Aromatization energy

Ab initio electron correlated calculations of the equilibrium geometries, dipole moments, and static dipole polarizabilities were reported for oxadiazoles <1996JPC8752>. The various measures of delocalization in the five-membered heteroaromatic compounds were obtained from MO calculations at the HF/6-31G level and the application of natural bond orbital analysis and natural resonance theory. The hydrogen transfer and aromatic energies of these compounds were also calculated. These were compared to the relative ranking of aromaticity reported by J. P. Bean from a principal component analysis of other measures of aromaticity <1998JOC2497>. 

While the neutral 1,2,3,4-oxatriazoles (1) still await synthesis, some of their properties have been predicted by theoretical calculations. AMI calculations combined with a principal component analysis loading data from other related heteroaromatics have been used to estimate geometric characteristics, aromaticity, energy of formation, and N chemical shifts <90JPR885>. The oxatriazoles (1) and (7) and the 1,2,3,5-thiatriazoles, which also have not been prepared, are calculated to be in the group with the lowest classical and magnetic aromaticity. 

The aromatic energy differences between the aminopyridine and pyridinone imine form (jE yridine -Epyridineimine) can be found as detailed previously (see Section 2.04.4.2) (72JCS(P2)1295). The pyridinone imine form retains much aromaticity, but less so than in the case of the oxygen compounds, as can be seen from the following figures for the above quantity 2-aminopyridine/pyridin-2-one imine, 42 4-aminopyridine/pyridin-4-one imine, 61 2-aminoquinoline/quinolin-2-one imine, 21 1-aminoisoquinolinone/isoquinolin-l-one imine, 26 kJ mol-1. 

The sensitized photo-Fries rearrangement of 55 in benzene, toluene, and in concentrated polystyrene solution in dioxane is effectively quenched with biacetyl.4 This phenomenon must again be attributed to quenching of the aromatic energy donor, because in pure dioxane the photorearrangement of 55 is not influenced by biacetyl (vide supra). 

The it energy of a non-classical conjugated hydrocarbon can be compared directly with that of a classical analogue by the PMO method.14 Consider an even monocyclic polyene. This can be formed by fusion of methyl with an odd AH with one atom less. These components can also be fused to form an acyclic polyene. Comparison gives the aromatic energy of the cyclic system by difference. In this way we find that rings with An + 2 atoms are more stable, and those with An atoms less stable, than analogous acyclic compounds. The same method can be used for the bicyclic systems XVII, XIX, XXI, XXII, XXIII. The procedure is indicated below... 

Hence heptafulvene is non-aromatic but cyclooctatetraene and pentalene (XXI) are antiaromatic (aromatic energy —26/5). 

We have devoted two other papers to heteropentalenes, one to azapentalenes with one ring junction nitrogen atom (B series) and the other to azapentalenes with two ring junction nitrogen atoms (C series) (unpublished results from this group). In the first case we have focused our attention on tautomerism and aromaticity (energies and NICS). 

Another detailed treatment for pyridine, pyridazine, pyrimidine, pyrazine, 1,2,4,5-tetrazine, and a number of five-membered aromatic heterocycles is based on AMI semiempirical calculations <89H(28)ll35>. Dewar and Holder s values for the aromatic energies are all lower than Wiberg s benzene (28.3 kcal mol ), pyridine (25.6 kcal mol ), pyridazine (22.7 kcal mol ), pyrimidine (25.0 kcal mol ), pyrazine (24.6 kcal mol ), and 1,2,4,5-tetrazine (12.4 kcal mol ) again the tetrazine system seems to be least stabilized. 

The correlation of the bonding situation with the PE spectra has been discussed <95JST(347)57>. Dewar and Holder compared the aromatic energies of 1,2-, 1,3- and 1,4-diphosphinins according to their heats of formation obtained from AMI semiempirical methods <89H(28)ll35>. HMO calculations of phosphorus heterocycles with 2V -phosphorus were reported as early as 1968 <68RRC1623>. 

The ZFS parameters clearly suggest that the unpaired electrons are more delocalized and the central angle is more expanded in the 1-isomer (l-51a) than in the 2-isomer (2-51a). Taking into account the smaller loss of aromatization energy in the resonance structure for the 1 -isomer than the 2-isomer, it is obvious... 

Semiempirical and ab initio calculations have also been carried out to determine aromatic energies of thiophene and other heterocycles. This will be discussed later in the section on aromaticity. 

According to their spectral properties,2-amino- and 2-methylamino-benzothiazoles exist chiefly in the enamino-forms (38a and 39a). Their dissociation constants indicate that the amino-forms (38a and 39a) predominate over the imino-forms (38b and 39b) by factors of the order of 2—3 X 10 in 80% methyl cellosolve and 50% methanol. The free-energy difference between the tautomers of both (38) and (39) is found, from the relation AG = - iJrin Artaut to be approximately 4.4 kcal mol", which is indicative of the loss of aromatic energy in passing from the amino- into the imino-form. As expected, this energy loss is smaller than... 

A method for the synthesis of dialkyl carbonates makes use of l,l -carbonyl-bis(4-benzylidene-l,4-dihydropyridine) 993 as a reagent. The required activation energy for this reaction is provided by the aromatization energy of the 1,4-dihydropyridine system forming the 4-substituted pyridine 995. Di-tert-butyl carbonate 994 can be obtained in 66% yield [715]. 

From these data, it is evident that electrophilic activity of nitroarenes is a superposition of two factors—electronic effects of additional substituents in the ring and aromatization energy, since the addition process is connected with dearomatization of nitroarenes. 

In the intramolecular union of 1,3,5-hexatriene to give (5)-(8), the expression for the change in 7r-electron energy is the same in all cases, namely, (2.13). The 2—5 and 1—4 bond orders in 1,3,5-hexatriene are negative, so 5E would be positive in the cases (5) and (1). The 1—5 bond order is zero, which accounts for the fact that fulvene (6) behaves like a strained polyene. The 1—6 bond order is positive, which accounts for the aromatic energy of benzene (8). These calculations are discussed in detail in Chapter 3. 

It will be seen that the aromatic energy of naphthalene (4dp) is double that of the monocyclic systems (2dp) naphthalene should therefore be a bicyclic aromatic system, and its chemistry is consistent with this. Phenyl-butadiene and [10]annulene are predicted to be similar in stability in practice [10]annulene is unstable because it cannot exist in a planar geometry, due to mutual interference by the central hydrogen atoms (18). The internal methylene derivative (19) of (18) is, however, stable, being almost planar. 

FIGURE 4.6. Calculation of aromatic energies of individual rings in aromatic hydrocarbons. 

The ready conversion of heptafulvene (15) to methyltropylium (16) by acid is another example of an equilibrium involving nonalternant HCs. Heptafulvene is easily seen to be nonaromatic, being derived by intramolecular union between positions of like parity in octatetraene (17). The conversion of (15) to (16) is therefore assisted by the aromatic energy of the product (16). 

A large number of pericyclic reactions are now known in which the transition states have analogous cyclic delocalized structures and the ease with which they occur is determined in all cases by the aromaticity or antiaromaticity of the transition states. Since the aromatic energies of these are unrelated to the heats of reaction, the BEP principle fails. 

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