Aromaticity systems differences

The independence of tt on the molecular system endows it with the characteristic of a substituent constant. This property is a direct consequence of the first approximation, namely that it reflects only the different solvation of substituent X compared to a reference substituent H. The validity of this approximation can be tested by obtaining tt from different molecular systems. It is well known (112) that irx values derived from aromatic systems differ from those derived from aliphatic systems tt The following table presents a few selected examples (112j>T ... 

The results of the derivation (which is reproduced in Appendix A) are summarized in Figure 7. This figure applies to both reactive and resonance stabilized (such as benzene) systems. The compounds A and B are the reactant and product in a pericyclic reaction, or the two equivalent Kekule structures in an aromatic system. The parameter t, is the reaction coordinate in a pericyclic reaction or the coordinate interchanging two Kekule structures in aromatic (and antiaromatic) systems. The avoided crossing model [26-28] predicts that the two eigenfunctions of the two-state system may be fomred by in-phase and out-of-phase combinations of the noninteracting basic states A) and B). State A) differs from B) by the spin-pairing scheme. 

The significance of frontier electron densities is limited to the orientation of substitution for a given aromatic system, but this approach has been developed to give two more complex reactivity indices termed superdelocalizabilities and Z values, which indicate the relative reactivities of different aromatic systems. 

Fluorocylatwn of enarnines and enamides has been intensively studied by different groups [78, 79, 80 SI] The effectiveness of this particular electrophilic substitution reaction becomes obvious when the nitrogen atom of the enamine moiety is engaged in an aromatic system [82 S3] or when the olefinic system is part of an aromatic nucleus [84] (equations 37 and 38) A further extension of this reaction is demonstrated by the tnfluoracetylation of aldehyde dialkyl hydrazones [S5 86] (equation 39)... 

Trifluoromethanesulfonic (triflic) anhydride is commercially available or can be prepared easily by the reaction of triflic acid with phosphorus pentoxide [66] This moderately hygroscopic colorless liquid is a useful reagent for the preparation of various organic derivatives of triflic acid A large variety of organic ionic triflates can be prepared from triflic anhydride A recent example is the preparation of unusual oxo-bridged dicatiomc salts of different types [SS, 89, 90, 91, 92, 93] (equations 38-44) Stabilized dication ether salts of the Huckel aromatic system and some other systems (equations 38 and 39) can be prepared in one step by the... 

In the benzene series, an approximately linear relationship has been obtained between the chemical shifts of the para-hydrogen in substituted benzenes and Hammett s a-values of the substituents. Attempts have been made, especially by Taft, ° to use the chemical shifts as a quantitative characteristic of the substituent. It is more difficult to correlate the chemical shifts of thiophenes with chemical reactivity data since few quantitative chemical data are available (cf. Section VI,A). Comparing the chemical shifts of the 5-hydrogen in 2-substituted thiophenes and the parahydrogens in substituted benzenes, it is evident that although —I—M-substituents cause similar shifts, large differences are obtained for -j-M-substituents indicating that such substituents may have different effects on the reactivity of the two aromatic systems in question. Differences also... 

So-called aryl values have been introduced by Imoto et ai. - for the piu pose of systematizing the reactivities of different aromatic systems by means of the linear free-energy relationship, which however, could not be applied to the decomposition rates of heterocyclic acyl azides in toluene. ... 

The mechanisms of the electrophilic substitutions in the isoxazole nucleus have not yet been studied. They should not differ fundamentally from those usually accepted for the substitution of aromatic systems but the structural specificity of the isoxazole ring might give rise to some peculiarities, as recently specially discussed.One important point is that isoxazole shows a clearcut tendency to form coordination compounds. Just as pyridine and other azoles, isoxazoles coordinate with halogens and the salts of heavy metals, for example of cadmium,mercury,zinc. Such coordination... 

Other precatalysts that are structurally related to 71a have recently been described. Structures and references are given in Table 5. Complex 71c is obviously even more reactive than 71b. The variation in these complexes compared to the parent compound 71a appears to be mainly steric. In contrast, complexes 71d and 71e differ significantly in the electronic properties of the aromatic system. 

In connection with normal substituent constants it must finally be mentioned that S02Me was included in an elaborate statistical analysis of a large body of data relating to substituent effects in aromatic systems ". What are claimed to be normal substituent constants emerge as different from those determined by Yukawa and coworkers or in the case of t7p from the ordinary value. 

We hope that the results outlined in this review have demonstrated that the chemistry of the tetrahalogenobenzynes is sufficiently different from the chemistry of benzyne to be worthy of study. That four electron with drawing substituents are essential to the high reactivity of arynes in reactions with aromatic systems has been demonstrated by generating the isomeric trifluorobenzynes from the aryl-lithium compounds (135) and (136) in the presence of an excess of benzene 169>. Whereas tetra-fluorobenzyne reacts under similar conditions to give the 1,4-cycloadduct in greater than 50% yield the compounds (137) and (138) are produced in 16% and ca. 1.5% yields respectively. 

Moving on to multisubstituted aromatic systems, the real value of Table 5.4 soon becomes apparent. In dealing with such systems, it will not be long before you encounter a 1,4 di-substituted benzene ring. This substitution pattern (along with the 1,2 symmetrically di-substituted systems) gives rise to an NMR phenomenon that merits some explanation - that of chemical and magnetic equivalence and the difference between them. Consider the 1,4 di-substituted aromatic compound shown in Structure 5.1. 

Some substituents induce remarkably different electronic behaviors on the same aromatic system (8). Let us consider, for example, the actions of substituents on an aromatic electron system. Some substituents have a tendency to enrich their electronic population (acceptors), while others will give away some of it (donors). Traditionaly, quantum chemists used to distinguish between long range (mesomeric) effects, mainly u in nature, and short range (inductive) effects, mainly a. The nonlinear behavior of a monosubstituted molecule can be accounted for in terms of the x electron dipole moment. Examples of donor and acceptor substituents can be seen on figure 1. 

Certainly, the role of photochemical excitation may be displayed in different ways19 and even in those cases when no aromatic system is formed, topologically... 

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