Substituent effects LUMO energy

A carbocation is strongly stabilized by a C substituent (Figure 7.1c) through n-type interactions which involve substantial delocalization into the substituent. The LUMO energy is relatively unchanged, but the reactivity of the electron-deficient center toward attack by nucleophiles is reduced because the orbital coefficients are smaller. Allyl and benzyl carbocations are prototypical of C -substituted carbo-cations. The effects of substitution are cumulative. Thus, the more C -type substituents there are, the more thermodynamically stable is the cation and the less reactive it is as a Lewis acid. A prime example is triphenyl carbocation. 

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. 

In the model compounds, this red shift has been ascribed to a combination of silicon backbone with the x-orbitals of the aromatic substituent coupled with a decrease in the LUMO energy due to x -(a, d) interactions (15,16). Further examination of the data in Table III shows that the absorption maximum of the cyclohexylmethyl derivative, 9, is also somewhat red-shifted relative to the other alkyl polymers suggesting that the steric bulk of the substituents and/or conformational effects may also influence the polysilane absorption spectrum. 

A large number of accurate rate constants are known for addition of simple alkyl radicals to alkenes.33-33 Table 2 summarizes some substituent effects in the addition of the cyclohexyl radical to a series of monosubstituted alkenes.36 The resonance stabilization of the adduct radical is relatively unimportant (because of the early transition state) and the rate constants for additions roughly parallel the LUMO energy of the alkene. Styrene is selected as a convenient reference because it is experimentally difficult to conduct additions of nucleophilic radicals to alkenes that are much poorer acceptors than styrene. Thus, high yield additions of alkyl radicals to acceptors, such as vinyl chloride and vinyl acetate, are difficult to accomplish and it is not possible to add alkyl radicals to simple alkyl-substituted alkenes. Alkynes are slightly poorer acceptors than similarly activated alkenes but are still useful.37... 

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. ... 

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