Phenacyl radicals

Phenacyl radicals are produced by photodecomposition of initiators containing the phenone moiety (Scheme 3.74). These initiators include benzoin derivatives and acylphosphine oxides (see 3.3.4.1.1). Acyl radicals can be formed by... 

The general chemistry of acyl radicals has been recently reviewed/88 Acyl radicals have nucleophilic character. Absolute rate constants for substituted phenacyl radical addition to BA have been reported to be in the range 1.3-5.5xl05... 

Acyl radicals undergo dccarbonylation. For aliphatic acyl radicals the rate constant for decarbonylation appears to be correlated with the stability of the alkyl radical formed. Values of the dccarbonylation rate constant range from 4 s for CH3C 0) to 1.5x10s s l [lor (CH.,)2C(Ph)C( )0] at 298 °C.3M The loss of carbon monoxide from phenacyl radicals is endothermic and the rate constant is extremely low (ca 10 8 s 1 at 298 nC).388 Consequently, the reaction is not observed during polymerization experiments. 

Phenacyl radicals can be generated from the corresponding xanthates and add in good yield to various substituted propenes. The products of the reaction can then be cyclized to tetralones using an equivalent of a peroxide.313... 

Time-resolved CIDEP and optical emission studies provide further definitive characterization of the triplet and excited singlet states followed by their primary photochemical reactions producing transient radicals in individual mechanistic steps in the photolysis of a-guaiacoxylacetoveratrone. Both fluorescence and phosphorescence are observed and CIDEP measurements confirm the mainly n,n character of the lowest triplet state. The results indicate a photo triplet mechanism involving the formation of the ketyl radical prior to the P-ether cleavage to form phenacyl radicals and phenols. Indirect evidence of excited singlet photo decomposition mechanism is observed in the photolysis at 77 K. 

Figure 4. a, CIDEP spectrum observed during the photolysis of a flowing GAV solution (flow rate at 5 mlVmin). b, The time-resolved polarization profiles monitored at selected magnetic fields 1, ketyl radicals and 2, phenacyl radicals. 

First, the polarization pattern of the 3-line phenacyl radicals appeared very different. Actually the difference is only due to the increased contribution of the E/A Radical-Pair polarization which now increases the relative emissive intensity of the low-field hyperfine line and essentially cancels the high-field line. As mentioned earlier E/A Radical-Pair polarization has no effect on the centre line. The relative Radical-Pair... 

Further revealing information is obtained from individual monitoring of the polarization evolution profiles of the separate components of the CIDEP spectrum. These are given in Figure 4b. The results clearly indicate that the "new" radical is the primary intermediate, formed before the phenacyl radical. Furthermore, the relative relaxation rate of the spin polarization of the phenacyl radical is faster than that of the "new" radical. A final critical experiment is that when very dry acetonitrile was used as solvent, no CIDEP observations could be obtained for any radicals. From all these observations we are now ready to formulate the primary phototriplet reactions mechanism to account for all these facts. 

With the establishment of the primary phototriplet reduction mechanism we now turn to the explanation for the effect of flow rates and the formation of polarized phenoxy radicals. Since reaction [2] is a relatively fast secondary process it is readily understood that the observation of the primary ketyl radicals will be dependent upon flow rate. The triplet polarization (E) of the secondary phenacyl radical should not have been affected but the increased contribution of the E/A Radical-Pair polarization altered the overall appearance of the polarization pattern. The diffusion model of the Radical-Pair theory relates the E/A polarization magnitude to the radical concentration within the reaction zone. Since the phenacyl radical is considered to be very chemically reactive, and the product phenol "accumulated" within the reaction zone is also a much better hydrogen donor, the following reactions will proceed within the reaction zone ... 

Benzoin and its derivatives (9) undergo Norrish type I cleavage on irradiation to afford a phenacyl radical and a a-hydroxy or a-alkoxy radical. The... 

Upon irradiation, a-(aryloxy)acetophenones cleave at the C-phenoxy bond generating phenoxy and phenacyl radicals [143,151], as shown in Figure 3.20 for a-(4-methoxyphenoxy)-4-methoxyacetophenone. The phenacyl radical has two absorption bands the most intense band occurs at about 380 nm and a broader, weaker band appears at about 540 nm [127]. The 540 mn band is attributed to the canonical form that has the unpaired electron on the carbonyl oxygen. 

FIGURE 3.20 C-phenoxy cleavage of a-(aryloxy)acetophenones. The canonical form of the phenacyl radical with the nnpaired electron on oxygen absorbs at about 540 nm. 

SCH EME 16.2 Recombination of phenacyl radical with resonance stabilizede mesomers of the guaiacoxy radical. (Redrawn from Fukagawa. N. and Ishizu, A., J Wood Chem Technol, 11, 263-289, 1991.)... 

The photochemistry of a-phenoxy acetophenones has been studied.Cleavage occurs to generate phenoxy and phenacyl radical pairs which suffer several fates, Including formation of compounds such as (329) which can arise from attack of the phenacyl group upon the aryl ring of the phenoxy radical. 

Reactivity of a Series of Anions Toward the Phenacyl Radical... 

Mass spectral and low-temperature (4K) e.s.r. data suggest that the phenacyl radical is best represented as spirocyclopropanone radical (409). Mass spectral fragmentation patterns have also involved cyclopropanones. ... 

Phenylmethylphenacylsulfonium p-toluenesulfonate (PMPS), an onium salt, is known as a self-destructive electron acceptor which decomposes rapidly into phenylmethylsulfide and a phenacyl rachcal upon accepting an electron (59). When PMPS is added to an aqueous solution of ZnTPP-functionalized unimer micelles, the electron acceptor is localized on the anionic surface of the unimer micelle. The election transfer from the 2HnTPP triplet to PMPS produces a ZnTPP cation radical (ZnTPP ) and a phenacyl radical as transient species (Scheme 1), but reaction between these two transient species, which usually follows the photoinduced forward election transfer in homogeneous solutions, is prevented in the unimer micelle system because the ZnTPP " and phenacyl radical species are separated. Thus, the porphyrin cation radicals can be accumulated in the unimer micelle. 

As discussed above (Section 66.3), phenacyl ester radical anions fragment readily to phenacyl radicals and carboxylate anions. The radical anions of benzoate esters also fragment to carboxylate anions and alkyl radicals, and this process is faster if the alkyl radical is stabilized (3°, benzyhc). This concept has been applied successfully to the deoxygenation of carbohydrate alcohols by photoinduced electron transfer (PET) sensitization. For instance, irradiation of 38 in water/2-propanol, using a substituted... 

In 1995, we began a comprehensive exploration of a variety of p-substituted phenacyl phosphates for their efficacy toward releasing phosphate. Among the substituents examined, the p-acetamido, methyl p-carbamoyl, and -butyl p-carbamoyl groups proved untenable because they gave a plethora of products, most of which resulted from coupling or reduction of an intermediate phenacyl radical [Eq. (69.11)] > ... 

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