Iminium salts, photochemistry

The allylsilane-iminium salt photochemistry has been found useful for the development of novel spirocyclization methodologies (equation 69)124,125. 

Electron-poor olefins with higher oxidation potentials may decrease the rate of electron transfer and other processes competing for deactivation of the iminium salt excited states may increase. Alternate reaction pathways involving olefin-arene 2 + 2 cycloaddition may take place in the photochemistry of 133 with electron-poor olefins (equation 62)120,121. 

From the foregoing discussion, it is clear that DPM rearrangements are very general for a variety of 1,4-unsaturated systems, such as, 1,4-dienes, (3,7-unsaturated aldehydes and ketones, and different 1-aza-1,4-diene derivatives. Surprisingly, the literature was devoid of studies describing the photoreactivity of the closely related 2-aza-1,4-diene derivatives. For many years, the only studies in this area were carried out by Mariano and his co-workers [60] on the photochemistry of iminium salts derived from 2-aza-1,4-dienes. The results obtained demonstrated the synthetic utility of the photocyclizations of iminium salts to different heterocycles, in reactions that are initiated by intramolecular single electron transfer [60]. 

P. S. Mariano, in W. M. Horspool (ed.), Synthetic Organic Photochemistry, Plenum (1984. This is a comprehensive account of the photochemical reactions of iminium salts and related species. 

Imides. Although the photochemistry of imides is quite diverse (203), there exists solid reason to believe that electron transfer can control the reactivity of some imide/olefin pairs. For example, addition reactions analogous to those discussed above for iminium salts can also be observed for imides, either in an intermolecular sense, eq. 67 (204),... 

Mariano [46] has investigated arene-iminium salt PET photochemistry where the photoexdted electron accepting iminium salt has been used to oxidize toluene... 

Addition Reactions.—As the years go by, the importance of electron transfer processes is becoming increasingly apparent, and hardly a month passes without the reinterpretation of a reaction as involving such a process. This has stimulated the publication of review articles such as that by Mattes and Farid on the electron transfer reactions of alkenes, and the more specific reviews by Mariano on the application of electron transfer photochemistry to iminium salts. In this area Mariano and his co-workers have reported further on the electron-transfer-initiated photochemistry of iminium salts (1), and in detail on the spiro-cyclization methodology of iminium salt cyclization. ... 

Numerous reviews have appeared which are relevant to aromatic photochemistry and are also of general interest and application these include accounts of the chemistry of excited-state complexes by Davidson, the photochemical electron-transfer reactions of ethylenes and related compounds by Mattes and Farid, photosolvolyses and photoreactions involving carbo-cations by Cristol and Bindel, conformational flexibility and photochemistry by Wagner, asymmetric photochemistry in solution by Rau, and the photochemistry of iminium salts and related heteroaromatic systems by Mariano." The photochemistry of fragrance materials has been considered by Shibamoto and Mihara, and in their second article they deal with aromatic compounds and phototoxicity. ... 

Photochemistry of heterocyclic iminium salts 83ACR130, 83T3845. Photochemistry of heterocyclic N-oxides 84CRV43. 

Borg, R.M., Heuckenroth, R.O., Lan, A.J.Y., Quillen, S.L., and Mariano, P.S., Arene-iminium salt electron transfer photochemistry. Mechanistically interesting photoaddition processes, /. Am. Chem. Soc., 109, 2728,1987. 

Mariano, P.S., Stavinoha, J., and Bay, E., Photochemistry of iminium salt. Electron transfer mechanism for singlet quenching and photoaddition of n-donating ethers and alcohols. Tetrahedron, 37, 3385,1981. 

Somekawa, K., Haddaway, K., Mariano, R.S., and TosseU, J.A., Electron-transfer photochemistry of allene-iminium salt systems. Rrobes of aUene cation radical structure by theoretical and chemical techniques, 7. Am. Chem. Soc., 106, 3060, 1984. 

The SET photochemistry of a wide variety of electron acceptors has been explored since the early recognition that SET pathways play prominent roles in excited-state reactions of organic substances. Included in this group are cyanoarenes, arylketones, and iminium and related N-heteroaromatic salts. Owing to their large positive excited-state reduction potentials (E (-) ca. 1.6-2.1 V), phthahmides participate as electron acceptors in photoinduced SET processes with donors having ground-state oxidation potentials lower than ca. 2 V. 

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