Fission of Weak Bonds

As indicated earlier, heat may be used to form radicals derived from the fission of weak bonds. A very common example of this is the thermal decomposition of metal alkyls. For example, tetraethyl lead, Pb(C2H5)4, readily gives ethyl... 

Carbohydrate-derived radicals are generated by direct electron transfer, hydrogen abstraction or fission of weak bonds. Direct electron transfer from the enediolate of reducing sugars is the basis of most reducing sugar assays. 

The reaction was formerly considered to involve a radical mechanism initiated by the non-ionic fission of the very weak N Br bond. 

For isoxazoles the first step is the fission of the weak N—O bond to give the diradical (51) which is in equilibrium with the vinylnitrene (52). Recyclization now gives the substituted 2//-azirine (53) which via the carbonyl-stabilized nitrile ylide (54) can give the oxazole (55). In some cases the 2H-azirine, which is formed both photochemically and thermally, has been isolated in other cases it is transformed quickly into the oxazole (79AHC(2.5)U7). 

Because of certain misconceptions with regard to the choice of solvent and the occurrence of sulfur-oxygen bond fission in hydroxylic solvents - , it is important to emphasize that one can greatly reduce the rate of this competing process by the use of weak bases. In systems which can undergo facile C—O as well as S—O bond fission, it is possible to control the type of bond cleavage by choosing the appropriate base . A remarkable illustration of this behavior is found in the ethanolysis of sulfinate 6a. In anhydrous ethanol at 90.0° with acetate ion as the added base, 6a yielded ethyl 2, 6-dimethylbenzenesulfinate plus a trace of sulfone 7a. Under the same conditions but with 2,6-lutidine the reaction was slower and sulfone 7a was the only detectable reaction product . ... 

This stems from the weakness, i.e. ease of thermal fission, of the Pb—R bond, and radicals may be generated in solution in inert solvents, as well as in the vapour phase, through such thermolysis of weak enough bonds, e.g. those with a bond dissociation energy of < w 165 kJ (40kcal)mol 1. Such bonds very often involve elements other than carbon, and the major sources of radicals in solution are the thermolysis of suitable peroxides (O+O) and azo compounds (C+N). Relatively vigorous conditions may, however, be necessary if the substrate does not contain substituents capable of stabilising the product radical, or... 

In Chap. 2, the analysis of diffusion-limited reaction rates of Smolu-chowski, Collins and Kimball, and that of Noyes is followed. The considerable literature on reaction rates between solute species is also presented. Additional and important other factors which influence the rate of reaction are a coulomb interaction between reactants, long-range energy or electron transfer and an angular dependence of the rate of reaction. These topics are considered in the Chaps. 3—5. The experimental and theoretical work are compared and contrasted. When the reactants are formed in pairs (by bond fission of a precursor), the rate or probability of recombination can be measured and is of considerable interest. Chapters 6 and 7 discuss the theoretical aspects of the recombination of neutral and ionic radical pairs and also appeal to the extensive literature on the experimentally measured rate of recombination. The weaknesses of this theoretical... 

Isoxazoles, like other five-membered heterocycles, can undergo a variety of transformations under the influence of light, heat, or electron impact.35,36 For isoxazoles, most of the transformations can be rationalized in terms of the pathways shown in Scheme 15. The first step is the fission of the weak N—O bond to give the diradical 73 (or its ionic equivalent). This can then recyclize to give the substituted 2H-azirine 74. Direct evidence for the diradical (73) or its equivalent is so far lacking, but some circumstantial evidence is available. The pyrazines (75) isolated from some reactions could arise by dimerization of the intermediate.183,184 And the ready interchange... 

Dialkyl peroxides decompose at measureable rates in gas phase systems at temperatures greater than about 400 °K. The reactions are always initiated by simple bond fission of the weak peroxide linkage, viz. 

Mass spectrometry is a powerful tool in structure determination of withanolides. The in electron impact mass spectra (EI-MS) is either weak or absent in various types of withanolides. The EI-MS spectra of withanolides are more informative, as compared to those having modified withanolide skeleton. A base peak at miz 125 is formed by fission of the C-20/C-22 bond and is considered to be a diagnostic feature for withanolides with an unsubstituted a,) -unsaturated -lactone moiety [37] (Scheme 114.1). However, the presence of a hydroxyl group at C-20 facilitates the cleavage of the C-17/C-20 bond and gives rise to a peak at mIz 169... 

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