Photochemical reactions, organized

Campos, P.J., Soldevilla, A., Sampedro, D., and Rodriguez, M.A. (2001) N-Cyclopropylimine-1 -pyrroline rearrangement. A novel photochemical reaction. Organic Letters, 3, 4087 -089. 

The chemical uses of tungsten have increased substantially in more recent years. Catalysis (qv) of photochemical reactions and newer types of soluble organometaUic complexes for industrially important organic reactions are among the areas of these new applications. 

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 photochemistry of carbonyl compounds has been extensively studied, both in solution and in the gas phase. It is not surprising that there are major differences between the photochemical reactions in the two phases. In the gas phase, the energy transferred by excitation cannot be lost rapidly by collision, whereas in the liquid phase the excess energy is rapidly transferred to the solvent or to other components of the solution. Solution photochemistry will be emphasized here, since both mechanistic study and preparative applications of organic reactions usually involve solution processes. 

We cover each of these types of examples in separate chapters of this book, but there is a clear connection as well. In all of these examples, the main factor that maintains thermodynamic disequilibrium is the living biosphere. Without the biosphere, some abiotic photochemical reactions would proceed, as would reactions associated with volcanism. But without the continuous production of oxygen in photosynthesis, various oxidation processes (e.g., with reduced organic matter at the Earth s surface, reduced sulfur or iron compounds in rocks and sediments) would consume free O2 and move the atmosphere towards thermodynamic equilibrium. The present-day chemical functioning of the planet is thus intimately tied to the biosphere. 

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... 

A review10 with eight references describes the photochemical reactions of the binuclear palladium(0) complex [Pd2(dppm)3] (dppm = bis(diphenyl)phosphinomethane) with organic halides. 

It is now more than 13 years ago that the second edition of A. Schon-berg s Preparative Organic Photochemistry appeared, a work representing a thorough compilation of photochemical reactions of preparative concern to the organic chemist. 

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. 

Photochemistry as topic is covered in several introductory textbooks 101 -107) yhe annuai literature is surveyed in a specialist periodical report108). Two series of monographs have to do with selected chapters from organic photochemistry 109) or photochemistry in general110). A series on molecular rearrangements also covers photochemical reactions U1). 

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. 

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