HOMO-LUMO interaction photochemical

Consideration of the HOMO-LUMO interactions also indicates that the [2n + 2ti] additions would be allowed photochemically. The HOMO in this case is the excited alkene 71 orbital. The LUMO is the ti of the ground-state alkene, and a bonding interaction is present between the carbons where new bonds must be formed ... 

Bearing these qualifications in mind, the important frontier orbitals in a photochemical reaction are HOMO/ HOMO and LUMO/ LUMO . We now see why so many photochemical reactions are complementary to the corresponding thermal reactions. Photochemical reactions often seem to do the opposite of what you would expect of the equivalent thermal reaction, when there is one. In the latter it is the HOMO/LUMO interactions which predominate in bond-making processes, and in the former it is HOMO/ HOMO and LUMO/ LUMO . 

Under thermal conditions, the TS of the [2 + 2] cycloaddition is made up of if/1 (the LUMO) of one component and i[io (the HOMO) of the other. Positive overlap between the orbitals at both termini of the 1t systems can be obtained only if one of the components reacts antarafacially. This orientation is very difficult to achieve geometrically, and hence [2 + 2] cycloadditions do not normally proceed under thermal conditions. However, under photochemical conditions, one of the components has an electron promoted from fo to //1. Now the HOMO-LUMO interaction is between ) / of the photoexcited component and i// of the unexcited component, and thus both components can be suprafacial in the TS. The [2 + 2] cycloaddition of most alkenes and carbonyl compounds do in fact proceed only upon irradiation with light. 

For photochemical reactions, HOMO-MHQMG" and lUMO-TUMO 1 interactions dominate, in contrast to the HOMO-LUMO interactions involved in thermal reactions, The rules tor numbers of electrons involved in photochemical pericydic reactions are the reverse of those for thermal reactions. 

The concepts of frontier orbital HOMO LUMO interactions, the idea of an aromatic transition state, and the alternative concept of conservation of orbital symmetry (not developed in this chapter) all lead to the same result for pericyclic reactions which involve a cyclic overlap of orbitals in the transition slate, thermal reactions are allowed for reactions involving 4n + 2 electrons in Hiickel systems (no change in phase between overlapped orbitals in the cyclic transition state) or for 4/j electrons in Mobius systems (phase between overlapped orbitals in the cyclic transition state changes once on going round the ring). For photochemical systems, these rules are reversed. 

In thermal reactions. HOMO LUMO interactions are the most important, for the reasons expounded in Answer 7.1. In photochemical reactions, since an electron has been promoted, HOMO- HOMO" and LUMO- LUMO" interactions will be favourable, and since the energy gap is small or zero, these interactions will dominate. Thus for reactions that have the wrong number of electrons for a thermal reaction, the photochemical reaction will lake place conversely, if the correct number of electrons for a thermal reaction is present, the photochemical reaction will not take place. 

What about the photochemical Diels-Alder reaction The observation that this reaction is most uncommon leads us to the immediate suspicion that there is something wrong with it. Usually, the absorption of a photon will promote an electron from the HOMO to the LUMO. In this case, the lower energy HOMO-LUMO gap is that in the diene partner. Absorption of light creates a new photochemical HOMO for the diene, 3, and now the HOMO-LUMO interaction with the dienophile partner involves one antibonding overlap. Both new bonds cannot be formed at the same time (Rg. 20.22). So this photochemical Diels-Alder reaction is said to be forbidden by orbital symmetry. ... 

What about the photochemical dimerization of alkenes Promotion of a single electron creates a new photochemical HOMO (Fig. 20.26a), and now the symmetries are perfect for a HOMO-LUMO interaction involtdng two bonding... 

In a photochemical cycloaddition, one component is electronically excited as a consequence of the promotion of one electron from the HOMO to the LUMO. The HOMO -LUMO of the component in the excited state interact with the HOMO-LUMO orbitals of the other component in the ground state. These interactions are bonding in [2+2] cycloadditions, giving an intermediate called exciplex, but are antibonding at one end in the [,i4j + 2j] Diels-Alder reaction (Scheme 1.17) therefore this type of cycloaddition cannot be concerted and any stereospecificity can be lost. According to the Woodward-Hoffmann rules [65], a concerted Diels-Alder reaction is thermally allowed but photochemically forbidden. 

Interaction of excited-state HOMO and LUMO in photochemical [2 -L 2] cycloaddition reactions. The reaction occurs with suprafacial geometry. 

The important frontier orbitals in a photochemical reaction are therefore HOMO/ HOMO and LUMO/ LUMO , where the inverted commas remind us that these orbitals are not the actual HOMO and LUMO at the time of the reaction, but were the HOMO and LUMO in the ground state, before the excitation took place. The HOMO/ LUMO and LUMO/ HOMO interactions... 

The preferred (disiotatory) mode of SOMO-HOMO ard SOMO-LUMO interactions in the photochemical ling opening of cyclobutene... 

In contrast with the thermal process, photochemical [2 + 2] cycloadditions me observed. Irradiation of an alkene with UV light excites an electron from i /, the ground-slate HOMO, to which becomes the excited-slate HOMO. Interaction between the excited-state HOMO of one alkene and the LUMO of the second alkene allows a photochemical [2 + 2j cycloaddition reaction to occur by a suprafacial pathway (Figure 30.10b). 

Figure 30.10 (a) Interaction of a ground-state HOMO and a ground-state LUMO in a potential [2 - 2] cycloaddition does not occur thermally because the antarafacial geometry is too strained, (b) Interaction of an excited-state HOMO and a ground-state LUMO in a photochemical [2 r 2] cycloaddition reaction is less strained, however, and occurs with suprafacial geometry. 

Irradiation of pyridinium-3-olates gives a dimer of different structure (454). In a photochemical process the important frontier orbital interactions are those between HOMO-HOMO and LUMO-LUMO. Figure 6b demonstrates that for photodimerization, a different orientation favors transition-state stabilization and leads to the novel adduct 454. 

Frontier orbitals also explain why the rules change so completely for photochemical reactions. In a photochemical cycloaddition, one molecule has had one electron promoted from the HOMO to the LUMO, and this excited-state molecule reacts with a molecule in the ground state. The interacting orbitals that most effectively lower the energy of the transition structure are... 

The easiest explanation is based on the frontier orbitals—the highest occupied molecular orbital (HOMO) of one component and the lowest unoccupied orbital (LUMO) of the other. Thus if we compare a [2 + 2] cycloaddition 6.133 with a [4 + 2] cycloaddition 6.134 and 6.135, we see that the former has frontier orbitals that do not match in sign at both ends, whereas the latter do, whichever way round, 6.134 or 6.135, we take the frontier orbitals. In the [2 + 2] reaction 6.133, the lobes on C-2 and C-2 are opposite in sign and represent a repulsion—an antibonding interaction. There is no barrier to formation of the bond between C-l and C-l, making stepwise reactions possible the barrier is only there if both bonds are trying to form at the same time. The [4 + 4] and [6 + 6] cycloadditions have the same problem, but the [4 + 2], [8 + 2] and [6 + 4] do not. Frontier orbitals also explain why the rules change so completely for photochemical reactions, as we shall see in Chapter 8. 

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