Five-Membered Meso-ionic Heterocycles of Type

Five-Membered Meso-ionic Heterocycles of Type B 

This second class of five-membered heterocyclic meso-ionic compounds is represented by the type formula 14 20. So far, only eight representatives (Table II) of type B have been described, whereas acceptable combinations of the groupings a, b, c, d, e, and f derived from suitably substituted carbon, nitrogen, oxygen, or sulfur atoms lead, in principle, to 84 possibilities. However, not all these 84 possible structures are necess y well based because valence tautomerism (see Section VI) might well be associated with thermodynamic preference for the acyclic covalent valence isomer (46) rather than the cyclic meso-ionic alternative (20). 

Forty-four five-membered heterocycles of type A (13, 19) have been described (Table I). If the atoms or groups a, b, c, d, e, and f are selected from suitably substituted carbon, nitrogen, oxygen, and sulfur atoms, then with these conditions it can be shown that 144 structural possibilities are provided by the general formula 19. The number of structural possibilities can be deduced in various ways, but a very useful approach is to regard type A meso-ionic molecules (19) as being derived by the union ( —u— ) of 1,3-dipoles (34) and heterocumulenes (35). 

In this approach, the term union is used in the sense defined by Dewar 18 union is a process in which two conjugated molecules combine in such a way that their two -systems unite into one larger one. For example, biphenyl can be formed by the union of two molecules of benzene  

The conditions given above for a, b,.c, d, e, and f provide nine 1,3-dipoles (36-44), which can be involved in union with ten heterocumulenes (e = C a and f variously chosen from O, S, 

The relation between the structures of the 44 known five-membered meso-ionic heterocycles (Table I) and the 1,3-dipoles (36—44) from which they can be formally derived by union is as follows  

---

IV. Five-Memb ed Meso-ionic Heterocycles of Type A V. Five-Membered Meso-ionic Heterocycles of Type B... 

IV. Five-Membered Meso-ionic Heterocycles of Type A... 

Regarding the valence tautomerism 20 46, which is, in principle, possible for five-membered meso-ionic heterocycles of type B, it may be noted that whereas the triketone (60) is obviously die more stable... 

It is now evident that it is desirable to divide five-membered meso-ionic heterocycles into two general classes type A and type B. This review is mainly concerned with the presently known 44 members of type A (Table I) and eight members of type B (Table II). Brief reference is made at the end of the review to various six-membered heterocycles and polycyclic systems that have been described as meso-ionic, but it is now firmly proposed that this practice should be discontinued. 

The betaine-type formula (6) has not been found to be acceptable for the general representation of meso-ionic heterocycles, but in view of the recent advocacy of the symbolism depicted in formula 7 some further comment is necessary. Although we originally used this symbolism in the representation of the sydnones, we subsequently recommended its replacement by 5 because this is in accord with current practice. Our present view is that the use of a special symbol (7) is not justified it is too vague to be useful, and in any case it can cause misunderstanding if formulas of type 8 with e=f are used as well as those of type 7 with e—f. The general formula 5 has much to commend it for discussion of type A and type B five-membered meso-ionic heterocycles and for the correlation of corresponding structural features of these two types (Tables I and II). 

III. The Two Types of Five-Membered Meso-ionic Heterocycles... 

One hundred and forty-four meso-ionic heterocycles of type A (13, 19) and 84 meso-ionic heterocycles of type B (14, 20) are possible. The numbers of these two types which are now known (Table I type A, 44 representatives) and (Table II type B, 8 representatives) encourage us to put forward the proposal that the term meso-ionic should in future be restricted to five-membered heterocycles belonging to type A (13, 19) and type B (14,20). This clear restriction upon the use of the term meso-ionic has obvious advantages. It still embraces 228 different classes of heterocycles with a common structural characteristic, and the many types of meso-ionic compounds included in the authoritative review by Ohta and Kato " are included. Needless to say, the restriction upon the definition of the term meso-ionic to five-mem red heterocycles of type A and type B still includes, for example, benz derivatives such as the compounds 67, 71, 110, 123, 133, 151, 206, 209, 226, 255, and 258. 

If heterocyclic chemists can be persuaded to restrict the use of the term meso-ionic only to five-membered heterocycles of type A and type B, then this wilt necessarily exclude some compounds that have... 

For the five-membered heterocycles (Fig. 1), it is important that the term meso-ionic is reserved for members of the general types 19 and 20. This point needs to be emphasized because recently several structures have been described as meso-ionic although they were in fact ylides 17 and 18. Petersen and Heitzer" have referred to Ae intermediate isolated... 

---