Photochemistry photoisomerization

The predominant, if not exclusive, formation of 5/7-fused hydroxy ketones was observed in the case of 4-alkylated dienones [(204) (205) (R = CH3) 6 1 from (201) (R = CH3)] ° and of prednisone 21-acetate [(206)-> (207)]. It appears therefore likely that intermediates which represent the conjugate acids of the postulated zwitterionic intermediates in the dienone photoisomerizations [c/. (202), (203)] participate both in the acid-catalyzed transformations of (200) and in the dienone photochemistry in protic solvents. 

Investigation of the photochemistry of protonated durene offers conclusive evidence that the mechanism for isomerization of alkyl-benzenium ions to their bicyclic counterparts is, indeed, a symmetry-allowed disrotatory closure of the pentadienyl cation, rather than a [a2a -f 7r2a] cycloaddition reaction, which has been postulated to account for many of the photoreactions of cyclohexadienones and cyclohexenones (Woodward and Hoffmann, 1970). When the tetramethyl benzenium ion (26) is irradiated in FHSO3 at — 90°, the bicyclo[3,l,0]hexenyl cation (27) is formed exclusively (Childs and Farrington, 1970). If photoisomerization had occurred via a [(r2a-t-772 ] cycloaddition, the expected... 

An interesting alternative mechanism of activation is the photochemical reduction of Pt(IV) to Pt(II) (Fig. 3). In addition to photoreduction, photosubstitution and photoisomerization can also occur, making the photochemistry of Pt complexes difficult to predict and a careful analysis of the photoproducts imperative (21). We have been involved particularly in the development of photochemotherapeutic agents based on Pt(IV) and the study of their photodecomposition and (subsequent) interactions with... 

A quick survey of the photochemistry of the different complexes described above shows that the mechanism of photoactivation and the subsequent nature of the observed photoproducts varies from complex to complex and from one geometric isomer to another. Photochemical pathways often involve a combination of photosubstitution, photoisomerization, and photoreduction steps. In general, photolysis is rather slow in water and many different products are obtained if the complex is irradiated alone. The presence of nucleophilic biomolecules, on the other hand, can have a major influence, as photoreduction is usually rapid and accompanied by simpler reaction pathways. NMR methods... 

The photochemistry of ally 1-substituted Group 14 metal compounds can be divided into two sections, namely photoisomerization reactions, and those involving some kind of intermolecular reaction. 

Direct ,Z-photoisomerization is sufficiently ubiquitous in the photochemistry of all polyene systems that it will be considered first in its own separate section, although the... 

Intriguingly, the conical intersection model also suggests that E,Z-isomerization of acyclic dienes might be accompanied by conformational interconversion about the central bond, reminiscent of the so-called Hula-Twist mechanism for the efficient ,Z-photo-isomerization of the visual pigment rhodopsin in its rigid, natural protein environment101. A study of the photochemistry of deuterium-labelled 2,3-dimethyl-l,3-butadiene (23-d2) in low temperature matrices (vide infra) found no evidence for such a mechanism in aliphatic diene E,Z -photoisomerizations102. On the other hand, Fuss and coworkers have recently reported results consistent with the operation of this mechanism in the E,Z-photoisomerization of previtamin D3 (vide infra)103. 

Abstract After a brief introduction and summary of various methods of asymmetric induction in organic photochemistry, the main part of the review covers the solid-state ionic chiral auxiliary approach to asymmetric photochemical synthesis. Application of this technique to the Norrish type II reaction, as well as to the di-n-methane and oxa-di-n-methane photorearrangements, and the cis,trans-photoisomerization of diarylcyclopropane derivatives is presented and discussed. 

Eberbach and coworkers have made an extensive study of the thermal chemistry and photochemistry of ethenyloxiranes (81CB1027 and refs, therein). Thermal isomerization can involve mainly either C—O (Scheme 8) or C—C (Scheme 9) cleavage. The reluctance of (11) to generate an ylide may be due to the fact that allowed, conrotatory opening would give a strained species (12) (cf. OEO-cycloheptene). The photoisomerization of ethenyloxiranes can involve either C—O (Scheme 10) or C—C (Scheme 11) cleavage. Photolysis of (13) led to the synthesis of the ketone tautomer (14) of a phenol (Scheme 12) (80TL463). 

Valence isomer formation is a feature also of the photochemistry of naphthalenes (3.33) and anthracenes for naphthalenes, as for benzenes, the extent ol steric crowding helps to determine which type of valence isomer predominates, since there is more severe interaction in the bicyclohexadiene products than in the benzvalene products. Amongst five-membered heteroaromalic compounds there are many known ring photoisomerizations that involve conversion of a 2-substituled to a 3-substituted system (e.g. 3.34). In some cases non-aromatic products can be isolated, such as bicyclo[2.1.0]pentene analogues from thiophenes 13.35). or acylcycfopropenes from furans (3.36) related species may be... 

Substituted benzenes have been shown to undergo various valence photoisomerizations to yield bicyclo[2.2 Ojhexadienes, benzvalenes, and prismanes.223 The photochemistry of benzene itself is, at present, rather ill defined. Recently, it has been shown that benzene excited by 2537-A irradiation adds to olefins224-226 and to dienes227 to form 1,2-, 1-3-, and 1,4-bridged products. 

In short, photochemical reactions should be classified as photoisomerization, photodissociation, photosubstitution, etc. Definite efforts should be made to use the terms already developed by inorganic and organic chemists to describe transformations involving the particular stoichiometric changes. There already exists enough descriptive names to classify reactions and it would be a pity to allow photochemistry to spawn another, independent family of reaction names. ... 

---