Photochemical reactions general pathways

There are many ways in which the excited species can react in general the photochemical reactions may be classified as intramolecular and intermolecular, depending on their mono- and bimolecular pathways, respectively (Figure 6.2). The former processes predominate in diluted phases, whereas the latter are observed mostly in condensed phases. 

Three types of cycloaddition products are generally obtained from the photochemical reaction between aromatic compounds and alkenes (Scheme 31). While [2 + 2] (ortho) and [3 + 2] (meta) cycloaddition are frequently described, the [4 + 2] (para or photo-Diels-Alder reaction) pathway is rarely observed [81-83]. Starting from rather simple compounds, polycyclic products of high functionality are obtained in one step. With dissymmetric alkenes, several asymmetric carbons are created during the cycloaddition process. Since many of the resulting products are interesting intermediates for organic syntheses, it is particularly attractive to perform these reactions in a diastereoselective way. 

A diversion of the general pathway in the photochemical di-7t-methane rearrangement has been found to lead to dihydrofurans. When one of the alkene moieties carries one or two carbonyl groups, this reaction pathway becomes dominant (Equation 66) <20020L1155>. 

Oxidation of Aliphatic Compounds. - A general review of the use of supra-molecular systems as microreactors for photochemical reactions contains a section dealing with the photosensitized oxidation of alkenes included in zeolites, nation membranes and vesicles. Particular consideration is given to the possibility of controlling the form and environment of the sensitizer and substrate so that the reaction selectively follows an energy-transfer or an ET pathway. The same authors have also provided a more substantial review on the same theme. Recent developments in relation to the stereochemistry and mechanism of the ene photooxygenation of alkenes by O2 have also been reviewed. ... 

Thermal or photochemical decomposition of cyclopropyl azides may give rise to 1-azetines and/or stereospecific fragmentation to olefin plus nitrile. In contrast to the photochemical reaction, substituents have a pronounced effect on the thermal reaction pathway. In general, substitution in the 2-position of the cyclopropyl azide... 

The photoproduct generally reverts to the initial system via thermal pathways, but there are examples where the reverse reaction is induced by radiation of a different wavelength from that driving the forward reaction, or by both thermal and photochemical processes. Generally, sensitivity to thermal effects controls the concentrations obtained from the forward reactions and, therefore, their effectiveness in producing the product. 

Mixed-ligand complexes of chromium(III) also undergo photoaquation reactions. In general, two or more photoreactions occur simultaneously, and the predominant product from the photochemical reaction may be different from the product that is obtained under thermal conditions. An example of such difference in regioselectivity is observed with the halo complexes CrX(NH3)s. Under thermal conditions, aquation of the halide ion (X") is the preferred pathway ... 

Consider Fig. 3.i6 it is seen that k + 2) electrons are involved in the reaction, and further that two electrons are delivered from the hybrid (sp") orbital of Xyz. This orbital, considered alone, can be acted upon in the supra-facial or antarafacial senses, and so can the k 7r-electron olefin. Therefore, there are the four usual combinations, namely supra-supra, antara-antara, antam-supra, and supra-antara (respectively Fig. 3.16(a) (i), (b) (ii), (a) (ii), and (b) (i)). When (/c + 2) - (4 + 2) electrons it will be expected, because of the general Wood ward-Hoffmann rule, that the supra-supra or antara-antara interactions will occur in the thermal cheletropic reactions. These respectively correspond to a linear cheletropic reaction with disrotatory cleavage, and to a non-linear cheletropic reaction with conrotatory cleavage. The alternative pathways are reserved for the cases (/ -I- 2) = An electrons. In the photochemical reactions the usual cross-over relationship should apply. 

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