Spin-trapping

We continue our discussion of the identification of radical intermediates in electrolytic reactions by describing the technique of spin trapping, which is particularly valuable when the radicals are not sufficiently long-lived to be observable directly. Spin trapping, which was introduced by Janzen and Blackburn in makes use of a diamagnetic compound (the spin trap) 

To illustrate the possibilities of spin trapping in electrochemical ESR, let us consider the oxidation of various organoborides, BR4, in acetonitrile as studied by Blount and coworkers. Direct oxidation of BBU4 under in situ conditions revealed no ESR signals. However, in the presence of the spin trap phenyl N-tert-butylnitrone (PBN), signals were recorded, and these were attributed to the adduct of PBN with butyl radicals  

Conversely, tetraphenylboride, BPh4, gave no spectrum, and although biphenyl was observed among the reaction products, it was concluded that this could not be formed via the intermediacy of free phenyl radicals. 

Various applications of spin trapping in both electrode oxidations and reductions have been reported. The widely used spin traps fall into two classes of compounds, the nitrones and the nitroso compounds. The former are typified by PBN, which has the following structure  

The advantage of nitrones is that they show a wide potential range over which they are inactive electrochemically. For PBN in acetonitrile at platinum electrodes, this stretches from +1.4 V versus SCE to -2.4 V. (These limits are obviously solvent and electrode dependent thus, for example, PBN is inert cathodically in water at platinum only to -1.9 V.) The potential range may be readily extended in either direction by substitution of the phenyl group. The less attractive feature of nitrones is that the trapped radical is rather distant from the location of the spin in the spin adduct. Thus, spectral parameters do not change much as the structure of the trapped species varies, and as a consequence, unambiguous radical identification is difficult without independent synthesis of the presumed adduct. 

Many of these free radical intermediates have been detected directly with ESR. Others are too reactive to detect directly, but a method to stabilize these free radicals called spin trapping has proved successful. Spin trapping is a technique in which a short-lived reactive free radical (R ) combines with a diamagnetic molecule ( spin trap ) to form a more stable free radical ( radical adduct ) which can be detected by electron spin resonance  

Spin traps come in basically two types nitroso compounds and nitrone compounds. Reactive free radicals react with the carbon of the nitrone functional group to form a radical adduct that always has a nitroxide group, which is an unusually stable type of free radical. Nitrones are the most useful spin traps for the in vivo detection of free radical metabolites because of the stability of the resulting radical adduct. However, identification of the parent radicals can be difficult because adducts derived from different radicals often have very similar EPR spectra. A comprehensive review of this area through 1992 has recently been published [48]. 

Reactive free radicals also react with the nitrogen of nitroso groups, forming a nitroxide one atom closer to the trapped radical than is the case with nitrone spin traps. This results in ESR spectra containing more chemical structural information. While nitroso spin traps provide radical identification, the resultant adducts are often less stable than those derived from nitrone traps. In particular, nitroso traps are unreliable for oxygen-centered radicals even in vitro. 

Chlorpromazine [2-chloro-A-(3-dimethylaminopropyl)phenothiazine] is a frequently prescribed antipsychotic drug which causes both phototoxic and photoallergic reactions [49,50], UV irradiation of chlorpromazine (C1PZ) in solution is known to produce the chlorpromazine cation radical [51], as well as other photoproducts, which may go through radical intermediates [52], Under UV irradiation, the aryl radical resulting from dechlorination of chlorpromazine has been trapped using 2-methyl-2-nitrosopropane (MNP), 

A spin-trapping investigation of chlorpromazine sulfoxide (CPZSO), a chlorpromazine metabolite formed in man and several other mammalian species, showed that when irradiated with near-UV light, the excited sulfoxide does not dechlorinate. Instead it produces large amounts of the highly reactive hydroxyl radical ( OH) as well as the cation radical (CPZ +) [54], The following mechanism is proposed  

The techniques and applications described in this chapter so far are restricted by the lifetime of the radical being investigated. If the radical has a particularly short lifetime, it may not be possible to observe it by direct in-situ methods. Instead, the technique of spin trapping may have to be used. The technique of spin trapping was first introduced by Janzen and Blackburn [104,105] short-lived free radicals react with a diamagnetic compound, e.g. a nitrone, to produce a relatively stable paramagnetic species. Where the spin trap is a nitrone, the corresponding nitroxide is found 

From the ESR parameters of the spin adduct, the structure of the original radical may be deduced. 

