Aromatic compounds frontier orbitals

The effect of cryptands on the reduction of ketones and aldehydes by metal hydrides has also been studied by Loupy et al. (1976). Their results showed that, whereas cryptating the lithium cation in LiAlH4 completely inhibited the reduction of isobutyraldehyde, it merely reduced the rate of reduction of aromatic aldehydes and ketones. The authors rationalized the difference between the results obtained with aliphatic and aromatic compounds in terms of frontier orbital theory, which gave the following reactivity sequence Li+-co-ordinated aliphatic C=0 x Li+-co-ordinated aromatic C=0 > non-co-ordinated aromatic C=0 > non-co-ordinated aliphatic C=0. By increasing the reaction time, Loupy and Seyden-Penne (1978) showed that cyclohexenone [197] was reduced by LiAlH4 and LiBH4, even in the presence of [2.1.1]-cryptand, albeit much more slowly. In diethyl ether in the absence of... 

The absorption of the benzo[z]heterocycles 135 (Table 22) at longer wavelengths than their benzol ] counterparts is a reflection of the lower aromaticity of the former compounds and the consequent differences in the energies of the HOMOs. The smaller energy separation between the frontier orbitals is demonstrated quantitatively by the AMI calculated HOMO-LUMO gaps shown in Table 1 (Section 2.3.2.1). 

Aromatic substrates are by far the most commonly used substrates in the rapidly expanding area of photoinduced electron transfer [1,2]. This is obviously due to the favourable location of the frontier molecular orbitals in such compounds. The same factor facilitates the formation of electron transfer donor-acceptor (EDA) complexes both in the ground state (these possibly are intermediates in some thermal reactions, e.g. selected electrophilic substitutions), and in the excited state (exciplexes). 

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