Photochemistry and Organic Synthesis

A cursory look to the literature shows that only about 1% of the published papers classed as organic syntheses by Chemical Abstracts involve a photochemical step. On the other hand, in photochemistry courses it is often stated that excitation by light multiplies by 3 the accessible reaction paths, because the chemistry of the excited singlet and triplet states are added to that of the ground state. It thus appears that photochemical reactions are less used as they may be. As it has been again recently remarked, this limited diffusion may be due to ill-founded prejudices [1], 

Two conditions should be verified in order that the potential of photochemical reactions is more extensively exploited. These are  

The connection between synthesis and photochemistry is vital. As long as photochemistry is felt as a sanctuary of the small group of professional photochemists, many synthetic perspectives will be ignored, and this is a negative impact also on mechanistic photochemistry that loses part of its interest. As a matter of fact, this remark is not new. In a talk in Leipzig in 1908, Hans Stobbe, a pioneer of photochemistry (well known for his innovative studies on the photochromism of 

Stobbe s wish has been only partially fulfilled in the century which has elapsed in the meantime. Whilst many photochemical reactions have been discovered, certainly many more wait to be uncovered, and it still holds true that more photochemistry carried out by synthetic chemists would contribute to the growth of photochemistry as a whole. This Handbook represents a modest attempt to contribute towards this aim and to foster the synthetic use of photochemistry. The presentation is referred to the small-scale laboratory synthesis of fine chemicals. In this aspect, the photochemical literature does not differ from the large majority of published synthetic work, most of which is carried out on the 100 mg scale for exploratory studies. However, there is no reason to think that a photochemical reaction is unfit for scaling up. As will shown below, an increase up to the 100 g scale can be obtained in the laboratory by simple arrangements. Furthermore, while the presently running industrial applications are limited in number, they are nonetheless rather important [3]. Some of these are well established, an example being the synthesis of vitamin D3 which has been produced at the several tons level each year for several decades, and for which dedicated plants continue to be built. This indeed demonstrates that photochemical syntheses are commercially viable. 

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Library of Congress Cataloging in Publication Data. Main entry under title Photochemistry and organic synthesis. 

J.-F. Labarre, Photochemistry and Organic Synthesis, Springer-Verlag, Berlin, 1985. 

Turro N.J., Cox G.S., Paczkowski., Photochemistry in micelles, Current Chemistry — Photochemistry and organic synthesis . 129, 57-97. (1985)... 

J. M. Coxon and B. Halton (1986). Organic Photochemistry. London Cambridge University Press N. J. Turro (1981). Modern Molecular Photochemistry. New York Benjamin-Cummings A. Padwa (1979-87). Organic Photochemistry, Vols 1-9. New York Dekker Photochemistry in Organic Synthesis. Ed. J. D. Coyle. Royal Society of Chemistry. Special publication No. 57 (1986). 

Wender, P. A. and von Geldern, T. W., Aromatic compounds isomerisation and cycloaddition, in Photochemistry in Organic Synthesis, Coyle, J. D., Ed., The Royal Society of Chemistry, London, 1986, 226. 

Photo-de-diazoniation has found relatively little application in organic synthesis, as is clearly evident from the annual Specialist Periodical Reports on Photochemistry published by the Royal Society of Chemistry. Since the beginning of these reports (1970) they have contained a section on the elimination of nitrogen from diazo compounds, written since 1973 by Reid (1990). In the 1980s (including 1990), at least 90% of each report is concerned with dediazoniations of diazoalkanes and non-quinon-oid diazo ketones, the rest being mainly related to quinone diazides and only occasionally to arenediazonium salts. 

Photochemical reactions have the principal advantage of clean chemistry , as they use light of defined energy [72, 74], Synthesis of vitamin D and photocleavage of protection groups, for example, are accepted organic synthesis routes. Nevertheless, no widespread use of photochemistry has been made so far as this technique... 

An elegant example of the use of photochemistry in complex organic synthesis is the preparation the bollweevil phenomone (sex attractant). The key step in both the Zoecon Corporation synthesis and the USDA synthesis involves the formation of a cyclobutane ring by a photoaddition reaction ... 

Simultaneous application of UV and MW irradiation has found widespread use in industry. The techniques are based on the conventional UV lamps and MW-powered electrodeless lamps and MW devices [28], The following paragraphs discuss several patents and papers that describe industrial microwave photochemistry, such as treatment of waste water, sterilization, or industrial photo induced organic synthesis. 

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