Alkoxyl radicals, homolytic cleavage

Instead of alkyl nitrite, other alkoxyl radical precursors such as ROOH, ROOR, ROI, ROC1, etc. can also be used for the same type of reaction. The high reactivity of these compounds comes from the weak bond dissociation energies in O-O, 0-1, and O-Cl bonds. Another simple method is as follows. Photolytical treatment of alcohol (5) with NIS (AModosuccinimide) provides the tetrahydrofuran skeleton (6), through the formation of alkyl hypoiodite (ROI), homolytic cleavage of the 0-1 bond to form an alkoxyl radical, 1,5-H shift to form a carbon-centered radical, reaction with ROI to form 8-iodoalcohol, and finally ionic cyclization to form a tetrahydrofuran skeleton, together... 

Step 1 is the simple homolytic cleavage of the peroxide molecule to produce two alkoxyl radicals. The oxygen-oxygen bond of peroxides is weak, and such reactions are known to occur readily ... 

Antioxidants also lose their efficiency at elevated temperatures because of homolytic decomposition of hydroperoxides formed by reversing reaction (4) and by producing a chain-carrying peroxyl radical. However, the homolytic decomposition of hydroperoxides producing alkoxyl radicals by reaction (10) is more important than reaction (-4), because it requires less energy to cleave an LO-OH bond (- 44 kcal/mol) than an LOO-H bond (90 kcal/mol). Cleavage by reaction (10) is also facilitated by metal catalysts and by heat (Chapter 4). 

In the field of enzyme catalysis, heme-proteins such as cytochrome P450, for example, exhibit both types of 0-0 bond cleavages in organic hydroperoxides and peroxy acids (178). Heterolytic cleavage of HOOH/ROOH yields H20 or the corresponding alcohol, ROH and a ferryl-oxo intermediate (Scheme 4). Homolytic 0-0 bond cleavage results in the formation of a hydroxyl (HO ) or an alkoxyl (RO ) radical and an iron-bound hydroxyl radical. 

When a molecule is irradiated, particularly with ultraviolet light, some bonds within the molecule can absorb this energy and undergo homolyt-ic cleavage to generate free radicals. These reactions include the formation of alkoxyl (RO ) radicals from alkyl hypoiodites (RO) or nitrites (RONO) or the generation of bromine atoms from bromine or A-bro-mosuccinimide. 

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