has been described by the same authors for the synthesis of related cyclic peptidomimetics [44]. A set often nucleophiles was employed for the substitution of the chlorine atom of the cyclic triazinyl-peptide bound to the cellulose membrane. By virtue of the aforementioned rate enhancement effects for nucleophilic substitution of the solid-supported monochlorotriazines, these reactions could be rapidly carried out by microwave heating. All products were obtained in high purity, enabling systematic modification of the molecular properties of the cyclic peptidomimetics. 

An 8000-member library of trisamino- and aminooxy-l,3,5-triazines has been prepared by use of highly effective, microwave-assisted nucleophilic substitution of polypropylene (PP) or cellulose membrane-bound monochlorotriazines. The key step relied on the microwave-promoted substitution of the chlorine atom in monochlorotriazines (Scheme 12.7) [35]. Whereas the conventional procedure required relatively harsh conditions such as 80 °C for 5 h or very long reaction times (4 days), all substitution reactions were found to proceed within 6 min, with both amines and solutions of cesium salts of phenols, and use of microwave irradiation in a domestic oven under atmospheric reaction conditions. The reactions were conducted by applying a SPOT-synthesis technique [36] on 18 x 26 cm cellulose membranes leading to a spatially addressed parallel assembly of the desired triazines after cleavage with TFA vapor. This concept was later also extended to other halogenated heterocycles, such as 2,4,6-trichloropyrimidine, 4,6-dichloro-5-nitropyrimidine, and 2,6,8-trichloro-7-methylpurine, and applied to the synthesis of macrocyclic peptidomimetics [37]. 

Scheme 12.7 High-speed nucleophilic substitution reactions on polypropylene or cellulose membranes.

Ionisation of the hydroxy groups in cellulose is essential for the nucleophilic substitution reaction to take place. At neutral pH virtually no nucleophilic ionised groups are present and dye-fibre reaction does not occur. When satisfactory exhaustion of the reactive dye has taken place, alkali is added to raise the pH to 10-11, causing adequate ionisation of the cellulose hydroxy groups. The attacking nucleophile ( X ) can be either a cellulosate anion or a hydroxide ion (Scheme 7.8), the former resulting in fixation to the fibre and the latter in hydrolysis of the reactive dye. The fact that the cellulosic substrate competes effectively with water for the reactive dye can be attributed to three features of the reactive dye/ cellulosic fibre system ... 

Fiber-Reactive Dyes. These dyes can enter intu chemical reaction with the fiber and form a covalent bond to become an integral part of the liber polymer. They therefote have exceptional wetfastness. Their main use is oil eellulosie fibers where they are applied neutral and then chemical reaction is initialed by the addition of alkali. Reaction with the cellulose can be by either nucleophilic substitution, using, for example, dyes containing activated halogen substituents, or by addition to the double bond in. for example, vinyl sulfone. -SCfCH=CH2, groups. 

The most important reactive groups are those based on haloiriazine or halopyrintidine systems, where an activated halogen substituent undergoes a nucleophilic substitution reaction with ionized cellulose, or dyes based on sulfatoethylsulfonyl groups... 

Alkali cellulose from cotton linters or wood pulp, usually prepared in a way similar to the first step in the viscose process (see Section 9.7), is used as raw material. Alkylation is carried out by using alkyl chlorides. The reaction proceeds according to the SN2 mechanism (bimolecular nucleophilic substitution) ... 

Cellulose Ethers. Cellulose ethers are formed when cellulose, in the presence of alkali or as alkali cellulose, is treated with alkyl or arylalkyl halides. Two types of reaction are employed in the preparation of cellulose ethers. The most common is nucleophilic substitution. Methylation of alkali cellulose with a methyl halide is an example of this type. The other type of etherification reaction is Michael addition. This reaction proceeds by way of an alkali-catalyzed addition of an activated vinyl group to the cellulose. The reaction of acrylonitrile with alkali cellulose is a typical example. The general reaction is outlined in Scheme 4. 

Here we discuss new types d cellulose derivatives synthesized by nucleophilic substitution of mixed polysaccharides containing repeating units of 2,3- and 3,6-anhydro sugars, units d amino sugars, their N-alkyl(aryl)- and N-carboxylalkyl(aryl) derivatives, alkyl derivatives and phenylbarenyl derivatives d deoxygliux)se, elimination reactions of unsaturated derivatives containing multiple C-C bonds in the... 

We give below the basic data obtained from a systematic study of the factors (tetermining the mechanism and stoichiometry of the nucleophilic substitution reactions used for the synthesis of cellulose derivatives. 

The synthesis of cellulose propiolate has also been accomplished by way of a nucleophilic substitution reaction involving the interaction... 

The present work summarizes the some results of these studies. The article describes some new types of cellulose ethers and esters, a number of mixed polysaccharides based on cellulose, which have been synthesized by the authors and their coworkers, and also discusses the principles governing a number of reactions for effecting the chemical transformation of cellulose, i.e. nucleophilic substitution, trans-esterification, ionic and free-radical addition, which enable new types of cellulose derivatives to be synthesized. 

These nucleophilic substitution reactions can in theory proceed intra- or intermolecularly (4). The structure of the repeating unit of the cellulose macromolecule is such that in an intramolecular reaction, two types of anhydro derivatives may form mixed polysaccharides, whose repeating units contain a-oxide rings (2,3-anhydro rings), and anhydro rings of the tetrahydrofuran type (3,6-anhydro rings). 

Another important commercial utilization of cotton etherification is in coloration of fabrics with reactive dyes [338 340]. Reactive dyes contain chromophoric groups attached to moieties that have functions capable of reaction with cotton cellulose by nucleophilic addition or nucleophilic substitution to form covalent bonds. In the nucleophilic addition reaction, an alkaline media transforms the reactive dye to an active species by converting the sulfatoethyl-... 

The mechanism is reported to be a bimolecular nucleophilic substitution. Kolosh and Eriksson have examined the penetration of CS2, into alkali cellulose sheets [153] and demonstrated conditions leading to nonuniform reaction and poor viscose quality. 

The nucleophilic substitution reaction may take place when a durable synthetic polymer is mixed with agar or yeast, cellulose, amides and water. The hydroxyl groups present in the cellulose become attached to the hydroxyl groups of agar and yeast in a link resembling a glycosidic linkage. 

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