Nitronates general reactivity

Rate constants for spin trapping of alkyl radicals measured by the procedures outlined here, are collected with other spin-trapping rate data in Table 5. It will be seen that most nitrone and nitroso traps scavenge reactive radicals of diverse types with rate constants generally in the range 10s-10 1 mol-1 s l. Of the nitroso-compounds, the nitroso-aromatics (except for the very crowded TBN) are particularly reactive, whilst MBN and DMPO are the most reactive nitrones. Much of the data for spin trapping by nitrones has been accumulated by Janzen and his colleagues, who have discussed in a short review how steric and electronic factors influence these reactions (Janzen et ai, 1978). 

Nitrones, reactive 1,3-dipoles, react with alkenes and alkynes to form isoxazolidines and isoxazolines, respectively. With monosubstituted olefinic dipolarophiles, 5-substituted isoxazolidines are generally formed predominantly however, with olefins bearing strongly electron-withdrawing groups, 4-substituted derivatives may also be formed.631... 

Atom abstraction from the cr-carbon atom in spin adducts, which leads to nitrones, and thence in some instances to spin adducts of these nitrones (e.g. Scheme 9), is undoubtedly a general phenomenon. For this sequence to be observed it is necessary that some, or all, of the following conditions are fulfilled. (0 The nitrone is more reactive towards radical addition than is the original spin trap and/or the latter is substantially consumed in the reaction. (h) The initial spin adduct is particularly susceptible to hydrogen-atom loss (and probably reacts this way by disproportionation). ( 7) The secondary spin adduct is particularly persistent. 

Compounds of structures (14)-(16) are in general less studied than all other classes of (oxa/thia)-2-azoles, either because of difficulty in their preparation or their thermal instability. Some of them are stable, but others are nonisolable reactive intermediates <75S205). The dioxazolidine (137) is reversibly dissociated to the nitrone and pivalaldehyde <80JCR(S)122> (Equation (2)) the dioxazolidine (138) is more stable, but decomposes on heating at 300°C to yield the pivalaldehyde (Bu CH=0) and the benzimidazolone (139) (Equation (3)) <86JOC732). 

Cyclic chiral nitrones generally offer better stereoselectivity than their acyclic counterparts. A more efficient shielding of one of the nitrone faces is often obtained due to the more rigid conformation of the cyclic nitrones. Furthermore, in this approach, ( /Z)-interconversion is avoided and cyclic nitrones are often more reactive since they, depending on the substitution pattern, are usually locked in the... 

Nitrile oxides are generally more reactive than nitrones in 1,3-dipolar cycloadditions. Nitrile oxides are most often generated in situ by dehydrochlorination of hydroximoyl chlorides. Similar to nitrones, the oxygen preferentially adds to the more substituted ethylenic carbon. 

The ways of getting around this problem involve increasing the lifetime of the radicals by some physical or chemical means. One such approach involves stabilizing the radicals by immobilization, for example, by freeze-quenching a reaction mixture [80]. The disadvantage of this method is that an immobilized radical is generally much harder to characterize and identify than one in fluid solution. Other approaches make use of the chemical reactivity of radicals, for example, their ability to add to the double bonds in nitrones and nitroso compounds. This has led to the development of the spin-trapping procedure [81,82], in which a transient radical is reacted with the... 

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