The use of spin trapping has been reported for both electrode reductions and oxidations [107-110], A good example of the technique using an in-situ method is the work of Volke and co-workers [110] on the electro-oxidation of substituted 1,4-dihydropyridines 

It was previously shown that oxidation produced a radical cation which could be detected by ESR [111, 112] provided the 4 position was fully substituted. If the 4 position contained a hydrogen atom, then the initial oxidation was followed by deprotonation with the formation of a neutral radical which could undergo further oxidation or dimerisation. In-situ spin trapping using PBN demonstrated the presence of radical intermediates in the electro-oxidation of substituted 1,4-dihydropyridines. That the trapped radicals were the deprotonated neutral radical, and not the primary radical cation, was demonstrated by comparison of the ESR parameters with those of the spin adduct produced by electroreduction of A-methylpyridine ion cations, where only the neutral dihydropyridyl radical would be produced. The work of Volke and co-workers clearly demonstrates how spin trapping may be applied to the study of more complex organic electrode reactions and how comparison of ESR spectra generated from different precursors may be used to reveal the nature of the trapped radical. 

The nitroso compounds do not suffer from this disadvantage the nature of the trapped radical is readily identified as it is close to the unpaired spin 

The first observations of the additions of transient radicals to nitroso compounds (equation 60) and nitrones (equation 61) to form stable nitroxyl radicals, which could be conveniently detected by ESR for the identification of the transient radicals, were reported in the mid-1960s. This technique has been extensively applied in chemical and biological systems.  

The name spin trapping was coined by Janzen, and derives from analogy with the use of stable nitroxyls as spin labels (or spin probes ) that provide spectroscopic information regarding their microscopic environment, a procedure pioneered by McConnell et alP  

Chemically induced dynamic nuclear polarization (CIDNP) was first reported in 1967 in independent work from three different laboratories. The effects of free radicals on NMR spectra were revealed (Fig. 6) in studies of radicals from peroxides (equation 62) and azo compounds, as well as radicals generated from the reaction of alkyl halides and organolithium compounds.  

The generation of silyl radicals, species which have proven to be of great value in both mechanistic and synthetic studies, was reported independently in 1947 by Sommer and Whitmore (equation 63), and by Barry et al. at Dovvf Chemical. These are useful for many purposes, including halide reductions.  

Tin radicals were generated by Noltes et al. in 1956, and their applications in organic chemistry were pioneered by Kuivila and co-workers. These species have proven to be of tremendous utility in organic chemistry, although their toxicity and other unfavorable properties have led to a search for substitutes. Interestingly, in the initial publication it was proposed that the reaction (equation 64) did not involve a free radical mechanism, as no inhibition by hydroquinone was detected. 

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The thiazolyl radical has never been directly observed. Torsell (284), however, could indirectly prove its existence by the method of spin trapping in the photolysis of 2-iodothiazole ... 

The nature of the intermediates impHcated in the photooxidation of water with Ti02 has been identified in several reports using spin traps by the electron spin resonance (esr) technique under ambient conditions (53). No evidence for OH species, even at 4.2 K, was found (43), but the esr signal... 

Bis(isoxazoline) decomposes to a number of products depending on the substitution (77H(6)1599) On photolysis using NiS04, two major products were formed. ESR spin trapping demonstrated the intermediacy of imino and 2-isoxazolinyl radicals (77TL4619). 

ESR studies on the initial free radicals were carried out by using MNP(2-methyl-2-nitrosopropane) or DMPO (5,5-dimethylpyrroline N-oxide) as the spin-trapping agent. The reactions are shown as ... 

Spin trapping (e.s.r) and C -labeling techniques were used to study the structure of the alkyloxy radicals produced and to show that these radicals isomerize to... 

