Photochemical arylation

Several types of nitrogen substituents occur in known dye stmetures. The most useful are the acid-substituted alkyl N-substituents such as sulfopropyl, which provide desirable solubiUty and adsorption characteristics for practical cyanine and merocyanine sensitizers. Patents in this area are numerous. Other types of substituents include N-aryl groups, heterocycHc substituents, and complexes of dye bases with metal ions (iridium, platinum, zinc, copper, nickel). Heteroatom substituents directly bonded to nitrogen (N—O, N—NR2, N—OR) provide photochemically reactive dyes. 

Photochemical [2 + 2] cycloaddition of benzonitrile and of 1- and 2-naphthonitriles to electron-rich alkenes such as 2,3-dimethyIbut-2-ene gives the corresponding 2-aryl-l-azetines in poor yield 72JA5929,76CC729, 77JOC4238). This does not appear to be a versatile route to 1-azetines. 

Benzo[b]furan, 2-aryl-2,3-dihydro- H NMR, 4, 570 Benzo[b]furan, 2-bromo-nitration, 4, 604 Benzo[b]furan, 5-cinnamoyl-properties, 4, 708 Benzo[b]furan, 2-cyano-photochemical reactions, 4, 636 Benzo[b]furan, 2,3-dialkyl-synthesis, 4, 710 Benzo[b]furan, 2,3-dihydro-aromaticity... 

A second general reaction that proceeds by an SrnI mechanistic pattern involves aryl halides. Aryl halides undergo substitution by eertain nueleophiles by a ehain mechanism of the SrnI class.Many of the reactions are initiated photochemically, and most have been conducted in liquid ammonia solution. 

Depending on structure, photolysis of films of arenediazonium fluoroborates and hexafluorophosphates at room temperature gives aryl fluorides m 10-75% yield [32] In situ photochemical decomposition of arenediazonium fluoroborates... 

Another way to prepare fluorinated sulfides is the photochemical alkylation of sulfides or disulfides by perfluoroalkyl iodides [69, 70, 71] (equations 62-64). Reaction of trifluoromethyl bromide with alkyl or aryl disulfides in the presence of a sulfur dioxide radical anion precursor, such as sodium hydroxymethanesulfi-nate, affords trifluoromethyl sulfides [72] (equation 65). 

The photocyclization of N-aryl enamines derived from cyclic or acyclic ketones proceeds under mild conditions to produce 2,3-dihydroindole derivatives (178b). The stereochemistry of the products is predominantly trans, which follows from a photochemical electrocyclic process which should take place in a conrotatory manner (178c,I78d). However, the presence of some cis products is not as easily explained. 

The 1,2,3,4-thiatriazoles are unstable. They decompose on heating— in some cases even at room temperature—and in many cases they melt with detonation. Accordingly the Ng-group has not been stabilized much by ring closure. The compounds behave in this respect similarly to azides and this fact doubtlessly delayed the recognition of their true nature. On heating with a solvent the thermal decomposition of 5-aryl-1,2,3,4-thiatriazoles proceeds according to Eq. (4). By the photochemical decomposition small amounts of the isothiocyanate, RNCS, are formed in addition to the nitrile. ... 

Interest in the mechanism and product distribution of thermal and photochemical transformations of aryl azides led to the isolation of some nitrogen-containing derivatives of heptafulvalene. Based on elemental analysis and spectroscopic data it has been suggested tentatively that the compound isolated following vapor-phase pyrolysis of azidopentafluoro-... 

An interesting observation (Becker and Israel, 1979) is that the photochemical de-diazoniation gives the same product ratio as the thermal reaction in a given solvent. Therefore both types of reaction probably proceed via the same intermediate, i. e., the aryl cation (see Sec. 10.13). 

Samsonova and Nikiforov, 1984), and porphyrin and phthalocyanine metal complexes (Becker et al., 1985a, 1986b Becker and Grossmann, 1990) were tested. That a series of relatively simple anions such as the oxalate monoanion, tetraphenyl bor-anate (Ph4B ), bromide, chloride, and even tetrafluoroborate can act as donors is, at least for the last mentioned anion, surprising, but Becker et al. (1985 b) were able to trap aryl radicals and in some cases also donor radicals (Cl, COO ) by spin trapping with nitrosodurene and phenyl-tert-butylnitrone. The photochemical effect is postulated to be due to ion pairs ArNJ X-. 

Photochemical arylations of ethene derivatives by arenediazonium salts, i.e., photo-Meerwein reactions, were carried out by Becker and Israel (1975), but were not studied or applied later. 

Intramolecular arylations can also be carried out photochemically. Huisgen and Zahler (1963 a) prepared fluoren-9-one by irradiation of benzophenone-diazonium salts in dilute sulfuric acid. The classical Pschorr synthesis of phenanthrene-9-carb-... 

Partial photochemical decomposition of racemic alkyl aryl sulphoxides in the presence of chiral amines as sensitizers gave non-decomposed sulphoxides in optically active form with optical purity of about 3%339. The report340 on the use of cholesteric liquid crystalline reaction media to change the enantiomeric composition of racemic sulphoxides at high temperatures could not be reproduced341. 

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