Oxo-acyl radicals

Transitions representative of oxo-acyl radical b are not generally observed in high-temperature TREPR experiments, although they often appear at room temperature and 

Upon expansion of the highest temperature spectrum to a sweep width of 50 G, it can be seen that the signal in Fig. 14.5 is actually a triplet of triplets (Fig. 14.6a). A simulation of this spectrum is shown in Fig. 14.6b. For this simulation, a F hyperhne 

FIGURE 14.6 Experimental (top) and simulated (bottom) TREPR spectra of oxo-acyl radical 6b from laser flash photolysis of PEOMA in the solvent system EC-70 (3M Corporation). Sweep width is 50 G. Simulation parameters are listed in Table 14.1. 

Our values agree reasonably well with the values of 2.6 and 0.1G reported by Kmsic et al. for y- and 8-hyperfine couplings, respectively, in perfluoroalkyl radicals. The hyperfine constants reported here are slightly larger, probably a consequence of having a a-type radical rather than a 7i-type radical as was the case in Krusic s work. 

The spectra in Fig. 14.2 and Figs. 14.4—14.6 show strong net emission from the TM of CIDER It is revealing that the dominant polarization mechanism is the TM 

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SCHEME 14.1 The general stmcture of an acrylic polymer and the estahhshed photodegradation mechanism via Norrish I a-cleavage of the carhonyl side chain, leading to main-chain polymeric radical a and oxo-acyl radical b. The secondary P-scission rearrangement reaction leading to the propagating radical c is also shown. 

All these spectra were acquired at elevated temperatures ( 100°C), that is, where the observation of fast motion spectra is expected. In Fig. 14.4A, the TREPR spectrum of the main-chain polymer radical from photolysis of /-PMMA is repeated from the bottom left side of Fig. 14.2, as it is the starting point for comparisons of spectral features such as hnewidths and hyperfine coupling constants. The nomenclature used throughout this section is derived using the notations indicated in Scheme 14.1 and Chart 14.1. For example, a main-chain radical from PMMA will be denoted la, whereas the oxo-acyl radical from PFOMA will be designated as radical 6b, and so on. For all radicals simulated, the parameters used are listed in Table 14.1. 

The magnitudes of both SCRP and RPM polarization patterns depend on the rate of encounters taking between the two radical centers. Polymeric radical a and oxo-acyl radical b have drastically different diffusional properties in solution. Radical b is small and will undergo much more rapid diffusion than a. An immediate consequence of this is that RPM and SCRP polarization mechanisms may be quenched by relatively slow reencounter rates and will therefore be obscured in this case by the very strong TM. In all TREPR spectra detected here, emissive TM is always observed, which is unusual for aliphatic carbonyl compounds, although not unprecedented for esters. 

A final noteworthy feature of these spectra is the lack of RPM polarization, which for these radicals would appear as low-field emissive, high-field absorptive transitions. It is curious that such polarization never develops at any delay time, even out to 20 xs where we have observed only TM polarization. The creation of RPM polarization requires reencounters of radicals on a suitable timescale and modulation of the exchange interaction between the unpaired electrons. This is normally accomplished by diffusion of the radicals between weak and strong exchange regions. That it never develops indicates that either these radicals do not make a significant number of reencounters, or perhaps it is due to the fast spin relaxation in the oxo-acyl radical. It may also be that the TM is simply so dominant that the RPM intensity is always much weaker and is never observed. At lower temperatures (cf. Fig. 14.1, top spectrum at 25°C), there does appear to be a slight superposition of an EIA pattern on top of the emissive TM polarization, but it has a very small effect. 

The photodegradation of poly(alkylacrylate)s and poly(methacrylate)s under UV irradiation (248 nm) in solution was studied for the first time by TR EPR by Harbron et Well-resolved spectra of oxo-acyl radicals from the ester side chain and of main-chain polymeric alkyl radicals were used to show the side-chain cleavage via the Norrish I process. The methacrylate spectra are strongly influenced by the stereoregularity of different polymer tacticity, the temperature and the solvent. The relations of these dependences on the conformational motion in the polymer chain are discussed. 

When another group having higher priority for citation as principal group is also present, the ketonic oxygen may be expressed by the prefix 0x0-, or one can use the name of the carbonyl-containing radical, as, for example, acyl radicals and oxo-substituted radicals. Examples are... 

A radical process was proposed for this transformation. Firsdy, a radical species could be generated through the reaction of acetophenone with terf-butyloxy radical and I2. Then the radical species 12 reacts with water and is subsequently oxidized to 2-oxo-2-arylacetaldehyde 14. Ddecarbonylation of this intermediate could form the desired acyl radical 16. Finally, the formed acyl radical 17 is further oxidized to give the desired benzamide derivatives in ammonia solution (Scheme 3.5). 

Af-alkyl-Af-phenylacrylamides and aryl aldehydes undergo oxidative cascade coupling reaction to 3-(2-oxo-2-arylethyl)mdolin-2-ones in good yields using CUCI2/TBHP as catalyst and oxidant, respectively. Both the C(sp2)-H of the Af-alkyl-Af-phenylacrylamides and C(sp )-H bonds of the aldehyde were activated in one step under this oxidative system. The acyl radical which is generated from... 

Reduction of the carbethoxy group to the hydroxymethyl group with lithium aluminum hydride at — 35° and Claisen condensation with ethyl acetate are known to take place with pyridazinecar-boxylic acids. 6-Oxo-l,6-dihydro-2-pyridazinyl aliphatic acids, having the pyridazinonyl residue attached at the a-position of the aliphatic radical, readily undergo decarboxylative acylation with acid anhydrides in the presence of pyridine to form the corresponding 2-alkanones (107). 

The silver-catalyzed decarboxylation of a-oxo acids (carboxylic acids " ) by peroxy-disulfate leads to acyl " (alkyl radicals, which can effect selective homolytic acylation (alkylation of quinoxaline. This procedure is effective in monoacylation when multiple positions of high nucleophilic reactivity are available in the heterocyclic ring. " ... 

Oxidation causes the formation of hydroperoxides and conjugated compounds, which by cleavage give aldehydes, alcohols, ketones, lactones, acids, esters, and hydrocarbons. Radical mechanisms lead to the formation of dimers, other oligomers, and oxidized TAG. The latter have one or more acyl group with an extra oxygen (hydroxy, keto, epoxy derivatives). Other oxidation products are TAG with short-chain fatty acyl and n-oxo fatty acyl groups. 

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