Synthetic Organic Photochemistry

R. Binkley and T. Elechtner in W. Horspool, ed.. Synthetic Organic Photochemistry, Plenum Press, New York, 1984, Chapt. 7. 

The photochemical reactions of organic compounds attracted great interest in the 1960s. As a result, many useful and fascinating reactions were uncovered, and photochemistry is now an important synthetic tool in organic chemistry. A firm basis for mechanistic description of many photochemical reactions has been developed. Some of the more general types of photochemical reactions will be discussed in this chapter. In Section 13.2, the relationship of photochemical reactions to the principles of orbital symmetry will be considered. In later sections, characteristic photochemical reactions of alkenes, dienes, carbonyl compounds, and aromatic rings will be introduced. 

The low solubility of fullerene (Ceo) in common organic solvents such as THE, MeCN and DCM interferes with its functionalization, which is a key step for its synthetic applications. Solid state photochemistry is a powerful strategy for overcoming this difficulty. Thus a 1 1 mixture of Cgo and 9-methylanthra-cene (Equation 4.10, R = Me) exposed to a high-pressure mercury lamp gives the adduct 72 (R = Me) with 68% conversion [51]. No 9-methylanthracene dimers were detected. Anthracene does not react with Ceo under these conditions this has been correlated to its ionization potential which is lower than that of the 9-methyl derivative. This suggests that the Diels-Alder reaction proceeds via photo-induced electron transfer from 9-methylanthracene to the triplet excited state of Ceo-... 

In this chapter the topochemical [2+2] photoreactions of diolefin crystals are reviewed from the viewpoints of organic photochemistry, analysis of reaction mechanism, and crystallography as well as in terms of synthetic polymer chemistry and polymer physics. 

We expect the development of the mechanistic aspects of organic photochemistry to continue at the present pace as new methods are developed to probe in increasing detail and shorter time scales the photochemical dynamics of both old and new photoreactions. Since photochemistry is no longer the sole domain of the specialist, it is relatively safe to predict a dramatic increase in the near future of the synthetic and industrial uses of organic photochemistry. 

The photocycloaddition of an aldehyde or ketone with an olefin to yield an oxetane was reported by Paterno and Chieffi in 1909. 58> Contemporary studies on the synthetic utility and mechanistic features were initiated nearly 50 years later by Biichi et al. 59) Two review articles summarizing synthetic aspects of Paterno-Biichi reactions have been published 6.12)) and mechanistic studies have been reviewed several times. 6,38,60-62) The reaction involves the addition to olefin of a photo-excited carbonyl moiety. This circumstance makes it advantageous to review this reaction before a discussion of olefin-olefin additions, because the solution photochemistry of carbonyl compounds is probably better understood than any other aspect of organic photochemistry. Many of the reactions of carbonyl compounds have been elucidated during studies of the important phenomena of energy transfer and photosensitization. 63-65)... 

Research in organic photochemistry over this period has not only produced a vast accumulation of factual knowledge, but the understanding of how these reactions proceed has also dramatically increased. The high rate of development in synthetic applications of photochemical reactions has — at least in part — only become possible due to this theoretical and mechanistic know-how. Organic photochemistry is today a multidisciplinary field itself. 

Organic photochemistry has typically been approached from a physical/mechanistic perspective, rather than a synthetic one. This may result from a certain reluctance on the part of synthetic chemists to employ reactions which often produce many products. Nonetheless, careful design of substrates can lead to high levels of chemoselectivity. In fact, there have been many recent examples of the successful application of photochemical reactions to the synthesis of complex targets1. In many cases, these processes provide access to unique modes of reactivity, or offer unrivaled increases in molecular complexity2. 

Clennan, E.L. (2005). Photo-oxygenation of the ene-type. In Synthetic Organic Photochemistry, pp. 365-390. Marcel Dekker, New York... 

A.L. Weedon, in "Synthetic Organic Photochemistry", W.M. Horspool (Editor), Plenum Press, New York and London, 1984, pp. 61-143 and references therein. 

Photoinduced electron transfer (PET) has attracted considerable interest and has been intensively studied as a fundamental step in mechanistic and synthetic organic photochemistry and appears to be involved in key biological processes. Cyclore version of oxetans by PET is important for the photoenzymatic repair of the photoproducts of the DNA dipyrimidine sites by photolyase244. The oxidative version of this reaction has been achieved using cyanoaromatics, chloranil, or (thia)pyrylium salts as electron-transfer photosensitizers245 246. 

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