Photochemistry compounds

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. 

Despite the increasing information on the photochemistry of 2,4-dienones and other unsaturated ketones, as well as on the ring-chain valence isomerism of halogen-substituted pyran and dihydi opyran systems,the data are still very scarce. The intermediate formation of pyrans valence-isomeric with unsaturated carbonyl compounds in the pyridine syntheses based on reactions of ammonia with aldehydes or ketones, advocated by various authors (cf. Section II,B,2,f), is still rather speculative. (See also Section II,B,2,e for the valence isomerism of 5-chloro-2,4-dienones with pyrylium chlorides.)... 

Compared with other metal coordination compounds, relatively little is known about chelate photochemistry. 

Azo-compounds and peroxides undergo photodecomposition to radicals when irradiated with light of suitable wavelength. The mechanism appears similar to that of thermal decomposition to the extent that it involves cleavage of the same bonds. The photodecomposition of azo-compounds is discussed in Section 3.3.1.1.2 and peroxides in Sections 3.3.2.1.2 (diacyl peroxides) and 3.3.2.3.2 (peroxyesters). Specific photoinitiators are discussed in Section 3.3.4. It is also worth noting that certain monomers may undergo photochemistry and direct photoinitiation on irradiation of monomer is possible. 

Many reviews have been written on the photochemistry of aromatic carbonyl compounds269 and on the use of these compounds as photoinitiators.270 272 Primary radicals are generated by one of the following processes ... 

Benzoin and a wide variety of related compounds (e.g. 12, 70-74) have been extensively studied both as initiators of polymerization and in terms of their general photochemistry.271 2 3 The acetophenone chromophore absorbs in the near UV (300-400 nm). In the absence of hydrogen atom donors the mechanism of... 

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. 

Photochemistry of transition metal coordination compounds. A. W. Adamson, W. L. Waltz,... 

Second-sphere photochemistry and photophysics of coordination compounds. V. Balzani, N. Sab-batini and F. Scandola, Chem. Rev., 1986, 86, 319 (173). 

The photochemistry of sulfoxides and sulfones, which was first comprehensively reviewed in 19691, continues to be an area of active research interest. In this early review some 30 to 40 primary publications on the photochemistry of sulfoxides and sulfones were described. Since that date, interest in this field has continued at a steady, rather than accelerated, pace but further reviews of the general area of photochemistry of organic sulfur compounds have appeared2,3. The present review will focus on the main areas of interest for both sulfoxides and sulfones which, in spite of their apparent similarity, exhibit quite different photochemical behavior. 

Warmuth R The Inner Phase of Molecular Container Compounds As a Novel Reaction Environment J. Inclusion Phenom. Macrocyclic Chem. 2000 37 1-38 Keywords inciusion reaction, photochemistry, photoinduced eiectron transfer, fuiierenes... 

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