Hypohalites photolysis

The primary product of hypohalite photolysis is a 1,5-halohydrin, the stability of which increases on passing from iodo- to bromo- to chloro-hydrins. If spontaneous elimination of hydrogen halide does not occur, the halohydrins can be converted into tetrahydrofurans by base treatment. In several instances it has been possible to isolate the intermediate... 

Since this early work, studies from the late 1950s until the 1960s by Walling, Green, Kochi and their collaborators using t-alkyl hypohalites disclosed some fundamental characteristics of the principal radical chain reactions, such as intermolecular hydrogen abstraction and P-scission of intermediate alkoxyl radicals in hypohalite photolysis. This work was summarized in a review article by Kochi."... 

In most cases the carbon radical formed in the hydrogen abstraction step 2 will react with the radical R formed in the homolysis of the X—R bond. However, a cage reaction does not seem to be involved in this step. This has been established in the nitrite photolysis and probably applies to hypohalites as well. In the lead tetraacetate reaction, the steps following the oxyradical formation leading to tetrahydrofuran derivatives are less clear. 

Cycloalkoxy radical intermediates are readily generated from a parent alcohol by various methods (e.g., nitrite ester photolysis, hypohalite thermolysis, lead tetraacetate oxidation) (83MI1). Once formed, reactive cycloalkoxy radicals undergo /3-scission to produce a carbonyl compound and a new carbon-centered radical. 

Until recently, in the chemistry of hypochlorites, there has been particular emphasis on the chemistry of -butyl hypochlorite. Hypochlorites in general were ignored. However, in their classical paper on the photolysis of organic nitrites,48 Barton et al. adumbrated a photochemical rearrangement of hypohalites of the type (A) —> (B) (X = any halogen). A year later applications of this principle were described... 

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... 

Alkoxyl radicals can be generated by a variety of methods including peroxide reduction, nitrite ester photolysis, hypohalite thermolysis, and fragmentation of epoxyalkyl radicals (for additional examples of alkoxyl radical generation, see Section 4.2.S.2). Hypohalites are excellent halogen atom donors to carbon-centered radicals, and a recent example of this type of cyclization from the work of Kraus is illustrated in Scheme 43.182 Oxidation of the hemiketal (57) presumably forms an intermediate hypoiodite, which spontaneously cyclizes to (58) by an atom transfer mechanism. Unfortunately, the direct application of the Barton method for the generation of alkoxyl radicals fails because the intermediate pyridine-thione carbonates are sensitive to hydrolytic reactions. However, in a very important recent development, Beckwith and Hay have shown that alkoxyl radicals are formed from N-alkoxypyridinethiones.183 Al-... 

The photolysis of alkyl hypohalites, especially alkyl hypoiodites [29], can be used in a manner similar to the organic nitrites for functionalization of an unactivated carbon atom. Scheme 25 outlines a general scheme for remote functionalization by the hypoiodite reaction. Readers requiring more information on this subject are advised to refer to review articles [2c, 16, 20],... 

Alkoxy radicals for ring expansion can be generated from alcohols by oxidative methods such as hypohalite thermolysis/photolysis [19a] and lead tetraacetate oxidation [19b], or peroxide reduction [19c]. The recent development of the hyper-valent organoiodine reagent (diacetoxyiodo)benzene (DIB) provides another way for efficient generation of alkoxy radicals (Scheme 11) [19d]. Additional oxidative methods to prepare cyclopropyloxy radicals include reaction of tertiary cyclopropanols or their silyl ether derivatives with various reagents such as manganese(III) tris(pyridine-2-carboxylate) [Mn(pic)3] [20a], Fe(III) salts [20b], and vanadyl ace-tylacetate [20c] (Scheme 12). 

The photolysis of a tertiary hypohalite (readily prepared from the corresponding alcohol) generates alkoxy radical, which abstracts hydrogen from 5-carbon to produce an alkyl radical. The resulting alkyl radical abstracts halogen atom from a second molecule of the hypohalite to give 5-haloalcohol, which is cyclized in presence of a base to form a tetrahydrofuran derivative [9]. These reactions are very similar to the Barton reaction and proceed through a six-membered cyclic transition... 

Homolysis is also initiated by azoisobutyronitrile or by thermolysis. This characteristic of alkyl hypohalites differs from those of the corresponding nitrites in solution homolysis of the ON bond in solution can only be achieved by photolysis. 

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