Fully substituted arenes

Diprotonated, superelectrophilic intermediates were suggested to be involved in both conversions. Considering protonated aldehydes, benzal-dehyde gives a carboxonium ion that is significantly resonance stabilized and thus unreactive towards aromatic substrates such as o-dichlorobenzene or nitrobenzene. Pyridinecarboxaldehydes, however, show much higher electrophilic reactivities due to their ability to form via TV-protonation the superelectrophile (5, eq 8).10 A similar situation is seen in the hydroxyalkylation reactions of acetyl-substituted arenes. Acetophenone is fully protonated in excess triflic acid, but the resulting carboxonium ion (6) is... 

While the potential for these species in nucleophile substitution for chloride has been demonstrated, the processes have not been fully developed nor applied. The two-stage process of addition-substitution for chloride and arene detachment is exemplified for the iron system in equation (21). 

Many similarities exist between metallabenzenes and conventional arenes. Among these similarities are structural features such as ring planarity and the absence of bond length alternation, spectroscopic features such as downfield chemical shifts for ring protons, and chemical reactions such as electrophilic aromatic substitution and arene displacement from (arene)Mo(CO)3. All of these features, taken together, strongly support the thesis that metallabenzenes represent a new class of aromatic compounds, one in which metal d orbitals participate fully with carbon p orbitals in the formation of ring 7r-bonds. 

Due to the increased number of OH functionalities, 29 pyridino homologues of p-tert-butylcalix[8]arene 21 can exist one mono-, hepta-, and octa-substituted derivatives, four di-, five tri-, eight tetra-, five penta- and four hexasubstituted regioisomers. The products of the peutial alkylation of 21 with PicCl-HCl are hitherto unknown, while fully alkylated octakis[(2-pyridylmethyl)oxy]calix[8]arene 22 has been obtained in 24% yield by using conditions similar to those described for calix[6]arene 20. From solvent extraction data, Shinkai has shown that compounds 20 and 22 display a high affinity for UO " at elevated temperature (100°C, o-dichlorobenzene) [26]. 

New P-cyclodextrin ethers having the carbohydrate linked through 0-2 to calix[4]arene by way of o- and /7-(aminomethyl)benzyl alcohol bridges have been reported, and some of the complexing characteristics described. Complexes formed between metals and a related benzylically-linked otherwise fully methylated P-cyclodextrin bonded to 2,2 -bipyridyl were reported in a 1992 symposium. P-Cyclodextrins carrying alkyl substituents with terminal amino functions substituted with terephthaloyl acyl groups were examined. Intramolecular inclusion was observed, and in the case of the compound with a pentyl link this novel type of association was complete. ... 

The fully intermolecular variant of this concept has also been realized. A variety of propiolates were coupled with allenes to afford a regioselective aromatic cycloadduct (Scheme 2.12) [9]. A symmetric ester bearing an alkyne was coupled successfully with butyl allene to afford arene 46 in high yields. Unsymmetrical internal alkynes (i.e., methyl- and phenyl-substituted propiolates) were also coupled with butyl-, cyclohexyl-, and phenyl-substituted allenes in high yields (47 to 50, Scheme 2.12). The current scope of the reaction is confined to the use of activated alkyne substrates in this cycloaddition. 

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