The high 96 Heterocyclic Chemistry value of the dipole moment 3. The delocalization energies of a-pyrone and y-pyrone are very close 2. The aromaticity is related with the stability which, in turn, may be considered to be related with the reactivity. However, the reactivity may not be considered to be a criterion of aromaticity. The presence of heteroatom in the heteroaromatic system usually affects the reactivity.
The basic principles governing the degree and the type of reactivity in the heteroaromatic systems are : i Nucleophilic attack : The heteroatom oxygen, nitrogen or sulfur bonded to the carbon atom with multiple bond can accept shared pair of Ti-electrons and permits the attack of nucleophilic reagent. The attack of nucleophile is facilitated when heteroatom is positively charged Fig. Aromatic Heterocycles 97 Fig. Nucleophilic attack in heteroaromatic ring system ii Electrophilic attack : A pair of electrons on the heteroatom oxygen, nitrogen or sulfur attached to an unsaturated system can be made available for the electrophilic attack through the following system.
This can also be operative when the heteroatom is negatively charged Fig. Electrophilic attack in heteroaromatic ring system iii Retention of aromaticity : Aromatic heterocycles tend to retain their aromatic character. However, ring oxygen atom, increasing number of ring heteroatoms and benzannelation reduce the aromaticity.
The introduction of heteroatom in an aromatic system causes considerable changes in the electronic properties and, therefore, the parameter relating to the heteroatom is used in the molecular orbital calculations in the heteroaromatic system. The reactivity indices are : 98 Heterocyclic Chemistry 4. The n-electron densities directly determine the orientation of electrophilic and nucleophilic substitutions. Electrophilic attack is considered to occur at the position where the electron density is highest, while nucleophilic attack takes place at the centre of low electron density.
The Tc-electron densities in homolytic substitution are not appreciably affected by the electron density distribution. The frontier electron distribution is much more uneven and the relative order of the frontier electron densities is not changed when approached by the reagent. The frontier electron densities are associated with the electron exchange rather than with electrostatic interaction and if electron exchange charge transfer is significant in stabilizing transition state, the frontier electron densities influence the reactivity.
In electrophilic substitution, the frontier electron density is the density in the highest filled molecular orbital HOMO and these electrons are considered to be analogous to the valence electrons of an atom. The electrophilic substitution reaction involves the interaction of highest occupied molecular orbital frontier orbital of heteroaromatic system with the lowest unoccupied molecular orbital of electrophile.
The activation energy depends on the extent of mixing which, in turn, depends on the difference in the energy between these two types of orbitals.
If smaller is the difference in the energy, lower will be the activation energy and faster will be the reaction with more efficient mixing of the orbitals. In nucleophilic substitution, the frontier molecular orbital is the lowest unoccupied molecular orbital LUMO. Nucleophilic substitution reaction involves the miytrtg of lowest unoccupied molecular orbital of the heteroaromatic system with a filled molecular orbital of nucleophile.
The heteroaromatic compound tends to accept the electron pair in the transition state and the frontier electron density at the carbon atom is then the electron density in this molecular orbital as if it were occupied by two electrons. The electrophilic and nucleophilic reactions occur at the carbon with the greatest appropriate frontier electron density Fig. Diagrammatic representation of frontier orbitals for electrophilic and nucleophilic substitutions 4.
The relative stabilities of the different transition states in the same molecule are in the same order as the localization energies. The localization energy in the heteroaromatic system reflects the relative stability of the transition state complex and therefore predicts the orientation in substitution reactions.
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Thus, the following inferences on orientation can be drawn on the basis of reactivity indices in the heteroaromatic systems : i The 7c-electron distribution is the main factor controlling the orientation. The transition states very closely resemble to the unpurturbed molecule and the electrostatic interactions contribute significantly to the orientation. Under acidic conditions, nitrogen of pyridine is protonated and further decreases reactivity of the ring carbon atoms toward electrophiles Table The comparison of reactivity indices for both pyridine and pyridinium ion accounts for the less reactivity of carbon atoms of azines.
The preferential electrophilic and nucleophilic attacks in the azines are represented Fig.
Selective positions for electrophilic and nucleophilic substitutions These effects can also be applied to the benzo analogs of the azines where electrophilic attack preferentially occurs at the carbon atoms of the benzo-fused ring Fig. While the presence of partial negative charge on the carbon atoms facilitates the electrophilic attack at the ring carbons. This charge distribution follows from the valence bond approach as a contribution to the resonance hybrid of the resonating structures Fig. Charge distribution in five-membered heteroaromatic rings The valence bond approach provides a general method of predicting relative reactivities of the different positions of a particular heteroaromatic compound.
Molecular orbital calculations also lead to the similar predictions. The preferential attack of electrophile at a-position rather than at P-position can be rationalized in terms of more effective delocalization of charge in the intermediates or 0 -complexes. The relative stabilities of the 0 -complexes are correlated with frie localization energies and thus predict the relative reactivity of the positions in the aromatic heterocycles Fig.
Relative stabilities of cr-complexes Benzo[b] heterocycles are expected to favour P-substitution in the heterocyclic ring over a-substitution which is based on the cr-complex stability.
However, benzo[b]furan undergoes mainly a-substitution due to the strong directing effect of the oxygen atom to the a-position Fig. H a-substitution P-substitution Fig. Cook, A. Katritzky and P. Linda in A. Katritzky Ed. HeterocycL Chem. Bird and G. Cheeseman in A. Aromatic Heterocycles 2 A. Katritzky, P. Barczynski, G. Musmnarra, D. Pisano and M. Szafran, J. Ill, 7 ; A. Katritzky, V. Frygelman, G.
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Advances in Heterocyclic Chemistry, Volume 57 - 1st Edition
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The chemistry of furazans fused to six- and seven-membered heterocycles with one heteroatom
Haddon and L. Jackman, Top. Aihara, J Am. Hasan and F. Fowler, J. Elvidge and L.