Nitroxide radicals from photolysis

Under photo-stimulation, isoindolyloxyl radical (5) abstracts primary, secondary, or tertiary hydrogens from unactivated hydrocarbons including cyclohexane, isobutane, or n-butane (Scheme l).23 The nitroxide (5) traps the resultant carbon-centred radical (R ) and so afford the A -aI koxyisoindo les (6). Blank photolysis experiments with no added hydrocarbon have shown some unprecedented / -fragmentation of (5) to afford the nitrone (7). A number of C60 nitroxide derivatives have been synthesized and characterized by ESR spectroscopy which show features common to nitroxide radicals.24 Reaction of nitroxide and thionitroxide radicals with thiyl radicals have been observed, from which sulfinyl, sulfonyl, and sulfonyloxy radicals were generated.25 The diisopropyl nitroxide radical was generated in the reaction of lithium diisopropylamide with a-fluoroacetate esters.26... 

Kavun and Buchachenko (247) have found that the "primary" amino radicals derived from thioamines in the presence of hydroperoxide are very stable and are not readily converted into "secondary" nitroxide radicals. Kommandeur and Wiersma (107) studied the photodecomposition of tetraphenylhydrazine in rigid solution at 77°K, and the observed ESR spectrum was assigned to the dimer of the diphenyl amino radical (248). Shida et al. (249) have also studied the photodimer of tetraphenylhydrazine and the photochromic dimer of triphenylimidazolyl at low temperature by optical and ESR techniques. These authors noted that photolysis of these compounds induces the homolysis while y-irradiation leads to heterolytic dissociation. Blinder et al. (250) reported the ESR spectrum of the monomer tetraphenylpyrryl radical in... 

Further evidence for Type I cleavage has been obtained by the use of diamagnetic radical scavengers as spin traps for radicals produced on photolysis of benzoin and benzoin methyl ether (18). In both < ses PhCO and PhCHOR (R = H, Me) radicals were trapped and characterised from the e.s.r. spectra of the stable nitroxide radicals formed e.g. 

Photolysis of benzoin and benzoin methyl ether in the presence ctf the phenoxynitrone gave rise to e.sj. signals from both phenoxy and nitroxide radicals. Hydrogen abstraction by benzoin and benzoin ethers would yield radicals of the type... 

In a laser flash-photolysis study, 2-phenyladamantene was generated in benzene at room temperature from 3-noradamantyl(phenyl)diazomethane. This strained cycloalkene decays with second-order kinetics to give a dimer, and reacts much faster with O2 and Bu3SnH than with methanol, thus revealing a substantial radical character. Diphenyldiazomethanes possessing stable /er -butylaminoxyl and Ullman s nitronyl nitroxide radicals, e.g. (25), have been prepared by photolysis of the parent diazomethanes. Analysis of ESR fine structures showed that the carbene and radical centres couple ferromagnetically in these molecules, as expected. 

The tert-butoxy radical is liberated by photolysis of XXXVIa (Scheme 9). Its formation was confirmed by reactions which produce more stable radicals detectable by the ESR method135. The heptane solution of XXXVIa was irradiated by light 360—480 nm in the presence of 2,6-di-tert-butyl-4-methylphenol or 2,4,6-tri-tert-butylphenol. These sterically hindered phenols were transformed by the liberated tert-butoxyl into phenoxyls. Their identification by the ESR method is the indirect evidence of tert-BuO formation. The direct evidence was achieved by means of nitro-sobenzene CVI (R=H) and nitrosodurene CVI (R=Me) as spin-traps. The nitroxide radical CVII (R=H or Me) is formed in both cases and was identified by ESR spectrum135. This spin-adduct is unstable. The intensity of its spectrum gradually decreases during irradiation and the spectrum of another radical appears which is less intense but invariable in time. Judging from its hyperfine structure, it may be formulated as CVIII (R=H or Me). More detailed data on the character of substituents RJ-R2 is not yet known. 

There is certainly strong experimental evidence for the existence of radical-solvent complexes. For instance, Russell and co-workers collected experimental evidence for radical-complex formation in studies of the photochlorination of 2,3-dimethylbutane in various solvents. In this work, different products were obtained in aliphatic and aromatic solvents, and this was attributed to formation of a Jl-complex between the Cl atom and the aromatic solvent. Complex formation was confirmed by flash photolysis. Complex formation was also proposed to explain experimental results for the addition of trichloromethane radical to 3-phenylpropene and to 4-phenyl-1-butene and for hydrogen abstraction of the t-butoxy radical from 2,3-dimethylbutane. Furthermore, complexes between nitroxide radicals and a large number of aromatic solvents have been detected. " Evidence for complexes between polymer radicals and solvent molecules was collected by Hatada et al., in an analysis of initiator fragments from the polymerization of MMA-d with AIBN and BPO initiators. They discovered that the ratio of disproportionation to combination depended on the solvent, and interpreted this as evidence for the formation of a polymer radical-solvent complex that suppresses the disproportionation reaction. 

