B Nucleophilic aromatic substitution

B Nucleophilic Aromatic Substitution through an Elimination-Addition Mechanism Benzyne... 

The efficient convergent synthesis developed by Semmelhackef is based on construction of the two sections of cephalotaxinone molecule as dissected in representation (i), and their combination for creation of the central 7-membered azepine ring. Several variations of the key cyclization step have been developed, involving (a) a nucleophilic attack on a transient benzyne derivative, (b) nucleophilic aromatic substitution via an aryl nickel species, and (c) an intramolecular reaction. ... 

It looks as though we can get B from A (which is used in frame 247) and so the nitro group is the obvious source of the amino group. It will also allow us to hydrolyse one ether specifically by nucleophilic aromatic substitution. 

There are several mechanisms by which net nucleophilic aromatic substitution can occur. In this section we will discuss the addition-elimination mechanism and the elimination-addition mechanism. Substitutions via organometallic intermediates and via aryl diazo-nium ions will be considered in Chapter 11 of Part B. 

The available experimental results are completely in accord with this formulation. Both of these limiting conditions have been observed experimentally, and plots of both k versus [B]0 and k versus [R2NH]0 have been shown to have characteristics consistent with this proposed mechanism. These observations thus constitute very convincing evidence for the intermediate complex mechanism in nucleophilic aromatic substitution. 

The utilization of polar polymers and novel N-alkyl-4-(N, N -dialklamino)pyridinium sedts as stable phase transfer catalysts for nucleophilic aromatic substitution are reported. Polar polymers such as poly (ethylene glycol) or polyvinylpyrrolidone are thermally stable, but provide only slow rates. The dialkylaminopyridininium salts are very active catalysts, and are up to 100 times more stable than tetrabutylammonium bromide, allowing recovery and reuse of catalyst. The utilization of b is-dialkylaminopypridinium salts for phase-transfer catalyzed nucleophilic substitution by bisphenoxides leads to enhanced rates, and the requirement of less catalyst. Experimental details are provided. 

As previously discussed, L-deprenyl (2) is a selective suicide inhibitor of MAO B. As part of a program to develop [ F]-labeled L-deprenyl for PET studies, Fowler and coworkers prepared [ C]-labeled o- and L-4-fluorodeprenyl (37) to study the effects of fluorine substitution on kinetics of uptake and localization [105]. Subsequently, no-carrier-added o,L-4-[ F]-fluoroderprenyl (38) was made by a nucleophilic aromatic substitution reaction [106],... 

Johnson, C. R. Zhang, B. Fantauzzi, P. Hocker, M. Yager, K. M. Libraries of/V-Alkylaminoheterocycles from Nucleophilic Aromatic Substitution with Purification by Solid Supported Liquid Extraction, Tetrahedron 1998, 54, 4097. 

This chapter covers reactions in which coordination of a transition metal to the ir-system of an arene ring activates the ring toward addition of nucleophiles, to give V-cyclohexadienyl-metal complexes (1 Scheme 1). If an electronegative atom is present in the ipso position, elimination of that atom (X in 1) leads to nucleophilic aromatic substitution (path a). Reaction of the intermediate with an electrophile (E+) can give disubstituted 1,3-cyclohexadiene derivatives (path b). If a hydrogen occupies the ipso posi-... 

Scheme 34 shows a two-step mechanism (pathway A), similar to the general addi-tion/elimination mechanism of nucleophilic aromatic substitution reactions155. Pathway B of Scheme 34 shows the addition-elimination mechanism. When nucleofuge expulsion is difficult (or the leaving group is absent) the saturated derivative 74 of Scheme 34 is the final product. Addition of amines to 1-methyl-4-vinylpyridinium cation (75) of reaction 13 is an instance of saturation of a C=C double bond by a nucleophilic attack156. R2NH of reaction 13 may be a primary or a secondary amine. 

B-2. Rank the following in order of decreasing rate of reaction with ethoxide ion (CH3CH2CT) in a nucleophilic aromatic substitution reaction ... 

B-6. Which one of the following compounds can be efficiently prepared by a procedure in which nucleophilic aromatic substitution is the last step ... 

Figure 3 Direct pathways of ozone reaction with organics. (A) Criegge mechanism. (B) Electrophilic aromatic substitution and 1,3-dipolar cycloaddition. (C) Nucleophilic substitution.

---