Electron-poor aromatic systems

Similarly, the Stoddart catenanes based on Ji-electron rich-n-electron poor aromatic systems have been incorporated into polymer backbones (e.g. Figure 5). These systems give a degree of polymerisation of l 7. 

Method of cleavage Cleavage is generally performed by protodesilylation. The conditions depend very much on the nature of the aromatic system, as well as on the substitution. Either neat TFA, TFA vapor or TFA/CH2CI2 can be used. Electron-poor aromatic systems require treatment with CsF in DMF/ water (4 1) at 110 °C. 

An excellent alternative to the classical Hunsdiecker reaction and its variants, which totally avoids the use of heavy metal salts and potent electrophilic reagents, consists of the simple photolysis or thermolysis of Barton esters in refluxing bromotri-chloromethane for the bromides or tetrachloromethane for the chlorides [4], The analogous decarboxylative iodination can also be achieved using iodoform as the reagent in a benzene/cyclohexene solvent system (Scheme 5). For the cases of vinylic and aromatic acids, where the usual problems of chain efficiency are encountered, the addition of azobisisobutyronitrile (AIBN) is also required [10]. Nevertheless, since this method can operate on both electron-rich and electron-poor aromatic systems, and moreover does not suffer from the competitive electrophilic aromatic bromination found with electron rich aromatics under normal Hunsdiecker conditions, this route to synthetically useful aryl iodides and bromides should find widespread application. 

Aromatic Chlorination. Many aromatic and heteroaromatic chlorinations using NCS are catalyzed by acetic acid. Ferric chloride and ammonium nitrite have also been used to catalyze the chlorination of various heterocycles with NCS. Although NCS has been used for halogenation of electron-rich aromatics, the halogenation of electron-poor aromatic systems with NCS has been difficult to achieve. However, the chlorination of various deactivated aromatic systems can be achieved when NCS is acid catalyzed with boron trifiuoride monohydrate. The reaction is impressive in that even the deactivated 1-fiuoro-2-nitrobenzene is chlorinated to afford 4-chloro-l-fiuoro-2-nitrobenzene in 81% yield after 18 h at 100 °C (eq 24). ... 

Direct perfluoroalkylatwn of electron poor aromatic and heterocyclic systems with perfluorocarboxylic acids is mediated by xenon difluonde [165] (equation 142)... 

So far only Pd-based systems have been highlighted in this section however, the use of other metals such as Ni has clear economic advantages. In this regard, Chiu and co-workers have used a bis-carbene tetradentate ligand to catalyse the coupling of aryl bromides and chlorides with both electron rich and electron poor aromatic rings however, the reaction with electron poor aryl bromides lead to superior yields (Scheme 6.30) [113]. 

The electrophiles in such reactions can be either aryl halides or triflates, possessing electron-rich, neutral or electron-poor ring systems, whereas amines can range from aliphatic to aromatic and primary to tertiary amines. The Pd-catalyzed C—N bond formation works both inter- and intramolecularly. 

Almost all of the studies of zeolite-catalyzed FC acylations have been conducted with electron-rich substrates. There is clearly a commercial need, therefore, for systems that are effective with electron-poor aromatics. In this context, the reports [52] on the acylation of benzene with acetic acid over H-ZSM-5 in the gas phase are particularly interesting. These results suggest that the adsorption ratios of substrate, acylating agent and product are more favorable in the gas phase than in the liquid phase. 

In contrast to Si and Ge linkages (traceless linkers) of the aryl ring to the resin where electron-rich aromatic ring systems favor rapid cleavage, the reductive cleavage of arylsulfonates works well with electron-poor aromatic rings. 

The term n-n stacking signifies a class of weak interactions between electron-rich and electron-poor aromatic rings. The attractive force is proportional to the contact surface between both r-systems. This interaction takes place between the negatively charged r-electron cloud... 

Because the pyridine ring is electron poor, the system undergoes electrophilic aromatic substitution only with great difficulty, several orders of magnitude more slowly than benzene, and at C3 (see Section 15-8). 

Arylethenes are inner-outer-ring dienes in which the vinyl group is linked to an aromatic system. These dienes are poorly or moderately reactive the presence of electron-donating substituents in the diene moiety markedly increases their reactivity. Their cycloadditions are usually accelerated in order to be carried out under mild conditions. 1-Vinylnaphthalene is more reactive than 2-vinyl-naphthalene and styrenes. 

Imines derived from aniline and glyoxylic acid esters can be regarded as electron-poor 2-azadienes, in which an aromatic carbon—carbon double bond takes part of the diene system. In this context, Prato and Scorrano et al. were able to achieve the [4 + 2] cycloaddition of ethyl N-phenyl glyoxylate imines with dihydrofuran and indene leading to hexahydrof-uro[3,2-c]- and tetrahydro-7//-indeno[2,l-c]quinolines, respectively, in moderate to good yields (88JHC1831). Similarly, tetrahydroquinoline derivatives were formed by [4 + 2] cycloaddition of 1,2-bis(trimethylsily-... 

Addition of an alkali metal ion to 23 should also make this host molecule a better binder of aromatic guest molecules. In the aa conformation unlike the as conformation-the 7r-electron rich naphthalene walls are capable of sandwiching rc-electron poor molecules (see Fig. 12). Indeed, upon addition of a potassium salt to a solution of 23, the for binding of 1,3-dinitrobenzene increases by a factor of 2 to 6, depending on the solvent systems [26]. Consistent with the proposed structure of the complex of 23 with sodium ions, addition of these ions has no effect on the of 23 with 1,3-dinitrobenzene. 

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