It has been suggested that the amine radical cation (46) is not directly involved in initiating chains and that most polymerization is initiated by benzoyloxy radicals.179 However, Sato et a ." employed spin trapping (3.5.2.1) to demonstrate that the anilinomethyl radical (45) was formed from the radical cation (46) by loss of a proton and proposed that the radical 45 also initiates polymerization. Overall efficiencies for initiation by amine-peroxide redox... 

Various oligomers formed by Diels-Alder/ene reactions are observed.333 334 For S-MA11 polymerization Sato et ci//31 used spin trapping to identify the initialing species. On the other hand, in the case of S-AN copolymerization, Ihe... 

In spin trapping, radicals are trapped by reaction with a diamagnetic molecule to give a radical product.476 This feature (i.e. that the free spin is retained in the trapped product) distinguishes it from the other trapping methods. The technique involves EPR detection of the relatively stable radicals which result front the trapping of the more transient radicals. No product isolation or separation is required. The use of the technique in studies of polymerization is covered in reviews by Kamachi477 and Yamada ft a/.478... 

The two most commonly employed spin traps are 2-methyl-2-nitrosopropane... 

Chalfont ei a m were the first to apply the spin trapping technique in the study of radical polymerization. They studied radicals produced during S polymerization initiated by r-butoxy radicals with 108 as the radical trap. Since... 

There are several limitations on the use of the spin trapping technique when quantitative results are required. These are ... 

The application of RPR in the detection and quantification of species formed by spin-trapping the products of radical-monomer reactions is described in Section 3.5.2.1, The application of time-resolved F.PR spectroscopy to study intermolecular radical-alkene reactions in solution is mentioned in Section 3.5.1. 

Many nitrones and nitroso-compounds have been exploited as spin traps in elucidating radical reaction mechanisms by EPR spectroscopy (Section 3.5.2.1). The initial adducts are nitroxides which can trap further radicals (Scheme 5.17). 

Morkovnik et al. (1989) found experimentally that the addition of an equimolar amount of 4-morpholino- or 4-dimethylaminoaniline to a suspension of nitrosyl perchlorate in 100 % acetic acid, dioxan, or acetonitrile yields a mixture of the diazonium perchlorate and the perchlorate salt of the amine radical cation, with liberation of gaseous nitric oxide. Analogous results in benzene, including evidence for radicals by ESR spectroscopy and by spin trapping experiments, were obtained by Reszka et al. (1990). 

Samsonova and Nikiforov, 1984), and porphyrin and phthalocyanine metal complexes (Becker et al., 1985a, 1986b Becker and Grossmann, 1990) were tested. That a series of relatively simple anions such as the oxalate monoanion, tetraphenyl bor-anate (Ph4B ), bromide, chloride, and even tetrafluoroborate can act as donors is, at least for the last mentioned anion, surprising, but Becker et al. (1985 b) were able to trap aryl radicals and in some cases also donor radicals (Cl, COO ) by spin trapping with nitrosodurene and phenyl-tert-butylnitrone. The photochemical effect is postulated to be due to ion pairs ArNJ X-. 

In related work, the reactions of hydrogen peroxide with iron(II) complexes, including Feu(edta), were examined.3 Some experiments were carried out with added 5.5"-dimethyl-1-pyrroline-N-oxide (DMPO) as a trapping reagent fa so-called spin trap) for HO. These experiments were done to learn whether HO was truly as free as it is when generated photochemically. The hydroxyl radical adduct was indeed detected. but for some (not all) iron complexes evidence was obtained for an additional oxidizing intermediate, presumably an oxo-iron complex. 

Spin trap, 102 Statistical kinetics, 76 Steady-state approximation, 77-82 Stiff differential equations, 114 Stoichiometric equations, 12 Stopped-flow method, 253-255 Substrate titration, 140 Success fraction approach, 79 Swain-Scott equation, 230-231... 

The reaction of OH radicals with dimethyl sulfoxide in aqueous solution was studied already in 1964 by Norman and coworkers37 38. They used the system T1m-H202 to produce OH radicals and using ESR/rapid mixing techniques they were able to demonstrate elimination of a methyl radical during the OH induced oxidation. Further studies showed the formation of sulfmic radicals in this reaction either directly or by spin trapping experiments39-44. 

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