Pyrrole localization energies

Brown has also predicted, from localization energy calculations, that pyrrole and glyoxaline should react with radicals mainly at the 2-position, whereas pyrazole should be most reactive at the 3-position. Browm and Heffernan s calculation that the orientation in pyrimidine substitution should be 4 > 2 > 5 is in agreement with the results from the p-nitrophenylation of pyrimidine. ... 

Alekseeva et al. (1972b) have carried out a comparison of calculated values of localization energies and free valency indices of pyrrolo[ 1,2-a] imidazole, pyrrolo[l,2-a] benzimidazole and indolizine. In all these molecules the a-position of the pyrrole ring is calculated to be more reactive than the / -position. The free valence indices increase in the order indolizine < pyrrolobenzimidazole < pyrroloimidazole, which is also the order of increasing basicity. 

As stated already, the observed orientations of electrophilic substitution can be accounted for, in the pyrrole molecule, in terms of electrophilic localization energies, without invoking an auxiliary inductive parameter, for positive values of A. However, pyrrole is very reactive, and for this reason orientation would be expected to follow 7r-electron densities. These fall into line (when the acceptable value A = + 2 is used) if A >0 19. The orientation of electrophilic substitution is satisfactorily accounted foi s when A = 2 and A = 0 25. 

HMO calculations, based on localized polyenes instead of isolated alkenes, can account for the heats of atomization of furan (41.64 observed, 41.69 eV) and of dibenzofuran (109.09 observed, 108.92 eV). For resonance energies (quoted as resonance energy per electron, REPE) they give furan, 0.007 oxepin, -0.006 benzo[6]furan, 0.036 benzo[c]furan, 0.002 and dibenzofuran, 0.047 /3 (72T3657). That furan emerges as hardly more aromatic than a diene while pyrrole (REPE 0.039/3) is clearly aromatic is in line with other results, including those from MINDO/3 and topological methods (see Section... 

HOMO 1,2-dimethoxyacetylene comparable to the FMO changes for acetylene addition to pyrrole. As stated previously, the addition of acetylene to the pyrrolium ion should be examined separately because of its charge the full localization of double bonds on the pyrrole ring (an ideal diene for a Diels-Alder reaction), and the charge in the pyrrolium cation result in an exceptionally low LUMO energy. 

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