Photochemistry pericyclic reactions

Photochemistry offers the possibility of simple syntheses of some materials that would be very difficult to synthesize by other means. Stereoselective syntheses of four-membered rings are an excellent example of this (Scheme 7.3). As discussed previously, pericyclic reactions, such as... 

The photochemistry of conjugated polyenes has played a central role in the development of modern molecular photochemistry, due in no small part to its ultimate relevance to the electronic excited state properties of vitamins A and D and the visual pigments, as well as to pericyclic reaction theory. The field is enormous, tremendously diverse, and still very active from both experimental and theoretical perspectives. It is also remarkably complex, primarily because file absorption spectra and excited state behavior of polyene systems are strongly dependent on conformation about the formal single bonds in the polyene chain, which has the main effect of turning on or off various pericyclic reactions whose efficiencies are most strongly affected by conformational factors. 

The organization of the book is traditional. We have, however, been selective in our choice of topics in order to be able to devote a significant portion J of the book to the pericyclic reaction theory and its applications and to include a chapter on photochemistry. 

We now turn to a different type of photochemistry and different mechanistic questions, namely photochemical pericyclic reactions and the utility of the Mobius-Hiickel treatment of these transformations. 

The photochemistry of alkenes, dienes, and conjugated polyenes in relation to orbital symmetry relationships has been the subject of extensive experimental and theoretical studyThe analysis of concerted pericyclic reactions by the principles of orbital symmetry leads to a complementary relationship between photochemical and thermal reactions. A process that is forbidden thermally is allowed photochemically and vice versa. The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the reaction types discussed in Chapter 10 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2Tr- -2Tr] cycloaddition of two alkenes, which was classified as a forbidden thermal reaction (see Section 10.1), can serve as an example. The correlation diagram (Figure 12.17) shows that the ground state molecules would lead to a doubly excited state of cyclobutane, and would therefore involve a prohibitive thermal activation energy. 

Some of the most exciting new types of organic polymers have applications in the electronics industry. In Chapter 17 we will return to polymers again. There, it will be the electronic structure of the polymers rather than the functionality and topology that will be key. However, in order to understand the electronic structure of these polymers, we need a more in-depth vmderstanding of molecular orbital theory. Thus, we now turn to Part III of this book, where molecular orbital theory is the starting point from which several topics are launched, including pericyclic reactions, photochemistry, and electronic materials. 

As such, we will not consider photochemical processes in this chapter, deferring such topics to Chapter 16, which is devoted entirely to photochemistry. When we make tables to present rules for various types of reactions, describing them as allowed or forbidden, we will only be addressing thermal conversions. The photochemical part of such tables has always been redundant you just reverse the thermal predictions. However, on a more basic level we feel that predictions about photochemical reactions based on the level of analysis presented in this chapter are risky and fail to take into account the many subtleties of photochemistry. If you want to consider a photochemical pericyclic reaction, it is best to consider it in the context of the entire field of photochemistry, rather than as the opposite of a thermal process. 

We noted in Chapter 15 that, for the most part, the orbital symmetry rules are not directly applicable to photochemistry. However, some photochemical reactions of simple tt systems do give products that are consistent with expectations based on orbital symmetry, although this does not prove that these are concerted, pericyclic processes, The photochemical selection rules for pericyclic reactions are opposite of those for thermal pericyclic reactions. For example, there are many examples of [1,3] and [1,7] sigmatropic shifts that appear to go by the photochemically "allowed" suprafacial-suprafacial pathway Eqs. 16.22 and 16.23 show two (recall that the thermal reactions would be suprafacial-antarafacial). These reactions occur upon direct irradation, while sensitized photolysis produces products more consistent with biradical-type reactions. 

Singh J (2009) Photochemistry and pericyclic reactions 3rd edn. New Academic Science... 

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