The Photochemistry of Alkenes

The photochemistry of alkenes and dienes has already been mentioned in connection with the principles of orbital symmetry control in electrocyclic and cycloaddition processes in Section 13.2. Cycloadditions are considered, from a synthetic viewpoint, in Chapter 6 of Part B. This section will emphasize unimolecular photoreactions of alkenes and dienes. 

Understand the importance and role of the lowest (n,n ) excited state of alkenes in the photochemistry of alkenes. 

P. Kropp, in A. Padwa (ed.). Organic Photochemistry, vol. 4. Dekker (1979). The photochemistry of alkenes in solution is reviewed, particularly the various rearrangement reactions of monoalkenes. 

The electronic structure of alkynes is related to that of alkenes, and the photochemistry of the two classes of compound reflects this similarity. Because the photochemistry of alkenes has received greater attention and has already been described in systematic form - it is not unexpected that the present account should point out the ways in which alkyne photochemistry parallels, or is markedly different from, that of alkenes. There is a considerable difference, however, in the range of compounds which has been studied in each class. Reports of photochemical reactions of alkynes very often refer to mono- or disubstituted acetylenes in which the substituents are alkyl, aryl or alkoxycarbonyl. There have been studies on diyne and enyne systems, but as yet there has emerged nothing in alkyne chemistry to match the wealth of photochemistry reported for dienes and polyenes. This reflects in part the greater tendency of the compounds containing the C=C bond to undergo photopolymerization rather than any other reaction on irradiation. Within this limitation there is a wide variety of reactions open to the excited states of alkynes, and quite a number of the processes have synthetic application or potential. 

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. 

This chapter deals with the photochemistry of alkenes, alkynes, dienes, polyenes, and related compounds through a choice of the literature published during the period January 2010 — December 2011. Furthermore, recently many researchers are developing the photochemistry of these compounds for energy conversion, e.g. through nanotechnology applications, such as molecular devices, chemomechanics, molecular switches, etc. This chapter also covers the nanotechnology aspects that are based upon the utilization of isomerization/electrocyclization/cycloaddition reactions of the title compounds. 

Related topics that He outside the scope of this chapter include thermal reactions in matrices between alkenes and reactive species such as F atoms and NH, even when these have been generated photochem-icaUy, the photochemistry of alkene radical cations, and matrix studies of azaalkenes, silaalkenes, and other alkene analogues. 

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