Photochemical aromatic

The identical stoichiometries and the color changes that are observed in thermal and photochemical aromatic osmylations point to the ion-radical pair Ar+, OsO T as the seminal intermediate in both activation processes. It is similarly possible that the osmylation of olefinic donors may proceed via the same types of reactive intermediates as delineated for the aromatic osmylation. 

The next three chapters are by Inoue and Mori, Albini, and Rossi, and deal with alkene photoisomerization reactions, the modification of benzylic positions and photochemical aromatic substitution reactions. (E)-2-cyclo-heptenone is produced upon irradiation of the Z-isomer at — 50 °C and can be trapped by cyclopentadiene to afford the adduct 13 [13]. Benzyl-substituted dihydroisoquinolinium derivatives can be used for the photochemical synthesis of tetrahydroisoquinolines. The corresponding... 

Photochemical aromatic substitution initiated by a reductive step as in SRN1 reactions can be used for the synthesis of cephalotaxinone (15). The corresponding iodoketone precursor cyclizes in liquid ammonia under photolysis [15]. 

Methoxy-l-nitronaphthalene (73a) and 1-nitronaphthalene (73b) undergo photochemical aromatic substitution reactions with cyanide (Scheme XXVIII). A two-fold increase in the quantum yield for the reaction is observed for (73a) when the reaction occurs in HDTC1 compared to aqueous solution 73). However, a 6800-fold catalytic increase in quantum yield is observed for (73b). SDS micelles decrease the quantum yield compared to aqueous solutions. The higher local concentration of cyanide near the HDTC1 micelles can explain a least partially the increase in quantum yield. However, the 6800-fold increase for (73b) is also due to a polarity effect on the reaction. This was demonstrated by an increase in the quantum yield of the reaction with decreasing polarity. 

R.L. Danheiser and co-workers generated a key vinylketene intermediate via tandem Wolff rearrangement-ketene-alkyne cycloaddition to utilize it in a photochemical aromatic annulation reaction Danheiser benzannulation) for the total synthesis of the phenalenone diterpene salvilenone. ... 

Photochemical Aromatic Substitution Reactions 6.4.1 Nucleophilic Substitution... 

Fig. 6-7 Rationalization of Photochemical Aromatic Substitution (The brackets identify the less common or unobserved reaction.)...

Photochemical aromatic substitution, in Handbook of Organic Photochemistry and Photobiology, 2nd edn (eds W. Horspool and F. Lend), CRC Press, Boca Raton, pp. 37/1-37/14. 

The use of polyfluoroalkyl substituents in positions 4 and 5 (compound 489), however, enabled a mechanistic pathway to pyrazine 490 substituted at positions 2 and 5, to be suggested (Scheme 92) Individual para-bonded species 491 and 492 have been isolated in this and other cases, and converted into the next component along the reaction pathway by photo or thermal reactions [260]. hi a case of 4,6-disubstituted pyridazine 493 only pyrazine substituted at positions 2 and 6 494 was observed. A very unusnal mechanistic pathway may be drawn from the structures of the isolated and characterised valence isomers (Scheme 92). This appears to be the first case where snbstitnent labelling has allowed each stage in a photochemical aromatic rearrangement to be identified through various intermediate valence isomers. 

Cornelisse, J., Photochemical Aromatic Substitution, in CRC Handbook of Photochemistry and Photobiology, CRC Press, Boca Raton, FL, 1994, 250. 

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