Spin trapping reagents

Spin-Trapping Reagents - These can scavenge short-iived radicais to produce free radical nitroxides of ionger lifetime. The short-lived radical can thus be prevented from interfering kinetically (as in this case) or be characterised by the epr of the nitroxide. 

The trapped nitric oxide becomes a stable nitroxide radical that is the same chemical moiety present in most common spin-trapping reagents. Nitric oxide production from activated macrophages has been directly assayed by this method (Korth et al., 1992). 

Short HO radical lifetimes in the dark prevent the collection of ambient air samples for later analysis, except perhaps for the case of filter collection techniques that use a spin-trapping reagent (28). Furthermore, rapid reactivity of HO with surfaces mandates careful attention to the sampling train used to move ambient air into a more amenable analysis environment. 

The addition of free radicals to nitrones and nitroso compounds gives very stable nitroxyls. In fact, nitrones are commonly used as spin trapping reagents for the study of free radicals by electron paramagnetic resonance (EPR), the electronic analog of NMR. Free-radical intermediates in reactions have too fleeting a lifetime to be examined directly, but their nitroxyl derivatives are long-lived and can be studied by EPR. 

Several studies have been done to establish the chain-carrying species in the oxidation of alcohol (29). The absence of peroxide in the photooxidation of 2-propanol suggests either that the chain carrier is a hydroperoxy radical and not the 2-hydroxy-2-propylperoxy radical or that 2-hydroperoxy-2-propanol is too unstable to accumulate, if formed under these conditions. Only the hydroperoxy radical could be trapped by the spin-trapping reagent phenyl-N-f-butylnitrone in the oxidation of 2-propanol at 25°C (28). The reported high rate constant (30) for the decomposition of the 2-hydroxy-2-propylperoxy radical to acetone and hydroperoxy radical also implies that the hydroperoxy radical is the predominant chain carrier. On the other hand, Schenck and his co-workers (31) isolated 2-hydroperoxy-2-propanol in the photooxidation of 2-propanol with 313-nm light. 

The nature of the intermediate species formed on photolysis of mixtures of M(CO)g and CCI4, (a useful initiating system for olefin metathesis and acetylene polymerization have been further investigated using spin-trapping reagents. 

An intermediate that cannot be observed spectroscopically might be trapped by added reagents. In Chapter 5 we discussed the use of spin trap reagents to capture transient radicals for analysis by EPR spectrometry. Another example can be seen in the studies that led to the bromonium ion mechanism for the addition of bromine to frans-2-butene (14) to produce meso-2,3-dibromobutane (15, equation 6.7). Adding a nucleophile such as methanol to the reaction mixture led to a product incorporating the nucleophile (16) as shown in equation 6.8, which suggested that a cation may be an intermediate in the reaction. Additional evidence is necessary to determine the structure of the intermediate (i.e., bromonium ion or bromine-substituted carbocation), but that is a matter of detail once the existence of an intermediate of some kind is established. 

A convenient synthesis of the spin-trap reagent, t-butyl nitroxide, has been reported a cautionary note on the use of such traps has been sounded, nitroxyl radicals being produced by reaction of the reagents with anions. 

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