HOMO and LUMO coefficients

The regioselectivity in radical addition reactions to alkenes in general has successfully been interpreted by a combination of steric and electronic effects1815,47. In the absence of steric effects, regiochemical preferences can readily be explained with FMO theory. The most relevant polyene orbital for the addition of nucleophilic radicals to polyenes will be the LUMO for the addition of electrophilic orbitals it will be the HOMO. Table 10 lists the HOMO and LUMO coefficients (without the phase sign) for the first three members of the polyene family together with those for ethylene as calculated from Hiickel theory and with the AMI semiempirical method48. 

The orbital coefficients obtained from Hiickel calculations predict the terminal position to be the most reactive one, while the AMI model predicts the Cl and C3 positions to be competitive. In polyenes, this is true for the addition of nucleophilic as well as electrophilic radicals, as HOMO and LUMO coefficients are basically identical. Both theoretical methods agree, however, in predicting the Cl position to be considerably more reactive as compared to the C2 position. It must be remembered in this context that FMO-based reactivity predictions are only relevant in kinetically controlled reactions. Under thermodynamic control, the most stable adduct will be formed which, for the case of polyenyl radicals, will most likely be the radical obtained by addition to the C1 position. 

TABLE 10. HOMO and LUMO coefficients for ethylene and selected polyenes... 

There are molecules that have two-electron deficient centers capable to react with nucleophiles. Such molecules are called ambident electrophiles. The reactivity profile is susceptible to the same kind of analysis as the one we have had above for the reaction of ambident nucleophiles with electrophiles. In the reaction of an electrophile with a nucleophile, it is the LUMO of the electrophile that interacts with the HOMO of the nucleophile. As such, the higher the HOMO and/or the lower the LUMO to reduce energy gap between the two, the faster will be the reaction. Alternatively, the better the match of the HOMO and LUMO coefficients, the more effective will be the reaction. 

In addition to ds-1 positions, which are forbidden for sterically demanding addends, the I,el-bisadduct has enhanced HOMO and LUMO coefficients in... 

HOMO and LUMO coefficients. The effects of the electron-donating or electron-withdrawing groups on the shapes of the frontier orbitals of dipo-larophiles are depicted in Figure 5.15. 

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