Electrophilic aromatic substitution symmetrical

An interesting example of a triple electrophilic aromatic substitution between an oxonium ion, generated from a trisubstituted dihydrofuran, and phloroglucinol was exploited for the total synthesis of the C3-symmetric xyloketal A, as depicted in the scheme below C06OL1427 06JOC1620>. 

Scheme 16. Comparison of TADDOL with BINOL and Future Goals. Derivatives of (R,R)-TADDOLs and of (P)- or (S)-BINOL, when employed in the same reaction, often give the same product enantiomer preferentially (cf. the Cj-symmetrical structural similarity). While TADDOLs are much easier to prepare and to modify, they are many orders of magnitude less acidic than BINOLs (TADDOLates are less polar ligands to a metal). Some TADDOL derivatives contain CPh2X groups which are labile to (undesired) nucleophilic substitution the dioxolane group of TADDOLs, on the other hand, is surprisingly stable to hydrolysis. BINOL (a naphthol derivative ) can be very sensitive to oxidation and (undesired) electrophilic aromatic substitution, and there are conditions under which it may racemize. Some goals to increase the usefulness of the TADDOL system are shown at the bottom of the Scheme other P derivatives, more acidic derivatives, reagents for enantioselective protonation, deprotonation,...

The spectroscopic manifestation of symmetry effects in cyclic hyperconjugation was reported in electron spin resonance (ESR) spectra. " Symmetric LUMOs show cooperative effects to hyperconjugative interactions, while interactions with antisymmetric LUMOs are cancelled by symmetry (Figure 8.28). Such effects can be extended to spin-paired molecules, Wheland intermediates of electrophilic aromatic substitution, metal cation/arene complexes, and n -cyclopentadieneylmetal compounds. ... 

Mono-substituted sumanenes 125 can easily be obtained by electrophilic aromatic substitution reaction [143]. However, this protocol is unsuitable for preparation of di- and trisubstituted sumanenes due to the low regioselectivity. Trisubstituted sumanenes, such as C3 symmetric triformylsumanene and its derivatives, were synthesized regioselectively by using suitable reaction intermediates syn-tri(norbomeno)benzenes (cf. Scheme 36) [144]. 

As regards the structure of the 7r-complex XLVIII, the calculations [136] did not detect a minimum on its PES. An analysis [98] of the results of a number of other nonempirical and semiempirical calculations on ion adducts in the reactions of electrophilic aromatic substitution (X = H,F,CH3) has led to the conclusion that generally the symmetric type XLVIII structures of Tc-complexes are not stable. This conclusion has been supported by semiempirical and ab initio calculations of the mechanisms governing the gas-phase reactions of nitrosation [137, 138] and nitration [139-141] of benzene. 

In earlier chapters we revealed how some reactive intermediates can be prepared, usually under special conditions rather different from those of the reaction under study, as a reassurance that some of these unlikely looking species can have real existence. Intermediates of this kind include the carboca-tion in the S l reaction (Chapter 17), the cations and anions in electrophilic (Chapter 22) and nucleophilic (Chapter 23) aromatic substitutions, and the enols and enolates in various reactions of carbonyl compounds (Chapters 21 and 26-29). We have also used labelling in this chapter to show that symmetrical intermediates are probably involved in, for example, nucleophilic aromatic substitution with a benzyne intermediate (Chapter 23). 

Type G syntheses are typified by the 1,3-dipolar cycloaddition reactions of nitrile sulfides with nitriles. Nitrile sulfides are reactive 1,3-dipoles and they are prepared as intermediates by the thermolysis of 5-substituted-l,3,4-oxathiazol-2-ones 102. The use of nitriles as dipolarophiles has resulted in a general method for the synthesis of 3,5-disubstituted-l,2,4-thiadiazoles 103 (Scheme 11). The thermolysis is performed at 190°C with an excess of the nitrile. The yields are moderate, but are satisfactory when aromatic nitrile sulfides interact with electrophilic nitriles. A common side reaction results from the decomposition of the nitrile sulfide to give a nitrile and sulfur. This nitrile then reacts with the nitrile sulfide to yield symmetrical 1,2,4-thiadiazoles <2004HOU277>. Excellent yields have been obtained when tosyl cyanide has been used as the acceptor molecule <1993JHC357>. 

Another approach for the preparation of either symmetrical or unsymmetrical iodonium salts used organolithium or organomercury compounds and (dichloroiodo)arenes [12]. The problem of the formation of unwanted isomers during reactions involving aromatic electrophilic substitution may also be overcome by the condensation of iodosylarenes with iodylarenes [12]. Several iodonium triflates were prepared in high yield from activated or mildly deactivated arenes with iodosylbenzene and triflic anhydride or triflic acid [13,14] or sulphur trioxide [15]. Some of these compounds are shown in Table 8.2. 

The benzo-l,3-dithiole derivative 153, a precursor of a 1,3-dithiolium cation, was used to synthesize substituted 155 and 3,4-disubstituted-pyrrole-2,5-dicarbaldehydes 156 (Scheme 14) <1996J(P1)2365>. Thus, in the aromatic electrophilic substitution reaction of pyrrole with 153, the symmetrical 2,5-disubstituted pyrrole 154 was obtained, from which, following functionalization of positions 3 and/or 4, then the final removal of benzo-l,3-dithiole moieties under hydrolytic conditions using the HgO-HBF4-DMSO, the corresponding 155 and 156 were formed, respectively (DMSO = dimethyl sulfoxide). 

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