Azides frontier orbitals

The 1,3-dipolar cycloaddition of organic azides with nitriles could give rise to two regioisomers. Since organic azides are Type II 1,3-dipoles on the Sustmann classification (approximately equal HOMO-LUMO gaps between the interacting frontier orbital pairs) the reactions could be dipole HOMO or LUMO controled and the regioselectivity should be determined by the orbital coefficients for the dominant HOMO-LUMO interaction. Such systems show U-shaped kinetic curves in their... 

Fig. 6.35 Frontier orbitals for phenyl azide and representative dipolarophiles...

Fig. 4-64 Frontier orbitals for phenyl azide, styrene and phenylacetylene...

The rates of 1,3-dipolar addition of mesitonitrile oxide to cis- (36a) and frans-cyclooctene (37a) and some other cyclic olefins have been discussed with respect to strain effects (271). It was concluded that frontier orbital energies of the alkenes and differences in olefinic strain energies do not correlate with the observed reactivities. The relative rates for addition of phenyl azide to several alkenes and the olefinic strain energies are shown in Table 31 (272). 

Some 1,3-dipoles, such as azides and diazoalkanes, are relatively stable, isolable compounds however, most are prepared in situ in the presence of the dipolarophile. Cycloaddition is thought to occur by a concerted process, because the stereochemistry E or Z) of the alkene dipolarophile is maintained trans or cis) in the cycloadduct (a stereospecihc aspect). Unlike many other pericycUc reactions, the regio- and stereoselectivities of 1,3-dipolar cycloaddition reactions, although often very good, can vary considerably both steric and electronic factors influence the selectivity and it is difficult to make predictions using frontier orbital theory. 

Type II (ambiphilic dipole) In such cases, the HOMO of the dipole can interact with the LUMO of the dipolarophiles or the HOMO of the dipolar-ophile can interact with the LUMO of the dipole. Therefore, any substituent on the either dipolarophile or dipole would accelerate the reaction by decreasing the energy gap between the two interacting frontier orbitals, i.e., an electron withdrawing group would decrease the energy of the LUMO while an EDO would increase the energy of the HOMO. For example, azides react with various electron-rich or electron-poor dipolarophiles with about a similar reactivity rate. 

The regioselectivity of the 1,3-dipolar cycloadditions of azides to alkenes is usually difficult to predict due to the similar energies for the transition states which involve either the HOMO (dipole) or the LUMO (dipole). The results of a study which utilized 5-alkoxy-3-pyrrolin-2-ones as dipolar-ophiles in reactions with a variety of aryl azides seemed to reflect this problem the results suggested that the low regioselectivity observed was due to the frontier molecular orbital interactions between dipole and dipolarophile, and not any steric hindrance offered by the 5-alkoxy function <84H(22)2363>. 

This distinction is related to the fact that in the Si state ofAPA the aNN -MO with the antibonding character with respect to the N-N2 bond is occupied, it becomes HSOMO (Figure 17). Occupation of this orbital is a prerequisite for the dissociation of this bond. In the Si state of cation APAH the 7t-MO bond localized on the acridine moiety is occupied and the Gnn -MO remains unoccupied (not shown in Figure 17). The difference in structures of the lowest singlet-excited states of two azides explains the difference in their photochemical properties (compare with frontier MOs of the neutral and protonated 9-azidoacridine, Figure 11). 

Theoretical basis was also foimd for experimentally observed interrelation between azide photoactivity (photodissociation quantum yield) and spectral sensitivity both these properties are determined by the structure and energy characteristics of the frontier molecular orbitals. At the same time, the problem of relative reactivity of different azido groups in polyazidopyridines is not yet solved because of scarce data on the selective photolysis of these azides. 

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