Isoconjugate systems

Absorption Spectra of Isoconjugated Systems of the Indole Anion... 

It follows at once that all the rules derived above for even AHs apply equally well to isoconjugate systems containing heteroatoms. Essential single and double bonds are localized, the rules for aromaticity are unchanged, bond lengths in aromatic systems tend not to alternate while those in antiaromatic systems alternate, and compounds for which one can write... 

FIGURE 3.21. The total 7c-bond energies of an even AH and of an isoconjugate system containing hetero-atoms are the same (E,). 

We need consider only the case where the conjugated system is an AH, because it can be shown that the conclusions we reach will also hold qualitatively for isoconjugate systems containing heteroatoms. Let us then consider the effect of attaching two substituents R and S to atoms i and fc, respectively, in an even AH, T ... 

FIGURE 6.14. Orbital energies and transitions in (a) an even AH, and (b) an isoconjugated system containing a heteroatom. 

FIGURE 6.15. MO energies in an odd AH anion R and an isoconjugate system Rijei containing a heteroatom at an unstarred position. 

FIGURE 6.16. MO energies of an odd AH anion R and of an isoconjugate system Rhet containing a heteroatom of a starred position. 

The oxoaporphine alkaloids Teliglazine (126), Corunnine (127), Nandazurine (128), PO-3 (129), A-Methylliriodendronine (130), and A,(9-Dimethylliriodendronine (131) contain the 6-methyl-7-oxo-dibenzo [Je,g]quinolinium-l-olate ring system 125 which is isoconjugate with the l-methyl-7-methylene-7H-benzo[Je]anthracene anion (Scheme 46). Therefore, these alkaloids belong to class 1, i.e., heterocyclic mesomeric betaines isoconjugate with odd alternant hydrocarbon anions. Another... 

Hydroxytriphenylmethane derivatives give rise to anionic charge-resonance systems that are isoconjugate with the corresponding amino-substituted dyes. Thus, for example, the oxonol analogue of malachite green is benzaurine (Scheme 6.33). The quinonoid neutral form (6.184), which is pale yellow, produces a violet anion (6.185) on the addition of alkali in... 

Applying these rules in pericyclic reactions it has been shown and a generalization given that thermal reactiom occur via aromatic transition states while photochemical reactions proceed via antiaromatic transition state. A cyclic transition state is considered to be aromatic or isoconjugate with the corresponding aromatic system if the member of conjugated atoms and that of the n... 

A further example is given by consideration of the preferred conformation of anti n-butane. This system is isoconjugate to trans 2,3-dimethyl-butadiene or trans 2,3-divinylbutadiene. Now, trans 2,3-dimethyl-butadiene is expected to have the conformation shown below. 

There remain in this Section to be considered p-aminobenzyl-ideneaniline and />-aminoazobenzene and their derivatives. In these molecules two nitrogen basic centres are conjugated through a benzene ring, and the two systems are isoconjugate. 

In addition to these systems, application of the PMO method leads to the prediction of the possible existence of many new and interesting classes of betaines. This predictive approach can be demonstrated by considering perturbations of the 1,3-dipoles 500, which are isoconjugate with the... 

Two systems (246,247) will be discussed in this subsection. The nomenclature of these compounds, as of other borazaromatics, has been a point of controversy. Whereas the authors in this field generally followed the procedure of naming these compounds after the isoconjugate parent aromatic hydrocarbon with prefixes like bora and aza indicating the replacement of a methine group by these atoms, in Chemical Abstracts the principal names are derived by reference to the azaborine system. In this Section and in Section... 

The chemistry of thienoquinolines has been explored to a limited degree. Two types (378,379) will be described in this subsection. The pharmacological interest in thieno[2,3-Z>]quinoline (378) and derivatives thereof stems from their isosteric and isoelectronic resemblance to acridine furthermore, (378) constitutes the sulfur analog of furoquinoline (Section 3.17.2.1.5), which is the parent system of a number of alkaloids. Thieno[3,4-6]quinoline (379) is an o-quinonoidal heterocycle which is of interest both for theoretical reasons compared with its isoconjugate analogues and as a synthon in Diels-Alder reactions for the preparation of other condensed heterocycles. 

The philosophy of perturbation theory is to consider the whole group of reactants undergoing a given reaction as variations on a central theme. We have seen how this principle is applied to heteroatomic systems by regarding them as perturbed forms of the isoconjugate hydrocarbons likewise it is convenient to treat as many such molecules as possible in terms of a fixed kernel with varying substituents attached to it. This is indeed the procedure commonly followed in chemistry, confirming the view that chemistry is in effect an exercise in perturbation theory. Our next problem then is to consider how substituents may influence reactivity. 

The charge density is therefore not a measure of reactivity per se, but of the difference in reactivity between a given hetero-atomic system and the isoconjugate hydrocarbon. The expression... 

These arguments can of course be extended to heterocyclic systems by using the perturbation methods introduced by Longuet-Higgins (Section IX above). In general a heterocyclic system will be aromatic if and only if the isoconjugate hydrocarbon is aromatic. A similar perturbation treatment can also be used to show that odd monocyclic systems should be most stable if they contain An + 2 electrons this of course completes the derivation of Hiickel s rule. 

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