Isoconjugate molecules

We can extend the above theoretical approach to isoconjugate molecules such as acrolein and glyoxal. Arguing as before, we conclude that the relative order of conformational stability will be gauche > tram > cis if pi nonbonded interactions dictate the preferred conformation, cis > gauche > trans if sigma interactions are dominant, and trans > gauche > cis if steric effects are the most important factor. 

To a first approximation, the 7r-electron distributions in two isoconjugate molecules are the same the 7r-electron densities and bond orders will therefore be approximately the same in an alternant heteroatomic molecule as in the isoconjugate AH. The 7T energy ( J of the former can then be calculated in terms of that (( J0) °f f e AH from Eq. (30) ... 

The discussion of radical cations (see Fig. 7.7) indicates that the energy of union of an odd AH radical and an odd AH cation to form a radical cation is just half the energy of union of the two corresponding odd AH radicals to form a neutral AH. As long as the resulting AH is alternant, the rules for aromaticity for it and for the radical cation derived from it will be the same and it is easily shown that this is also true for isoconjugate molecules containing heteroatoms. The course of pericyclic reactions where the transition... 

In our discussion of pi nonbonded attractive interactions we have sought to provide representative cases where these interactions obtain. We can further systematize the mass of experimental evidence regarding conformational preferences by seeking analogies among apparently unrelated molecules and geometries. One fruitful approach to this task involves the concept of the isoconjugate series exemplified 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. 

So far, this review has described betaine molecules that are isoconjugate with odd AH anions. We now consider betaines isoconjugate with even AH anions. 

A rigorous theoretical treatment of the non-alternant and heterocyclic indolizine is extremely difficult and, even for the related isoconjugate hydrocarbon, far from conclusive. Many questions, however, in which experimentalists are interested may be answered in a satisfactory way on the basis of a perturbational treatment. This approach has been used for a discussion of the electronic spectra of indolizine and some azaindolizines (63JCS3999). Following first-order PMO theory the 7r-stabilization which follows from aza substitution at the different positions of the model molecule depends on the ir-electron density qt as well as the change in electronegativity Sat (B-75MI30801). The perturbations caused by aza substitution of the indenyl anion are depicted in Scheme 1. 

Scheme 22. Definition of a Reference Structure, R, Which Is a. T-Isoconjugate to a Cyclic Molecule (CM) but with a Degree of Conjugation of an Open-Chain Polyene...

The difference in w energy between the isoconjugate heteroatomic molecule and the hydrocarbon is given by ... 

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. 

Two molecules having the same secular determinant (if all the atoms in conjugation are replaced by carbon atoms) are said to be isoconjugate. ... 

This chapter describes heterocyclic systems which are isoconjugate with the pentalenyl dianion (13) and which cannot be represented by classical Kekule structures. Molecules of this type (e.g. 9-12) can only be represented by dipolar structures, or possibly by structures involving tetracovalent sulfur atoms, and are described as conjugated mesomeric betaines or non-classical heteropentalenes. The structure and chemistry of this type of heterocycle have been the subject of two earlier reviews (77T3203, 77HC(30)317) and their classification as conjugated mesomeric betaines and relationship to other dipolar heterocycles has also been discussed (83UP43700). 

It has been demonstrated that there are 10 possible general types of neutral heteropen-talene which are isoconjugate with the pentalenyl dianion (13). Four of these general types (1-4) are mesomeric betaines and they are conveniently described as heteropentalene mesomeric betaines of type A (1), type B (2), type C (3) and type D (4) (77T3203). The difference between these four classes can be appreciated by considering the structures (5)-(8) in which a-h represent suitably substituted carbon or heteroatoms and the superscripts indicate the origin of the 10 7r-electrons. Structures (9)-(12) are known examples of these classes of molecule. 

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