Aromaticity scale

In carbocyclic chemistry, rather firm dividing lines usually exist between aromatic, non-aromatic, and anti-aromatic compounds, while in heterocyclic chemistry enormous variations in the extent of aromatic character are displayed.52 Furthermore, there is an enormous number of potential heterocycles as compared to carbocycles, as will be detailed in section 3 of this review. The degree of aromaticity has classically been judged qualitatively in connection with the diene character of heterocycles manifested in Diels— Alder reactions or polymerizations. In this regard for instance, furan (42) is less aromatic than benzene (43), as is isoindole (44) compared to indole (45) (Scheme 18). Therefore, a quantitative aromaticity scale would be useful. 

Structures and nomenclature for the most important five-membered monocycles with one or more heteroatoms are depicted in Scheme 1. The aromaticity scale in five-membered heterocycles has been long debated.97-101 The decreasing order of aromaticity based on various criteria is (DRE values in kcal/ mol) benzene (22.6) > thiophene (6.5) > selenophene > pyrrole (5.3) > tellurophene > fur an (4.3). Pyrrole and furan have comparable ring strains (Scheme 38). The aromaticity of furan is still controversial 100 the NMR shielding by ring current estimated it at about 60% of the aromaticity of benzene, and other methods reviewed earlier102 estimated it at less than 20%. 

Scheme 38. Aromaticity Scales in Five-Membered Heterocycles...

It is clear, as Katritzky et al. [7, 8] and ourselves [9] have pointed out, that aromaticity cannot be described with a single parameter. It is possible to select a parameter and classify aromatic compounds according to it and this approach is correct if one bears in mind that the aromaticity scale thus obtained is valid only for the chosen parameter. One of the most successful is Schleyer s NICS (nuclear independent chemical shifts) [10-12], a criterion we have used to separate aromatic and antiaromatic compounds [13], Cyranski et al. [14] as well as Sadlej-Sosnowska [15] have tried, with moderate success, to find an agreement between these different points of view. 

Structures and nomenclature for the most important five-membered monocycles with one or more heteroatoms are depicted in Figure 24. The aromaticity scale in five-membered heterocycles has long been debated <2001PCA5486, 1996CHEC-II(2)471, 1984CHEC(4)28, 1974AHC(17)255, B-1986MI2>. 

This last remark gives us the opportunity to develop what is perhaps one of the main criticisms against the use of the word aromaticity if we postulate that aromaticity is an intrinsic, mysterious, non-observable characteristic of the so-called aromatic compounds, it becomes obvious that all we can do is to collect the various answers provided by this myth to each of the external perturbations, and nobody can say whether the aromaticity scales so obtained will be consistent or not. Moreover, it must be added that each individual way subconsciously create a picture of the myth which is convenient for him. If one accepts this line of argument, it is clear that aromaticity is not a scientific word at all, but fundamentally an esthetic one (an assertion which is supported by the euphonious character of the word aromaticity). 

A consideration of the results of molecular structural determinations of furan,194 pyrrole,196 thiophene,196 and selenophene197-198 shows that bond alternation as exemplified by the ratio (R) of the C-2-C-3 to C-3-C-4 bond lengths decreases in the order, furan (R = 0.950), selenophene (R = 0.956), pyrrole (R = 0.959), and thiophene (R=0.964). Whether this corresponds to an aromaticity scale,189 considering that the bond angles at the heteroatom are widely divergent, is a matter for debate. The X-ray structure determination199 of 1,2,5-triphenylphosphole is discussed later. 

Although there is a large amount of data available, it is still difficult to construct an accurate aromaticity scale for the following reasons ... 

A great deal of effort has gone into the construction of quantitative aromaticity scales, as summarized above. However, it is quite clear that there have been very considerable difficulties, which have arisen for a variety of reasons that include the following ... 

Stimulated by the first studies applying principal component analysis to aromaticity scales, Krygowski et al. studied 32 polybenzenoid compounds with five different aromaticity indices. Included as... 

More recently, Krygowski, Katritzky, and co-work-ers have reviewed this question. They point out again that mutual relationships between aromaticity scales depend strongly on the selection of molecules in the sample. Schleyer et al. found a collinearity between NICS and the aromatic stabilization energy (ASE) as well as ASE and the diamagnetic susceptibity exaltation A for a limited set of monocyclic five-membered rings with one heteroatom. 

From these aromaticity scales some important rules can be formulated (Putz etal., 2010) ... 

The above reactivity indices-based aromaticity scales are now computed within the presented quantum chemical schemes for a limited yet significant series of benzenoids containing the life atoms of Table 4.4 (see Table 4.5). The AIM of aromaticity scales are those of (Putz, 2010a) ... 

FIGURE 4.3 Electron ativity-based aromaticity scales of Tables 4.6 and 4.7 computed... 

FIGURE 4.4 The chemical hardness-based aromaticity scales of Tables 4.6 and 4.7 computed within semi-classical schemes in (a) and within ab initio schemes in (b), respectively (Putz, 2010a). 

TABLE 4.9 The Same Check for the Present Aromaticity Rules As in Table 4.8 - Yet Here for the Chemical Hardness Based-Aromaticity Scale (Putz, 2010a)... 

For the atomic level, the experimental values based on the ionization potential and electron affinity definitions for electronegativity and chemical hardness were considered, see Eqs. (3.346) and (3.362), respectively. Nevertheless, the AIM level was formed by appropriate averaging of atoms-in-molecule summation for each of the considered reactivity indices, see Eqs. (4.15b), (3.252) and (3.248), and along of their MOL counterparts of Eqs. (4.15c), (3.346), and (3.362) the polarizability-, electronegativity- and chemical hardness- based aromaticity definitions were formulated with the associate qualitative trends established by Eqs. (4.15a), (4.16), and (4.17), respectively. Yet, for MOL level of computations all major quantum chemical methods for orbital spectra computation were considered, and implemented in the current application for some basics. Because of the quantum observable character of polarizability the related aromaticity scale was considered as benchmark for actual study and it offered the possibility in formulating... 

From quantum computational perspective, the consecrated Hartree-Fock method seems to get more marks in fulfillment of above Aromal-to-5 rules, cumulated for electronegativity and chemical hardness based- aromaticity scales it leads with the important idea the correlation effects are not determinant in aromaticity phenomenology, an idea confirmed also by the fact the density functional without exchange and correlation produces not-negligible fits with Aromal, 2, and 4 rules in electronegativity framework (Putz, 2010a). 

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