Electron rich aromatic rings

Phenols are polar compounds but less polar than alcohols They resem ble arylammes m having an electron rich aromatic ring... 

Phenylalanine and tryptophan have side chains that incorporate aromatic rings which are large and hydrophobic The aromatic portion of tryptophan is bicyclic which makes it larger than phenylalanine Tryptophan also has a more electron rich aromatic ring and is more polarizable than phenylalanine Its role is more specialized and it is less abundant m proteins than most of the other ammo acids... 

Pyrrole, furan, and thiophene, on the other hand, have electron-rich aromatic rings and are extremely reactive toward electrophilic aromatic substitution— rnore like phenol and aniline than benzene. Like benzene they have six tt electrons, but these tt electrons are delocalized over five atoms, not six, and ar e not held as strongly as those of benzene. Even when the ring atom is as electronegative as oxygen, substitution takes place readily. 

Aryl diazanium ions, ArNa, react with electron-rich aromatic rings, Ar H, to give azo dyes , ArN=NAr. A dye s color depends on its electronic structure and may change with substituents and solvent polarity. 

Step 3 of Figure 27.14 Third Cyclization The third cationic cyclization is somewhat unusual because it occurs with non-Markovnikov regiochemistry and gives a secondary cation at C13 rather than the alternative tertiary cation at C14. There is growing evidence, however, that the tertiary carbocation may in fact be formed initially and that the secondary cation arises by subsequent rearrangement. The secondary cation is probably stabilized in the enzyme pocket by the proximity of an electron-rich aromatic ring. 

The postulated mechanism for the reaction involves activation of the alkyne by jt-coordination to the cationic (IPr)Au% followed by direct nucleophilic attack by the electron-rich aromatic ring to form product 111. Alternatively, two 1,2-acetate migrations give the activated aUene complex, which can be cyclised to product 110 by nucleophilic attack of the aromatic ring on the activated aUene (Scheme 2.21) [92]. 

Expected structure-property trends from current data or prior experience (e.g., increasing lipophilicity leads to increasing clearance electron-rich aromatic ring in Series X leads to increasing clearance)... 

Interaction of Hg2 groups with electron-rich aromatic rings has been observed in the structure of [Hg2(C6Me6)](AlCl4)2 (r(Hg-Hg) 251.5 pm).406... 

The results obtained with different amines cannot be explained merely on the effects of amine basicity. Thus, to obtain complete hydrogenation of Q to DHQ, the basicity has to be tailored by other factors such as the steric hindrance of the amine and its electronic interaction with the catalyst active sites this seems to be favored by the presence of an electron-rich aromatic ring. Of note, the positive effect of substituted aromatic amines, with a 49% DHQ yield being obtained for ethylanilines, is independent of the substituent position of the alkyl group. 

A more recent approach, which also profits from the synthetic versatility of stabilized thionium ions, has been elaborated by Berard and Piras [22]. These authors observed that the cyclobutane thionium ions 1-76 obtained from the cyclopropyl phenyl sulfides 1-75 by treatment with pTsOH under anhydrous conditions can be trapped by an adjacent electron-rich aromatic ring to give the chromane derivatives 1-77 in good to excellent yields (Scheme 1.20). As expected, 1-77 were obtained as single diastereoisomers with a ds-orientation of the methyl and the phenylthio group as a consequence of steric constraints. 

Scheme 4.1 Oxidative cleavage of an electron-rich aromatic ring was a key component of Woodward s proposal for a biogenesis of the Strychnos alkaloids and a key step in his subsequent synthesis of strychnine...

An exploration of structural modifications on the activity of prolinol catalysts has been published <06T12264>. More electron-rich aromatic rings on the prolinol scaffold improve the activity in the epoxidation of a, 3-enones. The reaction of 10 with an enone and f-BuOOH provides the epoxy-ketones with moderate levels of enantioselectivity. 

The benzylic C-H activation has been effectively applied to the enantioselective synthesis of (+)-imperanene (Equation (16)).80 The key step was the Rh2(i -DOSP)4-catalyzed functionalization of the benzylic methyl C-H bond in arene 2. An impressive feature of this transformation was that both the carbenoid and substrate contained very electron-rich aromatic rings, which were compatible with the highly electrophilic carbenoids because they were still sterically protected. 

These substrates possess several oxidiz-able sites. Racemic laudanosine (71), for example, could be oxidized at nitrogen or in one or both of the electron-rich aromatic rings. In fact, it displays ve volta-metric peak potentials at 0.63, 0.81, 1.13, 1.30,andl.47 V(vsAg/AgN03). Yet,using potential control, it proved a simple matter to oxidize a dimethoxyaryl unit in the presence of the Ai-methylamine subunit. 

Pandey and co-workers have generated arene radical cations by PET from electron-rich aromatic rings [119]. The photoreaction is apparently initiated by single-electron transfer from the excited state of the arene to ground state 1,4-dicyanonaphthalene (DCN) in an aerated aqueous solution of acetonitrile. Intramolecular reaction with nucleophiles leads to anellated products regio-specifically. The author explains the regiospecifidty of the cyclization step from... 

A commonly used strategy for the higher nitration of phenolic substrates is to sulfonate the electron-rich aromatic ring before nitration. Sulfonic acid groups are electron withdrawing and... 

Wade expanded the 1971 hypothesis to incorporate metal hydrocarbon 7T complexes, electron-rich aromatic ring systems, and aspects of transition metal cluster compounds [a parallel that had previously been noted by Corbett 19) for cationic bismuth clusters]. Rudolph and Pretzer chose to emphasize the redox nature of the closo, nido, and arachno interconversions within a given size framework, and based the attendant opening of the deltahedron after reduction (diagonally downward from left to right in Fig. 1) on first- and second-order Jahn-Teller distortions 115, 123). Rudolph and Pretzer have also successfully utilized the author s approach to predict the most stable configuration of SB9H9 (1-25) 115) and other thiaboranes. 

The X-ray crystal structures (Figure 7) of 6-[Diquat] and 9-[Paraquat] demonstrate that complex formation is aided not only by [C—H O] hydrogen bonding and [N O] electrostatic interactions but also by charge transfer stabilization between the ir electron rich aromatic rings in the molecular receptors and the ir electron deficient bipyridinium rings in the substrates. 

Theme 3 recognizes electron-rich aromatic rings and is located to the right of Theme 1 between Tm4 and Tm5. The prototypical Theme 3 Motif... 

C-H activation at a primary benzylic site was the key step in very short syntheses of lig-nans 206 and 207 (Scheme 14.27) [138]. Even though both the substrate 203 and the vinyl-diazoacetate 204 contain very electron-rich aromatic rings, C-H activation to form 205 (43% yield and 91% ee) is still possible because the aromatic rings are sterically protected from electrophilic aromatic substitution by the carbenoid. Reduction of the ester in (S)-205 followed by global deprotection of the silyl ethers completes a highly efficient three-step asymmetric total synthesis of (-i-)-imperanene 206. Treatment of (R)-205 in a more elaborate synthetic sequence of a cascade Prins reaction/electrophilic substitution/lacto-nization results in the total synthesis of a related lignan, (-)-a-conidendrin 207. 

Y.S. Ding, C.Y. Shiue, J.S. Fowler, A.P. Wolf, A. Plevenaux, No carrier added (nca) aryl [F-18]fluorides via the nucleophilic aromatic-substitution of electron-rich aromatic rings, J. Fluor. Chem. 48 (1990) 189-205. 

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