Of mesomeric betaines

Mesomeric betaine structure 7, which is a rough simplification of the real situation indescribable by a single structure, represents another degree of saturation. The real structure of mesomeric betaines is a resonance hybrid of several dipolar structures <1977T3203>. Additional unsymmetrical substitution or aza substitution increases the number of possible distinct dipolar structures. 

No single structure can uniquely describe the bonding of mesomeric betaines and their true structure is a resonance hybrid of several dipolar structures. The possible dipolar structures for basic compounds of this class were discussed in CHEC-II(1996) <1996CHEC-II(8)747>. 

Nitration of a series of mesomeric betaines was extensively studied in connection with their potential use as explosives (Scheme 3). Nitration of l,2,3-triazolo[2,l- ]benzotriazole 74 can be achieved selectively, occurring first at the 7-position which is followed by nitration at the 3- and 5-positions. Thus, nitration with 45% nitric acid gives a mixture of 7-nitro derivative 75 (39%) and dinitro derivative 76 (58%), while 70% nitric acid yields a mixture of 3,7- (52%), 5,7- (23%), and 3,5-dinitro (5%) isomers 76-78. Clean trinitration to 3,5,7-trinitro-l,2,3-triazolo[2,l- ]benzotriazole 79... 

Pyrazolo[l,2- ][l,2,3]triazole mesomeric betaines are generally available by an electrophilic attack of singlet nitrenes on the pyrazole nitrogen atom. When phthalazone derivative 252 is used and the nitrene is generated by reduction with triethyl phosphite, 59% yield of mesomeric betaine 253 is obtained (Equation 40) <2000T5523>. 

Since only atoms of opposite parity are covalently bonded, it follows that double bonds in these species must necessarily link starred and unstarred atoms. This fundamental feature of double bonding in alternant systems is an important factor governing the representation of mesomeric betaines. The relationship between mesomeric betaines and AHs will be developed later in the section. 

Spite of being represented by dipolar structures, these molecules are quite distinct from the family of mesomeric betaines that we have just described. 

Little is known about the chemistry of this class of mesomeric betaines. 

The effect of heteroatoms on the LUMO energy of mesomeric betaines can be treated similary, and this leads to an estimate of the HOMO-LUMO splitting. If E is the splitting in an AH anion, the change (A ) upon substitution of a heteroatom at position r is given by Eq. (2). 

The HOMO-LUMO separation of mesomeric betaines is relatively small. Heteroatoms and conjugated substituents at inactive positions will usually reduce this separation by lowering the energy of the LUMO without perturbing the HOMO. 

In the absence of precise thermochemical data it is difficult to draw firm conclusions about the relative stability of mesomeric betaines and their isomers (e.g., 213 and 215), but some estimate can be obtained by comparing their pK values. 

The structural requirements of the mesomeric betaines described in Section III endow these molecules with reactive -electron systems whose orbital symmetries are suitable for participation in a variety of pericyclic reactions. In particular, many betaines undergo 1,3-dipolar cycloaddition reactions giving stable adducts. Since these reactions are moderately exothermic, the transition state can be expected to occur early in the reaction and the magnitude of the frontier orbital interactions, as 1,3-dipole and 1,3-dipolarophile approach, can be expected to influence the energy of the transition state—and therefore the reaction rate and the structure of the product. This is the essence of frontier molecular orbital (EMO) theory, several accounts of which have been published. 16.317 application of the FMO method to the pericyclic reactions of mesomeric betaines has met with considerable success. The following section describes how the reactivity, electroselectivity, and regioselectivity of these molecules have been rationalized. 

Fig. 7. Exemplifying two distinct modes of symmetry allowed cycloaddition of mesomeric-betaines (a) conventional 1,3-dipolar cycloaddition (b) addition across peri positions.

For these deprotonation procedures, the solubility of the ionic species present in the reaction mixture is of crucial importance. Although their solubility in water and in organic solvents might vary to some extent with their structure, the problem of isolation of pure target compounds of type 1 may sometimes be serious. In this connection, two examples of mesomeric betaines 10 reported by Dorofeenko and co-workers (78KG944) have been rechecked (87JOC5009, Table V). 

Little is known about the chemistry of this class of mesomeric betaines. Catalytic reduction of the 2-nitro betaine (186 R = NO2, = H) gives... 

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