One other aspect of the photolysis of coordinate spin labeled derivatives is of interest. Nitroxides are good free radical scavengers (123). As a result, when methyl-cobalamin is photolyzed in the presence of a nitroxide, the methyl radical generated will react with the free nitroxide and cause disappearance of the ESR spectrum (123). However, once the nitroxide is coordinated it is no longer susceptible to attack by free radicals. Thus the nitroxyl function is quite well protected from approach by other species. 

Experiments designed to utilize spin trapping to monitor free-radical chemistry in the gas phase were first reported by Janzen and Gerlock (1969). In these, radicals generated by photolysis in a stream of carrier gas were passed over solid PBN. The PBN was then dissolved in benzene, and the solution was found to contain spin adducts of radicals present in the gas stream. Photolysis of t-butyl hypochlorite vapour in this way leads to a nitroxide whose spectrum reveals splitting from two chlorine atoms. This proved to be due to butyl nitroxide (Janzen, 1971 Janzen et al., 1970), and recalls the observation of other nitroxides which apparently result from further reaction of the initial spin adducts. 

Laser flash photolysis at wavelengths within the charge-transfer absorption bands of 2,2,6,6-tetramethylpiperidine-./V-oxyl (TEMPO) and carbon tetrachloride yields theoxoam-monium chloride of TEMPO 291 (Xmax = 460 nm) and the trichloromethyl radical in an essentially instantaneous 18 ps) process152. The primary photochemical reaction is an electron transfer from TEMPO to carbon tetrachloride followed by immediate decomposition of the carbon tetrachloride anion radical to chloride and trichloromethyl radical (equation 140). The laser flash photolysis of TEMPO and of other nitroxides in a variety of halogenated solvents have confirmed the generality of these photoreactions152. 

The dimer of (74) is isolated as one of the products. In the flow ESR spectra, nitroxides (76) and (77) can be recognized. These are proposed to arise by H-abstraction from (74) by a cyclobutoxy radical, giving (75), which either reacts with a further molecule of (74), yielding (76), or undergoes, 5-exo ring closure to (77). The steroidal nitrite (78) (Scheme 17) yields 50% of the ketone (79) and 16% of the alcohol (80) when photolysed in the solid state (X > 300 nm), but only 5% of (79) and 52% of the Barton-type product (81) when photolysed in toluene solution. It is usual for ketones to be produced only in low yields from photoreactions of nitrites in solution, and so the promotion of this reaction pathway in solid-state photolysis is of considerable interest. Similar results were obtained for the solid-state photolyses of a number of other steroidal nitrites, but nitrites prepared from acyclic alcohols showed much less selectivity in favour of the corresponding ketones. 

Sulfamethoxazole failed to produce any trappable radicals with an array of different spin traps, but naproxen afforded the EPR spectrum shown in Figure 2.11 when irradiated with 330 nm UV-R in the instrument cavity in the presence of 2-methyl-2-nitroso-propane (MNP). The spectrum contains contributions from di-t-butyl nitroxide, a known photoproduct of MNP. The H-atom adduct MNP-H also evident can arise by several different mechanisms, including the trapping of an H atom by MNP the reaction of MNP with an electron followed by protonation and the direct reduction of MNP by an excited state species. In view of the flash photolysis results, it was concluded that photoionization was the major precursor of MNP-H. The third radical corresponded to a C-centered radical carrying a single H atom, leading to the postulate of a decarboxylation reaction as the primary photochemical step. Confirmation of the participation of free radical intermediates came from the initiation of the free radical polymerization of acrylamide with rates as shown in Table 2.1. 

Many nitrosoalkane dimers in solution are in equihbrium with nitrosoalkane monomers at room temperature. De Boer and collaborators demonstrated that the photolysis of nitrosoalkane dimers in solution results in the formation of the corresponding nitroxides, as proved by ESR spectroscopy (Scheme 49)." The formation involves a combination of alkyl radicals generated from excited nitrosoalkanes with ground-state nitrosoalkanes. 

The chromophore must be attached close to the nitroxide moiety to ensure an efficient intramolecular energy transfer from the chromophore to the NO-C bond which should also undergo homolysis in the macroalkoxyamine. However, direct attachment of the aromatic chromophore to the alkoxyamine nitrogen, as in 44, leads to competitive cleavage of the N-OC bond because the resulting aminyl radical is resonance stabilized (see Section 6.3). On the other hand, alkoxyamine 45 undergoes a selective NO-C photolysis. Linear increase of Mn with conversion was observed during NMPP of -butyl acrylate with 45, as expected for controlled polymerization (Scheme 4.18). ... 

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