Pyrrole Stabilization energy

The amino form is usually much more favored in the equilibrium between amino and imino forms than is the hydroxy form in the corresponding keto-enol equilibrium. Grab and XJtzinger suggest that in the case of a-amino- and a-hydroxy-pyrroles, structure 89 increases the mesomeric stabilization and thus offsets the loss of pyrrole resonance energy, but the increase due to structure 90 is not sufficient to offset this loss. Similar reasoning may apply to furans and... 

The planar form of phosphole is a first-order saddle point on the potential energy surface, 16—24 kcal/ mol above the minimum (at different levels of the theory). ° (The calculated barriers are the highest at the HF level, which underestimates aromatic stabilization of the planar saddle point, while the MP2 results are at the low end.) It has been demonstrated by calculation of the NMR properties, structural parameters, ° and geometric aromaticity indices as the Bird index ° and the BDSHRT, ° as well as the stabilization energies (with planarized phosphorus in the reference structures) ° and NIGS values ° that the planar form of phosphole has an even larger aromaticity than pyrrole or thiophene. 

The following important conclusions can be drawn from the above results [88JST(163)173]. First, the values of [2A ]n are nearly equal for furan and pyrrole hence the correct aromaticity trend can be ascertained only if the [XAE], contributions are also taken into account. Thus, the relative aromatic character of the compounds under discussion is determined by the sum of the stabilizing effects of the two electron interactions. These are the stabilization energy AE, referring to the interaction be-... 

Pyrrole is believed to be more aromatic than furan, with an aromatic stabilization energy estimated to be 100-130 kj mole-1, thus only with such extremely powerful dienophiles as tetrakis(trifluoromethyl)Dewar thiophene were the IEDA adducts isolated. Therefore, several attempts to achieve an IEDAR with pyrroles using high pressure have been made [9-14]. 

We have used a different approach to compare the aromaticities of phosphole (8) and pyrrole (10) [23, 24], From literature data on derivatives of 8 and 9 it is known that the inversion barrier of phosphole is about 67 kJ mol-1 (70.2 kJ mol-1 at the B3LYP/aug-cc-pVTZ level) [25] while that of tetrahydrophosphole amounts to 163 kJ mol-1. This is explained by the fact that the planar transition state of 8 is highly aromatic. Pyrrole (10) is planar and pyrrolidine has a calculated inversion barrier of 15-17 kJ mol-1. Several aromaticity indices were used in this study, based on different criteria of aromaticity energetic (aromatic stabilization energy, ASE), geometric (harmonic oscillator model of aromaticity, HOMA, and /5), and magnetic (NICS). 

Another theoretical criterion applied to estimation of aromaticity of homo- and heteroaromatic ring system is aromatic stabilization energy (ASE). Based on this approach, the aromatic sequence of five-membered ring systems (ASE in kcal mol-1) is pyrrole (20.6) > thiophene (18.6) > selenophene (16.7) > phosphole (3.2) [29], According to geometric criterion HOMA, based on the harmonic oscillator model [30-33], thiophene is more aromatic than pyrrole and the decreasing order of aromaticity is thiophene (0.891) > pyrrole (0.879) > selenophene (0.877) > furan (0.298) > phosphole (0.236) [29],... 

On the other hand, photogenerated closed-ring isomers of diarylethenes with pyrrole, indole, or phenyl rings, which have rather high aromatic stabilization energy, are thermally unstable.1221 The photogenerated, blue, closed-ring isomer of l,2-bis(2-cyano-l,5-dimethyl-4-pyrrolyl)perfluorocyclopentene 11a disappeared in 37 s (= t1/2 ) at 25 °C. 

Regarding the closed-ring isomers, the difference in behavior between those diarylethenes with furan, thiophene, or thiazole rings and those with pyrrole, indole, or phenyl rings agrees well with the theoretical prediction that the thermal stability depends on the aromatic stabilization energy of the aryl group.1201... 

Using MP2/6-31G calculations, the aromatic stabilization energy (ASE) has been obtained from the energies of isodesmic reactions (Equation 1) for many derivatives <1995JST57>. Thus, for arsole, the heat of bond separation reaction is calculated to be 16.63 kcalmoU. This is ca. 36% of the corresponding value for pyrrole. 

Recently, it was found that poly(phospholes) provide an even better perspective for such devices (06CRV4681). Indeed, the calculated aromaticity of the unsubstituted phosphole is quite low in comparison with that of pyrrole in terms of aromatic stabilization energy (ASE) and nucleus-independent chemical shift (NICS) pyrrole, ASE = 25.5 (kcal/mol), NICS =—15.1 (ppm) phosphole, ASE 7.0 (kcal/mol), NICS = -5.3 (ppm) (06CRV4681). 

Figure 7 The variation of (a) NICS(1) and (b) ASE with the number and position of ring nitrogen atoms in pyrrole and its aza-derivatives. ASE, aromatic stabilization energy NICS, nucleus-independent chemical shift. Reprinted with permission from Ramsden (20i0T40i). Copyright 2009 Elsevier Ltd.

The somewhat similar, but nonaromatic, compounds, 1,2- and 1,4-dihydropyridine, are both much stronger bases, which undergo protonation as also shown in Scheme 10. The differences between the basicities of pyrrole and the model compounds can be used to calculate the aromatic stabilization energy of pyrrole, according to the method of Scheme 11... 

Beside the bigger size of the phosphorus atom, as compared to that of nitrogen, the lack of aromaticity is due to the P-pyramide the criterion of coplanarity is not fulfilled and so the lone electron pair of the phosphorus cannot overlap with the pz orbitals of the sp2 carbon atoms (Fig. 2). While in the case of pyrrole, the aromatic stabilization covers the energy requirement of planarization, in the case of phospholes, there is a bigger barrier for the inversion. 

We can draw Frost circles (see Section 2.9.3) to show the relative energies of the molecular orbitals for pyridine and pyrrole. The picture for pyridine is essentially the same as for benzene, six jt electrons forming an energetically favourable closed shell (Figure 11.1). For pyrrole, we also get a closed shell, and there is considerable aromatic stabilization over electrons in the six atomic orbitals. 

Stabilization (AE ) and Destabilization (A 4f) Energies (in kcal/mol) Obtained for Furan, Pyrrole, Thiophene, and Benzene at the STO-3G (STO-3G for Thiophene) Computational Level [88JST(163)173]... 

The differences in aromaticity follow the results of theoretical analyses on the acidity of the NH proton of the pyrrole fragment of furo[ ]pyrroles <2000PJC207> and are nicely reflected in the observed stability of both systems. The total energy difference between methyl 4f/-furo[3,2- ]pyrrole-5-carboxylate 8a and methyl 6f/-furo[2,3- ]pyr-role-5-carboxylate 31a is rather small (—7.2 kj mol ) indicating the higher stability of the former system. However, if the increase of energy of the appropriate anions is compared (relative to the parent molecules), then it indicates that formation of 67/-furo[2,3-7]pyrrole-5-carboxylate anion is much easier (by —22.5 kJ mol ) than formation of 4//-furo[3,2- ]pyrrole-5-carboxylate anion. 

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