Modification electrophilic aromatic substitution

The classical Friedel-Crafts approach toward attaching a phosphorus site directly to an aromatic ring would seem a promising route. Phosphorus-centered acid halides would be anticipated to participate in electrophilic aromatic substitution much in the manner of ordinary acyl halides. Early efforts confirmed this concept.48-52 However, difficulties have been encountered in the use of the classical conditions,53 and modifications to the approach have been necessary. 

Although the chemical modification of tyrosine residues has enjoyed a long history, this residue remains an underused target for bioconjugation reactions. It is typically modified through electrophilic aromatic substitutions (EAS), which makes its reactivity distinct from other amino acid side chains. This reaction... 

Mechanical addressing can be also used for combinatorial postsynthetical modifications of conductive polymers. Postsynthetical modification was applied to formations of a number of different derivates of polyaniline (Fig. 13.4), the modification was performed by nucleophilic addition (Fig. 13.4), coupling with diazonium salts and by electrophilic aromatic substitution.46... 

The widespread applications of polystyrene derived resins is due to the fact that styrene consists of a chemically inert aUcyl backbone carrying chemically reactive aryl side chains that can be easily modified. As discussed earlier, a wide range of different types of polystyrene resins exhibiting various different physical properties can be easily generated by modification of the crosslinking degree. In addition, many styrene derived monomers are commercially available and fairly cheap. Polystyrene is chemically stable to many reaction conditions while the benzene moiety, however, can be funtionalised in many ways by electrophilic aromatic substitutions or lithiations. As shown in Scheme 1.5.4.1 there are principally two different ways to obtain functionalised polystyrene/DVB-copolymers. 

Electrophilic aromatic substitution of hydrogen (for which the symbol SnAr is usually used, while H is omitted) is a well-developed synthetic procedure which is widely used for structural modification of aromatic compounds [1]. 

JCei/u ofds Polysulfones, polysulfonylation, electrophilic aromatic substitution, post-polymerization, chemical modification... 

The study of polymer-supported reactions in organic chemistry (Ref 1-6) is a field which has enjoyed rapid growth in the past two decades and has required the preparation of a large number of specialty polymers carrying various functionalities much of this preparative work has been carried out through the modification of a few reactive polymers derived from polystyrene (Ref 7) The chemical modification route is particularly attractive in this instance as polystyrene resins contain aromatic rings which can be modified readily, often by electrophilic aromatic substitutions, while the rest of the molecule (polymer backbone) is relatively... 

In the case of phenazine, substitution in the hetero ring is clearly not possible without complete disruption of the aromatic character of the molecule. Like pyrazine and quinoxa-line, phenazine is very resistant towards the usual electrophilic reagents employed in aromatic substitution reactions and substituted phenazines are generally prepared by a modification of one of the synthetic routes employed in their construction from monocyclic precursors. However, a limited range of substitution reactions has been reported. Thus, phenazine has been chlorinated in acid solution with molecular chlorine to yield the 1-chloro, 1,4-dichloro, 1,4,6-trichloro and 1,4,6,9-tetrachloro derivatives, whose gross structures have been proven by independent synthesis (53G327). 

Fig. 10.3-5 Tyrosine modification using a when proteins are treated alone with either three component Mannich-type reaction. component, (b) The reaction conversion is (a) Aldehydes and anilines condense to listed for a number of anilines and aliphatic form imines in situ, which react with tyrosine amines using a-chymotrypsinogen A as the residues through an electrophilic aromatic substrate and formaldehyde as the aldehyde substitution reaction. No reaction occurs component.

The results in table 2.6 show that the rates of reaction of compounds such as phenol and i-napthol are equal to the encounter rate. This observation is noteworthy because it shows that despite their potentially very high reactivity these compounds do not draw into reaction other electrophiles, and the nitronium ion remains solely effective. These particular instances illustrate an important general principle if by increasing the reactivity of the aromatic reactant in a substitution reaction, a plateau in rate constant for the reaction is achieved which can be identified as the rate constant for encounter of the reacting species, and if further structural modifications of the aromatic in the direction of further increasing its potential reactivity ultimately raise the rate constant above this plateau, then the incursion of a new electrophile must be admitted. 

In our research, three chemical modification approaches were investigated bromination, sulfonylation, and acylation on the aromatic ring. The specific objective of this paper is to present the chemical modification on the PPO backbone by a variety of electrophilic substitution reactions and to examine the features that distinguish modified PPO from unmodified PPO with respect to gas permeation properties, polymer solubility and thermal behavior. 

Chemical modifications of PPO by electrophilic substitution of the aromatic backbone provided a variety of new structures with improved gas permeation characteristics. It was found that the substitution degree, main chain rigidity, the bulkiness and flexibility of the side chains and the polarity of the side chains are major parameters controlling the gas permeation properties of the polymer membrane. The broad range of solvents available for the modified structures enhances the possibility of facile preparation of PPO based membrane systems for use in gas separations. 

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