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Nucleophilic Aromatic Substitution SNAr

Aromatic nucleophilic substitution, SNAr, by azide ion is a significant exception to the generalization that second-order rate constants are similar in water and micelles (37). Values of k2m and kw in cationic micelles are similar for deacylation, an SN2 reaction and addition to (2,2, 4,4, 4"-pentamethoxy)-triphenylmethyl cation (Table II and references 37 and 41). These results suggest that the nucleophilicity of N3 is not increased by interaction with cationic micelles. But k2m is considerably larger than kw for aromatic nucleophilic substitution in reactions of N3 with 2,4-dinitrochlorobenzene,... [Pg.418]

Two other important modes of substitution require mention here. They are the SNAr and elimination-addition reactions. Actually, it is sometimes difficult to distinguish between true aromatic nucleophilic substitutions and addition-elimination processes. The second group involves pyridyne intermediates (Scheme 53). Both of these reaction types are discussed fully under substituent reactions (Chapter 2.06). [Pg.216]

The most important experimental data about NMR, IR, and UV spectroscopy have been reported in CHEC-I. In addition, an AMI SCF-MO study has been published <88JOC3900>. The relaxed reaction profile for aromatic nucleophilic substitution of some chloropyrimido[4,5-J]pyridazine has been investigated using the MNDO procedure <90JST(63)45>. Kinetic measurements and MNDO calculations show that the C-8 position of the pyridazine ring is more reactive than C-5 in nucleophilic substitution reactions, and these follow a two-step SNAr mechanism <89T4485>. [Pg.744]

Secondary steric effects could become significant in aromatic nucleophilic substitution in activated halogenoben-zenes. This can be ascribed to steric inhibition of resonance. In contrast, secondary steric effects are not important in SNAr reactions of thiophene derivatives this is due to the geometry of five-membered ring derivatives, which strongly lowers the steric interactions between the substituents on the thiophene ring. This has been reconfirmed by kinetic data in methanol on SNAr reactions of the two pairs of substrates 210 and 211 with different nucleophiles (piperidine and sodium benzenethiolate) <1997J(P2)309>. [Pg.813]

The basic concepts of nucleophilic substitution reactions appeared in the first semester of organic chemistry. These reactions follow SN1 or SN2 mechanisms. (In aromatic nucleophilic substitution mechanism, we use the designation SNAr.) In SN1 and SN2 mechanisms, a nucleophile attacks the organic species and substitutes for a leaving group. In aromatic systems, the same concepts remain applicable, but with some differences that result from the inherent stability of aromatic systems. [Pg.111]

Notes-. Most used reactions are SEAr (iododemetallation) and SNAr (halogen exchange, copper assisted). SeI, unimolecular electrophilic substitution Se2, bimolecular electrophilic substitution SeAp aromatic electrophilic substitution SeI, intramolecular electrophilic substitution S l, unimolecular nucleophilic substitution 3 2, bimolecular nucleophilic substitution S Ar, aromatic nucleophilic substitution. [Pg.749]

Scheme 7.14 A representation of an aromatic nucleophilic substitution reaction (SNAr) involving the intermediacy of a Meisenheimer complex. The curved arrows on the one resonance structure show a way that chloride ion might be lost. Scheme 7.14 A representation of an aromatic nucleophilic substitution reaction (SNAr) involving the intermediacy of a Meisenheimer complex. The curved arrows on the one resonance structure show a way that chloride ion might be lost.
Aromatic nitro compounds undergo nucleophilic aromatic substitutions with various nucleophiles. In 1991 Terrier s book covered (1) SNAr reactions, mechanistic aspects (2) structure and reactivity of anionic o-complexes (3) synthetic aspects of intermolecular SNAr substitutions (4) intramolecular SNAr reactions (5) vicarious nucleophilic substitutions of hydrogen (VNS) (6) nucleophilic aromatic photo-substitutions and (7) radical nucleophilic aromatic substitutions. This chapter describes the recent development in synthetic application of SNAr and especially VNS. The environmentally friendly chemical processes are highly required in modem chemical industry. VNS reaction is an ideal process to introduce functional groups into aromatic rings because hydrogen can be substituted by nucleophiles without the need of metal catalysts. [Pg.302]

Nucleophilic substitutions with [ F]fluoride have been largely developed both in aromatic (SNAr) and aliphatic (generally SN2) series. Nucleophilic additions remain rare. F-Nucleophilic radiofluorinations usually do not require any carrier and thus enable the synthesis of products with high specific radioactivity. The SN can be performed either directly on a suitable and generally complex precursor of the target molecule or indirectly via a small labelled precursor. Both approaches present drawbacks the first one generally leads to poor yields and the second requires multistep synthesis and more sophisticated automation processes. [Pg.218]

Cycloamidation has been used extensively to prepare 17-membered cycloisodityrosines. The acyclic biaryl ether precursors were prepared by methods including the Ullmann reaction 2-5 and nucleophilic aromatic substitution (SNAr)J6 7 Since these methods have all been used intramolecularly in cyclization reactions, they will be discussed in Sections 9.5.3 and 9.5.4. Evans and co-workers employed the pentafluorophenyl ester method of macrolactamization 8] to prepare 11, an intermediate in their total synthesis of OF4949-III (7) (Scheme 2)J3 In this case, the acidic removal of a Boc group was employed to release the cyclization substrate, although hydrogenolysis of a Z group is also effective 3 ... [Pg.195]

Nucleophilic aromatic substitution (SnAr) reactions are not, in general, facile and usually require the presence of at least one strongly electron-withdrawing group, such as a nitro group, or otherwise a very good leav-... [Pg.34]

Now you have three basic mechanisms for aromatic rings — electrophilic aromatic substitution, SNAr, and elimination/addition. How do you choose among these The first consideration is what types of other reagents are present. If the reagents include an electrophile, then the reaction will be electrophilic aromatic substitution. The presence of a nucleophile may lead to either SNAr or elimination/addition. If the system meets the three requirements for SNAr, then the reaction will follow that mechanism. If not, it will be an elimination/addition. [Pg.115]

The rate enhancements observed in these nucleophilic aromatic substitution reactions when using crown-complexed ions and tetraalkylam-monium ions are indeed not surprising. Many examples are known of increased reactivity in nucleophilic substitutions due to complexation with crown compounds (24), also in SNAr reactions (25, 26). In our system, however, this normal effect is accompanied by an inhibiting effect on the competing reduction path, which is discussed under Reduction Channel. [Pg.333]


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See also in sourсe #XX -- [ Pg.156 , Pg.181 , Pg.309 ]




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Aromatic nucleophiles

Aromatic substitution nucleophilic

Nucleophile aromatic substitution

Nucleophilic aromatic

Nucleophilic aromatic substitution (the SNAr mechanism)

Nucleophilic aromatic substitution nucleophiles

SNAr

SNAr reaction nucleophilic aromatic substitution

SNAr substitution

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