Photolysis of nitrite

Photochemical transformations of conjugated cyclohexenones, 317 Photochemical transformations of non-conjugatea ketones, 292 Photochemistry of cyclobutanones, 293 Photolysis of nitrites, 253... 

There is quite some evidence for a mechanism as formulated above,especially for the six-membered transition state—the Barton reaction is observed only with starting materials of appropriate structure and geometry, while the photolysis of nitrite esters in general seldom leads to useful products formed by fragmentation, disproportionation or unselective intermolecular hydrogen abstraction. 

Alkoxy radicals are also the active hydrogen-abstracting species in a procedure that involves photolysis of nitrite esters. This reaction was originally developed as a method for functionalization of methyl groups in steroids.379... 

The photolysis of nitrite esters has been found to be of considerable synthetic utility, particularly in functionalizing steroid methyl groups. The most spectacular application of this reaction has been a three-step synthesis of aldosterone.<59) Irradiation of corticosterone-11-nitrite (30) followed by nitric acid treatment gave aldosterone (32) in a 15% overall yield ... 

For the substitution of the angular methyl groups in steroids five methods are known (a) homolysis of N-chloramines [Loffier-Freytag reaction (only C-18)] (b) oxidation of alcohols with lead tetraacetate (c) photolysis of nitrite esters (d) homolysis of hypochlorites (e) the hypoiodite reaction. ... 

The photolysis of nitrites which do not contain g-hydrogen atoms usually results in the elimination of nitric oxide and the formation of hydroxy and carbonyl compounds. When g-hydrogen atoms are present than the product is an oxime or the corresponding nitroso dimer. This reaction is known as Barton S reaction. 

As expected based on our knowledge of gas-phase chemistry, in addition to the Fenton type chemistry involving iron, photolysis of Os, H202, HONO, and HNO-, are all potential OH sources in clouds and fogs. In addition, the photolysis of nitrite, nitrate, and HOJ in aqueous solutions can also form OH. In short, there are many potential sources of OH in clouds and fogs. 

An unambiguous choice between the two mechanisms is not yet possible. A point in favor of a Michael type of addition is that irradiation of nitrites which generate an intermediate radical of the type 57 gave exclusively, and in high yield, products formed by the collapse of 57 products from 58 have not been detected. Results of the photolysis of nitrites of type 59 might be instructive. 

II. Functionalization of the Angular Methyl Groups / 237 General principles of substitution / 237 Lead tetraacetate oxidations / 240 Hypohalite reactions / 246 Photolysis of nitrites (Barton reaction) / 253 A-Chloroamine reactions (Hoffmann-Lofller reaction) / 257 Ketone irradiations / 260 Summary / 264... 

H. Suginome, Remote functionalization hy alkoxyl radicals generated hy the photolysis of nitrite esters The Barton reaction and related reactions of nitrite esters in CR. Handbook of Organic Photochemistry and Photobiology, 2nd Edition,... 

Fischer and Warneck [175] investigated the photolysis of nitrite over the wavelength range of 280 and 390 nm and determined that the quantum yield for hydroxyl radical formation 0oh was decreased with increasing wavelength from 0.069 at 280 nm to 0.022 at 390 nm, in agreement with previous... 

Transformation of Phenol upon Photolysis of Nitrite and Nitrate. 230... 

The irradiation of dimethylamine and nitrite was shown to yield the well-known carcinogenic species nitroso dimethylamine, indicating that carcinogenic compounds may form upon photolysis of nitrite in natural waters [141]. Probably nitrosating agents for amines in irradiated nitrite solution are NO2 and N2O3 [142],... 

The mechanistic situation is complicated by the competing photolysis of nitrite, which results in the formation of nitrogen monoxide and oxide radical anions (Eq. 6-9). The latter are immediately protonated in water at pH <12 to yield hydroxide and hydroxyl radicals according to Eq. 6-8. 

Other applications of nitrite photolysis include the functionalisation of the i4a-methyl group of the lanosterol skeleton by irradiation of the nitrite of a, y a-alcohol [20], attack on C(i9> by irradiation of 5a-halo-6ji mtrito derivatives [2X,22] or nitrites of 2/ alcohols [23], intramolecular addition of a 20 alkoxy radical to a Ai -double bond [24], and some reactions involving radical-induced fragmentation of adjacent C-C bonds [23]. A comprehensive review by Nussbaum and Robinson [26] surveys the wide variety of reaction paths open to alkoxy radicals generated by photolysis of nitrites. 

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