Heteroatom, aromatic ring

Aromatic character in isoxazoles has been studied from a number of viewpoints, and these studies indicate that although isoxazole may be formally considered an aromatic system, the disposition of the ring heteroatoms modifies this character to an appreciable extent. From a qualitative viewpoint, thermal stability and electrophilic attack at the 4-position may be considered consistent with an aromatic character. Furthermore, NMR chemical shifts of the ring protons are consistent with those of an aromatic compound. References related to these studies may be found in Section 4.16.2.3.4. 

Thiepin, as a seven-membered conjugated system with sulfur as heteroatom, is a member of the 8 7t-electron heteroannulenes which are antiaroinatic according to Hiickel s rule. In contrast to oxepin, thiepin is not stable at room temperature and no valence isomerism with an arene sulfide has been observed. Stable thiepins are obtained only when two bulky substituents, e.g. /ert-butyl, are introduced into positions 2 and 7. In benzothiepins the annellation effect of the aromatic rings contributes decisively to the stability of these compounds stability increases with an increasing number of fused benzene rings. 

Nearly all of the polymers produced by step-growth polymerization contain heteroatoms and/or aromatic rings in the backbone. One exception is polymers produced from acyclic diene metathesis (ADMET) polymerization.22 Hydrocarbon polymers with carbon-carbon double bonds are readily produced using ADMET polymerization techniques. Polyesters, polycarbonates, polyamides, and polyurethanes can be produced from aliphatic monomers with appropriate functional groups (Fig. 1.1). In these aliphatic polymers, the concentration of the linking groups (ester, carbonate, amide, or urethane) in the backbone greatly influences the physical properties. 

When a heteroatom, such as N, O, or a halogen, is present in a molecule containing an aromatic ring or a double bond, lithiation is usually quite regio-selective. The lithium usually bonds with the sp carbon closest to the hetero atom, probably because the attacking species coordinates with the hetero atom. Such reactions with compounds such as anisole are often called directed metala-tions. In the case of aromatic rings, this means attack at the ortho position.Two examples are... 

Iron (III) chloride is a common catalyst used in electrophilic aromatic substitutions. In addition to those applications outlined above for the construction of aromatic C-C bonds, such salts have also been used for the introduction of heteroatom-based functional groups at the aromatic ring [47]. 

Formation of the least-hindered alkylzirconium complex during the course of the hydrozirconation-isomerization process of alkenes can be altered by the presence of aromatic rings, additional conjugative double bonds, chelating heteroatoms [81, 94] or groups that can be lost by elimination [95, 96]. 

Molecular modelling (AMI) indicated that the bridge hetero-atom separations in 137 were as follows N-N = 8A, N-0 = 4.6A, 0-0 = 2.7 A. In addition, the invertomer preference of the N-benzyl groups position them over the aromatic rings thereby ensuring that the lone pairs on the heteroatoms are concentrated within the cavity section of the molecule. 

Chiral 10 to 12-membered nitrogen and oxygen heterocycles, fused to isoxazo-line rings have been prepared with high regio- and stereoselectivity by INOCs of tethered N- and O-allyl carbohydrate derivatives. The use of a -Y-Ar-CH2 tether, containing a 1,2-disubstituted aromatic ring between the heteroatom attached to... 

The second salient feature of heterocycles is the marked activation at positions a- and y- to the heteroatom. For N-containing heterocycles, the presence of the N-atom polarizes the aromatic ring, thereby activating the a and y positions, making them more prone to nucleophilic attack. The order of SNAr displacement of heteroaryl halides with EtO is [14] ... 

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