Electron-deficient aromatic rings

Nucleophilic attack by a trialkyl phosphite on a significantly electron-deficient aromatic ring involving addition-elimination of a nitro group (as nitrite anion), which completes the final step of the normal Michaelis-Arbuzov reaction (Figure... 

The functionalization of electron rich aromatics rings is often accomplished by electrophilic aromatic substitution. However, electrophilic substitutions require stringent conditions or fail entirely with electron deficient aromatic rings. Nucleophilic aromatic substitutions are commonly used but must usually be conducted under aprotic conditions. In contrast, nucleophilic radicals can add to electron deficient aromatic rings under very mild conditions. 

Figure 6.40 Paraquat, the electron deficient blue box developed by Stoddart and co-workers and a later derivative. Electron deficient aromatic rings are coloured blue, red is used for electron rich rings, neutral hydrocarbons are coloured black while green is used for other moieties such as tetrathiafulvalene.

Pseudobase formation by nucleophilic addition to heteroaromatic cations is closely related to the long-known Meisenheimer complex formation by nucleophilic addition to an electron-deficient neutral aromatic molecule.20-25 In both cases nucleophilic attack on an electron-deficient aromatic ring produces a c-complex—an anionic Meisenheimer complex or a neutral pseudobase molecule. Despite the intense interest over the past few years in Meisenheimer complexes as models for er-complex intermediates in nucleophilic aromatic substitution reactions, there has been little overt recognition of the relationship between Meisenheimer complexes and pseudobases derived from heteroaromatic cations. In this regard, it is interesting that the pseudobase 165, which can be regarded as the complex intermediate that would be expected for an SNAr reaction between the l-methyl-4-iodoquinolinium cation and hydroxide ion, has been spectroscopically characterized.89... 

The enamides of TV-aroylenamine type (143-146), which contain an electron-deficient aromatic ring caused by the introduction of some electron-withdrawing factor, undergo a facile cyclization not only photochemi-cally but also thermally to afford the dehydrolactams (147-150) in good yields (53,67). The dinitro-substituted enamide (143) was thermally cyclized when refluxed in benzene to afford the corresponding dehydrolactam (147) in 95% yield. Compound 147 was also obtained from the 1-methylisoquino-... 

The problem of controlling the bead on its thread can be addressed by using the charge transfer interaction between Ti-electron-rich and r-electron-deficient aromatic rings. In particular the interaction of cyclobis(paraquat-p-phenylene) 14 with TT-electron-rich aromatic rings [73] and the interaction of bis(/ -phenylene)-34-crown-10 ether 22 with paraquat [74, 75] have been used. 

Tetrafluoropyridine derivatives, synthesized by nucleophilic substitution reactions of pentafluoropyridine are, of course, electron-deficient aromatic ring systems and further nucleophilic substitution can occur. However, few studies exploring the effects of the 4-substituent upon the regiochemistry of further nucleophilic substitution processes have been reported and examples of reactions of various 2,3,5,6-tetrafluoropyridine derivatives with diethylamine are shown in Table 8.3 to illustrate the synthetic possibilities. 

Since 2002 several groups have studied in detail the interaction of anions with electron deficient aromatic rings using theoretical methods. These studies can be considered to derive from earlier fundamental work carried out by Dougherty [14], Besnard [15] and Alkorta [16] on the interaction between hexafluorobenzene and the heteroatom in molecules such as H2O, HCN and HF wherein the negative end of the dipole is directed toward the ji-system and aligned with the Ce axis of the ring. 

Br", 30 M for Cl" and 34 for I". On the other hand, the association constant for receptor A lacking the electron-deficient aromatic ring required for the anion-JT interaction, were too small to be determined using the same titration methodology. The enhanced affinity exhibited by receptor B over receptor A with the array of halides is attributed to the existence of anion- it interaction in the anion complex. 

Two other types of protons resonate in the region around 9-10 ppm some aromatic protons and some protons attached to heteroatoms like OH and NH. We will deal with NH and OH protons in the next section, but first we must look at some electron-deficient aromatic rings with distinctively large shifts. 

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