Other Photochemically Induced Reactions

In nonaqueous solutions, two other types of reactions have been observed with polycyclic arenes condensation via free-radical reactions and oxidative ring fission. 

The photochemical transformation of phenanthrene sorbed on silica gel (Barbas et al. 1996) resulted in a variety of products including c -9,10-dihydrodihydroxyphenanthrene, phenanthrene-9,10-quinone, and a number of ring fission products including biphenyl-2,2 -dicarboxaldehyde, naphthalene-l,2-dicarboxylic acid, and benzo[c]coumarin. 

The products of the photooxidation of naphthylamines adsorbed on particles of silica and alumina were putatively less toxic than their precursors (Hasegawa et al. 1993). 

FIGURE 1.14 Photooxygenation of (a) 7,12-diniethylbenz[a]anthracene, (b) benzo[a]pyrene, and (c) photo-cylization of cw-stilbene. 

C HgfC H2)7-C HCI-C H-(C H2)7-C02CH3 FIGURE 1.15 Product of reaction between DDT and methyl oleate. 

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Insertion of the carbene Me3SiCH into C—H and Si—H bonds has been investigated following photolysis of MeaSiCHN2.404 Other photochemically induced reactions include the addition of H2S to dimethyldiallylsilane,4 6 and the chlorination of alkyltrichlorosilanes.404... 

This principle has also been applied to several other photochemically induced redox reactions, especially with ozone as an oxidant. A good example is the oxidation of the high-temperature species OPCl. The infrared spectrum of a matrix-isolated mixture of... 

Many other photochemically induced decompositions arise by carbon-halogen bond cleavage. The majority of these reactions are radical processes with little photochemical significance and are not therefore included in this Report. In some instances, ionic... 

Many other photochemically induced decompositions arise by carbon-halogen bond cleavage. The majority of these reactions are radical processes with little photochemical significance and are therefore not reviewed in detail in this Report. Interest is evident in photochemically generated vinyl radical cyclization. Irradiation in benzene of the vinyl azetidinone (140), for example, gave the 1a-methylcarbapenam (141) together with the reduction product (142). Products derived by both radical... 

Many other photochemically induced decomposition reactions arising by carbon-halogen bond homolysis have been reported, but these are essentially radical processes having no special photochemical significance and so are not reviewed in detail in this report. Attention should be drawn, however, to the accumulating evidence for both radical and ionic intermediates in such transformations. 165... 

MALDI-TOF may have an inherent upper mass limit due to the availability of suitable matrices and ionization agents and suitable solvents. A matrix may perform well for one class of polymers but have undesirable chemical and photochemically induced reactions with other polymer systems. For some classes of polymers there is a lack of suitable matrices, cationizing agents, and/or solvent systems for sample preparation. Polyethylene is an example of a polymer not readily amenable to MALDI [27]. 

During the past few years, increasing numbers of reports have been published on the subject of domino reactions initiated by oxidation or reduction processes. This was in stark contrast to the period before our first comprehensive review of this topic was published in 1993 [1], when the use of this type of transformation was indeed rare. The benefits of employing oxidation or reduction processes in domino sequences are clear, as they offer easy access to reactive functionalities such as nucleophiles (e. g., alcohols and amines) or electrophiles (e. g., aldehydes or ketones), with their ability to participate in further reactions. For that reason, apart from combinations with photochemically induced, transition metal-catalyzed and enzymatically induced processes, all other possible constellations have been embedded in the concept of domino synthesis. 

Usually the change in colour in the forward direction is to longer wavelength, i.e. bathochromic, and reversibility of this change is key to the many uses of photo-chromism. In many systems, e.g. spiropyrans, spirooxazines and chromenes, the back reaction is predominantly thermally driven but in others the photochemically induced state is thermally stable and the back reaction must be driven photochemically e.g. fulgides and diarylethenes). The assistance of heat in the reversion of colour can be regarded as an example of thermochromism, but in this text the term is reserved for those systems where heat is the main cause of the colour change (see section 1.3). 

Trimethyloxazole 257 undergoes photochemically induced [2 + 2] cycloaddition with aromatic and aliphatic aldehydes to provide bicyclic oxazolines 258 with excellent regiochemical and stereochemical control. Diastereoselec-tivities from 75-99% can be achieved, which is the first reported example of a Paterno-Biichi reaction involving an oxazole. The oxetane cycloadducts can be hydrolyzed to a-amino-(3-hydroxy ketones. Other oxazoles have not been evaluated to determine if they undergo the photochemical cycloaddition (Scheme 8.71). 

A hydroxymethyl group can be introduced (ArH —> ArCH2OH) by several variations of this method.345 Alkylation of these substrates can also be accomplished by generating the alkyl radicals in other ways from hydroperoxides and FeS04,346 from alkyl iodides and H Oi-FefH),347 from carboxylic acids and lead tetraacetate, or from the photochemically induced decarboxylation of carboxylic acids by iodosobenzene diacetate.348 The reaction has also been applied to acetophenone and ferrocene.349... 

We have emphasized that the Diels-Alder reaction generally takes place rapidly and conveniently. In sharp contrast, the apparently similar dimerization of olefins to cyclobutanes (5-49) gives very poor results in most cases, except when photochemically induced. Fukui, Woodward, and Hoffmann have shown that these contrasting results can be explained by the principle of conservation of orbital symmetry,895 which predicts that certain reactions are allowed and others forbidden. The orbital-symmetry rules (also called the Woodward-Hoffmann rules) apply only to concerted reactions, e.g., mechanism a, and are based on the principle that reactions take place in such a way as to maintain maximum bonding throughout the course of the reaction. There are several ways of applying the orbital-symmetry principle to cycloaddition reactions, three of which are used more frequently than others.896 Of these three we will discuss two the frontier-orbital method and the Mobius-Huckel method. The third, called the correlation diagram method,897 is less convenient to apply than the other two. 

Shaw AA, Voituriez L, Cadet J, Gregoli S, Symons MCR (1988) Identification of the products from the direct effect of y-radiation of thymidine. J Chem Soc Perkin Trans 2 1303-1307 Shetlar MD (1976) The photoreactions and free radical induced reactions of 1,3-dimethyluracil in methanol and other alcohols. Photochem Photobiol 24 315-319 Shetlar MD (1979) Photochemical and free radical initiated reactions of 1,3-dimethylthymine with isopropanol. Photochem Photobiol 29 253-259... 

Oxidative addition, such as that exemplified by addition of H-S1R3 to photo-generated Fe(C0)4, reaction (17), is a very important, large thermal reaction class. Generally, it can be said that oxidative addition to low valent, coordinatively unsaturated metal complexes is common. Recently, Geoffroy, Hammond, and Gray188 have established that reductive elimination can be photochemically induced. A typical example is shown in reaction (67). Other small molecules can be reductively elimi-... 

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