Substituent effects HOMO energy

Both the reactivity data in Tables 11.3 and 11.4 and the regiochemical relationships in Scheme 11.3 ean be understood on the basis of frontier orbital theory. In reactions of types A and B illustrated in Seheme 11.3, the frontier orbitals will be the diene HOMO and the dienophile LUMO. This is illustrated in Fig. 11.12. This will be the strongest interaction because the donor substituent on the diene will raise the diene orbitals in energy whereas the acceptor substituent will lower the dienophile orbitals. The strongest interaction will be between j/2 and jc. In reactions of types C and D, the pairing of diene LUMO and dienophile HOMO will be expected to be the strongest interaction because of the substituent effects, as illustrated in Fig. 11.12. 

A simple qualitative model of the three-electron hemibond in [X.. X], based on the Hiickel approximation, has been proposed by Gill and Radom [122]. This qualitative model predicts that the strength of the hemibond should vary in proportion to the Hiickel parameter a, which can be replaced by the HOMO energy in X because a good correlation is found between Eho-Mo(X) and De(X-X ). This model readily rationahzes the marked substituent effect on the strength of the hemibond. In particular, electron-withdrawing substituents are found to have a strengthening effect. 

It is known that the effects of substituent groups on a diene or dienophile vary between different types of parents [23]. A function, r(Y), has been determined for several functional groups, with Y corresponding to their electron donating or withdrawing capability such that a reasonable estimate of the HOMO energy could be obtained by use of the equation [15] ... 

If steric effects of substituents do not affect the formation of the complex and A (e.g. TCNE) is constant, coefficients a and b in equation 19 depend on D only130. In approximation 18, hvcr and Ip depend on the inductive and resonance effects of the X substituents for constant D128,129. As the donor properties of X become stronger, the HOMO energy increases and the hvcr and Ip values decrease. 

The first term of Equation 15.3 is responsible for most of the transition state stabilization of a Diels-Alder reaction with normal electron demand. In this case, the first term is larger than the second term because the denominator is smaller. The denominator of the first term is smaller because the HOMO of an electron-rich diene is closer to the LUMO of an electron-poor dienophile than is the LUMO of the same electron-rich diene with respect to the HOMO of the same electron-poor dienophile (Figure 15.24, column 2). Acceptors lower the energy of all 7F-type MOs irrespective of whether these MOs are bonding or antibonding. This is all the more true the stronger the substituent effects and the more substituents are present. 

However, this interaction should also be increased by alkyl substituents, which lower the alkene IP, or, equivalently, raise the alkene HOMO energy. Experimentally, there is either no change in rate, or a small decrease, as the IP of the alkene decreases. Thus, an apparent contradiction is revealed in these examples dipole LUMO-alkene HOMO control nicely accounts for regioselectivity and the nitrile oxide substituent effect, but does not explain the decrease in rate for increasing alkyl substitution. More potent electron-donors do, indeed, accelerate the reaction, but only feebly. For example, butyl vinyl ether reacts 2.1 times faster than ethylene with BNO at 0 °C, while styrene reacts only 1.2 times faster than ethylene with BNO, in spite of the low IP of styrene (8.48 eV)72. ... 

A substituent effect on electrochemical and xerographic properties of triarylamines has recently been reported16. The oxidation potentials correlate with both the Hammett substituent constants and the calculated HOMO energies of the molecules under investigation, and this approach was used16 to rationalize xerographic properties, including transport phenomena of solid-state solutions in polycarbonate-Z. 

A study employing a combined QSAR-CoMFA (comparative molecular field analysis) used observed [ H]diazepam pICso values, calculated HOMO and LUMO energies and total dipole moments hydrophobic, steric, and field/inductive substituent effects were also considered (116). Equation 5.13 was derived for a set of 30 compounds that varied only in substitution at the Cl, C2, and N1 positions (the latter varied only between H and CH3). 

Overall, the acidities of the substituted phenols are largely determined by the stabilization of the corresponding phenolate ions, i.e. the energies of the phenolate HOMOs. There is a similarity between the substituent effect on the latter and the LUMOs of substituted benzenes both can be understood by simple perturbative PMO treatment. ... 

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