Nucleophilic agents, effect

Metal carboxyiates have been considered as nucleophilic agents capable of removing aHyUc chlorine and thereby affording stabilization (143). Typical PVC stabilizers, eg, tin, lead, or cadmium esters, actually promote the degradation of VDC polymers. The metal cations in these compounds are much too acidic to be used with VDC polymers. An effective carboxylate stabilizer must contain a metal cation sufftcientiy acidic to interact with aHyUc chlorine and to facihtate its displacement by the carboxylate anion, but at the same time not acidic enough to strip chlorine from the polymer main chain (144). 

The effectiveness of magnesium enolates as nucleophilic agents limits the interest of the reaction. With less substituted substrates (R = H), the aldol reaction is faster than the sily-lation. Moreover, due to solubility limitations, the authors are unable to determine whether the high thermodynamic kinetic ratio of silylenol ethers obtained accurately represents the magnesium enolate composition. Nonetheless, this method is an excellent procedure to selectively prepare the thermodynamic silylenol ether from an unsymmetrical ketone. ... 

Lactams can act as acylating agents to those nucleophiles that effect the cleavage of the (3-lactam Ni C2 bond. Indeed, irreversible acylation of an enzyme amino acid unit belonging to a bacterial cell is the most common type of molecular mechanism for the (3-lactam antibiotics mode of action. Similarly, the process is on grounds of... 

Systemization of experimental data on the syntheses of heterocyclic compounds with perfluoroalkyl groups from perfluoroolefins is based on reactions with various 1,1-, 1,2-, 1,3-, and 1,4-binucleophilic reagents. While the main features of nucleophilic reactions are preserved, further transformations of the primary products (or adducts, or the products of substitution of the functional groups at the internal multiple bond) occur under the influence of the added functional group containing a heteroatom. Here one can expect dramatic differences in the effect of the nature of the nucleophilic reagent between cyclizations by new nucleophilic centers and centers already available in the molecule. Another important aspect is isomerization of the primary internal olefin into the terminal olefin or internal olefin with a different structure under the action of the nucleophilic agent. This may be critical to the structure of the heterocycle formed. 

Et) which could be converted into the thiol (35 R = H, = Et) with a mixture of trifluoroacetic acid and phenol. Compound (35 R = H, R = Et) was used for the preparation of novel derivatives, c.g. [35, R = 5-(l-methyl-4-nitro)imidazoyl, R = Et]. Other 6-substituted purine derivatives, e.g. [36 R = Et, R = NH(CH2)3Me] were prepared by effecting nucleophilic displacement reactions on the thiomethylguanine, (35 R = Me, R = Et). The same reaction was applied to the synthesis of 6-substituted 9-vinylpurine systems, e.g. (36 R = CH=CH2, R == NEt2). Finally, 6-substituted purine N-oxides have been obtained by treatment of 6-chloropurine AT-oxides with nucleophilic agents in a reaction which is mechanistically analogous." ... 

Heteroatoms in five-membered 7i-excessive heteroaromatic compounds are responsible for the chemical behaviour of the molecules as a whole. The heteroatoms not only give electrons to form an aromatic n-electron system but also determine the direction of the attack of electrophilic or nucleophilic agents. In fused 7i-excessive heterocycles containing two or more heteroatoms, the reactivity of compounds and their physical properties are substantially affected by both the mutual arrangement of the heteroatoms and the electronic effects associated with their nature. 

For development of practically convenient and effective methods of introduction of urea rests and its derivatives in the first position of the carbohydrate ring we have addressed to N-glycosides, which, as the kinetic studies showed, possess much greater reactional ability to the nucleophilic agents. From the point of view of electronic representations the replacement of glycoside hydrocsyl HCj-OH on N-aglycon should not increase... 

Figure 4 Effect of the Dha residue on the planarity of the peptide backbone. Top The double bond of the Dha, residue causes a region of extended planarity that spans across two residues, and the ability of the polypeptide backbone to swivel at the a-catbon of the Dha is lost. Bottom The thioerher link between Dha, and Cys, is. shown. The region of extended planarity around Dha, creares an unusual steric conftguiation in which the ability of nucleophilic agents to approach and add to the double bond of the Dha, residue is strongly influenced by the size and shape of R, and...

In order to explain the toxic, carcinogenic and mutagenic effects of these bioactivated intermediates many factors have to be taken into consideration (i) the rate of formation, (ii) the high reactivity towards nucleophilic agents such as DNA, RNA and proteins and (iii) sufficient stability to reach target sites in the organism (5, 8). It becomes therefore important to identify the presence of these intermediates and to study their effects in the different sites where their toxic action may occur. 

A decisive influence on the effectiveness of the anionic polymerization has the chemical structure of used monomer. In the extreme case, very reactive monomers can be polymerized using very weak bases as nucleophilic agents. 

The high acidity of superacids makes them extremely effective pro-tonating agents and catalysts. They also can activate a wide variety of extremely weakly basic compounds (nucleophiles) that previously could not be considered reactive in any practical way. Superacids such as fluoroantimonic or magic acid are capable of protonating not only TT-donor systems (aromatics, olefins, and acetylenes) but also what are called (T-donors, such as saturated hydrocarbons, including methane (CH4), the simplest parent saturated hydrocarbon. 

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