The Electronic Structure of Mesomeric Betaines

This review demonstrates that representatives of all four major classes of heterocyclic mesomeric betaines were isolated from natural sources. The profound differences in the electronic structures of these distinct classes can be realized by a closer look at the canonical formulae, the frontier orbital profile, the isoconjugate relationships, physico-organic properties, and the... 

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. 

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. 

Mesoionic compounds - These are defined as five-membered heterocycles that cannot be represented satisfactorily by any one covalent or polar structure and possess a sextet of electrons in association with the five atoms comprising the ring <1976AHC(19)1>. Two types of mesoionic compounds have been recognized and these are described as Type A and Type B. Mesoionic heterocycles are a subclass of heterocyclic mesomeric betaines <1985T2239>. For examples see Sections 2.41.2, 2.4.14, and 2.4.54. 

Heterocyclic mesomeric betaines 10 and 11, aza analogues of the A -ylide 9 (I, Scheme 2), are suitable for studying their behavior as dipoles, where the dipolar moiety contains more than four ir electrons. Moreover, their reactions with dipolarophiles should be a potentially attractive route for the synthesis of a variety of heterocyclic structures, and can also give entry into novel polycyclic ring systems. 

The best known mesoionic compounds have five-membered rings, and initially it was advocated by Baker, Ollis, Ramsden and other authors that only five-membered heterocycles which cannot be satisfactorily represented by any one covalent or ionic structure possessing a sextet of TT-electrons in association with the five atoms comprising the ring may be called "mesoionic". Here, following Katritzky, mesoionic means a mesomeric betaine. The first such compounds to be discovered were sydnones, followed by miinchnones and then by diazolones. In all these compounds the Z-type atom is part of a carbonyl group, and two Y-type atom chains separate two odd-numbered chains of X- and Z-atom chains. Only the main resonance structures are displayed in formulas (Figure 11). 

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... 

The betaine l,4-dimethyl-3,6-dioxo-l,2,4,5-tetrazin-l-ium-5(4//)-ide (3) has been prepared 25 a mesomeric structure with 6rr-electrons can be formulated. The X-ray structure analysis shows a planar ring with remarkably short C-0 bonds (1.215 A) which correspond to a pure C-0 double bond similar to aldehydes and ketones (1.215 A), The N-N bonds (1.310 A) are close to the N-N double bond (1.25 A) and the C-N bonds were found to be 1.365(1) A. Therefore, the structure of 3 is best represented by the mesomeric structure 3. 26... 

Ten mesomeric betaines, 430-432, of pyridinium azolates have been prepared from the corresponding pyridinium azole salts. Dipole moments, and NMR spectra. X-ray structures and theoretical calculations (MNDO) have been used to determine their molecular and electronic structures (87JOC5009). 

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