Substitution reactions SNAr mechanism

The reaction labelled IPSO substitution is only applicable to species like OH - and NH2" and corresponds to a special case of the SNAr mechanism. 

The neat resin preparation for PPS is quite complicated, despite the fact that the overall polymerization reaction appears to be simple. Several commercial PPS polymerization processes that feature some steps in common have been described (1,2). At least three different mechanisms have been published in an attempt to describe the basic reaction of a sodium sulfide equivalent anddichlorobenzene these are S Ar (13,16,19), radical cation (20,21), and Bunnett s (22) S l radical anion (23—25) mechanisms. The benzyne mechanism was ruled out (16) based on the observation that the para-substitution pattern of the monomer, j -dichlorobenzene, is retained in the repeating unit of the polymer. Demonstration that the step-growth polymerization of sodium sulfide and A dichlorobenzene proceeds via the SNAr mechanism is fairly recent (1991) (26). Further complexity in the polymerization is the incorporation of comonomers that alter the polymer structure, thereby modifying the properties of the polymer. Additionally, post-polymerization treatments can be utilized, which modify the properties of the polymer. Preparation of the neat resin is an area of significant latitude and extreme importance for the end user. 

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>. 

Chromium tricarbonyl-complexed aryl fluorides undergo nucleophilic substitution reactions. The substitution is not a straightforward SNAr mechanism as can be seen using, for example, 4-methoxy-l-fluorobenzene complex (71). Reaction of (71) with acetylide (72) gives a 1 2 mixture of the 1,2 and 1,3 products (73) and (74) (Scheme 115). Other leaving groups include halogens, alkoxides, and amines. Indazoles can be prepared by reaction with hydrazine followed by acidic deprotection-decomplexation (Scheme 116). 

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. 

Just as the benzyne mechanism was proposed in order to explain apparent anomalies in reactions considered initially to be SNAr reactions, another mechanism for nucleophilic aromatic substitution was developed because of results of studies under conditions in which the benzyne mechanism was expected. Kim and Burmett investigated the reaction of halogen-substituted isomers of pseudocumene (1,2,4-trimethylbenzene, 94) with KNH2 in liquid NH3. ° As shown in Figure 8.68, ehmination of HX from both the 5-halo-pseudocumenes (95a,b,c) and the 6-halopseudociunenes (96a,b,c) should produce the same aryne intermediate, 97. Therefore, the distribution of... 

In addition to the SNAr mechanism, several other mechanisms are known for nucleophilic aromatic substitutions. For example, an SnI mechanism is relevant for nucleophilic substitution reactions which encounter aromatic diazonium salts. Radical-nucleophilic aromatic substitutions (SrnI) are known in reactions where no electron-withdrawing group is available, whereas a mechanism via a benzyne intermediate is of relevance for substitutions employing NHJ as a nucleophile. 

The most widely used approach to the preparation of PESs in both academic research and technical production is a polycondensation process involving a nucleophilic substitution of an aromatic chloro- or fluorosulfone by a phenoxide ion (Eq. (3)). Prior to the review of new PESs prepared by nucleophilic substitution publications should be mentioned which were concerned with the evaluation and comparison of the electrophilic reactivity of various mono- and difunctional fluoro-aromats [7-10]. The nucleophilic substitution of aromatic compounds may in general proceed via four different mechanism. Firstly, the Sni mechanism which is, for instance, characteristic for most diazonium salts. Secondly, the elimination-addition mechanism involving arines as intermediates which is typical for the treatment of haloaromats with strong bases at high temperature. Thirdly, the addition-elimination mechanism which is typical for fluorosulfones as illustrated in equations (3) and (4). Fourthly, the Snar mechanism which may occur when poorly electrophilic chloroaromats are used as reaction partners will be discussed below in connection with polycondensations of chlorobenzophenones. 

Heavy atom kinetic isotopic effects. Kinetics of consecutive reactions. Nucleophilic substitution. SNAr mechanism. 

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. 

The SNAr reactions of heteroarenes can be realized in a similar manner, as in the series of arenes [71, 82]. These nucleophilic reactions are analogous to electrochemical Sn transformations of arenes [22, 24, 25]. Terrier and co-workers considered an opportunity for electrochemical methoxylation of 4,6-dinitrobenzofuroxan by action of the methoxide ion via the SnAt mechanism [21]. The intermediate o -complex formed was oxidized successfully into the corresponding substitution product. Analogously, the formation of heteroaromatic amines has been suggested to occur via intermediacy of the corresponding amino adducts, as exemplified by the oxidation of the o -complex derived fi om the reaction of pyrimidine with NH2 (Scheme 26) [3, 102-104]. 

In SnAr reactions, thiophene reacts more slowly than furan the SnAr reactivity of thiophene corresponds to that of anisole. The mechanism of electrophilic substitution is the same as described for furan (cf. p. 63) monosubstitution occurs predominantly in the 2-position, disubstitution in the 2,5-positions. 

Substrates 4 have a fluorine and a hydrogen atom located in similarly activated positions of the same nitroarene molecule. The reaction of substrates 4 with nucleophiles would result in the substitution of either the hydrogen or the fluorine atom, leading to a mixture of SNAr (6) and VNS (7) products (Scheme 36.5). In consequence, compounds 4 are ideal to evaluate the effect of base concentrations on the VNS mechanism. As the SwAr mechanism is not influenced by base concentration (a reasonable assumption in most of the cases), the study of the VNS/ SNAr product ratio gives information about the effect of the base on the VNS reaction rate. 

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