Electrophilic aromatic substitution derivatives, process

Hence the positional selectivity is different from that of the furan additions to 417 (Scheme 6.90). Assuming diradical intermediates for these reactions [9], the different types of products are not caused by the nature of the allene double bonds of 417 and 450 but by the properties of the allyl radical subunits in the six-membered rings of the intermediates. Also N-tert-butoxycarbonylpyrrole intercepted 450 in a [4 + 2]-cycloaddition and brought about 455 in 29% yield. Pyrrole itself and N-methylpyr-role furnished their substituted derivatives of type 456 in 69 and 79% yield [155, 171b]. Possibly, these processes are electrophilic aromatic substitutions with 450 acting as electrophile, as has been suggested for the conversion of 417 into 442 by pyrrole (Scheme 6.90). 

For electrogenerated sulfenium cations, not only the addition to multiple carbon-carbon bonds was shown but also examples of electrophilic aromatic substitution. Carried out in a low-nucleophiKc media (CH2CI2), this process provides a new route to arylthio derivatives allowing methylthiation of diverse phenols and... 

Trifluoromethyl)benzene (benzotrifluoride, 15) was the first organic fluoride to incorporate a trifluoromethyl group. By a standard nitration process, it formed l-nitro-3-(trifluoromethyl)-benzene (16) which was reduced to the 1-amino derivative, 17. This we a-directive influence on electrophilic aromatic substitution contrasted with that for fluorobenzene, which gave 4-and 2-nitro products. 

The primary method of direct palladation begins with Pd and proceeds by either electrophilic aromatic substitution or oxidative addition of an arene C-H bond (equation 2) In both cases, loss of H-X leads to an aryl-Pd-X derivative. Simple arenes can undergo palladation, but lead to isomers in the absence of a strongly directing substituent. The process is usually done in a stepwise manner, with isolation of the aryl-Pd-X intermediate. It is not easily made catalytic various reoxidation recipes are used for conversion of Pd° to Pd in other applications, but none has been found satisfactory here. 

Although a good start, these reports underscored the fact that attaching two disparate aromatics to isatin in a single step based on electrophilic aromatic substitution would be quite challenging to accomplish in a reasonable yield. Moreover, since an oxazole and a phenol have far different reactivities, this approach might not work at all. How, then, should one proceed on the basis of this information The answer that these researchers devised was based on the hypothesis that if they could form a compound such as 38 first, then perhaps the second aromatic moiety could be introduced through this same acid-catalyzed eletrophilic aromatic substitution process that Baeyer and Olah had used so productively. As shown in Scheme 3, this idea translated into the initial nucleophilic addition of a dianion derived from oxazole 34 to the more reactive... 

Electrophilic attack on coordinated polyenyl ligands is uncommon, but electrophilic attack on the Ti -cyclopentadienyl ligands in certain complexes has become synthetically valuable. For example, the acetylation of ferrocene (Equation 12.75) is a reliable Friedel-Crafts process " and generates derivatives used to make many ferrocenyl ligands. Similar electrophilic aromatic substitution reactions of haloboranes on ferrocene (shown in Equation 12.76) and on the anion generated from deprotonation of tungstenocene dihydride (Equation 12.77) leads to borylcyclopentadienyl complexes. " ... 

In another study, Arkin and Hazer modified the PHA-Cl into quaternary ammonium salts, thiosulfate moieties and phenyl derivatives. In addition, they cross-linked the modified PHA-Cl with benzene by electrophilic aromatic substitution using a Friedel-Crafts reaction. The random composition of PHA-Cl was calculated from its H NMR spectrum by comparing the relative peak areas of the methine protons on the polymer backbone. Hence, increased chlorination of the methyl protons caused the peak of methine protons to be moved further downfield. In addition, the PHA-Cl mole fractions were calculated by comparing the peak areas of protons on chlorinated a-carbons and protons on p-carbons. Samsuddin et al. ° described a process for the direct fluorination of PHBHHx at elevated pressure in the... 

Electrophilic aromatic substitution reactions are common with phenols and in these processes, the hydroxyl group is retained. However, as pointed out above, preparation of suitable derivatives can induce loss of that hydroxyl group too. 

A novel [3-I-3]-benzannulation process occurs in the coupling of alkynylcarbene complexes (e.g., 160, Scheme 17.27) and heteroaromatic aldehyde-derived imines (e.g., 201) [83]. Furan, benzofuran, and N-substituted indole derivatives led to benzannulation products, heteroaromatic-fused benzenes (e.g., 204). The mechanism proposed involves electrophilic aromatic substitution at the 3-position of the heterocyclic ring, resulting in the complex 202, followed by a simultaneous 1,2-shift of the metal and nucleophilic addition to the iminium salt. 

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